Transsacral screw safe zone size by sacral segmentation variations

Guidance for dysmorphic sacrum fixation with upper sacroiliac screw based on imaging anatomy study: techniques and indications

OBJECTIVE

This study aimed to investigate the techniques and indications of upper sacroiliac screw fixation for the dysmorphic sacrum.

METHODS

The dysmorphic sacra were selected from 267 three-dimensional pelvic models. The dysmorphic sacra which couldn't accommodate a 7.3 mm upper trans ilio-sacroiliac screw were classified as the main dysmorphic sacra. Then, the size of the bone corridor, the length of the screw in the corridor, and the orientation of the screw were measured. The insertion point on the sacrum was identified by two bone landmarks.

RESULTS

totally, 30.3% of sacra were identified as the main dysmorphic sacra. The inclinations of the screw oriented from posterior to anterior were (21.80 ± 3.56)° for males and (19.97 ± 3.02)° for females (p < 0.001), and from caudal to cranial were (29.97 ± 5.38)° for males and (28.15 ± 6.21)° for females (p = 0.047). The min diameters of the corridor were (16.31 ± 2.40) mm for males and (15.07 ± 1.58) mm for females (p < 0.001). The lengths of the screw in the Denis III zone were (14.41 ± 4.40) mm for males and (14.09 ± 5.04) mm for females (p = 0.665), and in the Denis II+III zones were (36.25 ± 3.40) mm for males and (38.04 ± 4.60) mm for females (p = 0.005). The rates of LP-PSIS/LAIIS-PSIS were (0.36 ± 0.04) for males and (0.32 ± 0.03) for females (t = 4.943, p < 0.001). The lengths of LPM were (8.81 ± 5.88) for males and (-4.13 ± 6.33) for females (t = 13.434, p < 0.001).

CONCLUSION

When the sacrum has the features of "sacrum not recessed" and/or "acute alar slope", the conventional trans ilio-sacroiliac screw couldn't be placed safely. The inclination oriented from posterior to anterior and from caudal to cranial are approximately 20° and 30°, respectively. The bone insertion point locates in the rear third of the anterior inferior iliac spine to the posterior superior iliac spine. The sacroiliac screw is not recommended to fix the fractures in Denis III zone.

Percutaneous iliosacral screw insertion with only outlet and inlet fluoroscopic view for unstable pelvic ring injuries: Clinical and radiological outcomes

Background:

Percutaneous iliosacral screw fixation in posterior pelvic ring fractures is challenging and commonly performed under fluoroscopy or navigation techniques. This study aimed to assess the safety and efficacy of percutaneous iliosacral screw implantation under fluoroscopy guidance with only inlet and outlet views.

Methods:

This retrospective study included 58 consecutive unstable posterior pelvic injury patients (36 sacral fractures and 22 sacroiliac joint disruptions) treated with percutaneous iliosacral screws between January 2015 and November 2019. Acceptable inlet radiographs show the anterior cortex of the S1 body superimposed on the S2 body. Acceptable outlet radiographs show the superior pubic symphysis at the level of the S2 foramen and visualize the S1 and S2 sacral foramina. In our technique, the screw was inserted at the inferior half of the outlet view and the posterior half of the inlet view. The time needed for screw insertion and the radiation exposure time was recorded. Intra and postoperative complications were documented. Postoperative computed tomography (CT) scans assessed screw position.

Results:

In total, 69 iliosacral screws were inserted in 58 patients. In postoperative CT scans, the screw position was assessed, 89.8% were in a secure position, and 10.2% had malposition. The mean operation time per screw was 21.18 min and the mean fluoroscopy time per screw was 112 s. There was no evidence of wound infection or iatrogenic neurovascular injury. No reoperation was performed.

Conclusion:

Percutaneous iliosacral screws can be placed using the only outlet and inlet fluoroscopic views with comparable radiological and clinical outcomes to the conventional method.

Reliability of Fossae Lumbales Laterales and Pelvic Incidence for Estimating Transsacral Corridors Assessed Using Reconstruction Computed Tomography

Background

The present study aimed to evaluate the effect of fossae lumbales laterales and pelvic incidence (PI) on transsacral corridors.

Methods

Patients who underwent pelvic computed tomography (CT) during routine therapy in a single center between 2015 and 2020 were retrospectively reviewed. The patients’ age and sex were documented during CT examination. Measurements were performed for both the upper and second sacral segments. Height and weight of the patients were determined using appropriate tools and body mass index (BMI) was calculated. Transsacral corridors were identified in true coronal and true sagittal planes and their width was determined as the maximum gap measured so that no screws could come out of the transacral corridors. PI was measured.

Results

Our study included 244 (57%) male and 184 (43%) female patients, who had a mean age of 49.3 ± 14.15 years (range, 18–89 years) and a mean BMI of 26.57 ± 2.38 kg/m². No statistically significant correlation was found between the detection of the dimple sign in physical examination and the presence of an adequate corridor. The PI was statistically significantly higher in the patients with dimples (p < 0.001). PI of the female patients was higher than that of the male patients (p = 0.026). The correlation between PI and the existence of adequate corridors for S1 and S2 screws was not statistically significant (p = 0.858 and p = 0.129, respectively). On the relationship between the presence of adequate S1 and S2 corridors where transsacral screws could be sent, an inverse relationship was detected: if the S1 transsacral corridor was adequate, the S2 corridor was inadequate or vice versa.

