ABSTRACT STUDY DESIGN: An _in vitro_ and _in vivo_ study in rats. OBJECTIVES: To design a novel rat spinal fixation device and investigate its biomechanical effectiveness in stabilizing the

spine up to 8 weeks post injury. METHODS: A fixation device made of polyetheretherketone was designed to stabilize the spine via bilateral clamping pieces. The device effectiveness was

assessed in a Sprague–Dawley rat model after it was applied to a spine with a fracture–dislocation injury produced at C5–C6. Animals were euthanized either immediately (_n_=6) or 8 weeks

(_n_=9) post-injury and the C3-T1 segment of the cervical spine was removed for biomechanical evaluation. Segments of intact spinal columns (C3-T1) (_n_=6) served as uninjured controls. In

(average±s.d.) were 18.1±3.3°, 19.9±7.5° and 1.5±0.7°, respectively for the intact, injured/fixed, and injured/8-week groups, with the differences being highly significant for the

injured/8-week group (_P_=0.0002). Flexion/extension NZs were 3.4±2.8°, 5.0±2.4°, and 0.7±0.5°, respectively for the intact, injured/fixed, and injured/8-week groups, with the differences

being significant for the injured/8-week group (_P_=0.04). CONCLUSION: The device acutely stabilizes the spine and promotes fusion at the site of injury. SIMILAR CONTENT BEING VIEWED BY

OTHERS BIOMECHANICAL PROPERTIES OF A NOVEL NONFUSION ARTIFICIAL VERTEBRAL BODY FOR ANTERIOR LUMBAR VERTEBRA RESECTION AND INTERNAL FIXATION Article Open access 29 January 2021 BIOMECHANICAL

INFLUENCE OF THE SURGICAL APPROACHES, IMPLANT LENGTH AND DENSITY IN STABILIZING ANKYLOSING SPONDYLITIS CERVICAL SPINE FRACTURE Article Open access 16 March 2021 SPINAL INSTRUMENTATION LENGTH

AFFECTS ADJACENT SEGMENT RANGE OF MOTION AND INTRADISCAL PRESSURE Article Open access 16 December 2024 INTRODUCTION _In vivo_ experimental SCI studies involve animal models with survival

post-injury and recovery. In many of these models, the spinal column is substantially destabilized. Examples include single or double laminectomies for dorsal contusion injuries,1, 2

septuple hemi-laminectomy for dorsal rhizotomies,3 or anterior and posterior column damage after a fracture–dislocation2, 3, 4, 5 or distraction injury.2, 6 Failure to stabilize the spinal

column may complicate the progression of subsequent secondary damage at the site of injury and likely exacerbate pain; therefore it is important to properly stabilize the spinal column

post-injury. Different methods have been used to achieve stabilization of the spinal column post-injury in rat models.7, 8, 9 Spinous process wires as internal fixation of the vertebral

column is a time-consuming technique and it is difficult to achieve a consistent degree of fixation due to the unknown tensile load applied to the wires. Transpedicular screws and rods are

theoretically advantageous, but not feasible in the rat spine due to the small size of the rat vertebrae.7, 8 Other methods have used casting of the spinal column with dental cement or

tissue adhesive (Vet-bond),10 however, this method compromises circulation, introduces potentially neurotoxic chemicals (for example, methacrylate) and is prone to failure. There appears to

be a need for a more repeatable and validated method of spinal fixation in the rat spine. There is a well-established methodology to assess the biomechanical effectiveness of fixation

devices in stabilizing the spinal column. Panjabi11 first described the basic principles and there are many examples of these being applied to a wide range of devices for the human spine.11,

12, 13, 14, 15, 16 The main concept is that a fixation device is intended to decrease the relative motion between adjacent vertebrae. To assess this biomechanically, one applies known loads

to the specimen and measures the resultant displacements, either immediately after injury or after a certain healing time. A substantially reduced magnitude of displacement is generally

desirable, particularly in a situation where vertebral dislocation has been performed and fusion is a clinical goal. We are unaware of any studies that have evaluated a fixation device for

the rat spine in such a manner. The objectives of this study were to design a custom MRI-compatible rat cervical spinal stabilization device and to evaluate the biomechanical fixation

provided by this device at 8 weeks post-injury. It is hypothesized that the stiffness of the stabilized spinal column is higher than the unstabilized spine immediately after the injury, and

that the overall stiffness of the spinal column increases after 8 weeks owing to healing and scar tissue formation. MATERIALS AND METHODS DESIGN The fixation device for the rat spine met a

