Form Truly Follows Function
  • Dr. Thomas Falls, MD

Form Truly Follows Function

There is good reason that form often follows function. In many cases, what is aesthetically pleasing is also good for biomechanical function. This holds true in leg alignment, particularly at the knee. Two very common deformities (orientations) of the leg are bow legs (genu varum) and knock knees (genu valgum). While these are generally described as knee problems, they do affect the overall function of the leg. However, what I am talking about today is how these alignments can lead to early wear and tear of the knee joint and potentially the need for premature total knee replacement. But have no fear - there is a way to fix this problem before irreversible damage is done!

I recently saw a patient who presented to the office with a significant bilateral knock knee deformity and severe knee pain. She stated that she never liked the way her legs looked, but was told there wasn't really anything that could be done about it. When the pain really began bothering her about 5 year ago, she was referred to physical therapy and told that "muscle imbalance" was the cause of the problem, and strengthening her thigh muscles would do the trick. In the end she presented to the office with end stage arthritis in one knee that necessitates total knee replacement, and early/moderate arthritis in the other knee that can potentially be saved. However, if she had been seen when she first really noticed the deformity, she may well have never developed arthritis, and with appropriate correction would have had straight legs that didn't cause her embarrassment when wearing shorts or skirts (again, form follows function). This is clearly no fault of her own, but a problem in the medical community. Deformity correction surgery is still an up and coming field, and while the type of surgery she would have needed has been described for many years, few surgeons do this type of procedure. This is one of the reasons I have this blog - to try and spread the word that there is a way to prevent the "inevitable arthritis" that comes with these deformities.

Let me briefly touch on why it is that these deformities cause problems. If you take an x-ray of the entire leg from hip to ankle, a line from the center of the hip to the center of the ankle should fall right through the middle of the knee. Slight deviation to the inside is technically within normal limits, but significant deviation to the inside and pretty much any deviation to the outside is pathologic. This is because this line, known as the 'mechanical axis' represents the line of of force distribution in the leg and through the knee. When it is too far to one side or the other, half the knee is being overloaded while the other half is not seeing any load at all. Over time this leads to increased wear and tear and eventually early arthritis. If the femur and/or tibia are re-aligned with bone cuts so that this line falls through the center of the knee again, then proper mechanics are restored and arthritis can be prevented. The image below is a graph from a cadaver study done by Hillstrom et al. which demonstrates how deviation to one side or the other quickly causes increased forces on one side of the knee, and little or no forces on the other. Image courtesy of Drs. S Rob Rozbruch and Austin Fragomen.

So, the conclusion here is knock knees and bow legs can both lead to early arthritis - but something can be done! For about 90% of cases bow legs can be corrected with that is called a 'high/proximal tibial osteotomy' and knock knees corrected with a 'distal femur osteotomy'. However, in some cases the osteotomies need to be mixed and matched - and since these deformities don't always follow the rules I am always careful to analyze the entire leg to see where the bone should be cut to address the problem. Below is the pre-op image of a young male who has a significant knock knee deformity bilaterally. The blue line shows where his mechanical axis currently is, and the red line shows where it should be - so in this case he had deformity in both the femur and tibia.

So, we corrected the femur with a distal femur osteotomy with a plate, and then corrected the tibia with a proximal tibia osteotomy with an intrameduallary nail (rod). We also decompressed the common peroneal nerve to prevent a foot drop, released tight lateral tissues around his knee cap so that it would track appropriately and released the fascia overlying the muscle in his leg to prevent swelling. It was quite the surgery, but his leg was nice and straight when we were done. Once he has gotten back on his feet we will have another long x-ray showing this nice correction. Then he will need to have his other leg corrected as well. By addressing this early we will have an impact on the future health of his knees!

Case courtesy of Dr. Austin Fragomen, MD

#Bowlegs #Knockknees #Valgus #Varus #osteotomy