Introduction
Degenerative lumbosacral stenosis (DLSS) is a disease primarily observed in medium- and large-breed dogs.1–5 DLSS induces compression and damage to the spinal cord terminal and spinal nerve due to narrowing of the spinal canal and intervertebral foramen caused by degenerative changes in the disks, bones, and soft tissues of the lumbosacral junction (LSJ).1 It may cause various clinical signs related to the cauda equina, such as pain, lameness, paresis of the hind limbs and tail, urinary incontinence, and fecal incontinence.6 Abnormal mobility and instability in the LSJ are involved in the onset of DLSS.2,3,7–13
Surgical options include LSJ decompression performed alone or in combination with fixation.10 LSJ dorsal laminectomy performed alone or dorsal laminectomy in combination with anulectomy are the most common procedures.2,14,15 Reports demonstrate that dorsal laminectomy improves postoperative clinical signs in 69% to 93% of cases in the short term2–4,11; however, in the long-term, clinical signs recur in 3% to 37% of cases.2,4,16,17
Treatment with decompression alone may increase the instability of the LSJ after surgery.14,18 If LSJ instability is the cause of DLSS, treatment with dorsal laminectomy alone is insufficient, and decompression of the LSJ should be performed in combination with fixation to stabilize the LSJ.19 The following LSJ fixation methods have been reported: transarticular fixation technique with a pin or screw, dorsal vertebral fixation with a pin or pedicle screw and polymethylmethacrylate (PMMA), dorsal vertebral fixation with a pedicle screw and rod, and fixation with a locking plate.2,6–10,14,15,20–23 A previous autopsy study14 indicated that a combination of dorsal laminectomy and dorsal vertebral fixation using both pedicle screws and rods could achieve stabilization of the LSJ. Inness et al23 reported that for dorsal laminectomy and dorsal anulectomy, distracted fixation with positive thread pins and PMMA could improve clinical signs of DLSS in the short and long term.
Few studies have reported on the long-term prognosis of dorsal laminectomy using a combination of pedicle screws and PMMA. Thus, the aim of this study was to evaluate the efficacy of the combined surgery of dorsal laminectomy and dorsal fixation with screws and PMMA for DLSS. We hypothesized that a combination of dorsal laminectomy and dorsal vertebral body fusion with transarticular screws, pedicle screws, and PMMA would be effective in the long term.
Materials and Methods
Cases
We retrospectively evaluated the clinical records of dogs weighing > 15 kg diagnosed with DLSS and surgically treated. DLSS was diagnosed based on clinical signs and MRI.24 For all dogs included in this study, we preoperatively performed a full neurological examination,6–8 orthopedic examination of the hind limbs, radiography of the spine and hind limbs, and CT and MRI (including the brain and spinal cord from C1 to sacrum) to rule out any orthopedic disease or other neurological disease except DLSS. The dogs with concurrent orthopedic disease and other neurological abnormalities were excluded. LSJ dorsal laminectomy was performed in combination with dorsal fixation surgery using screws and PMMA. Clinical signs, the presence or absence of complications, and the type of complications were recorded. Evaluations were conducted before surgery, after surgery, and 3, 6, 12, 24, and 36 months after surgery.
Clinical signs and evaluation of neurological grade
Evaluation of hind limb lameness, locomotive function (difficulties standing up from a prone position, avoidance when jumping or climbing steps), exercise intolerance, LSJ tenderness, and neurological impairment (proprioceptive deficits, urinary incontinence, fecal incontinence) were recorded preoperatively and postoperatively.
