The hip joint ( coxa in Latin) is the articulation connecting the pelvis and the femur. It is an encapsulated synovial joint with a ball and socket architecture in which the femoral head is the ball and the acetabulum is the socket. Although its structure may seem simple, it is actually very complicated with more than 20 muscles spanning the joint and a three-dimensional (3D) bony morphology that may vary widely among subjects. The acetabulum may be described as retroverted or anteverted, shallow or deep, and with high or low inclination of the weight-bearing surface. The femoral head may be overcovered by the acetabulum or undercovered, and the offset of the femoral head neck junction may be reduced or normal. The proximal femur may have a short or long neck, a high or low femoral neck-shaft angle, and be retroverted or anteverted. To further complicate matters, the head is not a completely round sphere; the socket is horseshoe shaped and the center of the femoral head moves relative to the socket, and as such it is not a true ball-in-socket joint.
The main function of the hip is to support the weight of the body in both static (e.g., standing) and dynamic (e.g., walking or running) situations. To address the biomechanical principles involved in the function of the human hip, it is essential to understand the anatomy of the proximal femur and pelvis, as well as the muscles, ligaments, and bony structures, which all contribute to the equilibrium of forces needed for controlled hip joint motion.
Prenatal Hip Joint Development
Knowledge regarding hip joint development is beneficial to the understanding of hip joint anatomy and biomechanics. Limb formation begins by the fourth week of the embryonic life. By the sixth week primitive chondroblasts accumulate at the proximal, center, and distal ends of the cellular femur template, forming chondrification centers, and a club-shaped cartilage model of the future femur arises from those centers. At the same time a shallow acetabulum begins to form proximal to the femoral head by the future ilium, ischium, and pubis precursor cells. Later, the chondrification process of these bones continues until fusion occurs. By the seventh week of development, the cartilage models of both the femur and the acetabulum are complete. By the eight week, the capsule, the acetabular labrum, the ligamentum teres, and the transverse ligament can be identified microscopically, and 3 weeks later they can be identified macroscopically. At 16 weeks, the ossification of the femur is complete up to the lesser trochanter, and primary ossification centers have appeared in the three pelvic bones; however, ossification centers of the acetabulum do not appear until adolescence. Through the twentieth week of gestation, the differentiation of the hip joint ends and the process shifts to growth and maturation.
The acetabulum has two components: the triradiate cartilage in the center ( Fig. 79-1 ) and the acetabular cartilage complex, which is formed by fusion of the ilium, ischium, and pubis. The triradiate cartilage forms the nonarticular medial wall of the acetabulum, and its growth is crucial for acetabular height and depth. The acetabular cartilage complex, which is composed mainly of hyaline cartilage, forms the cup-shaped articular portion of the acetabulum. Around the age of 8 to 9 years, secondary ossification centers appear at the acetabular rim (os acetabuli). In 1922, Perna identified three distinct os acetabuli—anterior, posterior, and superior. Those ossification centers have an important role in the development of the acetabular rim and its depth. In most cases, the superior os acetabulum fuses by adulthood; however, occasionally its fusion is delayed, and radiographically it may mimic a fracture of the acetabular rim. Normal acetabular development depends heavily on the interaction with the spherical femoral head as a template about which it forms. Complete absence of the proximal femur yields an absent acetabulum.
The normal acetabulum is angled 15 to 20 degrees anteriorly, or anteverted. The acetabular version can be estimated by the appearance of the anterior and posterior acetabular walls on a straight (not tilted) anteroposterior pelvis radiograph, whereas computed tomography (CT) can be used to measure the acetabular version. The anteverted acetabulum allows for hip flexion that is greater than hip extension. A retroverted acetabulum occurs when the acetabulum is angled less than 15 degrees anteriorly. The acetabulum may have a relative retroversion (still anteverted, but less than 15 degrees) or be truly retroverted, angled posteriorly. Additionally, cranial retroversion may be present, in which the anterior acetabular wall crosses over the posterior wall only superiorly (demonstrating a positive “cross-over sign” on radiographs).
