• View in gallery
    Figure 1—

    Diagram of the IDS used in the study. The screw had a diameter of 5 mm and a length of 13 or 14 mm. Because it had no screwhead, the IDS could be completely inserted between the vertebral bodies (into the intervertebral disk).

  • View in gallery
    Figure 2—

    Photographs of a Beagle positioned for CT imaging with its LSJ in a flexed (A) and extended (B) position. A—The dog was positioned on the imaging table in dorsal recumbency. The hind limbs were pulled cranially and secured so that a straight line joining the wing of the ilium (white asterisk) and ischial tuberosity (black asterisk) was perpendicular to the imaging table. B—The dog was positioned on the imaging table in dorsal recumbency. A 6.5 × 6.5 × 14-cm3 spacer (highlighted in the square) was placed at the edge of the cranial aspect of the wing of the ilium, and pressure was applied to the knee region until the caudal aspect of each hind limb (arrow) and plantar surface of each hind foot touched the imaging table. The hind limbs were then secured in that position.

  • View in gallery
    Figure 3—

    Multiplanar reconstruction midsagittal CT images of the LSJ in a Beagle in which the joint is in a flexed (A) and extended (B) position. The intervertebral angle was measured by determining the intersection points of tangent lines drawn on the dorsal aspect of the vertebral bodies of L7 to S1, L6 to L7, and L5 to L6. The ROM was then calculated as the intervertebral angle measured in the flexed position minus the intervertebral angle measured in the extended position.

  • View in gallery
    Figure 4—

    Multiplanar reconstruction midsagittal CT image of the LSJ in a Beagle in which the joint is in an extended position. For measurement of the ID between L7 and S1, tangent lines for the L7 vertebral body were drawn on the dorsal and ventral aspects (white solid lines) and caudal aspect (dashed lines; both caudal and cranial lines shown), and the points where they intersected (white circles) were determined. The central point between these 2 intersection points (black circle on caudal aspect of L7) was then identified. On the S1 vertebral body, the central point between the dorsoventral and cranial intersection points was also identified (black circle on the cranial aspect of S1). The ID was calculated as the distance separating the central points between the 2 vertebral bodies (black line).

  • View in gallery
    Figure 5—

    Multiplanar reconstruction midsagittal (A), coronal (B), and parasagittal (C) CT images of the LSJ in a Beagle showing measurement of the lumbosacral IFA. The height of the vertebral canal (arrows in panel A) was measured on a midsagittal image. Proper positioning was confirmed on a coronal image obtained at the center of the vertebral canal, in which the L7 pedicle appeared to be elliptical (B). The area enclosed by bone as identified by a straight line (black lines in panel B) on the sagittal image that overlapped with the long axis of this ellipse was defined as the lumbosacral IFA (C; diagonally shaded area).

  • View in gallery
    Figure 6—

    Multiplanar reconstruction coronal CT images of the LSJ in a Beagle showing 3 IVF subdivisions based on the order of appearance on sagittal CT images. The entrance zone was defined as the position in which the IVF was first completely delineated from the midsagittal plane to the outer side of the vertebral canal (A). The middle zone was measured on sagittal images from a straight line that overlapped the center of the elliptical pedicle (B). After the entrance and middle zones were measured, the same distance between these 2 sections was extended externally from the middle zone. The exit zone was defined as the IVF in sagittal images in this position (C).

  • View in gallery
    Figure 7—

    Lateral (A and B) and dorsoventral (C) radiographic views of the LSJ in a Beagle following IDS insertion into the L7-S1 disk, with the joint in an extended (A), flexed (B), and neutral (C) position.

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Effects of intervertebral distraction screw fixation of the lumbosacral joint on the adjacent lumbar segments in Beagles

Hirofumi Tanoue1Laboratory of Veterinary Surgery, Nippon Veterinary and Life Science University, 1-7-1, Kyounan, Musashino, Tokyo 180-8602, Japan.

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Rika Uchiyama1Laboratory of Veterinary Surgery, Nippon Veterinary and Life Science University, 1-7-1, Kyounan, Musashino, Tokyo 180-8602, Japan.

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Hideyuki Fukuda2Platon Japan Co Ltd, Ookura-machi Machida-city, Tokyo 195-0062, Japan.

