Procedural considerations for photographic-based joint angle measurements


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Abstract

Background and PurposeMeasuring static joint angles is important to clinicians involved in assessing, diagnosing and treating musculoskeletal disorders. New measurement techniques such as the Uillinn Method© (UM) employ the relatively new technology of digital photography and software to form a virtual goniometer. It is of central importance that the errors associated such new measurement techniques are known. Precision in joint angle measurement is a challenge and errors can arise from three separate categories: equipment error, examiner error or biological error. The aim of the present study was to discover the amount of equipment error associated with the UM and to present guidelines for the optimal use of a photographic based measurement technique.MethodThis was a non-clinical agreement study design that attempted to describe the best possible agreement between a mathematical control dataset and the angles calculated from the virtual goniometer, which in this case was the UM. When this was established, the effect of rotation and placing the angle at the periphery of the camera's field of view were tested.ResultsThe repeatability coefficient (RC) between the UM and the control data under optimal conditions was 0.81°; the typical error (TE) was 0.29° (n = 120). When the angle appeared at the edge of the photograph the RC increased to 2° and the TE to 0.73° (n = 48). When 5° rotation was introduced between the camera and the angle no increase in error was detected. However, increasing amounts of rotation above 5° was proportional to increases in the RC (RC at 10° = 2.3°; 20° = 3.86°; 30° = 14.8°; 40° = 27.27°) and the TE (Teat10° = 0.83°; 20° = 2.7°; 30° = 5.3°; 40° = 9.8°) scores.ConclusionPhotographic-based joint angle measurement techniques are subject to error if careful procedures are not observed. Best procedures include photography from a perpendicular viewpoint and centring the lens on the target angle.

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