The intra- and inter-rater reliability of X-ray radiological measurements for patellar instability

Patellar instability is a complex, multi-factorial disorder. Radiological assessment is regarded as an important part of the management of this population. The purpose of this study was to determine the intra- and inter-rater reliability of common radiological measurements used to evaluate patellar instability.

One hundred and fifty X-rays from 51 individuals were reviewed by five reviewers, two orthopaedic trainees, a radiological trainee, a consultant radiologist and an orthopaedic physiotherapist. Radiological measurements assessed included patellar shape, sulcus angle, congruence angle, lateral patellofemoral angle, lateral patellar displacement (LPD), lateral displacement measurement (LDM), boss height, and patellar height ratios. All assessors were provided with a summary document outlining the method of assessing each measurement. No further formal teaching was provided. Bland-Altman analyses were adopted to assess intra- and inter-rater reliability.

The results indicated generally low measurement error on intra-rater reliability assessment, particularly for LPD, LDM and boss height measurements. Whilst the inter-rater reliability between assessors indicated a low mean difference for assessments of patellar height measurements, patellar angle, lateral patellar dislocation and lateral displacement, wide 95% limits of agreement for all measurements indicated poor precision.

Many of the standard measurements used to assess the patellofemoral joint on plain radiographs have poor precision. Intra-rater reliability may be related to experience but it seems likely that to achieve good inter-rater reliability, specific training may be required to calibrate observers. More formal training in the technique of radiological measurement for those who were inexperienced might have improved the inter-rater reliability.