To our knowledge, this is the largest series of DMCs derived from a national arthroplasty register. Of all DMC patients, 44% had other diagnosis than OA, compared with 26% of patients in the Exeter group, which might indicate that Lithuanian surgeons have tended to use DMCs for more complex cases, which may have negatively affected outcomes for this group. However, at five years, the CRR for all reasons of revision was 4% for the 620 primary hips using DMCs compared with 5.4% for the Exeter group.
DMCs are reported to have a high rate of loosening [10, 13] and a higher rate of osteolysis and cup loosening in younger patients and cases of childhood disease sequelae . Worries about these issues have limited the use of DMCs. Our short-term comparison of DMCs to that of a well-documented THA system (Exeter) shows that the DM THA performed at least as well in the short term. We also found that surgery for diagnoses other than OA had an increased risk of revision, suggesting that dual articular cups could be considered for non-OA patients, e.g. for THA due to femoral neck fractures.
DMCs have mainly been used worldwide in patients considered prone to dislocation or in revisions for dislocations [8, 9]. Thus, it is likely that our nationwide database of patients receiving DMCs included patients with a higher-than-average risk of postoperative dislocation. Still, among the 620 primary DM hips, four revisions only were due to dislocation (of which three were intraprosthetic), compared with 52 revisions for dislocation in the Exeter group of 2170 hips.
Also to be considered is that after a primary arthroplasty, a revision for dislocation in conventional primary THA is usually not done before a patient has dislocated two to three times; thus, the RR for dislocation is lower than the true dislocation rate. However, dislocations of DMCs are often intraprosthetic, requiring surgery after the first event. Thus, the RR for dislocation is probably not much lower than the true dislocation rate.
A study from a single hospital in Lithuania found low RRs for dislocation with the DM cup in high-risk femoral neck fracture patients . Similarly, Combes at al. investigated 2480 primary THA with DMCs and reported 0.88% dislocation rate, which is far below the rates reported with conventional THA . Similar results were observed in our current register study, where revision rates due to dislocation in DMCs were 0.7% compared with 2.4% in the Exeter group.
A limitation of our study was selection bias due to the tendency to select DM systems for high-risk patients, especially those with femoral neck fractures (Table 1). This should theoretically have a negative effect on outcomes in DMCs; however, our findings were contradictory, finding that DMCs had a lower RR compared with the Exeter THA. Another limitation of our study is the relatively short follow-up; however, even these results are promising.
We conclude that DM implant had a low short-term complication rate, comparable with that of a conventional, well-defined THA. The especially low dislocation rate makes it a reasonable choice for high-risk patients, while longer-term results are needed before it can be recommended for general use.