Patella (Kneecap) Instability

The medial patellofemoral ligament (MPFL) is the primary restraint that stops the kneecap from dislocating toward the outside of the knee. It is a strong band of tissue that tethers the inner edge of the patella to the inner side of the femur, acting like a check-rein that holds the kneecap within its groove.

When the kneecap dislocates for the first time — usually with a twisting injury or direct blow — the MPFL is almost always torn or stretched. This is the critical event that changes the knee from a stable joint to one that is prone to further episodes. Without a functioning MPFL, the patella has no reliable soft-tissue brake against lateral displacement, and the risk of recurrent dislocation rises dramatically.

Why this matters: once the MPFL has been damaged, the patella becomes free to slip or dislocate with far less force. Simple activities such as getting out of a car, turning in bed, or stepping off a curb can trigger another episode. Repeated dislocations cause further damage to the articular cartilage on the back of the patella and the edge of the femoral groove, which can lead to arthritis over time.

This is why I place such importance on identifying MPFL injury early. In patients with recurrent instability, the torn or attenuated MPFL is the key structural problem that must be addressed — whether through careful rehabilitation in select cases, or through reconstruction when the ligament has failed to heal and instability persists.

Other factors that contribute to patellar instability

While the MPFL is the primary soft-tissue restraint, several anatomical factors work together to make dislocation more likely. Understanding these is essential when planning treatment, because correcting the MPFL alone may not be enough if the underlying bony architecture leaves the patella vulnerable.

Patella alta — a high-riding kneecap — means the patella does not sit deep enough within the trochlear groove of the femur. In early flexion, when the knee begins to bend, the patella is essentially sitting above the groove rather than within it. This leaves it free to slide sideways with little resistance, and is one of the strongest predictors of recurrent dislocation. Normalising patellar height through distalisation at the time of surgery is often a key part of achieving long-term stability.

Trochlear dysplasia refers to an abnormally shallow or flat groove at the end of the femur where the kneecap should track. In a normal knee, the bony walls of the trochlear groove rise up on either side and guide the patella as the knee bends. When these walls are underdeveloped or flattened, there is nothing to hold the kneecap in place. Trochlear dysplasia is a powerful risk factor for instability and, in severe cases, may require deepening the groove (trochleoplasty) as part of a comprehensive stabilisation procedure.

Tight lateral structures — including the lateral retinaculum and iliotibial band — can pull the kneecap persistently toward the outside of the knee. When these structures are too tight, they create a constant lateralising force that the weakened or torn MPFL cannot counteract. Releasing these tight structures (lateral release) during MPFL reconstruction restores a more balanced force environment around the patella and improves tracking.

Rotational malalignment of the femur and tibia is an often-overlooked contributor. When the thigh bone is excessively rotated inward (increased femoral anteversion) or the shin bone is rotated outward (external tibial torsion), the entire mechanical axis of the leg changes. This alters the line of pull of the quadriceps and patellar tendon, directing force laterally across the patellofemoral joint with every step. These rotational abnormalities are assessed on specialised imaging and may influence whether a tibial tubercle osteotomy or even a derotational osteotomy is needed alongside soft-tissue reconstruction.

These factors rarely exist in isolation. A patient with trochlear dysplasia may also have patella alta and a torn MPFL — the so-called "miserable malalignment" pattern. This is why a thorough assessment of all contributing elements is essential, and why treatment must be individualised to address not just the torn ligament, but the entire structural environment that allowed the dislocation to occur in the first place.

Valgus describes a knock-kneed posture where the lower leg angles outward relative to the thigh, so that the knees come together or even touch when the ankles are kept apart. This alters the line of pull of the quadriceps and patellar tendon, increasing the lateralising forces on the kneecap and making dislocation more likely.

Significant valgus malalignment is an important but sometimes overlooked contributor to patellar instability. When the mechanical axis of the leg falls medial to the knee centre, the patella is effectively pulled laterally with every step and squat. In these cases, a distal femoral osteotomy (DFO) may be discussed to correct the coronal plane alignment, realign the extensor mechanism and protect the patellofemoral joint.