specific set of design criteria: structural stiffness, ease of surgical handling, versatility, size and cost. Several proposed designs were considered, all of which consisted of two parts

that clamp the vertebrae laterally. The two serrated edges on the vertebral clamps fit on the spine at the narrowing of the rat vertebrae that was anterior to the lateral masses and

posterior to the transverse processes (Figure 1a). The device–spine attachment was a friction-fit; thus, it was essential to create a sufficiently high normal force at this interface. Two

slots on the posterior aspect of the device (Figure 1b) accommodated attachment to different sizes of spinal columns and also enabled the user to apply a normal force to the clamps before

fixation. The clamping force was applied to the outside faces of the fixation device using custom-designed instrumented forceps. A preliminary analysis of the device-structural stiffness

predicted that it could provide adequate mechanical stability for the injured spine. The posterior-screw design was compact and easy for the surgeon to work within the microscopic field of

vision. The design was made of polyetheretherketone (PEEK), which is widely used in medical and clinical applications.17, 18 PEEK is a rigid, bio-inert polymer with excellent

biocompatibility and MRI-compatibility characteristics.19 INJURY PRODUCTION AND SPINE STABILIZATION All procedures were approved by the University of British Columbia Animal Care Committee

in accordance with the guidelines published by the Canadian Council on Animal Care. Fixation of the device was biomechanically evaluated in a rat cervical spine model in two stages; first

_in vitro_ and second _in vivo_ (Figure 2). The _in vitro_ assessments were conducted on intact specimens (control) (_n_=6, mass=293±12 g) and specimens that were injured and fixed

immediately post-injury (injured/fixed) (_n_=6, mass=293±12 g). The _in vivo_ experiment was done in one group, injured and fixed with a survival time of 8 weeks (injured/8-week) (_n_=9,

mass=291±11 g) (Figure 2). A small subgroup of animals was evaluated at 3 weeks post-injury and fixation (injured/3-week) (_n_=3). In the injured groups, animals were anesthetized (2–4%

isofluorane; 1 l/min O2) and prepared for surgery and administered a subcutaneous injection of lactated-Ringer's (Hospira, Lake Forest, IL, USA) and Buprenorphine (0.03 mg kg−1). The

cervical spine of the animal was exposed between C2 and C7, the dorsal ligaments between C5 and C6 were transected, and the C5/C6 facet joints were removed to mimic the type of posterior

element fracture and ligament injury seen in bilateral facet fracture–dislocation20 and to produce consistent injuries.21 The intact (control) group animals underwent identical surgical

procedures and were secured within the stereotactic frame. The caudal vertebral clamp was coupled to the actuator without the displacement being applied. The dislocation injury at C5/C6 was

created with an electromagnetic linear actuator in a multimechanism SCI system at 364 mm s−1 velocity and 1.5 mm displacement.2 The displacement level of 1.5 mm was based on pilot study

findings that produced an SCI with distinct histopathological damage and created mild–moderate deficits in the animals. The injured groups of specimens received spinal column stabilization

provided by the fixation device post-injury. The fixation device was inserted across the C5/C6 segments using custom-instrumented surgical forceps to ensure the application of a consistent

clamping force. A small stretching load was applied on the spinal column to straighten it while implanting the device. While clamping the device on the vertebrae, strain measurements from

the instrumented forceps were monitored and recorded. Once the appropriate range of force magnitude was reached, the screws were tightened and the fixation device was secured to the spinal

column. Following the injury and fixation, the _in vitro_ group of animals were euthanized while deeply anesthetized by an over-dose of mixed ketamine hydrochloride (72 mg kg−1) and xylazine

hydrochloride (9 mg kg−1) at the corresponding time point. The spinal column was harvested from C3 to T1. In the animals designated to the _in vivo_ survival study, the muscle and the skin

was sutured over the fixation clamp. The animals were allowed to recover normally, and appropriate post-operative procedures were taken. Specific quantitative behavioral evaluation was not

conducted as it was outside the scope of this study. At 8 weeks post injury, the _in vivo_ group of animals was euthanized and the spinal columns were harvested similar to the _in vitro_

group. Note that three additional injured animals were euthanized and tested at 3 weeks post-injury for a qualitative analysis at this time point. The number of specimens in this group was

not statistically sufficient; therefore, the results at this time point were not used to compare the spine stabilization across the other groups at different time points. BIOMECHANICAL

