The effects of magnesium particles in posterolateral spinal fusion: an experimental in vivo study in a sheep model.

Object: Magnesium has recently become a material of interest as a biocompatible and biodegradable implant metal. Authors of several reports have noted the potential bone-cell activating or bone-healing effect of high Mg ion concentrations. The classic method for achieving intertransverse process fusion involves using an autologous iliac crest bone graft. Several studies have been performed to investigate enhancement of this type of autograft fusion. To the authors' knowledge, no research has been conducted in which the efficacy of pure Mg particles in posterolateral spinal fusion has been investigated. The objective of this study was to determine whether Mg particles enhance the effectiveness of intertransverse process lumbar fusion in a sheep model.

Methods: Sixteen skeletally mature female sheep were subjected to intertransverse process spinal fusions with pedicle screw fixation at L2-3 and L5-6. Each animal was given a 5-cm3 bone autograft at one fusion level, and a combined 5-cm3 bone autograft with the addition of 1 cm3 Mg at the other level. Six months after surgery, bone formation was evaluated by gross inspection and palpation, and by radiological, histological, scanning electron microscopic, and x-ray diffraction analyses. Radiological results were graded from 0 to 4 according to the status of the bridging bone, which was determined by evaluating both x-ray films and computed tomography scans. The quality of the spinal fusion was assigned a histological score of 0 to 7, in which a score of 0 represented an empty cleft and a score of 7 represented complete bridging of bone between the transverse processes. The trabecular bone formation at each fusion level and the Ca hydroxyapatite crystalline structure in core biopsy specimens were evaluated using scanning electron microscopy and x-ray diffraction analyses, respectively. The rate of rigid bone fusion, according to both palpation and radiological assessment, in the combined Mg and autologous bone treatment group was higher (81.25%) than in the autograft bone treatment group (62.5%), but this difference was not statistically significant. The quality of bone fusion, according to the histological grading system and scanning electron microscopy inspection, was higher in the bone fusion segments of the Mg and autologous graft combined group than in the group with autograft-only arthrodesis, and this difference was statistically significant. The x-ray diffraction analyses further confirmed the effect of Mg in promoting the formation of the crystalline portion of the bone (hydroxyapatite).

Conclusions: Based on the results of this study, adding Mg particles to autologous corticocancellous bone in a posterolateral intertransverse process fusion enhances the quality of bone formation. However, radiological findings did not reveal a statistically significant effect of Mg on the rate of solid bone fusion formation between the two transverse processes.