The hip is the articulation between the large spherical head of the femur and the deep socket provided by the acetabulum of the pelvis (Figure 12-1). Because of the joints’ central location within the body, the logical question arises: do the hips serve as “base” joints for the lower extremities, or basilar joints for the entire superimposed pelvis and trunk? As this chapter unfolds, it will become clear that the hips serve both roles. For this reason the hips play a dominant kinesiologic role in movements across a large part of the body. Pathology or trauma affecting the hips typically causes a wide range of functional limitations, including difficulty in walking, dressing, driving a car, lifting and carrying loads, and climbing stairs.

FIGURE 12-1. The anterior aspect of the pelvis, sacrum, and right proximal femur. Proximal attachments are indicated in red, distal attachments in gray. A section of the left side of the sacrum is removed to expose the auricular surface of the sacroiliac joint. The pelvic attachments of the capsule around the sacroiliac joint are indicated by dashed lines.

The hip joint has many anatomic features that are well suited for stability during standing, walking, and running. The femoral head is stabilized by a deep socket that is surrounded and sealed by an extensive set of connective tissues. Many large and forceful muscles generate the necessary torques needed to accelerate the body upward and forward, or decelerate the body in a controlled fashion. Weakness in these muscles can have a profound impact on the mobility and stability of the body as a whole.

Hip disease and injury are relatively common, particularly in the very young and in the elderly. An abnormally formed hip in an infant may be prone to dislocation. The hip in the aged adult is vulnerable to degenerative joint disease. Increased osteoporosis coupled with increased risk of falling also predisposes the elderly to a higher incidence of hip fracture.

This chapter describes the structure of the hip, its associated capsule and ligaments, and the actions of the surrounding musculature. This information is the basis for treatment and diagnosis of musculoskeletal problems in this important region of the body.

OSTEOLOGY

Innominate

Each innominate (from the Latin innominatum, meaning nameless) is the union of three bones: the ilium, pubis, and ischium (see Figures 12-1 and 12-2). The right and left innominates connect with each other anteriorly at the pubic symphysis and posteriorly at the sacrum. These connections form a complete osteoligamentous ring, referred to as the pelvis (from the Latin, meaning basin or bowl). The pelvis is associated with three important and very different functions. First, the pelvis serves as a common attachment point for many large muscles of the lower extremity and the trunk. The pelvis also transmits the weight of the upper body and trunk either to the ischial tuberosities during sitting or to the lower extremities during standing and walking. Last, with the aid of the muscles and connective tissues of the pelvic floor, the pelvis supports the organs involved with bowel, bladder, and reproductive functions.

FIGURE 12-2. A lateral view of the right innominate bone. Proximal attachments of muscle are indicated in red, distal attachments in gray.

The external surface of the pelvis has three striking features. The large fan-shaped wing (or ala) of the ilium forms the superior half of the innominate. Just below the wing is the deep, cup-shaped acetabulum. Just inferior and slightly medial to the acetabulum is the obturator foramen—the largest foramen in the body. This foramen is covered by an obturator membrane (see Figure 12-1).

While a person stands, the pelvis is normally oriented so that when viewed laterally a vertical line passes between the anterior-superior iliac spine and the pubic tubercle (see Figure 12-2).

ILIUM

The external surface of the ilium is marked by rather faint posterior, anterior, and inferior gluteal lines (see Figure 12-2). These lines help to identify attachment sites of the gluteal muscles. At the most anterior extent of the ilium is the easily palpable anterior-superior iliac spine (see Figures 12-1 and 12-2). Below this spine is the anterior-inferior iliac spine. The prominent iliac crest, the most superior rim of the ilium, continues posteriorly and ends at the posterior-superior iliac spine (Figure 12-3). The soft tissue superficial to the posterior-superior iliac spine is often marked by a dimple in the skin. The less prominent posterior-inferior iliac spine marks the superior rim of the greater sciatic notch. The opening of this notch is converted to the greater sciatic foramen by the sacrotuberous and sacrospinous ligaments.

FIGURE 12-3. The posterior aspect of the pelvis, sacrum, and right proximal femur. Proximal attachments of muscles are indicated in red, distal attachments in gray.

The internal aspect of the ilium has three notable features (see Figure 12-1). Anteriorly, the smooth concave iliac fossa is filled by the iliacus muscle. Posteriorly, the auricular surface articulates with the sacrum at the sacroiliac joint. Just posterior to the auricular surface is the large, rough iliac tuberosity, which marks the attachments of sacroiliac ligaments.

Osteologic Features of the Pubis

• Superior pubic ramus

• Pubic symphysis joint and disc

• Inferior pubic ramus

PUBIS

The superior pubic ramus extends anteriorly from the anterior wall of the acetabulum to the large flattened body of the pubis (see Figure 12-1). The upper border of the body of the pubis is the pubic crest, serving as an attachment for the rectus abdominus muscle. On the upper surface of the superior ramus is the pectineal line, marking the attachment of the pectineus muscle. The pubic tubercle projects anteriorly from the superior pubic ramus, serving as an attachment for the inguinal ligament. The inferior pubic ramus extends from the body of the pubis posteriorly to the junction of the ischium.

The two pubic bones articulate in the midline by way of the pubic symphysis joint (see Figure 12-1). This relatively immobile joint is typically classified as a synarthrosis. Hyaline cartilage lines the opposing surfaces of the articulation; the surfaces are not completely flat but possess small raised ridges, likely designed to resist shear.¹⁶⁰ The joint is firmly bound by a fibrocartilaginous interpubic disc and ligaments. The interpubic disc is strengthened by an interlacing of collagen fibers, combined with distal attachments made by the rectus abdominis muscles.¹⁶⁰ Up to 2 mm of translation and very slight rotation occur at the pubic symphysis joint.¹⁶⁸ The pubic symphysis provides stress relief throughout the anterior ring of the pelvis during walking and, in women, during childbirth.

