Anatomy of the thoracic pedicle

A Retrospective Analysis of Pedicle Screw Placement Accuracy Using the ExcelsiusGPS Robotic Guidance System: Case Series

BACKGROUND

Robotic guidance has become widespread in spine surgery. Although the intent is improved screw placement, further system-specific data are required to substantiate this intention for pedicle screws in spinal stabilization constructs.

OBJECTIVE

To determine the accuracy of pedicle screws placed with the aid of a robot in a cohort of patients immediately after the adoption of the robot-assisted surgery technique.

METHODS

A retrospective, Institutional Review Board-approved study was performed on the first 100 patients at a single facility, who had undergone spinal surgeries with the use of robotic techniques. Pedicle screw accuracy was graded using the Gertzbein-Robbins Scale based on pedicle wall breach, with grade A representing 0 mm breach and successive grades increasing breach thresholds by 2 mm increments. Preoperative and postoperative computed tomography scans were also used to assess offsets between the objective plan and true screw placements.

RESULTS

A total of 326 screws were analyzed among 72 patients with sufficient imaging data. Ages ranged from 21 to 84 years. The total accuracy rate based on the Gertzbein-Robbins Scale was 97.5%, and the rate for each grade is as follows: A, 82%; B, 15.5%; C, 1.5%; D, 1%; and E, 0. The average tip offset was 1.9 mm, the average tail offset was 2.0 mm, and the average angular offset was 2.6°.

CONCLUSION

Robotic-assisted surgery allowed for accurate implantation of pedicle screws on immediate adoption of this technique. There were no complications attributable to the robotic technique, and no hardware revisions were required.

Robot-Assisted Thoracolumbar Fixation After Acute Spinal Trauma: A Case Series

BACKGROUND

Pedicle screw fixation has become the workhorse for the stabilization of the thoracolumbar spine. Since accurate pedicle screw placement is necessary for a successful surgery, three-dimensional navigation has become a mainstay for placing pedicle screws. However, the published studies have an overrepresentation of lumbar screws despite the prevalence of thoracic fractures. Furthermore, no robotic-assisted pedicle screw study has focused solely on traumatic fractures. The goal of this study was to address whether (1) robot-assisted pedicle screw placement had comparable accuracy in the thoracic and thoracolumbar region and (2) robot-assisted spine surgery was feasible in an acute, traumatic setting.

METHODS

We performed 14 consecutive, thoracolumbar spinal stabilization procedures in which 126 pedicle screws were placed using the Globus ExcelsiusGPS® spine robot in an acute, traumatic setting. Operative times were measured, and the accuracy of pedicle screws was assessed with the Gertzbein and Robbins classification system by two board-certified neuroradiologists.

RESULTS

A total of 60-thoracic (T3-T11), the 24-thoracolumbar junction (T12-L1), 40-lumbar (L2-L5), and two-sacral pedicle screws were placed. Pedicle screw placement was accurate with a < 1% (1/126) pedicle breach rate. Thoracolumbar robotic spine surgery in an acute, traumatic setting was demonstrated to have a good safety profile with only one minor neurological deficit which was related to positioning. Furthermore, surgical times were inversely related to the case number.

CONCLUSIONS

These results together suggest that robot-assisted spine surgery is accurate in the thoracic spine. Furthermore, placement of thoracolumbar screws in an acute trauma is non-inferior to other methods when based on accuracy.

Morphometric analysis of thoracolumbar junction (T11-L2) in central Indian population: A computerized tomography based study of 800 vertebrae

OBJECTIVE

To determine various morphometric parameters like transverse and sagittal pedicle width; interpedicular distance; antero-posterior and transverse canal diameter and canal surface area at thoracolumbar junction (T11, T12, L1, L2) in central Indian population and compare results with similar studies available in literature.

MATERIAL AND METHODS

A prospective, computerized tomography scan based morphometric analysis of thoracolumbar junction was conducted at medical college and tertiary care centre in central India. All asymptomatic cases more than 18 years age with normal lateral radiograph and CT scan of thoracolumbar junction and free from any spinal pathology or trauma were included in the study. Parameters measured were transverse and sagittal pedicle width; interpedicular distance; antero-posterior and transverse canal diameter and canal surface area at thoracolumbar junction (T11, T12, L1, L2).

RESULTS

Mean transverse pedicle width was maximum at T11 and minimum at L1 in both males and females, whereas sagittal width was maximum at T11 and minimum at L2 in both the groups. Interpedicular distance was largest at L1 in both the groups. All the measurements were significantly different (P < 0.05) in males and females. Mean antero-posterior and transverse diameter was maximum at T12 and L2 respectively in both male and female study population. Canal surface area was maximum at L1 among males (230.10 mm²) as well as females (209.02 mm²).

CONCLUSION

There is significant variation in morphometric parameters of thoracolumbar junction in different races and population. Thorough knowledge of morphometry of a particular population is essential for dealing with pathology or trauma of thoracolumbar junction.

Freehand pedicle screw fixation: A safe recipe for dorsal, lumbar and sacral spine

Objective:

To determine outcome of freehand pedicle screw fixation for dorsal, lumbar and sacral fractures at a tertiary care centre in the developing world.

Methods:

A retrospective review was performed of 150 consecutive patients who underwent pedicle screw fixation from January 1, 2012 to 31^st December 2017. A total of 751 pedicle screws were placed. Incidence and extent of cortical breach by misplaced pedicle screw was determined by review of intra-operative and post-operative radiographs and/or computed tomography.

Results:

Among the total 751 free hand placed pedicle screws, four screws (0.53%) were repositioned due to a misdirected trajectory towards the disc space. six screws (0.79%) were identified to have cause moderate breach while four screws (0.53%) cause severe breach. There was no occurrence of iatrogenic nerve root damage or violation of the spinal canal.

Conclusion:

Free hand pedicle screw placement based on external landmarks showed remarkable safety and accuracy in our center. The authors conclude that assiduous adherence to technique and preoperative planning is vital to success.

Nutrient foramen location on the laminae provides a landmark for pedicle screw entry: a cadaveric study

Background

Nutrient foramina are often encountered around the entry point of pedicle screws. Further, while probing the pedicle for pedicle screw insertion around the nutrient foramen, bleeding from the probe insertion hole is often observed. The purpose of this study was to investigate the frequency of occurrence of nutrient foramina, the association between the nutrient foramen and pedicle, and the safety and accuracy of cervical and thoracic pedicle screw placement using the nutrient foramen as the entry point.

Methods

We identified the location of the nutrient foramina for the dorsal branches of the segmental artery and their anatomical association to the pedicles and bony landmarks in the vertebrae for C3–T12 in seven cadavers. We also determined the frequency with which the nutrient foramina were present in 119 cadaveric vertebrae. We identified the pedicle location, base of the superior articular facet, and lateral border of laminae with respect to the nutrient foramen.

Results

The overall presence of the nutrient foramina was 63% (150/238) in the specimens, with 60% (42/70) and 64% (108/168) identifiable in the cervical and thoracic vertebrae, respectively. In the cervical vertebrae, the nutrient foramen was located on the outer wall of the pedicle and was positioned between the cephalad and caudal walls. In the thoracic spine, 98% (106/108) nutrient foramina were located inside the pedicle walls.

