Abstract
Introduction
Dislocation is one of the leading causes of revision after primary total hip arthroplasty (THA). Polyethylene wear is one of the risk factors for late dislocations (>2 years). It can induce an inflammatory response resulting in distension and thinning of the pseudocapsule, predisposing the hip to dislocation. Alternatively, eccentric seating of the femoral head in a worn out socket may result in an asymmetric excursion arc predisposing the hip to impingement, levering out and dislocation. Highly cross linked polyethylene has a significantly lower wear rate as compared to conventional polyethylene. Incidence of late dislocations has been shown to be significantly greater with conventional polyethylene bearings as compared to ceramic bearings. However, there is no literature comparing the risk of dislocation between ceramic- on- ceramic (CoC) bearings with metal/ceramic- on- cross linked polyethylene (M/CoP) bearings and this was the aim our study
Methods
Data regarding revision for dislocation after primary THA for osteoarthritis (OA) between September 1999 and December 2013 was obtained from the Australian Orthopaedic Association National Joint Replacement Registry (AOA NJRR). Revision risk for dislocation was compared between CoC, CoP, and MoP bearings. Only those THAs with 28 mm, 32 mm, or 36 mm heads were included in the study.
Results
The numbers at risk (0 years) in the CoC, CoP, and MoP groups were 53,648; 23,746; and 90,040 THAs respectively. The overall revision (dislocation)/100 observed years was 0.13. Revision rate/100 observed years in the CoC, CoP, and MoP groups was 0.12, 0.12, and 0.16 respectively. The cumulative percentage revision for dislocation (CPRD) for the three bearing types is shown in figure 1. The CPRD at 12 years in the CoC, CoP, and MoP groups was 0.9, 1.0 and 1.2 respectively. The rise in CPRD between 2 and 12 years (late dislocations) was 0.4, 0.6, and 0.6 in the CoC, CoP, and MoP groups respectively (Figure 1). The age and gender adjusted hazards ratio (HR) for revision for dislocation in CoC vs. CoP groups was 1.05 (p=0.684, 1month+); in MoP vs. CoP group was 1.45 (p<0.001, entire period) and MoP vs. CoC group was 1.55 (p<0.001, 6 month+). Computation of age and gender adjusted HR comparing the three bearing groups after stratification according to head sizes (28 mm, 32 mm and 36 mm) showed no significant difference in HR after 3 months (p≥0.061).
Conclusion
The revision (dislocation)/100 observed years was greater in the MoP group as compared to CoC and CoP groups (MoP > CoC = CoP). The age and gender adjusted HR (revision for dislocation) was significantly greater in the MoP group as compared to CoC and CoP groups after 6 months. However, when the data was stratified according to head sizes, the age and gender adjusted HR was similar between the three bearing groups after 3 months. In conclusion, there is no significant difference in the risk of revision for dislocation between CoC, CoP, and MoP bearings after adjusting for age, gender, and head size after 3 months.
