Resisting subsidence with a truss Implant: Application of the "Snowshoe" principle for interbody fusion devices.

The primary objective was to compare the subsidence resistance properties of a novel 3D-printed spinal interbody titanium implant versus a predicate polymeric annular cage. We evaluated a 3D-printed spinal interbody fusion device that employs truss-based bio-architectural features to apply the snowshoe principle of line length contact to provide efficient load distribution across the implant/endplate interface as means of resisting implant subsidence. Devices were tested mechanically using synthetic bone blocks of differing densities (osteoporotic to normal) to determine the corresponding resistance to subsidence under compressive load. Statistical analyses were performed to compare the subsidence loads and evaluate the effect of cage length on subsidence resistance. The truss implant demonstrated a marked rectilinear increase in resistance to subsidence associated with increase in the line length contact interface that corresponds with implant length irrespective of subsidence rate or bone density. In blocks simulating osteoporotic bone, comparing the shortest with the longest length truss cage (40 vs. 60 mm), the average compressive load necessary to induce subsidence of the implant increased by 46.4% (383.2 to 561.0 N) and 49.3% (567.4 to 847.2 N) for 1 and 2 mm of subsidence, respectively. In contrast, for annular cages, there was only a modest increase in compressive load when comparing the shortest with the longest length cage at a 1 mm subsidence rate. The Snowshoe truss cages demonstrated substantially more resistance to subsidence than corresponding annular cages. Clinical studies are required to support the biomechanical findings in this work.

Finite Element Comparison of the Spring Distraction System and the Traditional Growing Rod for the Treatment of Early Onset Scoliosis.

STUDY DESIGN: Finite element analysis (FEA). OBJECTIVE: The aim of this study was to determine biomechanical differences between traditional growing rod (TGR) and spring distraction system (SDS) treatment of early-onset scoliosis. SUMMARY OF BACKGROUND DATA: Many "growth-friendly" implants like the TGR show high rates of implant failure, spinal stiffening, and intervertebral disc (IVD) height loss. We developed the SDS, which employs continuous, dynamic forces to mitigate these limitations. The present FEA compares TGR and SDS implantation, followed by an 18-month growth period. METHODS: Two representative, ligamentous, scoliotic FEA models were created for this study; one representing TGR and one representing SDS. initial implantation, and up to 18 months of physeal spinal growth were simulated. The SDS model was continuously distracted over this period; the TGR model included two additional distractions following index surgery. Outcomes included differences in rod stress, spinal morphology and iVD stress-shielding. RESULTS: Maximum postoperative von Mises stress was 249MPa for SDS, and 205MPa for TGR. During the 6-month TGR distraction, TGR rod stress increased over two-fold to a maximum stress of 417MPa, compared to a maximum of 262 MPa in the SDS model at 6-month follow-up. During subsequent follow-up periods, TGR rod stress remained consistently higher than stresses in the SDS model. Additional lengthenings in the TGR model led to a smaller residual curve (16.08) and higher T1-S1 growth (359 mm) at 18-month follow-up compared to the SDS model (26.98, 348 mm). During follow-up, there was less stress-shielding of the IVDs in the SDS model, compared to the TGR model. At 18-month follow-up, upper and lower IVD surfaces of the SDS model were loaded more in compression than their TGR counterparts (mean upper: +112 ±â€Š19N; mean lower: +100 ±â€Š17N). CONCLUSION: In the present FEA, TGR treatment resulted in slightly larger curve correction compared to SDS, at the expense of increased IVD stress-shielding and a higher risk of rod fractures. LEVEL OF EVIDENCE: N/A.

The Transverse Process Trajectory Technique: An Alternative for Thoracic Pedicle Screw Implantation-Radiographic and Biomechanical Analysis.

BACKGROUND: This study evaluates the accuracy, biomechanical profile, and learning curve of the transverse process trajectory technique (TPT) compared to the straightforward (SF) and in-out-in (IOI) techniques. SF and IOI have been used for fixation in the thoracic spine. Although widely used, there are associated learning curves and symptomatic pedicular breaches. We have found the transverse process to be a reproducible pathway into the pedicle. METHODS: Three surgeons with varying experience (experienced [E] with 20 years in practice, surgeon [S] with less than 10 years in practice, and senior resident trainee [T] with no experience with TPT) operated on 8 cadavers. In phase 1, each surgeon instrumented 2 cadavers, alternating between TPT and SF from T1 to T12 (n = 48 total levels). In phase 2, the E and T surgeons instrumented 1 cadaver each, alternating between TPT and IOI. Computed tomography scans were analyzed for accuracy of screw placement, defined as the percentage of placements without critical breaches. Axial pullout and derotational force testing were performed. Statistical analyses include paired t test and analysis of variance with Tukey correction. RESULTS: Overall accuracy of screw placement was comparable between techniques (TPT: 92.7%; SF: 97.2%; IOI: 95.8%; P = .4151). Accuracy by technique did not differ for each individual surgeon (E: P = .7733; S: P = .3475; T: P = .4191) or by experience level by technique (TPT: P = .1127; FH: P = .5979; IOI: P = .5935). Pullout strength was comparable between TPT and SF (571 vs 442 N, P = .3164) but was greater for TPT versus IOI (454 vs 215 N, P = .0156). There was a trend toward improved derotational force for TPT versus SF (1.06 vs 0.93 Nm/degrees, P = .0728) but not for TPT versus IOI (1.36 vs 1.16 Nm/degrees, P = .74). Screw placement time was shortest for E and longest for T for TPT and SF and not different for IOI (TPT: P = .0349; SF: P < .0001; IOI: P = .1787) but did not vary by technique. CONCLUSIONS: We describe the TPT, which uses the transverse process as a corridor through the pedicle. TPT is an accurate method of thoracic pedicle screw placement with potential biomechanical advantages and with acceptable learning curve characteristics. CLINICAL RELEVANCE: This study provides the surgeon with a new trajectory for pedicle screw placement that can be used in clinical practice.

