The knee-joint is a ginglymus or hinge-joint. It consists of three articulations : two condyloid joints between the condyles of the femur and the semilunar cartilages (menisci) and condyles of the tibia; and a third between the patella and the femur, partly plane, but not completely so, since the articular surfaces are not mutually adapted to each other, so that the movement is not a simple gliding one. This view of the construction of the knee-joint receives confirmation from a study of the articulation in some of the lower mammals, where, corresponding to these three subdivisions, three synovial cavities are found, either distinct from one another or connected by small communications. This view is further rendered probable by the existence in the middle of the joint of the two cruciate ligaments, which may be regarded as the collateral ligaments of the medial and lateral joints. The existence of the infrapatellar fold of synovial membrane indicates that the femoropatellar joint was originally distinct from the lateral and medial femorotibial joints.
The joint is partly subdivided by two semilunar fibrocartilages, which are placed between the femur and the tibia.
The ligaments of the joints are; capsular, anterior and posterior cruciate, ligamentum patellae, transverse, oblique posterior and arcuate, coronary, and medial and lateral collateral.
The articular capsule (figs. 560, 561) consists of a capsular ligament, which is strengthened in almost its entire extent by bands inseparably connected with it, but above and in front, under cover of the tendon of the quadriceps femoris, it is represented only by the synovial membrane. Its chief strengthening bands are derived from the fascia lata and from the tendons surrounding the joint. In front, expansions from the vasti and from the fascia lata and its iliotibial tract fill the intervals between the collateral and anterior ligaments, constituting the medial and lateral patellar retinacula. Behind, the capsular ligament consists of vertical fibers, which are attached to the condyles of the femur and tibia and to the lower border of the popliteal surface of the femur. The posterior part of the capsular ligament lies behind the cruciate ligaments, which, however, are excluded from the joint-cavity by the synovial membrane. The oblique posterior ligament; which is augmented by fibers derived from the tendon of the semimembranosus, strengthens the posterior aspect of the capsule. Laterally a prolongation from the iliotibial tract fills the interval between the- oblique posterior and the lateral (fibular collateral) ligaments, and partly covers the latter. Medially expansions from the sartoruis and semimembranosus pass upwards to the medial (tibial collateral) ligament and strengthen the capsule.
The arcuate ligament of the knee (arcuate popliteal ligament) (fig. 561.) is an arched bundle of fibers which varies somewhat in strength and appearance. It is attached to the lateral condyle of the femur and passes downwards to fuse with the capsular ligament. Two bands, an anterior and a posterior, converge from the upper and lower extremities of the arcuate ligament; they unite below to form the retinacmlum of the ligament, which is fixed to the styloid process of the head of the fibula. The anterior band of this retinaculum is sometimes described as the short lateral (fibular collateral) ligament and the popliteus emerges from under cover of the posterior band (fig. 561). In fig. 561, the oblique zipper border of the arcuate ligament shows an upward concavity, but in many cases it is straight and blended with the capsule. Some anatomists describe the ligament as a A-shaped band, consisting of femorotibial and femorofibular parts, which straddle the emerging tendon of the popliteus muscle.
The lateral (fibular collateral) ligament (fig. 562) is a strong, rounded cord, attached, above to the lateral epieondyle of the femur, immediately above the groove for the tendon of the popliteus; below, to the lateral side of the head of the fibula, in front of the styloid process. The greater part of it is hidden by the tendon of the biceps femoris, but the tendon divides at its insertion into two parts, which are separated by the ligament. Deep to the ligament are the tendon of the popliteus and the inferior lateral genicular vessels and nerve. The ligament has no attachment to the lateral semilunar cartilage.
The cruciate ligaments are of considerable strength, and are. situated in the middle of the joint, nearer to its posterior than its anterior surface. They axe called cruciate because they cross each other somewhat like the limbs of the letter X; and have received the names anterior and posterior, from the position of their attachments to the tibia.
The posterior cruciate ligament (fig. 563) is stronger, but shorter and less oblique in its direction, than the anterior. It is attached to the posterior part of the intercondylar area of the tibia, and to the posterior extremity of the lateral semilunar cartilage; it passes upwards, forwards and medially, to be attached to the lateral surface of the medial condyle of the femur.
