Chapter 11 The Knee

The knee is the largest joint in the body. The lower portion of the femur and upper aspect of the tibia articulate at only two points where the rounded femoral condyles bear weight on the flat tibial plateaus. The knee joint is subject to a wide variety of traumatic, mechanical, and inflammatory disorders.

Anatomy

The knee joint consists of medial and lateral, femoral and tibial condyles; and the patella. It is essentially a round bone sitting on a flat bone with no intrinsic bony stability and depends completely on its ligaments, muscles, menisci, and capsule for support. The most important ligaments of the knee are the medial and lateral collateral ligaments (LCLs) along with their associated posterior capsular structures and the anterior and posterior cruciate ligaments (Fig. 11-1). The medial collateral ligament originates below the adductor tubercle and attaches to the upper medial tibia. It limits abduction and assists in controlling rotation. The LCL attaches to the lateral epicondyle of the femur and head of the fibula and controls adduction. The cruciate ligaments attach to the intraarticular portions of the femur and tibia. The anterior cruciate ligament (ACL) prevents anterior displacement of the tibia and helps control rotation of the tibia on the femur. The posterior cruciate ligament (PCL) prevents backward displacement of the tibia on the femur.

FIG. 11-1 Ligaments of the knee. A, Anterior cruciate ligament; P, posterior cruciate ligament; L, lateral collateral ligament; M, medial collateral ligament; T, patellar tendon; ME, medial and lateral menisci.

The muscles about the knee also play important roles in its function. The quadriceps group is the most important. These muscles control extension and prevent dislocation of the patella. The medial and lateral hamstrings provide posterior support to the knee and control flexion. Additional support is provided by the popliteus muscle and the iliotibial band.

Normal knee motion consists of a combination of rotation and either extension or flexion. Normally, as the knee flexes, the tibia internally rotates. Extension of the knee is accompanied by lateral or external rotation of the tibia. These rotational motions are controlled by the ligaments and menisci of the knee. This rotation is reflected in the course that the patella takes with flexion and extension movements (Fig. 11-2). Thus, damage to the knee (such as a torn meniscus, which prevents normal tibial rotation) can cause patellar symptoms resulting from abnormal patellar excursion. These patellar symptoms are typically aggravated by walking up and down stairs, an activity that puts the greatest strain on the patella and knee extensors.

FIG. 11-2 Excursion of a patella with knee flexion.

Roentgenographic Anatomy

Anteroposterior and lateral views are essential in the diagnosis of knee disorders (Fig. 11-3). A tunnel view visualizes the intercondylar notch, and tangential views are helpful in diagnosing patellar disorders. With patients over the age of 40, the anteroposterior view should always be performed with the patient standing. This may reveal subtle joint space narrowing if osteoarthritis is present.

FIG. 11-3 Anteroposterior (A) and lateral (B) roentgenograms of a normal knee. M, Medial femoral condyle; L, lateral femoral condyle; T, tubercles of the intercondylar eminence; P, medial and lateral tibial plateaus; F, head of the fibula. Note that the medial femoral condyle projects more distally on both views.

Lesions of the Meniscus

INJURIES OF THE MENISCUS

The menisci, or semilunar cartilages, are two C-shaped structures composed of fibrocartilage that help act as cushions between the femur and tibia. They also assist in the control of normal knee motion. If the normal rotation of the tibia is forcibly prevented as the knee is flexed or extended (that is, if flexion occurs with external rotation or extension occurs with internal rotation), a tear in the meniscus can occur. The injury may be isolated or in conjunction with ligamentous ruptures. Meniscus tears are the most common of all knee injuries, and the pathologic characteristics of the tear are variable (Fig. 11-4). When an injury occurs that produces free fragments or tears, healing to the main body of the meniscus may not take place. The fragment often remains permanently detached but viable because nourishment for the meniscus is provided by the joint fluid. This joint fluid circulates around the tear and often prevents healing from taking place. Persistent symptoms are the result. The medial meniscus is injured 10 times more frequently because it is more firmly attached and less mobile than the lateral meniscus. In long-standing disease, articular cartilage erosion of the tibiofemoral and patellofemoral joints may even result.

FIG. 11-4 A, Normal meniscus. B, Longitudinal, or “bucket-handle,” tear. C, Tear of the posterior horn.

