The effect of increasing pedicle screw size on thoracic spinal canal dimensions: an anatomic study

The widths of the medial and lateral pedicle walls in adolescent idiopathic scoliosis with major thoracic curves

BACKGROUND CONTEXT

The widths of medial and lateral pedicle walls in the normal spine of middle-aged and elderly adults have been investigated and these studies found that the medial pedicle wall was thicker than the lateral pedicle wall. However, none had evaluated the widths of medial and lateral pedicle walls on adolescent or young adult scoliotic spines.

PURPOSE

This study aims to identify the distribution and variation of medial and lateral pedicle wall widths throughout the different vertebral levels of the scoliotic spine and its differences according to age, gender, body mass index (BMI), maturity, curve types and curve severity in adolescent idiopathic scoliotic (AIS) patients with major thoracic curves.

STUDY DESIGN

Retrospective study.

PATIENT SAMPLE

A total of 6,230 pedicles (right: 3,064, left: 3,166) from 191 patients were included in this study, with 264 (right: 183, left: 81) fully corticalized pedicles excluded from analysis.

OUTCOME MEASURES

Demographic data were age, gender, height, weight, BMI, Risser grade, Lenke curve types and Cobb angles. The main outcome measures were medial and lateral pedicle wall widths. Associations between pedicle wall widths and demographic data were calculated.

METHODS

This was a subanalytical retrospective study done on the same patient population as the previously published study on pedicle grading. The data was obtained from the main computed tomography (CT) scan pedicle study dataset. Medial and lateral pedicle wall widths were measured in the axial slices of CT scans from T1 to L5 vertebrae.

RESULTS

A total of 6,230 pedicles (right: 3,064, left: 3,166) from 191 patients were included in this study with 264 (right: 183, left: 81) fully corticalized pedicles excluded from analysis. Right-sided medial pedicle wall widths were narrower from T4-T10 (0.75±0.23 mm) compared to T1-T3 (0.89±0.28 mm) and T11-L5 (0.92±0.30 mm). Left-sided medial pedicle wall widths were narrower from T4 to T7 (0.76±0.24 mm) compared to T1-T3 (0.88±0.26 mm) and T8-L5 (0.90±0.27 mm). Medial cortical wall widths were significantly thicker compared to lateral cortical wall widths for all vertebras from T1 to L5 (right medial 0.85±0.28 mm vs lateral 0.64±0.26 mm (p<.001), left medial 0.86±0.26 mm vs lateral 0.64±0.26 mm (p<.001)). The left medial pedicle wall widths were marginally significantly (p<.001) thicker than the right side (right medial 0.85±0.28 mm vs left 0.86±0.26 mm). The main notable significant differences were located at the periapical region of the thoracic curve between T7 to T10 with the left concave medial pedicle width being thicker than the right convex medial pedicle width. The thinnest medial pedicle walls were located at right concave T7 (0.73±0.24 mm) and T8 (0.73±0.23 mm). We generally found no significant associations between the medial and lateral pedicle wall widths with age, gender, BMI, Risser grade, Cobb angle and curve types.

CONCLUSIONS

Knowledge on the widths of medial and lateral pedicle walls, their distribution and differences in a scoliotic spine is important for pedicle screw fixation, especially during pedicle probing to find the pedicle channel. The medial pedicle wall widths were significantly thicker than the lateral pedicle wall widths in AIS patients with major thoracic curves. The right concave periapical region had the thinnest medial pedicle walls.

Analysis of accuracy of pedicle screw placement in dysplastic pedicles in adolescent idiopathic scoliosis using the pedicle expansion technique with CT-based navigation

BACKGROUND

This study aimed to study the accuracy of pedicle screw (PS) insertion into dysplastic pedicles in adolescent idiopathic scoliosis (AIS) comparing cannulated screw using the pedicle expansion technique (PET) versus conventional technique.

METHODS

Forty-two AIS patients with 766 PSs were evaluated. In total, 236 screws were inserted into dysplastic pedicles: 138 and 98 screws were inserted using the PET (PET group) and standard technique (conventional group), respectively. Both methods used CT-based navigation to determine the insertion point. In the PET, a rigid ball tip feeler was tapped with a mallet to create an insertion route, a guide wire was passed through the tap, the pedicle was enlarged, and then a cannulated PS with a diameter of 4.35 mm was inserted. Postoperative CT was used to compare the accuracy of PS insertion.

RESULTS

In total, 23/236 (9.7%) perforations occurred. Regarding overall perforation, there were six (4.3%) and 17 (17.3%) cases in the PET and conventional group, respectively (P = 0.008). In terms of medial perforation, the PET group (n = 2, 1.4%) was significantly better than the conventional group (n = 7, 7.1%) (P = 0.021). In terms of lateral perforation, the PET group (n = 4, 2.9%) was significantly better than conventional group (n = 10, 10.2%) (P = 0.030). Only grade 1 perforation had occurred in the PET group, whereas grades 2 and 3 perforation occurred in the conventional group.

CONCLUSION

Use of the PET with CT-based navigation significantly increased the accuracy and safety of PS insertion in dysplastic pedicles in AIS.