On a cadaveric study of 154 hips, the mean values for the acetabular depth and diameter were 29.49 ± 4.2 mm and 54.29 ± 3.8 mm, respectively. The maximum and minimum measurements of acetabular diameter were 65.5 mm and 44.8 mm, respectively, and acetabular depth ranged from 38.6 to 22.6 mm, respectively.
Another reported measure of acetabular depth can be determined from axial magnetic resonance or CT views of the hip joint as the distance between the center of the femoral head and the line connecting the anterior acetabular rim to the posterior acetabular rim. The value is positive if the center of the femoral neck is lateral to the line connecting the acetabular rim and negative if medial to it. Murtha et al. have studied the acetabular anatomy using 3D surface models of the normal hemipelvis derived from volumetric CT data on 42 patients. For the 22 female subjects, the mean acetabular depth was 0.79 mm (0.56 to 1.04), and for the 20 male subjects it was 0.85 mm (0.65 to 0.99).
Zeng et al. studied acetabular morphologic differences between genders in a Chinese population; they measured the acetabular width as the inferior distance between the superolateral and lowermost points of the acetabulum and the acetabular depth as the perpendicular distance between the top and bottom of the acetabulum on an anteroposterior tomogram. Both width and depth were significantly smaller in women than in men, but the difference was not significant when adjusted for body height.
The acetabular depth can be also quantified radiographically on an anteroposterior pelvis view by the center edge angle (of Wiberg). The center edge angle is formed by a line drawn from the center of the femoral head to the outer edge of the acetabular roof and a vertical line drawn from the center of the femoral head directly superior. Although currently somewhat a matter of controversy, the normal values of the center edge angle are between 25 and 35 degrees. A center edge angle of 20 to 25 degrees is often considered “borderline dysplasia,” whereas the upper limits of the center edge angle may be reported as up to 40 degrees.
A deep acetabulum (profunda or protrusio) may result in pincer-type impingement, whereas on the other end of the spectrum, the acetabulum may not be deep enough. Failure of the secondary ossification centers to develop the acetabular rim and depth results in a shallow socket, also known as hip dysplasia.
At birth the ossification of the femoral shaft reaches the greater trochanter and the femoral neck. A few months later, two ossification centers appear, one in the center of the femoral head and one in the greater trochanter. Three growth plates are defined: longitudinal (between the femoral head and the neck), trochanteric (between the femoral neck and the greater trochanter), and the femoral neck isthmus. These growth plates are essential for the growth and shape of the proximal femur.
The pressure exerted on the femoral head by the acetabulum is necessary to result in a spherical femoral head. Overall, the development of both the proximal femur and the acetabulum are related to correct development and positioning of each other.
The main artery of the lower limb is the femoral artery, which is a continuation of the external iliac artery distal to the ilioinguinal ligament. The profunda femoris is a large lateral branch of the femoral artery that appears about 3.5 cm below the inguinal ligament. Branches from the profunda include the lateral circumflex, the medial circumflex, perforating arteries to the femur, muscular branches, and the descending genicular artery.
The proximal femur is supplied by three main blood sources: (1) the nutrient artery of the shaft that arises from the perforating arteries; (2) the retinacular vessels of the capsule that arise from the circumflex arteries; and (3) the foveal artery of the ligamentum teres.
The nutrient artery enters the midshaft of the femur and may be single or double; its superior branch runs in the medullary cavity and anastomoses with the retinacular vessels in the metaphysis. In adults over 13 years, the nutrient artery has been found to cross the epiphyseal plate from the metaphysis to the epiphysis.
