Hip Joint

Introduction

Introduction to the Hip Joint

The hip joint (coxafemoral joint) is a synovial ball-and-socket joint between the head of the femur and the acetabulum of the hip bone. It is the largest and most stable joint in the body, designed for weight-bearing, locomotion, and a wide range of motion, including flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction. Its great stability is provided by the depth of the acetabulum, the strong joint capsule, and three major extracapsular ligaments (iliofemoral, pubofemoral, and ischiofemoral), as well as the intra-articular ligamentum teres. The hip joint is crossed by powerful periarticular muscles that act as dynamic stabilisers. Its blood supply is primarily from the medial and lateral circumflex femoral arteries, with a significant contribution from the artery to the head of the femur within the ligamentum teres, which is particularly important in children. The hip joint is clinically significant for fractures (especially femoral neck fractures in the elderly), osteoarthritis, avascular necrosis of the femoral head, congenital dysplasia, and traumatic dislocation.

Study

1. Articular Surfaces

The hip joint is formed by the hemispherical head of the femur articulating with the cup-shaped acetabulum of the hip bone. The acetabulum is formed at the fusion of the ilium, ischium, and pubis. It consists of a horseshoe-shaped articular lunate surface covered with hyaline cartilage, which is deficient inferiorly at the acetabular notch. The non-articular floor of the acetabulum is the acetabular fossa, filled with a fat pad and the ligamentum teres. The acetabular labrum, a fibrocartilaginous rim, deepens the socket, increasing the coverage of the femoral head. The femoral head is covered with hyaline cartilage except at the fovea capitis, where the ligamentum teres attaches. The diameter of the femoral head is matched to the acetabulum, providing intrinsic stability. The angle of inclination of the femoral neck is normally 120–135 degrees in adults, and the angle of anteversion is 12–15 degrees.

2. Joint Capsule

The fibrous capsule is strong and dense, attached proximally to the acetabular margin, the outer surface of the labrum, and the transverse acetabular ligament. Distally, it attaches to the intertrochanteric line anteriorly, the femoral neck (about 1 cm above the intertrochanteric crest) posteriorly, and the bases of the greater and lesser trochanters. The capsule encloses the femoral neck and is reinforced by three main ligaments. The synovial membrane lines the inner surface of the capsule and covers the femoral neck up to the articular margin of the head, reflecting onto the labrum and the ligamentum teres. The retinacular vessels (from the circumflex arteries) run deep to the synovial membrane along the femoral neck, supplying the head.

3. Ligaments

The hip joint has three principal extracapsular ligaments, one intra-articular ligament, and the acetabular labrum. The iliofemoral ligament (Y-ligament of Bigelow) is the strongest ligament in the body. It arises from the anterior inferior iliac spine and the acetabular rim, and inserts onto the intertrochanteric line. It has two bands: the medial band limits hyperextension and abduction, the lateral band limits hyperextension and adduction. It prevents the trunk from falling backward during standing. The pubofemoral ligament originates from the iliopubic eminence and superior pubic ramus, blending with the medial part of the iliofemoral ligament, and inserts onto the intertrochanteric line and neck of the femur. It limits excessive abduction and extension. The ischiofemoral ligament arises from the ischium posterior and inferior to the acetabulum, spiraling superolaterally to attach to the posterior femoral neck. It limits internal rotation and extension. The zona orbicularis is a deep circular layer of the capsule that encircles the femoral neck like a collar and contributes to stability. The ligamentum teres (ligament of the head of the femur) is a flat, triangular band within the joint, running from the acetabular notch and transverse acetabular ligament to the fovea capitis. It contains the artery to the head of the femur (branch of the obturator or medial circumflex femoral artery) and is important for the vascular supply of the femoral head in children.

