Differences in early osteogenesis and bone micro-architecture in anterior lumbar interbody fusion with rhBMP-2, equine bone protein extract, and autograft

Animal Model for Anterior Lumbar Interbody Fusion: A Literature Review

Lumbar interbody fusion (LIF) is a surgical procedure for treating lumbar spinal stenosis and deformities. It removes a spinal disc and insert a cage or bone graft to promote solid fusion. Extensive research on LIF has been supported by numerous animal studies, which are being developed to enhance fusion rates and reduce the complications associated with the procedure. In particular, the anterior approach is significant in LIF research and regenerative medicine studies concerning intervertebral discs, as it utilizes the disc and the entire vertebral body. Several animal models have been used for anterior LIF (ALIF), each with distinct characteristics. However, a comprehensive review of ALIF models in different animals is currently lacking. Medium-sized and large animals, such as dogs and sheep, have been employed as ALIF models because of their suitable spine size for surgery. Conversely, small animals, such as rats, are rarely employed as ALIF models because of anatomical challenges. However, recent advancements in surgical implants and techniques have gradually allowed rats in ALIF models. Ambitious studies utilizing small animal ALIF models will soon be conducted. This review aims to review the advantages and disadvantages of various animal models, commonly used approaches, and bone fusion rate, to provide valuable insights to researchers studying the spine.

Effect of Trabecular Microstructure of Spinous Process on Spinal Fusion and Clinical Outcomes After Posterior Lumbar Interbody Fusion: Bone Surface/Total Volume as Independent Favorable Indicator for Fusion Success

OBJECTIVE

We assessed the trabecular microarchitecture of the spinous process as an autograft and investigated its correlations with fusion success and clinical outcomes for patients undergoing posterior lumbar interbody fusion.

METHODS

Micro-computed tomography reconstruction techniques were used to scan cancellous bone specimens from spinous processes. We then measured the microarchitectural parameters for 105 subjects.

RESULTS

The patient cohort included 44 older men and 61 postmenopausal women with a minimum of 2-year follow-up data available. The complete fusion rate was 87.6% (92 of 105) at the last follow-up. When stratified by fusion status, the union group had significantly greater bone surface/total volume (BS/TV) and trabecular number but significantly lower trabecular separation than the nonunion group. No statistically significant differences were observed between the 2 groups in the clinical variables, except for the bone mineral density at the femoral neck (P = 0.028). On binomial logistic regression analysis, BS/TV was identified as an independent predictor for fusion success (odds ratio, 8.532; P = 0.032). The receiver operating characteristic curve showed that BS/TV had excellent performance in predicting successful fusion (area under the curve, 0.807). Using a cutoff value for BS/TV of 3.145, a greater BS/TV was significantly associated with visual analog scale scores for lower back pain 6 months postoperatively and lower Oswestry disability index scores at 12 and 24 months postoperatively but not with visual analog scale scores for leg pain.

CONCLUSIONS

Our data suggest that microstructural deterioration of the spinal process as an autograft has detrimental effects on spinal fusion and clinical outcomes for patients undergoing instrumented posterior lumbar interbody fusion. Specifically, the microstructural parameter BS/TV has good potential for assessing lumbar bone quality and predicting fusion success.

In Vivo Bone Formation Within Engineered Hydroxyapatite Scaffolds in a Sheep Model

Large bone defects still represent a major burden in orthopedics, requiring bone-graft implantation to promote the bone repair. Along with autografts that currently represent the gold standard for complicated fracture repair, the bone tissue engineering offers a promising alternative strategy combining bone-graft substitutes with osteoprogenitor cells able to support the bone tissue ingrowth within the implant. Hence, the optimization of cell loading and distribution within osteoconductive scaffolds is mandatory to support a successful bone formation within the scaffold pores. With this purpose, we engineered constructs by seeding and culturing autologous, osteodifferentiated bone marrow mesenchymal stem cells within hydroxyapatite (HA)-based grafts by means of a perfusion bioreactor to enhance the in vivo implant-bone osseointegration in an ovine model. Specifically, we compared the engineered constructs in two different anatomical bone sites, tibia, and femur, compared with cell-free or static cell-loaded scaffolds. After 2 and 4 months, the bone formation and the scaffold osseointegration were assessed by micro-CT and histological analyses. The results demonstrated the capability of the acellular HA-based grafts to determine an implant-bone osseointegration similar to that of statically or dynamically cultured grafts. Our study demonstrated that the tibia is characterized by a lower bone repair capability compared to femur, in which the contribution of transplanted cells is not crucial to enhance the bone-implant osseointegration. Indeed, only in tibia, the dynamic cell-loaded implants performed slightly better than the cell-free or static cell-loaded grafts, indicating that this is a valid approach to sustain the bone deposition and osseointegration in disadvantaged anatomical sites.

Polyelectrolyte Complex Carrier Enhances Therapeutic Efficiency and Safety Profile of Bone Morphogenetic Protein-2 in Porcine Lumbar Interbody Fusion Model

STUDY DESIGN

Porcine lumbar interbody fusion model.