The retinacular vessels penetrate the capsule near its distal attachment and are the main blood supply to the femoral epiphysis and femoral head at all ages. The three main groups of retinacular vessels—superior, inferior, and anterior—are all intracapsular and covered with a synovial membrane, sometimes in a mesenteric-like fold of synovial membrane. The anterior retinacular vessels are the smallest of the three, branching from the lateral femoral circumflex artery, and are less consistent. Overall, the lateral femoral circumflex artery contributes little to the vascularity of the femoral head. The superior and inferior retinacular vessels arise from the deep branch medial femoral circumflex artery and run along the upper and lower borders of the femoral neck. The medial circumflex femoral artery supplies blood to the neck of the femur and femoral head. It arises from the medial and posterior aspects of the profunda femoris artery and winds around the medial side of the femur, passing first between the iliopsoas and pectineus muscles and then between the obturator externus and adductor brevis. These two groups are fairly large and consistent between specimens. The superior group, which runs in the lateral retinacular fold, is larger, supplies the weight-bearing part of the femoral head, and may be the sole blood supply to the epiphysis. Anastomoses of the deep branch of the medial femoral circumflex artery with other arteries have been described; a significant and consistent anastomosis with a branch of the inferior gluteal artery is found along the piriformis, and in some cases the inferior gluteal artery was found to be the main blood supply of the hip.
The foveal artery, running within the ligamentum teres, is formed by the acetabular branches of the obturator or the medial circumflex or from both, and often its contribution to the femoral head blood supply is minute. In some cases it was found to be anastomosing with the epiphyseal arteries, whereas in other cases it was found to supply only the area of insertion of the ligamentum teres to the fovea. A recent study found no significant vascular contribution by the foveal artery.
Femoral Neck Version
The femoral neck version is the angle between the femoral neck and the axis that crosses the distal femoral condyles; this angle can be measured through CT or magnetic resonance imaging (MRI) that includes both the knee and the hip. Normally the proximal femur is anteverted. Femoral anteversion is greatest at birth and decreases with growth. Normal average anteversion ranges between 35 and 45 degrees at the time of birth, is 31 degrees at the age of 1 year, and decreases to 15.4 degrees by skeletal maturity (16 years of age). Clinically, increased femoral anteversion can be seen as an “in-toed” appearance of the lower limb in a standing person, or squinting patellae, where the patellae point toward the midline.
Neck Shaft Angle
The neck shaft angle is the angle measured between the axis of the femoral neck and the femoral shaft. This angle can be measured on plain anteroposterior pelvis radiographs, but internal or external rotation of the hip may increase the measured angle. Similar to the version angle, the neck shaft angle is highest at birth and declines with growth. The normal neck shaft angle is approximately 136 degrees at 1 year of age and decreases to 127 degrees by age 18 years.
Femoral Head-Neck Junction
Normally the femoral head-neck junction is waist-shaped, with the femoral neck narrower than its head. The head-neck junction morphology can be quantified by the anterior offset or the alpha angle. The offset is the difference between the anterior contour of the head and femoral neck on axial MRI or CT scans. On axial radiographs (cross-table or Dunn view), it is defined as the distance between the widest diameter of the femoral head and the most prominent part of the femoral neck. The offset can be measured as the ratio between the femoral head and neck radii or as an absolute distance, which is normally measured as around 10 mm.
The alpha angle was described by Notzli et al. in 2002 as a measurement on axial MRI. The alpha angle is a simple method to quantify the concavity at femoral head-neck junction, and it was shown to correlate with anterior hip impingement. The alpha angle is composed of a line that connects the center of the femoral neck at its narrowest diameter to the center of the femoral head. The second line that composes the alpha angle is from the center of the femoral head to a point where the femoral head loses its sphericity, and thus where femoral head exceeds the normal radius of the femoral head. The alpha angle can also be measured on plain radiographs and was found to correlate well with the MRI values. In general, normal alpha angle perimeters have fluctuated since the original report, being less than 50 or 55 degrees.
