Anatomy biomechanics & kinematics of the knee

Knee Anatomy

Femoral Anatomy The largest and most complicated joint in the body Consists of three joints ( compartment) medial & lateral tibio-femoral joints patello-femoral joint Sustains large forces between the body’s two longest lever arms

Femoral Anatomy

Femoral Anatomy

Femoral Anatomy The medial & lateral femoral condyles have different Sagittal radii The distal medial condyle is shorter, narrower and more oblique than the lateral condyle Medial Lateral Lateral Medial

The Patella Oval shape wider medial to lateral Diameter 30 – 55mm Thickness 19 – 26mm Bi-concave posterior surface with 4 – 5mm thick articular cartilage Articulates with the trochlear groove

Tibial Anatomy

Tibial Anatomy

Tibial Anatomy The medial condyle is concave making the medial compartment more stable than the convex lateral side The metaphysis is angled posteriorly and the plateau slops posteriorly from 3° – 15° Lateral Lateral Medial

Tibial Anatomy The intercondylar eminence divides the tibial plateau contributes to M/L stability and provides attachment for the menisci and the ACL The lateral side is more circular than the longer medial side The patella ligament inserts into the tibial tuberosity

The Menisci Maintain contact between the femur and the tibia and bear 60% of the loads in the knee Lateral: moves 10 – 12mm A/P Medial: moves 4 – 5mm A/P Posterior cruciate ligament Lateral co-lateral ligament Medial co-lateral ligament Lateral meniscus Synovial membrane Anterior cruciate ligament Infrapatellar fat pad Patellar ligament

The Menisci Coronal cross section Medial Lateral

Knee Stabilisers Static: Congruent Articular Geometry Co-lateral ligaments Cruciate ligaments Capsule Dynamic: Muscles Menisci

Cruciate Ligaments So called because they cross in the coronal and sagittal planes Provide antero-posterior and some medio-lateral stability Interact with the MCL LCL and the menisci to control motion In flexion the ACL is almost horizontal and the PCL vertical this reverses in extension

The Anterior Cruciate Ligament Originates in the intercondylar notch on internal aspect of the lateral femoral condyle The tibial insertion is anterior and medial and consists of three distinct groups of fibres Prevents anterior displacement of the tibia

The Posterior Cruciate Ligament Originates in the intercondylar notch on the postero-medial aspect of the femoral condyle The tibial insertion is long extending from the intercondylar eminence on the posterior tibial plateau inferiorly for 1 – 2cms Consists of four distinct groups of fibres Prevents posterior displacement of the tibia

Lateral co-lateral ligament Medial co-lateral ligament Collateral Ligaments Taut in extension to provide medio-lateral stability and looser in flexion to allow rotation of the tibia Lateral co-lateral ligament Medial co-lateral ligament

The Medial Collateral Ligament Broad & fan-shaped originates on the medial femoral epicondyle inserts 4 – 5cm distal to the tibial plateau Consists of two bundles Anterior free of capsular attachment Posterior blends with the medial meniscus and the joint capsule

The Lateral Collateral Ligament Narrow and cord like originates on lateral femoral epicondyle Inserts on the head of the fibula free of any meniscal or capsular attachments

Limb and Joint Alignments

Limb & Joint Alignment Anatomic Axis A line connecting the centre of a bone proximally to the centre of a bone distally Mechanical Axis A line connecting the point of input of a load on a bone to its output to an associated structure e,g. The centre of the femoral head to the centre of the knee

Anatomic Axis Mechanical Axis Limb & Joint Alignment Anatomic Axis Mechanical Axis

The HKA Axis A line connecting the centre of the femoral head the centre of the knee and the centre of ankle This line runs inferiorly medial forming an angle of approx. 3° to the midline in normal stance The joint line is perpendicular to the midline and therefore lies approx. 3° medially oblique to the HKA axis

Limb & Joint Alignment Varus = Towards the Midline Valgus = Away from the Midline The tibia & femur do not form a straight line but form an obtuse angle of 170° – 175° the average being 173° which is the physiological Valgus of the knee 173°

Limb & Joint Alignment Valgus deformity Varus deformity

Femoral Alignment Neutral Alignment of the femoral A/P cut will usually produce a trapezoidal Flexion Gap 3° external rotation of the femoral A/P cut will usually produce a parallel flexion gap

