AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

INTRODUCTION

Section 2 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

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.

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 primarily occur with indirect mechanisms, and the fractures can be displaced or nondisplaced (see Images 1-4). 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.¹ 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. 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.²

Grogan et al classified other types of patellar avulsion fractures.³ 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.⁴ 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.^{5, 6, 7}

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,^{8, 9} 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.¹


• 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.¹⁰ 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)


3. Lateral or vertical (20% of patients)


5. Superolateral (most common type; 75% of patients)

Clinical Details

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.

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.

RADIOGRAPH

Section 3 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

Radiographic examination for the detection of patellar fractures optimally includes anteroposterior (AP), lateral, and tangential or Merchant views.

AP radiographs may obscure the findings of patellar fractures. Lateral views can be useful in evaluating the trabecular arrangement of the patella, as well as comminution and separation of fracture fragments. Tangential views are especially helpful in assessing vertical fractures, as well as in distinguishing a fracture from a partitioned patella.

Transverse fractures are characterized by a lucent fracture line that courses medially to laterally across the middle or distal third of the patella. Transverse fracture fragments may be displaced. Vertical fractures demonstrate a fracture line that courses superiorly to inferiorly, and these fractures can also be displaced. Comminuted fractures demonstrate a stellate pattern of fracture. Osteochondral sleeve fractures are characterized by a small avulsion fragment from the inferior pole of the patella, which is best demonstrated on the lateral view; these findings are usually accompanied by the presence of an effusion and a high-riding patella.

Degree of Confidence

Other imaging modalities (eg, MRI) are more useful than radiographs in fully characterizing the cartilaginous injury associated with an osteochondral patellar fracture and can define radiographically occult fractures. Because a sleeve fracture is in the coronal plane of the patella, this injury may be difficult to diagnose based on plain radiographs.

False Positives/Negatives

The differentiation of an acute fracture from a partitioned patella may be difficult on radiographs. Usually, the features of bipartite patella include a wide radiolucent line that courses across the superolateral margin of the patella, as well as smooth, well-corticated, opposing margins. These features are well depicted in the tangential projection. Because a bipartite patella is often bilateral, views of the opposite knee can be helpful for comparison. A sleeve fracture occurs in the coronal plane of the patella and may be difficult to diagnose on plain radiographs.

CT SCAN

Section 4 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

In patellar fractures, CT scanning is primarily performed for occult or osteochondral injuries. The patient is placed in the supine position with his or her feet externally rotated 15° and pressed against a perpendicular footrest. CT scan sections are obtained with the knee at rest, with the knee extended and quadriceps contracted, and with the knee in 15° of flexion with a relaxed quadriceps.

The position of the fracture fragments can be determined by identifying the fracture line. The reconstructed images in the sagittal, coronal, and axial planes can aid in localization of the fragments.

Degree of Confidence

CT scanning is limited in the evaluation of soft-tissue injury, but MRI may be performed to further evaluate this condition. CT scanning may be more useful than MRI in the identification of loose bodies.

MRI

Section 5 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

MRI is useful in the diagnosis of patellar injuries when making the clinical diagnosis is difficult, as with cases of sleeve fractures. On MRIs, the appearance of the normal patella includes signal intensities that are consistent with those of bone marrow and cortex. Articular cartilage has a lower signal intensity than marrow spin-echo images that are obtained with a short repetition time and echo time spin-echo. The signal intensity slightly increases on images obtained with a longer repetition time and echo time.

MRI is advantageous in the assessment of imaging signs related to patellar fracture, including bone bruises, soft-tissue injury, and loose bone fragments. The anatomy of the patellofemoral joint can be assessed in several planes. For example, axial imaging is useful for evaluating patellofemoral joint alignment, retinacular attachments, and overlying patellar cartilage. Sagittal images are useful for evaluating the quadriceps muscles and tendons, and the patellar tendon can be imaged in all 3 planes.

Traumatic dislocation of the patella may result in patellar fracture, medial retinaculum damage, lateral femoral condyle contusion, and effusion. These injuries can be evaluated with T1-weighted , T2-weighted fast spin-echo (FSE), or short-tau inversion recovery (STIR) images in all 3 planes. Avulsions of the medial retinaculum are best imaged with fat-saturated T2-weighted FSE for detection of bone fragments, edema, and hemorrhage. Edema and hemorrhage appear as areas of increased signal intensity on T2-weighted, fat-saturated T2-weighted FSE, and STIR images. Increased signal intensity in the patella after dislocation can be seen on T2-weighted images (see Image 5).

A dorsal defect of the patella, which is a benign defect of the posterior patella covered by articular cartilage, can mimic osteochondral pathology. MRI is useful in characterizing this lesion, which has intermediate or low signal intensity on T1-weighted MRIs, sometimes with central regions of increased signal intensity, and is usually a centimeter in diameter, well defined, and located in the superolateral part of the patella.

Fractures of the inferior pole of the patella may be associated with tears of the patellar tendon. Patellar tendon tears are characterized by increased signal intensity on T2-weighted, fat-saturated T2-weighted FSE, or STIR images. The tendon may be retracted or thickened.

