Knee Injuries

Chapter 45 Knee Injuries





ANATOMY 






MECHANICS 


MENISCI 
















ACUTE TRAUMATIC LESIONS OF LIGAMENTS 


























ANTERIOR CRUCIATE LIGAMENT INJURIES 















POSTERIOR CRUCIATE LIGAMENT 











TRAUMATIC DISLOCATIONS 










SYNOVIAL PLICAE 


ARTICULAR CARTILAGE INJURIES 



OSTEOCHONDRITIS DISSECANS 








DISORDERS OF THE PATELLA 









EXTRAARTICULAR ANKYLOSIS OF THE KNEE 




OPEN WOUNDS OF THE KNEE JOINT 



Anatomy


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. Basic to an understanding of knee injuries is an understanding of the normal knee anatomy. Although much emphasis has been placed on the ligaments of the knee, without the supporting action of the associated muscles and tendons, the ligaments are not enough to maintain knee stability. The structures around the knee have been classified into three broad categories: osseous structures, extraarticular structures, and intraarticular structures.



Osseous Structures


The osseous structures of the knee consist of three components: the patella, the distal femoral condyles, and the proximal tibial plateaus, or condyles. The knee is called a hinge joint, but actually it is more complicated than that, because in addition to flexion and extension its motion has a rotary component. The femoral condyles are two rounded prominences that are eccentrically curved. Anteriorly, the condyles are somewhat flattened, which creates a larger surface for contact and weight transmission. The condyles project very little in front of the femoral shaft but markedly so behind. The groove found anteriorly between the condyles is the patellofemoral groove, or trochlea. Posteriorly, the condyles are separated by the intercondylar notch. The articular surface of the medial condyle is longer than that of the lateral condyle, but the lateral condyle is wider. The long axis of the lateral condyle is oriented essentially along the sagittal plane, whereas the medial condyle usually is at about a 22-degree angle to the sagittal plane.


The expanded proximal end of the tibia forms two rather flat surfaces, condyles or plateaus, that articulate with the femoral condyles. They are separated in the midline by the intercondylar eminence with its medial and lateral intercondylar tubercles. Anterior and posterior to the intercondylar eminence are the areas that serve as attachment sites for the cruciate ligaments and menisci. The posterior lip of the lateral tibial condyle is rounded off where the lateral meniscus slides posteriorly during flexion of the knee.


The articular surfaces of the knee are not congruent. On the medial side, the femur meets the tibia like a wheel on a flat surface, whereas on the lateral side, it is like a wheel on a dome. Only the ligaments acting in concert with the other soft tissue structures provide the knee with the necessary stability.


The patella is a somewhat triangular sesamoid bone that is wider at the proximal pole than at the distal pole. The articular surface of the patella is divided by a vertical ridge, resulting in a smaller medial and a larger lateral articular facet, or surface. With the knee in extension, the patella rides above the superior articular margin of the femoral groove. In extension, the distal portion of the lateral patellar facet articulates with the lateral femoral condyle, but the medial patellar facet barely articulates with the medial femoral condyle until complete flexion is approached. At 45 degrees of flexion, contact moves proximally to the midportion of the articular surfaces. In complete flexion, the proximal portions of both facets are in contact with the femur; and during flexion and extension, the patella moves 7 to 8 cm in relation to the femoral condyles. With complete flexion, more pressure is applied to the medial facet.


Trauma that affects these osseous structures and their relationship with each other frequently causes derangement of the joint. Restoration of these structures is essential to restoration of the function of the knee.



Extraarticular Tendinous Structures


The important extraarticular structures supporting and influencing the function of this joint are the synovium, capsule, collateral ligaments, and musculotendinous units that span the joint. The musculotendinous units are principally the quadriceps mechanism, the gastrocnemius, the medial and lateral hamstring groups, the popliteus, and the iliotibial band.


