Anatomic Considerations

The patella is the largest sesamoid bone in the body and serves to increase the leverage and efficiency of the quadriceps muscle. The patella is generally triangular, with the apex pointing distally. The rectus femoris and vastus muscles (lateralis, medialis, intermedius) all insert on the proximal pole. The patellar ligament attaches to the distal pole of the patella and inserts on the tibial tuberosity. The medial patellofemoral ligament is the primary restraint to lateral subluxation of the patella, particularly in early flexion. Other medial stabilizers include the medial retinaculum, the medial patellotibial ligament, and the vastus medialis obliquus muscle. A thin layer of quadriceps tendon passes over the patella and joins the patellar ligament distally. The medial and lateral retinacula, which are extensions of the quadriceps muscle, bypass the patella and insert directly on the tibia. If the retinacula are intact after a patellar fracture, the patient is able to extend the knee actively. The primary blood supply of the patella enters the central portion and the distal pole. Fractures through the midportion of the patella compromise the vascular supply to the proximal pole and leave it at risk for avascular necrosis (AVN).

Fractures of the patella are broadly classified as transverse, stellate or comminuted, vertical, and osteochondral. Any of these fracture types can be displaced or nondisplaced. Transverse fractures are the most common type followed by stellate fractures. The majority of transverse fractures are in the lower third of the patella. Vertical fractures usually occur along the margins of the patella. An osteochondral fracture involving the medial patellar facet can occur after a patellar dislocation or subluxation.

Mechanism of Injury

Patellar fractures result from either direct or indirect forces. Most fractures are due to direct forces such as a fall on the anterior aspect of the knee or striking the knee on the dashboard in a motor vehicle accident (MVA). Indirect injuries occur when patients stumble and attempt to prevent themselves from falling. The quadriceps muscle contracts forcefully, exceeding the intrinsic strength of the patella. The patella actually fractures before the patient strikes the ground. Continued quadriceps muscle contraction tears the medial and lateral retinacula, which results in fracture displacement. Fractures caused by direct trauma are usually stellate and nondisplaced; fractures caused by indirect forces are usually transverse and displaced.

Clinical Presentation

The patient has tenderness and swelling over the anterior knee. When the fracture is displaced, a defect or separation is usually palpable if swelling is not excessive. The most important aspect of the examination is to check for the patient’s ability to actively extend the knee against gravity. If this capability is absent, the quadriceps mechanism is disrupted. Local anesthetic can be injected into the knee joint to relieve pain before testing extensor function.

Imaging

Anteroposterior (AP), lateral, and sunrise views of the knee should be adequate for diagnosing most patellar fractures ( Figure 12-1 ). The lateral view is the most useful view in delineating fracture lines and determining displacement ( Figure 12-2 ). A separation of more than 3 mm between fragments or an articular stepoff of more than 2 mm constitutes a displaced fracture. Marginal vertical fractures are best visualized on the sunrise view. A bipartite patella may be mistaken for an acute fracture. This variation of ossification is best seen on the AP view and can be distinguished from an acute fracture by the well-defined zone of separation usually in the superolateral aspect of the patella ( Figure 12-3 ). A radiograph of the opposite knee can confirm this diagnosis because bipartite patella is usually a bilateral finding. Another common finding on lateral radiographs of the knee is a small sesamoid bone in the tendons posterior to the knee joint ( Figure 12-4 ). This normal variant is called the fabella and is of no clinical significance.

Anteroposterior ( A ) and lateral ( B ) views of a nondisplaced transverse fracture of the patella.

Lateral view of the knee demonstrating a markedly displaced transverse fracture of the patella.

(From Browner BD, Jupiter JB, Levine AM, Trafton PG [eds]. Skeletal Trauma: Fractures, Dislocations, Ligamentous Injuries. Philadelphia, WB Saunders, 1992.)

Anteroposterior view showing a bipartite patella with the characteristic finding of a well-demarcated lucent zone in the superolateral aspect of the patella ( arrows ).

(From Mettler FA. Essentials of Radiology . Philadelphia, WB Saunders, 1996.)

Fabella. A lateral view shows a normal variant, a small sesamoid bone in the tendons posterior to the knee joint ( arrow ).

(From Mettler FA. Essentials of Radiology . Philadelphia, WB Saunders, 1996.)

