On having metal in your body (surgical implants, weather pains, and more)

Posted on December 8, 2014



I’ve found any discussion on surgical hardware sorely lacking. For years I assumed I was looking in the wrong places. At this point, I don’t think a solid discussion is out there. I’m near positive it doesn’t all exist in a comprehensive manner, which I’m hoping to ameliorate. My feeling is what may be obvious to the surgeons is not, at all, obvious to the recipients of surgeries. And, as we’ll see, there are certain things nobody is sure of, including the surgeons. Hopefully we can answer these questions.

When you place a foreign object in your body, there is a lot to discuss. Here is what we’re going to hit, in order. (You can click the section to be taken directly to it. Although, I recommend reading in order.)

This was an extraordinarily hard post to write. I’m not sure anything else I’ve worked on has ended up so mentally expansive. Unlike the majority of my writing, which comes solely from my own head, on this I had to enlist the help of quite a few different people, from quite a few different disciplines. I either talked to people directly, or indirectly used knowledge from areas such as:

  • Biology
    • Exercise Science
    • Physiology
    • Psychology
    • Orthopedics
  • Carpentry
  • Materials science
  • Mechanical engineering
  • Meteorology and Biometerology
  • NASA
  • Physics
    • Thermodynamics
  • Veterinary medicine

I know I have a good amount of readers who are engineers, doctors, researchers, and some just all around smart people. If you feel I’ve missed something, please let me know and I will correct it. I’ve tried to strike a balance between accuracy and explaining things in a way all the everyday people with metal in their body can understand.

As the index above suggests, there is a lot in this beyond just having a foreign object in your body. Going through this reminded me of some particulars about how the body adapts, along with exposing me to things I didn’t know. I never thought I’d be reading documents from NASA about pressurized suits and boiling blood, then finding applications in that for everyday people.

I hope this is clear, that you learn something, and can put the information to use. Please share it if you find it helpful.

When hardware goes awry

“However, one would think stress concentrations must be of significance in orthopedic surgery, especially when the surgeon fits a stiff metal prosthesis to a relatively flexible bone.”

From Structures: Or Why Things Don’t Fall Down

I work with a lot of people whose injury histories are longer than their life story. Obviously, this is usually older people. But I’ve trained one younger guy, 25 when I met him, who I too often think about.

Dan had a traumatic injury where he ended up needing quite a bit of surgical hardware in his body. At one point he had a metal rod and four metal screws in his leg. (He also had reconstructive ACL surgery, skin grafts, a bone graft, and more. Guy basically had a boulder fall on his legs.) By the time he got to me, some of the hardware had been removed, but a good amount remained.

After many months, I had to accept I wasn’t getting anywhere with Dan. His body wasn’t responding, and things just seemed off with him. He had been having a ton of issues exactly where one screw was located. Dan decided to get an X-Ray done on his mom’s, a veterinarian, machine. There was a ton of lysis (bone dissolution) going on right around the screw. Dan’s surgeon kept trying to avoid going back in, but this convinced them otherwise. (Don’t ask why Dan had to perform his own X-Ray.)

Turns out, the screw had broken in half, inside his bone. His bone was in a sense dying, and he had metal floating around within it. No wonder he had so many issues in that area.

He got the screw removed and within a few weeks was doing exponentially better.

Dan is one of those people who I like to say, “You have a body which hates foreign objects.” Some people go decades with all types of materials in them, never having a problem. Google “shrapnel in body” and you’ll come across loads of stories of soldiers going years and years with hundreds of bomb remnants inside them. Meanwhile, Dan had been progressively removing one thing after the other. His bones in particular did not agree with things. (We have to also acknowledge the possibility he had a surgeon who didn’t get things right.)

The hiccup with Dan was knowing the screw was broken. When Dan initially complained, nothing was unusual in his X-Rays. The screw was somehow broken in a way it wasn’t visible on radiographs. (Or someone missed it.) The bone dissolution also wasn’t apparent yet. This is one of the ways hardware really plays with people. You may have issues in the bone(s) but it doesn’t show up radiographically for months. It takes time until you can see the bone decaying on an X-Ray. (This interim period was one in which Dan tried things like mirror therapy. Along with too many painkillers.) [1]

Why hardware, and the material, matters

When it comes to placing screws in the body, we usually go with one of three options:

  • Metal (often steel or titanium)
  • Bioabsorable
  • Plastic

(Biabsorbable starts out similar to plastic. They then dissolve- absorb- within the body over time.)

Rather than detail the pros and cons of these devices, I’m going to borrow the following table from the paper Complications Associated With Use of Anterior Cruciate Ligament Fixation Devices. I will be focusing on the leg and often using reconstructive ACL surgery as a reference. (This is the surgery I’ve had.) The principles extend to the entire body though.

ACL fixation devices screws table

So, if you’re someone who is going to be having a lot of follow up imaging done, someone like Dan, metal may be a problem as it can cause issues. Whereas plastic is friendlier radiologically. Although, if a plastic screw moves around, plastic doesn’t show up on an X-Ray. It can be tough to know if it’s moved. If for some reason you wanted to get it removed, you’re looking at potential exploratory surgery.

Quick tangent, I’m not so sure plastic is completely unobservable on an X-Ray. You may not be able to see the actual screw, but if you look at my X-Rays, you can see where the screw is. Notice the tunneling?

ACL X Rays Left And Right

Look at the tibial plateu on the right leg. (Left side in image.)

ACL X Rays Close Up Right 2

ACL X Rays Close Up Right 1

ACL X Rays Close Up Right 2 with circle

ACL X Rays Close Up Right 1 with lines

On the other hand, if you’re someone who needs a strong material to help hold your leg up, like Dan, what’s better, metal or plastic? What’s stronger? Metal, of course.

This is why we use metal to fixate broken bones. The purpose of the internal fixation is not just to hold the bones in place, it’s to do this and allow a person to move around more than they would when casted up, hopefully speeding up the rehabilitative process. [2] However, in the interim period of the bone healing, you need to insure it’s together. Plastic will not get the job done.

Looking at the table, we can see metal is actually so strong it can damage the graft used in anterior cruciate ligament reconstructions.