The four components of the quadriceps mechanism form a three-layered quadriceps tendon that inserts into the patella. The tendon of the rectus femoris flattens immediately above the patella and becomes the anterior layer, which inserts at the anterior edge of the proximal pole. The tendon of the vastus intermedius continues downward as the deepest layer of the quadriceps tendon and inserts into the posterior edge of the proximal pole. The middle lamina is formed by the confluent edges of the vastus lateralis and vastus medialis. The fibers of the medial retinaculum formed from the aponeurosis of the vastus medialis insert directly into the side of the patella to help prevent lateral displacement of the patella during flexion. The patellar tendon takes its origin from the apex or distal pole of the patella and inserts distally into the tibial tuberosity.


The gastrocnemius, the most powerful calf muscle, spans the posterior aspect of the knee in intimate relationship with the posterior capsule to insert on the posterior aspect of the medial and lateral femoral condyles.


Pes anserinus is the term for the conjoined insertion of the sartorius, gracilis, and semitendinosus muscles along the proximal medial aspect to the tibia. These primary flexors of the knee have a secondary internal rotational influence on the tibia and help protect the knee against rotary and valgus stress. Their counterpart on the lateral side of the knee is the strong biceps femoris insertion into the fibular head, lateral tibia, and posterolateral capsular structures. This muscle is a strong flexor of the knee that also produces simultaneous strong external rotation of the tibia. It provides rotary stability by preventing forward dislocation of the tibia on the femur during flexion. Its contributions to the arcuate ligament complex at the posterolateral corner of the knee also provide varus and rotary stability. The iliotibial tract, the posterior third of the iliotibial band, inserts proximally into the lateral epicondyle of the femur and distally into the lateral tibial tubercle (Gerdy tubercle). It thus forms an additional ligament that is contiguous anteriorly with the vastus lateralis and posteriorly with the biceps. The iliotibial band moves forward in extension and backward in flexion but is tense in both positions. During flexion, the iliotibial band, the popliteal tendon, and the lateral collateral ligament cross each other, whereas the iliotibial band and biceps tendon remain parallel to each other as in extension, all serving to enhance lateral stability (Fig. 45-1).



The popliteus muscle has three origins, the strongest of which is from the lateral femoral condyle. Other important origins are from the fibula (popliteofibular ligament) and from the posterior horn of the lateral meniscus. The femoral and fibular origins form the arms of an oblique Y-shaped ligament, the arcuate. The arms are joined together by the capsule and meniscal origin. The arcuate ligament is not a separate ligament but is a condensation of the fibers of the origin of the popliteus (Fig. 45-2). With electromyographic studies, Basmajian and Lovejoy found that the popliteus muscle is a prime medial rotator of the tibia during the initial stages of flexion and also acts to withdraw the meniscus during flexion. In addition, it supplies rotary stability to the femur on the tibia and aids the posterior cruciate ligament in preventing forward dislocation of the femur on the tibia.



The semimembranosus muscle is especially important as a stabilizing structure around the posterior and posteromedial aspects of the knee. It has five distal expansions (Fig. 45-3). The first is the oblique popliteal ligament, which passes from the insertion of the semimembranosus on the posteromedial aspect of the tibia obliquely and laterally upward toward the insertion of the lateral gastrocnemius head (Fig. 45-4A). It acts as an important stabilizing structure on the posterior aspect of the knee. The semimembranosus helps tighten this ligament with contraction (see Fig. 45-4B). When the oblique popliteal ligament is pulled medially and forward, it tightens the posterior capsule of the knee. This maneuver can be used to tighten the posterior capsule in the posteromedial corner of the knee in surgical repair. A second tendinous attachment is to the posterior capsule and posterior horn of the medial meniscus. This tendinous slip helps tighten the posterior capsule and pulls the medial meniscus posteriorly during knee flexion. The anterior or deep head continues medially along the flare of the tibial condyle and inserts beneath the superficial tibial collateral ligament just distal to the joint line. The direct head of the semimembranosus attaches to the tubercle on the posterior aspect of the medial condyle of the tibia just below the joint line. This tendinous attachment provides a firm point in which sutures can be anchored for posteromedial capsular repair. The distal portion of the semimembranosus tendon continues distally to form a fibrous expansion over the popliteus and fuses with the periosteum of the medial tibia. The semimembranosus, through its muscle contraction, tenses the posterior capsule and posteromedial capsular structures, providing significant stability. Functionally, it acts as a flexor of the knee and internal rotator of the tibia.




