Starting at the top of the leg, the first joint is the hip joint. The hip joint is a synovial joint with a ‘ball-and-socket’ configuration.
This gives it strength (each hip joint has to be able to take the weight of the body when the person stands on one leg) and the depth of the acetabulum, encasing the head of the femur, gives stability (unlike the shoulder joint that has a much flatter ‘socket’).
The configuration of the hip joint allows movement in a number of planes, although not to the extent of the shoulder, its corresponding joint in the upper limb. The hip can perform flexion and extension, adduction and abduction and internal and external rotation.
On the pelvic side of the joint, the bone is initially made up of three separate bones:
- The ilium
- The pubis
- The ischium
These separate bones fuse in childhood, and into this acetabulum fits the head of the femur.
The gluteus medius and gluteus minimus muscles abduct the femur if the pelvis is stable. However, much more important than this function is the fact that they stabilize the pelvis when the opposite leg is lifted. When patients with weak glutei are asked to stand on one leg, there will be a dip in the pelvis on the side of the raised leg – the Trendelenburg sign (the gluteus maximus is a hip extensor).
Usually, the abductors protect the pelvis and lower back when one hip is flexed. However, in the unconscious patient, these muscles are relaxed. Putting the feet into stirrups one at a time can therefore cause twisting of the pelvis and potentially strain the sacroiliac or lower lumbar facet joints. For this reason, both legs should be lifted into stirrups together.
There are a number of bony landmarks that are useful to the surgeon.
The ilium makes up the large lateral component of the pelvis. At the top is the iliac crest which ends, anteriorly, with the anterior superior iliac spine.
The inguinal ligament is attached to the anterior superior iliac spine laterally and the pubic tubercle medially. Thus, the spine and the tubercle are important points of reference for nerve blocks and intravascular line placement.
The Sciatic Foramen
The exit from the back of the pelvis is split by two ligaments.
The larger sacrotuberous ligament runs from the ischial tuberosity to both posterior iliac spines on the ipsilateral side, and the margins of the sacrum and coccyx on that side.
Deep to this, the smaller sacrospinous ligament stretches between the ischial spine and the pelvic surface of the sacrum and coccyx.
This latter ligament is the fibrous part of the coccygeus muscle which, as we have no tail and therefore cannot wag or twitch it, has virtually no function in humans.
These ligaments help form the greater sciatic foramen (through which exit the terminal branches of the sacral plexus, blood vessels and the piriformis muscle) and the lesser sciatic foramen.
The Sciatic Foramen
The course of the sciatic nerve can be easily traced as it exits the pelvis through the greater sciatic foramen, midway between the posterior superior iliac spine (under the sacral dimple) and the tip of the ischial tuberosity.
The nerve then continues down and out to cross slightly medial to the midpoint of a line between the ischial tuberosity and the greater trochanter.
Error in deciding the position of the greater trochanter is the most common reason for failure to locate the sciatic nerve.
The greater trochanter is not the most lateral point that can be palpated, but slightly superior to this – try flexing and extending the hip. The top of the greater trochanter, in a normal hip, lies on a line between the anterior superior iliac spine and the ischial tuberosity.