The Influence of Increased Pedicle Screw Diameter and Thicker Rods on Surgical Results in Adolescents Undergoing Posterior Spinal Fusion for Idiopathic Scoliosis

Background: Modern surgical techniques allow for the correction of spinal deformity, stopping its progression and improving pain relief and social and physical functioning. These instruments have different implant designs, screws, and rod diameters and can be composed of different metal alloys with different hardnesses, which can have a significant impact on the effect of correcting spinal deformities. We designed a retrospective cohort study based on the same surgical technique and spine system using different implant sizes, and compared the results across them. Methods: This is a retrospective review of adolescent idiopathic scoliosis (AIS) patients who underwent posterior spinal fusion (PSF) between 2016 and 2022 with a minimum two-year follow-up (FU) using two spinal implant systems: 5.5 and 6.0 mm diameter screws with double 5.5 mm titanium rods (Group 1 (G1)), and 6.0 and 6.5 mm diameter pedicle screws with double 6.0 mm cobalt-chromium rods (Group 2 (G2)). The evaluated data were as follows: preoperative personal data, radiographic outcomes, complications, and health-related quality of life questionnaire (HRQoL). The parameters were reviewed preoperatively, after the final fusion, and during the FU. Results: The mean age of all 260 patients at surgery was 14.8 years. The average BMI was also similar in both groups and was noted as 21. The mean levels of fusion and screw density were similar in both groups. The mean preoperative major curves (MCs) were 57.6° and 62.5° in G1 and G2, respectively. The mean flexibility of the curves was noted as 35% in G1 and 33% in G2. After definitive surgery, the mean percentage correction of the MC was better in G2 vs. G1, with 74.5% vs. 69.8%, respectively (p < 0.001). At the final FU, the average loss of correction was 5.9° for G1 and 3.2° for G2 (p < 0.001). The mean preoperative (TK) thoracic kyphosis (T2-T5) was 12.2° in G1 and 10.8° in G2. It was corrected to 15.2° in G1 and to 13° in G2. At the FFU, we noted a significant difference in the TK (T2-T5) between the groups, with 16.7° vs. 9.6° for G1 vs. G2, respectively (p < 0.001). Statistical significance was observed between the preoperative sagittal balance and the final follow-up for both groups (p < 0.001). Conclusions: AIS patients surgically treated with screws with a larger diameter and thicker and stiffer rods showed greater correction and postoperative thoracic kyphosis without implant failure. The complication rates, implant density, and clinical outcomes remained similar. The radiographic benefits reported in this cohort study suggest that large-sized screws and stiffer rods for the correction of pediatric spinal deformities are safe and very effective.

3D printed pedicle screw guides reduce the rate of intraoperative screw revision in adolescent idiopathic scoliosis surgery

BACKGROUND CONTEXT

Pedicle screw fixation has become common in the treatment of adolescent idiopathic scoliosis (AIS). Malpositioned pedicle screws have significant complications and identifying surgical techniques to optimize screw placement accuracy is imperative.

PURPOSE

To compare the rate of intraoperative revision, replacement, or removal of pedicle screws placed utilizing 3D printed guides compared with pedicle screws placed utilizing a freehand technique.

STUDY DESIGN/SETTING

Retrospective cohort study/single academic center.

PATIENT SAMPLE

Thirty-two patients aged 10 to 18 with AIS.

OUTCOME MEASURES

Revision rate of pedicle screws and operative time between groups.

METHODS

A retrospective study was performed on patients 10 to 18 years of age who underwent posterior spinal instrumented fusion for AIS from February 2021 to July 2022. The study received an IRB exemption. Patient demographics, intraoperative measures, and outcome variables were recorded. Intraoperatively, all patients underwent a 3-dimensional fluoroscopic "check scan," which included axial, sagittal, and coronal images, to assess for screw accuracy. A secondary outcome of operative time was compared between groups. The p-values <.05 were considered significant.

RESULTS

A total of 32 patients were included in this study. There were 17 cases in the 3D guided and 15 cases in fluoroscopy-guided freehand cohort. There was a total of 254 pedicle screws using 3D guides and 402 screws using freehand technique. Between cohorts, there were no significant differences in a number of levels fused (p=.54) or length of surgery (p=.36). The total revision rate of 3D guided screw placement was 5.5% and that of the freehand technique was 8.5%. The freehand screw placement group had significantly higher revision rates per vertebral level compared with 3D guided (p=.0096). Notably, 3D printed guides had fewer screws that were removed/revised for being too anterior (7.1%) compared with freehand (23.5%). Surgical time was not significantly different between the 3D guided and freehand cohort (p=.35).

CONCLUSIONS

3D printed guides reduce intraoperative revision rate compared with freehand techniques. Total operative time is comparable to freehand technique.

What is the prevalence and distribution of narrow dysplastic and fully corticalized pedicles in Asian adolescent idiopathic scoliosis patients with major main thoracic curves? A computed tomography scan analysis of 6,494 pedicles

BACKGROUND CONTEXT

Patients with adolescent idiopathic scoliosis (AIS) have higher prevalence of abnormal or dysplastic pedicles.

PURPOSE

To investigate the prevalence and distribution of narrow dysplastic and fully corticalized pedicles in Asian AIS patients with major main thoracic curves.

DESIGN

Retrospective study.

PATIENT SAMPLE

A total of 6,494 pedicles in 191 patients were measured and evaluated.