Conclusions

Our study findings confirm that the location of the nutrient foramen can be used for identifying the entry point for pedicle screws. In the cervical vertebrae, the nutrient foramina are located lateral to pedicle but within the cranial and caudal margins. In the thoracic vertebrae, the nutrient foramina are located in the medial and caudal regions of the pedicle. Thus, to decrease the risk of overshoot, the entry point for thoracic pedicle screws should be positioned a few millimeters cephalad and lateral to the nutrient foramen.

Computed tomographic-based morphometric study of thoracic spine and its relevance to anaesthetic and spinal surgical procedures

BACKGROUND

To collect a baseline computer software aided normative morphometric data of thoracic spine in the Indian population and analyze it to give pre-procedural guidelines to clinicians for safe surgical and anaesthetic procedures in the thoracic spine.

METHODS

CT scans of thoracic spine of patients free from spinal disorders were reviewed in a total of 600 vertebrae in 50 patients. Parameters recorded with the help of computer software were pedicle width, length and height, transverse pedicle angles, chord length, canal dimensions, body width and height, spinous process angle and transverse process length.

RESULTS

Pedicle width decreased from T1 (9.27 ± 1.01) to T4 (4.5 ± 0.93) and increased to T12 (8.31 ± 1.83). At T4 76% and at T5 62% of the pedicles were smaller than 5 mm and would not accept 4 mm screw with 1.0-mm clearance. However, at T1 2%, at T11 7% and at T12 8% would not accept a 4 mm screw. Chord length gradually increased in upper thoracic vertebrae and was relatively constant in middle and decreased in lower thoracic vertebrae. Shortest estimated chord length was at T1 (30.30 ± 2.11). On an average, from T1 to T6 and at T11 and T12, a screw length of 25-30 mm could be accommodated and from T7 to T10, 30-35 mm screw length could be accommodated. Transverse pedicle angle decreased from T1 (35.4 ± 2.21) to T12 (-9.8 ± 2.39). Canal dimensions were narrowest at T4/T5 (20.02 ± 1.23) in anteroposterior and 21.12 ± 1.23 in interpedicular diameters. Spinous process angle increased from T1 (30.11 ± 6.74) to T6 (57.89 ± 9.31) and decreased to 16.21 ± 7.38 at T12. Transverse process length increased from T1 to T7 (23.54 + 2.12 to 31.21 + 1.91) and then decreased to 12.11 + 2.3 at T12. Vertebral body dimensions showed increasing trends from T1 to T12.

CONCLUSIONS

A thorough knowledge of anatomical and radiological characteristics of the spine and their variations is essential for the clinicians. Data collected in the present study provides baseline normative values in Indian population and will help in guiding safe and effective completion of both surgical and anaesthetic procedures in the thoracic spine. Computer software aided morphometric data can help in selecting appropriate size and optimal placement of the implant with minimal procedural difficulties and complications during spine surgery.

Freehand Thoracic Pedicle Screw Placement: Review of Existing Strategies and a Step-by-Step Guide Using Uniform Landmarks for All Levels

Pedicle screw fixation in the thoracic spine presents certain challenges due to the critical regional neurovascular anatomy as well as the narrow pedicular corridor that typically exists. With increased awareness of the dangers of intraoperative radiation, the ability to place pedicle screws with anatomic landmarks alone is paramount. In this study, we reviewed the literature from 1990 to 2015 for studies that included freehand pedicle screw placement in the thoracic spine with special emphasis on entry points and the trajectories of the screws. We excluded studies that used fluoroscopy guidance, navigation techniques, cadaveric and biomechanical articles, case reports, and experimental studies on animals. The search retrieved 40 articles, and after careful selection, seven articles were analyzed. Over 8,000 screws were placed in the different studies. The mean accuracy for placement of the thoracic screws was 93.3%. However, there is little consensus between studies in entry points, sagittal, and axial trajectories of the screws.

We complete this review by presenting our step-by-step technique for the placement of freehand pedicle screws in the thoracic spine.

Morphology study of thoracic transverse processes and its significance in pedicle-rib unit screw fixation

BACKGROUND

Thoracic transverse process is an important anatomic structure of the spine. Several anatomic studies have investigated the adjacent structures of the thoracic transverse process. But there is still a blank on the morphology of the thoracic transverse processes. The purpose of the cadaveric study is to investigate the morphology of thoracic transverse processes and to provide morphology basis for the pedicle-rib unit (extrapedicular) screw fixation method.

METHODS

Forty-five adult dehydrated skeletons (T1-T10) were included in this study. The length, width, thickness, and the tilt angle (upward and backward) of the thoracic transverse process were measured. The data were then analyzed statistically. On the basis of the morphometric study, 5 fresh cadavers were used to place screws from transverse processes to the vertebral body in the thoracic spine, and then observed by the naked eye and on computed tomography scans.

RESULTS

The lengths of thoracic transverse processes were between 16.63±1.59 and 18.10±1.95 mm; the longest was at T7, and the shortest was at T10. The widths of thoracic transverse processes were between 11.68±0.80 and 12.87±1.48 mm; the widest was at T3, and the narrowest was at T7. The thicknesses of thoracic transverse processes were between 7.86±1.24 and 10.78±1.35 mm; the thickest was at T1, and the thinnest was at T7. The upward tilt angles of thoracic transverse processes were between 24.9±3.1 and 3.0±1.56 degrees; the maximal upward tilt angle was at T1, and the minimal upward tilt angle was at T7. The upward tilt angles of T1 and T2 were obviously different from the other thoracic transverse processes (P<0.01). The backward tilt angles of thoracic transverse processes gradually increased from 24.5±2.91 degrees at T1 to 64.5±5.12 degrees at T10. The backward tilt angles were significantly different between each other, except between T5 and T6. In the validation study, screws were all placed successfully from transverse processes to the vertebrae of thoracic spine.

CONCLUSIONS

The length, width, and thickness of the thoracic transverse processes are suitable for screw placement. And the obvious upward and backward tilt angles provide an excellent screw passage from transverse process to the vertebral body. Screw placement from the transverse processes to the vertebral body is feasible in the thoracic spine. However, there is still some place for improvement of the pedicle-rib unit screw fixation method.

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.

Freehand thoracic pedicle screw technique using a uniform entry point and sagittal trajectory for all levels: preliminary clinical experience

Object

Experience with freehand thoracic pedicle screw placement is well described in the literature. Published techniques rely on various starting points and trajectories for each level or segment of the thoracic spine. Furthermore, few studies provide specific guidance on sagittal and axial trajectories. The goal of this study was to propose a uniform entry point and sagittal trajectory for all thoracic levels during freehand pedicle screw placement and determine the accuracy of this technique.

Methods

The authors retrospectively reviewed postoperative CT scans of 33 consecutive patients who underwent open, freehand thoracic pedicle-screw fixation using a uniform entry point and sagittal trajectory for all levels. The same entry point for each level was defined as a point 3 mm caudal to the junction of the transverse process and the lateral margin of the superior articulating process, and the sagittal trajectory was always orthogonal to the dorsal curvature of the spine at that level. The medial angulation (axial trajectory) was approximately 30° at T-1 and T-2, and 20° from T-3 to T-12. Breach was defined as greater than 25% of the screw diameter residing outside of the pedicle or vertebral body.

Results

A total of 219 thoracic pedicle screws were placed with a 96% accuracy rate. There were no medial breaches and 9 minor lateral breaches (4.1%). None of the screws had to be repositioned postoperatively, and there were no neurovascular complications associated with the breaches.

