An artificial hip joint serves as a replacement for a natural hip that has become damaged or diseased. The natural hip is a ball-and-socket joint, connecting the thigh bone (femur) to the pelvis and allowing a wide range of motion. Conditions such as osteoarthritis, rheumatoid arthritis, or severe injuries can cause deterioration of the hip’s cartilage and bone. When these issues cause significant pain and reduced mobility, an artificial hip joint can restore function.
Components and Materials of an Artificial Hip
An artificial hip joint is composed of several distinct parts, each designed to replicate the function of the natural hip. The femoral stem is the component inserted into the hollow center of the thigh bone. This stem is often made from strong metal alloys like titanium or cobalt-chromium, chosen for their durability and biocompatibility, meaning they are well-tolerated by the body. Some uncemented stems feature a special coating, such as hydroxyapatite, which encourages bone growth directly onto the implant for secure fixation.
Connected to the femoral stem is the femoral head, also known as the “ball” of the joint. It can be made from polished metal, typically cobalt-chromium, or from advanced ceramic materials like Biolox delta ceramic, which offer a very smooth surface. The smooth surface of the femoral head helps to reduce friction as the joint moves.
The acetabular cup is the “socket” component, which is positioned within the pelvis, replacing the natural hip socket. This cup is frequently made from titanium or other metal alloys. Inside the acetabular cup sits a liner, which acts as the new cartilage surface. Liners are commonly made from highly cross-linked polyethylene, a specialized plastic engineered to minimize wear and provide a low-friction surface for the femoral head to articulate against. Ceramic liners are also used, particularly with ceramic femoral heads, to create a ceramic-on-ceramic bearing surface known for its durability and low wear rate.
Types of Hip Prostheses
Hip replacement procedures involve different approaches, depending on the extent of joint damage and patient characteristics. A total hip replacement involves replacing both the femoral head (ball) and the acetabular cup (socket) components. In contrast, a partial hip replacement, or hemiarthroplasty, replaces only the femoral head, leaving the natural socket intact. This latter option is considered for specific types of hip fractures or conditions affecting only the ball part of the joint.
Beyond the extent of replacement, artificial hip components are secured in place using one of two primary methods. Cemented fixation involves using bone cement to secure the components directly to the prepared bone. This method allows for immediate weight-bearing after surgery and is chosen for older patients who may have weaker bone quality, providing immediate stability.
Uncemented fixation, often referred to as “press-fit,” does not use bone cement. Instead, these implants have porous surfaces that allow the patient’s own bone to grow into and around the implant over time, creating a biological bond. This approach is favored for younger, more active individuals with good bone density, as it relies on the natural healing process for long-term stability. Some implants may even combine both methods, with one component cemented and another uncemented, tailored to the patient’s specific bone condition and activity level.
The Implantation Procedure
The surgical procedure to implant an artificial hip joint is performed by an orthopedic surgeon. Initially, an incision is made on the side, back, or front of the hip, to access the joint. The specific location of the incision depends on the surgical approach chosen by the surgeon, suchs as a posterior or anterior approach. The goal is to gain clear access to the damaged hip joint while minimizing disruption to surrounding tissues.
Once the joint is exposed, the damaged femoral head is removed from the top of the thigh bone. The surgeon then prepares the remaining bone surfaces, including the inside of the thigh bone and the acetabulum (hip socket) in the pelvis. This preparation involves shaping the bone to ensure a precise fit for the artificial components. Specialized instruments are used to ream and broach the bone, creating the exact contours needed for stable implant seating.
After the bone preparation is complete, the artificial components are inserted. The femoral stem is placed into the thigh bone, followed by the attachment of the new femoral head. The acetabular cup is then positioned into the prepared pelvis, and the liner is inserted into the cup. The surgeon ensures that the new ball and socket articulate smoothly, verifying the joint’s stability and range of motion before closing the incision.
Longevity and Revision of an Artificial Hip
The lifespan of an artificial hip joint has improved with advancements in materials and surgical techniques. Modern hip implants are expected to last for 15 to 20 years, with some lasting up to 25 years. Several factors influence how long an individual implant will last. The patient’s age, body weight, and activity level contribute to wear over time.
The materials used in the implant also affect its durability. For instance, ceramic-on-ceramic bearings exhibit lower wear rates compared to metal-on-polyethylene designs. Over time, the components of an artificial hip can experience wear and tear, or the implant may loosen from the bone. These issues can lead to pain, reduced function, and instability, requiring attention.
When an artificial hip joint reaches the end of its functional life or encounters issues like loosening or excessive wear, a procedure known as revision surgery is performed. Revision surgery involves replacing worn or failed components of the implant. This procedure is more complex than the initial hip replacement because it involves removing components and addressing bone loss.