OUTCOME MEASURES

The primary outcomes measures were the pedicle width measurements (total transverse pedicle width, transverse cancellous width, total transverse cortical width) and classification of pedicles. Demographic data (age, gender, height, weight, body mass index), proximal thoracic Cobb angle, main thoracic Cobb angle and lumbar Cobb angle were also obtained.

METHODS

AIS patients with major (largest Cobb angle) main thoracic curves and had computed tomography (CT) scans prior to corrective spine surgery were reviewed. The pedicles were classified as Grade A: cancellous channel >4 mm; Grade B: cancellous channel 2 to 4 mm; Grade C: cancellous channel <2 mm or corticalized pedicle >4 mm; Grade D: corticalized pedicle ≤4 mm. Grades B, C, and D were dysplastic pedicles while grades C and D were narrow dysplastic pedicles.

RESULTS

The prevalence of dysplastic pedicles (grades B, C, and D) was 61.7%. There were 22.6% narrow dysplastic pedicles (grades C and D) and 4.1% fully corticalized pedicles (grade D). In the thoracolumbar region, there was a sharp transition from larger and less dysplastic pedicles at T11 and T12 to narrower and more dysplastic pedicles at L1 and L2 (narrow dysplastic pedicles at T11: 3.1%, T12: 3.1%, L1: 39.8% and L2: 23.6%). Higher prevalences of narrow dysplastic pedicles were located at right T3 to T5 (71.2%-83.7%) and left T7-T9 (51.3%-61.2%). Higher prevalences of fully corticalized pedicles were located at right T3 to T5 (20.9%-34.0%) and left T7 to T8 (11.0%-12.0%). These were the concave pedicles of proximal thoracic and main thoracic curves, respectively.

CONCLUSION

There were 95.9% pedicles with cancellous channels (grades A, B, and C) can allow pedicle screw fixation and only 4.1% fully corticalized pedicles (grade D) that require an alternative method of fixation. For grade C pedicles (18.5%), pedicle screws can still be attempted with caution. Precautions should also be observed at the L1 and L2 levels as there was a transition to narrower pedicles.

Effect of Pedicle Screw Size on Surgical Outcomes Following Surgery for 412 Adolescent Idiopathic Scoliosis Patients

STUDY DESIGN

Retrospective Review.

OBJECTIVE

The objective of this study was to determine differences in surgical and post-operative outcomes in AIS patients undergoing spinal deformity correction surgery using standard or large pedicle screw size.

SUMMARY OF BACKGROUND

Use of pedicle screw fixation in spinal deformity correction surgery is considered safe and effective. Still, the small size of the pedicle and the complex 3D anatomy of the thoracic spine makes screw placement challenging, with improper pedicle screw fixation leading to catastrophic complications including injuries to nerve roots, spinal cord, and major vessels. Thus, insertion of larger diameter screw sizes has raised concerns amongst surgeons, especially in the pediatric population.

MATERIALS AND METHODS

AIS patients undergoing PSF between 2013 and 2019 were included. Demographic, radiographic, and operative outcomes collected. Patients in the large screw size group (GpI) received 6.5 mm diameter screw sizes at all levels while standard screw size group (GpII) received 5.0 to 5.5 mm diameter screw sizes at all levels. Kruskall-Wallis and Fisher's exact test performed for continuous and categorical variables respectively.Subanalyses included (1) screw accuracy in patients with available CT scans, (2) stratified analysis of large- and standard-screw patients with ≥60% flexibility rate, (3) stratified analysis of large- and standard-screw patients with <60% flexibility rate, and (4) matched analysis of large- and standard-screw patients by surgeon and year of surgery.

RESULTS

GpI patients experienced significantly higher overall curve correction ( P <0.001), with 87.6% experiencing at least one grade reduction of apical vertebral rotation from preoperative to postoperative visit( P =0.008).Patients with larger screws displayed higher postoperative kyphosis. No patient experienced medial breaching.

CONCLUSION

Large screw sizes have similar safety profiles to standard screws without negatively impacting surgical and perioperative outcomes in AIS patients undergoing PSF. Additionally, coronal, sagittal, and rotational correction is superior for larger-diameter screws in AIS patients.

Influences of Increasing Pedicle Screw Diameter on Widening Vertebral Pedicle Size during Surgery in Spinal Deformities in Children and Adolescents without Higher Risk of Pedicle and Vertebral Breaches

BACKGROUND

A very common technique for treating spinal deformities in children and adolescents is the use of segmental screws. In order to obtain proper stability and the best possible correction, the screws must first be precisely inserted. Additional factors influencing the quality and success of the operation are the size and quality of the bone, the skills of the surgeon, and biomechanical factors, i.e., the width and length of the screws used during surgery. Our study was focused on evaluating the effect of increasing the diameter of the instrumented pedicles by pedicle screws and assessing the safety of expanding these pedicles with screws of various sizes in children with spinal deformities during the growth period, using preoperative magnetic resonance imaging and postoperative computed tomography (CT) to assess and compare preoperative size measurements from MRI to postoperative CT measurements.

METHODS

We obtained data for evaluation from the available medical records and treatment histories of patients aged 2 to 18 who underwent surgical treatment of spinal deformities in the years 2016-2023. In 230 patients (28 male and 202 female), 7954 vertebral bodies were scanned by preoperative MRI, and 5080 pedicle screws were inserted during surgery, which were then assessed by postoperative CT scan. For the most accurate assessment, patients were classified into three age groups: 2-5 years (Group 1), 6-10 years (Group 2), and 11-18 years (Group 3). In addition, we studied implant subgroups: vertebral bodies with inserted pedicles of screw sizes 5.0 mm and 5.5 mm (Group S), and pedicles of screw sizes 6.0 mm, 6.5 mm, and 7.0 mm (Group L).

