Patella fractures

INTRODUCTION — The patella is the largest sesamoid bone and serves several important functions. It improves the mechanics of knee extension, protects the knee joint from direct trauma, and assists in providing nourishment for the articular cartilage of the distal femur [1]. Patella fractures account for approximately 1 percent of all skeletal injuries in both adults and children.

The anatomy, presentation, diagnosis, and non-operative management of patella fractures is reviewed here. Other injuries of the knee and lower extremity are discussed separately. (See "Approach to the adult with unspecified knee pain" and "Approach to the adult with knee pain likely of musculoskeletal origin" and "Recognition and initial management of patellar dislocations" and "Patellofemoral pain" and "Proximal tibial fractures in adults" and "Midshaft femur fractures in adults".)

CLINICAL ANATOMY

Patella and surrounding tissue — The patella is triangular with the apex directed distally (picture 1 and figure 1 and figure 2). The superior pole of the patella serves as the site for the insertion of the quadriceps tendon, which is the confluence of the four individual quadriceps muscle tendons (rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis). The quadriceps tendon envelopes the patella and becomes the patella tendon distally, inserting on the tibial tuberosity. Forces exerted by the quadriceps muscles on the patella (eg, when landing on one's feet after a fall from a moderate height) can lead to indirect fracture and proximal retraction of the superior fracture piece, if there is complete disruption of bone or tendon.

The medial patellar retinaculum is an extension of the vastus medialis tendon that attaches to the superomedial border of the patella and distally to the medial condyle of the tibia. The lateral patellar retinaculum is an extension of the vastus lateralis and attaches to the superolateral border of the patella and distally to the lateral condyle of the tibia. If the medial and lateral retinacula are intact following a patella fracture, the patient may retain the ability to actively extend the knee.

The anastomoses of the superior, middle, and inferior geniculate arteries serve as the primary blood supply to the patella (picture 2). Branches of these arteries enter the bone through the central patella and distal pole. Fractures through the mid-patella can thus compromise blood supply to the superior pole and increase the risk for avascular necrosis [1]. Knee anatomy is discussed in greater detail separately. (See "Physical examination of the knee", section on 'Anatomy'.)

Bipartite patella — The patella develops with a single center of ossification 80 percent of the time. In the remaining 20 percent, two or three separate centers of ossification may exist. The patella develops into two separate pieces (ie, bipartite patella) when one of these ossification centers fails to fuse with the main patella. The incidence of bipartite patella is reported to range from 0.2 to 6 percent [2]. The incidence of unilateral bipartite patella is unclear. Some claim it to be a rare phenomenon, while others report the incidence to approach 60 percent [2-4]. It is more common in males than females [2,4,5].

Bipartite patella can be differentiated from acute patella fractures by radiographic appearance and location. Both pieces of a bipartite patella have smooth, well-corticated borders, and there is minimal separation between them. Furthermore, secondary ossification centers appear in specific locations. The Saupe classification of secondary ossification centers identifies three types in ascending order of frequency (figure 3):

●Type 1 – Inferior pole (5 percent)

●Type 2 – Lateral or vertical (20 percent) (image 1)

●Type 3 – Superolateral (75 percent) (image 2)

MECHANISM OF INJURY AND FRACTURE PATTERN — Most fractures of the patella result from direct forces, such as a fall onto a flexed knee or the knee striking the dashboard during an automobile accident [3]. In cases of high-energy blunt trauma (eg, motor vehicle collision [MVC]), the clinician must carefully assess the femur, hip, and pelvis for injury, in addition to the patella.

Fractures can also result from an indirect force applied to the patella through sudden, forceful contraction of the quadriceps. This might occur when someone lands on their feet after jumping from a moderate height or comes to a sudden stop from a full sprint. Should the force generated by the quadriceps exceed the strength of the patella, an avulsion fracture often results. Such a fracture is likely to be displaced if the medial and lateral retinacula are also torn as a result of the indirect trauma.

Patella fractures are classified as displaced if there is greater than 2 mm of articular step-off or 3 mm of separation of the fracture fragments (ie, diastasis) (image 3). Displaced fractures are referred for treatment. (See 'Indications for orthopedic referral' below and "General principles of fracture management: Bone healing and fracture description", section on 'Fracture description'.)

