1 Introduction

Because of the growing geriatric population, cases of fragility fractures of the pelvis (FFPs) are increasing.¹ The elderly population is growing rapidly in Japan as well.² Rommens and Hofmann classified FFPs into the following four main types and recommended treatments.³ Types 1 and 2 are considered nondisplaced fractures, and types 3 and 4 are considered displaced fractures. Surgical reconstruction with percutaneous fixation is recommended, especially for nondisplaced fractures.¹ For percutaneous fixation, iliosacral (IS) and trans-iliac trans-sacral (TITS) screws (Fig. 1) are the choices. Percutaneous IS screw fixation is one of the most effective methods for stabilizing the posterior pelvic ring. These screws are inserted from the outer cortex of the ilium perforating the iliosacral joint, through the fracture line, to the sacral vertebra or near the ipsilateral sacral foramen. Owing to its minimal invasiveness, it is considered a useful procedure for the elderly. However, the bones of these patients are osteoporotic, and the risk of implant loosening is high. To avoid this, many techniques were improved, such as augmentation with polymethylmethacrylate by placing it through the cannulated screw hole.⁴ Several cement-related complications have been reported.⁵

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Fig. 1 Clinical X-ray images of a patient with TITS screw (white arrows) (a) antero-posterior X-ray, (b) The inlet view of pelvis (the patients not included in this study).

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TITS screws are inserted from the outer cortex of the ilium through the sacral vertebra and the contralateral outer cortex of the ilium. These screws penetrate six cortices; thus, they have stronger stabilization than IS screws, and for their strong stabilization, they are useful tools for managing geriatric patients with osteoporotic bones.⁶ A fluoroscopic guide method is the gold standard for TITS screw insertion; however, visualizing the accurate trans-sacral corridor of the sacrum is difficult. The trans-sacral corridor has sacral foramens and has an anatomically complicated morphology, and some neurovascular bundles run through the pelvic cavity side near the trans-sacral corridor. Therefore, a computed tomography (CT) guide is more useful, as it accurately recognizes the bone morphology and can help insert TITS screws safely. However, because of the expensiveness of intraoperative CT guide systems, they are not widely available in every trauma hospital in Japan. Major complications of percutaneous TITS screw fixation include neurovascular damages, which can be fatal; therefore, accurate positioning of the screw is crucial. The morphology of the sacrum has racial variations.⁷ Studies on the effectiveness of TITS screw fixation in patients with osteoporosis are published worldwide; however, only few studies have focused on Japanese elderly patients.

Therefore, this study was designed to validate the possibility of TITS screw fixation by analyzing the morphology of the sacrum in Japanese individuals with osteoporotic vertebral fractures and to measure the width of the trans-sacral corridor using CT.

2 Methods

This is a retrospective, single-center, observational study. Seventy patients were enrolled in this study. Patients who underwent CT between January 2015 and February 2019 for diagnosing osteoporotic vertebral fractures and were hospitalized at our institution were included. At our hospital, patients suspected of having osteoporotic vertebral fractures are prescribed to undergo CT to confirm the diagnosis; however, at that time, in cases where the patient's symptoms are located in the lower back, CT examination including the pelvic bone is performed to determine the presence of FFP.

The exclusion criteria were as follows:1.Age of less than 65 years.2.Medical history of pelvic fractures3.Osteolytic pelvic lesions (e.g., metastatic bone tumor)4.Cases in which the CT imaging range did not include the sacrum and ilium.

CT scans were performed during clinical routine. The indications for CT were different from the aim of this study. Accordingly, the patients were not exposed to additional radiation for the purpose of this study. A 64-section CT scanner (PHILIPS Brilliance) was used to perform the CT scans. The CT images were taken with a 0.6-mm slice depth and saved in slices of 1.0 mm in thickness. With the consent of the patients for the analysis of these CT dates, this study was approved by the Ethics Committee of University of Tsukuba Hospital.

