Clinique vétérinaire

du Dr Bardet

Caudal cervical arthrodesis using a distractable fusion cage

This case report describes a cervical fusion cage, surgical technique and the long-term outcome of caudal cervical arthrodesis used to stabilise dynamic spinal cord compression at the sixth and seventh cervical intervertebral disc space (C6, C7) in a dog. A seven-year-old, 41 kg, entire male Doberman Pinscher was admitted for progressive ataxia of two weeks duration. Neurological examination revealed ambulatory tetraparesis. Computed tomographic myelogram scans in neutral and traction positions of the neck were performed and were used to determine presence of a dynamic component. A C6-C7 surgical distraction and stabilisation using a distractable intervertebral fusion cage was performed. There was not any deterioration of neurological status was observed on postoperative neurological evaluation. Within eight weeks after surgery, gait and postural abilities had re-turned to normal. Computed tomography evaluation indicated a complete bridging callus within and outside the cage at 15 weeks after surgery. There were not any complications or recurrences of initial neurological deficits observed during the 40 month follow-up period. Based on the follow-up period data, a C6-C7 dynamic spinal cord compression with disc protrusion was successfully treated by a distractable cervical fusion cage.

Introduction

Canine caudal cervical spondylomyelopathy (CCSM) affects large and giant-breed dogs, particularly Dobermann Pinschers and Great Danes. In the Dobermann, the lesions are frequently located at the fifth to sixth cervical (C5-C6) and C6-C7 intervertebral disc spaces (1–3). Spinal cord compression due to degenerative disc disease, hypertrophy of the dorsal aspect of the annulus fibrosis, and hypertrophy of the dorsal longitudinal ligament is often dynamic; the extent of cord compression can vary with flexion, extension, and linear traction (distraction) applied to the cervical spine (1–3). Based on the results of dynamic myelographic studies or traction computed tomography (CT) studies, it is possible to distinguish between static (traction non-responsive) lesions, dynamic (traction-responsive) lesions, and positional lesions (1, 2).

The treatment for dynamic CCSM lesions is still controversial. Medical management of dynamic CCSM may result in clinical improvement, but that improvement is often transient, and progression to an unacceptable neural status is common (2). A recent clinical study of CCSM found that a beneficial outcome was associated with nonsurgical treatment in 54% of dogs, and that the neural status of 81% of dogs treated surgically was improved, however the difference between these two outcomes was not significant (4). A variety of surgical techniques have been proposed for CCSM, with many of the authors claiming success rates between 70 and 90 % (1, 3). The goals of surgical intervention are relief of spinal cord compression, stabilisation of the cervical vertebral column, and reversal of neural deficits (1, 2). The selection of a surgical technique should take into consideration the success rate as well as the potential risks of complication such as implant failure, in-complete bony fusion, iatrogenic spinal cord injury, surgical site infection, and adjacent segment disease (1–3).

Various types of interbody fixation systems are currently available for use in the treatment of cervical disc pathology in humans. Since the time when Bagby developed the first interbody stainless-steel basket, a variety of other cage designs have been developed (5). The use of intervertebral body fusion cages to achieve interbody arthrodesis in humans is rapidly gaining acceptance, and several studies have re-ported fusion and satisfactory clinical out-come with the use of these devices (6–8). Our aim was to describe a purpose-de-signed, distractable intervertebral fusion cage to be used for caudal cervical arthrodesis in a dog that had dynamic CCSM at the C6-C7 intervertebral disc spaces. The surgical preparation, implantation and long-term follow-up in this case are reported.

Case report

A seven-year-old, 41 kg entire male Dobermann Pinscher dog was referred to our clinic because it had progressive tetra-ataxia of two-week duration that was more evident in the pelvic limbs, as well as knuckling of the thoracic limbs. Treatment of the dog with prednisone (60 mg, orally once daily for 4 days) prior to presentation had been ineffective, although the degree of rigidity in the thoracic limbs had temporarily decreased.

