Hip Anatomy and Biomechanics


The human hip consists of a stable but very mobile skeletal framework for the surrounding capsule, ligaments, muscles, nerves, and vasculature. Understanding the anatomy and the development of intra- and extra-articular pathologies is critical in any patient population, and careful consideration must go into the evaluation of a painful hip in the female athlete. The anatomy, biomechanics, and pathologies specific to the female athlete are explored in this chapter.

The hip is a ball-and-socket joint that is formed by the articulation of the femoral head and the acetabulum, between which there is a high level of congruence. The joint's configuration allows for multiplanar movement; however, due to the joint being an integral part of lower limb motions such as walking, running, jumping, and kicking, a high degree of stability is also required. It is markedly stable because of not only its osseous and articular architecture but also the encompassing soft tissues. Numerous structures provide stability to the joint. The capsule is one of these essential structures, and it is reinforced by the embedded capsular ligaments. The labrum helps stabilize the joint by deepening the socket, while the ligamentum teres acts to tether the femur to the acetabulum. Furthermore, the musculature surrounding the joint provides both static and dynamic stability. This chapter explores how each of these structures contributes to the function of the hip joint and its physical implications, with a focus on the female athletic population.


Basic Anatomy

Bony and articular anatomy

The hip joint is a synovial joint that can be further categorized as a ball-and-socket-type of joint. The hip joint is composed of the femoral head and the acetabulum, and these two components are in high congruence with one another. The acetabulum is formed by the junction of three bones, namely, the ilium, the ischium, and the pubis, that intersect to form the triradiate zone. The ilium is a broad, fanlike bone that expands superiorly from the acetabulum. The ischium extends posteroinferiorly from the acetabulum, while the pubis does so anteroinferiorly. These bones expand peripherally during growth to give the acetabulum its depth, and the concavity of the acetabulum develops around the sphericity of the head of the femur. The acetabulum is a relatively deep cavity, which contributes significantly to the stability of the joint; its depth has been measured to be approximately 30 mm. The articular cartilage of the acetabulum varies throughout the cavity, and it typically ranges between 1.3 and 3.0 mm in thickness, with the greatest thickness in the superolateral quadrant. Although the acetabulum is commonly described as a hemisphere, it is devoid of cartilage in the central-inferior portion, with an opening at its inferior aspect spanned by the transverse acetabular ligament. The bare area is where the ligamentum teres originates.

The diameter of a native femoral head can range from 40 to 54 mm, with smaller sizes usually found in females. All areas of the femoral head that articulate with the acetabulum are covered with hyaline cartilage, therefore covering 60%–70% of the spherical head. The articular cartilage of the femoral head can be between 0.8 and 2.8 mm in thickness. The inferomedial part of the head, the fovea capitis, lacks cartilage, as that is where the ligamentum teres inserts.

Capsular and ligamentous anatomy

The capsule consists of dense fibers that are cylindrically arranged around the joint to form a sleeve; they insert proximally along the acetabular periosteum, just proximal to the labrum, and distally to the anterior aspect of the femur, along the intertrochanteric line. The capsule functions to both constrain the hip joint and maintain congruence. Posteriorly, the capsule is composed mainly of the ischiofemoral ligament proximally and the zona orbicularis (ZO) distally. The ZO forms an arched free border that partially covers the femoral neck; it attaches just medial to the intertrochanteric crest.

Most fibers of the capsule are longitudinally oriented, as is the strongest of the capsular ligaments, the iliofemoral ligament, also known as the Y-ligament of Bigelow. This ligament is located anteriorly and lies between the anterior inferior iliac spine and the ilial portion of the acetabular rim and the intertrochanteric line. It fans across the front of the joint, dividing into superior and inferior bands, taking the shape of an inverted Y. Its fibers are taut in extension and external rotation and lax in flexion, thus this ligament is thought to be essential in maintaining erect posture and reducing the requirement for active muscle contribution. ,

The ischiofemoral ligament lies posterior to the joint and is spiral in shape. The fibers of the ligament are oblique, but the ischiofemoral ligament is less defined than the pubofemoral and iliofemoral ligaments. The ischiofemoral ligament extends from the ischial rim of the acetabulum to the posterior aspect of the femoral neck, at the base of the greater trochanter. The literature describes two distinct bands in this ligament: the superior and the inferior bands. , The superior band inserts at the base of the greater trochanter, where it intermingles with the fibers of the ZO. The inferior band spreads downward to insert more posteriorly on the intertrochanteric crest. , The ischiofemoral ligament is thought to be at maximal tautness primarily during internal rotation, and also during adduction when the hip is flexed, thus limiting the extent of these actions. ,

The pubofemoral ligament is slinglike in appearance; it originates proximally at the obturator crest and the superior pubic ramus. Distally, it blends anteriorly with the inferior part of the iliofemoral ligament and wraps posteriorly to insert inferior to the ischiofemoral ligament. It works in conjunction with the iliofemoral ligament to control external rotation of the hip and has been found to be maximally taut in hip abduction and lax in adduction.

The fibers of the ZO are circular and resist distraction of the hip. It is primarily a posterior and inferior structure, forming the free border of the posterior capsule. Recent studies indicate that the ZO has a role in synovial fluid circulation within the joint; it has been postulated that it acts as a bellow to unidirectionally force fluid from the peripheral compartment to the central compartment when the hip moves in flexion and extension.

