Anatomy and Biomechanics, Evaluation, Clinical Examination, and Imaging of the Hip

Hany Bedair, MD

Maureen K. Dwyer, PhD, ATC

Dr. Bedair or an immediate family member serves as a paid consultant to or is an employee of Smith & Nephew; has stock or stock options held in DEF Medical and Osteon Holdings; and has received research support from Zimmer. Neither Dr. Dwyer nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.

ABSTRACT

The hip is a complex joint which allows for motion in multiple planes and serves as the axis to propel the body forward into motion. Extensive knowledge of anatomy and normal function of the hip is critical to identifying pathologies and developing successful treatment strategies. The hip is an inherently stable joint, supported by its bony morphology, the strong capsuloligamentous complex surrounding it, and the multitude of muscles acting upon it. Any alteration to normal joint morphology can place the hip at risk for pathology, because of the effect on joint loading and function. A comprehensive clinical examination of the hip should include a thorough history, gait analysis, palpation, and special tests. Multiple imaging modalities are available to assess patients with hip pain; however, findings from imaging studies should be used in accordance with clinical examination findings to confirm a suspected diagnosis.

Keywords: biomechanics; hip anatomy; hip clinical examination; imaging

Introduction

Osseous and Ligamentous Anatomy

The hip is a multiaxial joint formed by the articulation between the pelvis and femur, which connects the axial skeleton and the lower extremity. The hemipelvis comprises three bones, the ilium, ischium, and pubis, which unite at the triradiate cartilage within the concave acetabulum. The shape and depth of the acetabulum is formed by the appearance of ossification centers around the end of the first decade of life, with complete fusion occurring around 18 to 19 years of age.¹ The ilium is a large flat bone that forms the majority of the coxal bone. Forming the ilium’s superior margin, the iliac crest terminates anteriorly at the anterior superior iliac spine (ASIS) and posteriorly at the posterior superior iliac spine (PSIS). Inferior to the ASIS and PSIS are the anterior and posterior inferior iliac spines (AIIS and PIIS). These bony landmarks are clinically important areas of the physical examination and serve as attachment points for various muscles. Directly below the PIIS is the greater sciatic notch through which the large sciatic nerve travels to pass out of the pelvis, and into the thigh. The ischium, a small L-shaped bone, forms the posteroinferior margin of the pelvis. The thickened portion of its body, the ischial tuberosity, is easily palpable posteriorly and also serves as the large attachment site for multiple muscle groups. Anteriorly, the pubis bone consists of a body and two rami that connect superiorly to the ilium and inferiorly to the ischium to form the obturator foramen, an important anatomical landmark that serves as a conduit for arteries and nerves. The obturator foramen is covered by a strong membrane that provides surface area for muscle attachments. The hemipelvises unite anteriorly at the pubic symphyses and articulate posteriorly with the sacral ala to form the sacroiliac (SI) joint.

The acetabulum comprises an articular crescent-moon-shaped lunate surface and a nonarticular central fossa that serves as the attachment point for the ligamentum teres. The acetabulum is incomplete inferiorly, forming a notch through which vital blood vessels and nerves pass to supply the joint. Attached to the rim of the acetabulum is the acetabular labrum, a fibrocartilaginous ring that extends the articulating surface area and increases femoral head coverage.² The labrum is triangular in cross section,²^,³^,⁴ which contributes to its ability to create a pressurized seal of the central compartment of the hip during loading.⁵^,⁶ Only the external one-third of the labrum contains blood vessels, leaving the majority of the structure avascular,⁷ limiting its healing ability following injury. The labrum is highly innervated, with the presence of both mechanoreceptors and nociceptors.⁸ The labrum is absent in the area of the inferior acetabular notch, where the transverse acetabular ligament serves as the continuation of the labrum, connecting the anterior and posterior lunate surfaces of the acetabulum.⁷^,⁹

The femoral head forms two-thirds of a sphere, with a small depression at its center from which the ligamentum teres extends to connect to the acetabular notch, blending with the transverse ligament. This connection provides a means by which the femoral head receives its blood supply via the acetabular branch of the obturator artery, named the artery of the ligamentum teres. The head is covered with articular cartilage and is connected to the shaft by the femoral neck, with the long axis of head and neck projecting superomedially at an angle to that of the obliquely oriented shaft. The neck-shaft angle averages 125°,¹⁰ which allows for greater mobility as it places the head and neck more perpendicular to the acetabulum in a neutral position. Normal version, the head-neck angle in the frontal plane, averages 15 to 20°.¹¹ The angular projection of the femoral head and neck in relation to the obliquely placed acetabulum allows for the rotary movements at the hip and prevents impingement. At the junction of the neck and shaft are the greater and lesser trochanters, which are connected by the intertrochanteric line anteriorly and the intertrochanteric crest posteriorly. These bony prominences serve as important bony landmarks as well as attachment sites for thigh and pelvic muscles.