EVALUATION OF THE IMPLANT A custom-made experimental apparatus was designed to biomechanically evaluate the excised spinal column. Because of the small size of the rat spinal column, small

and wireless visual markers were used to not obstruct or interfere with the spinal motion. The markers consisted of two small circular ‘+’ signs (3 mm diameter) glued to pins (Figure 3a),

that were inserted in the C5 and C6 vertebrae. The spinal column (C3-T1) was mounted on a screw with a similar diameter as the spinal canal (Figure 3a) such that it did not interfere with

the motion at C5/C6. A shorter screw was inserted into the cranial two vertebrae. The top screw was attached to a balancing weight (9.68 g) connected via a string to hold the column

vertically. Another string attached to the top screw applied known shear forces at distance _d_ (moment arm) from C5/C6 (Figure 3a). The shear forces of 0.24, 0.50, 0.74 and 0.98 N created

bending moments of flexion or extension at C5/C6, depending upon the direction of the application (Figure 3b). The moment arm ‘_d_’ was (mean±s.d.) 13±1 mm, producing bending moments of 3.1,

6.5, 9.6 and 12.7 Nmm. Moment arm ‘_d_’ was measured when the spine was vertically oriented. Stepwise loads were applied from lowest to highest, per standard protocols for flexibility

testing11, 12, 13 (Figure 4). The maximum load applied for the injured/fixed group was 0.74 N to avoid producing additional tissue damage in specimens that had been dislocated. Between each

loading step, a time window of 30 s was given to allow the specimen to creep.11, 13 To reduce cyclic viscoelastic effects, loading in flexion and extension was repeated three times and the

last cycle was selected for the motion measurements (Figure 4). The position of markers was tracked using camera snapshots (Phantom V9 camera, Vision Research, Wayne, NJ, USA). The change in

angle between the markers placed into the two vertebrae represented their relative motion under different loads. The accuracy and precision of measuring 2D rotations using this motion

capture method was found to be 0.5° and 0.2°, respectively. Rotation–moment graphs for individual specimens were generated to find neutral zone (NZ) and range of motion (ROM) values. NZ was

defined as the region within which the spinal motion was produced with no external load at the beginning of the third loading cycle.11 The entire motion measured from the neutral position

was defined as ROM. These values were analyzed for both flexion and extension motion on the third cycle of loading to obtain the total NZ and ROM of the specimen (Figure 4). STATISTICAL

ANALYSIS Our hypothesis that the device-spinal column motion over time was less than the injured spinal column immediately after the injury was tested using a one-way analysis of variance at

a 95% level of significance (parameters: ROM and NZ). The Kolmogorov–Smirnov test was performed to determine the normality of the ROM and NZ data sets. A _post hoc_ test, Student

Newman–Keuls test was used for pairwise comparisons.22 All the statistical analyses were analyzed using Statistica 7 (StatSoft, Tulsa, Oakland, CA, USA). RESULTS Typical moment rotation

curves for one specimen in the three experimental groups are shown in Figure 5, and the summary data for the study groups is shown in Table 1. The magnitudes of intervertebral motion for the

three experimental groups are shown in Figure 6. The intact moment–rotation curve is highly nonlinear as is observed for human specimens (Figure 5). After injury and fixation with the

custom device, the response was a similar motion magnitude to intact, which represents substantial stabilization given that this represents the response of a fixed joint after dislocation.

The intact group was not different from the injured/fixed group for either NZ or ROM (NZ, _P_=0.2; ROM, _P_=0.5) (Figure 6). After 8 weeks of healing, note that the motion of this joint was

decreased substantially to less than 20% of the intact (Figure 5). For both NZ and ROM, the motions after 8 weeks were significantly less than the other two groups (NZ, _P_=0.04; ROM,

_P_=0.0002) (Figure 6). Motion of the spine after the device was removed remained small (Table 1; NZ, 0.7deg, ROM, 1.5deg). For comparison, the injured/3-week specimen NZ and ROM values

(with the device removed) were 1.8±1.5 and 4.1±3.1, respectively (Mean±s.d.). Therefore, there was already substantial stabilization due to healing at this early timepoint. DISCUSSION This

study highlights the design and biomechanical evaluation of a novel fixation device for rodents. The two overall objectives of this project were to design a custom MRI-compatible spinal

stabilization device for the unstable cervical spinal column and to evaluate the biomechanical fixation provided by this device immediately after injury and at 8 weeks post-injury. The

performance of the implant was evaluated by measuring the degree of biomechanical fixation that it provided to the injured spine. The results of this study demonstrated that the

custom-designed fixation device was effective in stabilizing the dislocated rat cervical spine at both time points of observation. In this study, a two-dimensional biomechanical analysis was

implemented. To fully characterize the device, a three-dimensional assessment is required. However, the rat's spine did stabilize within the 8-week healing period; thus, the initial