The medial extensor expansion, or medial retinaculum, is a distal expansion of the vastus medialis aponeurosis. It attaches along the medial border of the patella and patellar tendon and distally inserts into the tibia. It functions as the medial tracking support of the patella in the patellofemoral groove. It covers and may blend into the anteromedial capsular ligament. Contraction of the vastus medialis helps tighten the anterior portion of the medial capsular ligament.


The lateral extensor expansion, or lateral retinaculum, is an extension of the vastus lateralis attaching to the iliotibial band, which helps tense this band as the knee extends and the iliotibial band moves forward. Imbalance between the lateral and medial retinacular structures often is present in patellar subluxations and dislocations.


In addition to these musculotendinous units that directly span the knee, abnormalities in the orientation and alignment of the foot as well as deficiencies in the hip flexors and abductors can influence the alignment and function of the knee and must be considered in evaluation and rehabilitation of this joint.



Extraarticular Ligamentous Structures


The joint capsule and the collateral ligaments are the principal extraarticular static stabilizing structures. The capsule is a sleeve of fibrous tissue extending from the patella and patellar tendon anteriorly to the medial, lateral, and posterior expanses of the joint. The menisci are attached firmly at the periphery to this capsule, especially so medially and less so laterally. Laterally, the passage of the popliteal tendon through the popliteal hiatus to its origin on the femoral condyle produces a less secure meniscal attachment than is present medially. The medial capsule is more distinct and well defined than its lateral counterpart. The capsular structures, along with the medial and lateral extensor expansions of the powerful quadriceps musculature, are the principal stabilizing structures anterior to the transverse axis of the joint. The capsule is especially reinforced by the collateral ligaments and the medial and lateral hamstring muscles as well as by the popliteus muscle and the iliotibial band posterior to the transverse axis. The medial and lateral “quadruple complexes” have been identified as principal stabilizers of the knee (Fig. 45-5). The medial quadruple complex is made up of the medial collateral ligament, the semimembranosus, the tendons of the pes anserinus, and the oblique popliteal ligament portion of the posterior capsule. The lateral quadruple complex is made up of the iliotibial band, the lateral collateral ligament, the popliteal tendon, and the biceps femoris. The capsule is reinforced posteriorly by the oblique popliteal ligament, at the posteromedial corner by the ramifications of the semimembranosus, and posterolaterally by the structures contributing to the arcuate complex.



The anteromedial and anterolateral portions of the capsule are relatively thin structures but are reinforced by the medial and lateral patellar retinacular expansions and also laterally by the iliotibial band and medially by reinforcing bands extending from the patella as the patelloepicondylar ligament and the patellotibial ligament. The anteromedial and anterolateral portions of the capsule are significant in protecting the anteromedial and anterolateral aspects of the knee against subluxation and rotational excesses.


The medial capsule has been divided into three distinct regions: the anteromedial capsule, as just discussed; the midmedial capsule; and the posteromedial capsule.


The midmedial capsule is reinforced and thickened by vertically oriented fibers and has often been referred to as the deep layer of the medial collateral ligament. It originates from the femoral condyle and epicondyle and inserts just below the tibial articular margin. It is divided into a meniscofemoral portion, extending from the meniscal attachment to the femoral origin, and the meniscotibial portion, extending as the coronary ligament of the meniscus to its tibial insertion. The meniscofemoral portion is the much longer and stronger of these two divisions. The midmedial capsule resists valgus and rotary stresses.


The posteromedial region of the medial capsule extends from the posterior edge of the medial collateral ligament posteriorly to the insertion of the direct head of the semimembranosus. Hughston described this posterior oblique ligament as a thickening of the medial capsular ligament attached proximally to the adductor tubercle of the femur and distally to the tibia and posterior aspect of the capsule. The distal attachment is composed of three arms: (1) the prominent central, or tibial, arm, which attaches to the edge of the posterior surface of the tibia close to the margin of the articular surface and central to the upper edge of the semimembranosus tendon; (2) the superior, or capsular, arm, which is continuous with the posterior capsule and the proximal part of the oblique popliteal ligament; and (3) the poorly defined inferior, or distal, arm, which attaches distally both to the sheath covering the semimembranosus tendon and to the tibia just distal to the direct insertion of the semimembranosus tendon (Figs. 45-6 to 45-9).






