Biomechanics of Knee Joint PDF

Dr. Bassem Khalifa Assistant Professor of Orthopedic Physical Therapy Biomechanics of Knee Joint STRUCTURE OF THE KNEE -The knee joint is one of the largest and most complex joints in the body. It is constructed by 4 bones and an extensive network of ligamentsand muscles....

Dr. Bassem Khalifa Assistant Professor of Orthopedic Physical Therapy Biomechanics of Knee Joint STRUCTURE OF THE KNEE -The knee joint is one of the largest and most complex joints in the body. It is constructed by 4 bones and an extensive network of ligamentsand muscles. It is a bi-condylar type of synovial joint, which mainly allows for flexion and extension (and a small degree of medial and lateral rotation). -The structure of the knee permits the bearing of tremendous loads, as well as the mobility required for locomotor activities. The knee is a large synovial joint, including three articulations within the joint capsule. The weight-bearing joints are the two condylar articulations of the tibiofemoral joint, with the third articulation being the patellofemoral joint. Although not a part of the knee, the proximal tibiofibular joint has soft-tissue connections that also slightly influence knee motion. Articulating Surfaes -The thigh bone (femur), the shin bone (tibia) and the kneecap (patella) articulate through tibiofemoral and patellofemoral joints. These three bones are covered in articular cartilage which is an extremely hard, smooth substance designed to decrease the friction forces. The medial and lateral condyles of the femur articulate with the tibia to form tibiofemoral joint. Similarly, the anterior and distal part of the femur articulate with the patella to form patellofemoral joint. -The tibiofemoral joint is the weight bearing joint of the knee. The patella lies in an indentation of the femur known as the intercondylar groove -The smaller fibula runs alongside the tibia and is attached via the superior tibiofibular joint is not directly involved in the knee joint, but provides a surface for important muscles and ligaments to attach to. -The distal aspect of the femur forms the proximal articulating surface for the knee, which is composed of 2 large condyles. The medial and the lateral. These two condyles are separated inferiorly by the intercondylar notch although they are connected anteriorly by a small shallow groove which is known as either the femoral sulcus or the patella groove or patella surface. This engages the patella in early flexion. 1 -The tibia also has 2 asymmetrical condyles (medial and lateral) of which are relatively flat, These are also known as the tibial plateau. The medial tibial plateau is much longer than the lateral anteroposteriorly, and the diameter of the proximal tibia is much greater than the shaft posteriorly which is sloped at approximately 7 to 10 o to facilitate flexion of the femoral condyles on the tibia. -The two tibial condyles are separated by the intercondylar tubercles, these are two bony spines which are roughened and their role lies within knee extension. They become lodged in the intercondylar notch of the femur, adding to the stability of the joint. Overall the tibiofemoral joint is a relatively unstable joint as the plateaus are slightly convex anteriorly and posteriorly. This emphasizes the importance of the other structures of the knee such as the menisci. 2 Tibiofemoral Joint The tibiofemoral joint is a modified hinge joint, which has six degrees of freedom. The bony configuration of the knee joint complex is geometrically incongruous and lends little inherent stability to the joint. Joint stability of the knee is dependent upon the static restraints of the joint capsule, ligaments, and menisci,and the dynamic restraints of the quadriceps, hamstrings, and gastrocnemius Patellofemoral Joint The patellofemoral joint is composed of the articulation of the patella with the femoral condyles of the femur. The patella is a passive component of the knee extensor mechanism, where the static and dynamic relationships of the underlying tibia and femur determine the patellar-tracking pattern. Menisci -The menisci, also known as semilunar cartilages after their half-moon shapes, are discs of fibrocartilage firmly attached to the superior plateaus of the tibia by the coronary ligaments and joint capsule. -The surface of each meniscus is concave superiorly, providing a congruous surface to the femoral condyles and is flat inferiorly to accompany the relatively flat tibial plateau. The horns of the medial meniscus are further apart and meniscus appears ‘C’ shaped, than those of the lateral one where meniscus appears more ‘O’ shaped - They are also joined to each other by the transverse ligament. The menisci are thickest at their peripheral borders, where fibers from the joint capsule solidly anchor them to the tibia. -The medial semilunar disc is also directly attached to the medial collateral ligament. Medially, both menisci taper down to paper thinness, with the inner edges unattached to the bone. -The menisci receive a rich supply of both blood vessels and nerves. The blood supply enables inflammation, repair, and remodeling. The outer portion of each meniscus is innervated, providing proprioceptive information regarding knee position, as well as the velocity and acceleration of knee movements. -The menisci deepen the articulating depressions of the tibial plateaus and assist with load transmission and shock absorption at the knee. The internal structure of the medial two-thirds of each meniscus is particularly well suited to resisting compression. 