Stability examination of the knee: a simple anatomic approach.

The knee is a complex joint and its stability depends upon many factors, but static stablizing structures are the most critical. The anatomic integrity of these structures correlates well with both their functional integrity and the instability observed during manual stress-testing of the knee. This makes accurate diagnosis possible. Stability of the knee is most easily defined in terms of straight planes, varus, valgus, anterior and posterior displacement. Rotory instabilities are syndromes which represent combinations of straight-plane instabilities. The stability in each plane is supplied by a primary static stabilizer and secondary and at times tertiary reinforcing structures. The information required to accurately interpret the pattern of injury is readily obtained by stressing the knee in the planes described.