Cutaneous vascular supply and lymphatic drainage

The arterial supply of the skin covering the knee is derived from genicular branches of the popliteal artery, the descending genicular branch of the femoral artery, and the anterior recurrent branch of the anterior tibial artery, with small contributions from the arteries to vastus medialis and the hamstrings (Fig. 82.1). For further details consult Cormack & Lamberty (1994).

Fig. 82.1 The arterial anastomoses around the knee joint.

Cutaneous veins are tributaries of the vessels that correspond to the named arteries. Cutaneous lymphatic drainage is initially to the superficial inguinal nodes, and possibly also to the popliteal nodes and thence to the deep inguinal group of lymph nodes.

Cutaneous innervation

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.)

Fig. 82.2 Left popliteal fossa.

(From Sobotta 2006.)

Fig. 82.3 Muscles of the calf: superficial view including boundaries of 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.

Fig. 82.4 Left popliteal, posterior tibial and fibular arteries: posterior aspect.

(From Sobotta 2006.)

Articulating surfaces

The articulating surfaces vary in size, form and inclination. The joint line may be transverse or oblique. The fibular facet is usually elliptical or circular, almost flat or slightly grooved. The surfaces are covered with hyaline cartilage.

Fibrous capsule

The capsule is attached to the margins of the tibial and fibular articular, and is thickened anteriorly and posteriorly.

Ligaments

The ligaments of the superior tibiofibular joint are not entirely separate from the capsule. The anterior ligament is made up of two or three flat bands which pass obliquely up from the fibular head to the front of the lateral tibial condyle: it is closely related to the tendon of biceps. The posterior ligament is a thick band that ascends obliquely between the posterior aspect of the fibular head and the lateral tibial condyle: it is covered by the popliteal tendon.

Synovial membrane

The synovial membrane of the superior tibiofibular joint is sometimes (in approximately 10% of subjects) continuous with that of the knee joint via the subpopliteal recess.

Vascular supply and lymphatic drainage

The superior tibiofibular joint receives an arterial supply from the anterior and posterior tibial recurrent branches of the anterior tibial artery. Lymphatics follow the arteries and drain to the popliteal nodes.

Innervation

The superior tibiofibular joint is innervated by branches from the common fibular nerve and from the nerve to popliteus.

Factors maintaining stability

Stability of the superior tibiofibular joint is maintained by the fibrous capsule and the anterior and posterior ligaments, assisted by the biceps tendon and the interosseous membrane.

Movements

Very little movement other than limited gliding takes place at the superior tibiofibular joint. Some movement must occur in conjunction with movement occurring at the inferior tibiofibular joint, however, surgical fusion (arthrodesis) of the superior tibiofibular joint seems to have no effect on movements of the ankle joint.

Relations

The common fibular nerve runs posterior to the fibular head, medial to the tendon of biceps, which is closely associated with the anterior capsule. The anterior and posterior tibial branches of the popliteal artery, and the fibular artery, are all vulnerable inferomedial to the joint.

Articulating surfaces

The articular surface of the patella is adapted to that of the femur. The latter extends onto the anterior surfaces of both femoral condyles like an inverted U. Since the whole area is concave transversely and convex in the sagittal plane, it is an asymmetrical sellar surface. The ‘odd’ facet on the articular surface of the patella contacts the anterolateral aspect of the medial femoral condyle in full flexion, when the highest lateral patellar facet contacts the anterior part of the lateral femoral condyle. As the knee extends, the middle patellar facets contact the lower half of the femoral surface; in full extension only the lowest patellar facets are in contact with the femur. In summary, on flexion the patellofemoral contact point moves proximally. The contact area also broadens to cope with the increasing stress that accompanies progressive flexion.

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.

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.

Fig. 82.7 The left tibial plateau. 1. Tibial tuberosity. 2. Attachment of anterior horn, lateral meniscus. 3. Lateral condyle. 4. Attachment of posterior horn, lateral meniscus. 5. Attachment of anterior horn, medial meniscus. 6. Attachment of anterior cruciate ligament. 7. Medial condyle. 8. Intercondylar eminence. 9. Attachment of posterior horn, medial meniscus. 10. Attachment of posterior cruciate ligament.

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).

Fig. 82.8 The intercondylar notch at arthroscopy, showing the anterior cruciate and intermeniscal ligaments.

(By courtesy of Smith and Nephew Endoscopy.)

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.

Fig. 82.9 Superior aspect of the left tibia, showing the menisci and the attachments of the cruciate ligaments.

(From Drake, Vogl, Mitchell, Tibbitts and Richardson 2008.)

