Evaluation of a screw insertion landmark for a minimally invasive repair technique in induced bilateral sacroiliac luxation in feline cadavers

Abstract

Objectives

The aim of this study was to describe an alternative landmark for screw insertion into the body of the ilium with bilateral sacroiliac luxation in cats.

Methods

Seven cat cadavers with artificially induced bilateral sacroiliac luxation were used. The screw insertion point was determined using the caudal iliac crest and cranial acetabular rim. These two points make the first guideline; a second guideline ran perpendicular to the caudal iliac crest point. The screw insertion point was halfway along the second guideline across the ilium body. Surgery was performed in a minimally invasive manner using fluoroscopy.

Results

Postoperative radiographs and CT were performed. In the postoperative evaluation, the sacroiliac joint reduction percentage was almost 90% and there was no significant difference in pelvic canal diameter ratio before and after surgery. Screw depth/sacral width was >60% in all cadavers. On CT, the angle between the screw and sacrum wing was within the normal range of 96.24° to the left and 98.65° to the right, except in one case.

Conclusions and relevance

In previous studies, surgical repair was based on having an intact contralateral ilium. However, this method is not applicable to patients with bilateral sacroiliac luxation and is mostly performed using open reduction methods. The screw insertion point suggested in this study offers a potential alternative repair technique for patients with bilateral sacroiliac luxation.

Introduction

Sacroiliac luxation is the second most common pelvic injury in cats, accounting for 16–59% of pelvic injuries in cats.^1,2 Unilateral sacroiliac luxation is always accompanied by other pelvic fractures or symphyseal separations. ³ Bilateral sacroiliac luxation can occur without other pelvic injuries.^2,4 Treatment of sacroiliac luxation can be broadly categorised into conservative treatment and surgical stabilisation. When there is no articular fracture and minimal displacement, conservative treatment can be opted for.^1,5,6

However, marked pain, inability to ambulate, unstable or marked displacement of the hemipelvis, narrowing of the pelvic canals and neurological deficits attributed to sacroiliac luxation are some of the indications for surgical stabilisation of the sacroiliac luxation/separation.^2,5,7,8 Sacroiliac joint luxation can be repaired using an open or closed reduction technique.^9–11 The open reduction technique allows direct visualisation of the surgical site, the landmarks of which have been studied in cadaveric specimens; however, these landmarks can be challenging to identify accurately during surgery.^9,12 In human medicine, minimally invasive closed reduction and percutaneous lag screw fixation of the sacroiliac joint using intraoperative fluoroscopy has increasingly gained importance in recent years and has also been successfully performed in dogs.^13–15 Moreover, the advantages of this technique are more accurate and safer screw positioning, improved sacral purchase, reduced blood loss, shorter operating and hospitalisation times, faster weightbearing and pain control, as well as lower complication rates and lower costs. ¹⁶

Usually, bilateral sacroiliac joint luxation is reduced by using a lag screw bilaterally, but it can also be reduced by using a single trans-sacral screw, a single transiliosacral pin, transiliac pin/bolt/screw internal fixation and one lag fashion screw.^{4,10,11,17,18} Several techniques have been described for sacroiliac luxation repair in cats.^10,11,17,18 Lag screw fixation is currently one of the preferred methods for sacroiliac joint stabilisation after luxation. ¹⁹

After lag screw fixation of sacroiliac luxation, screw loosening could occur for inappropriate screw sizes and insertion points. In one study, postoperative screw loosening occurred in cats treated with a screw depth <60% of the sacral width, whereas screw loosening occurred much less frequently in cats treated with screw depths of >60% of the sacral width. ¹¹ Determining the appropriate screw insertion sites are also critical and influences prognosis. The lag screw has to travel through a narrow safe drill corridor to remain intraosseous. ³ In cats, the safe drill corridor is small, often <0.5 cm², and the appropriate drill angles are difficult to achieve. ¹⁸ Screws exiting the sacrum risk iatrogenic damage to the cauda equina dorsally, the lumbosacral intervertebral disc cranially or the lumbosacral plexus and median sacral vessels ventrally. ¹¹

In unilateral sacroiliac joint luxation, sacroiliac joint reduction is judged based on the alignment of the iliac wing, superimposition of the L7 transverse process and parallelism of the L7–S1 endplates in the sagittal plane.^9,17 However, this method is only available if the contralateral sacroiliac joint, iliac body and surrounding bone structures are intact. Because bilateral sacroiliac joint luxation occurs in 33–37% of cases and >50% of cats that had either bilateral or unilateral sacroiliac joint luxation had contralateral associated orthopaedic injuries, these assessment techniques can be difficult to use in surgery for bilateral luxation. ² Moreover, these techniques can be challenging and potentially hazardous to achieve in a cat presenting with bilateral sacroiliac joint luxation, given the loss of orientation from disrupted pelvic landmarks, the small safe anatomical window and narrow dorsoventral corridor within the sacrum for the placement of long-reaching implants. ³ Because of these difficulties, it is difficult to find the screw insertion point during surgery. Therefore, the lag screw fixation method is very difficult in cats with bilateral sacroiliac joint luxation.