Conclusions

We could not obtain meaningful results on the use of the dimples of Venus or PI instead of CT to evaluate the adequacy of transverse corridors. Nevertheless, we confirmed that an increased PI was associated with the presence of dimples of Venus.

Iliosacral screw corridors in Japanese subjects: a study using reconstruction CT scans

Objectives:

To investigate the characteristics of iliosacral (IS) screw corridors of Japanese pelves.

Methods:

Computer tomography images of 42 adult Japanese subjects without any pelvic injury were analyzed at a workstation. Using the manual reconstruction function, the width of a simulated horizontal corridor for an IS screw on the true coronal and true axial planes in the upper (S1), second (S2), and the third (S3) sacral segments was measured. For pelves without an adequate S1 corridor, a cranially tilted corridor was sought. A corridor was defined as “adequate” if its width on both planes was 10 mm or more.

Results:

An adequate horizontal corridor was found in S1 in 17 (40.5%) subjects, in S2 in 29 (69.0%) subjects, and in S3 in no subject. An independent factor affecting the adequacy of the S1 corridor was the adequacy of the S2 corridor (OR: 0.09). Similarly, an independent factor affecting S2 adequacy was S1 adequacy (OR: 0.10). A tilted, 10 mm diameter corridor was found in all 25 subjects who did not have an adequate horizontal corridor in the S1 segment. The angle required to obtain a 10 mm diameter corridor inversely correlated with the diameter of a horizontal corridor on the true coronal plane (R = −0.713, P = .000).

Conclusions:

The characteristics of IS screw corridors in the 42 Japanese subjects were similar to those reported in previous studies conducted in the West. The importance of preoperative planning using reliable techniques, such as three-dimensional reconstruction, should be emphasized.

Level of evidence: Diagnostic Level III. See Instructions for Authors for a complete description of level of evidence.

Corridor-diameter-dependent angular tolerance for safe transiliosacral screw placement: an anatomic study of 433 pelves

BACKGROUND

The purpose of this study was to determine the angular tolerance of the S1 and S2 segments to accommodate a transiliosacral screw across both sacroiliac joints.

HYPOTHESIS

We hypothesized that the angular tolerance for transiliosacral screw placement would be more constrained than the angular tolerance for iliosacral fixation in pelves where a safe osseous corridor was measured.

MATERIALS AND METHODS

The cortical boundaries of the S1 and S2 sacral segments in 433 pelvic CTs were digitally mapped. A straight-line path was placed within each osseous corridor and extended across both SI joints past the outer iliac cortices. The diameter of the path was increased until it breached the cortex, geometrically determining maximum diameter (Dmax). Angular tolerance for screw placement was calculated with trigonometric analysis of the Dmax value of the corridor, and the average distance from the termination of the osseous corridor to the site of percutaneous insertion. Gender, age, and BMI were evaluated as independent predictors using binomial logistic regression.

RESULTS

The transiliosacral angular tolerance for the S1 and S2 osseous corridors was 1.53 ± 0.57 degrees and 1.02 ± 0.33 degrees, respectively. 68.9% of S1 corridors and 81.1% of S2 corridors had a safe zone (corridor diameter ≥ 10 mm) for transiliosacral placement, 48.3% of the pelves had a safe zone for both corridors, while 5.1% had no safe zones. Females had a less frequent Dmax ≥ 10 mm at S1, 52% vs 67% (p = 0.001), and at S2, 64% vs 86% (p < 0.001).

DISCUSSION

In conclusion, the angular tolerance of 1.53 and 1.03 degrees for the S1 and S2 segments, respectively, creating a narrow interval for safe passage of the trans-iliac and trans-sacral, with approximately 31.1% of patients not having a viable corridor for screw passage. A correlation exist between S1 and S2 corridors with Dmax ≥ 10 mm and the resulting increase in angular tolerance for safe passage of a transilioscral screw.

LEVEL OF EVIDENCE IV

Level Retrospective Cohort.

A useful intraoperative technique for transiliac-transsacral screws: a point-to-point coaxial guide apparatus

Background

The transiliac-transsacral screw placement is a clinical challenge for surgeons. This study explored a point-to-point coaxial guide apparatus assisting the transiliac-transsacral screw insertion and aimed to investigate the feasibility and accuracy of the guide apparatus in the treatment of posterior ring unstable pelvic fracture compared with a free-hand technique.

Methods

A retrospective study was performed to evaluate patients treated with transiliac-transsacral screws assisted by the point-to-point coaxial guide apparatus or free-hand technique. The intraoperative data of operative time and radiation exposure times were recorded. Postoperative radiographs and CT scans were performed to scrutinize the accuracy of screws position. The quality of the postoperative fracture reduction was assessed according to Matta radiology criteria. The pelvic function was assessed according to the Majeed scoring criteria at 6 months postoperatively.

