Wear and corrosion of modular interfaces in total hip replacements.

Modular components allow for the customization of hip replacements to the individual patient. Modular head-neck components allow for mixed material systems to minimize polyethylene wear as well as provide the ability to vary neck length and head size independent of the stem. Modular interfaces, however, result in an increased susceptibility to interface corrosion and wear debris generation. One hundred eight uncemented femoral stems with modular heads retrieved for reasons other than loosening with modular heads were examined for interface corrosion. In addition, in an effort to quantify the amount of wear debris generated at modular interfaces due to cyclic loading, mechanical testing and electrozone particle analysis was used to study various surface, material, and design combinations. Detectable degrees of corrosion were observed in ten of 29 (34.5%) mixed alloy systems and seven of 79 (9%) single alloy components at an average of 25 months in situ. There was no correlation between presence or extent of corrosion or surface damage with time in situ, initial diagnosis, reason for removal, age, or weight. Stems with corrosion were less likely to have bone ingrowth histologically. The results of mechanical testing showed a significant number of wear particles were generated by all head-neck combinations. The wear debris was almost totally in the size range less than 5 microns. As many as 2.5 million particles were generated the first million cycles loading, with as many as eight million particles generated at ten million cycles. The results indicate that surface preparation and material affect particle generation. Head-neck tolerance mismatch appears to be significantly variable in the number of particles generated.

Corrosion and wear at the modular interface of uncemented femoral stems.

We examined 108 uncemented femoral stems with modular femoral heads which had been retrieved for reasons other than loosening. There were detectable amounts of wear and corrosion in 10 of 29 (34.5%) mixed-alloy components and 7 of 79 (9%) single-alloy components after a mean implantation time of 25 months. We found no correlation between the presence or extent of corrosion or surface damage and any of time in situ, initial diagnosis, reason for removal, age, or weight. Stems with wear and corrosion were less likely to show histological bony ingrowth. The interface between the head and stem of modular total hip components is a possible source of ion release and wear debris, but wear and corrosion were totally absent in most specimens. This suggests that this problem could be avoided, and that further research is required to develop manufacturing methods which would minimise such changes.

Fretting corrosion in orthopaedic alloys.

Fretting corrosion, a mechanical-chemical phenomenon, most often occurs at screwhead-plate countersink junctions of internal fixation devices. An apparatus was constructed which would simulate the conditions of fretting corrosion in vivo. Fretting corrosion was studied as a function of the number of cycles and the solution in which the fretting occurred. The solutions studied were 0.9% physiological saline and a saline plus 0.5% albumin solution. The implant materials tested were Co-Cr-Mo alloy, 316L stainless steel, and Ti-6A1-4V alloy. The results demonstrated that weight loss increased with the number of fretting cycles but reached a plateau where further weight loss was negligible. Co-Cr-Mo alloy showed less weight loss than 316L stainless steel at any number of cycles. Weight loss for Ti-6A1-4V alloy was similar to Co-Cr-Mo alloy although marked abrasion was noted. All of the materials showed a marked decrease in weight loss when tested in the saline plus albumin solution as compared to the saline only solution.

Retrieval and analysis of intramedullary rods.

UNLABELLED: We examined ten intramedullary rods of similar design after routine retrieval from patients. Of these ten rods, four were found to exhibit cracking around their proximal third. This behavior could not be attributed to the surgical techniques employed or to the length of time in vivo of these rods. Rather, the cracking was a function of both the metal alloy used and the method of manufacture, which occasionally allowed a weld zone to be located at the point of maximum stress with the result that cracking occurred. A change in alloy composition to a low-carbon form of 316 stainless steel probably would reduce the risk of cracking. CLINICAL RELEVANCE: In the treatment of orthopaedic disorders, it is important for the operating physician to appreciate the problems that may be encountered in using implants. One of the most important of these problems is the possibility of implant failure. The present report illustrates how a combination of both metallurgical and fabrication factors may cause such an event to occur.

Interface mechanics of porous titanium implants.

The interfacial shear properties of bone tissue growth into porous coated Ti-6-A1-4V femoral implants have been examined as a function of the pore size of the porous surface. Three particle size range powders (297 microns, 420-500 microns, 595-707 microns) were used to fabricate cylindrical implants which were inserted into the femoral medullary canal of dogs for 6 months. Push-out tests on the removed femurs are reported and reveal: (i) that those implants residing in cortical bone exhibited significantly higher shear properties than the equivalent implants in cancellous bone and (ii) that the interfacial shear strength and stiffness decreased with increasing pore diameter within the range 175-325 microns. The extent of bone ingrowth into the surface of the implants was measured using quantitative optical microscopic techniques. This indicated that the percentage of bone which had grown into the surface was inversely proportional to the square root of the pore size and that further the shear properties of the interface were proportional to the extent of bone ingrowth.

Solution treatment behavior of Co-Cr-Mo alloy.

Current practice in the manufacture of Co-Cr-Mo alloy total hip prostheses is the use of a solution treatment to increase the ductility of the as-cast alloy. This study is concerned with the reactions encountered during solution treatment at temperatures between 1165-1270 degrees C. These reactions, including incipient melting, a carbide transformation from M23C6 to M6C and sigma-phase formation, have been examined using both qualitative and quantitative metallographic techniques, and are shown to influence the production of a single phase microstructure. As a result, an optimum temperature for solution treatment of 1220 degrees C has been determined. It is further proposed that a reduction in the carbon content of this alloy would improve its solution treatment behavior.