To the best of our knowledge, in cats with bilateral sacroiliac joint luxation, no reports have described alternative landmarks to use as a lag screw insertion point in the iliac body. The purpose of this study was to describe alternative screw insertion landmarks in the iliac body using the cranial acetabular rim and caudal iliac crest point, viewed under fluoroscopy, to treat feline bilateral sacroiliac luxation.

Materials and methods

Cadaver and pelvic bone fracture, luxation model

Seven cat cadavers (five Russian Blue, one Korean Shorthair and one Abyssinian) were donated to the Jeonbuk Animal Medical Center. The cats died from disease. It was fully explained to the owners that the bones, muscles and ligaments of the cadaver would be used in researching new treatment methods and prevention of various orthopaedic diseases, and – if they agreed – they signed the hospital’s donation pledge form and their cat was stored in a freezer at –2°C. Informed consent was obtained from the owners of the cadavers prior to the interview and for the use of data and pictures. For the artificial bilateral sacroiliac joint luxation model, a pelvic symphysis fracture was made with an osteotome and mallet. Bilateral sacroiliac joint luxation was then achieved by using an osteotome through the abdominal approach. Artificial bilateral sacroiliac joint luxation was identified using radiographic images (HF-525PLUS; EcoRay).

Implant selection

The outer diameter size of the implant screw was determined by measuring the cranial sacral endplate height, with a screw outer diameter 30–40% of the height. Screw length was determined by measuring the sacral width on the ventrodorsal radiographic image, and then selecting a screw length >60% of the sacral width. In all seven cats, 1.5 mm cortical screws were applied (Able Biomedical).

Surgical procedure

All experiments and measurements were performed by two surgeons who were the lead authors of the paper. With cats in a lateral recumbent position, a 1 cm skin incision was made along the line between the iliac crest and ischiatic tuberosity to perform the reduction using Kern forceps. Two needles were inserted percutaneously at the caudal iliac crest and cranial acetabular rim, respectively, under fluoroscopic guidance. Ideally, the needle was inserted vertically. However, it was practically impossible to keep it vertical during the reduction process. This marked the first guideline that joined the caudal iliac crest and cranial acetabular rim. The second guideline was made vertically from the caudal iliac crest, crossing over the body of the ilium. The halfway point of the line crossing the body of the ilium was determined as a screw insertion point (Figure 1).

Figure 1.

The first guideline was drawn from the cranial acetabular rim to the caudal iliac crest. The second guideline was drawn from the caudal iliac crest perpendicular to the first guideline. The screw insertion point is halfway along the second guideline crossing over the body of the ilium

After determining the screw insertion point, a 0.8 mm temporary Kirschner wire in this screw insertion point was inserted only up to the body of the ilium. The wire was inserted into the desired sacroiliac joint point using fluoroscopic guidance in a position where the patient’s lumbar vertebrae transverse processes were superimposed. A second 0.8 mm temporary Kirschner wire was inserted next to the first temporary pin. After the first temporary pin was removed, the screw insertion surgical procedure was the same as the typical procedure for minimally invasive treatment of sacroiliac joint luxation. Then, the second temporary pin was removed. Contralateral sacroiliac joint luxation was reduced by using the contralateral body of the ilium. The surgical procedure was performed in the same way as unilateral sacroiliac joint luxation, with careful opposition of the inserted screw (Figure 2).

Figure 2.

(a,b) Determining the screw insertion point; (c,d) inserting two temporary Kirschner wires; and (e,f) fluoroscopy after screw insertion

Radiographic evaluation

Postoperative radiographic evaluation included sacroiliac joint reduction percentage, pelvic canal diameter ratio and screw depth/sacral width percentage. Sacroiliac joint reduction percentage was measured based on the length of the ilium in contact with a sacral wing, which was divided by the normal length of the sacroiliac joint articular surface on a ventrodorsal radiograph. ¹¹

The pelvic canal diameter ratio was measured at two locations on the pelvis (sacrum points A and B and acetabulum points C–E in Figure 3) postoperatively. The pelvic canal diameter ratio was calculated as the ratio of the cranial aspect of the acetabulum width divided by the sacral width. The hemipelvic canal width ratio was calculated by dividing the hemipelvic canal width of the side of the sacroiliac fracture luxation by the contralateral hemipelvic canal width (points C and D and points D and E, respectively, in Figure 3). ²⁰ The screw depth/sacral width was measured on the ventrodorsal pelvic radiograph (Figure 4).