Results

From July 2017 to December 2019, a total of 38 patients were included in this study, 20 from the point-to-point guide apparatus group and 18 from the free-hand group. There were no significant differences between the two groups in gender, age, injury causes, pelvic fracture type, screws level, and follow-up time (P > 0.05). The average operative time of the guide apparatus group for each screw was significantly less than that in the free-hand group (25.8 ± 4.7 min vs 40.5 ± 5.1, P < 0.001). The radiation exposure times were significantly lower in the guide apparatus group than that in the free-hand group (24.4 ± 6.0 vs 51.6 ± 8.4, P < 0.001). The intraosseous and juxtacortical rate of screw placement (100%) higher than in the free-hand group (94.4%).

Conclusion

The point-to-point coaxial guide apparatus is feasible for assisting the transiliac-transsacral screw in the treatment of posterior unstable pelvic fractures. It has the advantages of simple operation, reasonable design and no need for expensive equipment, and provides an additional surgical strategy for the insertion of the transiliac-transsacral screw.

Gender-Associated Differences in Sacral Morphology Do Not Affect Feasibility Rates of Transsacral Screw Insertion. Radioanatomic Investigation Based on Pelvic Cross-sectional Imaging of 200 Individuals

STUDY DESIGN

Retrospective radioanatomic single-center cohort study.

OBJECTIVE

To investigate sex-specific differences in transsacral corridor dimensions, determine feasibility rates of transsacral screw placement without extended safety zones around planned screw positions, and develop an index defining sacral dysmorphism (SD) irrespective of transsacral corridor diameters.

SUMMARY OF BACKGROUND DATA

Previously reported SD definitions used radiologically identifiable pelvic characteristics or predefined minimum diameter thresholds of transsacral corridors in the upper sacral segment including safety zones for screw placement. Technical progress of surgical 3D image guidance improved sacral screw insertion accuracy questioning established minimum diameter threshold-based SD definitions.

METHODS

Datasets from cross-sectional pelvic imaging of 100 women and 100 men presenting to a general hospital from July 2018 through August 2018 were included in a database to evaluate transsacral trajectory rates, and dimensions of transsacral corridor lengths, widths (TSCWs), and heights (TSCHs) in sacral segments I to III (S1-3). SD was assumed, if no transsacral trajectory was found in S1 with a corridor diameter of at least 7.5 mm.

RESULTS

Women presented significantly higher rates of transsacral trajectories in the inferior sector of S1 (P = 0.03), and larger transsacral corridor lengths in S2 (superior sector, P = 0.045), and S3 (central position, P = 0.02). In men, significantly higher feasibility rates were found for the placement of two transsacral screws in S2 (P = 0.0002), and singular screws in S3 (P = 0.006), with larger S1- (P = 0.0002), and central S2-TSCWs (P = 0.006). SD was prevalent in 17% of women, and 16% of men (P = 0.85). Calculating TSCW ratios of S1 and S2 was significantly indicative for SD at values below a threshold of 0.8 in women (P < 0.00001), and men (P = 0.0004).

CONCLUSION

SD is independent of sex despite significant differences in sacral morphology. An index defining SD irrespective of absolute transsacral corridor dimensions is presented to reliably differentiate dysmorphic from nondysmorphic sacra in women and men.

LEVEL OF EVIDENCE

2.

The usefulness of reformatting CT scanning plane to distinguish sacral dysmorphism and introducing the variable of elevated height for predicting the possibility of trans-sacral screw fixation

INTRODUCTION

Ilio-sacral screw fixation has been used for sacral fractures and sacroiliac joint dislocations. However, it is associated with significant complications including screw loosening with backing-out and loss of reduction. Trans-sacral screw fixation is indicated for rigid fixation in specific circumstances including bilateral posterior ring injuries, osteoporotic bone, or to supplement other types of posterior ring fixation.

HYPOTHESIS

The reformatting the CT scanning plane may distinguish sacral dysmorphism and predict the possibility of trans-sacral screw fixation by introducing the new indicator.

MATERIAL AND METHODS

The CT data of 112 adult cadavers (61 males and 51 females) were imported into Mimics® software and a 7.0 mm-sized trans-sacral screw was virtually placed in the ideal position of S₁. The osseous widths around screw in the axial images (AxW_S1) were measured. By reformatting the CT scanning plane parallel to the superior endplate of S₁, the AxWR_S1 was measured and the height of elevated segment in the upper sacrum (elevated height) was assessed as a new preoperative indicator.

RESULTS

Cortical violation around screw was detected in 26 models and was considered as sacral dysmorphism. The average AxW_S1 was measured as 13.70±2.76mm in the non-dysmorphism group, and 5.81±2.19mm in the dysmorphism group, with statistical difference (p<0.001). By reformatting the CT scanning plane, the average AxWR_S1 increased to 16.61±2.79mm in the non-dysmorphism group, and 8.04±2.62 in the dysmorphism group, and the difference was statistically significant (p<0.001). The elevated height was 3.29±4.19mm in the non-dysmorphism group, and 17.52±3.09mm in the dysmorphism group, and the differences were statistically significant (p<0.001). Based on the ROC curve analysis, the cut-off value of elevated height was 12.90mm (sensitivity 1.0 and specificity 1.0).