Internal Bony Architecture of the Femoral Neck
The first description of the bony trabecular orientation in the femoral neck is attributed to Ward in Human Anatomy, which was published in London in 1838; in 1961 it was cited by Garden, who likened the trabecular structure within the femoral neck to that of a lamp bracket. Many other analogies of the trabecular pattern to other 3D weight-bearing subjects such as cranes are common as well. Because the proximal femur is exposed to tensile and compressive forces during weight bearing, those forces lead to functional internal bony architecture of the femoral neck trabeculae lines as stated by Wolff’s law of bone remodeling. These trabeculae consist of a primary compressive group, which arises from the medial subtrochanteric cortex and ascends superiorly into the weight-bearing femoral head, and a primary tensile group, which spans from the foveal area of the femoral head, through the superior femoral neck, and into the lateral subtrochanteric cortex ( Fig. 79-2 ). Secondary compressive, secondary tensile, and a greater trochanteric group complete the pattern of trabecular orientation. The calcar femorale was precisely defined anatomically by Merkle in 1874 as a dense plate of bone extending laterally from the posteromedial femoral cortex to the posterior aspect of the greater trochanter. This bone spur is thickest medially and gradually thins as it extends laterally. However, although defined anatomically as a cancellous bone spur, the term “calcar” is frequently (and some say mistakenly) used to describe the medial cortical bone of the femoral neck, which is the thickest cortex of the femur bone and the strongest bone in the hip. The medial cortex is also known as Adam’s arch.
The greater trochanter can be divided into four different facets—anterior, lateral, posterior, and superoposterior. Those facets are the insertions of the abductor complex; the gluteus medius muscle attaches to the superoposterior and lateral facets, and the gluteus minimus muscle attaches to the anterior facet. A bald spot was described on the lateral facet of the greater trochanter, devoid of tendon insertion, and bordered anteriorly and distally by the gluteus minimus, posteriorly by the gluteus medius, and proximally by the piriformis tendon ( Fig. 79-3 ). Blood supply to the greater trochanteric area arises from the trochanteric branch of the medial femoral circumflex artery.
The lesser trochanter is a pyramidal process that projects from the lower and posterior part of the base femoral neck. Medially it is in continuation with the lower border of the femoral neck, laterally with the intertrochanteric crest, and inferiorly with the linea aspera. The lesser trochanter is the insertion point of the iliopsoas muscle. The lesser trochanter varies in size in different subjects: its height is approximately 1.2 cm, and its width can range between 2 and 4.5 cm.
The acetabular labrum is a horseshoe-shaped structure attached to the acetabular rim. Inferiorly the labrum joins the transverse ligament to bridge the acetabular notch, forming a complete circle. It is triangular in its cross-sectional shape, with its base attached to the acetabulum and its apex forming a free edge ( Fig. 79-4 ). Petersen et al. looked at the labrum under light microscopy and found that the majority of the collagen fibers have a circumferential orientation. The capsular side of the labrum is composed of dense connective tissue mainly consisting of collagen types I and III, whereas the articular side is composed of fibrocartilage. At the articular side, the labrum is often separated from the articular cartilage by a physiologic cleft that is seen more frequently posteriorly, whereas anteriorly this cleft is usually absent and the transition between the labrum and articular cartilage is smooth.
Seldes et al. also studied the labrum histologically and found it to be widest in its inferior half and thickest at its superior half. On average, the acetabular labral size ranges from 4 to 8 mm. The average labral width as reported by Seldes et al. ranged from 3.8 mm posterosuperiorly to 6.4 mm posteroinferiorly. However, the labral width may vary according to the forces that act on it. It appears that the labral size may be inversely proportional to the depth of the bony acetabular socket contribution to femoral head coverage. Thus the labrum may be small, with a width of less than 3 mm in coxa profunda, or it may be large/hypertrophic, with a width of up to 14 mm in a dysplastic hip.
Inferiorly, the labrum appears to be continuous with the transverse acetabular ligament over the cotyloid notch; however, a distinction is seen between the labrum and the transverse ligament. A bony acetabular tongue exists within the labrum with the labrum firmly attached to it with a well-defined tidemark. Histologically, the acetabular labrum merges with the articular hyaline cartilage of the joint surface of the acetabulum through a transition zone of 1 to 2 mm, particularly anteriorly.
