Ball & Socket
Artificial Hip Joint
hip joint consists of a ball in a socket. The ball (femoral head)
is the upper end of the leg bone (femur). The femoral head sits
inside a socket (acetabulum) of the pelvis.
Wrapped around the hip joint is a tough membrane like material
called the joint capsule. On top of that, there are large leg muscles
and tendons around the hip joint. These muscles and tendon power
the motion of the hip joint. So, the hip is a very stable 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.
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.
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
seen on x-rays is gone; hip replacement may be considered as
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
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.
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