Riding can produce a bowlegged deformity. This happens by stretching the lateral ligament in the knee and compressing the medial ligament and the medial meniscus, the disc like cartilage between the femur (thigh bone) and tibia (leg bone below the knee) This takes a lot of riding to occur, but the funny photos of cowboys with bowed legs are real.
This is lessened by using an appropriately wedged stirrup. Here's how it works: bend your knee slightly as if you were in the stirrup, then slightly evert ( roll outward ) your foot. You will notice a very slight distraction or opening up of the inside of the knee, relieving some of the pressure on the inside of the knee joint. This also relaxes the ligament on the outside of the knee slightly. The wedged stirrup works in this manner to counteract the forces that result in bowlegs from our legs going around the horse's barrel.
Wedged stirrups are not new. You may have seen them in tack stores and wondered about them. There are 2 reasons why the old ones were not appropriate: first, the wedge angles used in these stirrups were too high, and second, they did not have an impact absorptive surface for the foot to rest on. A high angle results in jamming of the ankle bone against the base of the tibia in the ankle joint, resulting in cartilage wear and damage. This is relieved by a lower angle and by an impact absorptive pad. The pad allows micro-movements of the ankle reducing jamming and increasing joint fluid circulation between the joint surfaces.
The best combination I've found of angled support, plus shock absorption is produced by the company ThinLine, known for their impact absorbent saddle pads (also great for keeping riders spines healthy for decades longer than riding without a shock absorbing saddle pad). In addition to the angle and impact protection, their stirrups feature a third benefit by offering a stabilizing non-slip surface, potentially lessening the probability of a knee or ankle wrench in a “bad moment” on the horse.
As a surgeon who has seen countless accidents and long term damage in brains, backs hips and knees I am pleased riders now use helmets most of the time but believe the audience is unaware how much damage they continue to do to backs and knees by not using a simple shock absorber.