Automatic determination of an anatomical coordinate system for a three-dimensional model of the human patella.

Measuring the in vivo 3-D kinematics of the patella requires a repeatable anatomical coordinate system (ACS). The purpose of this study was to develop an algorithm to determine an ACS using the patella's unique morphology. An ACS was automatically constructed that aligned the proximal/distal (PD) axis with the posterior vertical ridge. Inter-subject ACS repeatability was determined by registering all patellae and their associated ACSs to a reference patella. The mean angle between the reference patella ACS and each subject's axes was less than 2.5° and the 95% CI was 1.0°-4.0°. Here, we presented an anatomical coordinate system that is independent of the observer's subjective judgement or orientation of the knee within the scanner.