DISCUSSION

By reformatting the CT scanning plane, the osseous width around screw was widened, and the new preoperative indicator of elevated height could be introduced to predict the possibility of trans-sacral screw fixation into S₁. If the elevated height exceeded 13mm, the pelvis was assigned to sacral dysmorphism and thus, could not apply the trans-sacral screw fixation into S₁.

LEVEL OF EVIDENCE

III, controlled laboratory study.

Feasibility of iliosacral screw placement in patients with upper sacral dysplasia

Background

Exact knowledge of the sacral anatomy is crucial for the percutaneous insertion of iliosacral screws. However, dysplastic anatomical patterns are common. In addition to a preoperative computed tomography (CT) analysis, conventional radiographic measures may help to identify upper sacral dysplasia and to avoid damage to surrounding structures. Aiming to further increase safety in percutaneous iliosacral screw placement in the presence of sacral dysmorphism, this study examined the prevalence of previously established radiographic signs and, in addition, defined the “critical SI angle” as a new radiographic criterion.

Methods

Pelvic CT scans of 98 consecutive trauma patients were analysed. Next to assessment of established signs indicating upper sacral dysplasia, the critical sacroiliac (SI) angle was defined in standardized pelvic outlet views.

Results

The critical SI angle significantly correlates with the presence of mammillary bodies and an intraarticular vacuum phenomenon. With a cut-off value of − 14.2°, the critical SI angle detects the feasibility of a safe iliosacral screw insertion in pelvic outlet views with a sensitivity of 85.9% and a specificity of 85.7%.

Conclusions

The critical SI angle can support the decision-making when planning iliosacral screw fixation. The clinical value of the established signs of upper sacral dysplasia remains uncertain.

Development of generic Asian pelvic bone models using CT-based 3D statistical modelling

Background/Objective

Artificial bone models (ABMs) are used in orthopaedics for research of biomechanics, development of implants and educational purposes. Most of the commercially available ABMs approximate the morphology of Europeans, but they may not depict the Asian anatomy. Therefore, our aim was to develop the first Asian ABM of the pelvis and compare it with the existing pelvic ABM (Synbone®; Caucasian male).

Methods

One hundred clinical computed tomography (CTs) of adult pelvises (male n ​= ​50, female n ​= ​50) of Malay, Chinese and Indian descent were acquired. CTs were segmented and defined landmarks were placed. Three 3D statistical pelvic model and mean models (overall, male, female) were generated. Anatomical variations were analysed using principal component analysis. To measure gender-related differences and differences to the existing ABM, distances between the anterior superior iliac spines (ASIS), the anterior inferior iliac spines (AIIS), the promontory and the symphysis (conjugate vera, CV) as well as the ischial spines (diameter transversa, DT) were quantified.

Results

Principal component analysis displayed large variability regarding the pelvic shape and size. Female and male statistical models were similar in ASIS (225 ​± ​20; 227 ​± ​13 ​mm; P ​= ​0.4153) and AIIS (185 ​± ​11; 187 ​± ​10 ​mm; P ​= ​0.3982) and differed in CV (116 ​± ​10; 105 ​± ​10 ​mm; P ​< ​0.0001) and DT (105 ​± ​7; 88 ​± ​8 ​mm; P ​< ​0.0001). Comparing the unisex mean model with the pre-existing ABM, the ASIS (226; 275 ​mm; P ​< ​0.0001), the AIIS (186; 209 ​mm; P ​< ​0.0001) and the CV (111; 105 ​mm; P ​< ​0.0001) differed significantly. Both models were similar regarding DT (97; 95 ​mm; P ​= ​0.6927). The analysis revealed notable gender- and size-dependent anatomical variations within the Asian population. Chinese, Malay and Indian descents did not differ notably. The overall Asian model was smaller than the existing ABM.

The translation potential of this article

Owing to the large differences between the Asian ABM and the pre-existing ABM, as well as differences between genders, the use of an Asian- and gender-specific ABM is important to consider in research, biomechanics and implant development for this population.

Space available for trans-sacral implants to treat fractures of the pelvis assessed by virtual implant positioning

INTRODUCTION

The use of trans-sacral implants to treat fractures of the sacrum is limited by the variable pelvic anatomy. We were interested in how many trans-sacral implants can be placed per pelvis? If a trans-sacral implant cannot be placed in S1, where is the cortex perforated, and is the use of sacroiliac screws safe in these pelves?

MATERIALS AND METHODS

3D pelvic models were created from CT scans of 156 individuals without fractures (92 European and 64 Japanese, 79 male and 77 female, mean age 66.7 ± 13.7 years). Trans-sacral implants with a diameter of 7.3 mm were positioned virtually with and without a surrounding safe zone of 12 mm diameter.

