Combined anteroposterior spinal fixation provides superior stabilisation to a single anterior or posterior procedure

Abstract

Fusion is the main goal in the surgical management of the injured and unstable spine. A wide variety of implants is available to enhance this. Our study was performed to evaluate the stabilising characteristics of several anterior, posterior and combined systems of fixation. Six thoracolumbar (T11 to L2) spines from 13-week-old calves were first tested intact. Then the vertebral body of T13 was removed and the defect replaced and supported by a wooden block to simulate bone grafting. Dorsal implants consisting of a Universal Spine System (USS) fracture system and an AO Fixateur interne (AOFI), and ventral implants comprising of a Kaneda Classic, a Kaneda SR, a prototype of the VentroFix single clamp/single rod construct (SC/SR) and the VentroFix single clamp/double rod construct (SC/DR) were first implanted individually to stabilise the removal of the vertebral body. Simulating the combined anteroposterior stabilisations, all ventral implants were combined with the AOFI. The range of motion (ROM) was measured under loads of up to 7.5 Nm. The load was applied in a custom-made spine tester in the three primary directions while measuring the intervertebral movements using a goniometric linkage system. The dorsal systems limited ROM in flexion below 0.9 degrees and in extension between 3.3 degrees and 3.6 degrees (median values). The improved Kaneda System SR yielded a mean ROM of 1.8 degrees in flexion and in extension. The median rotation found with the VentroFix (SC/DR) was 3.2 degrees for flexion and 2.8 degrees for extension. Reinforcement of the ventral constructs with a dorsal system reduced the ROM in flexion and extension in all cases to 0.4 degrees and lower. In rotation, the median ROM of the anterior systems ranged from 2.7 degrees to 5.1 degrees and for the posterior systems from 3.9 degrees to 5.7 degrees, while the combinations provided a ROM of 1.2 degrees to 1.9 degrees. In lateral bending, the posterior implants restricted movement to 1.1 degrees, whereas the anterior implants allowed up to 5.2 degrees. The combined systems provided the highest stability at less than 0.6 degrees. Our study revealed distinct differences between posterior and anterior approaches in all primary directions. Also, different stabilisation characteristics were found within the anterior and posterior groups. Combinations of these two approaches provided the highest stability in all directions.