1 Bones, Ligaments, and Joints
1.1 The Lower Limb: General Aspects
A Unique features and specialized function of the human lower limb
The evolution of the lower limb into a mechanism specifically adapted for bipedal locomotion, along with the specialization of the upper limb for visually guided manipulation, is a distinctive feature of the anatomy of the human primate. The uniquely human conformation of shapes and proportions is the end result of a process that rearranged the primate center of gravity and the positions of internal organs, dramatically altering the form and biomechanics of the trunk to produce a progressively more efficient bipedal gait. Other primates have the capacity to assume an erect body posture and to walk upright, but only for short periods and at a much greater relative expenditure of energy. The habitual upright gait of humans has been achieved through a series of anatomic adaptations of the musculoskeletal system. The most critical of these adaptations occurred in the vertebral column and pelvis. The design of the human vertebral column differs markedly from that of other primates: the simple “arch and cord” construction of the chimpanzee spine has been abandoned in favor of the human double-S-shaped curve, which allows the human axial skeleton to act as a shock-absorbing spring (see p. 105), while shifting the entire weight of the trunk over the load-bearing surface of the feet. This shift to an upright posture has imposed the full weight of the abdominal viscera upon the pelvis. Concomitantly, the iliac wings of the pelvis have spread farther apart, and the sacrum has broadened, to generate a structure in humans that is now specialized for bearing the load of the viscera. The efficiency of upright gait has been improved further by stabilization of the pelvis and secure anchoring to the spine via the sacrum. The unique proportions of the human lower limb provide a dramatic demonstration of the extent of this specialization. Because their function is more exclusively directed toward support and locomotion, the legs are exceptionally long and powerful in humans. While the leg length is 111% of trunk length in orangutans and 128% in chimpanzees, it measures 171% of trunk length in humans. The specialization of the human lower limb for bipedal gait is also reflected in the substantial changes in function of certain muscles, particularly the gluteal muscles, the knee-joint extensors, and the muscles of the calf.
B Overview of the skeleton of the lower limb
a Right lower limb, anterior view.
b Right lower limb, posterior view (the foot is in maximum plantar flexion in both views).
As in the upper limb, the skeleton of the lower limb consists of a limb girdle and the attached free limb.
• The pelvic girdle in adults is formed by the paired hip bones (ossa coxae, innominate bones). They differ from the shoulder girdle in that they are firmly integrated into the axial skeleton through the sacroiliac joints (see p. 144). The two hip bones combine with the sacrum and pubic symphysis to form the pelvic ring (see p. 415).
• The free limb consists of the thigh (femur), the leg (tibia and fibula), and the foot. It is connected to the pelvic girdle by the hip joint.
C Palpable bony prominences of the right lower limb
a Anterior view, b posterior view.
Almost all the skeletal elements of the lower limb have bony prominences, margins, or surfaces (e.g., the medial tibial surface) that can be palpated through the skin and soft tissues. The only exceptions are structures that are largely covered by muscle, such as the hip joint, the neck and shaft of the femur, and large portions of the fibular shaft. Several standard anatomic landmarks have been defined in the lower limb for use in measuring the length of the leg and certain skeletal elements. They are the anterior superior iliac spine, the greater trochanter of the femur, the medial joint space of the knee (superior margin of the medial tibial condyle), and the medial malleolus. The clinical evaluation of leg length discrepancy is important because “true” shortening of the leg (a disparity of anatomic leg lengths), as well as functional leg shortening (e.g., due to muscle contractures), can lead to pelvic tilt and scoliotic deformity of the spine (see p. 135).
D Leg length measurement in the standing position
Leg length discrepancy can be measured with reasonable accuracy in the standing patient by placing wooden blocks of known thickness (0.5 cm, 1 cm, 2 cm) beneath the foot of the shorter leg until the pelvis is horizontal. Horizontal position is confirmed when noting that both iliac crests are at the same level when palpated from behind and the anal fissure is vertical. If the pelvis cannot be leveled by placing blocks under the apparently shorter limb, then a “functional” leg length discrepancy is present rather than a “true” discrepancy. Most cases of this kind are caused by a fixed pelvic tilt secondary to a hip joint contracture or scoliosis. The measured leg lengths in these cases may actually be equal, and the pelvic tilt only mimics a length discrepancy.
