The knee is the largest joint in the body. It is rightly described as a ginglymoid joint, but there is also an arthrodial element; for, in addition to flexion and extension, there is a sliding backward and forward of the tibia upon the femoral condyles, as well as slight rotation round a vertical axis. It is one of the most superficial, and, as far as adaptation of the bony surfaces goes, one of the weakest joints, for in no position are the bones in more than partial contact.

Class. - Diarthrosis.

Subdivision. - Ginglymus.

Its strength lies in the number, size, and arrangement of the ligaments, and the powerful muscles and fascial expansions which pass over the articulation and enable it to withstand the leverage of the two longest bones in the bodj\ It may be said to consist of two articulations with a common synovial membrane - the patello-femoral and the tibio-femoral, the latter being double. It is composed of the condyles and trochlear surface of the femur, the condyles of the tibia, and the patella, united by the following ligaments, which may be divided into an external and internal set:

External

  1. Fibrous expansion of the extensors.
  2. Articular capsule.
  3. Oblique popliteal ligament.
  4. Fibular collateral.
  5. Tibial collateral.
  6. Ligamentum patellae:

Internal

  1. Anterior crucial.
  2. Posterior crucial.
  3. Medial meniscus.
  4. Lateral meniscus.
  5. Coronary.
  6. Transverse.

 

External Ligaments

Superficial to the fibrous expansion of the quadriceps extensor tendons the fascia lata of the thigh covers the front and sides of the knee-joint.

The deep fascia of the thigh, as it descends to its attachment to the tuberosity and oblique lines of the tibia, not only overhes but blends with the fibrous expansion of the extensor tendons. The oblique lines of the tibia curve upward and backward from the tuberosity on each side to the postero-lateral part of the condyles. The process of fascia attached to the lateral ridge of the tibia and to the head of the fibula descends from the tensor fascise latas and is very thick and strong. It is firmly blended with the tendinous fibers of the vastus laterahs. The fascia lata, on the medial side of the patella, besides being attached to the medial oblique ridge of the tibia, sends some longitudinal fibers lower down to become blended with the fibrous expansion of the sartorius. The fascia is much thinner on the medial side of the patella than on the lateral, and blends much less with the tendon of the vastus niedialis than the lateral part of the fascia does with the vastus lateralis. A thin layer of the fascia lata in the form of transverse or arci-form fibers passes over the front of the joint. These fibers are speciaUy well marked over the ligamentum patellae, and blend here with the central portion of the quadriceps extensor fibers.

The Lower Extremity of the Femur (Posterior View), to show the Relation of the Articular Capsule of the Knee-joint (in red) to the Epiphysial Line.

The fibrous expansion of the extensor tendons consists:

  1. of a central portion, densely thick and strong, 3.7 cm. (1 in.) broad, which is inserted into the anterior two-thirds of the upper border of the patella, many of its superficial fibers passing over the subcutaneous surface of the bone into the ligamentum patellfe;
  2. of two side portions thinner, but strong.

The side portions are inserted into the patella along its upper border on either side of the central portion and also into its medial and lateral borders, nearer the anterior than the posterior surface, as low down as the attachment of the ligamentum patellar; passing thence along the sides of the ligamentum patelte to the tibia, they are attached to the obhque lines which extend from the tuberosity to the medial and lateral condyles, and reach as far as the tibial and fibular collateral ligaments. On the lateral side, the fibers blend with the ilio-tibial band of the fascia lata, and on the medial they extend below the oblique line to blend with the periosteum of the shaft. Thus there is a large hood spread over the whole of the front of the joint, investing the patella, and reaching from the sides of the ligamentum pateUse to the collateral ligaments, attached below to the tibia, and separated everywhere from the synovial membrane by a layer of fatty tissue.

The ligamentum patellae is the continuation in line of the central portion of the conjoined tendon, some fibers of which are prolonged over the front of the patella into the ligament. It is an extremely strong, flat band, attached above to the lower border of the patella; below, it is fixed to the lower part of the tuberosity and upper part of the crest of the tibia, somewhat obliquely, being prolonged downward further on the lateral side, so that this border is fully 2.5 cm. (1 in.) longer than the medial, which measures 6.7 cm. {2\ in.) in length.

Behind, it is in contact with a mass of fat which separates it from the synovial membrane, and a small bursa intervenes between it and the head of the tibia.

In front, a large bursa separates it from the subcutaneous tissue, and at the sides it is continuous with the fibrous expansion of the extensors.

