Acute and Chronic Knee Injuries: A Clinical Review

With the already high prevalence of knee complaints now soaring, primary care physicians need to be prepared for an array of problems, including damage to the extensor mechanism, osteochondritis dissecans, meniscal tears, ligament injuries, and dislocation. The authors explain the anatomic, diagnostic, and therapeutic keys to each.

By Sameer Mathur, MD, and Shepard Splain, DO

The knee is one of the most frequently injured joints because of its anatomical structure, its exposure to external forces, and the functional demands placed on it. A recent survey has shown that approximately 2.5 million people seek medical assistance for knee complaints. The knee is a common site of trauma, degenerative disease, and rheumatologic conditions. The popularity of skiing, jogging, and long-distance running has markedly increased the prevalence of acute and recurrent knee complaints. This review will highlight the most prevalent bony and soft-tissue pathologies of the knee joint.
 

ANATOMY OF THE KNEE JOINT

The knee joint is classified as a hinge joint, but in addition to flexion and extension it has a rotary component. The osseous anatomy of the knee consists of the proximal tibia, distal femur, and patella. The distal femur consists of the medial and lateral condyles, medial and lateral epicondyles, femoral trochlear groove, and intercondylar notch. Both condyles are covered with articular cartilage and flattened anteriorly for maximal weight transmission and surface contact. The trochlear groove lies on the anterior aspect of the distal femur, between the medial and lateral femoral condyles. This surface is also covered by articular cartilage and serves as the site of articulation with the patella.

The epicondyles serve as the site of insertion of several important structures. The medial collateral ligament (MCL) attaches to the medial epicondyle, and the lateral collateral ligament (LCL) attaches to the lateral epicondyle. The proximal tibial surface consists of the medial and lateral plateaus and the intercondylar eminence. The medial plateau is larger and extends farther posteriorly compared to the lateral plateau. The intercondylar eminence is the site of attachment between the menisci and the cruciate ligaments.

The patella is a triangular sesamoid bone within the tendon of the quadriceps muscle. It provides protection to the knee joint as well as a mechanical advantage in knee extension by increasing the moment arm of the quadriceps muscle.

The osseous anatomy of the knee contributes only minimally to its stability. A complex soft tissue envelope contributes more significantly to stability as well as function. The soft tissue elements can be divided into static restraints (ligaments), dynamic restraints (muscles and tendons), and the menisci. The static restraints are the MCL, LCL, anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL).

The menisci are two crescent-shaped cartilaginous structures attached to the proximal tibial surface. They serve three primary purposes: to increase the surface area for weight-bearing, to reduce the stress per unit area on the articular cartilage, to augment stability by changing the flat tibial articular surface to a cupped surface. In addition, the menisci play an important role in joint fluid distribution and nutrition.
 

HISTORY: KEY CONSIDERATIONS

The history should begin with the chief complaint and how long the patient has suffered from the problem. The specific location of the pain, any radiation, the nature of the pain (aching, burning, or stabbing), and any aggravating or ameliorating factors should be noted. In particular, the relationship of the pain to activity and rest is important. Pain at rest may suggest neoplasm or sepsis.

Frequently, knee problems begin with an injury. A detailed history describing the injury can be extremely helpful in identifying the affected structures (see table, below). The nature of any external force acting on the knee as well as the position of the knee at the time of the injury are important considerations. Did an audible or palpable pop occur? Was the patient able to bear weight? Could the patient flex or extend the knee afterward?

Besides reporting pain, patients may complain of mechanical problems. When locking of the knee occurs, patients lose the ability to fully straighten the leg. Locking can be a result of a loose body or a meniscal injury. Instability is another common complaint. Patients will report that the knee shifts or buckles with activity. Instability is generally caused by injury to a ligament.

Most knee conditions are aggravated by activity. Patients will have difficulty climbing and descending stairs. Frequently, descending stairs will be more symptomatic because it places more stress across the patellofemoral joint. Patients with meniscal tears will have difficulty squatting; they may feel a snapping sensation or pain when getting up from a chair or climbing stairs. Activities that involve stopping, turning, or cutting will result in the knee shifting or giving way if there is ligamentous insufficiency.