Fig. 82.10 Coronal T1-weighted magnetic resonance image (MRI) of the knee in an adult male. 1. Posterior cruciate ligament. 2. Medial collateral ligament. 3. Medial meniscus. 4. Epiphysial line. 5. Anterior cruciate ligament. 6. Lateral collateral ligament. 7. Lateral meniscus. 8. Head of fibula.

(By courtesy of Dr Justin Lee, Chelsea and Westminster Hospital, London.)

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.

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.

Fig. 82.11 A, Sagittal T1-weighted and B, coronal STIR magnetic resonance images (MRI) of the knee in an adult male showing the anterior meniscofemoral ligament. A: 1. Patella. 2. Anterior meniscofemoral ligament of Humphry. 3. Epiphysial line. 4. Posterior cruciate ligament. B: 1. Anterior meniscofemoral ligament of Humphry. 2. Medial collateral ligament. 3. Medial meniscus. 4. Posterior cruciate ligament. 5. Lateral meniscus. 6. Lateral collateral ligament. 7. Head of fibula.

(By courtesy of Dr Justin Lee, Chelsea and Westminster Hospital, London.)

Fig. 82.12 A, Sagittal T1-weighted and B, coronal STIR magnetic resonance images (MRI) of the knee in an adult male showing the posterior meniscofemoral ligament. A: 1. Epiphysial line. 2. Posterior cruciate ligament. 3. Posterior meniscofemoral ligament of Wrisberg. B: 1. Medial femoral condyle. 2. Posterior meniscofemoral ligament of Wrisberg. 3. Lateral femoral condyle. 4. Lateral meniscus. 5. Head of fibula.

(By kind permission from Dr Justin Lee, Chelsea and Westminster Hospital, London.)

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.

Soft tissues

Recent advances in knee ligament surgery have contributed to a better understanding of the anatomy of the medial and lateral soft tissues of the knee.

Medial soft tissues

The medial soft tissues (see Fig. 80.25, Fig. 82.13) are arranged in three layers (Warren & Marshall 1979).

Fig. 82.13 Posterior dissection of the knee. A, Capsule intact. B, Capsule removed.

(From Sobotta 2006.)

Layer 1

Layer 1 is the most superficial and is the deep fascia that invests sartorius. The saphenous nerve and its infrapatellar branch are superficial to the fascia. Sartorius inserts into the fascia as an expansion rather than as a distinct tendon. The layer 1 fascia spreads inferiorly and anteriorly to lie superficial to the distinct and readily identifiable tendons of gracilis and semitendinosus and their insertions. The latter two tendons are commonly harvested for surgical reconstruction of damaged cruciate ligaments. To gain access to them the upper edge of sartorius can be identified. The sartorius (layer 1) fascia is then incised to reveal the tendons. Deep to the tendons is the pes bursa, which overlies the superficial medial collateral ligament: this bursa is sometimes the seat of inflammation, especially in track and field athletes. Posteriorly, layer 1 overlies the tendons of gastrocnemius and the structures of the popliteal fossa. Anteriorly, layer 1 blends with the anterior limit of layer 2 and the medial patellar retinaculum. More inferiorly, layer 1 blends with periosteum.

A condensation of tissue passes from the medial border of the patella to the medial epicondyle of the femur (the medial patellofemoral ligament), the anterior horn of the medial meniscus (the meniscopatellar ligament), and the medial tibial condyle (the patellotibial ligament).

Layer 2

Layer 2 is the plane of the superficial medial collateral ligament, which means that the tendons of gracilis and semitendinosus lie between layers 1 and 2. The superficial medial collateral ligament has vertical and oblique portions. The former contains vertically orientated fibres that pass from the medial epicondyle of the femur to a large insertion on the medial surface of the proximal end of the tibial shaft. It extends to an area about 5 cm distal to the joint line. Its anterior edge is rolled and easily seen just posterior to the insertions of gracilis and semitendinosus once layer 1 has been opened. The posteriorly placed oblique fibres run posteroinferiorly from the medial epicondyle of the femur to blend with the underlying layer 3 (capsule), effectively to insert on the posteromedial tibial articular margin and posterior horn of the medial meniscus. This area is reinforced by a part of the insertion of semimembranosus. There is a vertical split in layer 2 anterior to the superficial medial collateral ligament. The fibres anterior to the split pass superiorly to blend with vastus medialis fascia and layer 1 in the medial patellar retinaculum. The fibres posterior to the split pass superiorly to the medial epicondyle and thence anteriorly as the medial patellofemoral ligament.

