Thoracic pedicle morphometry in different body height population: a three-dimensional study using reformatted computed tomography

Analysis on the related factors of misplacement of freehand pedicle screws via posterior approach in degenerative scoliosis

BACKGROUND

To study the risk factors associated with misplacement of freehand pedicle screws through a posterior approach for degenerative scoliosis.

METHODS

A total of 204 patients who underwent posterior pedicle screw-rod system surgery for degenerative scoliosis in our hospital from December 2020 to December 2023 were retrospectively analyzed. Patient demographics, radiographic accuracy, and surgery-related information were recorded.

RESULTS

A total of 204 patients were included. A total of 2496 screws were placed. 2373 (95.07%) were in good position. Misplacement screws were 123 (4.93%). None of the patients had postoperative spinal nerve symptoms due to screw malposition. The misplacement rate of thoracic (T10-T12) pedicle screws was 11.11% (60/540). Misplacement of pedicle screws in the lumbar spine (L1-L5) was 3.22% (63/1956). Age, gender, surgeon, and operation time had no significant effect on misplacement of pedicle screws (P>0.05). Body mass index, Hu value, number of screw segments, Cobb angle, vertebral rotation, and spinal canal morphology had some correlation with pedicle screw misplacement. Among them, BMI, Hu value, number of screw segments, Cobb angle, and vertebral rotation grade were independent risk factors for PS misplacement (P<0.05). The height of the posterior superior iliac spine had a significant effect on pedicle screw misplacement in the lower lumbar spine (L4/5) (P<0.05).

CONCLUSION

BMI, Hu value, number of screw levels, Cobb angle, and vertebral rotation grade were independent risk factors for pedicle screw misplacement in patients with degenerative scoliosis. Posterior superior iliac spine height has a large impact on PS placement in the lower lumbar spine. Patients with degenerative scoliosis should be preoperatively planned for the size and direction of the placed screws by X-ray and CT three-dimensional, to reduce the misplacement rate of pedicle screws.

Three-dimensional morphological analysis of the thoracic pedicle and related radiographic factors in adolescent idiopathic scoliosis

Background

This study aimed to investigate the laterality of the pedicle morphology at the apical vertebra (AV) level and identify the radiographic factors associated with the laterality ratio of the pedicle morphology at the AV level in patients with adolescent idiopathic scoliosis (AIS).

Methods

Overall, 684 pedicles in 57 AIS patients aged 10–20 years, who underwent preoperative computed tomography (CT) and had Lenke type 1 or 2 with right convex main thoracic curves (MTC), were evaluated. Pedicle diameters of the MTC were assessed. We defined and compared the region containing two vertebrae adjacent to the AV (APEX±1) and the region containing two vertebrae adjacent to the neutral vertebra. We analyzed the pedicle diameter and laterality ratio of APEX±1 and performed multiple linear regression analysis to identify the radiographic factors associated with the laterality of the pedicle diameter.

Results

On the concave side of APEX±1, the pedicles of 15 patients (26.3%) did not accept a 4-mm-diameter pedicle screw (PS), even with 25% cortical bone width expansion. Laterality ratio differences in the pedicle diameters of the cortical bone width in APEX±1 were large in patients with more proximal AV level (p < 0.001) and smaller apical vertebral rotation (AVR) (p = 0.029).

Conclusions

Preoperative planning to accurately select and insert the PS in AIS should be based on the anatomical limitations in APEX±1, AV level, and AVR degree. In APEX±1, the correlation between AVR and the laterality ratio of the pedicle diameter may be useful for pathoetiological interpretation of the AIS deformity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05799-4.

Pedicle Screw Placement Using Intraoperative Computed Tomography and Computer-Aided Spinal Navigation Improves Screw Accuracy and Avoids Postoperative Revisions: Single-Center Analysis of 1400 Pedicle Screws

OBJECTIVE

Intraoperative computed tomography and navigation (iCT-Nav) is increasingly used to aid spinal instrumentation. We aimed to document the accuracy and revision rate of pedicle screw placement across many screws placed using iCT-Nav. We also assess patient-level factors predictive of high-grade pedicle breach.

