Comparison of two-transsacral-screw fixation versus triangular osteosynthesis for transforaminal sacral fractures.

Transforaminal pelvic fractures are high-energy injuries that are translationally and rotationally unstable. This study compared the biomechanical stability of triangular osteosynthesis vs 2-transsacral-screw fixation in the repair of a transforaminal pelvic fracture model. A transforaminal fracture model was created in 10 cadaveric lumbopelvic specimens. Five of the specimens were stabilized with triangular osteosynthesis, which consisted of unilateral L5-to-ilium lumbopelvic fixation and ipsilateral iliosacral screw fixation. The remaining 5 were stabilized with a 2-transsacral-screw fixation technique that consisted of 2 transsacral screws inserted across S1. All specimens were loaded cyclically and then loaded to failure. Translation and rotation were measured using the MicroScribe 3D digitizing system (Revware Inc, Raleigh, North Carolina). The 2-transsacral-screw group showed significantly greater stiffness than the triangular osteosynthesis group (2-transsacral-screw group, 248.7 N/mm [standard deviation, 73.9]; triangular osteosynthesis group, 125.0 N/mm [standard deviation, 66.9]; P=.02); however, ultimate load and rotational stiffness were not statistically significant. Compared with triangular osteosynthesis fixation, the use of 2 transsacral screws provides a comparable biomechanical stability profile in both translation and rotation. This newly revised 2-transsacral-screw construct offers the traumatologist an alternative method of repair for vertical shear fractures that provides biplanar stability. It also offers the advantage of percutaneous placement in either the prone or supine position.

Biomechanical evaluation of an expandable cage in single-segment posterior lumbar interbody fusion.

STUDY DESIGN: Controlled laboratory study. OBJECTIVE: To evaluate the biomechanical characteristics of a new expandable interbody cage in single-segment posterior lumbar interbody fusion (PLIF) using cadaveric lumbar spines. SUMMARY OF BACKGROUND DATA: One of the popular methods of treating lumbar spine pathologies involves a posterior lumbar interbody fusion using bilateral interbody nonexpandable cages. However, this method can require extensive bony removal and nerve root retraction. Expandable interbody cages may decrease the risk associated with PLIFs. METHODS: Biomechanical testing was performed on 5 fresh frozen L4/L5 mobile functional spinal units using a custom testing system that permits 6 df and a digital video digitizing system. The specimens were tested intact, postdiscectomy, after interbody cage placement, and after cage placement and pedicle screw fixation. Each specimen was tested from 0.5 to 8.0 N·m for extension, flexion, lateral bending, and rotation, and from 5 to 300 N for axial compression. The angular displacement, stiffness, disc height, and sagittal alignment were determined. RESULTS: When the cage was supplemented with pedicle screw fixation, the mean angular displacement for rotation and lateral bending was significantly less than all other conditions (P < 0.05). The percentage range of motion (% ROM) showed a statistically significant decrease in lateral bending (P < 0.05) for cage alone vs. postdiscectomy. For the pedicle screw construct, rotation showed a significantly lower percentage ROM compared with all other constructs (P < 0.05), and lateral bending and extension-flexion showed a significantly lower percentage ROM compared with postdiscectomy (P < 0.05). For all motions, stiffness of the cage and pedicle screw construct was greater than intact, with only rotation showing a statistically significant increase (P < 0.05). Anterior disc height was restored to intact after cage alone (P < 0.05). Sagittal alignment did not show statistically significant differences. CONCLUSION: PLIF using expandable lumbar interbody cage requires pedicle screw fixation.

Biomechanical evaluation of short-segment posterior instrumentation with and without crosslinks in a human cadaveric unstable thoracolumbar burst fracture model.