Patellar fractures may also be associated with damage to the quadriceps tendon and/or extensor musculature. Coronal or sagittal images are helpful in illustrating the longitudinal extent of the muscular injury, whereas axial views are helpful in defining the anatomic relationships among the involved muscle groups. Axial T1-weighted imaging is appropriate for evaluation of muscular atrophy. Increased signal intensity in the quadriceps tendon on images obtained with any sequence is highly associated with the presence of a muscular tear.

Sinding-Larsen-Johansson syndrome, which is defined as osteochondrosis of the distal pole of the patella at the tendinous insertion, is of uncertain etiology but related to chronic traction injury. This condition may mimic a stress fracture of the patella, an osteochondral sleeve fracture, or an un-united ossification center (see Images 6-8). The injury is characterized by a focal area of decreased signal on T1-weighted images and increased signal intensity on gradient-echo or fat-saturated T2-weighted FSE images.

Occult stress or insufficiency fractures are characterized by the presence of a linear band of low signal intensity on images obtained with all sequences; surrounding edema is also depicted (see Images 9-11). These fractures may also be detected in the healed phase (see Images 12-13).

ULTRASOUND

Section 6 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

The effectiveness of ultrasonography in the setting of patellar fractures is limited. Because of the close association of the articular cartilage posterior to the patella with the femur, an adequate scanning angle that is unimpeded by the overlying bone is not attainable. Therefore, ultrasonography is rarely used in clinical practice.

Secondary signs may be present in an acute patellar fracture, and sonograms may demonstrate these well. For example, ultrasonography can be useful in defining an effusion within the suprapatellar bursa, a well-defined fluid-filled space that is superior to the patella and posterior to the quadriceps tendon. The normal moderate echogenicity of the patellar ligament and quadriceps tendon may be interrupted by focal areas of echogenicity, or they may be discontinuous in a partial tear.

NUCLEAR MEDICINE

Section 7 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

Bone scanning is useful for evaluating a stress fracture of the patella, especially with an injury that is superimposed on a bipartite patella. Early after an injury to the patella, increased radionucleotide uptake can be present. This finding is indicative of hyperemia and edema. In the acute phase (first 3-4 wk), uptake in the patella is more diffuse. In the subacute phase (2-3 mo), radionucleotide uptake becomes more intense and localized, and then in the healing phase, the uptake gradually decreases. Within 2 years, the results of 90% of bone scan studies return to normal.

Degree of Confidence

A normal patellar bone scan that is obtained 1 week after trauma excludes an occult patellar fracture. Consider the use of other imaging modalities (eg, CT scanning, MRI) for suspected injuries because the resolution and evaluation of soft tissues is limited with bone scanning.

INTERVENTION

Section 8 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Treatment options for patellar fractures include nonsurgical and surgical options as follows:  

• Nonsurgical treatment is appropriate in the setting of nondisplaced vertical, peripheral, comminuted, and transverse patellar fractures that have an articular step-off of 2 mm or less. Results of long-term studies have shown that arthrosis increases with increasing levels of malalignment beyond 2 mm. Nonsurgical treatment consists of the use of immobilization cylinders or an above-the-knee cast with the knee in extension for 4-6 weeks.


• Surgical treatment is advised for displaced fractures, which are defined as fractures that have an articular step-off of more than 2 mm or a separation of more than 3 mm. The goal of surgical treatment is to preserve patellar function. Open reduction and internal fixation techniques are most useful for transverse fractures or minimally comminuted fractures. The popular modified tension-band technique consists of placing 2 Kirschner wires (2 mm each) across the fracture, as well as a looped 18-gauge wire over the Kirschner wires and the anterior patella (see Images 14-16). Alternatively, instead of Kirschner wires, cannulated screws may be used to traverse the fracture fragments in conjunction with the tension band. This technique may result in improved stability (see Images 17-18). Fragments not amenable to fixation may be removed.

MULTIMEDIA

Section 9 of 10  

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Media file 1:  Radiograph of a displaced transverse fracture of the patella.
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Media file 2:  Radiograph of a displaced transverse fracture of the patella.
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Media file 3:  Radiograph of a nondisplaced transverse fracture of the patella.
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Media file 4:  Radiograph of a nondisplaced transverse fracture of the patella.
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Media file 5:  Magnetic resonance image following a patellar dislocation.
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Media file 6:  Axial T2-weighted magnetic resonance image of a bipartite patella.
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Media file 7:  Sagittal T1-weighted magnetic resonance image of a minimally displaced patella fracture.
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Media file 8:  Magnetic resonance image of a "jumper's knee."
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Media file 9:  Magnetic resonance image of an occult patellar fracture.
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Media file 10:  Magnetic resonance image of an occult patellar fracture.
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Media file 11:  Magnetic resonance image of an occult patellar fracture.
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Media file 12:  Magnetic resonance image of a healed patellar fracture.
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Media file 13:  Magnetic resonance image of a healed patellar fracture.
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Media file 14:  Radiograph following modified tension band fixation.
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Media file 15:  Radiograph following modified tension band fixation.
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Media file 16:  Radiograph following modified tension band fixation.
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Media file 17:  Radiograph following screw fixation.
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Media file 18:  Radiograph following screw fixation.
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REFERENCES

Section 10 of 10

• Authors and Editors
• Introduction
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

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18. McCarroll JR. Acute dislocation of the patella resulting in a comminuted patella fracture. A case report. Am J Sports Med. Mar-Apr 1981;9(2):117-8. [Medline].
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Article Last Updated: May 24, 2007