Hip Anatomy

The hip joint is a synovial ball-and-socket joint formed between the femoral head and the acetabulum of the pelvis. It is built for two competing demands, a wide range of motion across three planes and the structural stability needed to transmit body weight during standing, walking and running.

This chapter breaks the joint down into four parts: the articulating surfaces, the bony anatomy, the capsule and ligaments, and the neurovascular supply.

• Articulating surfaces

The acetabulum and the femoral head are the two surfaces that actually articulate with one another, and neither is completely spherical.

The acetabulum’s articular surface is the lunate surface, a horseshoe-shaped rim of cartilage that covers only the periphery of the socket. The center of the acetabulum is a non-articular depression called the acetabular fossa, filled with fat and the ligamentum teres.

The femoral head is roughly two-thirds of a sphere. At its center is the fovea capitis, a small pit that anchors the ligamentum teres and has no cartilage covering.

• Bony Anatomy

The acetabulum forms where three separate bones meet: the ilium, ischium and pubis. The native acetabulum is oriented anterolaterally with approximately 15-20* of anteversion in the axial plane. The femoral neck connects the femoral head to the shaft at an angle of inclination, typically 125–135°, and at a version angle relative to the femoral condyles.

Acetabular and femoral version together determine the combined anteversion of the hip and affect the available terminal range of motion before impingement. Increased acetabular anteversion provides relatively greater clearance for terminal flexion and adduction, but decreases posterior coverage and may reduce tolerance of terminal extension and abduction. In contrast, acetabular retroversion increases anterior coverage and can produce earlier anterior contact during flexion and adduction, while relatively favouring extension and abduction. Similarly, increased femoral anteversion generally favours flexion, adduction, and internal rotation, whereas femoral retroversion tends to favour extension, abduction, and external rotation, albeit with reduced clearance during flexion

• Capsule and Ligaments

The hip capsule is a sleeve of dense fibrous tissue. Proximally, it attaches to the acetabular rim, the labrum and the transverse acetabular ligament. Distally, it attaches to the femur, anteriorly at the intertrochanteric line and posteriorly at a point on the femoral neck roughly one finger-breadth above the intertrochanteric crest.

The acetabular labrum rims the acetabulum and deepens the socket. It is fibrocartilaginous, roughly 2–3 mm thick, and merges inferiorly with the transverse acetabular ligament, closing the socket into a complete ring

The labrum creates a suction seal, a negative pressure effect that resists femoral head distraction and helps distribute joint forces evenly. This is why a labral tear is mechanically significant, not just structurally: it breaks the seal and changes how load moves across the cartilage.

Three ligaments reinforce the capsule from outside the joint, and one ligament sits inside the joint connecting bone to bone directly:

• Iliofemoral ligament: anterior, Y-shaped, the strongest ligament in the body; tightens on hip extension
• Pubofemoral ligament: anteroinferior; resists excessive abduction and extension
• Ischiofemoral ligament: posterior, the weakest of the three; spirals to blend with the zona orbicularis
• Ligamentum teres: the only intracapsular ligament, running from the acetabular notch to the fovea capitis; also carries a minor blood supply to the femoral head

The zona orbicularis is a distinct circular band of deep capsular fibers that wraps around the femoral neck like a collar, rather than running lengthwise between two bones. It resists joint distraction and is the exact structure surgeons measure for thickness on MR arthrography when screening for hip microinstability.

• Neurovascular Supply

Arterial supply to the femoral head comes mainly from the medial and lateral circumflex femoral arteries, both branches of the profunda femoris with lesser contributions from the obturator and gluteal arteries.

The medial femoral circumflex artery is the dominant supply in adults, primairly through its deep branch. After passing posteriorly around the proximal femur, the deep branch courses in close relation to the short external rotators, crossing posterior to the obturator externus and then anterior to the conjoined tendon of the obturator internus and gemelli. It penetrates the posterior capsule near the superior gemellus and then ascends along the posterosuperior femoral neck, immediately deep to the synovium, where it divides into approximately two to four superior retinacular vessels. These vessels supply the weight-bearing superolateral portion of the femoral head and represent the most surgically important component of its vascular supply.

The artery of the ligamentum teres, a branch of the obturator artery, provides a secondary supply through the fovea capitis. It matters more in children, where it can partly sustain the femoral head if the retinacular vessels are damaged, and becomes less reliable with age.

Innervation of the capsule is segmental, with each nerve supplying a distinct region

• Femoral Nerve → anterior capsule
• Obtrutraor nerve → inferior/medial capsule
• Superior gluteal nerve → superior capsule

The capsule is an important pain-sensitive structure but it is not the only innervated structure associated with the hip joint. Although articular cartilage is aneural, the acetabular labrum, ligamentum teres, transverse acetabular ligament, synovium and subchondral bone contain nociceptive nerve endings and can all contribute to hip pain.

The anterosuperior capsule and superior labrum are particularly rich in nociceptive innervation. Therefore, pre-operative symptoms may arise from capsular distension or inflammation, synovitis, labral pathology, ligamentum teres injury, or subchondral bone disease, and do not necessarily correlate with the degree of cartilage damage seen on imaging.

Movements and Associated Muscles:

The hip moves in three planes around three perpendicular axes passing through the center of the femoral head: flexion/extension, abduction/adduction, and internal/external rotation.

Flexion: Iliopsoas (single most powerful hip flexor), rectus femoris, sartorius, pectineus

Extension: gluteus maximus, and the hamstring muscle group → biceps femoris, semitendinosus, semimembranosus

Abduction: Gluteus medius, gluteus minimus, tensor fasciae latae

Adduction: Adductor longus, adductor brevis, adductor magnus

External Rotation: gluteus maximus, piriformis, obturator internus, obturator externus, superior gemellus, inferior gemellus

Internal Rotation: anterior fibers of gluteus minimus and gluteus medius, tensor fasciae latae