METHODS

Medical records of patients who underwent iCT-Nav pedicle screw placement between 2015 and 2017 at a single center were retrospectively reviewed. Screw placement accuracy was individually assessed for each screw using the 2-mm incremental grading system for pedicle breach. Predictors of high-grade (>2 mm) breach were identified using multiple logistic regression.

RESULTS

In total, 1400 pedicle screws were placed in 208 patients undergoing cervicothoracic (29; 13.9%), thoracic (30; 14.4), thoracolumbar (19; 9.1%) and lumbar (130; 62.5%) surgeries. iCT-Nav afforded high-accuracy screw placement, with 1356 of 1400 screws (96.9%) being placed accurately. In total, 37 pedicle screws (2.64%) were revised intraoperatively during the index surgery across 31 patients, with no subsequent returns to the operating room because of screw malpositioning. After correcting for potential confounders, males were less likely to have a high-grade breach (odds ratio [OR] 0.21; 95% confidence interval [CI] 0.10-0.59, P = 0.003) whereas lateral (OR 6.21; 95% CI 2.47-15.52, P < 0.001) or anterior (OR 5.79; 95% CI2.11-15.88, P = 0.001) breach location were predictive of a high-grade breach.

CONCLUSIONS

iCT-Nav with postinstrumentation intraoperative imaging is associated with a reduced need for costly postoperative return to the operating room for screw revision. In comparison with studies of navigation without iCT where 1.5%-1.7% of patients returned for a second surgery, we report 0 revision surgeries due to screw malpositioning.

Morphometric analysis of the proximal thoracic pedicles in Lenke II and IV adolescent idiopathic scoliosis: an evaluation of the feasibility for pedicle screw insertion

PURPOSE

Pedicles on the concave side of the proximal thoracic (PT) curve in adolescent idiopathic scoliosis (AIS) patients with Lenke II and IV deformities tend to be narrow and dysplastic, making pedicle screw (PS) insertion challenging. The aim of this study was to evaluate the feasibility for PS placement in these patients using pedicle chord length, diameter, and channel morphology.

METHODS

In this retrospective study, 56 consecutive AIS patients with Lenke II or IV curves who underwent instrumented posterior spinal fusion (PSF) were studied. The mean age at surgery was 14.8 years and the mean PT curve measured 45°. Two independent investigators evaluated all visible pedicles from T₁ to T₆ vertebral levels using axial images from intraoperative computed tomography-guided navigation recording the pedicle: (1) maximum transverse diameter 'd' at the isthmus, (2) maximum chord length 'l', and (3) qualitative assessment of the channel morphology (types A-D).

RESULTS

Two hundred and sixty-eight concave and 264 convex pedicles were measured. The mean 'd' of the concave pedicles at T₃ and T₄ was < 3.0 mm, compared to > 5.0 mm for the convex counterparts (p < 0.001). Of all concave pedicle channels, 48% had morphology characteristics that were riskier for PS cannulation (type C or D) compared to 2% of all convex pedicle channels (type A or B) (p < 0.001).

CONCLUSION

Almost half of all concave pedicles have morphologic characteristics that make them too small to accommodate a PS. Though PSs could be inserted using an in-out-in technique in these patients, alternative fixation anchors may improve strength and safety.

Morphological Parameters of the Thoracic Pedicle in an Asian Population: A Magnetic Resonance Imaging-Based Study of 3324 Pedicles

STUDY DESIGN

A cross-sectional magnetic resonance imaging (MRI)-based anatomical study.

OBJECTIVES

Instrumentation of the thoracic spine may be challenging due to the unique pedicle morphology and the proximity of vital structures. As prior morphological studies have mostly been done in Caucasians, our study aims to determine the optimal pedicle screw size for transpedicular fixation in an Asian population.