Layer 3

Layer 3 is the capsule of the knee joint and can be separated from layer 2 everywhere except anteriorly close to the patella, where it blends with the more superficial layers. Deep to the superficial medial collateral ligament it is thick and has vertically orientated fibres that make up the deep medial collateral ligament. It sends fibres to the medial meniscus. Anteriorly, the separation of the superficial and deep parts of the medial collateral ligament is distinct. Posteriorly, layers 2 and 3 blend to form a conjoined posteromedial capsule.

Lateral soft tissues

The lateral soft tissues (Figs 82.13, 82.15) are also arranged in three layers (Seebacher et al 1982). Most superficial is the lateral patellar retinaculum. The middle layer consists of the lateral collateral ligament, popliteofibular ligament, fabellofibular ligament and arcuate ligament. The deep layer is the lateral part of the capsule.

Fig. 82.15 Knee joint (lateral aspect); the synovial cavity is distended and synovial membrane appears grey.

(From Sobotta 2006.)

The lateral patellar retinaculum consists of superficial oblique and deep transverse portions. The former runs from the iliotibial band to the patella. The latter is thicker and subdivided into three parts: the lateral patellofemoral ligament, running from the lateral patellar border to the lateral epicondyle of the femur; the transverse retinaculum, running from the iliotibial band to the mid patella; and the patellotibial band, running from the patella to the lateral tibial condyle.

The fascia lata and the iliotibial band lie posterior to the lateral retinaculum. They come together distally to insert onto the tibia at Gerdy’s tubercle on the anterolateral proximal tibia, and some fibres continue to the tibial tuberosity. Proximally the fascia lata merges with the lateral intermuscular septum. Posteriorly it blends with the biceps fascia. Here, as it emerges from behind the biceps tendon, the common fibular nerve lies in a thin layer of fat bound by the fascia.

The lateral collateral ligament arises from the lateral epicondyle of the femur posterior to the popliteus insertion and just proximal to the popliteus groove. It is a cord-like structure that passes distally, superficial to the popliteus tendon and deep to the lateral retinaculum, to attach to the fibular head, where it blends with the biceps tendon just anterior to the apex of the fibular head. It is separated from the capsule by a thin layer of fat and the inferior lateral genicular vessels.

The single most important stabilizer of the posterolateral knee is the popliteofibular ligament. It passes from the popliteus tendon at a level just below the joint line, posteriorly, laterally and inferiorly, to the fibular head. It is probably what was previously described as the short lateral genual ligament. As a passive ‘tether’ combined with the popliteus tendon proximal to it, it resists external rotation of the tibia. Its connection to the tendon of popliteus also allows ‘dynamic’ tensioning.

The fabellofibular ligament is a condensation of fibres that runs from the fabella, or from the lateral head of gastrocnemius if the fabella is absent, to the fibular styloid. The arcuate ligament is a condensation of fibres that runs from the fibular styloid, posteromedially over the emerging tendon of popliteus below the level of the tibial joint surface, to the tibial intercondylar area. The lateral joint capsule is thin and blends posteriorly with the arcuate ligament. Anteriorly it forms the weak, lax coronary ligament, which attaches the inferior border of the meniscus to the lateral tibia.

Ligaments

Cruciate ligaments

The cruciate ligaments, so named because they cross each other, are very strong intracapsular structures. The point of crossing is located a little posterior to the articular centre. They are named anterior and posterior with reference to their tibial attachments (Fig. 82.14). Synovial membrane almost surrounds the ligaments but is reflected posteriorly from the posterior cruciate to adjoining parts of the capsule: the intercondylar part of the posterior region of the fibrous capsule therefore has no synovial covering.

Fig. 82.14 The left knee joint. A, Anterior aspect in full flexion. B, Posterior aspect in extension.

(From Drake, Vogl, Mitchell, Tibbitts and Richardson 2008.)

Anterior cruciate ligament

The anterior cruciate ligament is attached to the anterior intercondylar area of the tibia, just anterior and slightly lateral to the medial tibial eminence, partly blending with the anterior horn of the lateral meniscus (Figs 82.8, 82.9). It ascends posterolaterally, twisting on itself and fanning out to attach high on the posteromedial aspect of the lateral femoral condyle (Girgis et al 1975). The average length and width of an adult anterior cruciate ligament are 38 mm and 11 mm respectively. It is formed of two, or possibly three, functional bundles that are not apparent to the naked eye, but can be demonstrated by microdissection techniques. The bundles are named anteromedial, intermediate, and posterolateral, according to their tibial attachments (Amis and Dawkins 1991).

Absent anterior cruciate ligament

Congenital absence of the anterior cruciate ligament is rare. The condition is usually associated with lower limb dysplasia (Thomas et al 1985), and may be a cause of instability of the knee.

