Description of the knee anatomy and related conditions.
1. Bony landmarks
The patella, the condyles of the femur, the condyles and tuberosity of the tibia, the head of the fibula, are all easily examined.
1.1. The patella
The limb being supported in the straight position of the knee, and the extensor muscles relaxed, the natural range of mobility laterally of the patella can be estimated. This is interfered with by muscular action in inflammatory conditions or by early tuberculous ulceration of the contiguous cartilages. The numerous longitudinal striae or sulci on the anterior surface of this bone can now also be detected. In these are embedded tendinous bundles of the rectus, so as to give firmer leverage. The fact that these fibers, thus tied down, are liable after stretching and tearing to fold in between the ends of the bone after fracture, is a ready explanation of the difficulty of ensuring bony union here. (Macewen.) The patella is separated from the tibia by a pad of fat and a deep bursa, save at its insertion. Owing to the lowest part of the patella being thus separated from the joint by fat, fracture here does not, necessarily, open the joint.
1.1.1. The bone has the following relation to the femur in different positions:
In extension, the patella rises over the condyles, and in full extension only the lower third of its articular surface rests upon that of the condyles; its upper two-thirds lies upon the bed of fat which covers the lower and front part of the femur.
In extreme flexion, as the prominent anterior surface of the condyles affords leverage to the quadriceps, the patella needs to project very httle; thus, only its upper third is in contact with the femur, its lower two-thu-ds now resting on the pad of fat between it and the tibia.
In semiflexion the middle third of the patella rests upon the most prominent part of the condyles. (Humphry.) While the bone now affords the greatest amount of leverage to the quadriceps, it is also submitted to the greatest amount of strain from this muscle, which is acting almost at a right angle to the long axis of the patella. This position may therefore be called the 'area of danger,' as, in a sudden and violent contraction, the patella may be snapped across by muscular action, aided by the resistance given by the condyles, in the same way as a stick is snapped across the knee. The amount of separation of the fragments in a fracture of the patella is due chiefly to the extent to which the lateral tendinous expansions of the vasti are torn; to a less degree to the hemorrhage from the numerous articular vessels and synovial effusion. The lower fragment is usually the smaller, and its fractured surface tilted forward; that of the upper one usually looks backward.
The patella, the largest of the sesamoid bones, ossifies by a center which appears from the third to the fifth year. The process is completed about puberty. The rareness with which necrosis and caries occur here, when the exposed situation of the bone is remembered, is partly explained by the density of its tissue, especially in front, and the intimate blending of the rectus fibers with its periosteum. When the knee-joint is bent, the trochlear surface of the femur can be made out, with some difficulty, underneath the quadriceps expansion. The upper and lateral angle of this surface forms a useful landmark (Godlee) as a line drawn from it to the adductor tubercle marks the level of the lower epiphysis of the femur.
1.1.2. Dislocation of the patella
The following anatomical facts account for this taking place much more frequently laterally:
The medial edge of the patella is more prominent, and thus more exposed to injury; it is also well supported, as is seen when, the parts being relaxed, the fingers are insinuated beneath each border.
The pull of the extensor upon the patella, ligamentum patellae, and tibia is somewhat laterally, as the tibia is directed a little laterally to the femur, to meet the medial direction of this bone; the femora being directed medially here, to bring the knee-joints nearer the center of gravity, and, so, counterbalance their wide separation above at the pelvis. The lateral pull of the quadriceps upon the patella is, in all normal action of the muscle, counteracted by the space taken in the trochlear surface by the lateral condyle, this being wider and creeping up higher, and having a more prominent and thus protective lip. In violent contraction, however, these counteracting points may be overcome.
The condyles of the femur and tibia. It should be noted that on the medial side the prominence of the medial epicondyle of the femur is well marked, and that of the tibia is less so, while on the lateral side this condition is reversed. Descending to the lateral condyle of the tibia, the ilio-tibial band of the fascia lata can be traced. The more distinct lateral condyle is a good landmark for opening the joint in amputation and excision. It also indicates the lower level of the synovial membrane of the knee-joint.
Farther back are the biceps and fibular collateral (long external lateral) ligament. The gap on the medial side between the femoral and tibial condyles is the place for feeling for a displaced medial fibro-cartilage in 'internal derangement' of the knee, and also for 'lipping' in suspected osteoarthritis. On each femoral epicondyle, posteriorly, in a thin subject, can be felt its tubercle, which gives attachment to the collateral ligament. Owing to their being placed behind the – center of the bone, these ligaments become tight in extension. On the upper and posterior part of the medial femoral epicondyle the adductor tubercle and the vertical tendon of the adductor magnus can be felt during flexion. This bony point is a guide to the lower epiphysis. The medial aspect of this epicondyle faces practically in the same direction as the head of the femur.
