Biomechanics Explains Variability of Response of Small Hypertropia to Graded Vertical Rectus Tenotomy

Abstract

PurposeSmall angle hypertropia in sagging eye syndrome is conveniently treated by graded vertical rectus tenotomy, yet adjustable technique under topical anesthesia has been recommended due to variability of effect. We performed graded tenotomy in an experimental model to elucidate the effect reason for variability of response to this surgical procedure. DesignExperimental study: Methods: 32 fresh bovine rectus musculotendon specimens were prepared including continuity with insertional sclera, and extending for a total 40mm length to the proximal muscle bellies, and trimmed to 16mm width. Specimens were anchored by the clamps at the scleral insertion and muscle belly ends within a physiological chamber. After preconditioning and elongation to 10% strain was imposed by a linear motor, tensile force was allowed to stabilize at a plateau state. Then 25, 50, 75, 90, 100% marginal tenotomies were perormed progressively as remnant forces were measured. ResultsTendon thickness averaged 0.29±0.05mm and width 19.71±2.25mm. On average, remnant force decreased linearly (R2=0.985) from 4.23±1.34, 2.76±0.88, 1.70±0.73, 1.01±0.49, 0.39±0.10, and 0 N, at 0, 25, 50, 75, 90, 100% tenotomy. However, there was marked individual variability in effect among specimens, with coefficient of variation 32, 32, 43, 49, and 27%, respectively ConclusionOn average there is a linear relationship between graded rectus tenotomy and percentage force reduction, but the effect among individual tendons is large, paralleling the reported variation in surgical effect. This explains and implies continued advisability of adjustable technique in this procedure.