Biochemistry of fusion mass consolidation in the sheep spine

Trimming of sheep spinal cord by waterjet; an experimental study

The spinal cord is a structure of nervous tissue that primarily transmits nerve signals from the motor cortex to the body and from the afferent fibers of the sensory neurons to the sensory cortex. It is enveloped by three layers of meninges. Covering provides a supportive framework for the cerebral and cranial vasculature and protects the central nervous system (CNS) from mechanical damage. Surgical operation in the vicinity of the spinal cord is complicated and risky because it exposes it to probably irreversible damage. To reduce the risk of these operations, attempts have been made to remove the tumor using safer methods like waterjet operation. In these methods, the waterjet and spinal cord interaction are inevitable. To secure interaction of operation, a standard development of waterjet criteria is necessary. In this study, a system of waterjet is designed to perform sheep spinal cord as a tissue with a good resemblance to the human spinal cord. Effects of interaction between waterjet and sheep spinal cord are investigated to define a safe operation threshold. The impact of the liquid density of waterjet on failure criteria of spinal cord surgery is also investigated. Results show that meninges are stiff enough to protect the sheep spinal cord from rupture for pressures up to 8 bar; however internal spinal cord tissue cannot be guaranteed any damage. Three essential parameters represent the spinal cord meninges and spinal cord deformation during the tests. These parameters lead us to provide standard criteria for damage prevention of the spinal cord.

A novel finite element model of the ovine lumbar intervertebral disc with anisotropic hyperelastic material properties

The Ovine spine is an accepted model to investigate the biomechanical behaviour of the human lumbar one. Indeed, the use of animal models for in vitro studies is necessary to investigate the mechanical behaviour of biological tissue, but needs to be reduced for ethical and social reasons. The aim of this study was to create a finite element model of the lumbar intervertebral disc of the sheep that may help to refine the understanding of parallel in vitro experiments and that can be used to predict when mechanical failure occurs. Anisotropic hyperelastic material properties were assigned to the annulus fibrosus and factorial optimization analyses were performed to find out the optimal parameters of the ground substance and of the collagen fibers. For the ground substance of the annulus fibrosus the investigation was based on experimental data taken from the literature, while for the collagen fibers tensile tests on annulus specimens were conducted. Flexibility analysis in flexion-extension, lateral bending and axial rotation were conducted. Different material properties for the anterior, lateral and posterior regions of the annulus were found. The posterior part resulted the stiffest region in compression whereas the anterior one the stiffest region in tension. Since the flexibility outcomes were in a good agreement with the literature data, we considered this model suitable to be used in conjunction with in vitro and in vivo tests to investigate the mechanical behaviour of the ovine lumbar disc.

Development of an in vivo experimental model for percutaneous vertebroplasty in sheep

INTRODUCTION

Several studies have described 'open' approach techniques for cementation of sheep and goat vertebrae; however, no percutaneous technique has been developed so far for use in non-primates. The aim of this study was to develop an animal model for percutaneous vertebroplasty under clinical conditions.

METHODS

In a pilot study with dissected cadaveric ovine vertebrae, the technique and instruments as well as the optimal needle position were determined. In an in vivo animal study using 33 lumbar vertebrae of 11 sheep, a percutaneous vertebroplasty was performed under general anaesthesia. Needle position and cement volume were evaluated from high resolution, quantitative computed tomography imaging.

RESULTS

The percutaneous technique for vertebroplasty was applicable to the vertebral bodies (L1 to L5) of the ovine lumbar spine without any related adverse effects for the animals. The procedure showed a steep learning curve represented by the reduction of the distance between the actual and planned needle positioning (7.2 mm to 3.7 mm; median value) and shorter surgery times (21.3 min to 15.0 min, average) with progression of the study.

CONCLUSION

The described technique is feasible and repeatable under clinical conditions. This is the first percutaneous vertebroplasty technique for non-primates and we conclude that the sheep is a valid animal model to investigate the effects of cement augmentation in vivo.