The tibial (internal) collateral ligament is a strong, flat band, which extends from the depression on the tubercle on the medial side of the medial

Posterior View fo the Knee-joint.

 

epicondyle of the femur, to the medial border and medial surface of the shaft of the tibia, 3.7 cm. (1| in.) below the condyle. It is 8.7 cm. (3| in.) long, well defined anteriorly, where it blends with the expansion of the conjoined extensor tendons; but not so well defined posteriorly, where it merges into the oblique popliteal ligament.

Some of the lower fibers blend with the descending portion of the semimembranosus tendon. Its deep surface is firmly adherent to the edge of the medial meniscus and coronary ligament, while part of the semimembranosus tendon and inferior medial articular vessels and nerve pass between it and the bone. Superficially, a bursa separates it from the tendons of the gracilis and semitendinosus muscles and from the aponeurosis of the sartorius muscle.

The fibular (external) collateral ligament consists of two portions: the anterior, which is the longer and better marked, is a strong, rounded cord, about 5 cm. (2 in.) long, attached above to the tubercle on the lateral side of the lateral epicondyle of the femur, just below and in front of the origin of the lateral head of the gastrocnemius, whilst the tendon of the popliteus arises from the groove below and in front of it. Below, it is fixed to the middle of the lateral surface of the head of the fibula, 1.25 cm. (J) in. or more anterior to the apex.

Superficially is the tendon of the biceps, which sphts to embrace its lower extremity; while beneath it pass the popliteus tendon in its sheath, and the inferior lateral articular vessels and nerve.

Some fibers of the peroneus longus occasionally arise from the lower end of the ligament. The posterior portion is 8 mm. (\ in.) behind the anterior. It is broader and less defined; fixed below to the apex of the fibula, it inclines upward and somewhat backward, and ties down the popliteus against the lateral condyle of the tibia, blending beneath the lateral head of the gastrocnemius with the oblique popliteal ligament of the knee, of which it is really a portion.

The oblique popliteal ligament or ligamentum Winslowii is a broad dense structure of interlacing fibers, with large orifices for vessels and nerves. It is attached above to the femur close to the articular margins of the condyles, stretching across the upper margin of the intercondyloid fossa, to which it is connected by fibro-fatty tissue; it thus reaches across from the tibial to the fibular collateral ligaments. Below, it is fixed to the border of the lateral condyle of the tibia, to the bone just below the posterior intercondyloid notch, and to the shaft of the tibia below the medial condyle, blending with the descending slip of the semimembranosus and tibial collateral ligament.

The Lower Extremity of the Femur (Anterior View) to show the Relation OF THE Articular Capsule of the Knee-joint (in red) to the Epiphysial Line. 

Superficially, an oblique fasciculus from the semimembranosus runs across the center, passing upward and laterally from near the back part of the medial condyle of the tibia to the lateral epicondyle of the femur, where it joins the lateral head of the gastrocnemius, a sesamoid plate being sometimes developed at the point of junction. This slip greatly strengthens the oblique popliteal ligament, of which, if not the chief constituent, it is at least a very important part.

Its deep surface is closely connected with the semilunar menisci (especially the medial) and coronary ligaments, and in the interval between the cartilages with the posterior crucial ligament and fibro-fatty tissue within the joint. Superficially it forms part of the floor of the popliteal space. A special band, the arcuate ligament, is sometimes found extending from the lateral epicondyle to the oblique ligament.

The articular capsule is thin but strong, covering the synovial membrane, and looking like a loose sac. It is attached to the femur near the articular margin on the medial side, but further away on the lateral; it passes beneath the fibular collateral ligament to join the sheath of the popliteus. Medially it joins the tibial collateral ligament. Below, it is fixed to the upper as well as the medial and lateral borders of the patella and the anterior border of the head of the tibia. It is strengthened superficially between the femur and patella by an expansion from the articularis genu {suh-crureus) and is separated from the fibrous expansion of the extensor tendon by a layer of fatty tissue. The synovial membrane lines its deep surface, and holds it against the borders of the semilunar menisci; it is also attached to the coronary ligaments.

Internal Ligaments

The anterior crucial ligament is strong and cord-like. It is attached to the medial half of the fossa in front of the intercondyloid eminence of the tibia, and to the lateral border of the medial articular facet as far back as the medial intercondyloid tubercle. It passes upward, backward, and laterally to the back part of the medial surface of the lateral condyle of the femur. To the tibia, it is fixed behind the anterior extremity of the medial semilunar meniscus. Behind and to the lateral side it has the anterior extremity of the lateral meniscus, a few fibers of which blend with the lateral edge of the ligament.

