Novel Instrument for Clinical Evaluations of Active Extraocular Muscle Tension

Strabismus can be caused by abnormal tension of the extraocular muscles (EOMs) attached to the eyeball in superior, inferior, lateral, medial, superior oblique, and inferior oblique positions. Evaluating the tension in each EOM is crucial for surgical planning in strabismus, which is conducted by adjusting the tension on the EOM. The purpose of this study was to develop a compact measuring device to non-invasively evaluate the active EOM tension. The proposed device employed a cotton-tipped medical swab to transfer the EOM tension connected to the force sensor as a non-invasive medium. The tilting angle of the swab and the force of active EOM tension were wirelessly transferred to a laptop computer for recording and real-time displaying of the measured values. The active EOM tensions for the four recti muscles were 101.7 ± 15.0 g (mean ± SD) for the lateral rectus; 88.0 ± 15.4 g for the medial rectus; 61.3 ± 6.8 g for the inferior rectus; and 121.3 ± 38.5 g for the superior rectus. These values were higher than the reported values of 45–60 g measured in previous studies. In the previous studies, however, the EOM was detached from the globe and attached to a strain gauge, and, thus, there were no passive elastic forces from ocular connective tissue, resulting in lower values compared with the current study. The previous methods were also complex and not suitable for clinical measurement. Thus, the proposed method, which is non-invasive and mimics the conventional force generation test with a cotton-tipped swab, could facilitate the evaluation of active EOM tension, both clinically in strabismus management and in research into understanding its pathophysiology.