Conclusions

It is feasible to place freehand thoracic pedicle screws using a uniform entry point and sagittal trajectory for all levels. The entry point does not have to be adjusted for each level as reported in existing studies, although this technique was not tested in severe scoliotic spines. While other techniques are effective and widely used, this particular method provides more specific parameters and may be easier to learn, teach, and adopt.

ASTM F1717 standard for the preclinical evaluation of posterior spinal fixators: Can we improve it?

Preclinical evaluation of spinal implants is a necessary step to ensure their reliability and safety before implantation. The American Society for Testing and Materials reapproved F1717 standard for the assessment of mechanical properties of posterior spinal fixators, which simulates a vertebrectomy model and recommends mimicking vertebral bodies using polyethylene blocks. This set-up should represent the clinical use, but available data in the literature are few. Anatomical parameters depending on the spinal level were compared to published data or measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing implant design were considered, and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by considering either the combination of the average values for each parameter or their worst-case combination depending on the spinal level. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The stress on the pedicular screw is significantly influenced by the lever arm of the applied load, the unsupported screw length, the position of the centre of rotation of the functional spine unit and the pedicular inclination with respect to the sagittal plane. The worst-case combination of parameters demonstrates that devices implanted below T5 could potentially undergo higher stresses than those described in the standard suggestions (maximum increase of 22.2% at L1). We propose to revise F1717 in order to describe the anatomical worst case condition we found at L1 level: this will guarantee higher safety of the implant for a wider population of patients.

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.

Pedicle screw reinsertion using previous pilot hole and trajectory does not reduce fixation strength

STUDY DESIGN

Fresh-frozen human cadaveric biomechanical study.

OBJECTIVE

To evaluate the biomechanical consequence of pedicle screw reinsertion in the thoracic spine.

SUMMARY OF BACKGROUND DATA

During pedicle screw instrumentation, abnormal appearance on fluoroscopic imaging or low current reading with intraoperatively evoked electromyographic stimulation of a pedicle screw warrants complete removal to reassess for pedicle wall violation or screw malposition. However, screw fixation strength has never been evaluated biomechanically after reinsertion using a previous pilot hole and trajectory.

METHODS

Thirty-one thoracic individual fresh-frozen human cadaveric vertebral levels were instrumented bilaterally with 5.5-mm titanium polyaxial pedicle screws, and insertional torque (IT) was measured with each revolution. A paired comparison was performed for each level. Screw reinsertion was performed by completely removing the pedicle screw, palpating the tract, and then reinserting along the same trajectory. Screws were tensile loaded to failure "in-line" with the screw axis.

RESULTS

There was no significant difference for pedicle screw pullout strength (POS) between reinserted and control screws (732 ± 307 N vs. 742 ± 320 N, respectively; P = 0.78). There was no significant difference in IT between initial insertion for the test group (INI) (0.82 ± 0.40 N·m) and control (0.87 ± 0.50 N·m) (P = 0.33). IT for reinserted screws (0.58 ± 0.47 N·m) had significantly decreased compared with INI and control screws (29% decrease, P = 0.00; 33% decrease, P = 0.00, respectively). The test group screws in the thoracic spine had significant correlations between initial IT and POS (r = 0.79, P = 0.00), and moderate correlations between reinsertion IT and POS in the thoracic spine (r = 0.56, P = 0.00).

CONCLUSION

Despite a significant reduction in pedicle screw IT, there was no significant difference in pedicle screw POS with reinsertion. Therefore, when surgeons must completely remove a pedicle screw for tract inspection, reinsertion along the same trajectory may be performed without significantly compromising fixation strength.

LEVEL OF EVIDENCE

N/A.

CT provides precise size assessment of implanted titanium alloy pedicle screws

BACKGROUND

After performing instrumented spinal fusion with pedicle screws, postoperative imaging using CT to assess screw position may be necessary. Stainless steel implants produce significant metal artifact on CT, and the degree of distortion is at least partially dependent on the cross-sectional area of the implanted device. If the same effect occurs with titanium alloy implants, ability to precisely measure proximity of screws to adjacent structures may be adversely affected as screw size increases.

QUESTIONS/PURPOSES

We therefore asked whether (1) CT provides precise measurements of true screw widths; and (2) precision degrades based on the size of the titanium implant imaged.

METHODS

CT scans performed on 20 patients after instrumented spinal fusion for scoliosis were reviewed. The sizes of 151 titanium alloy pedicle screws were measured and compared with known screw size. The amount of metal bloom artifact was determined for each of the four screw sizes. ANOVA with Tukey's post hoc test were performed to evaluate differences in scatter, and Spearman's rho coefficient was used to measure relationship between screw size and scatter.

RESULTS

All screws measured larger than their known size, but even with larger 7-mm screws the size differential was less than 1 mm. The four different screw sizes produced scatter amounts that were different from each other (p < 0.001).The amount of metal bloom artifact produced does increase as the size of the screw increases (rho = 0.962, p < 0.001).

CONCLUSIONS

CT of titanium alloy pedicle screws produces minimal artifact, thus making this the preferred imaging modality to assess screw position after surgery. Although the amount of artifact increases with the volume of titanium present, the degree of distortion is minimal and is usually less than 1 mm.

Establishment of parameters for congenital thoracic stenosis: a study of 700 postmortem specimens

BACKGROUND

Congenital thoracic stenosis (CTS) occurs when the bony anatomy of the canal is smaller than expected in the general population. The diagnosis currently is made based on the clinical impression from subjective radiographic studies, and the normal values for CTS have not been established.

QUESTIONS/PURPOSES

We provided a statistical definition for CTS based on objective measurements of thoracic spine specimens and explored parameters that might predict CTS.

METHODS

We selected 700 adult skeletal specimens from the Hamann-Todd Collection in the Cleveland Museum of Natural History (Cleveland, OH, USA). We used calipers to measure the sagittal canal diameter (SCD), interpedicle distance (IPD), and pedicle length (PL). At each level, canal area was calculated using a geometric formula, a standard distribution was created, and values two SDs below the mean were considered congenitally stenotic. Corresponding values of SCD and IPD of the stenotic specimens were studied. The values of SCD and IPD predicting CTS with highest sensitivity and specificity were tabulated.

RESULTS

At each level, CTS was defined as: T1, 160 mm(2); T2, 135 mm(2); T3, 131 mm(2); T4, 130 mm(2), T5, 129 mm(2), T6, 127 mm(2); T7, 127 mm(2); T8, 129 mm(2); T9, 130 mm(2); T10, 132 mm(2); T11, 140 mm(2); and T12, 173 mm(2). A SCD less than 15 mm and an IPD less than 18.5 mm were predictive of CTS at each level with sensitivities and specificities of 80% to 100%.

CONCLUSIONS

We statistically defined CTS at each level. A SCD less than 15 mm or IPD less than 18.5 mm predicted the presence of CTS at all levels.

CLINICAL RELEVANCE

In a symptomatic patient, on routine radiologic examination, a physician should suspect stenosis of the thoracic canal if the SCD and IPD are less than 15 and 18.5 mm respectively. As a spinal deformity surgeon, the canal area is especially relevant when considering a possible canal intrusion by implants.

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

STUDY DESIGN

A three-dimensional study of the thoracic pedicle (T1-T12) morphometry in Chinese patients with different body height, using reformatted computed tomography (CT).

OBJECTIVE

To quantify the dimensions of the thoracic pedicles and to analyze the relationship between body height and thoracic pedicle parameters.