Anterior View of the Internal Ligaments of the Knee-joint.

 

Its anterior fibers at the tibial end are strongest and longest; being fixed highest on the femur; while the posterior, springing from the intercondyloid eminence, are shorter and more oblique. Near the spine, a slip is sometimes given off to the posterior crucial ligament.

The posterior crucial ligament is stronger and less oblique than the anterior. It is fixed below to the greater portion of the fossa behind the intercondyloid eminence of the tibia, especially the lateral and posterior portion, and then medially along the posterior intercondyloid fossa; being joined by fibers which arise between the intercondyloid tubercles, it ascends to the anterior part of the lateral surface of the medial condyle of the femur, having a wide crescentic attachment 1.5 cm. (f in.) in extent just above the articular surface.

Behind, it is connected at the tibia directly with the posterior Ligament, and a little higher up by means of a quantity of interposed areolar tissue. In front it rests upon the posterior horn of the medial semilunar meniscus, and receives a large slip from the lateral meniscus, which ascends along it, either in front or behind, to the femur; higher up in front it is connected with the anterior crucial ligament.

Until they rise above the intercondyloid eminence of the tibia the two crucial ligaments are closely bound together, so that no interspace exists between their tibial attachments and the point of decussation; the only space between them is therefore a v-shaped one corresponding to the upper half of their x-shaped arrangement, and this is a mere chink in the undissected state, and can be seen from the front only, owing to the fatty tissue beneath the synovial membrane which sm-rounds their femoral attachment.

The interarticular menisci or semilunar fibro-cartilages are two crescentic discs resting upon the circumferential portions of the articular facets of the tibia, and moving with the tibia upon the femur. They somewhat deepen the tibial articular surfaces, and are dense and compact in structure, becoming looser and more fibrous near their extremities, where they are firmly fixed in front of and behind the intercondyloid eminence of the tibia. The circumferential border of each is convex, thick, and somewhat loosely attached to the borders of the condyles of the tibia by the coronary ligaments and the reflexion of the synovial membrane. The inner border is concave, thin, and free.

Half an inch (1.3 cm.) broad at the widest part, they taper somewhat toward their extremities, and cover rather less than two-thirds of the articular facets of the tibia. Their upper surfaces are slightly concave, and fit on to the femoral condyles, while the lower are flat and rest on the head of the tibia; both surfaces are smooth and covered by synovial membrane.

 

Structures lying on the Head of the Tibia. (Right knee.)

The lateral meniscus is nearly circular in form and less firmly fixed than the medial, and consequently slides more freely upon the tibia. Its anterior cornu is attached to a narrow depression along the lateral articular facet, just in front of the lateral intercondyloid tubercle of the tibia, close to, and on the lateral side of, the anterior crucial ligament; a small slip from the cornu is often fixed to the tibia in front of the crucial ligament. The posterior cornu is firmly attached to the tibia behind the lateral intercondyloid tubercle, blending with the posterior crucial ligament, and giving off a well-marked fasciculus, which runs up along the anterior border of the ligament to be attached to the femur (ligament of Wrisberg). It also sends a narrow slip into the back part of the anterior crucial ligament. Its outer border is grooved toward its posterior part by the popliteus tendon, which is held to it by fibrous tissue and synovial membrane, and separates it from the fibular collateral ligament. From its anterior border is given off the transverse ligament.

The medial meniscus is a segment of a larger circle than the lateral, and has an outline more oval than cuxular. Its anterior cornu is wide, and has a broad and oblique attachment to the anterior margin of the head of the tibia. It reaches from the margin of the condyle toward the middle of the fossa in front of the intercondyloid eminence, being altogether in front of the anterior crucial ligament. The posterior cornu is firmly fixed by a broad insertion in an antero-posterior line along the medial side of the posterior intercondyloid fossa, from the medial tubercle to the posterior margin of the head of the tibia. Its convex border is connected with the tibial collateral ligament and the seviimeinhrmiosus tendon.

The transverse ligament is a rounded, slender, short cord, which extends from the convex border of the lateral meniscus to the concave border or anterior cornu of the medial, near which it is sometimes attached to the bone. It is an accessory band of the lateral meniscus, and is situated beneath the synovial membrane.

