A biomechanical analysis of double screw, double button and screw button fixation constructs in PSI guided Latarjet procedure.

Abstract

The Latarjet coracoid transfer procedure reliably stabilizes the glenohumeral joint for shoulder instability. However, complications such as graft osteolysis, non-union and fracture continue to affect patient clinical outcomes. The double screw (SS) construct is regarded as the gold standard method of fixation. SS constructs are associated with graft osteolysis. More recently, a double button technique (BB) has been suggested to minimize graft related complications. However, BB constructs are associated with fibrous non-union. To mitigate this risk, a single screw combined with a single button (SB) construct has been proposed. It is thought that this technique incorporates the strength of the SS construct and allows micromotion superiorly to mitigate stress shielding related graft osteolysis. The primary aim of this study was to compare the failure load of SS, BB and SB constructs under a standardized biomechanical loading protocol. The secondary aim was to characterize the displacement of each construct throughout testing. Computed tomography scans of twenty matched-pair cadaveric scapulae were performed. Specimens were harvested and dissected free of soft tissue. SS and BB techniques were randomly assigned to specimens for matched-pair comparison with SB trials. A patient specific instrument (PSI) guided Latarjet procedure was performed on each scapula. Specimens were tested using a uniaxial mechanical testing device under cyclic loading (100 cycles, 1Hz, 200N/s) followed by a load-to-failure protocol (0.5mm/s). Construct failure was defined by graft fracture, screw avulsion and/or graft displacement of more than 5mm. Forty scapulae from twenty fresh frozen cadavers with a mean age of 69.3 years underwent testing. On average, SS constructs failed at 537.8N (SD 296.8), while BB constructs failed at 135.1N (SD 71.4). SB constructs required a significantly greater load to fail compared to BB constructs (283.5N, SD 162.8, p=0.039). Additionally, SS (1.9mm, IQR 0.87) had a significantly lower maximum total graft displacement during the cyclical loading protocol compared to SB (3.8mm, IQR 2.4, p=0.007) and BB (7.4mm, IQR 3.1, p<0.001) constructs. These findings support the potential of the SB fixation technique as a viable alternative to SS and BB constructs. Clinically, the SB technique could reduce the incidence of loading-related graft complications seen in the first three months of BB Latarjet cases. The study is limited to time-specific results and does not account for bone union or osteolysis.