1.2 The Anatomic and Mechanical Axes of the Lower Limb
A The mechanical axis of the leg (the Mikulicz line)
a Normal mechanical axis, anterior view.
b Mechanical axis in genu varum, posterior view.
c Mechanical axis in genu valgum, posterior view.
In an individual with normal axial alignment, the large joints of the lower limb (the hip, knee, and ankle) lie on a straight line that represents the mechanical longitudinal axis of the leg (the Mikulicz line). This mechanical axis extends from the center of rotation of the femoral head through the intercondylar eminence of the tibial plateau and down through the center of the ankle mortise (the pocket created by the fibula and tibia for the talus in the ankle joint, from Arabic, murtazz, fastened). While the mechanical axis and anatomic axis coincide in the tibial shaft, the anatomic and mechanical axes of the femoral shaft diverge at a 6° angle. Thus, the longitudinal anatomic axes of the femur and tibia do not lie on a straight line but form a laterally open angle of 174° at the level of the knee joint in the coronal plane (the femorotibial angle). In genu varum (b) the center of the knee joint is lateral to the mechanical axis, and in genu valgum (c) it is medial to the mechanical axis. Both conditions impose abnormal, unbalanced loads on the joints (see B) that gradually cause degenerative changes to develop in the bone and cartilage (osteoarthritis of the knee) accompanied by stretching of the associated joint capsule, ligaments, and muscles. In genu varum (b), for example, the medial joint complex of the knee is subjected to abnormal pressure, while the lateral joint structures (e.g., the lateral collateral ligament), iliotibial tract, and biceps femoris muscle are subjected to abnormal tension. Genu varum also places greater stress on the lateral border of the foot, resulting in a fallen pedal arch.
B Position of the mechanical axes with the feet slightly apart and together
a In upright stance with the feet placed slightly apart, the mechanical axis runs almost vertically through the center of the three large joints.
b The legs are generally considered “straight” if, when the feet are together, the opposing medial malleoli and knees are touching. Accordingly, the intercondylar distance and the intermalleolar distance between the legs provide an index for the measurement of genu varum and genu valgum. When this stance is attempted, an intercondylar distance greater than 3 cm or an intermalleolar distance greater than 5 cm is considered abnormal (see C).
C The normal leg axes at different ages
a Infant, b small child, c school-age child.
Up to about 20° of genu varum is considered normal during the first year of life. Up to about 10° of genu valgum is also considered normal through 2 years of age. By the time the child enters school, the legs are essentially straight as a result of musculoskeletal growth.
D Normal anatomic position in relation to the line of gravity
Right lateral view. The line of gravity runs vertically from the whole-body center of gravity to the ground. In normal upright humans, it intersects the external auditory canal, the dens of the axis (dental process C 2), the inflection points between the normal curves in the vertebral column (between the cervical and thoracic curves, and thoracic and lumbar curves), the whole-body center of gravity, and the hip, knee, and ankle joints. Chronic deviation of any reference point from this line imposes abnormal stresses on different clusters of musculoskeletal elements.
E Skeleton of the right lower limb
Right lateral view.
1.3 The Bones of the Pelvic Girdle
A The right hip bone
B The right hip bone
C Column principle of the hip bone
a Lateral view, b medial view.
Major external trauma (e.g., traffic accidents) often result in fractures of the pelvis or acetabulum. In order to classify such fractures, the hip bone or acetabulum (according to Letournel) is divided into a short posterior column and a significantly larger anterior column. The major lines of force of the pelvis run through these two columns. In case of major trauma, the impact is first transmitted through the femoral neck to the acetabulum and then to the columns. The type of fracture depends on the position of the femoral neck at the time of forceful impact (compare page 435).
