PEDICLE FRACTURES OF C2, AND PULLOUT STRENGTH OF SUBSEQUENT SCREW FIXATION IN PORCINE VERTEBRAE

Abstract

In this biomechanical study of isolated porcine C2 vertebrae, antero-posterior forces were applied to potted specimens. Bipedicular fractures were generated in 25.4% of fifty-nine specimens (others fractured mostly through laminae). Bipedicular fractures were subsequently fixed under direct vision with trans-pedicular lag screws. These fixed specimens were then tested again. The mean ultimate tensile strength of the construct was 27.1% compared to the native bone’s fracture strength in the same specimen. This is a good model for hangman’s fracture, and is relevant to newer constructs that utilize C2 pedicle screws as part of a larger instrumentation procedure.

Hangman’s fracture rarely needs internal fixation. However, if fixation is indicated the fracture can sometimes be directly fixed with trans-pedicular screws rather than fusing C1-C2 and committing to a great loss of neck range of motion. To our knowledge, the actual pullout strength of pedicle screws in C2 is previously unreported. Hence, we developed a laboratory model of hangman’s fracture by applying antero-posterior tensile force in fifty-nine porcine specimens that were stripped of soft tissues. The failure strength of the bone averaged approximately 3200 N, and subsequent fixation averaged to 27.1% of this value. Regardless of pre-hoc expectations, these values indicate that the screw purchase is strong enough to consider continued use of the procedure in repair of hangman’s fracture, fixation to C2 of multi-level constructs, and so forth. Multiple anterior and posterior screw procedures have been tested in the past, but to our knowledge the pullout strength of C2 pedicle screws has not been examined. Because the usage of C2 pedicle screws is growing in popularity, this biomechanical information is pertinent to work in traumatic, degenerative, and reconstructive procedures. Some limitations of the present study are that the specimens were porcine rather than human, and that only fixation to single vertebrae was examined.

Funding: Dr Kaspar holds academic research grants from McMaster University and from The Physicians’ Services Incorporated (PSI) Foundation, the former of which was used to finance this project. There are no commercial grants or conflicts of interest.