sagittal plane analysis was deemed to be sufficient for device evaluation. Currently, there is no study in the literature reporting the change of intervertebral motion in a rat model. There

are many previous human and animal _in vitro_ studies that report post-surgical spinal motion and generally, the spinal motion post-surgery is lower than the intact level.23, 24 Obviously

this result depends upon the injury model and the type of fixation device used. In an _in vivo_ ovine model, a destabilizing procedure was performed in the lumbar spine followed by the

application of a fixation device, and results showed that 16 and 43% reductions in ROM were achieved with the fixation device and with fixation along with the band compared with the

intact.24 In a human cadaveric model, the C4/C5 ROM of the intact specimens and those with the arthroplasty device were similar and about 75% more than the specimens with the fusion

construct.23 These studies show that a wide range of reduction in intervertebral motion is achieved immediately after fusion operation, up to 75% reduction compared with the intact motion,

and indicate that the severity of the injury model is an important variable. The device design is clearly another determining factor in restabilizing the injured spine to the intact level.

The current study found the reductions in NZ and ROM at 8 weeks post-injury to be 79 and 92%. These values are comparable to the change in intervertebral motion in other animal models. In an

_in vivo_ rabbit model,25 the NZ and ROM of the fused specimens were significantly decreased from that of intact specimens in flexion and extension (60–80%). In a bovine model with 16 weeks

of healing,26 it was found that after removing a fixation plate, subjects with allograft bone were about 30% stiffer than the subjects with a titanium cage implant, possibly indicating

fusion at the construct. In a similar study on interbody fusion, results demonstrated a trend of increased fusion stiffness from 3 to 24 months.27 A novel spinal fixation device for rats was

designed and its performance in providing biomechanical fixation to an injured spine confirmed that the device provided stabilization of the cervical spine post-injury, both acutely and

after 8 weeks. This device is suitable for dorsal dislocation injury and enables long-term survival studies. Potential benefits of this device could include a reduction in secondary damage

and the volume of scar tissue in the spinal cord post-injury.9 It will be used in future survival studies to further investigate mechanisms of spinal cord injury, characterize cord

deformation using MRI, and study the effects of different treatment methods over time. The device design may have utility in a distraction injury model, but further assessment is needed. We

have identified that the sharpness of the teeth and the tightness of the screw fixation on the PEEK material may be important factors in the effectiveness of the device in this injury model.

It can be customized for use in the thoracic or lumbar regions of the spine, as well as for other strains of rats or mice depending on their size and vertebral structure. This study

furthers fundamental understanding of SCI mechanics in survival studies and helps facilitate future studies establishing a link between injury-specific observations and clinical treatment of

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Sciences and Engineering Research Council of Canada and the Canadian Institutes for Health Research. The technical assistance of Peggy Assinck Femke Streijger, and Jennifer Douglas-Mills is

gratefully acknowledged. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Orthopaedic and Injury Biomechanics Group (Departments of Orthopaedics and Mechanical Engineering), The University of

British Columbia and Vancouver Coastal Health Research Institute, Vancouver, British Columbia, Canada M Shahrokni, Q Zhu & T R Oxland * International Collaboration On Repair Discoveries,

The University of British Columbia and Vancouver Coastal Health Research Institute, Vancouver, British Columbia, Canada M Shahrokni, Q Zhu, J Liu, W Tetzlaff & T R Oxland Authors * M

Shahrokni View author publications You can also search for this author inPubMed Google Scholar * Q Zhu View author publications You can also search for this author inPubMed Google Scholar *

J Liu View author publications You can also search for this author inPubMed Google Scholar * W Tetzlaff View author publications You can also search for this author inPubMed Google Scholar *

T R Oxland View author publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHORS Correspondence to W Tetzlaff or T R Oxland. ETHICS DECLARATIONS

COMPETING INTERESTS The authors declare no conflict of interest. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Shahrokni, M., Zhu, Q., Liu, J. _et al._

Design and biomechanical evaluation of a rodent spinal fixation device. _Spinal Cord_ 50, 543–547 (2012). https://doi.org/10.1038/sc.2011.185 Download citation * Received: 23 August 2011 *

Revised: 17 November 2011 * Accepted: 01 December 2011 * Published: 31 January 2012 * Issue Date: July 2012 * DOI: https://doi.org/10.1038/sc.2011.185 SHARE THIS ARTICLE Anyone you share the

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Nature SharedIt content-sharing initiative KEYWORDS * spinal cord injury * biomechanics * implant * fixation device * rat model * dislocation