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Tom Ichinohe3Veterinary Teaching Hospital, Azabu University, 1-17-71, Fuchinobe, Chuo, Sagamihara, Kanagawa 252-0206, Japan.

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Nobuo Kanno1Laboratory of Veterinary Surgery, Nippon Veterinary and Life Science University, 1-7-1, Kyounan, Musashino, Tokyo 180-8602, Japan.

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Shuji Suzuki1Laboratory of Veterinary Surgery, Nippon Veterinary and Life Science University, 1-7-1, Kyounan, Musashino, Tokyo 180-8602, Japan.

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Yasuji Harada1Laboratory of Veterinary Surgery, Nippon Veterinary and Life Science University, 1-7-1, Kyounan, Musashino, Tokyo 180-8602, Japan.

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Yasushi Hara1Laboratory of Veterinary Surgery, Nippon Veterinary and Life Science University, 1-7-1, Kyounan, Musashino, Tokyo 180-8602, Japan.

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Abstract

OBJECTIVE

To investigate the effects of intervertebral distraction screw (IDS) fixation of the lumbosacral joint (LSJ) on the intervertebral foraminal area (IFA) and intervertebral stabilization of the LSJ and adjacent lumbar segments in dogs.

ANIMALS

7 healthy Beagles.

PROCEDURES

Dorsal laminectomy was performed at the LSJ in each dog to expose the intervertebral disk. The IDS was then inserted into the L7-S1 disk. Computed tomography was performed before and after laminectomy and after IDS insertion (intact, laminectomy, and IDS conditions, respectively) to measure the intervertebral range of motion (ROM) and intervertebral distance (ID) at L7-S1, L6-7, and L5-6 with the LSJ in a flexed and extended position. The intervertebral foramina stenosis rate was calculated from the intervertebral foramina area in entrance, middle, and exit zones. Results were compared among conditions.

RESULTS

The ROM at L7-S1 after IDS insertion was lower than that observed before and after laminectomy; no other differences were identified among conditions. With the LSJ in the flexed position, the ID at L7-S1 was larger after IDS insertion than before and after laminectomy; no other differences in ID were identified. In all evaluated zones, the stenosis rate was lower after IDS insertion than before and after laminectomy. No differences in ROM, ID, and stenosis rate were identified among conditions at L6-7 or L5-6.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that IDS fixation of the LSJ restricted lumbosacral ROM and prevented decreases in lumbosacral ID and IFA in healthy dogs. There were no changes at L6-7 and L5-6.

Abstract

OBJECTIVE

To investigate the effects of intervertebral distraction screw (IDS) fixation of the lumbosacral joint (LSJ) on the intervertebral foraminal area (IFA) and intervertebral stabilization of the LSJ and adjacent lumbar segments in dogs.

ANIMALS

7 healthy Beagles.

PROCEDURES

Dorsal laminectomy was performed at the LSJ in each dog to expose the intervertebral disk. The IDS was then inserted into the L7-S1 disk. Computed tomography was performed before and after laminectomy and after IDS insertion (intact, laminectomy, and IDS conditions, respectively) to measure the intervertebral range of motion (ROM) and intervertebral distance (ID) at L7-S1, L6-7, and L5-6 with the LSJ in a flexed and extended position. The intervertebral foramina stenosis rate was calculated from the intervertebral foramina area in entrance, middle, and exit zones. Results were compared among conditions.

RESULTS

The ROM at L7-S1 after IDS insertion was lower than that observed before and after laminectomy; no other differences were identified among conditions. With the LSJ in the flexed position, the ID at L7-S1 was larger after IDS insertion than before and after laminectomy; no other differences in ID were identified. In all evaluated zones, the stenosis rate was lower after IDS insertion than before and after laminectomy. No differences in ROM, ID, and stenosis rate were identified among conditions at L6-7 or L5-6.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that IDS fixation of the LSJ restricted lumbosacral ROM and prevented decreases in lumbosacral ID and IFA in healthy dogs. There were no changes at L6-7 and L5-6.

