Brian W. Su

Correlation of posterior ligamentous complex injury and neurological injury to loss of vertebral body height, kyphosis, and canal compromise.

Study Design. Retrospective, case-control study. Objective. The purpose of this study was to determine if thoracolumbar vertebral body collapse, translation, or canal compromise (CC) is associated with injury to the posterior ligamentous complex (PLC) or neurological elements. Summary of Background Data. Radiographical parameters, including loss of vertebral body height (LOVBH), vertebral body translation, local kyphosis (LK), and CC, are often used as indicators of spinal instability. The hypothesis of this study was that LOVBH greater than 50%, LK greater than 20°, translation greater than 3.5 mm, or CC greater than 50% is associated with ligamentous and neurological injury. Methods. Retrospective review of prospectively collected spinal cord injury database was performed. Inclusion criteria include consecutive patients with thoracolumbar burst fractures. Exclusion criteria include flexion-distraction injuries and pathological fractures. Computed tomographic scan measurements of the spine were performed by 2 experienced spine surgeons blinded to magnetic resonance imaging results. On magnetic resonance imaging, the supraspinous ligament, interspinous ligament, ligamentum flavum, facet joints, and disc were graded as intact, indeterminate, or disrupted. American Spinal Injury Association (ASIA) score and Frankel Scale score were recorded. Spearman correlation coefficients were calculated to evaluate relationships between vertebral body measurements, ligamentous injury, and neurological injury. Results. Forty-six patients were included in the study. Ten patients had kyphosis greater than 20°, 1 patient had kyphosis greater than 30°, and 9 patients had LOVBH greater than 50%. There were 34 patients with vertebral body translation greater than 3.5 mm and 15 patients with CC greater than 50%. Sixteen patients had ligamentous injury. There was a significant correlation between subjacent segment translation greater than 3.5 mm and ligamentous injury (R = 0.323, P = 0.029) and ASIA motor score (R = −0.379, P = 0.009). There was no significant correlation between ligamentous injury or neurological injury and the following threshold parameters: LOVBH greater than 50%, vertebral body kyphosis greater than 20°, caudal or cephalad interspinous widening greater than 7 mm, CC greater than 50%, and sagittal transverse ratio less than 0.48. Conclusion. The results of this study indicate that LOVBH greater than 50% and LK greater than 20° are not predictive of PLC injury in thoracolumbar burst fractures. Translation greater than 3.5 mm was associated with PLC injury. The PLC and neural elements should be directly assessed with magnetic resonance imaging if there is clinical concern.

An Anatomical Study of the Mid-Lateral Pars Relative to the Pedicle Footprint in the Lower Lumbar Spine

Study Design. An anatomic study that describes the relationship of the pedicle center to the mid-lateral pars (MLP) in the lower lumbar spine as a guide to pedicle screw placement. Objective. Describe morphometric data of the lower lumbar pedicles, the unique coronal pedicle footprints of L4 and L5, and their impact on the relationship of the pedicle center to the MLP. Summary of Background Data. Traditional medial-lateral starting points for lumbar pedicle screws use the facet as an anatomic reference for all lumbar levels. The facet is often a difficult landmark to use secondary to degenerative changes and the desire to minimize damage to the facet capsule in the most cephalad level. These techniques can also result in pedicle violation particularly in the lower lumbar spine. Use of the nonarthritic MLP is proposed in this study as an alternative anatomic reference point for the pedicle center. Methods. Seventy-two pedicles (L3–S1) from embalmed cadaveric spines were used. Linear and angular dimensions of the pedicle were measured, including the degree of coronal pedicle tilt of L4 and L5. The center of the pedicle relative to the MLP and relative to the midline of the base of the transverse process was measured. The axial superior facet angle and angle of pedicle screw insertion were also measured. Results. The minimum pedicle width was 10.9 and 12.4 mm and the coronal pedicle tilt was 36° and 55° for L4 and L5, respectively. A classification of 2 types of L5 pedicles relevant to pedicle center location was developed. In the medial-lateral direction, the pedicle center is 2.9 mm lateral to the MLP at L3 and L4. At L5, it is 1.5 and 4.5 mm lateral to the MLP for a type I and type II pedicle, respectively. In the superior-inferior direction, the pedicle center is 1 mm superior to the midline of the transverse process base for all lower lumbar levels. Significant differences between a type I and II L5 pedicle were a larger pedicle width and distance of the pedicle center to the MLP for a type II pedicle. The difference between the axial pedicle screw insertion angle and anatomic superior facet angles was 8° from L4–S1. Conclusion. The MLP is a reliable anatomic reference point for the center of the pedicle in the lower lumbarspine. Consideration needs to be taken when inserting pedicle screws at L4 and L5 because of the degree of their coronal tilts and unique pedicle footprints. It is important to distinguish a type I from type II L5 pedicle as a type II pedicle is wider, has a more lateral pedicle center relative to the MLP, and has the potential for lateral screw placement while still remaining within the pedicle.

