Summary

The graft used in SCR is stretched well beyond its intra-operative length and the length exceeds the graft ultimate strain.

Abstract

Introduction

Superior capsular reconstruction (SCR) has led to favorable clinical outcomes [1,2], but the in vivo behavior of the SCR graft remains unclear. The aims of this study were to determine regional graft elongation after SCR, its relationship to graft healing, and the effects on kinematics. It was hypothesized that anterior and posterior regions of the SCR graft would elongate uniformly, greater graft elongation would be associated with graft failure, and the graft would affect shoulder kinematics by decreasing the distance between graft anchor points after surgery.

Methods

Ten patients with irreparable rotator cuff were enrolled in this IRB approved study . Synchronized biplane radiographs of the shoulder were collected before (PRE) and 1 year after dermal SCR (POST) at 50 frames/s while patients performed 3 trials of both scapular plane abduction and internal/external rotation at 90° of humerothoracic abduction. Kinematics were determined with sub-millimeter accuracy by matching subject-specific digitally reconstructed radiographs of the CT-based models of the humerus and scapula to radiographs using a validated volumetric tracking technique [3]. Intra-op graft lengths were recorded from surgical notes. MRIs were acquired POST to identify anchor locations on bines and to evaluate whether the graft was healed at the four anchor locations. The graft length between anchors was calculated based upon bone motions recorded using biplanar radiography. Differences between the abduction angle at which the graft reached the intra-op length in healed and not healed grafts were evaluated using a Mann-Whitney U test, with significance set at p<0.05.

Results

All SCR grafts elongated beyond their intra-op length. During abduction, the anterior and posterior regions elongated up to 340% and 118% compared to intra-op, while during rotation the anterior and posterior regions elongated up to 222% and 171%. On average, graft elongation occurred when the GH joint passed below 66° of abduction. During rotation, the posterior regions elongated during internal rotation while the anterior region did not elongate (p<0.01). Grafts that were healed at both anterior anchors reached the intra-op length at lower abduction angles than grafts that were not healed at one of the anchors. The posterior anchor points were 2.1±1.5mm and 2.1±3.6mm farther apart POST compared to PRE during abduction (p<0.01) and rotation (p=0.05), respectively, but no difference was found in the distance between anterior anchor points for either motion (all p>0.38).

Discussion

The main finding of this study was that the graft used in SCR is stretched well beyond its intra-operative length and the length exceeds the graft ultimate strain of 24% [4]. The graft regions did not elongate uniformly during rotation, contradicting our first hypothesis. Grafts that were healed in the anterior region became elongated at lower abduction angles than grafts that were not healed, supporting our second hypothesis. The graft lengths were larger in the posterior region during abduction POST than they would have been in the simulated PRE graft, contradicting our third hypothesis.