Length-tension recording system for strabismus surgery

Changes in intraocular pressure during strabismus surgery

OBJECTIVE

To determine whether the alterations in intraocular pressure (IOP) after cutting off a lateral rectus muscle during exotropia surgery could reflect the degree of muscle contracture.

DESIGN

Prospective study.

PARTICIPANTS

Sixty eyes of 31 patients that had undergone lateral rectus recession surgery for correction of exotropia.

METHODS

IOP was measured using a Tono-pen XL before and after detaching lateral rectus muscles during the surgery. The changes in IOP were measured and their correlations with age, duration of disease, intermittency of exotropia, amount of exodeviation, laterality of the deviated eye, and tendon width of the muscle operated on were analyzed.

RESULTS

The mean baseline IOP was 16.3 (SD 2.2) mm Hg when measured 5 minutes after tracheal intubation and showed a significant decline to 13.5 (SD 2.8) mm Hg when measured immediately before detachment of a muscle. However, the IOP decline over this period of preparation for muscle detachment was not correlated with any clinical variable tested. The mean IOP also showed a significant reduction, to 10.8 (SD 2.3) mm Hg, immediately after the muscles were cut off the globe. The peridetachment IOP reduction was positively correlated with constant exodeviation (p < 0.001), whereas there was no significant correlation with age, duration of exotropia, or tendon width.

CONCLUSIONS

The changes in IOP seen after detaching an extraocular muscle during strabismus surgery may serve as a surrogate parameter to reflect the muscle contracture.

Dynamics of primate oculomotor plant revealed by effects of abducens microstimulation

Despite their importance for deciphering oculomotor commands, the mechanics of the extraocular muscles and orbital tissues (oculomotor plant) are poorly understood. In particular, the significance of plant nonlinearities is uncertain. Here primate plant dynamics were investigated by measuring the eye movements produced by stimulating the abducens nucleus with brief pulse trains of varying frequency. Statistical analysis of these movements indicated that the effects of stimulation lasted about 40 ms after the final pulse, after which the eye returned passively toward its position before stimulation. Behavior during the passive phase could be approximated by a linear plant model, corresponding to Voigt elements in series, with properties independent of initial eye position. In contrast, behavior during the stimulation phase revealed a sigmoidal relation between stimulation frequency and estimated steady-state tetanic tension, together with a frequency-dependent rate of tension increase, that appeared very similar to the nonlinearities previously found for isometric-force production in primate lateral rectus muscle. These results suggest that the dynamics of the oculomotor plant have an approximately linear component related to steady-state viscoelasticity and a nonlinear component related to changes in muscle activation. The latter may in part account for the nonlinear relations observed between eye-movement parameters and single-unit firing patterns in the abducens nucleus. These findings point to the importance of recruitment as a simplifying factor for motor control with nonlinear plants.

Evidence for wide range of time scales in oculomotor plant dynamics: implications for models of eye-movement control

Oculomotor-plant dynamics are not well characterised, despite their importance for modelling eye-movement control. We analysed the time course of the globe's return after horizontal displacements in three rhesus monkeys lightly anaesthetised with ketamine. The eye-position traces were well fitted by a sum of four exponentials (time constants 0.012, 0.099, 0.46, 7.8 s). The two long time-constant terms accounted for 25% of plant compliance, and led to a model that accounted for hitherto unexplained features of ocular motoneuron firing such as (i) hysteresis, and (ii) the inability of a 2 time-constant model to fit data for both fast and slow eye-movements.

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

PURPOSE

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

SUBJECTS AND METHODS

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.

RESULTS

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.

CONCLUSION

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.

Quantitative forced ductions in an animal model--characterization of passive forces

PURPOSE

Our purpose was to characterize the passive tissue forces involved in ocular rotation in a controlled animal model and to evaluate the influence of manual versus mechanized ductions, repeated measurements, speed of rotation, and the influence of the nondepolarizing muscle relaxant mivacurium.

METHODS

Forced ductions were performed under general anesthesia on 20 eyes of 10 pigs, with or without mivacurium, with use of a highly sensitive force gauge attached to the eye by a traction suture. The eye was moved either manually or at constant speed with a motorized platform. Eyes were rotated a total of 8 mm from their resting position under anesthesia. The force-displacement relationship was analyzed and compared between groups.

RESULTS

A linear (elastic) relationship between force and displacement was noted, with a slope of 0.4 g per degree with use of the mechanized technique. Neither speed of rotation, use of mivacurium, nor repeated ductions significantly influenced the shape or slope of the relationship. Hysteresis averaged 2 to 4 g. Measurements performed with use of the motorized platform showed significantly improved reliability over those made manually.

CONCLUSIONS

The passive length-tension data correlate well with data reported by others in humans. Within a wide range of eye movement, this force is elastic in nature. For relatively low angular velocities, such as might be produced in smooth pursuit, the passive forces do not change appreciably with changes in velocity. The nondepolarizing muscle relaxant mivacurium has no effect on accurate performance of passive forced ductions under general anesthesia. Studies collecting quantitative data on passive orbital forces should be performed, when feasible, with an automated duction and recording apparatus.