The influence of sagittal profile on the S2 alar-iliac (S2AI) screw trajectory in degenerative lumbar scoliosis patients: A computed tomography (CT) study

doi:10.21203/rs.3.rs-3104532/v1

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The influence of sagittal profile on the S2 alar-iliac (S2AI) screw trajectory in degenerative lumbar scoliosis patients: A computed tomography (CT) study

https://doi.org/10.21203/rs.3.rs-3104532/v1

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Background

The usage of S2 alar-iliac (S2AI) screw for spino-pelvic fixation had been gaining increasing popularity in degenerative lumbar scoliosis (DLS) patients owing to its considerable advantages. However, the recommended ideal angles for S2AI screw insertion varied in the literature. The purpose of this study was to investigate whether the optimal S2AI screw trajectory was influenced by sagittal profile in DLS patients.

Methods

Computed tomography (CT) scans of pelvis were performed in 47 DLS patients for 3-dimensional reconstruction of S2AI screw trajectory. Five S2AI screw trajectory parameters were measured in CT reconstruction images, including: 1) angle in the transverse plane (Tsv angle); 2) angle in the sagittal plane (Sag angle); 3) maximal screw length; 4) screw width; and 5) skin distance. The lumbar Cobb angle, lumbar apical vertebral translation (AVT); global kyphosis (GK); thoracic kyphosis (TK); lumbar lordosis (LL); sagittal vertical axis (SVA); sacral slope (SS); pelvic tilt (PT); and pelvic incidence (PI) were measured in standing X-ray films of the whole spine and pelvis.

Results

Both Tsv angle and Sag angle had significant positive associations with SS (P < 0.05) but negative associations with both PT (P < 0.05) and LL(P < 0.05) in all cases. Patients with SS less than 15°had both smaller Tsv angle and Sag angle than those with SS equal to or more than 15°(P < 0.05).

Conclusions

The decreased LL would lead to the backward rotation of the pelvis, resulting in a more cephalic and less divergent trajectory of S2AI screw in DLS patients.

sagittal

spino-pelvic

S2 alar-iliac screw

scoliosis

Solid sacropelvic arthrodesis is key to successful surgical treatment of degenerative lumbar scoliosis (DLS) patients, especially for those with long segment constructs, high body mass index, low bone density and so on [1–2]. Previously, iliac screw was widely used in the sacropelvic fixation is DLS patients. However, it had some disadvantages, including wide soft tissue dissection, skin prominence over the posterior superior iliac spine, wound infection and requirement of additional offset connectors [3–4]. To overcome these drawbacks, S2 alar-iliac (S2AI) screw had been developed and gained increasing popularity as a routine technique for sacropelvic fixation since it allowed limited dissection of soft tissue and eliminated the need for additional offset connector, decreasing the incidence of instrument-related complications [5–8].

Although S2AI screws had been widely used in pelvic fixation due to its merits over traditional iliac screw, some concerns regarding the potential vascular complications form S2AI screw displacement had been generated. The internal iliac artery was at risk of injury from anterior screw perforation while the superior gluteal artery was at risk of injury from caudal screw violation [9–11]. Therefore, surgeons should ensure accurate S2AI insertion to avoid these major vessels injuries, which can be life-threatening complications.

The S2AI screw had been successfully inserted under fluoroscopy, navigation assistance or robotic guidance [12–14]. However, these techniques increased both the radiation exposure to spine surgeons and operation time. Some scholars reported that free-hand technique was also reliable for S2AI screw placement in DLS patients, which obviated the need for intraoperative fluoroscopy [5, 15–16]. However, the recommended ideal trajectory angles for S2AI screw placement varied in the literature [5, 9, 11, 17–20]. The S2AI screw traverse both the ilium and sacrum. Therefore, there is no doubt that the S2AI screw trajectory is influenced by morphological characteristic of pelvis. It has been revealed that pelvic rotation plays a key role in compensating sagittal imbalance in human beings [21–22]. In patients with thoracolumbar kyphosis, the pelvis rotates backward to maintain an upright posture [23–24]. In theory, the backward rotation of pelvis may change the orientation of S2AI screw trajectory in DLS patients. To date, few studies specifically concerning on this topic had been reported. The objective of this current study is to investigate the influence of pelvic rotation on orientation of S2AI screw trajectory in DLS patients, with the purpose of helping spine surgeon accurately insert the S2AI screw with free-hand technique.