The semilunar cartilages (menisci) (fig. 564) are two crescent-shaped lamellae, which serve to deepen the surfaces of the upper end of the tibia for articulation with the condyles of the femur. The peripheral border of each cartilage is thick and convex; the opposite border is thin, concave, and free. The upper surfaces of the cartilages are smooth and concave, and in contact with the condyles of the femur; their lower surfaces are smooth and flat, and rest upon the tibia.
Each covers approximately the peripheral two-thirds of the corresponding articular surface of the tibia.
The medial semilunar cartilage is nearly semicircular in form and is broader behind than in front; its anterior end is attached to the anterior part of the intercondylar area of the tibia, in front of the anterior cruciate ligament, its posterior fibers being continuous with the transverse ligament; its posterior end is fixed to the posterior hart of the intercondylar area of the tibia; between the attachments of the lateral semilunar cartilage and the posterior cruciate ligament. Its peripheral border is attached to the capsular ligament and is firmly adherent to the deep surface of the medial ligament of the knee joint.
The transverse ligament (fig. 564) connects the anterior convex margin of the lateral to the anterior end of the medial semilunar cartilage; its thickness varies considerably in different subjects, and it is sometimes absent.
The coronary ligaments are merely portions of the capsule, connecting the periphery of each semilunar cartilage with the margin of the head of the tibia.
Bursae -The bursae near the knee-joint are the following:
In front there are four bursae : a large one (subcutaneous prepatellar bursa) is interposed between the lower part of the patella and the skin, a small one (deep infrapatellar bursa) between the upper part of the tibia and the ligamentum patellae, a third (subcutaneous infrapatellar bursa) between the lower part of the tubercle of the tibia and the skin, and a fourth, of large size (suprapatellar bursa), which usually communicates with the knee-joint, between the anterior surface of the lower part of the femur and the deep surface of the quadriceps femoris (fig. 565). Laterally there are four bursae : (1) one (which sometimes communicates with the joint) between the lateral head of the gastrocnemius and the capsule; (2) one between the lateral ligament and the tendon of the biceps femoris; (3) one between the same ligament and the tendon of the popliteus (this is sometimes only an expansion from the next bursa) : (4) one between the tendon of the popliteus and the lateral condyle of the femur, usually an extension from the synovial membrane of the joint. Medially, there are five bursae : (1) one between the medial head of the gastrocnemius and the capsule this sends a prolongation between the tendon of the medial head of the gastrocnemius and the tendon of the semimembranosus and often communicates with the joint; (2) one (tibial intertendinous bursa) superficial to the medial ligament, between it and the tendons of the sartorius, gracilis, and semitendinosus; (3) one deep to the medial ligament, between it and the tendon of the semimembranosus (this is sometimes only an expansion from the next bursa) (4) one between the tendon of the semimembranosus and the medial condyle of the tibia; (5) occasionally there is a bursa between the tendons of the semimembranosus and semitendinosus.
The arteries supplying the joint are the descending genicular (a. genu suprema), the genicular branches of the popliteal, the recurrent branches of the anterior tibial, and the descending branch from the lateral circumflex femoral branch of the arteria profunda femoris.
Movements.-The movements which take place at the knee-joint are flexion and extension of the leg, and, in certain positions of the joint, medial and lateral rotation. The movements of flexion and extension differ from those of a typical hinge-joint, such as the elbow, in that (a) the axis round which motion takes place is not a fixed one, but shifts forwards during extension and backwards during flexion of the leg on the thigh; (b) the end of extension is accompanied by lateral rotation, and the beginning of flexion by medial rotation, of the leg. When the leg is fully flexed the posterior parts of the tibial surfaces are in contact with the posterior parts of the articular surfaces of the femoral condyles. When the leg is extended the tibia and its semilunar cartilages glide forwards on the femoral condyles, and the axis on which the movement takes place gradually shifts forwards. The parts of the femoral condyles on which this movement takes place are parallel to each other and have similar curvatures.
The lateral tibial surface is brought almost to rest posterior cruciate ligament. In this position, however, the collateral and the oblique posterior ligaments and possibly the anterior fibers of the posterior cruciate ligament have not reached their limit of tension and the muscular action is continued in order to ensure the stability of the joint in the position of extension. No further movement is possible round a transverse axis, but the tibia is able to rotate laterally round a vertical axis, which passes, approximately, through the center of the lateral condyle. On account of the position of the axis the traverse of the medial condyle is greater than that of the lateral at this stage, and the articular surface of the medial femoral condyle is therefore, the more extensive of the two. This last phase of extension is described as the ‘screwing home’ or locking movement of the joint; when it is completed the collateral, the oblique posterior and, probably, also the, cruciate ligaments are all at their limit of tension, and the anterior edges of the semilunar cartilages rest in the grooves which separate the patellar articular surface from the tibial articular surfaces of the femur. The movement of flexion must be initiated by medial rotation of the tibia in order to unlock the joint. If the foot be placed firmly on the ground, rotation of the tibia is impossible, and the femur then rotates, medially during the last phase of extension, and laterally previous to flexion.
