Innovation & Evolution in Hip Replacement Surgery: Highlights from the Keggi–Rubin Hip Implant Collection at Yale University
Bearing Surfaces and Tribology
The evolution of bearing surfaces has always determined the performance of replaced joints. The progressive study of bearing surfaces, or tribology, has resulted in the continued advancement of the total hip replacement. In the beginning, surgeons used metal-on-polyethylene (MOP) and metal-on-metal (MOM) implant surfaces, but both of these had a high risk of catastrophic failure in the Charnley era. To address these concerns, surgeons began replacing hips with ceramic-on-ceramic (COC) bearings that demonstrated excellent wear resistance but were brittle and were prone to fracture, especially in younger and active patients. Because of those early COC fractures and squeaking, surgeons returned to using a new generation of MOM implants, which were sold as “unbreakable” surfaces designed for even the most active and athletic patients. Unfortunately, some of these MOM implants had unfavorable outcomes due to high wear rates, metal debris accumulation, adverse tissue reactions, and cobalt poisoning. While the MOM implants were experiencing high failure rates, the German company CeramTec continued to improve upon its COC bearing products through a number of implant generations. Thanks to CeramTec’s ongoing work, current 4th generation Biolox ceramic technology has improved to show superior wear rates, lower infection rates, and negligible fracture rates.
A selection of 3rd generation COC bearings are displayed. There are different acetabular shell designs shown, including early threaded titanium models. An unusual “sandwich” ceramic-in-polyethylene is included in this group, where the plastic acted as a peripheral shock-absorbing cushion around the more brittle ceramic liner. Examples of a COC bearing with a rim fracture and complete liner fracture are also shown here.
Allo Pro booklet on Operative Technique, Zweymüller cementless threaded cup, 1985
Below are a selection of MOM bearings. From left to right, the smallest is an early “sandwich” design where the metal liner was embedded within a polyethylene plastic cushion (not displayed). In the center is the Biomet Magnum MOM Implant, where the acetabulum was a single monoblock component with the shell and liner as one piece. The final example is the DePuy Ultamet modular MOM bearing whereby the cobalt chrome metal liner was fit into an outer titanium acetabular shell.
Below, examples of hip resurfacing arthroplasty are demonstrated. On the left is the early “Indiana Hip Resurfacing” Prosthesis, a MOP construct prone to failure both from plastic wear and loosening. On the right is the Smith & Nephew “Birmingham Hip Resurfacing” prosthesis made of cast cobalt-chrome, which is still in limited use today.
A selection of early acetabular shells are shown below. The shells have various designs that have also evolved over time. Top Row, from left to right: cemented Howmedica metal-backed acetabular cup with a modular plastic liner, Diamond plate grit-blasted shell design, bead-coated shell design. Bottom Row, from left to right: Early porous-coated acetabular shell design, Smith & Nephew Reflection I acetabulum and modular plastic liner, eccentric “double bubble” revision acetabular shell.
Evolution of Ceramtec Products
Since the 1970s, the fundamental science of ceramics has allowed a number of improvements to the chemical structure of the material itself, which in turn has help to improve the strength, longevity, and durability of the material. The material is manufactured by CeramTec for nearly all orthopaedic implant manufacturers worldwide, and has gone through four generations of development, including Alumina (1st Generation, 1974), Zirconia (2nd Generation, 1985), Biolox forte (3rd Generation, tan color, 1995), and Biolox delta (4th Generation, pink color, 2003–Present). The earlier generation materials were extremely smooth but quite brittle and were prone to catastrophic failure from fracture in vivo. The current generation of Biolox delta ceramics are composed of zirconia toughened alumina (ZTA) composites. These use new elements that convey resistance to crack propagation, which has greatly enhanced the strength and durability of the material under peak stress conditions. The hardness, smoothness, and success of this material have now renewed surgeons’ interest in COC bearings for highly active patients once again.
Smith & Nephew Oxinium Dual mobility implant
Ceramic on Ceramic Bearing Couple Featuring the Biolox “Contoura” Femoral Head
74mm Revision Shell from DePuy, with a 44 mm Inner Diameter Polyethylene and a 44mm Biolox Option Femoral Head
Biolox Option 40mm Head with 4 Titanium Sleeve Neck Length Options
Case 6 photograph of physical exhibition in Cushing Rotunda, Cushing/Whitney Medical Library- Bearing Surfaces and Tribology