175 – DISPLACEMENT OF THE RADIAL HEAD ABOUT THE CAPITELLUM: THE RADIO-CAPITELLUM RATIO (RCR), A NEW VALIDATED METHOD TO MEASURE IT

Abstract

Purpose: The literature contains little information on an objective method of measuring radiocapitellar joint translations, as would be seen with joint instability. The purpose of this study was to develop and validate a measurement method that was simple and that could be easily reproducible in a clinical setting or intra-operatively to assess radiocapitellar joint translations.

Method: We performed a radiological study on a synthetic elbow specimen in order to quantify radial head translations as related to the capitellum: the Radio-capitellum ratio (RCR). Thirty (30) lateral elbow x-rays were taken in different magnitude of subluxation of the radial head. The subluxation was created randomly by manipulation. X-rays where taken by fluoroscopy to obtain a perfect lateral view of the distal humerus. First, the evaluators determined the long axis of the radius and the center of the capitellum. The displacement of the radial head (in mm) was obtained by measuring the distance of the line perpendicular to the long axis of the radius passing through the center of the capitellum. Then, in order to adjust for variation of magnification, a ratio of the displacement of the radial head about the diameter of the capitellum was done. The RC ratio would be of zero because the long axis of the radius always crosses the center of the capitellum in a perfectly aligned joint. A five mm translation of the radial head and a capitellum diameter of twenty (20) mm would give a RCR of 25% and would be positive if anterior and negative if posterior. The measurements were done two times at one week intervals by three independent evaluators to test inter-observer agreement and intra-observer consistency. The radiological incidences were randomly ordered to minimize observer recall bias. Intra/inter-observer reliability was calculated using Intra-Class Correlation (ICC) and paired T-tests.

Results: The mean translation in the trial group was of 6,06% (SD 70.7%) from – 167% to 125%. A result over 100% means that it is a complete dislocation ie – the axis of the radius is outside of the capitellum. Negative values signify posterior translation and positive values an anterior translation. Intra-observer reliability was excellent for the Radio-capitellum ratio (ICC 0.988 and 0.995) and inter-observer reliability was excellent (ICC 0.984 in average). Paired T-test results confirm a high intra-observer repeatability (p=0.97 and p=0.99) as well as a large inter-observer reproducibility (p=0.98 in average).

Conclusion: The proposed measurement of radial head translation about the capitellum (in percent): radio-capitellum ratio (RCR) has excellent inter – and intra-observer reliability when using our measurement method.