Symphysis pubis dysfunction can occur in some women during pregnancy or just after birth. This painful condition is associated with increased instability in the symphysis pubis caused by the physiologic relaxation of the joint’s supporting ligaments.⁸⁹

ISCHIUM

The sharp ischial spine projects from the posterior side of the ischium, just inferior to the greater sciatic notch (see Figure 12-3). The lesser sciatic notch is located just inferior to the spine. The sacrotuberous and sacrospinous ligaments convert the lesser sciatic notch into a lesser sciatic foramen.

Osteologic Features of the Ischium

• Ischial spine

• Lesser sciatic notch

• Lesser sciatic foramen

• Ischial tuberosity

• Ischial ramus

Projecting posteriorly and inferiorly from the acetabulum is the large, stout ischial tuberosity (see Figure 12-3). This palpable structure serves as the proximal attachment for many muscles of the lower extremity, most notably the hamstrings and part of the adductor magnus. The ischial ramus extends anteriorly from the ischial tuberosity, ending at the junction with the inferior pubic ramus (see Figure 12-1).

ACETABULUM

Located just above the obturator foramen is the large cup-shaped acetabulum (see Figure 12-2). The acetabulum forms the socket of the hip. All three bones of the pelvis contribute to the formation of the acetabulum: the ilium and ischium contribute about 75%, and the pubis contributes the remaining approximately 25%. The specific features of the acetabulum are discussed in the section on arthrology.

Femur

The femur is the longest and strongest bone of the human body (Figure 12-4). Its shape and robust stature reflect the powerful action of muscles and contribute to the long stride length during walking. At its proximal end, the femoral head projects medially and slightly anteriorly for an articulation with the acetabulum. The femoral neck connects the femoral head to the shaft. The neck serves to displace the proximal shaft of the femur laterally away from the joint, thereby reducing the likelihood of bony impingement against the pelvis. Distal to the neck, the shaft of the femur courses slightly medially, effectively placing the knees and feet closer to the midline of the body.

FIGURE 12-4. The anterior aspect of the right femur. Proximal attachments of muscles are indicated in red, distal attachments in gray. The femoral attachments of the hip joint capsule and the knee joint capsule are indicated by dashed lines.

The shaft of the femur displays a slight anterior convexity (Figure 12-5, A). As a long, eccentrically loaded column, the femur bows very slightly when subjected to the weight of the body. Consequently, stress along the bone is dissipated through compression along its posterior shaft and through tension along its anterior shaft. Ultimately this bowing allows the femur to bear a greater load than if the femur were perfectly straight.

FIGURE 12-5. The medial (A) and posterior (B) surfaces of the right femur. Proximal attachments of muscles are indicated in red, distal attachments in gray. The femoral attachments of the hip joint capsule and the knee joint capsule are indicated by dashed lines.

Anteriorly, the intertrochanteric line marks the distal attachment of the capsular ligaments (see Figure 12-4). The greater trochanter extends laterally and posteriorly from the junction of the femoral neck and shaft (see Figure 12-5, B). This prominent and easily palpable structure serves as the distal attachment for many muscles. On the medial surface of the greater trochanter is a small pit called the trochanteric fossa (see Figures 12-5, A and 12-6). This fossa marks the distal attachment of the obturator externus muscle.

FIGURE 12-6. The superior aspect of the right femur. Distal attachments of muscles are shown in gray.

Posteriorly, the femoral neck joins the femoral shaft at the raised intertrochanteric crest (see Figure 12-5, B). The quadrate tubercle, the distal attachment of the quadratus femoris muscle, is a slightly raised area on the crest just inferior to the trochanteric fossa. The lesser trochanter projects sharply from the inferior end of the crest in a posterior-medial direction. The lesser trochanter serves as the distal attachment for the iliopsoas muscle, an important hip flexor and vertical stabilizer of the lumbar spine.

The middle third of the posterior side of the femoral shaft is clearly marked by a vertical ridge called the linea aspera (Latin words linea, line + aspera, rough). This raised line serves as an attachment site for the vasti muscles of the quadriceps group, many of the adductor muscles, and the intermuscular fascia of the thigh. Proximally, the linea aspera splits into the pectineal (spiral) line medially and the gluteal tuberosity laterally (see Figure 12-5, B). At the distal end of the femur, the linea aspera divides into the lateral and medial supracondylar lines. The adductor tubercle is located at the extreme distal end of the medial supracondylar line.

SHAPE OF THE PROXIMAL FEMUR

The ultimate shape and configuration of the developing proximal femur are determined by several factors, including differential growth of the bone’s ossification centers, the force of muscle activation and weight bearing, and circulation.¹⁷¹ Abnormal growth and development resulting in a misshaped proximal femur is referred to generically as femoral dysplasia (from the Greek dys, ill or bad, + plasia, growth). Trauma or other acquired factors can also affect the shape of the proximal femur. The shape and configuration of the proximal femur have important implications on the congruity and stability of the joint, as well as the stress placed on the joint structures. This topic will be revisited throughout this chapter.

Two specific angulations of the proximal femur help define its shape: the angle of inclination and the torsional angle.

Angle of Inclination: The angle of inclination of the proximal femur describes the angle within the frontal plane between the femoral neck and the medial side of the femoral shaft (Figure 12-7). At birth this angle measures about 140 to 150 degrees. Primarily because of the loading across the femoral neck during walking, this angle usually reduces to its normal adulthood value of about 125 degrees.¹⁶^,¹³² As depicted by the pair of red dots in Figure 12-7, this angle optimizes the alignment of the joint surfaces.

FIGURE 12-7. The proximal femur is shown: A, normal angle of inclination; B, coxa vara; and C, coxa valga. The pair of red dots in each figure indicates the different alignments of the hip joint surfaces. Optimal alignment is shown in A.

A change in the normal angle of inclination is referred to as either coxa vara or coxa valga. Coxa vara (Latin coxa, hip, + vara, to bend inward) describes an angle of inclination markedly less than 125 degrees; coxa valga (Latin valga, to bend outward) describes an angle of inclination markedly greater than 125 degrees (see Figure 12-7, B and C). These abnormal angles can significantly alter the articulation between the femoral head and the acetabulum, thereby affecting hip biomechanics. Severe malalignment may lead to dislocation or stress-induced degeneration of the joint.