E The pelvic girdle and pelvic ring
Anterior view. The paired hip bones that make up the pelvic girdle are connected to each other at the cartilaginous pubic symphysis and to the sacrum at the sacroiliac joints (see p. 144). This creates a stable ring, the bony pelvic ring (shaded in red), that permits very little motion. This stability throughout the pelvic ring is an important prerequisite for the transfer of trunk loads to the lower limb necessary for normal gait.
F The triradiate cartilage of a right hip bone: the junction of the ilium, ischium, and pubis.
G Schematic radiograph of the right acetabulum of a child
Lateral view (lateral projection). The bony elements of the hip bone come together in the acetabulum, with the ilium and ischium each comprising two fifths of the acetabulum and the pubis one fifth. Definitive fusion of the Y-shaped growth plate (triradiate cartilage) occurs between the 14th and 16th years of life.
1.4 The Femur: Importance of the Femoral Neck Angle
A The right femur
B The right femur
C The arrangement and prominence of tension trabeculae and compression trabeculae as a function of the femoral neck angle
Right femur, anterior view.
a Coronal section through the right hip joint at the level of the fovea on the femoral head. The angle between the longitudinal axis of the femoral neck and the axis of the femoral shaft is called the femoral neck angle or CCD angle (centrum-collum-diaphysis angle). This angle normally measures approximately 126° in adults and 150° in newborns. It decreases continually during growth due to the constant bone remodeling that occurs in response to the changing stress patterns across the hip.
b The trabecular pattern associated with a normal femoral neck angle.
c–e Radiographs in the sagittal projection.
c Normal femoral neck angle with a normal bending load.
d A decreased femoral neck angle (coxa vara) leads to a greater bending load with higher tensile stresses, thereby stimulating the formation of more tension trabeculae.
e An increased femoral neck angle (coxa valga) leads to a greater pressure load with higher compressive stresses, stimulating the formation of more compression trabeculae.
D Compressive and tensile stresses in a bone model
a An axial (centered) weight placed atop a Plexiglas model of a pillar creates a uniform pressure load that is evenly distributed over the cross section of the pillar and whose sum is equal to the applied weight.
b A nonaxial (eccentric) weight placed on an overhang creates a bending load that generates both tensile and compressive stresses in the pillar.
E The principle of the tension band (after Pauwels)
a The bending load acting on an I-beam model can be reduced by placing a high-tensile-strength member (chain) on the side opposite the bending force. This added member transforms the bending load into a pure compressive load.
b In the leg, the fascia lata on the lateral side of the thigh is thickened to form the iliotibial tract (see p. 485). By functioning as a tension band, the iliotibial tract reduces the bending loads on the proximal femur.
1.5 The Femoral Headand Deformities of the Femoral Neck
A The right femur
Proximal view. For clarity, the acetabulum has been sectioned in the horizontal plane. The distal end of the femur (with patella) has been added in light shading.
a Hip joint with centered femoral head, b hip joint externally rotated, c hip joint internally rotated.
Note the orientation of the acetabulum, which is angled forward by approximately 17°. This anterior angle affects the stability and “seating” of the femoral head in the hip joint (see p. 429). With the femoral head centered in the acetabulum and the femur slightly medially rotated (a), the distal femur and thus the knee joint point slightly inward (physiologic internal rotation of the knee). The position of the foot is additionally influenced by the external rotation of the tibia (see p. 423) (see D).
B The right femur
Note the reversal of perspective from A.
C The right femur.
Note the transverse condylar axis and the femoral neck axis. When the axes are superimposed, the two lines intersect each other at a 12° angle in adults (anteversion angle, see also D and A). This angle is considerably larger at birth, measuring 30° to 40°, but decreases to the normal adult value by the end of the 2nd decade.