CLINICAL FEATURES

The history is usually one of a twisting injury to the knee with the foot in the weight-bearing position. Occasionally, the injury is slight. A “popping” or “tearing” sensation is often felt, followed by severe pain. The pain is frequently well localized medially or laterally, depending on which meniscus is injured. Locking from mechanical blockage of motion by the meniscus may occasionally occur, but restricted motion after meniscus injury is usually caused by other factors, such as hamstring guarding or swelling, that produce a pseudolocking effect. Swelling from joint effusion gradually occurs over several hours. This is in contrast to ligamentous injury, where the swelling is immediate because of hemorrhage. The swelling from meniscus injury is frequently maximal on the day following the injury. The acute symptoms may subside within a few days, only to be replaced by intermittent episodes of locking, buckling, giving out, swelling, and mild pain. Walking up and down stairs is frequently difficult, and squatting may be painful.

The examination usually reveals a joint effusion (Fig. 11-5). Its presence indicates acute or chronic synovial irritation. Its absence by history or examination should cause another diagnosis to be considered. This fluid may cause the patella to be ballotable. A click may also be present when the patella is pressed against the femur.

FIG. 11-5 Checking for joint effusion. Milking the suprapatellar pouch with downward and distal pressure will frequently reveal fluid that might not otherwise be apparent (arrow). Pressure against the patella may produce a “click” as the patella strikes the femur.

Pain and tenderness at the joint line either medially or laterally may be present. The range of motion is frequently limited. This may be caused by swelling, pseudolocking, or occasionally the interposition of the torn meniscus. In long-standing disease, atrophy of the quadriceps muscle, especially the vastus medialis, occurs rapidly (Fig. 11-6).

FIG. 11-6 The circumference of the thigh is measured 10 cm above the proximal pole of the patella (the medial joint line is marked).

The McMurray test is sometimes helpful in detecting a meniscus tear, although it is difficult to perform on a painful, swollen knee, and the results are often inconsistent (Fig. 11-7). Full knee flexion, which increases pressure on the posterior horns, may cause pain with many meniscus tears.

FIG. 11-7 McMurray test. A, To test for medial meniscus injury, the hip and knee are flexed maximally, and a valgus (abduction) force is applied to the knee. B, The foot is externally rotated, and the knee is passively extended. An audible or palpable snap during extension suggests a tear of the medial meniscus. To test for lateral meniscus injury, the procedure is performed with a varus, internal rotation stress applied.

DISCOID MENISCUS

Because of a failure in normal development, a meniscus, usually the lateral, may be elliptical rather than semilunar. The most common clinical finding in this disorder is an audible click that occurs with motion of the knee joint. The click may be present in infancy, but the disorder is usually not otherwise symptomatic at this age. The meniscus frequently wears away, so the clicking disappears. Occasionally, however, symptoms persist into adulthood, with clicking and aching pain at the lateral joint margin being common complaints. A palpable degenerative cyst may even form in the meniscus. Local tenderness at the lateral joint space is usually present.

Partial meniscectomy may be necessary in an adult with persistent symptoms. Infants usually require no treatment.

CALCIFICATION OF THE MENISCI

The meniscus can become calcified from a variety of causes (Fig. 11-10), the most common being degeneration and trauma. However, calcification can also occur in several other conditions, including degenerative arthritis, ochronosis, and pseudogout. The calcification itself is usually not painful. The symptoms are those that result from the primary disorder, and treatment is directed at that disorder.

FIG. 11-10 Calcification of the meniscus.

Cysts

Only two types of cystic lesions, the popliteal cyst and the cyst of the semilunar cartilage (meniscus), occur with any frequency around the knee joint. The popliteal or Baker’s cyst is an enlargement of the semimembranous bursa that is normally present in the medial aspect of the popliteal space. This bursa usually connects with the knee joint. Small asymptomatic popliteal cysts are common in the general population and are often discovered incidentally, usually during testing for other problems.

Baker’s cysts develop in any age group. In children, the cyst appears to be a primary lesion, in contrast to adults, in whom most of these cysts are secondary to some intraarticular pathology involving the knee. This abnormality, frequently a posterior tear of the medial meniscus or rheumatoid or degenerative arthritis, causes an increase in joint fluid. This chronic effusion opens the normal anatomic communication between the joint and cyst and allows fluid to escape into the semimembranous bursa. The cyst may reach an enormous size in rheumatoid patients with severe synovitis and even dissect distally into the calf.

Occasionally, the Baker’s cyst may rupture in the adult, allowing the escape of the fluid into the soft tissues. The fluid can be irritating and can cause the development of a clinical picture (pain, swelling, and tenderness) resembling thrombophlebitis, referred to as the “pseudothrombophlebitis syndrome.” Homan’s sign may even be positive. Sometimes, blood from rupture of the cyst will dissect distally toward the ankle, where it can reach the surface, creating an ecchymotic area around the malleoli. Differentiation of cyst rupture from venous thrombosis is critical in that thrombophlebitis may require anticoagulation, which is contraindicated in the ruptured cyst (or rupture of the medial head of the gastrocnemius muscle, which can also present with similar findings). Venous ultrasound, MRI, or venography may be needed to distinguish the disorders.

