Anatomy of the Acetabulum
The column principle divides the acetabulum into an anterior and posterior column, which becomes important when considering acetabular fractures and their management. The anterior column is composed of the anterior ilium, the anterior wall and dome of the acetabulum, and the superior pubic ramus. The posterior column extends from the obturator foramen through the posterior aspect of the weight bearing dome of the acetabulum, and then obliquely through the greater sciatic notch. The ischium pubic ramus is a complex structure that consists of the inferior pubic ramus and the inferior ramus of the ischium. It forms the inferior border of the obturator foramen. The pelvis is oriented to form an inverted “Y” shape.
An important artery is the obturator artery, which is a branch of the anterior division of the internal iliac artery. It arises in the pelvis and enters the obturator canal. The obturator artery then divides into two branches: the anterior and the posterior branches of the obturator artery, which form a vascular circle around the outer surface of the obturator membrane. An acetabular branch reaches the hip joint and joins the ligamentum teres to supply the head of the femur, however usually only supplies a small portion.
The corona mortis is a connection between the internal iliac branch (obturator) and the external iliac or its branch, the inferior epigastric. The corona mortis is predominantly a venous connection, with the arterial connection being much less. Its location in the superior pubic ramus is variable, but is usually about 3–7 cm from the symphysis pubis. It is located behind and on top of the superior pubic ramus. Care must be taken with lateral dissection of the superior pubic ramus due to the location of the corona mortis. It is susceptible to injury in pelvis trauma and in pelvic surgery, especially during the ilioinguinal approach. Injury to the corona mortis may lead to significant hemorrhage, which can be difficult to control.
The superior gluteal artery is another important vascular aspect of the acetabulum. The superior gluteal artery passes through the greater sciatic notch. Injury to the superior gluteal artery can be associated with acetabular fractures, especially fractures that involve the posterior column. This artery can be damaged by aggressive retraction of the abductor muscle during a posterior approach to the hip.
The medial femoral circumflex artery is the main blood supply to the femoral head. It can be damaged due to dislocation of the femoral head or from taking down the quadratus femoris from the femur, instead of the ischium. A tag of 1 cm needs to be left for the piriformis and the obturator internus from the greater trochanter in order to preserve the deep branch of the medial circumflex artery. If these two tendons are detached too close to the trochanter, it could injure the deep branch of the medial femoral circumflex artery.
The sciatic nerve is close to the acetabulum and can be injured with an acetabulum fracture. In fact, sciatic nerve injury is the most common post-traumatic and iatrogenic nerve injury related to the acetabulum. Sciatic nerve injury is seen in about 10% of posterior hip dislocations, but the incidence may be higher with posterior acetabular fractures. Patients with an acetabular fracture should always be examined and checked for sciatic nerve function. To do so, dorsiflexion of the ankle and the toes should be examined, because it is the peroneal division of the sciatic nerve that will be affected. Numbness at the top of the foot should also be checked for. Partial sciatic nerve injury can get worse from acetabular surgery, and therefore should be checked again right before surgery.
The sciatic nerve anatomy is variable, but well described. There may be variation in its anatomy as well. The diagrams below show the most common patterns of the relationships between the sciatic nerve and the piriformis muscle and their incidence. It should be noted that the nerve can split, and that this is normal.
In order to protect the sciatic nerve during surgery using a posterior approach, the knee should be flexed and the hip should be extended. The sciatic nerve is posterior to the obturator internus muscle and anterior to the piriformis muscle, and because of this, when using the sciatic nerve retractor in the lesser sciatic notch, the muscle and tendon of the obturator internus protects the sciatic nerve. It acts as a buffer layer between the retractor and the nerve, because the nerve is posterior to the muscle.Sliding trochanter osteotomy allows exposure of the dome and the superior aspect of the acetabulum. This type of osteotomy will keep the muscles intact, which will balance its pulling forces. There will be less of a chance of displacement of the greater trochanter this way.
The superior gluteal nerve is close to the superior gluteal artery at the greater sciatic notch. The superior gluteal nerve can be injured from approaches that involve more than 5 cm above the acetabulum. Excessive traction or attempts to control a bleeding from a superior gluteal artery at the greater sciatic notch may injure the nerve, because a suture or a vascular clip used to stop the bleeding may entangle the nerve. An injury to the superior gluteal nerve may affect the gluteus medius and gluteus minimus. An injury to this nerve also affects the abductors of the hip joint, which may cause the patient to end up with a Trendelenburg gait.
The inferior gluteal nerve may also be injured. This nerve innervates the gluteus maximus muscle, and therefore this muscle will be affected.
The lateral femoral cutaneous nerve can become injured during the ilioinguinal approach for acetabular fixation. It usually passes under the ilioinguinal ligament approximately 2 cm medial to the anterior superior iliac spine (ASIS).