D Rotational deformities of the femoral neck
Right hip joint, superior view. Increased or decreased torsion of the femoral shaft results in torsion angles of varying size. When the hip is centered, this leads to increased internal or external rotation of the leg with a corresponding change in gait (a “toeing-in” or “toeing-out” gait). When the condylar axis is taken as the reference point, femoral torsion may be described as normal (a), increased (b), or decreased (c).
a A normal anteversion angle of approximately 12° with the foot directed forward (taking into account tibial torsion of 23°, see p. 423).
b An increased anteversion angle (coxa anteverta) typically leads to a toeing-in gait accompanied by a pronounced limitation of external rotation.
c The femoral neck is retroverted (points backward in relation to the condylar axis). The result is coxa retroverta with a toeing-out gait.
1.6 The Patella
A Location of the patella
Right knee joint, lateral view. The red line indicates the plane of section in C.
B Right patella
a Anterior view, b posterior view, c distal view.
Note that the apex of the patella points downward.
C Cross section through the femoropatellar joint
Right knee, distal view. The level of the cross section is shown in A. The femoropatellar joint is the site where the patellar surface of the femur, often called the femoral trochlea (by analogy with the distal humerus), articulates with the posterior articular surface of the patella. The patella is a sesamoid bone (the largest sesamoid) embedded in the quadriceps tendon. The patella is well centered when the ridge on the undersurface of the patella is seated within the groove of the femoral trochlea. The main functional role of the patella is to lengthen the effective lever arm of the quadriceps femoris muscle (the only extensor muscle of the knee), thereby reducing the force required to extend the knee joint (see also p. 488).
D Bipartite patella
Because the patella develops from multiple ossification centers, the failure of an ossification center to fuse results in a two-part (bipartite) patella. The upper lateral quadrant of the patella is most commonly affected. A fracture should always be considered in the radiographic differential diagnosis of a bipartite patella.
E The evaluation of patellar shape
Diagrams of tangential radiographs of the patella (“sunrise” view: supine position, knee flexed 60°, caudocranial beam directed parallel to the posterior patellar surface). Each diagram shows the relation of the patella to the femoral trochlea in a horizontal plane through the right knee joint. The posterior articular surface of the patella bears a vertical ridge dividing it into a lateral facet and a medial facet. Generally, the lateral facet is slightly concave, while the medial facet is slightly convex. The angle between the lateral and medial facets, called the patellar facet angle, is normally 130° ± 10°. Wiberg, Baumgart, and Ficat devised the following scheme for the classification of patellar shape based on the facet angle:
a Patella with medial and lateral facets of approximately equal size and a facet angle within the normal range.
b Most common patellar shape with a slightly smaller medial facet.
c A distinctly smaller medial facet (“medial hypoplasia”).
d Patellar dysplasia with a very steep medial facet (“hunter’s hat” configuration).
Besides the various patellar shapes, the patellar surface of the femur (the femoral trochlea) has a variable morphology (described in the Hepp classification system). Developmental dysplasias of the patella and femoral trochlea lead to patellar instability marked by recurrent lateral or medial subluxation or dislocation of the patella.
1.7 The Tibia and Fibula
A The tibia, fibula, and crural interosseous membrane
Right leg, anterior view. The tibia and fibula articulate at two joints that allow only very limited motion (rotation). Proximally, near the knee, is the synovial tibiofibular joint; distally, at the ankle, is the tibiofibular syndesmosis (fibrous joint with bony elements united by ligaments). The crural interosseous membrane (see also F) is a sheet of tough connective tissue that serves as an origin for several muscles in the leg. Additionally, it acts with the tibiofibular syndesmosis to stabilize the ankle mortise.
B The tibia, fibula, and crural interosseous membrane
Right leg, posterior view. In adults the tibial plateau has a 5-7 degree posterior to anterior inclination (known as the “tibial slope”).
