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Acute Knee Injuries: When to Refer

Bernard R. Bach, Jr, MD

Emergencies Series
Editor: Warren B. Howe, MD


In Brief: Primary care physicians are being called on more frequently to treat patients who have musculoskeletal injuries. This article discusses mechanisms of injury, diagnosis, and treatment, with emphasis on determining when to refer patients to an orthopedist. Discussed here are several types of knee injuries that require referral, including extensor mechanism injuries, most subluxations and dislocations, ligament injuries, and meniscus injuries.

Because of changes in healthcare delivery, primary care physicians in sports medicine are being called on increasingly to treat patients who require musculoskeletal care. It is important that these physicians recognize which patients should be referred to an orthopedist. Nonorthopedic physicians who provide musculoskeletal care will be held to the same standards as orthopedic surgeons are, and therefore timely recognition and appropriate referral are essential.

Classification and Triage

Acute knee injuries are classified as osseous or soft-tissue injuries, and injury mechanisms as low or high velocity. Although there are several classification systems for periarticular fractures of the tibia and femur, they are generally labeled as either open or closed and either displaced or nondisplaced fractures. Acute knee injuries are further characterized by the involvement of one or both condyles, whether there is extension into the metaphysis or diaphysis, or whether the patient's knee is neurovascularly intact.


Osseous injuries with displacement involving the distal femur (figure 1) or tibial plateau (figure 2) should be urgently referred to an orthopedic surgeon. Nondisplaced fractures should be referred and evaluated within 1 week following injury. With any fracture, the most important initial issue is whether it is open or closed. An open fracture, in which a puncture, laceration, or soft-tissue defect communicates with the fracture, requires emergent irrigation and debridement to minimize the likelihood of osteomyelitis. An abrasion overlying the prepatellar region does not constitute an open injury.


Radiographic Basics

An appropriate radiographic series is critical in evaluating knee injuries. X-rays will reveal fractures, whereas x-rays of the majority of soft-tissue injuries about the knee are unremarkable. Anteroposterior (AP), lateral, and patellar skyline views are the three minimum essential radiographs. Subtle fractures of the tibial plateau may necessitate oblique views of the knee to facilitate diagnosis. Osteochondritis dissecans lesions involving the medial femoral or lateral femoral condyles may not be recognized on plain AP radiographs, and are more likely to be seen on flexed-knee "tunnel" AP radiographs. Dynamic or variable-angle skyline views for assessment of patellar instability, although used in chronic conditions, are unnecessary for the initial evaluation of patellar problems.

Extensor Mechanism Injuries

Acute extensor mechanism injuries may be divided into five types representing a proximal to distal continuum, and all mandate referral of the patient to an orthopedic surgeon. These injuries include quadriceps tendon ruptures, patellar fractures, patellar tendon ruptures, patellar subluxations and dislocations, and tibial tubercle avulsion fractures.

Quadriceps tendon ruptures. These injuries generally occur in older recreational patients—usually people in their 40s and 50s—and are a result of an eccentric quadriceps load. On physical examination, the patient is unable to actively extend the quadriceps or perform a straight-leg raise, and a palpable gap can be detected proximal to the patella. This gap may approach 3 to 4 in. wide medially to laterally (figure 3a). Imaging studies can be helpful for confirming the diagnosis of quadriceps tendon rupture (figures 3b and 3c). Quadriceps tendon ruptures should be referred for surgical management.


Patellar fractures. Patellar fracture patterns may vary from simple to complex, and may be displaced or nondisplaced (figure 4). They occur in sports, usually as a result of a fall onto the knee, a direct contact blow to the knee, or an eccentric contraction of the quadriceps.


Generally, 2 mm of articular incongruity signals a displaced fracture. In the majority of fractures, displacement is obvious both radiographically and clinically. On physical examination, a portion of the patella is retracted proximally and there will be a visible and palpable defect between the fragments, which are mobile. Patterns may vary from the simple transverse midthird patellar fracture to the more complex, markedly comminuted stellate fracture, and may occur at the distal or proximal pole of the patella.

Physicians should be aware of ossification anomalies that produce a bipartite or tripartite patella in 1% to 4% of patients (figure 5). A characteristic pattern in the superolateral corner of the patella may appear to be a fracture, but the fact that it is almost always nontender differentiates it from an acute fracture. An injury to this region might result in localized pain as the fibrous union of a bipartite patella becomes mobile. A patient who has an asymptomatic bipartite or tripartite patella does not require urgent referral, provided the physician feels comfortable with this diagnosis.