Neurovascular anatomy

Vascular anatomy (blood supply)

The primary source of blood to the hip joint is the medial femoral circumflex artery (MFCA). It arises as a branch of the deep femoral artery, and its superficial branch supplies the adductor musculature. Direct branches of the MFCA and the lateral femoral circumflex artery (LFCA), which also arises from the deep femoral artery, predominantly supply the anterior capsule; they enter from the femoral aspect and run superficially along the capsule, encircling it from distal to proximal. The superior and inferior gluteal arteries divide into supra-acetabular and acetabular branches that terminate as capsular vessels; these run from proximal to distal to form anastomoses with the MFCA and LFCA and are the major supply to the posterior capsule.

The vascular supply of the labrum is formed by branches of the medial and lateral circumflex arteries, the superior and inferior gluteal arteries, and the vascular system within the pelvis. The primary blood supply to the labrum is postulated to be the connective tissue interposed between the capsule and the capsular region of the labrum; therefore the capsular side is more vascular than the articular region ( Fig. 9.1 ). The bone adjoining the labrum is also a major contributor of blood to the structure. In one study, vascular channels originating in the osseous acetabulum were found to cross into the labrum, demonstrating that the bone-adjacent labrum has greater vascularity than its more peripheral aspects. This can have significant implications in labral healing and repair.

Fig. 9.1, Capsular blood supply ( arrow ) as shown on a high-power sagittal section.

The blood supply to the femoral head is variable. A minor branch off of the posterior division of the obturator artery supplies the ligamentum teres and thus plays a small role in vascularizing the proximal part of the head. Ascending cervical branches that arise from the extracapsular arterial ring perforate the capsule to form retinacular arteries; these are the main supply to the femoral head. The retinacular arteries have classically been divided into three main groups: posterosuperior, posteroinferior, and anterior. The posterosuperior and posteroinferior arteries are supplied primarily by the MFCA, with the anterior most commonly from the LFCA. However, Ganz et al. determined that the deep branch of the MFCA can give rise to two to four superior retinacular vessels and, occasionally, to inferior retinacular vessels ( Fig. 9.2 ). The head can be completely perfused by the superior retinacular vessels alone. This can have significant consequences in the development of avascular necrosis of the femoral head, particularly after dislocation or during surgical management of femur fractures.

Fig. 9.2, (A) The perforation of the terminal branches into the bone (right hip, posterosuperior view). The terminal subsynovial branches are located on the posterosuperior aspect of the neck of the femur and penetrate the bone 2–4 mm lateral to the bone-cartilage junction. (B) A diagram showing (1) the head of the femur, (2) the gluteus medius, (3) the deep branch of the medial femoral circumflex artery (MFCA), (4) the terminal subsynovial branches of the MFCA, (5) insertion of and the tendon of gluteus medius, (6) insertion of the tendon of piriformis, (7) the lesser trochanter with nutrient vessels, (8) the trochanteric branch, (9) the branch of the first perforating artery, and (10) the trochanteric branches.

Nerve anatomy

The obturator nerve is considered to be the primary source of innervation to the hip joint; however, the femoral nerve and the sciatic nerve also contribute sensory and motor innervation. The hip capsule is believed to be regionally innervated; the anterior region of the hip capsule receives innervation from the femoral and obturator nerves, while the posterior region receives innervation from the superior gluteal nerve, the nerve to the quadratus femoris, and direct branches of the sciatic nerve.

The labrocapsular complex receives innervation from the sciatic, inferior gluteal, and femoral nerves, but more substantially from the nerve to the quadratus femoris and the obturator nerve. Sensory end organs, such as Pacinian, Golgi-Mazzoni, Ruffini, and Krause corpuscles, in addition to free nerve endings, have been found in the hip labrum. Pacinian, Golgi-Mazzoni, and Ruffini corpuscles aid in proprioception, and the free nerve endings sense pain. The highest concentration of nociceptive and proprioceptive fibers is found along the attachment site of the labrum to the acetabulum. There is also a fair amount of these fibers in the center of the ligamentum teres.

As for muscular innervations, the femoral nerve provides innervation to the psoas, iliacus, pectineus, sartorius, and quadriceps muscles and provides sensory innervation to the anterior thigh via cutaneous branches. The sciatic nerve arises from spinal nerves L4 through S3 to form part of the sacral plexus and consolidates to course through the greater sciatic foramen, just inferior to the piriformis. It then divides into the tibial and the peroneal branches; the tibial branch supplies the semitendinosus, the semimembranosus, and the long head of the biceps femoris. The peroneal branch of the sciatic nerve innervates the short head of the biceps femoris. The sciatic nerve does not have any direct sensory functions.

The obturator nerve, originating from L2-4, provides innervation to the obturator externus, gracilis, and adductor muscle group. It provides sensory innervation to the inferomedial thigh via its cutaneous branches. The superior and inferior gluteal nerves arise from the ventral rami of the L4-S1 and L5-S2 sacral spinal nerves, respectively, and transverse through the greater sciatic foramen from the sacral plexus. The superior gluteal nerve exits the pelvis at or just above the piriformis and innervates the gluteus medius, gluteus minimus, and tensor fascia latae. The gluteus maximus receives its innervation from the inferior gluteal nerve, which exits the greater sciatic foramen just inferior to the piriformis muscle. Neither nerve has a sensory role.

The short external rotators, with the exception of the obturator externus, receive innervation from the sacral plexus that descends from the ventral rami of the L4-S2.

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