Variables Affecting Fusion Rates in the Rat Posterolateral Spinal Fusion Model with Autogenic/Allogenic Bone Grafts: A Meta-analysis

The effect of cannabinoids on single-level lumbar arthrodesis outcomes in a rat model

BACKGROUND CONTEXT

The opioid epidemic is a public health crisis affecting spine care and pain management. Medical marijuana is a potential nonopioid analgesic yet to be studied in the surgical setting since its effects on bone healing are not fully understood. Studies have demonstrated analgesic and potentially osteoinductive properties of cannabinoids with endocannabinoid receptor expression in bone tissue.

PURPOSE

We hypothesize that tetrahydrocannabinol (THC) and cannabidiol (CBD) will not decrease bone healing in spinal fusion.

STUDY DESIGN

Seventy-eight adult Sprague-Dawley rats were used for this study. Utilizing allogenic bone grafts (6 donor rats), posterolateral inter-transverse lumbar fusion at the L4-L5 level was performed. The animals were equally divided into four treatment groups, each receiving 0.1 ml intraperitoneal injections weekly as follows: placebo (saline), 5 mg/kg THC, 5 mg/kg CBD, and a combination of 5 mg/kg THC and 5mg/kg CBD (Combo).

METHODS

Callus tissue was harvested 2- and 8-weeks postsurgery for qPCR assessment to quantify changes in the expression of osteogenic genes. Manual palpation was done to assess the strength of the L4-L5 arthrodesis on all rats. μCT image-based callus analysis and histology were performed. One-way ANOVA followed by post hoc comparisons was performed.

RESULTS

μCT demonstrated no significant differences. Treatment groups had slightly increased bone volume and density compared to control. qPCR at 2 weeks indicated downregulated RANKL/OPG ratios skewing towards osteogenesis in the CBD group, with the THC and CBD+THC groups demonstrating a downward trend (p>.05). ALPL, BMP4, and SOST were significantly higher in the CBD group, with CTNNB1 and RUNX2 also showing an upregulating trend. The CBD group showed elevation in Col1A1 and MMP13. Data at eight weeks showed ALPL, RUNX2, BMP4, and SOST were downregulated for all treatment groups. In the CBD+THC group, RANK, RANKL, and OPG were downregulated. OPG downregulation reached significance for the THC and CBD+THC group compared to saline. Interestingly, the RANKL/OPG ratio showed upregulation in the CBD and CBD+THC groups. RANKL showed upregulation in the CBD group. At 2 and 8 weeks, the CBD treatment group showed superior histological progression, increasing between time points.

CONCLUSION

This study demonstrates that CBD and THC have no adverse effect on bone healing and the rate of spinal fusion in rats. Osteogenic factors were upregulated in the CBD-treated groups at 2 weeks, which indicates a potential for bone regeneration. In this group, compared to control, the RANKL/OPG ratio at the early healing phase demonstrates the inhibition of osteoclast differentiation, enhancing bone formation. Interestingly, it shows promoted osteoclast differentiation at the later healing phase, enhancing bone remodeling. This aligns with the physiological expectation of a lower ratio in the early phases and a higher ratio in the later remodeling phases.

CLINICAL SIGNIFICANCE

CBD and THC showed no inhibitory effects on bone healing in a spinal fusion model. Moreover, histologic and gene expression analysis demonstrated that CBD may, in fact, enhance bone healing. Further research is needed to confirm the safe usage of THC and CBD in the postoperative setting following spinal fusions.

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.

Blockade of Osteoclast-Mediated Bone Resorption With a RANKL-Inhibitor Enhances Bone Formation in a Rat Spinal Fusion Model

STUDY DESIGN

Rat spine fusion model.

OBJECTIVE

The present study aimed to determine whether administration of osteoprotegerin (OPG) in a rat model of spinal fusion increases bone volume, bone density, and decreases osteoclasts in the fusion mass.

SUMMARY OF BACKGROUND DATA

OPG is a soluble RANK-ligand inhibitor that blocks osteoclast differentiation and activation. This makes it a potential agent to control the remodeling process and enhance bone mass during spinal fusion.

