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Anatomy of the Knee
Knee Joint
The knee is primarily a hinge joint that permits flexion and extension. In flexion, there is sufficient looseness to allow a small amount of voluntary rotation; in full extension, some terminal medial rotation of the femur (conjunct rotation) achieves the close-packed position. The condyles of the femur provide larger surfaces than those of the tibial condyles, and there is a component of rolling and gliding of the femoral surfaces that uses up this discrepancy. As the extended position is approached, the smaller lateral meniscus is displaced forward on the tibia and becomes firmly seated in a groove on the lateral femoral condyle, which tends to stop extension. However, the medial femoral condyle is still capable of gliding backward, thus bringing its flatter, more anterior surface into full contact with the tibia. These movements of conjunct rotation bring the cruciate ligaments into a taut, or locked, position. The collateral ligaments become maximally tensed, and a full, close-packed, and stable position of extension results. The tension of the ligaments and the close approximation of the flatter parts of the condyles make the erect position relatively easy to maintain.
The sequence of actions in flexion is reversed in extension. Flexion can be carried through about 130 degrees and is finally limited by contact between calf and thigh. The muscles concerned in the movements at the knee are primarily thigh muscles.
There are three articulations in the knee—the femoropatellar articulation and two femorotibial joints. The latter two are separated by the intra-articular cruciate ligaments and the infrapatellar synovial fold. The three joint cavities are connected by restricted openings.
The articular surfaces of the femur are its medial and lateral condyles and the patellar surface, also known as the trochlea of the knee. The condyles are shaped like thick rollers diverging inferiorly and posteriorly. Their surfaces gradually change from a flatter curvature anteriorly to a tighter curvature posteriorly and are separated from the patellar surface by a slight trochlear groove.
On the superior surface of the tibia, or the tibial plateau, there are two separate, cartilage-covered areas. The surface of the medial condyle is larger, oval, and slightly concave; that of the lateral condyle is approximately circular, concave from side to side, but concavoconvex from before backward. The fossae of the articular surfaces are deepened by disclike menisci. The composition and morphology of the menisci make them important in distribution of load about the knee during weight bearing.
The articular capsule of the knee joint is scarcely separable from the ligaments and aponeuroses apposed to it. Posteriorly, its vertical fibers arise from the condyles and intercondylar fossa of the femur; inferiorly, these fibers are overlain by the oblique popliteal ligament. The capsule attaches to the tibial condyles and, incompletely, to the menisci. The external ligaments reinforcing the capsule are the fascia lata and the iliotibial tract; the medial patellar and lateral patellar retinacula; and the patellar, oblique popliteal, and arcuate popliteal ligaments. The medial (tibial) collateral ligament also closely reinforces the capsule on the medial side.
The aponeurotic tendons of the vastus muscles attach to the sides of the patella and then expand over the front and sides of the capsule as the medial and lateral patellar retinacula. Below, they insert into the front of the tibial condyles and into their oblique lines as far to the sides as the collateral ligaments. Superficially, the fascia lata overlies and blends with the retinacula as it descends to attach to the tibial condyles and their oblique lines. Laterally, the iliotibial tract curves forward over the lateral patellar retinaculum and blends with the capsule anteriorly. Its posterior border is free, and fat tends to be interposed between it and the capsule.
The patellar ligament is the continuation of the quadriceps femoris tendon to the tuberosity of the tibia. An extremely strong and relatively flat band, it attaches above the patella and continues over its front with fibers of the tendon, ending somewhat obliquely on the tibial tuberosity. A deep infrapatellar bursa intervenes between the tendon and the bone. A large, subcutaneous infrapatellar bursa is developed in the tissue over the ligament.
The oblique popliteal ligament is one of the specializations of the tendon of the semimembranosus muscle, which reinforces the posterior surface of the articular capsule. As this tendon inserts into the groove on the posterior surface of the medial condyle of the tibia, it sends this oblique expansion lateralward and superiorly across the posterior aspect of the capsule.
Collateral Ligaments
These ligaments prevent hyperextension of the joint and any abduction-adduction angulation of the bones because they are essential in resisting varus and valgus forces to the knee. Both collateral ligaments are tighter in extension and progressively relaxed as the knee is brought into flexion. The inferior genicular blood vessels pass between them and the capsule of the joint, but only the lateral (fibular) collateral ligament stands clearly away from the capsule.
The medial (tibial) collateral ligament is a strong, flat band that extends between the medial condyles of the femur and tibia. It can be broken down into superficial and deep layers that may be separated by a thin bursa that facilitates the slight movement between these layers. The medial collateral ligament (MCL) is well defined anteriorly, blending with the medial patellar retinaculum. The pes anserinus tendon overlies the ligament below, the two being separated by the anserine bursa. The posterior portion of the ligament is characterized by obliquely running fibers, which converge at the joint level from above and below and give the ligament an attachment into the medial meniscus. The principal inferior attachment of the ligament is about 5 cm below the tibial articular surface immediately posterior to the insertion of pes anserinus.