Posterior cruciate ligament

The posterior cruciate ligament is thicker and stronger than the anterior cruciate ligament (Fig. 82.9), the average length and width of an adult posterior cruciate ligament being 38 mm and 13 mm respectively. It is attached to the lateral surface of the medial femoral condyle and extends up onto the anterior part of the roof of the intercondylar notch, where its attachment is extensive in the anteroposterior direction. Its fibres are adjacent to the articular surface. They pass distally and posteriorly to a fairly compact attachment posteriorly in the intercondylar region and in a depression on the adjacent posterior tibia. This gives a fan-like structure in which fibre orientation is variable. Anterolateral and posteromedial bundles have been defined: they are named (against convention) according to their femoral attachments. The anterolateral bundle tightens in flexion whilst the posteromedial is tight in extension of the knee. Each bundle slackens as the other tightens. Unlike the anterior cruciate ligament, it is not isometric during knee motion, i.e. the distance between attachments varies with knee position. The posterior cruciate ligament ruptures less commonly than the anterior cruciate; rupture is usually better tolerated by patients than that of the anterior cruciate ligament.

Synovial membrane, plicae and fat pads

The synovial membrane of the knee is the most extensive and complex in the body. It forms a large suprapatellar bursa between quadriceps femoris and the lower femoral shaft proximal to the superior patellar border (Fig. 82.16). The bursa is an extension of the joint cavity. The attachment of articularis genu to its proximal aspect prevents the bursa from collapsing into the joint. Alongside the patella the membrane extends beneath the aponeuroses of the vasti, especially under vastus medialis. It extends proximally a hand’s breadth above the superior pole of the patella. Distal to the patella, the synovial membrane is separated from the patellar tendon by an infrapatellar fat pad. Where it lies beneath the fat pad, the membrane projects into the joint as two fringes, alar folds, which bear villi. The folds converge posteriorly to form a single infrapatellar fold or plica (ligamentum mucosum), which curves posteriorly to its attachment in the femoral intercondylar fossa (Fig. 82.17). This fold may be a vestige of the inferior boundary of an originally separate femoropatellar joint. The extent of the infrapatellar plica ranges from a thin cord to a complete sheet that can obstruct the passage of instruments during knee arthroscopy. When substantial, it has been mistaken for the anterior cruciate ligament, which is directly posterior to it. The medial plica extends in the midline anteriorly from the medial alar fold medially to the suprapatellar pouch. Occasionally it can be thickened and inflamed, usually following acute or chronic trauma.

Fig. 82.16 Sagittal section through the left knee joint: lateral aspect.

Fig. 82.17 Left knee joint in full flexion: the quadriceps tendon has been sectioned and the patellar flap retracted distally. Compare with Fig. 82.14A.

The suprapatellar plicae are remnants of an embryonic septum that completely separates the suprapatellar pouch from the knee joint. Very occasionally a septum persists, either in its entirety, or perforated by a small peripheral opening. Loose bodies can lie hidden above this septum.

The infrapatellar fat pad is the largest part of a circumferential extrasynovial fatty ring which extends around the patellar margins (Newell 1991).

At the sides of the joint the synovial membrane descends from the femur and lines the capsule as far as the menisci, whose surfaces have no synovial covering. Posterior to the lateral meniscus the membrane forms a subpopliteal recess between a groove on the meniscal surface and the tendon of popliteus which may connect with the superior tibiofibular joint. The relationship of the synovial membrane to the cruciate ligaments is described above.

Bursae

Numerous bursae are associated with the knee. Anteriorly, there is a large subcutaneous prepatellar bursa between the lower half of the patella and skin; a small deep infrapatellar bursa between the tibia and patellar tendon; a subcutaneous infrapatellar bursa between the distal part of the tibial tuberosity and skin; and a large suprapatellar bursa which is the superior extension of the knee joint cavity (Fig. 82.16). Posterolaterally, there are bursae between the lateral head of gastrocnemius and the joint capsule (this bursa is sometimes continuous with the joint cavity); the fibular collateral ligament and the tendon of biceps femoris; the fibular collateral ligament and the tendon of popliteus; the tendon of popliteus and the lateral femoral condyle, which is usually an extension of the synovial cavity of the joint. The last two bursae may communicate with each other.

Medially, the arrangement of bursae is complex. The bursa between the medial head of gastrocnemius and the fibrous capsule is prolonged between the medial tendon of gastrocnemius and the tendon of semimembranosus (the semimembranosus bursa) and usually communicates with the joint. The bursa between the tendon of semimembranosus and the medial tibial condyle and the medial head of gastrocnemius may communicate with this bursa. There is a bursa between the medial collateral ligament and the tendons of sartorius, gracilis and semitendinosus (the ‘pes bursa’). Bursae that vary both in number and position lie deep to the medial collateral ligament between the capsule, femur, medial meniscus, tibia or tendon of semimembranosus. Occasionally there may be a bursa between the tendons of semimembranosus and semitendinosus.