METHODS

A retrospective analysis of 400 patients who had undergone MRI of the thoracic spine was performed. A total of 3324 pedicles were included. Pedicle morphology was graded qualitatively based on the size of its cancellous channel, and quantitatively with the following parameters: pedicle transverse diameter, pedicle screw path length, and pedicle angle. Subgroup analysis based on gender was performed.

RESULTS

Mean pedicle transverse diameter was the narrowest at the T4 (2.9 ± 1 mm) and T5 (3.1 ± 1.1 mm) level. The mean pedicle screw path length progressively increased from T1 (34 ± 4.6 mm) to T12 (47 ± 4.6 mm). The mean pedicle angle was the largest at T1 (34° ± 7.9°) and decreased caudally, to 9.4° ± 3.8° at the T12 level. Females had significantly lower mean pedicle diameter and screw path length than males at every vertebral level; however, they had a larger pedicle angle at T8 to T10. The most common size of the pedicle cancellous channel was more than 4 mm.

CONCLUSION

Morphological differences in the Asian pedicle suggest that caution needs to be taken during thoracic spine instrumentation, particularly in Asian females who have significantly smaller pedicles. In such cases, the use of alternative techniques or intraoperative navigation may be useful.

Snake-Eye Screwing: A Novel Free-Hand Technique of Pedicle Screw Placement in Cervicothoracic Spine and Preliminary Clinical Results

Objective

To propose a novel technique of free‐hand pedicle screw placement in cervicothoracic spine (snake‐eye method) and evaluate the preliminary effects and safety in clinical practice.

Methods

This is a retrospective study and we defined the period of this study as from December 2017 to April 2019 in our institution. Forty patients were included in this study who underwent cervicothoracic internal fixation in our hospital, and all patients undergoing implantation of 200 pedicle screws were divided into two groups. Twenty‐two patients (108 screws) had screw placement using traditional method, while 18 patients (92 screws) had screw placement using snake‐eye method. To reduce the possible selection bias, the patients we recruited in this study was originally performed on by one radiological doctor who was blind to the objective of this study. Patient demographics, including patient age, sex, obesity, smoking, and hypertension, were evaluated to figure out baseline differences between groups. Medical information was recorded including time, accuracy, and immediate (within 30 days after surgery) postoperative complications of pedicle screw placement (including pulmonary embolism or other thromboembolic events, surgical site infection, neurovascular injury, and mortality).

Results

There were 24 males and 16 females, with an average age of 52.2 years (range, 24–77). Finally, a total of 200 screws were successfully inserted in these patients, including fifteen patients with four pedicle screws, four patients with six screws, three patients with eight screws in traditional method group, and 12 patients with four pedicle screws, two patients with six screws, four patients with eight screws in snake‐eye method group. Patient demographic and comparison of two surgery methods are shown in Tables 1 and 2. The data baselines of the two groups were comparable because no impact of the two groups on population characteristics was demonstrated in the presented experiment. Also, we noticed that time and accuracy of the two groups were different with statistical significance at the level of P = 0.05. We observed that immediate (within 30 days after surgery) postoperative complications, including pulmonary embolism (PE), surgical site infection (SSI), neurovascular injury (NI), and mortality, in the two groups did not differ.

Conclusion

This study highlights a safe and effective technique for pedicle screw placement in cervicothoracic spine named snake‐eye method, and this technique may be particularly useful in emergency conditions with limited resources.

Magnetic Resonance Imaging Study of Oblique Corridor and Trajectory to L1-L5 Intervertebral Disks in Lateral Position

OBJECTIVE

This study investigated the retroperitoneal oblique corridor and trajectory of L1-L5 as the lateral surgical access to the intervertebral disks in the Chinese population and detected the potential relationship between the corridor or trajectory and vertebral parameters, including disk axis, psoas muscle, and retroperitoneal vessel.

METHODS

Seventy magnetic resonance imaging studies performed from January 2017 to January 2019 were investigated. The oblique corridor was defined as the distance between the left lateral border of the retroperitoneal vessel and the anterior border of psoas. The trajectory was defined as the distance between the retroperitoneal vessel and lumbar plexus.

