Naik, S. -Biomechanics of Knee Complex
Short Description
Biomechanics of the knee and hip...
Description
KNEE COMPLEX
Sagar Naik, PT
KNEE COMPLEX PT
ll.. .
Sagar Naik,
sio
4a
Knee complex plays a major role in supporting the body during dynamic and static activities. In a closed kinematic chain the knee joint works in conjunction with the hip joint and ankle to support the body weight in the static erect posture. Dynamically, the knee complex is responsible for moving and supporting the body in sitting and squatting activities and for supporting and transferring the body weight during locomotor activities. In an open kinematic chain the knee provides mobility for the foot in space. The knee is not only one of the largest joints in the body but also the most complex. The knee complex is composed of two distinct articulations within a single joint capsule: y Tibiofemoral joint The tibiofemoral joint is the articulation between the distal femur and the proximal tibia. y Patellofemoral joint The patellofemoral joint is the articulation between the patella and the femur.
U Tibiofemoral Joint:
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The tibiofemoral, or knee joint, is a double condyloid joint with 2° of freedom of motion. Flexion and extension occur in the sagittal plane around a coronal axis; medial and lateral rotation occurs in the transverse plane about a vertical axis.
Ö Femoral Articular Surface: The large medial & lateral condyles on the distal femur form the proximal articular surfaces of the knee joint. The condyles have a large and very obvious curvature anteroposteriorly but are also each slightly convex in the frontal plane. The two condyles are separated by the intercondylar notch or fossa through most of their length, but are joined anteriorly by an asymmetrical, shallow, saddle-shaped groove called the patellar groove or surface; the patellar 2
KNEE COMPLEX
Sagar Naik, PT
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surface is separated from the tibial articular surface by two slight grooves that run obliquely across the condyles. The shaft of the femur is not vertical but is angled in such a way that femoral condyles do not lie immediately below the femoral head, but somewhat medial. Given the obliquity of the shaft of the femur, the lateral condyle lies more directly in line with the shaft than does the medial condyle. The articular surface of the lateral condyle is also not as long as the articular surface of the medial femoral condyle. When the patellofemoral surface is excluded, it can be seen that the lateral tibial surface stops before the medial. The medial condyle extends further distally than the lateral so that, despite the angulation of the shaft of the femur, the distal end of the femur is essentially horizontal.
Ö Tibial Articular Surface:
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The articulating surfaces on the tibia that correspond to the femoral articulating surfaces are the two concave, asymmetrical medial and lateral tibial condyles or plateaus. The proximal tibia is enlarged as compared to the shaft and overhangs the shaft posteriorly. The articulating surface of the medial tibial condyle is 50% larger than that of the lateral condyle and the articular cartilage of the medial tibial condyle is three times thicker. A roughened area and two bony spines called the intercondylar tubercles separate the two tibial condyles. These tubercles become lodged in the intercondylar notch of the femur during knee extension.
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Ö Tibiofemoral Articulation:
When the large articular condyles of the femur are placed on the shallow concavities of the tibial condyle, the incongruence of the knee joint is evident. Each of the condyles of the knee joint has its own accessory joint structure, together known as the menisci of the knee. D Menisci: Two asymmetrical fibrocartilaginous joint discs called menisci are located on the tibial condyles. 3
KNEE COMPLEX
Sagar Naik, PT
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The medial meniscus is a semicircle; the lateral meniscus is four-fifths of a ring. Both menisci are open toward the intercondylar area, thick peripherally and thin centrally, forming concavities into which the respective femoral condyles can sit. The wedge-shaped menisci increase the radius of curvature of the tibial condyles and, therefore, joint congruence. By increasing congruence, the menisci also play an important part in distributing weight-bearing forces, in reducing friction between the joint segments, and serving as shock absorbers. The menisci have multiple attachments to surrounding structures, some common to both and some unique to each. Each meniscus is connected around its periphery to the tibial condyle by the coronary ligaments, which are composed of fibres from the knee joint capsule. Both menisci are also attached directly or indirectly to the patella via the so-called patellomeniscal or patellotibial ligaments, which are anterior capsular thickenings. The open ends of the menisci, which are attached to their respective tibial intercondylar tubercles, are called horns. Each meniscus has an anterior and a posterior horn. The anterior horns of the two menisci are joined to each other by the transverse ligament, which may be connected to the patella via the joint capsule. The attachment site of the posterior horn of the more mobile lateral meniscus had a greater zone of uncalcified fibrocartilage than the attachment site of the posterior horn of the medial meniscus. The attachment site of the anterior horn of the lateral meniscus had a thicker zone of cortical calcified cartilage than the attachment site of the anterior horn of the medial meniscus. The lateral meniscus, in addition to the connections it shares with the medial meniscus, is attached to the posterior cruciate ligament (PCL) and popliteus muscle via the coronary ligaments and posterior capsule, and to the somewhat variable posterior meniscofemoral ligaments. Some fibres from the anterior cruciate ligament (ACL) may also join the anterior and posterior horns.