Physical examination revealed pro-found cervical stiffness. Neurological examination revealed ambulatory tetra-paresis with diminished postural responses in all four limbs and stiffness of the thoracic limbs. Spasticity of the thoracic limb extensor muscles, but not the flexor muscles, in response to noxious stimuli was evident. Signs of pain were obvious on evaluation of the cervical region. The results of hematology, serum biochemical analyses and urinalysis were unremarkable. On the basis of these findings, a lesion located between the C6 and second thoracic spinal cord segments was suspected.

Following induction of general anaesthesia, lateral and ventro-dorsal survey radiographs of the cervical spine showed signs of mild narrowing of the C6-C7 disc space and sclerotic changes in the vertebral endplates. A CT (a) myelogram (b) with the neck in neutral and traction positions was per-formed. The neutral position CT myelogram revealed substantial ventral, extradural compression of the spinal cord at the level of the C6-C7 intervertebral disc space that was associated with intervertebral disc protrusion (Fig. 1 and 2). In the traction views this compressive component was no longer evident. There were not any degenerative changes observed in the adjacent disc spaces. A C6-C7 dynamic ventral spinal cord compression with intervertebral disc protrusion (disc-associated wobbler syndrome) was diagnosed.

Arthrodesis of the C6-C7 was perform-ed using a purpose-designed intervertebral fusion cage (c). The small, fenestrated, hollow titanium-alloy cage used in this patient was 15 mm in length, 8 mm in height and 9.9 mm in thickness (Fig. 3). It was trapezoidal in shape and composed of large cranial and caudal distractable wings that were supported by a central cross peg. The central cross peg was connected to the ventral aspect of the cage by a screw with a locking mechanism. Tightening the screw moved the central cross peg up, causing distraction of the cranial and caudal wings. Because of the cage expansion, the implant worked as an inlay which prevented dislocation or migration of the cage. In addition, the toothed outer walls prevented implant back-out.

Fig. 1 Preoperative sagittal computed tomography myelogram image, in neutral position. Notice the substantial attenuation of the ventral and dorsal contrast columns between the sixth and seventh cervical vertebrae (ventral compression of spinal cord). There is also significant end-plate disease with ventral reactive bone present and narrowed disc space.
Fig. 2 Preoperative tridimensional bone window computed tomography image (right half) in neutral position. Notice the decreased ventro-dorsal spinal canal height at the level of C6-C7 intervertebral space.
Fig. 3 Titanium alloy fusion cage.

A) When the screw is turned, the central cross peg moves up (vertical arrow), promoting distraction of the cranial and caudal wings (horizontal arrows).
B) Horizontal view of the intervertebral fusion cage.

The dog was positioned in dorsal recumbency for the surgery. Cancellous bone autograft was harvested from the proximal metaphysis of the right humerus and stored on a blood-soaked gauze sponge to prevent drying. A standard ventral approach to the sixth and seventh cervical vertebrae was made with the neck in neutral position. The C6-C7 intervertebral disk was fenestrated using an 11-scalpel blade. Using a high-speed pneumatic drill, a ventral slot was created, preserving as much of the caudal edge of C6 as possible. The slot was ex-tended in cranial and caudal directions by removal of cortical bone in the midsagittal plane from the endplates of C6 and C7 to the level of cancellous bone, and following the oblique angle of the intervertebral space. The dimensions of the slot were made 0.5 mm to 1 mm smaller than the dimensions of the fusion cage. The ventral aspect of the slot was made wider than the dorsal aspect (inverted V-shape) to avoid migration of the implant once the cage was distracted. The dorsal part of the annulus fibrosus was not removed. The cage was packed with cancellous bone autograft from the humerus and press fitted into the slot space that was being distracted by manual application of linear traction to the dog's head. Some additional cancellous bone autograft was packed around the ventral surface and endplate regions of the vertebrae adjacent to the cage, before manual linear traction ceased. Finally, the screw on the ventral aspect of the cage was tightened to open the fusion cage eccentrically by 1– 2 mm in a cranio-caudal direction to pro-mote additional intervertebral distraction. Routine closure of the surgical site was per-formed. Cephalexin (d) (20 mg/kg, orally three times daily for 5 days), meloxicame (0.1 mg/kg, orally once daily for 4 days) and tramadolf (3 mg/kg, orally twice daily for 7 days) were administered postoperatively.

