Biomechanics of Knee Joint

Study Notes

Tibiofemoral Joint Function
- Flexion/Extension:
  - Axis of flexion and extension passes through the femoral epicondyles
  - ACL causes femoral condyle to glide posteriorly during flexion and anteriorly during extension
  - PCL causes femoral condyle to glide anteriorly during flexion and posteriorly during extension
Rotation
- Occurs about a longitudinal axis through the medial tibial intercondylar tubercle
- Limited rotation possible when knee is in full extension due to taut ligaments and menisci
- Increased rotation possible when knee is flexed to 90°, with a total medial/lateral rotation of approximately 35°
- Medial tibial condyle and intercondylar tubercle act as pivot points during rotation
Patellofemoral Joint Motion
- 0-20° of flexion:
  - Tibia internally rotates, decreasing lateral pull on the patella
  - Patella moves into intercondylar groove and follows it until 90° of knee flexion
  - Contact area is mainly at the inferior pole of the patella
- 20-90° of flexion:
  - Patella moves in intercondylar groove
  - Contact areas are along lateral and medial patellar facets with lateral and medial condyles
- 90-135° of flexion:
  - Patella moves laterally and tilts medially into intercondylar groove
  - Odd facet contacts medial femoral condyle at about 135° of knee flexion

Trabecular Pattern
- Distal end of femur: contains vertical and horizontal trabeculae
- Proximal end of tibia: contains similar sets of trabeculae
Load Transmission
- Force applied to the superior aspect of the tibia is resolved into a vertical component (v) and a transverse component (t)
- Transverse component (t) tends to tilt the joint medially and exacerbate physiological valgus
- Medial ligaments prevent dislocation

Twisting Movement of the Knee
- Mechanism of injury: twisting of the body around a fixed flexed leg
- Consequences:
  - Longitudinal splitting of the meniscus
  - Complete detachment of the meniscus from the capsule
  - Complex tear of the meniscus
- Knee "locks" in a position of flexion
Violent Extension of the Knee
- Mechanism of injury: violent extension of the knee (e.g. kicking a football)
- Menisci fail to follow the movements of the femoral condyles on the tibial condyles
- Consequences:
  - Transverse tears or detachment of the anterior horn

Anterior Cruciate Ligament (ACL) Injury
- Common in sports, approximately 70% of ACL injuries are non-contact
- Mechanism of injury: femur rotates on the planted leg with the knee close to full extension
- Sudden changes in direction combined with acceleration or deceleration of the body
Posterior Cruciate Ligament (PCL) Injury
- Most commonly result from sport participation or motor vehicle accidents
- Mechanism of injury: hyperflexion of the knee with the foot plantarflexed
- Impact with the dashboard during motor vehicle accidents can result in combined ligamentous damage