Femoral Torsion: Femoral torsion describes the relative rotation (twist) between the bone’s shaft and neck. Normally, as viewed from above, the femoral neck projects about 15 degrees anterior to a medial-lateral axis through the femoral condyles.⁴³ This degree of torsion is called normal anteversion (Figure 12-8, A). In conjunction with the normal angle of inclination, an approximate 15-degree angle of anteversion affords optimal alignment and joint congruence (see alignment of red dots in Figure 12-8, A).

FIGURE 12-8. The angle of torsion is shown between the neck and shaft of the femur: A, normal anteversion; B, excessive anteversion; and C, retroversion. The pair of red dots in each figure indicates the different alignments of the hip joint surfaces. Optimal alignment is shown in A.

Femoral torsion that is markedly different from 15 degrees is considered abnormal. Torsion significantly greater than 15 degrees is called excessive anteversion (see Figure 12-8, B). In contrast, torsion significantly less than 15 degrees (i.e., approaching 0 degrees) is called retroversion (see Figure 12-8, C).

Typically a healthy infant is born with about 40 degrees of femoral anteversion.⁴³ With continued bone growth, increased weight bearing, and muscle activity, this angle usually decreases to about 15 degrees by 16 years of age. Excessive anteversion that persists into adulthood can increase the likelihood of hip dislocation, articular incongruence, increased joint contact force, and increased wear on the articular cartilage.⁵⁷ These factors may lead to secondary osteoarthritis of the hip.¹⁴⁷

Excessive anteversion in children may be associated with an abnormal gait pattern called “in-toeing.” In-toeing is a walking pattern with exaggerated posturing of hip internal rotation. The amount of in-toeing is generally related to the amount of femoral anteversion. This gait pattern apparently is a compensatory mechanism used to guide the excessively anteverted femoral head more directly into the acetabulum (Figure 12-9). In addition, Arnold and colleagues have shown that the exaggerated internally rotated position during walking serves to increase the moment arm of the important hip abductor muscles—leverage that is substantially reduced with excessive femoral anteversion.⁹ Regardless of the reason for the internal rotated position, children may, over time, develop shortening of the internal rotator muscles and various ligaments, thereby reducing external rotation range of motion. Fortunately, most children with in-toeing eventually walk normally.¹⁶³ The gait pattern typically improves with time because of a natural normalization of the anteversion or a combined structural compensation in other parts of the lower extremity, most commonly the tibia.⁶⁰ There is no evidence that nonoperative treatment can reduce excessive femoral anteversion.

FIGURE 12-9. Two situations show the same individual with excessive anteversion of the proximal femur. A, Offset red dots indicate malalignment of the hip while a subject stands in the anatomic position. B, As evidenced by the alignment of the red dots, standing with the hip internally rotated (“in-toeing”) improves the joint congruity.

Excessive femoral anteversion of 25 to 45 degrees is common in persons with cerebral palsy, and even anteversion as high as 60 to 80 degrees has been reported.⁸^,¹⁶ In-toeing typically persists in children with cerebral palsy who are ambulatory and usually does not resolve.¹⁴⁹

SPECIAL FOCUS 12-1 Natural Anteversion of the Femur: a Reflection of the Prenatal Development of the Lower Limb

During prenatal development, the upper and lower extremities both undergo significant axial rotation. By about 54 days after conception, the lower limbs have rotated internally (medially) about 90 degrees.¹¹⁵ This rotation turns the kneecap region to its final anterior position. In essence, the lower limbs have become permanently “pronated.” This helps to explain why the “extensor” muscles—such as the quadriceps and tibialis anterior—face anteriorly, and the “flexor” muscles—such as the hamstrings and gastrocnemius—face posteriorly after birth. The torsion angle between the shaft and the neck of the femur at birth partially reflects the degree of this medial rotation.

The functional consequence of the medial rotation of the lower limbs is that the plantar surfaces of the feet assume a plantigrade position suitable for walking. The fixed pronated position is evidenced by the medial position of the great toe of the lower limb, similar to the thumb in the fully pronated forearm. Additional anatomic features that may reflect this developmental medial rotation include the spiraled path of the lower extremity dermatomes (see Appendix IV, Part C), the twisted or spiraled ligaments of the hip (described ahead), and the oblique course of the sartorius muscle.

INTERNAL STRUCTURE OF THE PROXIMAL FEMUR

Compact and Cancellous Bone: Walking produces tension, compression, bending, shear, and torsion on the proximal femur. Many of these forces are large, exceeding one’s body weight. Throughout a lifetime, the proximal femur typically resists and absorbs these repetitive forces without incurring injury. This is accomplished by two strikingly different compositions of bone. Compact bone is very dense and unyielding, with an ability to withstand large loads. This type of bone is particularly thick in the cortex, or outer shell, of the lower femoral neck and entire shaft (Figure 12-10). These regions are subjected to large shear and torsion forces. Cancellous bone, in contrast, is relatively porous, consisting of a spongy, three-dimensional trabecular lattice, as shown in Figure 12-10. The relative elasticity of cancellous bone is ideal for repeatedly absorbing external forces. Cancellous bone tends to concentrate along lines of stress, forming trabecular networks. A medial trabecular and an arcuate trabecular network are visible within the femur shown in Figure 12-10. The overall pattern of the trabecular network changes when the proximal femur is subjected to abnormal forces over an extended time.

FIGURE 12-10. A frontal plane cross-section showing the internal architecture of the proximal femur. Note the thicker areas of compact bone around the shaft, and the cancellous bone occupying most of the medullary (internal) region. Two trabecular networks within the cancellous bone are also indicated. (From Neumann DA: An arthritis home study course. The synovial joint: anatomy, function, and dysfunction, LaCrosse, Wisc, 1998, Orthopedic Section of the American Physical Therapy Association.)

