Patellar instability occurs when the patella fails to engage securely in the trochlea at the start of flexion; it slips laterally and either dislocates completely or slips back medially to its correct position as flexion continues. In some patients this happens because the trochlear groove is too shallow or uneven (trochlear dysplasia).
Conservative treatment includes physiotherapy and exercises to strengthen the quadriceps. Surgical approaches include direct reconstruction of the dysplastic trochlea or correction of associated factors by procedures such as medial patellofemoral ligament reconstruction. Trochleoplasty aims to reshape the bony anatomy of the trochlea: it may involve deepening the groove or elevating the lateral wall of the trochlea (which should be higher than the medial wall). Trochleoplasty is usually done as an open procedure, which is associated with risks such as arthrofibrosis and, rarely, infection.