Stronger isn’t always better

Think of a rubber band being pinned on a wall, and pulling on the rubber. Now think about whether we use steel or plastic to pin the rubber band. Which material is more likely to tear the rubber band? The steel. If we pull on a plastic pin, the plastic has some give to it. Whereas steel is going to have much less. The steel pin is more likely to tear through the rubber than the plastic.

The steel is almost assuredly sharper too. When you pin a badge on a shirt, you’d rather have metal than plastic. The metal pushes through the material more easily.

Badge with metal pin

An ACL graft isn’t quite fixated like this analogy suggests. Rather than the screw be inserted through the graft, the screw is often inserted more along the graft. It “interferes” with the graft. It’s an “interference screw.” Making things quite snug, so the graft stays put.

ACL Surgery Screw 1

ACL tunnel with screw about to be inserted along graft. (Graft is behind screw in image.)


ACL Surgery Screw with graft

ACL Surgery Screw with graft with lines

The analogy still holds though. If you are screwing damn near into rubber, what is more likely to tear the rubber, steel or plastic? The steel, because it’s so much harder and stronger. When pushing plastic in against the graft, there is going to be a bit more give.

Stressing vs Straining bone

“However, one would think stress concentrations must be of significance in orthopedic surgery, especially when the surgeon fits a stiff metal prosthesis to a relatively flexible bone.”

I bolded flexible bone because it’s important we remember bone does bend. All materials bend, we can’t always see it, but they do.

In something like reconstructive ACL surgery, and in many surgeries, the purpose of the screw is not to perform supportive work for the bones. The purpose of the material is to hold the graft in place until the bones can do it on their own. That’s all. In contrast to a broken bone fixation where the purpose is to help support the bone, the purpose of a screw fixation with a graft is to help the bone hold the graft in place. Eventually, the bone will remodel and secure the graft, but it needs help and time before that happens.

The fact a screw can help hold a graft in place is all well and good, but we’re still leaving a screw in the body. We’ve already discussed how a screw can damage the graft, but what about a screw’s impact on the loading of the bone? If you put too big a screw into a piece of wood, or you screw too violently, the wood cracks. The screw is often fine. The metal is stronger and stiffer than the wood.

Even if the screw doesn’t crack the wood on insertion, if you place too high a load on the wood, the wood will crack. Again, the screw may end up fine. The breaking point for the wood is much lower than the breaking point for the, say, steel. Furthermore, the wood may crack right around the screw.

Notice the vertical cracks above and below the screw.

Notice the vertical cracks above and below the screw.

One way to not break is to bend more. Like a ziploc bag. Rather than tear, it first stretches. The plastic will deform quickly, but it can take a lot more before it fractures. However, when a really stiff object is placed where another object would like to bend, the object can’t deform like it would prefer. Less ability to deform = greater tendency to crack.

This is especially true with bone. In the book Bone in Clinical Orthopedicsthere is a fascinating discussion regarding how bone adapts. We tend to think of bone adapting to stress, the amount of force we place on it. This book makes the argument it’s not stress; it’s strain -the amount of bone deformation. (Something like a ziploc bag can’t take a lot of stress before it deforms, but it can handle quite a bit of strain.)

If we think back to Dan, we gain a better understanding of why he’s had so many issues with hardware. I mentioned he also had a metal rod in him. One aspect of the rod is to help handle the force of his body, instead of his bone, while his bone heals. (It’s also to help the bone stay in place while it handles load. Rather than move around like a fractured bone would.)

Tibia Fibula Screws and Plates

An example. (From tetongravity.com)

However, with such a stiff material, the amount his bone can strain -deform- may be minimized. In Dan, it was. You could see his bone dying because it wasn’t being strained where the rod was. Above the rod and below the rod, his bone had become more dense, but between the rod it was becoming more porous. (Had more holes in it -less dense.)

When he had the rod removed, he immediately laid down a bunch of new bone. It appears without cyclically straining bone, it literally dies. As people die they move less, but the converse is also true. When you move less, parts of you begin to die. [3]

Some pictures

Say we have a piece of wood, and we put a screw in it. Then we place this piece of wood on top of the ground.

Wood 1 better


Next, we place a force on top of the wood, pushing into the ground:

Wood 2 better

The ground is then going to oppose that force:

Wood 3 better

While all this is going on, the screw is going to receive these forces as well:

Wood 4 better

And it too will have an opposing action:

Wood 5 better

Bony defects

Let’s take a step back for a moment. The first important surgical implication of all this is the fact the wood needs to receive this force from the screw. Not just when the wood is loaded, but upon immediate insertion of the screw. The wood needs to make room for the screw.

Wood with outward screw stress

The implication is the wood will widen. You may not be able to eye ball it, but it happens. Here’s an exaggerated version (the yellow box represents where the wood originally was):

Wood expanding

If you’re someone who has a screw in your body, you may have noticed your bones are a little differently shaped than they used to be. Bone expands like the wood.

For me, I swear my distal femur and proximal tibia, the bones making up the knee joint, have expanded. In all likelihood, they have. Nobody else would be able to tell or feel it, it’s not visually obvious, but I can feel it. These areas have been power drilled once by having a tunnel put through them -again, radially stressing the bone- and two by having hardware placed inside them.

Femur and tibia after surgery

The dashed lines represent where the femur and tibia were before being drilled through.

In ACL surgery, you drill tunnels through the bones for the graft to be placed in, then you screw that graft in the tunnel. It’s going to take time for that tunnel to close. In the meantime, through the drilling, we have removed a chunk of bone, yet are still going to be placing a lot, if not the same amount, of stress and strain on the bone. That is, we’ve taken some bone away yet want a person to get walking pretty quickly after surgery. The bone takes a good couple of months to come back, whereas we want someone walking within a couple of weeks or so. (Depending the surgery and other factors.)

So, on the remaining bone, we may actually be placing more stress and strain. Less bone but same amount of forces = greater amount of forces on remaining bone. It’s conceivable the remaining bone, that which hasn’t been drilled, hypertrophies to some extent as well, helping to handle the loads and strains placed on it.

In a couple of different ways, it makes sense why your bones look and feel different after a surgery.

While it can feel and look weird, I don’t think this is anything to worry about. Joints widening in response to traumatic events is pretty common. I’ve dislocated my fingers; in response they’ve widened, a “periosteal reaction,” and it’s never been a problem.