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Sagar Naik, PT
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The connections of the lateral meniscus are considered to be fairly loose, leaving the lateral menisci a fair amount of mobility on the lateral tibial condyle. The medial meniscus is attached to the medial collateral ligament and to the semimembranosus muscle through its capsular connections. The medial meniscus is more firmly attached and less movable on the tibial condyle than the lateral meniscus. The menisci and meniscoligamentous complex are well established in the 8-week-old embryo and during the first year of the life the menisci are well vascularized throughout. The vascularity of the meniscal body gradually reduces from 18 months to 18 years. Over age 50 years only periphery of meniscal body is vascularized. The horns remain completely vascularized throughout life. In young children whose menisci have ample blood supply, the incidence of meniscal injuries is low. In adult the only the peripheral vascularized region of the meniscal body is capable of inflammation, repair, and remodeling following a tearing injury. The horns of the menisci and the peripheral vascularized portion of the meniscal bodies are well innervated with free nerve endings and three different mechanoreceptors. The meniscal innervation pattern indicates that the menisci are a source of information about joint position, direction of movement, and velocity of movement as well as information about tissue deformation. D Tibiofemoral Alignment & Weight-Bearing Forces: The anatomic (longitudinal) axis of the femur is oblique, directed inferiorly and medially from its proximal to its distal end. The anatomic axis of the tibia is directed almost vertically. Consequently, the femoral and tibial longitudinal axes normally form an angle medially at the knee joint of 185° to 190°; i.e., the femur is angled off vertical 5° to 10°, creating a physiologic (normal) valgus angle at the knee. The mechanical axis of the lower extremity is the weight-bearing line from the center of the head of the femur to the center of the superior surface of the head of talus. This line normally passes through the center of the knee joint between the intercondylar tubercles and averages 3° from the vertical given the width of the hip joints as compared to spacing of the feet. 5
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Sagar Naik, PT
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Because the weight-bearing line (ground reaction force) follows the mechanical rather than the anatomic axes, the weight-bearing stresses on the knee joint in bilateral static stance are equally distributed between the medial and lateral condyles, without any concomitant horizontal shear forces. This is not necessarily the case in unilateral stance or once dynamic forces are introduced to the joint. If the medial tibiofemoral angle is greater than 195° (165° or less measured laterally), an abnormal condition called genu valgum (knock knees) exists. This condition will increase the compressive force on the lateral condyle while increasing the tensile stresses on the medial structures. If the medial tibiofemoral angle is 180° or less (exceeding 180° as measured laterally), the resulting abnormality is called genu varum (bow legs). In this condition, the compressive stresses on the medial tibial condyle are increased, whereas the tensile stresses are increased laterally. In genu valgum or genu varum, constant overloading of, respectively, the lateral or medial articular cartilage may result in damage to the cartilage. The menisci of the knee are important in distributing and absorbing the large forces crossing the knee joint. Although compressive forces in the dynamic knee joint ordinarily may reach two to three times body weight in normal gait and five to six times body weight in activities such as running and stair climbing, the menisci assume 40% to 60% of the imposed load. If the menisci are removed, the magnitude of the average load per unit area on the articular cartilage nearly doubles on the femur and is six to seven times greater on the tibial condyles. Elimination of any angulation between the femur and tibia (a mild genu varum) will increase the compression on the medial meniscus by 25%. Five degrees of genu varum (medial tibiofemoral angle of 175°) will increase the forces by 50%.
Ö Knee Joint Capsule: Given the incongruence of the knee joint, even with the compensation of the menisci, stability is heavily dependent on the surrounding joint structures. In knee flexion when surrounding passive structures tend to be lax, the incongruence of the joint permits at least some anterior displacement, posterior displacement, and rotation of the tibia beneath the femur. 6
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Sagar Naik, PT
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The knee joint capsule and its associated ligaments are critical to restricting such motions to maintain joint integrity and normal joint function. Although muscles clearly play a role in stabilization, it is almost impossible to effectively stabilize the knee with active muscular forces alone in the presence of substantial disruption of passive restraining mechanisms. The joint capsule that encloses the tibiofemoral and patellofemoral joints is large, complexly attached, and lax with several recesses. y Posteriorly, the capsule is attached proximally to the posterior margins of the femoral condyles and intercondylar notch and distally to the posterior margins tibial condyle. The capsule is reinforced posteriorly by a number of muscles and by oblique popliteal and arcuate ligaments. y Medially & laterally, the capsule begins proximally above the femoral condyles to continue distally to the margins of the tibial condyle. The collateral ligaments reinforce the sides of the capsule. y Anteriorly, the patella, the tendon of the quadriceps muscles superiorly, and the patellar ligament inferiorly completes the anterior portion of the joint capsule. y Anteromedially and anterolaterally, expansions from the vastus medialis and vastus lateralis muscles extend from the patella and patellar ligament to the corresponding collateral ligaments and tibial condyles. The anteromedial and anterolateral portions of the capsule are known as the extensor retinaculum or the medial and lateral patellar retinacula. D Extensor Retinacula: Extensor retinaculum appear to be two layers, y The deeper of the two layers having longitudinally oriented fibres connecting the capsule anteriorly to the menisci and tibia via the coronary ligaments. These connections may be called the patellomeniscal or patellotibial bands. y The more superficial second layer consists of transversely oriented fibres of which the more proximal blend with fibres of the vastus medialis and lateralis muscles and the more distal continue to the posterior femoral condyles. The transverse fibres connecting the patella and the femoral condyles are known as the patellofemoral ligaments.