The labrum is continuous with the articular cartilage of the acetabulum; however, differences exist in the transition between the anterior and posterior transition zones. The anterior labral-chondral transition is sharp and abrupt with minimal interdigitation of fibers, whereas the posterior labral-chondral transition is smooth and gradual. This appearance is due to differences in the orientation of collagen fibers of the labrum; the anterior fibers are parallel to the labral-chondral junction, whereas the posterior fibers are oriented perpendicularly.
The main vascular supply of the labrum arises from radial vessels on its capsular side embedded in a loose connective tissue between the labrum and the capsule ( Fig. 79-5 ). These vessels seem to supply only the outer third of the labrum. Kalhor et al. used a silicon injection technique to show a periacetabular vascular ring supplying the labrum, originating from the superior and inferior gluteal vessels, the medial and lateral femoral circumflex arteries, and the intrapelvic vascular system.
The acetabular bony rim that is embedded in the base of the labrum is another source of blood supply to the labrum. McCarthy et al., in an immunohistochemical study, showed abundant vessels within the bony acetabulum that reach the junction with the labrum. Seldes et al. identified a group of three to four vessels located in the substance of the labrum that travels circumferentially around the labrum at its attachment site on the outer surface of the bony acetabular extension.
The labrum has been shown to be richly populated by many neurologic structures. Kim and Azuma found sensory nerves and organs such as Vater-Pacini, Golgi-Mazzoni, Ruffini, and Krause corpuscles within the acetabular labrum. Most of these sensory nerve end organs (86%) are in the articular side of the labrum. The corpuscles observed are receptors of pressure, deep sensation, and temperature sense. In addition, no differences in number or type of nerves and organs were found based on the age of the specimens, but more unmyelinated nerve endings, which function to sense pain, were identified in the superior and anterior quarters of the labrum. Additionally, Gerhardt et al. found that the anterior zone of the labrum contained the highest concentration of mechanoreceptors and sensory fibers, specifically Ruffini corpuscles. Thus the labrum may function to provide proprioceptive input, and a damaged labrum may be a source of hip pain.
The labrum deepens the acetabulum and acts as a suction seal, adding stability to the joint and protecting the articular cartilage. Seldes et al. have shown that the labrum increases the acetabular surface area and volume by 22% and 33%, respectively. Furthermore, the labrum creates a seal that opposes the flow of synovial fluid in and out of the central compartment. Safran et al. have shown that the labrum has strain at rest, which increases and decreases in different locations of the labrum as the hip is taken through range of motion. According to biomechanical studies, with an intact labrum, the acetabular and femoral cartilage surfaces do not come into direct contact with each other because of a film of fluid contained by the labrum. Ferguson et al. have found that hydrostatic fluid pressure within the intraarticular space was greater within the labrum than without, which may enhance joint lubrication, whereas labrum resection resulted in faster cartilage consolidation. Song et al. also found that the acetabular labrum plays a role in maintaining a low-friction environment, possibly by sealing the joint from fluid exudation, because both complete and focal labral debridement resulted in increased joint friction, a condition that is thought to be detrimental to articular cartilage and leads to osteoarthritis. The sealing function of the labrum also helps maintain the negative intraarticular pressure that occurs in all joints. This negative intraarticular pressure helps resist distraction of the femoral head from the socket; this function is called the “suction effect” and is thought to improve the stability of the joint.
The ligamentum teres, otherwise known as the round ligament of the femur, is a triangular double-band ligament with a length of 30 to 35 mm that attaches the femoral head to the acetabulum. Medially, it is attached to either side of the acetabular notch by two bands that originate from the acetabular transverse ligament and the pubic and ischial margins. Laterally, its apex extends to the anterosuperior portion of the femoral head, merging with the fovea capitis femoris.
The ligament is composed of thick, well-organized, parallel, and slightly undulating or wavy fibers of collagen bundles that are composed of collagen types I, III, and V. Embryonically, the ligamentum teres is defined around 8 weeks of intrauterine life as the joint space expands and is seen to attach to the medial border of the acetabular fossa, separating from the transverse acetabular ligament.