Degenerative lumbosacral stenosis is a neural disease commonly observed in large-breed dogs, particularly in German Shepherd Dogs, Bernese Mountain Dogs, and Golden Retrievers.1–3 This pathological condition is characterized by intervertebral disk degeneration, disk herniation, loss of disk height, proliferation of soft tissue and bone, and foraminal stenosis. The lesions are dynamic, and for diagnosis, images are required with the LSJ in flexed and extended positions.

Surgical treatment for DLSS in dogs includes direct decompression, indirect decompression (which increases LSJ stability), and a combination of these 2 methods. Direct decompression, which releases pressure on the terminal region of the spinal cord, may include procedures such as dorsal laminectomy, unilateral laminectomy, or dorsal partial diskectomy.1–7 In certain cases of vertebral canal stenosis, additional procedures such as unilateral or bilateral facetectomy or foraminotomy may be required.1 Dorsal laminectomy and partial diskectomy reportedly result in clinical improvement in 66.7% to 96.5% of treated dogs.8 However, insufficient relief of nerve compression contributes to a lack of or decreased clinical improvement after dorsal laminectomy,9 and clinical signs recur in 18% of dogs treated in this manner.3 Such conditions may be the result of postdecompression LSJ instability and an acceleration in degenerative changes.3,10,11 Moreover, although dorsal laminectomy can release pressure on the terminal region of the spinal cord (ie, the vertebral canal of the LSJ), it cannot release pressure on spinal nerve L7 in the IVF.6

Several indirect decompression procedures have been reported, including LSJ transarticular fixation with a pin in the LSJ,12 traction fusion of the LSJ with a screw,12,13 and dorsal vertebral fixation with a pedicle screw, bone cement, or rod.4,14–16 Dorsal fixation and fusion can be used to improve LSJ stability in dogs by restoring the height of the L7-S1 disk and opening the foraminal apertures, thereby relieving the pressure on neural tissues.16,17 Biomechanical evaluation of the lumbosacral portion of the vertebral column in canine cadavers has revealed increased stability of the LSJ after pedicle screw-rod fixation following dorsal laminectomy.16,17 The disadvantages of this technique include the risk of complications such as implant breakage and fracture of the articular processes of L7, which may be partially due to the lack of stabilized structures on the ventral aspect of the intervertebral joint, such as the intervertebral disk.17 It has also been reported that osseous LSJ healing cannot be promoted by fusion with a vertebral arch pedicle screw and rod alone.2

Insertion of a Fitz intervertebral traction screw, developed by Solano et al,18 between the cervical vertebrae can lengthen the distance between the vertebral end plates while maintaining correct positioning of vertebrae. This technique reportedly contributes to a reduction in thickening of the yellow ligament and condylar process joint capsule, along with other components of dorsal side decompression such as articular facet deformities.18 Spacers, such as the Fitz intervertebral traction screw, might also be used for decompression of the LSJ. In some cases, compression of the entrance, middle, and exit zones of the IVF cannot be released with a dorsal fixation device alone and concomitant use of an intervertebral spacing device, such as the Fitz intervertebral traction screw, can achieve greater expansion of areas within the IVF.13 Moreover, distraction in the intervertebral joint with a stand-alone cage in combination with dorsal fixation reportedly provides LSJ stabilization.17

An IDS has been developed to release compression of the cauda equina nerve region and, in particular, the L7 nerve root that runs through the IVF of the LSJ. The purpose of the study reported here was to investigate the change in motion and foramen area at L7-S1 and adjacent lumbar segments by IDS insertion into the L7-S1 disk as a preliminary step, with the ultimate goal of stabilizing the LSJ by means of dorsal fixation and IDS insertion in the future. We hypothesized that the ROM at L7-S1 would decrease while increasing in the adjacent lumbar segments. Moreover we hypothesized that the IFA during vertebral motion would decrease at L7-S1 and increase in the adjacent lumbar segments.

Materials and Methods

Animals

Seven healthy research Beagles scheduled for euthanasia were used in the study. Mean ± SD age was 22.6 ± 6.9 months, and mean body weight was 10.6 ± 1.4 kg. The dogs had comparable physiques and bone sizes. All dogs were examined via CT and MRI to confirm the absence of neural or orthopedic diseases such as intervertebral disk displacement or vertebral body deformities that might have impacted the study results. The study protocol and euthanasia method were approved by the Institutional Review Board of the Nippon Veterinary and Life Science University (approval No. S26S73).