Displaced patellar fractures are generally associated with disruption of the quadriceps retinaculum, and require surgical treatment involving open reduction and internal fixation. A nondisplaced fracture can be managed in a closed fashion with either a knee immobilizer or long leg cylinder cast. If a patient who has a nondisplaced patellar fracture demonstrates good quadriceps control and can perform a straight-leg raise, I usually manage these fractures conservatively. Nevertheless, because of potential problems associated with arthrofibrosis, it is advisable that patellar fractures, whether displaced or nondisplaced, be referred to an orthopedic surgeon. Furthermore, primary care physicians should not assume responsibility for applying a cylindrical or long leg cast on an extremity.

Patellar tendon ruptures. These injuries occur infrequently, and are generally seen in athletic patients. The mechanism of injury is usually an eccentric quadriceps load, and, in my experience, basketball is the sport in which they most commonly occur. The patient may have had antecedent symptoms of patellar tendinitis or "jumper's knee." Patients who have patellar tendon ruptures generally have significant pain initially, with rapid onset of swelling that later subsides.

As with a quadriceps tendon rupture, patients will be unable to actively extend the lower leg or perform a straight-leg raise, but there may be less soft-tissue disruption, and the typical patient is younger than with a quadriceps tendon rupture. A palpable defect at the distal pole of the patella and a high-riding patella (patella alta) that is asymmetric to the opposite knee are generally noted on physical examination (figure 6). A lateral radiograph will demonstrate the high-riding patella. The quadriceps retinaculum is usually disrupted approximately 1 1/2 to 2 in. medially and laterally. Occasionally a fleck of bone may be noted in the soft tissues, which is consistent with a small avulsion fracture. Patellar tendon ruptures should be referred to an orthopedic surgeon for management.


Patellar instability. Patellar instability represents a spectrum ranging from microinstability to subluxation to gross dislocation. Although there are controversies regarding indications and timing for surgical treatment of recurrent patellar subluxation or chronic recurrent patellar dislocation, primary care physicians caring for a patient who sustains an acute patellar subluxation or dislocation should consider orthopedic consultation.

Radiographs should be obtained, since up to 20% of patients demonstrate a radiographic loose body secondary to patellar instability. Osteochondral sleeve fractures of the patella may mimic patellar instability, demonstrating a loose body in the patellofemoral articulation on a radiograph. Patients may develop mechanical locking symptoms necessitating arthroscopic removal of the loose body.

Patients who have acute patellar subluxation or dislocation generally present with an episode of instability and have localized tenderness along the medial extensor retinaculum or possibly at the adductor tubercle, which is the origin of the medial patellofemoral ligament. Also, the patient generally has localized tenderness along the peripheral edge of the lateral femoral condyle where impaction from the patella occurs with flexion of the knee.

With acute patellar subluxation or dislocation, an effusion associated with a hemarthrosis is frequently present. Aspiration need not be done routinely, and does not affect the natural history of the subluxation or dislocation. Aspirating the hemarthrosis may, however, help make a patient more comfortable, and may enable physical examination if the hemarthrosis is severe. Examination of the aspirate for fat globules may help to determine whether there is an intra-articular fracture, which may not be visible on a radiograph.

Acute patellar subluxation often appears no different from an acute patellar dislocation that has spontaneously reduced. Distinguishing one from the other is based on patient history. With a dislocation, the patient reports that the kneecap moved and had to be pushed back into place; with subluxation, the patient reports that the kneecap slipped out, then went back into place spontaneously.

An acute patellar subluxation is generally not considered an indication for surgical treatment, and there are varied opinions with regard to the surgical indications for an acute patellar dislocation. When evaluating patellar instability acutely, most sports medicine orthopedists treat acute patellar subluxations or dislocations in a closed fashion and emphasize a graded rehabilitation program after a period of immobilization. A palpable rent in the medial extensor mechanism, however, may be considered a surgical indication.

Patellar instability can occur in conjunction with ligamentous injuries about the knee. In particular, valgus mechanisms of injury resulting in a medial collateral ligament (MCL) sprain may also be associated with instability. Occasionally, acute patellar instability may occur in conjunction with an acute anterior cruciate ligament (ACL) injury.

Tibial tubercle avulsion fractures. Acute trauma in a skeletally immature patient may result in a fracture of the tibial tubercle growth plate (figure 7). The fracture fragments may vary in size and may also involve the tibial physes. These fractures generally occur secondary to a fall or in a jumping sport, and usually cause severe pain.