In addition to the rotatory movements associated with the completion of extension and the initiation of flexion, medial and lateral rotation of the leg can be effected when the joint is partly flexed; these movements are freest when the leg is bent at right angles with the thigh.
Movements of the patella.-The articular surface of the patella, is indistinctly divided into seven facets-upper, middle, and lower horizontal pairs, and a medial perpendicular facet (fig. 567). In extreme flexion of the knee-joint the patella is in contact with the semilunar surface on the lateral part of the medial condyle of the femur, and the highest of the three lateral facets of the patella with the front part of the lateral condyle. As the leg is carried from the flexed to the extended position, horizontal facets are successively brought into contact with the patellar surface of the femur. In the extended position, when the quadriceps femoris is relaxed, the patella lies loosely on the front of the lower end of the femur.
In the attitude of ‘attention’ the weight of the body falls in front of a line carried across the centers of the knee-joints, and therefore tends to produce over-extension of the articulation; but this is prevented by the tension of the cruciate, oblique posterior and collateral ligaments.
Muscles producing the movements:
- Flexion.-Biceps femoris; Semitendinosus, Semimembranosus, Popliteus; Gracilis, Sartorius, Gastrocnemius, Plantaris.
- Extension.–Quadriceps femoris.
- Medial rotation of the leg.-Popliteus, Semitendinosus, Semimembranosus, Gracilis, Sartorius.
- Lateral rotation of the leg.-Biceps femoris.
Applied Anatomy.-From a consideration of the construction of the knee-joint, it would at first sight appear to be one of the least secure. joints in the body. It is formed between the two longest bones, and therefore the amount of leverage which can be brought to bear upon it is considerable; the articular surfaces are but ill-adapted to each other, and the range of motion which it enjoys is great. All these circumstances tend to render the articulation insecure; nevertheless, on account of the powerful ligaments which bind the bones together, the joint is one of the strongest in the body, and dislocation from traumatism is a rare occurrence.
One or other of the semilunar cartilages may be torn or detached, tearing being the commoner accident; when a cartilage is torn it is the thin portion which is separated; the torn part projects into the interior, and leads to locking of the joint in the semiflexed position. The accident is produced by a twist of the leg when the leg is flexed, and is accompanied by a sudden pain, and fixation of the knee in the flexed position. The semilunar cartilage may be displaced either towards the tibial intercondylar eminence, so that it becomes lodged in the intercondylar notch, or to one side, so that it projects beyond the margin of the two articular surfaces. The medial semilunar cartilage is much more commonly affected than the lateral because (1) it is firmly adherent to the medial ligament; (2) it is the more firmly attached to the tibia; (3) during the slight rotation of the joint it moves through a greater interval than the lateral cartilage.
The cruciate ligaments are sometimes ruptured, but great violence is necessary to produce this injury. When the anterior is torn the tibia can be pushed forwards; when the posterior is torn the tibia can be pulled backwards.
Acute synovitis, the result of traumatism, is of frequent occurrence in the knee-joint. When the cavity is distended with fluid, the swelling shows itself above and at the sides of the patella, reaching about 2.5 cm., occasionally 5 cm. or more, above the patellar surface of the femur, and extending a little higher under the vastus medialis than under the vastus lateralis. The lower level of the synovial membrane is just below the upper end of the tibia.
The close relationship of the head of the fibula to the synovial membrane of the knee-joint explains the risk of opening that joint in removing the head of the fibula.
The bursae about the knee-joint are sometimes the seat of enlargement. The bursa between the front of the patella and the skin is frequently affected in those who are in the habit of kneeling, and the condition is known as ‘housemaid’s knee.’ The bursa beneath the semimembranosus tendon also occasionally becomes enlarged, and forms a fluctuating swelling at the back of the knee. During extension the swelling is firm and tense; but during flexion it becomes soft, and, as the bursa often communicates with the synovial cavity of the joint, the fluid it contains can be made to disappear by pressure when the knee is flexed.
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