IDS

The IDSa used in this study was made from titanium alloy (ASTMF136). Choice of implant size was made on the basis of the ID of the study dogs and data from a previous study17 involving a titanium cage as a stand-alone device. The same size was used for all dogs given their comparable size. The screw was cylindrical in shape, with a diameter of 5 mm and length of 13 to 14 mm. The depth from crest to root of the screw was 0 to 0.42 mm, with 0.65-mm pitches along the surface. Because the IDS had no screwhead, it could be completely inserted between the vertebral bodies (into the intervertebral disk) during application (Figure 1).

Figure 1—
Figure 1—

Diagram of the IDS used in the study. The screw had a diameter of 5 mm and a length of 13 or 14 mm. Because it had no screwhead, the IDS could be completely inserted between the vertebral bodies (into the intervertebral disk).

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

Surgical procedures

All surgical procedures were performed by a diplomate of the Japanese College of Veterinary Surgeons. All surgical techniques were performed while dogs were anesthetized because of concerns that postmortem rigor might affect measurements and to allow assessment of any potential complications associated with IDS insertion, such as excessive bleeding. Anesthesia was induced with propofol (6 mg/kg, IV), dogs were endotracheally intubated, and anesthesia was maintained with isoflurane. Before and every 2 hours during surgery, buprenorphine was administered (0.02 mg/kg, IV) for analgesia. After induction of anesthesia and 30 minutes prior to skin incision, epidural anesthesia was administered with bupivacaine (0.5 mg/kg) via L7-S1.

Each dog was positioned in ventral recumbency and restrained so that the hind limbs were bent at the knee, with the toes pointing cranially. The lower back region was shaved, and standard asepsis procedures were carried out. A skin incision was made above the dorsal midline and along the spinous processes from L5 to S3, and a bipolar electrical surgical knife was used to separate soft tissue until the spinous processes of L6 to S3 were exposed. A rongeur was then used to resect the spinous process from the caudal aspect of L7 to the cranial aspect of S1, and the dorsal aspect of the LSJ was exposed. An electrical surgical burb was used to perform an 8 × 20-mm dorsal laminectomy along the center of the dorsal aspect of the L7 and S1 vertebral bodies. The width of the dorsal laminectomy was determined as a maximum width within the range in which the joint process could be preserved (ie, the joint capsule remained attached to the joint process). The length of the dorsal laminectomy was determined so that a nerve hook could be used to probe into the pedicle. Previous research19 has shown that bilateral damage to the joint capsule of the articular process joint can affect intervertebral mobility; consequently, sufficient care was taken to ensure that the resection range did not extend into the joint capsule.

From the site at which laminectomy was performed, a nerve hook was used to pull the spinal nerve of the cauda equina and expose the annulus fibrosus on the dorsal aspect of the L7-S1 disk and the dorsal surface. A 3.8-mm-diameter trephine drillc for bone collection was applied to the center of the exposed intervertebral disk to remove the dorsal and ventral aspect of the annulus fibrosus and intervertebral disk and create a cylindrical screw hole. A manual screw handle was then used to insert the IDS into the vertebral canal until it could no longer be felt to be protruding. Proper IDS insertion was radiographically confirmed during and after surgery. After all procedures had concluded, dogs were euthanized by IV administration of an overdose of pentobarbital.

CT imaging

Each dog underwent CT imaging prior to surgery (intact condition), after dorsal laminectomy (laminectomy condition), and after IDS insertion (IDS condition) with the LSJ in flexed and extended positions (because positioning greatly affects the shape of the LSJ IVF20–22). For the flexed position, dogs were positioned in dorsal recumbency with the hind limbs pulled cranially and secured so that a straight line joining the wing of the ilium and ischial tuberosity was perpendicular to the imaging table (Figure 2). For the extended position, dogs were positioned in dorsal recumbency, a 6.5 × 6.5 × 14-cm3 spacer was placed at the edge of the cranial aspect of the wing of the ilium, and pressure was applied to the knee region until the plantar surface of the hind feet touched the imaging table. The hind limbs were then secured in that position. For CT imaging, an 80-row 160-slice CT scannerd was used, with a slice width of 0.5 mm and field of view of 300 mm. Workstation softwaree was used to process the images.