Posteriorly, bursae associated with the knee are variable.

The clinically important bursae are the anterior group, the pes bursa, and the semimembranosus bursa. Inflammation of the subcutaneous prepatellar bursa and infrapatellar bursa are referred to colloquially as ‘housemaid’s knee’ and ‘clergyman’s knee’, respectively. The pes bursa can become inflamed in athletes. In adults, bursal inflammation producing a popliteal fossa swelling commonly occurs secondary to degeneration within the knee joint: regardless of size and position, it almost always arises from the plane between semimembranosus and the medial tendon of gastrocnemius.

Movements

Movements at the knee are customarily described as flexion, extension, internal (medial) and external (lateral) rotation. Flexion and extension differ from true hingeing in that the articular surface profiles of the femoral and tibial articular surfaces produce a variably placed axis of rotation during the flexion arc, and when the foot is fixed, flexion entails corresponding conjunct (coupled) external (lateral) rotation. These conjunct rotations are a product of the complex geometry of the articular surfaces and, to an extent, the disposition of the associated ligaments. There is differential motion in the medial and lateral tibiofemoral compartments. Laterally, there is considerable displacement of the femur on the tibia, with rolling as well as sliding at the joint surface. In contrast, medially, for most of the flexion arc there is minimal relative motion of the femur and tibia, and the motion almost exclusively involves one joint surface sliding on the other. In full flexion, the lateral femoral condyle is close to posterior subluxation off the lateral tibial articular surface. Medially, significant posterior femoral displacement only occurs when flexion exceeds 120°. The menisci move with the femoral condyles, the anterior horns more than the posterior, and the lateral meniscus considerably more than the medial.

The axial rotations have a smaller range than the arc of flexion and extension. These rotations are conjunct, and integral with flexion and extension, i.e. they are obligatory. They can also be adjunct and independent, i.e. voluntary, and are best demonstrated with the knee semi-flexed. The degree of axial rotation therefore varies with flexion and extension.

The range of extension is 5–10° beyond the ‘straight position’. Active flexion is approximately 120° with the hip extended, 140° when it is flexed, and 160° when aided by a passive element, e.g. sitting on the heels. Voluntary rotation is 60–70°, but conjunct rotation only 20°.

Conjunct medial rotation of the femur on the tibia in the later stages of extension is part of a ‘locking’ mechanism, the so-called ‘screw-home movement’, which is an asset when the fully extended knees are subjected to strain. Full extension results in the close-packed position, with maximal spiralization and tightening of the ligaments. The roles of the articular surfaces, musculature and ligaments in generating conjunct rotations remain controversial (Girgis et al 1975, Rajendran 1985), but the following points can be made. The lateral combined meniscotibial ‘receiving surface’ is smaller, more circular and more deeply concave. Since the articular surface is virtually convex in sagittal section the depth of the receiving surface is largely due to the presence of the lateral meniscus. The lateral femoral articular surface is also smaller. Consequently, the lateral femoral condyle approaches full congruence with the opposed surface some 30° before full extension (well before the medial condyle). Simple extension cannot continue, but medial rotation of the femur occurs on a vertical axis through its head and medial condyle: the medial femoral condyle moves very little in the sagittal plane and is stabilized by the ‘upslope’ of the anterior half of the medial tibia, while rotation of the lateral femoral condyle and meniscus brings the anterior horn of the latter onto the anterior ‘downslope’ of the lateral tibial condyle. Rotation and extension follow simultaneously and smoothly until final close packing of both condyles is accomplished. At the beginning of flexion from full extension (with the foot fixed) lateral femoral rotation occurs, which ‘unlocks’ the joint. While joint surfaces and many ligaments are involved, electromyographic evidence reveals that contraction of popliteus is important, and that it pulls down and backwards on the lateral femoral condyle, lateral to the axis of femoral rotation. It also retracts the posterior horn during lateral rotation and continuing flexion, via its attachment to the lateral meniscus, and so prevents traumatic compression.

Any position of extension adopted is a balance between forces (torque) extending the joint and passive mechanisms resisting them. The range near to close packing is functionally important. In symmetrical standing, the line of body weight is anterior to the transverse axes of the knee joints, but the passive mechanisms noted above preserve posture with minimal muscular effort (Joseph 1960). Active contraction of the extensors and a close-packed position only occur in asymmetrical postures, e.g. in leaning forward, heavy loading, or when powerful thrust is needed.

