Ball & Socket Artificial Hip Joint

The surface of the femoral head and the lining of the acetabulum are both covered by a soft layer of material called cartilage. Cartilage is very smooth and gives the hip lubrication. So the hip joint has smooth and painless motion.

When the surface cartilage wears out like the tires of a car, the surfaces become rough and moving the hip becomes painful and difficult. When joint pain happens, the patient will use the hip less and becomes less active. When every step becomes painful and the pain is also there at night in bed; the doctor will prescribe medicines and walking aids. When the pain can no longer be controlled by medicines and the cartilage seen on x-rays is gone; hip replacement may be considered as an option.

The artificial hip joint consists of two sides. One side is the socket (acetabular component) and it has two parts. The first is a half metal sphere that has an outside surface that favors bone to grow onto or attach. So, it may be a rough surface such as plasma sprayed titanium. It may also be coated with a material called hydroxylapatite which has a chemistry similar to that of bone. Usually, the acetabular shell is pressed into bone to get a press fit. Then screws are placed through it for additional security. The second part of the socket is a liner. This liner fits inside the half metal sphere. The liner material is usually a plastic one.

The other side of the hip joint (femoral component) also has two parts. The first part is a stem that fits into the canal of the femur. The stem may be press fitted into the canal or secured in place with bone cement. The second part is a ball with a female neck taper. The ball fits over a top of the stem which has a male taper. The female neck taper of the ball comes in different neck lengths so that the stability of the joint can be adjusted.

The femoral implant used by Dr. Menendez in the muscle sparing technique is called the Citation design. It has a very complex geometry based on a statistical study of human femurs. So, it will follow the patient’s own canal and seat itself.

When both parts of the hip joints are installed, they are linked together. The ball on the stem will fit into the liner of the socket and the hip joint is put backed together.

There are instances when the shape of the femur and/or the acetabulum are so different from the normal patients that the socket and the femoral components do not press fit well. In those instances, a grouting space filling agent called ‘bone cement’ may have to be used to securely fix them in bone. The bone cement has been used for forty years in hip surgery and has also given good results.