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Sagar Naik, PT
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The lateral patellofemoral ligament is connected not only to the vastus lateralis muscle but also to the iliotibial band either directly or indirectly via an iliopatellar band. The tendon of the biceps femoris muscle to provide superficial reinforcement to the capsular and retinacular layers accompanies the iliotibial band and its associated fascia lata posteriorly. D Synovial Lining: The intricacy of the fibrous layer of the knee joint capsule is surpassed by its synovial lining, the most extensive, and involved in the body. The synovium adheres to the inner wall of the fibrous layer except posteriorly where the synovium invaginates anteriorly following the contour of the femoral intercondylar notch. The invaginated synovium adheres to the anterior aspect and sides of the anterior cruciate ligament and the posterior cruciate ligament. Thus, anterior cruciate ligament and posterior cruciate ligament are intracapsular but extrasynovial. Embryonically, the synovial lining of the knee joint capsule is actually divided by septa into three separate compartments. There is initially a superior patellofemoral compartment and two separate medial and lateral tibiofemoral compartments. By 12 weeks of gestation, the synovial septa are resorbed to some degree, resulting in a single joint cavity, but retaining the posterior invagination of the synovium that forms some separation of the condyles. The superior compartment continues to be recognizable as a superior recess of the capsule known as the suprapatellar bursa. Posteriorly, the synovial lining may invaginate laterally between the popliteus muscle and lateral femoral condyle. It may also invaginate medially between the semimembranosus tendon, the medial head of the gastrocnemius muscle, and the medial femoral condyle. When the synovial septa, which exist embryonically, are not completely resorbed but persist into adulthood, they exist as folds or pleats of synovial tissue known as plicae or patellar plicae. These vestiges have been observed in 20% to 60% of the normal population and are referred to, in order of most frequently to least frequently found, as the inferior plica (infrapatellar plica), the superior plica (suprapatellar plica), and the medial plica (mediopatellar plica).
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Sagar Naik, PT
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The inferior plica, which also has been described as the infrapatellar fold or ligamentum mucosum, is located below the patella anterior to the anterior cruciate ligament. The inferior plica extends from the anterior portion of the intercondylar notch to attach the infrapatellar fat pad. The superior plica is located between the suprapatellar bursa and the knee joint. This plica is often bilateral and symmetrical and extends from synovial pouch at the anterior aspect of the femoral metaphysis area to attach to the posterior aspect of the quadriceps tendon above the patella. The medial plica arises from the medial wall of the pouch of the retinaculum and runs parallel to the medial edge of the patella to attach to the infrapatellar fat pad and synovium of the inferior plica. Occasionally, however, the plica may become irritated and inflamed, which leads to pain, effusion, and changes in joint structure and function. The plica syndrome generally does not arise from the most common infrapatellar plica, but from the medial or superior plicae. The knee joint capsule is reinforced by a number of ligaments that play an important part not only in knee joint stability but also in knee joint mobility.
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Ö Knee Joint Ligaments:
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Given the lack of bony restraint to virtually any of the knee motions, the ligaments are credited with resisting or controlling: y Excessive knee extension y Varus and valgus stresses at the knee y Anterior or posterior displacement of the tibia beneath the femur y Medial or lateral rotation of the tibia beneath the femur y Combinations of anteroposterior displacements and rotations of the tibia, known as rotatory stabilization It is also possible that the stresses may occur on the femur while the tibia is fixed (weight-bearing). In such instances, the anteroposterior displacements and rotations will reverse; that is, anterior displacement of the tibia is equivalent to posterior displacement of the femur and so forth. D Collateral Ligaments: The medial (tibial) collateral ligament (MCL) attaches to the medial aspect of the medial femoral epicondyle, sloping anteriorly to insert into the medial aspect of the proximal tibia. 9
KNEE COMPLEX
Sagar Naik, PT
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The posterior medial fibres of the ligament blend with fibres of the joint capsule and some fibres extend medially to attach to the medial meniscus. The lateral (fibular) collateral ligament (LCL) is a strong cordlike structure extending from the lateral femoral epicondyle and attaching posteriorly to the head of the fibula. Unlike the medial collateral ligament, the lateral collateral ligament has no attachment either to the meniscus or to the joint capsule. Both collateral ligaments are taut in full extension and, therefore, help resist hyperextension of the knee joint. Medial Collateral Ligament (MCL): The medial collateral ligament resists valgus stresses (attempted abduction of the tibia) across the knee joint, being especially effective in the extended knee when the ligament is taut. However, it may play a more critical role in resisting valgus stresses in the slightly flexed knee when other structures make a lesser contribution. The medial collateral ligament is also aligned in such a way as to check lateral rotation of the tibia. The medial collateral ligament is also a backup restraint to pure anterior displacement of the tibia when the primary restraint of the anterior cruciate ligament is absent.