1.2. Ligamentum patellae and tuberosity of tibia
These, in a well-formed leg, should, with the center of the ankle-joint, be all in the same straight line, a useful point in the adjustment of fractures. (Holden.) Behind the upper half of the ligament is the infrapatellar pad of fat; below, the lower half is separated from the tibia by a deep bursa. The tuberosity (tubercle) of the tibia is on a level with the head of the fibula.
1.2.1. Prepatellar bursa
This usually protects the lower part of the patella and upper part of the ligamentum patellae. It is liable to be enlarged in those who habitually kneel much, the enlargement being either fluid or solid, and occasionally, in tertiary syphilis. Its close connection with the patella and, at the sides, with the joint itself, is to be remembered in infective inflammations of the bursa. Usually the deep fascia, passing off from the sides of the patella upward to the thigh and downward to the leg, serves to conduct inflammation away from the joint.
1.2.2. Synovial membrane
This, the largest of the synovial membranes, forms a short cul-de-sac above the patella, between the quadriceps extensor and the front of the femur, this process reaching about 2.5 cm. (1 in.) above the trochlear surface of the femur. At its highest point this cul-de-sac communicates with an other synovial, bursa-like sac lying between the quadriceps and front of the femur. Thus, synovial membrane will usually be met with 6.2 cm. (2.5 in.) or more above the trochlear surface or the upper border of the patella when the limb is extended. Flexing the joint draws the membrane down very slightly. During extension, the above pouch is supported by the articularis genu (subcrureus). Traced downward, the membrane reaches the level of the head of the tibia, being separated in the middle line from the upper part of the ligamentum patellae by fat. It here gives off to the intercondyloid notch the patellar synovial fold (ligamentum mucosum), with its free lateral prolongations, the alar folds (ligamenta alaria). These three so-called ligaments contain fat, the processes not only padding gaps, but also meeting concussions.
The enlargement of these processes, under conditions not yet understood, may certainly be a cause of 'internal derangement,' and simulate a loosened meniscus. But the synovial membrane of this joint is not only the largest: it is also the most complicated, a fact accounting for the grave peril of infective arthritis, and the well-known difficulty of effective drainage and cleansing this joint. Thus it passes over the greater portion of the crucial ligaments, but the posterior surface of the posterior crucial, which is connected by means of fibro-areolar tissue to the front of the ligamentum postioum, and the lower portions of both crucial ligaments, where they are united together, of course cannot receive a complete covering from the membrane., (Morris.)
From the above ligaments, the membrane is conducted, lining the lower part of the capsule and other ligaments, to the semilunar cartilages, first over their upper surfaces to their free borders, and then along their under surfaces to the tibia. Between the lateral of these and the upper and back part of the tibia is a prolongation of the synovial membrane to facilitate the play of the popliteus tendon.
Finally, amid the complications of this synovial membrane, its communication with some of the bursae mentioned below, and occasionally with the superior tibio-fibular joint, is to be borne in mind. In effusion, the bony prominences are obliterated, and the patella 'floats.' The knee-joint is easily opened by free lateral incisions lying midway between the margins of the patella and the tuberosities of the condyles, drainage-tubes being passed so as to meet above the patella. The above-mentioned complications of the synovial membrane show that such drainage will be often inadequate. By passing a director to the back of the joint and cutting down upon it carefully from the popliteal space, better drainage will be given, but opening the joint by an anterior flap is needed where the above fail, and, even then, cleansing of the numerous deep recesses is obviously difficult.
1.3. Structures on the head of the tibia
From before backward these are:
Anterior end of medial meniscus (fibro-cartilage).
Lower attachment of anterior crucial.
Anterior end of lateral meniscus blending with (3).
Posterior extremity of lateral meniscus giving off a strong process to posterior crucial.
Posterior extremity of medial meniscus.
Posterior crucial ligament. Menisci. These serve as buffer-bonds and cushions between the contiguous bones. The more frequent displacement of the medial is explained by (a) its greater fixity, and, therefore, its feeling strains more. Thus, in addition to weaker attachments to the coronary and transverse ligaments, it is connected all along its convex border with the inside of the capsule, and strongly with the tibial collateral ligament. The lateral meniscus, on the other hand, is more weakly attached to the capsule, especially opposite to the popliteus tendon, and has no tie to the fibular collateral ligament. (b) When, in the erect position, the knee-joint is rotated laterally and slightly flexed, a common position, an especial strain is thrown upon the very important tibial collateral ligament, and from the above-mentioned connection, on the medial meniscus also.
