Diagnosing Posterior Cruciate Ligament Injuries
Eric A. Morgan, MD; Randall R. Wroble, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 25 - NO. 11 - NOVEMBER 97
In Brief: Posterior cruciate ligament (PCL) injuries are difficult to detect because patients rarely present with findings that suggest a severe ligament injury. The keys to diagnosis include learning the mechanism of injury and performing a posterior drawer test. A complete knee exam rules out associated injuries. Nonoperative treatment is indicated for low-grade, isolated PCL injuries, but patients should be monitored for any degenerative changes. Combined injuries, high-grade injuries, and avulsion fractures require surgery. Rehabilitation goals for all PCL injuries include regaining full range of motion and quadriceps strength.
In the medical literature, posterior cruciate ligament (PCL) injuries have attracted far less attention than anterior cruciate ligament (ACL) injuries because they occur less often and cause much less functional disability. Despite this, it's important to detect PCL injuries. Though athletes can often function at a high level after an undiagnosed PCL injury, untreated injuries may result in disability years later.
Another important difference between ACL and PCL injuries is that the natural history data on PCL injuries are too sparse to provide clear guidance for treatment decisions and assessment of prognosis. As a result, treatment protocols remain undefined and controversial. With that in mind, the focus of this paper is on injury assessment and detection. Emphasis is given to physical examination technique, as well as what is known and unknown about the natural history and the prognostic indicators in patients who have PCL-deficient knees. Background on the anatomy of the ligament is also provided (see "Posterior Cruciate Anatomy," below).
Incidence and Mechanism
PCL injuries occur in athletes and multiple-trauma victims. Incidence data are limited. One study (1) estimates that PCL injuries make up as many as 20% of all knee ligament injuries. A recent study (2) of all patients admitted to a regional trauma center with severe knee injuries (ie, dislocations) found that 38% had PCL injuries; 56% of the PCL injuries were unrelated to sports, and 32.9% were sports-related.
Many believe that injuries to the PCL are underreported and often missed. Parolie and Bergfeld (1) found that 2% of the participants in the National Football League predraft physical examination had PCL injuries that were discovered incidentally.
Regardless of whether the injury is due to a sports activity or a motor vehicle accident, the mechanism of injury remains fairly consistent: posterior force on the proximal tibia. In motor vehicle accidents this occurs as the proximal tibia strikes the dashboard. In sports, a PCL injury can occur when an athlete falls to the ground on a flexed knee with the foot plantar flexed, so that the proximal tibia strikes the ground first (figure 1). Less common mechanisms of injury include severe twisting with valgus or varus force, hyperextension, or hyperflexion. A prominent tibial tubercle resulting from Osgood-Schlatter disease may enhance and focus the impact when the tibia strikes the ground.
Isolated PCL injuries do occur, but combined ligamentous injuries are more common. Associated injuries may involve the ACL, medial collateral ligament, lateral collateral ligament, or posterolateral complex. Meniscal and articular cartilage injuries are also commonly seen in association with both acute and chronic PCL injuries, with incidences of 27% and 49%, respectively (3). Although much less common, fractures may also occur with PCL injuries.
The diagnosis of PCL injuries is best made with a thorough history and physical along with appropriate radiographic studies.
History. The patient history typically consists of a fall with a blow to the anterior aspect of the proximal tibia. Unlike those with ACL injuries, patients who have PCL injuries usually do not have incapacitating pain, and they report vague symptoms such as unsteadiness or insecurity of the knee. Patients who have chronic injuries may report patellofemoral symptoms.
Physical exam. The physical exam usually does not reveal findings typical of a severe ligamentous injury, and often only a mild hemarthrosis will be noted. Most patients with an isolated PCL injury will have nearly full range of motion. Abrasions or lacerations to the anterior proximal tibia should raise the physician's suspicion for a PCL injury.
Several tests aid in the diagnosis of PCL injuries. The most commonly performed tests include the posterior drawer, posterior sag, Lachman's, quadriceps active, and the reverse pivot shift. It is important to perform a complete knee examination, given the frequency of associated injuries. The physician should examine the normal knee for comparison.
