Biomechanical study and digital modeling of traction resistance in posterior thoracic implants.

ABSTRACT

STUDY DESIGN: Posterior implants were used to test the resistance of seven human thoracic spines to traction, and results were compared with those of a digital study. OBJECTIVES: To compare the use of hook and screw fixation methods for posterior thoracic surgery in a study supplemented by a digital analysis using the finite-element method. SUMMARY OF BACKGROUND DATA The maximum break strength of pedicular screws on bone has already been studied several times. Far fewer studies have investigated the stresses that may be applied to pedicular and laminar hooks or compared these hooks with pedicular screws in the thorax. METHODS: For this study, seven human thoracic spines were used, identifying 49 groups of two vertebrae each. The assemblies used a bolt in the medullary cavity. The central diameter of this bolt could be varied using tubes to suit the individual medullary cavity, spreading the stresses over the entire centrum during traction. The assembly was placed on an Instron machine. Vertical traction was applied up to the maximum break strength, that is, the strength recorded at the top of the traction curve read out directly on the Instron machine. Traction speed in all the tests was 5 mm per minute. Four pedicular screws and two pedicular-laminar clips were used alternately. In these tests, 25 values were obtained with hooks and 24 with screws. The group with screw fixation was tested in two halves, with 12 tests using 4-mm diameter screws and 12 tests using 5-mm diameter screws. For the digital study, the vertebra model comprised nearly 63,000 nodes and 14,000 elements. Elastic field calculations were carried out using the Abaqus finite-element calculation method. RESULTS: Each time pedicular-laminar traction was used, the pedicles broke at the base. Where screws were used, a medial fissure was found at the base of the pedicle. For the hooks, the maximum break strength was 1150 +/- 388 N. It was 820 +/- 418 N when 4-mm diameter screws were used and 1395 +/- 435 N when 5-mm screws were used. The most fragile vertebrae were found to be at T5-T6 and T7-T8. The screw-instrumented model showed that stresses were concentrated at the medial part of the pedicle, inside the medullary cavity. Use of a long screw did not lead to noticeable stress reduction. The hook-instrumented model showed that the stresses were greatest in the lower part of the pedicles. CONCLUSIONS: Screw fixation is the most appropriate from a mechanical point of view. During tests, however, screw fixation falls short of its promises. This leads to the hypothesis that in most cases screws pull out because the bony anchoring of the pedicle screwing fails. Screws are less effective if their anchoring in the pedicles cannot be guaranteed, which is probably the reason for their relative weakness. Screw diameter should be chosen according to the size of each pedicle. The findings showed that hooks impose additional stresses on the vertebrae.