3 -The menisci correct the lack of congruence between the articular surfaces of tibia and femur, increase the area of contact and improve weight distribution and shock absorption. They also help to guide and coordinate knee motion, making them very important stabilizers of the knee. -The arrangement of the fibres in the menisci allows for axial loads to be dispersed radially decreasing the wear on the hyaline articular cartilage. This is essential as the compressive loads through the knee can reach 1-2 times body weight during gait and stair climbing and an astonishing 3-4 times body weight during running. -The menisci are connected with the tibia by coronary ligaments. The medial meniscus is much less mobile during joint motion than the lateral meniscus owing in large part to its firm attachment to the knee joint capsule and medial collateral ligament (MCL). On the lateral side, the meniscus is less firmly attached to the joint capsule and has no attachment to the lateral collateral ligament (LCL). -In fact, the posterior horn of the lateral meniscus is separated entirely from the posterolateral aspect of the joint capsule by the tendon of the popliteus muscle as it descends from the lateral epicondyle of the femur 4 Joint Capsule -The joint capsule has thick and fibrous layer superficially and thinner layers deeper. This along side the capsule ligaments enhances she stability of the knee. As with all of the structures that from the knee they are under most tension therefore more stable in an extended (closed packed) position in comparison to the laxity present in a flexed position (open packed). -Inside this capsule is a specialized membrane known as the synovial membrane which provides nourishment to all the surrounding structures. The synovial membrane produces synovial fluid which lubricates the knee joint. - Other structures include the infrapatellar fat pad and bursa which function as cushions to exterior forces on the knee. -The synovial fluid which lubricates the knee joint is pushed anteriorly when the knee is in extension, posteriorly when the knee is flexed and in the semi flexed knee the fluid is under the least tension therefor being the most comfortable position if there is a joint effusion. Ligaments -Many ligaments cross the knee, significantly enhancing its stability. The location of each ligament determines the direction in which it is capable of resisting the dislocation of the knee. 5 Medial Collateral Ligament (MCL): -This ligament can be divided into two sets of fibres - the superficial and the deep fibres. The general location of this band runs from the medial epicondyle of the femur to the medial condyle and the superior part of the medial surface of the tibia. The superficial fibres originates from medial femoral condyle and attaches to the medial aspect of the proximal tibia distally to the pes anserinus. The deep fibres are continuous to the joint capsule and originates from the inferior aspect of the medial femoral condyle, and inserts to the proximal aspect of the medial tibial plateau. In the middle of the ligament the deep fibres are attached to the medial meniscus. - The MCL primarily resists forces acting from the outer surface of the knee, valgus forces, but also resists the lateral rotation of the tibia on the femur. -The MCL is able to resist a valgus stress more effectively in the closed pack position (extension) due to the laxity of the ligament in the open packed position (flexed). -The MCL does have another role in restraining anterior translation of the tibia on the femur. Therefore when someone has an MCL injury the protection of the anterior cruciate ligament needs to be considered. 6 Lateral Collateral Ligament(LCL): -A cord like ligament that begins on the lateral epicondyle of the femur and joins with the tendon of the biceps femoris (hamstring muscle) to form the conjoined tendon. -This ligament is different to the MCL and is considered to be an extracapsular ligament. Its main role is resisting varus forces on the knee, and similarly to the MCL is most effective in full extension. another similarity of the MCL and the LCL is the ability of the LCL to also resist lateral rotation of the tibia on the femur. Anterior Cruciate Ligament(ACL): -The ACL is an important structure in the knee for resisting anterior translation of the tibia on the femur. -This ligament is a very well known ligament due to the high injury rate of athletes, which has resulted in a lot of research being done in the field of the ACL. -The cruciate ligaments are so called because they form a cross in the middle of the knee joint. The ACL runs from anterolateral aspect of the medial intercondylar tibal spine superolaterally and posteriorly to the posteromedial aspect of the lateral femoral condyle. The ACL twists medially as it travels proximally. - There