The central portion is the thickest and probably the most important arm of the ligament, originating in the region of the adductor tubercle and coursing posteriorly and obliquely to insert at the posteromedial corner of the tibia near the insertion of the direct head of the semimembranosus tendon. The superior, or more proximal, arm of the posterior oblique ligament passes posteriorly, blending with the posterior capsule and the oblique popliteal ligament as it separates from the semimembranosus tendon. The inferior and distal groups of fibers pass superficially over the insertion of the semimembranosus tendon, attach to the tibia and fascia inferiorly, and probably have little functional importance.


The posteromedial portion of the medial capsular ligamentous complex is especially important for valgus and rotational stability to the knee. The posteromedial capsule and posterior oblique ligaments become progressively relaxed as the knee flexes; however, with active contraction of the semimembranosus muscle, each of the three arms of the posterior oblique ligament is tense. Therefore, both kinetic and static stabilizing effects are obtained from this portion of the medial capsular ligament, even with the knee flexed. In knee ligament reconstruction, this important part of the posteromedial complex is as essential as any other structures requiring attention if stability is to be restored. A precise understanding of anatomy and function is required for repair or reconstruction of this posteromedial complex. The central arm of the posterior oblique ligament must be tightened in surgical repair or reconstruction, or passive stability cannot be attained regardless of any other surgical procedures.


The medial collateral ligament is a long, rather narrow, well-delineated structure lying superficial to the medial capsule and capsular ligaments, originating on the medial epicondyle and inserting 7 to 10 cm below the joint line on the posterior half of the medial surface of the tibial metaphysis deep to the pes anserinus tendons. It has been referred to as the superficial tibial collateral ligament or the superficial portion of the medial collateral ligament. Biomechanical studies have shown that it provides the principal stability to valgus stresses. It glides forward over the side of the femoral condyle in extension and posteriorly in flexion (Fig. 45-10). The long fibers of the medial collateral ligament are the primary stabilizers of the medial side of the knee against valgus and external rotary stress. The anterior fibers of the ligament tighten as the knee flexes, with fibers more posteriorly becoming slack (Fig. 45-11).




In their classic study of knee anatomy, Warren and Marshall divided the knee into three layers. Layer I includes the deep fascia or crural fascia; layer II is composed of the superficial medial collateral ligament, various structures anterior to this ligament, and the ligaments of the posteromedial corner; and layer III is made up of the capsule of the knee joint and the deep medial collateral ligament. Layer I is the first fascial plane encountered after a skin incision. Its plane is defined by the fascia that invests the sartorius muscle (Fig. 45-12). Proceeding posteriorly, layer I is a thin sheet that overlies the two heads of the gastrocnemius and the structures of the popliteal fossa. If a vertical incision is made in layer I posterior to the parallel fibers of the medial collateral ligament and the anterior portion of layer I is reflected forward, the entire superficial medial collateral ligament is exposed (Fig. 45-13). Inferiorly, the tendons of the gracilis and semitendinosus can be seen as distinct structures that can be separated from layer I superficially and layer II beneath (Fig. 45-14). Layers I and II are separated by these tendons as they cross to their insertions on the tibia.