An anterior branch of the posterior division of the obturator artery provides the blood supply to the ligamentum teres ( Fig. 79-6 ). Vascular canals extend from the fovea capitis of the femoral head to supply the epiphysis of the femoral head; however, these arteries are not patent in a third of the population. These vessels do not anastomose with the distal arterial terminals in the femoral head until around age 15 years, when ossification of the head is nearly complete.
The biomechanical role of the ligamentum teres has been debated in the medical literature since the nineteenth century, with proposed functions including that of a stabilizer, a fluid and force distributor in the acetabulum, and an embryonic remnant with no specific role in adults. The ligamentum teres has also been previously described as a possible transmitter of somatosensory signals that act to help the hip avoid painful and excessive ranges of motion. More recently, the ligamentum teres in the hip has been thought to provide functions comparable with the anterior cruciate ligament in the knee; with similar tensile strength, it has been proposed to provide some degree of stability in the hip, resisting dislocation and microinstability. However, many other studies report that the ligamentum teres plays little role in the stability of the hip joint and suggest it is possibly a mere embryonic remnant.
The ligamentum teres has been found to be taut in flexion, adduction, and external rotation, and thus it may play a role in stability of the hip joint in those positions. In a recent study, Domb et al. have found that the arthroscopic presence of ligamentum teres tears was associated with acetabular bony morphology and age. Ligamentum teres tears were less frequent in hips with a high lateral coverage index (center edge angle minus acetabular inclination) and also less frequent in patients younger than 30 years.
Two histologic studies have found type IVa free nerve endings in the ligamentum teres, which are nociceptors and mechanoreceptors. Leunig et al. suggested that in addition to its mechanical and structural functions, the ligamentum teres may be involved in transmitting specific somatosensory afferent signals to the spinal and cerebral regulatory systems. Hence the ligamentum teres may be part of an integral reflex system involved in joint protection, acting as a rein to avoid excessive motion that may be potentially harmful to the joint. Alternatively, Gerhardt et al. identified a paucity of neural fibers in the ligamentum and did not find any sensory nerve fibers within it.
Although the function of the ligamentum teres has yet to be determined, its role as a source of hip pain after a full or a partial tear has been more clearly elucidated.
The articular cartilage of the hip, both on the acetabular side and the femoral side, has been shown to be highly inhomogeneous in thickness distribution. Von Eisenhart et al. studied the cartilage thickness and pressure on eight fresh cadaveric hip specimens during the phases of gait cycle. Maximum cartilage thickness was found ventrosuperiorly in the acetabulum and in the femoral head. The location of maximum thickness corresponded with the ventrosuperior location of maximum pressure recorded during the walking cycle; the maximal thickness ranged from 2.6 to 4.3 mm in the acetabulum (average, 3.3 mm) and from 2.4 to 5.3 mm in the femoral head (average, 3.5 mm). In general, cartilage thickness decreased with age. No statistical difference was found between the values for maximum thickness on both surfaces, although the mean thickness of the femoral cartilage (1.5 to 2.0 mm, with an average of 1.7 mm) was higher ( P < .01) than that of the acetabulum (1.1 to 1.7 mm, with an average of 1.4 mm).
Hip Joint Capsule
The hip capsule is made up of internal fibers (within the joint) and external fibers (outside or away from the joint). The external fibers run longitudinally and comprise the iliofemoral ligament, ISFL, and pubofemoral ligament. The inner fibers comprise the zona orbicularis, which forms a collar around the femoral neck. The capsule inserts on the bony acetabulum proximal to and distinct from the labrum, forming a recess between the two that ranges between 6.6 and 7.9 mm from the anteroinferior and posteroinferior quadrants, respectively. From its acetabular attachment, the capsule extends to surround the femoral head and neck in a spiral fashion and is attached anteriorly to the intertrochanteric line, superiorly to the base of the femoral neck, superomedially to the intertrochanteric crest, and inferiorly to the femoral neck near the lesser trochanter. One theory regarding the spiral architecture of the capsule is that it originated as humans began to walk upright. As humans transitioned from quadruped to biped, the hips were brought into relative extension,thus causing the capsular fibers to twist into a spiral pattern. In normal stance, if the upper body is leaned slightly posteriorly, stability is provided primarily by the static restraints of the anterior capsule (mainly the iliofemoral ligament). If the anterior capsule is damaged or lax, maintaining the upright position may be difficult because the anterior muscle strength is not as powerful as the posterior muscle strength.