SUMMARY OF BACKGROUND DATA

The thoracic pedicle morphometry has been studied extensively in different populations using various techniques. Previous studies have shown a significantly smaller size of the thoracic pedicles in women than in men and in Asians than in Caucasians. Some authors postulated that it is the body height that contributes to the variation in the pedicle size. To our knowledge, however, no study has specifically analyzed the relationship between body height and thoracic pedicle parameters in detail.

METHODS

In this study, T1 to T12 vertebrae were imaged in 126 Chinese patients by a Lightspeed Vct CT (General Electric, Bridgeport, Connecticut, USA). After reformatting the original images, the following parameters were calculated: outer pedicle width, outer pedicle height and pedicle cortical thickness of the pedicle isthmus, pedicle length, and transverse pedicle angle. All measured data were statistically analyzed by the independent t test and Pearson correlation test using SPSS software (SPSS Inc, Chicago, IL).

RESULTS

The thoracic pedicle parameters were significantly smaller in women than in men except for the transverse pedicle angles and the pedicle cortical thickness. The percentage of outer pedicle widths less than 4.5 mm was extremely high at T3 to T9 in females and T4 to T7 in males. There was a much higher percentage of pedicle width of 4.5 mm or lesser, 4.0 mm or lesser, and 3.5 mm or lesser when body height was less than 160 cm. Body height is probably one of the main factors that contribute to the variation in pedicle size since a significant positive correlation was observed between pedicle size and body height.

CONCLUSION

Body height is probably one of the main factors that contribute to the variation in pedicle size among different ethnic and sex groups. Transpedicular procedures using a 4.5-mm screw may not be applicable to much of the Chinese population at the upper and middle thoracic segments, especially for patients less than 160 cm in height. A reformatted CT evaluation is routinely recommended not only for choosing the proper screw but also for determining the feasibility of a true transpedicular procedure.

Morphometric measurements of cadaveric thoracic spine in Indian population and its clinical applications

STUDY DESIGN

Analysis of morphometric data obtained from direct measurements of 100 cadaveric thoracic spines in Indian population.

PURPOSE

To collect a base line morphometric data and analyze it in reference to the musculoskeletal anatomy and biomechanics of the spine; implants and instrumentations; and to suggest the requisite modification in spinal surgery instrumentations.

OVERVIEW OF LITERATURE

Most of the previous studies in the world literature have focused primarily on the parameters of the pedicle and to the authors' knowledge; no study has been published from the Indian subcontinent reporting a detailed morphometry of the thoracic spine.

METHODS

One thousand and two hundred thoracic vertebrae were studied by direct measurements for linear and angular dimensions of the vertebral body, spinal canal, pedicle, and spinous and transverse processes in 100 human cadavers.

RESULTS

Thirty-five point five percent of all the pedicles; 71% of T5 pedicles; 54.6% of all the female pedicles; and 94.4% of the T5 pedicles in females were smaller than 5 mm in mid-pedicle width dimension. Transverse pedicle angle was more at all levels and pedicles were sagittaly angulated in cephalad direction in comparison to other studies. Minimum value of interpedicular distance was at T5 (15.48 ± 1.24). Vertebral body width showed slight decrease from T1 to T4. The transverse process length was relatively constant between T2 to T10. The spinous process angle showed increasing trend from T1 to T6 and then gradually decreased to T12.

CONCLUSIONS

Most of the trends in changes of the parameters from T1 to T12 can be explained on the basis of local musculoskeletal anatomy and biomechanical stresses. The smallest diameter screw and shortest available screw for adults may not be safe in majority of the Indian population in mid-thoracic region. The results of the present study can help in designing implants and instrumentations; understanding spine pathologies; and management of spinal disorders in this part of the world.

Thoracic pedicle screw insertion in Asian cadaveric specimen: does radiological pedicle profile affect outcome?

PURPOSE

Pedicle screw instrumentation has superior biomechanical as well as clinical outcome. Thoracic pedicles show great variation in different population groups, particularly in Asians who have been shown to have smaller pedicle dimensions. Although plain radiographs are widely performed prior to spine surgery, no studies have been done so far to investigate whether the thoracic pedicle profile on plain radiographs affect thoracic pedicle screw insertion. Therefore, this is a cadaveric study aimed to determine the relationship between plain radiographic thoracic pedicle profile in Asians and the outcome of pedicle screw insertion in the thoracic spine.

METHODS

A pre-insertion radiograph with an enlargement reference scale was performed and surgeons were blinded to the plain radiographic morphometry of the thoracic pedicles. From the pre-insertion radiograph, the normalized pedicle width and height (which controls for any magnification error) as well as the pedicle width:body width (PWBW) and pedicle width:pedicle height (PWPH) ratio was derived. 240 pedicle screws were inserted in ten Asian cadavers from T1 to T12 using the funnel technique. 5.0 mm screws were used from T1 to T6 while 6.0 mm screws were used from T7 to T12. Perforations were detected by direct visualization via wide laminectomies after pedicle screw insertion. The outcome of thoracic pedicle screw insertion was correlated with the radiological profile using independent t-test. Pearson correlation coefficient was used to investigate the correlation between the ratios and the normalised pedicle width and height.

RESULTS

The narrowest pedicle width is from T3 to T6 determined from normalized measurement of the pedicle width. T5 pedicle width is the smallest measuring 4.1 ± 1.3 mm. The overall perforation rate is 10.4% (25 perforations). There is only one significant perforation. There were twice as many lateral and inferior perforations compared to the medial perforations. 48% of the perforations occurred at T1, T2 and T3. Pedicles <4.0 mm in width and upper thoracic pedicles are risk factors for pedicle perforation. The normalised pedicle width has a high degree of linear correlation with PWBW and a normalised pedicle width of 4.0 mm correlated with a PWBW ratio of 0.3.

CONCLUSIONS

Plain radiographic thoracic pedicle morphometry has an influence on the outcome of thoracic pedicle screw insertion in Asian cadavers. Pedicle width <4.0 mm is associated with higher risk of pedicle perforation. This critical value corresponds to a PWBW ratio of 0.3. Identification of such pedicle profile warrants full evaluation of the morphometry of the thoracic pedicles and possibly extra-pedicular techniques should be employed to avert the risk of critical pedicle perforation.

Pedicle screw placement in the thoracic spine: a comparison study of computer-assisted navigation and conventional techniques

The technique of computer-assisted pedicle screw installation and its clinical benefit as compared with conventional pedicle screw installation was evaluated. Twenty-two patients had thoracic screw insertion under 3-dimentional computer-assisted navigation (92 screws) and 20 patients under conventional fluoroscopic control (84 screws). The 2 groups were compared for accuracy of screw placement, screw insertion time by postoperative thin-cut computed tomography scans, and statistical analysis. The cortical perforations were graded by 2-mm increments. In the computer group, 88 (95.65%) were grade I (good), 4 (4.35%) were grade II (<2 mm), and 0 were grade III (>2 mm) violations. There were 4 cortical violations (3.57%). In the conventional group, there were 14 cortical violations (16.67%), 70 (83.33%) were grade I (good), 11 (13.1%) were grade II (<2 mm), and 3 (3.57%) were grade III (>2 mm) violations (P<.001). The number (19.57%) of upper thoracic pedicle screws (T1-T4) inserted under 3-dimensional computer-assisted navigation was significantly higher than that (3.57%) by conventional fluoroscopic control (P<.001). Average screw insertion time in the conventional group was more than in the computer group (P<.001). Three-dimensional computer-assisted navigation pedicle screw placement can increase accuracy, reduce surgical time, and be performed safely and effectively at all levels of the thoracic spine, particularly the upper thoracic spine.