Rubinstein et al (4) assessed the accuracy of PCL injury diagnoses. The posterior drawer was the most sensitive test (90%) and was highly specific (99%). While the quadriceps active test and reverse pivot shift both had high specificity (97% and 95% respectively), they both had low sensitivity (58% and 26% respectively). The authors found an overall clinical exam accuracy of 96%.
Posterior drawer test. Proper technique in performing the posterior drawer test is critical. The examiner should sit on the patient's foot to keep it from sliding and to allow the patient to relax the thigh muscles. The test is done with the patient supine, the knee flexed 90°, and the foot flat on the table. The physician first observes the resting position of the tibial plateau in relation to the femoral condyles. With the knee flexed 90°, the medial tibial plateau normally lies approximately 1 cm anterior to the medial femoral condyle. This can be easily felt by running the thumb or index finger down the medial femoral condyle toward the tibia. However, in a PCL-deficient knee this relationship will not be present.
The posterior drawer exam is graded by the amount of posterior subluxation. The relationship of the tibia to the femoral condyles is noted as the examiner posteriorly translates the tibia (figure 2). Tibial translation between 1 mm and 5 mm is considered a grade 1 injury. The tibial condyle translates posteriorly to a position in which the tibia remains slightly anterior to the femoral condyle. A grade 2 injury exists when posterior tibial translation is 5 mm to 10 mm. This allows the tibia to lie in a position flush with the femoral condyles. Any further posterior subluxation is considered a grade 3 injury. This is seen when the tibia translates more than 10 mm posteriorly, and the tibia lies posterior to the femoral condyles.
Posterior sag test. The tibiofemoral relationship can also be observed with the hip and knee flexed 90°. This often produces a more obvious sag in a PCL-deficient knee and is referred to as the posterior sag test (figure 3).
Lachman's test. The resting position of the tibia should also be noted when performing Lachman's test. In a PCL-deficient knee, Lachman's test may falsely appear positive. The tibia lies in a posteriorly subluxated position, and as anterior force is applied, it translates anteriorly to its anatomic position. Although there is translation, Lachman's test will reveal a solid endpoint if the ACL is intact.
Quadriceps active test. If a PCL injury is present, an active contraction of the quadriceps muscle—performed when the patient's knee is flexed 60° to 90°—will visibly and palpably eliminate the posterior sag (figure 4).
Reverse pivot shift test. The reverse pivot shift test helps identify posterolateral rotatory instability from associated injury to posterolateral structures (figure 5). The test helps the examiner discern combination injuries from isolated PCL injuries.
Radiographs. Plain radiographs are routinely obtained because they help rule out avulsion fractures. Appropriate radiographs include anteroposterior and lateral views.
Magnetic resonance imaging (MRI) is also useful in confirming the diagnosis (figure 6), with a reported sensitivity and specificity of 100% (5). MRI proves especially helpful when a good physical exam is difficult to obtain, and it aids in ruling out concomitant injuries. However, when the injury is chronic, MRI may show an apparently normal PCL though the patient has increased posterior translation (6).
Natural History and Prognosis
Treatment of PCL injuries is controversial because the natural history is uncertain. Fowler and Messieh (7) published the only true natural history study, ie, a study focusing exclusively on acute, isolated PCL injuries. The other studies have investigated patients with combined ligament injuries or patients who sought treatment for problems associated with PCL laxity or both. After more true natural history studies are published, physicians will have better information on treatment and outcome. Until then, conclusions drawn from the existing literature must guide treatment decisions.
The long-term outcome of patients who have untreated PCL injuries is unclear (1,7-12). As many as 80% to 90% of patients with untreated PCL injuries report occasional pain on follow-up. Two groups (10,12) found that up to 43% of the patients considered walking a problem, and as many as 65% decreased their activity level after the injury.
Progressive degenerative changes have been identified in as many as 90% of untreated PCL-deficient knees 4 to 5 years after injury (13,14). Geissler and Whipple (3) found that 49% of chronic isolated PCL injuries and 12% of acute injuries (3 to 21 days) had associated articular cartilage defects. Recently, Boynton and Tietjens (12) showed that degenerative joint changes continue more than 15 years after injury. However, other authors (1,9) have found no significant progression of degenerative changes.
