Intervertebral Biomechanics of Locking Compression Plate Monocortical Fixation of the Canine Cervical Spine

Cervical spine reconstruction after total vertebrectomy using customized three-dimensional-printed implants in dogs

BACKGROUND

Sufficient surgical resection is necessary for effective tumor control, but is usually limited for vertebral tumors, especially in the cervical spine in small animal neurosurgery.

OBJECTIVE

To evaluate the primary stability and safety of customized three-dimensional (3D)-printed implants for cervical spine reconstruction after total vertebrectomy.

METHODS

Customized guides and implants were designed based on computed tomography (CT) imaging of five beagle cadavers and were 3D-printed. They were used to reconstruct C5 after total vertebrectomy. Postoperative CT images were obtained to evaluate the safety and accuracy of screw positioning. After harvesting 10 vertebral specimens (C3-C7) from intact (group A) and implanted spines (group B), implant stability was analyzed using a 4-point bending test comparing with groups A and C (reconstituted with plate and pins/polymethylmethacrylate after testing in Group A).

RESULTS

All customized implants were applied without gross neurovascular damage. In addition, 90% of the screws were in a safe area, with 7.5% in grade 1 (< 1.3 mm) and 2.5% in grade 2 (> 1.3 mm). The mean entry point and angular deviations were 0.81 ± 0.43 mm and 6.50 ± 5.11°, respectively. Groups B and C significantly decreased the range of motion (ROM) in C3-C7 compared with intact spines (p = 0.033, and 0.018). Both groups reduced overall ROM and neutral zone in C4-C6, but only group B showed significance (p = 0.005, and 0.027).

CONCLUSION

Customized 3D-printed implants could safely and accurately replace a cervical vertebra in dog cadavers while providing primary stability.

Mapping Subchondral Bone Density Distribution in the Canine C6-C7 Vertebral Endplates: A CT-OAM Study

Intervertebral cage subsidence is a common complication in treating disc-associated cervical spondylomyelopathy in dogs. The mechanical stability of the vertebral endplate in contact with the cage is crucial to preventing subsidence. This study aims to assess subchondral bone mineral density (sBMD) in the canine vertebral endplate (specifically, the C6-C7 vertebral motion unit) as a measure of its mechanical stability. The sBMD distribution was mapped for the C6 caudal and C7 cranial vertebral endplates in 15 middle- to large-breed dogs using computed tomography osteoabsorptiometry. The sBMD distribution in the canine C6 and C7 vertebral endplates exhibited a heterogeneous pattern, with lower density observed in the central and dorsal contact areas of the nucleus pulposus, where common subsidence occurs. Our results suggest a potential need to redesign intervertebral cages to ensure that contact areas align with regions of higher bone density. A broad-based design extending toward the lateral and dorsal aspects of the annulus fibrosus contact area may enhance stability.

Instrumented cervical fusion using patient specific end-plate conforming interbody devices with a micro-porous structure in nine dogs with disk-associated cervical spondylomyelopathy

Objective

To report the medium and long-term outcome of nine dogs with disk-associated cervical spondylomyelopathy (DA-CSM), treated by instrumented interbody fusion using patient specific end-plate conforming device that features a micro-porous structure to facilitate bone in-growth.

Study design

A retrospective clinical study.

Animals

Nine medium and large breed dogs.

Methods

Medical records at two institutions were reviewed between January 2020 and 2023. Following magnetic resonance imaging (MRI) diagnosis of DA-CSM, pre-operative computed tomography (CT) scans were exported to computer software for in-silico surgical planning. Interbody devices were 3D-manufactured by selecting laser melting in titanium alloy. These were surgically implanted at 13 segments alongside mono-or bi-cortical vertebral stabilization systems. Follow-up included neurologic scoring and CT scans post-operative, at medium-term follow up and at long-term follow-up where possible. Interbody fusion and implant subsidence were evaluated from follow-up CT scans.

