Summary

This animal study observes the effect of "inverse tubularization" on tendon-to-bone healing in a rat biceps tenodesis model, demonstrating improved healing in the experimental group via 1) an increased pull-out strength with biomechanical testing and 2) increased direct fascicular healing to bone on histology.

Abstract

Introduction

Tendon-to-bone repair (surgical tenodesis) is an imperfect science. Bone tunnel tenodesis is at risk for graft pull-out, re-rupture, and tunnel widening. In this study, we hypothesized that inside-out tendon inversion, whereby the epitenon is longitudinally incised and inverted, may improve bone tunnel healing by obviating epitendinous lubrication interference at the tendon-bone interface. Tendon inversion also permits exposure of intra-tendinous fascicles to the healing interface, which may improve healing strength by enabling direct fascicular healing against the bone tunnel walls.

Methods

40 adult Sprague-Dawley rats underwent suprapectoral long head of biceps (LHB) tenodesis of the shoulder. 20 underwent tendon inversion (inversion group) and 20 did not (control group). The tendon is fixed by suspensory fixation in a bicortical bone tunnel using a 1.6mm microvascular clip. All animals were permitted ad lib cage activity until 8-week sacrifice. 14 animals from each group underwent biomechanical testing, and 6 underwent paraffin embedding for histology and immunohistochemistry.
Biomechanics: All specimens were frozen in -80C for one freeze-thaw cycle prior to specimen preparation for biomechanical testing. Musculature was removed from an otherwise intact humerus except for the tenodesed LHB. The humerus was potted in 1 inch copper tubing with Bondo. Specimens were kept moist at all times using 1x phosphate buffered saline. The tendon was mounted in a 320-grit sandpaper sandwich with Krazy Glue. The specimens were mounted in an Instron 8872 materials testing system (Instron, Norwood, MA) with a 50N load cell for pull-to-failure testing. All specimens were preloaded to 1N prior to initiation of testing. Peak failure forces are recorded and averaged. Failure mode was recorded.

Results

All 40 animals successfully underwent surgery. There were no surgical complications. For biomechanical testing, it was apparent on specimen preparation that two tendon inversion specimens had an early tendon failure leading to a popeye deformity. One control group specimen had uncharacteristic adhesion formation to the pectoralis insertion which interfered with failure testing. These three specimens were excluded from analysis.
Biomechanics: The inversion group demonstrated a failure force of 10.97±2.90 N, versus control at 8.61±1.76N (p=0.0247).Interestingly, by failure mode, there were 5 tendon pullouts in the inversion group, versus 2 tendon pullouts in the control group, as opposed to surface or midsubstance failures, though this difference was not statistically significant by Chi-square testing (p=0.14).
Histology and immunohistochemistry: H&E histology demonstrated evidence of direct intra-tendinous fascicular healing against the bone tunnel walls, with evidence of formation of Sharpey fibers, mechanically oriented fibril insertion into bone, and possible layered transitional tissues reminiscent of native entheses.

Conclusions

Using a rat biceps tenodesis model, we have demonstrated that longitudinal, inside-out tendon inversion (inverse tubularization) may improve tendon-to-bone healing in a bone tunnel. This work may inform future improvements in surgical techniques for tendon-to-bone fixation.