Does Spanning a Lateral Lumbar Interbody Cage Across the Vertebral Ring Apophysis Increase Loads Required for Failure and Mitigate Endplate Violation.

STUDY DESIGN: Randomized Biomechanical Cadaveric Study-Level II. OBJECTIVE: We aimed to elucidate that placing lateral lumbar interbody cages that span the stronger ring apophysis will require increasing loads for failure, decreasing rates of subsidence, regardless of bone density or endplate integrity. SUMMARY OF BACKGROUND DATA: There are several reports regarding the rates and grades of cage subsidence when utilizing the lateral lumbar interbody fusion technique. However, there is limited data on how spanning the lateral cage across the ring apophysis can prevent it. METHODS: Eight fresh-frozen human spines (L1-L5) were utilized. Each vertebra was placed with their endplates horizontal in an MTS actuator. A total of 40 specimens were randomized into Groups:Load displacement data was collected at 5 Hz until failure. RESULTS: Longer cages spanning the ring apophysis provided more strength in compression with less subsidence relative to shorter cages, regardless of endplate integrity.Longer cages, spanning the ring apophysis, resting on intact endplates (G2) had a significant (P < 0.05) increase in strength and less subsidence when compared with the smaller cage group resting on intact endplates (G1) (P = 0.003).Longer cages spanning the ring apophysis of intact endplates (G2) showed a significant (P < 0.05) increase in strength and resistance to subsidence when compared with similar length cages resting on decorticated endplates (G4) (P = 0.028). CONCLUSION: Spanning the ring apophysis increased the load to failure by 40% with intact endplates and by 30% with decorticated endplates in this osteoporotic cadaveric model. Larger cages that span the endplate ring apophysis could improve the compressive strength and decrease subsidence at the operative level despite endplate violation or osteoporosis. LEVEL OF EVIDENCE: 2.

Morphological changes of the caudal cervical intervertebral foramina due to flexion-extension and compression-traction movements in the canine cervical vertebral column.

BACKGROUND: Previous studies in humans have reported that the dimensions of the intervertebral foramina change significantly with movement of the spine. Cervical spondylomyelopathy (CSM) in dogs is characterized by dynamic and static compressions of the neural components, leading to variable degrees of neurologic deficits and neck pain. Studies suggest that intervertebral foraminal stenosis has implications in the pathogenesis of CSM. The dimensions of the cervical intervertebral foramina may significantly change during neck movements. This could have implication in the pathogenesis of CSM and other diseases associated with radiculopathy such as intervertebral disc disease. The purpose of this study was to quantify the morphological changes in the intervertebral foramina of dogs during flexion, extension, traction, and compression of the canine cervical vertebral column. All vertebral columns were examined with magnetic resonance imaging prior to biomechanic testing. Eight normal vertebral columns were placed in Group 1 and eight vertebral columns with intervertebral disc degeneration or/and protrusion were assigned to Group 2. Molds of the left and right intervertebral foramina from C4-5, C5-6 and C6-7 were taken during all positions and loading modes. Molds were frozen and vertical (height) and horizontal (width) dimensions of the foramina were measured. Comparisons were made between neutral to flexion and extension, flexion to extension, and traction to compression in neutral position. RESULTS: Extension decreased all the foraminal dimensions significantly, whereas flexion increased all the foraminal dimensions significantly. Compression decreased all the foraminal dimensions significantly, and traction increased the foraminal height, but did not significantly change the foraminal width. No differences in measurements were seen between groups. CONCLUSIONS: Our results show movement-related changes in the dimensions of the intervertebral foramina, with significant foraminal narrowing in extension and compression.

Effects of flexion and extension on the diameter of the caudal cervical vertebral canal in dogs.

OBJECTIVE: To quantify changes in the diameter of the vertebral canal with flexion and extension in the cervical vertebral column. STUDY DESIGN: Cadaveric biomechanical study. SAMPLE POPULATION: Cadaveric canine cervical vertebral column (n = 16 dogs). METHODS: All vertebral columns were evaluated with MRI. Group 1 consisted of 8 normal vertebral columns. Group 2 included 8 vertebral columns with intervertebral disc degeneration. Flexion, extension, compression, and tension were applied to the caudal cervical region (C4-5, C5-6, C6-7). Sagittal vertebral canal diameters (VCD) were obtained by measuring the distance between the ventral and dorsal aspects of vertebral canal. RESULTS: No differences were seen between groups, thus the results are for both groups. Comparison of VCD between flexion and extension with no load revealed a difference of 2.2 mm (28.9%; P < .001). Comparison between neutral position and extension revealed a reduction of 1.5 mm (16.5%; P < .001), whereas comparison between neutral and flexion showed an increase of 0.7 mm (7.7%; P = .001) in VCD. Comparison between neutral with no load and neutral with compression showed a difference of 0.5 mm, with reduction of 5.5% in the vertebral canal (P = .006). Comparison of extension with no load versus extension with tension revealed an increase of 0.7 mm (9.2%) in the vertebral canal (P < .001). CONCLUSIONS: Cervical vertebral canal diameter decreased significantly with extension and increased with flexion. The results support the presence of dynamic impingement possibly playing a role in diseases characterized by vertebral canal stenosis, such as cervical spondylomyelopathy.