It must be kept in mind that when a patient complains of a knee problem, diagnostic considerations must not be limited to the knee itself. A review of systems and an evaluation of the hip and back is necessary to rule out referred pain from disc disease or hip pathology.

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PHYSICAL EXAMINATION

A complete physical examination must be performed because many systemic illnesses can present with knee pain. The knee examination should begin with inspection of the leg. The skin should be examined for rash, clubbing, psoriatic changes, rheumatoid nodules, pallor, alopecia, and tophi. In addition, a thorough check of all joints and neurologic testing should be performed. The patient's gait should be observed to see if there is any limping or lurching or an inability to bear weight. Swelling in the knee may be localized (bursitis) or generalized (intra-articular). Bursal swelling over the knee is prepatellar bursitis. Swelling in the popliteal fossa may be associated with a cyst.

Swelling medial to the tibial tubercle is consistent with inflammation of the pes anserine bursa. Intra-articular hemorrhage or irritation of the synovium (synovitis) causing secretion of synovial fluid can precipitate a generalized swelling that affects the entire knee. Generalized swelling can partially or fully obscure the knee's normal contour; the knee is usually flexed to accommodate the swelling.

With the patient standing, the leg should have a normal valgus angulation of seven degrees. From the side, the knee should look slightly extended.

The knee should then be examined with the patient sitting with his or her legs over the edge of the table. The position of the patella should be anterior and symmetric. Patella tracking can then be followed by asking the patient to flex and extend the knee as the patella is palpated. There should be little lateral movement; crepitus may be noted between the patella and the femoral trochlear groove. The knee should then be examined with the patient lying supine. It should be palpated to determine the site of maximum tenderness. It should then be taken through a passive range of motion from full flexion to full extension. Any painful positions, snaps, clicks, crepitus, or joint incongruity should be noted. The extensor mechanism can be examined by having the patient perform a straight leg raise.
 

EXTENSOR MECHANISM INJURIES

The extensor mechanism consists of the quadriceps muscles, quadriceps tendon, medial and lateral retinacula, patella, patellar tendon, and tibial tubercle. Disruptions of the extensor mechanism may occur at any level from the quadriceps muscle to the insertion on the tibial tubercle. Injury generally results from vigorous contraction of the quadriceps muscle with the knee in a flexed position. Rupture of the quadriceps tendon usually occurs at or just proximal to the patellar insertion. Most patellar tendon ruptures occur at the site of origin on the inferior pole of the patella.

Tendons of the extensor mechanism are extremely resistant to tensile loads and usually do not rupture under normal physiologic conditions. However, chronic systemic conditions, such as rheumatoid arthritis, gout, systemic lupus erythematosus, and hyperparathyroidism, may predispose the tendon to early rupture.

The quadriceps and patellar tendon disruption is associated with intense pain. The patient usually cannot walk without assistance and seeks immediate medical attention. Physical findings include a palpable defect over the superior pole of the patella for a quadriceps tear or a defect at the inferior pole of the patella for a patellar tendon rupture. In addition, swelling and subcutaneous ecchymosis may be evident. With complete rupture, the knee cannot be extended from a flexed position, and the patient cannot perform a straight-leg raise. With incomplete rupture, the patient may be able to raise the leg from a supine position but unable to extend it from a flexed position.

Standard anteroposterior and lateral x-rays may reveal characteristic findings. In patellar tendon rupture, patella alta may be found on the lateral x-ray using a ratio of patellar tendon length to patellar length. If this ratio is greater than 1:2, then patella alta is present. Although there are many x-ray findings that point toward tendon injury, diagnosis can still be difficult. Magnetic resonance imaging (MRI) can delineate the entire extensor mechanism; however, it should be reserved for patients with incomplete tears of the quadriceps or patellar tendon.

Partial tears of the quadriceps tendon may be treated without surgery. If the tear is partial, immobilization in full extension for four to six weeks, depending on the extent of the tear and return of quadriceps strength, is the treatment of choice. For a complete quadriceps tear, early surgical repair has yielded the best results. Patellar tendon ruptures should also be treated surgically.
 

TEARS OF THE MENISCI

A meniscus is usually torn by a rotational force that occurs while the joint is partially flexed. The medial meniscus, being far less mobile on the tibia than the lateral meniscus, can become caught between the condyles, resulting in a tear. The most common site of injury is the posterior horn of the meniscus, with longitudinal tears involving the posterior segment of the medial or lateral meniscus.