RESULTS

The morphology of pedicles (Lenke classification) analyzed before surgery using MRI was 55.2% type A, 33.8% type B, 4.7% type C, and 6.3% type D. The postoperative lateral and medial breaches were noted, and these did not cause any complications requiring revision surgery. The mean pedicle diameter before surgery for T1-L5 vertebral pedicles was between 3.79 (1.44) mm and 5.68 (1.64) mm. The mean expanding diameter of pedicles after surgery for T1-L5 vertebral pedicles ranged from 1.90 (0.39) mm to 2.92 (0.28) mm, which corresponds to the extension of the pedicle diameter in the mean range of 47% (4.1)-71% (3.0). We noted that the mean vertebral pedicle expansion was 49% in Group 1, 52% in Group 2, and 62% in Group 3 (N.S.), and the mean expansion for 7.0 mm screw pedicles was 78%.

CONCLUSIONS

Our study confirms that there is a wide range of expansion of the vertebral pedicle during screw insertion (up to 78%) with a low risk of lateral or medial breaches and without an increased risk of complications. The larger the diameter of the screw inserted into the pedicle, the more the pedicle expands. Pedicle measurements by preoperative MRI may be helpful for sufficient reliability in preoperative planning.

Factors Affecting Pedicle Screw Insertional Torque in Spine Deformity Surgery

STUDY DESIGN

Retrospective observational study of consecutive patients.

OBJECTIVE

We sought to: (1) clarify the key factors predominantly associated with the insertional torque of pedicle screws; (2) compare the optimal factors for pedicle screw insertion to obtain rigid screw fixation in patients with adult spinal deformity (ASD) and in those with adolescent idiopathic scoliosis (AIS); (3) determine the optimal screw/pedicle ratio (S/P) to obtain rigid pedicle screw fixation.

SUMMARY OF BACKGROUND DATA

Rigid pedicle screw fixation is mandatory to perform corrections for spinal deformities properly and to allow successful fusion after surgery. The fixation depends mainly on screw position accuracy and patient bone quality. Traditionally, spinal surgeons have decided the screw size, trajectory, and tapping size based on their intuition. Insertional torque has been indicated as useful to predict screw fixation strength, and is correlated with screw pullout strength and frequency of postoperative screw loosening.

METHODS

We compared insertion torque at L1-L3 levels of 324 screws in 68 patients with ASD and 58 screws in 32 patients with AIS. We assessed the association between screw/pedicle ratio and insertion torque by constructing a spline curve.

RESULTS

Pedicle and screw diameter correlated positively with insertion torque in patients with either ASD or AIS. The optimal screw/pedicle ratio to obtain rigid pedicle screw fixation in patients with ASD was close to, but less than one, and, by contrast, was about 1 to 1.25 in patients with AIS.

CONCLUSION

We propose the concept of an optimal S/P ratio for obtaining rigid pedicle screw fixation during spinal corrective surgery, which is different for patients with ASD and patients with AIS. The S/P ratio is useful for deciding the appropriate diameter screw for each case in preoperative planning.Level of Evidence: 4.

Micro-computed tomography analysis of the lumbar pedicle wall

Background

Although the pedicle is routinely used as a surgical fixation site, the pedicle wall bone area fraction (bone area per unit area) and its distribution at the isthmus of the pedicle remain unknown. The bone area fraction at the pedicle isthmus is an important factor contributing to the strength of pedicle screw constructs. This study investigates the lumbar pedicle wall microstructure based on micro-computed tomography.

Methods

Six fresh-frozen cadaveric lumbar spines were analyzed. Left and right pedicles of each vertebra from L1 to L5 were resected for micro-computed tomography scanning. Data was analyzed with custom-written software to determine regional variation in pedicle wall bone area fraction. The pedicular cross-section was divided into four regions: lateral, medial, cranial, and caudal. The mean bone area fraction values for each region were calculated for all lumbar spine levels.

Results

The lateral region showed lower bone area fraction than the medial region at all spinal levels. Bone area fraction in the medial region was the highest at all levels except for L4, and the median values were 99.8% (95.9–100%). There were significant differences between the lateral region and the caudal region at L1, L2 and L3, but none at L4 and L5. The bone area fraction in the lateral region was less than 64% at all spinal levels and that in the caudal region was less than 67% at the L4 and L5 levels.

Conclusions

This study provides initial detailed data on the lumbar pedicle wall microstructure based on micro-computed tomography. These findings may explain why there is a higher incidence of pedicle screw breach in the pedicle lateral and caudal walls.

