Haines Paik

Low lumbar burst fractures

BACKGROUND CONTEXT The most common location for burst fractures occurs at the thoracolumbar junction, where the stiff thoracic spine meets the more flexible lumbar spine. With our current military conflicts in Iraq and Afghanistan, we have seen a disproportionate number of low lumbar burst fractures. PURPOSE To report our institutional experience in the management of low lumbar burst fractures. STUDY DESIGN Retrospective review. METHODS We performed a retrospective review of medical records and radiographs for all patients treated at our institution with combat-related injuries and thoracolumbar fractures. We included all patients who had sustained a burst fracture from T12 to L5 and had at least 1-year clinical follow-up. RESULTS Thirty-two patients sustained burst fractures. Nineteen patients (59.4%) had low lumbar (L3-L5) burst fractures, and 12 patients (37.5%) had thoracolumbar junction (T12-L2) burst fractures as their primary injury. Additionally, seven patients sustained less severe burst fractures at an additional level. One patient sustained burst fractures at both upper and lower lumbar levels. Of the low lumbar fractures, 52.6% had evidence of neurologic injury, two of which were complete. Similarly, in the upper lumbar group, 58.2% sustained a neurologic injury, two of which were complete. Twenty-two patients underwent surgical intervention, complicated by infection in 18%. At most recent follow-up, all but one patient with presenting neurologic injury had persistent deficits. CONCLUSION Low lumbar burst fractures are the predominant combat-related spine injury in our current military conflicts. The rigidity offered by current body armor may effectively lower the transition zone that normally occurs at the thoracolumbar junction, thereby, transferring forces into the lower lumbar spine. Increased awareness of this fracture pattern is warranted by all surgeons because of unique clinical challenges associated with its treatment. Although the incidence is increased in the military population, other surgeons may be involved with long-term care of these patients on completion of their military service.

Low lumbar burst fractures: a unique fracture mechanism sustained in our current overseas conflicts

BACKGROUND CONTEXT The most common location for burst fractures occurs at the thoracolumbar junction, where the stiff thoracic spine meets the more flexible lumbar spine. With our current military conflicts in Iraq and Afghanistan, we have seen a disproportionate number of low lumbar burst fractures. PURPOSE To report our institutional experience in the management of low lumbar burst fractures. STUDY DESIGN Retrospective review. METHODS We performed a retrospective review of medical records and radiographs for all patients treated at our institution with combat-related injuries and thoracolumbar fractures. We included all patients who had sustained a burst fracture from T12 to L5 and had at least 1-year clinical follow-up. RESULTS Thirty-two patients sustained burst fractures. Nineteen patients (59.4%) had low lumbar (L3-L5) burst fractures, and 12 patients (37.5%) had thoracolumbar junction (T12-L2) burst fractures as their primary injury. Additionally, seven patients sustained less severe burst fractures at an additional level. One patient sustained burst fractures at both upper and lower lumbar levels. Of the low lumbar fractures, 52.6% had evidence of neurologic injury, two of which were complete. Similarly, in the upper lumbar group, 58.2% sustained a neurologic injury, two of which were complete. Twenty-two patients underwent surgical intervention, complicated by infection in 18%. At most recent follow-up, all but one patient with presenting neurologic injury had persistent deficits. CONCLUSION Low lumbar burst fractures are the predominant combat-related spine injury in our current military conflicts. The rigidity offered by current body armor may effectively lower the transition zone that normally occurs at the thoracolumbar junction, thereby, transferring forces into the lower lumbar spine. Increased awareness of this fracture pattern is warranted by all surgeons because of unique clinical challenges associated with its treatment. Although the incidence is increased in the military population, other surgeons may be involved with long-term care of these patients on completion of their military service.

Do Stand-Alone Interbody Spacers with Integrated Screws Provide Adequate Segmental Stability for Multilevel Cervical Arthrodesis?