ARTHROLOGY

Functional Anatomy of the Hip Joint

The hip is the classic ball-and-socket joint of the body, secured within the acetabulum by an extensive set of connective tissues and muscles. Thick layers of articular cartilage, muscle, and cancellous bone in the proximal femur help dampen the large forces that routinely cross the hip. Failure of any of these protective mechanisms because of disease, congenital or developmental malalignment or malformation, or trauma often leads to a deterioration of the joint structure.

FEMORAL HEAD

The femoral head is located just inferior to the middle one third of the inguinal ligament. On average, the centers of the two adult femoral heads are 17.5 cm (6.9 inches) apart from each other.¹²⁷ The head of the femur forms about two thirds of a nearly perfect sphere (Figure 12-11). Located slightly posterior to the center of the head is a prominent pit, or fovea (see Figure 12-5, A). The entire surface of the femoral head is covered by articular cartilage, except for the region of the fovea. The cartilage is thickest (about 3.5 mm) in a broad region above and slightly anterior to the fovea (see highlighted region in Figure 12-11).⁸⁶

FIGURE 12-11. The right hip joint is opened to expose its internal components. The regions of thickest cartilage are highlighted (in blue) on the articular surfaces of the femoral head and acetabulum.

The ligamentum teres (also known as the ligament to the head of the femur) is a tubular sheath of synovial-lined connective tissue that runs between the transverse acetabular ligament and the fovea of the femoral head (see Figure 12-11). Although the ligament is stretched during flexion and adduction, it likely contributes only a small amount of stability to the articulation.¹⁶⁰ Interestingly, the ligament functions primarily as a protective conduit, or sheath, for the passage of the small acetabular artery (a branch from the obturator artery) to the femoral head. The small and inconstant acetabular artery provides only a minor source of blood to the femur.²⁸^,¹⁶⁰ The primary blood supply to the head and neck of the femur is through the medial and lateral circumflex arteries, which pierce the capsule of the joint adjacent to the femoral neck.

ACETABULUM

The acetabulum (from Latin, meaning “vinegar cup”) is a deep, hemispheric cuplike socket that accepts the femoral head. About 60 to 70 degrees of the rim of the acetabulum are incomplete near its inferior pole, creating the acetabular notch (see Figure 12-2).

The femoral head contacts the acetabulum only along its horseshoe-shaped lunate surface (see Figure 12-11). This surface is covered with articular cartilage, thickest along the superior-anterior region of its dome.³⁸^,⁸⁶ The region of thickest cartilage (about 3.5 mm) corresponds to approximately the same region of highest joint force during walking.³¹ During walking, hip forces fluctuate from 13% of body weight during mid-swing phase to over 300% of body weight during the mid-stance phase. During the stance phase—when forces are the greatest—the lunate surface flattens slightly as the acetabular notch widens slightly, thereby increasing contact area as a means to reduce peak pressure (Figure 12-12).³⁸^,¹⁰¹ This natural dampening mechanism represents yet another design that strives to keep the stress on the subchondral bone within physiologic tolerable levels.

FIGURE 12-12. Graph shows a computer model’s estimate of the hip joint compression force as a multiple of body weight during the gait cycle. The stance phase is between 0% and 60% of the gait cycle, and the swing phase is between 60% and 100% of the gait cycle (vertical stippled line separates these major divisions of the gait cycle). The images above the graph indicate the approximate area of acetabular contact at three selected magnitudes of hip joint force, estimated by data published in the literature.³⁸ The area of joint contact increases from about 20% of the lunate surface during the swing phase to about 98% during mid stance phase.

The acetabular fossa is a depression located deep within the floor of the acetabulum. Because the fossa does not normally contact the femoral head, it is devoid of cartilage. Instead, the fossa contains the teres ligament, fat, synovial membrane, and blood vessels.

ACETABULAR LABRUM

The acetabular labrum is a flexible ring of primary fibrocartilage that surrounds the outer circumference (rim) of the acetabulum (see Figure 12-11).¹⁴² Adjacent to the acetabular notch, the labrum widens as it is transformed into the transverse acetabular ligament.¹⁶⁰

The acetabular labrum is nearly triangular in cross-section, with its apex projecting outward about 5 mm toward the femoral head.¹⁶⁴ The base of the labrum attaches along the internal and external surfaces of the acetabulum rim. The part of the labrum that attaches to the internal surface gradually fuses with the articular cartilage within the acetabulum.

The acetabular labrum provides significant stability to the hip by “gripping” the femoral head and by deepening the volume of the socket by approximately 30%.¹⁶⁴ The seal formed around the joint by the labrum helps maintain a negative intra-articular pressure, thereby creating a modest suction that resists distraction of the joint surfaces. The circumferential seal also holds the synovial fluid within the joint; therefore the labrum indirectly enhances the lubrication and load dissipation functions of the articular cartilage.⁴⁴ The labrum directly protects the articular cartilage by reducing contact stress (force/area) by increasing the surface area of the acetabulum.¹⁶⁴

Consisting primarily of fibrocartilage, the labrum is poorly vascularized, receiving only modest blood supply to its outer one third.¹⁰⁹^,¹⁴² For this reason, a torn labrum has a very limited ability to heal. In contrast to its poor vascularization, the labrum is well supplied by afferent nerves capable of providing proprioceptive feedback and, when the labrum is acutely injured, the sensation of pain.⁸²

ACETABULAR ALIGNMENT

In the anatomic position the acetabulum typically projects laterally from the pelvis with a varying amount of inferior and anterior tilt. Congenital or developmental conditions may cause an abnormally shaped acetabulum. A malshaped, dysplastic acetabulum that does not adequately cover the femoral head may lead to chronic dislocation and increased stress, often leading to degeneration or osteoarthritis. Two measurements are commonly used to describe the extent to which the acetabulum naturally covers and helps secure the femoral head: the center-edge angle and the acetabular anteversion angle.

