eMedicine Specialties > Radiology > Musculoskeletal

Patella, Fractures

Author: Christine Lamoureux, MD, Diagnostic Radiologist, Department of Radiology, Rocky Mountain Medical Imaging
Contributor Information and Disclosures

Updated: Jan 21, 2010

Introduction

Background

Several primary types of patellar fractures have been identified, each with separate diagnostic, imaging, and management considerations. The primary types include transverse, vertical, marginal, and osteochondral fractures. Transverse patellar fractures, displaced and nondisplaced, are seen in the images below.

Wright et al studied the use of a reinforced braided polyblend suture (FiberWire) for tension-band fixation of displaced transverse patellar fractures. They found that compared to stainless-steel wire, FiberWire maintained its initial stiffness until failure and had a significantly higher failure load when the suture was tied and locked under the tension of a modified Wagoner hitch.²

Meding et al identified the factors associated with patellar component failure in posterior cruciate ligament-retaining total knee arthroplasty (TKA) with all-polyethylene patellar components. Patellar fracture was identified in 5.2% of patients, and patellar component loosening occurred in 4.8%. A body mass index greater than 30 kg/m² was associated with the greatest risk of fracture, along with male gender, preoperative varus alignment greater than 5º, and a large patellar component. TKA performed with a lateral release was associated with the greatest risk of patellar loosening, along with medial patellar component position, tibial component thickness greater than 12 mm, preoperative valgus alignment of 10º or more, and preoperative flexion of 100º or more.³

Pathophysiology

Traumatic fractures of the patella occur with both direct and indirect mechanisms. A direct mechanism, such as a fall, focuses the mechanical forces directly on the patella and results in a higher degree of comminution, less displacement of fracture fragments, and more damage to the articular cartilage compared with an indirect mechanism. Indirect mechanisms, such as jumping (rapid flexion against a fully contracted quadriceps), increase tension and compression on the patella and result in less comminution, increased fracture fragment displacement, and less damage to articular cartilage.

Transverse fractures of the patella occur primarily with indirect mechanisms, and the fractures can be displaced or nondisplaced, as demonstrated in the images in the previous section. Transverse fractures tend to occur in the central aspect of the patella or in its distal one third. Comminution may also be present.

Vertical fractures are rare and course superiorly to inferiorly in the sagittal plane.

Marginal fractures involve the edge of the patella and do not extend across the bone. They are not associated with disruption of the extensor mechanism.

Osteochondral fractures occur as a result of a direct or indirect blow and/or patellar dislocation. Compared with adults, children are more vulnerable to this type of fracture because they have more patellar mobility. Osteochondral fractures may occur in 5% of patients with acute patellar dislocation.⁴ The fracture occurs at the point of contact, with a separate fracture fragment that contains articular cartilage, subchondral bone, and supporting trabecular bone. These fragments may be displaced intra-articularly and become loose bodies, or they remain in place and heal.

Patellar dislocation occurs when the weightbearing knee twists into a valgus position and a snap can be felt or heard. The indirect mechanism can occur, for example, when one stumbles with the knee partly flexed, causing a strain to be placed on the actively contracting extensor mechanism. The resultant fracture is transverse and involves the lower pole of the patella.

A separate type of osteochondral patellar fracture occurs in children and adolescents. Known as the sleeve fracture, this is an uncommon injury consisting of avulsion of a portion of the articular surface.⁵ A sleeve fracture may involve the superior, inferior, medial, or lateral aspect of the patella. When it involves the inferior pole, usually with indirect trauma (eg, forceful contraction of the quadriceps tendon against the flexed knee), a portion of the patellar bone and retinaculum and a large portion of articular cartilage are displaced inferiorly such that the larger, superior fragment is high compared with the contralateral side.⁶

Grogan et al classified other types of patellar avulsion fractures.⁷ These types include avulsions of the superior pole, medial aspect (after acute lateral dislocation), and lateral aspect of the patella caused by chronic stress at the site of insertion of the vastus lateralis muscle.

Patellar fractures may be secondary to anterior cruciate ligament reconstruction with an autogenous patellar tendon in which a middle-third patellar bone block is removed. The remaining portion of patella is at an increased risk of fracture, especially in the setting of accelerated knee rehabilitation programs.⁸ Such fractures are relatively rare, associated with superimposed direct or indirect trauma, and most likely to occur during the first 8-10 weeks of knee rehabilitation.

Pathologic fracture of the patella has been reported. Broad categories of disease that affect the patella include the following:

• Infection
• Degenerative or metabolic disease
• Benign or malignant primary or metastatic tumors

Tumor types can comprise the following:

• Chondroblastoma
• Histiocytosis
• Giant cell tumor
• Simple bone cyst
• Hemangioma
• Osteochondroma
• Lipoma
• Brown tumor of hyperparathyroidism
• Osteoblastoma

Malignant tumors include lymphoma and hemangioendothelioma; however, metastatic tumors and myeloma rarely affect the patella. Another rare cause of pathologic fracture of the patella is gout.^9,10,11

Frequency

United States

Fractures of the patella are responsible for approximately 1% of all skeletal injuries in both adults and children.

Mortality/Morbidity

Many of the complications of a patellar fracture can be recognized radiographically.

