A Method for Intervertebral Space Distraction Before Stabilization

Combined with Complete Ventral Slot for Treatment of

Disc-Associated Wobbler Syndrome in Dogs

MERAV H. SHAMIR, DVM, Diplomate ECVN, ORIT CHAI, DVM, and EMMANUEL LOEB, DVM

Objective—To evaluate the use of a modified K-wire spacer for maintaining intervertebral distrac-tion after ventral decompression and during stabilization as a treatment for disc-associated wobblersyndrome in large breed dogs.Study Design—A retrospective study.Animals—Dogs (n¼ 7) with disc-associated wobbler syndrome.Methods—Medical records (2003–2006) of dogs treated by a modified surgical method were eval-uated. Data retrieved were signalment, onset and duration of clinical signs, neurologic abnormal-ities, diagnostic methods, surgical procedure, immediate, and long-term (�1 year) postoperativeclinical and radiographic outcome.Results—Mean duration of clinical signs was 4.8 months. Neurologic signs included ataxia (2),ambulatory tetraparesis (2), and non-ambulatory tetraparesis (3). Three dogs had disc protrusion in2 sites, 2 dogs had the procedure in 1 location and stabilization of both affected sites. All dogsimproved dramatically and remained for 1–3 years. One dog had recurrence of cervical discomfort13 months later.Conclusions—Despite the limited number of dogs, overall initial successful outcome with only 1 doghaving mild recurrence 13 months later supports further use and evaluation of this technique.Clinical Relevance—Distraction using a K-wire spacer after ventral decompression followed bystabilization should be considered in dogs with disc-associated wobbler syndrome to preventcollapse of the intervertebral space.r Copyright 2008 by The American College of Veterinary Surgeons

INTRODUCTION

DISC PROTRUSION in the caudal cervical spine oflarge breed dogs is known to be the most common

of all 5 forms of caudal cervical spondylomyelopathy(CCSM) also known as wobbler syndrome.1 CCSM oc-curs primarily in large-breed dogs with Doberman andGreat Danes most commonly represented.2–4 Other formsof CCSM are hypertrophy of the ligamentum flavum,congenital vertebral malformation, vertebral tipping, andhourglass compression caused by hypertrophy of thearticular facets and ligamentum flavum.5,6 These formsdiffer in pathogenesis and direct cause of cord compres-

sion and thus require different surgical approaches formanagement. Clinical signs are typical to cervical mye-lopathy of varying degrees; ataxia and paresis of thepelvic limbs with hypometria of the thoracic limbs pre-dominate but minor neurologic abnormalities like ataxiaof the pelvic limbs alone or complete non-ambulatorytetraparesis are also seen. These signs are usually chronicand progressive but can deteriorate suddenly.

Various surgical techniques have been reported fortreatment of disc-associated wobbler syndrome includingcomplete ventral slot, partial slot followed by distractionand stabilization, and distraction stabilization aloneusing different devices and implants.6,7 Most studies

Address reprint requests to Dr. Shamir, DVM, Diplomate ECVN, Koret School of Veterinary Medicine, The Hebrew University of

Jerusalem, PO Box 12, Rehovot 76100 Israel. E-mail: shamir@agri.huji.ac.il.

Submitted March 2007; Accepted November 2007

From the Koret School of Veterinary Medicine, Hebrew University of Jerusalem, Israel.

r Copyright 2008 by The American College of Veterinary Surgeons

0161-3499/08

doi:10.1111/j.1532-950X.2007.00360.x

186

Veterinary Surgery

37:186–192, 2008

report 70–90% success rate with recurrence ranging from20–28% because of either postoperative collapse of theoperated disc space, implant failure, or collapse of theadjacent disc termed the ‘‘domino’’ effect.1,3,4,6–10

All previously reported techniques either decompressedthe cord by removal of disc material from the canal ex-posing the patient to subsequent collapse of the involveddisc space or attempted decompression by distraction andstabilization, which may result in inadequate spinal de-compression. Our purpose was to report the use of mod-ified U-shaped K-wire as a spreader to maintain thedesired intervertebral disc space as part of a combinedsurgical technique for treatment of disc-associated wobllersyndrome in large breed dogs. We describe the use of adistracting device as a part of a surgical technique thatincludes complete ventral slot with removal of degener-ated disc material from the spinal canal, insertion of can-cellous bone graft, followed by stabilization using screwsand polymethylmethacrylate (PMMA), and report out-come (clinical and radiographic) over 1–3 years.

