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


OBJECTIVE
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.


CONCLUSIONS
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.
Imaging Research Laboratories, Robarts Research Institute, 100 Perth Drive, London, ON, N6A 5K8, Canada. mteeter@imaging.robarts.ca
Matthew G
MG
Teeter
Douglas D R
DD
Naudie
Robert B
RB
Bourne
David W
DW
Holdsworth
D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects / Q000295:instrumentation*; D001696:Biomechanical Phenomena; D017076:Computer-Aided Design; D019544:Equipment Failure Analysis; D007720:Knee Prosthesis; D020959:Polyethylene; D011474:Prosthesis Design; D011475:Prosthesis Failure; D015203:Reproducibility of Results; D013314:Stress, Mechanical; D013499:Surface Properties; D055114:X-Ray Microtomography
D020959:Polyethylene