Fingers side by side with outlines

Right side thicker than left.

I’ve actually also dislocated my right elbow, which is now larger than my left, and that also has never been an issue. During my football playing days, these types of things were quite common amongst my teammates and I, and I see it with clients regularly. I think this is a positive adaptation more than anything else.

As someone with a screw in their body, what I’ve been most concerned about is that opposing force of the screw.

Wood 5 better

Is it strong enough to cause the wood (bone) issues? Like a fracture? Again, like this:

Wood cracking by screw

There are two ways to think about this. First, we’re not made of wood. We’re made of bone. Which, as we’ll see, is a great thing.

Strengths of different materials

When it comes to deforming a material, you push or pull on it. When you push you compress; when you pull you tension.

Image credit: http://hendrix2.uoregon.edu/~imamura/102/images/tension-compression.gif

Although, this isn’t always true. Depending on where you push or pull, you may compress and tension at the same time. If we take a beam and push down on it at the center:

Tension Compression Same Time 1

Then we’re actually compressing one end of the beam and tensioning the other at the same time:

Point being, we need to worry about the tensile and compressive strengths of the material(s) we put in our body. This is important because these two strengths are often different. Just because you can tension something a certain amount does not mean you can compress it an equal amount.

If you need to stack things, brick is great. But if you need to prevent tension, brick is terrible.

What about the materials we’re most concerned with? For metal, we’re going to use the most common, steel. For plastic, according to the ACL paper I linked above, PEEK and PET are the most commonly used plastic screws. Finally, our most important material, our bones.

These numbers are not the easiest to find. Amazingly (to me), none of this is addressed in any paper I can find on using materials inside the body. The ACL paper I linked was great…but mentioned none of this. (There may be a decent reason for this, which I’ll get to.) So, I kind of had to hodge podge these numbers.

For PET plastic, according to its Wikipedia page, the tensile strength is 55-75 MPa. (MegaPascals. I’m going to keep all the units in this form.) We’ll call it 65. According to this page, its compressive strength is 80. However, according to plastics international, the numbers are more like 80 and 100 respectively. We’ll go with the higher numbers for now.

For PEEK plastic, the tensile strength is ~95; its compressive strength is ~117, according to plastics international.

For steel, the numbers seem to be~475 tensile; 250 compressive.

For bone, the numbers jump around quite a bit. This is due to the fact the strength of bone can vary between people, different people will have different densities, and other factors such as different bones in different areas of the body are going to have different strengths. Think a leg bone versus a finger bone. Wikipedia lists the tensile and compressive strengths at ~112 and ~170 respectively. From other sources, this seems right in the neighborhood. (Some sources are higher; some are lower.)

So, let’s say:

  • Plastic (using PEEK) = 95 tensile; 117 compressive.
  • Steel =475 tensile; 250 compressive.
  • Bone = 112 tensile; 170 compressive.

Getting the numbers to be perfectly precise is not the point here. The point is:

Plastic < Bone < Steel

Another visualization

Take a really thick book, like a textbook. Open the book at the halfway point. Place a really strong material inside the pages. Like a metal screw. Now close the book.

Steel in book 1

Because the strength of the material is so much stronger than the strength of the book’s materials, the book deforms. Even if you place some force on top of the book, that steel is going to push back significantly:

Steel in book 2

Take a weaker material though. Like a pen.

Pen in book 1

Pen in book 2

The book doesn’t deform nearly as much. Metal on left; plastic on right:

Pen Scre book side by sidePen Scre book side by side with lines

Depending on the material, the book may not deform at all. It’s all relative with this.

Steel > Bone > Plastic

With steel, we’re placing a really, really strong material inside the book (bone). With plastic, our bones aren’t going to deform as much. The plastic is what will deform, as our bones are much stronger than a textbook.

Entering volume into the equation

I mentioned there were two ways to look at this. One, what we just went over, is the strengths of the materials. The second aspect is the volumes of materials. When we’re screwing a steel screw into our bone, our bones are bringing a lot more soldiers to the fight. In ACL reconstruction, we’re inserting a relatively small screw into a relatively large tibial plateau.

Tibial interference screw x ray

Tibial interference screw x ray with text

Tibial interference screw x ray with shading

So, even though the steel screw’s material is so much stronger than our bones, our bones can make up for things by having a lot more of itself.

Thinking back to Dan though, the opposite occurred. So much metal was brought to the fight that his bone was deforming more than it usually would in certain areas, and less than it normally would in other areas. It’s tough to have hard rules here. The type of surgery, amount of metal, material, orientation of fixation, bone density, all come into play.

I’ve talked to a decent amount of people about this. An orthopedic surgeon, veterinarian surgeon, engineers, people with some carpentry / woodworking experience, in all cases nobody thinks something like the ACL interference screw is going to present a bone loading issue. The size of the screw is a big factor, but so is the incredible ability of our bones to adapt.

In a pig, the radius and ulna are both weight bearing bones. There are documented accounts where if the ulna is removed, the radius will accommodate. All the way to the point where the strain on the osteotomized (bone removed) side will not be different than the opposite side. To be clear, one bone remodels enough to take the place of two bones!

In humans, there are circumstances, such as in bone grafts, where the fibula is removed, causing the tibia to have to increase its load and strain bearing.

tibia fibula basic

In a study showing the awe-inspiring ability of the body to adapt and survive, the fibulas were removed from kids who needed a humeral reconstruction due to cancer. That is, take a fibula from the leg and place it in the arm to make a new humerus. Look at what happened to their tibias in response:

From the paper: Tibia Adaptation after Fibula Harvesting: An in Vivo Quantitative Study.

From the paper: Tibia Adaptation after Fibula Harvesting: An in Vivo Quantitative Study.

Fibula removed x rays 2

Notice the tibia on the left (right side of body) progressively increase in size in (B) and (C).

Fibula removed cross section

In these cases, the cross-sectional area for the side of the tibia without a fibula, ended up matching exactly that of the side of the tibia and fibula. That is, the tibia on one side became, on its own, the size of the tibia and fibula combined on the other side. The adaptations were that specific.