The diagnosis of a meniscal tear can be difficult even for an experienced orthopedic surgeon. Tears can result from a specific injury involving a painful twisting during athletic activities or while getting up from a kneeling position. Pain along the joint line is felt immediately after the injury. Movement increases the pain, and range of motion is limited. Pain on squatting or getting up from a sitting position is commonly reported.

Tears of the menisci do not always result from a specific injury, however. In a middle-aged or elderly person with abnormal menisci, no such injury may be reported. In these patients, the tear occurs as a result of the meniscal tissues softening and the edges becoming frayed and trapped between the edges of bone. There may be minimal swelling, no symptoms of locking, and moderate joint line pain.

Click, snaps, or catches, either audible or palpable during flexion, extension, or rotary motion of the joint, may indicate meniscal pathology. Numerous manipulative tests have been described but the McMurray and Apley tests are the most commonly used. For the McMurray test, the patient is placed supine and the knee acutely and forcibly flexed. The examiner can check the medial meniscus by palpating the posteromedial margin of the joint with one hand while grasping the foot with the other hand. Keeping the knee completely flexed, the leg is externally rotated as far as possible and the knee is slowly extended. During the extension phase, a click or snap may be audible. For the Apley test, the patient is prone, the knee is flexed 90 degrees, and the leg is internally and externally rotated with pressure applied to the heel. Downward pressure eliciting pain suggests meniscal pathology.

Radiographs will not diagnose a torn meniscus but are essential to exclude other causes of knee pain such as loose bodies, fracture, or osteochrondritis dissecans. Anteroposterior, lateral, and tunnel views should be routine. Magnetic resonance imaging is the noninvasive gold standard for diagnosing meniscal injury.

Acute phase treatment involves reducing the inflammatory response with anti-inflammatory medication and rest. After the inflammatory response has subsided, arthroscopic debridement or repair may be indicated, depending on the patient's symptoms. The most important aspect of nonsurgical treatment is restoring the strength of the muscles around the knee with a regular program of progressive exercise.
 

OSTEOCHONDRITIS DISSECANS

In osteochondritis dissecans, the most common source of loose bodies in the knee joint, an area of subchondral bone becomes necrotic, and degenerative changes usually occur in the cartilage overlying it. Unless there is medical or surgical intervention, the necrotic bone and cartilage may separate from the femoral condyle and become a loose body. Although the cause is unknown, most experts agree that trauma initiates the pathology.

There are two different affected populations: the child or young adolescent with an open physis and the adult patient. Symptoms are usually vague, aching discomfort in the knee, with minimal effusion.

Chondral injuries are difficult to diagnose because they mimic meniscal injuries. Patients often have pain, swelling, and buckling episodes without a history of trauma. Localized tenderness of the condyle is often the only physical finding. Pain can be localized or diffuse. Intermittent locking, crepitus, and persistent pain may all be associated with chondral injuries. Clicking and popping may be prominent if there is a loose body in the joint.

Plain x-rays, including anteroposterior, lateral, and tunnel views, should be obtained. In addition, to determine whether the osteochondral fragment has separated from the condyle, an MRI scan should be performed. The osteochondral fragment may appear separated from the condyle by a thin line of low-signal intensity on MRI.

Treatment varies from observation in children to surgery in adults. Most children with an open physis can be treated with observation or immobilization as symptoms require. In adults, the prognosis is less favorable; surgical intervention may include drilling or excising the fragment, debriding the crater, and various forms of fixation and grafting. Loose pieces of articular cartilage should be removed.

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CRUCIATE LIGAMENT INJURIES

The incidence of knee ligament injuries in the United States has increased over the past two decades. Ligament injuries may involve any or all of the ligaments—the MCL, LCL, ACL, and PCL—and may range from a mild sprain to complete tears.

The cruciate ligaments are the primary stabilizers against anterior and posterior displacement of the tibia on the femur. The ACL prevents excessive anterior displacement and helps control rotation and hyperextension of the knee during cutting, twisting, and turning activities. The PCL prevents excessive posterior displacement, especially during flexion. The PCL is extremely strong; injuries are relatively uncommon.

