Download TSM97 - The Knee Joint

TSM97: THE KNEE JOINT
03/12/08
LEARNING OUTCOMES
Describe the functional anatomy of the knee joint
OVERVIEW OF THE KNEE JOINT
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The knee joint is the site of articulation between the distal femur, proximal tibia and the patella
o Synovial hinge joint between the two long bones
o Two articulations between the femur and tibia; one between the femur and patella
The distal femur does not articulate directly with the lateral leg bone the fibula (compare to elbow)
o Posteriorly the linea aspera divides into the medial and lateral supracondylar lines
o Also protruding posteriorly are the medial and lateral condyles and intercondylar fossa
o Anteriorly the medial and lateral epicondyles are divided by the patellar surface
o Just above the medial epicondyle is the adductor tubercle where adductor magnus attaches
The proximal tibia has a small supero-lateral articular facet posteriorly for the fibular head
o Superiorly the medial and lateral condyles have two broad flat articular surfaces (plateaux)
o Also superiorly the intercondylar eminence is elevated with two intercondylar tubercles
o Anteriorly the tibial tuberosity protrudes centrally just below the condyles
o Posteriorly the soleal line descends and crosses medially from the superior tibio-fibular joint
The patella is the largest sesamoid bone (‘intra-tendinous’) in the body within quadriceps femoris
o Roughly triangular with its apex facing inferiorly for attachment to the patellar ligament
o Broad base superiorly for attachment of the quadriceps femoris tendon
o Posterior surface has medial and lateral facets that articulate with patellar surface of femur
JOINT CAPSULE AND ASSOCIATED LIGAMENTS
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A fibrous capsule extends around the articular margins of the femoral and tibial condyles
o Covers the posterior and lateral regions of the joint but is replaced by the patella anteriorly
o Extracapsular medial and lateral collateral ligaments limit lateral displacement on either side
 Medial collateral ligament is continuous with the capsule
 Lateral collateral ligament joins to the fibula
o Intracapsular anterior and posterior cruciate ligaments limit antero-posterior displacement
 Arise from the tibia (area of origin determines name) and cross over
 Insert onto the lateral and medial femoral condyles respectively
 Anterior ligament is tensed in extension; posterior is tensed in flexion
 Posterior ligament is much thicker and stronger
The femoral condyles are rounded whereas they articulate with the flat superior facets of the tibia
o Normal articular (hyaline) cartilage covers the facets on the tibial plateaux
o Lateral and medial fibro-cartilaginous interarticular rings called menisci also line the facets
 Wedge-shaped – broader at the edges; thinner but more firmly attached centrally
 Mostly avascular so do not repair well – complete excision prompts regeneration
 Medial meniscus is continuous with the capsule and hence prone to rupture
Explain the anatomical basis of injuries to the collateral ligaments, cruciate ligaments and the menisci of the knee
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The knee joint is adapted for weight-bearing and stability but is essentially not a very strong joint
The associated ligaments and muscles are mostly responsible for strengthening the joint
The collateral ligaments can be damaged in lateral displacement at the knee joint
o Blow to the lateral knee can rupture the medial collateral ligament, capsule and meniscus
o Tearing of the lateral collateral ligament can cause damage to the common peroneal nerve
 Supplies the dorsiflexors of the foot – damage results in ‘foot drop’
The cruciate ligaments can be damaged in antero-posterior displacement at the knee joint
o Injury to the extended knee can easily damage the anterior cruciate as it will be tensed
o Injury to the flexed knee can similarly damage the posterior cruciate
Demonstrate the range of normal movements of the knee joint
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The majority of movement at the knee is flexion facilitated by the hamstrings
o Normal range of knee flexion is up to 150˚
o Limited by posterior thigh
Small amount of hyperextension possible up to 10˚ facilitated by quadriceps femoris
During extension of the knee to a ‘locked’ stable position (i.e. standing) the femur laterally rotates
o Passive rotation of about 5˚ – a normal physiological mechanism
o In unlocking of the knee the femur is actively medially rotated by the popliteus muscle
 Arises from the lateral femoral condyle within the fibrous capsule then pierces it
 Inserts on the postero-medial surface of the proximal tibia (crosses the midline)
Describe the tests for instability of the knee joint
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The Q-angle can be measured as the angle between the two imaginary lines:
o From ASIS through the middle of the patella (roughly along the angle of the femur – oblique)
o From the tibial tuberosity through the middle of the patella (vertical)
This angle is around 14˚ in males and 17˚ in females
o Reduced values indicate genu varum (varus – twisting towards midline) – ‘bow-legged’
o Elevated values indicate genu valgum (valgus – twisting away from midline) – ‘knock-knee’)