Center-Edge Angle: The center-edge angle is highly variable but on average measures about 35 degrees in radiographs from adults (Figure 12-13, A).⁴^,⁵⁰ As described in the legend of Figure 12-13, a significantly lower central-edge angle reduces the acetabular coverage of the femoral head. This reduced coverage increases the risk of dislocation and, equally important, reduces the contact area within the joint.¹⁰⁸ A central-edge angle of only 15 degrees, for example, reduces normal contact area by as much as 35%.⁵⁰ During the single-limb–support phase of walking, for instance, this reduced surface area would theoretically increase joint pressure (force/area) by about 50%. Over many years of walking, this scenario may lead to premature hip osteoarthritis, often starting with degeneration of the acetabular labrum.^26,94,111

FIGURE 12-13. A, The center-edge angle measures the fixed orientation of the acetabulum within the frontal plane, relative to the pelvis. This measurement defines the extent to which the acetabulum covers the top of the femoral head. The center-edge angle is measured as the intersection of a vertical, fixed reference line (stippled) with the acetabular reference line (bold solid line) that connects the upper lateral edge of the acetabulum with the center of the femoral head. A more vertical acetabular reference line results in a smaller center-edge angle, providing less superior coverage of the femoral head. B, The acetabular anteversion angle measures the fixed orientation of the acetabulum within the horizontal plane, relative to the pelvis. This measurement indicates the extent to which the acetabulum covers the front of the femoral head. The angle is formed by the intersection of a fixed anterior-posterior reference line (stippled) with an acetabular reference line (bold solid line) that connects the anterior and posterior rim of the acetabulum. A larger acetabular anteversion angle creates less acetabular containment of the anterior side of the femoral head. (A normal femoral anteversion of 15 degrees is also shown.)

Acetabular Anteversion Angle: The acetabular anteversion angle measures the extent to which the acetabulum projects anteriorly within the horizontal plane, relative to the pelvis. Such a perspective can be measured through computed tomography. Observed from above, the acetabular anteversion angle is normally about 20 degrees (Figure 12-13, B).⁴^,¹⁴⁸ Even when normal, this orientation exposes part of the anterior side of the femoral head. The thick anterior capsular ligament of the hip and the iliopsoas tendon naturally cover and support this vulnerable side of the joint. A hip demonstrating excessive acetabular anteversion is more exposed anteriorly: when anteversion is severe, the hip is more prone to anterior dislocation and associated lesions of the anterior labrum, especially at the extremes of external rotation. The likelihood of these associated pathologies increases when acetabular anteversion is combined with excessive femoral anteversion.⁹⁵

An acetabulum that projects directly laterally, or even slightly posterior-laterally, within the horizontal plane is described as being abnormally retroverted.

CAPSULE AND LIGAMENTS OF THE HIP

A synovial membrane lines the internal surface of the hip joint capsule. The iliofemoral, pubofemoral, and ischiofemoral ligaments reinforce the external surface of the capsule (Figures 12-14 and 12-15). Passive tension in stretched ligaments, the adjacent capsule, and the surrounding muscles help define the end-range of movements of the hip (Table 12-1).⁴⁷ Increasing the flexibility in various parts of the capsule is an important component of manual physical therapy for restricted motion of the hip.⁶⁶^,¹⁰³

TABLE 12-1.

Connective Tissues and Selected Muscles That Become Taut at the End-Ranges of Passive Hip Motion

End-Range Position	Taut Tissue
Hip flexion (knee extended)	Hamstrings
Hip flexion (knee flexed)	Inferior and posterior capsule; gluteus maximus
Hip extension (knee extended)	Primarily iliofemoral ligament, some fibers of the pubofemoral and ischiofemoral ligaments; psoas major
Hip extension (knee flexed)	Rectus femoris
Abduction	Pubofemoral ligament; adductor muscles
Adduction	Superior fibers of ischiofemoral ligament; iliotibial band; and abductor muscles such as the tensor fasciae latae and gluteus medius
Internal rotation	Ischiofemoral ligament; external rotator muscles, such as the piriformis or gluteus maximus
External rotation	Iliofemoral and pubofemoral ligaments; internal rotator muscles, such as the tensor fasciae latae or gluteus minimus

FIGURE 12-14. The anterior capsule and ligaments of the right hip. The iliopsoas is cut to expose the anterior side of the joint. Note that part of the femoral head protrudes just medial to the iliofemoral ligament. This region may be covered by a bursa.

FIGURE 12-15. The posterior capsule and ligaments of the right hip.

The iliofemoral ligament (or Y-ligament) is a thick and strong sheet of connective tissue, resembling an inverted Y. Proximally, the iliofemoral ligament attaches near the anterior-inferior iliac spine and along the adjacent margin of the acetabulum. Fibers form distinct medial and lateral fasciculi, each attaching to either end of the intertrochanteric line of the femur (see Figure 12-14). Full hip extension stretches the iliofemoral ligament and anterior capsule. Full external rotation also elongates fibers of the iliofemoral ligament, especially those within the lateral fasciculus.⁴⁷^,¹⁰⁶

The iliofemoral ligament is the strongest and stiffest ligament of the hip.⁶¹^,¹⁶² The mean maximal force required to disrupt either fasciculus is approximately 330 N (75 lb).⁶¹ When a person stands with the hip fully extended, the anterior surface of the femoral head presses firmly against the iliofemoral ligament and superimposed iliopsoas muscle.¹⁸¹ From a position of standing, passive tension in these structures forms an important stabilizing force that resists further hip extension. Persons with paraplegia often rely on the passive tension in an elongated and taut iliofemoral ligament to assist with standing (Figure 12-16).

FIGURE 12-16. A person with paraplegia is shown standing with the aid of braces at the knees and ankles. Leaning the pelvis and trunk posteriorly orients the body weight vector (red arrow) posterior to the hip joints (small green circle), thereby stretching the iliofemoral ligaments. This stretch provides a passive flexion torque at the hip, which helps to balance the extension torque generated by gravity. Once counterbalanced, these opposing torques can stabilize the pelvis and trunk, relative to the femur, during standing. (Modified from Somers MF: Spinal cord injury: functional rehabilitation, Norwalk, 1992, Appleton & Lange.)