C Normal orientation of the tibia and its role in stability
When the transverse axes of the upper tibia (tibial plateau) and lower tibia (ankle mortise) are superimposed, they form an angle of approximately 23°; i. e., the transverse axis of the ankle joint is rotated 23° laterally relative to the transverse axis of the tibial plateau (normal tibial orientation, a). As a result of this, the longitudinal anatomic axis of the foot does not lie in the sagittal plane, and the toes point outward when the upper tibia is directed forward (b). This significantly improves the stability of bipedal stance by placing the line of gravity close to the center of the area of support (see p. 411).
D The right tibial plateau
E The right ankle mortise
F Cross section through the middle third of the right leg
1.8 The Bones of the Footfrom the Dorsal and Plantar Views
A The bones of the right foot
B The right talus and calcaneus
Dorsal view. The two tarsal bones have been separated at the subtalar joint to demonstrate their articular surfaces.
C Anatomic subdivisions of the pedal skeleton
Right foot, dorsal view. In the nomenclature of descriptive anatomy, the skeletal elements of the foot are subdivided into three adjoining segments:
• The tarsus, composed of seven bones.
• The metatarsus, composed of five bones.
• The antetarsus, composed of 14 phalanges.
Compare this diagram with the functional subdivisions in D.
D Functional subdivisions of the pedal skeleton
Right foot, dorsal view. The skeleton of the foot is often subdivided as follows based on functional and clinical criteria:
• The hindfoot (calcaneus and talus)
• The midfoot (cuboid, navicular, and cuneiforms)
• The forefoot (the metatarsals and phalanges)
E The bones of the right foot
F The right talus and calcaneus
Plantar view. The two tarsal bones have been separated at the subtalar joint to demonstrate their articular surfaces.
1.9 The Bones of the Foot from the Lateral and Medial Views; Accessory Tarsal Bones
A The right talus and calcaneus
Medial view. The two tarsal bones have been separated at the subtalar joint to demonstrate their articular surfaces.
B The bones of the right foot
C Sustentaculum tali (“supports talus”)
• The sustentaculum tali is a bony protuberance of the medial calcaneus.
• It is palpable approximately 1.5 cm below the tip of the medial malleolus.
• It supports the talus on its balcony-like protrusion.
• It marks the end of the tarsal canal, which divides the two chambers of the lower ankle joint (see p. 463).
• It serves as a support for the tendon of the flexor hallucis longus (thereby supporting the calcaneus in upright position, see p. 470).
• It has a longitudinal groove for the flexor digitorum longus (see p. 470).
• Two ligaments insert at the sustentaculum tali: the spring ligament and part of the deltoid ligament (see p. 464).
• It is a common location for fractures associated with snowboarding.
C The right talus and calcaneus
Lateral view. The two tarsal bones have been separated at the subtalar joint to demonstrate their articular surfaces.
D The bones of the right foot
E Accessory tarsal bones
Right foot, dorsal view. A number of accessory (inconstant) ossicles are sometimes found in the foot. While they rarely cause complaints, they do require differentiation from fractures. A clinically important accessory bone is the external tibial bone, which can be a source of discomfort when tight shoes are worn.
1.10 The Hip Joint: Articulating Bones
A The right hip joint from the anterior view
In the hip joint, the head of the femur articulates with the acetabulum of the pelvis. Owing to the shape of the two articulating bones, the joint is a special type of spheroidal (ball-and-socket) joint. The roughly spherical femoral head, which has an average radius of curvature of approximately 2.5 cm, is largely contained within the acetabulum (see also C).
B The right hip joint from the posterior view
C Transverse angle of the acetabular inlet plane in the adult
Right hip joint, anterior view. Coronal section at the level of the acetabular fossa. The acetabular inlet plane, or bony acetabular rim, faces inferolaterally (transverse angle) and also anteroinferiorly (sagittal angle; see D). The inferolateral tilt of the acetabulum can be determined by drawing a line from the superior acetabular rim to the inferior acetabular rim (lowest point of the acetabular notch) and measuring the angle between that line and the true horizontal. This transverse angle normally measures approximately 51° at birth, 45° at 10 years of age, and 40° in adults (after Ullmann and Sharp). The value of the transverse angle affects several parameters, including the degree of lateral coverage of the femoral head by the acetabulum (the center-edge angle of Wiberg, see p. 441).