Results

Nine dogs were diagnosed with DA-CSM between C5-C7 at a total of 13 operated segments. Medium-term follow up was obtained between 2 and 8 months post-operative (3.00 ± 1.82 months). Neurologic scoring improved (p = 0.009) in eight of nine dogs. Distraction was significant (p < 0.001) at all segments. Fusion was evident at 12/13 segments. Subsidence was evident at 3/13 operated segments but was only considered clinically relevant in one dog that did not improve; as clinical signs were mild, revision surgery was not recommended. Long-term follow up was obtained between 9 and 33 months (14.23 ± 8.24 months); improvement was sustained in 8 dogs. The dog that suffered worsened thoracic limb paresis at medium-term follow up was also diagnosed with immune-mediated polyarthropathy (IMPA) and was euthanased 9 months post-operative due to unacceptable side-effects of corticosteroid therapy.

Conclusion

End-plate conforming interbody devices with a micro-porous structure were designed, manufactured, and successfully implanted in dog with DA-CSM. This resulted in CT-determined fusion with minimal subsidence in the majority of operated segments.

Clinical significance

The technique described can be used to distract and fuse cervical vertebrae in dogs with DA-CSM, with favorable medium-and long-term outcomes.

Comparison of Cervical Stabilization with Transpedicular Pins and Polymethylmethacrylate versus Transvertebral Body Polyaxial Screws with or without an Interbody Distractor in Dogs

OBJECTIVE

 The main aim of this study was to compare the biomechanical properties of caudal cervical vertebral stabilization using bicortical transpedicular pins with polymethylmethacrylate (PMMA) versus transvertebral body polyaxial screws and connecting rods with or without an interbody distractor.

STUDY DESIGN

 Ten canine cervical vertebral columns (C2-T3) were used. Four models (intact, transvertebral body polyaxial screw with interbody distractor [polyaxial + distractor], transvertebral body polyaxial screw without interbody distractor [polyaxial - distractor] and bicortical transpedicular pins/polymethylmethacrylate [pin-PMMA]) were applied to C6-7 sequentially on the same specimens. Angular range of motion (AROM) in the form of flexion and extension was measured at C4-5, C5-6 and C6-7 in all groups.

RESULTS

 Treated vertebral specimens had significantly less AROM than unaltered specimens. There was no significant difference in AROM between the experimental groups at C6 and C7. Angular range of motion ratio in flexion-extension was 80.8, 72.7 and 78.3% for polyaxial + distractor, polyaxial - distractor and pin-PMMA groups, respectively, which were less than the intact group. There was no significant increase in the range of motion of the adjacent vertebrae after stabilization.

CONCLUSION

 Stabilization obtained with transvertebral body polyaxial screws was comparable to that from the well-established bicortical pins/PMMA construct. Association of an intervertebral distractor did not change AROM of the polyaxial screw constructs.

Kinematics of a Novel Canine Cervical Fusion System

OBJECTIVE

 The aim of this study was to determine the biomechanical behaviour of a novel distraction-fusion system, consisting of an intervertebral distraction screw, pedicle locking screws and connecting rods, in the canine caudal cervical spine.

STUDY DESIGN

 Biomechanical study in cadaveric canine cervicothoracic (C3-T3) spines (n = 6). Cadaveric spines were harvested, stripped of musculature, mounted on a four-point bending jig, and tested using non-destructive four-point bending loads in extension (0-100 N), flexion (0-60 N) and lateral bending (0-40 N). Angular displacement was recorded from reflective optical trackers rigidly secured to C5, C6 and C7. Data for primary and coupled motions were collected from intact spines and following surgical stabilization (after ventral annulotomy and nucleotomy) with the new implant system.

RESULTS

 As compared with the intact spine, instrumentation significantly reduced motion at the operated level (C5-C6) with a concomitant non-significant increase at the adjacent level (C6-C7).