The lateral (fibular) collateral ligament is a more rounded, pencil-like cord, which is entirely separate from the capsule of the knee joint. It is attached to a tubercle on the lateral condyle of the femur above and behind the groove for the popliteus muscle. It ends below on the lateral surface of the head of the fibula, about 1 cm anterior to its apex. The tendon of the popliteus muscle passes deep to the ligament, and the biceps femoris tendon divides around its fibular attachment, with a small inferior subtendinous bursa intervening. Another bursa lies under the upper end of the ligament, separating it from the popliteus tendon. The synovial membrane of the joint, protruding as the subpopliteal recess, separates the popliteus tendon from the lateral meniscus.
Cruciate Ligaments
The cruciate ligaments prevent forward or backward movement of the tibia under the femoral condyles. These ligaments also play a large role in providing rotator stability about the knee joint. They are somewhat taut in all positions of flexion but become tightest in full extension and full flexion. They lie within the capsule of the knee joint, in the vertical plane between the condyles, but are excluded from the synovial cavity by coverings of synovial membrane. Both ligaments spread linearly at their bony attachments, especially at the femoral condyles.
The anterior cruciate ligament (ACL) arises from the rough, nonarticular area in front of the intercondylar eminence of the tibia and extends upward and backward to the posterior part of the medial aspect of the lateral femoral condyle. The ACL can be divided into an anteromedial bundle and a posterolateral bundle. The anteromedial bundle is tight in flexion, and the posterolateral bundle is tight in extension.
The thicker and stronger posterior cruciate ligament (PCL) passes upward and forward on the medial side of the anterior ligament. It extends from an extra-articular attachment over the back of the tibial plateau to the lateral side of the medial condyle of the femur. The PCL also consists of an anterolateral bundle and a posteromedial bundle. The posteromedial bundle is tight in extension, and the anterolateral band is tight in flexion. Both ligaments receive their primary blood supply from the medial genicular artery and their innervations from branches off the tibial nerve.
Menisci
These crescent-shaped wafers of fibrocartilage surmount the peripheral parts of the articular surfaces of the tibia. Thicker at their external margins and tapering to thin, unattached edges in the interior of the articulation, they deepen the articular fossae for the reception of the femoral condyles. They are attached to the outer borders of the condyles of the tibia and at their ends, anterior and posterior, to its intercondylar eminence.
The medial meniscus is larger and more nearly oval in outline. Broader posteriorly, it narrows anteriorly as it attaches in the intercondylar area of the tibia in front of the origin of the PCL. The lateral meniscus is more nearly circular. Although smaller than the medial meniscus, it covers a somewhat greater proportion of the tibial surface. Anteriorly, it attaches in the anterior intercondylar area, lateral to and behind the end of the ACL. Posteriorly, it ends in the posterior intercondylar area in front of the end of the medial meniscus. The medial meniscus is also attached to the MCL, making it significantly less mobile than the lateral meniscus. The lateral meniscus is weakly attached around the margin of the lateral tibial condyle and lacks an attachment where it is crossed and notched by the popliteus tendon. At the back of the joint, it gives origin to some of the fibers of the popliteus muscle; and close to its posterior attachment to the tibia, it frequently gives off a collection of fibers, known as the posterior meniscofemoral ligament. This may join the PCL or may insert into the medial femoral condyle behind the attachment of the PCL. An occasional anterior meniscofemoral ligament has a similar but anterior relationship to the PCL. The transverse ligament of the knee connects the anterior convex margin of the lateral meniscus to the anterior end of the medial meniscus.
The blood supply to the medial and lateral menisci come from the superior and inferior branches of the medial and lateral geniculate arteries, respectively. There are three commonly referred to zones of the menisci based on their respective blood supply. Starting from the most vascularized peripheral (outermost) portion of the meniscus, these are the red-red, red-white, and white-white zones. These zones play a large role in therapeutic decision making owing to the role that increased vascularity will play in the likelihood of healing. Vascularity is also variable among patients of different ages, because younger patients tend to have a more robust blood supply.
Synovial Membrane and Joint Cavity
The articular cavity of the knee is the largest joint space of the body. It includes the space between and around the condyles, extends upward behind the patella to include the femoropatellar articulation, and then communicates freely with the suprapatellar bursa between the quadriceps femoris tendon and the femur. The synovial membrane lines the articular capsule and the suprapatellar bursa. Recesses of the joint cavity are also lined by synovial membrane; the subpopliteal recess has been described. Other recesses exist behind the posterior part of each femoral condyle; at the upper end of the medial recess, the bursa under the medial head of the gastrocnemius muscle may open into the joint cavity.
The infrapatellar fat body or pad represents an anterior part of the median septum, which, with the cruciate ligaments, separates the two femorotibial articulations. From the medial and lateral borders of the articular surface of the patella, reduplications of synovial membrane project into the interior of the joint and form two fringelike alar folds, which cover collections of fat. The fat pad is a normal structure but in many cases it may become inflamed or impinge within the patella and femoral condyle and become problematic.