OBJECTIVE

This study evaluates the effect of polyelectrolyte complex (PEC) carrier in enhancing the therapeutic efficiency and safety profile of bone morphogenetic protein-2 (BMP-2) in a large animal model.

SUMMARY OF BACKGROUND DATA

Extremely large amounts of BMP-2 are administered to achieve consistent spinal fusion, which has led to complications. Heparin-modified PEC carrying reduced BMP-2 doses of 0.5 μg was demonstrated to achieve consistent spinal fusion with reduction of complications in rodent model. The purpose of this study was to evaluate whether PEC could improve the therapeutic efficiency of BMP-2 in porcine model.

METHODS

Three-segment (L3-L6) anterior lumbar interbody fusions with instrumentation were performed on 6 pigs using 3 different doses of BMP-2, namely, (1) 50 μg, (2) 150 μg, and (3) 300 μg. The BMP-2 was delivered using heparin-modified alginate microbeads loaded into biodegradable cage. Fusion performance was evaluated after 3 months.

RESULTS

Manual palpation and micro-computed tomography showed consistent fusion in all experimental groups. Heterotopic bone formation beyond the cage implant area was more evident in group 2 and group 3 than in group 1. Similarly, superior bone microstructure was observed in the new bone with the lowered BMP-2 dose. Biomechanical evaluation revealed enhanced stiffness of the operated segments compared with nonoperated segments (P < 0.05). Mechanical stability was maintained despite dose reduction of BMP-2. Although the mineral apposition rate was higher in group 3, unsatisfactory bony microstructure with decreased trabecular number was observed in group 3 compared with group 1.

CONCLUSION

PEC carrying low doses of BMP-2 achieved consistent interbody fusion. We observed dose-related reduction in heterotopic ossification without compromising the stability of the fused segments. PEC carrier reduces the efficacious doses of BMP-2. This could enhance the safety profile of BMP-2 and reduce dose- and carrier-related complications.

LEVEL OF EVIDENCE

N/A.

Sequestration of rhBMP-2 into self-assembled polyelectrolyte complexes promotes anatomic localization of new bone in a porcine model of spinal reconstructive surgery

Efficient and therapeutically safe delivery of recombinant human bone morphogenetic protein 2 (rhBMP-2) continues to be a central issue in bone tissue engineering. Recent evidence indicates that layer-by-layer self-assembly of polyelectrolyte complexes (PECs) can be used to recreate synthetic matrix environments that would act as tuneable reservoirs for delicate biomolecules and cells. Although preliminary in vitro as well as small-animal in vivo studies support this premise, translation into clinically relevant bone defect volumes in larger animal models remains unreported. Here we explored the use of native heparin-based PEC, deposited on a hydrated alginate gel template, to load bioactive rhBMP-2 and to facilitate lumbar interbody spinal fusion in pigs. We observed that triple PEC deposits with the highest protein sequestration efficiency and immobilization capacity promoted higher volume of new bone formation when compared with single PEC with low sequestration efficiency and immobilization capacity. This also resulted in a significantly enhanced biomechanical stability of the fused spinal segment when compared with PEC carriers with relatively low protein sequestration and immobilization capacities (p<0.05). Most importantly, PEC carriers showed a more orderly pattern of new bone deposition and superior containment of bone tissue within implant site when compared to collagen sponge carriers. We conclude that this growth factor sequestration platform is effective in the healing of clinically relevant bone defect volume and could overcome some of the safety concerns and limitations currently associated with rhBMP-2 therapy such as excessive heterotopic ossification.

Critical size bone defect reconstruction by an autologous 3D osteogenic-like tissue derived from differentiated adipose MSCs

For critical size bone defects and bone non-unions, bone tissue engineering using osteoblastic differentiated adipose mesenchymal stem cells (AMSCs) is limited by the need for a biomaterial to support cell transplantation. An osteoblastic three-dimensional autologous graft made of AMSCs (3D AMSC) was developed to solve this issue. This autograft was obtained by supplementing the osteoblastic differentiation medium with demineralized bone matrix. Two surgical models were developed to assess the potential of this 3D osteogenic AMSC autograft. A four-level spinal fusion using polyetheretherketone cages was designed in six pigs to assess the early phase of ossification (8-12 weeks postimplantation). In each pig, four groups were compared: cancellous bone autograft, freeze-dried irradiated cancellous pig bone, 3D AMSC, and an empty cage. A critical size femoral defect (n = 4, bone non-union confirmed 6 months postoperatively) was used to assess the 3D AMSCs' ability to achieve bone fusion. Pigs were followed by CT scan and explanted specimens were analyzed for bone tissue remodeling by micro-CT scan, micro-radiography, and histology/histomorphometry. In the spine fusion model, bone formation with the 3D AMSC was demonstrated by a significant increase in bone content. In the critical-size femoral defect model, the 3D AMSC achieved new bone formation and fusion in a poorly vascularized fibrotic environment. This custom-made 3D osteogenic AMSC autograft is a therapeutic solution for bone non-unions and for critical-size defects.

