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Abstract

Objectives

Methods

Twenty-one male feline cadavers were enrolled in the study. All feline cadavers were evaluated by CT. Examinations were performed with the cadaver in a neutral position and dorsal and ventral recumbency. Sagittal vertebral canal diameters (VCDs) were obtained by measuring the distance between the ventral and dorsal aspects of the vertebral canal in the middle of the intervertebral space.

Results

A comparison of the VCDs between L6 and L7, L7 and S1, S3 and Co1 and Co1 and Co2 in neutral position vs dorsal recumbency revealed a reduction of 0.27 mm (14.6%; P <0.001) between S3 and Co1 and 0.26 mm (18.1%; P <0.001) between Co1 and Co2. No differences were seen when comparing L6–L7 and L7–S1. The VCDs were decreased in all segments when comparing neutral with ventral recumbency. This study revealed a reduction of 0.13 mm between L6 and L7 (3.3%; P = 0.003), 0.14 mm between L7 and S1 (4.1%; P = 0.003), 0.61 mm between S3 and Co1 (32.5%; P <0.001) and 0.63 mm between Co1 and Co2 (44.1%; P <0.001). Comparison of the VCD between dorsal and ventral recumbency in L6–L7, L7–S1, S3–Co1 and Co1–Co2 revealed a decrease in the VCDs in ventral recumbency of 0.13 mm (3.3%; P <0.001), 0.12 mm (3.6%; P <0.001), 0.34 mm (21.0%; P <0.001) and 0.37 mm (31.7%; P <0.001), respectively.

Conclusions and relevance

The results provide evidence that, from an anatomical point of view, perineal urethrostomy performed in dorsal recumbency is superior to ventral recumbency, but further clinical studies to verify these findings are necessary.

Introduction

Perineal urethrostomy is a surgical procedure performed in male cats to create a permanent opening between the pelvic urethra and the skin in the perineal region. Indications include recurrent urethral obstruction unresponsive to nutritional or medical therapy and urethral obstruction that cannot be relieved by catheterisation and retrograde flushing, including urolithiasis, urethral strictures, urethral trauma and neoplasia.¹

Several techniques and modifications of perineal urethrostomy have been described since the early 1960s.^2–6 The most common perineal urethrostomy technique presently performed is a variation of the procedure described by Wilson and Harrison in 1971.⁶ This technique involves mobilisation of the pelvic urethra and creation of a new stoma by suturing the urethral mucosa to the perineal skin and the amputation of the narrow penile urethra.^6,7 Multiple studies have been published since, describing the short- and long-term outcome of this surgical technique.^8–14 The most frequent complication after perineal urethrostomy in cats is a bacterial urinary tract infection with an incidence of 17–57%.^{8,10,14,15,16} This condition seems to be multifactorial.¹² Decreased intraurethral pressure after surgery has been postulated as an important predisposing factor caused by nerve injury during surgery.¹⁷

A dorsal or, more commonly, ventral recumbency is recommended for this surgical procedure.¹⁸ However, none of the aforementioned publications investigated the surgical positioning as a potential outcome-influencing factor. Consequently, the question to be asked is: does the dorsal or ventral positioning during surgery have an influence on the anatomy of the lumbococcygeal spine? Considering the hyperextension and dorsal pulling of the tail during ventral positioning, parallels to mild ‘tail-pull injuries’ can be drawn, an injury that is well accepted as causing urinary tract dysfunction.¹⁹ To our knowledge the effect of the intraoperative positioning on the caudal vertebral canal diameter has not yet been evaluated.

The objective of this study was to quantify the changes in the diameter of the vertebral canal in the lumbosacral and sacrococcygeal column (L6–Co2) in cats in dorsal and ventral recumbency simulating real body positioning during a perineal urethrostomy. We hypothesised that the established ventral and dorsal positioning for perineal urethrostomy in cats would lead to a reduction in the diameter of the lumbococcygeal spinal canal, bearing the potential for nerve root irritation.

Materials and methods

Animals

Twenty-one feline cadavers were collected from adult male European Shorthair cats euthanased for reasons unrelated to this study. CT was performed immediately (within 15 mins) after euthanasia to evaluate the vertebral morphology. This research adheres to the European Union convention regarding the protection of animals used for experimental and scientific purposes (Revised Directive 86/609/EEC).

Testing procedure

Non-contrast CT studies were obtained using a four-slice helical scanner (GE LightsSpeed QX/i; GE Healthcare). Slice thickness was set at 0.625 mm. Images were obtained from L6–L7 to Co1–Co2 using an axial scanning mode. Technical settings used were: 120 kV, automatic mA, 512 × 512 scan matrix, detail algorithm and 6.25 mm/s table speed.