Blood Vessels and Nerves
In the region of the knee there is an important genicular anastomosis. This consists of a superficial plexus above and below the patella, plus a deep plexus on the capsule of the knee joint and the adjacent bony surfaces. The anastomosis is made up of terminal interconnections of 10 vessels. Two of these descend into the joint—the descending branch of the lateral circumflex femoral artery and the descending genicular branch of the femoral artery. Five are branches of the popliteal artery at the level of the knee—the medial superior genicular, lateral superior genicular, middle genicular, medial inferior genicular, and lateral inferior genicular arteries. Three branches of leg arteries ascend to the anastomosis—the posterior tibial recurrent, circumflex fibular, and anterior tibial recurrent arteries. Veins of the same names accompany the arteries. The lymphatics of the knee joint drain to the popliteal and inguinal node groups.
The nerves of the knee joint are numerous. Articular branches of the femoral nerve reach the knee via the nerves to the vastus muscles and the saphenous nerve. The posterior division of the obturator nerve ends in the joint, and there are also articular branches of the tibial and common peroneal nerves.
Patella
The large sesamoid is developed in the tendon of the quadriceps femoris muscle. It bears against the anterior articular surface of the inferior extremity of the femur and, by holding the tendon off the lower end of the femur, improves the angle of approach to the tendon to the tibial tuberosity. The convex anterior surface of the patella is striated vertically by the tendon fibers. The superior border is thick, giving attachment to the tendinous fibers of the rectus femoris and vastus intermedius muscles. The lateral and medial borders are thinner; they receive the fibers of the vastus lateralis and vastus medialis muscles. These borders converge to the pointed apex of the patella, which gives attachment to the patellar ligament. The articular surface is a smooth oval area, divided by a vertical ridge into two facets. The ridge occupies the groove on the patellar surface of the femur, the medial and lateral facets corresponding to facing surfaces of the femur. The lateral facet is broader and deeper than the medial. Inferior to the faceted area is a rough nonarticular portion from which the lower half of the patella ligament arises.
The patella maintains a shifting contact with the femur in all positions of the knee. As the knee shifts from a fully flexed to a fully extended position, first the superior, then the middle, and lastly the inferior parts of the articular surface of the patella are brought into contact with the patellar surfaces of the femur. The largest amount of contact between the patella and the trochlea is at about 45 degrees of knee flexion.
Ossification develops from a single center, which appears early in the third year of life. Complete ossification occurs by age 13 in the male and at about age 10 in the female.
Arthrocentesis of Knee Joint
Swelling, ecchymosis, and tenderness signal a significant injury to the knee. Clinical findings may also include joint effusion, limitation of motion, and instability. Arthrocentesis is often performed to help define the nature of the intra-articular pathologic processes. A large-bore needle is used via an anteromedial or anterolateral approach (usually superior with knee in extension or at joint line with knee in flexion), with care to avoid injury to the articular cartilage.
Fluid obtained from the joint is often sent for white blood cell count with differential, Gram stain, and culture and examined microscopically under polarized light to detect any crystals. Healthy joints usually yield less than 5 mL of fluid. Normal synovial fluid is clear, pale yellow, and more viscous than water. The average number of leukocytes is about 65/mm 3 , and most are lymphocytes and monocytes. Acute inflammation increases the ratio of polymorphonuclear leukocytes to lymphocytes and monocytes.
Synovial effusions are categorized as group I, noninflammatory; group II, inflammatory; group III, septic; and group IV, hemorrhagic. Group I synovial fluid has high viscosity, is pale to dark yellow, and is transparent. Leukocyte count is generally under 200/mm 3 , of which about 25% are polymorphonuclear leukocytes. Glucose concentration is similar to that in serum. Group I fluid is typically found in joints with osteoarthritis.
Group II synovial fluid has low viscosity, may be yellow to light green, and is translucent. Leukocyte counts of 2,000 to 75,000/mm 3 are common, and about 50% of the cells may be polymorphonuclear leukocytes. Glucose concentration is generally lower than that in serum. Group II synovial fluid is found in joints with rheumatoid arthritis.
Group III synovial fluid is obtained from a native septic joint and has a variable viscosity and color but is opaque. The leukocyte count is frequently greater than 50,000/mm 3 , and polymorphonuclear leukocytes predominate (75%). The glucose level is significantly lower than that in serum. Group IV synovial fluid is bloody, has variable viscosity, and often looks like whole blood on gross examination. A knee joint effusion consisting principally of blood (hemarthrosis) is often associated with rupture of the ACL.
An effusion containing numerous fat droplets along with blood indicates an intra-articular fracture. The volume of fat may be so great that the fat layer is visible on a lateral radiograph of the knee; after aspiration, the fat appears as a distinct layer floating on the synovial fluid and blood in the syringe. Other injuries, such as avulsion of a ligament at its insertion into bone, may produce a hemarthrosis with a few fat droplets. A large tear of the joint capsule may result in little or no detectable effusion because the blood and joint fluid leak into the periarticular tissues and they cannot be aspirated from the joint.