are thought to be 2 bundles of fibres that form the ACL - the anteromedial bundle (AMB) and the posterolateral bundel (PLB). -The ACL is responsible for resisting anterior sheering forces on the knee. So when the knee close to full extension the PLB will be taut and resisting the force, but as the knee moves into a flexed position the PLB become lax and the AMB becomes taut taking over the role of resisting the anterior sheering forces. -At approximately 30o of the flexion neither of the bundles of the ligament are taut leading to the most anterior translation available at this range. It is most commonly injured in twisting movements. - The ACLis also an accessory ligament in resisting rotary forces medially and laterally as well as valgus and varus forces. The PLB of the ACL is theorised to be most effective at providing rotary stability of the knee. In addition to this the AMB is under most tension at approximately 10-15o of knee flexion with medial rotation. -Posterior Cruciate Ligament(PCL): This ligament runs from the posterior surface of the tibia between the two posterior horns of the menisci it then runs superiorly and anteriorly and attaches to the lateral aspect of the medial femoral condyle. 7 -The PCL is much shorted and less oblique with a much larger cross sectional area in comparison to the ACL. As the PCL blends with the posterior capsule as it crosses to the tibial attachment. - Factors such as the size, shape and location possibly contribute to the increased strength of the PCL in comparison to the ACL and is much less frequently injured. -The PCL similarly has 2 bundles of fibres the posteromedial (PMB) and the anterolateral bundle (ALB). When the knee is in near full extension the ALB which is much larger and stronger are lax and the PMB are taut whereas in 80-90o of flexion the PMB are lax and the ALB are taut. -The PCL is more adept for resisting posterior translation / sheering forces in knee when it is flexed despite there being the most posterior translation available at 75-90o flexion. The secondary stabilisers at this point in the range are ineffective and relay upon the PCL. -The PCL also plays an important role in resisting rotation and valgus / varus forces on the knee. -The PCLbest resists medial tibial rotation at 90o flexion rather than extension, but is not very good at resisting lateral tibial rotation. If the PCL becomes damaged the popliteus muscle plays an important role in stabilising the knee from posterior sheering forces. In the PCL deficient person hamstring contraction can destabilise the knee joint alongside a gastrocnemius contractions (at angles greater than 40o knee flexion), whereas quadriceps contractions degrees the strain on the PCL between angles of 20 and 60o flexion. Bursae -A bursa is synovial fluid filled sac, found between moving structures in a joint – with the aim of reducing wear and tear on those structures. -There are four bursae found in the knee joint.: 8  ▪ Suprapatellar bursa: An extension of a synovial cavity of a knee, located between the quadriceps femoris as well as the femur. ▪ Prepatellar bursa: Found between an apex of a patella & the skin. ▪ Infrapatellar bursa: Split into deep as well as superficial. A deep bursa lies between a tibia & the patella ligament. A superficial lies between a patella ligament & the skin. ▪ Semimembranosus bursa: Located posteriorly in a knee joint, between a semimembranosus muscle & a medial head of the gastrocnemius. MOVEMENTS AT THE KNEE Muscles Crossing the Knee Like the elbow, the knee is crossed by a number of two-joint muscles. There are four main movements that a knee joint permits: ▪ Extension: Produced by a quadriceps femoris, which inserts into a tibial tuberosity. ▪ Flexion: Produced by the hamstrings, gracilis, sartorius as well as popliteus. ▪ Lateral rotation: Produced by a biceps femoris. ▪ Medial rotation: Produced by five muscles; semimembranosus, semitendinosus, gracilis, sartorius as well as popliteus. Locking of the knee joint ▪ Closed kinematic chain extension from 30-degree knee flexion. ▪ A larger medial femoral condyle continue rolling & gliding posteriorly when the smaller lateral side stopped. This results in medial rotation of a femur on the tibia, in the last 5 degrees of an extension. The medial rotation of the femur at the final stage of extension is 9  not voluntary or even produced by muscular force, which is referred to as “Automatic” or “Terminal Rotation”. ▪ The rotation within a knee joint brings the joint into a closed packed or locked position. The consequences of automatic rotation are also known as “locking mechanism” or “screw home mechanism.” ▪ Open kinematic chain: Lateral rotation of a tibia on a femur. True Knee Locking ▪ True locking at the knee is where a knee gets physically stuck as well as a patient physically cannot move a knee for a person timeue knee locking is caused by a mechanical block where something gets caught inside a knee joint, preventing movement. ▪ A truly locked knee is fairly rare and typically happens as a patient moves the knee into full extension, for instance towards being fully straight. Pseudo Knee Locking ▪ Pseudo-knee locking is much more common than true