Histological evaluation of the sheep's cervical spine segment 3/4--a histomorphological and histomorphometrical study

The 2-year-old sheep is a frequently used model in spinal orthopaedic research; however, so far no investigations have been performed on physiological tissue of untreated animals. In this study, the cervical spine segment 3/4 of eight 2-year-old female Merino sheep was evaluated histologically and histomorphometrically. Specimens were stained with Masson-Goldner-Trichrome, Safranin-Orange/Lightgreen, Safranin-Orange/von Kossa and Astrablue. Analysis of bone mass, mass of cartilage and mineralized cartilage in a defined region of interest was performed. A new finding of the study was that--although 2-year-old sheep is regarded as adult--tissue of the growth plate was still detected. Moreover, this study revealed that in the nucleus pulposus of the 2-year-old sheep notochordal cells are still existent, suggesting a complex remodelling of the nucleus pulposus in sheep.

In vivo strain measurements from hardware and lamina during spine fusion

Currently, spine fusion is determined using radiography and clinical evaluation. There are discrepancies between radiographic evidence and direct measurements of fusion, such as operative exploration and biomechanical or histological measurements. In order to facilitate the rapid return of patients to normal activities, a monitoring technique to accurately detect fusion in vivo and to prevent overload during the postoperative period would be useful. The objectives of this study were to develop an implantable monitoring system consisting of CPC-coated strain gauges and a radio transmitter to detect the onset of fusion and measure strain during postsurgical activities. A patient underwent anterior release and fusion, followed by posterior instrumentation and fusion with segmental spinal instrumentation. Four strain gauges were placed during surgery. One was attached to the left-side rod and one to each of the lamina at T9, T10, and T11. An externally powered implanted radio transmitter attached to the gauges was placed in a subcutaneous pouch. Strains were monitored weekly and tabulated during various activities for 7 months. Peak strains during twisting and bending were tabulated to detect the onset of fusion. Strains were also recorded during activities such as climbing off an examination table, rising from a chair, and climbing stairs. Strains collected from the left rod indicated that, immediately postoperatively, it was loaded at acceptable levels. The largest and most consistent strain changes measured from the lamina were recorded during twisting.

Biomechanical comparison of bioabsorbable cervical spine interbody fusion cages

STUDY DESIGN

In vitro biomechanical study of bioabsorbable cervical spine interbody fusion cages using a sheep model.

OBJECTIVES

The purpose of this study was to evaluate the segmental stability provided by 2 new developed bioabsorbable cervical spine interbody fusion cages and to compare it with a tricortical iliac crest bone graft and a titanium meshed interbody fusion cage. Further, the biomechanical effect of an additional anterior plate instrumentation was determined.

SUMMARY AND BACKGROUND DATA

Despite the initial favorable results, the long-term effects of metallic cage devices on spinal motion segments are still unknown. Furthermore, shortcomings of metallic cages like migration, adjacent level degeneration, stenotic myelopathy, and artifacts in postoperative radiologic assessment have already been reported. Bioabsorbable cages have been designed to avoid these complications. Currently, no information is available about the biomechanical properties of bioabsorbable cervical spine interbody fusion cages.

METHODS

Forty sheep cervical spines (C2-C5) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive stiffness method using a nonconstrained testing apparatus. First, the motion segment C3-C4 was tested intact. After complete discectomy, the following groups were evaluated: autologous iliac crest bone graft, titanium mesh cylinder (Harms, DePuy AcroMed), bioabsorbable PDLLA-cage (experimental), and bioabsorbable Resorbon cage (Biomet Merck). Further, all implants were tested with an additional anterior plate instrumentation. The mean apparent stiffness, range of motion, neutral zone, and elastic zone were calculated from the corresponding load-displacement curves.

RESULTS

No significant difference in range of motion and segmental stiffness among the tricortical iliac crest bone graft, meshed titanium Harms cage, and PDLLA-cage could be determined. The Resorbon cage significantly (P < 0.05) decreased range of motion and increased stiffness in rotation and flexion in comparisonto all tested implants and the intact motion segment. An additional anterior plate significantly (P < 0.05) decreased range of motion and increased stiffness in flexion and extension.

CONCLUSION

In this study, bioabsorbable cages demonstrated biomechanical in vitro properties equal or superior to metallic cages. From the biomechanical point of view, bioabsorbable cages, especially the Resorbon cage, may be a viable alternative to current metallic interbody cage devices. However, animal experimental in vivo evaluation of bioabsorbable cervical spine interbody fusion cages still has to be performed.