CONCLUSION

 The combination of a locking pedicle screw-rod system and intervertebral spacer provides an alternative solution for surgical distraction-stabilization in the canine caudal cervical spine and supports the feasibility of using this new implant system in the management of disc-associated cervical spondylomyelopathy in dogs. The increase in motion at C6-C7 may suggest the potential for adjacent level effects and clinical trials should be designed to address this.

Optimal safe implantation corridors in feline cervical vertebrae (C2-T1): CT study in 16 domestic shorthair cats

OBJECTIVES

The objective of this study was to define safe corridors for the optimal placement of bicortical implants in the feline cervical spine (C2-T1) using CT.

METHODS

CT images of feline cervical spines (n = 16) were reviewed retrospectively. Multiplanar reconstructions were used to define the optimal safe corridors. Safe corridors were defined by their angle of insertion, width and length. The insertion point within the vertebral body was also described. Vertebral measurements were compared between vertebrae using multilevel linear regression, and left and right measurements within vertebrae were compared with the paired samples Wilcoxon signed-rank test. A P value <0.05 was considered significant for all analyses.

RESULTS

The safe corridor insertion points were located within the caudal third of the vertebral body in C2 (mean cranial vertebral ratio 0.73) and in the cranial third of the vertebral bodies from C3-T1 (mean cranial vertebral ratios 0.34-0.38). Mean safe corridor widths ranged from 1.04 mm in C2 to 2.30 mm in C7 and T1. The mean right and left optimal angles of implantation were, respectively, 21.79° and 21.49° for C2, 45.26° and 46.19° for C3, 51.48° and 51.04° for C4, 53.52° and 54.30° for C5, 56.36° and 56.65° for C6, 63.40° and 64.92° for C7, and 53.90° and 52.90° for T1. There were statistically significant differences between vertebrae in almost every measurement.

CONCLUSIONS AND RELEVANCE

Cervical vertebral safe corridors in cats are narrow and differ to those reported in dogs. Safe corridors are located in the caudal third of C2 and cranial third of the C3-T1 vertebral bodies. Current recommendations for implant sizes should be reviewed, as 1.5-2 mm implants would be oversized for bicortical implantation in most of the feline cervical vertebrae.

Spinal fracture in a dog with diffuse idiopathic skeletal hyperostosis

A six-year-old, spayed female Weimaraner dog was first presented with the complaint of hindlimb paresis and then hindlimb paralysis two years later after colliding with a tree. Radiographs and computed tomography revealed spinal fractures at lumbar vertebrae (L)2-3 and at L4-5. In addition, the spinal column was affected by new bone formation along the vertebral bodies, bridging the disc spaces, as seen in diffuse idiopathic skeletal hyperostosis (DISH). Open reduction and internal fixation was achieved with standard vertebral body plating. This is the first report of DISH-associated spinal fractures after minor trauma in a dog. Surgery resulted in return of the full function after the first, and in improvement of neurologic function after the second incident.

Biomechanical comparison between pins and polymethylmethacrylate and the SOP locking plate system to stabilize canine lumbosacral fracture-luxation in flexion and extension

OBJECTIVE

To determine the stability of a simulated complete L7-S1 fracture-luxation immobilized with SOP locking plate system, compared to pins and polymethylmethacrylate (PMMA).

STUDY DESIGN

In vitro biomechanical study.

ANIMALS

Cadaver specimens of 18 skeletally mature large-breed dogs.

MATERIALS AND METHODS

Specimens were randomly stabilized with one of the two fixation techniques. Lumbosacral spine specimens (L5-S3) were subjected to a bending moment applied to the caudal and cranial ends of the specimen. The biomechanical parameters (ie, range of motion [ROM], neutral zone [NZ], and elastic zone stiffness [EZS]) were compared between fixation techniques.

RESULTS

No difference was found between the means of the NZ in flexion (P = .3458), extension (P = .1255), and total value (P = .3458) of L7-S1 stabilized with the two fixation techniques. Mean ROM in flexion (P = .2386), extension (P = .1255), and mean of EZS in extension (P = .4094) did not differ between fixations. The only significant differences were in the means of total ROM and means of the EZS in flexion, with the means being smaller with SOP fixation.