In extension, parts of both cruciate ligaments, the tibial and fibular collateral ligaments, the posterior capsular region, the oblique popliteal ligament, skin and fasciae are all taut. Passive and sometimes active tension exists in the hamstrings and gastrocnemius, and the anterior part of the medial meniscus is compressed between the femoral and tibial condyles. During extension the patellar tendon is tightened by quadriceps femoris but is relaxed in the erect attitude. When the knee flexes, the fibular collateral ligament and the posterior part of the medial collateral ligament relax but the cruciate ligaments and the anterior part of the medial collateral ligament remain taut: the posterior parts of the menisci are compressed between the femoral and tibial condyles. Flexion is checked by quadriceps femoris, anterior parts of the capsule, posterior cruciate ligament and compression of soft tissues behind the knee. In extreme passive flexion, contact of the calf with the thigh may be the limiting factor and parts of both cruciate ligaments are also tense. In addition to conjunct rotation with terminal extension or initial flexion, relaxed collateral ligaments also allow independent medial and lateral rotation (adjunct rotation) when the joint is flexed.

Articular kinematics

We now consider knee motion at a smaller scale, within the joint. This, of course, is difficult to separate from the actions of the ligaments that act as passive restraints to tibiofemoral joint movements, and are discussed below.

Sagittal sections of the knee reveal that the arcs of the femoral condyles are much longer than the anterior–posterior length of the tibial plateau. This means that if the knee flexed with a purely rolling motion, then the femur would roll off of the back of the tibia long before the knee reaches full flexion (Fig. 82.20). This does not happen because the femur slides anteriorly at the same time as it rolls posteriorly, and thus remains in correct articulation. If the knee were to possess a fully conforming roller-in-trough geometry, as in the humeroulnar joint, then flexion would occur by pure sliding movement between the joint surfaces. This conformity is not possible at the knee because it would inhibit the tibial internal-external rotation that is needed during locomotion.

In the locked, fully-extended knee, the anterodistal femoral articular surfaces press onto the anterior horns of the menisci. This tends to cause the femur to slide posteriorly, tensing the ACL and slackening the PCL. With knee flexion, the femur lifts off of the anterior horns of the menisci, leading to contact between the smaller radii of the posterior parts of the femoral condyles, and the tibial plateau plus posterior horns of the menisci. This means that the centre of contact moves posteriorly in early knee flexion. After this, the femoral condyles have approximately circular sagittal sections. The femur is now prevented from rolling back any further by tension in the ACL. The action of the ACL is similar to that of a rope attaching the back of a car to the ground: when the car tries to drive away, the wheels roll until the rope tightens, and then spin in one position (sliding motion).

Articular mechanics

The combined effect of the external and internal loads on the knee is to impose considerable forces on the articular cartilage. This may be analysed into a vertical (compressive) component and a horizontal (shear or friction) component. The friction component is a combination of the compressive force and the friction characteristics of the joint surfaces.

Compression

The compressive force is distributed over an area to produce a contact pressure (contact stress). The contact pressure is, therefore, dependent on the area of contact as well as the load itself. Fully-conforming articular surface geometry would allow the greatest area of contact, and thus would minimize contact pressure, however, this is not present in the knee. The medial compartment of the knee is semi-conforming with a convex femoral condyle articulating over a concave medial tibial plateau. The lateral compartment has less conformity: the lateral tibial plateau is flat or slightly convex in sagittal sections. These different shapes reflect the differential movement of the medial and lateral compartments. In normal movement, the screw home rotation of the tibia occurs about a medial axis which means that most of the rotation of the tibia is due to an anterior–posterior translation of the lateral compartment (Fig. 82.21).

Fig. 82.21 The shape of the articular surfaces providing mobility of the lateral compartment.

In order to maintain some degree of conformity, and thus minimize contact pressure, the menisci have wedge-shaped cross-sections that fit between the femur and tibia around the bones. This serves to increase the area over which the compressive force on the knee is distributed. In the absence of the menisci, the load is carried by a much smaller area of cartilage, resulting in higher contact stresses on the articular cartilage. This explains the prevalence of osteoarthrosis following meniscectomy (Fig. 82.22).

Fig. 82.22 The function of the menisci in increasing articular conformity, with increased peak pressure after meniscectomy.

The menisci are most firmly attached at the tibial intercondylar eminence and so they can translate anteriorly and posteriorly to ‘follow’ the femoral rollback. The lateral meniscus is more mobile, because it is attached to the capsule less tightly than the medial meniscus. The anterior–posterior movement of the menisci is approximately half the magnitude of the anterior–posterior movement of the femur, suggesting that the conformity of the joint changes during knee joint flexion.

