Medial Pedicle Wall Research Articles

An Anatomic and Radiographic Study of Lumbar Facets Relevant to Percutaneous Transfacet Fixation

An anatomic study of lumbar facet anatomy for transfacet fixation. Describe the ideal starting point and trajectory for percutaneous transfacet fixation. Percutaneous transfacet fixation is gaining popularity for posterior stabilization after anterior lumbar interbody fusion. Despite biomechanical and clinical studies, there are no anatomic guidelines for safe placement of percuatenous transfacet screws. Eighty L3-S1 facet joints from embalmed cadaveric spines were analyzed. Linear and angular measurements of the facets were recorded. Under direct visualization, the segments were pinned with an ipsilateral transfacet technique. The degrees of angulation in the sagittal and axial plane were recorded. The distances of the starting point relative to landmarks of the superior body were measured. Under fluoroscopy, radiographic parameters for ideal visualization of the pin and pin ending points were determined. Inferior and superior facet heights ranged from 15.7 to 17.5 mm at all levels. The percentage of inferior facet extending below the L3 and L4 end plates was 84% and 86% respectively and decreased at L5 to 72%. The percentage of superior facet extending above the end plate ranged from 36% to 44% at all levels. The transverse facet angle progressively increased from L3 to S1. The L2-L3 segments could not be instrumented from the ipsilateral side due to the vertical facet orientation. For L3-S1 segments, the starting point in the coronal plane is based on the superior body of the instrumented segment and should be in line with the medial border of the pedicle in the medial-lateral direction and in line with the inferior end plate in the cranial-caudal direction. The screw should be laterally angulated approximately 15 degrees in the axial plane approximately 30 degrees caudally in the sagittal plane. The screw should end in the inferolateral quadrant of the pedicle on the AP radiograph and at the pedicle-vertebral body junction on the lateral radiograph. 35 degrees of axial rotation is the optimal fluoroscopic view for confirming screw placement. Ipsilateral transfacet fixation can be successfully performed in the L3-S1 segments by using the inferior end plate and medial pedicle wall of the superiorly instrumented level as anatomic landmarks in conjunction with axial and sagittal angles of insertion.

Reproduzierbarkeit der Befundung von lumbalen Pedikelschraubenpositionen mit ISO C 3D und CT

This study was conducted to evaluate the reproducibility of the reading of lumbar pedicle screw scans using a C-arm-based imaging system in comparison to computed tomography. The influence of the technique and the experience of the rater should be determined. The lumbar spines of 23 patients were stabilized using 102 pedicle screws. The position of the screws was controlled intraoperatively using an Arcadis Orbic 3D scanner. All scans were evaluated independently by three raters. The position of the implants in reference to the pedicle walls was described. Additionally, another 100 lumbar pedicle screws in 16 patients were evaluated postoperatively with a multirow CT. Kappa according to Fleiss was calculated for the reproducibility of the rater statements. Each rater repeated the analysis of 24 screws to assess the intraobserver variance. The reports of the CT scans showed significantly less variation. The consent of all 3 raters was achieved in 79.4 vs. 65.1 % of cases. The Kappa values were 0.56 and 0.29, respectively. Poor results were obtained especially for the medial pedicle wall (consent 70.0 vs. 50.0 %). The influence of the experience of the rater was not able to be verified. The image quality of the ISO C 3D is worse than that of multirow CT scans for the evaluation of lumbar pedicle screws. This causes greater variance among the rater reports. We stopped using the ISO C 3D technique intraoperatively for the implantation of lumbar pedicle screws.

Accuracy of Pedicle Screw Placement Using Intraoperative Neurophysiological Monitoring and Computed Tomography

