Defining the Cortical Purchase Zone in New Minimally Invasive Bunion Surgery. A Retrospective Study of 638 Cases.

Abstract

Introduction The stability of the screws in the cortical purchase zone (CPZ) with new minimally invasive bunion surgery (MIBS) is of utmost importance in creating a stable construct for maintaining position and providing a scaffold for bone healing. The distance between the osteotomy and where the screw(s) exit the lateral cortex (a distance we herein coin the “Cortical Runway”) within the CPZ has yet to be studied, and is the focus of this retrospective study. Methods A retrospective review was preformed of a single surgeons first and consecutive cases of patients who had undergone MIBS from January 2018 to November 2022. The CPZ measurement was obtained on early postoperative radiographs. The CPZ was divided into five stability regions using the statistical empirical rule to define the boundaries of each of the regions. Results We included 427 patients and 638 feet in this study. Radiographic measurements were performed at an average 20.7±24.2 days (95% CI, 15.3–15.8 days) from surgery. The mean age of the patients was 46.5±15.0 years (95% CI, 45.3–47.6 years). There were 330 (51.7%) right feet and 308 (48.3%) left feet. Of the 427 patients, 211 (49.4%) had surgery on both feet, with 97 (46%) having both feet operated on the same day. A 1-screw construct was performed in 526 feet (82.4%) and a 2-screw construct in 112 feet (17.6%) of the total 638 feet. The mean anchor screw (primary screw) distance was 10.4±3.7mm (95% CI, 10.1-10.7mm). The mean collateral screw (secondary screw) distance was 1.2±2.0mm (95% CI, 0.8-1.5) and the mean cortical bridge (distance between the two screws) was 10.3±3.2mm (95% CI, 9.7-10.9), in 112 feet. We were able to define the CPZ stability region boundaries, which are: danger (< 2.9mm), vulnerable (3.0 - 6.6mm), standard (6.6 - 14.1mm), safety (14.2 - 17.8mm) and the security (> 17.9mm). Conclusion These CPZ boundaries and newly delineated stability regions will serve not only as an intraoperative visual tool for surgeons to perform MIBS successfully, but will also provide a much more stable and safer construct, thus minimizing risks and complications.