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 m aterial systems to minimize polyethylene wear as well as provide the a bility to vary neck length and head size independent of the stem. Modu lar interfaces, however, result in an increased susceptibility to inte rface corrosion and wear debris generation. One hundred eight uncement ed femoral stems with modular heads retrieved for reasons other than l oosening 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 e lectrozone particle analysis was used to study various surface, materi al, and design combinations. Detectable degrees of corrosion were obse rved in ten of 29 (34.5%) mixed alloy systems and seven of 79 (9%) sin gle 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 weig ht. Stems with corrosion were less likely to have bone ingrowth histol ogically. The results of mechanical testing showed a significant numbe r of wear particles were generated by all head-neck combinations. The wear debris was almost totally in the size range less than 5 mu m. As many as 2.5 million particles were generated the first million cycles loading, with as many as eight million particles generated at ten mill ion cycles. The results indicate that surface preparation and material affect particle generation. Head-neck tolerance mismatch appears to b e significantly variable in the number of particles generated.