RESULTS

The oblique corridor analysis to L1-L5 disks have the following mean distances: L1-2 13.36 mm, L2-3 13.36 mm, L3-4 12.37 mm, and L4-5 10.36 mm. There was no difference in the L1-L5 corridor between genders. And the position of retroperitoneal vessel was negatively correlated with the corridor width. The trajectory measurements to L1-L5 disks have the following mean distances: L1-2 27.44 mm, L2-3 30.86 mm, L3-4 30.73 mm, and L4-5 24.36 mm. Moreover, the vertebral parameters, including the disk axis and psoas muscle, were positively correlated with the trajectory width. Otherwise, the position of retroperitoneal vessel was negatively correlated with the trajectory width.

CONCLUSIONS

Compared with previous studies, the safe surgical area of the Chinese is generally smaller than that of Caucasian. The position of the retroperitoneal vessel is the vital potential to limit the corridor and trajectory. Preoperative assessment of vertebral parameters, especially vascular structure, is essential for planning surgical process.

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.

Cortical Bone Trajectory Screw Fixation in the Upper and Middle Thoracic Spine (T1-T8): An Anatomic and Radiographic Assessment

OBJECTIVE

To assess an alternative method of cortical bone trajectory (CBT) screw placement in the thoracic spine from T1 to T8 and to investigate the proper entry point, screw insertion angle, screw length, and diameter.

METHODS

Computed tomography images of 80 patients were used to measure all related parameters. Ten cadaveric spines were studied and 4.5-mm screws were inserted into both sides of the vertebrae. Computed tomography of all vertebrae previously inserted with CBT screws was performed and lateral or medial pedicle wall violation was checked.

RESULTS

Of the inserted 160 CBT screws, 41.25% (66/160) penetrated the lateral pedicle wall, and the proportion of medial pedicle wall violation accounted for 0.625% (1/160). Maximal screw length tended to gradually increase from T1 to T8 (from 24.54 mm at T1 to 29.53 mm at T8). Compared with T4 and T5, maximal screw diameters of other thoracic levels were wider, ranging from 4.63 mm at T4 to 5.53 mm at T8. The greatest lateral angle was 8.66° in men and 8.39° in women at T8, whereas the smallest lateral angle was 4.97° in men and 4.67° in women at T1. The cephalad angle ranged from 17.23° at T7 to 31.50° at T1 and it was significantly larger in men at T7 (P < 0.05), T6 and T8 (P < 0.01).

CONCLUSIONS

Based on the results of this study, we suggest that patients could be placed with 4.5 × 25.0-30.0 mm CBT screws from T1 to T8 through the pedicle or pedicle rib unit.

Effect of Pedicle Fill on Axial Pullout Strength in Spinal Fixation After Rod Reduction

Rod reduction to pedicle screws is used for a variety of spinal fixation procedures; however, it can alter the integrity of the screw-bone interface. The authors investigated the effect of pedicle fill (ratio of pedicle screw diameter to pedicle diameter) on the strength of the screw-bone interface after simulated rod reduction on 17 vertebrae (3 thoracolumbar spine specimens). Pedicle diameter was measured with standard clinical computed tomography scan protocols. The authors determined the minimum pedicle diameter for each level. Polyaxial pedicle screws were surgically placed bilaterally with a freehand technique with standard clinical anatomic landmarks. The pedicle pairs were instrumented with pedicle screws of predetermined diameter, 1 with greater than 80% fill and 1 with less than 80% fill. A simulated reduction maneuver was performed with a 5-mm gap followed by an axial pullout test to assess screw interface strength. Comparison of insertion torque between less than 80% fill and greater than 80% fill did not show significant increases. A significant difference in pullout load (P=.043) occurred with greater than 80% fill (791±637 N) compared with less than 80% fill (636±492 N). No significant difference in stiffness was noted (P=.154) with pedicle fill of greater than 80% (427±134 N/mm) compared with less than 80% (376±178 N/mm). The current findings support the use of greater than 80% pedicle fill for optimal screw anchoring in pedicle screw-based constructs involving rod reduction. Surgeons should consider placing screws that can safely fill vertebral pedicles, especially at the apex of the curve and the proximal and distal levels of constructs, where excessive forces are imparted to the screws. [Orthopedics. 2017; 40(6):e990-e995.].