Thoracic pedicle subtraction osteotomy for fixed sagittal spinal deformity

STUDY DESIGN.: A retrospective clinical study. OBJECTIVE.: To find the corrective capacity of a thoracic pedicle subtraction osteotomy (PSO), determine if segmental correction is dependent on level, and to compute the impact of thoracic PSO on regional and global spinal balance. SUMMARY OF BACKGROUND DATA.: PSO is a technique popularized in the lumbar spine primarily for the correction of fixed sagittal imbalance. Despite several studies describing the clinical and radiographic outcome of lumbar PSO, there is no study in literature reporting its application in the thoracic spine. METHODS.: We retrospectively analyzed patients with fixed thoracic kyphosis who underwent thoracic PSOs for sagittal realignment. Segmental pedicle screw instrumentation and intraoperative neurophysiologic monitoring was used in all patients. Data acquisition was performed by reviewing medical charts and radiographs to determine sagittal correction (segmental/regional/global) and complications. Clinical outcome using the Scoliosis Research Society-22 (SRS-22) instrument was determined by interview. RESULTS.: A total of 25 thoracic PSOs were performed (mean: 1.7 PSOs/patient, range: 1-3) in 15 patients (9 M/6 F). The study population had an average age of 56 years (range, 36-81 years) and was followed up after surgery for a mean of 3.5 years (range, 24-75 months). The osteotomies were carried out in the proximal thoracic spine (T2-T4, n = 6), midthoracic spine (T5-T8, n = 12), and distal thoracic spine (T9-T12, n = 7). Mean correction at the PSO for all 25 levels was 16.3 degrees +/- 9.6 degrees . Stratified by region of the spine, thoracic PSO correction was as follows: T2-T4 = 10.7 degrees +/- 15.8 degrees , T5-T8 = 14.7 degrees +/- 4.6 degrees , and T9-T12 = 23.9 degrees +/- 4.1 degrees . Mean thoracic kyphosis (T2-T12 Cobb angle) was improved from 75.7 degrees +/- 30.9 degrees to 54.3 degrees +/- 21.4 degrees resulting in a significant regional sagittal correction of 21.4 degrees +/- 13.7 degrees (P < 0.005). Global sagittal balance was improved from 106.1 +/- 56.6 to 38.8 +/- 37.0 mm yielding a mean correction of 67.3 +/- 54.7 mm (P < 0.005). One patient, in whom there was segmental translation during osteotomy closure, had a decline in intraoperative somatosensory-evoked potentials. No patient sustained a temporary or permanent neurologic deficit after surgery. The mean SRS-22 Questionnaire score at final follow-up was 82.4 +/- 10.2. CONCLUSION.: Thoracic PSO can be performed safely. Segmental sagittal correction appears to vary based on the region of the thoracic spine the PSO is performed. The distal thoracic segments, which more closely resemble lumbar segments in morphology, rendered the greatest sagittal correction after PSO, approximately 24 degrees . There was no case of neurologic injury associated with thoracic PSO, and clinical outcomes according to the SRS-22 instrument were generally favorable.

Accuracy of upper thoracic pedicle screw placement using three-dimensional image guidance

BACKGROUND CONTEXT

Pedicle screw malposition rates using conventional techniques have been reported to occur with a frequency of 6% to 41%. The upper thoracic spine (T1-T3) is a challenging area for pedicle screw placement secondary to the small size of the pedicles, the inability to visualize this area with lateral fluoroscopy, and significant consequences for malpositioned screws. We describe our experience placing 150 pedicle screws in the T1-T3 levels using three-dimensional (3D) image guidance.

PURPOSE

The aim of this study was to assess the accuracy of 3D image guidance for placing pedicle screws in the first three thoracic vertebrae.

STUDY DESIGN

The accuracy of pedicle screw placement in the first three thoracic vertebrae was evaluated using postoperative thin-section computed tomography (CT) scans of the cervicothoracic region.

PATIENT SAMPLE

Thirty-four patients who underwent cervicothoracic fusion were included.

OUTCOME MEASURES

Radiological investigation with CT scans was performed during the postoperative period.

METHODS

Thirty-four consecutive patients underwent cervicothoracic instrumentation and fusion for a total of 150 pedicle screws placed in the first three thoracic vertebrae. All screws were placed using 3D image guidance. Medical records and postoperative imaging of the cervicothoracic junction for each patient were retrospectively reviewed. An independent radiologist reviewed the placement of the pedicle screws and assessed for pedicle breach. All cortical violations were reported as Grade 1, 0 to 2 mm; Grade 2, 2 to 4 mm; and Grade 3, greater than 4 mm.

RESULTS

Overall, 140 (93.3%) out of 150 screws were contained solely in the desired pedicle. All 10 pedicle violations were Grade 1. The direction of pedicle violation included three medial, four inferior, two superior, and one minor anterolateral vertebral body. No complication occurred as a result of screw placement or the use of image guidance.

CONCLUSIONS

Upper thoracic pedicle screw placement is technically demanding as a result of variable pedicle anatomy and difficulty with two-dimensional visualization. This study demonstrates the accuracy and reliability of 3D image guidance when placing pedicle screws in this region. Advantages of this technology in our practice include safe and accurate placement of spinal instrumentation with little to no radiation exposure to the surgeon and operating room staff.

Thoracic pedicle screws: comparison of start points and trajectories

STUDY DESIGN

Experimental design using cadaveric computerized tomography (CT) scans and a computer-assisted image guidance system to compare various thoracic pedicle screw start points and trajectories.

OBJECTIVE

To compare described thoracic pedicle screw start points and trajectories to determine which allows strictly intrapedicular screw placement with the most margin of error.

SUMMARY OF BACKGROUND DATA

Thoracic pedicle screws are being used in a variety of spinal conditions to include fracture, tumor, and deformity. Optimal thoracic pedicle screw start points have received increasing attention in the literature. Optimal thoracic pedicle trajectory is still undetermined.

METHODS

Using fine cut CT scans of 3 cadaveric male specimens (aged 65-70 years) loaded onto a computer-assisted image guidance system, 966 pedicle screws, were virtually inserted. The effective pedicle diameter (EPD) and maximum insertional arc (MIA) was assessed using 3 different trajectories and start points: (1) straight ahead, (2) straight forward, and (3) anatomic. EPD was measured by placing a maximum-sized virtual screw, using a specific trajectory, without cortical violation of the pedicle and/or the vertebral body. The MIA was assessed by measurement of the angle formed by the most superiorly and inferiorly directed 0.1-mm virtual screw through a given start point without violation of the pedicle cortex and obtaining at least 50% vertebral body purchase.

RESULTS

Mean EPD in the sagittal plane was 7.6 +/- 0.3 (SEM) mm for the straight forward trajectory and 9.1 +/- 0.3 (SEM) mm for the anatomic trajectory, a 20% increase (P < 0.0005). Mean EPD in the axial plane was 4.1 +/- 0.2 (SEM) mm for the straight ahead trajectory and 5.0 +/- 0.2 (SEM) mm for the anatomic trajectory, a 22% increase (P < 0.0005). EPD was found to be statistically different based on the trajectory used for placement in both the axial and sagittal planes in the upper (T1-T4), middle (T5-T8), and lower (T9-T12) thoracic spine. Mean MIA in the sagittal plane was 18.7 +/- 1.1 (SEM) for straight ahead start points, 25.8 degrees +/- 0.8 degrees (SEM) for straight forward start points, and 30.2 degrees +/- 0.8 degrees (SEM) for anatomic start points, a 38% increase (P < 0.0005) in MIA compared with straight ahead and a 17% increase (P < 0.0005) in MIA compared with straight forward. Mean MIA in the axial plane was 17.8 degrees +/- 0.6 degrees (SEM) for straight ahead and anatomic start points, and 18.6 degrees +/- 0.6 degrees (SEM) for straight forward start points. This difference was not statistically significant (P = 0.086). MIA was found to be statistically different based on start points used in the sagittal, but not the axial plane, in the upper, middle, and lower thoracic spine.

