Monoaxial Pedicle Screws Are Superior to Polyaxial Pedicle Screws and the Two Pin External Fixator for Subcutaneous Anterior Pelvic Fixation in a Biomechanical Analysis

Purpose. Comparison of monoaxial and polyaxial screws with the use of subcutaneous anterior pelvic fixation. Methods. Four different groups each having 5 constructs were tested in distraction within the elastic range. Once that was completed, 3 components were tested in torsion within the elastic range, 2 to torsional failure and 3 in distraction until failure. Results. The pedicle screw systems showed higher stiffness (4.008 ± 0.113？Nmm monoaxial, 3.638 ± 0.108？Nmm Click-x; 3.634 ± 0.147？Nmm Pangea) than the exfix system (2.882 ± 0.054？Nmm) in distraction. In failure testing, monoaxial pedicle screw system was stronger (360？N) than exfixes (160？N) and polyaxial devices which failed if distracted greater than 4？cm (157？N Click-x or 138？N Pangea). The exfix had higher peak torque and torsional stiffness than all pedicle systems. In torsion, the yield strengths were the same for all constructs. Conclusion. The infix device constructed with polyaxial or monoaxial pedicle screws is stiffer than the 2 pin external fixator in distraction testing. In extreme cases, the use of reinforcement or monoaxial systems which do not fail even at 360？N is a better option. In torsional testing, the 2 pin external fixator is stiffer than the pedicle screw systems. 1. Introduction A technique of subcutaneous anterior pelvic fixation for anterior pelvic ring fractures has been recently reported and has been termed “infix” [1]. It involves two supra-acetabular pins [2–5] and a subcutaneous rod, tunneled under the skin, at the top of the “bikini area” [6, 7]. In a multicenter study, infix has been shown to be effective in the treatment of pelvic fractures when combined with the appropriate posterior fixation. It has good patient tolerance, avoids the traditional complications of external fixation [8], and is useful to reduce pelvic injuries [9, 10]. When constructed with traditional polyaxial pedicle screws, infix is biomechanically as effective at posterior SI compression as a femoral distracter [11] and stiffer than a traditional 2 pin anterior external fixator in single stance gait testing in synthetic pelvic models [11, 12]. Pedicle screw implants use a rod screw construct which can be made with monoaxial screws where the head of the pedicle screw is immobile or polyaxial screws where the head of the pedicle screw can rotate in several directions. Polyaxial screws allow surgeons maneuverability when applying these devices, so that the screw heads do not have to line up exactly to attach the rod. The maneuverability of these screws is at the expense of some strengths of the