Three-dimensional distribution of CT attenuation in the lumbar spine pedicle wall

This study investigated in vivo the three-dimensional distribution of CT attenuation in the lumbar spine pedicle wall measured in Hounsfield Unit (HU). Seventy-five volunteers underwent clinical lumbar spine CT scans. Data was analyzed with custom-written software to determine the regional variation in pedicle wall attenuation values. A cylindrical coordinate system oriented along the pedicle's long axis was used to calculate the pedicular wall attenuation distribution three-dimensionally and the highest attenuation value was identified. The pedicular cross-section was divided into four quadrants: lateral, medial, cranial, and caudal. The mean HU value for each quadrant was calculated for all lumbar spine levels (L1-5). The pedicle wall attenuation was analyzed by gender, age, spinal levels and anatomical quadrant. The mean HU values of the pedicle wall at L1 and L5 were significantly lower than the values between L2-4 in both genders and in both age groups. Furthermore, the medial quadrant showed higher HU values than the lateral quadrant at all levels and the caudal quadrant showed higher HU values at L1-3 and lower HU values at L4-5 than the cranial quadrant. These findings may explain why there is a higher incidence of pedicle screw breach in the pedicle lateral wall.

Investigation of the "Superior Facet Rule" Using 3D-Printed Thoracic Vertebrae With Simulated Corticocancellous Interface

BACKGROUND

Pedicle screw placement is the most common method of fixation in the thoracic spine. Use of the "superior facet rule" allows the operator to locate the borders of the pedicle reliably using posterior landmarks alone. This study investigated the ability of 3-dimensionally (3D)-printed thoracic vertebrae, made from combined thermoplastic polymers, to demonstrate pedicle screw cannulation accurately using the superior facet as a reliable landmark.

METHODS

An anonymized computed tomography scan of the thoracic spine was obtained. The T1-T12 thoracic vertebrae were anatomically segmented and 3D-printed. The pedicle diameters and distance from the midpoint of the superior facet to the ventral lamina were recorded. A total of 120 thoracic pedicles in 60 thoracic vertebral models were instrumented using a freehand technique based only on posterior landmarks. The vertebral models were then coronally cut and examined for medial or lateral violations of the pedicle after screw placement.

RESULTS

A total of 120 pedicle screws were placed successfully within the 3D-printed thoracic vertebral models. Average measurements fell within 1 standard deviation of previous population studies. There were no pedicle wall violations using standard posterior element landmarks for instrumentation. There were 3 lateral violations of the vertebral body wall during screw placement, all attributable to the insertion technique.

CONCLUSIONS

3D-printed thoracic vertebral models using combined thermoplastic polymers can accurately demonstrate the anatomical ultrastructure and posterior element relationships of the superior facet rule for safe thoracic pedicle screw placement. This method of vertebral model prototyping could prove useful for surgical education and demonstrating spinal anatomy.

Thoracic Pedicle Morphometry of Dry Vertebral Columns in Relation to Trans-Pedicular Fixation: A Cross-Sectional Study From Central India

Introduction

Trans-pedicular screw fixation is one of the main modalities of spinal instrumentation today. It is particularly challenging in the thoracic spine due to the narrow pedicle dimensions especially in the upper and mid-thoracic levels. We aimed to study the anatomical variations like pedicle dimensions and angulation in transverse and sagittal planes.

Material and methods

We conducted an anatomical investigation on 20 dry vertebral columns (14 male and six female), from T1 to T12 levels. The measurements included pedicle width, height, and transverse and sagittal angles of the pedicle. Numerical variables were summarized using mean and standard deviation.

Results

T12 vertebra was found to have the widest pedicle width (mean 7.89 ± 0.70 mm) and the widest pedicle height (mean 15.45±0.78 mm) while T5 vertebra (mean 3.65±0.40 mm) had the narrowest pedicle width. T1 vertebra had the maximum transverse angle of the pedicle (mean 30.37±2.56 degree); whereas, T2 vertebra had the maximum sagittal angle (mean 19.22±2.24 degree).

Conclusion

We have reported detailed pedicle measurements including their angulation for the thoracic spine in dry vertebral columns of central India. The pedicles are directed more medially from T1 to T10 levels and are almost neutral at T11 and T12 levels. These findings would not only be of immense help to the spinal surgeons but also help in designing implants and instrumentations specific for the thoracic spine for the central Indian population as well as aiding surgeons to perform more precise and, therefore, safe surgical procedures.

Utility of Thoracolumbar Low-Dose CT With Model-Based Iterative Reconstruction for Measuring Pedicle Diameter Using a Radiation Dose Less Than a One-Time Lumbar X-Ray

STUDY DESIGN

Retrospective.

OBJECTIVE

To evaluate the image quality of low-radiation-dose computed tomography (LD-CT) of the thoracolumbar spine, using model-based iterative reconstruction (MBIR) for measuring pedicle diameter.

SUMMARY OF BACKGROUND DATA

MBIR can drastically reduce radiation dose but its utility in spine surgery planning is unknown.

METHODS

We identified patients (mean age, 70.5 ± 13.3 yrs) who incidentally underwent both standard-radiation-dose CT (SD-CT) with hybrid iterative reconstruction and LD-CT with MBIR of the thoracolumbar spine within 2 years. We compared radiation dose, subjective image sharpness, signal-to-noise ratio, and contrast-to-noise ratio for the two tests. Additionally, inner pedicle diameters were measured on SD-CT (DSD) and LD-CT (DLD), and statistically compared.