MATERIAL AND METHODS

Inclusion Criteria

We retrieved medical records (2003–2006) of large breeddogs diagnosed with disc-associated wobller syndrome andtreated using a combination of complete ventral slot, K-wirespreader for distraction, and screws with PMMA for stabi-lization. Dogs selected for the study had a myelographic di-agnosis, surgical treatment, and � 1 year follow-up. Bothambulatory and non-ambulatory dogs were included.

Diagnostic Imaging

Myelographic diagnosis of spinal cord compression be-cause of disc protrusion was made using 3 lateral and 1 vent-rodorsal projections. The lateral projections were performedin traction, flexion, and neutral position of the neck and wereused to determine presence of a dynamic component. In 3dogs, myelography was followed by computed tomography(CT) of the involved vertebrae. Immediate postoperative sur-vey radiographs were obtained in all dogs and postoperativemyelography and CT were performed in 1 dog.

Follow-up

Neurologic and radiographic evaluation was performed atleast once for all dogs. Owners were invited for an additionalfollow-up examination of their dog before study completion toprovide current information on clinical status and radio-graphic changes of the cervical spine. Long-term follow-upwas 1–3 years.

Necropsy

One dog euthanatized 10 days after the surgery because ofthe detection of lung neoplasia was necropsied. The surgically

involved vertebrae (C4–C5) were harvested, fixed in 10%buffered formaldehyde, and then decalcified for 72 hours.Tissues were embedded in paraffin, and processed by routinemethods for microscopic examination using a hematoxylinand eosin stain.

Surgical Technique

Dogs were positioned in dorsal recumbency with the cau-dal part of the neck supported to achieve the desired degree ofcervical extension. The head was secured to the table and thethoracic limbs were directed caudally with sufficient tension toapply linear traction on the cervical spine. Using a ventralapproach to the cervical spine with extended elevation of thelongus colli muscles from the vertebral bodies, the affectedintervertebral disc and the entire ventral aspect of adjacentvertebral bodies was exposed. A high-speed drill was usedto create a longitudinal full-thickness slot at the site of theintervertebral disc to remove the dorsal part of the anulusfibrosus and the hypertrophied dorsal longitudinal ligament toexpose the spinal cord. The size of the slot did not exceedone-third the width and length of the involved vertebrae.

Two 2.7 or 3.5mm screws were inserted monocortically ineach vertebra on both sides of the operated disc. Screw sizeand hole depth were selected individually for each dog basedon CT or survey radiograph measurements of the involvedvertebrae. The holes were drilled close to the midline on 1 sideand then directed obliquely to the other side through the ver-tebral body to achieve maximal hole length. An angle of 301 tothe midline axis was attempted for drilling and special carewas taken to ensure that hole depth did not exceed the per-pendicular height of the vertebral body to avoid penetration ofthe vertebral canal or the transverse foramina where thevertebral arteries are located (Fig 1).

