A total hip replacement (THA) removes damaged bone and cartilage from the hip joint and replaces them with prosthetic components. This procedure is performed to relieve severe pain and restore function, often due to conditions like osteoarthritis. The success and longevity of the new joint depend significantly on the choice of materials used in its construction. No single material is considered “best” for every patient, as selection requires balancing the implant’s durability, resistance to wear, and specific patient health factors.
Anatomy of a Total Hip Replacement
A total hip replacement mimics the natural ball-and-socket structure of the hip joint using four components. The femoral stem, a metal alloy piece, is inserted down the hollow center of the femur to provide stable fixation. Attached to the top of this stem is the femoral head, which acts as the prosthetic ball.
The acetabular cup, a metal shell (typically titanium or tantalum), is secured into the patient’s pelvis and functions as the new socket. The fourth component is the liner, or bearing surface, which fits inside the metal cup and articulates with the femoral head. Friction and wear occur at this interface, making the material choice for the bearing surface crucial to the implant’s lifespan.
Core Bearing Surface Materials: Properties and Trade-offs
The selection of materials for the articulating surfaces is determined by their physical characteristics. Highly cross-linked polyethylene is a widely used plastic engineered for improved wear resistance. The cross-linking process creates a stronger molecular structure, reducing wear by up to \(90\%\) compared to conventional polyethylene. The main concern is the eventual production of microscopic wear debris, which can trigger an immune response and lead to bone loss around the implant (osteolysis).
Ceramic materials, typically made from alumina or zirconia, are extremely hard and smooth. This low-friction surface produces far less wear debris than plastic, benefiting long-term implant survival. Ceramic components are also hypoallergenic, avoiding material sensitivity issues associated with metal alloys. However, ceramic is brittle, carrying a risk of catastrophic fracture, and may produce an audible “squeaking” sound during movement.
Metal alloys, most often cobalt-chromium, have a long history of use in orthopedics. They were historically used in both the ball and socket, creating Metal-on-Metal (MoM) bearings. Friction in MoM systems generates metal wear debris, which can leach into the bloodstream. These metal ions have been linked to adverse tissue reactions and systemic issues, causing MoM bearings to be largely phased out. Today, cobalt-chromium is primarily used as the femoral head when paired with a polyethylene socket.
Evaluating Common Material Combinations and Wear Performance
The most common pairing is Metal-on-Polyethylene (MoP), which combines a cobalt-chromium femoral head with a highly cross-linked polyethylene liner. This combination has a proven track record, is cost-effective, and provides reliable function for many years. Polyethylene wear remains the primary mode of failure, which can lead to osteolysis over time.
Ceramic-on-Polyethylene (CoP) bearings substitute the metal head with a hard, smooth ceramic one. The ceramic head is resistant to scratching and maintains a smoother surface, significantly reducing abrasive wear on the polyethylene liner. CoP bearings with highly cross-linked liners have substantially lower wear rates than older MoP systems, improving the likelihood of long-term survival. This combination is a common choice, balancing the durability of ceramic and the established track record of polyethylene.
The Ceramic-on-Ceramic (CoC) pairing provides the highest resistance to wear, often below the threshold for causing osteolysis. The low friction and wear make CoC an appealing option for younger patients seeking the longest possible implant lifespan. However, the risks of ceramic fracture and the possibility of squeaking remain concerns. CoC bearings also require high surgical precision for placement, as misalignment can increase the risk of chipping or premature failure.
Determining the Best Material for the Individual Patient
Patient age and activity level are primary considerations, as they dictate the anticipated stress and lifespan required of the implant. Younger, highly active individuals place greater demands on the joint, making low-wear options like Ceramic-on-Ceramic or Ceramic-on-Polyethylene more appropriate to reduce the chance of revision surgery.
For older patients with lower activity levels, a Metal-on-Polyethylene bearing may provide sufficient longevity and a lower risk of complications, such as fracture. A patient’s medical history, particularly known sensitivities to metals like nickel or cobalt, can contraindicate the use of certain metal components. In these cases, ceramic or coated metal heads are often preferred to mitigate the risk of an allergic reaction.