The mechanism of injury determines the fracture pattern. Direct, blunt trauma often causes comminuted, stellate fractures (image 4). These are more frequent in the elderly given their weakened bone and increased propensity to fall. Indirect trauma most commonly causes a transverse fracture, but it can also lead to lower pole or upper pole avulsion fractures. Transverse fractures usually involve the central or distal third of the patella, and there is often associated disruption of the medial and lateral retinacula. Patients with these fractures tend to be younger. Diastasis often results when contraction of the quadriceps pulls the superior fracture fragment proximally.

Vertical fractures are much less common than either transverse or stellate fractures and can be produced by both indirect and direct forces. Osteochondral and chondral injuries can be caused by direct trauma and, in younger adults, can result from patella subluxation or dislocation. Stress fractures are uncommon but can be seen in athletes.

CLINICAL PRESENTATION AND EXAMINATION — A patella fracture should be considered whenever a patient presents with an acutely swollen knee and patella pain following direct or indirect trauma to the anterior knee. In such cases, it is most important that the clinician carefully assess the function of the knee extensor mechanism and look for any signs of an open fracture. Any suspicion for significant concomitant internal injury (eg, abdominal or thoracic) following direct trauma warrants evaluation in the emergency department. (See "Initial management of trauma in adults".)

Examination of the patient with a patella fracture typically reveals a knee joint effusion or hemarthrosis and focal tenderness of the patella. The patient may not be able to extend the knee against gravity, and a gap in the extensor mechanism may be palpable.

Examination of the knee extensor mechanism is important because disruption requires surgical repair. The knee extensor mechanism can be assessed by having the patient bend the knee, either by allowing the injured leg to dangle over the side of the examination table or by placing towels under the injured knee, and then having the patient extend the knee against gravity. This maneuver should be done gently after pain is relieved. Another, possibly less painful, way to assess the extensor mechanism is a straight leg raise. This is performed by having the supine patient maintain their knee in full extension, and then raise the involved leg about 6 inches (15 cm). Joint aspiration may be necessary to relieve the pain from a tense effusion or hemarthrosis but is generally unnecessary unless pain is severe. Intra-articular injection of an analgesic (eg, bupivacaine) may be necessary to determine whether a patient’s inability to extend the knee is due to pain rather than disruption of the extensor mechanism.

Overlying lacerations and abrasions are often present when the mechanism of injury involves a fall onto the knee. The possibility of intra-articular communication (ie, open fracture) must be excluded in this circumstance. This can be done by injecting saline with a small amount of methylene blue dye into the joint away from the laceration or puncture wound in question and looking for extravasation from the wound.

DIAGNOSTIC IMAGING

Plain radiographs — A patella fracture is diagnosed by radiograph. Standard radiographic evaluation includes anteroposterior (AP), lateral, and sunrise views (image 5 and image 6 and image 4). The lateral view is generally most helpful for assessing displacement (image 7). The sunrise view may be difficult to obtain if there is a large effusion or severe pain, and may be unnecessary if the fracture is clearly visible on other views. A tangential view can be helpful in identifying osteochondral fragments and in assessing vertical fractures (image 8). Contralateral views can confirm bipartite patella (8 percent of population). However, it is important to remember that the incidence of bilateral bipartite patella may be as low as 40 percent (image 2 and image 1). (See 'Bipartite patella' above.)

Musculoskeletal ultrasound — Ultrasound may be used to evaluate the knee extensor mechanism and to screen for patellar fractures. The ultrasound examination of the knee is discussed in detail separately. (See "Musculoskeletal ultrasound of the knee".)

To assess the extensor mechanism, a high-frequency (10 mHz) linear transducer is positioned in the long-axis configuration, cephalad to the superior pole of the patella and directly over the quadriceps tendon. It is then moved caudally over the distal quadriceps tendon (image 9), patella (image 10), and patellar tendon (image 11), allowing visualization of the entire extensor mechanism.