Meniscal Variations and Tears
Discoid Meniscus
The meniscus is normally a crescentic structure, although several forms of discoid lateral menisci have been described. These range from a complete disc to a very rare ring-shaped meniscus with abnormal thickness. The common explanation for these variant discoid forms assumes that the normal meniscus is formed from an original discoid shape and that the discoid lateral meniscus is a congenital variant in which the central portion does not degenerate with time. This theory would explain the variously shaped menisci found at surgery. However, no discoid menisci have been found in fetuses and a review of comparative anatomy shows no mammal with such a pattern of formation.
A second theory is a developmental one. Many discoid lateral menisci have abnormal attachments to the tibia. When the attachment to the posterior tibial plateau is deficient, there is a strong attachment to the medial femoral condyle by the meniscofemoral ligament (Wrisberg's ligament). This pattern of attachment may allow abnormal movement of the lateral meniscus: the posterior horn of the lateral meniscus moves into the center of the lateral compartment during full extension of the knee. With time, scarring and fibrosis of the lateral meniscus occur, with resultant thickening. These changes may account for the popping on flexion and extension that is usually noticed during childhood or early adolescence.
Treatment. Many discoid menisci are asymptomatic, and the mere presence of one is not an indication for treatment. The popping itself is not harmful unless it is accompanied by pain of swelling of the knee. Pain, swelling, and a history of trauma are relative indications for arthroscopy. Tears of the meniscus or degenerative changes on the articular surfaces may necessitate resection. Arthroscopic techniques allow for partial resection or saucerization of the discoid lateral meniscus, leaving a peripheral rim that may function properly. Resection may be difficult because of the increased thickness in such menisci. Prognosis for patients with discoid menisci is good. Discoid menisci without degenerative changes have been found in the joints of elderly persons. Therefore, every attempt should be made to salvage function of the meniscus by avoiding complete excision simply to eliminate the snapping, clicking sensation.
Meniscus Tears
Tears of the meniscus are common findings in a patient with an acutely injured knee, especially in situations in which a traumatic twisting event has occurred. Tears may occur in either meniscus or in both menisci at the same time. A meniscus tear often becomes symptomatic if its torn portion is mobile and slides into an abnormal position between the articular surfaces of the femur and tibia. Patients with a displaced meniscus tear often report pain at the joint line and blocked extension, flexion, or both. The affected knee frequently gives way and exhibits recurrent effusions.
A bucket-handle tear is a longitudinal tear through the substance of the meniscus. The torn portion remains attached to the anterior and posterior horns of the meniscus. A small radial tear initially causes very few symptoms, but if not treated it may progress to a deeper, more symptomatic parrot-beak tear. The unstable flap of meniscus may cause mechanical signs in the injured knee such as recurrent effusions, giving way, and a catching sensation. Horizontal tears of the meniscus appear to be a delamination of the substance of the meniscus. Neglected horizontal tears frequently result in an unstable flap of meniscal tissue, which can also cause mechanical signs.
When an unstable portion of meniscus displaces into the intercondylar notch and becomes incarcerated, it will cause the knee to lock. Manipulation of the knee may be possible and often occurs with a loud, audible and palpable “clunk.” This sound and the temporary resolution of symptoms indicate reduction of the displaced portion into its normal anatomic position. A persistently locked knee requires urgent intervention. If it is neglected, attempts at weight bearing and knee movement cause severe, irreversible erosion of the articular cartilage surfaces of the femur and tibia.
Physical Examination and Special Tests
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Joint line tenderness: Tenderness along the medial or lateral joint lines is among the most sensitive findings for a meniscal tear.
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McMurray test: The patient is supine and relaxed. The patient is asked to flex the knee maximally with external tibial rotation (medial meniscus) or internal tibial rotation (lateral meniscus). While maintaining rotation, the patient brings the knee into full extension. A positive test is indicated by a painful pop occurring over the medial joint line (medial meniscus) or lateral joint line (lateral meniscus).
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Apley compression test: The patient is prone. The knee is flexed to 90 degrees with external tibial rotation (medial meniscus) or internal tibial rotation (lateral meniscus). Axial compression is applied to the tibia while the patient flexes and extends the knee. A positive test is indicated by a painful pop over the medial joint line (medial meniscus) or lateral joint line (lateral meniscus).
McMurray and Apley test results may vary considerably from one examination session to the next owing to patient apprehension and chronicity of injury. A joint effusion will often be present after an acute tear. Chronically, atrophy of the quadriceps muscle may occur. With peripheral meniscus detachment and positive anterior drawer test, a loud “clunk” may be elicited as the meniscus displaces during anterior drawer testing.
Imaging. Plain films are usually normal, unless a meniscus tear has been present for a significant time. After that time, they may show joint line spurring, narrowing, or other arthritic changes. MRIs have now supplanted arthrograms for diagnosis of meniscal injury. MRI has a sensitivity as high as 95% for demonstration of medial meniscus tears but has somewhat less sensitivity in detection of lateral meniscus tears.