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Lateral Collateral Ligament (LCL): The lateral collateral ligament resists varus stresses (attempted adduction of the tibia) across the knee. Given its alignment, it also appears to limit lateral rotation of the tibia, making its most substantial contribution at about 35° of flexion, in conjunction with the posterolateral capsule. The lateral collateral ligament also resists combined lateral rotation with posterior displacement of the tibia in conjunction with the tendon of the popliteus muscle. Iliotibial Band: The iliotibial band (ITB) or iliotibial tract is formed proximally from the fascia investing the tensor fascia lata, the gluteus maximus, and the gluteus medius muscle. The iliotibial band continues distally to attach to the linea aspera of the femur via the lateral intermuscular septum and inserts into lateral 10
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Sagar Naik, PT
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tubercle of the tibia, reinforcing the anterolateral aspect of the knee joint. The iliotibial band appears to be consistently taut regardless of position of the hip joint or knee joint, although it falls anterior to the knee joint axis in extension and posterior to the axis in flexion. The fibrous connections of the iliotibial band to the biceps femoris and vastus lateralis muscles through the lateral intermuscular septum form a sling behind the lateral femoral condyle, assisting the anterior cruciate ligament in preventing posterior displacement of the femur when the tibia is fixed and the knee joint is near extension. With knee flexion iliotibial band moves posteriorly, while with knee extension iliotibial band moves anteriorly. The iliotibial band sends fibres from its anterior margin to attach to the patella, forming an iliopatellar band. When the iliotibial band moves posteriorly in knee flexion it exert a lateral pull on the patella resulting in a progressive laterally tilting as flexion increases. This is prevented by vastus medialis muscle. D Cruciate Ligaments: The anterior cruciate ligament and posterior cruciate ligament are intracapsular but extrasynovial ligaments. These ligaments are named according to their tibial attachments. The anterior cruciate ligament arises from the anterior aspect of the tibia; the posterior cruciate ligament arises from the posterior aspect of the tibia. Usually both ligaments are described to have main posterolateral and smaller anteromedial bands that behave differently in different movements. Anterior Cruciate Ligament (ACL): The anterior cruciate ligament attaches to the anterior tibia, passes under the transverse ligament, and extends superiorly and posteriorly to attach to the posterior part of the inner aspect of the lateral femoral condyle. Generally, the numerous fascicles of the anterior cruciate ligament are grouped into an anteromedial band (AMB) and a posterolateral band (PLB). Changes in the lengths of the various bands or fibres during joint motion are used as indicators of the ligaments functions. 11
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At 0° of knee flexion the anteromedial band is at its shortest length (lax) while the posterolateral band is at it longest length (taut). Therefore, at 0° the lax anteromedial band would be able to offer the least restraint and the taut posterolateral band would be able to offer the most restraint. Under valgus loading the length of both bands of the anterior cruciate ligament increases as knee flexion increases. Anterior loading alone or combined with valgus loading causes an increase in length of all portions of the anterior cruciate ligament with increases in knee flexion. In anterior loading some portion of the anterior cruciate ligament is tight throughout the knee joint range. In knee flexion, the anteromedial band is taut and posterolateral band is lax. The anterior cruciate ligament is generally considered the primary restraint to anterior displacement of the tibia on the femoral condyles. There would appear to be essentially no anterior translation of the tibia possible in full extension when many of the supporting passive structures of the knee are taut. Forces producing anterior translation of the tibia will result in maximal excursion of the tibia at about 30° of flexion when neither of the anterior cruciate ligament bands is particularly tensed. The posterolateral band tends to be injured with excessive knee hypertension, whereas the anteromedial band tends to be injured with trauma to the flexed knee. The anterior cruciate ligament would also appear to make at least a minor contribution to restraining both varus and valgus stresses across the knee joint. When the medial collateral ligament is damaged and knee is flexed, the anterior cruciate ligament will make a more major contribution to restraining varus and valgus stresses. Both cruciate ligaments appear to play a role in producing and controlling rotation of the tibia. The anterior cruciate ligament appears to twist around the posterior cruciate ligament in medial rotation of the tibia, thus checking excessive medial rotation.
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Sagar Naik, PT
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Injury to the anterior cruciate ligament appears to occur most commonly when the knee is flexed and tibia rotated in either direction. In flexion and medial rotation, the anterior cruciate ligament is tensed as it winds around the posterior cruciate ligament. In flexion and lateral rotation, the anterior cruciate ligament is tensed as it is stretched over the lateral femoral condyle. When attempting to determine whether there has been a tear of the anterior cruciate ligament, the presence of both anteromedial and anterolateral instability is the most diagnostic. Hamstrings can be considered to act synergistically with the anterior cruciate ligament.