locking, & knee motion is limited by temporary muscle spasming as the body tries to protect itself in response to pain. Unlocking the knee ▪ To initiate flexion, a knee should be unlocked. ▪ A flexion force will automatically result in a lateral rotation of the femur. ▪ Owing to the larger medial condyle will move before the shorter lateral condyle. ▪ Popliteus is the primary muscle to unlock a knee. Osteokinematics and range of motion -The ligaments and menisci provide static stability and the muscles and tendons dynamic stability -The main movement of the knee is flexion - extension. For that matter, knee act as a hinge joint, whereby the articular surfaces of the femur roll and glide over the tibial surface. During flexion and extension, tibiaand patella act as one structure in relation to the femur. -The quadriceps muscle group is made up of 4 different individual muscles. They join together forming one single tendon that inserts into the anterior tibial tuberosity. Embedded in the tendon is the patella, a triangular sesamoid bone, as well as its function, is to improve the efficiency of the quadriceps contractions. Contraction of the quadriceps pulls the patella upwards & extends a knee. Range of motion of Knee joint -Knee extension 0 degrees. The hamstring muscle group consists of the biceps femoris, semitendinosus as well as semimembranosus. They are situated at the back of a thigh as well as their function is flexing or even bending a knee & providing stability on either side of a joint line. -Range of motion: Knee flexion 135 degrees. 10 -Secondary movement is internal & external rotation of the tibia in relation to the femur, but it is possible only when the knee is flexed. -Extended position ▪ With the knee extended, both lateral & medial collateral ligaments, as well as an anterior part of the anterior cruciate ligament, is taut. ▪ During extension, the femoral condyles glide as well as roll into a position that causes the complete unfolding of the tibial collateral ligament. ▪ While the last 10 degrees of extension, an obligatory terminal rotation is triggered in which the knee is rotated medially 5 degrees. A final rotation is produced by a lateral rotation of the tibia in a non-weight-bearing leg, and by a medial rotation of the femur in a weight- bearing leg. A terminal rotation is made possible by the shape of a medial femoral condyle, assisted by the contraction of a popliteus muscle as well as the iliotibial tract & is caused by a stretching of the anterior cruciate ligament. Both cruciate ligaments are slightly unwinded as well as both lateral ligaments become taut. -Flexed position ▪ In a flexed position, collateral ligaments are relaxed while the cruciate ligaments are taut. Rotation is controlled by twisted cruciate ligaments; the two ligaments get twisted around each other whilst a medial rotation of the tibia which reducing the amount of rotation possible while they become unwound during lateral rotation of the tibia. Because of the oblique position of cruciate ligaments, at least part of one of them is always tense as well as these ligaments control the joint as the collateral ligaments are relaxed. ▪ Moreover, the dorsal fibers of the tibial collateral ligament become tensed while extreme medial rotation, as well as a ligament, also reduces a lateral rotation to 45–60 degrees. Arthrokinematics: -Viewed in the sagittal plane, an articulating surface of a femur is convex whereas an articulating surface of a tibia is concave. -The knee arthrokinematics is based on the rules of concavity & convexity as well as is described in terms of open & closed chain: ▪ Open kinetic chain: While the knee extension, a tibia glides anteriorly on a femur. More precisely, from 20 degrees knee flexion to full extension, a tibia rotates externally. While a knee flexion, a tibia glides posteriorly on a femur & from full knee extension to 20 degrees flexion, a tibia rotates internally. ▪ Closed kinetic chain: While knee extension, a femur glides posteriorly on a tibia. To be more specific, from 20 degrees knee flexion to full extension, while that time the femur rotates internally on a stable tibia. While knee flexion, a femur glides anteriorly on a tibia, and from full knee extension to 20 degrees flexion, the femur rotates externally on the stable tibia. 11  ▪ The “screw home mechanism” ▪ The “screw-home” mechanism, considered to be a key element to knee stability, is the rotation between a tibia & the femur. It occurs at the end of a knee extension, between full extension (0 degrees) & 20 degrees of the knee flexion. A tibia rotates internally while an open-chain motions (swing phase) & externally while a closed-chain movement (stance phase). The external rotation occurs during a terminal degree of a knee extension and results in a tightening of both cruciate ligaments, which locks a knee. The tibia is then in the position of maximal stability with respect to the femur. LOADS ON THE KNEE -Because the knee is positioned between the body’s two longest bony levers (the femur and the tibia), the potential for torque development at the joint is large. The knee is also a major weight- bearing joint. Forces at the Tibiofemoral Joint -The