Distraction of the disc space was achieved by linear trac-tion through pulling on the head and body and the use of amodified Gelpi retractor inserted into the slot. To maintainthe intervertebral disc space width at 3–5mm during stabili-zation, we inserted a K-wire spacer 2 or 3mm diameter de-pending on vertebral size. Using 2 wire twisters, the wire wasbent into a ‘U’ shape so that the length of the horizontal partinserted into the slot was enough to provide the desired in-tervertebral space. One vertical arm was longer so that thespacer could be incorporated into the PMMA (Fig 1). Gel-foam soaked in morphine (0.5mg/kg body weight) wasinserted ventral to the K-wire to cover the spinal cord andsupply sustained release local analgesia (Fig 1C). Cancellousbone graft harvested from the proximal humerus was insertedinto the slot (above and around the K-wire) to enhance fusionof adjacent vertebrae (Fig 1C). Free fat graft was placed overthe bone graft to avoid heat damage to the graft and provideadditional protection to the spinal cord during the exothermicphase of acrylic hardening.

PMMA was placed over the entire ventral aspect of bothvertebrae with care to ensure that all 4 screws and the longarm of the modified U-shaped K-wire were incorporated andcovered by acrylic. Saline (0.9% NaCl) solution was used tocool the PMMA as it hardened (Fig 1B,C).

187SHAMIR, CHAI, AND LOEB

Radiographic Assessment

Postoperative lateral and ventrodorsal projections weretaken to document final screw position and the amount of

distraction achieved. Follow-up radiographs were obtainedevery 6–12 months.

Neurologic Assessment

Neurologic assessment was scored by 1 neurologist (M.S.)initially and at each subsequent examination. Postoperativeneurologic evaluation was done at 24 hours after surgery andon day 10. Dogs that had a longer recovery were evaluatedmonthly until outcome was considered satisfactory. Five dogswere reexamined 1–3 years after surgery by the same neurol-ogist (M.S.) to acquire long-term outcome information where-as for 2 dogs (1 and 1.5 years after surgery) clinical status wasdetermined by telephone communication with the owners.

Neurologic Scoring

The scoring method for cervical myelopathy was: 0¼nor-mal performance (may have slight low head carriage or verymild ataxia of the pelvic limbs); 1¼ pain or discomfort onmanipulation of head and neck, stiff head carriage, cervicalmuscles spasm (with mild ataxia of the pelvic limbs); 2¼ atax-ia or mild paresis of the pelvic limbs with normal thoraciclimbs function; 3¼ ataxia involving all limbs with hypometricthoracic gait and hypermetric paresis in the pelvic limbs;4¼weakly ambulatory tetraparesis—the dog was able to riseand make few steps before collapsing; 5¼non-ambulatorytetraparesis (no ability to rise or walk); and 6¼ non-ambula-tory tetraparesis (tetraparesis with minimal to no voluntarymovement detected either in the pelvic or thoracic limbs).Each dog was scored on admission and on each subsequentpostoperative examination.

RESULTS

Seven large breed dogs (5 males, 2 females; medianage, 7.79 years; range, 5.5–10 years; mean weight, 33.4 kg;range, 30–50kg) were identified. Breeds wereWeimaraner(4), Doberman (2), and Dog De Bordeaux (1). Four dogs(1, 4, 5, and 6) had disc protrusion at 1 site only whereas3 dogs (2, 3, and 7) had an additional degenerated discidentified on CT myelogram, with disc space narrowingand sclerotic changes of the involved vertebrae but withmilder compression (Fig 3A). In 2 of these dogs bothdiseased disc spaces were distracted and stabilized and themore severely affected site had surgical decompression asdescribed. The third dog (2) had surgical decompressioncombined with distraction and stabilization for the moreseverely affected disc only.

All dogs had satisfactory postoperative recovery withmean follow-up of 1.79 years (range, 1–3 years). No re-currence or deterioration of neurologic status was re-ported for 6 dogs whereas 1 dog (2) that had surgery atC5–C6 deteriorated 13 months later because of disc pro-trusion at C6–C7. This was the disc that had apparentslight degeneration on preoperative radiographs. This

Fig 1. Preoperative (A) and postoperative (B) lateral radio-

graphs and schematic drawing (C) of a technique using a mod-

ified K-wire spacer (C—black line) to maintain intervertebral

space after complete ventral slot until the polymethylmethac-

rylate (PMMA) hardens. Note the position of morphine soaked

Gelfoam (C—dark gray rectangle) and cancellous bone graft

(C—dotted area). The PMMA (C—light colored ellipse) cover

the screws and the long arm of the K-wire spacer completely.

