Assessing the stiffness of spinal fusion in animal models

Biomechanical consequences of cement discoplasty: An in vitro study on thoraco-lumbar human spines

With the ageing of the population, there is an increasing need for minimally invasive spine surgeries to relieve pain and improve quality of life. Percutaneous Cement Discoplasty is a minimally invasive technique to treat advanced disc degeneration, including vacuum phenomenon. The present study aimed to develop an in vitro model of percutaneous cement discoplasty to investigate its consequences on the spine biomechanics in comparison with the degenerated condition. Human spinal segments (n = 27) were tested at 50% body weight in flexion and extension. Posterior disc height, range of motion, segment stiffness, and strains were measured using Digital Image Correlation. The cement distribution was also studied on CT scans. As main result, percutaneous cement discoplasty restored the posterior disc height by 41% for flexion and 35% for extension. Range of motion was significantly reduced only in flexion by 27%, and stiffness increased accordingly. The injected cement volume was 4.56 ± 1.78 ml (mean ± SD). Some specimens (n = 7) exhibited cement perforation of one endplate. The thickness of the cement mass moderately correlated with the posterior disc height and range of motion with different trends for flexions vs. extension. Finally, extreme strains on the discs were reduced by percutaneous cement discoplasty, with modified patterns of the distribution. To conclude, this study supported clinical observations in term of recovered disc height close to the foramen, while percutaneous cement discoplasty helped stabilize the spine in flexion and did not increase the risk of tissue damage in the annulus.

Abaloparatide Enhances Fusion and Bone Formation in a Rabbit Spinal Arthrodesis Model

STUDY DESIGN

Prospective randomized placebo controlled animal trial.

OBJECTIVE

Determine the effect of daily subcutaneous abaloparatide injection on the intervertebral fusion rate in rabbits undergoing posterolateral fusion.

STUDY OF BACKGROUND DATA

Despite the wide utilization of spine fusion, pseudarthrosis remains prevalent, and results in increased morbidity. Abaloparatide is a novel analog of parathyroid hormone-related peptide (1-34) and has shown efficacy in a rat posterolateral spine fusion model to increase fusion rates. The effect of abaloparatide on the fusion rate in a larger animal model remains unknown.

MATERIALS AND METHODS

A total of 24 skeletally mature New Zealand White male rabbits underwent bilateral posterolateral spine fusion. Following surgery, the rabbits were randomized to receive either saline as control or abaloparatide subcutaneous injection daily. Specimens underwent manual assessment of fusion, radiographic analysis with both x-ray and high-resolution peripheral quantitative computed tomography, and biomechanical assessment.

RESULTS

Rabbits that received abaloparatide had a 100% (10/10) fusion rate compared with 45% (5/11) for controls ( P <0.02) as assessed by manual palpation. Radiographic analysis determined an overall mean fusion score of 4.17±1.03 in the abaloparatide group versus 3.39±1.21 for controls ( P <0.001). The abaloparatide group also had a greater volume of bone formed with a bone volume of 1209±543 mm 3 compared with 551±152 mm 3 ( P <0.001) for controls. The abaloparatide group had significantly greater trabecular bone volume fraction and trabecular thickness and lower specific bone surface and connectivity density in the adjacent levels when compared with controls. Abaloparatide treatment did not impact trabecular number or separation. There were no differences in biomechanical testing in flexion, extension, or lateral bending ( P >0.05) between groups.

CONCLUSIONS

Abaloparatide significantly increased the fusion rate in a rabbit posterolateral fusion model as assessed by manual palpation. In addition, there were marked increases in the radiographic evaluation of fusion.