tibiofemoral joint is loaded in both compression and shear during daily activities. Weight bearing and tension development in the muscles crossing the knee contribute to these forces, with compression dominating when the knee is fully extended. The muscles that cross the knee are primary contributors to tibiofemoral compression, although the gluteus medius also contributes substantially to compression on the medial tibial plateau. -Compressive force at the tibiofemoral joint is slightly greater than three times body weight during the stance phase of gait, increasing up to approximately four times body weight during stair climbing. The medial tibial plateau bears most of this load during stance when the knee is extended, with the lateral tibial plateau bearing more of the much smaller loads imposed during the swing phase. Because the medial tibial plateau has a surface area roughly 60% larger than that of the lateral tibial plateau, the stress acting on the joint is less than if peak loads were distributed medially. -The fact that the articular cartilage on the medial plateau is three times thicker than that on the lateral plateau also helps protect the joint from wear. -The menisci act to distribute loads at the tibiofemoral joint over a broader area, thus reducing the magnitude of joint stress. The menisci also directly assist with force absorption at the knee, bearing as much as an estimated 45% of the total load. Because the menisci help protect the articulating bone surfaces from wear, knees that have undergone either complete or partial meniscectomies are more likely to develop degenerative conditions. Forces at the Patellofemoral Joint 12 -Compressive force at the patellofemoral joint has been found to be one-half of body weight during normal walking gait, increasing up to over three times body weight during stair climbing. patellofemoral compression increases with knee flexion during weight bearing. -There are two reasons for this. A. The increase in knee flexion increases the compressive component of force acting at the joint. B. Flexion increases, a larger amount of quadriceps tension is required to prevent the knee from buckling against gravity. -The squat exercise is known for being particularly stressful to the patellofemoral joint, and joint reaction forces increase with the depth of the squat, as well as with load.Training within the 0– 50° knee flexion range is recommended for those who wish to minimize knee forces, however.The squat has been shown to be an effective exercise for use during rehabilitation following cruciate ligament or patellofemoral surgery. Sample Problem : the relationship between quadriceps force and patellofemoral joint compression. Compression at the patellofemoral joint is the vector sum of tension in the quadriceps and the patellar tendon. A. In extension,the compressive force is small because tension in the muscle group and tendon act nearly perpendicular to the joint. B. As flexion increases, compression increases because of changed orientation of the force vectors and increased tension requirement in the quadriceps to maintain body position. 13 Knee Axis: 14 Pathomechanics of Knee Joint: 15 Knee Angles: The Q angle of the knee : -is a measurement of the angle between the quadriceps muscles and the patella tendon. It provides useful information about the alignment of the knee joint. -The Knee Q angle (also known as Quadriceps Angle) is defined as the angle between the quadriceps muscle (primarily the rectus femoris) and the patellar tendon. It sometimes called quadriceps pull angle. -It represents the dynamic “instability” of patella, the greater it is the more unstable patella will be. 16 -How to measure the Q angle? You will need a long-arm goniometer. You can measure the Q angle either laying down or standing up. Standing is usually more suitable. This is due to the normal weight-bearing forces applied to the knee joint during daily activity. Place the centre of the goniometer over the centre of the patella. Position the bottom arm in line with the patella tendon and tibial tuberosity. -A line is drawn from the anterior superior iliac spine to the midpoint of the patella, corresponding to the quadriceps’ tensile direction. -Another line is drawn from the tibial tubercle to the midpoint of the patella, corresponding to the patellar tendon. The angle formed by the crossing of these two lines is called the Q-angle of the knee. 17 -Normal for men is 13 degrees and for women is 18 degrees. -Women usually have a higher Q angle due to their naturally wider pelvis. If measured laying down the angle will be 1-3 degrees lower. -Any angle less than 13° may be associated with patellofemoral dysfunction or patella alta. -An angle greater than 18° is often associated with subluxing patella, increased femoral anteversion, genu valgum , or increased lateral tibial torsion or mal-tracking of the patella. -Over time this causes microtrauma to the cartilage on the rear of the patella. Eventiually this becomes pain, often known as anterior knee pain, patellofemoral pain or chondromalacia patella. -Having over-pronated feet also places additional strain on the Q angle due to excessive internal rotation of the tibia. 18