Caudal cervical dorsoventral and lateral radiographs were taken immediately after surgery (Fig. 4). Following recovery, the bladder was intermittently catheterized every eight hours and emptied completely. Twenty-four hours after surgery the dog was able to urinate spontaneously and completely empty its bladder. There was not any deterioration in neurological status observed postoperatively. On the second day after surgery, the dog did not appear to exhibit any signs of pain, and the rigidity of the thoracic limbs had decreased. The dog was discharged on the fourth day after surgery, and then evaluated four weeks after surgery. The dog walked and ran with agility, but would intermittently bear weight on the dorsal surface of the paws (knuckling) on the thoracic limbs. No signs of pain or discomfort were detected on manipulation of the head and neck. Gait and postural abilities returned to normal within eight weeks after surgery. There was not any recurrence or deterioration of neurological status observed during this period. Radiographic and CT examinations were performed at four, 10, 15, and 24 weeks postoperatively, then every six months during an 18 month period to allow evaluation of the arthrodesis for evidence of cage migration and quality of fusion. The last CT examination was made at 34 months after surgery to rule out any cervical problem. Radiographic examination revealed new bone formation around the cage at 10 weeks postoperatively (Fig.5). Evaluation by CT imaging at 15 weeks after surgery indicated complete bridging callus within and around the cage – a continuous bony bridging from endplate to endplate and through the cage fenestrations was observed (Fig. 6).

Fig. 4 Immediate postoperative lateral radiographic view after implantation of the distractable fusion cage to maintain distraction of the C6-C7 inter- vertebral space and to promote cervical stability and arthrodesis.
Fig. 5 Lateral radiographic view 10 weeks post-operatively of the cervical vertebral column. Note the optimal implant location and vertebral sagittal alignment; a bone bridge from endplate to endplate (C6-C7) was observed.
Fig. 6 Axial (transverse) bone window computed tomography image 15 weeks after C6-C7 intervertebral cage implantation surgery, on which the endoprosthetic bone bridge is well visualised.

The apparent bone density measured by quantitative CT in the intervertebral disc space (780 Hounsfield units) was similar to the adjacent bone within the vertebral bodies (660 Hounsfield units; normal range 200 to 1000 Hounsfield units). Based on these data, the C6 and C7 intervertebral disc space appeared completely fused. Nar-rowing of the disc spaces or sclerotic changes in the vertebral endplates were not observed in the adjacent disc spaces on CT examinations before surgery nor during the 34 month CT follow-up in this case.

Forty months after surgery, the owners were contacted by telephone for follow-up. During this conversation, the owners indicated that the dog remained clinically nor-mal without any recurrence of abnormal gait or signs of cervical pain.

Discussion

To our knowledge, the use of a cervical intervertebral distractable fusion cage has not been described previously for intervertebral distraction and fusion in the dog.

Before cage implantation, a ventral slot must be created in both adjacent endplates so that the middle part of the cage is in con-tact with cancellous bone. This preparation is generally accepted to favour the formation of bone tissue. It has been speculated that stress shielding, limited vascular penetration, or a combination of both might inhibit bone graft incorporation in the interior of cage (6, 9). Therefore, an optimally designed cage should have a contact area that ensures sufficient space for bio-logical bone ingrowth as well as permitting adequate loading of the tissue growing within the cage (6, 7, 10). This hollow and fenestrated cage can be packed with cancellous bone autograft or bone graft substitute materials which facilitate vascular in-growth and load sharing. In contrast to the middle part, the lateral sides of the cage are in contact with intact parts of the endplates. This is intended to minimise the subsidence risk, and to stabilise the affected segment, especially in lateral bending of the spine (11).

