Please help me explain something to my 71-year-old grandpa. He needs a hip replacement. The doctor told him that part of his thigh bone (the neck?) is short and twisted. They will have to “fix” this during the operation. What do they do to fix this problem?

The hip joint is made up of two basic parts: the femur (thigh bone) and the acetabulum (hip socket). At the top of the femur is a round ball of bone called the femoral head. This ball fits into the socket and rotates in all directions to give us hip motion.

Between the long shaft of the femur and the femoral head is the femoral neck. The neck is like a short bridge between these two parts. The “twist” the physician mentioned refers to the angle of this “neck” between the shaft of the long bone and the round head at the top.

The femoral neck angle is usually about 150 degrees at birth. With time, growth and development, and weight-bearing, this angle decreases. By adulthood, it is more in the range of 125 to 135 degrees. Some experts suggest coxa vara occurs when the angle between the ball and the shaft of the femur is less than 120 degree (in adults). The condition may or may not be painful but usually results in a limp because one leg is shorter than the other.

When one leg is shorter (or longer) than the other leg, the condition is referred to as a leg-length discrepancy. Placement of a hip implant in conditions like your grandpa’s requires some special planning on the part of the surgeon. A shorter neck length can place the femoral head up higher in the hip socket (rather than in the center). The two anatomical changes (short neck with a twist) will be addressed during the hip replacement procedure.

The surgeon will use imaging studies before the surgery to study the hip shape and alignment. Sometimes, the actual surgical techniques to correct problems aren’t decided until during the procedure when the surgeon can see all of the structures in three-dimensions. The ultimate goal is to provide restoration of the proper limb length and center the implant correctly.