The ACL is most often injured during sports activities, particularly skiing and football. Numerous mechanisms can cause ACL injuries, especially activities with a plant-and-pivot or stop-and-jump sequence.

The diagnosis of ACL injury can often be made from the history alone. Most patients with acute ACL injury report immediate disability and an inability to continue activity. Pain is the most common complaint with ACL rupture; it is usually immediate and often accompanied by acute hemarthrosis. The triad of an audible pop, immediate swelling, and a sensation of an unstable knee is pathognomonic of an acute ACL tear.

Mechanisms of PCL injury include a fall to the ground with the foot plantarflexed, a direct posterior blow to a flexed knee, hyperflexion, and hyperextension. Pain and swelling are common complaints with isolated PCL tears and with tears and concomitant capsuloligamentous disruption. Popping or tearing sensations are infrequently noted. Instability is usually not immediately apparent, but the patient may complain of it later.

The anterior drawer test can be used to evaluate anterior translation of the tibia on the femur. This test is performed with the patient in a supine position, the hip flexed at 45 degrees, and the knee flexed at 90 degrees. The examiner stabilizes the foot and leg positions by sitting on the foot, then determines the degree of step-off between the femoral condyle and the tibial plateau by placing his or her thumbs over the joint line while exerting a smooth, gentle pull anteriorly on the tibia. The amount of forward displacement is compared with the other leg.

Lachman's test is currently the best test for determining the integrity of the ACL and one of the few reliable tests in a patient with acute hemarthrosis. The test is especially useful for acute injuries, in which muscle spasm and an effusion often limit knee flexion. Lachman's test is performed with the knee flexed 20 to 30 degrees. The examiner grasps the thigh to stabilize the femur, then pulls the tibia forward with an anteriorly directed force. Any tibial excursion should be noted. The examiner records "firmness" or a "soft endpoint." The endpoint can be graded as 1+ (0 to 5 mm displacement), 2+ (5 to 10 mm), or 3+ (more than 10 mm).

The posterior drawer test remains the gold standard for evaluating PCL injury. This can be accomplished with the patient's knee flexed at 90 degrees and the foot stabilized by the examiner's thigh. A smooth backward force is applied to the tibia. Posterior displacement of the tibia by more than 5 mm, or a soft endpoint, indicates injury to the PCL. A normal knee should exhibit no significant posterior excursion.
 

COLLATERAL LIGAMENT INJURIES

The medial stabilizers of the knee are the joint capsule and the MCL. These structures resist valgus laxity and medial rotary instability. The MCL is a two-part structure with a long superficial and a deep capsular component that attaches to the medial meniscus and acts as a stabilizer for this structure. This ligament is usually injured by a direct blow or impact to the lateral aspect of the knee, which imposes a valgus stress. Overall, MCL injury remains the most common isolated knee ligament injury, and it also is the injury most commonly associated with ACL injury. Isolated MCL injuries do not require surgical treatment in most patients.

The lateral stabilizers of the knee are the LCL and the lateral joint capsule. Resistance to varus stress is provided mainly by the LCL. Fibers descend from the lateral femoral condyle and insert at the fibula head. The lateral ligaments are under tension during standing and walking, when they are at or near maximum extension. The LCL is usually injured by hyperextension, with varus stress commonly accompanied by a direct blow or rotation. Injuries to the LCL are less common but more disabling than those on the medial side. The forces necessary to produce LCL injury are usually greater than those required for medial injury, which partly explains the high frequency of associated injuries accompanying LCL injury.

The knee should be examined as soon as possible after an injury to exclude ligamentous damage. Focal tenderness at the origin or insertion sites suggests collateral ligament trauma, but it can also occur with muscular injury, osseous pathology, or a meniscal tear. Range of motion should be documented and stability testing done to assess ligamentous injury, as discussed previously. Particular attention must be given to peroneal nerve function.

The collateral ligament stress test is used to test the integrity of the medial and lateral collateral ligaments. With the patient lying supine, the examiner applies varus and valgus stress with the knee at 0 and 30 degrees of flexion. Joint line opening is the degree of movement produced between the tibia and femur. This can be palpated and estimated in millimeters. The other knee should then be subjected to the same degree of valgus and varus stress and joint line opening compared with the injured knee.