MATERIALS AND METHODS

Forty-eight male Sprague-Dawley rats received a one-level spinal fusion of L4-L5 with bone allograft. Rats were then divided into four groups according to initiation of treatment: (1) saline on day 0 (saline), (2) OPG on day 0 (OPG D0), (3) OPG on day 10 (OPG D10), and (4) OPG on day 21 (OPG D21) postsurgery. After their initial injection, rats received weekly subcutaneous injections of OPG (10 mg/kg) and were euthanized six weeks postsurgery. MicroCT analysis of the fusion site and histological analysis of bone surface for quantification of osteoclast lining was performed.

RESULTS

Increased bone volume in the fusion site and around the spinous process was seen in OPG D0 and OPG D10 when compared with saline. Mean trabecular thickness was greater in all groups receiving OPG compared with saline, with OPG D0 and OPG D10 having significantly greater mean trabecular thickness than OPG D21. All OPG groups had less bone surface lined with osteoclasts when compared with Saline, with OPG D0 and OPG D10 having fewer than OPG D21.

CONCLUSIONS

This study indicates that OPG inhibited osteoclast bone resorption, which led to greater bone at the fusion site. Future studies investigating OPG on its own or in combination with an osteogenic factor to improve spinal fusion outcomes are warranted to further elucidate its potential therapeutic effect.

Effect of Local Tranexamic Acid on the Quality of Bone Healing in a Rat Spinal Fusion Model

Introduction

The use of the antifibrinolytic agent tranexamic acid has positive effects on bleeding control, but our knowledge is still limited regarding how fibrinolysis suppression changes the process of bone formation and the quality of bone. Because of the several side effects of systemic tranexamic acid, topical usage has been established in several procedures. This study aimed to investigate the effect of local tranexamic acid on vertebral fusion by using macroscopic, radiologic, and microscopic techniques. We also attempted to determine the safe dose range in case some doses had negative effects on fusion.

Methods

Twenty-eight Wistar albino rats underwent intertransverse fusion. All rats were randomized into four groups: groups treated with local tranexamic acid doses of 1 mg/kg (D1), 10 mg/kg (D10), and 100 mg/kg (D100) and the control group with no drug (D0). At the end of the eighth week, all rats were sacrificed for evaluation in terms of palpation, mammography, and histopathologic analysis.

Results

The manual palpation results presented with lower fusion rates in D10 and D100 groups than in the control group. Radiological examination results were significantly higher in the control group. The histopathologic examination revealed no significant differences between groups in the percent of new bone formation.

Conclusions

Our results showed that local administration of tranexamic acid reduced the quality and stability of fusion without a delay in bone formation. However, doses of 1 mg/kg did not reduce the stability in the palpation test. Our findings suggest that 1 mg/kg dose is a critical threshold above which tranexamic acid reduced the bone healing process of fusion and that surgeons should consider the doses of local tranexamic acid during surgery.

Human perivascular stem cells prevent bone graft resorption in osteoporotic contexts by inhibiting osteoclast formation

The vascular wall stores mesenchymal progenitor cells which are able to induce bone regeneration, via direct and paracrine mechanisms. Although much is known regarding perivascular cell regulation of osteoblasts, their regulation of osteoclasts, and by extension utility in states of high bone resorption, is not known. Here, human perivascular stem cells (PSCs) were used as a means to prevent autograft resorption in a gonadectomy-induced osteoporotic spine fusion model. Furthermore, the paracrine regulation by PSCs of osteoclast formation was evaluated, using coculture, conditioned medium, and purified extracellular vesicles. Results showed that PSCs when mixed with autograft bone induce an increase in osteoblast:osteoclast ratio, promote bone matrix formation, and prevent bone graft resorption. The confluence of these factors resulted in high rates of fusion in an ovariectomized rat lumbar spine fusion model. Application of PSCs was superior across metrics to either the use of unpurified, culture-defined adipose-derived stromal cells or autograft bone alone. Under coculture conditions, PSCs negatively regulated osteoclast formation and did so via secreted, nonvesicular paracrine factors. Total RNA sequencing identified secreted factors overexpressed by PSCs which may explain their negative regulation of graft resorption. In summary, PSCs reduce osteoclast formation and prevent bone graft resorption in high turnover states such as gonadectomy-induced osteoporosis.

