Posterior Knee Pain and Its Causes
A Clinician's Guide to Expediting Diagnosis
THE PHYSICIAN AND SPORTSMEDICINE - VOL 32 - NO. 3 - MARCH 2004
In Brief: Because posterior knee pain is a relatively uncommon patient complaint, its etiology is challenging and often elusive. The differential diagnosis for posterior knee pain can be vast, so clues for distinguishing causes are important. Many clinicians are unfamiliar with this complicated anatomic area and may not have a standard clinical evaluation to establish a cause of the patient's pain. Review of several known causes of knee pain can provide the examiner with a more comprehensive list of potential disorders to consider as differential diagnoses when patients present with posterior knee pain.
It is critical that the examiner obtain a good history when evaluating patients who have posterior knee pain. Information regarding the onset, duration, location and quality of pain (using the visual analogue scale), aggravating and alleviating factors, past injuries, operations, and other treatments, including medications, procedures, rehabilitation, and orthotic use, can aid with diagnosis. Also significant is knowing whether the pain truly arises from a local source or is being referred from a more distant source, such as in sacroiliac dysfunction or radicular pain.
Soft-tissue and tendon injuries are perhaps more common causes of posterior knee pain than are vascular, neurologic, and iatrogenic injuries, but these less common origins should not be overlooked in patients who present with posterior knee pain (table 1).
|Clinicians should be cognizant of soft-tissue disorders and tumors when examining patients who report posterior knee pain.
Baker's cyst (popliteal synovial cyst). The popliteal synovial cyst, more commonly known as Baker's cyst, is a frequently documented source of posterior knee pain. The condition is caused by a posterior herniation of the synovial membrane or by a communicating semimembranous bursa into the popliteal space and usually indicates underlying pathology. This cyst is seen in disorders such as osteoarthritis, rheumatoid arthritis, and internal derangement of the knee, including meniscal tears.1 However, degenerative arthritis or meniscal pathology alone may be a potential source of posterior knee pain. In fact, posterior horn meniscal tears often present with ill-defined posterior knee pain, especially during deep flexion. Thus, clinicians should examine patients who have posterior knee pain for meniscal pathology.
Magnetic resonance imaging (MRI) or ultrasound can aid the diagnosis of Baker's cyst. MRI is advantageous, because it may identify the underlying cause, such as a concomitant meniscal tear. Focus should be in the most common area for Baker's cyst—along the medial aspect of the popliteal fossa beneath the medial head of the gastrocnemius.
Even though Baker's cysts are often asymptomatic, they can enlarge or dissect and become symptomatic, producing joint swelling, pain, or a feeling of fullness in the popliteal fossa. Occasionally, dissection or rupture may lead to lower-limb swelling, simulating venous thrombosis. A ruptured cyst usually displays a "crescent sign"—an ecchymotic area around the malleoli—that may help distinguish this disorder from venous thrombosis.2 Venography or ultrasonography should be performed if any doubt persists about the diagnosis.
Treatment should address the precluding problem, such as associated meniscal tear or inflammatory arthritis, but if the cause is unknown, conservative management with the RICE protocol (rest, ice, compression, and elevation) and nonsteroidal anti-inflammatory medication can be helpful. Although the cyst may disappear without intervention, some rare cases may require excision. Unfortunately, surgery may not always provide a cure, because the cyst can recur and refill, particularly if the underlying derangement is not addressed.
Tumors. Both benign and malignant soft-tissue and bone tumors can also cause posterior knee pain. Diagnosis of soft-tissue tumors is often delayed, because patients may not come in, especially in the early stages of tumor growth, and the tumors are difficult to diagnose. Presenting symptoms of tumors in the posterior knee include pain from pressure of the mass on adjacent nerves,3 limitation of knee flexion,4 and knee locking with an effusion.5 Some tumors that cause posterior knee pain include osteochondromas, endochondromas, chondroblastomas, osteosarcomas, pigmented villonodular synovitis, and synovial chondromatosis.