The hip is surrounded by a dense fibrous capsule extending from the periphery of the acetabulum to the intertrochanteric line of the femoral neck. The capsule enhances joint stability by preventing translation of the femoral head in the acetabulum.¹² Reinforcing the capsule are the three main ligaments that support the hip. Anteriorly, the Y-shaped iliofemoral ligament is the thickest and strongest of the three. The medial portion of the ligament connects the anterior inferior iliac spine to the anterior intertrochanteric line, while the lateral portion originates slightly superior to the medial arm and attaches to the anterior greater trochanter. The iliofemoral ligament functions to limit external rotation,³⁹^,⁴⁰^,⁴¹ while, in isolation, the lateral arm limits extension of the joint.³⁹ Extending from the ischial margin of the acetabulum to the greater trochanter of the femur, the ischiofemoral ligament provides support posteriorly and restricts internal rotation motion.³⁹ Inferiorly, the pubofemoral ligament extends from the obturator crest of the pubic bone to the femoral neck and acts to limit abduction of the joint. Deep fibers from all three ligaments merge to form the zona orbicularis, which circumvents the femoral neck, adding vital stability to the anterosuperior portion of the joint capsule.¹¹ The zona orbicularis provides the greatest contribution to resisting dislocation forces.¹³

Muscular Anatomy

Multiple muscles cross the hip, providing motion in all three cardinal planes and enhancing joint stability. The muscles are organized into four compartments based on their location and function—the gluteal region, anterior compartment, medial compartment, and posterior compartment.

Anterior Compartment

The anterior compartment consists of the sartorius, iliopsoas, quadriceps, and pectineus muscles. The most superficial muscle, the sartorius is the longest muscle in the body, running obliquely from the ASIS to insert on the proximal medial tibia at the pes anserine. The sartorius acts to flex, abduct, and laterally rotate the femur at the hip and flexes the leg at the knee. The iliopsoas muscle is a composite of the iliacus, psoas major, and psoas minor muscles, whose fibers merge to form a conjoined tendon that attaches to the lesser trochanter of the femur. The iliacus muscle originates from the iliac crest, iliac fossa, and sacrum, whereas the psoas major originates from the transverse processes, bodies, and disks of the T12-L5 vertebrae. The psoas minor originates from the body and intervertebral disks of T12 and L1 vertebrae. The iliopsoas muscle is the prime flexor of the femur at the hip and contributes to lateral rotation, whereas the psoas major contributes to spinal stabilization. The quadriceps muscle group consists of four muscles—the vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris muscles. The rectus femoris muscle originates on the ASIS, whereas the vasti muscles originate along the anterior proximal shaft of
the femur. Together, they form a conjoined tendon that attaches directly to the base of the patella and indirectly to the tibial tuberosity via the patellar ligament. As a group, the quadriceps muscle flexes the leg at the knee. The rectus femoris is the only one of the four to contribute to hip motion, assisting with hip flexion. With the exception of the psoas major and psoas minor muscles, which are innervated by the anterior rami of lumbar nerves, all muscles in the anterior compartment are innervated by the femoral nerve.

Gluteal Region

The gluteal region is organized into two layers of muscles—superficial and deep. The largest and most superficial of the gluteal muscles, the gluteus maximus originates from the outer surface of the ilium, the sacrum, coccyx, and sacrotuberous ligament and inserts into the iliotibial tract and gluteal tuberosity. As the strongest extensor and lateral rotator of the hip, the gluteus maximus is most active against resistance and when rising from sitting. The gluteus medius originates from the external ilium between the superior and posterior gluteal lines and attaches to the greater trochanter, whereas the gluteus minimus extends from the external ilium between the anterior and inferior gluteal lines and attaches to the greater trochanter, anterior to the gluteus medius. The tensor of fascia lata extends from the ASIS to insert into the iliotibial tract. The tensor of fascia lata contributes to abduction and medial rotation of the thigh. The primary function of the gluteus medius and minimus is to stabilize the pelvis during single limb support.¹⁴ With the exception of the gluteus maximus, which is innervated by the inferior gluteal nerve, the muscles of the superficial gluteal region are innervated by the superior gluteal nerve.

The deep layer of the gluteal region consists of a group of small, deep muscles which primarily serve to stabilize the head of the femur in the acetabulum. This group of muscles includes the piriformis, the superior and inferior gemelli muscles, the obturator internus, and the quadratus femoris muscles. Most of these muscles insert on the medial surface of the greater trochanter, whereas the quadratus femoris originates laterally on the ischial tuberosity and attaches to the quadrate tubercle of the femur. The obturator internus and superior gemelli muscles are innervated by the nerve to the obturator internus, whereas the inferior gemelli and quadratus femoris are innervated by the nerve to the quadratus femoris. The superior and inferior gemelli muscles, piriformis muscle, and obturator externus muscle also assist in external rotation of the femur. The obturator internus muscle assists in internal rotation of the femur at the hip.

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