We’re talking removing an entire bone here. If bone can adapt to this, I doubt a little screw, or most implants, present a problem in this regard. Insuring the bone is able to be strained, but still held in place, like a broken bone fixation, seems to be much more the concern. You want the fracture to have a stable healing environment, but if the bone isn’t strained, we have issues. Too little strain is more a concern than too much.

Reconstructive ACL simulation

I actually took the time to get some bones, power drill a tunnel, screw some metal into the bone, then hammer it. (If you want to cause some discomfort, ask the guy at Home Depot how strong of a power drill you should buy to drill through bone.) I’m not sure how clear this will be, but I can tell you doing this gave me a better appreciation for how unlikely a screw is to be a concern in this regard. [4]

That bone in the video is not going to be as resilient as live bone because, well, it’s dead. I weakened the bone around the screw placement pretty significantly due how I drilled. (Had some trouble getting the right size.) And, that bone hasn’t been given anytime to remodel accordingly. Because the surrounding bone is going to be loaded a little more -if nothing else there is a little less bone now, due to the screw taking up that room- the bone is going to remodel accordingly. It’s going to get a bit stronger.

Through a myriad of perspectives, this doesn’t seem like something worth worrying about.

That doesn’t mean you’re in the clear (screw migration)

We have to keep in mind here, everything I’ve gone over is considering ideal circumstances. Going back to our book analogy, what if we place the screw not in the middle, but much closer to the top of the book?

Image credit: www.orthopaedics.com.sg

We don’t have nearly as much material to offset the strength of the screw. Despite too little strain being more a concern than too much, that picture above makes me very nervous.

We already do this with many screw placements due to necessity, such as where a bone is broken or where you’re fixating a graft. (You wouldn’t fix an ACL graft at the mid-shin.) Also, say your surgeon is having an off day and accidentally screws too high or too low. We start getting to where the level of concern may grow.

Another thing to consider is screw migration. A screw moving around on someone is a common enough occurrence. The screw may start off where there is enough bone to offset it, but it may migrate to where a stress riser occurs. That is, the stress from the screw moves to a point where there’s not enough bone to counteract.

Why will a screw move?

Notice anything funky about the screws in this X-Ray?



One is bent:

TibiaPlateScrew with circle

TibiaPlateScrew with line

This is one of the biggest issues with internal fixations: When you compound the strength of bone, with its ability to remodel, on top of having something like a 200 pound person walking, on top of thousands of cycles, even in something like steel, the material can start to deform and bend. It may even break.

This is what happened to Dan. Over time, the material can only handle so much. It literally will fatigue. Especially when you account for the fact bone can remodel -break a bit apart then get put back together- where as metal cannot.

If you google some X-Rays of screws and plates (internal fixations), you’ll quickly find quite a few examples of a screw looking not straight. This is fairly common.

A screw may move because of this, or it may end up moving due to the nature of the surgery. You know how screws have threading?

screw threading

There’s no guarantee a person’s bone is going to catch that threading. In something like reconstructive ACL surgery, the screw is going to be placed not only against bone, but against tendon (the graft).

The other aspect of a screw is it’s conical-like shape.

Interference screw close up

Interference screw close up with outline

Because of this, not all of the screw is even going to be touching bone.

Let’s look at the tibia from the side. We’ll have our tibial tunnel, the graft (green), and our interference screw:

ACL tibia tunnel simulation

Look at the distal aspect of the screw:

ACL tibia tunnel simulation with distal screw lines

Due to the anatomy of a screw, when we place it inside the tunnel, some of it isn’t going to be touching anything.

In both circumstances, the screw touching tendon and the screw touching nothing, it’s tough for bone to catch the screw completely. So, what can happen then is, as the bone remodels, it can push the screw around. Some of the bone may remodel faster than the other, some may touch one part of the screw sooner, etc.

And this is again, assuming perfect screw placement. In reality, a surgeon isn’t always going to get things as snugly as they want. If you have a slightly loose screw that you start placing an entire human bodyweight on, it’s going to move around. What happens to a slightly loose screw that gets banged around on? It can become an even more loose screw.

This is where a metal screw’s strength can be an advantage. The threading of a metal screw is going to be more likely to nicely slice into bone, causing a stronger fixation. (However, notice in the picture how it can lacerate the graft!) With plastic, the bone is going to be better able to push the threading away, due to plastic’s lesser strength.

This is where the timelines during the rehabilitative process is crucial. The body needs time to remodel in accordance with the graft. You simply cannot rush this aspect. Doing so only increases your odds of ripping the graft, causing the screw to migrate, or imprudently loading your bones (risk of fracture), as they haven’t had time to adapt to their new loading pattern.

Regardless of the material or how carefully things are done though, this is where things simply suck. Whether the screw moves around or not is in some sense a crap shoot. And knowing whether the screw has moved, as well as finding the screw, is often quite hard. Remember Dan?

There are a few things I think one can be on the lookout for, to know whether the screw is moving or causing problems.

  • For someone like Dan, laying down his symptoms were fine. But as soon as he placed a load on his bones, he had issues. If you’re someone where immediately upon standing you’re having problems, that’s a decent sign something is up with the hardware. (We’re talking a significant amount of time post-op though. This isn’t a day after surgery.) Opposed to if you’re someone who only has issues during certain activities. Where in that case it may be something else going on.
  • If the symptoms are quite localized, that’s another good sign it’s the hardware causing the problem.
  • If you’re able to feel where the screw is, has it moved? Is it changing positions?

Unfortunately, beyond an arthroscope, not a whole lot can be known for sure here. Radiology can’t pick up plastic, and even with metal there’s no guarantee you can see it. Despite Dan having metal and X-Rays, nobody knew the screw was broken until they went inside him. And his surgeon had to dig through his bone to find the screw’s remnants.

Not to mention, just because a piece of hardware has moved doesn’t mean you need to get it removed. Reminder: See all those soldiers with shrapnel in their body for years.

Only you can know if a piece of hardware is causing you enough issues that the costs of another surgery outweigh the issues you’re having.



If and why the weather influences things like metal implants or achy joints turns out to be quite a hard question to answer.

Part of the issue with this is how variable and individualistic pain can be. Just think about cold weather in general. So many people have so many different responses to it. Some love it, some hate it, some will move to other states to avoid it, some vacation in it. Psychologically, it’s impossible to say, “People have this response.”