Figure 2—
Figure 2—

Photographs of a Beagle positioned for CT imaging with its LSJ in a flexed (A) and extended (B) position. A—The dog was positioned on the imaging table in dorsal recumbency. The hind limbs were pulled cranially and secured so that a straight line joining the wing of the ilium (white asterisk) and ischial tuberosity (black asterisk) was perpendicular to the imaging table. B—The dog was positioned on the imaging table in dorsal recumbency. A 6.5 × 6.5 × 14-cm3 spacer (highlighted in the square) was placed at the edge of the cranial aspect of the wing of the ilium, and pressure was applied to the knee region until the caudal aspect of each hind limb (arrow) and plantar surface of each hind foot touched the imaging table. The hind limbs were then secured in that position.

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

Measurements

The CT images were reconstructed in 3-D (multiplanar reconstruction processing) from L5 to the sacrum, and several measurements were performed on these images (window width, 1,500 HU; window length, 300 HU) as follows.

Intervertebral ROM—The intervertebral angle was measured by determining the intersection points of tangent lines drawn on the dorsal aspect of the vertebral bodies from L7 to S1, L6 to L7, and L5 to L6 in the flexed and extended positions (Figure 3). The ROM was then calculated as the intervertebral angle measured in the flexed position minus the intervertebral angle measured in the extended position.

Figure 3—
Figure 3—

Multiplanar reconstruction midsagittal CT images of the LSJ in a Beagle in which the joint is in a flexed (A) and extended (B) position. The intervertebral angle was measured by determining the intersection points of tangent lines drawn on the dorsal aspect of the vertebral bodies of L7 to S1, L6 to L7, and L5 to L6. The ROM was then calculated as the intervertebral angle measured in the flexed position minus the intervertebral angle measured in the extended position.

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

ID—For measurement of the ID between L7 and S1, tangent lines for the L7 vertebral body were drawn on the dorsal and ventral aspects and caudal aspects, and the points where they intersected were determined (Figure 4). The central point between these 2 intersection points was then identified. On the S1 vertebral body, the central point between the dorsoventral and cranial intersection points was also identified. The ID was then calculated as the distance separating the central points between the 2 vertebral bodies. The same method was used to perform measurements of the ID between L6 and L7 and between L5 and L6.

Figure 4—
Figure 4—

Multiplanar reconstruction midsagittal CT image of the LSJ in a Beagle in which the joint is in an extended position. For measurement of the ID between L7 and S1, tangent lines for the L7 vertebral body were drawn on the dorsal and ventral aspects (white solid lines) and caudal aspect (dashed lines; both caudal and cranial lines shown), and the points where they intersected (white circles) were determined. The central point between these 2 intersection points (black circle on caudal aspect of L7) was then identified. On the S1 vertebral body, the central point between the dorsoventral and cranial intersection points was also identified (black circle on the cranial aspect of S1). The ID was calculated as the distance separating the central points between the 2 vertebral bodies (black line).

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

IFA—For measurement of the IFA of L7-S1, the height of the vertebral canal was measured with the dog positioned at the center of the length of the L7 vertebral body (Figure 5). Coronal images obtained at the center of the vertebral canal were used to confirm this positioning and showed that the L7 vertebral arch pedicle appeared to be elliptical. The area enclosed by bone as identified by a straight line on the sagittal image that overlapped with the long axis of this ellipse was defined as lumbosacral IFA. The IFA was measured in 3 IVF subdivisions—entrance, middle, and exit zones—as previously defined,7 with the LSJ in flexed and extended positions (Figure 6).

Figure 5—
Figure 5—

Multiplanar reconstruction midsagittal (A), coronal (B), and parasagittal (C) CT images of the LSJ in a Beagle showing measurement of the lumbosacral IFA. The height of the vertebral canal (arrows in panel A) was measured on a midsagittal image. Proper positioning was confirmed on a coronal image obtained at the center of the vertebral canal, in which the L7 pedicle appeared to be elliptical (B). The area enclosed by bone as identified by a straight line (black lines in panel B) on the sagittal image that overlapped with the long axis of this ellipse was defined as the lumbosacral IFA (C; diagonally shaded area).