CLINICAL FEATURES

In children, the symptoms are usually related to the effects of direct pressure of the cyst on the adjacent soft tissues. Local discomfort may be present. In adults, the symptoms are related not only to the effects of the pressure of the cyst but also to the primary intraarticular abnormality. (Many cysts are asymptomatic, however.) The cyst commonly changes in size, depending on the activity of the patient and the amount of swelling in the knee.

Examination will reveal a cystic mass of variable size lateral to the medial hamstrings in the popliteal fossa (Fig. 11-11). The lesion may be locally tender. Other findings of primary joint disease may be present, especially in adults.

FIG. 11-11 The popliteal cyst in its usual medial location inferior to the knee crease and lateral to the medial hamstrings.

Except for occasional osteoarthritis, the roentgenogram is usually normal. Ultrasound studies are easy and can document the cystic nature of the lesion when the diagnosis is in doubt. They are also cost effective. MRI can identify underlying joint pathology as well. Occasionally, the cyst may even form multiple calcific loose bodies (Fig. 11-12).

FIG. 11-12 Multiple calcific loose bodies in a Baker’s cyst.

TREATMENT

In children with primary cysts, treatment should be conservative. There is a high rate of spontaneous disappearance of the cyst in this age group and an equally high rate of recurrence following surgical excision. Aspiration and injection of the cyst may be attempted, but they are usually not necessary because the cyst frequently disappears in 1 to 2 years. The easily performed ultrasound study can reassure the family of the benign nature of the lesion.

Adult patients are primarily treated nonsurgically. The cyst may be aspirated to reassure the patient of its benign nature. Aspiration, sometimes with injection of 1 mL of steroid, is often performed to relieve symptoms, and although recurrence is common, the symptoms are usually more tolerable. An intraarticular injection may also be needed to suppress the synovitis. If surgery is being considered, every attempt should be made to detect any underlying joint abnormality. Cyst excision without correction of the intraarticular abnormality is followed by a high rate of recurrence of the cyst. Correction of the intraarticular condition also frequently renders cyst excision unnecessary because the cyst becomes asymptomatic following elimination of the cause of the chronic effusion. Most patients are treated successfully by aspiration alone, or simple observation if the cyst is not symptomatic.

Comments

Popliteal cysts are usually separated from the midline neurovascular structures by the medial head of the gastrocnemius muscle. Whether or not the typical soft cyst is capable of compressing these structures is controversial.

CYSTS OF THE MENISCUS

Cysts may also develop in a meniscus, usually the lateral, as a result of degeneration or trauma. The patient is generally a young adult who has a history of pain and a gradually enlarging mass over the lateral joint line (Fig. 11-13). A knee effusion may be present. Treatment consists of aspiration or meniscectomy and cyst excision if the patient is symptomatic.

FIG. 11-13 Cyst of the lateral meniscus.

Lesions of the Ligaments

Ligamentous injuries to the knee are among the most serious of all knee disorders. The management of these injuries has evolved over the years and continues to evolve. These injuries may occur alone or in combination and are sometimes associated with meniscal tears. Although men have the greatest number of ACL injuries, the female athlete is at much greater risk, possibly because of a combination of hormonal and neuromuscular factors.

CLINICAL FEATURES

The mechanism is usually one of forceful stress against the knee when the extremity bears weight. Direct contact may be involved, although the ACL in particular is often injured without contact. A valgus stress against the knee may sprain or tear the medial collateral ligament, and a varus stress will injure the LCL. Cruciate injuries often occur as a result of a twisting injury, and a “pop” or tearing sensation is often described, especially with ACL ruptures.

After the injury, the ability to bear weight on the extremity is often lost. Swelling from an acute ligament or capsular tear is usually immediate because of a hemorrhage. If a cruciate injury has occurred, the joint fills rapidly with blood. If a collateral or capsular tear has occurred, localized ecchymosis may become visible in a few days. Incomplete tears or sprains are often more painful than complete ligamentous ruptures.

The examination is of utmost importance in the acute injury. Any swelling or discoloration is noted. The lesion can frequently be localized by palpation alone. Palpation should begin away from the suspected area to promote cooperation. A point of maximum tenderness is often present along the course of the collateral ligament or capsule.