Treatment. In young, active persons, arthroscopic repair of torn menisci should always be considered. In these younger patients the loss of a large portion of a meniscus can be devastating because meniscal deficiency can lead to earlier-onset arthritis and, potentially, total joint arthroplasty.
At the time of the surgery, with the patient under anesthesia, a locked knee may spontaneously unlock. The knee is then examined manually to determine any ligament instability, and an arthroscopic examination is performed. To help preserve the articular cartilage, the displaced part of the meniscus can be removed during arthroscopy.
Repairs in the well-vascularized peripheral third (“red zone”) of the medial and lateral menisci have been quite successful. With proper technique and stabilization of the knee joint with ligament repair where necessary, repair of the meniscus can be successful in 90% of cases. Multiple studies have looked at establishing which tears outside the vascular peripheral third can be repaired and whether there are biologic or pharmacologic means that may improve the potential for healing. Although these tears have less chance of healing, repair may well be indicated in younger patients. Approaches to repair include all-inside, outside-in, and inside-out techniques, with most surgeons now preferring the all-inside approach when possible.
Rehabilitation programs after arthroscopy and partial meniscectomy generally include minimal immobilization of the knee, immediate weight bearing, and early physical therapy. Therapy consists of gait training and active and passive range-of-motion and quadriceps-strengthening exercises. Ice or heat may be applied as needed. After repair of meniscus tears, vigorous rehabilitation and range-of-motion exercises may be delayed a few weeks.
Knee Ligament Injury
Sprains of Knee Ligaments
Ligament injuries (sprains) of the knee are very common in athletes. In first-degree sprains, the ligament is trenched, with little or no tearing. These injuries produce mild point tenderness, slight hemorrhage, and swelling. Erythema may develop over the painful area but resolves in 2 or 3 weeks after injury. Joint laxity is not present, and the injury does not produce any significant long-term disability. Appropriate treatment consists of rest and muscle rehabilitation. Second-degree sprains are characterized by partial tearing of the ligament, resulting in joint laxity, localized pain, tenderness, and swelling. When stress is placed on a joint during examination, the examiner should still feel a definite “end point” to the joint movement. Because the ligament is only partially injured, the joint remains stable; thus, vigorous rehabilitation alone will likely be sufficient treatment. Third-degree sprains produce complete rupture of a ligament, making the joint unstable. Tenderness, instability, absence of a definite end point to stress testing, and severe ecchymosis are the hallmarks of third-degree sprains. Surgical intervention may be needed.
Sprains of the medial (tibial) collateral ligament are caused by a valgus force to the knee. Patients frequently report a snapping or tearing sensation and pain on the medial aspect of the knee. If only the MCL is injured, patients can usually continue to walk and may be able to continue the activity that causes the injury.
Physical examination reveals tenderness along the course of the MCL, and careful palpation can isolate the precise level of injury: at the origin of the ligament, on the medial femoral condyle, at the joint line (midsubstance), or along the long distal insertion of the ligament into the medial aspect of the tibia. Patients are more comfortable if examined lying supine on the examining table with the thigh supported. The physician cradles the lower leg in both hands off to the side of the table and alternately applies varus and valgus stresses to the knee (varus and valgus stress tests). When the leg is fully extended, the PCL is the structure most responsible for mediolateral stability. However, placing the knee in 30 degrees of flexion takes the PCL “out of play” so the MCL can be tested by applying a valgus force.
Third-degree sprains of the MCL may require direct surgical repair. However, an isolated third-degree sprain may be successfully treated by controlling swelling, by increasing range of motion, and with rehabilitation of the quadriceps femoris and hamstring muscles.
Marked medial (valgus) laxity may indicate that the posteromedial corner of the knee capsule is also injured. Surgical repair is needed to prevent residual rotational instability. A football clipping injury may result in the “unhappy triad” of O'Donoghue, which includes ruptures of the MCL and the ACL plus a tear of the medial meniscus. However, in the recent literature it has been shown that the lateral meniscus is more likely to be acutely torn at the time of ACL injury, whereas the medial meniscus is more often compromised in the chronically ACL-deficient knee. These injuries often require arthroscopically aided repair of the ligaments as necessary and repair of the injured meniscus if possible.
Rupture of the Anterior Cruciate Ligament
The ACL is the primary restraint to anterior translation of the tibia and it also contributes to internal rotation and varus/valgus instability with the knee extension. The anatomic configuration of its two bundles ensures functional tautness throughout the arc of motion, with the anteromedial bundle taut in flexion and the posterolateral component taut in extension. Although it may be torn by a contact injury, the ACL is most commonly injured without contact by a decelerating valgus angulation and external rotation force. In basketball, the ACL is commonly torn when a player lands from jumping with the knee in hyperextension and the tibia in internal rotation. The player hears a “pop,” feels a tear and acute pain in the knee, and may not be able to continue playing. The knee may feel very unstable during weight bearing and is often felt as a “giving way” of the knee. Patients complain that their knee slips or slides when they turn right or left with the foot planted. This sliding reflects the tibia subluxating anteriorly on the femur. Rupture of the ACL is a common cause of acute traumatic hemarthrosis.