STUDY DESIGN: This study evaluates the biomechanical characteristics of spinal instrumentation constructs in a human unstable thoracolumbar burst fracture model simulated by corpectomy. OBJECTIVE: To compare the biomechanical characteristics of short-segment posterior instrumentation, with and without crosslinks, in a human unstable burst fracture model simulated by corpectomy. SUMMARY OF BACKGROUND DATA: Unstable thoracolumbar burst fractures are serious injuries, and their management remains controversial. Some authors advocate the use of short-segment posterior instrumentation for certain burst fractures. Whether crosslinks contribute additional stability has not been determined. METHODS: Six fresh frozen human spines (T10-L2) were potted to isolate the T11-L1 segments, and biomechanically tested in axial rotation, lateral bending, flexion, and extension. A custom spine testing system was used that allows motion with 6 degrees of freedom. After testing was completed on intact specimens, a corpectomy was performed at T12 to simulate an unstable burst fracture with loss of anterior and middle column support. Short-segment transpedicular instrumentation was then performed from T11 to L1. Each specimen was retested with 1, 2, or no crosslinks. Construct stiffness and motion data were analyzed with each intact specimen serving as its own internal control. RESULTS: Torsional stiffness in axial rotation was significantly increased (P < 0.05) in short-segment fixation constructs with 1 and 2 crosslinks, but none was restored to the preinjury baseline level. Significant reductions in standardized motion were also achieved with 1 and 2 crosslinks compared to no crosslinks (P < 0.05), but they remained greater than baseline. Crosslinks significantly increased stiffness and decreased motion in lateral bending, beyond the baseline level (P < 0.05). In flexion, all constructs had significantly decreased stiffness and increased motion compared to the intact specimen (P < 0.05), with crosslinks providing no additional benefit. Conversely, none of the constructs demonstrated a significant change in extension compared to baseline (P > 0.05). When attempting to load the constructs to failure, screw pullout was seen in all specimens. CONCLUSION: Crosslinks, when added to short-segment posterior fixation, improve stiffness and decrease motion in axial rotation, but do not restore baseline stability in this corpectomy model. Short-segment posterior fixation is also inadequate in restoring stability in flexion with injuries of this severity. Short-segment posterior instrumentation alone can achieve baseline stability in lateral bending, and crosslinks provide even greater stiffness.

Site specificity of transverse crista terminalis conduction in patients with atrial flutter.

INTRODUCTION: The causes of transcristal conduction (TC) in patients with atrial flutter (AFL) are unknown. METHODS AND RESULTS: In two groups of patients referred for AFL ablation, 36 had cavotricuspid isthmus (CTI) dependent flutter (Group I) and 24 had lower (n = 21) or upper loop reentry (n = 5) (Group II). After ablation, isthmus block was evaluated by pacing from the coronary sinus (CS) and low lateral right atrium and by alternative techniques, including mapping with electrodes spanning the CTI or electroanatomic mapping. After bidirectional CTI block was verified, 21/36 (58%) in Group I showed TC with CS pacing, including low TC in 16 (including 11 showing "pseudo" CTI conduction), higher TC in 6 and multiple breaks in 3. However, 8 with low TC during CS pacing showed unidirectional block by pacing outside of the CS os. Twelve (50%) in Group II had TC during CS pacing after bidirectional CTI block, with low TC in 5 (2 mimicking residual CTI conduction) and higher breaks in 9. There was no significant difference in the incidence of TC during CS pacing after CTI block between groups. In seven autopsied hearts, the muscle orientation between the proximal CS musculature and Eustachian ridge were examined. Muscular connections between the CS and Eustachian ridge coursing toward the orifice of inferior vena cava were found in one of the hearts. CONCLUSIONS: It is concluded that in patients with bidirectional CTI block, pacing from the CS may be associated with TC mimicking a conduction leak through the isthmus. Pacing just outside the CS os helps distinguish pseudo from true isthmus block.

Intervertebral disc cell therapy for regeneration: mesenchymal stem cell implantation in rat intervertebral discs.

This study explores the use of mesenchymal stem cells (MSCs) for intervertebral disc regeneration. We used an in vivo model to investigate the feasibility of exogenous cell delivery, retention, and survival in the pressurized disc space. MSC injection into rat coccygeal discs was performed using 15% hyaluronan gel as a carrier. Injections of gel with or without MSCs were performed. Immediately after injection, fluorescently labeled stem cells were visible on sections of cell-injected discs. Seven and 14 days after injection, stem cells were still present within the disc, but their numbers were significantly decreased. At 28 days, a return to the initial number of injected cells was observed, and viability was 100%. A trend of increased disc height compared to blank gel suggests an increase in matrix synthesis. The results indicate that MSCs can maintain viability and proliferate within the rat intervertebral disc.