Testing the impact of discoplasty on the biomechanics of the intervertebral disc with simulated degeneration: An in vitro study

Percutaneous Cement Discoplasty has recently been developed to relieve pain in highly degenerated intervertebral discs presenting a vacuum phenomenon in patients that cannot undergo major surgery. Little is currently known about the biomechanical effects of discoplasty. This study aimed at investigating the feasibility of modelling empty discs and subsequent discoplasty surgery and measuring their impact over the specimen geometry and mechanical behaviour. Ten porcine lumbar spine segments were tested in flexion, extension, and lateral bending under 5.4 Nm (with a 200 N compressive force and a 27 mm offset). Tests were performed in three conditions for each specimen: with intact disc, after nucleotomy and after discoplasty. A 3D Digital Image Correlation (DIC) system was used to measure the surface displacements and strains. The posterior disc height, range of motion (ROM), and stiffness were measured at the peak load. CT scans were performed to confirm that the cement distribution was acceptable. Discoplasty recovered the height loss caused by nucleotomy (p = 0.04) with respect to the intact condition, but it did not impact significantly either the ROM or the stiffness. The strains over the disc surface increased after nucleotomy, while discoplasty concentrated the strains on the endplates. In conclusion, this preliminary study has shown that discoplasty recovered the intervertebral posterior height, opening the neuroforamen as clinically observed, but it did not influence the spine mobility or stiffness. This study confirms that this in vitro approach can be used to investigate discoplasty.

Comparative Efficacy of Commonly Available Human Bone Graft Substitutes as Tested for Posterolateral Fusion in an Athymic Rat Model

Background

Insufficient data exist on bone graft substitute materials efficacy; two thirds lack any clinical data.1,2 This prospective animal study identified efficacy differences among commercially available materials of several classes.

Methods

Historically validated muscle pouch osteoinduction study (OIS) and posterolateral fusion (PLF) were performed in an athymic rat model. Grafting material products implanted were demineralized bone matrix (DBM)-based allografts (Accell EVO3, DBX Mix, DBX Strip, Grafton Crunch, Grafton Flex, Grafton Matrix, Grafton Putty, Magnifuse, and Progenix Plus), allografts (OsteoSponge, MinerOss), cellular allograft (Osteocel Plus), ceramics (Mozaik Strip), or activated ceramics (Actifuse ABX Putty, Vitoss BA). After 4 weeks, OIS specimens were evaluated ex vivo by histologic osteoinductivity. After 8 weeks, PLF ex vivo specimens were evaluated for fusion by manual palpation (F_MP), radiography (F_XR), and histology (F_HISTO).

Results

OIS: No materials exhibited a rejection reaction on histology. All DBM-based materials exhibited osteoinductive potential as new bone formation at > 88% of implanted sites. One plain allograft (OsteoSponge) formed bone at 25% of sites. No bone formed for one ceramic (Mozaik Strip), three activated ceramics (Actifuse ABX Putty), or one cellular allograft, regardless of human bone marrow aspirate (hBMA) when added. PLF: Among the 10 DBMs, 6 had F_MP of 100% (Accell EVO3, DBX Mix, DBX Strip, Grafton Flex, Grafton Putty, Magnifuse), 2 had F_MP of 94% (Grafton Crunch, Grafton Matrix), and 2 conditions had F_MP of 0% (Progenix Plus, Progenix Plus + athymic rat iliac crest bone graft [arICBG]). Ceramics (Mozaik Strip), activated ceramics (Actifuse ABX Putty, Vitoss BA), plain allograft (OsteoSponge, MinerOss (PLF study), and cellular allograft (Osteocel Plus) demonstrated 0% F_MP. ArICBG demonstrated 13% F_MP.

Conclusions

Eight DBM-based materials (Accell EVO3, DBX Mix, DBX Strip, Grafton Crunch, Grafton Flex, Grafton Matrix, Grafton Putty, Magnifuse) demonstrated excellent (> 90% F_MP) efficacy in promoting fusion via bone healing. Two DBM conditions (Progenix Plus, Progenix Plus + arICBG) showed no manual palpation fusion (F_MP). Systematically, over the 2 studies (OIS and PLF), cellular (Osteocel Plus), plain allografts (OsteoSponge, MinerOss; PLF study), ceramic (Mozaik Strip), and activated ceramics (Actifuse ABX Putty, Vitoss BA) demonstrated poor F_MP efficacy (< 10%).

Clinical Relevance

When selecting DBMs, clinicians must be cognizant of variability in DBM efficacy by product and lot. While theoretically osteoinductive, cellular allograft and activated ceramics yielded poor in vivo efficacy. Whole allograft and ceramics may provide osteoconductive scaffolding for mixed-material grafting; however, surgeons should be cautious in using them alone. Direct clinical data are needed to establish efficacy for any bone graft substitute.