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Posterior Cruciate Ligament (PCL): The posterior cruciate ligament, which runs superiorly and somewhat anteriorly from its posterior tibial origin to attach to the inner aspect of the medial femoral condyle, is shorter and less oblique than the anterior cruciate ligament. The posterior cruciate ligament blends with the posterior capsule and periosteum as it crosses to its tibial attachment. Usually posterior cruciate ligament is divided into an anteromedial band (AMB) and a posterolateral band (PLB) named by the tibial origin. The anteromedial band is lax in extension, and the posterolateral band is taut. At 80° to 90° of flexion, the anteromedial band is maximally taut and the posterolateral band is relaxed. The posterior cruciate ligament is primary restraint to posterior displacement of the tibia beneath the femur, with little or no displacement possible in full extension. In the flexed knee, maximal displacement of the tibia with a posterior translational force occurs at 75° to 90° of flexion. The posterior cruciate ligament also has some role in restraining varus and valgus stresses at the knee. The posterior cruciate ligament appears to play a role in both restraining and producing rotation of the tibia. Posterior translatory forces on the tibia are consistently accompanied by concomitant lateral rotation of the tibia, with little or no rotation produced at the femur. 13
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Tension in the posterior cruciate ligament with knee extension may be instrumental in creating the lateral rotation of the tibia that is critical to locking of the knee for stabilization. The popliteus muscle shares the function of the posterior cruciate ligament in resisting posteriorly directed forces on the tibia and contributes to knee stability when the posterior cruciate ligament is absent. The posterior cruciate ligament, posterior joint capsule, lateral collateral ligaments, posterior oblique ligament, medial collateral ligament with meniscus attached, posterior medial and posterior lateral meniscotibial bands, and posterior meniscofibular ligament comprise a complex restraining system for knee extension. D Posterior Capsular Ligaments: The posteromedial aspect of the capsule is reinforced by the tendinous expansion of the semimembranosus muscle, which is known as the oblique popliteal ligament. This ligament passes from a point posterior to the medial tibial condyle and attaches to the central part of the posterior aspect of the joint capsule. The arcuate popliteal ligament reinforces the posterolateral aspect of the capsule. The arcuate ligament arises from the posterior aspect of the head of fibula and passes over the tendon of the popliteus muscle to attach to the intercondylar area of the tibia and to the lateral epicondyle of the femur. Both the oblique popliteal and the arcuate ligaments are taut in full extension and assist in checking hyperextension of the knee. The arcuate and oblique popliteal ligaments play an important role in checking varus and valgus stresses, respectively, in the extended knee, and in providing secondary restraint to other tibial motions. The popliteofibular ligament becomes taut at 0°, 30°, 45°, and 90° and acts as a restraint to lateral rotation of the tibia when posterior force is applied to the knee. The ligament also helps to limit posterior translation of the tibia. D Meniscofemoral Ligaments: The two meniscofemoral ligaments arise from the posterior horn of the lateral meniscus and insert on the lateral aspect of the medial femoral condyle near the insertion site of the posterior cruciate ligament. The ligament that runs anterior to the posterior cruciate ligament is called either the ligament of Humphrey or anterior meniscofemoral ligament. 14
KNEE COMPLEX
Sagar Naik, PT
Ö Knee Joint Bursae:
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The ligament that runs posterior to the posterior cruciate ligament is called the ligament of Wrisberg or posterior meniscofemoral ligament. It is also known as third cruciate ligament of Robert. The meniscofemoral ligaments work in conjunction with the popliteus muscle and become taut during femoral lateral rotation and may prevent posterior translation of the tibia.
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The extensive ligamentous apparatus of the knee joint and the large excursion of the bony segments set up substantial frictional forces between muscular, ligamentous, and bony structures. However, numerous bursae prevent or limit such degenerative forces. The suprapatellar bursa, the subpopliteal bursa, and the gastrocnemius bursa are not usually separate entities but are either invaginations of the synovium within the joint capsule or communicate with the capsule through small openings. The suprapatellar bursa lies between the quadriceps tendon and the anterior femur; the subpopliteal bursa lies between the tendon of the popliteus muscle and the lateral femoral condyle; and the gastrocnemius bursa lies between the tendon of the medial head of the gastrocnemius muscle and the medial femoral condyle. The gastrocnemius bursa may also continue beneath the tendon of the semimembranosus muscle to protect it from the medial femoral condyle. The lubricating synovial fluid contained in the knee joint capsule moves from recess to recess during flexion and extension of the knee, lubricating the articular surfaces. In extension, the posterior capsule and ligaments are taut and the gastrocnemius and subpopliteal bursae are compressed. This shifts the synovial fluid anteriorly. In flexion, the suprapatellar bursa is compressed anteriorly by tension in the anterior structures and the fluid is forced posteriorly. When the joint is in the semiflexed position, the synovial fluid is under the least amount of tension. When there is an excess of fluid in the joint cavity due to injury or disease, the semiflexed knee position helps to relieve tension in the capsule and therefore helps to reduce pain. Several other bursae are associated with the knee but do not communicate with the synovial capsule. 15
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The prepatellar bursa, located between the skin and the anterior surface of the patella, allows free movement of the skin over the patella during flexion and extension. The subcutaneous infrapatellar bursa lies between the patellar ligament and the overlying skin. The subcutaneous infrapatellar bursa and prepatellar bursa may become inflamed as a result of direct trauma to the front of the knee or through activities like kneeling (House Maid’s Knee). The deep infrapatellar bursa, which is located between the patellar ligament and the tibial tuberosity, is separated from the synovial cavity of the joint by the infrapatellar pad of fat. The deep infrapatellar bursa helps to reduce friction between the patellar ligament and the tibial tuberosity. There are also several small bursae that are associated with the ligaments of the knee joint. There is commonly a bursa between the lateral collateral ligament and the tendon of the biceps femoris muscle and between the lateral collateral ligament and the popliteus muscle. There is a bursa deep to the medial collateral ligament protecting it from the tibial condyle and one superficial to the medial collateral ligament protecting it from the tendons of the semitendinosus and gracilis muscles that cross the medial collateral ligament.