The severity of clinical signs was classified as follows: 0, no symptoms; 1, mild symptoms (mild pain in the dorsal lumbosacral region such as unwillingness for palpation, mild locomotive disorders such as difficulties rising up and reluctance to jump and climb stairs, occasional avoidance of excessive exercise, and no neurological abnormalities); 2, moderate symptoms (moderate to severe pain in the dorsal lumbosacral region such as biting and screaming during palpation, moderate hind limb lameness such as intermittent weight-bearing lameness, flexed posture in the lumbosacral region, moderate [ie frequent] locomotive avoidance, and mild neurological abnormalities); and 3, severe symptoms (in addition to moderate symptoms, dislike of all forms of locomotion and moderate to severe neurological abnormalities, including sciatic nerve paresis and urinary or fecal incontinence).2
Hind limb conscious proprioception was evaluated and scored as follows: 0, absent; 1, decreased; and 2, normal.6
Radiography
Lateral and ventral-dorsal radiographs of the lumbosacral region were taken to confirm the placement of the implant and to evaluate spinal morphology. To evaluate the postfixation status, the pre- and postoperative L7-S1 intervertebral distances (LS-IDs) were measured on the lateral X-ray image taken in a neutral position.8 ImageJ (version 1.52a; National Institutes of Health) was used to measure the LS-ID.
Computed tomography
A CT scanner (Aquilon Prime; Toshiba Medical Systems Corp) was used to capture 80 rows of 160 slices for CT. The imaging settings were 0.5-mm slice width and 300-mm field of view. Imaging software (Aze Virtual Place; Canon Medical Systems Corp) was used for image processing. Three-dimensional reconstruction (multiplanar reconstruction processing) of the LSJ was performed using captured CT images. The conditions for drawing CT images were a window width of 1,500 HU and a window level of 300 HU.
CT imaging was performed preoperatively to determine the locations of the pedicle screws that were to be fixed to L7 and the sacrum. Three-dimensional CT images were used to determine the size of the pedicle (length/width), and screws with the largest diameter that would fit within the pedicle width were selected accordingly.
Magnetic resonance imaging
Magnetic resonance imaging was performed using a 3.0-T unit (Signa HDtx 3.0T; GE Healthcare). The shooting conditions were slice thickness, 2 mm; slice spacing, 0 to 1 mm depending on the size of the individual; repeat time, 5,000 milliseconds; echo time, 87.5 milliseconds; and field of view, 30 X 30 cm. Using MRI, the preoperative degree of degeneration of the LSJ intervertebral disk was evaluated and graded from 1 to 4.24
Surgery
Anesthesia was induced through intravenous administration of 2 mg/kg of propofol, 0.3 mg/kg of midazolam, and an additional 4.5 mg/kg of propofol. After tracheal intubation, anesthesia was maintained through inhalation of 1% to 2.5% isoflurane. A mixed solution of 1 mg/kg of bupivacaine and 0.1 mg/kg of morphine hydrochloride was administered to L7-S1 for epidural analgesia. For pain management, intraoperative fentanyl was administered using a continuous infusion rate of 2 to 5 μg/kg/h. Postoperatively, a fentanyl patch (DuroTep MT patch; Janssen Pharmaceutical K.K) was applied every 72 hours (body weight of 10 to 20 kg, 8.4 mg/animal; body weight of 20 to 30 kg, 12.6 mg/animal; body weight of > 30 kg, 16.8 mg/animal). Before skin incision, 25 mg/kg of cefmetazole sodium was administered IV, and then 25 mg/kg of cefmetazole sodium was administered IV every 90 minutes during surgery.25
A single surgeon (Yasushi Hara) performed all surgeries. First, LSJ dorsal laminectomy was performed. The hip joint of the dog was flexed, and the toes were made to point to the head. The dog was fixed in the prone position while the LSJ was maintained in the flexed position. Dorsal laminectomy was performed across the caudal aspect of L7 and the cranial aspect of the sacrum, and the ligamentum flavum was removed to expose the cauda equina.8,23 The excision length of the dorsal vertebral arch was half the length of the L7 vertebral body. The excision width was one-third of the L7 spinal canal so that the caudal articular process of L7 could be preserved. The articular process joint between L7 and S1 was left untreated except in cases