Subjects

With the approval from the institutional review board in our hospital, DLS patients underwent posterior correction surgery from Sept. 2019 to Nov 2021 were retrospectively reviewed. The inclusion criteria were: 1) with age older than 40 years; 2) with 3-dimenional CT reconstruction of S2AI screw trajectory. The exclusion criteria were: 1) with previous pelvic trauma or surgery; 2) with primary or metastatic tumor of the pelvis; and 3) with obvious pelvic tilt. Finally, there were 47 case (5 males and 42 females) with an average age of 64.1 years (range, 48 to 78 years) included in this study. Informed consent was obtained from each patient.

Measurements

Radiographic parameters

The preoperative standing posteroanterior and sagittal X-ray films of the whole spine and pelvis were obtained in all cases. The radiographic parameters assessed included the following : 1) lumbar Cobb angle [25]; 2) lumbar apical vertebral translation (AVT) [25]; 3) global kyphosis (GK): angle between the superior end plate of the maximally tilted upper end vertebra and the inferior end plate of the maximally tilted lower end vertebra [25]; 4) thoracic kyphosis (TK): the angle between the upper endplate of the T5 and the lower endplate of the T12 [25]; 5) lumbar lordosis (LL) : the angle between the superior endplate of L1 and S1[25]; For GK,TK and LL, the angle was positive if the curve was kyphotic and was negative if the curve was lordotic; 6) sagittal vertical axis (SVA) :the distance between the C7 plumb line (C7PL) and the posterosuperior corner of S1[24]. The SVA was positive if the C7PL was located anterior to the posterosuperior corner of S1 and negative if located posterior to the posterosuperior corner of S1; 7) sacral slope (SS): the angle between the sacral plate and the horizontal line (Fig. 1a) [24]; 8): pelvic tilt (PT): the angle formed by the vertical line and the line joining the middle of the sacral plate and the hip axis (Fig. 1a) [24]. The PT was positive if the hip axis was located anterior to the middle of the sacral plate and negative if the hip axis was located posterior to the middle of the sacral plate; and 9) pelvic incidence (PI): the angle between the perpendicular of the sacral plate and the line joining the middle of the sacral plate and the hip axis (Fig. 1a) [24].

CT parameters

The CT scans of both spine and pelvis were performed by Brilliance CT 64-channel scanner (Philips Medical Systems, PC Best, Netherlands) with the following parameters: 320 mAs, 120 kvP, 3 mm slice thickness, with a 3 mm gap between slices. The CT images were reconstructed using Light speed workplace AW4.3 (General electric company, USA) with matched software used for three-dimensional interactive viewing and manipulation. On the 3-dimensional model of the pelvis, the entry point of S2AI was located at the midpoint between the lateral aspect of the S1 and S2 dorsal foramen. After the entry point was identified, the CT imaging planes were manipulated to determine a ideal S2AI screw trajectory with maximal length and width of the osseous channel in each patient. Five CT parameters describing S2AI screw trajectory parameters were measured: 1) angle in the transverse plane (Tsv angle) :the anteroposterior inclination angles for S2AI screw measured between the screw trajectory line and the perpendicular line (Fig. 1b) [18] ;2) angle in the sagittal plane (Sag angle): the craniocaudal inclination angle for S2AI screw measured between the screw trajectory line and the horizontal line (Fig. 1c) [18]; 3)maximal screw length: the maximal distance from the entry point to the anterior inferior iliac spine (Fig. 1b) [18]; and 4) screw width: the narrowest iliac width in the presumed trajectory measured between the inner cortices (Fig. 1b) [18]; 5) skin distance: the distance of the starting point lateral from the skin (Fig. 1b) [18].