Physical Examination and Special Tests
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Lachman test: This test is simple to perform and relatively painless for the patient with an acute injury. The examiner compares the amount of play in the injured knee with that in the normal one to determine if abnormal motion is present. The Lachman test is performed with the knee flexed 20 degrees to reduce the stability provided by the menisci. One of the examiner's hands stabilizes the femur while the other hand grasps the proximal tibia. With the patient relaxed, the examiner attempts to slide the proximal tibia anteriorly on the femur. An intact ACL prevents the tibia from sliding forward. When the ligament is injured, the tibia is moved from its normal position and can be subluxated anteriorly during the test. The examiner must note the quality of the end point of this stress test. If there is a solid mechanical stop at the most anterior extent of tibial motion, the ACL may be only partially torn. However, if the end point is soft and spongy, a complete rupture should be suspected. The integrity of the PCL must be ascertained before the result of this test can be considered valid. If the PCL is ruptured, the proximal tibia sags posteriorly, and the Lachman test will seem to be positive as the posterior subluxation is reduced.
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Anterior drawer test: The anterior drawer test is performed with the patient lying supine, resting comfortably with the knee flexed 90 degrees (see Plate 3-12 ). The patient's foot is stabilized during the test and may be held in place by the seated examiner's thigh. The examiner grasps the patient's calf near the popliteal fossa with both hands and attempts to slide the tibia anteriorly. When the ACL is ruptured, the tibia slides anteriorly with respect to the femur. The anterior drawer test is performed several times, with the patient's foot and leg positioned first in internal rotation, then in neutral rotation, and finally in external rotation. As in the Lachman test, the injured knee must be compared with the normal one. The anterior drawer test is useful for detecting complete ruptures of the ACL but is often less sensitive than the Lachman test in diagnosing partial injuries.
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Pivot shift and jerk tests: These tests identify most cases of clinically significant knee instability. The patient should be lying supine and relaxed, although this is often difficult in the acutely injured patient because this test can reproduce a feeling of discomfort. It has been found that this test is most accurate when performed on an anesthetized patient. The examiner stands beside the injured leg, facing it. With one hand grasping the patient's foot, the examiner places the other hand on the lateral aspect of the knee, with the thumb underneath the head of the fibula. With the knee starting in full extension (pivot shift) or flexed to 90 degrees (jerk test), a valgus force is applied at the knee while the tibia is internally rotated by the hand holding the foot. This maneuver causes the lateral tibial plateau to subluxate anteriorly on the femur. With the knee in extension, the iliotibial tract is anterior to the instantaneous center of rotation of the knee and acts as an extensor. The knee is then slowly flexed (pivot shift) or extended (jerk test), and the subluxation becomes more apparent. At a point between 20 and 40 degrees of flexion, the iliotibial tract slides posterior to the instantaneous center of rotation of the knee and acts as a flexor, causing reduction of the tibia. The reduction is palpable, visible, and frequently audible.
Treatment. Not all acute injuries of the ACL require surgery. If the careful physical examination, using anesthesia if necessary, reveals minimal ligamentous laxity and no sign of meniscus injury and the patient does not have sensations of instability or symptoms preventing full function, then prompt, vigorous rehabilitation is instituted. If the Lachman or anterior drawer test indicates mild ligament instability but the pivot shift test is negative and there are no other associated injuries, a nonoperative program may be instituted. However, significant instability may eventually develop if injury of the ACL is neglected or treated conservatively. It has been shown that a chronically ACL-deficient knee can also lead to degenerative tears of the meniscus and eventually an earlier onset of osteoarthritis. Patients whose knees give way during daily activities are candidates for delayed reconstruction of the ligament. If the instability is a problem only during intense physical activity, using a brace may provide relief.
Physical therapy focuses on rehabilitation of the quadriceps and hamstring muscles. Although the knee is reasonably stable, instability may develop gradually, eventually necessitating reconstruction of the ACL. Increased instability may lead to meniscus tears.
Surgical treatment is usually indicated in patients with complete ACL injury, symptomatic or clinical instability, and a positive pivot shift test. The procedure used is dictated by the patient's lifestyle, expectations and other medical conditions. Older, sedentary patients may need no surgery, whereas young, active patients should be considered for repair or reconstruction. The goals of surgery are to restore stability to the knee to permit return to activity, prolong the survival of the menisci, and delay the development of osteoarthritis.