Two-dimensional (2D) reconstructions and measurements were performed using a software for preoperative planning for total hip arthroplasty (ZedHIP®️ LEXI, Inc.). The true inlet view was made according to the manufacturer's instruction. First, the mid sagittal plane was made (Fig. 2a). Second, the true outlet view was made by adjusting the axes just parallel to the posterior wall and the lower endplate of S1 (Fig. 2b). Then, from the outlet view, we made the inlet view by making a plane connecting the cranial ends of S1 and vertical to the axis of the sacrum (Fig. 2c). Finally, the inlet view was adjusted to align the middle of the promontorium of the sacral body and the spinous process. The same reconstructions were performed in S2. The trans-sacral corridor was measured in the inlet view, as shown in Fig. 3. A line parallel to the posterior wall of S1 was drawn (blue line in Fig. 3 (a)) as a reference. The perpendicular distance from the line to the anterior cortex of the sacrum was measured on both sides of the sacroiliac (SI) joint (distance 1), the middle of the promontorium (distance 3), and the middle of the two points (distance 2). The trans-sacral corridor was defined as the width of these points and was assessed in the upper fourth, the middle, and the lower fourth. The same process was followed in S2.

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Fig. 2 (a) The mid sagittal plane of the sacrum. (b) The true outlet view was made by adjusting the axis just parallel to the posterior wall (blue line). (c) The inlet view was made by making a plane connecting the cranial ends of S1 in the inlet view (red line). (d) The inlet view.

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Fig. 3 (a) The measurement of the corridor in the inlet view. The distance were measured at the SI joint (distance 1), the middle of the promontorium (distance 3), and the middle of the distance between the SI joint and the middle of the promontorium from the line parallel to the posterior wall of the body of S1 (blue line). (b) Planes parallel to the lower endplate of S1 in the upper, middle, and lower quartiles of the body of S1 are shown in red lines.

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Once the measurement was done, the trans-sacral corridor was divided into three groups: “ascending,” “descending,” and “horizontal” types, defined by Goetzen.⁸ In the ascending trans-sacral corridor, distance 1 is smaller than distance 3 (Fig. 4a). In the descending trans-sacral corridor, distance 1 is larger than distance 3 (Fig. 4b). In the horizontal trans-sacral corridor, distances 1 and 3 are equal (Fig. 4c). A difference of more than 3 mm were considered relevant.

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Fig. 4 The trans-sacral corridor was divided into three groups. (a) Ascending type. (b) Descending type. (c) Horizontal type.

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The presence of a “notch” was also considered in this study. A notch is defined as an indentation in the anterior cortex of the sacrum. The nerve root runs down the notch, indicating that a misplacement of the IS screw may harm the nerve root. The mean diameter of distances 1 and 3 was calculated and compared with that of distance 2. If the calculated distance was smaller than that of distance 2 by 3 mm, the presence of the notch was defined. Fig. 5 shows an example of a trans-sacral corridor with a notch. The extraction method and measurement of this reconstructed image slice were performed according to Goetzen.⁸

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Fig. 5 If the calculated distance was smaller than that of distance 2 by 3 mm, the presence of the notch was defined. (a) With notch. (b) Without notch.

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Furthermore, when a TITS screw is insertable, the width (mm) of the insertable safe zone (minimum width of the trans-sacral corridor in the extracted slice) was measured (Fig. 6).

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Fig. 6 The minimum width (white double-headed arrow) of the trans-sacral corridor was measured for TITS screw (mm).

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Regarding the measured values, the intra-observer error (the author (TS) performed the measurement twice) and the inter-observer error (two orthopedic specialists (TS & YY)) were calculated. For scale variables, mean ± standard deviations are presented. All statistical analyses were performed using Statistical Package for the Social Sciences (SPPS) (version 25; IBM Corporation, Armonk, NY, USA). Correlations were quantified using the Pearson coefficient for metric scaled data and using the Spearman coefficient for non-parametric samples. Differences with p-values of less than 0.05 were considered statistically significant.

3 Results

Regarding the accuracy of this image analysis, intraclass correlation coefficients (ICC) (ICC [2, 1] = 0.970; ICC [1, 1] = 0.943) were calculated using SPSS (IBM Co., Armonk, USA). From the ICC results, it was found that the reliability of this measurement method by Goetzen[8] is high. The CT scans of the 70 patients were analyzed, and 2,100 measurements were obtained. Among the 70 patients, 26 were males and 44 were females, and the mean age of the patients was 76.5 ± 8.6 years. Twenty-one vertebral fractures were classified as AO Spine thoracolumbar classification type A1, thirty-eight as type A3, nine as type A4 and two as type B3.