D Sagittal angle of the acetabular inlet plane in the adult
Right hip joint, superior view. Horizontal section through the center of the femoral head.
The bony acetabular rim is angled anteroinferiorly relative to the sagittal plane (compare this with the horizontal plane in C). This aperture angle measures approximately 7° at birth and increases to 17° by adulthood (after Chassard and Lapine).
1.11 The Ligaments of the Hip Joint: Stabilization of the Femoral Head
A The ligaments of the right hip joint
a Lateral view, b anterior view, c posterior view.
The strongest of the three ligaments, the iliofemoral ligament, arises from the anterior inferior iliac spine and fans out at the front of the hip, attaching along the intertrochanteric line (see b). With a tensile strength greater than 350 N, it is the most powerful ligament in the human body and provides an important constraint for the hip joint: it keeps the pelvis from tilting posteriorly in the upright stance, without the need for muscular effort. It also limits adduction of the extended limb (particularly the lateral elements of the ligament) and stabilizes the pelvis on the stance side during gait; i.e., it acts with the small gluteal muscles to keep the pelvis from tilting toward the swing side.
B The ligaments of the hip joint
• Iliofemoral ligament
• Pubofemoral ligament
• Ischiofemoral ligament
• Zona orbicularis (anular ligament)*
• Ligament of head of femur**
* Not visible externally, it encircles the femoral neck like a buttonhole (see p. 433, C).
** Has no mechanical function, but transmits vessels that supply the femoral head (see also p. 433).
C Actions of the ligaments as a function of joint position
a Right hip joint in extension, lateral view. The capsular ligaments of the hip joint (see facing page) form a ringlike collar that encircles the femoral neck. When the hip is extended, these ligaments become twisted upon themselves (as shown here), pushing the femoral head more firmly into the acetabulum (joint-stabilizing function of the ligaments).
b Right hip joint in flexion, lateral view. During flexion (anteversion), the ligament fibers are lax and press the femoral head less firmly into the acetabulum, allowing a greater degree of femoral mobility.
c, d The twisting mechanism of the capsular ligaments can be represented by a model consisting of two disks interconnected by parallel bands. The situation in c represents the position of the ligaments when the hip joint is extended. When one of the two disks rotates (blue arrow), the bands become twisted and draw the two disks closer together (red arrows). Panel d models the situation in the flexed hip. The ligaments are no longer twisted, so the distance between the two disks is increased (after Kapandji).
D Weak spots in the capsule of the right hip joint
a Anterior view, b posterior view.
There are weak spots in the joint capsule (color-shaded areas) located between the ligaments that strengthen the fibrous membrane (see A). External trauma may cause the femoral head to dislocate from the acetabulum at these sites (see E).
The combination of great ligament strength and the close congruity of the femoral head in the acetabulum makes the hip joint very stable and dislocations relatively rare. The situation is different, however, following a hip replacement arthroplasty. The hip joint ligaments must be at least partially divided to implant the prosthesis, and the risk of dislocation is markedly increased.
E Traumatic dislocation of the hip
a It is most common for the femoral head to dislocate upward and backward from the acetabulum (iliac dislocation) between the iliofemoral ligament and ischiofemoral ligament. Typically, this is caused by a fall from a great height, a motor vehicle accident (front-end collision), etc. In this type of dislocation, the leg assumes a position of adduction and slight internal rotation.
b Lateral view. Position of the femoral head in various types of dislocation. The greater trochanter may be above or below the Roser–Nélaton line (line between the ischial tuberosity and the anterior superior iliac spine). In an intact hip with the thigh flexed 45°, the greater trochanter is projected precisely onto this line.