Anteroposterior and lateral knee radiographs may show gross formation of a mass. However, if suspicion for a tumor is high, an MRI with contrast should be obtained for further diagnostic workup and management. MRI is a useful imaging study, because it can help clinicians distinguish location, expansion, and characteristics of the tumor. For example, in pigmented villonodular synovitis, the tumor may clinically mimic a meniscal tear, but MRI can be used to distinguish between these two entities.6 Similarly, another advantage of a contrast-enhanced MRI is that one can differentiate a solid tumor from a ganglion cyst, which will only have rim enhancement.7 MRI can also aid with preoperative staging and planning as well as postoperative follow-up. In addition, angiography may reveal further anatomic information about the content of the mass and show any meaningful displacement of nearby vascular structures. Treatment options may include resection, amputation, radiation, and chemotherapy, depending on the stage and grade of the lesion.
Posterior knee pain can arise from acute tendon strain or chronic injury resulting in tendinitis of any of the musculotendinous structures in or about the popliteal fossa. Ganglion cysts in the presence of tendon injury may also contribute to the pain. Some of the more commonly injured structures posterolaterally include the biceps femoris and the popliteus tendons. Posteromedially, injuries to the semitendinosus and semimembranosus tendons are more common. Although they are unusual occurrences, strains or ruptures of the plantaris muscle may cause posterior knee pain.
Hamstring injury. Although the hamstring tendons consist of the semitendinosus, semimembranosus, and the long and short heads of the biceps femoris, the most commonly injured of these is the short head of the biceps femoris. Most hamstring injuries occur around the musculotendinous junction. However, injury to the tendon itself near the posterolateral corner of the knee may occur during rapid bursts of running or jumping or during sudden deceleration. Increased susceptibility to this injury may be from inadequate stretching during warm-up exercises, decreased flexibility, and muscle fatigue. Endurance sports, such as running or cycling, are also associated with injury to the biceps femoris tendon.
Physical examination may reveal tenderness at the distal aspect of the biceps femoris tendon as well as pain during knee extension. If the clinical diagnosis is in doubt, an ultrasound or MRI may be done. If peripheral neurologic symptoms are present, advanced imaging modalities may help to rule out a concomitant hematoma that may externally compress adjacent structures, such as the tibial nerve. MRI may also help physicians determine the prognosis for return to sport. If more than 50% of cross-sectional muscle or distal myotendinous tears occur, athletes usually require more than 6 weeks before they may return to sports-specific programs.8 An earlier return to play may be associated with subtle muscle strength abnormalities, which can lead to a recurrence of symptoms and possibly worsen the original tear.9
Gastrocnemius tendon calcification. A rare cause of tendon injury accompanied by posterior knee pain is calcification of the gastrocnemius tendon as calcium pyrophosphate dihydrate (CPPD) becomes deposited.10,11 Anteroposterior and lateral knee radiographs may reveal this phenomenon. CPPD deposition may be seen more often among the elderly; the involvement of the gastrocnemius tendon is relatively rare in younger patients.12
Popliteus injury. The examiner should also test the popliteus tendon as a possible pain generator. The popliteus muscle and tendon (figure 1) stabilize the posterolateral corner of the knee and prevent anterior translation, especially during downhill running. Injury to the popliteus tendon, therefore, is most commonly seen in athletes who run. The posterolateral corner is a complex area that is often misunderstood and underrepresented as a cause of posterior knee pain.
On examination, the popliteus muscle may be tender in the posterolateral corner of the knee. However, a provocative maneuver that typically provokes pain involves examining the patient in the prone position with internal rotation of the tibia. The patient then flexes the knee against resistance. Reproduction of symptoms during flexion suggests injury to the popliteus tendon.
Treatment of a popliteus tendon injury consists of the RICE protocol, gradual stretching exercises in multiple planes, closed-kinetic-chain eccentric strengthening exercises, such as slow, multidirectional lunges that patients progress to doing on nonlevel surfaces, and gradual return to athletic participation. Since these muscle fibers have a rotational component, rehabilitation should emphasize exercises with rotation.
Other Posterolateral Corner Components
Although the popliteus may be a frequently injured part of the posterolateral corner, other components include the lateral collateral ligament, the posterolateral capsule, and the popliteofibular ligament. During the initial 30° of knee flexion, these posterolateral structures in combination with the posterior cruciate ligament (PCL) are important in resisting excessive varus orientation, external rotation, and posterior translation of the knee.