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

Figure 6—
Figure 6—

Multiplanar reconstruction coronal CT images of the LSJ in a Beagle showing 3 IVF subdivisions based on the order of appearance on sagittal CT images. The entrance zone was defined as the position in which the IVF was first completely delineated from the midsagittal plane to the outer side of the vertebral canal (A). The middle zone was measured on sagittal images from a straight line that overlapped the center of the elliptical pedicle (B). After the entrance and middle zones were measured, the same distance between these 2 sections was extended externally from the middle zone. The exit zone was defined as the IVF in sagittal images in this position (C).

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

IVF stenosis rate—The IVF stenosis rate was calculated at each IVF subdivision as (IFA in flexed position – IFA in extended position)/IFA in flexed position × 100.

Statistical analysis

Statistical analyses were performed by use of statistical software.f The Shapiro-Wilk test was performed to assess all measurements for normality of distribution. For normally distributed data, 1-way ANOVA for repeated measures was performed to compare measurements among the intact, laminectomy, and IDS conditions. When a significant (ie, P < 0.05) difference was identified, post hoc analysis was conducted by means of the Tukey-Kramer test. For nonnormally distributed data, the Kruskal-Wallis test was performed; when a significant difference was identified, post hoc analysis was conducted by means of the Steel-Dwass test.

Results

Findings on postoperative radiographic images confirmed that the entire IDS had been completely inserted between the L7 and S1 vertebral bodies in the study Beagles and that there was no protrusion of the IDS into the vertebral canal (Figure 7). No signs of pain such as an increased heart rate or blood pressure fluctuations were noted during surgery.

Figure 7—
Figure 7—

Lateral (A and B) and dorsoventral (C) radiographic views of the LSJ in a Beagle following IDS insertion into the L7-S1 disk, with the joint in an extended (A), flexed (B), and neutral (C) position.

Citation: American Journal of Veterinary Research 81, 4; 10.2460/ajvr.81.4.367

Intervertebral ROM

Values for intervertebral ROM at L7-S1, L6-7, and L5-6 in the intact, laminectomy, and IDS conditions were summarized (Table 1). Although no significant differences were observed between the intact and laminectomy conditions, ROM at L7-S1 in the IDS condition was significantly lower than that in the intact (P = 0.005) and laminectomy (P = 0.02) conditions. No differences were identified among conditions at L6-7 or L5-6.

Table 1—

Mean ± SD values for ROM and ID in the lumbosacral region of 7 healthy Beagles before (intact) and after laminectomy and after IDS insertion into the LSJ.

 L7-S1L6-7
MeasurementIntactLaminectomyIDSIntactLaminectomyIDSIntactLaminectomyIDS
ROM (°)34.8 ± 4.233.3 ± 6.025.5 ± 3*9.7 ± 4.79.2 ± 3.49.4 ± 4.17.9 ± 7.06.8 ± 4.17.3 ± 4.7
ID in flexion (mm)2.2 ± 0.52.5 ± 0.33.3 ± 0.5*1.4 ± 1.01.7 ± 0.51.6 ± 0.31.3 ± 0.91.5 ± 0.51.4 ± 0.4
ID in extension (mm)3.0 ± 0.63.1 ± 0.63.6 ± 0.72.1 ± 0.52.0 ± 0.52.3 ± 0.51.7 ± 0.61.7 ± 0.71.3 ± 0.4

Within an intervertebral space, the indicated value differs significantly (P < 0.05) from the corresponding value for the intact condition.

Within an intervertebral space, the indicated value differs significantly (P < 0.05) from the corresponding value for the laminectomy condition.

ID

Values for ID at L7-S1, L6-7, and L5-6 with the LSJ in a flexed and extended position were summarized (Table 1). When the LSJ was flexed, the ID at L7-S1 was significantly larger in the IDS condition than in the intact (P = 0.004) and laminectomy (P = 0.01) conditions. No other differences were identified among conditions, including at L6-7 or L5-6, regardless of whether the LSJ was in a flexed or extended position.