Ö Knee Joint Function:
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D Osteokinematics of Knee Joint: The primary motions of the knee joint are flexion / extension and, to lesser extent, medial rotation / lateral rotation. The knee joint can also undergo tibial or femoral displacement anteriorly and posteriorly and some abduction and adduction through varus and valgus forces. The small amounts of anteroposterior displacement and valgus / varus forces that can occur in the normal flexed knee are the result of joint incongruence and variations in ligamentous elasticity. Excessive amounts of such motions are abnormal and generally indicate ligamentous incompetence. Flexion / Extension: The axis for flexion and extension at the tibiofemoral joint passes horizontally through the femoral condyles at an angle to the mechanical and anatomic axes. 16
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The obliquity of the axis causes the tibia to move from a position slightly lateral to the femur in full extension to a position medial to the femur in full flexion. The axis of motion for flexion and extension at the knee is not relatively fixed, but moves to a considerable extent through the ROM. The pathway of the instant axis of rotation (IAR) of the tibiofemoral joint for flexion and extension forms a semicircle, moving posteriorly and superiorly on the femoral condyles with increasing flexion. As many of the muscles associated with the knee are two-joint muscles that cross both the hip and the knee, hip joint position can influence knee ROM. Passive range of knee flexion is generally considered to be 130° to 140°. Knee flexion may be limited to 120° or less when the hip joint is simultaneously hyperextended and the stretched rectus femoris muscle becomes passively insufficient. Knee flexion may also reach as much as 160° in activities like squatting when the hip and knee are flexing at the same time and the body weight is superimposed on the joint. Normal gait on level ground requires approximately 60° of knee flexion. This requirement increases to about 80° for stair climbing and to 90° or more for sitting down into a chair and arising from it. Activities beyond simple mobility tasks require 115° of knee flexion or more. Knee joint extension (hyperextension) of 5° to 10° is considered within normal limits. Excessive knee hyperextension is termed genu recurvatum. When the lower extremity is weight bearing and the knee is part of a closed kinematic chain, range limitations at ankle joint may cause restriction in knee joint flexion or extension. Eg – A limitation in ankle dorsiflexion (due to tight plantarflexors) may prevent the knee from being flexed; a limitation in plantarflexion (due to tight dorsiflexors) may restrict the ability of the knee to fully extend.
Rotation: The knee joint rotates in two different ways that are quite different both structurally and functionally. Axial rotation provides the second degree of freedom to the tibiofemoral joint. 17
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There is joint rotation involved in the locking mechanism of the knee joint, also known as terminal or automatic rotation. Rotation associated with the locking mechanism occurs with close packing of the knee joint and does not contribute to degrees of freedom. Axial rotation of the knee joint occurs around a longitudinal axis that runs through or close to the medial tibial intercondylar tubercle. Medial and lateral rotations of the knee joint are named for the motion or relative motion of the tibia. The medial and lateral rotations available in axial rotation occur because of articular incongruence and ligamentous laxity. The range of knee joint rotation depends on the position of the knee. y When the knee is in full extension, it is in close-packed (locked) position and the ligaments are taut; no axial rotation is possible. The tibial tubercles are lodged in the intercondylar notch and the menisci are tightly interposed between the articular surfaces. y As knee flexes increasing toward 90°; the capsule and ligaments become more lax. The tibial tubercles are no longer in the intercondylar notch and the condyles of the tibia and femur are free to move on each other. y At 90° of knee flexion, approximately 60° to 70° of either active or passive rotation is possible. The range for lateral rotation (0° to 40°) is slightly greater than the range of medial rotation (0° to 30°). The maximum range of axial rotation is available at 90° of knee flexion, with the magnitude of axial rotation diminishing as the knee approaches both full extension and full flexion.
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D Arthrokinematics of Knee Joint: Flexion / Extension: The large articular surface of the femur and the relatively small tibial condyle create a potential problem as the femur begins to flex on the tibia. If the femoral condyles were permitted to roll posteriorly on the tibial condyle, the femur would run out of tibial condyle before much flexion had occurred. This would result in a limitation of flexion, or the femur would roll off the tibia. For the femoral condyles to continue to roll with increased flexion of the femur, the condyles must simultaneously glide anteriorly on the 18
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tibial condyle to prevent them from rolling posteriorly off the tibial condyle. y The first part of flexion of the femur from full extension (0° to 25°) is primarily rolling of the femoral condyles on the tibia, bringing the contact of the femoral condyles posteriorly on the tibial condyle. y As flexion continues, the rolling is accompanied by a simultaneous anterior glide just sufficient to create a nearly pure spin of the femur; that is, the magnitude of posterior displacement that would occur with the rolling of the condyles is offset by the magnitude of anterior glide, resulting in little linear displacement of the femoral condyles after 25° of flexion. The anterior glide of the femoral condyles results in part from the tension encountered in the anterior cruciate ligament as the femur rolls posteriorly on the tibial condyle. The menisci whose shape forces the femoral condyle to roll “uphill” as the knee flexes may further facilitate the glide. The menisci accompany the femoral condyles as the condyles move posteriorly on the tibial condyle, maintaining the increased congruence the menisci provide in the fully extended knee. The menisci cannot move in there entirely because they are attached at their horns to the intercondylar tubercles of the tibial condyle. Extension of the knee from flexion occurs initially as a rolling of the femoral condyles on the tibial condyle, displacing the femoral condyles anteriorly back to neutral position. After the initial forward rolling, the femoral condyles glide posteriorly just enough to continue extension of the femur as an almost pure spin (roll plus posterior glide) of the femoral condyles on the tibial condyles. Tension in the posterior cruciate ligament and the shape of the menisci facilitate the intra-articular movements of the femoral condyles during knee extension. The condyles are once again accompanied in displacement by distortion of the wedge-shaped menisci. As extension begins from full flexion, the posterior margins of the menisci return to their neutral position. As extension continues, the anterior margins of the menisci move anteriorly with the femoral condyles. 19
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The motion of the menisci with flexion and extension are an important component of the motions. Given the need of the menisci to reduce friction and absorb forces of the large femoral condyles on the small tibial condyle, the menisci must remain beneath the femoral condyles to continue their function. Failure of the menisci to distort in the proper direction can also result in limitation of joint motion. y If femur literally rolls up the wedge-shaped menisci in flexion (without either the anterior glide of the femur or the posterior distortion of the menisci), the increasing thickness of the menisci and the threat of rolling off the posterior margin will cause flexion to be limited. y Similarly, failure of the menisci to distort anteriorly with the femoral condyles in extension will cause the thick anterior margins to become wedged between the femur and tibia as the segments are drawn together in the final stages of extension. The interposition of the menisci will prevent extension from being completed.