Injury mechanism and exam. The most common mechanisms of posterolateral corner injury involve athletic trauma, motor vehicle collisions, and falls. Isolated injury to this complex usually derives from a posterolaterally directed force when the knee is in full extension. Although a patient's initial clinical presentation may involve minimal symptoms, a compromised posterolateral corner can lead to worsening local symptoms. Patients typically complain of knee pain while walking and may even develop a varus thrust. Examination may reveal swelling, abrasion, or ecchymosis. Point tenderness may occur over the fibular head as well as diffusely in the posterolateral corner. In chronic cases, there may be a varus thrust seen in stance (figure 2) or during ambulation.
Tests, accompanying injuries, and treatment. The external rotation recurvatum test (figure 3) can help confirm posterolateral rotary instability. The examiner performs the test by holding the patient by each great toe and observing any side-to-side differences in hyperextension, varus, and tibial external rotation.
The dial test also assesses posterolateral rotation of the tibia on the femur to detect posterolateral knee instability. The patient is supine with 30° of knee flexion and with the foot extended over the side of the examining table. The examiner externally rotates the foot while stabilizing the thigh and observes the amount of rotation of the tibial tubercles. Increased external rotation on the injured side indicates a posterolateral corner injury. If this maneuver is performed with the knee flexed to 90° and less rotation is seen than when performed at 30°, then an isolated posterolateral corner injury is probable. If the injured knee rotates more at 90°, then a concomitant PCL injury is likely. Since isolated posterolateral corner injuries are relatively uncommon and exam maneuvers are often negative, this injury is frequently missed. The posterior drawer test is more sensitive for detecting PCL-only injury.
Paresthesia and weakness from common peroneal nerve injury may also be present with a posterolateral corner injury. Researchers have documented that 15% of patients with a posterolateral knee injury also have a common peroneal nerve injury.13 In their review, Veltri and Warren14 noted that hemorrhage can be a contributing factor to peroneal nerve palsy in acute posterolateral corner injury despite an intact nerve. They also noted that in some cases of lateral and posterolateral corner knee injury, the concomitant varus thrust may lead to direct injury of the peroneal nerve.
Radiographs taken while the patient is standing may illustrate abnormal widening of the lateral joint space and arthritis. However, MRI is superior at delineating injury to the structures of the posterolateral corner.
Nonoperative treatment includes early mobilization with gait retraining and hip girdle strengthening. The focus should be on quadriceps strengthening, since the quadriceps are most likely to atrophy in chronic posterolateral instability. Some acute ligamentous injuries warrant operative repair in the first 3 weeks after injury to provide the optimal result.
Vascular and Nerve Injuries
Injuries to vessels and nerves should not be overlooked in patients with posterior knee pain.
Popliteal artery entrapment syndrome (PAES). This condition arises from hypertrophy of the medial gastrocnemius, soleus, plantaris, or semimembranosus muscles that compresses the popliteal artery as it courses through the popliteal fossa. Although the symptoms are most common in athletes, other cases have been reported in truck drivers, because the same mechanism causes direct arterial compression. Individuals with rheumatoid arthritis and associated knee pathology can also present with PAES.15
Symptoms include posterior knee pain and progressive lower-extremity arterial insufficiency causing claudication of the calf with ambulation or other exertion. Leg swelling, cramping, coldness, paresthesias, trophic changes, and blanching below the knee may also be present. On exam, distal pulses decrease or disappear when the knee is in hyperextension with active plantar flexion or passive dorsiflexion. Other conditions that can mimic PAES include accelerated atherosclerosis, thromboangiitis obliterans, adventitial cystic disease, adductor canal outlet syndrome, acute popliteal artery occlusion, microemboli, collagen vascular disease, Takayasu's arteritis, and coagulopathy.
Several imaging studies can help determine this unusual diagnosis. Duplex ultrasonography can be used for detection; however, because it is operator dependent, the technique may yield a high rate of false positives.16 The single most useful study is MRI, since it can illustrate the area of entrapment as well as determine the patency of the artery if the scan is combined with special imaging sequences. Although angiography with digital subtraction can be useful in determining the severity of stenosis, it cannot detect the source of extrinsic compression, and it is also an invasive study.
