Knee

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CHAPTER 82 Knee

The knee is the largest synovial joint in the body. It consists of three distinct and partially separated compartments that collectively form a complex ‘hinge’ joint. This arrangement offers a fulcrum for the powerful extensor and flexor muscles that act on the joint during propulsion. A number of stabilizing factors counter the considerable biomechanical demands that are imposed upon the joint: most important is a complex arrangement of intracapsular and extracapsular ligaments. Our understanding of the anatomy of the knee has improved considerably in recent years, due in large measure to significant technological advances in diagnostic imaging and surgery.

SKIN AND SOFT TISSUE

SKIN

SOFT TISSUE

Popliteal fossa

The popliteal fossa (Figs 82.2, 82.3) is a narrow intermuscular space posterior to the knee joint, with a diamond-shaped outline that is rendered more apparent when its boundaries are artificially separated during dissection. The boundaries are biceps femoris proximolaterally; semimembranosus and the overlying semitendinosus proximomedially; the lateral head of gastrocnemius with the underlying plantaris distolaterally, and the medial head of gastrocnemius distomedially. The anterior boundary (or floor) of the fossa is formed, in proximodistal sequence, by the popliteal surface of the femur, the oblique popliteal ligament (overlying the posterior surface of the capsule of the knee joint), and the posterior aspect of the proximal tibia covered by popliteus and the fascia overlying popliteus. The fossa is covered posteriorly by the popliteal fascia, which is referred to as the roof of the fossa. The popliteal fascia is continuous with the fascia lata proximally and with the fascia cruris distally. It is a dense layer that is strongly reinforced by transverse fibres and is often perforated by the short saphenous vein and sural nerve; these two structures are useful landmarks in the direct posterior approach to the knee joint. (Note that ‘popliteal fascia’ refers to the deep fascia that forms the ‘roof’ of the fossa, in contradistinction to the ‘fascia overlying popliteus’, which forms part of the floor.)

image

Fig. 82.2 Left popliteal fossa.

(From Sobotta 2006.)

Contents

With its boundaries undisturbed, the popliteal fossa is typically 2.5 cm wide. Its contents are largely hidden, especially in its distal part, where the heads of gastrocnemius are in contact with each other. When its boundaries are separated, its contents are revealed as the popliteal vessels (Figs 82.2, 82.4), tibial and common fibular nerves, short saphenous vein, sural nerve, posterior femoral cutaneous nerve, an articular branch from the obturator nerve, lymph nodes, fat and a variable number of collapsed bursal sacs. The tibial nerve descends centrally immediately anterior to the popliteal fascia, crossing the vessels posteriorly from lateral to medial. The common fibular nerve descends laterally immediately medial to the tendon of biceps femoris. Popliteal vessels are deeply located and held together by dense areolar tissue. They lie on the floor of the fossa, the popliteal vein being superficial to the artery. Proximally, the thick-walled vein lies lateral to the artery, crossing to its medial side distally. At times the popliteal vein is duplicated, and the artery lies between the two veins, which are usually bridged by connecting channels. An articular branch from the obturator nerve descends on the artery to the knee. Six or seven popliteal lymph nodes are embedded in the fat, one under the popliteal fascia near the termination of the short saphenous vein, one between the popliteal artery and knee joint, the others around the popliteal vessels.

BONE

PATELLA

Topography

The patella is the largest sesamoid bone (Figs 82.5, 82.6) and is embedded in the tendon of quadriceps femoris, anterior to the distal femur (femoral condyles). It is flat, distally tapered, proximally curved, and has anterior and posterior surfaces, three borders and an apex which is the distal end of the bone. With the knee in extension, the apex is just proximal to the line of the knee joint.

The subcutaneous, convex anterior surface is perforated by nutrient vessels. It is longitudinally ridged, separated from the skin by a prepatellar bursa, and covered by an expansion from the tendon of quadriceps femoris, which blends distally with superficial fibres of the patellar tendon (patellar ligament), the continuation of the tendon of quadriceps. The posterior surface has a proximal smooth, oval articular area, crossed by a smooth vertical ridge, which fits the intercondylar groove on the femoral patellar surface and divides the patellar articular area into medial and lateral facets; the lateral is usually larger. Each facet is divided by faint horizontal lines into equal thirds. A seventh ‘odd’ facet is present as a narrow strip along the medial border of the patella; it contacts the medial femoral condyle in extreme flexion. Distal to the articular surface, the apex is roughened by the attachment of the patellar tendon. Proximal to this, the area between the roughened apex and the articular margin is covered by an infrapatellar pad of fat. The articular cartilage is the thickest in the body, reflecting the magnitude of the stresses to which it is subjected.