The trans-sacral corridor types are shown in Table 1. As the level became lower, the percentage of the ascending type decreased, and the descending type increased. There were no ascending types in S2. The results of the presence of the notch are shown in Table 2. The frequency of the appearance of the notch increases as the level became lower, and over 80% of the patients had a notch below the lower quarter of S1.

There were 26 males and 44 females. The difference in morphology between men and women was only in S1 upper quartile. The breakdown of males was 57.5% Ascending type, 23.1% Horizontal type, and 19.2% Descending type, whereas females were 79.5%, 11.4%, and 9.1%, respectively. An analysis of population rate showed a significant difference between males and females (p < 0.05).

The average minimum width (mm) of the insertable safe zone for TITS screw (Fig. 6) was 7.89 ± 7.31 mm in the ascending type at S1 middle quartile and 21.61 ± 2.86 mm. The average minimum width of the descending type at S2 middle quartile was 15.25 ± 2.99 mm.

4 Discussion

Although the previous study analyzed 1,000 patients,⁸ the target disease was not limited to elderly patients with osteoporosis. This study involved a small number of cases. This is because this study included elderly patients and patients with a history of fragility fractures. The TITS procedure was intended to be used in patients with FFPs. Therefore, patients with fragility, osteoporotic vertebral fractures were included in the analysis. The number of patients with fragility fractures who were hospitalized and treated in our hospital and for whom CT data of the pelvic region including the iliac and sacral bones were available were retrospectively observed, resulting in a small number of patients.

TITS screws has been reported to be highly effective because of their strong stability and low invasiveness.⁹ For elderly patients, IS/TITS screws are considered useful.¹⁰ The dysmorphism of the sacrum has been reported by several authors, and due to this dysmorphism with a fixed probability, there are patients for whom TITS screw fixation cannot be indicated.¹¹ This study is the first one to report the possibility of TITS screw fixation due to sacral dysmorphism for the Japanese elderly population with osteoporotic vertebral fractures.

Racial and sexual differences in bone morphology have been reported at several sites.^12–14 Because of racial differences in the femoral anterior bow, the anterior curvature of the intramedullary nail should considered when selecting a model for each case.¹⁵ Similarly, racial differences in the pelvic ring may exist, and considering how many patients can be treated with TITS screw fixation is important.

In the three different types of the trans-sacral corridor of the sacrum, inserting TITS screws in the descending and horizontal types is possible; however, the risk of misplacement is higher in the ascending type. The trajectory of the screw is angled more sagittal, facing toward the anterior cortex of the sacrum (Fig. 4). In this study, the average diameter of the ideal trans-sacral corridor of the trajectory for an ascending-type sacrum is 7.89 mm. Usually, the diameter of the screw should be larger than 6.5 mm (in Japan, cannulated cancerous screws with an outside diameter of 6.5 or 7.3 mm are clinically available for TITS); thus, the trans-sacral corridor is narrow compared with the screw, which can also be a risk of mispositioning. Among the elderly Japanese subjects included in this study, 34% had an ascending-type bone morphology in the middle quartile of S1, and the average width of the bone corridor was 7.89 mm. This may not be eligible for treatment with percutaneous TITS screws. If TITS screw fixation is not indicated because of bone morphology, other fixation methods may need to be considered.¹⁶

5 Conclusions

This research is the sacral morphology investigation in osteoporotic elderly population with OVF whose average age of 76.5 years. At S1 middle quartile in ‘ascending’ type, there was no TITS screw transsacral corridor because the average width was 7.89 ± 7.31 mm 34.3% of seventy OVF patients had not the sacral morphology for which treatment with TITS screw insertion.

Patents

None.

Author contributions

Yohei Yanagisawa: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft Preparation. Takahiro Sunami: Investigation, Writing - Original Draft Preparation, Visualization, Masashi Yamazaki: Supervision.

Funding

None.