Localized bony segments characteristic of Osgood-Schlatter syndrome may mimic tibial avulsion fractures, and, frequently in patients who have unresolved Osgood-Schlatter syndrome, the examiner will see bumps over the tibial tubercle. A lateral knee radiograph will clearly demonstrate whether a tibial tubercle fracture has occurred.

Three general fracture patterns according to the classification system of Ogden et al (1) may be noted with tibial avulsion fractures (figure 8). Type I involves the tibial tubercle apophysis at the level of the tibial tubercle; type II involves the primary and secondary ossification centers but does not extend into the tibial articular surface; and type III extends from the apophysis intra-articularly. The fracture fragment is generally displaced. A hemarthrosis is usually seen, along with moderate-to-severe soft-tissue swelling. For patients who have a displaced fracture of the tibial tubercle, surgical treatment is recommended, and referral to an orthopedist is essential.


Tibial-Femoral Subluxations or Dislocations

The most severe soft-tissue injury to the knee joint is a knee dislocation. Knee dislocations are defined by the position of the tibia relative to the femur (figure 9). Anterior dislocations occur most frequently, followed by posterior dislocations. Medial or lateral dislocations may also occur, or there may be a rotatory component. These injuries are extremely uncommon, but they may occur as a result of low- or high-energy mechanisms.


Whether open or closed, tibial-femoral subluxations and dislocations are surgical emergencies. As these injuries are frequently associated with vascular disruption of the popliteal artery, the patient's neurovascular status must be determined on the playing field. Although experienced sports medicine orthopedists may attempt to perform a closed manipulation of a knee dislocation on the field, a primary care physician covering an athletic event should avoid attempting to reduce the dislocation and should send the patient by ambulance to the nearest emergency room.

Joint injuries and serious vascular injuries may occur with knee dislocations. Joint injuries may occur in up to 25% of patients who have knee dislocations, and neurologic injuries involving either the common peroneal or tibial nerve or both may occur in a similar percentage of patients (2). If a vascular injury is not recognized and is left untreated, loss of limb may occur. In some studies, the loss of a leg has been reported in as many as 20% of patients who had vascular injury with a knee dislocation (2).

Arteriography is recommended for all knee dislocations whether or not a pulse is present. Intimal lesions of the popliteal artery may occur and can be diagnosed by arteriography. Some trauma centers use Doppler ultrasonography to assess vascular status, but arteriography is preferable and is the accepted standard for determining vascular injury.

Immediate consultation with a vascular surgeon and an orthopedist is advisable if a primary care physician establishes a diagnosis of acute knee dislocation. It is important to differentiate clearly between a knee dislocation and patellar dislocation when communicating with an orthopedic surgeon.

Ligament Injuries

Anterior cruciate ligament injuries. These injuries may result from nearly any type of sports activity that involves deceleration or change-of-direction forces. Within the past 15 years, ACL injuries have increased in almost epidemic proportions. According to Moeller and Lamb (3), ACL injuries have increased among women as more of them have participated in all levels of sports. Methods of play in women's sports have also come to involve greater speed, precision, and power.

ACL injuries most frequently occur in noncontact situations but may also result from contact mechanisms. They may result from varus or valgus rotational forces or hyperextension mechanisms. In my practice, downhill skiing, basketball, football, and volleyball are the four sports activities that most often cause ACL injuries. In approximately 80% of ACL injuries, patients experience a popping or tearing sensation, and a similar percentage of patients note a rapid onset of swelling (hemarthrosis), usually within 3 hours. The absence of a hemarthrosis does not preclude an ACL injury.

Differential diagnosis of a hemarthrosis includes ACL injury (70% to 80% of all acute knee hemarthroses in the athletic population), followed by patellar dislocation, peripheral meniscal tears, intra-articular fractures, posterior cruciate ligament (PCL) and popliteal tendon avulsions, which are much less common causes of acute hemarthrosis.

A patient who has had an acute ACL tear is generally unable to continue participation at the time of the injury. Skiers, for example, usually need to be transported down the slope by sled to the local emergency room. Physical examination for a suspected ACL injury includes the Lachman, anterior drawer, and pivot shift tests The Lachman test is performed at 25° ± 5° of knee flexion, with anterior force applied to the tibia while securing the thigh. Increased tibial translation relative to the normal limb raises suspicion of an ACL injury. The absence of an abrupt endpoint should also be noted—a soft endpoint indicates a ligament tear. The anterior drawer test is less sensitive than the Lachman test and is performed at 90° of knee flexion. As in the Lachman test, the tibia is pulled anteriorly, and translation is assessed relative to the opposite extremity. A pivot shift test may be difficult to perform in the acute or semi-acute setting because of patient apprehension and guarding.

