This places more pressure on one side of the underlying cartilage than the other. In time this pressure can cause damage to the articular cartilage. Another type of imbalance may exist due to differences in how the bones of the knee are shaped. These differences, or anatomic variations , are something people are born with. Some people are born with a greater than normal angle where the femur and the tibia come together at the knee joint.
Women tend to have a greater angle here than men.
The patella sits at the center of this angle - where it sits in the femoral groove. When the quadriceps muscle contracts, the force tries to straighten this angle, pushing the patella to the outside of the knee. In cases where this angle is increased, the patella tends to shift outward with greater pressure. This leads to a similar problem as above. As the patella slides through the femoral groove, it shifts to the outside.