Fifty consecutive patients with posterior thoracolumbar spine fusion were included in a prospective study to determine the accuracy of intraoperative neurophysiological monitoring (IONM) for safe pedicle screw placement using postoperative computed tomography (CT). The patients were allocated into two equal groups. Pedicle screw placement was evaluated intraoperatively by using the image intensifier. In group A, the integrity of the pedicle wall was evaluated intraoperatively with monopolar stimulation of each screw head with a hand-held single-tip stimulator; the compound muscle action potentials were recorded. A constant current threshold of 7 mA was considered indicative of pedicle breach; < 7 mA was considered as direct contact with neural elements, and > 7mA was considered normal. In group B, pedicle screw placement was performed without IONM. Overall, 306 pedicle screws were inserted in both groups. Postoperatively, all patients underwent CT scans of the spine to evaluate pedicle screw placement. Intraoperatively, five screws in respective group A patients had to be repositioned after IONM (threshold of < 7 mA); in these patients, postoperative CT scans showed proper screw placement. Postoperative CT scans showed eight misdirected screws; two screws (1.26%) in group A patients and six screws (4%) in group B patients. Two screws were misdirected through the medial pedicle wall and six screws were misdirected through the lateral pedicle wall. Both medially misdirected screws were observed in group B patients (1.35%); these patients developed neurologic symptoms postoperatively and underwent revision surgery, with redirection of the misdirected screws and subsequent resolution of the neurologic symptoms. Two of the six laterally misdirected screws were observed in group A patients (1.26%); the remaining four laterally misdirected screws were observed in group B patients (2.7%). None of these patients had neurologic sequelae; no revision surgery was required. The cut-off value of 7 mA had a 98.73% (> 95%) positive predictive value for accurate pedicle screw placement.

Comparison of Unipedicular versus Bipedicular Kyphoplasty for Osteoporotic Vertebral Compression Fractures

Study Design: A retrospective study Objectives: To compare the radiological and clinical results of the unipedicular and bipedicular approach of kyphoplasty for osteoporotic vertebral compression fractures. Summary of Literature Review: A unipedicular rather than a bipedicular technique has been suggested to decrease the risks associated with surgical procedures. Materials and Methods: Between July 2005 and May 2006, 136 vertebrae of 97 patients, who underwent kyphoplasty for osteoporotic vertebral compression fractures, were analyzed. Group 1, with the bipedicular approach, consisted of 86 vertebrae of 67 patients with a mean age of 72.2 years. Group 2, with unipedicular approach, consisted of 50 vertebrae of 30 patients with mean age of 73.4 years. The plain radiographs, MRI and surgical records were reviewed. Results: The mean operation time of the single vertebral body in group 2 was statistically lower than in group 1(p 0.05). Conclusion: There were no significant differences in clinical satisfaction and radiological results between the unipedicular and bipedicular kyphoplasty. The advantage of a unipedicular approach is the shorter procedure time than the bipedicular approach. This is particularly useful in multi-level compression fractures. The rate of the unipedicular approach in upper and mid thoracic spine is higher because of the higher convergence of the pedicle and the lower volume of vertebral body despite the disadvantages of instrument insertion through the medial pedicle wall.

Insertion Torque Profile During Pedicle Screw Insertion of the Thoracic Spine With and Without Violation of the Pedicle Wall

Biomechanical study in human cadaveric thoracic spines. To compare the insertion torque profile of cylindrical and conical screws with normal insertion tract, and with violation of the pedicle. Previous studies revealed higher insertion torque for conical screws. No study has investigated the torque profile of both screws when there is violation of the pedicle. The pedicle screws were inserted intrapedicularly (Type 1), violating the lateral cortex of the pedicle (Type 2), and violating the medial cortex of the pedicle (Type 3). Type 2 screws were further subdivided into those having contact with the lateral vertebral body (2a) and those without (2b). Type 3 screws were subdivided into those having contact with the medial pedicle wall (3a) and without (3b). RESULTS.: During initial insertion, the torque increased as they penetrated into the pedicles. With violation of the lateral cortex of the pedicle, insertion torque for the cylindrical screw decreased as the screw advanced anteriorly out of the pedicle. The insertion torque for the conical screw continued to increase at a constant rate after violation of the lateral cortex. With medial breach of the pedicle wall, torque profile of the conical screws remained the same as those inserted in the normal trajectory. There was a decline in the insertion torque for the cylindrical screws when medial violation of the pedicle wall occurred, the decrease being greater for Type 3b as compared with Type 3a screws. Surgeons may be unable to perceive the occurrence of medial or lateral breach of the cortex when inserting a conical screw. Surgeons inserting cylindrical screws with Type 3a violation may not be able to perceive a significant decrease in torque.