The knee should always be tested for stability with the patient relaxed in the supine position. The injured knee is always compared with the opposite, uninvolved knee. The tests are performed in the following sequence:

1 Valgus–varus stress testing at 30 degrees of knee flexion. With the knee flexed 30 degrees, the cruciate ligaments are relaxed. This prevents them from producing a false-negative test result. The medial and lateral ligaments can then be tested by applying valgus and varus stresses to the knee (Fig. 11-14). If laxity exists in either direction with testing, the test reflects an injury to the involved collateral ligament. Sometimes, the test is graded according to the amount of laxity. A grade I injury is present when the joint opens 5 mm more than the normal, and a grade III or “complete” rupture is present when the joint opens more than 1 cm.

2 Valgus–varus stress testing at 0 degrees. Valgus and varus stresses are applied in the same manner as when the knee was tested at 30 degrees of flexion. If the knee was stable at 30 degrees, it will also be stable at 0 degrees because the collateral ligaments are intact. However, with the knee in extension, the cruciate ligaments tighten and by themselves can prevent the joint from opening despite a collateral ligament tear. Therefore, if the knee is unstable in extension, a more serious knee injury has occurred. Posterior capsular and cruciate damage may also be present in this case. (When the knee is extended, the cruciate and posterior capsular structures play a greater role in preventing varus and valgus opening.)

3 Drawer signs. Anteroposterior and rotatory instability are tested by determining how much abnormal excursion of the tibia is present when anterior and posterior stresses are applied to the tibia with the knee in a flexed position (Fig. 11-15). Anterior drawer testing is performed with the foot in external rotation, neutral rotation, and internal rotation. Abnormal forward excursion of the tibia with the foot in either position is highly suggestive of a significant injury to the ACL and joint capsule. The posterior drawer test is then performed by applying backward pressure against the tibia. Abnormal laxity with this test is present with posterior cruciate and posterior capsular injuries. The tibia will also “sag” posteriorly with the hip and knee flexed 90 degrees if the PCL is ruptured.

4 The ACL can also be assessed by the Lachman test (Fig. 11-16). This is essentially an anterior drawer test performed with the knee close to full extension. The femur is stabilized with one hand while firm pressure is applied to the proximal portion of the tibia in an attempt to translate it forward. A positive test result is one in which there is palpable and visual anterior movement of the tibia with a characteristic soft end point. This test is probably more accurate than the traditional anterior drawer sign and has the other advantage that it can be performed in the position of comfort of the acutely injured knee.

Roentgenographic examination may reveal avulsion fractures pulled off by the injured ligament (Fig. 11-17). Roentgenograms should always be obtained, especially in the growing child younger than 15 years of age with open epiphyses to rule out a fracture of the distal femoral epiphysis that may simulate collateral ligament injury (Fig. 11-18). MRI is helpful in assessing cruciate ligament status.

FIG. 11-14 Abduction (valgus) stress test at 30 degrees of knee flexion. To test the lateral collateral ligament, a varus stress is applied. The test is repeated in full extension. Collateral ligament injuries are graded by the amount of joint space opening.

FIG. 11-15 The drawer tests are performed with the hip flexed 45 degrees and the knee flexed 90 degrees. The hamstrings should always be relaxed.

FIG. 11-16 Lachman’s test. This is essentially an anterior drawer test performed with the knee in approximately 30 degrees of flexion. The femur is stabilized with one hand, and the tibia is drawn forward with the other. An increase in the forward motion and lack of a definite end point suggest a rupture of the anterior cruciate ligament.

FIG. 11-17 The lateral capsular fracture (arrow). A small avulsion fracture of the lateral ligament, which usually signals a severe joint injury commonly in combination with an ACL rupture.

FIG. 11-18 Stress roentgenography of the knee reveals an epiphyseal fracture of the lower portion of the femur that may be misdiagnosed as a ligamentous injury clinically.

TREATMENT

Most isolated collateral ligament injuries are now treated nonsurgically, even those that are complete (except severe LCL ruptures). Rest, ice, compression dressing, protection, and early rehabilitation are recommended. Functional hinged braces are used, and healing is usually good. Complete LCL injuries are often treated surgically to prevent late instability.

Treatment of cruciate injuries varies, depending on the age and activity of the patient and the presence of any additional injuries. Isolated ACL and PCL ruptures are generally treated nonsurgically, at least initially, in many cases. Reconstruction is usually required in patients with high demands. A well-constructed brace may provide an option in some patients. The treatment of combined cruciate/collateral injuries is unsettled. Surgery may be required to improve joint stability in the patient with multiple ligament involvement.

Minor ligament sprains are treated in the same manner as meniscus injuries (Fig. 11-19). Immobilization in a compression dressing with ice and elevation for 2 to 3 days are followed by exercises to promote absorption of swelling and restoration of motion and strength.