Kinematics

Kinematics The Study of Joint Motion The knee does not flex around a fixed centre it is capable of axial rotation and transverse movements During the first 20° of flexion the femur moves posteriorly on the tibia femoral " roll-back " Roll-back is initiated and controlled by the cruciate ligaments As flexion increases roll-back stops and the femoral condyles slide on the the tibial plateau allowing the knee to flex

Femoral Roll-Back Rolling only would cause the knee to dislocate as the distance around the femoral condyles is approximately twice the A/P width of the tibial plateau Sliding only would cause impingement of the posterior femoral shaft on the posterior tibial plateau and block flexion Rolling and sliding together allow the knee to remain stable and flex fully

Range of Motion Active Flexion Passive Flexion 160° Rotation When the hip is in extension 120° When the hip is in flexion 140° Passive Flexion 160° Rotation Is only possible in flexion 40° lateral 30° medial at 90° of flexion

Angle of flexion required for daily activities Walking : 0° – 67° Climbing stairs : 0° – 83° Descending stairs : 0° – 90° Sitting down : 0° – 93° Tying a shoe : 0° – 106° Lifting an object : 0° – 117°

Biomechanics

Forces during gait Heel strike Stance phase Toe off generates 2 – 3 x bodyweight associated with the contraction of the hamstrings Stance phase generates 2 x bodyweight and is associated with contraction the of the quadriceps Toe off generates 2 – 4 x bodyweight and is associated with contraction of gastrocnemius

Forces during gait Ground reaction force (GRF) occurs during gait from heel strike to toe off GRF is counterbalanced the joint reaction force and the patella tendon force For 1 bodyweight the GRF is 700N The patella ligament exerts a force of 2100N Therefore the tibio-femoral joint reaction force is 2800N

Biomechanics Loads transmitted across the knee Walking 2 – 4 BW Running 3 – 5 BW Stairs 5 – 7 BW Parachute jump 20 BW

The Extensor Mechanism Made up of the 4 quadriceps muscles and the patella The quadriceps muscles are responsible for knee extension Help to prevent posterior displacement of the tibia Rectus femoris Vastus intermedius Vastus lateralis Vastus medialis

The Extensor Mechanism The patella increases the efficiency and guides the pull of the quadriceps The patella stays with the femur when the tibia rotates it is stabilised by it’s congruent fit in the trochlear groove and the medial and lateral retinaculae Lateral retiaculum Medial retinaculum

Joint Reaction Force Patello-femoral joint reaction force is a vector force ranging from 0.5 BW at 9° of flexion to 7 – 8 BW at 130° of flexion

Joint Reaction Force The patellar moment arm r can be changed during patellar reconstruction Excessive bone resection will reduce r and the quadriceps will have to pull harder Insufficient bone resection will increase r producing high patello-femoral contact forces Both increase the PFJRF and may lead to patellar instability, pain, patella fracture, loosening, and excessive polyethylene wear

The ‘Q’ Angle The angle between a line drawn from the centre of the patella to the anterior superior iliac crest and a line drawn from the centre of the tibial tuberosity through the centre of patella normally 15° Any increase in the Q angle will predispose the patella to instability Tibial rotation has the greatest effect on the Q angle

Summary The knee is capable of complex motion and sustains high dynamic loads during daily activities Both tibio-femoral and patello-femoral articulations play a part in the function of the knee The knee is able to dissipate high loads through the muscles and ligaments as well as the more compliant tissues of the menisci and cartilage

Summary If the knee is damaged the biomechanics change the natural knee can compromise to an extent Prosthetic replacements must restore function and be capable of sustaining high dynamic loads in both the aligned and mal-aligned condition Prosthetic designs focus around load dissipation and lowering wear in the tibio-femoral and patello-femoral articulations

Anatomy Biomechanics & Kinematics of the Knee

Femoral Component 6° of Freedom Anterior / Posterior Anterior: Not enough posterior condyles, Patella Kinematics Posterior: Anterior Notch, elongation of Posterior Condyles = Tight in Flexion Medial / Lateral Proximal / Distal

Femoral Component 6° of Freedom 4. Varus / Valgus 5. Flexion / Extension: Gross flexion: The prosthesis has to hyper extend in extension Gross Extension: Tends to notch the anterior cortex 6. Internal / External

Tibial 6° of Freedom 1. Anterior / Posterior 2. Medial / Lateral 3. Proximal / Distal (Resection Level) 4. Varus / Valgus Rotation 5. Flexion / Extension (Posterior Slope) 6. Internal / External Rotation The Varus / Valgus position is the most important