RESULTS

Fifty-one percent of pelves accommodated trans-sacral implants in S1 with a safe zone. Twenty-two percent did not offer enough space in S1 for an implant even when ignoring the safe zone. Every pelvis had sufficient space for a trans-sacral implant in S2, in 78% including a safe zone as well. In S1, implant perforation was observed in the sacral ala and iliac fossa in 69%, isolated iliac fossa perforation in 23% and perforation of the sacral ala in 8%. Bilateral sacroiliac screw placement was always possible in S1.

CONCLUSIONS

The use of trans-sacral implants in S1 requires meticulous preoperative planning to avoid injury of neurovascular structures. S2 more consistently offers space for trans-sacral implants.

Re-examining the Spectrum of Lumbosacral Transitional Dysmorphisms: Quantifying Joint Asymmetries and Evaluating the Anatomy of Screw Fixation Corridors

Objective

Although a wide range of sacral dysmorphisms has been documented with lumbosacral transitional vertebrae (LSTV) variations, quantitative characterization of the upper segment morphology and articular anatomy across the array of lumbosacral transitions are hardly found in the literature. This study presents LSTV anomalies as a series of sequential morphological changes (the LSTV spectrum) and quantitatively compares 6 LSTV subtypes with normative sacral dimensions including the anatomy at the upper sacral segments used for percutaneous sacroiliac screw insertion.

Methods

Seven linear dimensions were measured from LSTV subtypes and normal sacral variants from dried adult sacral specimens. The auricular, superior articular and facet surface areas were quantified. Obliquity and thickness of osseous corridors used for sacroiliac screw fixation were measured. Data were statistically compared within and between LSTV subtypes and the normal variants.

Results

LSTVs presented a wide range of morphometric differences in comparison to the normal bones. Grouping LSTV according to auricular surface positions (high, normal, and low) demonstrated significant between-group differences in the obliquity and thickness at the S1 and S2 segmental corridors.

Conclusion

Frequent occurrence of LSTV in the general population may require evaluation of anatomical parameters in these variations for safe sacroiliac instrumentation around this region.

Accuracy of sacroiliac screw placement with and without intraoperative navigation and clinical application of the sacral dysmorphism score

INTRODUCTION

Percutaneously-placed sacroiliac (SI) screws are currently the gold-standard fixation technique for fixation of the posterior pelvic ring. The relatively high prevalence of sacral dysmorphism in the general population introduces a high risk of cortical breach with resultant neurovascular damage. This study was performed to compare the accuracy of SI screw placement with and without the use of intraoperative navigation, as well as to externally validate the sacral dysmorphism score in a trauma patient cohort.

PATIENTS AND METHODS

All trauma patients who underwent sacroiliac screw fixation for pelvic fractures at a level 1 trauma centre over a 6 year period were identified. True axial and coronal sacral reconstructions were obtained from their pre-operative CT scans and assessed qualitatively and quantitatively for sacral dysmorphism - a sacral dysmorphism score was calculated by two independent assessors. Post-operative CT scans were then analysed for breaches and correlated with the hospital medical records to check for any clinical sequelae.

RESULTS

68 screws were inserted in 36 patients, most sustaining injuries from road traffic accidents (50%) or falls from height (36.1%). There was a male preponderance (83.3%) with the majority of the screws inserted percutaneously (86.1%). Intraoperative navigation was used in 47.2% of the patient cohort. 30.6% of the cohort were found to have dysmorphic sacra. The mean sacral dysmorphism scores were not significantly different between navigated and non-navigated groups. Three cortical breaches occurred, two in patients with sacral dysmorphism scores >70 and occurring despite the use of intraoperative navigation. There was no significant difference in the rates of breach between navigated and non-navigated groups. None of the breaches resulted in any clinically observable neurovascular deficit.

CONCLUSION

The sacral dysmorphism score can be clinically applied to a cohort of trauma patients with pelvic fractures. In patients with highly dysmorphic sacra, reflected by high sacral dysmorphism scores, intraoperative navigation is not in itself sufficient to prevent cortical breaches. In such patients it would be prudent to consider instrumentation of the lower sacral corridors instead.

Is S3 a Viable Osseous Fixation Pathway?

OBJECTIVES

To report the incidence of patients with a third sacral segment (S3) osseous fixation pathway (OFP) that could accommodate a transiliac-transsacral screw.

DESIGN

Retrospective case series.

SETTING

Regional Level 1 Trauma Center.

PATIENTS/PARTICIPANTS

A total of 250 patients without pelvic trauma from January 2017 to February 2017 were included.

INTERVENTION

The axial and sagittal reconstruction images of each patient's computed abdomen and pelvis tomography (CT) scans were reviewed.

MAIN OUTCOME MEASUREMENTS

Each CT was evaluated for the presence of sacral dysmorphism and whether an S3 OFP that could accommodate an intraosseous transiliac-transsacral screw exists.

RESULTS

There were 130 of the 250 patients (52%) with sacral dysmorphism. Overall, 38 of the 250 patients (15.2%) had an S3 OFP that could accommodate a 7.0-mm transiliac-transsacral style screw. When narrowed to patients who had an S3 OFP, 38 of 153 patients (24.8%) could accommodate a 7.0-mm transiliac-transsacral screw. Specific to the 38 patients with an adequate S3 OFP, 34 of 38 patients (89.5%) were noted to have sacral dysmorphism.

