The Knee Joint By Prof. Dr. Muhammad Imran Qureshi Structurally, it is the Largest and the most complex joint in the body because of the functions that it performs: Allows mobility (flexion/extension) Weight bearing Shock absorbing Must have some intrinsic stability Articulating Bones: Femur (Lower end) Tibia (Upper end) Patella (Posterior aspect) Not part of the knee joint: Fibula - it does not articulate with the femur or the patella Important Structural Landmarks: Femur (lateral and medial condyles) convex surface Tibia (lateral and medial condyles)...concave surface Tibial tuberosity Patella Fibula (technically not part of knee, but it is an important related structure) Type Of the Joint: Simple? Compound? Complex? Modified Compound Synovial / diarthrotic Structurally considered compound and bi condyloid Two fibrocartilage menisci occur within the joint cavity (absorb stress) Variety Of the Joint: Primarily acts as a hinge joint. The Hinge is Modified. Passive ROTATION takes place during the normal process of full Extension (Screw Home or Locked Position). Active ROTATION occurs in a flexed knee (changing direction while walking / running). Articular Surfaces: They are Covered with Hyaline Cartilage, They are: 1. The TWO femoral condyles, separated by a deep notch posteriorly, are fused anteriorly into a trochlear groove. In lateral profile, the curve of the femoral condyles is Cam shaped. It is flatter on the end of the femur and more highly curved at the free posterior margin of each condyle.
The distal surface of the medial condyle is narrower, longer, and more curved than the lateral condyle. 2. TWO separate Concave articular facets on the upper surface (Plateau) of the tibial condyles. The medial facet is larger and lies wholly on the upper surface of the condyle. The lateral smaller facet becomes slightly convex at the back where it curves back over the posterior margin of the tibial condyle. 3. The articular surface of the Patella has a vertical ridge that divides it into: A Larger lateral and a smaller medial surface. The smaller medial surface is further divided by a vertical ridge into two smaller areas. The larger lateral surface remains in contact with the respective femoral condyle in all ranges of movement. However, in extension, most medial of the three surfaces is not in articulation with the femoral condyle. In flexion, this most medial of the three surfaces glides into articulation with the medial condyle, and the middle of the three surfaces lies free in the intercondylar notch of the femur. Fibrous Capsule: On the Femur: It is attached to the articular margins of the bone EXCEPT at TWO places: 1. At the back, it is attached to the intercondylar ridge at the lower limit of the popliteal surface of the bone, and 2. On the lateral surface of the lateral condyle, it is attached above the pit and
groove for the popliteus tendon, thus enclosing it i.e. rendering it intracapsular. On the Tibia: It is attached around the margins of the plateau EXCEPT at TWO places: 1. Posteriorly, it is attached to the ridge between the condyles at the lower end of the groove for Posterior Cruciate Ligament, and 2. Laterally, instead of attaching to the tibia, it is prolonged down over the popliteus tendon to the styloid process of the fibular head, and this part of the capsule is also known as the Arcuate Popliteal Ligament. On the medial side part of the capsule is thickened to form the deep part of the medial ligament (short internal lateral ligament); it is firmly attached to the medial meniscus. On the lateral side a much less marked thickening (short external lateral ligament) may occur under the lateral ligament, but such a thickening here is frequently absent. Elsewhere around the convexity of both menisci the capsule being attached to the femur above and tibia below, is thin and lax and sometimes known here as the coronary ligament. Synovial Capsule / Membrane: On the femur it is attached all around the articular margin. Thus it lines the intercondylar notch, and on the lateral condyle is separated from the capsule by the attachment of popliteus tendon, which lies between the two. On the tibia it is attached to the articular margins of medial and lateral
condyles, and from these margins it is reflected forwards over the anterior cruciate ligament. A fold extending from here to the inferior margin of the patella is known as the infrapatellar fold; an alar fold extends both medially and laterally from it. These infrapatellar fold and alar folds are produced by an extra synovial fat pad and they adapt their shape to the contours of the bones in different positions of the knee. By keeping the synovial membrane in contact with the articular surfaces of the femoral condyles, they act as Haversian fat pads. The synovial cavity of the knee joint communicates with certain bursae. i. It communicates with the suprapatellar bursa. ii. A herniation of synovial membrane beneath the rounded tendon of popliteus produces the banana-shaped popliteus bursa lying in the gutter between the tibia and the head of the fibula. iii. Bursae beneath the Gastrocnemii: The bursa beneath the medial head of gastrocnemius always and that beneath the lateral head usually, communicate with the joint. The bursa under the medial head of gastrocnemius usually also communicates with the semimembranosus bursa, thereby communicating it with the cavity of the knee joint. Ligaments of the joint: Extracapsular Ligaments: The patellar retinacula are fibrous expansions from the quadriceps tendon and from the lower margins of vastus medialis and lateralis. They extend from the patella to the lower margins of the tibial condyles; In front of the collateral ligaments they blend with the capsule; further anteriorly below the patellar attachment of the capsule, they are attached to the margins of the patellar ligament. Below the patella the capsule is replaced by the very strong and dense patellar ligament, which by its attachment to the tibial tuberosity keeps the patella at a constant distance from the tibia.