The plane of layer II is clearly defined by the parallel fibers of the superficial medial collateral ligament. As layer II is traced posteriorly from the anterior edge of this ligament, the fibers joining the femur to the tibia become more oblique in their orientation (Fig. 45-15). At the posteromedial corner of the knee, layer II merges with layer III and with the tendon sheath of the semimembranosus (Figs. 45-16 and 45-17; see also Fig. 45-14). The conjoined structure formed by layer II and layer III extends posteriorly to form the posteromedial capsule that envelops the medial condyle of the femur. This posteromedial capsule is augmented by fibers from the semimembranosus tendon sheath. Most of the semimembranosus tendon inserts onto bone through the direct insertion at the posteromedial corner of the tibia just below the joint line (see Fig. 45-16). A more anterior insertion, which is an extension of the direct insertion, proceeds around the medial side of the tibia just below the joint line. This anterior insertion lies deep to the superficial medial collateral ligament and layer II and distal to the tibial margin of the capsule, or layer III (see Figs. 45-14C and 45-16). These two insertions do not participate in any of the three layers, because they go directly to bone. The semimembranosus tendon sheath sends fibrous extensions upward and downward into layer II (see Figs. 45-15 to 45-17). The most clearly defined of these fiber tracts are the ones that extend directly upward over the medial femoral condyle and across the back of the knee to the lateral condyle, forming the oblique popliteal ligament (see Fig. 45-16). A third, smaller extension of the semimembranosus sheath runs distally to insert on the tibia posterior to the inferior oblique portion of the superficial medial collateral ligament and to blend with the oblique fibers of that ligament to a varying degree. Judging from their morphological features, these fibers do not appear to have much functional significance.





Anterior to the superficial medial collateral ligament, layer II is variable. Whereas it is a single layer posteriorly, it combines anteriorly with layer I, forming the parapatellar retinacular fibers and the patellofemoral ligaments.


Layer III is the true capsule of the knee joint, attached above and below the joint at the margins of the articular surfaces (Fig. 45-18). The anterior part of the capsule is thin. It does not appear to function as a stabilizing ligament and simply envelops the fat pad. Beneath the superficial medial collateral ligament, layer III becomes thicker and forms a vertically oriented band of short fibers, variably known as the deep medial collateral ligament, deep layer of the medial collateral ligament, or medial capsular ligament (see Figs. 45-14A and B and 45-18). This deep ligament extends from the femur to the midportion of the peripheral margin of the meniscus and tibia (see Fig. 45-17). Elsewhere the capsule is thin. The deep and superficial ligaments are readily separated where they are in direct contact, but farther posteriorly, 1 to 2 cm behind the anterior edge of the superficial medial collateral ligament, layers II and III blend. The result is that the meniscofemoral portion of the deep ligament tends to merge with the overlying superficial ligament near its proximal attachment (see Figs. 45-14A to C and 45-18). The meniscotibial ligament (coronary ligament) is consistently separated readily from the overlying superficial ligament. Farther posteriorly, layer III merges with layer II (see Fig. 45-14A to C), and their combined fibers envelop the posteromedial corner of the joint, forming a composite structure.



The lateral collateral ligament attaches to the lateral femoral epicondyle proximally and to the fibular head distally. The lateral collateral ligament has an average femoral attachment slightly proximal (1.4 mm) and posterior (3.1 mm) to the lateral epicondyle. Distally, it is attached 8.2 mm posterior to the anterior aspect of the fibular head. It is more of a tendinous structure than a wide ligamentous band. It is of prime importance in stabilizing the knee against varus stress with the knee in extension. As the knee goes into flexion, the lateral collateral ligament becomes less influential as a varus-stabilizing structure.


In addition to the lateral ligaments and lateral capsular structures, stability depends on the iliotibial band, the biceps tendon, and the popliteal tendon. The iliotibial band inserts into the lateral epicondyle of the femur and then passes in its broad expansion between the lateral aspect of the patella and the more posterior location of the biceps femoris to insert into the lateral tibial (Gerdy) tubercle. Thus it acts as a supplemental ligament across the lateral aspect of the joint. This band moves anteriorly as the knee extends and slides posteriorly as the knee flexes but remains tense in all knee positions. With flexion, the iliotibial band, the popliteal tendon, and the lateral collateral ligament cross each other, thereby greatly enhancing lateral stability. The biceps tendon functions as a lateral stabilizer by contributing to the arcuate complex and by being a powerful flexor and external rotator of the tibia on the femur. The popliteal tendon courses from the posterior aspect of the tibia through the popliteal hiatus and attaches deep to and somewhat anterior to the femoral insertion of the lateral collateral ligament.