All cadavers underwent the same CT scanning protocol. For a neutral position, the cadavers were placed in dorsal recumbency with the lumbar and coccygeal part of the body in a neutral position without tying down the limbs. Afterwards, the cadavers were secured to a custom-made testing apparatus simulating regular intraoperative positioning during perineal urethrostomy. The previous scans were repeated with the cadavers placed in dorsal (Figure 1) and ventral recumbency (Figure 2). For dorsal recumbency the thoracic limbs were tied in a caudal direction and the pelvic limbs in a cranial direction using gauze. The tail hung free over the apparatus’s trailing edge. Ventral recumbency was accomplished by tying the thoracic limbs in a cranial direction and the pelvic limbs over the edge of the apparatus in a ventrolateral position. The tail was hyperextended and secured with tape to the apparatus, so access to the perineal region was ensured.

Figure 1

(a) Dorsal recumbency. The cadaver is secured onto the testing apparatus in dorsal recumbency. The thoracic limbs are pulled in a caudal direction and the pelvic limbs in a cranial direction with gauze. (b) The tail hangs free over the apparatus’s trailing edge

Figure 2

(a) Ventral recumbency. The cadaver is secured onto the testing apparatus in ventral recumbency. The thoracic limbs are pulled in a cranial direction and the pelvic limbs are tied over the edge of the apparatus in a ventrolateral position. (b) The tail is hyperextended and secured with tape to the apparatus

Morphometric analysis of the CT images

All measurements were taken from sagittal slices by a single observer (PS), trained by the senior investigator (MB), using a CT-Workstation (Xtream Workstation for GE LightSpeed QX/i CT; GE Healthcare). The method described by Ramos²⁰ was modified for the use of intact cadavers and used to measure the vertebral canal diameter. The sagittal vertebral canal diameter (VCD) was obtained by measuring the distance between the outlines of the dorsal and ventral aspects of the vertebral canal in the middle of the intervertebral space, reflecting the intervertebral disc and ligamentum flavum levels, respectively (Figure 3). Measurements were performed at the L6–L7, L7–S1, S3–Co1 and Co1–Co2 intervertebral space during neutral, dorsal and ventral recumbency. All measurements were repeated three times with at least a 1 week interval between measurements, as carried out in the study by Ramos.²⁰

Figure 3

Sagittal vertebral canal diameter: measurement of the vertebral canal diameter (mm) in neutral position

Statistical analysis

A generalised linear model was used to compare the VCD between L6–L7, L7–S1, S3–Co1 and Co1–Co2 in neutral position, dorsal and ventral recumbency. Statistical significance was adjusted using the Bonferroni procedure and set at values of P <0.05. Intra-observer agreement was established using the proportion of total variance found between subjects (rho) using a variance components model based on a generalised linear model. All statistical models were analysed using a software programme for statistical analysis (IBM SPSS Statistic 22).

Results

Specimens were collected from mature feline male cadavers (n = 21) with a mean ± SD weight of 5.38 ± 2.02 kg and a mean ± SD age of 8.17 ± 4.68 years. Influence of age and weight on the VCDs yielded insignificant results (P = 0.591 and P = 0.596, respectively).

Comparison of the VCD between L6 and L7 in neutral position vs the VCD in dorsal and ventral recumbency

The comparison between the VCD in neutral position and dorsal recumbency yielded insignificant results (P = 1) (Figure 4). For the comparison between neutral position and ventral recumbency, the VCD decreased by 0.13 mm (3.3%), which was significantly different (P = 0.003) compared with the neutral position (Table 1).

Figure 4

Mean (and SD) vertebral canal diameter (mm) between L6 and L7 in neutral position, dorsal and ventral recumbency

Table 1

Comparison between the vertebral canal diameter in neutral position and dorsal and ventral recumbency (based on the generalised linear model)

	Difference – canal size		95% Wald confidence interval		Bonferroni- P value
Comparison	mm	%	Minimum	Maximum	Bonferroni- P value
L6–L7 (neutral) vs L6–L7 (dorsal)	0.00	0.0	−0.075	0.074	1.000
L6–L7 (neutral) vs L6–L7 (ventral)	0.13	−3.3	0.031	0.235	0.003
L7–S1 (neutral) vs L7–S1 (dorsal)	0.02	−0.5	−0.049	0.086	1.000
L7–S1 (neutral) vs L7–S1 (ventral)	0.14	−4.1	0.034	0.244	0.003
S3–Co1 (neutral) vs S3–Co1 (dorsal)	0.27	−14.6	0.149	0.396	<0.001
S3–Co1 (neutral) vs S3–Co1 (ventral)	0.61	−32.5	0.488	0.727	<0.001
Co1–Co2 (neutral) vs Co1–Co2 (dorsal)	0.26	−18.1	0.155	0.361	<0.001
Co1–Co2 (neutral) vs Co1–Co2 (ventral)	0.63	−44.1	0.499	0.759	<0.001

Comparison of the VCD between L7 and S1 in neutral position vs the VCD in dorsal and ventral recumbency

No significant differences between neutral position and dorsal recumbency were identified (P = 1) (Figure 5). Comparing the VCD between neutral and ventral recumbency, the VCD decreased by 0.14 mm (4.1%) with P = 0.003 (Table 1).

