Hip joint replacement surgery involves the resurfacing of the ball and socket portions of the hip joint. This is typically accomplished by the insertion of specific implants that are made of artificial materials that function as the original hip joint did. Hip prosthetic components are made of different materials, and their selection is determined by an orthopedic surgeon based on the needs of the patient.
The Acetabulum or Cup
The human hip consists of two main parts: the acetabulum, or the socket, and the femoral head, or ball portion of the thigh bone. Acetabular components in hip replacement procedures are manufactured from space-age metals, such as cobalt chrome, stainless steel and titanium. They can also have accessory parts made of ceramic material, metal or high-density plastic, all of which are used to interface with the ball of the thigh bone, or femur.
The acetabulum is typically a spherical shell configuration that can have several holes in it for metallic screw placement. It can also be coated on the back side with small metallic shavings, or sprayed-on coatings. These rough coatings are designed to stimulate the body's ability to produce new bone and literally heal itself to the component. This shell can also have a highly polished metallic inner surface that interfaces with the metallic ball of the femur. This is called a metal-on-metal prosthesis.
The Femoral Stem
On the thigh bone side of the hip, implants are inserted into the canal of the thigh bone and anchored by either cement fixation or press-fit. This stem has a small post on the top facing the acetabulum, called a trunion or neck. This trunion is the post that the ball is fixed to, by way of a press fit, which replaces the larger, anatomic ball of the human hip. These stems come in different lengths, diameters and shapes to accommodate the internal canal of the thigh bone and prevent it from sliding or rotating.
The Ball, or, Femoral Head
Replacing the femoral head is accomplished with the use of various diameter components called femoral heads. Examples of materials used in the femoral heads include cobalt chrome and ceramic. The diameter of this ball is in direct proportion to the size and diameter of the corresponding acetabular shell or liner. The ball comes in direct contact with the acetabular, or cup, side of the hip, which is lined with either metal, plastic or ceramic to reduce friction and provide a smooth marriage of materials.
The Spacer
If a surgeon is using a hip system other than a metal-on-metal system, spacers, or liners, are used. The liner that separates the metallic acetabulum from the metallic femoral component is frequently made of high-density plastic, and sometimes ceramic material.
References
- The American Association of Orthopedic Surgeons: Hip Implants
- The Anderson Orthopedic Research Institute: Total Hip Replacement
- The Hip and Knee Institite: Hip Replacement Surgery
- The University of Washington Medicine: What is Hip Replacement?
- The Hip and Knee Institute: Hip Implant Designs and Materials