Assessment of a rabbit posterolateral spinal fusion using movement between vertebrae: a modification of the palpation exam for quantifying fusions

Background

Manual palpation of rabbit spine levels has been used to assess fusion status. This method of testing is subject to inter-observer differences in assessment. We attempted to quantify fusion based on the amount of movement between rabbit vertebrae at the level of fusion.

Methods

Rabbits were divided into three groups. The first underwent a sham surgery; the second underwent a unilateral spinal fusion; and the third underwent a bilateral spinal fusion. All groups were sacrificed at either 5- or 10-week post-procedure. Each spine was tested for fusion using standard manual palpation techniques. The spines were also placed on a specially designed apparatus and moved through 10°, 20°, and 30° of extension/flexion.

Results

Out of 10 rabbits, 2 underwent sham surgery, 2 underwent a fusion procedure at L4-L5 and 6 underwent a fusion at L5-L6. We only included rabbits that underwent a L5-L6 fusion surgery. Our apparatus did not always rotate the spine the intended amount with up to 30% error. When rabbits graded as fused were compared to sham rabbits, there was a trend towards reduction in percent of overall measured angle within the fused group as compared to the sham group (8.77% vs. 13.84%, P=0.14).

Conclusions

Our model attempted to quantify the amount of displacement between vertebrae during the manual palpation exam. There is a trend towards reduced measured angle between vertebrae between fused and non-fused spines and no statistically significant difference in overall measured angle between unilaterally and bilaterally fused spines.

Biomimetic hydroxyapatite/collagen composite drives bone niche recapitulation in a rabbit orthotopic model

Synthetic osteoinductive materials that mimic the human osteogenic niche have emerged as ideal candidates to address this area of unmet clinical need. In this study, we evaluated the osteoinductive potential in a rabbit orthotopic model of a magnesium-doped hydroxyapatite/type I collagen ​(MHA/Coll) composite. The composite was fabricated to exhibit a highly fibrous structure of carbonated MHA with 70% (±2.1) porosity and a Ca/P ratio of 1.5 (±0.03) as well as a diverse range of elasticity separated to two distinct stiffness peaks of low (2.35 ​± ​1.16 ​MPa) and higher (9.52 ​± ​2.10 ​MPa) Young's Modulus. Data suggested that these specific compositional and nanomechanical material properties induced the deposition of de novo mineral phase, while modulating the expression of early and late osteogenic marker genes, in a 3D in vitro model using human bone marrow-derived mesenchymal stem cells (hBM-MSCs). When tested in the rabbit orthotopic model, MHA/Col1 scaffold induction of new trabecular bone mass was observed by DynaCT scan, only 2 weeks after implantation. Bone histomorphometry at 6 weeks revealed a significant amount of de novo bone matrix formation. qPCR demonstrated MHA/Coll scaffold full cellularization in vivo and the expression of both osteogenesis-associated genes (Spp1, Sparc, Col1a1, Runx2, Dlx5) as well as hematopoietic (Vcam1, Cd38, Sele, Kdr) and bone marrow stromal cell marker genes (Vim, Itgb1, Alcam). Altogether, these data provide ​evidence of the solid osteoinductive potential of MHA/Coll and its suitability for multiple approaches of bone regeneration.

Preclinical models for orthopedic research and bone tissue engineering

In this review, we broadly define and discuss the preclinical rodent models that are used for orthopedics and bone tissue engineering. These range from implantation models typically used for biocompatibility testing and high-throughput drug screening, through to fracture and critical defect models used to model bone healing and severe orthopedic injuries. As well as highlighting the key methods papers describing these techniques, we provide additional commentary based on our substantive practical experience with animal surgery and in vivo experimental design. This review also briefly touches upon the descriptive and functional outcome measures and power calculations that are necessary for an informative study. Obtaining informative and relevant research outcomes can be very dependent on the model used, and we hope this evaluation of common models will serve as a primer for new researchers looking to undertake preclinical bone studies. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:832-840, 2018.