RESULTS

We included 24 CT and 84 pedicles for each CT group. The radiation dose of LD-CT estimated by volume CT dose index was 1.21 ± 0.42 mGy, one-sixth the dose of SD-CT. The effective dose of LD-CT was 0.58 ± 0.31 mSv, equivalent to or less than that of a one-time lumbar X-ray in a previous report. LD-CT was significantly inferior in subjective image sharpness for the contour of vertebrae and trabecular structure, but superior for signal-to-noise ratio and contrast-to-noise ratio. The intra-rater reliability (intra-RR) and inter-RR for DLD were 0.985 and 0.892, respectively, comparable to those of DSD. DLD was consistently 0.30 mm smaller than DSD when compared within the same pedicle, regardless of pedicle diameter.

CONCLUSION

LD-CT with MBIR produced a radiation dose equivalent to a one-time lumbar X-ray and provided excellent images for measuring pedicle diameter. LD-CT can be a substitute for SD-CT when planning spine surgery if the relationship between DSD and DLD is sufficiently understood.

LEVEL OF EVIDENCE

3.

In Vivo Assessment of Thoracic Vertebral Shape From MRI Data Using a Shape Model

STUDY DESIGN

Feasibility study on characterizing thoracic vertebral shape from magnetic resonance images using a shape model.

OBJECTIVES

Assess the reliability of characterizing thoracic vertebral shape from magnetic resonance images and estimate the normal variation in vertebral shape using a shape model.

SUMMARY OF BACKGROUND DATA

The characterization of thoracic vertebra shape is important for understanding the initiation and progression of deformity and in developing surgical methods. Methods for characterizing shape need to be comprehensive, reliable, and suitable for use in vivo.

METHODS

Magnetic resonance images of the thoracic vertebrae were acquired from 20 adults. Repeat scans were acquired, after repositioning the participants, for T4, T8, and T12. Landmark points were placed around the vertebra on the images and used to create a shape model. The reliability was assessed using relative error (E%) and intraclass correlation (ICC). The effect of vertebral level, sex and age on vertebral shape was assessed using repeated measures analysis of variance.

RESULTS

Five modes of variation were retained from the shape model. Reliability was excellent for the first two modes (mode 1: E% = 7, ICC = 0.98; mode 2: E% = 11, ICC = 0.96). These modes described variation in the vertebral bodies, the pedicle width and orientation, and the facet joint position and orientation with respect to the pedicle axis. Variation in vertebral shape was found along the thoracic spine and between individuals, but there was little effect of age and sex.

CONCLUSIONS

Magnetic resonance images and shape modeling provides a reliable method for characterizing vertebral shape in vivo. The method is able to identify differences between vertebral levels and between individuals. The use of these methods may be advantageous for performing repeated measurements in longitudinal studies.

LEVEL OF EVIDENCE

N/A.

The Importance of the Pedicle Diameters at the Proximal Thoracic Vertebrae for the Correction of Proximal Thoracic Curve in Asian Patients With Idiopathic Scoliosis

STUDY DESIGN

A retrospective comparative radiographic study.

OBJECTIVE

The purpose of this study was to evaluate differences in the pedicle diameters of proximal thoracic vertebrae in relation to the severity of the proximal thoracic curve in Asian patients with idiopathic scoliosis.

SUMMARY OF BACKGROUND DATA

A small pedicle diameter at the proximal thoracic vertebra has been reported in normal population, but the changes of pedicle diameter in patients with a proximal thoracic curve have not been properly evaluated.

METHODS

One hundred eighty-two patients with adolescent idiopathic scoliosis involving a greater than 10-degree proximal thoracic curve were analyzed. Sixty-nine and 113 patients had a structural and nonstructural proximal thoracic curve, respectively. The pedicle width was evaluated from T1 to T4 using a reconstructed computed tomography (CT) scan. The pedicle widths were compared between the convex and concave side, and between the structural and nonstructural proximal thoracic curve groups.

RESULTS

The pedicle widths at T3 (0.76 ± 0.92 mm) and T4 (0.50 ± 0.69 mm) on the concave side for the structural proximal thoracic curves were extremely narrow compared with those for the nonstructural proximal thoracic curves (T3,1.17 ± 0.84 mm; T4,0.82 ± 0.72 mm) (P = 0.002, 0.003, respectively). However, the T2 pedicle width was comparable on the concave side in both groups (2.44 ± 0.94 mm for the nonstructural and 2.32 ± 0.97 mm for the structural proximal thoracic curve, P = 0.430).

CONCLUSION

A pedicle screw insertion at the T3 or T4 vertebra on the concave side is not always possible in cases of a structural proximal thoracic curve; however, the pedicle width of the T2 vertebra is very wide and safe for the standard pedicle screw insertion. These results should be considered when the surgeon decides the upper instrumented vertebra or the correction method for the structural proximal thoracic curve.

LEVEL OF EVIDENCE

3.

Currently Adopted Criteria for Pedicle Screw Diameter Selection

Background

Transpedicular screw insertion has become widely accepted for the correction of spinal deformity as well as degenerative and traumatic injury, but adoption of this technique has remained less widespread in the thoracic compared to the lumbar spine. This is thought to be associated with the relative technical difficulty of screw insertion into the narrower widths of the thoracic pedicles and the neurologic and mechanical risks associated with breach of the pedicle wall. The surgical decision making involves determining the appropriate sized screw for maximum fixation strength while simultaneously respecting the structural integrity of the vertebral pedicles to prevent a breach and provide better fixation. This paper presents a systematic review of criteria for thoracic pedicle screw diameter (SD) selection in order to orient inexperienced surgeons on the impact of this selection on pedicle breaching and fixation strength.

