RESUMEN
The straightness error of a coordinate measuring machine (CMM) is determined by measuring a rule standard. Thanks to a reversal technique, the straightness uncertainty of the CMM is theoretically dissociated from the straightness uncertainty of the rule. However, stochastic variations of the whole measurement system involve uncertainties of the CMM straightness error. To quantify these uncertainties, different sources of stochastic variations are listed with their associated probability density functions. Then Monte Carlo methods are performed first to quantify error and secondly to optimize measurement protocol. It is shown that a 5-measurement distance from 0.1 mm to each measurement coordinate allows a double reduction of uncertainties, principally due to the rule roughness amplitude (R(a) = 0.35 µm) and because this optimal distance of 0.1 mm is equal to the autocorrelation length of the rule roughness. With this optimal configuration, the final uncertainly on the straightness error of the CMM studied is less than 1 µm on the whole evaluated length of the rule (1 m). An algorithm, including Probe Tip Radius of the CMM and surface roughness of the piece, is finally proposed to increase CMM reliability by minimizing error measurements due to surface roughness of the measured piece.
RESUMEN
PURPOSE OF THE STUDY: The objective of this work was to identify the mechanisms leading to deterioration of the polyethylene acetabular surface in total hip prostheses. MATERIAL AND METHODS: We conducted an in vivo analysis of damage caused to articulating surfaces of metal-on-polyethylene total hip prostheses. This analysis was performed on three of the 45 explants of Harris-Galante prostheses available to use from our series of 1480 implantations performed between 1985 and 1996. These three pieces were selected because large-sized fibers of metallic debris were identified on the polyethylene surface and because the explants had been performed for reasons other than loosening in aseptic patients. Macroscopic examination was combined with scan electron microscopy to analyze morphological changes. RESULTS: We found that identical damage could be observed on the polyethylene explants despite the different sizes of the metallic heads and different metal-on-polyethylene combinations. Our analysis confirmed the deleterious effect of metallic foreign bodies trapped in contact with the articulate surfaces. These foreign bodies resulted in one case from rupture of a multifilament wire used for the trochanteric fixation and in two cases from the surfacing of the non-cemented Harris-Galante cups. The microscopic analysis demonstrated several successive localizations of the foreign bodies on the polyethylene surface before becoming deeply encrusted in the polyethylene where they deteriorated the femoral head surface (rough) and wore the polyethylene surface (abrasion). When the fibers came from the surfacing of the Harris-Galante cups, the metallic debris remained unrecognized during the revision procedure and were only identified later during the systematic examination of prosthetic explants. DISCUSSION AND CONCLUSION: These findings emphasize the importance of quality surface treatments for non-cemented prostheses and argue in favor of particular precautions during operative manipulation of these components. A systematic analysis of the prosthetic explants is advisable, particularly for components explanted because of wear or unexplained osteolysis. Use of multifilament trochanteric wires should be avoided because rupture raises the risk of intra-articulate migration of metallic debris. If the prosthesis involves a polyethylene acetabular surface, ceramic heads should be preferred to improve resistance to abrasion, particularly to limit the deleterious effect of a third body.