Have you ever been out on a run and half way through you feel a twinge in your knee every time you land? Have you felt that nagging ache somewhere around your knee cap that comes on at different points in your run and with varying intensity? Have you poked your thumb around your knee looking for a sore structure only to find that nothing hurts to the touch? This presentation is more common than many realize. In a given year, it is estimated that 20-79% of runners sustain an injury. The more unbelievable statistic is that 40-50% of those injuries occur at the knee! As far as sport specific statistics go, that is a very high percentage.
So what is going on at the knee joint when people run that is resulting in so many injuries? Let's look at the data to find out.
Taunton et al. British Journal of Sports Medicine 2002;36:95-101
From this research conducted on 2,002 runners we observe that of all the injuries sustained, patellofemoral pain syndrome (PFPS) is by far the most common injury. It occurred at double the rate of IT band syndrome and plantar fasciitis (also common injuries). Additionally, when we look at the biomechanical factors observed in the study, we can see that runners who had PFPS also exhibited a disproportionate amount of knee valgus, varus, high Q angle, and patellar squinting.
For clarity, the terms valgus, high Q angle, and patellar squinting are all synonymous and that particular movement is demonstrated in the image above. When your knees move too far inwards or outwards there is increased stress at the knee joint and the patellofemoral joint.
How Does the Patella Cause Pain
The patella (knee cap) articulates with the front part of the femur (thigh bone). The articulating face of the patella has two concavities allowing high congruency with the femur. By having a solid fit with the femur there is increased surface area of bone contacts. If we think back to our high school math or physics class we would remember that as surface area goes up pressure goes down (think of the amount of pressure felt pocking someone with one finger (low surface area, high pressure) versus a whole palm (high surface area, low pressure).
The issue occurs when there is altered alignment of the patella within the femoral groove. Typically the patella will shift laterally, towards the outside of your knee, causing the surface area of contact to decrease and pressure in the joint to rise. Over time, and through highly repetitive actions such as running, the joint cartilage under the patella and on the femur weaken causing chondromalacia and eventually osteoarthritis. It is necessary, therefore, to limit the shifting of the knee cap during weight bearing activities such as running, hiking, and jumping.
What Makes The Patella Shift?
The straight answer to this question is it depends which authority you ask. Podiatrists will say it is increased pronation at the foot, hands on practitioners will say it is a tight IT band pulling the patella laterally, and others might even say it is quadriceps weakness that is not holding the patella in the groove. To be fair, there is some valid research to back up each of these points. The newest most peer reviewed research; however, definitively points to poor lower extremity alignment as the main cause. The best research looks at how a structure behaves in an environment that most mimics where the pathology occurs. In the case of the knee joint that would be in weight bearing. When researchers (Souza, Draper, Fredericson, Powers) studied the patellofemoral joint using and MRI machine they discovered exactly what causes the patella to shift laterally. It turns out it is the rotation of the femur (thigh bond) that rotates UNDER the patella versus the patella shifting OVER the femur.
In the above photo you can observe the femur (horseshoe looking bone) rotating medially (to the right) while the patella (the topmost bone) stays still. The patella does end up in a lateral position but only because of the rotation of the femur.
Having seen the actual motion of the knee during a weight bearing squat we know controlling the movement of the femur is essential to decrease knee pain. The skill of running is essentially a single leg squat performed over and over again and therefore there is high correlation to this studies design.
Let me end this background discussion with a summary. Of all the running related injuries 40-50% occur at the knee and of those injuries a disproportionate amount occur at the patellofemoral joint. Over the last 5-10 years, research has clearly documented that patellofemoral joint pain occurs due to decreased surface area contact between the patella and femur and high joint forces. Biomechanical studies highlight medial rotation of the femur as a main culprit. Therefore, correcting lower extremity alignment is essential to eradicating knee pain in runners.
Now that we understand some history behind "runners' knee" we can delve into how to get rid of the pain and return to running. That post will be coming soon. There is some dense information in this post so please follow up with any questions you may have.