[Measurement of glenohumeral joint translation with a dynamic shoulder model]

Ex vivo experimental strategies for assessing unconstrained shoulder biomechanics: A scoping review

BACKGROUND

Biomechanical studies of the shoulder often choose an ex vivo approach, especially when investigating the active and passive contribution of individual muscles. Although various simulators of the glenohumeral joint and its muscles have been developed, to date a testing standard has not been established. The objective of this scoping review was to present an overview of methodological and experimental studies describing ex vivo simulators that assess unconstrained, muscular driven shoulder biomechanics.

METHODS

All studies with ex vivo or mechanical simulation experiments using an unconstrained glenohumeral joint simulator and active components mimicking the muscles were included in this scoping review. Static experiments and humeral motion imposed through an external guide, e.g., a robotic device, were excluded.

RESULTS

Nine different glenohumeral simulators were identified in 51 studies after the screening process. We identified four control strategies characterized by: (a) using a primary loader to determine the secondary loaders with constant force ratios; (b) using variable muscle force ratios according to electromyography; (c) calibrating the muscle path profile and control each motor according to this profile; or (d) using muscle optimization.

CONCLUSION

The simulators with the control strategy (b) (n = 1) or (d) (n = 2) appear most promising due to its capability to mimic physiological muscle loads.

Ultrasonic measurement of the anterior translation in the shoulder joint

A new method for measuring the anterior translation in the shoulder joint by dynamic ultrasound was evaluated. We placed a 3.5-MHz transducer on the shoulder anteriorly. By using 3 bony landmarks, we then measured the anterior translation of the humeral head with a force of 90 N applied posteriorly. We performed such measurements in 20 subjects with healthy shoulders and in 20 patients with unilateral shoulder instability. There was a mean translation of 1.9 mm in healthy shoulders and 4.9 mm in unstable shoulders (P < .01). The mean difference between the 2 sides in subjects with normal shoulders was 0.7 mm, whereas the mean difference in patients with instability was 2.8 mm (P < .01). The normal shoulders were examined by 2 examiners to determine the degree to which different examiners' measurements might vary. Although one examiner recorded higher values than the other, the new method seems suitable for measuring increased laxity in unstable shoulders.