How do CAD models compare with reverse engineered manufactured components for use in wear analysis?

Abstract

Background: To accurately quantify polyethylene wear in retrieved arthroplasty components, the original geometry of the component must be estimated accurately using a reference geometry such as a computer-aided design (CAD) model or a never-implanted insert. However, differences may exist between the CAD model and manufactured inserts resulting from manufacturing tolerances.

Questions/purposes: We quantified the deviations between CAD models and newly manufactured inserts and determined how these deviations compared with using a never-implanted insert as a reference geometry.

Methods: We obtained five cruciate-retaining (CR) and five posterior-stabilizing (PS) tibial inserts and their CAD models. The inserts were scanned and reconstructed using microcomputed tomography (micro-CT). Differences in volume and surface geometry were measured among (1) the individual inserts; (2) between the inserts and a CAD model; and (3) between the inserts and a reference geometry constructed from multiple scanned inserts averaged together.

Results: The micro-CT volumes were, on average, 0.4% smaller (34-178 mm(3)) than the CAD model volumes. The mean deviation between the CAD model and insert surface geometry was 25.7 μm smaller for CR and 36.8 μm smaller for PS. The mean deviation between the inserts and an averaged reference geometry was 1.4 μm larger for CR and 0.4 μm smaller for PS.

Conclusions: Deviations exist between manufactured tibial inserts and CAD models that could cause errors in wear measurements. Scanned inserts may better represent the preimplantation geometry of worn inserts than CAD models, depending on the manufacturing variability between lots.

Clinical relevance: The magnitude of the error in estimation of the preimplantation geometry of a retrieved component could add or subtract the equivalent of 1 year of wear.

Fig. 1A–D

Three-dimensional geometry of the CR inserts from the (A) CAD model and (B) micro-CT scan and of the PS inserts from the (C) CAD model and (D) micro-CT scan. CR = cruciate-retaining; CAD = computer-aided design; PS = posterior-stabilizing.

Fig. 2A–H

Maps of the three-dimensional surface deviations (in microns) for the CR inserts. Means are on the left and SDs are on the right for both the articular and backside surfaces. (A) Mean deviations for the CAD model versus individual inserts; (B) SDs for the CAD model versus individual inserts; (C) mean deviations for the scaled CAD model versus individual inserts; (D) SDs for the scaled CAD model versus individual inserts; (E) mean deviations between individual inserts; (F) SDs between individual inserts; (G) mean deviations between the averaged insert versus individual inserts; and (H) SDs between the averaged insert versus individual inserts. CR = cruciate-retaining; CAD = computer-aided design.

Fig. 3A–H

Maps of the three-dimensional surface deviations (in microns) for the PS inserts. Means are on the left and SDs are on the right for both the articular and backside surfaces. (A) Mean deviations for the CAD model versus individual inserts; (B) SDs for the CAD model versus individual inserts; (C) mean deviations for the scaled CAD model versus individual inserts; (D) SDs for the scaled CAD model versus individual inserts; (E) mean deviations between individual inserts; (F) SDs between individual inserts; (G) mean deviations between the averaged insert versus individual inserts; and (H) SDs between the averaged insert versus individual inserts. PS = posterior-stabilizing; CAD = computer-aided design.