A fracture is identified by disruption in the normally continuous, bright (hyperechoic) interface between the bone and soft tissue. Identification of a hypoechoic (dark) collection, suggestive of a hematoma, can help guide the clinician to the site of cortical disruption [6]. The results of some preliminary prospective studies performed in patients with acute knee trauma suggest that musculoskeletal ultrasound has relatively high sensitivity for diagnosing occult knee fractures [7]. Additional, well-performed clinical studies are needed to confirm these findings.

Advanced imaging — Advanced imaging studies such as computed tomography (CT) or magnetic resonance imaging (MRI) are rarely needed solely to evaluate a patella fracture. MRI can be used if necessary to assess injury to the articular cartilage or soft tissues (eg, cruciate ligaments) and can identify fractures not visible on plain radiograph. Bone scans can exclude the rare stress fracture in an athlete. (See "Overview of stress fractures", section on 'Diagnosis'.)

INDICATIONS FOR ORTHOPEDIC REFERRAL — Referral for operative repair is recommended in the following circumstances [1,8]:

●Fractures with greater than 2 mm of articular step-off (ie, displacement) (image 3)

●Fractures with greater than 3 mm of fragment separation (ie, diastasis) (image 3)

●Comminuted fractures, with or without displacement of the articular surface

●Disruption of the extensor mechanism

●Any open fracture or persistent neurovascular deficit requires immediate surgical referral

In addition, avulsion fractures involving the superior or inferior pole of the patella are essentially quadriceps and patellar tendon injuries respectively, and these are treated surgically. Thus, patients with avulsion fractures should be referred to an orthopedic surgeon.

Patients who are debilitated, poor surgical candidates for other reasons, or who have poor bone quality are often managed nonoperatively.

The goal of operative repair is rigid bone fixation that allows for early joint motion and rehabilitation, restoration of articular congruity to minimize the risk of degenerative arthritis, and ultimately restoration of knee function. The multiple techniques for internal fixation of transverse fractures are a topic of considerable debate among surgeons, and little high-quality evidence is available to guide decision-making [9]. The orthopedist assuming care should discuss the treatment options, including the relative risks and benefits of the different surgical techniques, with the patient.

DIAGNOSIS — A patella fracture is initially diagnosed by radiograph, or possibly ultrasound. In the large majority of cases, plain radiographs are sufficient, including antero-posterior, lateral, and sunrise views (although a "standard" sunrise view may not be obtainable if there is severe pain or a large effusion). A patella fracture is suspected on the basis of a history of direct or indirect trauma to the knee and the presence of suggestive clinical findings, including acute knee swelling and focal patellar tenderness.

DIFFERENTIAL DIAGNOSIS — As patella fractures are caused by trauma to the knee, the differential diagnosis consists primarily of other injuries that may be sustained. Such injuries include patella dislocation or subluxation, tears of the quadriceps or patellar tendon, fractures of the distal femur or proximal tibia, tears of knee ligaments or menisci, and soft tissue injuries (eg, quadriceps muscle contusion or tear). In cases of high energy trauma, such as a motor vehicle collision involving a knee striking the dashboard, concomitant injury is common and advanced imaging studies are often needed to determine the extent of injuries. With low energy trauma, concomitant injury does not usually accompany a patella fracture and plain radiographs that clearly identify a patella fracture, combined with an examination that does not reveal ligamentous instability or signs of additional injury, are typically sufficient to rule out other diagnoses. A notable exception to this principle is the geriatric patient, who can sustain multiple significant injuries from a seemingly minor mechanism. (See "Geriatric trauma: Initial evaluation and management".)

INITIAL TREATMENT — Operative management is recommended for displaced or complex fractures in patients able to tolerate both the procedure and postoperative rehabilitation (see 'Indications for orthopedic referral' above). Nondisplaced patella fractures with an intact extensor mechanism that do not meet operative criteria can be treated nonsurgically. Nondisplaced marginal vertical fractures do not require immobilization. They are treated with activity modification for four to six weeks and progressive range of motion and strengthening exercises.

Acutely, nonsurgical patients should be placed in a knee immobilizer or splint that is placed in full knee extension (not hyperextension). Patients are not to bear weight on the injured leg until a cylinder cast or brace locked in full extension is placed. Compression of the knee using an elastic bandage, ice applied to the patella area, and elevation of the leg above the level of the heart are important for reducing swelling and pain following the initial trauma.