Figure 5

Mean (and SD) vertebral canal diameter (mm) between L7 and S1 in neutral position, dorsal and ventral recumbency

Comparison of the VCD between S3 and Co1 in neutral position vs the VCD in dorsal and ventral recumbency

In the comparisons of the VCD between neutral position, dorsal and ventral recumbency the VCD decreased by 0.27 mm (14.6%) and 0.61 mm (32.5%), respectively (Figure 6). Both differences were significantly lower (P <0.001) (Table 1).

Figure 6

Mean (and SD) vertebral canal diameter (mm) between S3 and Co1 in neutral position, dorsal and ventral recumbency

Comparison of the VCD between Co1 and Co2 in neutral position vs the VCD in dorsal and ventral recumbency

The comparison between the VCD in neutral position, dorsal and ventral recumbency yielded significant results (P <0.01) in both positions (Figure 7). The VCD decreased in dorsal recumbency by 0.26 mm (18.1%) and in ventral recumbency by 0.63 mm (44.1%) (Table 1).

Figure 7

Mean (and SD) vertebral canal diameter (mm) between Co1 and Co2 in neutral position, dorsal and ventral recumbency

Comparison of the VCD between L6 and L7, L7 and S1, S3 and Co1, and Co1 and Co2 in dorsal vs ventral recumbency

For the comparison of the VCD between dorsal and ventral recumbency in L6–L7, L7–S1, S3–Co1 and Co1–Co2, the VCD decreased by 0.13 mm (3.3%; P <0.001), 0.12 mm (3.6%; P <0.001), 0.34 mm (21.0%; P <0.001) and 0.37 mm (31.7%; P <0.001), respectively (Table 2).

Table 2

Comparison of the vertebral canal diameter between L6 and L7, L7 and S1, S3 and Co1, and Co1 and Co2 in dorsal vs ventral recumbency (based on the generalised linear model)

	Difference – canal size		95% Wald confidence interval		Bonferroni P value
Comparison	mm	%	Minimum	Maximum	Bonferroni P value
L6–L7 (dorsal) vs L6–L7 (ventral)	0.13	−3.3	0.055	0.213	<0.001
L7–S1 (dorsal) vs L7–S1 (ventral)	0.12	−3.6	0.043	0.198	<0.001
S3–Co1 (dorsal) vs S3–Co1 (ventral)	0.34	−21.0	0.238	0.432	<0.001
Co1–Co2 (dorsal) vs Co1–Co2 (ventral)	0.37	−31.7	0.232	0.510	<0.001

Intra-observer agreement

The intra-observer agreement (rho) for the vertebral canal ratios was 0.836. This result indicates a fair agreement for all variables.

Discussion

The goal of this study was to compare the changes in the sagittal vertebral canal diameters in the lumbosacral and sacroccocygeal column (L6–Co2) secondary to the application of complex biomechanical forces in cats positioned in dorsal and ventral recumbency, simulating the positioning for perineal urethrostomy. This study demonstrated that the sagittal vertebral canal diameter is significantly decreased in dorsal recumbency between S3–Co1 and Co1–Co2; in ventral recumbency, a significant reduction in the VCDs was seen in all evaluated vertebral segments. The degree of reduction was significantly higher in ventral compared with dorsal recumbency.

To ensure adequate surgical access to the perineal region in ventral recumbency the tail had to be tied cranially in a relatively unphysiological manner (hyperextension). It was not necessary to tie the tail in dorsal recumbency, as it hung over the edge of the apparatus in only a mild extension due to gravitation. Based on the findings presented above, we assume that the higher degree of VCD reduction in ventral recumbency could be caused largely by tail hyperextension in this position. Further biomechanical studies quantifying this assumption are needed.