• Orthopedic hardware failure may result in malalignment of fractured patellar fragments; in these cases, further surgery may be necessary. Other complications related to hardware placement include sepsis, malunion or nonunion, and femoropatellar degenerative arthritis.
• A distance of 3 mm or more between fractured patellar fragments should be noted in the radiology report. This degree of separation may lead to an increased incidence of malunion and posttraumatic degenerative arthritis. Recognizing an osteochondral fracture is important because displacement of a fragment that contains cartilage, subchondral bone, and trabecular bone may occur, resulting in a loose body.
• In long-term follow-up studies, degenerative arthritis of the patella has been reported to be more common in knees that were injured previously than in noninjured knees. The arthritis may be due to surface irregularities that involve the fracture fragments, as well as damage to the articular cartilage, resulting in increased contact stresses.

Sex

No sex predilection for the incidence of patellar fracture is reported. In the case of un-united bipartite patella, which may be radiographically confused with an acute fracture of the patella,^12,13 the male-to-female ratio is 9:1.

Age

In both adults and children, patellar fracture represents 1% of all fractures. 

• Osteochondral fractures are more common in children and adolescents than in adults. The fractures are estimated to occur in approximately 5% of all acute dislocations of the patella in children.^4,5
• In a study by Flachsmann et al, mature, immature, and adolescent bovine cartilage bone laminates were loaded with pure shear forces under impact, and the resultant injuries were compared.¹⁴ Adolescent tissue was deemed more vulnerable to injury than either the more immature or the mature tissue.

Anatomy

The patella is the largest sesamoid bone in the body. Its superior pole is the major site of attachment of the quadriceps aponeurosis, which is also known as the trilaminar quadriceps tendon. The trilaminar quadriceps tendon consists of the superficial rectus femoris muscle and tendon, the vastus intermedius muscle and tendon, and portions of the muscles and tendons of the vastus medialis and the vastus lateralis.

The inferior pole of the patella is the major site of attachment for the patellar ligament or tendon, which inserts distally onto the anterior lip of the tibia and the tibial tubercle. A portion of the patellar ligament is composed of fibers of the rectus femoris that course over the surface of the patella. Medially, the patellar attachment is formed by the medial retinaculum, which is a confluence of the tendons of the vastus medialis and the rectus femoris. These tendons attach to the superomedial border of the patella and the medial condyle of the tibia. The lateral retinaculum of the patella is composed of the tendon of the vastus lateralis, which inserts into the superolateral border of the patella and the lateral tibial condyle. Anteriorly, only a thin layer of skin, subcutaneous tissue, and the prepatellar bursa cover the patella; posteriorly, it is lined by thick articular cartilage.

The patella's primary functional role is knee extension, in which tensile forces from the quadriceps muscles are transferred to the proximal patella and then distally via the patellar ligament's attachment to the tibia. Posteriorly, contact stresses develop when the patella articulates with the femur. This 3-point bending stress is concentrated in the anterior patella  and involves both tension and compressive forces. The stress is maximal at 45° of flexion (2-10 N/mm²).

Ossification of the patella occurs between the ages of 2 and 6 years; however, the patella may be congenitally absent or hypoplastic, as in nail-patella syndrome (Fong disease). In 77% of persons, only a single center of ossification exists. In the remaining 23% of patients, 2-3 separate centers of ossification may exist. These secondary centers typically coalesce by the time children reach age 12 years, but the centers may remain separate in 2% of children. Radiographically, the ossification centers that do not fuse with the major primary portion remain visible, as in bipartite patellae.

Of the 2% of the population with an un-united bipartite patella, only 2% are symptomatic. Bipartite patella occurs unilaterally in 57% of these individuals and bilaterally in 43%. A fracture or fibrous nonunion of a bipartite patella may be acute or a result of chronic stress.

The Saupe classification for partitioned patella includes the following: 

1. Inferior pole (5% of patients)
2. Lateral or vertical (20% of patients)
3. Superolateral (most common type; 75% of patients)

Presentation

A fracture of the patella should be considered when the patient presents with persistent patellar tenderness and pain or a joint effusion and a history of a direct or indirect injury.^15,16,17

An osteochondral sleeve fracture may cause swelling, tenderness and hemarthrosis, proximal displacement of the patella, difficulty with weightbearing, and an inability to actively and fully extend the knee. A palpable gap may also be noted in the extensor mechanism. On physical examination, this fracture may be difficult to distinguish from a patellar-ligament rupture. An osteochondral sleeve fracture may require surgical reduction if the separation among fragments is wide.

Bipartite patellae are usually asymptomatic. After a patient sustains this type of injury, the primary clinical sign of a fracture or fibrous nonunion of a bipartite patella is tenderness over the superolateral aspect of the patella.

Preferred Examination

In most patients, radiographs are the most useful imaging tool for the examination of patella fractures, followed by computed tomography (CT) scans, bone scans, and magnetic resonance images (MRIs).

CT scanning is useful when a suspected fracture is not visible on radiographs. The expeditious use of CT scanning can prevent a delay in treatment and help to identify the position of fracture fragments and the localization of intra-articular loose bodies.

Bone scans are also useful when a fracture is suspected yet the radiographic findings are normal. If the bone scan results are also normal, a fracture can be excluded. However, if the findings are positive, the age of the fracture cannot be accurately determined because bone scanning results can be positive in the setting of fractures for as long as 24 months.

MRI also can help detect abnormalities that are not identified on plain radiographs. Compared with bone scanning, MRI can be performed without delay, it does not use radiation, and it may be less expensive. This modality can show bone-marrow and soft-tissue injury with great detail.

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