Minimally invasive cortical bone trajectory screws placement via pedicle or pedicle rib unit in the lower thoracic spine: a cadaveric and radiographic study

PURPOSE

To evaluate the feasibility of cortical bone trajectory (CBT) screws fixation via pedicle or pedicle rib unit in the cadaveric thoracic spine (T9-T12).

METHODS

Computed tomography (CT) images of 100 patients are analyzed by multiplanar reconstruction. Ten cadaveric thoracic spines are used to insert 4.5 × 35.0 mm CBT screws at all levels from T9 to T12.

RESULTS

Maximal screw length obtained by CT has a tendency to gradually increase from T9 (29.64 mm) to T12 (32.84 mm), and the difference reaches significant level at all levels except T9 versus T10 (P < 0.01). Maximal screw diameter increases from T9 (4.92 mm) to T12 (7.47 mm) and the difference reaches significant level among all levels (P < 0.01). Lateral angle increases from T9 (7.37°) to T12 (10.47°), and the difference reaches significant level among all levels except T11 versus T12. Cephalad angle from T9 to T12 are 19.03°, 22.10°, 25.62° and 27.50° (P < 0.01), respectively. The percentage of the inner and outer pedicle breakage are 2.5 and 22.5 %, respectively. The violation of lateral pedicle wall occurs at T9 and T10, especially for women at T9.

CONCLUSIONS

Both radiographic and cadaveric studies establish the feasibility of CBT screws placement via pedicle or pedicle rib unit in the lower thoracic spine (T9-T12). Furthermore, our measurements are also useful for application of this technique.

Computer-Assisted Screw Size and Insertion Trajectory Planning for Pedicle Screw Placement Surgery

Pathological conditions that cause instability of the spine are commonly treated by vertebral fixation involving pedicle screw placement surgery. However, existing methods for preoperative planning are based only on geometrical properties of vertebral structures (i.e., shape) without taking into account their structural properties (i.e., appearance). We propose a novel automated method for computer-assisted preoperative planning of the thoracic pedicle screw size and insertion trajectory. The proposed method extracts geometrical properties of vertebral structures by parametric modeling of vertebral bodies and pedicles in three dimensions (3D), and combines them with structural properties, evaluated through underlying image intensities in computed tomography (CT) images while considering the guidelines for pedicle screw design. The method was evaluated on 81 pedicles, obtained from 3D CT images of 11 patients that were appointed for pedicle screw placement surgery. In terms of mean absolute difference (MAD) and corresponding standard deviation (SD), the resulting high modeling accuracy of 0.39±0.31 mm for 3D vertebral body models and 0.31±0.25 mm for 3D pedicle models created an adequate anatomical frame for 3D pedicle screw models. When comparing the automatically obtained and manually defined plans for pedicle screw placement, a relatively high agreement was observed, with MAD ±SD of 0.4±0.4 mm for the screw diameter, 5.8±4.2 mm for the screw length, 2.0±1.4 mm for the pedicle crossing point and 7.6±5.8(°) for screw insertion angles. However, a statistically significant increase of 48±26% in the screw fastening strength in favor of the proposed automated method was observed in 99% of the cases.

Lower thoracic pedicle morphometry: male, taller, and heavier specimens have bigger pedicles

STUDY DESIGN

An anatomic study of pedicle dimensions was performed for lower thoracic vertebrae from American human subjects.

OBJECTIVE

To quantify the dimensions of the lower thoracic pedicles and to better define the demographic factors that could ultimately govern the caliber selection of pedicle screws.

SUMMARY OF BACKGROUND DATA

Transpedicular screw fixation allows for segmental instrumentation into multiple vertebrae across multilevel fusion area, offering considerable biomechanical advantage over the conventional hook and lateral mass fixation. Large variations in morphology from previous studies may be related to differences in demographics, sample size, and methodology.