I don’t like using the psychological explanation as a sufficient answer though. One could easily say “So and so has issues with metal implants and cold weather due to some previous experience, existing beliefs, blah blah.” One of the great aspects of pain science is its recognition pain is comprised of various elements. The psychological aspect here, I believe, is just one aspect to take into account.

We cannot ignore the body does have some common responses to cold weather, low pressure, and the fact materials like metal have different thermal properties than the body. Understanding these responses can, I think, give us a framework for how to respond should an issue come up.

Some primers on thermal effects

When things gain or lose heat, they respectively expand or contract. Metal, plastic, etc. It can get bigger or smaller in response to the temperature. We call this “thermal expansion.”

The body isn’t as clear cut, but it does provide some typical responses. One of those is, when it’s cold, a shuttling of blood from the limbs to the core. When you get cold, you never think “Man, my stomach is freezing!” But you often think “My hands / feet / toes / ears are killing me!” This is because as it gets colder, different parts of the skin are different temperatures.

The body constricts itself distally for the tradeoff of keeping things like the vital organs warmer. If you have to lose something, it’s better survival wise to lose a hand than it is a liver. Thermal expansion is still a factor, but the body is just more clever about it.

Have you ever wondered why metal seems to always feel cold? For instance, you take a thermos out of a refrigerator compared to a different material. Both materials have been in the same fridge, why does one feel so much colder?

This is where the body is a little tricky. When you touch metal what you are feeling isn’t actually the temperature, because the metal and every other material in that fridge are the same temperature. (Assuming they’ve all been in there a while.) What’s different is how quickly the material takes the heat from your grabbing hand. Your skin in sensing a loss of heat from your body, not how cold something actually is. We call this “thermal conductivity,” or the ability to grab heat, and metal is quite good at it.

Some good TV has come from this phenomenon:

This is why when you get out of a hot shower you may suddenly feel cold as hell, despite your house not being cold. Were you cold before the shower? No. But when you get out of a hot shower, your body has some more heat to give away. You suddenly feel cold because your body is sensing a huge sense of heat loss. Your body is literally giving heat to the room which has lesser heat. (It’s not colder, it’s technically less heated.) Things tend to even out like this.

Now let’s think about someone with a big metal rod in their leg.

Steel rod tibia fixation

Notice the direction the metal is placed in relative to the leg. It’s not on top, it’s not below, it’s coming at the body from the side. The “transverse plane.” This happens to be the direction our bones most dislike. It’s the direction our bones are weakest in. [5]

If we get colder, we know we’re likely to start constricting the limb. At the same time, our body is generating heat in an attempt to get warm / maintain current temperature. Due to this, our body is going to be particularly sensitive to something attempting to take that heat.

The body is constricting itself against a material which is grabbing heat from us, a material which is potentially expanding due to increased heat, a material which is much stronger than our constricting skin. Furthermore, this material may be expanding against a brittle material, our bones, in the direction our bones most dislike, especially considering that specific area of bone may be weakened (due to trauma or the fixation). All in an environment where our skin / body / brain is extra sensitive to heat loss, as well as an environment where most people tend to not feel great period. (Cold, damp, overcast, etc.)

This is why I think it’s been so hard to give people an explanation for “Why does my surgically repaired leg ache when it’s cold out?” It’s likely not just one factor. But a confluence of them.

“But what about the size of the metal relative to the size of our leg? Shouldn’t our leg, being bigger than the metal rod, be able to offset this?” (Generate enough heat.)

This is where the properties of metal come even more into play. Because our body has so much water in it, it’s actually hard to heat it up. When it comes to everyday materials, water is the hardest to heat up. It has a great “heat capacity.” That is, it can withstand a ton of heat. (This is one reason it’s so hard for humans to overheat. We’re made of so much water. We do great in the heat; we don’t do nearly as well in cold weather.) Despite this, metal is so dense, about eight times that of most human tissue, it can make up for it’s relatively poor heat capacity by being so damn heavy. [6]

Meaning a relatively small metal rod in a relatively large leg, can still hold a ton of heat. Meaning it can take a lot of heat from our body. Meaning when you go outside, you may feel that metal for a while, because it can take heat from your body for a while.

This is where the location of the metal matters as well. If some of the metal is only covered by the skin, or it’s just not very deep in the body, then you have relatively thin skin being pushed against relatively thick metal. The metal is going to be able to take heat for quite a bit of time against that thin skin.

Just like a splinter feels odd / uncomfortable / painful under the skin, so too can metal. Particularly when the weather is certain conditions.

Next time it’s a bit cold out, just touch something made of metal. Like a pole. You will immediately feel how much colder the steel is relative to your body -how quickly it’s taking your heat. Just think if you then pressed that metal against your body perpetually. It wouldn’t feel great.

Now, say that metal is in your forearm. Or somewhere pretty far from your core. You then couple this all with the fact the body is constricting things distally. Meaning the body is being pulled against the steel / titanium / metal even more, and more than usual. It makes sense for people’s pain description in these scenarios to be customarily “aching” or “I can feel the cold metal under my skin.” The metal is constantly pushing outward on the contracting limb. If someone were to constantly push on any part of your body, it would begin to ache after a while.

Contrast this with warmer temperatures. The body isn’t constricting against the material like in the cold; it’s, if anything, expanding.

This is a perk of plastic. It’s more malleable than metal, doesn’t conduct heat to the same degree, and isn’t going to present the same level of issues with the weather. The plastic is going to be better able to go with the flow, sort of speak.

Bonus sections:

The meat of this post is about surgical hardware being inserted in the body. And because the weather is so often a remark from those with hardware in them, it was fitting to spend some time on it. Even without hardware in the body though, it’s customary for people to say the weather influences their joints, or makes them aware of an old injury. (An old injury which may have accompanied hardware at some point.) So, I think it’s fitting to continue some more on weather here.

Low pressure

The research on weather and joint pain is a bit all over the place. One thing that seems to have some consistency though is when it comes to arthritis, it’s not the temperature that’s a problem, it’s the barometric pressure. Anecdotally, metal is more an issue in colder weather; generalized joint pain is more an issue in lower pressure, and maybe colder weather.