Statistical Analysis

Statistical analysis was performed by SPSS 16.0 software (Chicago, IL, USA). Correlation analysis was used to determine a Pearson coefficient (r) between radiographic parameters and CT parameters in all cases. All these CT parameters were compared between patients with SS less than 15°and those with SS equal to or more than 15°by independent-t test. Significance was established at the P < 0.05 level.

The general information and radiographic characteristics of all these cases were illustrated in Table 1. Both left and right CT parameters for S2AI screw trajectory were shown in Table 2. The correlations between radiographic parameters and CT parameters for S2AI screw trajectory were shown in Table 3. Both left and right Tsv angles had significant positive associations with SS (P < 0.05) but negative associations with both PT (P < 0.05) and LL(P < 0.05). Similarly, both left and right Sag angle had significant positive associations with SS (P < 0.05) but negative associations with both PT (P < 0.05) and LL(P < 0.05). Patients with SS less than 15°had both smaller Tsv angle and Sag angle than those with SS equal to or more than 15°(P < 0.05) (Table 4).

Table 1

Patients Characteristics
	Mean value
Age (yrs)	64.1 ± 6.1
Sex (F/M)	42/5
Cobb angle (°)	37.6 ± 12.2
Lumbar AVT (mm)	7.8 ± 40.6
GK (°)	33.0 ± 14.8
TK (°)	16.3 ± 8.6
LL (°)	-9.0 ± 23.4
SVA (mm)	62.4 ± 49.5
SS (°)	14.6 ± 10.1
PT (°)	33.0 ± 8.5
PI (°)	47.6 ± 8.1

Table 2

CT parameters of S2AI screw trajectory
	Left	Right	P value
Sag angle (°)	16.6 ± 6.6	15.0 ± 5.8	0.001*
Tsv angle (°)	39.8 ± 2.9	39.5 ± 3.3	0.507
Iliac width(mm)	11.9 ± 1.7	13.1 ± 2.0	0.000*
Max-length(mm)	118.4 ± 6.1	115.2 ± 6.9	0.001*
Skin distance(mm)	39.6 ± 8.8	40.4 ± 10.1	0.328
* means the difference was statistically significant

Table 3

Correlation between radiographic parameters and the CT parameters for S2AI screw trajectory
Parameters		SVA	PT	SS	PI	LL	GK	TK	AVT	Cobb
Left
Sag angle (°)	r	-0.209	-0.622*	0.291*	-0.286	-0.458**	0.190	0.180	-0.030	0.074
Tsv angle (°)	r	-0.039	-0.470**	0.552*	0.131	-0.318*	-0.037	0.060	0.055	0.043
Iliac width (mm)	r	0.154	0.058	-0.280	-0.193	0.043	0.090	0.072	0.243	0.104
Max-length (mm)	r	0.109	0.186	-0.186	-0.018	0.053	-0.114	0.225	0.311*	-0.197
Skin distance (mm)	r	-0.211	-0.168	-0.039	-0.166	-0.217	0.182	0.210	-0.100	0.004
Right
Sag angle (°)	r	-0.134	-0.608*	0.341*	--0.219	-0.496**	0.166	0.204	0.070	-0.044
Tsv angle (°)	r	-0.162	-0.442*	0.668*	0.269	-0.494**	-0.282	-0.234	0.068	0.082
Iliac width (mm)	r	0.089	0.115	-0.210	-0.074	0.115	0.048	-0.001	0.164	0.292*
Max-length (mm)	r	-0.042	0.198	-0.016	0.157	0.019	-0.130	-0.067	0.365*	-0.031
Skin distance (mm)	r	-0.174	-0.105	-0.166	-0.224	-0.218	0.326*	0.330*	-0.069	-0.096
* means the difference was statistically significant

