The Knee

Published on 16/03/2015 by admin

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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.

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.

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.

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.

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).

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.

TREATMENT

The initial treatment is conservative, except for those rare cases in which the knee is truly locked. Many meniscus tears, especially peripheral ones, can heal spontaneously in a few weeks. A bulky compression dressing (often called the “Robert Jones” dressing) and ice are applied if the injury is acute, and the knee is elevated (Fig. 11-8). The patient is placed on crutches and started on quadriceps-strengthening exercises (Fig. 11-9). These exercises will help compress out the joint effusion and maintain strength, and they should always be performed with the knee near complete extension to prevent patellofemoral pain from developing. Gentle range-of-motion exercises are started in 2 to 3 days. Swimming is an excellent exercise for increasing motion and decreasing discomfort. As pain subsides and motion returns, weight-bearing activities are gradually resumed, but quadriceps exercises are continued for 2 to 4 weeks. There are few indications for aspiration of the knee and even fewer indications for the injection of steroids in the treatment of an acute injury. The protective responses of the patient should be maintained, and it is better to reduce swelling by quadriceps contractions and to rehabilitate the knee through exercises.

Surgery is reserved for those cases of true irreducible locking or cases with recurrent or persistent signs and symptoms of meniscus injury. The meniscus is removed or repaired, often arthroscopically. The results are usually excellent, especially in the young, and most patients are able to resume normal activities 3 to 6 weeks after surgery.

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.

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.

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.
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.