1.12 The Ligaments of the Hip Joint: Nutrition of the Femoral Head
A The ligaments of the right hip joint
a Lateral view. The joint capsule has been divided at the level of the acetabular labrum, and the femoral head has been dislocated to expose the divided ligament of the head of the femur. This ligament transmits important nutrient blood vessels for the femoral head.
b Anterior view. The fibrous membrane of the joint capsule has been removed at the level of the femoral neck to show the conformation of the synovial membrane. This membrane extends laterally from the acetabular rim, and about 1 cm proximal to the attachment of the fibrous membrane, it is reflected onto the femoral neck within the joint cavity. It continues up the femoral neck to the chondro-osseous junction of the femoral head (see also the coronal section in C).
c Posterior view.
B The acetabulum of the right hip joint with the femoral head removed
Lateral view. The cartilage-covered articular surface of the acetabulum is crescent-shaped (lunate surface) and is broadest and thickest over the acetabular roof. The lunate surface is bounded externally by the slightly protruding bony rim of the acetabulum, which is extended by a lip (the acetabular labrum) composed of tough connective tissue and fibrocartilage. The cartilaginous articular surface lines much of the acetabular fossa, which is occupied by loose, fibrofatty tissue and is bounded inferiorly by the transverse acetabular ligament in the area of the acetabular notch (not visible here). The ligament of the head of the femur, which has been sectioned in the drawing, transmits blood vessels that nourish the femoral head (see C).
C The blood supply to the femoral head
a Coronal section through the right hip joint, anterior view.
b Course of the femoral neck vessels in relation to the joint capsule (right femur, anterior view).
The femoral head derives its blood supply from the lateral and medial circumflex femoral arteries and the artery of the ligament of the head of the femur, which branches from the obturator artery (see p. 556). If the anastomoses between the vessels of the ligament of the head of the femur and the femoral neck vessels are absent or deficient due to the avulsion of blood vessels caused by a dislocation or femoral neck fracture, the bony tissue in the head of the femur may become necrotic (avascular necrosis of the femoral head).
1.13 Cross-sectional and X-ray Anatomy of the Hip Joint. Typical Medical Condition of the Elderly: Femoral Neck Fractures
A Coronal section through the right hip joint
Anterior view (drawing based on a specimen from the Anatomical Collection of Kiel University).
B MRI of the hip region: coronal T1-weighted spin-echo (SE) image at the level of the acetabular fossa (from Vahlensieck M, Reiser M. MRT des Bewegungsapparates. 3rd ed. Stuttgart: Thieme; 2006).
C Classification of proximal femoral fractures
Among the femoral fractures close to the hip joint, the medial femoral fractures (see F) are typical injuries of the osteoporotic bones of the elderly. Often, the cause is minimal trauma, such as falling on the greater trochanter or the outstretched leg.
D X-ray of the hip joint and diagnostic reference levels in case of acetabular fractures
a X-ray of the hip joint in a sagittal beam path (section of a pelvis survey radiograph; from Möller TB, Reif E. Taschenatlas der Röntgenanatomie. 2nd ed. Stuttgart: Thieme; 1998); b reference lines important for diagnostic radiology, mainly X-rays of the hip socket.
The pelvis survey radiograph is supplemented only when needed (i.e., when it is not sufficent for making a diagnosis, e.g., in undisplaced femoral fractures) with the help of particular positioning (e.g., ala and obturator view in which the healthy or injured side, respectively, is lifted at 45°) or MRI (see E) or CT. The CT is required as soon as an acetabular fracture has been diagnosed.
Particular reference lines in the anteroposterior (AP) view of the pelvis are important for diagnosis or for therapeutic surgical procedures in cases of acetabular fractures: anterior and posterior socket rim, acetabular roof, Köhler’s teardrop (equivalent to the depth of the acetabular fossa), and iliopectineal or ilioischial line.