Warren et al. identified a strong direct attachment of the popliteal tendon to the fibula, which has been called the popliteal fibular fascicle and the fibular origin of the popliteus muscle. These researchers called this structure the popliteofibular ligament because it connects the fibula to the femur through the popliteal tendon (Fig. 45-19). This ligament is located deep to the lateral limb of the arcuate ligament; it originates from the posterior part of the fibula and posterior to the biceps insertion and joins the popliteal tendon just proximal to its musculotendinous junction. Thus the popliteus muscle-tendon unit is a Y-shaped structure with a muscle origin from the posterior part of the tibia, a ligamentous origin from the fibula, and a united insertion on the femur. The popliteal tendon has a constant, broad-based femoral attachment at the most proximal and anterior fifth of the popliteal sulcus. The popliteal tendon attachment on the femur is always anterior to the lateral collateral ligament. The average distance between the femoral attachments of the popliteal tendon and the lateral collateral ligament is 18.5 mm. The popliteofibular ligament has two divisions, anterior and posterior. The average attachment of the posterior division is 1.6 mm distal to the posteromedial aspect of the tip of the fibular styloid process, and the anterior division attaches 2.8 mm distal to the anteromedial aspect of the tip of the fibular styloid process. Selective cutting studies confirmed that the popliteal tendon attachments to the tibia and the popliteofibular ligament are important in resisting posterior translation, varus rotation, and external rotation.



Seebacher, Inglis, Marshall, and Warren defined three distinct layers of the lateral structures of the knee. The most superficial layer, or layer I, has two parts: (1) the iliotibial tract and its expansion anteriorly and (2) the superficial portion of the biceps femoris and its expansion posteriorly (Figs. 45-20 and 45-21). The peroneal nerve lies on the deep side of layer I, just posterior to the biceps tendon. Layer II is formed by the retinaculum of the quadriceps, most of which descends anterolaterally and adjacent to the patella.




Posteriorly, layer II is incomplete and is represented by the two patellofemoral ligaments. The proximal ligament joins the terminal fibers of the lateral intermuscular septum; the distal one ends posteriorly at the fabella or at the insertions of the posterolateral capsular reinforcements and of the lateral head of the gastrocnemius on the femoral condyle (see Figs. 45-20 and 45-21). The patellomeniscal ligament also is part of layer II. It extends obliquely from the patella, attaches to the margin of the lateral meniscus, and terminates inferiorly on the lateral tibial (Gerdy) tubercle deep to the iliotibial tract. Layers I and II are adherent to each other in a vertical line at the lateral margin of the patella. Discrete attachments of the uppermost fibers of the patellofemoral ligament to the overlying iliotibial tract occur just below the termination of the lateral intermuscular septum at the lateral femoral epicondyle.


Layer III, the deepest layer, is the lateral part of the joint capsule (Fig. 45-22). It is attached to the edges of the tibia and femur circumferentially in horizontal planes at the proximal and distal ends of the knee joint. The capsular attachment to the outer edge of the lateral meniscus is called the coronary ligament. The popliteal tendon passes through a hiatus in the coronary ligament to attach to the femur. Just posterior to the overlying iliotibial tract, the capsule divides into two layers. It encompasses the lateral collateral ligament and ends posteriorly at the variably sized fabellofibular ligament (short external ligament) (see Figs. 45-20 and 45-22A). The deep lamina of the posterolateral part of the capsule passes along the edge of the lateral meniscus. The inner lamina terminates posteriorly at the Y-shaped arcuate ligament. These two capsular laminae always are separated from each other by the inferolateral geniculate vessels, which pass forward. Seebacher et al. noted three anatomical variations in their dissections: (1) the arcuate ligament alone reinforced the capsule in 13% of the knees; (2) the fabellofibular ligament alone reinforced the capsule in 20%; and (3) both of these ligaments reinforced the posterolateral aspect of the capsule in 67%. These variations were associated with variations in the size of the osseous cartilaginous fabella in the lateral head of the gastrocnemius. Most commonly, the fabellofibular and arcuate ligaments both were present and were of modest size. When the fabella was large, there was no arcuate ligament and the fabellofibular ligament was robust. Conversely, when the fabella or its cartilaginous remnant was absent, the fabellofibular ligament also was absent and only the arcuate ligament was present (Fig. 45-23; see also Figs. 45-20 and 45-22). Both the arcuate and fabellofibular ligaments insert on the apex of the fibular styloid process. They ascend vertically on the free edges of their respective capsular lamina to the lateral head of the gastrocnemius, where they are joined by the posterior termination of the oblique popliteal ligament. When present, the fabella was traversed by all of these ligaments.





