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.
Our results indicate that the jumbo cup technique results in hip center elevation despite placement of the cup adjacent to the inferior acetabulum. For a hypothetical increase from a 54-mm socket to a 72-mm socket, as one might see in the context of the revision of a failed THA, our model would predict an elevation of the hip center of approximately 5 mm and loss of approximately 15 mm of anterior column bone. This suggests that an increase in femoral head length may be needed to compensate for the hip center elevation caused by the use of a large jumbo cup in revision THA. A jumbo cup may also result in protrusion through the anterior wall.
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