Summary

Biomechanical evaluation of latarjet fixation techniques

Abstract

Introduction

The Latarjet procedure is a well-established intervention used to address shoulder instability1. Despite the overall excellent clinical outcome, complications related to coracoid graft resorption and fracture has been reported2. Different fixation techniques, using single or combined screws and cortical buttons, have been proposed to mitigate the rate of osteolysis. A single screw and single button construct has been employed to harness the merits of both fixation devices with an overall aim to reduce superior graft osteolysis without compromising on initial fixation stability.

Double suture-button constructs (BB) have been reported to fail at a significantly lower load as compared to double screw constructs (SS)3. However, there is currently no clinical or biomechanical research studies comparing the potential merit of a combined screw and suture-button (SB) technique. Thus, the aim of our study is to compare the failure load of SS, BB and SB fixation techniques under a standardised loading protocol

Material & Methods
Computed tomography scans of forty cadaveric scapulae (20 matched pairs) were performed prior to harvesting. Specimens with morphological abnormalities, obvious pathologies, or prior operative intervention were excluded. Customised 3D jigs were designed to create a reproducible 20% glenoid bone defect for each specimen. Further jigs were customised individually to standardise Coracoid osteotomy and a PSI graft alignment and fixation In addition, PSI jigs were designed to pot each specimen to standardise the biomechanical loading conditions.

SS and BB fixation techniques were randomly assigned to specimens for matched-pair comparison with SB trials.
Uniaxial mechanical testing device under cyclic loading (50-150N, 100 cycles at 1Hz) and load-to-failure protocols (0.5mm/s) was used to test potted scapulae. Construct failure was defined by graft fracture, screw avulsion or graft displacement of more than 5mm.

Results

& Discussion
Forty matched-pair scapulae from twenty fresh frozen cadavers with a mean age of 63.9 years (48-85) underwent testing. The mean failure load of the SB constructs was significantly higher than the BB technique (284.0N vs 135.1N, p<0.001) (Figure 1). However, the mean failure load of the SB constructs was less than the SS technique (284.0N vs 537.8N, p<0.01). A motion tracking system found that SS, SB and BB constructs displaced on average 0.3mm, 2.3mm and 5.2mm respectively at the 100th cycle of the preloading protocol. In addition, the BB construct failed within the first 23 seconds of loading compared to the SB (83sec) and BB (111sec) constructs.

This cadaveric study is the first to report biomechanical data on SB constructs for graft fixation at time zero. In line with the literature, BB failed at a lower load than SS constructs. In addition, our findings indicate that SB may be a stronger and more resilient alternative to BB. The study is limited to time-specific results and does not account for bone union or osteolysis. A clinical study is underway to evaluate the clinical outcome of SB construct.

References
1. Lafosse JSES 2010
2. Gupta Current Review in Musculoskeletal Medicine 2015
3. Williams JSES 2020