Evaluation of Pedicle Screw Placement in the Deformed Spine Using Intraoperative Plain Radiographs: A Comparison With Computerized Tomography

A retrospective study. To develop an accurate and reliable method to detect malpositioned pedicle screws during thoracic and lumbar spinal deformity operations using intraoperative plain radiographs. The reliability of pedicle screw assessment using plain radiographs is more difficult during scoliosis operations compared to nondeformed spine operations. Methodology is necessary to document and improve the accuracy of interpretation of intraoperative plain radiographs for deformity surgeries. A total of 789 pedicle screws, including 632 thoracic and 157 lumbar, inserted from T1 to L4 in 49 patients with spinal deformity with postoperative computerized tomography (CT) data were investigated. According to the diagnoses, the number of screws placed was 683 for scoliosis in 43 patients and 106 for kyphosis in 6 patients. The position of the pedicle screw inserted was graded with CT as an acceptable screw (n = 724) versus violated screw (n = 65), defined as the central axis of the inserted pedicle screw out of the outer cortex of the pedicle wall. There were 3 plain radiographic criteria used to judge the accuracy of screw position after minor screw tip position adjustment according to the relative length of the screws in the lateral radiograph: (1) violation of the harmonious segmental change of the tips of the inserted screws with reference to vertebral rotation using the posterior upper spinolaminar junction in the posteroanterior (PA) radiograph (medial or lateral out); (2) no crossing of the medial pedicle wall by the tip of the pedicle screw inserted with reference to the vertebral rotation using the posterior upper spinolaminar junction in the PA radiograph (lateral out); and (3) violation of the imaginary midline of the vertebral body using the posterior upper spinolaminar junction in the PA radiograph by the position of the tip of the inserted pedicle screw (medial out). Comparative analysis of these pedicle screws using postoperative CT and intraoperative plain radiographs confirmed 65 violated pedicle screws, including 15 medial violations and 50 lateral violations. Of 15 pedicle screws with medial wall violation, 13 showed a loss of harmonious segmental change in the screw tips and violation of the imaginary midline of the vertebral body (sensitivity 0.87, specificity 0.97, and accuracy 0.98). One case showed only a loss of harmonious change in the screw tip, and the other one case did not show any significant plain radiograph findings. Of the 50 pedicle screws with lateral wall violation, 47 cases showed a loss of harmonious segmental change in the screw tips and no crossing of medial pedicle wall by the pedicle screw inserted (sensitivity 0.94, specificity 0.90, and accuracy 0.96). Two cases did not show any significant plain radiograph findings, and the other one case showed only violation of the harmonious segmental change in the screw tips. Intraoperative plain radiographs alone using 3 radiographic criteria were very sensitive to detect lateral wall pedicle screw violations, specific for assessing for medial wall violations, and highly accurate for both. This result confirms the ability of careful intraoperative plain radiographic assessment after pedicle screw insertion to detect malpositioned screws, to allow for possible revision during the index operation.

Pedicle morphometry in the upper thoracic spine: limits to safe screw placement in older patients.

An anatomic study of pedicle dimensions and orientation was performed for upper thoracic vertebrae from elderly human subjects. To quantify dimensions of thoracic pedicles, and to determine the potential for safe transpedicular screw fixation in the upper thoracic spine. Clinical and anatomic reports support thoracic pedicle fixation as a safe, effective alternative to hook fixation in both normal and osteoporotic bone. Much available data, however, pertains to young, robust patients, thoracolumbar segments, and mixed placement techniques. For this study, T1-T6 vertebrae from 18 human cadavers were separated into individual vertebrae. Examiners measured each vertebra to determine medial-lateral pedicle width, cranial-caudal pedicle height, and coaxial depth from lamina to anterior vertebral cortex. Mean values were derived from repeated measures compared by level and side. Dimension a varied with individual and level, but not between the left and right pedicles. Pedicle diameter uniformly diminished as specimens proceeded caudally from T2. Findings showed that 25% of T1 pedicles, 17% of T2 pedicles, and 42% of T3 pedicles were narrower than 5.5 mm. At T4 61% of pedicles were too small, at T5 67% were too small, and at T6 75% were too small to accept a 5.5-mm screw. Dimension b remained relatively constant. Pedicles became increasingly narrow and oblong in the T4-T6 cross sections. Dimension c increased consistently from T1 to T6. Safe screw lengths ranged from 30 mm at T1 and T2, to 35 mm at T4 -T5, to 40 mm at T5 and T6. Even the largest patients had some pedicles that could not accommodate the smallest standard pedicle screw, and more than one half of the pedicles average patients were too small. Transpedicular screw placement is not safe in these patients. Proper placement must avoid penetration of the medial pedicle wall.