Table 4

Differences of CT images between patients with different SS
	SS < 15° (n = 23)	SS ≥ 15° (n = 24)	P value
Left
Sag angle (°)	14.4 ± 5.3	18.7 ± 7.1	0.024*
Tsv angle (°)	37.8 ± 2.1	41.8 ± 2.2	0.000*
Iliac width (mm)	12.4 ± 1.6	11.5 ± 1.8	0.068
Max-length (mm)	119.6 ± 6.2	117.3 ± 5.9	0.195
Skin distance (mm)	39.8 ± 8.5	39.4 ± 9.3	0.889
Right
Sag angle (°)	12.7 ± 4.2	17.1 ± 6.4	0.008*
Tsv angle (°)	37.3 ± 2.6	41.7 ± 2.4	0.000*
Iliac width (mm)	13.5 ± 2.0	12.8 ± 2.0	0.221
Max-length (mm)	114.9 ± 7.5	115.5 ± 6.3	0.742
Skin distance (mm)	41.9 ± 10.3	39.0 ± 9.9	0.344
* means the difference was statistically significant

Although the successful placement of S2AI screw with free-hand technique had been reported, it remained a challenge for spine surgeons since displacement of S2AI screw posed threaten to important neurovascular structures adjacent to pelvis [9–11]. Accurate S2AI screw placement depended on proper trajectory angles in both sagittal plane and axial plane. Various recommended angles of trajectory had been reported by previous studies, ranging from 8.3°to 34.5°in the sagittal plane, and 32.1°to 42.5°in the axial plane, which made the spine surgeons confused [5, 9, 11, 17–20]. We thought that several factors might contribute to the variations of S2AI trajectory angles, such as etiology, race, the spatial position of the pelvis, and so on. The trajectory of S2AI screw goes through the sacrum, the sacroiliac joint space, and the ilium. Therefore, the pelvic rotation would undoubtedly change the orientation of the S2AI screw trajectory.

It has been widely recognized that pelvic rotation plays a key role in maintaining the sagittal balance of human species. A proper sagittal alignment depends on the interactions between the spine and pelvis, which is called “spino-pelvic harmony” [22]. To better understand the effect of pelvic rotation on the restoration of human sagittal balance, three pelvic parameters, including pelvic incidence (PI), PT, and SS, had been introduced to describe the anatomic characteristics of the pelvis. PI is a constant morphological parameter with the value fixed regardless of pelvic rotation while both PT and SS are positional parameters with the value altered according to the extent of pelvic rotation. PT measures the sagittal pelvic version and SS quantifies the sagittal inclination of the sacral plateau [23–24]. Geometrically, PI equals the SS plus the PT. In patients with thoracolumbar kyphosis (decreased LL), the pelvis rotates backwards around the hips to maintain an erect position. In this case, the pelvis become vertical with SS decreased and PT correspondingly increased. The orientation of S2AI screw trajectory also become horizontal in the sagittal plane (Fig. 2) Therefore, it is easy to understand the positive associations between the Sag angle and SS, as well as the negative associations between the Sag angle and both PT and LL. Similar results were also reported by Vivace [19]. In our study, patients with SS less than 15°had smaller Sag angle (L:14.4° vs 18.7° ;R:12.7° vs 17.1°) than those with SS equal to or more than 15°. Our findings suggested a less caudal S2AI screw trajectory in DLS patients with thoracolumbar kyphosis than those with normal sagittal profile to avoid an inferior violation of sciatic notch, which could lead to devastating consequences with injury of superior gluteal neurovascular structures.