Four-point bending as a method for quantitatively evaluating spinal arthrodesis in a rat model

The most common method of evaluating the success (or failure) of rat spinal fusion procedures is manual palpation testing. Whereas manual palpation provides only a subjective binary answer (fused or not fused) regarding the success of a fusion surgery, mechanical testing can provide more quantitative data by assessing variations in strength among treatment groups. We here describe a mechanical testing method to quantitatively assess single-level spinal fusion in a rat model, to improve on the binary and subjective nature of manual palpation as an end point for fusion-related studies. We tested explanted lumbar segments from Sprague-Dawley rat spines after single-level posterolateral fusion procedures at L4-L5. Segments were classified as 'not fused,' 'restricted motion,' or 'fused' by using manual palpation testing. After thorough dissection and potting of the spine, 4-point bending in flexion then was applied to the L4-L5 motion segment, and stiffness was measured as the slope of the moment-displacement curve. Results demonstrated statistically significant differences in stiffness among all groups, which were consistent with preliminary grading according to manual palpation. In addition, the 4-point bending results provided quantitative information regarding the quality of the bony union formed and therefore enabled the comparison of fused specimens. Our results demonstrate that 4-point bending is a simple, reliable, and effective way to describe and compare results among rat spines after fusion surgery.

Effect of anterior cervical discectomy and fusion on adjacent segments in rabbits

This study is to investigate the effect of anterior cervical discectomy with internal fixation and fusion at different levels on adjacent segments in rabbits. Sixty New Zealand rabbits were randomly divided into four groups, one control group and three model groups, with 15 in each group. Each group underwent anterior cervical internal fixation and fusion at C3-4, C4-5, and C5-6 levels respectively. X-ray film was examined three, six and nine months after fusion to observe the changes in intervertebral space and endplate of adjacent segment. Immunohistochemistry was utilized to evaluate the effects of different fusion methods on adjacent segments of spine. As time went by, in model groups, the majority of cartilage endplates were calcified, as examined by X-ray. Immunohistochemical results of the intervertebral disc showed that the expression levels of collagen type II in nucleus pulposus were decreased significantly, while the expression levels collagen type I in annular fibrosus were increased. And collagen type I tends to replace collagen type II gradually in nucleus pulposus as time goes by. The change in collagen between upper and lower adjacent segments at C3-4 and C4-5 showed no statistical significance after fixation and fusion (p > 0.05). But for C5-6, the change showed statistical significance (p < 0.05). Cervical internal fixation and fusion can induce intervertebral disc degeneration of adjacent segment in rabbits, and cervical internal fixation and fusion operated at different levels may result in different effects on adjacent segments of cervical intervertebral disc.

Quantitative study of parathyroid hormone (1-34) and bone morphogenetic protein-2 on spinal fusion outcomes in a rabbit model of lumbar dorsolateral intertransverse process arthrodesis

STUDY DESIGN

A posterolateral rabbit spinal fusion model was used to evaluate the effects of recombinant human bone morphogenetic protein-2 (rhBMP-2) and teriparatide (PTH [1-34]) used individually and in combination on spinal fusion outcomes.

OBJECTIVE

To test the efficacy of parathyroid hormone on improving spinal fusion outcomes when used with BMP-2.

SUMMARY OF BACKGROUND DATA

Of the more than 250,000 spinal fusion surgical procedures performed each year, 5% to 35% of these will result in pseudarthrosis. Growing controversy on the efficacy and cost of rhBMP-2 for improving spinal fusion outcomes has presented a challenge for clinicians. Research into PTH as an adjunct therapy to rhBMP-2 for spinal fusion has not yet been investigated.

METHODS

Forty-eight male New Zealand white rabbits underwent bilateral posterolateral intertransverse process arthrodesis surgery at the L5-L6 level. Animals were divided into 6 groups. Two groups were treated with autograft alone or autograft and PTH (1-34), whereas the other 4 groups were treated with low-dose rhBMP-2 alone, high-dose rhBMP-2 alone, or either dose combined with PTH (1-34). All animals were euthanized 6 weeks after surgery. The L4-L7 spinal segment was removed and assessed using manual palpation, computed tomography (CT), and biomechanical testing.