CONCLUSIONS

Our study demonstrates that 15.2% of patients have an S3 OFP large enough to accommodate an intraosseous implant. Patients who have sacral dysmorphism are more likely to have an adequate S3 OFP. Additional studies are needed to quantify the S3 OFP, understand the bone quality of the S3 segment and accompanying biomechanical implications, and investigate the anatomical concerns associated with S3 screw placement.

LEVEL OF EVIDENCE

Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Critical dimensions of trans-sacral corridors assessed by 3D CT models: Relevance for implant positioning in fractures of the sacrum

Trans-sacral implants can be used alternatively to sacro-iliac screws in the treatment of osteoporosis-associated fragility fractures of the pelvis and the sacrum. We investigated trans-sacral corridor dimensions, the number of individuals amenable to trans-sacral fixation, as well as the osseous boundaries and shape of the S1 corridor. 3D models were reconstructed from pelvic CT scans from 92 Europeans and 64 Japanese. A corridor of <12 mm was considered critical for trans-sacral implant positioning, and <8 mm as impossible. A statistical model of trans-sacral corridor S1 was computed. The limiting cranio-caudal diameter was 11.6 mm (±5.4) for S1 and 14 mm (±2.4) for S2. Trans-sacral implant positioning was critical in 52% of cases for S1, and in 21% for S2. The S1 corridor was impossible in 26%, with no impossible corridor in S2. Antero-superiorly, the S1 corridor was limited not only by the sacrum but in 40% by the iliac fossa. The statistical model demonstrated a consistent oval shape of the trans-section of corridor S1. Considering the variable in size and shape of trans-sacral corridors in S1, a thorough anatomical knowledge and preoperative planning are mandatory using trans-sacral implants. In critical cases, S2 is a veritable alternative. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2577-2584, 2017.

Computational simulation study on ilio-sacral screw fixations for pelvic ring injuries and implications in Asian sacrum

OBJECTIVES

Despite a high possibility of technique-related complications, ilio-sacral (IS) screw fixation is the mainstay of operative management in posterior pelvic ring injuries. We aimed to make IS screw trajectory with fully intraosseous path that was optimal and consistent, and confirm the possibility of transiliac-transsacral (TITS) screw fixation in Asian sacrum.

METHODS

Eighty-two cadaveric sacra (42 males and 40 females) were enrolled and underwent continuous 1.0-mm slice computed tomography (CT) scans. CT images were imported into Mimics^® software to reconstruct three-dimensional model of the pelvis. To simulate IS screws, we inserted 7.0-mm-sized TITS cylinder for first (S₁) and second (S₂) sacral segment and 7.0-mm oblique cylinder for S₁. TITS cylinder could not be inserted into S₁ of 14 models (sacral variation models) but could be inserted into the S₂ of all models. The actual length of virtual IS screws was measured, and anatomic features of safe zone (SZ_S2) including the area, horizontal distance (HD_S2), and vertical distance (VD_S2) were evaluated by the possibility of TITS screw fixation in the S₁.

RESULTS

When the oblique cylinder was directed toward the opposite upper corner of S₁ at the level of the first foramen, there was no cortical violation regardless of sacral variation. The average length of TITS cylinder was 152.3 mm (range 127.9-178.2 mm) in S₁ and 136.0 mm (range 97.8-164.1 mm) in S₂, and for oblique cylinder it was 99.2 mm (range 82.4-132.2 mm). The average VD_S2, HD_S2, and the area of SZ_S2 were 15.5 mm (range 8.7-24.4 mm), 18.3 mm (range 12.7-26.6 mm), and 221.1 mm² (range 91.1-386.7 mm²), respectively. The VD_S2 and SZ_S2 of sacral variation were significantly higher than those of normal (both p = 0.001).

CONCLUSIONS

Considering the high variability of the S₁, it is better to direct the IS screw trajectory toward the opposite upper corner of the S₁ at the level of first sacral foramen. If a TITS screw is needed, the transverse fixation for the S₂ could be performed alternatively due to its sufficient osseous site even in Asian sacrum.

Can the possibility of transverse iliosacral screw fixation for first sacral segment be predicted preoperatively? Results of a computational cadaveric study

OBJECTIVES

The purpose of this study was to predict the possibility of transverse iliosacral (TIS) screw fixation into the first sacral segment (S₁) and introduce practical anatomical variables using conventional computed tomography (CT) scans.

MATERIALS AND METHODS

A total of 82 cadaveric sacra (42 males and 40 females) were used for continuous 1.0-mm slice CT scans, which were imported into Mimics^® software to produce a three-dimensional pelvis model. The anterior height (BH) and superior width (BW) of the elevated sacral segment was measured, followed by verification of the safe zone (SZ_S1 and SZ_S2) in a true lateral view. Their vertical (VD_S1 and VD_S2) and horizontal (HD_S1 and HD_S2) distances were measured. VD_S1 less than 7mm was classified as impossible sacrum, since the transverse fixation of 7.0 mm-sized IS screw could not be done safely.

