Background: The Constant score (CS) is one of the most frequently applied tools for the assessment of the shoulder joint. However, evaluation of strength is not standardized leading to potential bias when comparing different studies.
Methods: Seventy-six patients with fractures of the proximal humerus undergoing open reduction and internal fixation (ORIF) were assessed using standardized CS strength measurements at the deltoid muscle insertion and at the wrist in three different arm positions. Variation coefficients were evaluated for each patient and position.
Results: Forty women (57%) and 36 men (43%) were examined 96 months in mean after ORIF. We could state a maximum of 105.3 N difference if measurements were performed at the wrist or the insertion of the deltoid muscle in 90° forward flexion on the injured arm (167.9 ± 83.1 N; 62.6 ± 29.4 N). The lowest variation coefficient of the three performed measurements could be stated at the deltoid muscle insertion in a 90° abduction position in the scapula plane (6.94 ± 5.5).
Conclusion: Following our study results, different positions of force measurement can change the total CS by a whole category (e.g. "very good" to "good"). We recommend performing the measurement at the insertion of the deltoid muscle in a 90° abduction position in the scapula plane. Otherwise, even in the non-injured, it is hard to reach a "normal" shoulder function, based on the CS. When using the CS as outcome parameter, authors must give detailed information about the force measuring and use an exact measuring device.
Keywords: Constant score; Functional assessment; Proximal humeral fracture; Strength measuring.