When the hip joint doesn’t function, due to disease or fracture, it can be replaced by a prosthetic implant either as a hemi (half) replacement or total replacement.
Today, total hip replacement is a successful and widely used surgical procedure. The concept is still based on Sir Charnley’s design and composed of a femoral prosthesis with a small diameter head, a cup and a polymer-made socket between them, to favor a low friction and smooth articulatory motion, which is critical.
Normal hip joint morphology and hip implant. The low friction polymer (HDPE) is visible within the cup.
The stem is either cemented into the femur by the means of a polymethylmetacrylate (PMM) mixture (used as a grout between the bone and the implant) or fixed cementlessly, through bone ingrowth into the implant.
Even though they need one, patients often have to wait to get an implant because surgeons know it doesn’t last for a lifetime and revision surgeries are often more complicated than the initial orthopedic surgery. Most people can only go through 1-3 revision surgeries. Improving hip implant’s durability is crucial for patients. Mechanical fatigue failure is rare and the most common reason for revision surgery is “asceptic loosening”, which usually occurs 10 to 20 years after implantation and is caused by mechanical and biological factors.
An important biological factor is the biological response to wear debris, primarily generated from the prosthetic joint articular surface. The sustained chronic inflammatory response initiated at the implant-bone interface is manifested by recruitment of various cell types including most importantly osteoclasts, the principal bone resorbing cells. Signs and symptoms of the subtle progression of tissue destruction around the implant may not be clinically apparent until late stages of failure, which is insidious. Reducing as much as possible the friction coefficient of the material at the joint helps reduce biological factors.
Mechanical factors include implant-bone “micro-motions” and “stress shielding”. Micro-motions result from an inadequate initial fixation leading to a mechanical loss of fixation over time. Stress shielding is caused by the unnaturally high stiffness of the implant compared to the bone material. The stiff implant bears too large a share of the load and the surrounding living bone tissue adapts to the low stress level and resorbs.
Additive manufacturing offers a way to counteract aseptic loosening due to mechanical factors. In addition to the achievement of a better stability during the initial orthopedic surgery with patient-specific geometry, complex surface structures such as open cellular structures (for example acetabular cups that integrate trabecular structure) are designed for good bone ingrowth and improved flexibility to reduce or eliminate stress shielding.