Comparing the efficacy of syngeneic iliac and femoral allografts with iliac crest autograft in a rat model of lumbar spinal fusion

BACKGROUND

Despite widespread use of femoral-sourced allografts in clinical spinal fusion procedures and the increasing interest in using femoral reamer-irrigator-aspirator (RIA) autograft in clinical bone grafting, few studies have examined the efficacy of femoral grafts compared to iliac crest grafts in spinal fusion. The objective of this study was to directly compare the use of autologous iliac crest with syngeneic femoral and iliac allograft bone in the rat model of lumbar spinal fusion.

METHODS

Single-level bilateral posterolateral intertransverse process lumbar spinal fusion surgery was performed on Lewis rats divided into three experimental groups: iliac crest autograft, syngeneic iliac crest allograft, and syngeneic femoral allograft bone. Eight weeks postoperatively, fusion was evaluated via microCT analysis, manual palpation, and histology. In vitro analysis of the colony-forming and osteogenic capacity of bone marrow cells derived from rat femurs and hips was also performed to determine whether there was a correlation with the fusion efficacy of these graft sources.

RESULTS

Although no differences were observed between groups in CT fusion mass volumes, iliac allografts displayed an increased number of radiographically fused fusion masses and a higher rate of bilateral fusion via manual palpation. Histologically, hip-derived grafts showed better integration with host bone than femur derived ones, likely associated with the higher concentration of osteogenic progenitor cells observed in hip-derived bone marrow.

CONCLUSIONS

This study demonstrates the feasibility of using syngeneic allograft bone in place of autograft bone within inbred rat fusion models and highlights the need for further study of femoral-derived grafts in fusion.

Comparing cellular bone matrices for posterolateral spinal fusion in a rat model

INTRODUCTION

Cellular bone matrices (CBM) are allograft products that provide three components essential to new bone formation: an osteoconductive scaffold, extracellular growth factors for cell proliferation and differentiation, and viable cells with osteogenic potential. This is an emerging technology being applied to augment spinal fusion procedures as an alternative to autografts.

METHODS

We aim to compare the ability of six commercially-available human CBMs (Trinity ELITE®, ViviGen®, Cellentra®, Osteocel® Pro, Bio4® and Map3®) to form a stable spinal fusion using an athymic rat model of posterolateral fusion. Iliac crest bone from syngeneic rats was used as a control to approximate the human gold standard. The allografts were implanted at L4-5 according to vendor specifications in male athymic rats, with 15 rats in each group. MicroCT scans were performed at 48 hours and 6 weeks post-implantation. The rats were euthanized 6 weeks after surgery and the lumbar spines were harvested for X-ray, manual palpation and histology analysis by blinded reviewers.

RESULTS

By manual palpation, five of 15 rats of the syngeneic bone group were fused at 6 weeks. While Trinity ELITE had eight of 15 and Cellentra 11 of 15 rats with stable fusion, only 2 of 15 of ViviGen-implanted spines were fused and zero of 15 of the Osteocel Pro, Bio4 and Map3 produced stable fusion. MicroCT analysis indicated that total bone volume increased from day 0 to week 6 for all groups except syngeneic bone group. Trinity ELITE (65%) and Cellentra (73%) had significantly greater bone volume increases over all other implants, which was consistent with the histological analysis.

CONCLUSION

Trinity ELITE and Cellentra were significantly better than other implants at forming new bone and achieving spinal fusion in this rat model at week 6. These results suggest that there may be large differences in the ability of different CBMs to elicit a successful fusion in the posterolateral spine.

Effects of Intraoperative Intrawound Antibiotic Administration on Spinal Fusion: A Comparison of Vancomycin and Tobramycin in a Rat Model

BACKGROUND

Local, intrawound use of antibiotic powder, such as vancomycin and tobramycin, in spinal fusion surgery has become an increasingly common prophylactic measure in an attempt to reduce rates of postsurgical infection. However, the effects of localized antibiotic delivery on fusion remain unclear. The objective of this study was to examine the in vivo effects of intraoperative local delivery of 2 antibiotics commonly used in bone-grafting surgery on spinal fusion outcomes in a rat model.

