Knee Biomechanics PDF

BIOMECHANICS OF KNEE JOINT THE KNEE CONSISTS OF :- 1- TIBIOFEMORAL JOINT. 2- PATELLOFEMORAL JOINT.  TIBIOFEMORAL JOINT ANATOMY OF BONY STRUCTURE Distal femur Medial condyle is larger Lateral condyle is smaller but is prominent anteriorly this asymmetry...

BIOMECHANICS OF KNEE JOINT THE KNEE CONSISTS OF :- 1- TIBIOFEMORAL JOINT. 2- PATELLOFEMORAL JOINT.  TIBIOFEMORAL JOINT ANATOMY OF BONY STRUCTURE Distal femur Medial condyle is larger Lateral condyle is smaller but is prominent anteriorly this asymmetry affects knee kinematics specially during full extension. Proximal tibia The medial tibial plateau is larger in anteroposterior direction than the lateral tibial plateau and the lateral articular cartilage is larger than the medial one. A. Show differences in size and configuration between the medial and lateral tibial plateaus. B. The tibial The proximal tibia is larger than the tibial shaft that plateau overhangs the shaft of the tibia posteriorly forms a slope angle about 7 to 10 degree that help in and is inclined posteriorly 7° to 10° Figure(1) knee flexion.figure(1)  TIBIOFEMORAL JOINT ALIGNMENT The anatomical axis of the femur is directed inferiorly and medially from it’s proximal to distal end forming angle of inclination 125 degree. The anatomical axis of the tibia is almost vertical. That forms lateral angle of the knee which normally about 170 to 175 degrees.(normal genu valgum). Figure(3) Figure(3) TIBIOFEMORAL JOINT ALIGNMENT A lateral angle less than 170 is called excessive genu valgum. A lateral angle more than 180 is called genu varum. Figure(4) Figure(4)  KNEE JOINT STABILITY Joint stability is provided not by a tight bony fit, but by forces and physical containment provided by muscles, ligaments, capsule, menisci and body weight. So, any trauma of the knee will cause soft tissue injury.  KNEE JOINT STABILITY Closed packed position of the knee joint in which the two opposing joint surfaces are fully congruent is maximum extension and maximum lateral rotation. Maximum Loose packed position of the knee in which the opposing joint surfaces are not congruent is 25° of knee flexion.  CAPSULE AND REINFORCING LIGAMENTS The fibrous capsule of the knee encloses the medial and lateral compartments of the tibiofemoral joint and the patellofemoral joint. The proximal and distal attachments of the capsule to bone are indicated by the dotted lines. Figure(5) Figure(5) The capsule of the knee receives significant reinforcement from muscles, ligaments, and fascia. Five reinforced regions of the capsule are summarized in Table.1  SYNOVIAL MEMBRANE, BURSAE, AND FAT PADS The internal surface of the capsule of the knee is lined with a synovial membrane. The knee has many bursae that form at inter-tissue junctions that encounter friction during movement. Fat pads are often associated with bursae the most extensive fat pads are associated with the suprapatellar and deep infrapatellar bursae. Figure(10) Figure(10)  MENISCI Anatomical consideration crescent-shaped, fibrocartilaginous structures located within the knee joint. The external edge of each meniscus is attached to the tibia and the adjacent capsule by coronary (or menisco- tibial) ligaments. A slender transverse ligament connects the two menisci anteriorly. Figure(11) Figure(11) Quadriceps, semimembranosus and popliteus muscles have attachment to the menisci that help in stabilization of the meniscus. Medial meniscus attaches to the MCL and capsule while the lateral attach only to the capsule. The tendon of the popliteus passes between the lateral collateral ligament and the external border of the lateral meniscus. Figure(12) Figure(12) Blood comes from capillaries located within the adjacent synovial membrane and capsule. Blood supply to the menisci is greatest near the peripheral (external) border “red zone”. The internal border of the menisci, in contrast, is essentially avascular “white zone”. The narrow junction between red and white zones is often referred to as the “pink zone” because of its significantly reduced vascularity as compared to the “red zone.” Figure(13) Figure(13)  FUNCTIONAL CONSIDERATION 1. The primary function of the menisci is to reduce the compressive stress across the tibiofemoral joint. Compression forces at the knee joint routinely reach 2.5 to 3 times body weight while one is walking, and over 4 times body weight while one ascends stairs. By nearly tripling the area of joint contact, the menisci significantly reduce pressure (i.e., force per unit area) on the articular cartilage. 2. stabilizing the joint during motion 3. lubricating the articular cartilage 4. providing proprioception 5. helping to guide the knee’s arthrokinematics.  OSTEOKINEMATICS AT THE TIBIOFEMORAL JOINT ❑1) Flexion and Extension Flexion and extension at the knee occur around frontal axis. The axis is not fixed, migrating within the femoral condyles. The curve path of the axis is known as an “evolute”. Figure(15) Biomechanically, the migrating axis alters the length of the internal moment arm of the flexor and extensor muscles. Figure(15) ❑2) Internal and External (Axial) Rotation A knee flexed to 90 degrees can perform about 40 to 45 degrees of total axial rotation. External rotation range of motion generally exceeds internal rotation by a ratio of nearly 2:1. Figure(16)  ARTHROKINEMATICS AT THE TIBIOFEMORAL JOINT ❑Extension of the Knee Figure(17) “SCREW-HOME MECHANISM” ROTATION OF THE KNEE “screw-home” rotation is observable during the last 30 degrees of extension. This final position of extension increases joint congruency and stability. Figure(18) “SCREW-HOME MECHANISM” ROTATION OF THE KNEE The screw-home rotation mechanics are driven by at least three factors: 1. the shape of the medial femoral condyle. 