Methods

We performed a systematic literature review focused on studies reporting SD selection in relation to pedicle dimensions, measures of fixation strength, and breach rate.

Results

Twenty-nine articles that measured fixation strength, breach rate, and/or provided SD in relation to pedicle width were selected for inclusion.

Conclusions

A commonly accepted criteria for pedicle SD selection has not yet been proposed. Screw diameters approximately 80% of the pedicle width have been adopted, but this proportion is rarely reported in the midthoracic vertebrae for which smaller pedicles and inadequate hardware specificity result in higher breach rates. Depending upon the insertion technique adopted, greater specificity in diameter selection by vertebral level should be pursued in order to maximally target cortical bone purchase.

Clinical Relevance

Based on this review of the literature, we believe that proper selection of the SD for individual vertebral level directly affects the insertion technique and the potential breach.

Augmented reality-assisted pedicle screw insertion: a cadaveric proof-of-concept study

OBJECTIVE

Augmented reality (AR) is a novel technology that has the potential to increase the technical feasibility, accuracy, and safety of conventional manual and robotic computer-navigated pedicle insertion methods. Visual data are directly projected to the operator's retina and overlaid onto the surgical field, thereby removing the requirement to shift attention to a remote display. The objective of this study was to assess the comparative accuracy of AR-assisted pedicle screw insertion in comparison to conventional pedicle screw insertion methods.

METHODS

Five cadaveric male torsos were instrumented bilaterally from T6 to L5 for a total of 120 inserted pedicle screws. Postprocedural CT scans were obtained, and screw insertion accuracy was graded by 2 independent neuroradiologists using both the Gertzbein scale (GS) and a combination of that scale and the Heary classification, referred to in this paper as the Heary-Gertzbein scale (HGS). Non-inferiority analysis was performed, comparing the accuracy to freehand, manual computer-navigated, and robotics-assisted computer-navigated insertion accuracy rates reported in the literature. User experience analysis was conducted via a user experience questionnaire filled out by operators after the procedures.

RESULTS

The overall screw placement accuracy achieved with the AR system was 96.7% based on the HGS and 94.6% based on the GS. Insertion accuracy was non-inferior to accuracy reported for manual computer-navigated pedicle insertion based on both the GS and the HGS scores. When compared to accuracy reported for robotics-assisted computer-navigated insertion, accuracy achieved with the AR system was found to be non-inferior when assessed with the GS, but superior when assessed with the HGS. Last, accuracy results achieved with the AR system were found to be superior to results obtained with freehand insertion based on both the HGS and the GS scores. Accuracy results were not found to be inferior in any comparison. User experience analysis yielded "excellent" usability classification.

CONCLUSIONS

AR-assisted pedicle screw insertion is a technically feasible and accurate insertion method.

Variability Analysis of Manual and Computer-Assisted Preoperative Thoracic Pedicle Screw Placement Planning

STUDY DESIGN

A comparison among preoperative pedicle screw placement plans, obtained from computed tomography (CT) images manually by two spine surgeons and automatically by a computer-assisted method.

OBJECTIVE

To analyze and compare the manual and computer-assisted approach to pedicle screw placement planning in terms of the inter- and intraobserver variability.

SUMMARY OF BACKGROUND DATA

Several methods for computer-assisted pedicle screw placement planning have been proposed; however, a systematic variability analysis against manual planning has not been performed yet.

METHODS

For 256 pedicle screws, preoperative placement plans were determined manually by two experienced spine surgeons, each independently performing two sets of measurements by using a dedicated software for surgery planning. For the same 256 pedicle screws, preoperative placement plans were also obtained automatically by a computer-assisted method that was based on modeling of the vertebral structures in 3D, which were used to determine the pedicle screw size and insertion trajectory by maximizing its fastening strength through the underlying bone mineral density.

RESULTS

A total of 1024 manually (2 observers × 2 sets × 256 screws) and 256 automatically (1 computer-assisted method × 256 screws) determined preoperative pedicle screw placement plans were obtained and compared in terms of the inter- and intraobserver variability. A large difference was observed for the pedicle screw sagittal inclination that was, in terms of the mean absolute difference and the corresponding standard deviation, equal to 18.3° ± 7.6° and 12.3° ± 6.5°, respectively for the intraobserver variability of the second observer and for the interobserver variability between the first observer and the computer-assisted method.

CONCLUSION

The interobserver variability among the observers and the computer-assisted method is within the intraobserver variability of each observer, which indicates on the potential use of the computer-assisted approach as a useful tool for spine surgery that can be adapted according to the preferences of the surgeon.

LEVEL OF EVIDENCE

3.

Evaluation of pedicle screw placement by pedicle channel grade in adolescent idiopathic scoliosis: should we challenge narrow pedicles?

BACKGROUND

Surgeons often have concerns about whether to place screws in narrow pedicles for correction of scoliosis. The aim of this study was to use pedicle channel grades based on preoperative CT to evaluate pedicle screw placement in posterior surgery for adolescent idiopathic scoliosis.