188 INTERVERTEBRAL SPACE DISTRACTION IN DOGS

dog improved gradually after 4 weeks of confinementfollowed by 3 months of restricted activity. Two otherdogs (3, 7) with narrowing and protrusion of 2 adjacentdiscs, responded well to double space stabilization withthe most protruded disc decompressed. Outcome at 2.5years was good in 1 dog (score improved from 3 to 2) andwas excellent at 1 year for the other dog (score improvedfrom 5 to 1).

Five dogs had improvement within 48 hours of surgerywhereas the other 2 (dogs 2 and 3) had a slower recovery.These dogs were admitted 18 and 6 months after onset ofclinical signs and were scored 4 and 3 for neurologicdysfunction on admission.

Six dogs had complete recovery with return to normalactivity although some limitation of cervical range ofmotion was evident at follow-up neurologic evaluation in2 dogs (1, 2). One dog (6) was admitted with severe lateralrecumbent tetraparesis (score 6) and improved dramat-ically, and was standing unaided within 48 of surgery.This dog took longer to fully return to walking but grad-ually improved over the next 3 months. In the last follow-up telephone conversation with the owner 1 year later,the dog was still improving.

Radiographic Outcome

Beyond initial postoperative radiographs and at least 1set taken within the first year, 5 dogs had an additional 2–5 radiographic examinations during follow-up. Increasedopacity of the slot, indicating different levels of fusion foroperated vertebrae was detected at 9–12 months in alldogs. No collapse of the gap between vertebrae was de-tected indicating that the stabilization technique used wassufficient to maintain the required gap until the vertebralfusion (Fig 3B,C).

Slowly progressive degenerative changes representedas narrowing of the disc spaces and sclerotic changes inthe vertebral endplates were observed in the adjacent discspaces of all dogs examined. If surgery was performedinitially at the C5–C6 disc space, the next disc that haddegenerative changes was always C6–C7 whereas whenC6–C7 disc had initial surgery, the C5–C6 space was al-ways next to have postoperative degenerative change (Fig3C). Changes were progressive and included narrowing ofthe disc space, sclerotic changes of the vertebral end-plates, and osteophyte formation and were detected at1-year follow-up and clearly evident 1.5–3 years afterinitial surgery (Fig 3C).

DISCUSSION

The technique we report, a combination of completeventral slot that includes removal of the degenerated discfrom the spinal canal, distraction using a K-wire spacer

until the PMMA hardens, and stabilization with screwsand PMMA, is a modification of an earlier method fortreating disc associated wobbler syndrome in large breeddogs. Although our case experience was small, we believethat this combined procedure using a modified U-shapedK-wire to achieve and maintain the desired space betweenthe involved vertebrae was effective.

The combination of complete ventral slot with dis-traction and stabilization permits careful removal of discmaterial from the canal enabling the surgeon to view thespinal cord and subsequent distraction and stabilizationof adjacent vertebra to prevent collapse of the space orjoint instability. Insertion of a modified U-shaped K-wireinto the slot helps achieve and maintain the desiredamount of parallel distraction of the disc space until thePMMA hardens. The long arm of the K-wire implant isthen incorporated in the PMMA, eliminating the concernof future wire migration. The K-wire spreader is preparedto fit the specific slot size in each patient after distractionusing a modified Gelpi retractor inserted in the slot.Other methods of maintaining the intervertebral spacehave been reported such as stainless-steel or titaniumcages, or washer and screws11; however, these devices re-sult in continuous compressive loads on the vertebral endplates. The bone eventually remodels around the metalimplant and engulfs the device, which can result in col-lapse of the intervertebral space and recurring bulge andmay again compress the spinal cord.11,12 Our methodavoids this complication because the spacer is fully load-ed only during the time required for the PMMA toharden after which most of the load is taken by the screwsand PMMA.