Distraction and arthrodesis of traction-responsive CCSM lesions allows decompression of the spinal cord in regions of extradural compression created by hypertrophied annulus fibrosus or folded ligamentum flavum (12–14). The use of a purpose-designed fusion cage in this clinical case allowed long-term efficient spinal cord decompression through a combination of stretching of these soft tissues structures, and some mild degree of lordosis. There were not any intraoperative or postoperative complications during the 40 month follow-up period in this particular case. Several other intervertebral distraction techniques have been described. Among them, linear distraction combined with stabilisation with Steinmann pins and poly-methyl methacrylate is one of the most widely described and utilized techniques in the dog (1–3, 15). However, insertion of bi-cortical implants in the caudal cervical spine in Dobermann Pinschers for the management of CCSM carries a high risk of vertebral canal and intervertebral foramina violation (16). With this technique, the loss of vertebral column distraction has been associated with pin loosening, pull-out, and migration before bony fusion is complete (1–3, 17).

Other methods of intervertebral space distraction and fusion using implants such as stainless-steel or titanium closed cages, cancellous bone screws and bone graft, polymethyl methacrylate – interbody plug, Kirschner-wire spreader, and washer and screws all result in continuous and compressive loading of the vertebral endplates (18–22). The bone eventually remodels around the implant and engulfs it, but does not invade inside it. Long-term collapse of the intervertebral space and recurring compression of the spinal cord have been associated with these techniques, secondary to displacement or failure of the implants (1–3, 19). There was not any loss of vertebral distraction noticed in the case de-scribed herein on the CT examination per-formed 15 weeks postoperatively. The new bone was located at the ventral aspect of the vertebral bodies and inside the fusion cage, preventing collapse of the intervertebral space. The bone remodeling was also re-mote from the vertebral canal and nerve roots.

Although most dogs with CCSM improve clinically after vertebral distraction and fusion of dynamic lesions, 10% to 20% develop secondary lesions at adjacent sites (1– 3). There are two proposed aetiologies for these 'domino lesions'. The first is alteration of vertebral column biomechanics at intervertebral sites adjacent to the surgically treated site. Abnormal stresses on adjacent disc spaces are considered to be secondary to increased rigidity of the surgically stabilized vertebral motion unit (1, 2, 23). In addition, damage to muscles and ligaments created during the surgical expo-sure weakens the neighbouring intervertebral joints and may result in instability and recurrence of signs (1, 23). Multiple adjacent intervertebral discs could be simultaneously affected with CCSM. As one site becomes a clinical problem, it is surgically treated and stabilized, but continuation of the overall disease process subsequently results in clinical signs from additional affected sites adjacent to the initially treated site (2, 3, 15, 17). The technique of fusion described here may not have any advantage over previously reported techniques in pre-venting the domino effect. However, this technique could potentially be used to dis-tract and stabilise several intervertebral disk spaces simultaneously. Clinical and subclinical disc spaces could potentially be treated to prevent recurrence of spinal compression from the domino effect. In humans, multilevel discectomy and cage fusion provides good clinical results and good fusion rates for cervical degenerative disc disease. The lower complication rate and shorter hospital stay has proven that the procedure is safe and effective in humans (24).

To our knowledge, there have not been any published reports on the use of a dis-tractable fusion cage in dogs suffering from CCSM. This procedure was less invasive than other described techniques, and the device used to achieve intervertebral fusion and distraction was effective for the long-term treatment of this clinical case of C6-C7 dynamic CCSM. However, further clinical cases undergoing the described technique are needed to support these findings.

a CTe ProSpeed 3rd Generation: General Electric Medical Systems, Milwaukee, WI, USA
b Omnipaque®: GE Healthcare SA, Vélizy-Villacoublay, France
c Cervlock-cage®: Porte vet, Le Blanc – Mesnil, France
d Cefaseptin®: Vetoquinol SA, Lure, France
e Metacam®: Boehringer Ingelheim, Ingelheim, Germany f Topalgic®: Hoechst Houde Laboratory, Paris, France

Acknowledgements

The authors thank the PORTE. Vet company, located in Le Blanc-Mesnil, France (www.portevet.fr), for their invaluable contribution to the design of the implant used in this study.

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