Rongming Xu

Anatomic Considerations of Pedicle Screw Placement in the Thoracic Spine: Roy-camille Technique versus Open-lamina Technique

Study Design. In this cadaveric study, the outcomes of two techniques for pedicle screw placement in the thoracic spine were compared. Objectives. To assess the Roy‐Camille technique, and to determine whether pedicle screw placement, aided by partial laminectomy, could decrease the incidence of pedicle violations. Summary of Background Data. Pedicle screw fixation in the thoracic spine remains technically challenging. The Roy‐Camille method may be one of the leading techniques of thoracic pedicle screw placement. However, there are few studies evaluating this technique and determining methods to decrease the incidence of thoracic pedicle penetration with screw insertion. Methods. Ten cadaveric thoracic spines from T1 to T10 were used for pedicle screw placement. Two techniques of transpedicular screw placement were used, the Roy‐Camille technique (screw placed on the right side; used in 95 screw placements) and the open‐lamina technique screw placement with combined partial laminectomy (screw placed on the left side; used in 94 screw placements). After screw placement, all specimens were evaluated visually to determine violation of the pedicle. Results. The screw placement with the Roy‐Camille technique had a higher percentage of pedicle violation (54.7%) than did that with the open‐lamina technique (15.9%). No Grade III violation was seen in the screw placement with the open‐lamina technique. Conclusions. The Roy‐Camille technique was associated with a high incidence of pedicle violation, whereas screw placement with a partial laminectomy significantly reduced the incidence of pedicle violation. Pedicle screw fixation in the thoracic spine remains a technical challenge and should not be used routinely. Screw placement with the open‐lamina technique is recommended if pedicle screw fixation is strongly indicated in the thoracic spine.

Anatomic consideration of C2 pedicle screw placement.

Study Design This anatomic study tested placement of C2 pedicle screws using cadaver specimens. Objectives To further assess the safety of transpedicular screw placement in the axis by comparing two surgical techniques. Summary of Background Data Transpedicular screw fixation of traumatic spondylolisthesis of the axis has been described in the literature. Recently, anatomic studies and clinical applications of transpedicular screw fixation for traumatic lesions of middle and lower cervical spine have been described. No previous study assessing the safety of C2 pedicle screw placement is available. Methods Sixteen embalmed cadaveric specimens were used for this study. In the first eight specimens (Method A), the point of entry for screw placement was chosen to be about 5 mm inferior to the superior border of C2 lamina and 7 mm lateral to the lateral border of the spinal canal. The screw direction was chosen to be about 30° medial to the sagittal plane and 20° cephalad to the transverse plane. A 3.5‐mm cortical screw of appropriate length, determined with depth gauge, was placed bilaterally into the C2 pedicle. In the next eight specimens (Method B), the direction of the drill bit was guided directly by the medial and superior aspect of the individual C2 pedicle. Gross dissection was done to view violation of dura, nerve roots, vertebral artery, and penetration of medial, lateral, superior, and inferior cortex of the C2 pedicle. Radiographs and computed tomography scans were obtained to evaluate screw placement in the C2 pedicle. Results In Method A, four screws had lateral violations into the vertebral artery. In Method B, only two cases of minimal penetration of pedicle cortex were found. No medial, superior, or inferior violation of the pedicle cortex was found in the present study. Conclusions The present anatomic study suggests that transpedicular screw fixation may be performed safely in the C2 pedicle by using the second technique. Using the first technique is not safe.

Anatomic relations of the thoracic pedicle to the adjacent neural structures.

Study Design. This study analyzed anatomic parameters between the thoracic pedicles and the spinal nerve roots. Objectives. To quantitatively determine the anatomic relations of the thoracic pedicle to the adjacent neural structures. Summary of Background Data. Pedicular screw placement carries with it potential hazard to the surrounding neural structures, especially in the thoracic spine. No studies exist regarding the anatomic relations of the thoracic pedicle to the adjacent nerve roots. Methods. Fifteen cadavers were obtained for study of the thoracic spine. All soft tissue was dissected off the thoracic spine. Laminectomy and total removal of the superior and inferior articular facets was then performed on C7‐T1 through T12‐L1 to expose the pedicles, nerve roots, and dura. Measurements were taken from the pedicle to the nerve root superiorly and inferiorly as well as between the pedicles. Also, the superoinferior diameter of the nerve root and the frontal angle of the nerve root were measured. Symmetrical structures were measured bilaterally. Results. The results showed that no epidural space could be found between the dural sac and the pedicle in all 15 cadavers. The average distances from the thoracic pedicle to the adjacent nerve roots superiorly or inferiorly at all levels ranged from 1.9 to 3.9 mm and from 1.7 to 2.8 mm, with a minimum of 1.3 mm, respectively. The interpedicular distance increased from T1 (13.8 mm) to T3, slightly decreased in T4‐T5, then gradually increased to T12 (16.6 mm). The superoinferior diameter of the nerve root increased consistently from 2.9 mm at T1 to 4.6 mm at T11. The frontal nerve root angle decreased consistently from T1 (120.1°) to T12 (57.1°), except at T4‐T5. Conclusions. This study suggested that more care be taken into consideration in placing a transpedicular screw in the transverse plane than in placing a screw in the sagittal plane in the thoracic spine.