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Locking & Unlocking: In weight bearing closed chain motion as an example, extension of the femur on the relatively fixed tibia results in additional motions. As the femur extends to about 30° of flexion, the shorter lateral femoral condyle completes its rolling-gliding motion. As extension continues, the longer medial femoral condyle continues to roll and to glide posteriorly although the lateral condyle has halted. This continued motion of medial femoral condyle results in medial rotation of the femur on tibia, pivoting about the fixed lateral condyle. The medial rotatory motion of the femur is most evident in final 5° of extension. Increasing tension in the knee joint ligaments as the knee approaches full extension may also contribute to the rotation within the joint. As the medial rotation of the femur that accompanies the final stages of the knee extension is not voluntary or produced by muscular forces, it is referred to as automatic or terminal rotation of the knee joint. This rotation within the joint that accompanies the end of extension also brings the knee joint into the closed-packed or locked position.
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The tibial tubercles are lodged in the intercondylar notch, the menisci are tightly interposed between the tibial and femoral condyles, and the ligaments are taut. Consequently, automatic rotation is also known as the locking mechanism or screw home mechanism of the knee. To initiate flexion, the knee must first be unlocked; that is, the medially rotated femur cannot flex in the sagittal plane, but must laterally rotate before flexion can proceed. A flexion force will automatically result in lateral rotation of the femur because the longer medial side will move before the shorter lateral side of the joint. If there is an external restraint to unlocking or derotation of the femur, the joint, ligaments, and menisci can be damaged, as the femur is forced into flexion oblique to the sagittal plane in which its structures are oriented. Automatic rotation or locking of the knee occurs in both open chain and closed chain knee joint function. In an open kinematic chain, the freely moving tibia laterally rotates on the relatively fixed femur during the last 30° of extension. Unlocking, consequently, is brought about by medial rotation of the tibia on the femur before flexion can proceed.
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Axial Rotation: During axial rotation of the knee joint, the longitudinal axis for motion lies at the medial intercondylar tubercle. Consequently, the medial condyles act as the pivot point while the lateral condyles move through a greater arc of motion than the medial regardless of the direction of rotation. When lateral rotation of the tibia occurs at the knee joint, the medial tibial condyle moves only slightly anteriorly on the relatively fixed medial femoral condyle while the lateral tibial condyle moves a large distance posteriorly on the relatively fixed lateral femoral condyle. In medial rotation the direction of motion of the tibial condyles reverses, with the medial tibial condyle moving only slightly posteriorly while the lateral condyle moves anteriorly through a larger arc of motion. When tibia is fixed and the femur is free to move, lateral rotation of the femur occurs as the lateral femoral condyle moves posteriorly on 21
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the lateral tibial condyle while the medial femoral condyle moves slightly anteriorly. Lateral rotation of the femur on the tibia produces an opposite set of motions. When there is rotation between the femoral and tibial condyles (either in axial or automatic rotation), the menisci of the knee joint maintain their relationship to the femoral condyles just as they did in flexion and extension; that is, in rotation of the knee, the menisci will distort in the direction of movement of the corresponding femoral condyle. y In medial rotation, the medial meniscus will distort anteriorly on the tibial condyle to remain beneath the anteriorly moving medial femoral condyle, and lateral meniscus will distort posteriorly to remain beneath the posteriorly moving lateral femoral condyle. In this way, the menisci continue to reduce friction and distribute the forces the femoral condyles create on the tibial condyle without restricting motion.
Ö Muscles of the Knee Joint:
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D Flexors of Knee Joint: There are seven muscles which flexes the knee joint that are follows: y Semimembranosus y Semitendinosus y Biceps Femoris y Sartorius y Gracilis y Popliteus y Gastrocnemius All of the knee flexors, except for the short head of the biceps femoris and the popliteus, are two-joint muscles. As two-joint muscles, their ability to produce effective force can be influenced by the relative position of the two joints over which they pass. The popliteus, gracilis, semimembranosus, and semitendinosus muscles are considered to medially rotate the tibia on the fixed femur, whereas the biceps femoris is considered to be a lateral rotator of the tibia.