Intraarticular Structures


The principal intraarticular structures of importance are the medial and lateral menisci and the anterior and posterior cruciate ligaments. Numerous functions have been assigned to the menisci, some known and some hypothetical. Among these functions are distribution of joint fluid, nutrition, shock absorption, deepening of the joint, stabilization of the joint, and a load-bearing or weight-bearing function. The cruciate ligaments function as stabilizers of the joint and axes around which rotary motion, both normal and abnormal, occurs. They restrict the backward and forward motion of the tibia on the femur and assist in the control of both medial and lateral rotation of the tibia on the femur. External rotation of the tibia produces an unwinding of the ligaments, and internal rotation produces a winding up of the cruciate ligaments (Fig. 45-24). Further discussion of their specific functions is presented in the section on cruciate ligament injuries.




Mechanics


The mechanical axis of the femur does not coincide with its anatomical axis, because a line traversing the center of the hip joint and the center of the knee forms an angle of 6 to 9 degrees with the axis of the shaft of the femur. The mechanical axis generally passes near the center of the normal knee joint. Significant deviations from this mechanical axis may be present with genu varum or genu valgum deformity. In the erect position, the transverse axis through the knee joint lies in or near the true horizontal plane. Because of the disparity between the lengths of the articular surfaces of the femoral condyles and the tibial condyles, two types of motion during flexion and extension are produced. The knee thus possesses features characteristic of both a ginglymus (hinge joint) and a trochoid (pivot joint) articulation. The joint permits flexion and extension in the sagittal plane and some degree of internal and external rotation when the joint is flexed. No rotation is possible when the knee is in full extension. The complex flexion-extension motion is a combination of rocking and gliding. The rocking motion is demonstrable in the first 20 degrees of flexion, after which the motion becomes predominantly of the gliding type. This transition from one form of motion to the other is gradual but progressive. The rocking motion in the first 20 degrees of flexion better meets the requirements for stability of the knee in the relatively extended position, whereas the gliding motion as the joint unwinds permits more freedom for rotation.


The natural deflection outward of the tibia on the femur at the knee joint produces greater weight-bearing stresses on the lateral femoral condyle than on the medial condyle, but because the medial condyle of the femur is prolonged farther forward than the lateral condyle, the vertical axis of rotation falls in a plane near the medial condyle. During rotary movements, the medial condyle describes a smaller arc than the lateral condyle.


An accurate plot of the contact points between the femur and tibia reveals that the rate of rolling to gliding does not remain constant through all degrees of flexion. This ratio is approximately 1 : 2 in early flexion and about 1 : 4 by the end of flexion (Fig. 45-25).



The configuration of the osseous structures and the tension of the supporting ligaments and the menisci allow no rotary motion in the fully extended position. As flexion is initiated, the capsule and collateral ligaments as well as the cruciate ligaments become less tense, allowing rotary movements that progress increasingly as flexion increases from 0 to 90 degrees. Rotation ranges from 5 to 25 degrees with individual variation, internal rotation always being greater than external rotation.


Both menisci are displaced slightly forward in full extension and move backward as flexion proceeds. The anchorage of the medial meniscus permits less mobility than of the lateral meniscus, possibly explaining why injuries are more common to the medial meniscus than to the lateral meniscus. The action of the popliteus muscle laterally and the semimembranosus muscle medially retracting the menisci posteriorly also helps prevent the menisci from becoming entrapped during movements of the knee.


The menisci are described as moving with the femoral condyles with flexion and extension but moving with the tibia with rotary movements.