RESULTS

CT assessments revealed fusion in 50% of autograft controls, 75% of autograft PTH (1-34) animals, 87.5% in the 2 groups treated with low-dose rhBMP-2, and 100% in the 2 groups treated with high-dose rhBMP-2. CT volumetric analysis demonstrated that all groups treated with biologics had fusion masses that were on average significantly larger than those observed in the control group (P < 0.0001). Biomechanical data demonstrated no statistical difference between controls, PTH (1-34), and low-dose rhBMP-2 in any testing orientation. PTH (1-34) did not increase bending stiffness when used adjunctively with either low-dose or high-dose rhBMP-2.

CONCLUSION

Although intermittent teriparatide administration results in increased fusion mass volume, it does not improve biomechnical stiffness over use of autograft alone. When delivered concurrently with high- and low-dose rhBMP-2, teriparatide provided no statistically significant improvement in biomechanical stiffness.

LEVEL OF EVIDENCE

N/A.

The influence of alendronate on spine fusion in an osteoporotic animal model

STUDY DESIGN

A posterolateral lumbar fusion model in osteoporotic rats. OBJECTIVE.: To assess the effect of alendronate on spine fusion in an osteoporotic animal model.

SUMMARY OF BACKGROUND DATA

The effect of alendronate on spine fusion is still controversial. Also, there are no studies using an osteoporotic animal model to assess the effect of alendronate on spine fusion.

METHODS

Forty-six female Sprague-Dawley rats underwent either sham-operation (sham) (N = 24) or bilateral ovariectomy (OVX)(N = 22). Eight weeks after the first surgery, animals underwent intertransverse spine fusion at L4-L5. Animals received saline or alendronate 70 μg/kg/wk by subcutaneous administration once a week for 8 weeks after spinal arthrodesis. All animals were divided into four groups: sham-control, sham-alendronate, OVX-control, and OVX-alendronate. After that the animals were killed and the fusion mass was assessed by radiographic, peripheral quantitative computed tomography (pQCT) scanning, and biomechanical and histologic analysis.

RESULTS

In the radiographic study and the pQCT scanning, the area of fusion masses of animals treated with alendronate was considerably larger and denser than that of the control animals in both sham and OVX groups. In the biomechanical study, the ultimate load of the fusion mass of alendronate-treated animals was higher than that of control animals in the osteoporotic groups. Histologic analysis of sagittal sections of fusion mass revealed greater new bone formation in alendronate-treated animals in osteoporotic models and significant inhibition of osteoclasts among the grafted area of alendronate-treated animals compared with the control models.

CONCLUSION

Alendronate was effective for radiologic, biomechanical, and histologic success of spine fusion in an osteoporotic animal model. Though much of the graft bone was not resorbed, alendronate increased biomechanical strength with ingrowth of new bone formation in osteoporotic animals. This study suggests the alendronate may improve spine fusion healing in the presence of osteoporosis.

In vivo and in vitro analysis of rat lumbar spine mechanics

BACKGROUND

Rodent lumbar and caudal (tail) spine segments provide useful in vivo and in vitro models for human disc research. In vivo caudal models allow characterization of the effect of static and dynamic loads on disc mechanics of individual animals with time, but the lumbar models have required sacrifice of the animals for in vitro mechanical testing.

QUESTIONS/PURPOSES

We therefore developed a novel displacement controlled in vivo lumbar spine noninvasive induced angular displacement (NIAD) test; data obtained with NIAD were used to compare angular displacement between segmental levels (L4/L5, L5/L6 and L6/S1), interobserver radiograph measurement agreement, and intraobserver radiograph measurement repeatability. Measurements from NIAD were compared with angular displacement, bending stiffness, and moment to failure measured by an in vitro test.

METHODS

Anesthetized Lewis rats were xrayed in a 90° angled fixture, and NIAD was measured at lumbar levels L4 to S1 by two independent and blinded observers. After euthanasia, in vitro angular displacement (IVAD), stiffness, and failure moment were measured for the combined L4-L6 segment in four-point bending.