The iliofemoral ligament (ILFL), also known as the Y ligament of Bigelow, is shaped like an inverted “Y” and distally splits into two distinct arms, medial and lateral ( Fig. 79-7 ). The single proximal insertion abuts the anterior inferior iliac spine, wrapping around the base like a crescent, and extends within a few millimeters of the acetabular rim along the anterior and anterolateral acetabulum. Distally, the ILFL lateral arm crosses the joint obliquely and inserts on the anterior prominence of the greater trochanteric crest, just superior to the origin of the intertrochanteric line, with an elongated oval-shaped footprint. The medial arm passes almost vertically inferior and inserts on a subtle angulated prominence of the anterior-inferior femur, at the level of the lesser trochanter, with a circular-shaped footprint. The individual arms of the ILFL diverge 57 mm (range, 50 to 64 mm) distal to the most superior aspect of the proximal attachment footprint; the medial and lateral insertional footprints are a few millimeters apart on intertrochanteric line. The ILFL restricts external rotation in both flexion and extension and internal rotation in flexion.
The pubofemoral ligament (PFL) originates on the iliopectineal eminence of the superior pubic ramus with a triangle-shaped insertional footprint ( Fig. 79-7 ). The most inferomedial aspect of the insertional footprint extends to within a few millimeters of the acetabular rim. The PFL crosses inferoposteriorly under the medial arm of the ILFL and wraps around the femoral head like a sling or hammock, proximal to the zona orbicularis. The PFL terminates abruptly by blending with the proximal ischiofemoral ligament (ISFL), near the acetabular rim, beneath the inferior aspect of the femoral neck ( Figure 79-7, B ); the PFL lacks a bony femoral attachment. The PFL blends anteriorly with the medial ILFL. The PFL controls external rotation in extension.
The ISFL resembles a large asymmetric triangle with a long tapered apex and consists of a single band (see Fig. 79-7 ). The proximal insertional footprint on the ischial acetabular margin is shaped like a broad triangle, beginning near the root of the ischial ramus and extending to within a few millimeters of the acetabular rim. The ISFL spirals superolaterally to insert at the base of the greater trochanter at the femoral neck-trochanteric junction, slightly anterior to the femoral neck axis; the distal ISFL footprint does not have a consistent shape. The ISFL was noted to be the most significant restrictor of internal rotation both in internal and external rotation.
The zona orbicularis is a thickening of the capsule, just distal to the femoral head-neck junction, that runs around the femoral neck, perpendicular to the axis of the femoral neck. The zona orbicularis has leashlike fibers organized in a spiral configuration; together with the anterior capsular ligaments, they tighten in a “screw home” mechanism during terminal extension and external rotation, further stabilizing the joint. Additionally, the zona orbicularis may limit distraction of the femoral head from the acetabulum.
Sensory innervation of the hip capsule for proprioception and nociception has been studied extensively in the modern literature. The capsule is generally thought to receive its innervation from branches of the obturator, femoral, sciatic, and superior gluteal nerves, the nerve to quadratus femoris, and possibly from the accessory obturator nerve. The complexity of the hip joint innervation results in a nonhomogenic pattern of pain referred from the hip joint, with hip pathology causing pain in the groin, all around the thigh, in the buttock, below the knee, and even in the foot.
Birnbaum et al. examined 11 formalin-mounted human hips and described a separation between the anterior innervation of the capsule (obturator and femoral nerve) and its posterior innervation (sciatic nerve, superior gluteal nerve, and the nerve to the quadratus femoris).