Another important finding in our study was that the Tsv angles had significant positive associations with both SS but negative associations with both PT and LL (P < 0.05), which had not been reported before. It meant that the S2AI screw trajectory become less divergent in DLS patients with thoracolumbar kyphosis and pelvic retroversion (Fig. 2). The patient was in prone position during the operation. Hence, the distal end of S2AI screw trajectory moves ventrally in sagittal plane when the pelvis rotated backwards, which was like a pendulum swinging (Fig. 3). Correspondingly. the distal end of screw trajectory moves anteromedially in the axial plane, which may be explained the decreased Tsv angle (the orientation of the screw trajectory in relation to a vertical line in axial plane) in these patients. Patients with SS less than 15°had smaller Tsv angle (L:37.8° vs 41.8° ;R:37.3° vs 41.7°) than those with SS equal to or more than 15°. Such results of our study seemed inconsistent with that of Vivace’s [19]. In his study, the Tsv angle was not influenced by the pelvic parameters. The Tsv angle was comparable between patients with PT less than 20°and those with PT more than 20°. Such inconsistency was caused by the different grouping criteria between Vivace’s study and our study. Apparently, SS could more directly reflect the extent of pelvic rotation when compared with PT. Therefore, we subdivided the patients into 2 groups based on SS rather than PT. In addition, the associations between Tsv angle and pelvic parameters were not analyzed in Vivace’s study. For these patients with pelvic retroversion, the recommended Tsv angle by previous studies might lead to posterior violation of ilium, which reduce the pull-out strength of the S2AI screw. In our cases, the maximal screw length ranged from L102/R101 mm to L128/ R129 mm with an average of L118.4/ R115.2 mm and the iliac width ranged from L9 /R9 mm to L17/ R17 mm with an average of L11.9/ R13.1 mm. It meant that the most commonly used S2AI screw (diameter:7.5 mm; length:80–90 mm) can be safely used in all these patients. The mean skin distance was L39.6/ R40.4 mm, which was similar to the results of previous studies. Obviously, such distance was larger than that of traditional iliac screw, which ensured complete coverage of S2AI screw by intact layer of muscle and avoided skin prominence from implants.

There was one limitation which should be mentioned in the current study. The DLS patient was in a prone position in the operation. Unfortunately, the intra-operative X-ray films of both spine and pelvis, were not obtained in our cases. Although the measurements on standing X-ray films could not accurately reflect the sagittal profile of the patients in a prone position, the significant associations between spinopelvic parameters and S2AI screw trajectory parameters in our study demonstrated the influences of sagittal spinopelvic alignment on the orientation of S2AI screw trajectory in DLS patients.

In conclusion, we recommended both smaller Sag angle and smaller Tsv angle of S2AI screw trajectory in patient with thoracolumbar kyphosis which led to backward rotation of pelvis when compared with those with normal sagittal profile. For these patients, the recommended angles for S2AI screw placement by previous studies may lead to either a excessively caudally inserted screw which poses a great threaten to the vital neurovascular structures below the greater sciatic notch, or a more divergently inserted screw which causes posterior penetration of ilium and decreases the pull-out strength of the screw. Preoperative 3-dimensional CT images of pelvis is helpful for these patients to determine the proper orientation of S2AI screw trajectory.

Ethics approval and consent to participate

The Drum Tower Hospital’s review board approved this study and all participating subjects or their legal guardians signed the written informed consents. All methods were carried out in accordance with relevant guidelines and regulations.

Declaration of Conflicting Interests

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

Consent for publication

Not applicable.

Acknowledgements

Not applicable.

Availability of data and materials

The datasets used and analysed during the current study available from the corresponding author on reasonable request.

Funding

This work received funding from Jiangsu Provincial Key Medical Center (Grant No. YXZXA2016009).

Author contributions

JJ: Data analysis, drafting paper. SCY: Data collection, data analysis, drafting paper, paper modification. WH: Paper modification. QY: Paper modification. WB: Paper modification. ZZZ: Paper modification. YY: Proposed idea, paper modification. All authors read and approved the final manuscript.

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No competing interests reported.

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The influence of sagittal profile on the S2 alar-iliac (S2AI) screw trajectory in degenerative lumbar scoliosis patients: A computed tomography (CT) study

Status:

Version 1

Abstract

Background

Methods

Results

Conclusions

Figures

Introduction

Materials and Methods

Subjects

Measurements

Radiographic parameters

CT parameters

Statistical Analysis

Results

Discussion

Declarations

References

Additional Declarations

Status:

Version 1