Although thinner and more circular than the fibers of the iliofemoral ligament, the pubofemoral and ischiofemoral ligaments blend with and strengthen adjacent aspects of the capsule. The pubofemoral ligament attaches along the anterior and inferior rim of the acetabulum and adjacent parts of the superior pubic ramus and obturator membrane (see Figure 12-14). The fibers blend with the medial fasciculus of the iliofemoral ligament, becoming taut in hip abduction and extension and, to a lesser degree, external rotation.¹⁰⁶

The ischiofemoral ligament attaches from the posterior and inferior aspects of the acetabulum, primarily from the adjacent ischium (see Figure 12-15). Fibers from this ligament join circular fibers located deeper within the posterior and inferior capsule. Other more superficial fibers spiral superiorly and laterally across the posterior neck of the femur to attach near the apex of the greater trochanter (see Figure 12-14). These superficial fibers become taut in full internal rotation and extension¹⁰⁶; other more superior fibers become taut in full adduction.

Close-Packed Position of the Hip: Full extension of the hip (i.e., about 20 degrees beyond the neutral position) in conjunction with slight internal rotation and slight abduction twists or “spirals” most of the fibers within the capsular ligaments to their most taut position (Figure 12-17). This position is useful therapeutically during attempts to stretch the entirety of the hip’s capsular ligaments. Because the position of full extension, slight internal rotation and abduction elongates most of the capsule, it is considered the close-packed position at the hip.¹⁶⁰ The passive tension generated especially by full extension lends stability to the joint and reduces passive accessory movement or “joint play.” The hip is one of a very few joints in the body in which the close-packed position is not also associated with the position of maximal joint congruency. The hip joint surfaces fit most congruently in about 90 degrees of flexion with moderate abduction and external rotation. In this position, most of the capsule and associated ligaments have “unraveled” to a more slackened state, adding only little passive tension to the joint.

FIGURE 12-17. A, The hip is shown in a neutral position, with all three capsular ligaments identified. B, Superior view of the hip in its close-packed position (i.e., fully extended with slight abduction and internal rotation). This position elongates at least some component of all three capsular ligaments.

SPECIAL FOCUS 12-2 Intracapsular Pressure within the Hip

As described earlier, the intracapsular pressure within the healthy hip is normally less than atmospheric pressure. This relatively low pressure creates a partial suction that provides some stability to the hip.

Wingstrand and colleagues studied the effect of joint position and capsular swelling on the intracapsular pressure within cadaveric hips.¹⁷⁵ Except in the extremes of motion, pressures remained relatively low throughout most of flexion and extension. When fluid was injected into the joint to simulate capsular swelling, pressures rose dramatically throughout a greater portion of the range of motion (Figure 12-18). Regardless of the amount of injected fluid, however, pressures always remained lowest in the middle of the range of motion. These data help to explain why persons with capsulitis and swelling within the hip tend to feel most comfortable holding the hip in partial flexion. Reduced intracapsular pressure decreases distension of the inflamed capsule. Unfortunately, over time, the flexed position may lead to contracture caused by the adaptive shortening of the hip flexor muscles and capsular ligaments.

FIGURE 12-18. The intracapsular pressure in the hip joints of cadavers as a function of hip flexion angle. The four curved lines indicate the pressure-angle relationships after the injection of different volumes of fluid into the capsule of the hip.¹⁷⁵

Persons with an inflamed synovium, capsule, or bursa of the hip are susceptible to flexion contracture. It is important to reduce the inflammation through medicine and physical therapy so that activities that favor the extended position can be tolerated. When tolerated, exercises should be devised that strengthen hip extensor muscles while also stretching the hip flexor muscles and anterior capsular structures.

Osteokinematics

This section describes the range of motion allowed by the adult hip, including the factors that permit and restrict this motion. Reduced hip motion may be an early indicator of disease or trauma, either at the hip or elsewhere in the body.⁴¹ Limited hip motion can impose significant functional limitations in activities such as walking, standing upright comfortably, or picking up objects off the floor.

Two terms are used to describe the kinematics at the hip. Femoral-on-pelvic hip osteokinematics describes the rotation of the femur about a relatively fixed pelvis. Pelvic-on-femoral hip osteokinematics, in contrast, describes the rotation of the pelvis, and often the superimposed trunk, over relatively fixed femurs. Regardless of whether the femur or the pelvis is the moving segment, the osteokinematics are described from the anatomic position. The names of the movements are as follows: flexion and extension in the sagittal plane, abduction and adduction in the frontal plane, and internal and external rotation in the horizontal plane (Figure 12-19).

FIGURE 12-19. The osteokinematics of the right hip joint. Femoral-on-pelvic and pelvic-on-femoral rotations occur in three planes. The axis of rotation for each plane of movement is shown as a colored dot, located at the center of the femoral head. A, Side view shows sagittal plane rotations around a medial-lateral axis of rotation. B, Front view shows frontal plane rotations around an anterior-posterior axis of rotation. C, Top view shows horizontal plane rotations around a longitudinal, or vertical, axis of rotation.

Reporting the range of motion at the hip uses the anatomic position as the 0-degree or neutral reference point. Within the sagittal plane, for example, femoral-on-pelvic (hip) flexion occurs as the femur rotates anteriorly beyond the 0-degree reference position. Extension, the reverse movement, occurs as the femur rotates posteriorly toward and beyond the 0-degree reference position. The term hyperextension is not used to describe normal range of motion at the hip.

As depicted in Figure 12-19, each plane of motion is associated with a unique axis of rotation. The axis of rotation for internal and external rotation is often referred to as a “longitudinal” or vertical axis. (The vertical description assumes the subject is standing with the hip in the anatomic position.) This longitudinal axis of rotation extends as a straight line between the center of the femoral head and the center of the knee joint. Because of the angle of inclination of the proximal femur and the anterior bowing of the femoral shaft, most of the longitudinal axis of rotation lies outside the femur itself (see Figure 12-19, A and B). The extramedullary axis has implications on some of the actions of hip muscles, a point discussed later in this chapter.

Unless otherwise specified, the following discussions include passive ranges of motion. The connective tissues and selected muscles that limit motion are also described and are summarized in Table 12-1. The muscles used to produce and control hip motion are discussed later in this chapter. Although femoral-on-pelvic and pelvic-on-femoral movements often occur simultaneously, they are presented here separately.