A modified Gelpi retractor where the sharp ends werecut on both sides is commonly used to temporarily dis-tract the intervertebral space. Correct placement of thismodified Gelpi is not always simple and requires expo-sure of the ventral aspects of additional vertebrae andfenestration of the adjacent discs. Additional soft-tissuedamage to muscles and ligament weakens the neighbor-ing intervertebral joints and may result in instability andrecurrence of signs.11 The modified spacer is simple toconstruct from K-wire and is inexpensive and readily ac-cessible. Furthermore the spacer occupies only a smallamount of the slot space enabling the surgeon to fit thedevice into the slot while the modified Gelpi retractor is inposition. Once the spacer is positioned, the Gelpi retrac-tor can be gently removed leaving ample space for inser-tion of large quantities of cancellous bone graft with goodcontact to the exposed vertebral bone to ensure latervertebral fusion (Figs 1 and 3).

During the exothermic phase of PMMA hardening,heat is transmitted through the metal implants (screws,K-wire) and may damage the vertebral bone surroundingthe screws and the cancellous graft. Tissue from the dog

189SHAMIR, CHAI, AND LOEB

that died 10 days after surgery had mild coagulation ne-crosis marked with a sharp demarcation zone betweennecrotic and vital bone tissue in the area surroundingthe screws on both C5 and C6 vertebrae, and the boneallograft was composed of fair amount of fibrin anderythrocytes. In areas adjacent to the vertebral bone someintact, spindle-shaped cells, considered activated osteo-blasts were observed along woven bone trabeculae (Fig2B). These findings suggest that the bone damage causedby heat produced from the PMMA and transmittedthrough the K-wire and screws was minimal and basedon follow-up radiographs for other dogs did not preventvertebral fusion (Fig 3C). Care must be taken to ensurethat the size and shape of the PMMA are suitable toallow complete coverage of all screw heads and the longarm of the K-wire spacer. A larger piece of cement or aprotruding sharp end of the spacer should be avoidedbecause they might interfere with normal function of theesophagus and trachea.

Two dogs, one non-ambulatory (dog 7) and anotherweak tetraparetic (dog 1) were able to stand and walksteadily the morning after the surgery despite a relativelylong time between onset of clinical signs and surgical in-tervention (3 and 1 month, respectively). This may reflectmeticulous removal of foreign material from the spinal

Fig 2. Histologic examination of cancellous bone graft from

a dog that died 10 days after the surgery. The allograft is

composed of a fair amount of fibrin and erythrocytes (A and

B). Adjacent to the vertebral bone (A) some intact, spindle

shaped cells (A and B arrows) were observed along woven bone

trabeculae. Scalebar¼ 20 lm.

Fig 3. Preoperative (A), postoperative (B), and 15-month

follow-up (C) lateral radiographs of dog 1 (Table 1). Note the

amount of distraction achieved at C5–C6 intervertebral space

using the K-wire spacer (B) and the maintenance of implants

position at 15 months (C). Degenerative changes of the discs

spaces on both sides of the operated site are clearly evident (C)

although narrowing of the C6–C7 disc space was present pre-

operatively (A).

190 INTERVERTEBRAL SPACE DISTRACTION IN DOGS

canal followed by efficacious distraction of the disc spaceusing the K-wire spacer before stabilization.

