Legs bend at the knees. Being bowlegged is a condition of the legs with an outward curvature in the region of the knee (Webster’s II New Riverside University Dictionary). In some medical cases, the legs curve inwardly. Corrective surgery or wearing leg braces is done in some cases to straighten out and strengthen the legs.
Severed ligaments at the knee would keep the legs “bent.” A shattered knee joint will keep the legs “bent.”
Severe arthritis in the knees and gout in the knees will definitely obstruct the knees and keep the knees from straightening out and thus cause the legs to remain “bent”.