In vivo quantification of capsular end-point in the nonimpaired glenohumeral joint using an instrumented measurement system - Author response

Study Design: Single-group factorial. Objectives: Quantify glenohumeral joint translations as a function of force using an instrumented measurement system. Our first specific aim was to compare the magnitude of force (N) required to reach capsular end-point between the anterior, posterior, and inferior directions. Our second specific aim was to compare the magnitude of translation (mm) at capsular end-point between the anterior, posterior, and inferior directions. Background: Manual force-displacement techniques are subjective in nature, and the clinician must rely on "feel" to determine capsular end-point. Several investigators have attempted to utilize more objective measures at the glenohumeral joint, however none have quantified the force or displacements necessary to achieve a true capsular end-point. Methods and Measures: An arthrometric technique was used to measure anterior, posterior, and inferior glenohumeral translations in 20 nonimpaired shoulders (11 women and 9 men with a mean age of 20.9 ± 3.6 years). Results: The magnitude of applied force required to reach capsular end-point was significantly different between directions of translations. Anterior-directed translations required a significantly greater magnitude of applied force to reach capsular end-point than inferior-directed translations. The magnitude of translation was not significantly different between directions at capsular end-point. Conclusions: Applied forces in the range of 181 to 203 N are necessary to reach capsular end-point in subjects with nonimpaired shoulders. Anterior translation required significantly higher forces (203.1 ± 13.1 N) to reach capsular end-point than inferior translation (181.4 ± 31.9 N).