- Largest joint in the human body – formed by 3 articulations between 3 bones - femur, tibia and patella
- Synovial, bicondylar joint (although sometimes described as 2 condyloid joints between tibia and femur and a sellar joint between patella and femur)
- Motion permitted about 2 axes
- Flexion / Extension - sagittal plane, transverse axis and Rotation - transverse plane, longitudinal axis (only possible physiologically when the knee is flexed)
Articular Surfaces
3 articulations to be described
- Medial & Lateral tibio-femoral
- Patello-femoral
The Femur
- Medial & lateral femoral condyles patellar surface between them (differentiated by 2 faint ridges anteriorly)
- Covered in hyaline cartilage
- Note how concavity of patella surface changes from superior to inferior
The Femur
Femoral Condyles
- Biconvex - convex in AP and medio-lateral directions - covered in Hyaline Cartilage
- Note difference in shape - medial condyle being narrower and longer than lateral
- Also note orientation relative to each other - lines bisecting each condyle would converge anteriorly due to angulation of medial condyle
Lateral View - Femoral Condyles
- Note variation in radius of curvature of both condyles
- Consequently, motion at knee does not occur about a single stationary axis
- Axis of motion moves posterior as the knee passes into flexion
The Patella
- Posterior aspect of patella articulates with patellar surface on the femur
- Divided by a central ridge into the smaller medial and larger lateral facets
- In flexion central ridge moves into the concave intercondylar groove (most stable position)
- Covered in a thick layer of hyaline cartilage which is thickest over central ridge
The Tibia
- Superior aspect of tibial condyles - covered in hyaline cartilage
- Divided by intercondylar eminence which is deficient of hyaline cartilage
- Note difference between 2 condyles…
- Medial - larger, longer and possibly narrower (reflects shape of femoral condyle)
- Lateral - rounder
- Lateral View - Tibial Condyles
- Medial tibial condyle is biconcave (AP and medio-laterally)
- Lateral tibial condyle is concave medio-laterally but is convex in an antero-posterior direction
Joint Capsule
- Simple capsule passing from one articular margin to next
- Replaced anteriorly by extensor mechanism – capsule blends with patella, tendon & ligament
Synovial Membrane
- Most extensive in the body attaching to the articular margins and lines the capsule for the most part
- However a number of folds and bursae are also present:
- Suprapatellar Bursa - Extension of synovial membrane under lower part of quadriceps - attached to base of patella and articular margin in femur
- The Alar Folds - Two folds of synovial membrane extending from the lower border of the patella into the intercondylar region of the femur
- Posteriorly the synovial membrane passes into the intercondylar area and around the cruciate ligaments (covering the medial, lateral and anterior surfaces)
Non Communicating Bursae and Nutritional Fat Pad
- Prepatella bursa
- Superficial infrapatella bursa
- Deep infrapatella bursa
- Suprapatella bursa
- Fat Pad
Ligaments
- There are a number of ligaments around the knee
- The 4 most important ones are:
- Medial (Tibial) collateral ligament
- Lateral (Fibular) collateral ligament
- (both extra-capsular)
- Anterior cruciate ligament
- Posterior cruciate ligament
- (both intra-capsular but extra-synovial)
Medial (Tibial) Collateral Ligament
- Broad flat band of fibrous tissue (approx 10 cm long)
- Multilayered - deep part blending with capsule & medial meniscus of knee
- Note lower part crossed by tendons of sartorius, gracilis and semitendinosus (separated by pes anserine bursa)
- Limits abduction and lateral rotation
Lateral (Fibular) Collateral Ligament
- Shorter rounded cord of fibrous tissue (approx 5 cm long)
- Stands clear of the joint (no capsular attachment)
- Limits adduction of tibia
Anterior Cruciate Ligament
- Extremely strong stabiliser of the knee
- Note direction and obliquity of fibres - passing upward, backward and laterally from its tibial attachment
- Prevents anterior displacement of tibia
Posterior Cruciate Ligament
- Larger of two - Crosses ACL
- Prevents posterior displacement of tibia
- Note direction and obliquity of fibres - upward, forward and medially from its tibial attachment
- Both the ACL and the PCL are found within the boundaries of the articular capsule (intra-capsular) but outside of synovial membrane (extra-synovial)
The Menisci
- Poor congruency of the femoral and tibial condyles makes joint inherently unstable
- Congruency and stability is improved by the presence of the 2 Menisci
- 2 pieces of fibrocartilage - triangular in cross section and semilunar in shape
Functions of the Menisci
- Improve congruency between tibia and femur due to shape
- Shock absorption - due to tissue type
- Weight dissipation - circumferentially - due to collagen orientation
- Assist with lubrication
Medial Meniscus
Larger of the 2
- Semilunar in shape
- Wider posteriorly than anteriorly
Attachments
- Intercondylar area - anterior to ACL and PCL attachments
- Peripherally - via the coronary ligament
- Blends with capsule & medial collateral ligament (Very firmly bound down - less mobile than the lateral)
Lateral Meniscus
- Smaller of the 2
- 4/5 of a circle in shape
Attachments
- Anterior horn attached to intercondylar eminence posterior to the ACL
- Peripherally via coronary ligament
- Posteriorly to intercondylar eminence anterior to medial meniscus
- Very mobile due to lack of attachment peripherally to collateral of knee - less susceptible to injury
Surface Markings of the Knee Joint
- Can be done by direct palpation of the joint line
- Or a line perpendicular to the tibia passing through a point midway between the apex of the patella and the tibial tubercle (anterior)
- Or a line perpendicular to the tibia 1cm above apex of head of fibula (posterior)