Pressure in this instance is a metric of how much the earth’s air is pushing against our body. When the air doesn’t push as much, our bodies push against the air more.

This is one reason astronauts wear space suits. In space, there’s no atmospheric pressure against the body. The “pressurized” suit artificially generates this pressure. Without, the body would expand significantly, although NASA tells me it wouldn’t explode. Our skin is strong enough to hold things together. For NASA, the bodily expansion isn’t the biggest factor. Our blood boiling due to such low pressure is the more pressing concern.

Same idea when we get in airplanes. A cabin is “pressurized” because the further away from the earth’s center you get, the less air above you there is to push on you. This is why some of you may notice your feet swell when you fly. Less pressure on you = more pressure in your body = things swell.

Simple experiment to show this:

The best theory I’ve seen out there is when the pressure is lower, our joints expand. When you take an already inflamed / pissed off joint, like an arthritic one, and you have it expand more, some pain may result.

There is an important subtlety here though. It’s not that the joint expands like I’ve seen others state. It’s more the fluids (gas and liquid) within the joint expands. The joint is made of all types of things. Bone, fluids, ligaments, etc. If general expansion were an issue, we’d expect people to also have issues in the heat -things expand in heat too- which you basically never hear of. In the heat though, fluid can actually move more easily. Similar to magma from a volcano. As it cools, it doesn’t move as easily as when it’s on fire.

What I believe happens here is any expansion of a joint in the heat, and the problems that could cause, are offset by the easier flow of fluid through the joint.

Plus, when it comes to our environment, higher temperatures tend to go with higher pressures. So, if we increase the temperature -potential joint expansion- we usually increase the pressure -joint is constricted- offsetting one another.

In the cooler seasons, air molecules don’t move as quickly. If you remember back to elementary science, the Earth has different atmospheres, changing as you get further from the surface. The lowest level, the one humans primarily hang out in, adjusts its height based on the time of year. In the summer, it’s lower (closer to the Earth’s surface); in the winter, it’s higher.

When it’s lower, the pressure on us is higher, meaning the density of the atmosphere is higher (same amount of molecules in lesser amount of space), meaning the temperature is higher. When the atmosphere is higher, the pressure on us is lower, the density of the atmosphere is lower, the temperature of the atmosphere is lower.

This is where the distinction between weather and climate comes in. Much like a cold day here and there, or even a cold winter here and there (weather), doesn’t mean the earth isn’t warming on a longer scale (climate), just because the pressure is higher in the summer doesn’t mean there won’t be some days where it’s lower.

As a general rule though, lower pressure tends to accompany lower temperatures, which is the combination you most often hear influencing how people’s joints feel. However, there may be the occasional time where it’s decently warm out and you still have some issues. (The pressure dropped.) I live in San Diego and see this all the time. It’s 75 out, but suddenly someone doesn’t feel great. Or the occasional time where the temperature hasn’t changed much, but there was a good drop in pressure.

So, when someone asks you about the weather affecting how you feel, your memory goes immediately to “cold weather” because in your mind that’s much more often a negative influence on you. But it’s not so much “cold” weather as it is lower pressure. Particularly sudden drops, which can occur during any season, and in various geographies. That said, when it’s cold and the pressure is low, that’s probably when things feel the worst, so we think about that most, as how bad things get is a big part of our memories. [7]

Much like people handle temperature differently, people handle pressure differently. Some athletes respond very well to altitude training (increased red blood cell production), whereas others don’t get much out of it.

When the pressure lowers, some are going to have a negative effect where others are not. Some may boost blood circulation helping to remove the excess joint fluid, while others may not. Some are going to already have a higher starting pressure within their joint or body, like high blood pressure or excess fluid, where others won’t. Some are going to have a more sensitive nervous system. Some are going to hate cold weather which makes everything feel worse. You get the idea.

High humidity

There is a recent review on weather influencing osteoarthritis pain. It goes through how, essentially, none of the research on this topic has been satisfactory. The best study is amazingly 50 years old, but has had follow up studies which conflict its results. Part of this can be explained by the fact no other study was as rigorous. This 50 year old study was the only one strictly climate controlled. So, despite its age and other issues, what did this study find?

That it wasn’t one variable, such as low pressure, which increased people’s pain, but it was the mixture of low pressure and high humidity.

I still want to point out though, air pressure seems to be more of the consistent factor. However, I think this makes sense and fits in with the reasoning I’ve been going through.

Humidity is a measure of how much water vapor is in the air. Or how wet the air is.

When we sweat, we’re trying to get hot water out of our body and into the air. If there’s not much water vapor already in the air, this works great. The air has room to receive our heat / vapor. If there’s already a lot of water vapor in the air -high humidity- this system starts to falter.

This is why you feel like you’re sweating so much more when it’s humid. It’s not so much that you’re really sweating a ton more, it’s that you’re sitting in your own sweat more. Rather than go in the air, the moisture stays on you.

And this isn’t only a hot weather thing. Evaporation occurs even in cooler temperatures. It’s not as much, and it’s not as visual, but it’s happening.

Bringing our lower pressure scenario back into the mix: In low pressure we have potential to have increased fluid expansion. If we combine this with a scenario in which we have decreased ability to get rid of fluids, high humidity, we have an even better likelihood for something like too much fluid in our joints. That is, some discomfort.


Practical applications for surgical implants, cold weather, arthritis, low pressure, and more

Picking surgical hardware

If it hasn’t been made clear yet, plastic is a sure fire better choice for ACL reconstructions. The lesser chance of lacerating the graft, the fact plastic is more malleable, doesn’t conduct heat like metal (no issues with weather), are big wins for this surgery. If your surgeon plans on using metal, unless he has an extremely persuasive argument, you may have a good sign to go with another surgeon.

My assumption here is most surgeries involving fixating tendon are best with plastic. I believe the reason some surgeons go with metal is because metal is easier to screw than plastic. It drills more easily and is less likely to strip.

I didn’t go into this, but bioabsorbables look to be one of those great on paper, not so great in application ideas. Quite a few problems can arise.

For things like internal bone fixations, metal is the clear winner. The type of metal depends on the strength needed. Titanium is great as it has some more flexibility. It stresses and strains more similarly to bone than does steel. But, it’s not always strong enough.