The coronary ligaments connect the margins of the semilunar discs with the head of the tibia. The lateral is much more lax than the medial, permitting the lateral disc to change its position more freely than the medial.

They are not in reality separate structures, but consist of fibers of the several surrounding ligaments of the knee-joint which become attached to the margins of the discs as they pass over them.

The Uppee Extremity of the Tibia (Anterior View), to show the Relation Of THE Articular Capsule of the Knee-joint (in red) to the Epiphysial Line.

 

The synovial membrane of the knee forms the largest synovial sac in the body. Bulging upward from the patella, it follows the capsule of the joint into a large cul-de-sac beneath the tendon of the extensor muscles on the front of the femur. It reaches some distance beyond the articular surface of the bone, and communicates very frequently with a large bursa interposed between the tendon and the femur above the line of attachment of the articular capsule. After investing the circumference of the lower end of the femur, it is reflected upon the fibrous envelope of the joint formed by the capsular, posterior, and collateral ligaments.

The synovial membrane covers a great portion of the crucial ligaments, but leaves uncovered the back of the posterior crucial where the latter is connected with the posterior ligament, and the lower part of both crucial ligaments where they are united. Thus the ligaments are completely shut out of the synovial cavity. Along the fibrous envelope the synovial membrane is conducted down to the semilunar menisci, over both surfaces of which it passes, and is reflected off the under surface on to the coronary ligaments, and thence down to the head of the tibia, around the circumference of which it extends a short way. It dips down between the external meniscus and the head of the tibia as low as the superior tibio-fibular ligament, reaching inward nearly as far as the intercondyloid notch, and forming a bursa for the play of the popliteal tendon.

At the back of the joint two pouches are prolonged beneath the muscles, one on each side between the condyle of the femur and the origin of the gastrocnemius. Large processes of synovial membrane also project into the joint, and being occupied by fat serve as padding to fill up spaces. The chief of these processes, the patellar synovial fold (ligamentum mucosum), springs from the infrapatellar fatty mass. This so-called ligament is the central portion of the large process of synovial membrane, of which the alar folds form the free margins.

It extends from the fatty mass, below the patella, backward and upward to the intercondyloid notch of the femur, where it is attached in front of the anterior crucial, and lateral to the posterior crucial ligament. Near the femur it is thin and transparent, consisting of a double fold of synovial membrane, but near the patella it contains some fatty tissue. Its anterior or upper edge ia free, and fully 2.6 cm. (an inch) long; the posterior or lower edge is half the length, and is attached to the crucial ligaments above, but is free below.

Passing backward from the capsule on each side of the patella is a prominent crescentic fold formed by reduplications of the synovial membrane - these are the alar folds.

Their free margins are concave and thin, and are lost below in the patellar fold. There is a slight fossa above and another below each Ligament.

Anterior View of the Knee-joint, showing the Stnovial, Ligaments. (Anterior portion of capsule with the extensor tendon thrown downward.)

 

The Upper Extremity of the Tibia (Posterior View), to show the Relation of the articular capsule of the Knee-joint (in yellow) to the Epiphysial Line.

The arterial supply is derived from the art. genu suprema (anastomotica) ; the superior and inferior medial and lateral articular; the medial articular; the descending branch of the lateral circumflex; the anterior recurrent branch from the anterior tibial; and the posterior tibial recurrent.

The nerve-supply comes from the great sciatic, femoral, and obturator sources. The great sciatic pves off the tibial and common peroneal; the tibial sends tAvo, sometimes three branches - one with the medial articular artery; one with the inferior medial, and sometimes one with the superior medial articular artery; the common peroneal gives a branch which accompanies the superior, and another which accompanies the inferior articular artery, and a recurrent branch which follows the course of the anterior recm-rent branch of the anterior tibial artery.

The femoral sends an articular branch from the nerve to the vastus lateralis; a second from the nerve to the vastus mediaiis; and sometimes a third from that to the vastus intermedins. Thus there are three articular twigs to the knee derived from the muscular branches of the femoral.

The obturator by its deep division sends a branch through the adductor magnus on to the popliteal artery, which enters the joint posteriorly.

Sagittal Section of the Knee-joint. (The bones are somewhat drawn apart.)

