Overview
The hip bone (os coxae) is the largest and flattest bone of the appendicular skeleton. The two hip bones meet ventrally at the pelvic symphysis and dorsally with the sacrum, and together with it they form the bony pelvis, a closed ring that houses the rectum and the urogenital organs and transmits the propulsive force of the pelvic limb to the trunk. In the puppy the ilium, ischium and pubis are separate bones joined by cartilage that meets at the acetabulum, together with a small acetabular bone; they fuse during the first months of life.
Ilium (os ilium)
- Wing (ala ossis ilii): flattened cranial plate. Its cranial border is the iliac crest (crista iliaca); its ventral angle forms the tuber coxae, palpable as the "point of the hip", and its dorsal angle the tuber sacrale.
- Auricular surface (facies auricularis): on the medial face of the wing; articulates with the sacrum.
- Body (corpus ossis ilii): narrow portion that runs caudally towards the acetabulum. Its dorsal border is the greater ischiatic notch (incisura ischiadica major), along which the sciatic nerve passes.
Ischium (os ischii)
Forms the caudal part of the hip bone. Its body contributes to the acetabulum and carries dorsally the ischiatic spine (spina ischiadica) and the lesser ischiatic notch. Caudally it ends in the ischiatic tuberosity (tuber ischiadicum), origin of the hamstring muscles and palpable on either side of the tail base. The two tuberosities bound the ischiatic arch (arcus ischiadicus). The ramus of the ischium joins the pubis to close the obturator foramen.
Pubis (os pubis)
The smallest, cranioventral portion. Its body contributes to the acetabulum; the cranial ramus carries the pecten (pecten ossis pubis) and the iliopubic eminence; the caudal ramus joins the ischium. The two pubic bones meet at the pelvic symphysis (symphysis pelvina), which ossifies with age.
Obturator foramen
The obturator foramen (foramen obturatum) is a large opening bounded by the ischium and the pubis. It is closed by the obturator muscles and transmits the obturator nerve and vessels.
Acetabulum (acetabulum)
Articular cavity where the three bones meet. The crescent-shaped lunate surface (facies lunata) is the cartilage-covered portion that contacts the femoral head; the central, non-articular acetabular fossa (fossa acetabuli) houses the ligament of the femoral head. The acetabular notch (incisura acetabuli) interrupts the ventral rim and is bridged by the transverse acetabular ligament.
Clinical relevance
- Hip dysplasia: the shallow acetabulum and rounded dorsal rim are part of the lesion; pelvic osteotomy aims to reorient it over the femoral head.
- Pelvic fractures: very common after road traffic accidents. Because the pelvis is a ring, it usually breaks in several places; fractures of the weight-bearing axis (ilium, acetabulum, sacroiliac joint) are repaired surgically. A pelvic canal narrowed by malunion causes chronic constipation.
- Palpable landmarks: tuber coxae, ischiatic tuberosity and greater trochanter form the triangle used to assess coxofemoral luxation.