Perfect knees don't exist, but if they did, they'd be straight and symmetrical. The knee cap would be pointing forwards, there would be no pockets of fat or fluid around the knee, and the knee joint would be surrounded by strong quadriceps holding the patella in place.
Real knees, however, are much more varied than the ones we see in textbooks. Fat and fluid build up around the knees with age. Quadriceps are often weak or tight (or sometimes both), which causes the patella to point off to one direction or another. For most people, an off-centre patella will mean nothing more than a strange click, but for people who really use their knees a lot (gardeners, walkers/runners, golfers, etc.) then a lopsided patella can be a bigger, more painful problem.
The problem is this: the patella has a ridged bottom, which glides through a grove on the tibia (shin bone). When the patella sits off-side, or too high, then the ridge misses the groove, and the patella is forced to grind bone-on-bone onto the tibia or femur (thigh bone)
Lateral (outward) migration is the most common. This is when your 2 patellae point away from each other. A textbook would tell you that it's caused by tightness through the muscles of the outer thigh, which isn't wrong, but the underlying pathology is a weakness in the inner thigh - specifically vastus medialis (VM). To correct lateral migration, strengthen VM until its strength matches that of the muscles in the outer thigh. If there's no benefit in 2-3 weeks, then start using the foam roller on TFL.
Medial (inward) migration is the opposite of lateral migration - both in the direction of the patella movement and in the underlying cause. Weakness in the outer thigh and tightness in the inner thigh will cause the patella to move inwards. Using a foam roller to release VM will correct this migration.
Superior (upward) migration is a combination of medial and lateral movement - it is when the muscles of the outer thigh and VM are too tight, forcing the patella sits too high on the joint. Regular, quality quadriceps stretching will move the patella back to its normal position.
For more information on knee pain, email me directly via the contact page. For specific information on an injury you have, or for treatment, contact one of my clinics directly. You can find that information here.
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Dr Mitch Clark