IFA and IVF stenosis rate

Values for IFA and IVF stenosis rate with the LSJ in a flexed and extended position were summarized (Table 2). The stenosis rate at L7-S1 in the IDS condition was significantly (P < 0.001) lower than in the intact and laminectomy conditions in all IVF subdivisions. At L6-7 and L5-6, no significant differences in stenosis rate were identified among conditions for any IVF subdivision.

Table 2—

Mean ± SD values for IFA and stenosis rate in 3 IVF subdivisions of the LSJ in the Beagles of Table 1.

 L7-S1L6-7L5-6
Measurement by zoneIntactLaminectomyIDSIntactLaminectomyIDSIntactLaminectomyIDS
Entrance IFA in flexion (mm2)59.8 ± 5.756.9 ± 10.155.4 ± 7.952.3 ± 4.746.1 ± 6.446.3 ± 7.553.7 ± 5.851.5 ± 9.251.6 ± 11.6
IFA in extension (mm2)34.4 ± 6.033.0 ± 3.139.5 ± 6.835.7 ± 5.237.1 ± 7.334.6 ± 8.142.1 ± 5.740.5 ± 6.739.2 ± 6.0
Stenosis rate (%)42.6 ± 7.941.0 ± 8.328.6 ± 6.9*31.9 ± 7.824.8 ± 8.826.6 ± 10.921.5 ± 7.320.9 ± 8.022.5 ± 9.5
Middle IFA in flexion (mm2)56.8 ± 4.552.9 ± 5.751.5 ± 4.542.8 ± 3.940.0 ± 5.240.8 ± 4.145.4 ± 4.643.1 ± 5.142.9 ± 6.3
IFA in extension (mm2)27.5 ± 3.626.3 ± 3.333.6 ± 3.729.9 ± 2.529.7 ± 3.529.3 ± 2.435.1 ± 5.033.4 ± 6.032.8 ± 5.1
Stenosis rate (%)51.4 ± 6.650.0 ± 6.234.7 ± 3.9*30.2 ± 6.626.9 ± 8.328.1 ± 6.822.6 ± 9.922.1 ± 12.023.9 ± 10.4
          
Exit IFA in flexion (mm2)64.4 ± 7.860.2 ± 7.657.8 ± 6.043.2 ± 3.943.4 ± 5.043.5 ± 6.745.1 ± 4.642.1 ± 4.643.5 ± 7.6
IFA in extension (mm2)28.8 ± 4.226.8 ± 4.134.1 ± 4.029.7 ± 2.930.1 ± 3.429.2 ± 3.736.1 ± 4.633.8 ± 5.932.2 ± 4.4
Stenosis rate (%)54.6 ± 7.055.5 ± 4.640.9 ± 5.5*31.7 ± 4.831.0 ± 7.133.3 ± 6.720.5 ± 6.418.4 ± 14.515.1 ± 9.5

See Table 1 for key.

Discussion

In small animal neurosurgery, dorsal laminectomy is the most commonly used technique for decompression of the terminal region of the spinal cord in dogs with DLSS.3,4,6-9,16,23 However, this procedure has several limitations. Although other surgical decompression methods include unilateral and bilateral laminotomy and foraminotomy,1,3,7,10,14,24,25 those methods relieve stenosis of only the vertebral canal or the IVF and yet compression of the terminal region of the spinal cord and left and right L7 nerves occurs alone or simultaneously with DLSS. Several reports1,6,8,10,24 have indicated that in dogs with DLSS where abnormalities in these sites occur simultaneously, no improvement in clinical signs is achieved by treating only a single site. Research has been conducted to develop surgical procedures to achieve decompression of both of these sites and stabilize the vertebral bodies.17 As the next step, we aim to immobilize the LSJ by combining dorsal fixation and use of an IDS. The present study served as a preliminary step toward this aim by investigating the effect of IDS insertion alone on the LSJ and adjacent vertebrae in healthy dogs.

