Odontoid process type II and III fracture fixation using bone allograft screws versus cannulated screws: a biomechanical study

Abstract

Introduction

Fractures of the odontoid process are associated with high non-union rates, challenging treatment, and high incidence of screw-related complications. The aim of this study was to compare the biomechanical competence of a single biointegrative bone allograft screw versus two conventional cannulated screws for odontoid fracture fixation.

Materials and methods

The odontoid process of intact C2 vertebral specimens was subjected to quasi-static loading until fracture. Specimens with an Anderson and d’Alonzo type II or III fracture (n = 47) were fixated with either two conventional cannulated screws or with a single bone allograft screw. The constructs were biomechanically tested to failure in the same fashion as in their intact state. Stiffness, yield, and ultimate load were evaluated. The results were adjusted by age, sex, volumetric bone mineral density (vBMD), and the cross-sectional area ratio of cortical bone to total bone measured at the junction of the odontoid process with the vertebral body (Ct.Ar/Tt.Ar).

Results

Stiffness, yield and ultimate load were restored in the cannulated screws group by 44 ± 10%, 46 ± 7%, and 46 ± 5% and in the bone allograft group by 50 ± 12%, 30 ± 9%, and 34 ± 6% (mean ± SE). There were no significant differences between the groups regarding the three mechanical outcomes (0.104 ≤ p ≤ 0.223). Positive significant relation was found between vBMD and stiffness in each group (0.248 ≤ R²≤0.273, 0.018 ≤ p ≤ 0.038), as well as between Ct.Ar/Tt.Ar and stiffness (R²=0.218, p = 0.033), vBMD and ultimate load (R²=0.430, p = 0.001) and ultimate loadand vBMD (R²=0.315, p = 0.010) in the cannulated screws group.

Conclusions

The primary stability of odontoid fracture fixation is determined mainly by the quality of the local bone and independent of the fixation technique. From the biomechanical perspective, the lower mean values for the yield and ultimate load restored in the bone allograft group compared to the cannulated screws group should be compensated by lower peak load during the patient’s rehabilitation process.