Bone-graft harvesting from iliac and fibular donor sites: techniques and complications.

&NA; The ilium and the fibula are the most common sites for bone‐graft harvesting. The different methods for harvesting iliac bone graft include curettage, trapdoor or splitting techniques for cancellous bone, and the subcrestal‐window technique for bicortical graft. A tricortical graft from the anterior ilium should be taken at least 3 cm posterior to the anterior superior iliac spine (ASIS). Iliac donor‐site complications include pain, neurovascular injury, avulsion fractures of the ASIS, hematoma, infection, herniation of abdominal contents, gait disturbance, cosmetic deformity, violation of the sacroiliac joint, and ureteral injury. The neurovascular structures at risk for injury during iliac bone‐graft harvesting include the lateral femoral cutaneous, iliohypogastric, and ilioinguinal nerves anteriorly and the superior cluneal nerves and superior gluteal neurovascular bundle posteriorly. Violation of the sacroiliac joint can be avoided by limiting the harvested area to 4 cm from the posterior superior iliac spine (PSIS) and by not penetrating the inner cortex. The caudal limit for bone harvesting should be the inferior margin of the roughened area anterior to the PSIS on the outer table to keep from injuring the superior gluteal artery. Potential complications of fibular graft harvesting include neurovascular injury, compartment syndrome, extensor hallucis longus weakness, and ankle instability. The neurovascular structures at risk for injury during fibular bone‐graft harvesting include the peroneal nerves and their muscular branches in the proximal third of the fibular shaft and the peroneal vessels in the middle third.

Morphometric evaluation of lower cervical pedicle and its projection.

Study Design This study evaluated the lower cervical pedicle from C3 to C6 to provide information for accurate transpedicular screw fixation in this region. Objectives To measure the dimensions of the lower cervical pedicle and to determine the correct location of the pedicle axis on the posterior aspect of the lateral mass. Summary of Background Data Several anatomic studies and clinical applications of transpedicular screw fixation in the cervical spine have been documented, but little quantitative data concerning the lower cervical pedicle and its projection are available. Methods Forty dry cervical specimens from C3 to C6 (160 cervical vertebrae) were used for this study. Anatomic evaluation included pedicle height, width, effective length, and anguli. The distances from the projection point of the pedicle axis to reference lines related to the lateral edge of the lateral mass (vertical) and the inferior edge of the superior facet (horizontal) also were measured. The means, ranges, and standard deviations were calculated for all of the specimens and separately for male and female spines. Results Statistically significant differences in dimensions of males and females were found in one linear and one angular measurement, which included the pedicle height of C6 and the pedicle sagittal angle of C4. The greatest variation for males and females was found in the pedicle sagittal angle, with a range of 4.3‐9.8 degrees. The distances from the projection point to the horizontal line did not show any real pattern of change from C3 to C6, whereas the distances from the projection point to the vertical line consistently increased from cephalad to caudad. Conclusions Taking into consideration some variations between individuals, this information, combined with evaluation of results of preoperative axial computed tomography and conventional radiography, may enhance the safety of transpedicular screw fixation in the lower cervical spine.