Repetitive recombinant human bone morphogenetic protein 2 injections improve the callus microarchitecture and mechanical stiffness in a sheep model of distraction osteogenesis

Evidence suggests that recombinant human bone morphogenetic protein 2 (rhBMP-2) increases the mechanical integrity of callus tissue during bone healing. This effect may be either explained by an increase of callus formation or a modification of the trabecular microarchitecture. Therefore the purpose of the study was to evaluate the potential benefit of rhBMP-2 on the trabecular microarchitecture and on multidirectional callus stiffness. Further we asked, whether microarchitecture changes correlate with optimized callus stiffness. In this study a tibial distraction osteogenesis (DO) model in 12 sheep was used to determine, whether percutaneous injection of rhBMP-2 into the distraction zone influences the microarchitecture of the bone regenerate. After a latency period of 4 days, the tibiae were distracted at a rate of 1.25 mm/day over a period of 20 days, resulting in total lengthening of 25 mm. The operated limbs were randomly assigned to one treatment groups and one control group: (A) triple injection of rhBMP-2 (4 mg rhBMP-2/injection) and (B) no injection. The tibiae were harvested after 74 days and scanned by µCT (90 µm/voxel). In addition, we conducted a multidirectional mechanical testing of the tibiae by using a material testing system to assess the multidirectional strength. The distraction zones were tested for torsional stiffness and bending stiffness antero-posterior (AP) and medio-lateral (ML) direction, compression strength and maximum axial torsion. Statistical analysis was performed using multivariate analysis of variance (ANOVA) followed by student's t-test and Regression analysis using power functions with a significance level of P<0.05. Triple injections of rhBMP-2 induced significant changes in the trabecular architecture of the regenerate compared with the control: increased trabecular number (Tb.N.) (treatment group 1.73 mm/1 vs. control group 1.2 mm/1), increased cortical bone volume fraction (BV/TV) (treatment group 0.68 vs. control group 0.47), and decreased trabecular separation (Tb.Sp.) (treatment group 0.18 mm vs. control group 0.43 mm).The analyses of the mechanical strength of regenerated bone showed significant differences between treatment group (A) and the control group (B). The bending stiffness anterior-posterior (treatment group 17.48 Nm vs. control group 8.3 Nm), medial-lateral (treatment group 18,9 Nm vs. control group 7.92 Nm) and the torsional stiffness (treatment group 41.17N/° vs. control group 16.41N/°) are significantly higher in the treatment group than in the control group. The regression analyses revealed significant non-linear relationships between BV/TV, TB.N., Tb.Sp. and all mechanical properties. Maximal correlation coefficients were found for the Tb.Sp. vs. the bending stiffness AP and ML with R(2)=0.69 and R(2)=0.70 (P<0.0001). There was no significant relation between Connectivity and the compression strength and the maximum axial torque. This study suggests that rhBMP-2 optimizes the trabecular microarchitecture of the regenerate, which might explain the advanced mechanical integrity of newly formed bone under rhBMP-2 treatment.

An approximate model for cancellous bone screw fixation

This paper presents a finite element (FE) model to identify parameters that affect the performance of an improved cancellous bone screw fixation technique, and hence potentially improve fracture treatment. In cancellous bone of low apparent density, it can be difficult to achieve adequate screw fixation and hence provide stable fracture fixation that enables bone healing. Data from predictive FE models indicate that cements can have a significant potential to improve screw holding power in cancellous bone. These FE models are used to demonstrate the key parameters that determine pull-out strength in a variety of screw, bone and cement set-ups, and to compare the effectiveness of different configurations. The paper concludes that significant advantages, up to an order of magnitude, in screw pull-out strength in cancellous bone might be gained by the appropriate use of a currently approved calcium phosphate cement.

Use of carboxymethyl cellulose and collagen carrier with equine bone lyophilisate suggests late onset bone regenerative effect in a humerus drill defect - a pilot study in six sheep

We assessed the use of a filler compound together with the osteoinductive demineralized bone matrix (DBM), Colloss E. The filler was comprised of carboxymethyl-cellulose and collagen type 1. The purpose of the study was to see if the filler compound would enhance the bone formation and distribute the osteoinductive stimulus throughout the bone defect. Six sheep underwent a bilateral humerus drill defect. The drill hole was filled with a compound consisting of 100 mg CMC, 100 mg collagen powder, and 1 ccm autologous full blood in one side, and a combination of this filler compound and 20 mg Colloss E in the other. The animals were divided into three groups of two animals and observed for 8, 12 and 16 weeks. Drill holes was evaluated using quantitative computed tomography (QCT), micro computed tomography (µCT) and histomorphometry. Mean total bone mineral density (BMD) of each implantation site was calculated with both QCT and µCT. Bone volume to total volume (BV/TV) was analyzed using µCT and histomorphometry. Although not statistically significant, results showed increased bone BMD after 16 weeks in µCT data and an increased BV/TV after 16 weeks in both µCT and histology. Correlation between QCT and µCT was R² = 0.804. Correlation between histomorphometry and µCT BV/TV data was R² = 0.8935 and with an average overrepresentation of 8.2% in histomorphometry. In conclusion the CMC-Collagen + Colloss E filler seems like a viable osteogenic bone filler mid- to long term. A correlation was found between the analytical methods used in this study.