The patella is the largest sesamoid bone in the human body. The posterior aspect of the patella contains a thick cartilage layer, which is the thickest cartilage in the body. A vertical ridge separates the medial and lateral facets of the articular surface and articulates with the femoral trochlea. The patella protects the anterior aspect of the knee joint, serves as the insertion for the quadriceps tendon, and functions as a fulcrum to maximize the efficiency of the extensor mechanism. The inferior pole of the patella attaches to the patella tendon. The patellar retinaculum is formed by contributions from the fascia lata, vastus medialis, and vastus lateralis. The patella receives centripetal blood supply from the geniculate arteries, with the superior vessels lying anterior to the quadriceps tendon and the inferior vessels passing posterior to the patellar tendon. The most important blood supply to the patella penetrates the inferior pole along the fat pad below the patella.
Fractures of the patella may be due to direct or indirect forces, and the mechanism of injury often determines the fracture pattern. The patella most commonly fails indirectly under tension. Eccentric forces from the extensor mechanism may overpower the mechanical properties of the bone. Due to the patella’s subcutaneous position, a direct injury may result from a blow to the anterior knee, such as from a fall or impact from the dashboard in a motor vehicle accident.
Fractures of the patella account for approximately 1% of all fractures. They occur more commonly in males. Open injuries account for 6% to 9% of patellar fractures and are commonly associated with other injuries given the high energy mechanism. In a small percentage of the population, a secondary ossification center in the patella, most commonly at the superolateral patellar edge, may fail to fuse, resulting in a bipartite patella. On radiographs, they may be mistaken for a fracture. In 50% of patients with this condition, the bipartite patella is found in both knees.
The patella experiences complex, dynamic loading patterns. With the knee in extension, the pull of the quadriceps places the patella in tension. Conversely, flexing the knee places compressive forces along the posterior patella. Additionally, the patella experiences compression and 3-point bending forces exerted by the quadriceps and patellar tendons during knee flexion. If rapid knee flexion occurs during active contraction of the quadriceps, the 3-force bending forces may cause the patella to fail in compression. The indirect eccentric tension through the extensor mechanism often results in avulsion fractures of the inferior pole or transverse fractures extending into the extensor retinaculum, causing fracture displacement. With direct fractures of the patella, the retinaculum and extensor mechanism often remain intact despite the substantial fracture comminution and cartilage damage.
The original classification of patellar fractures characterizes them into seven common fracture patterns: nondisplaced, transverse with displacement, inferior pole, comminuted without displacement, comminuted with displacement, vertical, or osteochondral. The descriptive nature of this system provides information on the mechanism of injury but does not dictate treatment or provide prognostic information. In the AO/OTA classification of patellar fractures, extra-articular (avulsion or isolated body) fractures are classified as 34A. Vertical partial articular fractures are classified as 34B. Complete articular fractures are classified as 34C and graded in severity based on the amount of comminution.
A thorough history and physical examination are necessary to identify the injury mechanism and other potential injuries. Patients who sustain high-energy dashboard injuries may also present with fractures to the ipsilateral acetabulum and lower extremity. Initial inspection of the injured knee is aimed at assessing soft tissue trauma. A saline load test using 155 to 175 mL of sterile saline may be warranted if there is suspicion for open injury to the knee joint. Palpation of closed injuries may reveal crepitus, fracture gap, and swelling indicative of hemarthrosis. A straight-leg test must be performed to assess the extensor mechanism, although this may be limited secondary to pain.
Radiographic evaluation with anteroposterior (AP) and lateral knee radiographs should be obtained. Lateral views are useful to assess for displacement of the patellar fragments. Advanced imaging of patellar fractures is rarely warranted. Imaging of bipartite patella most commonly demonstrates a fragment at the superolateral pole. They can be differentiated from acute fractures by the appearance of smooth edges. Imaging of the contralateral knee may reveal a bilateral bipartite patella in up to 50% of cases.
Restoration of the extensor mechanism is the primary treatment goal for patellar fractures. Nonoperative management is indicated for minimally displaced fractures (less than 2 to 3 mm of step-off and less than 1 to 4 mm fracture gap) with the extensor mechanism in continuity. Patients with displaced patellar fractures who are not medically or psychologically fit for surgery may also undergo a trial of nonoperative treatment. A long-leg splint, knee immobilizer, or hinged knee brace locked in extension can be used to stabilize the fracture while allowing early weight-bearing. Physical therapy is recommended after the pain subsides to work on the active and active-assisted range of motion. Repeat radiographs should be obtained a week after initiation of motion exercises to evaluate for fracture displacement. Approximately six weeks after initial injury, the patient can be started on resistance exercises.
Open reduction and internal fixation are indicated for open fractures, substantially displaced fractures, and fractures with a loss of extensor function. Urgent irrigation, debridement, and antibiotic treatment are indicated for open fractures. The Arbeitsgemeinschaft fur Osteosynthesefragen (AO)/Association for the Study of Internal Fixation (ASIF) developed the technique of tension band wire fixation as the standard of care for patellar fractures. This technique converts the tension forces along the anterior fracture site into compression forces at the articular surface. As with any attempt at tension band technique, success relies heavily on a straight transverse, non-comminuted compression side. Evolutions of the surgical treatment led to the modified anterior tension band (MATB) technique, which combines an 18-gauge stainless-steel wire in a figure-of-eight configuration wrapped around two vertical Kirschner wires (K-wires). According to the AO, the MATB technique should be combined with an additional cerclage around the patella to supplement the fixation for comminuted patellar fractures. Wire migration and soft tissue irritation have prompted research into alternative materials and minimally invasive approaches. Despite the variety of techniques and instrumentations described in the literature, high union rates and clinical outcomes based on the Bostman grading scale have been reported after surgical fixation. Other methods of fixation include tension band plating along the anterior surface, as well as circumferential cerclage wiring or plating around the perimeter of the patella. These methods of fixation are typically reserved for highly comminuted fractures in which tension band principles cannot be used.
Every attempt should be made to preserve the patella; however, resection of all (total patellectomy) or part of the patella (partial patellectomy) may be indicated in certain situations. Biomechanical studies have demonstrated that removal of the patella decreases the quadriceps strength by 50%. Partial patellectomy is indicated for severely comminuted fractures of the superior or inferior pole that comprise less than 40% of the patellar height and are not amenable to fixation. This is performed by excising the fracture fragment and advancing the quadriceps or patellar tendon via suture tunnels or suture anchors.
Postoperatively, similar to nonoperative treatment, patients are placed in a hinged knee brace locked in extension and are allowed to weight-bear as tolerated after surgical fixation. Early knee motion is usually begun at the first follow-up visit to prevent stiffness. As the pain diminishes, knee motion is progressed, and isometric strengthening of the quadriceps is initiated. The rehabilitation protocol may vary after a partial patellectomy or a tenuous fixation by delaying knee motion to ensure adequate healing.
Postoperative complications include symptomatic hardware, implant failure and wire migration, and knee stiffness. Nonunion and infection are rare complications but are more likely in open fractures. The rate of osteonecrosis of the patella following fractures is reportedly as high as 25%; however, this has not significantly affected clinical outcomes.