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A threshold level of 4° of motion is commonly used to identify a pseudarthrosis. Our prospective study suggests that this value has a high PPV, but a low specificity and would miss many of the pseudarthroses that have angular motion less than 4° (sensitivity 23%). By lowering the threshold for angular motion to 1°, the sensitivity improves to 77%. CT scan has been touted as the gold standard, and it has a high positive predictive value of 100%. However, its NPV was slightly lower than using 1° of motion on QMA analyzed flexion-extension films (73% vs. 79%). In conclusion, although CT scan has long been regarded as the gold standard for determining a pseudarthrosis in the cervical spine, the interpretation is subjective and vulnerable to both type I and type II errors. Analysis of motion using Quantitative Motion Analysis is seemingly less subjective than CT and in our prospective study was more predictive of an operatively confirmed pseudarthrosis.

We used a mechanical method to locate the axis of rotation of the talocrural joint. A single constant axis of rotation was found just distal to the tips of the malleoli. The paths of light-emitting diodes mounted on a talar pin were recorded with time lapse photography during flexion-extension. These paths were analyzed both in the sagittal plane and in a plane perpendicular to the axis of rotation. Sagittal plane studies were distorted by perspective error. The plane used to analyze joint motion or shape must be the plane of motion. Models of the talocrural joint for gait analysis, reconstructive surgery and prosthetic design should have a single offset axis of rotation.