Characterization of a developing lumbar arthrodesis in a sheep model with quantitative instability

BACKGROUND CONTEXT

Mechanical forces have been considered responsible for stress shielding an arthrodesis, but the biology of a developing lumbar fusion has not been well characterized.

PURPOSE

A large animal model was used to test the hypothesis that mechanical forces modify the biological processes involved in a developing bony fusion.

STUDY DESIGN

Lumbar fusion was performed in an ovine model using custom instrumentation that permitted a controlled degree of anterior-posterior translation after surgery. Fusion sites were evaluated by radiography, microradiography, histology and histomorphometry at time points that corresponded with predicted early and later stages of bone healing.

METHODS

Fourteen skeletally mature ewes underwent lumbar spinal fusion under general anesthesia. In the control (stable) group, the spine was rigidly fixed with a cage anteriorly and pedicle screws posteriorly. In the experimental (unstable) group, the spine was destabilized by an annulectomy (with no anterior implant) and custom pedicle screws that allowed 2 mm of anterior-posterior translation. Animals were euthanized 6 and 12 weeks after surgery.

RESULTS

Radiographs confirmed that the fusion mass had not fully consolidated at either time point. Microradiographs revealed a trend toward increased bone formation at 6 weeks in the stable case as compared with the unstable, but by 12 weeks, this trend had reversed (p=.03). Intramembranous bone formation was the primary mechanism of healing near the transverse process in animals with both stable and unstable fixation. In the area between the two transverse processes, new bone formation occurred primarily through endochondral ossification. At 12 weeks, the stable case had significantly more cartilage formed (p=.023) but less newly formed bone (p=.07) as compared with the quantitatively unstable.

CONCLUSIONS

This clinically realistic animal model allowed characterization of the biology of the developing arthrodesis before fusion. Under stable or unstable conditions, endochondral ossification was the predominant mechanism of new bone formation within the intertransverse process region. This finding, which contrasts with previous reports from small animal models of spine fusion, may reflect a difference in biology that results from the increased size of the intertransverse space in sheep as compared with small animals. Interestingly, mechanical instability increased the formation of new bone within this region, but not at the transverse process. Endochondral ossification therefore appears to respond to mechanical factors in the fusion site. The ovine model shows promise as an alternative to the rabbit model and may provide a more stringent test for potential new surgical and nonsurgical strategies for spine fusion.

An implantable strain measurement system designed to detect spine fusion: preliminary results from a biomechanical in vivo study

STUDY DESIGN

The strain distribution on the thoracic vertebrae during anteroposterior bending and torsion was examined for use with an implantable strain gauge system and miniature radio transmitter, which also were evaluated.

OBJECTIVES

To identify strain gauge placement sites by testing cadaver spines in vivo, and to evaluate an implantable gauge bonding technique and subminiature radio transmitter for accurate strain monitoring.

SUMMARY OF BACKGROUND DATA

Fusion is determined currently through the use of radiographic techniques. Discrepancies exist between radiographic evidence and more direct measurements of fusion such as operative exploration4,5,12 and biomechanical or histologic measurements.12,15 To facilitate the return of patients to full unrestricted activity, it would be useful to develop a technique for accurate in vivo determination of fusion.

METHODS

Three cadaver spines were tested during anteroposterior bending and torsional loading in the control, instrumented, and instrumented plus polymethylmethacrylate states. The spines were instrumented with an ISOLA(R) (Acromed Corporation, Cleveland, Ohio) construct, and a simulated fusion was achieved through the application of polymethylmethacrylate. Strain gauges were attached in uniaxial, biaxial, and rosette configurations. The principal strains were calculated. Calcium phosphate ceramic-coated gauges were implanted in patients and recovered after up to 15 months in vivo. A radio transmitter was developed and tested for use in patients.

RESULTS

The largest and most consistent strain changes after simulated fusion were recorded during torsional loading on the laminae of a vertebra directly underneath a hook. Calcium phosphate ceramic-coated strain gauges showed excellent bone bonding to the lamina when fusion occurred. Radio telemetry accurately tracked strain magnitudes and strain rates expected in patients.