CONCLUSION

EPD and MIA are trajectory (EPD) and start point (MIA) dependent. In the axial plane, anatomic EPD was greater than straight ahead EPD. In the sagittal plane, anatomic EPD was greater than straight forward EPD. Using anatomic start points in the sagittal plane, a greater MIA is achievable. These data suggest that in the diminutive thoracic pedicle or when a larger screw is needed, an anatomic trajectory using anatomic start points may allow a larger bone channel for intrapedicular placement of instrumentation.

SEGMENTAL SPINAL INSTRUMENTATION IN THE MANAGEMENT OF SCOLIOSIS

SPINAL INSTRUMENTATION FOR the correction of spinal deformity began with Harrington and his rod system. The use of the Harrington rods was limited, however, because of the need for long-segment instrumentation, distraction, and the potential for hood dislodgment and construct failure. Luque subsequently introduced the next generation of spinal fixation techniques via the concept of segmental instrumentation; his use of sublaminar wires allowed the construct to be fixated to the spine at every level. This arrangement allowed greater control over correction of spinal deformities and significantly lessened the incidence of hardware dislodgment. Modern instrumentation systems, including the use of pedicle screws, permit even greater control of the spine in multiple planes during deformity correction. Newer strategies have decreased the incidence of neurological injury during implant application and provide greater stability. A review of segmental fixation, including surgical techniques, is provided in this article.

Computer-assisted pedicle screw placement for thoracolumbar spine fracture with separate spinal reference clamp placement and registration

BACKGROUND

The objective of the study was to improve the accuracy of computer-assisted pedicle screw installation in the spine. This study evaluates the accuracy of computer-assisted pedicle screw placement with separate spinal reference clamp placement and registration on each instrumented vertebra for thoracolumbar spine fractures.

METHODS

Postoperative radiographs and CT scans assessed the accuracy of pedicle screw placement in 21 adult patients on each instrumented vertebra. Screw placements were graded as good if the screws were placed in the central core of the pedicle and the cancellous portion of the body. Screw placements were graded as fair if the screws were placed slightly eccentrically, causing erosion of the pedicular cortex, and with less than a 2-mm perforation of the pedicular cortex. Screw placements were graded as poor if screws were placed eccentrically with a large portion of the screw extending outside the cortical margin of the pedicle and with more than a 2-mm perforation of the pedicular cortex.

RESULTS

A total of 140 image-guided pedicle screws were placed in 21 patients: 78 in the thoracic and 62 in the lumbar spine. Of the 140 pedicle screw placements, 96.4% (135/140) were categorized as good; 3.6% (5/140), fair; and 0% were poor. All 5 fair placement screws were placed in the thoracic spine without any mobility.

CONCLUSION

Separate registration increases accuracy of screw placement in thoracolumbar pedicle instrumentation. Separate spinal reference clamp placement in the instrumented vertebra provides real-time virtual imaging that decreases the possibility of downward displacement during manual installation of the screw.

Randomized clinical study to compare the accuracy of navigated and non-navigated thoracic pedicle screws in deformity correction surgeries

STUDY DESIGN

Randomized clinical trial (level I evidence).

OBJECTIVE

To compare the accuracy of non-navigation and Iso-C based navigation in pedicle screw fixation in thoracic spine deformities.

SUMMARY OF BACKGROUND DATA

Thoracic pedicle screw insertion for spinal deformity correction can be associated with increased pedicle breaches. Iso-C based navigation has been reported to improve the accuracy of pedicle screw placement, but its use in the presence of deformity has not been reported.

METHODS

Twenty-seven patients with scoliosis and 6 patients with kyphosis had a total of 478 thoracic pedicle screws. The average Cobb angle was 58.4 degrees +/- 8 degrees (range 50 degrees -80 degrees), and the mean kyphotic angle was 54.6 degrees +/- 4 degrees (range 51 degrees -76 degrees). By random allocation, 17 patients had screw insertion under navigation (242 screws) and 16 under fluoroscopic control (236 screws). The 2 groups were compared for accuracy of screw placement, time for screw insertion, and the number of times the C-arm had to be brought into the field. Two independent blinded observers determined accuracy using postoperative computed tomography assessments.

RESULTS

There were 54 (23%) pedicle breaches in the non-navigation group as compared to only 5 (2%) in the navigation group (P < 0.001). Thirty-eight screws (16%) in the non-navigation group had penetrated the anterior or lateral cortex compared to 2 screws (0.8%) in the navigation group. Average screw insertion time in the non-navigation group was 4.61 +/- 1.05 minutes (range 1.8-6.5) compared to 2.37 +/- 0.72 minutes (range 1.16-4.5) in navigation group (P < 0.01). The C-arm had to be moved into the operation field on an average of 1.5 +/- 0.25 times (range 1-3) per screw. With single screening data, an average of 11.4 pedicles (range 9-14) could be visualized without necessity to bring the C-arm into operating field again.

CONCLUSIONS

Iso-C navigation increases accuracy, and reduces surgical time and radiation in thoracic deformity correction surgeries.

Morphometric analysis of the thoracic and lumbar spine in Japanese on the use of pedicle screws

The pedicle screw and hook have become popular instruments in treating spinal deformity and disease. This study gathered morphological data on thoracic and lumbar spines in a Japanese population that should serve as useful reference for posterior instrumentation surgery. One hundred and three dry bones were used to investigate the morphology of pedicle and facet in thoracic and lumbar spines. Measurements included the diameter and axial length of pedicle from T8 to L5, height and width of facets and thickness of articular processes from T1 to T12, and axial angle of pedicle from T1 to L5. The diameter and axial length of pedicle were smallest at T8, diameter was largest at L5 and axial length was largest at L3. Height of facets and thickness of articular processes were largest at T12. Men tended to have larger pedicles and facets than women. Transverse angle of pedicle was smallest at T12. These precise data may provide useful information when performing posterior instrumentation surgery and when developing new spinal implant systems for Asians.

Probing for thoracic pedicle screw tract violation(s): is it valid?

BACKGROUND

Preparation of the thoracic pedicle screw tract is a critical step prior to the placement of screws. The ability to detect pedicle wall violation(s) by probing prior to insertion of thoracic pedicles screws, however, has not been studied. The purpose of this study was to evaluate the inter- and intraobserver agreement and the accuracy in detecting thoracic pedicle screw tract violation(s) among surgeons at various levels of training.

METHODS

With use of a straightforward trajectory, under direct visualization, 108 thoracic pedicle screw tracts (54 cadaveric thoracic vertebrae) were prepared in a standard fashion, followed by tapping with a 4.5-mm cannulated tap. A deliberate pedicle violation was randomly created by an independent investigator in either the anterior, the medial, or the lateral wall in 65 pedicles. Following this, four blinded, independent surgeons at various levels of training probed the specimens on three separate occasions to determine if a breach was present (1,296 discrete data points). Surgeon findings were then recorded as breach present or absent and, if present, breach location. The Cohen kappa correlation coefficient (kappa a) and 95% confidence interval were used to assess the accuracy of the observers and the inter- and intraobserver agreement.