CONCLUSION

The stability of the two fixation techniques in flexion and in extension was similar for the L7-S1 and adjacent L5-L6 junctions, while the mean of ROM of L6-L7 in flexion was smaller with SOP fixation.

CLINICAL RELEVANCE

Stability of the resulting construct should be considered when selecting an implant. Our results provide evidence that fixation via pin-PMMA or SOP provide similar stability for L7-S1 fracture-luxation. In this context, other factors become more important in selecting the fixation method.

Biomechanical assessment of the effects of vertebral distraction-fusion techniques on the adjacent segment of canine cervical vertebrae

OBJECTIVE To assess effects of vertebral distraction-fusion techniques at a treated segment (C5-C6) and an adjacent segment (C4-C5) of canine cervical vertebrae. SAMPLE Cervical vertebrae harvested from cadavers of 10 skeletally mature Beagles. PROCEDURES Three models (intact, titanium plate, and polymethylmethacrylate [PM MA]) for stabilization of the caudal region of the cervical vertebrae (C4 through C7) were applied to the C5-C6 vertebral segment sequentially on the same specimens. Biomechanical assessments with flexion-extension, lateral bending, and axial rotational tests were conducted after each procedure. Range of motion (ROM) for a torque load applied with a 6-axis material tester was measured at C4-5 and C5-6 and calculated by use of a 3-D video measurement system. RESULTS In both the plate and PMMA models, ROM significantly increased at C4-5 and significantly decreased at C5-6, compared with results for the intact model. The ROM at C5-6 was significantly lower for the plate model versus the PMMA model in lateral bending and for the PMMA model versus the plate model in axial rotation. Conversely, ROM at C4-5 was significantly higher in axial rotation for the PMMA model versus the plate model. No significant differences were identified in flexion-extension between the PMMA and plate models at either site. CONCLUSIONS AND CLINICAL RELEVANCE Results of this study suggested that vertebral distraction and fusion of canine vertebrae can change the mechanical environment at, and may cause disorders in, the adjacent segment. Additionally, findings suggested that effects on the adjacent segment differed on the basis of the fusion method used.

Comparison of the biomechanical properties of a ventral cervical intervertebral anchored fusion device with locking plate fixation applied to cadaveric canine cervical spines

OBJECTIVE

To evaluate fixation properties of a new intervertebral anchored fusion device and compare these with ventral locking plate fixation.

STUDY DESIGN

In vitro biomechanical evaluation.

ANIMALS

Cadaveric canine C4-C7 cervical spines (n = 9).

METHODS

Cervical spines were nondestructively loaded with pure moments in a nonconstraining testing apparatus to induce flexion/extension while angular motion was measured. Range of motion (ROM) and neutral zone (NZ) were calculated for (1) intact specimens, (2) specimens after discectomy and fixation with a purpose-built intervertebral fusion cage with integrated ventral fixation, and (3) after removal of the device and fixation with a ventral locking plate.

RESULTS

Both fixation techniques resulted in a decrease in ROM and NZ (P < .001) compared with the intact segments. There were no significant differences between the anchored spacer and locking plate fixation.

CONCLUSION

An anchored spacer appears to provide similar biomechanical stability to that of locking plate fixation.

Ventral stabilization and facetectomy in a Great Dane with wobbler syndrome due to cervical spinal canal stenosis

This report describes the management of clinically significant, single level cervical spinal canal stenosis associated with articular facet hypertrophy in a three-year-old Great Dane dog, by combined surgical decompression and spinal stabilization. Spinal column stabilization was achieved by ventral application of two String of Pearls locking plates and subsequent decompression was accomplished by unilateral facetectomy. The six-month postoperative reassessment examination confirmed satisfactory clinical outcome with a complete return to normal activity, resolution of neurological deficits and maintenance of implant positioning.