The technique of plasma spraying has been applied to deposit a thin, dense layer of hydroxylapatite onto a titanium substrate. Bond strength of such apatite coatings with the substrate have been measured, as well as the (absence of) influence of the coating process on fatigue properties of the substrate. Animal studies showed similar histological reactions to apatite coatings as to (well documented) apatite bulk materials.

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 size range less than 5 gm. As many as 2.5 million particles were generated the first million cycles loading, with as many as eight million particles generated a t 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.Modular stems are widely used in total hip replacement. Modular head-neck components provide the ability to vary neck length and head size independent of the stem while also reducing inventory. In addition, modular heads allow for mixed alloy systems such --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 averageof25 months insit,,. l h e r e was no correlation between presence or extent of corroas the combination of a titanium alloy stem with a cobalt-chrome head. Mears" predicted that cobalt-chrome and titanium alloy could be combined in a modular configusion or surface damage with time in siru, 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 ration without significantly enhancing the potential for corrosion. Likewise, studies by Lucas rt a/.14 and Kummer and Rose" predicted no exaggerated in vivo corrosion with such a mixed alloy system. Collier rl u / . ,~,~ however, have reported galvanic corrosion at this interface in human retrieved specimens and concluded that the process is time dependent and may be due to a form ofgalvanically accelerated crevice Corrosion.

There have been increasing concerns about the structural fatigue resistance of crosslinked UHMWPE devices due to deterioration of certain mechanical properties. However, due to the lack of clear correlation between specific mechanical properties and clinical performance, these concerns remain theoretical. In order to evaluate the potential benefits and risks of various crosslinked polyethylene materials for hip and knee bearing applications, two clinically relevant worst-case scenarios were simulated on functional devices. In the first worst-case scenario, cemented all-poly patellar components were tested under simulated stair-climbing conditions with rotational misalignment. In the second worst-case scenario, metal-backed thin acetabular liners were tested in a hip joint simulator under rim-loading conditions. Various types of crosslinked UHMWPE were prepared according to published process descriptions of commercial materials. While significant levels of volumetric wear reduction were confirmed by both the patellar and hip simulator tests, mixed results were obtained on the structural integrity of the devices. The latter was more significantly affected by the post-irradiation thermal treatment history than by the total dose of irradiation. Re-melting following irradiation led to catastrophic fractures of both rim-loaded liners and rotational-malaligned patellar pegs. The key mechanical property that was positively identified to correlate with the structural fatigue performance of crosslinked polyethylene materials was the ultimate tensile strength, whereas tensile elongation within 250% and 400% range had no effect on structural integrity. However, the results presented in this study should not be misconstrued with respect to the potential clinical performance of irradiation crosslinked and re-melted polyethylene liners with proper designs. In fact, the true outcome of all the crosslinked materials can only be revealed by long-term clinical follow-up.

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