Morphology of the Second Cervical Vertebra and the Posterior Projection of the C2 Pedicle Axis

Study Design. This study assessed numerous structural features of the second cervical verte bra (C2), describing the projection point of the pedicle on its posterior aspect. Objectives. Evaluation of the specimens included quantitative description of 18 linear and four angular parameters, which then were correlated between male and female specimens. The point of projection of the C2 pedicie axis was described with an emphasis on a perspective relevant to a posterior approach. Summary of Background Data The literature regarding the anatomy of the axis focuses mainly on the dens. Very little research regarding the quantitative study of the C2 pedicle has been reported. Methods. Fifty dry C2 cervical vertebrae (30 male, 20 female) were obtained for anatomic measurements. Anatomic evaluation focused on the pedicle, vertebral body, dens, superior facet, and vertebral canal. All measurements were made using calipers and a standard rule linear measurements and a goniometer for angular measures. Based on the mesurement of 50 specimens, including 18 linear and four angular parameters, the mean, range, and standard deviation were calculated for all of the specimens and for male and female seperately. Results. A significant difference was found to exist for 11 of 18 linear measurements and one of four angular parameters. The projection point of the pedicle axis on the posterior aspect of the lateral mass was described with an emphasis on a perspective relevant to a posterior surgical approach. The location of the projection point of the pedicle axis was found to be 5.4 ± 1.2 mm inferior to the horizontal line, and 7.2 ± 1.3 mm lateral to the vertical line. The predicle axis was found to lie at 33° in the medial direction and 20° in the superior direction form the point of pedicle axis projection. Conclusions. When the techniques described here are used, the findings may be helpful in cases involving C2 when surgical intervention and instrumentation are desired.

Projection of the Thoracic Pedicle and Its Morphometric Analysis

STUDY DESIGN This study defined the projection point of the thoracic pedicles on their posterior aspect and its relation to a reliable landmark. It also reported pedicle dimensions based on 43 thoracic spines. OBJECTIVES To determine the projection point of the pedicle axis on the posterior aspect of the thoracic spine, quantitatively describe relations of the projection point to some reliable landmarks, and evaluate linear and angular dimensions of the thoracic pedicle. SUMMARY OF BACKGROUND DATA Posterior segmental screw fixation is the current standard of internal fixation at the level of the second lumbar vertebrae or below. However, pedicular screw fixation in the thoracic spine, especially in the middle and upper thoracic region, is not common because the small dimensions of the pedicle in this region make screw insertion difficult. More information about pedicle axis projection (not pedicle zone) and its quantitative relationship to some reliable landmarks is essential. METHODS Forty-three dry thoracic specimens (516 vertebrae) were obtained for study of the thoracic pedicle. Anatomic evaluation focused on the determination of the projection point of the thoracic pedicle axis on its posterior aspect and the anatomic relationship of this point to the lateral edge of superior facet and the midline of the transverse process. Also, pedicle dimensions, including linear and angular, were measured. The mean, range, and standard deviation were calculated for all of the specimens and for male and female specimens separately. RESULTS Sexual difference was found to be significant statistically in more than half of parameters. For T1-T2, the projection point of the pedicle axis was approximately 7-8 mm medial to the lateral edge of the superior facet and 3-4 mm superior to the midline of the transverse process. For T3-T12, this point was 4-5 mm medial to the lateral margin of the facet and 5-8 mm superior to the midline of the transverse process. The transverse angle of the pedicle axis was found to be 30-40 degrees at T1-T2, 20-25 degrees at T3-T11, and 10 degrees at T12. CONCLUSIONS This information, in conjunction with preoperative computed tomography evaluation, may enhance our knowledge of transpedicular screw fixation in the thoracic pedicle.

The anatomic relation of lateral mass screws to the spinal nerves : A comparison of the Magerl, Anderson, and An techniques

STUDY DESIGN Analysis of the anatomic relation of the Magerl, Anderson, and An screws to the spinal nerve. OBJECTIVES To compare the potential incidence of nerve root (ventral and dorsal ramus) injury caused by the Magerl, Anderson, and An techniques. SUMMARY OF BACKGROUND DATA Posterior plating with lateral mass screw fixation is a common procedure for managing an unstable cervical spine. Comparative study of the Roy-Camille and Magerl techniques has been reported. However, the risk of nerve root injury for the Anderson and An techniques is not known. METHODS Three lateral mass screw insertion techniques were performed in this study: Magerl, Anderson, and An. Each technique involved two specimens and 20 screws inserted from C3 through C7. A 20-mm-long screw was used to overpenetrate the ventral cortex. The anterolateral aspect of the cervical spine was carefully dissected to allow observation of the screw-ramus relationship. RESULTS The overall percentage of nerve violation was significantly higher with the Magerl (95%) and Anderson (90%) techniques than with the An (60%) technique (P < 0.05). The largest percentages of nerve violation for the Magerl, Anderson, and An screws were found at the dorsal ramus (50%), the bifurcation of the ventral dorsal ramus (45%), and the ventral ramus (55%), respectively. CONCLUSIONS The results of this study indicate that the potential risk of nerve root violation is higher with the Magerl and Anderson techniques than with the An technique.