The thick superior border (surface) slopes anteroinferiorly. The medial and lateral borders are thinner and converge distally; the expansions of the tendons of vastus medialis and lateralis (medial and lateral patellar retinacula respectively) are attached to them. The lateral retinaculum receives contributions from the iliotibial tract. Ossification occasionally extends from the lateral margin of the patella into the tendon of vastus lateralis.

The shape of the patella can vary, and certain configurations are associated with patellar instability. Not infrequently a bipartite patella is seen on radiographs. The bone seems to be in two parts, usually with a smaller superolateral fragment: this has long been attributed to the presence of a separate ossification centre, but in some cases could represent failed union following a stress fracture.

JOINTS

SUPERIOR TIBIOFIBULAR JOINT

The superior (proximal) tibiofibular joint is an almost plane synovial joint between the lateral tibial condyle and fibular head.

PATELLOFEMORAL JOINT

The patellofemoral joint is a synovial joint and is part of the knee joint.

Patellar tendon sheath and patellar tendon

The patellar tendon (patellar ligament) is the central band of the tendon of quadriceps femoris, and is continued distally from the patella to the tibial tuberosity (see Fig. 80.25). It is strong, flat and 6 to 8 cm in length. Proximally it is attached to the patellar apex and adjoining margins, to roughened areas on the anterior surface and to a depression on the distal posterior patellar surface. Distally it is attached to the superior smooth area of the tibial tuberosity. This insertion is oblique, and is more distal laterally. Its superficial fibres are continuous over the patella with the tendon of quadriceps femoris, the medial and lateral parts of which descend, flanking the patella, to the sides of the tibial tuberosity, where they merge with the fibrous capsule as the medial and lateral patellar retinacula. The patellar tendon is separated from the synovial membrane by a large infrapatellar fat pad and from the tibia by a bursa, and lies within its own well-defined sheath.

In the procedure of tibial osteotomy, the tibia is cut transversely just above the patellar tendon insertion. Failure to appreciate the obliquity of the tibial attachment of the tendon may lead to inadvertent division of the tendon during this procedure, with unfortunate consequences.

All other aspects of the patellofemoral joint are described with the tibiofemoral joint.

TIBIOFEMORAL JOINT

The tibiofemoral joint is a complex synovial joint and is part of the knee joint.

Articulating surfaces

Proximal tibial surface

The proximal tibial surface (often referred to as the tibial plateau) slopes posteriorly and downwards relative to the long axis of the shaft (Fig. 82.7). The tilt, which is maximal at birth, decreases with age, and is more marked in habitual squatters. The tibial plateau presents medial and lateral articular surfaces (facets) for articulation with the corresponding femoral condyles. The posterior surface, distal to the articular margin, displays a horizontal, rough groove to which the capsule and posterior part of the medial collateral ligament are attached. The anteromedial surface of the medial tibial condyle is a rough strip, separated from the medial surface of the tibial shaft by an inconspicuous ridge. The medial patellar retinaculum is attached to the medial tibial condyle along its anterior and medial surfaces, which are marked by vascular foramina.

The medial articular surface is oval (long axis anteroposterior) and longer than the lateral articular surface. Around its anterior, medial, and posterior margins, it is related to the medial meniscus; the meniscal imprint, wider posteriorly and narrower anteromedially, is often discernible. The surface is flat in its posterior half and the anterior half slopes upwards about 10°. Much of the posterior surface is covered by the meniscus, so that overall a concave surface is presented to the medial femoral condyle. Its lateral margin is raised as it reaches the intercondylar region.

The lateral tibial condyle overhangs the shaft posterolaterally above a small circular facet for articulation with the fibula. The lateral articular surface is more circular and coapted to its meniscus. In the sagittal plane the articular surface is fairly flat centrally, and anteriorly and posteriorly the surface slopes inferiorly. Overall this creates a rather convex surface, so that, with the lateral femoral condyle in contact, there are anterior and posterior recesses (triangular in section), which are occupied by the anterior and posterior meniscal horns. Elsewhere the surface has a raised medial margin that spreads to the lateral intercondylar tubercle. Its articular margins are sharp, except posterolaterally, where the edge is rounded and smooth: here the tendon of popliteus is in contact with bone.