The distal aspect of the femur, resting on the menisci in the extended knee, has a large radius of curvature, and so it fits against the entire area of the menisci. However, as the knee flexes, the smaller radii of the posterior parts of the femoral condyles cause the femur to lift off the anterior horns of the menisci, and the femoral contact is now solely on the posterior horns in the flexed knee. This, combined with the large joint forces that are generated when arising from a seated position, explains why there are often spontaneous tears of the posterior horns in older patients, when standing up from a squatting position.

The load-carrying mode of the menisci is shown in Fig. 82.23. When the femoral condyle presses down on the meniscus, it tends to squeeze the meniscus out of the joint, because of its tapered cross-section. This causes the diameter of the circular shape of the meniscus (and therefore the meniscal circumference) to increase. This is resisted by ‘hoop tension’ in strong fibres that pass around the periphery of the meniscus, and transmit the tension to the tibial plateau via strong insertional ligaments. Because the tissue is adapted to resist hoop stresses, it has much greater hoop strength (approximately 100 MPa) than radial strength (approximately 3 MPa), which explains why bucket handle tears occur. It also explains why a circumferential tear does not have such a serious effect on meniscal function, because the circumferential fibres can still transmit the loads, whereas a radial tear breaks the load-carrying fibres.

Fig. 82.23 The load-carrying mode of the menisci – conversion of axial load into meniscal hoop stresses.

Friction

The knee acts as a bearing that transmits forces and movements between the limb segments. Load-bearing synovial joints move with remarkably little friction, and their surfaces must withstand many millions of impact loads which tend to cause fatigue failure and breakdown of the surfaces.

During walking, every step involves phases of action that vary the loading and velocity conditions at the knee joint. A variety of lubrication mechanisms normally prevent joint surface damage. Thus, in the swing phase, when the foot is off the ground, the joint surfaces are loaded lightly, and have high relative velocity while the knee flexes and extends. This allows the synovial fluid to separate the surfaces, giving fluid film lubrication, with very low friction and no wear (Fig. 82.24). At the time of heel strike, a large impact load acts to compress the surfaces together. At a microscopic level, the surfaces do not come into contact, because of the squeeze film effect: in essence, the impact occurs so rapidly (less than 0.1 sec) that the synovial fluid cannot all be squeezed out of the joint space, because of its viscosity and the narrowness of the space. In the mid stance phase, the flexion–extension motion again entrains the synovial fluid in between the joint surfaces, producing what is known as a hydrodynamic effect: the fluid is trapped between the surfaces by the motion, and therefore it acts to separate them. Finally, when the foot is pushing off the body weight, there is little motion, and the thin fluid film diminishes under the compressive load. If the squeeze-film effect is insufficient, then the joint surfaces would now come into direct contact, were it not for the fact that the synovial fluid contains large protein molecules that are trapped on the cartilage surfaces when the fluid is expelled. This molecular layer acts as a boundary lubricant, protecting the cartilage in the same way that grease protects a synthetic bearing.

Fig. 82.24 The different modes of knee joint motion and lubrication when walking.

Many aspects of the arthritic breakdown of the cartilage can be explained on the basis of lubrication biomechanics. Thus, in joints affected by osteoarthrosis, synovial fluid is known to have a lower viscosity than that in normal joints. The viscosity is lower still in rheumatoid joints, and unable to prevent attritional damage to the cartilage surfaces.

Primary and secondary restraints

On testing the laxity in any of the degrees of freedom of the joint (e.g. in the anterior drawer test, a bedside clinical test in which the subject is placed in the supine position with the knee to be tested flexed to 80–90° before passive forward traction is applied to the tibia), there are usually combinations of ligaments that are tensed. Some of these are better aligned to resist the applied load or displacement. These are termed primary restraints, and are exemplified by the cruciate ligaments and the medial and lateral collateral ligaments. Secondary restraints are less well-aligned, but still have a significant restraining effect. These are exemplified by the menisci, and by the meniscofemoral ligaments. In the example in Fig. 82.25, the ACL is well-aligned to resist the applied anterior drawer force. With an absent ACL, the superficial MCL can resist the applied force, however, it does so by being loaded to a much higher level than the original loading on the ACL. The size of the lines in the vector diagram demonstrate this principle: although joint laxity may remain normal initially following rupture of a primary restraint, it may subsequently result in the overload of a secondary restraint, and ultimately, in further soft-tissue failure.

Fig. 82.25 Primary and secondary restraints to anteroposterior forces – example of the ACL and superficial MCL.

The patellofemoral joint is most heavily loaded during weight-bearing activities when the knee is flexed. Analyses of rising from a chair have predicted that the patellar tendon tension at 90° flexion may then be 2.9 BW, and so the joint force will be 5.5 BW. This is higher than the tibiofemoral joint, which is loaded to 3.3 BW at the same time (Amis & Farahmand 1996).