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Hamstring Muscles: The semitendinosus, semimembranosus, and the biceps femoris muscles are known collectively as the hamstrings. These muscles all originate on the ischial tuberosity of the pelvis. The semimembranosus and the semitendinosus insert on the posteromedial and anteromedial aspects of the tibia, respectively. The semimembranosus muscle has fibres that attach to the medial meniscus. This attachment assists in knee flexion by facilitating posterior motion of the medial meniscus during active knee flexion. The semitendinosus muscle has a fibrous septum that separates it into distinct proximal and distal compartments. This may give it some specificity of action at the hip joint and at the knee joint. Most of the hamstrings, crossing the hip (as extensors) and the knee (as flexors), work most effectively at the knee joint if they are lengthened over the flexed hip. With active knee flexion with the body in the prone position, the hamstrings muscles are forced to attempt to shorten over both the hip (which will be extended) and over the knee. The hamstrings will weaken as knee flexion proceeds because the muscle group is approaching active insufficiency and must overcome the increasing tension in the rectus femoris, which is approaching passive insufficiency. Biceps Femoris: The biceps femoris muscle has two heads, both of which insert on the lateral condyle of the tibia and the head of the fibula. The biceps femoris tendon may be attached to the iliotibial band and retinacular fibres of the lateral joint capsule, a set of attachments that implies that the biceps femoris has a stabilizing role at the posterolateral aspect of the joint. The short head of the biceps femoris does not cross the hip joint and, therefore, has a unique action at the knee joint. Gastrocnemius: The gastrocnemius muscle arises from the posterior aspects of the medial and lateral condyles of the femur by two heads. It inserts into the calcaneus by way of the calcaneal tendon. Except for the plantaris muscle, the gastrocnemius is the only muscle at the knee that crosses the ankle and the knee. 23
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Although the gastrocnemius generates a large plantarflexor torque at the ankle, it makes a relatively small contribution to knee flexion. Rather than working to produce knee flexion, the gastrocnemius appears to be effective in preventing knee joint hyperextension. Paralysis of the plantarflexors is classically accompanied by a snapping back of the knee into hyperextension in the final stages of single-limb support during walking. The gastrocnemius appears to be less a mobility muscle at the knee joint than a dynamic stabilizer.
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Sartorius: The sartorius muscle arises anteriorly from the anterior superior iliac spine of the ilium and crosses the femur to insert the anteromedial surface of the tibial shaft posterior to the tibial tuberosity. Although a potential flexor and medial rotator of the tibia, activity in the sartorius is more common with hip motion than with knee motion. When attached just anterior to its more usual location, it may fall anterior to the knee joint axis, serving as a mild knee joint extensor rather than as a knee flexor. Gracilis: The gracilis muscle arises from the inferior half of the symphysis pubis arch and inserts on the medial tibia by way of a common tendon with the sartorius and the semitendinosus muscles. It is not only a hip joint flexor and adductor, but it can also flex the knee joint and produce slight medial rotation of the tibia. Pes Anserinus: The gracilis, semitendinosus, and sartorius muscles attach to the tibia by a common tendon on the anteromedial aspect of the tibia. The common tendon is called the pes anserinus because of its shape. The three muscles of the pes anserinus appear to function effectively as a group to stabilize the medial aspect of the knee joint. Popliteus: Popliteus muscle originates on the posterior aspect of the lateral femoral condyle and attaches on the medial aspect of the tibia. The popliteus muscle is a medial rotator of the tibia on the femur in an open kinematic chain (or a lateral rotator of the femur on the tibia in a closed kinematic chain). 24
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The active popliteus muscle is considered to play an important role in initiating unlocking of the knee because it reverses the direction of automatic rotation that occurred in the final stages of knee extension. The popliteus muscle is commonly attached to the lateral meniscus. The lateral meniscus is drawn posteriorly by tension in the popliteus expansion.
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D Extensors of Knee Joint: The four extensors of the knee, namely y Rectus Femoris y Vastus Medialis y Vastus Intermedius y Vastus Lateralis are known collectively as the quadriceps femoris muscle. The only portion of the quadriceps that crosses two joints is the rectus femoris, which originates on the inferior spine of the ilium. The vastus intermedius, vastus lateralis, and vastus medialis muscles originate on the femur and merge into a common tendon, the quadriceps tendon. The quadriceps tendon continues distally as the patellar ligament. The patellar ligament runs from the apex of the patella, across the anterior surface of the patella, into the proximal portion of the tibial tubercle. The vastus medialis and vastus lateralis also insert directly into the medial and lateral aspects of the patella by way of the retinacular fibres of the joint capsule. Together, the muscles of the quadriceps femoris extend the knee. The different orientation of lower fibres of the vastus medialis muscle has resulted in reference to the upper fibres as the vastus medialis longus (VML) and the lower fibres as the vastus medialis oblique (VMO). Mechanically, the patella affects the efficiency of the quadriceps muscle: the patella lengthens the moment arm (MA) of the quadriceps femoris by increasing the distance of the quadriceps tendon and patellar ligament from the axis of the knee joint. The patella, as an anatomic pulley, deflects the action line of the quadriceps femoris away from the joint, increasing the angle of pull and the ability of the muscle to generate a flexion torque. Interposing the patella between the quadriceps tendon and the femoral condyles also reduces friction between the tendon and condyles. 25
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The position of the patella relative to the joint axis varies as the instantaneous axis shifts and as the contour of the femoral condyles changes. The effect of patella on the moment arm (MA) of the quadriceps muscle, therefore, will vary through the knee joint ROM.
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