Many techniques are used to repair or reconstruct the ACL, with specific considerations to be taken into account based on each individual patient. In the skeletally mature patient, the ligament is traditionally reconstructed using an arthroscopically assisted approach. The graft choice in younger patients will usually be a hamstrings or bone-patella-bone autograft, whereas older patients or those with other contraindications to hamstrings harvest may use allograft tissue. In skeletally immature patients with open growth plates, there exist multiple modifications to the traditional reconstruction methods that avoid compromising the tibial, femoral, or both open physes. Protected weight bearing is allowed immediately, with some surgeons advocating the use of continuous passive motion machines and braces in the early postoperative period. Early and consistent physical therapy is important to achieve optimal results. Patients should avoid participation in high-demand sports for 6 to 12 months after surgery.
Rupture of Posterior Cruciate Ligament
The PCL is the chief stabilizer of the knee in full extension. The most common causes of rupture of this ligament are hyperextension of the knee and a direct blow to the anterior aspect of the flexed knee. Severe varus or valgus stress to the knee after injury to the collateral ligaments can cause rupture of the PCL.
Physical Examination and Special Tests
A knee lacking a functioning PCL may be hyperextended during examination. The examiner stands at the foot of the supine patient and simultaneously lifts both feet by the great toes, observing the amount of extension at each knee. A knee with rupture of a PCL exhibits noticeable hyperextension and greater joint laxity than its normal counterpart when varus and valgus stresses are applied with the knee in full extension.
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Posterior drawer test: The posterior drawer test is performed with the patient lying supine on an examining table and the knee in 90 degrees of flexion. The patient's foot is stabilized by the examiner's thigh on the table as for the anterior drawer test. The examiner uses both hands to push the proximal tibia posteriorly in an effort to displace it relative to the distal femur. By alternately pushing and pulling the tibia, the examiner can determine if the ACL is intact and if the proximal tibia is moving posteriorly. The examiner must recognize the starting point of the drawer test to determine accurately which of the two cruciate ligaments is injured.
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Posterior sag sign: With the patient supine and relaxed, a pad is placed under the distal thigh on the affected side; the heel is allowed to rest on the examining table, and the calf of the leg hangs unsupported. The examiner observes the knee from the patient's side. When a rupture of the PCL is present, the proximal tibia subluxates posteriorly and the anterior surface of the proximal leg appears to sag.
Treatment. Patients who have high-demand knee and severe instability are candidates for reconstruction of the PCL. This is routinely accomplished in an arthroscopically assisted fashion as with the ACL, with surgical options again being individualized to each patient. As with the ACL, when avulsion of the bony attachment of the PCL occurs at either end, primary repair of the avulsion fragment may be performed. If bone-to-bone repair is not possible, many surgeons may elect to treat the patient without resorting to surgery. Repairs of the PCL have historically been less successful than those of the ACL, with higher risk of recurrent instability after surgery and loss of motion. Injury to the posterolateral corner of the knee capsule must also be considered and addressed when necessary at the time of surgery to avoid a poor functional result.
After surgery, the knee may be immobilized in extension for a period of 2 weeks. Vigorous physical therapy is then instituted while avoiding activities that place a load on the knee when it is flexed past 90 degrees. Achieving full extension may be very difficult and should be a goal of therapy, although manipulation under anesthesia may eventually be required.
Medial (Tibial) Collateral Ligament Injury
Injury to the taut MCL is often caused by a valgus force applied to the knee with external tibial rotation. This may occur by a noncontact twist event or from a blow to the lateral side of the joint. The patient will initially describe pain on the medial side of the knee and, with a complete tear, complaints of the knee giving way into valgus.
Physical Examination. Injury to the MCL is noted with a positive valgus stress test with the knee in 30 degrees of flexion as compared with the opposite knee. An injured MCL along with disrupted ACL or PCL will result in more gapping that occurs when the knee is tested in full extension. Frequently, but not always, a positive anterior drawer sign results with external rotation of the tibia as the medial tibial condyle rotates anteriorly.
Imaging. An abduction stress film may be used to distinguish ligament injury from epiphyseal fracture in skeletally immature athletes as a fracture opens at a growth plate and a ligament tear opens at a joint line. This should be done in 30 degrees of knee flexion. MRI is also useful to help to diagnose disruptions or edema in the MCL.
Treatment. Grades I and II sprains are often treated with the RICE (rest, ice, compression, elevation) protocol, along with use of crutches while weight bearing and physical rehabilitation. Complete tears, unless present with other associated injuries or in a high-demand athlete, are rarely surgically repaired. Surgical options include primary repair, allograft reconstruction, and repair with anchor fixation for avulsion injuries. When attempting nonoperative treatment, immobilization should be used for a short period after acute injury with an unstable knee. However, in patients with only mild instability, rigid immobilization may not be necessary. The patient should begin a rehabilitation program as soon as possible.
Lateral Ligament and Posterolateral Corner Injuries
Injury to the lateral (fibular) collateral ligament (LCL) often occurs with a varus force or twisting moment at the knee. Injuries to this region of the knee may be associated with injuries to the popliteus tendon, iliotibial band, popliteofemoral ligament, and peroneal nerve. Posterolateral ligaments are often injured by a hyperextension mechanism, frequently with a blow to the anteromedial tibia.