CONCLUSIONS

The consistency obtained in torsional loading indicates that this type of loading will provide the most useful data from patients in vivo. Excellent bonebonding and accurate strain transmission using a long-term strain measurement system and miniature radio transmitter indicate that strains collected from patients with this system will be accurate.

Biomechanical comparison of cervical spine interbody fusion cages

STUDY DESIGN

An in vitro biomechanical study of cervical spine interbody fusion cages using a sheep model was conducted.

OBJECTIVES

To evaluate the biomechanical effects of cervical spine interbody fusion cages, and to compare three different cage design groups.

SUMMARY AND BACKGROUND DATA

Recently, there has been a rapid increase in the use of cervical spine interbody fusion cages as an adjunct to spondylodesis. These cages can be classified into three design groups: screw, box, or cylinder designs. Although several comparative biomechanical studies of lumbar interbody fusion cages are available, biomechanical data for cervical spine constructs are lacking. Additionally, only limited data are available concerning comparative evaluation of different cage designs.

METHODS

In this study, 80 sheep cervical spines (C2-C5) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive stiffness method using a nonconstrained testing apparatus. Three-dimensional displacement was measured using an optical measurement system (Qualysis). Complete discectomy (C3-C4) was performed. Cervical spine interbody fusion cages were implanted according to manufacturers' information. Eight spines in each of the the following groups were tested: intact, autologous iliac bone graft, two titanium screws (Novus CTTi; Sofamor Danek, Koln, Germany), two titanium screws (BAK-C 8 mm; Sulzer Orthopedics, Baar, Switzerland), one titanium screw (BAK-C 12 mm; Sulzer Orthopedics), carbon box (Novus CSRC; Sofamor Danek), titanium box (Syncage; Synthes, Bochum, Germany), titanium mesh cylinder (Harms; DePuy Acromed, Sulzbach, Germany), titanium cylinder (MSD; Ulrich, Ulm, Germany), and titanium cylinder (Kaden; BiometMerck, Berlin, Germany). The mean apparent stiffness values were calculated from the corresponding load-displacement curves. Additionally, cage volume and volume-related stiffness was determined.

RESULTS

After cervical spine interbody fusion cage implantation, flexion stiffness increased, as compared with that of the intact motion segment. On the contrary, rotation stiffness decreased after implantation of a cervical spine interbody fusion cage, except for the Novus CSRC, Syncage, and Kaden-Cage. If two screws were inserted (Novus CTTi and BAK-C 8 mm), there was no significant difference in flexion stiffness between screw and cylinder design groups. If one screw was inserted (BAK-C 12 mm), flexion stiffness was higher for cylinder designs (P < 0.05). Extension and bending stiffness were always higher with cylinder designs (P < 0.05). Volume-related stiffness for flexion extension and bending was highest for the Harms cage (P < 0.05). There was no difference for rotation volume-related stiffness between Harms and Syncage.

CONCLUSIONS

The biomechanical results indicate that design variations in screw and cylinder design groups are of little importance. In this study, however, cages with a cylinder design were able to control extension and bending more effectively than cages with a screw design.

Comparison between sheep and human cervical spines: an anatomic, radiographic, bone mineral density, and biomechanical study

STUDY DESIGN

The quantitative anatomic, radiographic, computerized tomographic, and biomechanical data of sheep and human cervical spines were evaluated.

OBJECTIVES

To compare the anatomic, radiographic, computerized tomographic, and biomechanical data of human and sheep cervical spines to determine whether the sheep spine is a suitable model for human spine research.

SUMMARY OF BACKGROUND DATA

Sheep spines have been used in several in vivo and in vitro experiments. Quantitative data of the normal sheep cervical spine are lacking, yet these data are crucial to discussion about the results of such animal studies.

METHODS

In this study, 20 fresh adult female Merino sheep cervical spines and 20 fresh human cadaver cervical spines were evaluated anatomically, radiographically, computerized tomographically, and biomechanically. Three linear and two angular parameters were evaluated on four digital radiographic views: anteroposterior, right lateral in neutral position, flexion, and extension. Quantitative computed tomography scans at the center of each vertebral body and 3 mm below both endplates were analyzed for bone mineral density measurements. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending by a nondestructive stiffness method using a nonconstrained testing apparatus. Range of motion and stiffness of each motion segment were calculated. Additionally, 10 linear anatomic parameters of each vertebra were measured using a digital ruler.

