Before the modern age of hip replacement, surgeons used a variety of materials to improve the lifestyles of patients with diseased hips. These attempts did little to help patients, who were forced to endure constant pain and sedentary lives. It wasn’t until the 1960s that medical advancements led to the development of the first hip implant. Since that time, orthopaedic surgeons, researchers, scientists, engineers and manufacturers have made extraordinary advances in hip implants in a relatively short time as they continually work to improve effectiveness, durability and patient outcomes.
Some of the first bearing surface options were metal-on-metal. Materials included stainless steel and cobalt chrome molybdenum alloys. The patient population at this point was relatively older and less active.
Success was mixed. Manufacturing processes were not precise enough to minimize wear as components rubbed together. Designs and cementing techniques were rudimentary, and components sometimes loosened and/or broke.
After using various materials over time, orthopaedic surgeon Sir John Charnley introduced a high grade plastic, called polyethylene, for the socket side of the hip. This was paired with cobalt chrome molybdenum for the ball side of the joint.
This design was easier to manufacture and had a stem design that resulted in fewer loose and broken stems. Polyethylene became widely used for the socket side of the hip and was successful in many patients, although some doctors and scientists continued working with
metal-on-metal and ceramic-on-ceramic designs.
While metal-on-polyethylene hips were successful for many patients, some complications continued or were emerging with the metal-on-polyethylene CHARNLEY Hip. The smaller femoral head size sometimes caused painful dislocations. In some cases, polyethylene components fractured and wore out and small particles of polyethylene wore away from the bearing's surface, causing tissue reactions and bone loss that could lead to complex revision surgeries. In particular, polyethylene components were less successful in younger, heavier, and more active patients, who were increasingly seeking hip replacement surgery.
Polyethylene was still widely used. But the complications with metal-on-polyethylene and concerns about performance in younger, heavier, and more active patients continued, leading the industry and surgeons to seek different options.
Concerns over polyethylene's lack of durability led physicians and engineers to reconsider metal-on-metal implant technology. Research showed excellent long-term results in some of the first generation metal-on-metal bearing options, with virtually no wear and a low frequency of bone loss.1, 2
Metal-on-metal bearings also allowed for larger femoral heads to be used, providing greater stability for younger, heavier and more active patients. In addition, manufacturing techniques and materials had advanced, allowing designs that reduce component wear.
- Schmalzried, Thomas P., et al. “Long-duration Metal-on-metal Total Hip Arthroplasties with Low Wear of the Articulating Surfaces.” The Journal of Arthroplasty 11 (3), 1996: 322-31.
- Zahiri, Christopher A., et al. “Lessons Learned from Loosening of the McKee-Farrar Metal-on-Metal Total Hip Replacement.” The Journal of Arthroplasty 14 (3), 1999: 326-32.
With low wear and good durability, ceramics were another alternative being considered. Potential risks with this material included fracturing and chipping, and a squeaking noise when the patient moved.
Enhanced polyethylenes, known as cross-linked polyethylenes, were also being developed. The manufacturing technique of ‘cross-linking’ polyethylene was done to make the material more wear resistant. This technology was promising, but did not yet have an established clinical track record.
SEVERAL TOTAL HIP OPTIONS AVAILABLE
Manufacturers offered bearing options made of metal, ceramic, traditional polyethylene and cross-linked polyethylene, each with its own unique benefits and risks. Surgeons chose the bearing option based on a variety of factors, including the age, weight, and activity level of the patient.
CROSS-LINKED POLYETHYLENE & CERAMICS
Advancements in bearing technology have continued and physician preferences have shifted. Cross-linked polyethylene is now the most used option, and continues to perform well in patients. Ceramic-on-polyethylene and ceramic-on-ceramic are other options used frequently today.
According to the American Academy of Orthopaedic Surgeons, hip replacement surgery is one of the most successful operations in all of medicine.3 The orthopaedic community continues to research and innovate, always seeking to give patients and doctors better options to relieve pain and increase mobility.