The tibial collateral (medial) ligament : It consists of superficial and deep parts. The superficial part is attached to the femoral epicondyle below the adductor tubercle and to the medial surface of the tibia about 4 cms below the knee (about 12 cms long). Its anterior margin lies free (not attached to the medial meniscus), while its posterior margins converge to be inserted into the medial meniscus; only these marginal fibres of the superficial part are attached to the meniscus. The condyle of the tibia is free to rotate beneath the upper part of the ligament. From its tibial attachment the ligament slopes back a little as it passes up to be inserted behind the axis of flexion of the femoral condyle It is thus drawn taut by (and limits) extension of the knee and its terminal 'screw-home rotation. The deep part of the medial ligament is the short internal lateral ligament. It is under cover of the superficial part and is attached above and below to the femur and tibia just beyond the articular margins and centrally to the periphery of the medial meniscus. The fibular collateral (Lateral) ligament: It is attached to the lateral epicondyle of the femur and slopes down and back to the head of the fibula. It lies free from the capsule and lateral meniscus. It is round and cord like, about 5 cm long. It is attached just behind the axis of flexion of the femoral condyle and is drawn taut by (and limits) extension and the terminal 'screw-home' movement of the knee. The oblique popliteal ligament: It is a thick, strong rounded band. It is a lateral expansion from the insertion of semimembranosus, which slopes up to the popliteal surface of the femur. It blends with the capsule above the lateral condyle of the femur, and in the intercondylar notch rather above its margin, so that a prolongation upwards of synovial membrane extends a little on the popliteal surface of the femur. The obliquity of this ligament limits rotation-extension in the 'screwhome' or locked position. The arcuate popliteal ligament: From its attachment to the styloid process of the head of the fibula it can be traced upwards to blend with the capsule, sometimes as far as the lateral
condyle of the femur. Derived from the tendon of the biceps femoris muscle, this ligament arches over the popliteus tendon as it emerges from the capsule, some popliteus muscle fibres also get attached to it. Intracapsular Ligaments: The Cruciate Ligaments consist of a pair of very strong ligaments connecting tibia to femur. They lie within the capsule of the knee joint, but not within the synovial membrane. It appears as if they had been herniated into the synovial membrane from behind, carrying forward over themselves a fold which invests their anterior and lateral surfaces but leaves their posterior surfaces uncovered. They are named from their tibial origins. The anterior cruciate ligament is attached to the anterior part of the tibial plateau in front of the tibial spine and extends upwards and backwards to a smooth impression on the lateral condyle of the femur well back in the intercondylar notch. (AL) The posterior cruciate ligament is attached to the posterior part of the tibia between the condyles and from the uppermost part of the adjacent posterior surface. It passes forwards medial to the anterior cruciate ligament and is attached to a smooth impression on the medial condyle of the femur well forward in the intercondylar notch. (PM) The two cruciate ligaments cross like the limbs of the letter X. They are essential to the antero posterior stability of the knee joint, especially in the flexed position. The posterior cruciate ligament prevents the femur from sliding forwards off the tibial plateau. In the weightbearing flexed knee it is the only stabilizing factor for the femur and its attached quadriceps. In walking downhill or downstairs the upper knee is flexed and weight-bearing while the lower knee is straight as its foot reaches down to find support. Thus with a ruptured posterior cruciate ligament the patient leads with the damaged leg at each downward step, keeping the weight-bearing knee extended. The anterior cruciate ligament prevents backward displacement of the femur on the tibial plateau, but this is unlikely to happen. The anterior cruciate ligament has a much more important role, that of limiting extension of the lateral condyle of the femur and then causing medial rotation of the femur in the 'screw-home' position of full extension. The menisci, ( semilunar cartilages), are composed of fibrocartilage. They are basically C-shaped, triangular in cross section and avascular except at their attachments.