In the frontal plane the quadriceps muscles and patellar tendon tensions combine to cause a lateralizing force vector termed the Q-angle effect. The Q angle is defined as the difference between the resultant force vector of the quadriceps, which is normally parallel to the femoral shaft, and the patellar tendon (Fig. 82.26). Clinically, the Q angle is changed by the position of hip rotation, tibial rotation, and quadriceps tension. The clinical Q-angle is 12–15° (males) and 15–18° (females), which means that there is a greater lateralizing force vector on the patellofemoral joint in females. Contraction of the quadriceps, therefore, tends to displace the patella laterally; this is resisted by the geometry of the joint and by the passive stabilizers. Vastus medialis obliquus (VMO) acts medially and posteriorly as much as it acts proximally, and so its tension helps to resist the Q angle effect.

Fig. 82.26 Patellofemoral joint force at extension (A) and at 90° flexion (B).

Popliteus

Popliteal artery

The popliteal artery is the continuation of the femoral artery and crosses the popliteal fossa (Fig. 82.4). It descends laterally from the opening in adductor magnus to the femoral intercondylar fossa, inclining obliquely to the distal border of popliteus, where it divides into the anterior and posterior tibial arteries. This division usually occurs at the proximal end of the asymmetrical crural interosseous space between the wide tibial metaphysis and the slender fibular metaphysis. The artery is relatively tethered at the adductor magnus hiatus and again distally by the fascia related to soleus, and is therefore susceptible to traction damage in knee injuries, e.g. dislocation.

Popliteal vein

The popliteal vein ascends through the popliteal fossa to the opening in adductor magnus where it becomes the femoral vein (see Fig. 79.9B). Its relationship to the popliteal artery changes as the vein ascends. Distally it is medial to the artery, between the heads of gastrocnemius it is superficial (posterior) to it, and proximal to the knee joint it is posterolateral to the artery. Its tributaries are the short saphenous vein, veins corresponding to branches of the popliteal artery, and muscular veins, including a large branch from each head of gastrocnemius. There are usually four or five valves in the popliteal vein.

Long saphenous vein

The course of the long saphenous vein is described in Chapter 80.

Short saphenous vein

The short saphenous vein (small saphenous vein) begins posterior to the lateral malleolus as a continuation of the lateral marginal vein (see Fig. 79.9B). In the lower third of the calf it ascends lateral to the calcaneal tendon, lying on the deep fascia and covered only by superficial fascia and skin. Inclining medially to reach the midline of the calf, it penetrates the deep fascia, within which it ascends on gastrocnemius, only emerging between the deep fascia and gastrocnemius gradually at about the junction of the intermediate and proximal thirds of the calf (usually well below the lower limit of the popliteal fossa). Continuing its ascent, it passes between the heads of gastrocnemius then proceeds to its termination in the popliteal vein in the popliteal fossa, 3–7.5 cm above the knee joint.

Popliteal lymph nodes

There are usually six small lymph nodes embedded in the fat of the popliteal fossa. One, near the termination of the short saphenous vein, drains the superficial region served by the vein. Another lies between the popliteal artery and the posterior aspect of the knee joint, receiving direct vessels from the knee joint and those accompanying the genicular arteries. The remainder flank the popliteal vessels, receiving trunks that accompany the anterior and posterior tibial vessels. Popliteal efferents ascend close to the femoral vessels to reach the deep inguinal nodes; some may accompany the long saphenous vein to the superficial inguinal nodes.

Inflammation of the popliteal nodes is often caused by lateral lesions of the heel.

Saphenous nerve

The saphenous nerve is the largest and longest cutaneous branch of the femoral nerve. It descends lateral to the femoral artery in the femoral triangle and enters the adductor canal, where it crosses in front of the artery to lie medial to the artery. At the distal end of the canal it leaves the artery and emerges through the aponeurotic covering with the saphenous branch of the descending genicular artery. As it leaves the adductor canal it gives off an infrapatellar branch that contributes to the peripatellar plexus (p. 1393) and then pierces the fascia lata between the tendons of sartorius and gracilis, becoming subcutaneous to supply the prepatellar skin. It descends along the medial tibial border with the long saphenous vein and divides distally into a branch which continues along the tibia to the ankle and a branch which passes anterior to the ankle to supply the skin on the medial side of the foot, often as far as the first metatarsophalangeal joint. The saphenous nerve connects with the medial branch of the superficial fibular nerve. Near midthigh, it gives a branch to the subsartorial plexus. The nerve may be subject to an entrapment neuropathy as it leaves the adductor canal.