METHODS

The subjects comprised 55 patients who underwent posterior correction and fusion, and a total of 810 pedicles were examined in which screw placement had been planned and probing had been performed. Pedicle channel grades were determined by measuring inner pedicle diameter on preoperative CT scans. The grades were defined as grade 1 with an inner diameter of ≥ 4 mm, grade 2 with an inner diameter of ≥ 2 mm and < 4 mm, grade 3 with an inner diameter of ≥ 1 mm and < 2 mm, and grade 4 for a "cortical channel" with an inner diameter of < 1 mm.

RESULTS

The failure rate of screw placement was 0.5 % for pedicle channel grade 1, 2.9 % for grade 2, 12.0 % for grade 3, and 31.5 % for grade 4, showing significant differences (p < 0.001). For the laterality of curvature, the failure rate was 5.9 % for the convex side, 8.0 % for the neutral vertebra, and 9.0 % for the concave side, showing no significant difference. There was also no significant difference in failure rate between degrees of curvature of < 60° (8.2 %) and ≥ 60° (5.6 %). Logistic analysis showed that the pedicle channel grade was a significant risk factor for failure (odds ratio 4.0, p < 0.001).

CONCLUSIONS

The failure rate of screw placement was 31.5 % for a cortical channel with a pedicle inner diameter of < 1 mm. Screw placement should be attempted in pedicles with an inner diameter of 1 mm or larger.

Accuracy and safety of fluoroscopic guided percutaneous pedicle screws in thoracic and lumbosacral spine: a review of 2000 screws

STUDY DESIGN

Retrospective study.

OBJECTIVE

To investigate the accuracy and safety of percutaneous pedicle screws placed using fluoroscopic guidance in the thoracic and lumbosacral spine.

SUMMARY OF BACKGROUND DATA

Several studies had examined the accuracy and safety of percutaneous pedicle screws but provided large variations in their results with small number of patients or few number of pedicle screws evaluated.

METHODS

Computerized tomography of patients who had surgery with fluoroscopic guided percutaneous pedicle screws were chosen from 2 centers: (1) European patients from University Medical Center Hamburg-Eppendorf, Germany and (2) Asian patients from University Malaya Medical Centre, Malaysia. Screw perforations were classified into Grade 0, Grade 1 (<2 mm), Grade 2 (2-4 mm), and Grade 3 (>4 mm).

RESULTS

In total, 2000 percutaneous pedicle screws from 273 patients were analyzed: 1290 screws from 183 European patients and 710 screws from 90 Asian patients. The mean age was 59.1 ± 15.6. There were 140 male patients and 133 female patients. The total perforation rate was 9.4% with 151 (7.5%) Grade 1, 31 (1.6%) Grade 2, and 5 (0.3%) Grade 3 perforations. The total perforation rates among Europeans were 9.4% and among Asians were 9.3%. There was no difference between the 2 groups (P > 0.05). There were 3 distinct peaks in perforation rates (trimodal distribution) at T1, midthoracic region (T4-T7), and lumbosacral junction (L5 and S1). The highest perforation rates were at T1 (33.3%), S1 (19.4%), and T4 (18.6%).

CONCLUSION

Implantation of percutaneous pedicle screws insertion using fluoroscopic guidance is safe and has the accuracy comparable to open techniques of pedicle screws insertion.

LEVEL OF EVIDENCE

4.

A comparison of feasibility and safety of percutaneous fluoroscopic guided thoracic pedicle screws between Europeans and Asians: is there any difference?

PURPOSE

To directly compare the safety of fluoroscopic guided percutaneous thoracic pedicle screw placement between Caucasians and Asians.

METHODS

This was a retrospective computerized tomography (CT) evaluation study of 880 fluoroscopic guided percutaneous pedicle screws. 440 screws were inserted in 73 European patients and 440 screws were inserted in 75 Asian patients. Screw perforations were classified into Grade 0: no violation; Grade 1: <2 mm perforation; Grade 2: 2-4 mm perforation; and Grade 3: >4 mm perforation. For anterior perforations, the pedicle perforations were classified into Grade 0: no violation, Grade 1: <4 mm perforation; Grade 2: 4-6 mm perforation; and Grade 3: >6 mm perforation.

RESULTS

The inter-rater reliability was adequate with a kappa value of 0.83. The mean age of the study group was 58.3 ± 15.6 years. The indications for surgery were tumor (70.3 %), infection (18.2 %), trauma (6.8 %), osteoporotic fracture (2.7 %) and degenerative diseases (2.0 %). The overall screw perforation rate was 9.7 %, in Europeans 9.1 % and in Asians 10.2 % (p > 0.05). Grade 1 perforation rate was 8.4 %, Grade 2 was 1.2 % and Grade 3 was 0.1 % with no difference in the grade of perforations between Europeans and Asians (p > 0.05). The perforation rate was the highest in T1 (33.3 %), followed by T6 (14.5 %) and T4 (14.0 %). Majority of perforations occurred medially (43.5 %), followed by laterally (25.9 %), and anteriorly (23.5 %). There was no statistical significant difference (p > 0.05) in the perforation rates between right-sided pedicle screws and left-sided pedicle screws (R: 10.0 %, L: 9.3 %).

CONCLUSIONS

There were no statistical significant differences in the overall perforation rates, grades of perforations, direction of perforations for implantation of percutaneous thoracic pedicle screws insertion using fluoroscopic guidance between Europeans and Asians. The safety profile for this technique was comparable to the current reported perforation rates for conventional open pedicle screw technique.