The medial meniscus is the larger and is really comma-shaped, with an open curve whose ends or horns enclose the horns of the lateral meniscus. The anterior horn of the medial meniscus (the narrow end of the comma) is attached to the intercondylar area of the tibia in front of the anterior cruciate ligament, while its broader posterior horn is similarly attached in front of the posterior cruciate ligament. The lateral meniscus is also C shaped but its horns are fairly close to one another, attached to the intercondylar area of the tibia immediately in front of and behind the intercondylar spine. They are not fused with the lateral ligament. Its posterior convexity is slung by fibrous tissue ligaments to the femur. These slings are attached to the medial condyle of the femur in front of and behind the attachment of the posterior cruciate ligament, forming the anterior and posterior meniscofemoral ligaments. The circumference of the meniscus is attached by very lax capsule (the coronary ligaments) to the articular margins of femur and tibia except beneath the tendon of popliteus. Here there is a gap in the coronary ligament; through it the popliteus tendon and bursa pass. The posterior convexity of the lateral meniscus receives the insertion of a flat tendon derived from the upper half of the popliteus muscle. The transverse ligament passes across between the anterior horns of the medial and lateral menisci. The delicate capsule is attached to it. Blood Supply of the joint: The capsule and joint structures are supplied from the anastomoses around the knee. The chief contributors are the five genicular branches of the popliteal artery, of which the middle genicular supplies the cruciate ligaments. Nerve Supply of the joint: In accordance with Hilton's law the joint is supplied from the femoral, through its branch to the vastus medialis, from the sciatic through genicular branches of the tibial and common peroneal, and from the obturator nerve through twig from its posterior division. The menisci are virtually devoid of sensory fibres. Movements of the joint: Extension: It is performed by the quadriceps and is limited by the tension of the anterior cruciate ligament, the oblique popliteal ligament and the collateral ligaments, but these four ligaments do not tighten simultaneously. As the knee moves into full extension the anterior cruciate ligament is the first to become taut. Extension of the lateral condyle of the femur is thus terminated. Further extension of the medial condyle is made possible by passive rotation forwards of the lateral condyle around the radius of the taut anterior cruciate ligament. This forces the medial condyle to glide backwards into its own full extension. That is why the medial condyle has a longer and more curved articular surface than the lateral condyle in order to obtain the final degrees of complete extension.
This medial rotation of the femur on the tibial plateau tightens the oblique popliteal ligament; the medial and lateral ligaments of the knee joint are set slightly obliquely and are tightened simultaneously. All three become taut and limit further rotation. This 'screw-home' movement is said to lock the joint. In this position the joint is slightly hyperextended and all four ligaments are taut - the anterior cruciate preventing further extension and the other three also preventing further rotation. The knee is completely rigid. These rotatory movements, which may amount to 15 or so, are purely passive and result from the slanting pull of the obliquely set ligaments. They occur whether the extending force on the knee is active (quadriceps contraction) or passive, and have been shown radiologically to begin at a much earlier stage than the final 'screw-home'. The lowest fibres of vastus medialis (those attached at right angles to the side of the patella) are of the very great importance Flexion: Flexion is performed by the hamstrings and this is limited to about 150 o by compression of the soft parts behind the knee. From the 'screw-home' or 'locked' position lateral rotation of the femur must precede flexion; this lateral rotation is produced by the popliteus. The 'untwisted' knee can now be flexed by the hamstrings. Rotation: In the flexed position all the above four ligaments are relaxed, and a smaller femoral surface articulates with the tibial plateau; thus active rotation is possible and is produced by the hamstrings contracting alternately. Stability Of the joint: It depends upon THREE factors: Bony Factors Ligamentous Factors, and Muscular Factors Bony Factors: Bony contours contribute nothing to the anteroposterior stability of the knee joint. However, the spine of the tibia prevents sideways gliding of femur on tibia. The lateral condyle of the femur extends slightly anterior to the medial condyle and thus prevents lateral displacement of the patella by the pull of the quadriceps femoris. Ligamentous Factors: The cruciate ligaments are indispensable to anteroposterior stability in flexion. The collateral and oblique popliteal ligaments provide lateral stability and stability in extension. Muscular Factors: Posteriorly, some stability is provided by the two heads of gastrocnemius.