An anatomic and radiographic study of lumbar facets relevant to percutaneous transfacet fixation.

Study Design. An anatomic study of lumbar facet anatomy for transfacet fixation. Objective. Describe the ideal starting point and trajectory for percutaneous transfacet fixation. Summary of Background Data. 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. Methods. 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. Results. 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° in the axial plane approximately 30° 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° of axial rotation is the optimal fluoroscopic view for confirming screw placement. Conclusion. 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.

Comparison of fatigue strength of C2 pedicle screws, C2 pars screws, and a hybrid construct in C1-C2 fixation.

Study Design. A biomechanical study comparing the fatigue strength of different types of C2 fixation in a C1–C2 construct. Objective. To determine the pullout strength of a C2 pedicle screw and C2 pars screw after cyclical testing and differentiate differences in stiffness pre– and post–cyclical loading of 3 different C1–C2 fixations. Summary of Background Data. Some surgeons use a short C2 pars screw in a C1–C2 construct, because it is less technically demanding and/or when the vertebral artery is high riding. Difference in construct stiffness between use of bilateral C2 pedicle screws, bilateral C2 pars screws, or a hybrid construct is unknown. Methods. Biomechanical testing was performed on 15 specimens. A bicortical C1 lateral mass screw was used in combination with 1 of 3 methods of C2 fixation: (1) bilateral long C2 pedicle screws (LL), (2) bilateral 14-mm C2 pars screws (SS), and (3) unilateral long C2 pedicle screw with a contralateral 14-mm C2 pars screw (LS). Each construct was subject to 16,000 cycles to simulate the immediate postoperative period. Changes in motion in flexion-extension, lateral bending, and axial rotation were calculated. This was followed by pullout testing. Results. The ability to limit range of motion significantly decreased after cyclical testing in flexion-extension, lateral bending, and axial rotation for all 3 groups. After loading, the LL and LS groups had less percentage of increase in motion in flexion-extension and lateral bending than the SS group. Overall, the average pullout strength of a pedicle screw was 92% stronger than a pars screw. Conclusion. C2 pedicle screws have twice the pullout strength of C2 pars screws after cyclical loading. In cases in which the anatomy limits placement of bilateral C2 pedicle screws, a construct using a unilateral C2 pedicle screw with a contralateral short pars screw is a viable option and compares favorably with a bilateral C2 pedicle screw construct. Level of Evidence: N/A

Quantitative Assessment of the Anatomical Footprint of the C1 Pedicle Relative to the Lateral Mass: A Guide for C1 Lateral Mass Fixation

Study Design: Anatomic study. Objectives: To determine the relationship of the anatomical footprint of the C1 pedicle relative to the lateral mass (LM). Methods: Anatomic measurements were made on fresh frozen human cadaveric C1 specimens: pedicle width/height, LM width/height (minimum/maximum), LM depth, distance between LM’s medial aspect and pedicle’s medial border, distance between LM’s lateral aspect to pedicle’s lateral border, distance between pedicle’s inferior aspect and LM’s inferior border, distance between arch’s midline and pedicle’s medial border. The percentage of LM medial to the pedicle and the distance from the center of the LM to the pedicle’s medial wall were calculated. Results: A total of 42 LM were analyzed. The C1 pedicle’s lateral aspect was nearly confluent with the LM’s lateral border. Average pedicle width was 9.0 ± 1.1 mm, and average pedicle height was 5.0 ± 1.1 mm. Average LM width and depth were 17.0 ± 1.6 and 17.2 ± 1.6 mm, respectively. There was 6.9 ± 1.5 mm of bone medial to the medial C1 pedicle, which constituted 41% ± 9% of the LM’s width. The distance from C1 arch’s midline to the medial pedicle was 13.5 ± 2.0 mm. The LM’s center was 1.6 ± 1 mm lateral to the medial pedicle wall. There was on average 3.5 ± 0.6 mm of the LM inferior to the pedicle inferior border. Conclusions: The center of the lateral mass is 1.6 ± 1 mm lateral to the medial wall of the C1 pedicle and approximately 15 mm from the midline. There is 6.9 ± 1.5 mm of bone medial to the medial C1 pedicle. Thus, the medial aspect of C1 pedicle may be used as an anatomic reference for locating the center of the C1 LM for screw fixation.