Nick Dong

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.

Do Jumbo Cups Cause Hip Center Elevation in Revision THA? A Computer Simulation

BackgroundAcetabular revision THA with use of a large (jumbo) cup is an effective treatment for many cavitary and segmental peripheral bone defects. However, the jumbo cup may result in elevation of the hip center and protrusion through the anterior acetabular wall as a result of the oversized geometry of the jumbo cup compared with the physiologic acetabulum.Questions/purposesThe purpose of this computer simulation was to determine how much elevation of the hip center and anterior wall protrusion occurs in revision THA with use of a jumbo cup technique in which the inferior edge of the jumbo cup is placed at the inferior acetabular rim and the superior edge of the jumbo cup is placed against host bone at the superior margin of a posterosuperior bone defect.MethodsTwo hundred sixty-five pelvic CT scans were analyzed by custom CT analytical software. The computer simulated oversized reaming. The vertical and anterior reamer center shifts were measured, and anterior column bone removal was determined.ResultsThe computer simulation demonstrated that the hip center shifted 0.27 mm superiorly and 0.02 mm anteriorly, and anterior column bone removal increased 0.86 mm for every 1-mm increase in reamer diameter.ConclusionsOur 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.