The maximum pull-out force of the sacral 1-2 translateral mass screw, an in vitro experiment

English Keywords:Sacrum  Lateral mass  Screw fixation  Biomechanics

English Abstract:

  【Abstract】 Objectives: To measure the maximum pull-out force of the sacral 1-2 translateral mass screw（STMS）, and to discuss the efficacy of this technique. Methods: Eight adult cadavers which were soaked and fixed in formalin liquid were used in this study. The bone mineral density(BMD) of S1 vertebra was measured by the dual-energy X-ray absorptiometry(DEXA). The average data of BMD were recorded. Three types of sacral screw were implanted into these specimens randomly. The experimental groups were divided as follows: group A(STMS fixation), group B(S1 pedicle screw fixation) and group C(S1 anterolateral screw fixation). Then the specimens were placed on a MTS 858 bionix testing machine, and a pull-out test was perfomed along the axis of the screw until the screw was completely pulled out of the sacrum. According to the experimental data, the curves of force(N)-displacement(mm) were drawed. The vertex of curves was taken as the maximum pull-out force. Then variance analysis was carried out by statistical software SPSS 16.0. Results: The BMD of 8 S1 vertebra was 0.43-0.74g/cm3(average 0.641±0.275g/cm3). The average maximum instant pull-out force for three fixations was 379.62±73.10N, 829.12±170.74N and 230.62±98.52N for group A, B and C respectively, with significant difference between each 2 groups(P＜0.05). Conclusions: The pull-out force of STMS is lower than that of S1 pedicle screw, but higher than that of the S1 anterolateral fixation. When S1 pedicle screw placement is unavailable, STMS instrumentation is an effective option.