Figure 3.

Dorsoventral view of the pelvis and sacrum. The pelvic canal diameter ratio is CE/AB. The hemipelvic canal width ratio is CD/DE

Figure 4.

Radiograph after inserting the opposite screw following Figure 2. a = width of the sacral body; b = screw length with purchase in the sacrum; screw depth/sacral width ratio = b/a × 100

CT evaluation

Postoperative CT (Alexion TSX-034A; Toshiba) was performed to measure the dorsoventral articular surface angulation of the screw, which was calculated based on the craniocaudal CT image. The measuring method followed that reported by Shales and Langley-Hobbs (Figure 5).^12,21

Figure 5.

Postoperative craniocaudal CT image of screw: a = angulation of the sacroiliac articular surface; b = angle of the inserted screw; and c = dorsoventral articular angle

Statistical analysis

The mean ± SD was calculated for all data. The Wilcoxon signed-rank test was used to evaluate the pelvic canal diameter ratio. Statistical analysis was performed using GraphPad Prism software. Statistical significance was set at P <0.05.

Results

The seven cats comprised five intact females, one neutered male and one intact male. Their mean body weight was 2.7 kg (range 1.8–3.76) and their mean age was 5.71 years (range 3–8).

Radiographic evaluation

Postoperative sacroiliac joint reduction percentage was measured. The mean left and right sacroiliac joint reduction percentages were 87.73 ± 5.53% and 89.78 ± 5.30%, respectively (Table 1).

Table 1.

Postoperative left and right sacroiliac (SI) joint reduction percentages in radiographic evaluation

Cadaver number	Left SI joint reduction percentage	Right SI joint reduction percentage
1	90.71	93.58
2	82.62	81.77
3	92.99	92.11
4	83.33	84.89
5	80.05	86.54
6	93.35	94.81
7	91.12	94.82
Mean ± SD	87.73 ± 5.53	89.78 ± 5.30

The pelvic canal diameter was measured preoperatively and postoperatively. The preoperative mean pelvic canal diameter ratio was 1.24 ± 0.087. The postoperative mean pelvic canal diameter ratio was 1.34 ± 0.085 (Table 2). Pelvic canal diameter ratios were higher postoperatively than preoperatively, although the difference was not statistically significant (P = 0.1563). The hemipelvic canal width ratio was measured postoperatively. The mean hemipelvic canal width ratio for all seven cats was 1.02 (Table 2).

Table 2.

Postoperative radiographic evaluation of the pelvic canal diameter ratio and hemipelvic canal width ratio

Cadaver number	Preoperative pelvic canal diameter ratio	Postoperative pelvic canal diameter ratio	Postoperative hemipelvic canal width ratio
1	1.14	1.42	1.17
2	1.15	1.41	0.97
3	1.19	1.31	1.06
4	1.37	1.44	0.90
5	1.31	1.31	1.07
6	1.30	1.20	0.90
7	1.23	1.30	1.11
Mean ± SD	1.24 ± 0.087	1.34 ± 0.085	1.02 ± 0.10

The screw depth/sacral width was measured on the left and right. The left mean screw depth/sacral width was 66.04 ± 3.67% and right mean screw depth/sacral width was 67.99 ± 5.84% (Table 3).

Table 3.

Left and right screw depth/sacral width ratio in radiographic evaluation

Cadaver number	Left screw depth/sacral width (%)	Right screw depth/sacral width (%)
1	65.16	70.43
2	63.07	77.82
3	61.35	60.10
4	64.39	67.55
5	72.09	66.95
6	67.32	70.69
7	68.94	62.39
Mean ± SD	66.04 ± 3.67	67.99 ± 5.84

CT evaluation

The mean dorsoventral articular angulations of the left and right screws were 96.24° and 98.65°, respectively (Table 4). Through the CT evaluation, it was confirmed that the screws inserted in this study did not penetrate the sacral cortex and invade the sacral canal.

Table 4.