requiring resection due to severe intraforaminal compression of the seventh spinal nerve.2 Subsequently, a dorsal vertebral body fixation was performed. At completion of the dorsal laminectomy, the position of the hind limbs was changed. The hip joint, stifle joint, and tarsal joint were all flexed, and the LSJ was returned to the neutral position. Transarticular fixation was performed on the left and right articular process joints of the LSJ. That is, 2 cortical bone screws were used from the center of the caudal articular process of L7 to the ipsilateral sacral cranial articular process. A 2.4-mm-diameter cortical bone screw (self-tapping cortical screw; Mizuho Corp) was used. After confirmation of the positions of the L7 and S1 pedicles based on preoperative CT transverse imaging, the screws were placed on the L7 and S1 pedicles so that they would not be inserted into the spinal canal, and they were fixed perpendicular to the floor of the spinal canal. Once the screws were placed, intraoperative horizontal radiography was performed to confirm the condition of the screw placement. Locking-head screws (Depuy Synthes VET) or self-tapping cortical screws (Mizuho Corp) with diameters of 2.7 to 3.5 mm were used. Two to 4 pedicle screws were used for L7 and S1. In addition to screws that were long enough to fix L7-S1 from the dorsal side of the vertebral arch to the ventral cortical surface, screws that were long enough to be exposed from the dorsal surface of the vertebrae (ie, 10 to 15 mm [to allow for fixation using bone cement]) were selected (Figure 1).6,14 After screw installation, free subcutaneous fat was transplanted to the laminectomy of the L7-S1 to avoid recompression of the cauda equina due to postoperative fibrotic scar membrane formation.26 Thereafter, PMMA (Surgical Simplex; P Stryker) impregnated with cefmetazole sodium (1 g/40 g) was installed so that the shafts and heads of all the installed bone screws were filled.23,27 To prevent heat generation during PMMA polymerization, PMMA was cooled with saline.27 Closure was performed as previously described27 (Figure 2).
Placement of screws based on 3-D CT. A—L7-S1 dorsal surface. B—L7-S1 sagittal section. C—L7 transverse section. D—S1 transverse section. Red: L7 pedicle screw. Blue: S1 pedicle screw. Yellow: transarticular screw. Two to 4 pedicle screws of L7, 2 to 4 pedicle screws of S1, and 2 transarticular screws were used. The red dotted line indicates the screw installation direction of the L7 vertebral body, and the blue dotted line indicates the screw installation direction of the S1 vertebral body.
Citation: Journal of the American Veterinary Medical Association 260, 14; 10.2460/javma.22.04.0176
Radiograph showing screw and polymethylmethacrylate fixation after surgery. A—Lateral. B—Ventral-dorsal.
Citation: Journal of the American Veterinary Medical Association 260, 14; 10.2460/javma.22.04.0176
After surgery, 25 mg/kg of cefmetazole sodium was administered IV every 12 hours until the 10th to 14th day, and then 15 to 25 mg/kg of cephalexin was administered orally at 12-hour intervals for 30 days. For inflammation and pain management, 2 mg/kg of robenacoxib was injected SC for 3 to 5 days after surgery.
Postoperative complications
Postoperative complications associated with clinical signs that required reoperation under general anesthesia, such as implant removal and lumbosacral refixation surgery, were defined as major complications. Minor complications were defined as those with clinical signs that could be controlled by treatment such as medication and asymptomatic implant failure without reoperation. The presence or absence of symptoms and the location and time of occurrence of the breakage were recorded.
Data analysis
Statistical analysis was performed using Stata (version 14; Stata Corp). The severity of clinical signs, proprioceptive deficits, and LS-ID immediately after surgery and 3, 6, 12, 24, and 36 months after surgery were compared with those before surgery. The Kruskal-Wallis test was performed; when a significant difference was identified, post hoc analysis was conducted by means of the Mann-Whitney U test. The threshold for a statistically significant difference was P < 0.05.
Results
The median and range of parameters were presented according to number of dogs (n) and number of hindlimbs (N) at each observation time point.
