Abstract
Introduction
Hip simulators proved to be valuable, pre-clinical tests for assessing wear. Preferred implant positioning has been with cup mounted above head, i.e. ‘Anatomical’ (Figs. 1a-c) 1,2 while the ‘Inverted’ test (cup below head) was typically preferred in debris studies (Figs. 1d-f).3,4 In an Anatomical study, wear patterns on cups and heads averaged 442 and 1668 mm² area, respectively, representing 8% and 30% of available hemi-surface (Table 1), i.e. the head pattern was ×3.8 times larger than cup. This concept of wear patterns is illustrated well in the ‘pin-on-disk’ test (Fig. 1) in which the oscillating pin has the ‘contained’ wear area (CWP) and the large wear track on the disk is the ‘distributed’ pattern (DWP). Hip simulators also create CWP and DWP patterns, site dependant on whether Anatomical (Fig. 1a-c) or ‘Inverted’ (Fig. 1d-f) test. However there is scant foundation as to clinical merits of either test mode. Retrieval studies of MOM bearings have indicated that cups have the larger wear patterns, i.e. contrary to simulator tests running Anatomical cups (Table 1).5 Therefore we compared Anatomical and Inverted cup modes using 38mm and 40mm MOM in two 5-million cycle simulator studies.
Methods
38mm and 40mm MOM bearings were run in Anatomical mode (study-1) and Inverted (study-2) mode, respectively, in a hip simulator. Lubricant was bovine serum diluted to provide protein concentration 17 mg/ml. Wear was measured gravimetrically and wear-rates calculated by linear regression. Wear patterns were assessed by stereomicroscopy and compared to algorithms using standard spherical equations.
Results
MOM wear-rates ranged 0.3 to 6 mm³/Mc by 5-million cycles duration. Contained wear patterns (CWP) averaged 410 mm² for cups in study-1 (Anatomical) and 397 mm² for heads in study-2 (Fig. 3: Inverted). Distributed wear patterns (DWP) averaged 945mm² in study-1 (heads, Anatomical) and 846mm² in study-2 (cups, Inverted). Cup Hemi-ratios averaged 18% and 38% in studies 1 and 2 respectively (Table 2).
Discussion
While vendor, implant and experimental differences were clearly present, study-1 (Anatomical) and study-2 (Inverted) produced almost identical CWP and DWP wear patterns, only reversed on heads versus cups. This unequivocal evidence demonstrated that there was no difference in wear mechanics for spherical CoCr bearings run in either test mode. In addition, wear patterns observed in MOM cup retrievals5 (1000–2700 mm²) were much larger than produced in Anatomical simulator tests (Fig. 2h: 400–500 mm²). Such large discrepancies in cup wear patterns between Anatomical simulator tests and retrieved MOM cups indicated that the Inverted cup mode (Fig. 2g) may be more clinically relevant.
