1.1 History

Nonoperative treatment of displaced patellar fractures prevailed until the end of the 19th century when Quenu [1] published a review of 26 cases treated surgically and recommended operative repair. In 1942, Gallie and Lemesurier [2] described a technique to repair a quadriceps tendon rupture. Following these reports, surgical management became increasingly accepted.

1.2 Epidemiology

Patellar fractures are relatively common, accounting for approximately 1% of all skeletal injuries [3]. The patella is prone to injury because of the subcutaneous anterior location. About half of these fractures are nondisplaced, with the extensor mechanism remaining intact. Patellar fractures may occur as direct or indirect trauma with dashboard injuries, and falls being the most common causes.

1.3 Special characteristics

The patella serves as the fulcrum of the extensor mechanism with two lever arms: the quadriceps tendon and the patellar tendon. Enormous forces are transmitted across the patellofemoral joint. This can reach up to seven times body weight, thus the load-bearing capacity required for an osteosynthesis is high. Some types of activities, such as stair climbing and squatting, may generate patellofemoral compressive forces more than seven times body weight; consequently, the tension strain at the anterior patellar surface is close to the values that result in fracture [4]. The shape of the patellofemoral joint, and hence, the posterior surface of the patella, varies widely. Patellar tracking also depends on the configuration of the extensor mechanism and on the balance of the quadriceps muscles. The congruity of the articulation of the patella with the femur changes considerably from extension to flexion. From full extension to 45° of flexion the articular surface of the patella is in contact with the anterior femur. In a knee flexed more than 45°, the posterior surface of the quadriceps tendon is in contact with the patellar facets of the femur and this increases the lever arm.

The increased lever arm of the extensor mechanism due to the patella acting as a fulcrum adds an additional 60% of the force needed to gain full extension (final 15°).

Thus, full extension power is markedly reduced following patellectomy.

2.1 Case history and physical examination

The mechanism of injury and the type of force determines the fracture pattern. A direct force from a direct blow to the front of the knee causes the patella to fail in compression and results in a multifragmentary or in a stellate fracture pattern with significant chondral damage. More commonly, the patella fails in tension, as force from the extensor mechanism exceeds the strength of the bone. Typically, this fracture type is transverse or an avulsion of the inferior pole. The injury often extends in a transverse fashion through the retinaculum and results in fracture displacement and loss of active knee extension [5]. In closed patellar fractures, typical clinical signs are swelling, tenderness, and limited or loss of function of the extensor mechanism.

Preservation of active knee extension does not rule out a patellar fracture if the auxiliary extensors of the knee (retinacula) are intact [5].

If displacement is significant, the physician can palpate a defect between the fragments. The examination must include evaluation for pure rupture of the quadriceps or patellar tendons and injury to other knee ligaments resulting in joint instability (see chapter 6.7.2).

2.2 Imaging

In addition to standard x-rays of the knee in two planes, a tangential view of the patella may be useful in some cases. In the AP view the patella normally projects into the midline of the femoral sulcus and usually delineates the primary nature and direction of the fracture lines and may reveal additional fracture fragments. A lateral view taken with the knee in 30° of flexion demonstrates the true extent of displacement. The proximal tibia must be visible to exclude a bone avulsion of the patellar tendon from the tibial tuberosity. A rupture of the patellar or quadriceps tendon results in an abnormal position of the patella with a high-riding patella (patella alta) or low patella (patella baja). The Insall-Salvati method [6, 7] is used to evaluate the position of the patella ( Fig 6.7.1-1 ). The tangential view (skyline view), which is obtainable in 45° knee flexion, can identify a longitudinal or osteochondral fracture. Computed tomography and ultrasonography may be helpful to further define osteochondral or sleeve fractures. Bone scanning may be indicated for detection of occult stress fractures. Magnetic resonance imaging is helpful to diagnose cartilage defects and ligamentous lesions; anterior cruciate ligament rupture is a common associated injury.

3.1 Key anatomical features

The anterior surface is surrounded by an extraosseous arterial ring, which receives inflow from branches of the genicular arteries. This anastomotic ring supplies the patella through midpatellar vessels, which penetrate the middle third of the anterior surface, and the polar vessels, which enter the apex. The infrapatellar branch of the saphenous nerve crosses from medial to the anterolateral aspect of the tibia close to the apex of the patella ( Fig 6.7.1-2 ). It runs in the subcutaneous tissue layer and is at risk with longitudinal incisions. The surgeon must also be aware of the integrity of the other important structures that make up the extensor mechanism, such as the quadriceps and patellar tendons and the patella-femoral ligament.

4.1 AO/OTA Fracture and Dislocation Classification

The patella bone is coded as 34 in the AO/OTA alphanumeric system with patellar fractures classified as extraarticular, partial articular, and complete ( Fig 6.7.1-3 ) with further subclassification shown in Fig 6.7.1-4 .

6.1 Implant selection

Tension band wiring is highly effective in transforming tension force into compression force (see chapter 3.2.3). A 1.25 mm wire in combination with 1.6, 1.8, or 2.0 mm K-wires are the implants of choice. Single 3.5 mm cortex screws, if applied properly as a lag screw, will add to stability, but should not be used without a tension band except in longitudinal type B fractures. Articular osteochondral fractures can be reduced and fixed with biodegradable pins (1.6–2.0 mm diameter). Heavy sutures (No. 5 nonabsorbable) may also serve to reduce bone and soft tissues. Cannulated screws (4 mm) can also be placed across the reduced fracture and the tension band wire placed through the screw. This has been shown to be biomechanically stronger than the K-wire construct [10]. Patellar plates can be used in complex multi planar fracture.