In suspected ACL injuries, radiographs should be obtained to rule out an associated intra-articular fracture. Avulsion fractures of the tibial eminence may occasionally be noted, particularly in patients over 35 who have some associated osteopenia. Additionally, a marginal avulsion fracture of the lateral tibial plateau (Segond fracture) may be present in fewer than 3% of patients (4). Magnetic resonance imaging is not generally used to establish a diagnosis of an ACL tear but may be used to determine if there is associated meniscal pathology. Meniscus tears occur in 50% to 70% of patients who have ACL tears; they most frequently involve the lateral meniscus in patients who have acute injuries and the medial meniscus in patients who have chronic injuries (5).

Recommendations regarding surgical reconstruction are based on the degree of laxity, sport-specific demands, hours per week of physical activity, intensity of activity, frequency of instability, associated repairable meniscal tear, and side-to-side differences measured by maximum manual testing with a KT-1000 arthrometer (MEDmetric Corporation, San Diego). Because of these various factors, patients who have acute ACL injuries should be evaluated by an orthopedic surgeon who is experienced and comfortable with the care of these injuries.

Posterior cruciate ligament injuries. As compared with ACL injuries, PCL injuries occur much less frequently, and establishing a diagnosis can be more difficult. The relative frequency of ACL to PCL injuries in most busy sports practices ranges from 10:1 to 20:1 (6). In a recent review (6), I found that typical sports medicine orthopedists performed an average of over 75 ACL reconstructions a year and fewer than 5 PCL reconstructions annually. This difference is related to the relative infrequency of the injury, controversies regarding the indications for surgical treatment, and inconsistent results achieved with PCL reconstructive surgery.

A variety of mechanisms may result in PCL injuries. In sports activities, they generally occur as the result of a fall on a flexed knee with the foot plantar flexed, resulting in a blow to the tibial tubercle that drives the tibia posteriorly.

On physical examination, PCL injuries are diagnosed with clinical tests such as the reverse Lachman, posterior drawer, posterior sag, external rotation-recurvation, and reverse pivot shift tests. The amount of hemarthrosis encountered with PCL injuries is generally less than in an ACL injury, and patients tend to have less instability. There may be associated posterior knee pain and popliteal ecchymosis. As in ACL injuries, there may be associated injury to either the MCL or lateral collateral ligament (LCL).

Additionally, injuries to the posterolateral lateral corner may complicate diagnosis and treatment. These injuries present complex problems that are still being studied, and they can result in long term instability and disability.

Radiographs should be obtained when PCL injuries are suspected, because a displaced avulsion fracture of the tibial insertion site may have occurred, and the fracture may be repaired by surgical fixation. In general, the amount of posterior translation noted acutely may diminish over time. Because PCL injuries are associated with future disability more than with chronic instability, it is important for patients who have had PCL injuries treated in acute settings to be referred to an orthopedist for evaluation and long-term follow-up.

Collateral ligament injuries. These injuries result from valgus forces to the MCL and varus forces to the LCL. The magnitude or severity of the injury may vary, and grading is determined relative to the opposite extremity. Injury grading is based on increased varus or valgus opening relative to the other knee: 0 to 5 mm, grade 1; 6 to 10 mm, grade 2; 11 to 15 mm, grade 3. With a grade 3 MCL or LCL injury, injury to the ACL or PCL must be ruled out, and physical examination in the acute setting may be difficult because of the degree of general laxity within the knee.

MCL injuries. The amount of effusion that occurs in a grade 3 MCL injury may be less than in an acute ACL injury because of extravasation of the hemarthrosis through capsular rents into the surrounding soft tissues. In this situation, there may be more swelling distally in the lower leg. There may be diffuse tenderness at either the femoral origin or tibial insertion of the ligament, and the patient may report experiencing joint-line tenderness.

