The patella and its role
The patella (kneecap) is the largest sesamoid bone in the human body, triangular in shape. On its posterior surface lies the thickest cartilage in the human body, reaching up to 7 mm. It sits in the intercondylar groove of the femur and forms part of the knee extensor mechanism. Its main function is to increase the moment arm of the quadriceps muscle, amplifying extension force by up to 50%.
Together with the femur it forms the patellofemoral joint. Due to the shallow and incongruent nature of this joint, its stability depends greatly on surrounding soft tissues. For this reason the patellofemoral joint is considered biomechanically one of the most complex joints in the human body.
Primary traumatic patellar dislocation
Primary traumatic patellar dislocation (first-time dislocation) is a traumatic displacement of the patella relative to the femur, usually laterally. After ACL rupture, it is the second most common cause of traumatic haemarthrosis and accounts for approximately 3% of all traumatic knee injuries. In children under 14 it is the most common serious knee injury with acute haemarthrosis. Female sex and younger age represent greater risk.
Predisposing factors
- Femoral trochlear dysplasia
- Patella alta (high-riding patella)
- Increased femoral anteversion
- Increased external tibial rotation
- Lateral patellar tilt
- Quadriceps hypoplasia
- Subtalar pronation
- Knee valgus (knock-knee)
- Generalised ligament laxity
Mechanism of injury
Approximately 90% of cases are non-contact injuries. The typical mechanism involves minimal knee flexion with the lower leg in valgus while the trunk begins to rotate. Direct blows to the patella are rarer.
Diagnosis
Patients describe the knee "popping out laterally" followed by a fall. Spontaneous reduction occurs in some patients on extending the knee. Clinical findings vary widely – from severe pain and swelling to a near-painless presentation.
Palpation reveals haemarthrosis, tenderness on the medial patellar border (MPFL tear site) and at the lateral femoral condyle. The apprehension test is positive.
X-ray in three projections (AP, lateral, axial) is mandatory to identify osteochondral fractures. MRI shows MPFL tear in over 90% of cases.
Treatment of first-time dislocation
Every child after primary traumatic patellar dislocation needs orthopaedic assessment. If there is a free osteochondral fragment in the joint, urgent surgery is needed – arthroscopic fixation with screws or removal of the fragment.
If no bony injury is present: immobilise in an extension orthosis for a few days, combined with quadriceps-strengthening physiotherapy; from one week post-injury, a patellar stabilising orthosis is applied.
Recurrent dislocation and patellar instability
Recurrence occurs in 15–44% of cases after first-time dislocation. Recurrent instability can cause:
- Articular cartilage damage
- Osteochondral fractures
- Chronic instability and pain
- Reduced physical activity
- Degenerative patellofemoral changes
MPFL reconstruction
When the patella is unstable and the child experiences functional limitations, surgery is indicated. Grafts are taken from tendons around the same knee. This may be performed as a standalone procedure or as part of a more complex operation, depending on additional findings.
Based on biomechanical research from his doctoral dissertation, Prim. dr. sc. Josip Vlaić developed a new surgical technique for treating patellar instability – achieving excellent stabilisation outcomes in children and adolescents.
Post-operatively: orthosis immobilisation for several weeks combined with a structured physiotherapy protocol. Return to sport is expected after 4–5 months. Results are excellent – the patella becomes stable and children can engage in the activities that fulfil them.