RESULTS

The mean accuracy over three iterations, the validity in detecting the breach location, and the intraobserver agreement varied by level of training and experience, with the most experienced observer (observer 1) scoring the best and the least experienced observer (observer 4) scoring the worst. The three most senior surgeons had good intraobserver agreement. Interobserver agreement was low between the four observers.

CONCLUSIONS

An observer's ability to accurately detect the presence or absence of a pedicle tract violation and the breach location, if present, is dependent on the surgeon's level of training. Probing the pedicle tract prior to placement of pedicle screws in the thoracic spine is likely a learned skill that improves with repetition and experience.

Morphometric study of the thoracic vertebral pedicle in an Indian population

STUDY DESIGN

Morphometric study of the thoracic vertebral pedicular parameters and comparison with the previous studies in the literature.

OBJECTIVES

To define pedicular dimensions and screw placement in the Indian population.

SUMMARY OF THE BACKGROUND DATA

Pedicular morphometric characteristics vary in different population groups. Thoracic pedicular screw fixation is being used more frequently with the advent of better imaging methods. Because of the small size and close proximity to the neurovascular structures, thoracic pedicle fixation has little margin of safety.

METHODS

T1-T12 vertebral pedicles were studied in 18 cadavers by direct, roentgenographic, and computerized tomographic scan methods. The following parameters were studied: transverse diameter, transverse angle, sagittal diameter, sagittal angle, chord length, interpedicular distance, pedicle entrance point, and pedicle cortical thickness (medial and lateral). RESULTS.: Transverse diameter was more than 6 mm at both ends of the thoracic spine (T1, T2, T11, and T12). Between T3-T9 levels, it was less than 5 mm at some levels. The transverse angle was widest at T1 (30 degrees) and was less than 5 degrees from T5 to T12. Pedicles were directed cephalad in the sagittal plane at all thoracic levels. Sagittal angle was less than 12 degrees at all thoracic levels. Chord length was largest at T11 (37.3 mm) and smallest at T1 and T2 (29.9 mm). Interpedicular distance was 29 mm at T12 and 21.3 mm at T1. Medial pedicular cortex was thicker than lateral cortex at all levels.

CONCLUSIONS

These results suggest that even a 4-mm screw should be used carefully at the midthoracic level; 5-mm screw seems to be safe at upper and lower thoracic spine. Because of very small sagittal and transverse angle at mid and lower thoracic levels, the pedicular screw should be inserted along almost perpendicular line in these planes; 25-mm and 30-mm screw length appears to be safe at upper thoracic and lower thoracic levels, respectively. Pedicle entrance point lies along the midtrans-verse line at upper thoracic levels and along the upper border of transverse process at lower thoracic levels.

Three-dimensional fluoroscopy-guided percutaneous thoracolumbar pedicle screw placement

✓ The authors sought to evaluate the feasibility and accuracy of three-dimensional (3D) fluoroscopic guidance for percutaneous placement of thoracic and lumbar pedicle screws in three cadaveric specimens.

After attaching a percutaneous dynamic reference array to the surgical anatomy, an isocentric C-arm fluoroscope was used to obtain images of the region of interest. Light-emitting diodes attached to the C-arm unit were tracked using an electrooptical camera. The image data set was transferred to the image-guided workstation, which performed an automated registration. Using the workstation display, pedicle screw trajectories were planned. An image-guided drill guide was passed through a stab incision, and this was followed by sequential image-guided pedicle drilling, tapping, and screw placement. Pedicle screws of various diameters (range 4–6.5 mm) were placed in all pedicles greater than 4 mm in diameter. Postoperatively, thin-cut computerized tomography scans were obtained to determine the accuracy of screw placement.

Eighty-nine (94.7%) of 94 percutaneous screws were placed completely within the cortical pedicle margins, including all 30 lumbar screws (100%) and 59 (92%) of 64 thoracic screws. The mean diameter of all thoracic pedicles was 6 mm (range 2.9–11 mm); the mean diameter of the five pedicles in which wall violations occurred was 4.6 mm (range 4.1–6.3 mm). Two of the violations were less than 2 mm beyond the cortex; the others were between 2 and 3 mm.

Coupled with an image guidance system, 3D fluoroscopy allows highly accurate spinal navigation. Results of this study suggest that this technology will facilitate the application of minimally invasive techniques to the field of spine surgery.

Measurements of the lumbar pedicles in the Eastern Anatolian population

Pedicle screw fixation of the spine has become one of the most stable and versatile methods of spine fixation, and knowledge of pedicle morphology is crucial for the safe application of these systems. We undertook this study because only a few reports have investigated Eastern populations. Lumbar pedicle anatomy, i.e., pedicle width (PW) and pedicle height (PH), transverse and sagittal pedicle angles (TPA, SPA), and pedicle length (PL), were assessed in the following two groups: (1) computed tomography scans of 29 normal adults, and (2) 16 dried lumbar spines obtained from the Anatomy Department. Interpedicular distance was different in each group. PW ranged from 4 mm to 14 mm. In both groups, the narrowest PH was 8.2 mm, the widest 19.7 mm. TPA ranged from 6(o) to 19(o) and increased from L1 to L5. In the sagittal plane, the pedicles angled caudally at L5. PL was longest at L1 and shortest at L5. In conclusion, pedicle dimensions and angles may show individual and structural differences. Our data were not significantly different from previous reported data. A detailed knowledge of these relationships is important for any surgery involving screw purchase via a pedicle, to prevent screw cutout and failure of fixation or neurological injury. Selection of the proper diameter of screw is an important issue for safe placement. Knowledge of the pedicle axis length is essential in choosing screw lengths but should always be checked intraoperatively with fluoroscopic control during screw insertion.

Accuracy of thoracic vertebral body screw placement using standard fluoroscopy, fluoroscopic image guidance, and computed tomographic image guidance: a cadaver study

STUDY DESIGN

A surgical simulation study in human cadaver spine specimens was conducted to evaluate the accuracy of thoracic vertebral body screw placement using four different intraoperative imaging techniques.

OBJECTIVE

To compare standard fluoroscopy, fluoroscopy-based image guidance with two different referencing methods, and computed tomography-based image guidance by the measuring the time required for screw placement, the radiation exposure to specimen and surgeon, and the accuracy of screw position in the thoracic spine.

SUMMARY OF BACKGROUND DATA

Image guidance provides additional anatomic information to the surgeon and may improve safety of technically difficult surgical procedures. The placement of screws in the thoracic spine is a technically demanding procedure in which inaccurate screw positioning places the spinal cord, nerve roots, and paraspinal structures such as the aorta and pleural space at risk for injury. Image-guided surgery may improve the accuracy of thoracic screw placement.

METHODS

Using four different intraoperative imaging methods, two experienced surgeons placed 337 vertebral body screws through the pedicles of thoracic vertebrae in 20 human cadaver thoracic spine specimens. The specimens then were examined with radiographs, computed tomography, and anatomic dissection to determine screw position. Measurements included procedure setup and screw insertion time, radiation exposure to the specimen, the surgeon's hand, the surgeon's body, frequency, direction, and magnitude of screw perforation through the cortical margins of thoracic vertebrae.