In general, only the most severe MCL injuries are currently being treated surgically (7). However, restoring normal stability to an MCL after a grade 2 or 3 injury requires considerable skill in management. The period of immobilization, the timing of weight bearing, and the use of appropriate rehabilitation guidelines are critical for an excellent result. Associated injuries to the posterior medial corner—an area with complexity and outcomes similar to posterolateral corner injuries—may warrant surgical treatment. Nonoperative measures include use of a splint or hinged knee brace—graded from non-weight bearing to full weight bearing—and supervised rehabilitation.

LCL injuries. General treatment guidelines for an LCL injury parallel those for the MCL. Far fewer LCL injuries than MCL injuries are seen. The more debilitating injuries associated with the LCL are to the posterolateral corner, involving the popliteus tendon and arcuate complex; these are also some of the most debilitating injuries that sports medicine orthopedists encounter (2). These may be extremely difficult to diagnose on physical examination, and guidelines for surgical treatment and the types of surgical procedures are still evolving. Significant injuries to the medial and lateral ligamentous structures must be evaluated by an orthopedic surgeon.

Meniscus Injuries

Meniscus tears may occur either acutely or chronically. In acute settings, a common mechanism is squatting or kneeling associated with a twisting of the knee. Patients often report a tearing or popping sensation. A meniscal tear is generally associated with a small effusion, except in situations where there is a peripheral tear, which may cause a hemarthrosis. Patients may report discomfort with rotational maneuvers such as rolling over in bed, crossing the legs, squatting, kneeling, and getting in and out of a car. Physical examination generally reveals joint-line tenderness on the affected side, usually posterior to the midline. Discomfort referred to the medial or lateral compartment—determined by meniscal rotation tests—raises the suspicion of a meniscal tear. These tests involve flexion, rotation, and extension of the extremity, which causes pain on the appropriate side.

Standard radiographs should be obtained to rule out associated degenerative joint disease, and standing AP films—which can demonstrate features related to load-bearing—should be obtained. A standing 45° posterior-anterior view also should be considered to rule out degenerative joint disease.

If the patient is unable to extend the knee completely, one should suspect a displaced bucket-handle tear. Displaced bucket-handle tears should be referred to an orthopedist urgently because they may be repairable. In general, meniscal tears do not require urgent referral to an orthopedist, and symptoms may improve over a few weeks. Persistence of symptoms, however, warrants orthopedic evaluation, as patients who have these injuries are predictably helped by arthroscopic surgical treatment.


  1. Ogden JA, Tross RB, Murphy MJ: Fractures of the tibial tuberosity in adolescents. J Bone Joint Surg (Am) 1980;62(2):205-215
  2. Holmes CA, Bach BR Jr: Posterolateral knee ligament injuries, in Torg J, Shephard R (eds): Current Therapy in Sports Medicine, ed 3. Philadelphia, Mosby, 1995, pp 359-367
  3. Moeller JT, Lamb MM: Anterior cruciate ligament injuries in female athletes. Phys Sportsmed 1997;25(4):31-54
  4. Bach BR Jr, Warren RF: Radiographic indicators of ACL injury, in Feagin JA (ed): The Crucial Ligaments: Diagnosis and Treatment of Ligamentous Injuries About the Knee, ed 2. New York City, Churchill Livingstone, 1994, pp 423-437
  5. Bellabarba C, Bush-Joseph CA, Bach BR Jr: Patterns of meniscal tears in ACL deficient knee: a review of the literature. Orthopedics 1997;26(1):18-23
  6. Bach BR Jr: Graft selection for posterior cruciate ligament surgery, in Drez D Jr, DeLee JC (eds): Operative Techniques in Sports Medicine. Philadelphia, WB Saunders Co, 1993, pp 104-109
  7. Bach BR Jr: Knee and leg: soft-tissue trauma, in Kasser J (ed), Orthopaedic Knowledge Update 5. Rosemont, IL, American Academy of Orthopedic Surgeons, 1996, pp 463-480

Dr Bach is a professor in the Department of Orthopedic Surgery and director of the sports medicine section at Rush-Presbyterian-St. Luke's Medical Center in Chicago. He is also a member of the editorial board of The Physician and Sportsmedicine. Address correspondence to Bernard R. Bach, Jr., MD, Rush-Presbyterian-St. Luke's Medical Center, 1725 W Harrison St, Suite 1063, Chicago, IL 60612; send e-mail to [email protected]. Dr Howe is a team physician at Western Washington University in Bellingham, Washington, and an editorial board member of The Physician and Sportsmedicine. Address correspondence to Roy T. Bergman, MD, 405 W Greenlawn, Suite 235, Lansing, MI 48910.



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