Suprascapular nerve injury after reverse total shoulder arthroplasty. Correlation with screw out of vault penetration and functional situation. Prospective study.

Abstract

Baseplate screws have been suggested as a possible cause of suprascapular neuropathy after reverse total shoulder arthroplasty (RTSA). This study aims to investigate the association between screw penetration out of the vault, electromyographic study and the clinical outcomes. 31 patients who underwent RTSA for cuff tear arthropathy were prospectively enrolled. They were followed up for a minimum of 24 months. All underwent computed tomography 6 months postoperatively in order to determine the extraosseous position of the screws (perforation of the second bone cortex and protrusion into the supra or infraspinatus fossa). Electrodiagnostic evaluation was performed preoperatively and postoperatively to stablish any relation between cortex perforation of the screw and SSN injury. Clinical outcomes pre and postoperatively (Constant score, ranges of motion, and VAS) of patients with and without documented injury were recorded. 14 patients (45.2%) had abnormal preoperative SSN electrodiagnostic study (chronic or disuse injuries) and 6 patients (19.4%) abnormal postoperative study (acute injury. Of these last 6 patients: 2 cases appeared over the pre-existing lesion and 4 appeared over an intact preoperative nerve, all of them affecting the infraspinatus branch of the SSN. Perforation of the second cortex was detected for 60% of superior screws and 40% of posterior screws. The mean lengths of the superior and posterior screws were 30 and 18.2 mm, respectively. Patients with screw perforation of the second cortex were assessed as having a high risk of nerve injury (40% vs. 9.5%). Preoperative SSN injuries do not have a significant clinical impact and do not predispose to an acute postoperative SSN lesion. The Constant Score and VAS scale for patients with acute SSN injuries were not statistically different than those without SSN injury. Extraosseous position of the screw increases the probability of a SSN injury to 31%. This risk is higher with the posterior screw, which leads us to question whether it is really necessary to use it.