Dorsoventral articular angulation of the left and right inserted screw on CT

Cadaver number	Left dorsoventral articular angulation (°)	Right dorsoventral articular angulation (°)
1	84	103
2	101.63	108.05
3	112	105
4	97.05	72.97
5	99.57	101.32
6	85	103.7
7	94.46	96.55
Mean ± SD	96.24 ± 9.73	98.65 ± 11.86

Discussion

In the treatment of patients with sacroiliac joint luxation, achieving 100% joint reduction did not significantly influence screw loosening,^14,22,23 and joint repair is likely to be influenced by the relatively large flat surface available on the medial aspect of the ilium for the development of a compressive force by the screw. ¹¹ In this study, the left and right mean sacroiliac joint reduction percentages were 87.73% and 89.79%, respectively. Because of the limitations of cadaveric studies, screw loosening could not be assessed after surgery. Therefore, further studies are needed to determine whether the sacroiliac joint reduction percentage affects screw loosening when applied to these surgical methods in a clinical study.

The pelvic canal diameter ratio provides an estimate as to whether the width of the pelvic canal has been re-established after repair of pelvic injuries. ¹⁴ The normal pelvic canal diameter ratio in dogs has been reported to be ⩾1.1, although no normal has been reported for cats.^17,20 In this study, as the normal pelvic canal diameter ratio has not been reported in cats, comparisons of the postoperative pelvic canal diameter ratio were based on the normal preoperative pelvic canal diameter ratio. The mean preoperative pelvic canal diameter ratio was 1.24 ± 0.08 and the mean postoperative pelvic canal diameter ratio was 1.34 ± 0.08. There was no significant difference in the pelvic canal diameter ratio before and after surgery (P = 0.1563). The hemipelvic canal width ratio was calculated to detect lateralisation or medialisation of the luxated hemipelvis. The postoperative hemipelvic canal width ratio ranged from 0.90 to 1.17, indicating symmetry between the hemipelvis and repaired sacroiliac luxation. These postoperative results indicate that surgery using the landmark described in this experiment can be a useful method.

Screw loosening after surgery occurred less frequently when the screw length was >60% of the sacral width. ¹¹ In this study, all inserted screws were >60% of the sacral width. Unilateral sacroiliac joint luxation can use only one screw of >60% of the sacral width without any resistance. However, in the case of bilateral sacroiliac joint luxation, the second contralateral screw could be interrupted by the first inserted screw. We assumed that the counteracting force generated in the process of inserting the second screw can affect the first inserted screw and become vulnerable to loosening. In this study, in order to prevent a counteracting force and to have a small safe drill corridor in the cat, a 1.5 mm cortical screw was selected from the applicable 1.5 mm and 2.0 mm cortical screws. ³

The dorsoventral angle of the inserting screw is important as it can damage cauda equina nerves if inserted too dorsally or median sacral vessels if inserted too ventrally. ¹² Moreover, there is an increased risk of screw loosening because of less stable fixation with suboptimal purchase within the sacral body.^21,22,24 The dorsoventral angle for screw insertion in cats has been studied. ¹² However, it is difficult to check the dorsoventral angle when inserting the screw during surgery. In this study, under fluoroscopic guidance, cats were positioned in perfect lateral recumbency. This means a complete overlap of the patient’s lumbar vertebral transverse processes. Drilling and screw insertion were performed in this position.

There were several limitations to this study. The direction of surgery was not first set during the experiment. If one of the left or right directions was set as the first operation direction, a more objective comparison could be performed, but this comparison could not proceed because the experimental population was insufficient.

The anatomical sacroiliac joint location and screw insertion point have been reported previously. ³ Although this method proposes an accurate and safe corridor, performing this measuring method under fluoroscopy is difficult. In this study, the screw insertion point was determined using the caudal iliac crest and cranial acetabular rim. These two points form the first guideline, and the second guideline is perpendicular to the caudal iliac crest point. The screw insertion point is halfway along the second guideline across the body of the ilium. This method is more accurate in bilateral sacroiliac joint luxation, which cannot use the opposite body of the ilium as an intact model.

Conclusions

In bilateral sacroiliac joint luxation, it is difficult to determine the accurate screw insertion point because the opposite joint does not have a normal anatomy. In addition, it is difficult to find the screw insertion point in the body of the ilium during surgery. Determining an accurate screw insertion point is important for stabilising the anatomical structure of the sacroiliac joint. In this study, an alternative screw insertion point using the cranial acetabular rim and caudal iliac crest was shown to be effective in treating bilateral sacroiliac luxation under fluoroscopy. Therefore, this minimally invasive technique that determines the screw insertion point in this study could be used an alternative method in bilateral sacroiliac joint luxation.