METHODS

For this study, T7-T12 vertebrae from 503 American human cadavers were directly measured with a digital caliper. Examiner measured each vertebra to determine medial-lateral pedicle width and cranial-caudal pedicle height. Demographic information regarding age, sex, and race, as well as body height and weight, was available for all 503 subjects.

RESULTS

Both pedicle height and pedicle width generally increased in size caudally down the lower thoracic spine. The highest pedicle height was at the T12 level with a mean of 17.08 mm. The widest pedicle width was at the T11 level with a mean of 9.31 mm. Males have larger pedicles than females for all upper thoracic levels. The tallest and heaviest groups had larger pedicles than the shorter and lighter groups, respectively. Age and race did not consistently affect pedicle dimension in a statistically significant manner.

CONCLUSION

Our large-scale study of American specimens characterized the relationship between pedicle dimensions and a variety of demographic factors such as age, sex, body height and weight. With substantial statistical power, this study showed that male, taller, and heavier individuals had larger pedicles.

LEVEL OF EVIDENCE

N/A.

Pedicle morphometry of upper thoracic vertebrae: an anatomic study of 503 cadaveric specimens

STUDY DESIGN

An anatomic study of pedicle dimensions was performed for upper thoracic vertebrae from American human subjects.

OBJECTIVE

To quantify the dimensions of the upper thoracic pedicles and to better define the demographic factors that could ultimately govern the caliber selection of pedicle screws.

SUMMARY OF BACKGROUND DATA

Transpedicular screw fixation allows for segmental instrumentation into multiple vertebrae across multilevel fusion area, offering considerable biomechanical advantage over the conventional hook and lateral mass fixation. Large variations in morphology from previous studies may be related to differences in demographics, sample size, and methodology.

METHODS

For this study, T1-T6 vertebrae from 503 American human cadavers were directly measured with a digital caliper. Examiner measured each vertebra to determine medial-lateral pedicle width and cranial-caudal pedicle height. Demographic information regarding age, sex, and race, as well as body height and weight, was available for all 503 subjects.

RESULTS

Pedicle height generally increased in size caudally down the upper thoracic spine, but the highest pedicle height was at the T3 level with a mean of 12.25 mm. Pedicle width displayed a narrowing pattern moving down. The widest pedicle width was at the T1 level with a mean of 8.66 mm. The 2 older age groups had larger pedicles than the 2 younger age groups. Males have larger pedicles than females for all upper thoracic levels. The tallest and heaviest groups had larger pedicles than the shorter and lighter groups, respectively. Race was not a significant factor in affecting pedicle dimension.

CONCLUSION

Our large-scale study of American specimens characterized the relationship between pedicle dimensions and a variety of demographic factors such as age, sex, body height, and weight. With substantial statistical power, this study showed that male, older, taller, and heavier individuals had larger pedicles.

LEVEL OF EVIDENCE

N/A.

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

STUDY DESIGN

Anatomic study.

OBJECTIVE

To determine whether the thoracic spinal canal diameter decreases when the pedicle is allowed to expand with increasing screw diameter. To observe whether osseous breach occurs medially or laterally.

SUMMARY OF BACKGROUND DATA

Insertion of a pedicle screw that is larger in diameter than that of the native pedicle has been shown to expand the pedicle and increase biomechanical fixation strength. With this technique, there is concern for medial expansion of the pedicle causing decrease in spinal canal diameter, especially in the concavity of scoliosis, resulting in spinal cord compression. Also, large pedicle screws that are inserted correctly may still cause undetected medial bony breach during surgery.

METHODS

A total of 162 pedicles from 81 thoracic vertebrae (T1-T12) of 7 fresh-frozen adult cadavers were analyzed. After undertapping the pedicle by 1 mm, pedicle screws were inserted in increasing diameter (range, 4.0-9.5 mm) bilaterally until there was an osseous breach in the pedicle. A total of 938 screws were used. Transverse spinal canal diameter and pedicle circumference were measured (in millimeters) before and after each pedicle screw placement. Photographs and fluoroscopic images of representative specimens were obtained for visual assessment.

