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//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.

  

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.

Femoral surface

The femoral condyles, bearing articular cartilage, are almost wholly convex. Opinions as to the contours of their sagittal profiles tend to vary. One view is that they are spiral with a curvature increasing posteriorly (‘a closing helix’); that of the lateral condyle being greater. An alternative view is that the articular surface for contact with the tibia on the medial femoral condyle describes the arcs of two circles. According to this view, the anterior arc makes contact with the tibia near extension and is part of a virtual circle of larger radius than the more posterior arc, which makes contact during flexion. The lateral femoral condyle is believed to describe a single arc and thus to possess a single radius of curvature.

Tibiofemoral congruence is improved by the menisci, which are shaped to produce concavity of the surfaces presented to the femur; the combined lateral tibiomeniscal surface is deeper. The lateral femoral condyle has a faint groove anteriorly which rests on the peripheral edge of the lateral meniscus in full extension. A similar groove appears on the medial condyle, but does not reach its lateral border, where a narrow strip contacts the medial patellar articular surface in full flexion. These grooves demarcate the femoral patellar and condylar surfaces. The differences between the shapes of the articulating surfaces correlate with the movements of the joint.

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.

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.

  

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.

Capsule and retinacula

The capsule is a fibrous membrane of variable thickness. Anteriorly it is replaced by the patellar tendon and does not pass proximal to the patella or over the patellar area. Elsewhere it lies deep to expansions from vasti medialis and lateralis, separated from them by a plane of vascularized loose connective tissue. The expansions are attached to the patellar margins and patellar tendon, extending back to the corresponding collateral ligaments and distally to the tibial condyles. They form medial and lateral patellar retinacula, the lateral being reinforced by the iliotibial tract.

Posteriorly the capsule contains vertical fibres that arise from the articular margins of the femoral condyles and intercondylar notch and from the proximal tibia. The fibres mainly pass downwards and somewhat medially. The oblique popliteal ligament is a well-defined thickening across the posteromedial aspect of the capsule, and is essentially an extension from the tendon of insertion of semimembranosus.

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.

Accessory movements

Wider rotation can be obtained by passive movements when the knee is semi-flexed. To a limited extent, the tibia can also be translated backwards and forwards on the femur. Abduction and adduction are prevented in full extension by the collateral ligaments and secondary restraints such as the cruciate ligaments. With the knee slightly flexed, limited adduction and abduction are possible, both passive and active. Slight separation of the femur and tibia can be achieved by strong traction on the leg with countertraction applied to the thigh.

Muscles producing the movements

Flexion

Biceps femoris, semitendinosus and semimembranosus, assisted by gracilis, sartorius and popliteus. With the foot stationary, gastrocnemius and plantaris also assist (See Fig. 82.3).

Extension

Quadriceps femoris, assisted by tensor fasciae latae.

Medial rotation of the flexed leg

Popliteus, semimembranosus and semitendinosus, assisted by sartorius and gracilis.

Lateral rotation of the flexed leg

Biceps femoris.

Relations and ‘at risk’ structures

Anteriorly are the tendon of quadriceps femoris (which encloses and is attached to the non-articular surfaces of the patella), the patellar tendon, tendinous expansions from vastus medialis and lateralis (which extend over the anteromedial and anterolateral aspects of the capsule respectively), and the patellar retinacula. Posteromedial is sartorius, and the tendon of gracilis which lies along its posterior border, both descending across the joint. Posterolaterally, the tendon of biceps and the common fibular nerve (which lies medial to the tendon) are in contact with the capsule, and thereby separated from the tendon of popliteus. Posteriorly the popliteal artery and associated lymph nodes lie posterior to the oblique popliteal ligament; the popliteal vein is posteromedial or medial, and the tibial nerve is posterior to both. The nerve and vessels are overlapped by both heads of gastrocnemius and laterally by plantaris. Gastrocnemius contacts the capsules on either side of the vessels. Semimembranosus lies between the capsule and semitendinosus, medial to the medial head of gastrocnemius.

Factors maintaining stability

The control of the stability of the knee is of considerable importance.

Patellofemoral joint

The alignment of the femoral and tibial shafts is such that the pull of the quadriceps on the patella imparts a force on the patella that is directed both superiorly and laterally. The static bony factors that counter this tendency to move laterally are the congruity of the patellofemoral joint and the buttressing effect of the larger lateral part of the trochlear groove. Instability may result if the patella is small, or resides too high above the trochlea, or if the trochlear groove is too shallow. The static ligamentous factors are the medial patellofemoral ligament and medial patellar retinaculum.

Dynamic muscular control is important. The most distal part of vastus medialis (vastus medialis obliquus) consists of transverse fibres that are attached directly to the medial edge of the patella: these pull the patella medially, countering the tendency to lateral movement. This is the muscle that is preferentially strengthened in a physiotherapy programme aimed at treating patellofemoral problems.

Tibiofemoral joint

The tibiofemoral joint surfaces are inherently unstable, especially laterally. Medially some stability is afforded by the relatively concave tibial surface and the relatively fixed posterior horn of the medial meniscus. Both medially and laterally the menisci are helpful, particularly as they move with the femoral condyles. Ligaments play a major role in providing stability: their function is partly static, in that they bind the bones in positions of extreme stress, but they also provide proprioceptive feedback which aids coordination of stabilizing muscle activity. Taking a somewhat ‘two-dimensional’ view, the medial and lateral collateral ligaments may be considered as resistors of valgus and varus forces on the knee respectively, and the anterior and posterior cruciate ligaments as resistors of anterior and posterior tibial translation respectively. However in reality the situation is more complex than this. The stresses are rarely applied in orthogonal planes and so a combination of forces, especially rotational, is involved. Moreover, many structures other than the collateral and cruciate ligaments are involved in stabilizing the joint. The ‘posterolateral corner’, which resists tibial external rotation, consists of the popliteofibular, fabellofibular, arcuate, and lateral collateral ligaments and iliotibial band, together with popliteus, the lateral head of gastrocnemius and biceps femoris. The ‘posteromedial corner’, which resists tibial rotation, consists of the posterior oblique portion of the superficial medial collateral ligament, the capsule (including the oblique popliteal ligament) and semimembranosus. Since stresses are often a combination of force plus rotation, structures usually operate together rather than in isolation.

Loading at the knee

During level walking, the force across the tibiofemoral joint for most of the cycle is between two and four times body weight, and can be more. In contrast, the force across the patellofemoral joint is no more than 50% body weight. Peak force transmission across the joint increases sequentially as the menisci, articular cartilage and subchondral bone are damaged or removed. Walking up or down stairs has little influence on tibiofemoral forces, but significantly increases patellofemoral forces to two (walking up) or three (walking down) times body weight, reflecting the changed angle of the quadriceps tendon and patellar tendon during flexion. There are two mechanisms for ameliorating forces transmitted across the patella: the extensor lever arm is lengthened as the axis of rotation moves posteriorly during flexion, and the contact area between the patella and femur almost triples between 30° and 90°. To cope with the potential large forces generated by activities such as running, the patella has the thickest articular cartilage in the body.

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