Experimental laparoscopic and thoracoscopic discectomy and instrumented spinal fusion. A feasibility study using a porcine model

Spinal fixation after laminectomy in pigs prevents postoperative spinal cord injury

Background

A safe, effective, and ethically sound animal model is essential for preclinical research to investigate spinal medical devices. We report the initial failure of a porcine spinal survival model and a potential solution by fixating the spine.

Methods

Eleven female Dutch Landrace pigs underwent a spinal lumbar interlaminar decompression with durotomy and were randomized for implantation of a medical device or control group. Magnetic resonance imaging (MRI) was performed before termination.

Results

Neurological deficits were observed in 6 out of the first 8 animals. Three of these animals were terminated prematurely because they reached the predefined humane endpoint. Spinal cord compression and myelopathy was observed on postoperative MRI imaging. We hypothesized postoperative spinal instability with epidural hematoma, inherent to the biology of the model, and subsequent spinal cord injury as a potential cause. In the subsequent 3 animals, we fixated the spine with Lubra plates. All these animals recovered without neurological deficits. The extent of spinal cord compression on MRI was variable across animals and did not seem to correspond well with neurological outcome.

Conclusion

This study shows that in a porcine in vivo model of interlaminar decompression and durotomy, fixation of the spine after lumbar interlaminar decompression is feasible and may improve neurological outcomes. Additional research is necessary to evaluate this hypothesis.

Experimental Model for Interlaminar Endoscopic Spine Procedures

BACKGROUND

Endoscopic spinal surgery is becoming quite popular, and the pursuit of a training model to improve surgeons' skills is imperative to overcome the limited availability of human cadavers. Our goal was to determine whether the porcine spine could be a representative model for learning and practicing interlaminar percutaneous endoscopic lumbar procedures (IL-PELPs).

METHODS

Lumbar and cervical segments of the porcine cadaver spine were used for the IL-PELP. We have described the technical notes on the difficulties of the procedure and the relevant anatomical features. To endorse the porcine cadaver for this procedure, 5 neurosurgeons underwent 1 day of training and completed a survey.

RESULTS

The porcine lumbar spine has small interlaminar windows, and laminectomy is necessary, mimicking the translaminar approaches for higher human lumbar spine levels. The porcine cervical spine has wide and high interlaminar windows and mimics the human L5-S1 interlaminar approach. Entering the spinal canal with the working sheath and endoscope and training the rotation maneuver to access the disc space is only possible in the lumbar segment. It was possible to perform flavectomy and to identify and dissect the dural sac and nerve root in both the lumbar and cervical spine. The neurosurgeons considered the porcine model of good operability and, although different, possible to apply in humans.

CONCLUSIONS

The porcine spine is an effective and representative model for learning and practicing IL-PELPs. Although the described anatomical differences should be known, they did not interfere in performing the main surgical steps and maneuvers for IL-PELPs in the porcine model.

Animal models for spinal fusion

Animal models for spinal fusion are essential for preclinical testing of new fusion methods and adjuncts. They allow for control of individual variables and quantification of outcome measures. Model characteristics are considered. Preclinical experiments to evaluate proof of concept, feasibility, and efficacy are generally studied in an orderly progression from smaller to larger animal models with an evolving cascade of evidence which has become known as the "burden of proof". Methods of fusion analysis include manual palpation, radiographs, computed tomography, histology, biomechanical testing, and molecular analysis. Models which have been developed in specific species are reviewed. This sets the stage for the interpretation of studies evaluating bone graft materials such as allograft, demineralized bone matrices, bone morphogenetic proteins, ceramics, and others with consideration of the variables affecting their success. As evidence accumulates, clinical trials and applications are defined.

Comparative anatomy of the porcine and human thoracic spines with reference to thoracoscopic surgical techniques

BACKGROUND

This study compared porcine and human thoracic spine anatomies for a better understanding of how structures encountered during thoracoscopy differ between training with a porcine model and actual surgery in humans.

METHODS

Parameters were measured including vertebral body height, width, and depth; disc height; rib spacing; spinal canal depth and width; and pedicle height and width.

RESULTS

Although most porcine vertebral structures were smaller, porcine pedicle height was significantly greater than that of humans because the porcine pedicle houses a unique transverse foramen. The longus colli and psoas attach, respectively, to T5 and T13 in swine and to T3 and T12 in humans. In swine, the azygos vein generally was absent. The intercostal veins drained into the hemiazygos vein.

CONCLUSIONS

Several thoracoscopically relevant anatomic differences between human and porcine spinal anatomies were identified. A thoracoscopic approach in a porcine model probably is best performed from the right side. The best general working area is between T6 and T10.

Endoscopic Instrumented Posterolateral Lumbar Fusion with Healos and Recombinant Human Growth/Differentiation Factor-5

OBJECTIVE

The goal of this study was to use a minimally invasive endoscopic surgical technique in a sheep model to evaluate the efficacy of an osteoinductive growth factor, recombinant human growth/differentiation factor-5 (also designated MP52), and an osteoconductive matrix formulation (Healos; DePuy AcroMed, Inc., Mountain View, CA) for inducing and facilitating bone formation.