RESULTS

Fourteen models (16.7%; six females, eight males) were assigned as the impossible sacrum. There was no statistical significance regarding gender (p=0.626) and height (p=0.419). The average values were as follows: BW, 31.4mm (SD 2.9); BH, 16.7mm (SD 6.8); VD_S1, 13.4mm (SD 6.1); HD_S1, 22.5mm (SD 4.5); SZ_S1, 239.5mm² (SD 137.1); VD_S2, 15.5mm (SD 3.0); HD_S2, 18.3mm (SD 2.9); and SZ_S2, 221.1mm² (SD 68.5). Logistic regression analysis identified BH (p=0.001) and HD_S1 (p=0.02) as the only statistically significant variables to predict the possibility. Receiver operating characteristic curve analysis established a cut-off value for BH and HD_S1 of impossible sacrum of 20.6mm and 18.6mm, respectively.

CONCLUSION

BH and HD_S1 could be used to predict the possibility of TIS screw fixation. If the BH exceeds 20.6mm or HD_S1 is less than 18.6mm, TIS screw fixation for S₁ should not be undertaken because of narrowed SZ.

Morphometry of the sacrum and its implication on trans-sacral corridors using a computed tomography data-based three-dimensional statistical model

BACKGROUND CONTEXT

Trans-sacral implants are increasingly used to treat fractures of the sacrum, especially for osteoporosis-associated fragility fractures. However, the complex and highly variable sacral anatomy limits their use. It is still not clear which morphologic characteristics are critical to determine the availability and dimensions of trans-sacral corridors.

PURPOSE

This study aims to assess sacral anatomy and its influence on trans-sacral corridors.

STUDY DESIGN

This study used a computed tomography (CT)-based three-dimensional (3D) statistical size and shape model of the sacrum with multiple morphometric measurements.

MATERIALS AND METHODS

A 3D statistical model was computed using clinical CT data of 92 intact pelvises (mean age 61.5 years). Multiple measurements of the sacrum and the trans-sacral corridors were taken. Descriptive statistics and linear regression were calculated. Shape and size were analyzed using principal component analysis.

RESULTS

The limiting craniocaudal diameter of the trans-sacral corridor was 13.1 mm (±5 mm) in S1 and 13.8 mm (±2.4 mm) in S2. In S1, the craniocaudal diameter correlated with larger sacral curvature (SC), pelvic incidence (PI), and cranially located auricular surfaces. The presence of an accessory articulation with L5 was associated with a larger trans-sacral corridor S1. In contrast, the craniocaudal diameter of the S2 corridor correlated with more caudally situated auricular surfaces and lower PI. The sacral shape, as demonstrated by the statistical model, was highly variable, which affected the size and availability of trans-sacral corridor S1. Important determinants of trans-sacral corridor S1 were the craniocaudal position of the auricular surfaces and the shape of the sacral ala, which were influenced by SC, sacral height, and PI.

CONCLUSIONS

The human sacrum is highly variable in size and shape. The dimensions of trans-sacral corridors depend on the sacral shape and specific morphologic characteristics. Understanding of morphologic variants helps with preoperative assessments of the trans-sacral corridors. When planning to use trans-sacral implants, because of variable sacral anatomy and dimensions of the trans-sacral corridors, thorough preoperative planning is mandatory.

Simple mathematical model of sacroiliac screws safe-zone-Easy to implement by pelvic inlet and outlet views

Percutaneous sacral screw fixation is the mainstay of posterior pelvic ring fixation. This study quantifies the accuracy of fluoroscopic screw placement using post-operative CT scans and redefines the fluoroscopic safe zone using a mathematical calculation obtained from Inlet and outlet images. The authors hypothesized that a mathematical calculation of screw placement within the ala will improve accuracy of screw placement. A retrospective review of consecutive patients admitted to a level 1 trauma center with pelvic fractures fixed with iliosacral screws from January 2011 to December 2014 was performed. Accuracy of screw placement was determined by comparing fluoroscopy to post-operative CT scans. A mathematical calculation of screw position within the sacral ala was applied to determine assess screw position and compared to CT findings. Ninety-four patients with 156 screws met inclusion criteria, of which 50 (32.0%) had a cortical breech on CT. The sensitivity and specificity of the inlet-outlet safe zone using mathematical calculation were 97.1% and 84.0%, respectively. The positive and negative predictive values were 92.7% and 93.3%, respectively. Overall accuracies of the radiographic inlet-outlet and lateral safe zones were 92.9% and 70.0%, respectively (p-value = 0.004). Sacral dysmorphism was not found to be associated with sacral cortical breech. A Simple mathematical calculation (screw position relative to percentage of bone width) on the inlet-outlet provides an accurate way to predict the accuracy of sacroiliac screws. The method is easy to implement, part of the surgery work-flow, and provides higher accuracy than relying on subjective interpretation of inlet, outlet, and lateral images. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1478-1484, 2017.