Signalment
Twenty-one dogs aged 60.2 ± 39.3 months (mean ± SD; range, 6 to 150 months) and weighing 29.9 ± 7.8 kg (15.2 to 43.0 kg) were examined, including 14 males (intact, 7; castrated, 7) and 7 females (intact, 3; spayed, 4). The breeds included Golden Retriever (n = 8), Labrador Retriever (n = 4), German Shepherd (n = 3), Bernese Mountain Dog (n = 3), Australian Shepherd (n = 1), Weimaraner (n = 1), and English Springer Spaniel (n = 1). In terms of severity of preoperative clinical signs, 8 animals were grade 1 and 13 animals were grade 2. For proprioceptive deficits, 5 limbs were scored as 0, 14 limbs were scored as 1, and 23 limbs were scored as 2. Hind limb lameness was observed in 13 of 21 dogs (61.9%). For locomotive impairment, 4 of 21 dogs (19.0%) were observed to avoid standing up from a prone position, and 6 of 21 (28.5%) were observed to avoid ascending and descending movements, such as jumping or undertaking a level change. Exercise intolerance was observed in 15 of 21 (71.4%), tenderness at the LSJ in 16 of 21 (76.2%), and urinary and fecal incontinence in 0 of 24 (0%). Knuckling of the hind limb was observed in 6 of 21 dogs (28.6%). The degrees of disk degeneration were as follows: grade one, 4 dogs; grade two, 2 dogs; grade three, 12 dogs; and grade four, 3 dogs. The fixation methods and screws used are summarized (Supplementary Table S1).
Outcomes
Twenty-one dogs were examined before surgery, 21 after surgery, 21 at 3 months after surgery, 16 at 6 months after surgery, 14 at 12 months after surgery, 10 at 24 months after surgery, and 8 at 36 months after surgery.
The median and range of the severities of clinical signs, proprioceptive deficits, and LS-ID in all cases are summarized (Table 1). The severity of clinical signs was significantly improved at 3 months after surgery relative to before surgery (P < 0.00). Proprioceptive deficits were significantly improved at 3 months after surgery compared with preoperatively (P < 0.00). All cases with decreased proprioceptive deficits before surgery (19 limbs) showed improvement. In addition, cases with normal proprioception before surgery (23 limbs) maintained normal proprioception after surgery. No change was observed in LS-ID during the entire observation period.
Longitudinal changes in the severity of clinical signs, proprioceptive deficits, and LS-ID.
| Duration after surgery (mo) | |||||||
|---|---|---|---|---|---|---|---|
| Variable | Before surgery | 0 | 3 | 6 | 12 | 24 | 36 |
| Severity of clinical signs | 2 (1–2) | 2 (1–2) | 0 (0–1)* | 0 (0–0)* | 0 (0–1)* | 0 (0–0)* | 0 (0–0)* |
| n = 21 | n = 21 | n = 21 | n = 16 | n = 14 | n = 10 | n = 8 | |
| Implant nonfailure cases | 2 (1–2) | 2 (1–2) | 0 (0–1) | 0 (0–0) | 0 (0–1) | 0 (0–0) | 0 (0–0) |
| n = 19 | n = 19 | n = 19 | n = 15 | n = 13 | n = 9 | n = 7 | |
| Implant failure cases | 2 (2–2) | 2 (2–2) | 0.5 (0–1) | 0 | 0 | 0 | 0 |
| n = 2 | n = 2 | n = 2 | n = 1 | n = 1 | n = 1 | n = 1 | |
| Proprioceptive deficits | 2 (0–2) | 2 (0–2) | 2 (1–2)† | 2 (2–2)† | 2 (1–2)† | 2 (2–2)† | 2 (2–2)† |
| n = 21, N = 42 | n = 21, N = 42 | n = 21, N = 42 | n = 16, N = 32 | n = 14, N = 28 | n = 10, N = 20 | n = 8, N = 16 | |
| Implant nonfailure cases | 2 (0–2) | 2 (0–2) | 2 (1–2) | 2 (2–2) | 2 (1–2) | 2 (2–2) | 2 (2–2) |
| n = 19, N = 38 | n = 19, N = 38 | n = 19, N = 38 | n = 15, N = 30 | n = 13, N = 26 | n = 9, N = 18 | n = 7, N = 14 | |
| Implant failure cases | 1 (1–2) | 1 (1–2) | 1.5 (1–2) | 2 (2–2) | 2 (2–2) | 2 (2–2) | 2 (2–2) |