2. the passive tension in the anterior cruciate ligament. 3. the slight lateral pull of the quadriceps muscle. Figure(19)  FLEXION OF THE KNEE. The arthrokinematics of knee flexion occur by a reverse action of knee extension. For a knee that is fully extended to be unlocked, the joint must first internally rotate slightly. This action is driven primarily by the popliteus muscle. The muscle can rotate the femur externally to initiate femoral-on-tibial flexion or can rotate the tibia internally to initiate tibial-on-femoral flexion. Internal and External (Axial) Rotation of the Knee ( as we mentioned before ) KNEE LIGAMENTS  MEDIAL AND LATERAL COLLATERAL LIGAMENTS Functional Considerations The primary function of the collateral ligaments is to limit excessive knee motion within the frontal plane. MCL has a role in : 1. Resists valgus stress (abduction stress) specially when knee joint is extended as it becomes taut. 2. Sustains a force of 115 kg/cm 2. 3. Resists axial rotation. 4. Restricts pure ant displacement of tibia when ACL is absent. 5. Resists excessive knee extension.  MEDIAL AND LATERAL COLLATERAL LIGAMENTS Functions of LCL 1. Resists varus stress (adduction stress ) 2. Sustains force of 276 kg /cm 2. 3. Resists axial rotation. 4. Resists post displacement of tibia. 5. Resists excessive knee extension. MEDIAL AND LATERAL COLLATERAL LIGAMENTS Flexion at 30 degrees relaxes the collateral ligaments (the position of immobilization following ligaments repair). Figure(20)  ANTERIOR AND POSTERIOR CRUCIATE The cruciate ligaments provide most of the resistance to anterior-posterior shear forces. These forces are inherent to the natural sagittal plane kinematics associated with ADL and sports activities. Tension in the anterior and posterior cruciate ligaments helps guide the knee’s arthrokinematics, because the cruciates contain mechanoreceptors, they indirectly provide the nervous systems with proprioceptive feedback.  ANTERIOR CRUCIATE LIGAMENT ❑Anatomy and Function Two sets of fiber bundles within the ACL: anterior-medial and posterior-lateral, named according to their relative attachments to the tibia. It attaches along an impression on the anterior inter ocular area of the tibial plateau. From this attachment, the ligament runs obliquely in a posterior, superior, and lateral direction to attach on the medial Figure(21) side of the lateral femoral condyle ANTERIOR CRUCIATE LIGAMENT During extension the resulting tension in the stretched fibers of the ACL helps limit the extent of this anterior slide. In normal knee, ACL provides about 85% of the total passive resistance to the anterior translation of the tibia. So, The quadriceps muscle is often referred to as an “ACL antagonist”. Figure(22) POSTERIOR CRUCIATE LIGAMENT It has two primary bundles (anterior-lateral) and (posterior-medial). Between full extension and approximately 30 to 40 degrees of flexion, most of the PCL is relatively slackened; tension peaks between 90 and 120 degrees of flexion. For this reason, the hamstrings are often referred to as a “PCL antagonist” With the knee flexed to about 90 degrees the PCL provides about 95% of the total passive resistance to the posterior translation Figure(24) of the tibia. The patellofemoral joint is a unique and complex structure consisting of static elements (bones and ligaments) and dynamic elements (neuromuscular system).The patella has a configuration of a triangle with its apex directed inferiorly. Superiorly, it articulates with the trochlea, the distal articulating surface of the femur, which are the main articulating surfaces of the patellofemoral joint  The patella It is the largest sesamoid bone in the body. It is a triangular bone embedded within the quadriceps tendon. Rounded vertical ridge runs across the posterior surface of the patella on both sides of this ridge there is lateral and medial facets. There is a third facet(odd) in the extreme medial facet. Figure (2) Function of patella : 1-The patella acts as a spacer protecting the quadriceps tendon from excessive friction from the femur, allows for smoother movements when bending and straightening the leg 2-To increase the moment arm of the quadriceps tendon. providing increased force facilitating knee extension 3- protect anterior surface of knee Quadriceps Angle (Q-Angle) or Patellofemoral Angle The Q-angle is defined as the angle between the quadriceps muscles (primarily the rectus femoris) and the patellartendon and represents the angle of quadriceps muscle force. A line is drawn from the ASIS to the midpoint of the patella on the same side and from the tibial tubercle to the midpoint of the patella. The angle formed by the crossing of these two lines is called theQ-angle Normally, the Q-angle is 13° for males and 18° for females when the knee is straight