A Useful Preoperative Planning Technique for Transiliac-Transsacral Screws

Stabilization of posterior pelvic ring injuries is increasingly performed using percutaneously placed iliosacral and transiliac-transsacral screws. Understanding the unique and specific anatomical variations present in each patient is paramount. Multiple methods of evaluating potential osseous fixation pathways for screw placement exist, but many require specific imaging protocols, specialized software, or modification of data. Not all surgeons and institutions have access to these options for a variety of reasons. A simple technique to preoperatively plan for safe transiliac-transsacral screws is proposed.

Transsacral Osseous Corridor Anatomy Is More Amenable To Screw Insertion In Males: A Biomorphometric Analysis of 280 Pelves

BACKGROUND

Percutaneous iliosacral screw placement is the standard procedure for fixation of posterior pelvic ring lesions, although a transsacral screw path is being used more frequently in recent years owing to increased fracture-fixation strength and better ability to fix central and bilateral sacral fractures. However, biomorphometric data for the osseous corridors are limited. Because placement of these screws in a safe and effective manner is crucial to using transsacral screws, we sought to address precise sacral anatomy in more detail to look for anatomic variation in the general population.

QUESTIONS/PURPOSES

We asked: (1) What proportion of healthy pelvis specimens have no transsacral corridor at the level of the S1 vertebra owing to sacral dysmorphism? (2) If there is no safe diameter for screw placement in the transsacral S1 corridor, is an increased and thus safe diameter of the transsacral S2 corridor expected? (3) Are there sex-specific differences in sacral anatomy and are these correlated with known anthropometric parameters?

METHODS

CT scans of pelves of 280 healthy patients acquired exclusively for medical indications such as polytrauma (20%), CT angiography (70%), and other reasons (10%), were segmented manually. Using an advanced CT-based image analysis system, the mean shape of all segmented pelves was generated and functioned as a template. On this template, the cylindric transsacral osseous corridor at the level of the S1 and S2 vertebrae was determined manually. Each pelvis then was registered to the template using a free-form registration algorithm to measure the maximum screw corridor diameters on each specimen semiautomatically.

RESULTS

Thirty of 280 pelves (11%) had no transsacral S1 corridor owing to sacral dysmorphism. The average of maximum cylindrical diameters of the S1 corridor for the remaining 250 pelves was 12.8 mm (95% CI, 12.1-13.5 mm). A transverse corridor for S2 was found in 279 of 280 pelves, with an average of maximum cylindrical diameter of 11.6 mm (95% CI, 11.3-11.9 mm). Decreasing transsacral S1 corridor diameters are correlated with increasing transsacral S2 corridor diameters (R value for females, -0.260, p < 0.01; for males, -0.311, p < 0.001). Female specimens were more likely to have sacral dysmorphism (defined as a pelvis without a transsacral osseous corridor at the level of the S1 vertebra) than were male specimens (females, 16%; males, 7%; p < 0.003). Furthermore female pelves had smaller-corridor diameters than did male pelves (females versus males for S1: 11.7 mm [95% CI, 10.6-12.8 mm] versus 13.5 mm [95% CI, 12.6-14.4 mm], p < 0.01; and for S2: 10.6 mm [95% CI, 10.1-11.1 mm] versus 12.2 mm [95% CI, 11.8-12.6 mm ], p < 0.0001).

CONCLUSIONS

Narrow corridors and highly individual, sex-dependent variance of morphologic features of the sacrum make transsacral implant placement technically demanding. Individual preoperative axial-slice CT scan analyses and orthogonal coronal and sagittal reformations are recommended to determine the prevalence of sufficient-sized osseous corridors on both levels for safe screw placements, especially in female patients, owing to their smaller corridor diameters and higher rate of sacral dysmorphism.

A feasibility study of pelvic morphology for curved implants

OBJECTIVES

We hypothesize that inserting a curved intramedullary internal fixation device which follows curved osseous fixation paths (OFPs) would be more versatile and mechanically stronger than straight screws for fixation of pelvic ring and acetabular injuries. This study characterizes the dimensions of curved OFPs of the pelvic ring and acetabulum and suggests design parameters for such a curved device.

METHODS

CT scans of intact pelves of 50 female and 50 male subjects were studied using MIM Maestro™ and Solidworks™ to determine the constriction points (smallest cross sections) and the tightest radii of curvature (RoC) in the anterior column, posterior column, iliosacral and pubic symphysis OFPs.

RESULTS

The constriction point diameters for the superior pubic ramus and supra-acetabular areas were 13±3mm and 12±3mm, respectively. The anterior column RoC was greater than 65mm in all cases. The minimum observed RoC for the path from one ilium, across the SI joint, the sacrum and to the other ilium was 71mm, with 99% of the cases having a RoC of at least 80mm, in both the inlet and outlet views.

CONCLUSION

This study shows that if a flexible implant which could be stiffened once in place was available, it would enable the use of larger and longer fixation taking advantage of the pelvis's curved intracortical spaces. Even for dysmorphic pelves, accessible tunnels support a long, strong, curved fixation device.