| n = 2, N = 4 | n = 2, N = 4 | n = 2, N = 4 | n = 1, N = 2 | n = 1, N = 2 | n = 1, N = 2 | n = 1, N = 2 | |
| LS-ID | 2.60 (1.80–3.80) | 2.66 (1.74–3.68) | 2.72 (1.34–3.92) | 2.75 (1.02–3.81) | 2.69 (1.61–3.55) | 2.72 (1.64–3.56) | 2.96 (2.16–3.42) |
| n = 21 | n = 21 | n = 21 | n = 16 | n = 14 | n = 10 | n = 8 | |
| Implant nonfailure cases | 2.64 (1.80–3.80) | 2.66 (1.74–3.68) | 2.72 (1.34–3.91) | 2.72 (1.02–3.81) | 2.58 (1.60–3.55) | 2.69 (1.64–3.56) | 3.03 (2.16–3.42) |
| n = 19 | n = 19 | n = 19 | n = 15 | n = 13 | n = 9 | n = 7 | |
| Implant failure cases | 2.18 (2.15–2.21) | 2.39 (2.09–2.69) | 2.51 (2.03–3.01) | 3.07 | 2.64 | 2.75 | 2.41 |
| n = 2 | n = 2 | n = 2 | n = 1 | n = 1 | n = 1 | n = 1 | |
*P < 0.05 versus the grade before surgery. †P < 0.05 versus the score before surgery.
LS-ID = L7-S1 intervertebral distance. n = Number of dogs. N = Number of limbs.
Postoperative complications
Minor complications observed were postoperative implant failure (n = 2; 9.5%), delayed healing of surgical wounds (2; 9.5%), seroma (1; 4.8%), and swelling of the affected area (1; 4.8%). For delayed healing of surgical wounds, 1 case was resutured without anesthesia on the 13th day after surgery, while the other case was resutured without anesthesia on the 16th day after surgery because a piece of skin came loose after the initial suture was removed. For seroma, puncture suction was performed on the 13th and 15th days after surgery. For swelling of the affected area, no additional treatment was required and the condition subsequently improved. Implant failure occurred in 1 case with the left and right L7 pedicle screws, left S1 pedicle screw, and left transarticular screw breaking 3 months after surgery. In another case, the left S1 pedicle screw broke 12 months after surgery, and the right S1 pedicle screw broke 36 months after surgery (Figure 3). However, implant failure did not cause recurrence or worsening of clinical signs. No major complications were detected.
Radiograph showing screw breakage. Case 1 lateral (A) and ventral-dorsal (B): the left and right L7 pedicle screws, the left S1 pedicle screw, and the left transarticular screws break 3 months after surgery. Case 2 lateral (C) and ventral-dorsal (D): the left S1 pedicle screw breaks 12 months after surgery, and the right S1 pedicle screw breaks 36 months after surgery.
Citation: Journal of the American Veterinary Medical Association 260, 14; 10.2460/javma.22.04.0176
Discussion
Clinical signs improved postoperatively in all 21 cases in the study, and no recurrence of clinical signs was detected during the observation period. Affected limbs that were normal before surgery were included among those analyzed for proprioception. Previous studies have found that most cases of suspected DLSS are not accompanied by neurological abnormalities, and only severely affected dogs exhibit reduced proprioception, such as knuckling of the hind limb.5 In the present study, all cases with decreased proprioception before surgery showed improvement. In addition, cases with normal proprioception before surgery maintained normal proprioception after surgery. The severity of clinical signs had improved by 3 months after surgery, with no further deterioration. However, it should be noted that only 66% of the total cases were able to be followed up for a year, which limits the ability to accurately determine long-term recurrence rate.
