A hip replacement is a surgical procedure that removes a diseased or damaged hip joint and replaces it with an artificial implant called a prosthesis. This prosthetic device is engineered to mimic the natural ball-and-socket function of the human hip, restoring mobility and reducing chronic pain. The modern hip replacement typically consists of several parts made from metal, plastic, and ceramic, with titanium alloys often forming the structural foundation of the replacement joint.
The Material: Why Titanium is Used
The appearance of a titanium hip implant is a dull, non-reflective gray metal, distinct from the polished stainless steel used in older implants. This color is characteristic of the titanium alloy, which is widely favored for orthopedic applications due to its exceptional properties. The metal offers a high strength-to-weight ratio, meaning it is light while being strong enough to endure the constant stresses of movement.
Titanium also exhibits excellent biocompatibility, allowing it to exist within the human body without triggering an adverse immune reaction. This material is highly resistant to corrosion from body fluids, ensuring its integrity over decades. The elastic modulus of titanium is closer to that of natural bone compared to other metals, which helps minimize the phenomenon known as “stress shielding.” This property ensures that the implant transfers stress more naturally to the surrounding bone.
The Design: Components of the Artificial Hip
A total hip replacement is a modular system composed of four distinct components that replace the damaged bone structures:
- The femoral stem is a long, curved piece inserted down the hollow center of the femur (thigh bone), providing the main structural anchor for the leg. Its shape is often tapered or anatomically contoured to fit snugly within the prepared bone cavity.
- The femoral head is a spherical component that replaces the natural ball of the joint. While the stem is typically titanium, the head is often made of highly polished ceramic or cobalt-chrome metal for a smooth articulation surface.
- The acetabular cup is a hemispherical metal shell designed to replace the natural hip socket in the pelvis.
- The liner is the actual bearing surface, inserted securely into the acetabular cup. It is commonly constructed from durable polyethylene plastic or ceramic, acting like new cartilage to minimize friction as the femoral head rotates within it.
Integration: How the Prosthesis is Fixed
The method used to secure the titanium implant to the patient’s bone significantly influences the final appearance of the implant. One primary method is cemented fixation, where a fast-curing acrylic bone cement is used as a grout to create a secure mechanical interlock between the bone and the prosthesis. On an X-ray, this cement appears as a distinct white line surrounding the stem and cup components.
The alternative, and increasingly common, method is cementless or biologic fixation, which relies on promoting new bone growth directly onto the implant surface. To achieve this, the titanium components, particularly the stem and cup, are manufactured with a porous surface texture. This surface often resembles a fine mesh or a rough, sandpaper-like coating that encourages existing bone cells to grow into the microscopic pores.
This process, called osteointegration, provides a stable, lasting biological bond between the bone and the implant. In some designs, this porous titanium surface is further enhanced with a coating of hydroxyapatite, a substance chemically similar to natural bone mineral, to actively stimulate bone ingrowth. The result is an implant that is integrated into the skeletal structure, designed to provide long-term stability without the use of bone cement.