4. Blood Supply

The blood supply of the femoral head and neck is of paramount clinical importance due to the risk of avascular necrosis following femoral neck fractures. The primary supply comes from the medial and lateral circumflex femoral arteries, usually branches of the profunda femoris artery. They form an extracapsular arterial ring at the base of the femoral neck. Ascending cervical (retinacular) branches pierce the capsule and run along the femoral neck within the synovial retinaculum, entering the head at the articular margin. The lateral epiphyseal artery (from the medial circumflex femoral artery) is the most important vessel for the femoral head. The artery of the ligamentum teres (foveal artery), a branch of the obturator artery (or sometimes the medial circumflex femoral artery), supplies the foveal region and is more significant in children. The nutrient artery of the femur from the profunda femoris supplies the shaft and metaphysis but has limited contribution to the epiphysis. Disruption of the retinacular vessels in subcapital fractures leads to avascular necrosis.

5. Nerve Supply

The hip joint receives innervation from multiple nerves, following Hilton’s law (the nerve that supplies a muscle crossing the joint also supplies the joint). The anterior aspect is supplied by the femoral nerve (via the nerve to rectus femoris) and the obturator nerve (anterior division). The posterior aspect is supplied by the sciatic nerve (nerve to quadratus femoris) and the superior gluteal nerve. Pain from the hip joint may be referred to the knee via the obturator nerve (the knee's infrapatellar branch also comes from the femoral nerve and obturator nerve), which is why hip pathology can present as knee pain, especially in children.

6. Movements and Muscles

Flexion (0–120 degrees with knee flexed, less with knee extended): iliopsoas (prime mover), assisted by rectus femoris, sartorius, and pectineus. Extension (0–30 degrees): gluteus maximus (prime mover), hamstrings (biceps femoris, semitendinosus, semimembranosus). Abduction (0–45 degrees): gluteus medius and minimus, assisted by tensor fasciae latae. Adduction (0–30 degrees): adductor magnus, longus, and brevis, gracilis, pectineus. Medial (internal) rotation (0–45 degrees): anterior fibers of gluteus medius and minimus, tensor fasciae latae. Lateral (external) rotation (0–45 degrees): gluteus maximus, short external rotators (piriformis, obturator internus, gemelli, quadratus femoris), and obturator externus. Circumduction is a combination of the above movements.

7. Clinical Relevance

Femoral neck fractures: Common in elderly osteoporotic patients after low-energy falls. Intracapsular (subcapital) fractures disrupt the retinacular vessels, risking avascular necrosis of the femoral head. Extracapsular (intertrochanteric) fractures have a better blood supply and heal more readily. Hip arthroplasty or internal fixation is determined by fracture type and patient age. Avascular necrosis of the femoral head: Caused by trauma, steroids, alcoholism, or sickle cell disease. Presents with gradual onset groin pain and limited movement. MRI is diagnostic. Osteoarthritis of the hip: Characterized by progressive cartilage loss, osteophyte formation, and pain with activity. Groin pain and limited internal rotation are early signs. Treatment ranges from conservative to total hip replacement. Congenital hip dysplasia: Shallow acetabulum leads to instability, subluxation, or dislocation. Detected by Ortolani and Barlow maneuvers in infants. Early treatment with a Pavlik harness can prevent long-term disability. Traumatic hip dislocation: Usually posterior (85–90%) due to dashboard injury; presents with leg shortened, internally rotated, and adducted. Anterior dislocation is less common, with leg externally rotated and abducted. Sciatic nerve injury may accompany posterior dislocation. Emergency reduction is essential to prevent avascular necrosis. Labral tears: Can cause mechanical symptoms, clicking, and pain. Diagnosed by MRI arthrogram. Treated with arthroscopic repair or debridement.

Summary

Summary of the Hip Joint

The hip is a stable ball-and-socket synovial joint formed by the femoral head and acetabulum. Its stability is enhanced by a deep socket with a labrum, a strong capsule, and three main ligaments (iliofemoral, pubofemoral, ischiofemoral). The ligamentum teres contains the artery to the head of the femur, contributing to blood supply particularly in children. The main vascular supply to the femoral head is from the medial and lateral circumflex femoral arteries via retinacular vessels, which are vulnerable in subcapital fractures, leading to avascular necrosis. Nerve supply is from femoral, obturator, sciatic, and superior gluteal nerves, explaining hip pain referral to the knee. The joint allows flexion, extension, abduction, adduction, and rotation, powered by large muscle groups. Common pathologies include fractures, osteoarthritis, avascular necrosis, congenital dysplasia, and dislocation.