In the present study, we confirmed that IDS insertion into the L7-S1 disk resulted in a decrease in the lumbosacral ROM and stenosis rate in 3 evaluated IVF zones. We also found an increase in the lumbosacral ID when the LSJ was in a flexed position. A previous report26 indicates that the IFA in dogs with DLSS is significantly lower when the LSJ is in an extended versus flexed position, and the authors speculated that the decrease in lumbosacral ROM with IDS insertion could result in a lower stenosis rate. Moreover, the authors also speculated that an increase in ID achieved with IDS makes the IVF larger. Use of a dorsal fixation device alone cannot completely release compression at the IVF entrance, middle, and exit zones12; however, results of the present study could be interpreted as indicating that LSJ extension with an IDS expands each region in the IVF, thereby increasing the ID and reducing pressure on the nerve roots and spinal cord.

Because DLSS involves dynamic compression and nerve compression within the IVF is marked when the LSJ is in an extended position, it appears that IDS insertion as performed in the present study could sufficiently restrict the degree of extension and thus serve as an effective method for treating nerve root compression. The increase in ID achieved with IDS insertion caused changes in the shape of the IVF that increased its area.

In the study reported here, ROM, ID, and IVF stenosis rate were also measured at L6-7 and L5-6. Results indicated that although the ROM of the LSJ decreased significantly with IDS insertion, there was no effect on the adjacent lumbar segments. Nevertheless, results also showed that the mean ± SD lumbosacral ROM following IDS insertion was 25.5 ± 3.8°, indicating that the LSJ was not completely immobilized with the IDS alone. In healthy dogs, the LSJ is the region of the vertebral column with the greatest mobility.27,28 Although ROM from flexion to extension in the dorsoventral direction of other lumbar intervertebral joints is approximately 10°, mean ± SD ROM of the LSJ is reportedly 34.8 ± 8.5° in hypochondroplastic dogs and 33.0 ± 4.7° in nonhypochondroplastic dogs.27 Values reported for other studies involving German Shepherd Dogs with DLSS include a median (range) of 29° (10° to 44°)29 and mean ± SD (range) of 26.4 ± 6.0° (11° to 43°).30 The fact that LSJ mobility following IDS insertion in the present study was maintained at approximately 25°, which is similar to reported ROM for healthy dogs, may explain why no IVF changes were observed at L6-7 and L5-6. We expect that the fixation strength achieved with IDS insertion into the L7-S1 disk would increase when dorsal fixation is also used, but additional research would be required to confirm this. Because no method has been established for accurate measurement of IFA via CT images, such a method would first need to be identified for measurement of the IFA in the LSJ of dogs.

The present study had several limitations that warrant consideration. First, flexion and extension of the LSJ were investigated in only the dorsoventral direction, but this joint also moves by rotation and lateral flexion. Therefore, one cannot conclude that IDS insertion would completely restrict LSJ mobility, and additional research would be needed to investigate that possibility. Under the investigated conditions, findings indicated that the IFA did not decrease statically but, rather, dynamically in the LSJ of dogs, and not in the adjacent lumbar segments. These preliminary results suggested that IDS fixation of the LSJ may be effective as a component of (and not sole) treatment for dogs with dynamic spinal cord compression and that the IVF stenosis rate is significantly greater with dorsal laminectomy alone. In addition to the aforementioned research needs, in vivo research is warranted to determine the safety and efficacy of IDS insertion into the L7-S1 disk for this purpose.

Acknowledgments

Mr. Fukuda is employed by Platon Japan Co Ltd, the manufacturer of the IDS evaluated in this study.

The authors declare that there were no other conflicts of interest.

ABBREVIATIONS

DLSS

Degenerative lumbosacral stenosis

ID

Intervertebral distance

IDS

Intervertebral distraction screw

IFA

Intervertebral foraminal area

IVF

Intervertebral foramen

LSJ

Lumbosacral joint

ROM

Range of motion

Footnotes

a.

Platon Japan Co Ltd, Tokyo, Japan.

b.

Epen, DePuy Synthes Inc, Pennsylvania, Pa.

c.

Trephine drill, Platon Japan Co Ltd, Tokyo, Japan.

d.

Aquilon Prime, Toshiba Medical Systems Inc, Tokyo, Japan.

e.

Virtual Place Aze, Virtual Place Aze Inc, Kanagawa, Japan.

f.

BellCurve for Excel, Social Survey Research Information Co Ltd, Tokyo, Japan.

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Contributor Notes

Address correspondence to Mr. Tanoue (tanouevet@jcom.home.ne.jp).