This combined surgical procedure may have potentiallong-term advantages. Any stabilization device appliedafter distraction of vertebrae in the caudal cervical areamay fail because of the load concentrated on relativelysmall areas of contact between the bone and implant.This load may result in local pressure necrosis of the bonesurrounding the stabilizing implant (e.g. screws/washer/cement plugs) on both sides of the gap and result in col-lapse of the disc space.13 If this occurs when completeventral slot was not performed, rebulging of any remain-ing degenerated anulus fibrosus and the hypertrophieddorsal longitudinal ligament will result in spinal cordcompression. This may account for the relatively highprevalence (10%) of early deterioration in neurologicstatus reported in several studies14,15 and some of themore delayed relapses of clinical signs that exceeded 30%in other studies.12,13,16, We performed complete ventralslot before distraction to eliminate this cause of recurrentspinal cord compression at the operated site.

All of our dogs improved within the first week aftersurgery and continued to improve during the first fewmonths. Six dogs had no recurrence of neurologic dys-function during follow-up of 1–3 years. Dog 2, admittedfor surgery 18 months after initial clinical signs, improveddramatically and functioned normally for 13 months be-fore recurrence of neurologic signs. CT myelography atthat time revealed disc protrusion at the intervertebraldisc space adjacent the operated space. This outcome isreferred to as the domino effect. This disc space had ev-idence of degeneration on preoperative radiographs butwe decided not to distract or stabilize this space.

The ‘‘domino’’ effect is believed to be caused by loadtransmission from the stabilized intervertebral disc to theinterspace immediately cranial or caudal to it. Loadtransmission exacerbates subclinical instability in anadjacent disc space enhancing the degenerative processesthat lead to disc protrusion and dorsal longitudinal

ligament hypertrophy.17 Although only 1 dog developeda domino effect, we do not believe our technique has anyadvantage over previously reported techniques in pre-venting domino effect. It would appear that distractingand stabilizing both the clinical and subclinical degener-ated disc space may be beneficial in preventing recurrenceof spinal compression from the domino effect.

In the other 2 dogs (3, 7) with degenerative changes inadjacent discs spaces, ventral decompression was per-formed at the most affected site but distraction and sta-bilization was performed at both clinical and subclinicaldisc spaces. Neither of these dogs had recurrence of clin-ical signs during follow-up of 1 and 3 years, respectively.Mild degenerative changes of C4–C5 were observed 2years later in dog 3 that had both C5–C6 and C6–C7stabilized (Table 1); however, no clinical deteriorationoccurred over the next 3 years. Extensive degenerativechanges in neighboring disc spaces on both sides of theoperated disc indicate excessive instability in interspacesthat are now carrying the load transmitted from the oper-ated intervertebral space.17 In some cases these degener-ative changes may result in ankylosis of the disc space orspaces without causing the dog any apparent discomfort.

Although we report a small number of cases, ourtechnique was beneficial in both ambulatory and non-ambulatory dogs with no recurrence in 6 of 7 dogs with amean follow-up of 1.9 years. Improved understanding ofthe forces and loads affecting the caudal cervical regionin large breed dogs will facilitate development of moresuitable methods to minimize development of furtherdegenerative changes that leads to recurrence of neuro-logic signs.

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Table 1. Summary Data for 7 Dogs with Disc-Associated Wobbler Syndrome Treated by Ventral Slot Decompression, Distraction, and Stabilization

Dog

Age (years), Sex,

Breed

Duration

(months)

Admission

Neurologic Score Lesion

Follow-up

(years)

Follow-up

Neurologic Score Recurrence

1 8 M Doberman 1 4 C5–C6 3 0 none

2 10 M Weimaraner 18 4 C5–C6

C6–C7

2.5 2 C6–C7

3 5.5 F Weimaraner 6 3 C5–C7

C6–C7

3 2 none

4 7 M Doberman 0.5 3 C6–C7 1.5 1 none

5 9 M Weimaraner 3 5 C5–C6 1 0 none

6 7M Weimaraner 1 6 C6–C7 1 3 none

7 8 F Dog de Bordeaux 1.5 5 C5–C6

C6–C7

1 1 none

M, male; F, female.

191SHAMIR, CHAI, AND LOEB

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192 INTERVERTEBRAL SPACE DISTRACTION IN DOGS