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Early motion following four-corner arthrodesis using headless compression screws: A biomechanical study
Andrew Greenberg, MD; Mark Shreve, MD; Daniel Bazylewicz, MD; Michael Cohn, MD; Anthony Sapienza, MD; The Hospital for Joint Diseases
The Hospital for Joint Diseases, New York City, NY, USA
Introduction: Four-corner arthrodesis is a popular surgical option for the treatment of scapholunate advanced collapse (SLAC) and scaphoid nonunion advanced collapse (SNAC) wrist deformities. Multiple fixation options exist, each with varying reports of success and complications. The variable pitch screw provides compression as well as a large surface area of bony purchase between sites of fusion, and has shown very promising results as their use has expanded to four-corner fusions. Screw constructs have enabled early post-operative motion when used for other procedures. Still, numerous surgeons utilizing screw fixation continue to employ cast immobilization until radiographic union because of concerns of nonunion related to potential motion at the carpal interfaces. The likely reason for this dichotomy is that there is no biomechanical study in the literature that evaluates the fixation strength of headless compression screw fixation for four-corner fusions. Moreover, there is no study focusing on the superiority of one screw construct relative to another. Our biomechanical study focuses on the ability of headless compression screws, assembled in two accepted screw constructs, to maintain immobile bony contact throughout wrist cycling. Our hypothesis is that neither screw construct will permit displacement of the carpal bones, and therefore, can tolerate early post-operative range of motion. Materials and Methods: Using 6 matched-paired cadaveric arms (Science Care, Phoenix AZ), a standard dorsal approach to the carpus was performed. The Scaphoid was excised and the remaining articulations decorticated. Using cannulated compression screws (Medartis, SpeedTip CCS) the carpal bones were either fused in a U-type construct (Captiolunate (CL), Lunotriquetral (LT), Triquetrohamate (TH)) or in an X-type construct (2 crossing CL screws, with the LT and TH interfaces immobilized using 2 diverging 0.045 Kirschner wires). The arms were mounted onto an MTS machine, which cycled each wrist 5000 times. Intercarpal distances were measured from fixed points using a digital caliper at 0, 100, 1000, and 5000 cycles at 0 degrees, 30 degrees, and maximum passive flexion. Noting that previously published studies considered intercarpal distances of greater than 1mm as significant, our definition of fixation failure was a distance of greater than 0.5mm. Results: Neither screw construct showed any significant intercarpal gapping at any point during the experiment (Average U-construct gap 0.07mm versus 0.02mm for the X-type construct). There were no significant differences between the constructs with regarding sub significant gapping at any point. Conclusions: Cannulated compression screws do not allow intercarpal gapping during early motion after four-corner arthrodesis.
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