Teaching Case

A 13 year old fell while playing on a scooter and landed directly on his right knee. The knee swelled immediately. On examination the knee was tender anteriorly and had a large joint effusion. No active knee movements were present and he was unable to perform a straight leg raise. Radiographs of the knee showed proximal migration of the patella and a small avulsion fracture of the inferior pole of the patella. Magnetic resonance imaging showed extensive soft tissue swelling and a joint effusion. There was a dissociation of the inferior patella from the patella tendon with a large cartilaginous fragment.

A 13 year old boy fell while playing on a scooter and landed directly on his right knee. The knee swelled immediately. On examination the knee was tender anteriorly and had a large joint effusion. He was unable to straight leg raise. The lateral radiograph of the knee showed proximal migration of the patella and a small avulsion fracture of the inferior pole. There was extenxive soft tissue swelling around the patella ligament. Magnetic resonance imaging (T2-weighted sequence) showed extensive soft tissue swelling and a joint effusion. The fracture passed transversely across the inferior patella with the avulsed fragment consisting mainly of the cartilage sleeve with a small fragment of bone. At operation the patella was replaced in its cartilaginous sleeve and the superficial fibres of the retinaculum repaired by suture. The knee was kept extended for 6 weeks in plaster before gradual mobilisation with physiotherapy. At 2 months post-operation he is fully mobile and has knee flexion from 0 to 100 degrees.

The patella is a sesamoid bone in the quadriceps tendon. The quadriceps tendon inserts into its upper surface and continues as the patella ligament. A few superficial fibres groove the anterior surface of the patella and from each side extensions from the vasti (patellar retinacula) pass down to the patella ligament and are in continuity with the joint capsule. The mechanism of injury is forced flexion of the knee against resistance causing tearing of the superficial fibres of the quadriceps tendon. A sleeve of cartilage from the inferior pole of the patella is avulsed from the main body. Diagnosis may be missed on X-ray, as the bony fragment may be so small as to be not detectable. Clinically the inability to straight leg raise and swelling and tenderness over the patella ligament suggested damage to the extensor mechanism. The radiographs showed a high riding patella and swelling around the patella ligament suggesting a patella sleeve fracture. An ultrasound scan may be used to confirm the diagnosis. However MRI was used as accurate images are produced helping with pre-operative planning and showing the damage to the articular surface. Treatment is by accurate replacement of the patella within the cartilaginous sleeve and repair of the quadriceps mechanism, which gives good results. Results with conservative management are poor and can cause patella alta or ossification in the patellar tendon.