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AN17.1-3 | Hip Joint — Part 1
CLINICAL SCENARIO
Three patients arrive at your orthopaedic unit on the same day:
- A 75-year-old woman with osteoporosis fell from a standing height. X-ray shows a displaced intracapsular fracture of the femoral neck. She is taken for hemiarthroplasty.
- A 30-year-old man was involved in a road traffic accident. He is brought in with his left lower limb held in flexion, adduction, and internal rotation. He cannot straighten it. The femoral head is not palpable in the groin.
- A 55-year-old woman with longstanding rheumatoid arthritis has severe hip pain and reduced range of motion. She is being listed for total hip replacement (THR).
All three cases centre on the same joint — the hip. Let us understand its anatomy so completely that these clinical pictures make instant anatomical sense.
WHY THIS MATTERS
The hip joint is one of the most tested topics in MBBS anatomy because:
- Neck of femur fractures — the most common osteoporotic injury globally; decision to fix vs replace depends on the capsular anatomy and blood supply
- Hip dislocation — posterior dislocation in RTAs (flexed, adducted, internally rotated limb) puts the sciatic nerve at risk
- Total hip replacement (THR) — one of the most common orthopaedic procedures; understanding why the normal hip anatomy fails guides the surgical approach
- Avascular necrosis — can also occur in sickle cell disease, steroid use, and alcoholism — common in India
RECALL
From your study of lower limb bones:
- Femoral head sits in the acetabulum (formed by ilium, ischium, pubis)
- The fovea capitis on the femoral head is where the ligament of the head inserts
- The femoral neck has an intracapsular and extracapsular portion
- Retinacular blood vessels run along the neck under the capsule
Now we will see how these structures come together to form the most stable joint in the body.
Type and Articular Surfaces
The hip joint is a ball-and-socket (spheroidal) synovial joint — one of only two in the body (the other being the glenohumeral joint of the shoulder). It is designed for stability over mobility, unlike the shoulder which prioritises range of motion.
Articular surfaces:
- Ball: the head of the femur — spherical, covered by hyaline cartilage except at the fovea capitis
- Socket: the acetabulum — formed by the ilium (superior), ischium (posteroinferior), and pubis (anteroinferior)
- The acetabular articular surface is C-shaped (lunate surface) — covered by hyaline cartilage; the central non-articular floor is the acetabular fossa (filled with fat and the ligament of the head)
- The acetabular rim is deepened by the acetabular labrum — a fibrocartilaginous ring that increases depth by ~20%; torn in hip dysplasia and impingement
- The acetabular notch (inferior gap in the rim) is bridged by the transverse acetabular ligament
The femoral head is covered more than half by the acetabulum — this accounts for the hip's inherent bony stability (unlike the shoulder where the humeral head is barely covered).
Capsule, Synovial Membrane, and Ligaments
Capsule:
- Proximally: attached to the acetabular margin (and transverse acetabular ligament)
- Distally (femoral side): anteriorly along the intertrochanteric line (so the entire neck is intracapsular anteriorly); posteriorly only attaches halfway along the neck (so the outer half of the posterior neck is extracapsular)
- This asymmetry means that the retinacular vessels running along the posterior neck can be torn even in "intracapsular" fractures where the fracture line is more distal posteriorly
Synovial membrane: lines the inner surface of the capsule; reflects up along the neck (as the retinacular folds) carrying the retinacular vessels
Four major ligaments of the hip joint (all thickenings of the capsule):
- Iliofemoral ligament (Y-ligament of Bigelow) — strongest ligament in the body; anterior surface; attaches from AIIS to the intertrochanteric line; two limbs form an inverted Y; resists extension and lateral rotation (prevents falling backward when standing)
- Pubofemoral ligament — anteroinferior; from the pubic part of acetabular rim to the intertrochanteric line; resists abduction and extension
- Ischiofemoral ligament — posterior; from ischial part of acetabular rim, spiralling around the neck; resists extension and medial rotation
- Ligament of the head of femur (ligamentum teres/round ligament) — intra-articular (but extrasynovial); from fovea capitis to the acetabular notch (transverse acetabular ligament); carries the foveal artery (branch of obturator artery); functionally minimal in adults (more important in children for blood supply)
All three extracapsular ligaments are arranged spirally and tighten in extension — this "screw-home" mechanism stabilises the hip in standing without muscle effort.
Movements, Muscles, and Nerve Supply
Movements of the hip joint:
| Movement | Range | Primary Muscles |
|---|---|---|
| Flexion | 0–120° (knee flexed) | Iliopsoas (primary), rectus femoris, sartorius, pectineus |
| Extension | 0–30° | Gluteus maximus (primary), hamstrings |
| Abduction | 0–45° | Gluteus medius, gluteus minimus (nerve: superior gluteal) |
| Adduction | 0–30° | Adductor longus, brevis, magnus, gracilis (nerve: obturator) |
| Medial rotation | 0–45° | Anterior fibres of gluteus medius and minimus |
| Lateral rotation | 0–45° | Short external rotators: piriformis, obturator internus/externus, gemelli, quadratus femoris |
Nerve supply of the hip joint (Hilton's law):
- Femoral nerve — anterior
- Obturator nerve — inferior
- Nerve to quadratus femoris (branch of sciatic) — posterior
- This explains why hip pathology can refer pain to the anterior thigh (femoral nerve), medial knee (obturator nerve), and buttock (sciatic)
Blood supply of the hip joint:
- Medial circumflex femoral artery (from profunda femoris) → retinacular branches — principal supply to femoral head in adults; runs in the retinacular folds of synovial membrane
- Lateral circumflex femoral artery — supplies femoral neck and trochanteric region
- Obturator artery → foveal artery (via ligamentum teres) — minor in adults
Bursae around the hip:
- Trochanteric bursa — between gluteus maximus and greater trochanter; inflamed in trochanteric bursitis (lateral hip pain, worse lying on side)
- Iliopsoas (iliopectineal) bursa — between iliopsoas tendon and hip capsule; the largest bursa in the body; can communicate with the hip joint cavity; may become swollen in hip synovitis
- Ischial bursa — between ischial tuberosity and gluteus maximus (not commonly inflamed)