The medial-lateral position of the patella on routine magnetic resonance imaging: when is normal not normal?

The purpose of this study was to determine the position of the normal patella during routine magnetice resonance imaging (MRI). The literature indicates that the normal patella is positioned laterally relative to the trochlea when the knee is fully extended. As such, a laterally positioned patella on MRI is often interpreted as normal. Yet, in our experience, patients with a normal extensor mechanism show a patella that is centered over the trochlea on MRI, and we set out to formally study this. The MRIs of 60 patients without knee extensor pathology were analyzed. In 59 patients, the patella was centered over the underlying femur. The discrepancy between these results and those in the literature can probably be accounted for by the following: (1) Knees in general are somewhat flexed during MRI, and (2) relative to patients in other studies, patients in this study were subject to stricter criteria of normality. The results of this study strongly suggest that the normal patella is centered over the underlying trochlea during routine MRI. A laterally positioned patella, although possibly common, should not be automatically dismissed as a (medically) normal finding.