Functional stretching can treat the condition, but if that is unsuccessful, then surgical intervention may be necessary. Surgery usually involves resection of the hypertrophied muscle to liberate the popliteal artery.
Nerve entrapment. Although rare, common peroneal and tibial nerve injury in the popliteal space should be suspected in patients who have unrelenting posterior knee pain. In a case report, Ekelund17 described idiopathic nerve entrapment in the popliteal space that caused posterior knee pain in a young patient during walking and running. The patient had a tender lateral popliteal space that was surgically explored, exposing a fibrous band that was compressing the common peroneal nerve. Decompression was performed, and 2 weeks later, the patient was asymptomatic. The same patient later returned to the clinic with complaints of pain and had tenderness in the central aspect of the popliteal space in the opposite knee. This area was also explored operatively, and fibers from the medial gastrocnemius were found to be the cause of tibial nerve entrapment. This area was also decompressed, and in 2 weeks, the patient was asymptomatic. In a case series, Saal et al18 also reported nine patients who had tibial nerve lesions in the popliteal space with local tenderness over the area of entrapment.
Traumatic injuries or soft-tissue injuries that have been surgically repaired may provoke posterior knee pain.
Postsurgical arthrofibrosis. Posterior knee pain can arise from posttraumatic arthrofibrosis, a condition in which scar tissue proliferates after trauma. Occasionally, patients with a history of injury or surgery may experience arthrofibrosis, and it usually produces limited range of motion, stiffness, and pain. Affected patients experience posterior knee pain that becomes worse with knee extension. A typical example may occur after an acute anterior cruciate ligament (ACL)-deficient knee is reconstructed before the patient regains adequate range of motion. In such cases, hypertrophic tissue may adhere to the ACL graft site or graft itself. This additional scar tissue contributes to posterior knee pain, because it can prevent the patient from regaining full range of motion postoperatively, particularly the terminal 5° of extension.19 Therefore, delaying the operation approximately 3 weeks after ACL injury should decrease the likelihood of arthrofibrosis and reduce the overall incidence of posterior knee pain. Aggressive, accelerated rehabilitation programs that emphasize passive extension, muscle reeducation, cryotherapy, and functional rehabilitation may decrease the incidence of this disabling condition.20
Bioabsorbable tacks. Another potentiator of postoperative posterior knee pain is placement of bioabsorbable tacks. Because the menisci are important for weight bearing within the knee, new systems of repair, such as bioabsorbable tacks, to prevent future degenerative joint disease have been employed in arthroscopy. Bioabsorbable tacks are T-shaped fasteners with barbed shafts. The tacks generally maintain structural integrity for approximately 4 to 6 months and fully resorb in 3 years. In one retrospective case series,21 a relatively high incidence (31%) of focal posterior knee pain was referred from the site of tack placement, despite a stable knee 6 weeks after surgery. However, pain resolved between 4 and 6 months postoperatively, about the time the tacks begin to resorb. The tack length, number used, and meniscal tear type were irrelevant to symptoms.
Knowledge of this transient phenomenon is important to the examiner evaluating patients who have postarthroscopic posterior knee pain. Symptoms may include tenderness of the posterior knee and sharp posterior knee pain that is exacerbated by knee extension. Physical therapy protocols should not be altered, as there is no difference in knee stability or return to activity in these patients. Reassurance is important during patient evaluation, since symptoms typically resolve as the tacks resorb.
Thoughts About Diagnosis
With these descriptions and diagnostic tips (see table 1), examiners should have a more comprehensive understanding of potential pain generators about the posterior knee. While many different sources can cause posterior knee pain, review of potential causes should give providers a firm understanding of disorders to consider in their diagnostic workup.
Dr Muché is a resident physician in the department of orthopedic surgery and rehabilitation, division of physical medicine and rehabilitation, at Loyola University Medical Center in Maywood, Illinois. Dr Lento is an attending physiatrist in the Rehabilitation Institute of Chicago and assistant professor at Northwestern Medical School in Chicago. Address correspondence to Julie A. Muché, MD, Dept of Orthopedic Surgery and Rehabilitation, 2160 S First Ave, Maywood, IL 60153;e-mail to [email protected].
Disclosure information: Drs Muché and Lento disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.