Use of a transpedicular drill guide for pedicle screw insertion in the thoracic spine

A transpedicular drill guide (TDG) was designed to assist in the safe placement of pedicle screws in the thoracic spine. In a preliminary study, pilot holes were drilled into the pedicles (T1 to T12) of eight anatomical models in order to compare the conventional anatomical technique to the TDG. Visual inspection of the drilled pedicles was performed. Subsequently in a cadaveric study, pilot holes were made using the TDG in the thoracic spine (T1 to T11) of one human cadaver before inserting 4.5 mm diameter screws. CT scan followed by visual inspection of the cadaveric spine was performed to evaluate the position of the screws. With the anatomical models, 19 of 96 (19.8%) holes drilled using the TDG and 64 of 96 (66.7%) holes drilled using the anatomical technique violated the pedicle wall (p<0.001). The TDG reduced the rate of medial perforation. Ninety-nine percent of the pilot holes made with the TDG were within 2 mm from the pedicle wall compared to 79.2% for the anatomical technique. In the cadaveric study, one of the 22 (4.5%) screws violated the medial wall of the right T1 pedicle by less than 1 mm. No screw penetrated the anterior vertebral cortex, nor the lateral, superior or inferior pedicle wall. The TDG is easy to use and can decrease the incidence of misplaced thoracic pedicle screws. The TDG could be used alone as an alternative to navigation systems in certain applications or with fluoroscopy during thoracic pedicle screw placement, especially for training surgeons.

Triggered electromyographic threshold for accuracy of thoracic pedicle screw placement in a porcine model.

A porcine model of thoracic pedicle screw insertion was used to determine the effect of screw position on triggered electromyographic response. To develop a model of intraoperative detection of misplaced thoracic pedicle screws. Triggered electromyographic stimulation has been a valuable aid in determining appropriate placement of lumbar pedicle screws. The use of pedicle screws is increasing in the thoracic spine. Misplaced thoracic pedicle screws may have significant implications if the spinal cord is injured. This study was an attempt to determine whether the established lumbar model can be used for thoracic pedicle screws. Five 120- to 150-lb domestic pigs had 85 pedicle screws placed bilaterally in the thoracic spine at each level from T6 to T15. Screws were inserted entirely in the pedicle (Group A). After removal of the medial pedicle wall, the screws were reinserted in the pedicle with no neural contact (Group B). The screws were then placed with purposeful contact with the neural elements (Group C). The screws were stimulated, eliciting an electromyographic response in the intercostal muscles for each instrumented level. The type of response noted was classified as either primary (response from appropriate nerve root), secondary (response at different root) or no response (response at different root, no response at appropriate root). Two hundred fifty responses were recorded. A primary response was noted in 72% of recordings. There was a relatively consistent decrease in the triggered electromyographic response from Group A (mean 4.15 +/- 1.80 mA) to Group C (mean 3.02 +/- 2.53 mA) screws (P = 0.0003). There was little difference in the response obtained from Group A to Group B (mean 4.37 +/- 2.48 mA) screws (P > 0.05). When a primary response was recorded, the mean threshold electromyographic response recorded was significantly lower than recordings with secondary and no response recordings (P < 0.05). Even though there was a consistent decrease between the A and C screws that was more definitively separated when a primary response was elicited, it was not possible to determine a cutoff trigger electromyographic level that would consistently differentiate intraosseous from epidural pedicle screw placement. Furthermore, this method could not differentiate screws clearly in the pedicle from screws with medial pedicle wall breakthrough. A more direct method of spinal cord monitoring must be established to provide the surgeon with early warning of the potential of neural injury in the placement of thoracic pedicle screws.

Anterior Transpedicular Fixation of the Lower Thoracic and Lumbar Spine

Currently, no anterior spinal implant provides a strong bone-screw interface because of the cancellous characteristics of the vertebral body. A more secure anchorage could be obtained by anterior transpedicular screw fixation. Four hundred transpedicular screws located between T7 and L5 were placed using the newly developed direction finder. Measurements were obtained directly from radiographs of the cadaveric specimens. In 10 cases (2.5%), the screws crossed the medial pedicle border, but never by more than 1.4 mm. A lateral protrusion was noted in another 41 screws (10%), with no protrusion greater than 2.2 mm. Encroachments beyond the superior or inferior border were not observed. The mean angle of the screws at each level measured between 7 and 19 in the transverse plane and between 2 and 4.5 in the sagittal plane. This technique should be reserved for vertebrae without significant arthritic changes. The rare screw with minimal infraction through the medial or lateral pedicle wall should not cause any vascular or neural compromise. The anterior transpedicular screw technique appeared relatively safe (88%) and encouraged the development of the new plate system for anterior spinal stabilization.