The medial and lateral femoral condyles have different configurations. The lateral condyle is broader in the anteroposterior and the transverse planes than the medial condyle, and the medial condyle projects distally to a level slightly lower than the lateral condyle. This distal projection helps compensate for the inclination of the mechanical axis in the erect position so that the transverse axis lies near the horizontal. The articular surface of the medial condyle is prolonged anteriorly, and as the knee comes into the fully extended position, the femur internally rotates until the remaining articular surface on the medial condyle is in contact. The posterior portion of the lateral condyle rotates forward laterally, thus producing a “screwing home” movement, locking the knee in the fully extended position. When flexion is initiated, unscrewing of the joint occurs by external rotation of the femur on the tibia. As previously mentioned, the rotary movement responsible for screwing and unscrewing of the knee joint occurs around an axis that passes near the medial condyle of the femur and is greatly influenced by the posterior cruciate ligament.


Normal flexion and extension are from 0 to 140 degrees, but 5 to 10 degrees of hyperextension is often possible. With the knee flexed to 90 degrees, passive rotation of the tibia on the femur can be demonstrated up to 25 or 30 degrees; this passive rotation varies with each individual. The extent of internal rotation always exceeds that of external rotation, and no rotation is possible with the knee fully extended. Sagittal displacement of the tibia on the fixed femur is detectable in both the anterior and posterior directions when the knee is flexed. Under normal conditions, the extent of the excursion should not exceed 3 to 5 mm. When the knee is extended, lateral (abduction-adduction) motion at the knee joint occurs to a limited extent; this motion varies with individual characteristics but should not exceed 6 to 8 degrees. In the hyperextended position, no lateral motion is present. In the flexed position, more lateral motion is possible but should never exceed 15 degrees.


Alterations in the vertical and transverse axes can occur with disruptions and derangements of the knee joint. When the medial ligaments are disrupted, the vertical axis of rotation shifts laterally and vice versa. This is discussed in greater detail in the section on simple and combined instabilities of the knee. Because of the eccentricity of the femoral condyles, the transverse axis of rotation constantly changes position (instant center of rotation) as the knee progresses from extension into flexion.



Menisci



Function and Anatomy


Meniscal function is essential to the normal function of the knee joint. As stated in the previous section on anatomy, various functions have been attributed to the menisci, some of which are known or proved and others that are theorized. The menisci act as a joint filler, compensating for gross incongruity between femoral and tibial articulating surfaces (Figs. 45-26 and 45-27). So located, the menisci prevent capsular and synovial impingement during flexion-extension movements. The menisci are believed to have a joint lubrication function, helping to distribute synovial fluid throughout the joint and aiding the nutrition of the articular cartilage. They undoubtedly contribute to stability in all planes but are especially important rotary stabilizers and are probably essential for the smooth transition from a pure hinge to a gliding or rotary motion as the knee moves from flexion to extension.




Radiographic changes apparent after meniscectomy include narrowing of the joint space, flattening of the femoral condyle, and formation of osteophytes. Narrowing of the joint space initially is caused by removal of the spacer effect of the meniscus (approximately 1 mm); it is further narrowed by a reduction in the contact area in the absence of the meniscus. When the medial meniscus is removed, the contact area is reduced by approximately 40%; in other words, the contact area is 2.5 times greater when the meniscus is present. The larger contact area provided by the meniscus reduces the average contact stress acting between the bones. The menisci are thus important in reducing the stress on the articular cartilage; they prevent mechanical damage to both the chondrocytes and the extracellular matrix. Increased contact stress resulting from decreased contact area may produce bone remodeling, producing a flattened femoral condyle. Softening of the joint cartilage also results in increased joint space narrowing and osteophyte formation.


The menisci have long been assumed to have shock- or energy-absorbing functions. Significant weight-bearing or load-transmitting forces are carried by the menisci, from 40% to 60% of the superimposed weight in the standing position. Thus, if normal and intact menisci spare the articular cartilage from compressive loads, then perhaps this partly explains the high incidence of osteoarthritis after removal of the meniscus.

Jun 5, 2016 | Posted by in ORTHOPEDIC | Comments Off on Knee Injuries
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