Intercondylar area

The rough-surfaced area between the condylar articular surfaces is narrowest centrally where there is an intercondylar eminence, the edges of which project slightly proximally as the lateral and medial intercondylar tubercles. The intercondylar area widens behind and in front of the eminence as the articular surfaces diverge (Fig. 82.8).

The anterior intercondylar area is widest anteriorly. Anteromedially, anterior to the medial articular surface, a depression marks the site of attachment of the anterior horn of the medial meniscus. Behind this a smooth area receives the anterior cruciate ligament. The anterior horn of the lateral meniscus is attached anterior to the intercondylar eminence, lateral to the anterior cruciate ligament. The eminence, with medial and lateral tubercles, is the narrow central part of the area. The raised tubercles are thought to provide a slight stabilizing influence on the femur.

The posterior horn of the lateral meniscus is attached to the posterior slope of the intercondylar area. The posterior intercondylar area inclines down and backwards behind the posterior horn of the lateral meniscus. A depression behind the base of the medial intercondylar tubercle is for the posterior horn of the medial meniscus. The rest of the area is smooth and provides attachment for the posterior cruciate ligament, spreading back to a ridge to which the capsule is attached.

Menisci

The menisci (semilunar cartilages) are crescentic, intracapsular, fibrocartilaginous laminae (Figs 82.9, 82.10). They serve to widen and deepen the tibial articular surfaces that receive the femoral condyles. Their peripheral attached borders are thick and convex and their free, inner borders thin and concave. Their peripheries are vascularized by capillary loops from the fibrous capsule and synovial membrane, while their inner regions are avascular. Tears of the menisci are common. Most occur in the avascular, inner zones and seldom heal spontaneously; if surgery is indicated, these menisci are best resected. Peripheral tears (i.e. in the vascularized zone) have the potential to heal satisfactorily if repaired surgically. The meniscal horns are richly innervated compared with the remainder of the meniscus. The central thirds are devoid of innervation (Gronblad et al 1985). The proximal surfaces are smooth and concave and in contact with the articular cartilage on the femoral condyles. The distal surfaces are smooth and flat, resting on the tibial articular cartilage. Each covers approximately two-thirds of its tibial articular surface. Canal-like structures open onto the surface of menisci in infants and young children and may transport nutrients to deeper avascular areas.

Two structurally different regions of the menisci have been identified. The inner two-thirds of each meniscus consists of radially organized collagen bundles, and the peripheral third consists of larger circumferentially arranged bundles (Ghadially et al 1983). The articular surfaces of the inner part are lined by thinner collagen bundles parallel to the surface, while the outer portion is covered by synovium. This structural arrangement suggests specific biomechanical functions for the two regions: the inner portion of the meniscus is suited to resisting compressive forces while the periphery is capable of resisting tensional forces. With ageing and degeneration, compositional changes occur within the menisci which reduce their ability to resist tensional forces. Outward displacement of the menisci by the femoral condyles is resisted by firm anchorage of the peripheral circumferential fibres to the intercondylar bone at the meniscal horns.

Menisci spread load by increasing the congruity of the articulation; give stability by their physical presence and as providers of proprioceptive feedback; probably assist lubrication; and may cushion the underlying bone from the considerable forces generated during extremes of flexion and extension.

Medial meniscus

The medial meniscus broader posteriorly, is almost a semicircle in shape (Fig. 82.9). It is attached by its anterior horn to the anterior tibial intercondylar area in front of the anterior cruciate ligament; the posterior fibres of the anterior horn are continuous with the transverse ligament. The anterior horn is in the floor of a depression medial to the upper part of the patellar tendon. The posterior horn is fixed to the posterior tibial intercondylar area, between the attachments of the lateral meniscus and posterior cruciate ligament. Its peripheral border is attached to the fibrous capsule and the deep surface of the medial collateral ligament. The tibial attachment of the meniscus is known as the ‘coronary ligament’. Collectively these attachments ensure that the medial meniscus is relatively fixed and moves much less than the lateral meniscus.