BACKGROUND CONTEXT Some postoperative complications after anterior cervical fusions have been attributed to anterior cervical plate (ACP) profiles and the necessary wide operative exposure for their insertion. Consequently, low-profile stand-alone interbody spacers with integrated screws (SIS) have been developed. Although SIS constructs have demonstrated similar biomechanical stability to the ACP in single-level fusions, their role as a stand-alone device in multilevel reconstructions has not been thoroughly evaluated. PURPOSE To evaluate the acute segmental stability afforded by an SIS device compared with the traditional ACP in the setting of a multilevel cervical arthrodesis. STUDY DESIGN In vitro human cadaveric biomechanical analysis. METHODS Thirteen human cadaveric cervical spines (C2-T1) were nondestructively tested with a custom 6 df spine simulator under axial rotation, flexion-extension, and lateral bending loading. After intact analysis, eight single-levels (C4-C5/C6-C7) from four specimens were instrumented and tested with ACP and SIS. Nine specimens were tested with C5-C7 SIS, C5-C7 ACP, C4-C7 ACP, C4-C7 ACP+posterior fixation, C4-C7 SIS, and C4-C7 SIS+posterior fixation. Testing order was randomized with each additional level instrumented. Full range of motion (ROM) data were obtained and analyzed by each loading modality, using mean comparisons with repeated measures analysis of variance. Paired t tests were used for post hoc analysis with Sidak correction for multiple comparisons. RESULTS No significant difference in ROM was noted between the ACP and SIS for single-level fixation (p>.05). For multisegment reconstructions (two and three levels), the ACP proved superior to SIS and intact condition, with significantly lower ROM in all planes (p<.05). When either the three-level SIS or ACP constructs were supplemented with posterior lateral mass fixation, there was a greater than 80% reduction in ROM under all testing modalities (p<.05), with no significant difference between the ACP and SIS constructs (p>.05). CONCLUSIONS The SIS device may be a reasonable option as a stand-alone device for single-level fixation. However, SIS devices should be used with careful consideration in the setting of multilevel cervical fusion. However, when supplemented with posterior fixation, SIS devices are a sound biomechanical alternative to ACP for multilevel fusion constructs.

The biomechanical consequences of rod reduction on pedicle screws: should it be avoided?

BACKGROUND CONTEXT Rod contouring is frequently required to allow for appropriate alignment of pedicle screw-rod constructs. When residual mismatch is still present, a rod persuasion device is often used to achieve further rod reduction. Despite its popularity and widespread use, the biomechanical consequences of this technique have not been evaluated. PURPOSE To evaluate the biomechanical fixation strength of pedicle screws after attempted reduction of a rod-pedicle screw mismatch using a rod persuasion device. METHODS Fifteen 3-level, human cadaveric thoracic specimens were prepared and scanned for bone mineral density. Osteoporotic (n=6) and normal (n=9) specimens were instrumented with 5.0-mm-diameter pedicle screws; for each pair of comparison level tested, the bilateral screws were equal in length, and the screw length was determined by the thoracic level and size of the vertebra (35 to 45 mm). Titanium 5.5-mm rods were contoured and secured to the pedicle screws at the proximal and distal levels. For the middle segment, the rod on the right side was intentionally contoured to create a 5-mm residual gap between the inner bushing of the pedicle screw and the rod. A rod persuasion device was then used to engage the setscrew. The left side served as a control with perfect screw/rod alignment. After 30 minutes, constructs were disassembled and vertebrae individually potted. The implants were pulled in-line with the screw axis with peak pullout strength (POS) measured in Newton (N). For the proximal and distal segments, pedicle screws on the right side were taken out and reinserted through the same trajectory to simulate screw depth adjustment as an alternative to rod reduction. RESULTS Pedicle screws reduced to the rod generated a 48% lower mean POS (495±379 N) relative to the controls (954±237 N) (p<.05) and significantly decreased work energy to failure (p<.05). Nearly half (n=7) of the pedicle screws had failed during the reduction attempt with visible pullout of the screw. After reduction, decreased POS was observed in both normal (p<.05) and osteoporotic (p<.05) bone. Back out and reinsertion of the screw resulted in no significant difference in mean POS, stiffness, and work energy to failure (p>.05). CONCLUSIONS In circumstances where a rod is not fully seated within the pedicle screw, the use of a rod persuasion device decreases the overall POS and work energy to failure of the screw or results in outright failure. Further rod contouring or correction of pedicle screw depth of insertion may be warranted to allow for appropriate alignment of the longitudinal rods.