Patients should begin performing strength exercises as soon as possible, including isometric contraction of the quadriceps and straight leg raises (picture 3A-B), but only while their knee is immobilized.

FOLLOW-UP CARE — Little published evidence is available to inform management [9]. Definitive treatment for patella fractures that are amenable to nonsurgical management typically consists of immobilization with a cylinder cast or brace from the groin (do not stop at mid-thigh) to the ankle (ending just proximal to the malleoli), with the knee in extension (not hyperextension), for four to six weeks. The cast is generally placed five to seven days following the injury to allow swelling to subside. In lieu of a cast, continued immobilization in a knee immobilizer or locking knee brace may be appropriate for highly compliant patients and elder patients, for whom it is less burdensome.

While there is some debate as to whether to allow weight-bearing once the cast is placed, many feel that weight-bearing as tolerated and strength exercises are important to limit bone loss and muscular atrophy. We agree with this approach and encourage our patients to begin weight-bearing and strengthening exercises as soon as definitive immobilization is in place.

For exercises, we begin with isometric quadriceps contractions and ask patients to perform two sets each day, maintaining the contraction for 10 seconds per set. Gradually, over the course of days to weeks, we ask them to increase the number of sets to three or four and to maintain the contraction for 30 seconds or longer for each set, as pain allows. In addition, we have patients perform straight leg raises (picture 3A-B). They begin by performing two sets of 10 repetitions. Again, gradually and as pain allows, they are encouraged to increase the number of sets to four and the number of repetitions to 20. To minimize atrophy of the hip abductors and extensors, we have patients perform hip abduction raises and prone leg raises. The progressions used for each of these exercises are the same as those used for the quadriceps exercises.

Follow-up radiographs should be taken two weeks after the injury, to ensure the fracture remains nondisplaced, and again four to six weeks after the injury, to look for callus formation. Surgical referral is made if displacement occurs. The same diastasis measurements used to determine the initial need for referral are used here as well, with the caveat that a slight increase in the fracture "gap" is expected at the fracture site early on due to the bone resorption that is a normal part of healing. The increase in the gap should be no more than approximately 1 mm. The cast is removed when callus is seen on radiograph. Complete healing requires 8 to 10 weeks. (See 'Indications for orthopedic referral' above.)

After cast removal, patients begin rehabilitation with knee range-of-motion exercises. Gradually, strengthening exercises are added to the regimen, usually as part of formal physical therapy. Healing typically requires 8 to 10 weeks. It is reasonable to restrict patients from participation in contact sports during that period. (See 'Return to work and sports' below.)

Nondisplaced, marginal vertical fractures do not require immobilization. They are treated with activity modification for four to six weeks and progressive range of motion and strengthening exercises. The large majority of nondisplaced or minimally displaced fractures suitable for nonsurgical management have good outcomes [3,10].

PEDIATRIC CONSIDERATIONS — Avulsion (ie, sleeve) fractures are more common in children. When examining plain radiographs for a possible avulsion fracture of the patella in children, remember that an unremarkable radiograph does not rule out a fracture, bone fragments can be difficult to see on standard radiographs. A distal fracture fragment can be much larger than it appears in the radiograph because it consists largely of cartilage. The management of avulsion fractures is similar to that for quadriceps or patellar tendon injuries, and thus surgery is typically required.

When assessing children, contralateral radiographs often play an important role. As an example, such studies can help the clinician distinguish among bipartite patella, apophysitis (Sinding-Larsen-Johansson syndrome), and avulsion fracture. A high-riding patella suggests a patellar avulsion fracture [2].

Patellar subluxation and dislocation are relatively common among children, but the mechanism of injury is different than that for patella fracture. Lateral patellar dislocation, the most common type, typically occurs when the foot is planted and an internal rotatory twisting force is applied to the flexed knee in valgus (eg, spinning or twirling maneuver in dance or gymnastics, swinging a baseball or softball bat, or quick lateral change of direction while running or ice skating). Focal bony tenderness is absent with patella subluxation or dislocation. (See "Recognition and initial management of patellar dislocations".)