METHODS

Twelve mature sheep underwent bilateral posterolateral lumbar fusion and pedicle screw fixation via a posterior endoscopic approach. Each sheep received two different types of graft material, autogenous iliac crest bone, or a bone graft substitute (MP52 with Healos), inserted into the right and left sides of the spine in an alternating fashion. Groups of four sheep were killed at 2, 4, and 6 months postoperatively for manual, radiographic, and histological evaluation.

RESULTS

No neurological impairments, infections, or other complications were observed. After 2 months, partial fusion on both sides was observed, but radiographic evaluation showed greater bone growth on the side that received the bone graft substitute. Solid posterolateral fusion was observed in both autograft and bone graft substitute sites at 4 and 6 months, and autograft and Healos MP52 fusion sites were essentially the same at histological examination. There was no abnormal overgrowth of new bone from either of these two materials.

CONCLUSION

Endoscopic posterolateral lumbar arthrodesis and instrumentation is feasible, safe, and effective in a sheep model. Healos is a useful bonding agent that mimics natural bone in posterolateral intertransverse fusion sites. Combined with MP52, it produced fusion comparable to that of autogenous bone graft. Minimally invasive techniques and bone graft substitutes could eliminate morbidity and increase the likelihood of successful fusion.

Thoracoscopic intradiscal spine fusion using a minimally invasive gene-therapy technique

BACKGROUND

Gene therapy has been utilized to achieve posterior intertransverse process fusion in rodents. To our knowledge, however, no one has previously reported on the use of this technique to achieve anterior spinal fusion in mammals. The purpose of the present study was to determine if a gene-therapy technique can be utilized to achieve anterior intradiscal fusion in pigs with use of minimally invasive techniques.

METHODS

Mesenchymal stem cells were isolated from each of three pigs, expanded in culture, and transduced with an adenovirus carrying either the gene for bone morphogenetic protein-2 (Adv-BMP2) or the control gene, beta-galactosidase (Adv-betagal). In vitro, assays were performed to detect BMP-2 expression as well as protein markers of bone formation. In vivo, four thoracic disc spaces in each of three pigs were injected thoracoscopically with cells after 1 cm (3) of the disc had been removed. In each of the three pigs, two discs were injected with autologous mesenchymal stem cells transduced with Adv-BMP2, the third disc was injected with cells transduced with Adv-betagal (control 1), and the fourth disc served as the sham-operated control (control 2). The three animals were killed six weeks after the implantation. Computerized tomographic scanning was performed on two of the specimens, and histological examination was performed on all specimens. The computerized tomographic scans and histological examinations were then interpreted in a blinded fashion.

RESULTS

In the in vitro study, a human BMP-2 protein band was detected in the medium of Adv-BMP2-transduced stem cells but not in that of the control cells. The Adv-BMP2-transduced stem cells were associated with a fivefold increase in alkaline phosphatase activity compared with the controls as well as with matrix mineralization and increased protein expression of type-I collagen, osteopontin, and bone sialoprotein. In the in vivo study, radiographic examination demonstrated anterior spinal fusion in all six disc spaces that had been treated with implantation of Adv-BMP2-transduced stem cells. In contrast, the six control disc spaces had little or no intervening bone. Histological examination demonstrated bridging bone from end plate to end plate in all six disc spaces that had been treated with implantation of Adv-BMP2-transduced stem cells. The six control disc spaces had no bridging bone.

CONCLUSION

The Adv-BMP2-transduced mesenchymal stem cells produced BMP-2 protein. Further, the cells differentiated into osteoblasts and induced anterior spinal fusion in six of six disc spaces in this pig model. Although many technical and practical challenges remain, the results of the present study suggest that it may eventually be possible to use similar techniques to achieve anterior spinal fusion in humans.

Minimally invasive spine surgery: an animal model for endoscopic approach to the anterior cervical and upper thoracic spine

BACKGROUND

Minimally invasive video-assisted techniques are currently used for thoracic and lumbar spine surgery with the aim of reducing the morbidity. Recently, an endoscopic approach has been used for endocrine neck surgery, with reduced pain and improved cosmetic results.

PURPOSE

To develop an animal model for an endoscopic neck approach to the anterior cervical and upper thoracic spine.

METHODS

Five pigs were used. A combination of one 5- and two 3-mm laparoscopic instruments was used to dissect the spine, and dedicated instrumentation was then used to perform discectomies. Carbon dioxide was insufflated at 10 mm Hg. Dissection was carried out upward and downward on the anterior aspect of the spine; discectomy was performed at various levels and evaluated at autopsy.

RESULTS

All pigs tolerated the procedure well. Visibility of the cervical spine was excellent, and exposure from C1 to T3 was obtained. For discectomy, an additional 10-mm trocar was inserted, and discectomy of C3-C4 and C4-C5 was performed. Proper location and adequacy were confirmed at autopsy.

CONCLUSION

The endoscopic neck approach allows exposure of the entire cervical spine and the upper thoracic spine in the porcine model. This approach has the potential to reduce the morbidity associated with the open cervical approach and provides a wider view and exposure than conventional open surgery. The availability of the porcine model allows the acquisition of the necessary technical skills before introducing this advanced procedure in humans.