Relations

Anteriorly and at the sides the knee-joint is merely covered and protected by skin, fascia, and the tendinous expansions of the quadriceps extensor muscle. Laterally and posteriorly it is crossed by the biceps tendon. Medially and posteriorly lie the sartorius and the tendons of the gracilis and seinitendinosus muscles. Posteriorly it is in relation with the popliteal vessels and nerves, the semimembranosus, the two heads of the gastrocnemius, and the plantaris. The tendon of the popliteus pierces the capsule behind and medial to the biceps tendon.

The movements which occur at the knee-joint are flexion and extension, with some slight amount of rotation in the bent position. These movements are not so simple as the corresponding ones at the elbow, for the knee is not a simple hinge joint. The movements of rotation instead of occurring between tibia and fibula, as between radius and ulna, are movements of the tibia with the fibula upon the condyles of the femur.

The Collateral Ligaments oF the Knee in Flexion and Extension.

The knee differs from a true hinge joint, like the elbow or ankle, in the following particulars:

  1. The points of contact of the femur with the tibia are constantly changing. Thus, in the flexed position, the posterior part of the articular surface of the tibia is in contact with the rounded bacli part of the femoral condyles; in the semiflexed position the middle parts of the tibial facets Articulate with the anterior rounded part of the condyles; while in the fully extended position the anterior and middle parts of the tibial facets are in contact with the anterior flattened portion of the condyles. So with the patella: in extreme flexion the medial articular facet rests on the lateral part of the medial condyle of the femur; in flexion the upper pair of facets rests on the lower part of the trochlear surface of the femur; in mid-flexion the middle pan rests on the middle of the trochlear surface; while in extension the lower pair of facets on the patella rests on the upper portion of the trochlear surface of the femur. This difference may be described as the shifting of the points of contact of the articular surface.
  2. It differs from a true hinge in that, in passing from a state of extension to one of flexion, the tibia does not revolve round a single transverse axis drawn through the lower end of the femur, as the ulna does round the lower end of the humerus. The articular surface of the tibia slides forward in e.xtension and backward in flexion; thus the axis round which the tibia revolves upon the femur is a shifting one, as is seen by reference to fig. B, C, D.
  3. 3. Another point of difference is that extension is accompanied by lateral rotation, and flexion by medial rotation. This rotation occurs round a vertical axis drawn through the middle of the lateral condyle of the femur and the lateral condyle of the tibia, and is most marked at the termination of extension and at the commencement of flexion. This rotation of the leg at the knee is a true rotation about a vertical axis, and thus differs from the obliquity of the flexion and extension movements at the elbow which is due to the oblique direction of the articular surfaces of the bones.
  4. The antero-posterior spiral curve of the femoral condyles is such that the anterior part is an arc of a greater circle than the posterior; hence certain ligaments which are tightened during extension are relaxed during flexion, and thereby a considerable amount of rotatory movement is permitted in the flexed position. The axis of this rotation is vertical, and passes through the medial intercondyloid tubercle of the tibia, so that the lateral condyle moves in the arc of a larger circle than does the medial, and is therefore required to move more freely and easily; hence the shape of the lateral articular facet and the loose connection of the lateral meniscus which is adapted to it.

Section of the Knee, showing the Crucial Ligaments in Extension.

 

Crucial Ligaments in Flexion.

 

 

In extension, all the ligaments are on the stretch with the exception of the ligamentum patellae and front of the capsule. Extension is checked by both the crucial ligaments and the collateral ligaments (figs. A, B).

In flexion the ligamentum patellae and anterior portion of the capsule are on the stretch; so also is the posterior crucial in extreme flexion, though it is not quite tight in the semiflexed state of the joint. All the other ligaments are relaxed (fig. C, D), although the relaxation of the anterior crucial ligament is slight in extreme flexion. Flexion is only checked during hfe by the contact of the soft parts, i. e., the calf with the back of the thigh.

Rotation medially is checked by the anterior crucial ligament; the collateral ligaments being loose.

Rotation laterally is checked by the collateral Ligaments; the crucial ligaments have no controlling effect on it, as they are untwisted by it.

Sliding movements are checked by the crucial and collateral ligaments - sliding forward especially by the anterior, and sliding backward by the posterior crucial.

Muscles which act upon the knee-joint.

Flexors

Biceps, semimembranosus, semitendinosus, sartorius, gastrocnemius, plantaris, and popliteus.

Extensor

Quadriceps extensor.

Medial Rotators

Sartorius, gracilis, semitendinosus, semimembranosus, popliteus.

Lateral Rotator

Biceps.

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