FEMORAL-ON-PELVIC OSTEOKINEMATICS

Rotation of the Femur in the Sagittal Plane: On average, with the knee flexed, the hip flexes to about 120 degrees (Figure 12-20, A).⁴¹^,¹⁵⁰ Tasks such as comfortably squatting or tying a shoelace typically require this amount of hip flexion.⁷⁷ Full hip flexion slackens the three primary capsular ligaments but stretches the inferior capsule and muscles such as the gluteus maximus. With the knee fully extended, hip flexion is typically limited to 70 to 80 degrees by increased tension in the hamstring muscles. Considerable variability can be expected in this movement because of high inter-subject variability in hamstring muscle flexibility.

FIGURE 12-20. The near maximal range of femoral-on-pelvic (hip) motion is depicted in the sagittal plane (A), frontal plane (B), and horizontal plane (C). Tissues that are elongated or pulled taut are indicated by straight black or dashed black arrows. Slackened tissue is indicated by a wavy black arrow.

The hip normally extends about 20 degrees beyond the neutral position.¹⁵⁰ Full hip extension increases the passive tension throughout the capsular ligaments—especially the iliofemoral ligament and the hip flexor muscles. When the knee is fully flexed during hip extension, passive tension in the stretched rectus femoris, which crosses both the hip and the knee, reduces hip extension to about the neutral position.

Rotation of the Femur in the Frontal Plane: The hip abducts on average about 40 degrees, limited primarily by the pubofemoral ligament and the adductor muscles (see Figure 12-20, B).¹⁵⁰ The hip adducts about 25 degrees beyond the neutral position.¹⁷ In addition to interference with the contralateral limb, passive tension in stretched hip abductor muscles, iliotibial band, and superior fibers of the ischiofemoral ligament limits full adduction.

Rotation of the Femur in the Horizontal Plane: The magnitude of internal and external rotation of the hip is particularly variable among subjects. On average, the hip internally rotates about 35 degrees from the neutral position (see Figure 12-20, C).¹⁵⁰^,¹⁵⁶ With the hip in extension, maximal internal rotation elongates the external rotator muscles, such as the piriformis, and parts of the ischiofemoral ligament.

The extended hip externally rotates on average about 45 degrees. Excessive tension in the lateral fasciculus of the iliofemoral ligament can limit full external rotation. In addition, external rotation can be limited by excessive tension in any internal rotator muscle.

PELVIC-ON-FEMORAL OSTEOKINEMATICS

Lumbopelvic Rhythm: The caudal end of the axial skeleton is firmly attached to the pelvis by way of the sacroiliac joints. As a consequence, rotation of the pelvis over the femoral heads typically changes the configuration of the lumbar spine. This important kinematic relationship is known as lumbopelvic rhythm, introduced in Chapter 9. This concept is revisited in this chapter with a focus on the kinesiology at the hip.

Figure 12-21 shows two contrasting types of lumbopelvic rhythms frequently used during pelvic-on-femoral (hip) flexion. Although the kinematics depicted are limited to the sagittal plane, the concepts can also be applied to pelvic rotations in frontal and horizontal planes. Figure 12-21, A shows an example of an ipsidirectional lumbopelvic rhythm, in which the pelvis and lumbar spine rotate in the same direction.⁹⁰ The effect of this movement is to maximize the angular displacement of the entire trunk relative to the lower extremities—an effective strategy for increasing reach of the upper extremities. The kinematics of the ipsidirectional lumbopelvic rhythm are discussed in detail in Chapter 9. In contrast, during contradirectional lumbopelvic rhythm, the pelvis rotates in one direction while the lumbar spine simultaneously rotates in the opposite direction (see Figure 12-21, B). The important consequence of this movement is that the supralumbar trunk (i.e., that part of the body located above the first lumbar vertebra) can remain nearly stationary as the pelvis rotates over the femurs. This type of rhythm is used during walking, for example, when the position of the supralumbar trunk—including the head and eyes—needs to be held relatively fixed in space, independent of the rotation of the pelvis. In this manner the lumbar spine functions as a mechanical “decoupler,” allowing the pelvis and the supralumbar trunk to move independently.¹⁵² A person with a fused lumbar spine, therefore, is unable to rotate the pelvis about the hips without a similar rotation of parts of the supralumbar trunk. This abnormal situation is readily apparent when the individual walks.

FIGURE 12-21. Two contrasting types of lumbopelvic rhythms used to rotate the pelvis over fixed femurs. A, An “ipsidirectional” rhythm describes a movement in which the lumbar spine and pelvis rotate in the same direction, thus amplifying overall trunk motion. B, A “contradirectional” rhythm describes a movement in which the lumbar spine and pelvis rotate in opposite directions. See text for further explanation.

Figure 12-22 shows pelvic-on-femoral osteokinematics at the hip, organized by plane of motion. These kinematics are all based on the contradirectional lumbopelvic rhythm. In many cases the amount of pelvic-on-femoral rotation is restricted by natural limitations of movement within the lumbar spine.

FIGURE 12-22. The near-maximal range of pelvic-on-femoral (hip) motion is shown in the sagittal plane (A), frontal plane (B), and horizontal plane (C). The motion assumes that the supralumbar trunk remains nearly stationary during the hip motion (i.e., kinematics based on a contradirectional lumbopelvic rhythm). The large colored and black arrows depict pelvic rotation and the associated “offsetting” lumbar motion (see text for further explanation). Tissues that are elongated or pulled taut are indicated by thin, straight black arrows; tissues slackened are indicated by thin wavy black arrows. The range of motions depicted in each figure has been estimated by observing photographs of healthy young adults.