Giving the hardware the best chance for success

Knowing the healing timelines of your various surgery are a huge help. Using ACL surgery as an example, the fact bone is being drilled through and needs to adapt tells one to go easy on it for 6-8 weeks, as that’s when bone heals.

Furthermore, depending on the graft, sometimes the bone has to heal to a tendon, rather than bone to bone. In the ACL world, it’s customary to add a month on top of the 6-8 weeks to insure the graft is secure. Where again, you’re careful how much you load the bones, along with how much range of motion you put the graft through.

Regardless of the procedure, there’s no point in pushing these types of things. Even if you have a broken bone put back together by steel, the bone itself is not back together. Biology needs time, and by rushing things all you do is increase your odds of complications.

In Bone in Clinical Orthopedicsa study is referenced stating upon inserting a screw, the ability of bone to store energy was reduced by 70%. Now, eight weeks later, the bone remodeled and this was alleviated. [8]

I referenced the incredible study showing how the tibia can adapt to be the size of the tibia and fibula, if the fibula is removed. In that study though, some of these adaptations were taking upwards of 3.5 years! The study was done on kids where part of this time involved chemotherapy, and we’re talking removing an entire bone, but it was also done on kids who have a faster ability to adapt. Point being sometimes adapting takes a long, long time. Give the body the time it needs.

At the same time, remember, in order to remodel, the bone does need to be strained. The “I’ll just do nothing for a few months” approach is just as bad as the “I’m going to come back faster than anyone else” approach. Strike a balance.

Screw removal

If you’re someone like Dan, where you’re having such intense issues due to hardware, the resolution is clear. Get the hardware removed. Although, even in Dan, this wasn’t so cut and dry. One reason his surgeons were apprehensive about removal was they wanted to give the bone as much time as possible to remodel. Removing the hardware meant a potential increased chance of the bone not staying together.

In other cases, you have to weigh how much the hardware is affecting things against going through another surgery. There are routine surgeries for surgeons. There are no routine surgeries for patients.

I referenced shrapnel at one point. The reason so many soldiers have tens, if not hundreds, of pieces of metal floating around in their body is not that a surgeon can’t remove a great deal of it. It’s the costs of doing that type of surgery don’t outweigh the effects of the shrapnel. If you merely have some discomfort every now and then, it’s probably best to just deal with it.

If you do get hardware removed, you now have to give your body the same level of respect to adapt as you hopefully did when the hardware was inserted. The body went through a bunch to adapt to how things were stressed and strained with hardware in it. It has to do that all over again without the hardware.

I referenced a study where a screw changed the energy storage capacity of bone by 70%, but the bone adapted after a couple of months. When removing this same screw, the bone’s ability to store energy was again reduced by 70%. 

Athletes really need to take note here. If you’re getting some hardware removed due to some discomfort, you have to acknowledge you are going to significantly alter your offseason due to needing to give your body two months to adapt. For many, that’s the majority of the offseason.

Metal implants and cold weather

Quick review: The things we’re looking to counteract here are 1) A constriction of the distal aspects of the body 2) The sensitivity of the skin in losing heat to the metal.

What we can do: Get ourselves warmer. Whether it’s extra layers or moving to get more blood flowing, the warmer we feel the less likely our body is going to be constricting the limbs. Thereby hopefully lessening how much the metal is being pressed against. (Again, assuming the metal is touching the skin. Some metal implants are completely encased in bone.) Also, the warmer our brain thinks we are, the less annoyed it will be to losing heat. In fact, you can get yourself to the point where it welcomes it.

Next, perhaps we can put an extra layer on the specific site of the implant. Something like an ACE bandage wrapped around the implant site. This way we give that specific area of skin some extra help in not losing heat.

Low pressure / high humidity and achey joints

Quick review: We’re looking to combat the expansion of the joints due to excess fluid accumulation.

What we can do: Compress the joint / site of achiness. This way we artificially increase the pressure on the joint. This is what a spacesuit does. Here is how I like to wrap the knee:

In terms of how tight to wrap, think of it like a blood pressure cuff. The tighter it is, the more pressure on the area. The more fluid you have or achy you are, the tighter you may want to wrap. Where you can then lessen the tautness after you get some relief.

You can also compress things more globally. Like leggings or tights. It’s common for people to feel achey when they first wake up. So, wearing something like this to bed could work.

Next, we want to get moving quite a bit here, helping to get any excess fluid into circulation and out of the site of discomfort.

This is counterintuitive to some. They may wake up, feel achy, and go “Eh, I’ll take it easy today.” The days where this happens is actually when you want to move more. Moving less will only make matters worse. [9]

There is also room for some nutritional considerations. When we eat carbohydrates we store more water. If you’re someone who eats a good amount of carbohydrates, there is potential for lowering your carb intake, leading to less water / bloating, which may help.

Bringing us to the final, somewhat obvious recommendation with all this. The better in shape you are, the less overweight you are, the better much of this will be. Being overweight is hard enough on the body already. Being in shape tends to help blood circulation and lessen inflammation. Etc. Etc. All that great stuff that comes with being in shape.

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Thanks to Dan, who is also a med-school student, for the inspiration on this, as well as his thoughts. My all around knowledgable Dad for his thoughts on the bone simulation. Tim, a surgeon, for answering many questions and drawing surgical pictures in the gym for me. My new roommate, whom I had no idea was a mechanical engineer until he saw me walk in the house with a power drill. Warren, a former structural engineer. My brother, who is studying physics / computer science / mathematics. And my girlfriend for filming and listening to me try to explain this in layman’s terms, which was the primary goal.

Pertinent links

-The book Bone in Clinical Orthopedics. This was lent to me by an orthopedist. It’s a graduate level text for orthopedic surgeons in training. This isn’t quick reading, but if you have some anatomy background, I think you’ll be good. I made my way through this and Structures: Or Why Things Don’t Fall Down at the same time. Where the “Bone” book falls short on explaining mechanical concepts the “Structure” book really clarifies. For instance, stress and strain was a blip in “Bone,” whereas it was covered continuously all throughout “Structure.”