RESULTS

As compared with surgery using standard fluoroscopy, fluoroscopy-based image guidance that uses multiple reference marks and computed tomography-based image guidance improves the accuracy of thoracic vertebral body screws, but increases the time required for screw placement and the specimen radiation exposure. Exposure to radiation is minimal at the surgeon's body level and dependent on surgical technique at the surgeon's hand level. Screw perforation occurs most frequently in the lateral direction.

CONCLUSIONS

Fluoroscopy-based image guidance that uses only a single reference marker for the entire thoracic spine is highly inaccurate and unsafe. Systems with registration based on the instrumented vertebrae provide more accurate placement of thoracic vertebral body screws than standard fluoroscopy, but expose the patient to more radiation and require more time for screw insertion.

Stereotactic navigation for placement of pedicle screws in the thoracic spine

OBJECTIVE

Pedicle screw fixation in the lumbar spine has become the standard of care for various causes of spinal instability. However, because of the smaller size and more complex morphology of the thoracic pedicle, screw placement in the thoracic spine can be extremely challenging. In several published series, cortical violations have been reported in up to 50% of screws placed with standard fluoroscopic techniques. The goal of this study is to evaluate the accuracy of thoracic pedicle screw placement by use of image-guided techniques.

METHODS

During the past 4 years, 266 image-guided thoracic pedicle screws were placed in 65 patients at the University of Michigan Medical Center. Postoperative thin-cut computed tomographic scans were obtained in 52 of these patients who were available to enroll in the study. An impartial neuroradiologist evaluated 224 screws by use of a standardized grading scheme. All levels of the thoracic spine were included in the study.

RESULTS

Chart review revealed no incidence of neurological, cardiovascular, or pulmonary injury. Of the 224 screws reviewed, there were 19 cortical violations (8.5%). Eleven (4.9%) were Grade II (< or =2 mm), and eight (3.6%) were Grade III (>2 mm) violations. Only five screws (2.2%), however, were thought to exhibit unintentional, structurally significant violations. Statistical analysis revealed a significantly higher rate of cortical perforation in the midthoracic spine (T4-T8, 16.7%; T1-T4, 8.8%; and T9-T12, 5.6%).

CONCLUSION

The low rate of cortical perforations (8.5%) and structurally significant violations (2.2%) in this retrospective series compares favorably with previously published results that used anatomic landmarks and intraoperative fluoroscopy. This study provides further evidence that stereotactic placement of pedicle screws can be performed safely and effectively at all levels of the thoracic spine.

Accuracy requirements for image-guided spinal pedicle screw placement

STUDY DESIGN

Accuracy requirement analysis for image-guided pedicle screw placement.

OBJECTIVES

To derive theoretical accuracy requirements for image-guided spinal pedicle screw placement.

SUMMARY OF BACKGROUND DATA

Underlying causes of inaccuracy in image-guided surgical systems and methods for quantifying this inaccuracy have been studied. However, accuracy requirements for specific spinal surgical procedures have not been delineated. In particular, the accuracy requirements for image-guided spinal pedicle screw placement have not been previously reported.

METHODS

A geometric model was developed relating spinal pedicle anatomy to accuracy requirements for image-guided surgery. This model was used to derive error tolerances for pedicle screw placement when using clinically relevant screw diameters in the cervical (3.5 mm), thoracic (5.0 mm), and thoracolumbar spine (6.5 mm). The error tolerances were represented as the permissible rotational and translational deviations from the ideal screw trajectory that would avoid pedicle wall perforation. The relevant dimensions of the pedicle model were extracted from existing morphometric data.

RESULTS

As anticipated, accuracy requirements were greatest at spinal levels where the relevant screw diameter approximated the dimensions of the pedicle. These requirements were highest for T5, followed in descending order by T4, T7, T6, T3, T12, L1, T8, T11, C4, L2, C3, T10, C5, T2, T9, C6, L3, C2, T1, C7, L4, and L5. Maximum permissible translational/rotational error tolerances ranged from 0.0 mm/0.0 degrees at T5 to 3.8 mm/12.7 degrees at L5.

CONCLUSIONS

These results, obtained by mathematical analysis, demonstrate that extremely high accuracy is necessary to place pedicle screws at certain levels of the spine without perforating the pedicle wall. These accuracy requirements exceed the accuracy of current image-guided surgical systems, based on clinical utility errors reported in the literature. In actual use, however, these systems have been shown to improve the accuracy of pedicle screw placement. This dichotomy indicates that other factors, such as the surgeon's visual and tactile feedback, may be operative.

Pedicle morphology in thoracic adolescent idiopathic scoliosis: is pedicle fixation an anatomically viable technique?

STUDY DESIGN

A radiographic study of thoracic pedicle anatomy in a group of adolescent idiopathic scoliosis (AIS) patients.

OBJECTIVE

To investigate the anatomic constraints of the thoracic pedicles and determine whether the local anatomy would routinely allow pedicle screw insertion at every level.

SUMMARY OF BACKGROUND DATA

In spite of the clinical successes reported with limited thoracic pedicle screw-rod constructs for thoracic AIS, controversy exists as to the safety of this technique.

MATERIAL AND METHODS

Twenty-nine patients with right thoracic AIS underwent preoperative thoracic CT scans and plain radiographs. Anatomic parameters were measured from T1 to T12.

RESULTS

Information on 512 pedicles was obtained. The transverse width of the pedicles from T1 through T12 ranged from 4.6-8.25 mm. The medial pedicle to lateral rib wall transverse width from T1 through T2 ranged from 12.6 to 17.9 mm. Measured dimensions from the CT scans showed the actual pedicle width to be 1-2 mm larger than would have been predicted from the plain radiographs. Age, Risser grade, curve magnitude, and the amount of segmental axial rotation did not correlate with the morphology or size of the thoracic pedicles investigated. In no case would pedicle morphology have precluded the passage of a pedicle screw.

CONCLUSION

Based on the data identified in this group of adolescent patients, it is reasonable to consider pedicle screw insertion at most levels and pedicle-rib fixation at all levels of the thoracic spine during the treatment of thoracic AIS.

Efficacy of pedicle screw fixation in the treatment of spinal instability and failed back surgery: a 5-year review

OBJECT

The goal of this study was to review retrospectively the outcome of 95 patients with various disorders leading to instability of the thoracolumbar and lumbar spine who were treated consecutively via a posterior surgical approach with pedicle screw fixation in which the Texas Scottish Rite Hospital system was used.

METHODS

All cases were managed according to the same protocol. Follow-up review averaged 29.6 months. Radiographic evidence of osseous union and the patient's current status were analyzed. Four screws were malpositioned, and there were two dural lacerations of a nerve root and one pedicle fracture. Deep wound infections developed in five patients (5.2%), and three patients had postoperative radicular pain. In one case, the rods disengaged from the screws; in four cases, hardware was removed but there were no broken screws. Neurological deficits improved in 85% of the surviving patients, and no patient was worse neurologically after surgery. The rate of osseous union was 96.8%. Three patients developed pseudarthrosis, one of whom was asymptomatic. Back pain improved in 80 patients. A solid bone fusion, however, was not necessarily associated with decreased back pain.

CONCLUSIONS

These results support the use of pedicle screw fixation as an effective and safe procedure for fusion of the thoracolumbar and lumbar spine and support the finding that complications can be minimal when a meticulous surgical technique is used. The proper selection of patients for surgery is probably the most important factor associated with good outcomes.