Pelvic Rotation in the Sagittal Plane: Anterior and Posterior Pelvic Tilting: Hip flexion can occur through an anterior pelvic tilt (see Figure 12-22, A). As defined in Chapter 9, a “pelvic tilt” is a short-arc, sagittal plane rotation of the pelvis relative to stationary femurs. The direction of the tilt—either anterior or posterior—is based on the direction of rotation of a point on the iliac crest. The anterior tilt of the pelvis occurs about a medial-lateral axis of rotation through both femoral heads. The associated increased lumbar lordosis offsets most of the tendency of the supralumbar trunk to follow the forward rotation of the pelvis. While sitting with 90 degrees of hip flexion, the normal adult can perform about 30 degrees of additional pelvic-on-femoral hip flexion before being restricted by a completely extended lumbar spine. Full anterior tilt of the pelvis slackens most of the ligaments of the hip, most notably the iliofemoral ligament. Marked tightness in any hip extensor muscle—such as the hamstrings—could theoretically limit the extremes of an anterior pelvic tilt. As depicted in Figure 12-22, A, however, because the knees are flexed, the partially slackened hamstring muscles would not normally produce any noticeable resistance to an anterior pelvic rotation. During standing (and with knees fully extended), however, the more elongated hamstrings are more likely to resist an anterior pelvic tilt, but the amount of resistance is usually insignificant unless the muscle is physiologically impaired and generating extreme resistance to elongation.⁹⁷

As depicted in Figure 12-22, A, the hips can be extended about 10 to 20 degrees from the 90-degree sitting posture via a posterior tilt of the pelvis. During sitting, this short-arc pelvic rotation would increase the length (and therefore tension) only minimally in the iliofemoral ligament and rectus femoris muscle. As depicted in the figure, the lumbar spine flexes, or flattens, as the pelvis posteriorly tilts.

Pelvic Rotation in the Frontal Plane: Pelvic-on-femoral rotation in the frontal and horizontal planes is best described assuming a person is standing on one limb. The weight-bearing extremity is referred to as the support hip.

Abduction of the support hip occurs by raising or “hiking” the iliac crest on the side of the nonsupport hip (see Figure 12-22, B). Assuming that the supralumbar trunk remains nearly stationary, the lumbar spine must bend in the direction opposite the rotating pelvis. A slight lateral convexity occurs within the lumbar region toward the side of the abducting hip.

Pelvic-on-femoral hip abduction is restricted to about 30 degrees, primarily because of the natural limits of lateral bending in the lumbar spine. Marked tightness in hip adductor muscles or the pubofemoral ligament can limit pelvic-on-femoral hip abduction. In the event of a marked adductor contracture, the iliac crest on the side of the nonsupport hip remains lower than the iliac crest of the support hip, which may interfering with walking.

Adduction of the support hip occurs by a lowering of the iliac crest on the side of the nonsupport hip. This motion causes a slight lateral concavity within the lumbar region on the side of the adducted hip. A hypomobile lumbar spine and/or reduced extensibility in the iliotibial band or hip abductor muscles, such as the gluteus medius, piriformis, or tensor fasciae latae, may restrict the extremes of this motion.

Pelvic Rotation in the Horizontal Plane: Pelvic-on-femoral rotation occurs in the horizontal plane about a longitudinal axis of rotation (see green circle at femoral head in Figure 12-22, C). Internal rotation of the support hip occurs as the iliac crest on the side of the nonsupport hip rotates forward in the horizontal plane. During external rotation, in contrast, the iliac crest on the side of the nonsupport hip rotates backward in the horizontal plane. If the pelvis is rotating beneath a relatively stationary trunk, the lumbar spine must rotate (or twist) in the opposite direction as the rotating pelvis. The small amount of axial rotation normally permitted in the lumbar spine significantly limits the full expression of horizontal plane rotation of the support hip. The full potential of pelvic-on-femoral rotation requires that the lumbar spine and trunk follow the rotation of the pelvis—a movement strategy more consistent with an ipsidirectional lumbopelvic rhythm.

Arthrokinematics

During hip motion, the nearly spherical femoral head normally remains snugly seated within the confines of the acetabulum. The steep walls of the acetabulum, in conjunction with the tightly fitting acetabular labrum, limit significant translation between the joint surfaces. Hip arthrokinematics are based on the traditional convex-on-concave or concave-on-convex principles (see Chapter 1).

Figure 12-23 shows a highly mechanical illustration of a hip opened to enable visualization of the paths of articular motion. Abduction and adduction occur across the longitudinal diameter of the joint surfaces. With the hip extended, internal and external rotation occur across the transverse diameter of the joint surfaces. Flexion and extension occur as a spin between the femoral head and the lunate surfaces of the acetabulum. The axis of rotation for this spin passes through the femoral head.

FIGURE 12-23. A mechanical drawing of the right hip. The joint surfaces are exposed by swinging the femur open like a door on a hinge. The articular paths of hip frontal and horizontal plane motion occur along the longitudinal (purple) and transverse (blue) diameters, respectively. Consider these paths of motion for both femoral-on-pelvic and pelvic-on-femoral motions.

MUSCLE AND JOINT INTERACTION

Innervation of the Muscles and Joint

INNERVATION OF MUSCLES

The lumbar plexus and the sacral plexus arise from the ventral rami of spinal nerve roots T¹² through S⁴. Nerves from the lumbar plexus innervate the muscles of the anterior and medial thigh, including the quadriceps femoris. Nerves from the sacral plexus innervate the muscles of the posterior and lateral hip, posterior thigh, and entire lower leg.

Lumbar Plexus: The lumbar plexus is formed from the ventral rami of spinal nerve roots T¹²-L⁴. This plexus gives rise to the femoral and obturator nerves (Figure 12-24, A). The femoral nerve, the largest branch of the lumbar plexus, is formed by L²-L⁴ nerve roots. Motor branches innervate most hip flexors and all knee extensors. Within the pelvis, proximal to the inguinal ligament, the femoral nerve innervates the psoas major and iliacus. Distal to the inguinal ligament, the femoral nerve innervates the sartorius, part of the pectineus, and the quadriceps muscle group. The femoral nerve has an extensive sensory distribution covering much of the skin of the anterior-medial aspect of the thigh. The sensory branches of the femoral nerve innervate the skin of the anterior-medial aspect of the lower leg, via the saphenous cutaneous nerve.