Laxity on full extension implies complete collateral ligament tear, with injury to the secondary restraints, such as the ACL, PCL, and posteromedial or posterolateral corners. When the knee is flexed to 30 degrees, the stability provided by the cruciate ligaments to valgus and varus stress is removed. Thus, if the knee is stable to valgus stress with the knee in full extension but lax at 30 degrees of flexion, the injury is limited to the superficial and deep medial collateral ligaments. Varus stress that produces no instability at 0 degrees of flexion but produces laxity at 30 degrees indicates that at least part of the lateral collateral ligament and lateral stability complex has been injured.

Plain films may be used in cases of suspected ligamentous injury to rule out the possibility of an associated fracture. The initial radiographic evaluation should include anteroposterior, lateral, intercondylar notch, and sunrise views. In certain cases, the patient may not be able to bend his or her knee to obtain a sunrise view. Each x-ray should be evaluated for osteochondral injuries, loose bodies, or avulsion injuries at the attachment sites of ligaments. The lateral capsular sign (Segond fracture) is highly suggestive of injury to the ACL (see x-ray, below).

Segond sign. Anteroposterior view of the knee demonstrating a small lateral capsular avulsion from the tibial plateau.

An MRI scan is an excellent modality for determining the status of the cruciate and collateral ligaments. It is highly accurate in diagnosing ACL rupture, with both sensitivity and specificity exceeding 90%. Generally, an MRI should be delayed for a minimum of two weeks until the acute phase of the injury has resolved.

There is a consensus that isolated grade I and II collateral ligament injuries should be managed conservatively, provided the ACL is intact. The focus should be on controlling pain, restoring range of motion, regaining muscle strength, and protecting the knee from further injury. Appropriate initial therapy includes ice-pack application, nonsteroidal analgesic medications, and use of a knee immobilizer, if necessary. Orthopedic follow-up is advised, and a rehabilitative exercise program for quadriceps and hamstring strengthening may be undertaken when the acute injury resolves.

In isolated grade III collateral ligament injuries, a conservative approach is also preferred. In cases in which there are concomitant tears of the ACL and the MCL or LCL, the treatment plan may change from conservative management to surgical repair.

Most young patients with a complete ACL tear who are active in sports require reconstructive surgery to stabilize the knee. Usually, this is performed arthroscopically after waiting two to three weeks to allow swelling to subside. Older patients and those who do not participate in active sports may be treated conservatively with muscle-strength training. If recurrent functional instability subsequently develops, reconstruction may be considered.

The history of patients with isolated PCL injury varies. Instability is a less frequent complaint after these injuries than after isolated ACL injuries. Isolated PCL injuries are more frequently associated with pain than instability. Most patients do relatively well with a good rehabilitative program that includes quadriceps strengthening and functional bracing. Nonsurgical treatment should focus on quadriceps rehabilitation.
 

DISLOCATION OF THE KNEE

The most severe soft tissue injury of the knee is a dislocation. In comparison to other injuries, dislocations are relatively uncommon. However, when they occur there is usually a great deal of soft tissue injury. The diagnosis is usually simple with an acutely dislocated knee. The patient will be in severe pain, and the knee will be grossly deformed and swollen. However, in an obese individual or patients with multiple trauma a knee dislocation may be easily missed.

The first priority in a traumatically dislocated knee is assessment of the patient's vascular status. Popliteal artery injury is common in anterior knee dislocations, occurring approximately 25% of the time. Other structures in the knee that may be injured include the anterior and posterior cruciate ligaments, medial and lateral collateral ligaments, and the posterolateral corner.

After vascular status has been assessed, the next priority is to quickly reduce the dislocation. Vascular status should then be reassessed. Femoral arteriography is routine for any patient with questionable circulation or absent peripheral pulses before or after reduction of the dislocation. The knee is immobilized in a posterior splint in 30 to 40 degrees of flexion. If vascular status remains stable for 72 hours, injured ligaments may be reconstructed and fractures stabilized in the operating room.
 

FREQUENT PRESENTING COMPLAINT

Knee injuries are a frequent presenting complaint in the primary care physician's office and the emergency department. The increasing popularity of recreational sports ensures that the incidence of knee injuries will continue to rise. A thorough knowledge of functional anatomy, a detailed history, and a thorough physical examination are the keys to making an accurate diagnosis and implementing proper treatment.
 

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