Pelvis — MRCS Part B Anatomy

The bony pelvis with a pelvic radiograph, leading into the hip: the bones of the acetabulum, the blood supply of the femoral head and why it fails, and the scale of the hip fracture problem.

The bones

Each hip bone is the fusion of ilium, ischium and pubis, which meet at the triradiate cartilage in the floor of the acetabulum and unite at around 15 years. The two hip bones meet in front at the pubic symphysis, a secondary cartilaginous joint, and behind at the sacro-iliac joints with the sacrum. The obturator foramen lies between the pubis and ischium, closed by the obturator membrane except for the canal carrying the obturator nerve and vessels.

Landmarks

Iliac crest, anterior superior and inferior iliac spines, posterior superior iliac spine (the dimple of Venus), the greater and lesser sciatic notches separated by the ischial spine, the ischial tuberosity, the pubic tubercle, the arcuate line running from the sacral promontory to the pubic crest as the pelvic brim, and the pectineal line along the superior pubic ramus. The groove below the anterior inferior iliac spine holds the tendon of iliopsoas as it passes to the lesser trochanter.

Greater sciatic foramen

The greater sciatic notch is closed into a foramen by the sacrospinous ligament below and the sacrotuberous ligament behind. Piriformis fills it. Above piriformis pass the superior gluteal nerve and vessels. Below it pass the sciatic nerve, the inferior gluteal nerve and vessels, the posterior cutaneous nerve of the thigh, the pudendal nerve and internal pudendal vessels, and the nerves to obturator internus and quadratus femoris. The pudendal bundle then turns back in through the lesser sciatic foramen, over the ischial spine.

Blood supply of the femoral head

Mostly the retinacular vessels from the medial circumflex femoral artery (with a smaller lateral circumflex contribution), which run up the neck inside the capsule from the trochanteric anastomosis. The artery of the ligamentum teres, from the obturator artery, is small and unreliable in adults. Nutrient vessels from the shaft cross the physis only after it closes. A displaced intracapsular fracture tears the retinacular vessels, which is why it is treated by replacement in older patients while an extracapsular fracture is fixed.

Capsule and stability

The capsule attaches to the acetabular margin and labrum, and on the femur to the intertrochanteric line in front but only halfway along the neck behind, so the posterior neck is partly extracapsular. Three ligaments spiral around it and tighten in extension: the iliofemoral (the strongest, the “Y” ligament of Bigelow), the pubofemoral and the ischiofemoral. The deep socket, the labrum, negative intra-articular pressure and the short rotators complete the joint's stability.

Hip fractures in the UK

Around 70,000 a year, at an average age over 80. About a third of patients die within a year, roughly a tenth within a month, and fewer than half return to their previous mobility. The National Hip Fracture Database standards — surgery within 36 hours, orthogeriatric review, early mobilisation — exist because each of those numbers improves when the standards are met.