Patella fractures make up approximately 1% of all fractures and are usually seen in active individuals between the ages of 20 and 50. Men are nearly twice as likely as women to sustain this injury. The mechanism is usually a direct blow to the patella and may be from a low energy fall or a high energy dashboard injury. In high energy mechanism, ipsilateral femur fracture or proximal tibia fracture should be sought. Indirect mechanisms can occur from forceful contraction of the quadriceps usually in an eccentric manner.
The patella is the largest sesamoid bone in the body. This bone lies in the substance of the quadriceps mechanism. The quadriceps tendon inserts into the superior pole, while the patellar ligament originates at the inferior pole.
The patella takes a triangular shape with the apex distally. On the undersurface, the proximal 75% of the patella is covered with articular cartilage. This articular cartilage is the thickest in the body and may be as thick as 1 cm. The articular surface is made up of medial, lateral and odd facets which are concavities that help patella tracking when articulating with the distal femur. The lateral facet is the largest and makes up ~50% of the articular surface. The medial and lateral facets are separated by a central ridge. The odd facet is medial to the medial facet and articulates with the distal femur in deep flexion.
The blood supply to the patella is important to consider in anterior approaches to the knee and during open patella fixation. The primary blood supply to the patella is from the anastomosis of the geniculate arteries. The arterial supply comes from the superior geniculate, medial, lateral superior, lateral inferior and recurrent tibial vessels. The superior portion of this anastomosis is anterior to the quadriceps tendon, while the inferior contribution passes posterior to the patellar ligament thought the fat pad.
The soft tissues that surround the patella are intergral to the function of the extensor mechanism. The patellar retinaculum comes from the deep fasica lata and combines with the tendons of the vastus medialis and vastus lateralis to form the medial and lateral retinaculi. The retinaculi insert in the proximal tibial and assist in knee extension.
Patella fractures are classified in descriptive terms. The fracture pattern can be described as nondisplaced, transverse, avulsion of the inferior or superior pole, comminuted or vertical.
The OTA classification breaks the fractures down into: Type A - extraarticular, Type B - partial articular and Type C - complete articular with disrupted extensor mechanism.
Patients typically present with a swelling, ecchymosis and tenderness to palpation directly over the patella. There is a large effusion and the patients are often unable to ambulate.
The diagnosis is made though a combination of history, physical examination and radiographs. Physical examination will reveal a largereffusion and if the fracture is displaced, a palpable defect. The skin should be carefully inspected for open fracture. A intraarticular saline load test may aide in this determination. It is also important to assess the continuity of the extensor mechanism. This is best assessed through active straight leg raise and will be important in determining if operative intervention is required. Active knee extension does not rule out a fracture, but simply determines that the extensor mechanism is intact. Aspiration of the hemarthrosis and instillation of local anesthetic will the diagnostic usefulness of this test. A thorough examination of the ipsilateral extremity should ensue to evaluate for associated injury.
Radiographs should include AP, lateral and axial views (Merchant). The lateral view is the best for determining fracture pattern and comminution. The Merchant view may demonstrate a vertical fracture and is the only axial view practical in the trauma setting as the patient remains supine with the knee flexed to 45 degrees. On the AP, the inferior pole of the patella should lie within 2 cm of a plane formed by the distal femoral condyles. On the lateral, the proximal pole of the patella should lie below the anterior cortex of the femur when the knee is flexed to 90 degrees. MRI and CT scans are rarely indicated for patella fractures.
Differential Diagnosis/Associated Injuries
If an open fracture is suspected NS should be injected to look for communicating lesions. Patients in trauma situations should be assessed for associated lower extremity injuries.
Nonoperative treatment is indicated for nondisplaced fractures with intact extensor mechanisms. Nondisplaced fractures have fragment separation less than 3mm and less than 1mm articular step off. Treatment consists of cylinder cast or long leg cast for 4-6 weeks. If treated in a cylinder cast, patients may be weight bearing as tolerated and quadriceps exercises in the form of straight leg raises can begin immediately.
Operative treatment is recommended for patients with displaced fractures who are medically stable to undergo surgery. Skin and soft tissues should be allowed 1-2 to heal prior to surgery. Tension band wiring is appropriate for most transverse fractures and some mildly comminuted fractures. The principle of tension band wiring is to covert the tension forces from the extensor mechanism into compression forces across the fracture. Two 1.6mm K wires or 4.0mm cancellous screws should be placed closer the anterior surface of the patella (tension side) and combining the tension band with interfragmentary screws will improve stability. The figure of eight wire confiruation has been shown to be more stable than circular wiring. For fractures with two or three large fragments, these fragments can be lagged and a tension band applied to the stabilized reduced fragments.
For comminuted fractures not immediately amendable to tension banding, cerclage wiring placed immediately adjacent to the bone to obtain indirect reduction can often be followed by anterior tension banding.
Due to soft tissue irritation from the 18gauge wire, a few studies have shown that heavy (No. 5) braided polyester suture was approximately 75% as strong as 18 gauge wire and was comparable to wire fixation.
For low demand patients with severe comminution of the superior or inferior pole of the patella, partial patellectomy with repair of the soft tissues can be effectively employed. However, all efforts to retain large fragments should be made.
Total patellectomy should be reserved for cases of severe comminution not amendable to cerclage wiring. Total patellectomy is technically less complication and can lead to shorter periods of immobilization, but extensor strength can be decreased by 50% leading to significant functional impairment. Soft tissue imbrication should be concurrently perfomed to minimize extensor lag. Post total patellectomy, the leg should be immobilized in slight flexion in a long leg cast for 6 weeks.
Soft tissue irritation from harware - Subcutaneus location of hardware and large joint range of motion lends itself to soft tissue irritation. This may necessitate hardware removal post fracture healing.
Nonunion - occurs in approximately 2%. However, low demand patients may achieve adequate function. Painful nonunions require revision fixation or partial patellectomy.
Infection - most often related to open fracture. Nonviable bone fragments should be removed at initial debridement.
Avasuclar necrosis - occurs when significant portions of the patellar blood supply are disrupted through the injury mechanism or excessive surgical dissection. Revascularization may occur over time.
Posttraumatic arthritis - occurs secondary to articular incongruity. Care must be taken to achieve anatomic reduction of the articular surface for displaced fractures. Despite careful reduction post-traumatic arthritis may ensure and may possible be related to the initial injury. Tibial tubercle realignment or patellofemoral arthroplasty may relieve symptoms.
Red Flags and Controversies
Bipartite patella may be confused with patella fracture. Bipartite patella is congenital and occurs when the when the ossification center fail to fuse. Most are stable synchondrosis. May occur in up to 1% of the population and be bilateral in 45%. Most remain asymptomatic. They are classified into three types: Type I - inferior pole, Type II - lateral margin, Type III: - superolateral. Type III is most common. Bipartite patella is usually easily distinguished from patella fracture by the lack of hemarthrosis, history, degree of tenderness and radiographic appearance.
The site of reattachment of the patella tendon (articular surfact vs. anterior surface in partial patellectomy is controversial. MRI and anatomic evaluations reveal that the native patella tendon originates anterior which is contrary to the classical partial patellectomy technique which sutured the tendon near the articular surface.
Most series of tension banding for transverse fractures reveal low nonunion rates and skin irritation in approximately 5%. Good results have been reported after partial patellectomy, however the results may depend on fracture pattern. The results of total patellectomy are generally poor due to the decreased extensor strength secondary to the loss of the patellar lever arm. Retaining some fragment of the patella may improve the lever arm and function.