METHODS

Single-level (L4-L5), bilateral posterolateral intertransverse process lumbar fusion surgery was performed on 60 female Lewis rats (6 to 8 weeks of age) using syngeneic iliac crest allograft mixed with clinical bone-graft substitute and varying concentrations of antibiotics (n = 12 each): (1) control without any antibiotics, (2) low-dose vancomycin (14.3 mg/kg), (3) high-dose vancomycin (71.5 mg/kg), (4) low-dose tobramycin (28.6 mg/kg), and (5) high-dose tobramycin (143 mg/kg). Eight weeks postoperatively, fusion was evaluated via micro-computed tomography (µCT), manual palpation, and histological analysis, with blinding to treatment group. In the µCT analysis, fusion-mass volumes were measured for each rat. Each spine specimen (L4-L5) was rated (manual palpation score) on a scale of 2 to 0 (2 = fused, 1 = partially fused, and 0 = non-fused).

RESULTS

The mean fusion-mass volume on µCT (mm) was as follows: control, 29.3 ± 6.2; low-dose vancomycin, 26.3 ± 8.9; high-dose vancomycin, 18.8 ± 7.9; low-dose tobramycin, 32.7 ± 9.0; and high-dose tobramycin, 43.8 ± 11.9 (control versus high-dose vancomycin, p < 0.05; and control versus high-dose tobramycin, p < 0.05). The mean manual palpation score for each group was as follows: control, 1.46 ± 0.58; low-dose vancomycin, 0.86 ± 0.87; high-dose vancomycin, 0.68 ± 0.62; low-dose tobramycin, 1.25 ± 0.71; and high-dose tobramycin, 1.32 ± 0.72 (control versus high-dose vancomycin, p < 0.05). The histological analyses demonstrated a similar trend with regard to spinal fusion volume.

CONCLUSIONS

Intraoperative local application of vancomycin, particularly at a supraphysiological dosage, may have detrimental effects on fusion-mass formation. No inhibitory effect of tobramycin on fusion-mass formation was observed.

CLINICAL RELEVANCE

When spine surgeons decide to use intraoperative intrawound antibiotics in spinal fusion surgery, they should weigh the reduction in surgical site infection against a possible inhibitory effect on fusion.

Effect of the type of electrical stimulation on spinal fusion in a rat posterolateral spinal fusion model

BACKGROUND CONTEXT

Posterolateral fusion (PLF) with autogenous iliac bone graft is one of the most common surgical procedures for lumbar spinal disease. However, its limited success demands new biologically competent graft enhancers or substitutes. Although the use of direct current (DC) electrical stimulation has been shown to increase rate of successful spinal fusions, little is known about the effect of the type of current in DC stimulation.

PURPOSE

To evaluate the effects of various DC stimulators on the strength and success rate of posterolateral fusion facilitated by using a nitinol mesh container, in rats.

STUDY DESIGN

This was an experimental animal study.

METHODS

A conductive, tubular nitinol mesh container was used to carry small pieces of bone grafts. The nitinol mesh container received electrical stimulation via a lead that connected the container to different types of DC stimulators. Sixty male Sprague-Dawley rats were divided into three groups (N=20 in each): a control group that underwent PLF with a nitinol container filled with autograft, a constant DC group that received a nitinol container and constant DC (100 μA), and a pulsed DC group that received a nitinol container and pulsed DC (100 μA, 100 Hz, 200 μs). The rats underwent PLF between L4 and L5, and transverse processes were grafted with bilateral iliac grafts. A stimulator was implanted subcutaneously. The rats were sacrificed 8 weeks postsurgery, and lumbar spines were removed. Spinal fusion was evaluated by microcomputed tomography, manual testing, biomechanical testing, histologic examination, and molecular analysis.

RESULTS

All animals in the DC stimulation groups displayed solid fusion, whereas only 70% of control animals showed solid fusion. Radiographic images, biomechanical testing, histologic examination, and molecular analysis revealed improved fusion in the order control group<constant DC group<pulsed DC group. The volume of new bone mass was significantly higher in the pulsed DC group (p<.05). Fusion was more solid in the pulsed DC group than in control group (p<.05). The pulsed DC group displayed the lowest inflammatory responses.

CONCLUSIONS

Pulsed DC electrical stimulation is efficacious in improving both strength and fusion rate in a rat spinal fusion model. In addition, tubular nitinol mesh, made of conductive suture, appears useful for holding small pieces of bone grafts and maintaining a good environment for bone fusion.

CLINICAL RELEVANCE

Pulsed DC electrical stimulation may be potentially useful to increase the fusion rate after spinal fusion in humans. Future research is required to evaluate the safety and efficacy of tubular nitinol mesh and pulsed DC electrical stimulation in humans.