Indications for Orthopedic Referral

Displaced patellar fractures and avulsion fractures involving a quadriceps or patellar tendon injury require operative fixation and referral to an orthopedic surgeon. Patients with severely comminuted fractures should also be referred for consideration of partial or complete patellectomy. Quadriceps tendon repair and preservation of as much of the patella as possible are important in restoring knee extension. Total patellectomy is indicated only for highly displaced, severely comminuted fractures in which no large fragments remain. The timing of the surgery depends somewhat on the integrity of the overlying skin, which is often abraded during the injury. If the abrasions are clean and the wound is less than 8 hours old, the operative risk of infection is low. Extensive abrasions or a contaminated wound may necessitate delaying surgery until the skin heals.

Initial Treatment

Table 12-1 summarizes management guidelines for nondisplaced patellar fractures.

Aspiration of the knee joint hemarthrosis with use of aseptic technique should be considered to reduce pain and swelling, especially if a tense effusion is present. The knee should be wrapped with a compressive elastic bandage and the patient placed in a knee immobilizer with the knee in full extension. Icing and elevation of the knee are important in keeping the swelling to a minimum. The patient should remain non–weight bearing until examined again 5 to 7 days later.

Follow-up Care

Patients who have nondisplaced patellar fractures that have a smooth articular surface and an intact quadriceps mechanism and who are capable of extending the knee against gravity can be treated effectively nonoperatively. A cylinder cast from the groin (not the midthigh) to just above the ankle malleoli with the knee in extension (not hyperextension) is the most reliable form of immobilization. Selected compliant patients can be treated with a removable hinged knee brace locked in extension for walking provided they wear the brace at all times except for bathing. Weight-bearing and straight-leg-raising exercises should be started in the cast or brace. The hinged brace is lighter than a cast and is an especially good choice for an elderly patient so early motion can be started to minimize loss of function.

A follow-up radiograph should be taken after 2 weeks to confirm that the fracture remains nondisplaced. Any displacement beyond the aforementioned limits warrants referral to an orthopedic surgeon. Immobilization should continue until radiographic evidence of union is apparent, which usually occurs in 4 to 6 weeks. Once the cast or brace is discontinued, the patient should be instructed in knee range of motion (ROM) exercises and quadriceps-strengthening exercises. A physical therapy referral is usually necessary to help the patient regain normal strength and motion.

Nondisplaced marginal vertical fractures do not have to be immobilized and can be treated with reduced activity for 3 to 6 weeks and progressive ROM and strengthening exercises.

Return to Work or Sports

The level of activity required by the patient’s job should be considered when deciding how soon a patient may return to work. Patients with relatively sedentary jobs could return to work after several days of elevating and icing the knee if their symptoms allow, but it may be best to wait until after the first follow-up visit (5 to 7 days after injury). Patients whose jobs involve squatting, climbing stairs, or other activities requiring knee flexion should not return until the cast or brace is discontinued and knee ROM has progressed to the point at which they can perform these activities with reasonable comfort. This level of recovery is likely to take a minimum of 6 to 8 weeks. Athletes may begin activities that require little knee flexion (e.g., walking or freestyle swimming with gentle turns) soon after immobilization ends. Patients may begin gentle running when they are able to walk with little or no discomfort and knee ROM permits. This activity can be advanced as symptoms permit, with jumping and cutting activities added last. Patients can return to active jobs or sports more quickly by performing regular weight bearing, straight-leg raises, and ankle ROM while the knee is immobilized and by working diligently on regaining leg ROM and strength after immobilization ends. Referral to a physical therapist can greatly facilitate this process.

Complications

Complications after treatment of nondisplaced patellar fractures are rare, although patellofemoral pain and post traumatic osteoarthritis may occur. Persistent patellofemoral pain after a patellar fracture should be managed with nonsteroidal antiinflammatory drugs (NSAIDs) and physical therapy. Prolonged immobilization may result in significant weakness in the extremity and loss of knee motion, and patients should work with physical therapists to restore function. Fracture fragment separation and dehiscence of fracture repair, infection, and AVN may complicate operative repair of patellar fractures.

Pediatric Considerations

The patella ossifies in early childhood, first becoming visible on plain radiographs between the ages of 3 and 6 years. Up to six separate ossification centers may be present, simulating the presence of a fracture. Palpation for tenderness and a comparison view of the normal knee may be used to distinguish a fracture from a growth plate.

Unlike injuries in adults, a knee effusion in a child is rarely caused by ligament injury. Patellar dislocation or fracture is far more likely. If a child has a traumatic knee effusion, a patellar dislocation that has spontaneously become reduced should be suspected. In a child with a traumatic knee effusion that was not caused by a patellar dislocation, an occult fracture is probably present. Such patients should either be referred or undergo aspiration of the joint. In the presence of normal radiographs, fat in the aspirate would indicate an occult fracture. Because the fracture may primarily involve cartilage, magnetic resonance imaging (MRI) is often required for adequate evaluation.