E Radiologic diagnosis of proximal femoral fractures (from Bohndorf K, Imhof H, Fischer W. Radiologische Diagnostik der Knochen und Gelenke. 2nd ed. Stuttgart: Thieme; 2006)
a Normal X-ray of a medial, undisplaced femoral fracture in a sagittal beam path; b MRI identifies fracture: T1-weighted coronal cross section of the same patient with fracture-related edema (red arrows). Whereas diagnosing displaced factures usually is not a problem since they are visible in AP views of the hip (see Da), both undisplaced femoral fractures and stress fractures often appear in conventional X-rays only as slight abnormalities of the trabecular bone structure, so an MRI is needed to identify a fracture (because fracture-related edema can be identified through the lesser signals it emits).
F Medial femoral fractures (classification according to Pauwels)
Medial femoral fractures are much more common than lateral femoral fractures (95% of cases compared to 5% of cases). Medial femoral fractures are always intracapsular and due to common complications associated with them (e.g., ischemic femoral head necrosis, delayed fracture healing, and development of pseudarthrosis) are of particular clinical relevance. In particular, the damage to epiphyseal vessels (see p. 433) as a result of intracapsular femoral head fractures leads to impaired blood flow to the femoral head. Medial femoral fractures are classified according to Pauwels by the inclination angle of the fracture line relative to the horizontal line (type I: 0–30°, type II: 30–70°, and type III: > 70°). The steeper the angle, meaning the steeper the course of the fracture line, the greater the risk of the femoral head sliding off and the greater the danger of pseudarthrosis.
1.14 Cross-sectional Anatomy of the Hip Joint: Sonographic Representation of Hip Joint Effusion
A Transverse section through the right hip joint
Superior view (drawing based on a specimen from the Anatomical Collection of Kiel University).
B MRI of the hip region: axial (transverse) T1-weighted SE image at the level of the femoral neck (from Vahlensieck and Reiser. MRT des Bewegungsapparates. 3rd ed. Stuttgart: Thieme; 2006).
The synovial bursae shown in A are not visible because in T1-weighted MRI, they always appear as low signal intensity and are barely distinguishable from muscles, which are also low signal intensity.
Note: With MRI, axial (transverse) cross sections are always inferior views.
C Diagostic ultrasonography of a longitudinal section of the hip: normal and showing hip joint effusion
a Position of the transducer from the front and in the longitudinal direction to the femoral neck; b sonogram of a normal finding (from Konermann W, Gruber G. Ultraschalldiagnostik der Bewegungsorgane. 2nd ed. Stuttgart: Thieme; 2006); c sonogram of hip joint effusion (from Niethard FU, Pfeil J. Orthopädie. 5th ed. Stuttgart: Thieme; 2005); d schematic view of the sonogram. Ultrasonography delivers real-time pictures. Structures at the top of the screen are close to the transducer; those at the bottom are farther away; those on the left side are proximal, those on the right side are distal.
In addition to the diagnostic ultrasound of the embryonic hip joint, the standardized diagnostic ultrasound of the infant hip joint (see p. 440) is of great significance as it is cost-effective and quickly done. Just as in diagnostic X-rays, two almost perpendicular sectional planes (transverse and longitudinal relative to the femoral neck) are recorded. The sonographic examination of the hip joint is performed with the patient lying down. The hip and knee joints are in neutral position (a). The longitudinal section allows for a very good evaluation of the anterior parts of the articular capsule and the osseous and periarticular structures of the hip joint. The surface contours of the anterior margin, the crescent-shaped femoral head, and the femoral neck appear as echoic structures. The articular capsule runs parallel to the femoral head and neck and is generally separated from the femoral neck by a narrow low echoic area (b). Hip disorders, which are accompanied by increased intra-articular volume (e.g., effusion as part of synovialitis or bacterial coxitis), can be very well shown with the help of an anterior longitudinal section because the effusion appears as a capsular distention at the anterior side of the femoral neck (see c). A lateral difference of more than 2 mm between the healthy and affected area (between the articular capsule and the femoral neck) is viewed as significant and implies increased intra-articular volume.
Note: Diagnostic ultrasound helps to show periarticular fluid buildup, e.g., in the case of trochanteric bursitis (= above the greater trochanter).