Difference in Spinal Fusion Process in Osteopenic and Nonosteopenic Living Rat Models Using Serial Microcomputed Tomography

Murine models of posterolateral spinal fusion: A systematic review

BACKGROUND

Rodent models are commonly used experimentally to assess treatment effectiveness in spinal fusion. Certain factors are associated with better fusion rates. The objectives of the present study were to report the protocols most frequently used, to evaluate factors known to positively influence fusion rate, and to identify new factors.

METHOD

A systematic literature search of PubMed and Web of Science found 139 experimental studies of posterolateral lumbar spinal fusion in rodent models. Data for level and location of fusion, animal strain, sex, weight and age, graft, decortication, fusion assessment and fusion and mortality rates were collected and analyzed.

RESULTS

The standard murine model for spinal fusion was male Sprague Dawley rats of 295g weight and 13 weeks' age, using decortication, with L4-L5 as fusion level. The last two criteria were associated with significantly better fusion rates. On manual palpation, the overall mean fusion rate in rats was 58% and the autograft mean fusion rate was 61%. Most studies evaluated fusion as a binary on manual palpation, and only a few used CT and histology. Average mortality was 3.03% in rats and 1.56% in mice.

CONCLUSIONS

These results suggest using a rat model, younger than 10 weeks and weighing more than 300 grams on the day of surgery, to optimize fusion rates, with decortication before grafting and fusing the L4-L5 level.

Pharmacological inhibition of DKK1 promotes spine fusion in an ovariectomized rat model

Osteoporosis is common in patients undergoing spine surgery, and carries a considerable risk of adverse outcomes. New methods to positively influence bone regeneration and spine fusion under osteoporotic conditions would be impactful. Neutralizing anti-Dickkopf-1 (DKK1) antibodies has been used as a bone anabolic agent, and recently reported by our group to aid in stem cell-mediated appendicular bone regeneration. Here, a small molecule designed as a DKK1 inhibitor, WAY-262611, was used to induce posterolateral spine fusion in an ovariectomized rat model. In vitro, pharmacological inhibition of DKK1 enhanced osteogenesis and Wnt signaling activity among rat bone marrow-derived stem/stromal cells (BMSCs). In vivo, systemic treatment with WAY-262611 promoted both chondrogenesis and osteogenesis within the spinal fusion site, and ultimately led to significant improvements in lumbar fusion as assessed by XR, μCT, histology and manual palpation assessments. No significant effect on osteoclast numbers or fusion site angiogenesis was detected, suggesting a primary direct effect on mesenchymal cells of the implantation site. Finally, evidence from human stem/stromal cells further demonstrated that pharmacologic inhibition of DKK1 promoted osteogenic differentiation in vitro. Taken together, our results suggest that targeting DKK1 promotes local bone formation and suggests potential clinical value for osteoporotic bone repair.

Depth of vertebral screw insertion using a cortical bone trajectory technique in lumbar spinal fusion: radiological significance of a long cortical bone trajectory

OBJECTIVE

Contrary to original cortical bone trajectory (CBT), "long CBT" directed more anteriorly in the vertebral body has recently been recommended because of improved screw fixation and load sharing within the vertebra. However, to the authors' knowledge there has been no report on the clinical significance of the screw length and screw insertion depth used with the long CBT technique. The aim of the present study was to investigate the influence of the screw insertion depth in the vertebra on lumbar spinal fusion using the CBT technique.

METHODS

A total of 101 consecutive patients with L4 degenerative spondylolisthesis who underwent single-level posterior lumbar interbody fusion at L4-5 using the CBT technique were included (mean follow-up 32.9 months). Screw loosening and bone fusion were radiologically assessed to clarify the factors contributing to these outcomes. Investigated factors were as follows: 1) age, 2) sex, 3) body mass index, 4) bone mineral density, 5) intervertebral mobility, 6) screw diameter, 7) screw length, 8) depth of the screw in the vertebral body (%depth), 9) facetectomy, 10) crosslink connector, and 11) cage material.

RESULTS

The incidence of screw loosening was 3.1% and bone fusion was achieved in 91.7% of patients. There was no significant factor affecting screw loosening. The %depth in the group with bone fusion [fusion (+)] was significantly higher than that in the group without bone fusion [fusion (-)] (50.3% ± 8.2% vs 37.0% ± 9.5%, respectively; p = 0.001), and multivariate logistic regression analysis revealed that %depth was a significant independent predictor of bone fusion. Receiver operating characteristic curve analysis identified %depth > 39.2% as a predictor of bone fusion (sensitivity 90.9%, specificity 75.0%).

CONCLUSIONS

This study is, to the authors' knowledge, the first to investigate the significance of the screw insertion depth using the CBT technique. The cutoff value of the screw insertion depth in the vertebral body for achieving bone fusion was 39.2%.