What is the best way to optimize thoracic kyphosis correction? A micro-CT and biomechanical analysis of pedicle morphology and screw failure.

Study Design. A human cadaveric biomechanical analysis. Objective. The purpose of this study was to evaluate the bone density/trabecular width of the thoracic pedicle and correlate that with its resistance against compressive loading used during correction maneuvers in the thoracic spine (i.e., cantilever bending). Summary of Background Data. As surgeons perform cantilever correction maneuvers in the spine, it is common to have pedicle screws pullout or displace while placing corrective forces on the construct. Currently, surgeons either compress against the cephalad aspect of the pedicle or vice versa. We set out to establish which aspect of the pedicle was the most dense and to determine the optimal direction for screw compression during kyphosis/deformity correction. Methods. Fifteen fresh-frozen cadaveric vertebrae (n = 15) were examined by micro–computed tomography to determine percent bone volume/total volume (%BV/TV) within the cephalad and caudad aspects of the pedicle. Specimens were sectioned in the sagittal plane. Pedicles were instrumented according to the straightforward trajectory on both sides. Specimens were then mounted and loading to failure was performed perpendicular to the screw axis (either the cephalad or the caudad aspect of the pedicle). Results. Mean failure when loading against the caudad aspect of the pedicle was statistically, significantly greater (454.5 ± 241.3 N vs. 334.79 1 ± 158.435 N) than for the cephalad pedicle (P < 0.001). In concordance with failure data, more trabecular and cortical bones were observed within the caudad half of the pedicle compared with the cephalad half (P < 0.001). Conclusion. Our results suggest that the caudad half of the pedicle is denser and withstands higher forces compared with the cephalad aspect. In turn, the incidence of intraoperative screw loosening and/or pedicle fracture may be reduced if the compressive forces (cantilever bending during deformity correction) placed upon the construct are applied against the caudad portion of the pedicle.

The ventral lamina and superior facet rule: a morphometric analysis for an ideal thoracic pedicle screw starting point.

BACKGROUND CONTEXT With the increasing popularity of thoracic pedicle screws, the freehand technique has been espoused to be safe and effective. However, there is currently no objective, definable landmark to assist with freehand insertion of pedicle screws in the thoracic spine. With our own increasing surgical experience, we have noted a reproducible and unique anatomic structure known as the ventral lamina. PURPOSE We set out to define the morphologic relationship of the ventral lamina to the superior articular facet (SAF) and pedicle, and describe an optimal medial-lateral pedicle screw starting point in the thoracic spine. STUDY DESIGN We conducted an in vitro fresh-frozen human cadaveric study. METHODS One hundred fifteen thoracic spine vertebral levels were evaluated. After the vertebral body was removed, Kirschner wires were inserted retrograde along the four boundaries of the pedicle. Using digital calipers, we measured width of the SAF and pedicle at the isthmus, and from the borders of the SAF to the boundaries of the pedicle. We calculated the morphologic relationship of the ventral lamina and the center of the pedicle (COP) to the SAF. RESULTS Two hundred twenty-nine pedicles were measured, with one pedicle excluded because of fracture of the SAF during disarticulation. The ventral lamina was clearly identifiable at all levels, forming the roof of the spinal canal and confluent with the medial pedicle wall (MPW). The mean distance from the SAF midline to the MPW was 1.36±1.23 mm medial. The MPW was lateral to SAF midline in 34 pedicles (14.85%) and, on average, was a distance of 0.52±0.51 mm lateral. The mean distance from the SAF midline to the COP was 2.17±1.38 mm lateral. The COP was medial to SAF midline in only 11 pedicles (4.80%). CONCLUSIONS The ventral lamina is an anatomically reproducible structure located consistently medial to the SAF midline (85%). We also found the COP consistently lateral to the SAF midline (95%). Based on these morphologic findings, the medial-lateral starting point for thoracic pedicle screws should be 2 to 3 mm lateral to the SAF midline (superior facet rule), allowing screw placement in the COP and avoiding penetration into the spinal canal.