Patients will complain of pain present over the lateral ligament complex. The knee may also give way when twisting, cutting, or pivoting. In chronic cases, posterolateral corner injury gives a feeling of giving way into hyperextension when standing, walking, or running backward.
Physical Examination and Special Tests
In acute cases, there may be increased gapping on a varus stress test at 30 degrees of flexion and a positive posterolateral drawer sign. Chronic cases often show a positive reverse pivot shift and external rotation recurvatum test. The dial test will likely present in all cases of severe posterolateral ligament disruption.
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Dial test: With the patient either prone or supine, the examiner will place an external rotation force to the knee through the ankle at 30 degrees of knee flexion. Increased external rotation of 10 to 15 degrees as compared with the opposite knee indicates an injury to the posterolateral corner. If positive, the test is repeated at 90 degrees of knee flexion, and increased external rotation at this point indicates a concurrent PCL injury.
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External rotation recurvatum test: With the patient supine and a stabilizing downward force placed on the femur, the externally rotated foot is lifted by the great toe. Increased recurvatum or hyperextension at the knee indicates an injury to the posterolateral corner. The external rotation recurvatum test may also be apparent on standing, giving an increased varus appearance to the knee.
Imaging. On plain radiographs, a “lateral capsular sign” shows avulsion of the midportion of the lateral capsular ligament with a small fragment of proximal lateral tibia. This is associated with a high incidence of an ACL tear and indicates anterolateral instability. The “arcuate sign” shows avulsion of the proximal fibula with the posterolateral ligament and is also associated with an ACL injury. As with MCL injuries, stress view radiographs and MRI may also be used for diagnosis of these injuries.
Treatment. Similar to injuries to the MCL, grade I and II sprains are treated conservatively with the RICE protocol, crutches, and physical rehabilitation. In complete tears, primary surgical repair or allograft reconstruction is usually preferable, especially if the injury involves more than just the LCL. Immobilization alone may be less successful for these injuries in patients with severe instability. Cases of mild instability may be treated nonoperatively similar to that for lesser-grade sprains.
Disruption of Quadriceps Femoris Tendon or Patellar Ligament
Damage to the quadriceps mechanism generally occurs when there is active contraction of the quadriceps femoris muscle against forced flexion of the knee. Most ruptures of this extensor mechanism occur in older patients. At the time of injury, the patient experiences sudden pain, which may be associated with a tearing sensation about the knee. The tendon may be weakened by age-related degenerative changes or by pathologic changes due to psoriatic arthritis, rheumatoid arthritis, arteriosclerosis, gout, hyperparathyroidism, diabetes, chronic renal failure, or corticosteroid therapy.
Physical Examination. Palpation of the knee often reveals a hematoma, which may make examination difficult. A high-riding patella may indicate rupture of the patellar ligament, whereas a patella that is riding lower than normal suggests a rupture of the quadriceps femoris tendon. A large defect may be palpable soon after injury, although if the ruptured ligament is not treated for weeks or months the sulcus may fill with scar tissue.
The most important finding during examination is the patient's inability to actively extend the knee fully against gravity. Also, the patient may not be able to maintain a passively extended knee against gravity. Patients with rupture of the quadriceps femoris tendon or patellar ligament without involvement of the medial or lateral retinaculum may be able to extend the injured knee actively to within 10 degrees of full extension. When there is a widely separated tear of either tendon or ligament combined with involvement of the medial and lateral retinacula, active extension is very difficult. Patients with chronic rupture of the quadriceps femoris tendon complain of giving way of the knee and marked weakness on attempting active extension.
Imaging. Whereas physical examination is often sufficient to diagnose disruptions of the extensor mechanism, it is recommended to obtain plain radiographs to assess for fracture and patellar positioning. MRI may be performed to assess the involved soft tissues in detail, although this is not often necessary for these patients.
Treatment. Rupture of the quadriceps femoris tendon generally occurs at its point of intersection into the superior pole of the patella, whereas rupture of the patellar ligament usually occurs at the inferior pole of the patella. In both cases, surgery is required to reestablish the continuity of the quadriceps mechanism. The tendon or ligament most often is reattached with sutures through drill holes in the patella. Then, the medial and lateral retinacula are sutured. After surgery, the knee is routinely immobilized in full extension for 6 to 8 weeks.
Patients who also have chronic metabolic disorders or receive long-term corticosteroid treatment may require a more complex repair that uses tendon, fascia, or wire to reinforce the damaged quadriceps mechanism. After postoperative immobilization for 8 to 10 weeks, patients gradually start protected range-of-motion exercises and should use a cane or walker for some time.
Rupture of the patellar ligament may also occur at its insertion on the tibia, with or without fracture of the tibial tuberosity. In children whose growth plates have not yet closed, the ligament should be sutured, because this injury may disturb the growth of the proximal tibia. In adults, avulsion of the ligament from the tibial tuberosity is repaired by suturing the avulsed ligament though drill holes in the tibia or securing it with a metal staple or screw. A displaced fracture of the tibial tuberosity may be treated with open reduction and fixation with a metal screw.
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