Innovation & Evolution in Hip Replacement Surgery: Highlights from the Keggi–Rubin Hip Implant Collection at Yale University
Early Press-Fit Implants
While Charnley’s cemented implants had become the gold standard for hip replacement surgery, implant wear and gradual cement degradation were growing problems. Over time, the cement would break and induce implant loosening and cause significant pain, which typically required revision surgery. Throughout the late 1980s and early 1990s, implant companies raced to discover new long-term ways to fixate implants in the bone. The companies tried to improve implant longevity and durability through various surface enhancements because implants with smooth metal surfaces, like Charnley’s prosthesis, did not offer support for bone ingrowth or ongrowth. Companies began to produce press-fit implant designs with various coatings, textures, and geometric shapes that intended to achieve a better fit into the femur and stimulate bone growth onto the implant for fixation.
Bead Coat Stems
Porous bead coated femoral stems were an early press-fit design originally made of a Vitallium alloy. The metallic beads were applied to the implant through a robust sintering process. The beads created a porous surface that facilitated bone growth onto the implant, interlocking the implant and the new bone. Companies developed stems with different bead coat patterns, such as Johnson & Johnson’s bead coat “pads.” A concern with bead coated exteriors was the potential breakdown of the beaded surface, or “bead shedding,” which could lead to an accumulation of “third-body” metal debris both within and around the joint space.
(top left) Richards Collared Opti-Fix Femoral Stem
(center left) Richards “Tri-Wedge” Porous Coated Femoral Stem
(bottom left) Howmedica Porous Coated Anatomic (PCA) Cup and Stem
(right) Johnson & Johnson Press-Fit Condylar (PFC) Femoral
Stem with bead pads
Grit Blasted Stems
Grit blasting is a process whereby small, hardened material (grit) is forcefully blasted against a metallic surface, roughening a smooth area. The resulting textured metal provides the prosthesis with a better exterior surface for bone adhesion and ongrowth. Some grit blasted stems evolved to incorporate ridges in their designs that further improved the implant’s grip and friction in the femur. The Howmedica Omnifit implant was one of the first femoral stems to showcase grit blasted ridges, and, after some modifications, there is now a modern version of this stem (Secur-Fit) available with a porous titanium coating.
(left) DePuy Ultima P.F.C. Grit Blasted Collared Fracture Stem
(center) Early Grit Blasted Omnifit Femoral Stem
(right) Howmedica Omnifit HA Coated Femoral Stem
Zweymüller Stem
The original Zweymüller stem was introduced in the 1970s, and has since undergone a series of minor geometric modifications. Designed by Prof. Karl Zweymüller in Austria, the first-generation femoral stem featured a unique rectangular cross section with nine holes forged out of a titanium alloy. His second generation Alloclassic Zweymüller SL Stem, produced in 1986, shared that same rectangular cross section geometry but had an adjusted neck angle and only four holes. In 1993, the Zweymüller SL Plus Stem appeared with five holes, a grit-blasted surface finish, and a modified shoulder design. The rectangular shape of Zweymüller stems was designed to fit precisely into the cylindrical femoral canal; the four corners of the stem dug into and became rooted in the proximal femur, thereby establishing rotational stability. Overall, Zweymüller’s designs achieved exceptional immediate and long-term fixation in the femur due to their unique shape and grit-blasted surfaces, but also could cause thigh pain from the weight-bearing stress being applied along the femoral canal.
(left) Zweymüller Revision Stem
(center) Alloclassic Zweymüller SL Offset Stem
(right) Zweymüller SL Plus Stem
Cylindrical Extensively Coated Implants
These stems achieve implant fixation through their cylindrical shape and extensive plasma titanium coating. Their cylindrical shape was forged out of cobalt chrome, which is stiffer than titanium, and designed to reduce shear stress and implant rotation. The stem was then sprayed with a titanium plasma coating on the implant’s surface with a plasma gun, which created a porous titanium finish. The porous coat increased the friction between the implant and the bone, allowing the bone to grow onto the implant and achieve long-term stabilization of the femoral stem. An original implant example was the DePuy Anatomic Medullary Locking (aml) Stem, and a similar stem was the Smith & Nephew Echelon implant, which featured smooth distal “flutes” to decrease micromotion of the stem.
(left) Echelon Porous Straight Standard Stem
(right) Echelon Porous Bowed +15 mm Calcar Replacing Revision
Stem
Echelon Revision Hip System Surgical Technique booklet, Smith & Nephew 1998
Case 3 photograph of physical exhibition in Cushing Rotunda, Cushing/Whitney Medical Library- Early Press-Fit Implants