Anatomic relation between the cervical pedicle and the adjacent neural structures.

STUDY DESIGN An evaluation of the anatomic relation between the cervical pedicles and the adjacent neural structures. OBJECTIVES To determine quantitatively the antomic relation of the cervical pedicles to the adjacent nerve roots and dural sac. SUMMARY OF BACKGROUND DATA Transpedicular screw fixation in the cervical spine has been reported, but little quantitative data regarding the anatomic relation between the cervical pedicles and the surrounding neural structures are known. METHODS Twenty cadavers were used for dissection to observe the relations of the cervical pedicles to the adjacent dural sac and nerve roots. After removal of whole posterior bony elements including the spinous processes, laminas, and lateral masses, the isthmus of the pedicles, the dural sac, and the nerve roots of C3-C7 were exposed. Direct measurements included the distance from the pedicle to the superior and inferior nerve roots and the dura. Also, the pedicle height and width were measured at its isthmus. RESULTS No distance was found between the pedicle and the superior nerve root, nor between the pedicle and the dural sac at C3-C7 for all specimens. The mean distances between the pedicle and the inferior nerve roots for all specimens ranged from 1.4 to 1.6 mm. The mean pedicle heights and widths for all specimens at C3-C7 ranged from 6.0 to 6.5 mm and 4.7 to 5.3 mm, respectively. A significant difference between male and female specimens was noted in the pedicle heights for all levels measured (P = 0.001), and in the pedicle widths for the levels of C4 and C6 (P = 0.05). CONCLUSIONS This study suggests that the incidence of neurologic injuries may be higher in screw penetration of the medial or superior cortex of the pedicle than in screw penetration of the inferior cortex of the pedicle.

Evaluation of Porous Biphasic Calcium Phosphate Ceramics for Anterior Cervical Interbody Fusion in a Caprine Model

Study Design. This study compared the efficacy of characterized 50/50 hydroxyapatite/β-tricalcium phosphate ceramics of 30%, 50%, and 70% porosity and autograft to promote interbody spinal fusion at C2-C3 and C5-C6 in 24 goats: 12 at 3 months and 12 at 6 months. Objectives. Radiographs, histology, dual energy x-ray absorptiometry analysis, and biomechanical testing were used to evaluate the ability of the 30%, 50%, and 70% porous 50/50 hydroxyapatite/β-tricalcium phosphate ceramics and autograft to promote cervical interbody fusion. Summary of Background Data. The conundrum in the use of calcium phosphates for interbody fusion is what porosity is most effective to promote ingrowth yet strong enough to resist compressive stresses found in the spine? It is known that the ability for bone ingrowth increases and the compressive strength decreases as porosity of the ceramic is increase. Dense ceramics remain intact but may be surrounded by fibrous tissue. Porous ceramics have good ingrowth but may fracture. Methods. Radiographs were evaluated for fusion and fracture or collapse of the ceramics or autograft. Dual energy x-ray absorptiometry was used to evaluate the fusion mass. Treated motion segments underwent biomechanical testing to quantify the flexibility of the segment. Undecalcified and decalcified histologic analysis were performed to evaluate the presence or absence of a bony union. Results. Thirty percent, 50% and 70% porous ceramics had better radiographic fusion scores than the autograft at 3 and 6 months. Incidence of ceramic fracture did not increase with porosity and was equivalent to the collapse of autograft, although ceramics maintained disc height when fracture occurred. No statistically significant differences were found between autograft and the porous ceramics with biomechanical testing and peri-implant bone mineral density values as measured by dual energy x-ray absorptiometry. At 3 months, histologic analysis showed a union rate of 0% for autograft and 30% porous ceramic, 67% for 50% porous ceramic, and 83% for 70% porous ceramic. At 6 months, the union rate was 67% for the 30%, 50%, and 70% porous ceramics and 50% for autograft. Conclusions. Thirty percent, 50%, and 70% porous ceramics performed equal to or better than autogenous bone after 3 and 6 months. There may be promise for the use of 50/50 hydroxyapatite/β-tricalcium phosphate in spine surgery as the need to harvest autograft from the iliac crest is obviated, and complications and cost associated with the harvest are avoided.