Detailed information on the long-term performance of dorsal laminectomy for DLSS is limited, and there is no consensus on prognosis.4 Previous studies reported that dorsal laminectomy improved clinical signs in 69% to 93.2% of dogs with DLSS, but recurrence was observed in 3% to 17.6% of cases (observation period, 16 to 60 months).2,4,15,17 Moreover, increased LSJ mobility after surgery was considered an important contributing factor to the worsening of clinical signs in cases with recurrence of clinical signs.2 Chambers et al19 showed that the pathology of DLSS may involve excessive mobility of the LSJ. Based on the results of the aforementioned studies, it can be inferred that the treatment of DLSS requires not only decompression of the cauda equina in the LSJ, but also stabilization of the unstable LSJ. A study of dorsal decompression combined with dorsal fixation using screws and rods also demonstrated the possibility of greater improvements in hind limb function compared with decompression alone over a long period of time.3 Inness et al23 reported that after dorsal laminectomy and anulectomy, distracted fixation using positive thread pins and PMMA improved clinical signs and was an effective long-term treatment. In the present study, clinical signs improved in all 21 cases, and no recurrence was observed 36 months after surgery.
Narrowing of the intervertebral disk space (IVDS) is a phenomenon reported in dogs with DLSS.1,28,29 The earliest-occurring degenerative change in the LSJ in DLSS-affected dogs without spondylitis or spondylosis is the narrowing of the IVDS due to the L7 caudal endplate and S1 rostral endplate drawing closer together,19 which would suggest the presence of LSJ instability.8 It was reported that dorsal laminectomy combined with transarticular fixation maintained the LSJ IVDS for at least 6 months and indicated that it is therefore an effective surgical method.8 Inness et al23 performed distracted fixation using positive thread pins and PMMA after dorsal laminectomy and anulectomy and reported that the LSJ IVDS increased by 50% postoperatively and that the effect lasted for 24 months. In the present study, anulectomy after dorsal laminectomy was not applied in all cases. The LS-ID did not change during the entire observation period. Thus, it is considered that dorsal fixation of the LSJ may maintain the LSJ IVDS and reduce LSJ instability.
If the implant used for LSJ fixation is damaged after surgery, there is a risk that the intended outcome is not achieved. Implant failure was observed in 9.5% of the animals that underwent our surgical procedure. However, no associated clinical signs or worsening of neurological severity was observed, no additional surgery was required, and there were no effects on the improvement of clinical signs. Our procedure involved the combination of multiple types of fixation methods using transarticular screws, pedicle screws, and PMMA. Given this, it is possible that the failure of a particular fixation device was compensated for by the other fixation devices, thereby maintaining a fixed LSJ state. It is also possible that this method prevented the recurrence of clinical signs by avoiding the invasion of surrounding tissues, which occurs with displacement of implant or bone by implant failure. Therefore, we believe that our method may be a more effective treatment that reduces the risk of implant failure–induced recurrence of clinical signs.
This study had some limitations. First, the observation periods and breed varied between cases, and the number of cases was insufficient when using retrospective data to evaluate complication rates (potential for underreporting of complications in records). Moreover, it should be noted that only 66% of the cases could be followed for 12 months, which limited the ability to determine long-term recurrence rate accurately, as described above. Second, the surgical procedures were not standardized with regard to the number of implants and the amount of PMMA. Third, subjective indices, such as clinical signs, were assessed in this study. Finally, this study did not include severe cases with urinary incontinence or fecal incontinence because it is reported that surgical decompression is less effective in DLSS-affected dogs with urinary incontinence.15 Moreover, the presence or absence of urinary incontinence or fecal incontinence before surgery was the only clinical sign that influenced the improvement of postoperative clinical signs. In particular, the duration of preoperative urinary incontinence has been reported to have a significant effect.4 Hence, the therapeutic effect of combined surgery could not be verified in such severe cases, and these details must be verified in future studies.
Nevertheless, the combined application of dorsal laminectomy and dorsal vertebral fixation using transarticular screws, pedicle screws, and PMMA was determined to be effective in the long term, even with implant failure.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org
Acknowledgments
The funding sources did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript. The authors declare that there were no conflicts of interest.
The authors thank everyone involved in this research. They also thank Editage for English-language editing.
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