Total Hip Replacement Implants

The implants used in the hip replacement may differ in a number of aspects. They may be made from different materials or may have different shapes. The type of implant best for the patient is usually decided by the surgeon after a discussion with the patient.

Total hip replacement is a surgical procedure to replace the diseased parts of the hip joint. The hip joint is formed by the upper end of the thigh bone (femur) and the bottom of the pelvis (acetabulum). The hip replacement surgery remains the most successful surgery in modern medicine.

Procedure

Different techniques can be used to access the hip joint. Most commonly, the hip joint is accessed from behind (posterior approach). It can also be accessed from the front (anterior approach) or the side (lateral approach).

A Minimally Invasive technique involves giving a smaller incision and less separation of tissues. Robotic Surgical assistance may be used in all these techniques. The merits and demerits of each technique are discussed with the patient so that the technique can be individualized.

• The damaged part of the acetabular socket is removed, and a cup made of metal alloy or ceramic is fixed with screws or press-fitted in the socket.
• The head of the femur is cut with a saw, and a stem made of metallic alloy is inserted in the upper part of the thigh bone.
• The stem may be press-fit or fixed using a special form of bone cement, allowing the bone to hold the implant.
• A prosthetic head made of metal alloy or ceramic is placed on the stem replacing the natural head of the femur.
• A special form of highly durable plastic called polyethylene is placed between the head and socket to allow smooth gliding.

Implants

An ideal implant should last duplicate the function of the natural joint. The artificial hip should allow natural movements for the patients to do the activities they enjoy. The implant should be non-allergic and have a good track record. The implant should have a low degree of friction and less generation of wear and tear particles.

An ideal implant should last a patient’s lifespan. Most implants last on an average for 15-20 years or more. The choice of implant used also depends upon the surgeon’s familiarity with the implant and expertise. Currently used implants may either be hard-on-soft bearing or hard-on-hard bearings.

The image shows a splined tapered modular distal femoral prosthetic component. The tapered end allows ease of insertion and the splined design provides rotational stability in the femoral canal. The modular implants are commonly used in revision hip replacement surgery. The special design allows to achieve greater stability in the event of encountering bone loss during extraction.

Hard-on-soft bearing

The ball is made of metal and the cup is made of specialized plastic called polyethylene. The polyethylene plastic is highly cross-linked to make it resistant to wear and tear. The metal may be made of a cobalt-chromium alloy or ceramic. The metal ball and a polyethylene liner is currently the most commonly used bearing surface.

Ceramic is a harder surface than the cobalt-chromium alloy. The ceramic surface is ultra-smooth and scratch-resistant. Due to these properties, ceramic has extremely low generation of wear and tear particles.

There are different types of acetabular liners and shells. Neutral liners are commonly used in primary surgeries. Lateralized liners and acetabular liners with elevated rims may be used in revision surgeries to prevent dislocation. Constrained liners and dual mobility heads are used in cases of total hip replacement dislocation.

Hard-hard bearing

These bearing surfaces can either be a metal ball on a metal cup or ceramic ball on a ceramic cup. The surface of the metal/ceramic implants may be made rough to allow the bone to grow on the implant. The metal alloy may be highly carbonized to decrease the generation of wear and tear particles.

The advantage of hard-on-hard bearing surfaces is reduced chances of dislocation. The hard-on-hard bearing surfaces are able to accommodate a large size of the ball (femoral head). A larger size of the ball reduces the chance of dislocation.

The hard bearing acetabular liner comes with an inserter ring to prevent micromotion. The large cavity of the metallic liner allows insertion of a large prosthetic head of femur therefore limiting the chances of dislocation.

A disadvantage of hard-on-hard metal alloy implants is the generation of wear and tear particles. The levels of the alloy (cobalt-chromium) may be increased in the blood. These implants are therefore avoided in patients with kidney failure. The implants are also avoided in patients of childbearing age group.

The metal alloy wear particles may cause aseptic vasculitis associated vascular lesion (ALVAL). The immune cells of the body may accumulate around the implant and cause erosion of the bone and inflammation of the tissues. These may also cause the formation of a solid mass known as a pseudotumor.

The ceramic hips are generally made of alumina ceramic. The alumina ceramic has a very high density and therefore has an extremely smooth surface. The smooth surface allows decreased friction. The implants are also hydrophilic (water retaining), that allows a small film of water on the surface leading to lubrication.

The ceramic implants are also scratch-resistant and are inert in the body. The ceramic implants are extremely sensitive to implant position. Malpositioned ceramic implants may lead to failure. Ceramic implants may also press against each other at the outer boundary to cause chip fracture. The implants may also produce a squeaking sound.

Conclusion

Although a number of different implant designs and hip systems may be used in joint replacement surgeries, the metal on polyethylene remains a very popular option. The type of implant and the material used may depend upon the patient’s anatomy and the orthopaedic surgeon’s experience.