During the mid-1990s when we were using a DePuy press-fit porous-coated cup without supplemental initial fixation combined with an extensively porous-coated stem for primary THA, the manufacturer transitioned from gamma irradiation to gas plasma for the terminal sterilization of their polyethylene liners. Although the elimination of gamma irradiation eliminated the generation of free radicals during the sterilization process, it also eliminated a source for polyethylene crosslinking. As a consequence, we transitioned from using mildly crosslinked polyethylene with residual free radicals to using noncrosslinked polyethylene without any radiation-induced free radicals. At minimum 10-year followup, we asked whether the fixation achieved by solely relying on a press-fit would be durable and how the different liner sterilization methods affected radiographic wear, osteolysis, and survivorship.
This retrospective cohort study has several limitations. First, the two surgeons whose cases we reviewed did not treat all their patients with Duraloc® 100 cups combined with extensively porous-coated stems. The series we are reporting on represents 90% (398 of 440) of the primary THAs performed over a 22-month period. Second, whether or not a revision has been performed by 10-year followup is unknown for 8% (32 of 398) of the THAs in this series. These 32 THAs are included in our survivorship analyses and censored at the time of their last known followup so that they add to the uncertainty by increasing the magnitude of the 95% CIs. Third, radiographs were available for only 63% (185 of 293) of the unrevised THAs among living patients eligible for 10-year followup. Although our radiographic followup is incomplete, patient sex and age at surgery among the sterilization groups are not different (Table ) so we believe comparisons among the groups are valid. We acknowledge the duration of followup was modestly longer for the gamma-barrier liners, but this group had the lowest incidence of osteolysis and wear-related complications. Fourth, the method used to sterilize the polyethylene liners was not explicitly randomized among the patients in our study population. However, we have no reason to suspect there was any surgeon preference for using liners with different sterilization methods since the physicians used the available inventory and did not select the sterilization method based on patient, implant, or surgical considerations.
Our results are consistent with previous reports describing the outcome of press-fit cups implanted without supplemental fixation [41
]. With the exception of early radiographic outcome data from a single institution [39
], the Duraloc®
100 cup has consistently demonstrated excellent fixation. Including the data from our study and three other clinical outcome series with 5- to 10-year followup, only one case of aseptic loosening has been reported among more than 700 Duraloc®
100 cups [2
]. In our study, the absence of cup loosening, despite a 31% (57 of 185) incidence of pelvis osteolysis, may be explained by a previous finding that the surface area of the cup occupied by osteolysis plateaued at 40% despite progressively increasing lesions sizes [8
]. As this prior research has demonstrated, when pelvic osteolytic lesions originate in DeLee and Charnley Zone 2 behind the dome hole, as they did among the majority of hips in our series, these lesions can expand into periacetabular trabecular bone without compromising the fixation achieved around the rim of the cup. Since other cementless cups that rely on bone ongrowth for fixation have demonstrated high rates of revision for osteolysis and loosening in conjunction with wear rates comparable to what we found in this study [10
], we also attribute the durability of fixation in our series to the ingrowth achieved with the rough, three-dimensional porous-coated surface of the Duraloc®
100 cup. On the femoral side, our results are consistent with other studies demonstrating excellent fixation of the AML®
stems at minimum 10-year followup [16
]. Fracture of the greater trochanter associated with osteolysis was our most common femoral complication. Including the three fractures among the 20 revised THAs and the six fractures associated with lysis among the 185 THAs with minimum 10-year radiographs, the incidence was 4.4% (nine of 205) in our series, which is similar to the 4.3% rate reported in a previous study [3
]. We presume these fractures may be due to the combination of osteolysis and the use of a straight-stemmed prosthesis that requires a larger entry hole through the greater trochanter.
Confirming studies with shorter followup [7
], we found the method of polyethylene sterilization can have a profound influence on wear. Noncrosslinked gas plasma liners had head penetration rates about twice those of liners moderately crosslinked by terminal sterilization with gamma irradiation. Among the irradiated liners, we also found gamma-barrier liners had lower head penetration rates than gamma-air liners [18
]. Consistent with other reports measuring wear and osteolysis, we found osteolytic lesion size was correlated with the amount of wear and the gamma-barrier liners with the lowest wear rates tended to have less osteolysis, but there can be considerable variation in the amount of osteolysis among individual patients with higher wear rates regardless of the polyethylene sterilization method [9
]. Although the gas plasma liners had higher wear rates than the gamma-air liners, the incidence and size of the osteolytic lesions for both groups were similar (Table ). With the trend toward reduced osteolysis associated with the gamma-barrier liners, we conclude the potential liability associated with oxidation of residual free radicals in vivo is outweighed by the decreased wear resulting from crosslinking associated with gamma irradiation, at least through 10-year followup.
Although we found a relatively high mean head penetration rate of 0.15 mm/year in this series, the combination of a press-fit Duraloc®
100 cup without supplemental fixation and an extensively porous-coated stem demonstrated durable implant fixation even in the presence of osteolysis [40
] and a low revision rate with 95.7% survivorship at 10-year followup using component revision for any reason as an end point. We conclude supplemental initial fixation is not required with a press-fit porous-coated cup [36
]. Despite a similar incidence of osteolysis in the pelvis and femur, pathologic fractures have only occurred on the femoral side. While we have not observed any osteolysis-related component loosening to date, continued followup will be required to assess the long-term effects of osteolysis on component fixation, particularly in view of the relatively high head penetration rates we observed. Longer followup will also be required to determine whether the differences in wear among the liner sterilization methods will eventually lead to differences in implant survivorship or the incidence of wear-related complications.