Biomechanics of superior oblique Z-tenotomy: Is it different from unguarded tenotomy?

Abstract

IntroductionA recent report suggests that 70-80% Z-tenotomy of the superior oblique (SO) effectively treats A-pattern strabismus associated with overdepression in adduction. We examined the effect of Z-tenotomy on SO tendon biomechanics.MethodsFresh bovine SO tendons were reduced to the 16mm long x 8mm wide dimensions similar to the human SO tendon, and clamped in a custom micro-tensile load cell under physiological conditions of temperature and humidity. Minimal pre-load was applied to avoid slackness. Tendons were elongated by 20% of original length following Z-tenotomies, made with scissors from opposite sides of the tendon, spaced 6.4 mm apart and each encompassing 0, 40, 60, or 80% tendon width. Transient and sustained tensions were monitored using a precision strain gauge with digital sampling. Control experiments were performed in similar-sized specimens of bovine rectus tendon, and isotropic latex.ResultsZ-tenotomy of latex caused a linear reduction in force transmission reaching zero at ∼95% tenotomy. In contrast, Z-tenotomy of SO and rectus tendons caused progressive reduction in force transmission reaching a negligible value at only 80% tenotomy.DiscussionUnlike isotropic latex, the parallel fiber structure of tendons reduces shear force transfer across tendon width and enhances the biomechanical effect of Z-tenotomy. Z-tenotomy <80% progressively reduces force transmission in the tendon, but Z-tenotomy of ≥80% reduces force transmission to zero just as complete tenotomy. The marked SO tendon elongation observed surgically at the end point of 80% Z-tenotomy represents loss of all SO force transmission.Conclusions80% or greater SO Z-tenotomy is biomechanically equivalent to complete unguarded tenotomy. IntroductionA recent report suggests that 70-80% Z-tenotomy of the superior oblique (SO) effectively treats A-pattern strabismus associated with overdepression in adduction. We examined the effect of Z-tenotomy on SO tendon biomechanics. A recent report suggests that 70-80% Z-tenotomy of the superior oblique (SO) effectively treats A-pattern strabismus associated with overdepression in adduction. We examined the effect of Z-tenotomy on SO tendon biomechanics. MethodsFresh bovine SO tendons were reduced to the 16mm long x 8mm wide dimensions similar to the human SO tendon, and clamped in a custom micro-tensile load cell under physiological conditions of temperature and humidity. Minimal pre-load was applied to avoid slackness. Tendons were elongated by 20% of original length following Z-tenotomies, made with scissors from opposite sides of the tendon, spaced 6.4 mm apart and each encompassing 0, 40, 60, or 80% tendon width. Transient and sustained tensions were monitored using a precision strain gauge with digital sampling. Control experiments were performed in similar-sized specimens of bovine rectus tendon, and isotropic latex. Fresh bovine SO tendons were reduced to the 16mm long x 8mm wide dimensions similar to the human SO tendon, and clamped in a custom micro-tensile load cell under physiological conditions of temperature and humidity. Minimal pre-load was applied to avoid slackness. Tendons were elongated by 20% of original length following Z-tenotomies, made with scissors from opposite sides of the tendon, spaced 6.4 mm apart and each encompassing 0, 40, 60, or 80% tendon width. Transient and sustained tensions were monitored using a precision strain gauge with digital sampling. Control experiments were performed in similar-sized specimens of bovine rectus tendon, and isotropic latex. ResultsZ-tenotomy of latex caused a linear reduction in force transmission reaching zero at ∼95% tenotomy. In contrast, Z-tenotomy of SO and rectus tendons caused progressive reduction in force transmission reaching a negligible value at only 80% tenotomy. Z-tenotomy of latex caused a linear reduction in force transmission reaching zero at ∼95% tenotomy. In contrast, Z-tenotomy of SO and rectus tendons caused progressive reduction in force transmission reaching a negligible value at only 80% tenotomy. DiscussionUnlike isotropic latex, the parallel fiber structure of tendons reduces shear force transfer across tendon width and enhances the biomechanical effect of Z-tenotomy. Z-tenotomy <80% progressively reduces force transmission in the tendon, but Z-tenotomy of ≥80% reduces force transmission to zero just as complete tenotomy. The marked SO tendon elongation observed surgically at the end point of 80% Z-tenotomy represents loss of all SO force transmission. Unlike isotropic latex, the parallel fiber structure of tendons reduces shear force transfer across tendon width and enhances the biomechanical effect of Z-tenotomy. Z-tenotomy <80% progressively reduces force transmission in the tendon, but Z-tenotomy of ≥80% reduces force transmission to zero just as complete tenotomy. The marked SO tendon elongation observed surgically at the end point of 80% Z-tenotomy represents loss of all SO force transmission. Conclusions80% or greater SO Z-tenotomy is biomechanically equivalent to complete unguarded tenotomy. 80% or greater SO Z-tenotomy is biomechanically equivalent to complete unguarded tenotomy.