Built for pressure: How thick is kneecap cartilage?

Your kneecap is naturally under pressure. Quite a lot of pressure.

This is the most important thing to understand about patellofemoral pain — pain related to the joint between the kneecap and the end of the femur, often quite stubborn. Although the details of this kind of pain can be confounding — which is why I’ve written an entire book about it — it’s not hard to understand in general why the kneecap is vulnerable. Huge forces are applied to the underside of it constantly: every single time you climb stairs, squat, or even just sit with your knees bent, it squeezes the cartilage like vice grips.

Well, maybe it’s the second most important thing to understand. Likely playing an even larger role are the much less obvious physiological vulnerabilities (e.g. lipid profile, blood sugar, the effects of smoking or alcohol or sleep loss). These determine whether or not this joint — or any joint, or any tissue at all — is vulnerable to physical stress in the first place, and how much they struggle to recover from it. I believe this is the nature of most of our seemingly “mechanical” musculoskeletal conditions: overload + physiological vulnerability = trouble!

But the mechanical loading is part of that equation, and it’s especially clear in this case: it’s really pinchy under the kneecap! Just how pinchy? Pressures per square inch underneath the patella can be “greater than three times body weight during stair-climbing and eight times body weight during squatting and deep-knee-bending” (see Kisner et al, Reilly et al, Hartmann et al). So when you crouch down to give the cat some love … converting to metric, if you weigh 100 kilograms, pressures under the kneecap will skyrocket to >800 kgs per square centimetre.

And we can see this high-pressure functionality written into the anatomy in the form of thick cartilage.

The kneecap is all about leverage — we have kneecaps so that we can extend the knee more powerfully, as shown by this really nice little model (source unknown). But that leverage has a cost: the loading on the joint is perpetually more extreme, making the patellofemoral joint more vulnerable to fatigue & failure.

So how thick is the patellofemoral joint cartilage?

The whole knee joint has the thickest articular cartilage in the human body, and it’s thickest of all under the kneecap. The patellofemoral joint cartilage is at least 2mm thick in most people, and perhaps double that in some, probably the younger and larger. We get these numbers from mostly needle probes — inserting needles into cartilage to see how “deep” it is 😬 — and other methods.

I found one claim on a webpage that the patellar cartilage is up to 6mm thick, which was eyebrow-raising, unverifiable, and at odds with all the scientific sources I could find. But, even at 2-4mm, it’s still the thickest cartilage we’ve got … and maybe that’s also surprisingly thin, given the job it has to do.

This patellofemoral cartilage is also the spongiest of cartilage in the lower limb. Specifically, it is the least stiff, a lower “compressive modulus.” Cartilage in the lower limbs is either thinner and stiffer (ankle), or thicker and spongier (patellofemoral joint). The lower stiffness may give the cartilage more of a “shock absorbing” quality, because it’s more likely to have to stand up to more focal pressure in joints that don’t fit together as neatly (but I’m just speculating there).

Most of us would assume that cartilage thins as arthritis gets going, but in fact it’s likely the opposite, getting a bit thicker in the early stages, like a callus.

A photograph of the back of a dissected left patella & an illustrated right patella from Gray’s Anatomy (plate 353) highlighting cartilage in blue. The surface is divided into two zones or “facets,” one larger lateral & another smaller medial. (There’s also the “odd” facet on the innermost edge of the medial facet, not labelled here.) The facets are usually considered the part that gets sore in patellofemoral joint pain, but the bony edge beyond the cartilage of the bone might well also be tender. Either the medial and/or lateral edge could be sore.

And the thickness of the patella itself?

Another fun fact: it’s very thick, and consistently half as thick as it is wide. So most 4cm wide kneecaps are about 2cm thick. A curious bit of anatomical symmetry.

And one more! When surgeons add too much thickness to a patellar implant, they call it “overstuffing” the joint — which is a bad thing, I gather. You want a Goldilocks amount of stuffing.

The surprisingly consistent 2:1 ratio of patellar width to thickness. Want to know how thick a kneecap is? It’s much easier to measure the width, so just do that & divide by two.

To make the fairly simple claims about cartilage and patellar thickness in this post, I had to wrap my head around a fair bit of science, mainly from three studies: Shepherd and Seedhom, Cohen et al, and Iranpour et al.

This post is an adapted excerpt from my e-book about patellofemoral pain syndrome, one of ten books about common painful problems I have been publishing and maintaining for many years now. The book has more detail, especially some detailed footnotes (and so does the members-only audio for this post).