-A solid, albeit still lacking, article on weather and joint pain, from WebMD:


Some cool pictures and information about the skin and its response to heating / cooling

-That Veritasium video again:

-A shorter video, with some different info, regarding thermal conductivity and our perceptions:

-These discussions on Reddit were helpful regarding metal feeling cold:

-This site, http://www.itis.ethz.ch/itis-for-health/tissue-properties/database/database-summary/ , amazingly has an extensive list of tissues in the human body, and their corresponding densities, heat capacities, thermal conductivities, and more. The fact this exists is incredible.

-A word of caution, when looking at various values and comparing, make sure the units are the same. (www.engineeringtoolbox.com has a lot of the values.) They are often not. I used a couple different unit conversion sites.


[1] I think about Dan all the time because he is a reminder to me that localized abnormalities CAN cause pain. There has been this surge of pain science, all obsessing over our psychology’s influence on pain. In the pain science world, Dan was a classic case of a guy who is post surgery, having unusual pain, hmm…something has to be going on at his brain, right? Terms such as smudging, catastrophizing, sensitization, all that good stuff. Dan even did mirror therapy at one point, something typically reserved for phantom limb pain. The fact of the matter is the dude has a broken piece of metal floating around in his decaying bone, and that’s why he had pain. His brain was pissed because his leg was pissed.

[2] Humans have learned the hard way the worst thing to do after any injury is immobilization. You may significantly modify how you move, how much you move, how intensely you move…but you better be moving.

[3] This distinction between stressing and straining bone has some significant implications. How many people exercise by means of the pool, a reclining bike, or the elliptical? Now think about what’s happening to the bones in these scenarios. Sure, the bones are having to endure some stress, but is it enough stress that the bone strains?

There are actually solid examples of high level cyclists with bone density issues. While it’s obvious in these activities -cycling and such- there is not as much strain as something like walking, more intense weightlifting, etc. it isn’t obvious the level of strain is so low, or nonexistent, that people may lose bone.

The unfortunate irony here is those who are most worried about bone density tend to be those who are most prone to using the pool, bike, or elliptical!  I’m going to be succinct on this for now: If you want to improve your ability to be on land, work on your ability to be on land. Don’t work in a buoyed state, a seated state, or a gliding over the ground state. Get your feet on the ground. Your body needs some level of impact.

[4] I went to Home Depot and bought:

  • 20 volt power drill.
  • Drill bit kit. (Standard cannulated bits. You don’t need anything stronger than this.)
    • You will need something sharp. A regular bit did not get the job done for me.
  • A metal screw 20-30 mm in length and 7-12 mm in diameter (I bought a few just in case)
  • A fairly thick rubber band to place in the tunnel as a graft.
  • A big rubber hammer.

I also went to a butcher and got some soup bones.

From there I:

  • Drilled a pilot hole with a 2.4mm bit, at a 45 degree angle.
  • Moved to a larger, spiked drill bit to get a tunnel.
  • I then used a blank drill bit to enlarge the tunnel. (The spike bit wasn’t big enough.)
  • Placed the graft into the tunnel.
  • Screwed the screw, which should be 1mm smaller than the tunnel, into the tunnel. The screw can be smaller than the tunnel because once you place the graft (rubber band) in, the screw will end up being quite snug. (I had a lot of trouble with this part due to not having the right sized bit to drill the screw in. I ended up rubber hammering the screw in -I was losing daylight- which ended up fine.)

While I had to deviate, and no this isn’t an exact simulation, I followed the procedure for an ACL reconstruction quite closely. I followed guidelines such as these, and watched a good amount of videos, to organize all the above.

I wasn’t sure what was going to come of this. But I can tell you I’m really glad I did it. It really helps you grasp what this, or whatever surgery, entails. An engineering acquaintance remarked I should have tried to simulate this with software to be more precise. While I get the mentality, doing this with your own hands gives you a different level of understanding. Feeling how strong bone is cannot be replaced. This is one reason virtual anatomy and robotic surgery has faded out. It doesn’t reproduce all the senses. I’ve talked to actual surgeons about this. There is an element of feel you don’t get with a robot. Sometimes you don’t recognize how amazing your senses are until you try to do something without one of them.

Last piece of advice: The same engineering acquaintance mentioned to use some oil to clean the bits afterwards, to avoid corroding the bits from the bone remnants and whatnot. Advice I’d adhere to.

[5] The strengths of bone:

  • Compressive 170 MPa
  • Tensile 112 MPa
  • Transverse (shear) 51.6 MPa

Why is bone so weak in the transverse plane? Much like wood, you don’t want to load against the grain. Think of the grain of bone as:

Bone Grain


Anything that hits at a 90 degree angle to those grains and the bone can’t resist it well. This is one reason banging your shin on something hurts so damn much.

[6] The equation for heat storage is:

q = V ρ cp dt

    = m cp dt      


q = sensible heat stored in the material (J, Btu)

V = volume of substance (m3, ft3)

ρ = density of substance (kg/m3, lb/ft3)

m = mass of substance (kg, lb)

cp = specific heat capacity of the substance (J/kgoC, Btu/lboF)

dt = temperature change (oC, oF)

Keeping other variables constant, steel has a

-density 8,000 kg/m^3

-heat capacity 466 j/kg/c

=>Multiplying = 3,728,000

While the body has a

-density 1,000 kg/m^3 (rough average from here.)

-heat capacity 3,000 j/kg/c

=>Multiplying = 3,000,000

The point here is merely to show how much steel’s density can make up for the body’s much greater heat capacity (remember, different than storage).

[7] For more on how our memories trick us, particularly with pain, see Making your (memory of your) workout more enjoyable. 

[8] I do want to point out this was a study done in rabbits. That said, bone is one of the few instances where animal research and human research seem to relate very, very well.

[9] Fun fact: NASA actually uses exercise as a means of getting astronauts ready for spacewalks. It’s literally part of the protocol before leaving the space vehicle and entering their suits, where the pressure is much lower. The rationale being it increase the speed at which certain substances (nitrogen) are removed from the blood, insuring the astronaut is better and more quickly prepared for a lower pressure environment.

Fun fact two: There is some research out there suggesting women are more likely to give birth after the pressure lowers. See: Spontaneous delivery is related to barometric pressure.


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