RESULTS

NIAD was greater at L4/L5 and L5/L6 than at L6/S1. Combined coronal NIAD for L4-L6 was 42.8° ± 5.3° and for IVAD was 61.5° ± 3.8°. Reliability assessed by intraclass correlation coefficient (ICC) was 0.905 and 0.937 for intraobserver radiograph measurements, and interobserver ICCs ranged from 0.387 to 0.653 for individual levels. The interobserver ICC was 0.911 for combined data from all levels. Reliability for test-retest NIAD measurements had an ICC of 0.932. In vitro failure moment correlated with NIAD left bending.

CONCLUSIONS

The NIAD method yielded reproducible and reliable rat lumbar spine angular displacement measurements without required euthanasia, and allows repetitive monitoring of animals with time. For lumbar spine research studies performed during a course of time, the NIAD method may reduce animal numbers required by providing serial angular displacement measurements without euthanasia.

CLINICAL RELEVANCE

Improved methods to assess comparative models for disease or aging may permit enhanced clinical treatments and improved patient care.

Genetically modified mesenchymal stem cells induce mechanically stable posterior spine fusion

Most spine fusion procedures involve the use of prosthetic fixation devices combined with autologous bone grafts rather than biological treatment. We had shown that spine fusion could be achieved by injection of bone morphogenetic protein-2 (BMP-2)-expressing mesenchymal stem cells (MSCs) into the paraspinal muscle. In this study, we hypothesized that posterior spinal fusion achieved using genetically modified MSCs would be mechanically comparable to that realized using a mechanical fixation. BMP-2-expressing MSCs were injected bilaterally into paravertebral muscles of the mouse lumbar spine. In one control group BMP-2 expression was inhibited. Microcomputed tomography and histological analyses were used to evaluate bone formation. For comparison, a group of mouse spines were bilaterally fused with stainless steel pins. The harvested spines were later tested using a custom four-point bending apparatus and structural bending stiffness was estimated. To assess the degree to which MSC vertebral fusion was targeted and to quantify the effects of fusion on adjacent spinal segments, images of the loaded spine curvature were analyzed to extract rigidity of the individual spinal segments. Bone bridging of the targeted vertebrae was observed in the BMP-2-expressing MSC group, whereas no bone formation was noted in any control group. The biomechanical tests showed that MSC-mediated spinal fusion was as effective as stainless steel pin-based fusion and significantly more rigid than the control groups. Local analysis showed that the distribution of stiffness in the MSC-based fusion group was similar to that in the steel pin fusion group, with the majority of spinal stiffness contributed by the targeted fusion at L3-L5. Our findings demonstrate that MSC-induced spinal fusion can convey biomechanical rigidity to a targeted segment that is comparable to that achieved using an instrumental fixation.

Natural bone collagen scaffold combined with autologous enriched bone marrow cells for induction of osteogenesis in an ovine spinal fusion model

Autologous bone graft, the standard of bone grafting in achieving spinal fusion, is associated with several limitations and complications. The use of bone marrow cells (BMCs) as a potential cell source for spinal fusion, combined with a suitable scaffold to promote bone formation, may be a better choice. The aims of this study were to evaluate the efficacy of natural bone collagen scaffold (NBCS) combined with autologous-enriched BMCs for induction of osteogenesis in vitro and in vivo. Ovine-enriched BMCs were co-cultured with NBCS for 1, 2, 3, and 4 weeks to investigate whether NBCS would support the population expansion and differentiation of enriched BMCs. Using an ovine interbody fusion model, NBCS seeded with autologous enriched BMCs was implanted into the lumbar disc space. Fusion outcomes were compared with the use of the autograft, NBCS without BMCs, and BMCs without NBCS. In vitro results demonstrated that NBCS facilitated the population expansion and differentiation of ovine-enriched BMCs, promoting the expression of collagen type I and the formation of a mineralized matrix. The use of NBCS combined with enriched BMCs in vivo enhanced the spinal fusion rate (6 of 6 at 10 week) (p < 0.05), the biomechanical stiffness of fusion masses, and bone volume at the fusion site (p < 0.05). Histological findings also revealed that a combination of NBCS and BMCs induced new bone formation that integrated well with host bone tissue. In conclusion, NBCS is an effective scaffold that supports ovine-enriched BMCs. The combination of NBCS and BMCs may be a useful alternative for autograft in induction of spinal fusion.