Knee Biomechanics Joint Positions Resting position knee 25

Knee Biomechanics

Joint Positions Resting position knee 25 degrees flexion Close packed position knee Full extension Capsular pattern of the knee Gross limitation of flexion (e. g. 90 degrees), mild limitation of extension (e. g. 5 -10 degrees) ROM values Flexion Extension 150 degrees 0 -5 degrees

Arthrokinematics • Flexion and extension are a combination of rolling and gliding in the opposite direction • Convex femur articulating with a concave tibia

Flexion Arthrokinematics • From full extension, the initial phase of flexion is pure rolling. • The degree of pure rolling varies per condyle • The medial condyle rolls for 10 -15 degrees, the lateral condyle rolls for 20 degrees as it has a larger joint surface • The 15 -20 degrees of rolling corresponds with the normal range of flexion/extension in walking. • The tibia glides posterior after the initial phase of flexion

Extension Arthrokinematics • Tibia rolls and glides in anterior direction

Rotation Arthrokinematics • External rotation Lateral femur condyle glides anterior, medial condyle glides posterior • Internal rotation Lateral femur condyle glides posterior, medial condyle glides anterior

Function Cruciate Ligaments With the knee flexed at 90 degrees: • The PCL prevents backward sliding of the tibia on the femur • The ACL prevents forward sliding of the tibia In flexion • The PCL takes on a more vertical orientation • ACL becomes more horizontal In extension • Both cruciates are stretched • PCL becomes less vertical Some of the fibers of the cruciates are always in a stage of tension because: • There is no sliding motion in any position • The 2 ligaments do not change length when from flexion to extension, when the condyles stay in contact with the tibial plateau

Role of muscles assisting ligaments • Iliotibial band assists LCL • Sartorius, gracilis, semitendinosus assist MCL • Quadriceps influences stability medially and laterally • Straight and oblique fibers form a fibrous canopy over the anterior aspect of the joint. The straight fibers prevent opening on the same side, the oblique fibers prevent opening on the opposite side. • Flexor muscles (biceps femoris, gastrocnemius) play an active role in limiting extension

Rotatory stability of the knee • Axial rotation can only occur when the knee is flexed. • In full extension, rotation of the knee is prevented by tension in the collaterals and the cruciates • The cruciates are wound counter clockwise. Therefore if the tibia is rotated medially, they are wound up more, and will tighten. During lateral rotation of the tibia they unwind • The collaterals are wound clockwise. If the tibia is rotated laterally, the ligaments are wound up more and will resist further rotation • So, the collateral ligaments prevent lateral rotation, the cruciate ligaments prevent medial rotation.

The “screw home” mechanism • Terminal extension is associated with a small measure of external rotation. • Occurs automatically in the absence of any voluntary movement. • Flexion is associated with an automatic internal rotation of 20 degrees • This motion is conjunct, it cannot be performed independently

The “screw home” mechanism Reasons • The lateral femur condyle is greater than the medial; therefore it rolls over a greater distance. • The lateral femur condyle glides more freely on the convex tibial surface. • During extension, the MCL is stretched more rapidly than the LCL, which allows the lateral femur condyle to recede farther. • Tension of the cruciates at end of extension produces external rotation • Slight lateral pull of quadriceps

Patella • Sesamoid bone in the quadriceps tendon • Function: increases efficiency of the quadriceps by shifting the line of its muscular pull anteriorly. • Extensor efficiency of the quadriceps is increased 1. 5 times by the presence of the patella.

Patellar position • Patella should face anterior and slightly lateral • With a high patella (alta) you’ll see a double bump when patient is sitting, one from the high patella, and one from the infrapatellar fat pad below that.

Patellar mobility • The superior/lateral force of the quadriceps is turned into a vertical force by the central groove of the femoral patellar surface • The patella glides proximally with knee extension • It moves over a distance 2 times its length, approximately 8 cm.

Knee joint capsule and patella The knee joint capsule forms 3 recesses in relation to the patella: • Superiorly the suprapatellar fold with the suprapatellar bursa • On either side, the parapatellar recesses • These unfold in flexion and permit excursion of patella under the condyles. Inflammatory lesions can cause adhesions in the recesses, leading to restrictions in knee flexion

Q Angle • Angle between rectus femoris and patellar tendon • Normal: male 13 degrees; female 18 degrees • Angle>18 degrees associated with PFA, patellar subluxation, femoral anteversion, genu valgum, lateral displacement tibial tuberosity • Angle <13 degrees associated with PFA

Menisci • Make up for a lack of congruence of the articular surfaces by increasing the area of contact • Triangular in cross section • Crescent shaped • The lateral meniscus forms almost a complete circle, the medial meniscus is semi-lunar.

Attachments of the menisci CAPSULAR attached to the deep fibers of the capsule BONY each horn is attached to the intercondylar fossa meniscopatellar fibers, from the lateral edges of the patella to the lateral border of each meniscus anterior horns of the menisci are linked by transverse ligament the semimembranosus has fibers to the posterior medial meniscus; and the popliteus sends fibers to the posterior edge of the lateral meniscus MCL fibers attach to the medial meniscus; PCL fibers to posterior horn of lateral meniscus; ACL fibers to anterior horn of medial meniscus PATELLAR MENISCAL MUSCULAR LIGAMENTS

Meniscal Loading • By increasing the contact surface, transmit between 30 -70% of the load applied across the joint. • Meniscal loads increase with increasing flexion (85% at 90 degrees of flexion)

Menisci Role in Stability • Minimal to none in the ACL intact knee • Plays an important role in ACL deficient knee. • So, what is the possible clinical implication when an ACL injury occurs?