Normal values of the axial patellotrochlear overlap on MRI: Good correlation with patellotrochlear index in patients...

There are several imaging-based measurements for patellofemoral height, which are obtained from sagittal images. However, these methods can be misleading with sagittal oblique slices and when the patella is tilted and/or chronically subluxated. This study aimed to describe a simple method of measuring patellar height using axial patellotrochlear overlap (APTO) on MRI.

A retrospective observational study of 97 knees from 251 patients, excluding those with fractures, massive effusion, or patellofemoral conditions. APTO was measured as follows: (1) patellar length (P) - expressed as the number of axial images showing patellar articular cartilage; (2) trochlear overlap (T) - the number of axial images showing the overlap between patellar articular cartilage and articular cartilage of the lateral trochlea. APTO is the ratio T/P. All measurements were performed independently by six raters on two separate occasions. The raters were two orthopaedic consultants, one knee surgery fellow, two consultant musculoskeletal radiologists, and one radiology fellow. The conventional patellotrochlear index (PTI) was measured as a control for all patients by a senior musculoskeletal radiologist.

The mean APTO value was 36.7% (range 14.2-66.6; standard deviation 11.4). There was a positive correlation with the PTI, Pearson correlation coefficient: 0.76, P < 0.001. Intra-observer reliability was good (intraclass correlation coefficient(ICC): 0.66, 95% confidence interval (CI) 0.54, 0.76, P < 0.001). Inter-observer reliability was fair (ICC: 0.51, 95% CI 0.41, 0.6, P < 0.001).

APTO was shown to be a reliable measurement of patellar height and correlated with existing PTI for patellar height. Measurement of APTO on MRI could be a reliable alternative for the evaluation of patellar height. However, further studies are required to assess its validity in patients with patellofemoral pathology.