RESULTS

The average transverse spinal canal diameter was 17.7 mm. The average transverse canal diameter with the largest screw inserted before bony breach was detected was 17.6 mm (P = 0.92). The average diameter of the largest screw inserted before breach was 6.9 mm. Pedicle circumference increased from 41.8 mm before screw placement to 43.4 mm at maximal expansion before bony breach with the next sized screw. Twenty-eight pedicles did not break with 9.5-mm-diameter screws. There were 133 lateral and 1 medial breaches.

CONCLUSION

Increasing pedicle screw size caused pedicle expansion laterally but did not significantly alter transverse spinal canal dimensions. When there was an osseous breach, most were lateral (99.3%).

LEVEL OF EVIDENCE

N/A.

Feasibility of laminar screw placement in the upper thoracic spine: analysis using 3-dimensional computed tomographic simulation

STUDY DESIGN

Evaluation using 3-dimensional screw trajectory software and computed tomographic scans.

OBJECTIVE

To investigate the anatomic feasibility of laminar screw placement in the upper thoracic spine compared with pedicle screw placement.

SUMMARY OF BACKGROUND DATA

Although laminar screws have been suggested as an alternative to pedicle screws in the upper thoracic spine, previous anatomic feasibility studies have some limitations.

METHODS

Four types of screws were simulated from T1 to T6: unilaminar screw (US), superior bilaminar screw (SBS), inferior bilaminar screw (IBS), and pedicle screw (PS). Maximum allowable screw dimensions and the success rates of 4.5-mm screw placement were compared for each level. Laminar screw dimensions with more than 90% success rate at each level were determined for reference.

RESULTS

Computed tomographic scans of 132 patients were analyzed. Laminar screw diameters gradually increased from T1 (4.4-5.4 mm, for each type) to T6 (4.8-6.7 mm), whereas PS diameter steeply declined from T1 (5.9 mm) to T4 (3.4 mm) and then leveled off. At T1, PS had greater success rate of 4.5-mm screw placement than laminar screws (US > IBS > SBS); at T2, US had greater success rate than IBS, followed by PS and SBS; and at T3 to T6, laminar screws (US > IBS > SBS) had greater success rate than PS in all comparisons. Except for SBS at T1, laminar screw diameters with more than 90% success rates were between 3.5 and 5.0 mm.

CONCLUSION

In view of their anatomic feasibility, laminar screws can be a viable alternative to PSs in the upper thoracic spine. Particularly at T3 to T6 where the pedicle width is inherently small, the success rates of laminar screw placement were significantly and consistently higher than those of PS placement. The comparable success rates of laminar screws using commercially available screw sizes further emphasize their potential clinical use.

Evaluation of thoracic pedicle morphometry in a Chinese population using 3D reformatted CT

The aim of this study was to investigate the detailed three-dimensional morphology of the pedicles from T1 to T12 in 120 Chinese patients using a light-speed Vct CT (General Electric). After reformatting the original images, the following parameters were studied: outer pedicle width (OPW), outer pedicle height (OPH), pedicle chord length (PCL), pedicle cortical thickness (PCT) of the isthmus, and transverse pedicle angle (TPA). The mean outer pedicle width, outer pedicle height, and pedicle chord length were significantly smaller in females than in males at all levels (P < 0.01). The percentage of outer pedicle width ≤ 5.0 mm and ≤ 4.5 mm was high at mid-thoracic pedicles. No significant differences were found in transverse pedicle angle and pedicle cortical thickness in males and females. Pedicle cortical thickness was significantly thinner in patients over 50 years old compared with patients below 50 years old at most levels. The results showed that a screw of larger than 4.5 mm would be too large for mid-thoracic segments in Chinese population, especially for female patients. Considering the amount of variation between individuals and the complicated structure of the thoracic pedicles, the use of the transpedicular screw fixation must be individualized for each patient and based on detailed preoperative assessment. Reformatted CT assessment is essential before this procedure is performed.