Hip implants are artificial devices that form the essential parts of the hip joint for a hip replacement surgery. The hip implants vary by size, shape, and material. Implants are made of biocompatible materials that are accepted by the body without producing any rejection response. Implants can be made of metal alloys, ceramics, or plastics, and can be joined to the bone. The metals used include stainless steel, titanium, and cobalt chrome, whereas the plastic used is polyethylene. Various components of a hip implant may be used for hip replacement surgery. The components used may depend on the extent of damage to the hip joint, and the preference of the orthopedic surgeon performing the procedure.
Components of a Hip Implant
The hip joint is a ball-and-socket joint. The ball or the spherical head of the thighbone (femur) moves inside a cup shaped socket (acetabulum) of the pelvis.
The components of a hip implant replicate the natural shape and structure of the ball-and-socket joint. The components used depend on the size of the body and vary from patient to patient. A total hip replacement implant has three parts:
- Stem: The stem fits into the femur
- Ball: The ball replaces the spherical head of the femur
- Cup: The cup replaces the worn out hip socket
Types of Hip Implants
Based on the patient's activity level, any of the following types of hip implants may be used in a hip replacement surgery.
Metal-on-polyethylene implant: The ball is replaced with a metal ball and the socket is replaced with polyethylene or has a polyethylene lining.
Ceramic-on-polyethylene implant: The ball is replaced with a ceramic ball and the socket is replaced with polyethylene or has a polyethylene lining.
Metal-on-metal implant: The ball and socket of the hip joint are replaced with metal prosthesis. Metal-on-metal implants have greater durability compared to metal-on-polyethylene implants.
Ceramic-on-ceramic implant: The ball is replaced with a ceramic ball, and the socket has a ceramic lining. These wear less than metal-on-metal implants, and are the most durable among the available hip implants.
Ceramic-on-metal implant: The ball is replaced with a ceramic ball and the socket has a metal lining.
Types of Implant Fixation
Depending on the age and activity level of the patient undergoing hip replacement surgery, an orthopedic surgeon may recommend any of the available three types of implant fixation.
Cemented Fixation: The femoral and acetabular components are held together with special bone cement. The bone cement is made from a special polymer called polymethylmethacrylate (PMMA). Patients can often immediately be full weight bearing and walk after a cemented fixation. Cemented fixation is an option for less active patients. However, too much stress on cemented fixation can lead to fatigue fractures.
Cementless Fixation: Cementless implants are coated with a porous material. They attach to the new bone that grows to the surface of the implant via bone ingrowth. The implant may be fixed using screws or pegs until bone ingrowth. Patients need to limit weight bearing and use crutches or a walker following cemented fixation to allow the bone to attach itself to the implant. Cementless fixation is an option for more active patients with good bone quality.
Hybrid Fixation: Hybrid fixation uses a combination of cemented and cementless fixation. The acetabular socket is inserted without cement and the femoral stem is inserted with cement.