2. Position of knee-joint in disease
In inflammatory effusion, the position which best accommodates the collection of fluid is one of moderate flexion of the knee, the ligaments being now mainly relaxed. Later on, when the ligaments are softened, the hamstrings obstinately displace the leg backward, the tibia being rotated laterally by the biceps. The antero-posterior displacement is always more marked than the lateral. In straightening an anchylosed joint, the resistance of the shortened lateral, crucial, and posterior ligaments, and the facility with which a softened upper epiphysial line of the tibia may give way, must never be forgotten. Erasion and excision. The extent and complications of the synovial membrane render attention to the following points imperative: (1) Free exposure of the joint usually by an anterior curved incision, the medial extremity of which must not damage the great saphenous vein. (2) The extent of the pouch under the quadriceps, it may be for 5 cm. (2 in.) above the patella, and the lateral recesses under the vasti. The pouches at the back of the joint are far more difficult to deal with, viz., the partial covering of the posterior crucial ligament, the proximity of the popliteal artery, the pouches in relation to the popliteus, gastrocnemii, and back of the femoral condyles. In erasion, the cartilage and bone, where diseased, are removed with a gouge. Owing to the removal, in addition to the synovial membrane, of the fibro-cartilages, and crucial ligaments, and the damage to lateral and patellar ligaments, there is a most obstinate tendency to flexion afterward. In excision, to avoid injury to the epiphysis, the section of the femur should not pass higher than through the upper third of the trochlear surface. Of the tibia, only 12 mm. (1/2 in.) should be removed.
2.1. Genu valgum
Here the natural angle at which the femur inclines medially to the tibia is increased. As shown by the late v. Mikuliez, this is due to an abnormal growth downward of the medial part of the femoral diaphysis, the epiphysial line being gradually altered from one at right angles to the shaft to one which runs obliquely from without downward and medially. The femur is not only elongated on its medial side, but bent at its lower end, the concavity of the curve being lateral. Other changes have to be remembered. Pes valgus very commonly coexists, and in the tibia, there may be a compensatory curve, the concavity being medial, in the lower third, or an analogous alteration in the line of the upper epiphysis may be present, its direction being no longer at a right angle with the shaft, but oblique. In Sir W. Macewen's supra-condyloid osteotomy, a longitudinal incision, about 3.7 cm. (1.5 in.) long is made where the following lines meet, viz., one transverse, a finger's breadth above the upper margin of the lateral condyle, and one longitudinal, 1.2 cm. (0.5 in.) in front of the adductor magnus tendon. The bone is divided in front of the genu suprema and above the superior medial articular artery, above the epiphysial line and behind the upward extension of the synovial membrane under the quadriceps.
The following bursae about the knee-joint must be remembered. Some, it will be seen, are much more constant than others:
A. In front. (1) One between the patella and skin, the bursa prepatellaris subcutanea; (2) a deeper one between the ligamentum patellae and the upper part of the tibia; (3) between the skin and the lower part of the tuberosity of the tibia. This is not constant.
B. On the medial side of the knee. (1) One between the medial head of the gastrocnemius and medial condyle, often extending between the above muscle and the semi-membranosus. This is the largest of the bursae about the knee-joint, and, after adult life, usually communicates with the knee-joint. But, owing to the narrow communication, it is rarely possible, when the parts are relaxed by flexion of the joint, to empty the cyst. For its removal, a straight incision is made over the most prominent part of the swelling, its neck found by drawing aside the tendons. A ligature is then pushed high up around the neck, and the cyst cut away. (2) One superficial to the tibial (collateral) ligament, between it and the tendon of the sartorius, gracilis, and semi tendinosus (3) One beneath the ligament, between it and the tendon of the semi-membranosus. (4) One between the medial condyle of the tibia and the semi-membranosus. (5) One between the semi-membranosus and semi-tendinosus. Of the above bursae, the first two alone are constant. The second and third are often one bursa prolonged.
C. On the lateral side of the knee. (1) One between the lateral head of the gastrocnemius and the condyle; (2) one superficial to the fibular collateral ligament between it and the biceps tendon; (3) one under the ligament between it and the popliteus tendon; (4) one between the popliteus tendon and the lateral condyle of the femur. This is usually a diverticulum from the synovial membrane.
The following explanations may be given of an inflamed knee-joint usually taking the flexed position:
By experimental injections, Braune found that the capacity of the synovial sac reaches its maximum with a definite degree of flexion, i. e., at an angle of twenty-five degrees.
As the same nerves supply the synovial membrane and the muscles which act upon the joint, reflex spasm of the flexors will help to explain the flexed position. (Hilton.)
2.2. Anastomoses around the front and sides of the knee-joint
The most important of these take the form of three transverse arches. (1) The highest passes through the quadriceps fibers just above the upper edge of the patella. It is formed by a branch from the deep division of the genu suprema (anastomotica magna) and one from the lateral circumflex and superior lateral articular. The middle and lowest arches lie under the ligamentum patellae. (2) The middle arch, formed by branches from the genu suprema and superior medial articular on the medial side, and the inferior lateral articular, on the lateral, runs in the fatty tissue close to the apex of the patella. (3) The lowest arch lies on the tibia just above its tuberosity, and results from the anastomosis of the recurrent tibial and the inferior medial articular. Seven arteries thus take place in this series of anastomoses.
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