Most patella fractures in children are either osteochondral fractures or avulsion fractures. Osteochondral fractures occur in approximately 40% of childhood patellar dislocations. Patients with osteochondral fractures should be referred to an orthopedic surgeon.

In children, patellar avulsion fractures may appear deceptively small on radiographs. However, the fragment typically includes a portion of unossified patella, making the fracture much larger than it appears. In patellar sleeve fractures, for instance, much or all of the articular surface of the patella is avulsed along with a small fragment of bone from the inferior pole of the patella. This injury is characterized by a high-riding patella ( Figure 12-5 ), tenderness over the inferior pole, and an extensor lag when leg extension is tested. Patients with sleeve fractures should be referred, as should patients with avulsion fractures that are displaced or complicated by an inability to fully extend the knee. Nondisplaced fractures may be managed as previously described for adults. Healing is likely to be more rapid, allowing earlier discontinuation of the cast.

Sleeve fracture of the patella. On the lateral view, the patella is high riding, and a defect is noted on the inferior border of the patella, which represents the avulsed inferior pole. The fractured inferior portion of the patella overlies the femoral condyle and is not easily seen.

(From Green NE, Swiontkowski MF [eds]. Skeletal Trauma in Children , 2nd ed. Philadelphia, WB Saunders, 1998.)

Transverse fractures of the patella are rare in children. They may be managed as described for adults, with special emphasis on assessing the patient’s ability to actively extend the knee. If the patient is unable to fully extend the knee, referral is indicated.

Patellar Dislocation

Dislocation of the patella usually occurs in patients with underlying patellofemoral malalignment. Predisposing factors that put the patient at risk of subluxation or dislocation include a shallow femoral groove, hypoplasia of the lateral femoral condyle, ligament hyperlaxity, a small or high-riding patella, genu valgum, external tibial torsion, and atrophy of the vastus medialis muscle. Nearly all patellar dislocations are lateral.

Anatomic Considerations

The tibia and fibula are bound together proximally by the tibiofibular articulation, a synovial joint reinforced with strong anterior and posterior ligaments. Distally, the tibia and fibula are connected by the tibiofibular syndesmosis, which is composed of the anterior and posterior tibiofibular ligaments and the interosseous membrane. The bones and muscles of the leg are surrounded by the crural fascia. The interosseous membrane and fibrous extensions of the crural fascia separate the leg into four distinct compartments ( Figure 12-6 ).

Cross section of the lower leg demonstrating the four compartments.

The popliteal artery divides to form the anterior and posterior tibial arteries after it exits the popliteal fossa. The anterior tibial artery enters the anterior compartment just below the level of the fibular head and descends along the interosseous membrane. It is vulnerable to direct injury in proximal tibial fractures.

The medial and lateral tibial plateaus are the articular surfaces of the medial and lateral tibial condyles. The two plateaus are separated by the intercondylar eminence, which serves as the attachment for the anterior cruciate ligament (ACL). The outer aspect of each plateau is covered by the cartilaginous meniscus. The medial plateau and condyle are larger and stronger than on the lateral side, and fractures of the relatively weaker lateral plateau are more common.

Tibial Plateau Fractures

Imaging

AP, lateral, and internal and external oblique views of the knee should be obtained in the evaluation of a suspected proximal tibial fracture. A tunnel (notch) view is useful in visualizing the intercondylar eminence. The tibial plateau normally slopes from anterosuperior to posteroinferior. Findings on the AP view may be subtle and fail to define fracture depression because this view does not demonstrate the normal plateau slope ( Figure 12-7 ). An area of increased bone density from fracture depression may be the only finding on the AP view ( Figure 12-8 ). Split fractures of the tibial plateau are more common in younger patients with dense cancellous bone. Depressed fractures or split fractures with depression are more common in older patients with decreased bone density. Fractures can also occur in both plateaus simultaneously.

A, Anteroposterior view of the knee, showing subtle irregularity of the superior edge of the lateral tibial condyle. B, Lateral view clearly demonstrates a depressed fracture of the anterior aspect of the lateral plateau ( arrow ).

A, Anteroposterior view of the knee showing minimal findings of increased bone density of the lateral tibial condyle ( arrow ). B, The oblique view more clearly demonstrates a depressed, displaced fracture of the lateral plateau.

Additional imaging with computed tomography (CT) scanning is useful in delineating the extent of articular involvement and the degree of fracture depression. MRI has replaced CT scanning in many centers because of its superior ability to detect associated ligamentous and meniscal injuries.

Initial Treatment

Table 12-2 summarizes the management guidelines for nondisplaced tibial plateau fractures.

Table 12-2

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