Applied force and elongation in the medial rectus in esotropic patients with and without movement restriction

To study the different ways of expressing the force-elongation relationship in medial rectus muscles in esotropia with and without muscular restriction.

Twenty-nine passive force-elongation curves were obtained without restriction (group I, n = 13) and with restriction (group II, n = 10) by means of a manual pachymeter and a digital dynamometer.

In group I, the mean age was 14 years and 7 days and the mean esotropia was 53.88(Delta) while in group II the mean age was 35 years and 5 days and the mean esotropia was 60.5(Delta). Comparisons of structural muscular parameters between groups I and II were made for length (38.69 +/- 0.75 vs. 32.48 +/- 1.84 mm, p < 0.05), width (8.64 +/- 0.75 vs. 7.95 +/- 0.68 mm), thickness (0.67 +/- 0.07 vs. 0.71 +/- 0.14 mm), and area (5.79 +/- 0.73 vs. 5.62 +/- 1.23 mm2). The differences in width were statistically significant between both groups. As expected, the force-elongation relationship, whether normalized or not, followed an exponential curve. The constant c, which represents force when the elongation is zero, remained the same in all curves. In contrast, the constant b, which represents the slope of the curve, showed a significant difference between the two groups only for the curves of force-absolute elongation and tension-absolute elongation.

The results imply that the constant b is better for characterizing the difference between the behavior of the medial rectus in esotropia with and without restriction. In addition, the elongation normalization showed that the contractile component is similar between the two groups and, therefore, the classical way of analysis, which does not employ normalization, is appropriate to correlate muscle properties with clinical findings.