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What is the best position for coracoid fixation in the Latarjet procedure?
Guilherme Augusto Stirma1, Leandro Massini Ribeiro1, Evandro Dias Gaio2
1Federal University of São Paulo, Paulista School of Medicine - UNIFESP, São Paulo (SP), Brazil.
Hypothesis:
Multiple problems and complications associated with Latarjet fixation have been described; thus, this is the first study in the literature to identify the maximum allowed screw clamping force and best fixation screw position for Latarjet surgery.
Methods:
A variation of distal and proximal coracoid screw positions with and without a flat washer was evaluated through finite element analysis, at a minimum distance of 3 mm from the edge. A loading progression test was performed until the maximum stress reached a limit imposed by the bone yield. We identified the maximum allowed screw clamping force based on a von Mises and maximum principal stresses failure theory.
Results:
When using the flat washer, the cortical bone generally has only space for 1 piece. For this reason, as a primary study, it was observed that when the distal screw was more than 7 mm from the edge, the clamping force supported will be higher than that during the proximal fixation regardless of the proximal location screw. We have found that the best position is 7 mm from the distal edge, with the highest compression of 445 N (7 mm proximal distance, 5 mm distal distance) in due respect to the von Mises failure theory. To get around this lack of space situation, in this study, we have proposed a fixation plate to replace the flat washer. This plate has shown very interesting values when compared to the previously flat washer study, but now, for both screw holes. With those results, we can assure that using a fixation plate like this will ensure surgery safety and higher allowed compression force when clamping the bolts.
Conclusion:
The distal screw provided higher tensile strength values when located more than 7 mm from the coracoid edge. The geometry of the coracoid in its distal position supports higher stress loads than in the proximal position. When the flat washer was in the proximal position, the coracoid was submitted with a more distributed and uniform load, preventing localized bone damage as a crush.