Maximizing Anterior Vertebral Maximizing Anterior Vertebral Screw Fixation for Spinal Screw Fixation for Spinal Growth Tethering Growth Tethering 1 Eric S. Varley, DO 1 Eric S. Varley, DO 2 Christine L. Farnsworth, MS 2 Christine L. Farnsworth, MS 2 Tucker Tomlinson, MS 2 Tucker Tomlinson, MS Claire Robertson, BS 2 2 Claire Robertson, BS 2 Thomas N. Nunn, BS 2 Thomas N. Nunn, BS 1 Salil V. Upasani, MD 1 Salil V. Upasani, MD 1,2 Peter O. Newton, MD 1,2 Peter O. Newton, MD 1 University of California, San Diego, CA 2 Rady Children’s Hospital, San Diego, CA
Potential Fusionless Scoliosis Potential Fusionless Scoliosis Correction Correction • Proposed Etiology Proposed Etiology • Theory: Anterior spinal Theory: Anterior spinal 1 overgrowth 1 overgrowth • Propagated by: Heuter Propagated by: Heuter- - • 2 Volkmann effect 2 Volkmann effect • Spinal buckling Spinal buckling • • Fusionless treatments Fusionless treatments • – Tension posterior spine Tension posterior spine – – Compress anterior spine Compress anterior spine – Anterior Spinal Tethering Anterior Spinal Tethering
Anterior Spinal Growth Tethering Anterior Spinal Growth Tethering • Applies a compressive Applies a compressive • force to the physes on force to the physes on convexity of the curve convexity of the curve • Shown to create Shown to create • deformity in animal deformity in animal 3,4 models 3,4 models
4 Anterolateral Flexible Tether 4 Pilot Study - - Anterolateral Flexible Tether Pilot Study • 6 animals tethered over four levels • 6 animals tethered over four levels ± 8.3 ° at 6 month (n=12) 12.4 ± 8.3 ° Scoliosis creation: at 6 month (n=12) • Scoliosis creation: 12.4 • ± 14.4 ° at 12 month (n=6) 26.8 ± 14.4 ° at 12 month (n=6) 26.8 ± 1.3mm at 6 months* 4.4 ± • Vertebral wedging: 1.3mm at 6 months* • Vertebral wedging: 4.4 ± 3.9mm at 12 months* 8.5 ± 8.5 3.9mm at 12 months* Conclusion: Tethering Alters Vertebral Growth • • Conclusion: Tethering Alters Vertebral Growth Spinal Growth PA Lat PA Lat
Fixation Problems Fixation Problems st Generation Implant Previous Bovine Growth Study: 1 st Generation Implant Previous Bovine Growth Study: 1 Screws levered & plowed through the bone Screws levered & plowed through the bone
Effect of Intra- -Op Deformity Op Deformity Effect of Intra Correction on Screw Fixation? Correction on Screw Fixation? 1. Immediate deformity correction Immediate deformity correction 1. Intra- -Operative Tether Tensioning Operative Tether Tensioning Intra 5 2. Improve Screw Fixation Improve Screw Fixation 5 2. Hydroxyapatite Coating Hydroxyapatite Coating
Purpose Purpose To evaluate the effect of hydroxyapatite To evaluate the effect of hydroxyapatite (HA) coating of the vertebral body (HA) coating of the vertebral body screws and intra- -operative tensioning of operative tensioning of screws and intra the tether on screw integration as the tether on screw integration as measured by the screw extraction measured by the screw extraction torque. torque. Clinical Application Experimental Application
Study Design Study Design Yucatan mini-pigs (n=8) Instrumented 4 Levels Tensioned Tether Un-tensioned tether (n=4) (n=4) Implant screws: Implant screws: Implant screws: Implant screws: ( - ) HA Coating ( + ) HA Coating ( - ) HA Coating ( + ) HA Coating (n=2) (n=2) (n=2) (n=2) (n=8 screws) (n=8 screws) (n=8 screws) (n=8 screws)
Methods Methods • T8 T8- -T11 T11 – – UHMWPE Tether/Screw Construct UHMWPE Tether/Screw Construct • – Non Non- -Tensioned group Tensioned group (n=4): Slack taken out (n=4): Slack taken out – – Tensioned group Tensioned group (n=4): Intra (n=4): Intra- -op tensioning (250N) op tensioning (250N) – • Animals grow for 12 months Animals grow for 12 months • – Monthly Biplanar X Monthly Biplanar X- -rays rays – – Post Harvest 12 month 3T MRI Post Harvest 12 month 3T MRI – • Following Harvest: Uncoated screws placed Following Harvest: Uncoated screws placed • T7 & T8 T7 & T8 Time Zero (T0) Time Zero (T0) T12 &T13 T12 &T13 Controls Controls
Screw Extraction Analysis Screw Extraction Analysis • Focus on the bone Focus on the bone- - • 6 screw interface 6 screw interface • Screws rotated with Screws rotated with • custom jig custom jig • No Axial load No Axial load • • Data processed for Data processed for • 1.Yield Torque Yield Torque 1. 2.Yield Angle Yield Angle 2.