Lateral meniscus

The lateral meniscus forms approximately four-fifths of a circle, and covers a larger area than the medial meniscus. Its breadth, except at its short tapered horns, is more or less uniform. It is grooved posterolaterally by the tendon of popliteus, which separates it from the fibular collateral ligament. Its anterior horn is attached in front of the intercondylar eminence, posterolateral to the anterior cruciate ligament, with which it partly blends. Its posterior horn is attached behind this eminence, in front of the posterior horn of the medial meniscus. Its anterior attachment is contorted so that the free margin faces posterosuperiorly, and the anterior horn rests on the anterior slope of the lateral intercondylar tubercle. Near its posterior attachment it commonly sends a posterior meniscofemoral ligament superomedially behind the posterior cruciate ligament to the medial femoral condyle. An anterior meniscofemoral ligament may also connect the posterior horn to the medial femoral condyle anterior to the posterior cruciate ligament. The meniscofemoral ligaments are often the sole attachments of the posterior horn of the lateral meniscus. More laterally, part of the tendon of popliteus is attached to the lateral meniscus, and so mobility of its posterior horn may be controlled by the meniscofemoral ligaments and by popliteus. A meniscofibular ligament occurs in most knee joints. There is a tibial attachment via a coronary ligament, but the meniscus has no peripheral bony attachment in the region of popliteus: in surgical literature this gap is referred to as the popliteus hiatus.

Discoid lateral meniscus

A discoid lateral meniscus occurs in up to 5% of the population, often bilaterally. The distinguishing features of a discoid lateral meniscus are its shape and posterior ligamentous attachments. The following classification of the abnormality is based on Watanabe et al (1979). In its mildest form, the partial discoid meniscus is simply a wider form of the normal lateral meniscus. The acute, medial free edge is interposed between femoral and tibial condyles, but it does not completely cover the tibial plateau. A complete discoid meniscus appears as a biconcave disc with a rolled medial edge and totally covers the lateral tibial plateau. The Wrisberg type of meniscus has the same shape as a complete discoid meniscus, but its only peripheral posterior attachment is by the meniscofemoral ligaments. The normal tibial attachment of the posterior horn of the lateral meniscus is lacking, but the posterior meniscofemoral ligament persists. As a result, this type of meniscus is attached anteriorly to the tibia and posteriorly to the femur, which renders the posterior horn unstable. Under these circumstances, the meniscus is liable to get caught between the femur and tibia: this accounts for the classic presenting symptom of the ‘clunking knee’ in some patients. The aetiology of discoid meniscus is not clear. Most are asymptomatic, and are often found by chance at arthroscopy. However, they may cause difficulty in gaining access to the lateral compartment at arthroscopy.

Discoid medial meniscus is extremely rare.

Transverse [intermeniscal] ligament

The transverse ligament connects the anterior convex margin of the lateral meniscus to the anterior horn of the medial meniscus (Figs 82.8, 82.9). It varies in thickness and is often absent. Its exact role is conjectural; presumably it helps to decrease tension generated in the longitudinal circumferential fibres of the menisci when the knee is subjected to load. A posterior meniscomeniscal ligament is sometimes present.

Meniscofemoral ligaments

The two meniscofemoral ligaments (MFLs) connect the posterior horn of the lateral meniscus to the inner (lateral) aspect of the medial femoral condyle (Figs 82.11, 82.12). The anterior MFL (aMFL; ligament of Humphry) passes anterior to the posterior cruciate ligament. The posterior MFL (pMFL; ligament of Wrisberg) passes behind the posterior cruciate ligament and attaches proximal to the margin of attachment of the posterior cruciate.

Anatomical studies found that at least one meniscofemoral ligament was present in 92% of cadaveric knees examined, whilst both coexisted in 32% (Gupte et al 2003). Biomechanical studies have revealed the cross-sectional area and strength of the meniscofemoral ligaments to be comparable to those of the posterior fibre bundle of the posterior cruciate ligament.

The meniscofemoral ligaments are believed to act as secondary restraints, supporting the posterior cruciate ligament in minimizing displacement caused by posteriorly directed forces on the tibia. These ligaments are also involved in controlling the motion of the lateral meniscus in conjunction with the tendon of popliteus during flexion.