COMPLICATIONS — Potential complications after nonoperative treatment of nondisplaced fractures include decreased knee range of motion (generally loss of terminal extension), weakness due to prolonged immobilization, nonunion, and patellofemoral joint pain [1-3]. There is increased risk of the development of osteoarthritis of the patellofemoral joint. Loss of terminal extension or persistent extension lag does occur, but it usually does not compromise function or the ability to return to sport. Nonunion is rare. Degenerative arthritis often develops after severely comminuted fractures. Patellofemoral pain is treated in standard fashion: icing, stretching of the hamstrings and iliotibial band, and strengthening of the quadriceps and gluteal muscles as indicated by examination.

Complications of operative treatment include infection, failure of hardware (eg, wires breaking), decreased range of motion, nonunion, and osteonecrosis. (See "General principles of fracture management: Early and late complications".)

RETURN TO WORK AND SPORTS — Job requirements and symptoms determine the speed with which patients can return to work. Patients with sedentary jobs may be able to return to work after several days of rest, ice, and elevation if their symptoms are manageable. Patients with active jobs requiring knee flexion (eg, squatting, climbing stairs) should rest from work until knee immobilization is no longer required for treatment and they can perform work-related tasks without unreasonable discomfort. This generally requires six to eight weeks.

Athletes should perform rehabilitation exercises to minimize any loss in strength while their knee is immobilized. (See 'Follow-up care' above.)

Once immobilization is no longer needed, patients may begin activities that require minimal knee flexion, such as walking. Activity intensity and duration should progress gradually under the guidance of a physician, physical therapist, or athletic trainer. Those activities that place greatest stress on the quadriceps mechanism, such as vigorous jumping and abrupt changes in direction, are added last. We suggest that athletes not return to full competitive sport until knee motion is fully restored, strength is at least 80 to 90 percent of the uninjured leg, and they are able to pass a functional, sport-specific assessment supervised by a knowledgeable trainer, coach, or clinician. As an example, for field sports requiring sprints and sudden changes of direction, an athlete might be asked to perform a 20 yard (18 m) shuttle run at full speed.

ADDITIONAL INFORMATION — Several UpToDate topics provide additional information about fractures, including the physiology of fracture healing, how to describe radiographs of fractures to consultants, acute and definitive fracture care (including how to make a cast), and the complications associated with fractures. These topics can be accessed using the links below:

●(See "General principles of fracture management: Bone healing and fracture description".)

●(See "General principles of fracture management: Fracture patterns and description in children".)

●(See "General principles of definitive fracture management".)

●(See "General principles of acute fracture management".)

●(See "General principles of fracture management: Early and late complications".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Lower extremity (excluding hip) fractures in adults" and "Society guideline links: Acute pain management".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Knee pain (The Basics)")

●Beyond the Basics topic (see "Patient education: Knee pain (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Mechanism of injury – Patella fractures result from either blunt trauma or indirect force applied through a contracting quadriceps. (See 'Mechanism of injury and fracture pattern' above.)

●Physical examination – Assessment of the integrity of the knee extensor mechanism (the ability to extend the leg against gravity) and inspection for the presence of an open fracture are of the utmost importance. (See 'Clinical presentation and examination' above.)

●Diagnostic imaging – Standard radiographic evaluation includes anteroposterior (AP), lateral, and sunrise views of the knee. The lateral view is generally most helpful for assessing displacement. (See 'Diagnostic imaging' above.)

●Indications for surgical referral – Orthopedic referral is indicated for the following:

•Fractures with greater than 2 mm of articular step-off

•Fractures with greater than 3 mm of fragment separation

•Comminuted fractures with displacement of the articular surface

•Disruption of the extensor mechanism. (See 'Indications for orthopedic referral' above.)

●Management – Initial treatment includes immobilization of the knee in extension with a knee immobilizer. Definitive treatment for fractures without surgical indications consists of continued immobilization with a cylinder cast from the groin to the ankle (ending just proximal to the malleoli), with the knee in extension, for four to six weeks. (See 'Initial treatment' above and 'Follow-up care' above.)

●Return to work and sport – Job requirements and symptoms determine the speed with which patients can return to work. (See 'Return to work and sports' above.)