Biomechanical Data Collection Biomechanical Data Collection Force x moment Yield Torque (N-mm) s s e n f Yield f i t S Angle Degrees of Rotation ( ° )
Statistical Analysis Statistical Analysis ONE- -WAY ANOVA WAY ANOVA TWO- -WAY ANOVA WAY ANOVA ONE TWO Changes with time Effects of HA & Tensioning Changes with time Effects of HA & Tensioning • Each Group vs. Controls Each Group vs. Controls (1) Coating the screws • (1) Coating the screws (2) tensioning the tether (2) tensioning the tether (3) Interaction (3) Interaction 4 experimental groups: 4 experimental groups 1. Tensioned & HA Coating 1. Tensioned & HA Coating 2. 2. Tensioned & No Coating Tensioned & No Coating 3. Un- -tensioned & HA Coating tensioned & HA Coating 3. Un 4. Un- -tensioned & No Coating tensioned & No Coating 4. Un
Deformity Creation Deformity Creation Tensioned 35 Non-Tensioned 30 Created Coronal Deformity (degrees) 25 Vertebral body + Disc 20 * * * * 15 * Disc * 10 * 5 0 Pre- Post- 1 2 3 4 5 6 7 8 9 10 11 12 op op Months Post-op -5 • Tensioned Group: Greater initial deformity • Tensioned Group: Greater initial deformity • Equal total growth modulation to non Equal total growth modulation to non- -tensioned group tensioned group •
Post-harvest MR Analysis (3T) Non-Tensioned Tensioned •Nucleus Pulposus migrated toward tether in both surgical groups tensioned >> non-tensioned (p=0.02) 12 months post-op, all discs “healthy” except one tensioned and one non-tensioned motion segment with no T2 bright signal: 2/36 = 6% of all discs 12-mo post-op T2 12-mo post-op T2
Yield Angle Comparison Yield Angle Comparison 1-WAY ANOVA (p<0.05) • Tensioned, Coated Group 2-WAY ANOVA (p<0.05) • Tensioning the screw • Coating the screw
Yield Torque Comparison Yield Torque Comparison 1-WAY ANOVA (p<0.05) • Tensioned, Coated Group • Tensioned, Uncoated Group 2-WAY ANOVA (p<0.001) • Tensioning the screw � No screws had Torque < 500 Nmm � No screws had Torque < 500 Nmm
Conclusions Conclusions Deformity Creation Deformity Creation 1. 1. Greater initial deformity, Same ultimate deformity Greater initial deformity, Same ultimate deformity 2. NP Migration: Tension>>Non- -tensioned tensioned 2. NP Migration: Tension>>Non 3. 3. Disc Health: No difference Disc Health: No difference Screw out testing: bone- -implant interface integrity implant interface integrity Screw out testing: bone 1. Contrary to initial concerns 1. Contrary to initial concerns Tensioning appears to INCREASE INCREASE fixation fixation Tensioning appears to More vigorous response to the greater biomechanical forces transmitted mitted – – More vigorous response to the greater biomechanical forces trans to the bone- -screw interface screw interface to the bone 2. HA coating may moderately enhance osseous integration 2. HA coating may moderately enhance osseous integration – Osteoconductive properties – Osteoconductive properties
References References 1. Somerville EW. Rotational lordosis: the development of the single curve. J Bone Joint Surg (Br) 1952 2. Stokes IA et al., Biomechanical spinal growth modulation and progressive adolescent scoliosis-a test of the ‘vicious cycle’ pathogenetic hypothesis: summary of an electronic focus group debate of the IBSE, Scoliosis 2006 3. Newton PO et al., Spinal growth modulation with use of a tether in an immature porcine model, JBJS 2008 4. Newton PO et al., Multilevel Spinal Growth Modulation with an Anterolateral Flexible Tether in an Immature Bovine Model, Spine 2005 5. Upasani VV et al., Pedicle screw surface coatings improve fixation in nonfusion spinal constructs, Spine 2009 6. Sandén B et al., Improved extraction torque of hydroxyapatite- coated pedicle screws, Eur Spine J 2000
Recommend
More recommend