The lumbosacral and sacral plexus are the most important neuroanatomical structures in this region. The lumbosacral plexus innervates the muscles that are involved with pelvic limb movement and the cutaneous region of the pelvic limb. The sacral plexus innervates the muscles and skin of the perineal region. Motor neurons of the femoral and obturator nerves are present between L4 and L6. The motor neurons originate in the spinal cord segments between L6 and S1 for the cranial gluteal nerve, between L7 and S2 for the sciatic nerve and between S1 and S3 for the pudendal nerve.²¹ In cats, the motor neurons innervating the coccygeal muscles originate in the spinal cord segments S2–Co3.^22,23

Intraoperative nerve injuries caused by inappropriate positioning of the patient during surgery in the perineal region are well documented in human medicine,^24,25 and occasionally described in various regions in animals.^26–29 These are often transitory in nature and for this reason often remain clinically underdiagnosed.³⁰ A common method of quantitatively evaluating function disorders of the lower urinary tract in clinically unclear cases in humans, as well as in cats, is urethral pressure measurement.^17,31,32 Urodynamic studies conducted in cats undergoing perineal urethrostomy found significant correlation between the decrease in urethral pressure and the increase in urinary tract infections after surgery.^7,12 It was thought that the decrease in urethral pressure was caused by excessive soft tissue dissection during surgery and the associated iatrogenic nerve injuries.⁷ A study by Sackman et al disproves this hypothesis.³³ Neither sharp nor blunt extensive intrapelvic dissection significantly alters urethral pressure and sphincter electromyography after surgery.³³ Based on this finding it can be postulated that the mechanism of nerve injury leading to a decrease of urethral pressure after surgery is of another origin.

The present study has some potential limitations. The sample population was relatively small with a broad age difference. In our study a novel methodology to evaluate the VCD with CT was applied. The original idea of this study was based on the methodology of the work of Ramos et al,²⁰ where the investigators quantified the changes of the VCD in the cervical vertebral column in Great Danes after application of tension–compression and complex loads of flexion and extension forces. Compared with this study, Ramos et al used stripped vertebral columns; the dorsal longitudinal ligament and ligamentum flavum were marked with barium sulfate cream and the changes in the VCD were visualised with fluoroscopy.²⁰ Owing to the anatomical complexity in the lumbosacral and sacrococcygeal column and our intention to evaluate the intact cadavers under conditions as similar as possible to animals in anaesthesia, it was decided to use CT to visualise the vertebral canal. The advantage of this approach was the speed of examination, which allowed an examination before signs of rigor mortis appeared. Assessing cachectic cadavers with insufficient epidural fat tissue presented a challenge, as in these cases the anatomical borders were not always absolutely clear. Imaging methods such as CT with myelography or MRI are superior to non-contrast CT for the visualisation of the vertebral canal,³⁴ but both methods are more time consuming and changes due to rigor mortis would be more developed.

Extension forces on the extremities and tail were not quantified and unified in this study design. The extremities were tied down and the tail taped to the testing apparatus, representing common intraoperative positioning to access the perineal region, but only with application of minimal forces necessary for stable positioning.

Conclusions

This study shows that sagittal vertebral canal diameters decrease significantly in dorsal recumbency between S3 and Co1, and Co1 and Co2, and in ventral recumbency in all evaluated vertebral segments. Reduction in the diameter of the vertebral canal diameters was significantly higher in ventral compared with dorsal recumbency. Reduction of the vertebral canal diameter between L6 and Co2 arising from body positioning could potentially, directly or indirectly, cause iatrogenic nerve injuries and influence neuronal pathways in these regions. The results provide evidence that, from an anatomical point of view, perineal urethrostomy performed in dorsal recumbency is superior to ventral recumbency, but further clinical studies to verify these findings are necessary.

Footnotes

Acknowledgements

The authors express their gratitude to Alexander Haake for editing and proofreading the manuscript.

Accepted: 20 April 2017

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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Effect of intraoperative positioning on the diameter of the vertebral canal in cats during perineal urethrostomy (cadaveric study)

Abstract

Objectives

Methods

Results

Conclusions and relevance

Introduction

Materials and methods

Animals

Testing procedure

Morphometric analysis of the CT images

Statistical analysis

Results

Comparison of the VCD between L6 and L7 in neutral position vs the VCD in dorsal and ventral recumbency

Comparison of the VCD between L7 and S1 in neutral position vs the VCD in dorsal and ventral recumbency

Comparison of the VCD between S3 and Co1 in neutral position vs the VCD in dorsal and ventral recumbency

Comparison of the VCD between Co1 and Co2 in neutral position vs the VCD in dorsal and ventral recumbency

Comparison of the VCD between L6 and L7, L7 and S1, S3 and Co1, and Co1 and Co2 in dorsal vs ventral recumbency

Intra-observer agreement

Discussion

Conclusions

Footnotes

Acknowledgements

Conflict of interest

Funding

References