Recently, Kampa et al. dissected 20 formalin-fixed human hips to further explore the innervation of the capsule and to define its pattern more accurately. They chose to illustrate the capsular innervation arrangement by depicting the capsule as the face of a clock. The reference point from which measurements were taken was the inferior acetabular notch to depict the 6- o’clock position. Therefore the position between 12 and 6 o’clock represented the anterior aspect of the capsule and the position between 6 and 12 o’clock represented the posterior position. Their findings, discussed later in this chapter, were consistent with previous reports and demonstrate a richly innervated structure, innervated by five to seven nerves and a variable number of their branches (direct or muscular).
The femoral nerve is formed by the L2 to L4 nerve roots in the lumbar plexus. It courses along the iliacus muscle and alongside the psoas, descends under the inguinal ligament, innervates the anterior thigh musculature, and provides cutaneous innervation to the lower leg through its terminal branch of the saphenous nerve. The femoral nerve travels with (and lateral to) the femoral artery and vein. The capsular branches of the femoral nerve, which travel along the anterior margin of the capsule, pierce the capsule either medially or laterally over an arc of 75 degrees between the half-past 2- and 5-o’clock positions.
The obturator nerve roots from L2 to L4 descend through the fibers of the psoas major and emerge from the medial border and later enter the thigh through the obturator canal. The capsular branches of the obturator nerve travel along the anteroinferior margin of the capsule, entering the capsule primarily medially over an arc of 105 degrees between 3 o’clock and half past 6 o’clock, with an equal contribution from both the anterior and posterior branches. The obturator nerve also provides innervation to the knee joint.
Accessory Obturator Nerve
In the past the accessory obturator nerve has been described as being present in between 10% and 30% of people. However, Birnbaum et al. did not find it at all in 11 specimens, and Kampa et al. found it in only 1 of 20 hips (5%), where it crossed the anteroinferior margin of the capsule and entered it medially over an arc of 15 degrees between half past 5 and 6 o’clock.
Superior Gluteal Nerve
Originating at the sacral plexus from the L4 to S1 sacral nerves, the superior gluteal nerve leaves the pelvis through the greater sciatic foramen above the piriformis, accompanied by the superior gluteal artery and the superior gluteal vein. Kampa et al. found that the superior gluteal nerve branches to the capsule are small and have a leash of vessels; they cross the superior and posterolateral aspects of the capsule and enter it medially or more commonly laterally over an arc of 75 degrees between half past 10 and 1 o’clock.
Inferior Gluteal Nerve
The inferior gluteal nerve, a branch of the sacral plexus, leaves the pelvis through the lower part of the greater sciatic foramen, below the piriformis, and terminates by innervating the gluteus maximus. Kampa et al. have found a contribution to capsular innervation from the inferior gluteal nerve in only two specimens (10%), and noted that the nerve entered the capsule laterally at 8 o’clock.
The sciatic nerve is formed by the joining of the L4 to S3 roots. The nerve courses laterally through the pelvis, exits at the greater sciatic foramen, and travels distally deep to the piriformis and superficially to the short external rotators. The nerve divides along the course of the posterior thigh into the common peroneal trunk and the tibial trunk. The common peroneal trunk lies more laterally. The area of the capsule supplied by the sciatic nerve overlaps with that of the superior gluteal nerve, with its branches traveling along the posterior margin of the capsule and entering the capsule mainly medially but also laterally over an arc of 90 degrees between 9 and 12 o’clock.
Nerve to Quadratus Femoris
The nerve to the quadratus femoris is a sacral plexus nerve that arises from L4 to S1 and leaves the pelvis through the greater sciatic foramen. Its branches to the capsule supply the quadratus femoris posteroinferiorly and enter it predominantly medially and occasionally laterally over an arc of 105 degrees between half past 6 and 10 o’clock.
Safe Zone of the Capsule
Kampa et al. found that anterosuperiorly, between 1 o’clock and half past 2 o’clock, no nerves enter the capsule. This internervous plane, which is poorly innervated, was named the “safe zone” of the capsule ( Fig. 79-8 ).