RESULTS

Anterior and mean disc space height in the sheep cervical spine increased constantly from C2-C3 to C6-C7, whereas middle disc space height decreased and posterior disc space height remained unchanged. Anterior and mean disc space height were significantly higher in sheep. In both sheep and human cervical spines, intervertebral angles were not significantly different. Standard deviations of bone mineral density in the human cervical spine were fourfold higher than in the sheep cervical spine, yet no significant differences were found in bone mineral density values between the two species. Range of motion differed significantly between the two species except in flexion-extension of C3-C4, C5-C6, axial rotation of C2-C3, and lateral bending of C2-C3, C3-C4, and C4-C5. Stiffness also was significantly different except in flexion-extension of C2-C3, C4-C5, C5-C6, and lateral bending of C2-C3, C3-C4, and C4-C5. Anatomic evaluation showed no difference in upper endplate parameters for C4 and C5.

CONCLUSIONS

Although several differences were found between human and sheep cervical spines, the small intergroup standard deviations and the good comparability with the human spine encourage the use of the sheep cervical spine as a model for cervical spine research. On the basis of the quantitative data obtained in this study, the sheep motion segment C3-C4 seemed to be the most reliable model for the corresponding human motion segment.

Are sheep spines a valid biomechanical model for human spines?

STUDY DESIGN

Range of motion, neutral zone, and stiffness parameters of the complete cervical, thoracic, and lumbar sheep spine were determined in flexion and extension, axial left/right rotation, and right/left lateral bending.

OBJECTIVES

To determine quantitative biomechanical properties of the sheep spine and compare them with those from the human spine.

SUMMARY OF BACKGROUND DATA

Sheep spines often serve as a model for experimental in vivo and in vitro studies in spine research, but few quantitative biomechanical data from sheep spines for comparison with human specimens are available.

METHODS

Complete spines were sectioned into single-joint segments and tested in a spine tester under pure moments in the three main anatomic planes.

RESULTS

The craniocaudal variation in range of motion in all load directions was qualitatively similar between sheep spines and values reported in the literature for human specimens.

CONCLUSIONS

Based on the biomechanical similarities of sheep and human spines demonstrated in this study, it appears that the use of the sheep spine, which already includes evaluation of surgical techniques and bone healing processes, might be extended to spinal implants.

Anatomy of the sheep spine and its comparison to the human spine

BACKGROUND

The sheep spine is often used as a model for the human spine, although the degree to which these spines are anatomically comparable has yet to be categorically established. The purpose of this study was to investigate the characteristic anatomical dimensions of the sheep spine and to compare these with existing human data.

METHODS

Five complete spines were measured to determine 21 dimensions from the pedicles, spinal canal, transverse and spinous processes, facets, endplates, and disc.

RESULTS

The results showed that sheep and human vertebrae are most similar in the thoracic and lumbar regions, although they show substantial differences in certain dimensions. Morphological variations as a function of spine level typically were well matched in the two species.

CONCLUSIONS

Sheep spine may be a useful model for experiments related to the gross structure of the thoracic or lumbar spine, with certain limitations for the cervical spine. A thorough database has been provided for deciding the appropriateness of using the sheep spine as a model for the human spine.

Load-displacement properties of the thoracolumbar calf spine: experimental results and comparison to known human data

The availability of human cadaveric spine specimens for in vitro tests is limited and the risk of infection is now of vital concern. As an alternative or supplement, calf spines have been used as models for human spines, in particular to evaluate spinal implants. However, neither qualitative nor quantitative biomechanical data on calf spines are available for comparison with data on human specimens. The purpose of this study was to determine the fundamental biomechanical properties of calf spines and to compare them with existing data from human specimens. Range of motion, neutral zone, and stiffness properties of thoracolumbar calf spines (T6-L6) were determined under pure moment loading in flexion and extension, axial left/right rotation and right/left lateral bending. Biomechanical similarities were observed between the calf and reported human data, most notably in axial rotation and lateral bending. Range of motion in the lumbar spine in flexion and extension was somewhat less in the calf than that typically reported for the human, though still within the range. These results suggest that the calf spine can be considered on a limited basis as a model for the human spine in certain in vitro tests.