Case 18 – Cap it off

A 20 year old hockey player presents to your clinic with anterior knee pain, worse on stairs. 2 weeks ago, he was playing hockey and was hit into the boards. He retaliates by slashing his opponent. While sitting in the box he feels his right knee stiffen up, but still finishes the game.

On exam, a warm effusion is present in the right knee. The patellar is tender on palpation. Quadricep tendon and patellar tendon are non-tender. No laxity in the patellofemoral joint. The MCL, ACL, PCL, and LCL are intact. Hamstring power okay. Quadricep mechanism is weak, but intact.

Imaging:

Figure 1: Anteriorposterior radiograph of the right knee.

Figure 2: Lateral radiograph of the right knee.

Figure 3: Skyline radiograph of the right knee. Note the vertical fracture of the patella not visualized on AP or lateral radiographs.

Discussion:

The patella is the largest sesamoid bone in the human body. It lies between the quadriceps tendon and the patellar tendon. Aponeurotic fibres from the lateral and medial vastus muscles form the patellar retinaculum (see figure 4). Patellofemoral ligaments that radiate from the patella to femoral epicondyles also contribute the the patellar retinaculum and are important because they allow some degree of leg extension even in the presence of patellar fracture.

The patella functions as a lever for knee extension and increases the force of the quadriceps. As such, the quadriceps are weaker without an intact patella, there are large compressive forces at the patellofemoral joint and the patella is subject to high tension forces. Fractures of the patella can occur from either direct or indirect forces. The classic indirect mechanism is a fall on the feet that overwhelms the resistance to knee flexion causing the knee extension mechanism to fail. This leads to tendon rupture or, typically,  transverse patellar fracture. Direct mechanism injuries, by contrast, often lead to comminuted or stellate fractures.

Diagnosis is made on the basis of injury mechanism, physical exam, and imaging. Examination usually reveals a knee joint effusion or hemarthrosis and focal tenderness of the patella. The knee extensor mechanism is important to assess because disruption requires surgical repair. Complete inability to extend the knee suggests a tear to the medial and lateral retinaculum in addition to fracture. Anteroposterior and lateral radiographs should be obtained and an axial or sunrise view can also helpful for evaluation though it may be difficult to obtain if there is a large effusion or severe pain.

Surgical referral is recommended in the following circumstances: fractures with greater than 2mm of articular step-off, fractures with greater than 3mm of fragment separation, comminuted fractures, disruption of extensor mechanism, any open fracture, and avulsion fractures.

Diagnosis: Non-displaced Patellar fracture

Treatment:

Nondisplaced vertical fractures do not require surgical intervention. Therefore, conservative management is recommended for 4-6 weeks. Patient should be placed in a knee immobilizer or splint in full knee extension. Range of motion and strengthening exercises should be started as soon as possible. Vitamin D 2000 units daily is also recommended.

Figure 4: Patella anatomy and stabilization.

Author:  Lucas Nguyen Oct 16, 2018 (PRND)

References:

• Blout JG. (Last updated: 2016, Nov.21). Patella fractures. Retrieved from https://www-uptodate-com.
• Scolaro, J., Bernstein, J., & Ahn, J. (2011). In Brief: Patellar Fractures. Clinical Orthopaedics and Related Research, 469(4), 1213–
• Gwinner, C., Märdian, S., Schwabe, P., Schaser, K.-D., Krapohl, B. D., & Jung, T. M. (2016). Current concepts review: Fractures of the patella. GMS Interdisciplinary Plastic and Reconstructive Surgery DGPW, 5, Doc01.