Several studies have attempted to identify significant prognostic factors for clinical outcome in patients who have PCL-deficient knees. Associated ligament injuries have clearly been shown to contribute to PCL instability (15). Torg et al (9) found that of patients who had unidirectional instability, 85% had good or excellent outcomes and 7% had poor outcomes. But of those with multidirectional instability, only 44% had good or excellent results, while 34% had poor results.
Quadriceps strength may also affect the course of PCL injuries (9). Cross and Powell (16) found that excellent or good results occurred in 88% of patients with good quadriceps function versus only 13% of those with poor quadriceps function.
Some authors (10,12) have found a significant correlation between increased posterior translation and decreased function, while others (9,16) have not.
Recommendations for treating PCL injuries vary greatly. The success of nonoperative treatment has been reported to be as high as 80% to 85% for isolated PCL injuries (1,9), but in other reports a substantial number of patients had difficulty walking and reported occasional pain (10,12 ). Some authors (16) recommend nonoperative treatment, but say degenerative changes of the knee are probably inevitable.
Treatment indications for multiple-ligament injuries are more clear-cut. Fewer than half of these patients will achieve satisfactory results with nonoperative treatment. In our experience, early PCL reconstruction and repair or reconstruction of associated ligament injuries have yielded the best results.
Early surgical treatment should also be pursued for large avulsion fractures, which usually occur at the tibial insertion. Patients who have avulsion fractures should be treated with open reduction and internal fixation.
Patients who have grade 1 or 2 PCL injuries (less than 1 cm of posterior translation) should first receive nonoperative treatment that includes aggressive quadriceps strengthening and full range-of-motion maintenance. Patients may return to sports when quadriceps and hamstring strength reaches 90% of the contralateral side. This may take as long as 4 to 6 weeks.
The patient should be followed every 2 years with plain radiographs and a bone scan to monitor joint status. If early signs of degenerative joint disease appear on the bone scan, PCL reconstruction should be considered. Surgical treatment should also be considered if symptoms develop or if function declines.
Patients who have grade 3 injuries (greater than 1 cm of posterior translation) should be treated with PCL reconstruction. Before surgery the patient should undertake rehabilitation to regain full range of motion and quadriceps strength.
Building a Knowledge Base
Increased awareness of PCL injuries and knowledge of appropriate physical exam techniques for injury detection will decrease the number of missed injuries. Recording and documenting information about PCL injuries and carefully monitoring the outcomes of patients will lead to a greater understanding of the natural history and, hence, of appropriate treatment for this condition.
Posterior Cruciate Anatomy
An understanding of posterior cruciate ligament (PCL) anatomy helps explain why PCL injuries are so different from anterior cruciate ligament (ACL) injuries. The PCL, unlike the ACL, is an extrasynovial structure. It attaches to the lateral side of the medial femoral condyle. The tibial attachment begins 1 cm below the tibial plateau on the posterior surface of the proximal tibia.
The PCL is larger and stronger than the ACL (1). It consists of a large anterolateral and a smaller posteromedial bundle. Its orientation is from anterior, at the femoral attachment, to posterior, at the tibial attachment (figure A). This orientation fits the ligament's function: preventing posterior subluxation of the tibia.
A ruptured PCL will allow the tibia to assume a more posterior position relative to the femur if a posterior force is applied to the tibia. When the PCL is torn, the extensor mechanism, including the patella and the patellar tendon, forcefully holds the tibia in a reduced position, which results in increased patellofemoral pressure (2). Increased patellofemoral loading is also caused by a vector change resulting from posterior tibial displacement. This may explain complaints of patellofemoral pain in patients who have PCL-deficient knees.
Dr Morgan is an orthopedic surgeon at Maryville Orthopedic Clinic in Maryville, Tennessee. Dr Wroble is an orthopedic surgeon with SportsMedicine Grant in Columbus, Ohio, and is a member of the editorial board of The Physician and Sportsmedicine. Address correspondence to Randall R. Wroble, MD, SportsMedicine Grant, 323 E Town St, Columbus, OH 43215.