- Email: [email protected]
Biomechanical Aspects of Pedicle Screw Loosening in the Lumbar Spine
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Proceedings of the NASS 26th Annual Meeting / The Spine Journal 11 (2011) 1S–173S
vertebrae were divided into Black line or non-Black line. On lateral view at 6 months X-p, pseudarthrosis and kyphosis were evaluated. Dynamic motion or cleft sign in fractured vertebra was defined as pseudarthrosis. Kyphosis rate was obtained by subtracting the anterior vertebral height in the injured vertebra from the average anterior vertebral height of the normal vertebrae above and below the injured vertebra in the lateral images and dividing the value by the average vertebral height of the normal vertebrae. Kyphosis progression rate was obtained by subtracting the kyphosis rate on initial examination from that at 6 months and dividing the value by the initial examination. All measurements were performed with the investigator blinded concerning the patient’s name, clinical findings, and other imaging data. Visual analogue scale for back pain (VAS) was evaluated at 6 months examination. The chi-square test was used for pseudarthrosis rate, and the Student’s t-test was used for kyphosis progression rate and VAS. p!.05 was taken as a statistically significant difference. RESULTS: Fourteen of 63 vertebrae (22%) resulted in pseudarthrosis. Powder sign (n516) included 0 (0%) pseudarthrosis, and non-Powder sign (n547) included 14 (30%), and Powder sign included pseudarthrosis significantly less than non-Powder sign (p5.01). Sensitivity of bone union by Powder sign was 33%, and specificity of that was 100%. Black line (n524) included 11 (46%) pseudarthrosis, non-Black line (n539) included 4 (10%), and Black line included pseudarthrosis significantly more than non-Black line (p5.004). Sensitivity of pseudarthrosis by Black line was 71%, and specificity of that was 71%. Kyphosis progression rate of Powder sign was 11% (621%) and that of non-Powder sign was 23% (625%), and there was no significant difference (p5.08). Kyphosis progression rate of Black line was 32% (622%) and that of non-Black line was 13% (622%), and there was a significant difference (p5.003). The average value of VAS of powder sign was 26 mm (618 mm) and that of nonPowder sign was 28 mm (623 mm), and there was no significant difference. The average value of VAS of Black line was 35 mm (626 mm) and that of non-Black line was 23 mm (617 mm), and there was a significant difference (p5.04). CONCLUSIONS: On MRI STIR, Powder sign can predict bone union, and Black line was the risk factor of pseudarthrosis, kyphosis progression and back pain on osteoporotic vertebral fractures. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2011.08.237
Friday, November 4, 2011 4:45–5:45 PM Focused Paper Presentations: Biomechanics 183. In Vivo Evaluation of a Novel Expanding Pedicle Screw: A Six-Month Study in a Sheep Model Mark Levy, MD, ScD1, Ory Keynan, MD2, Orna Popper3, Elad Sapir3, Yair Spanier3, Michal Ruchelsman3, Jaffar Hleihil3; 1Technion Israel Institute of Technology, Haifa, Isreal; 2Kokhav Yair, Isreal; 3Expanding Orthopedics, Caesarea, Isreal BACKGROUND CONTEXT: Solid fixation of pedicle screws is the cornerstone of successful spinal fusion. It’s particularly important in compromised bone quality such as elderly population, various bone pathologies and revision surgery. Early fixation of pedicle screws as well as long term implant osteointegration, minimize the rate of failure optimizing the chances of fusion. PURPOSE: The objective of the study was to evaluate in-vivo a novel titanium expanding pedicle screw, comprising a screw section in the pedicle and a 4-wing expandable portion in the vertebral body. The study assessed the mechanical performance and biological fixation with time, including implantation and deployment, characterization of bone ingrowth on the
exterior and interior surface of the expandable portion and the ability to remove the screw after bone ingrowth. STUDY DESIGN/SETTING: A GLP-compliant animal study using standard and expanding pedicle screws for lumbar spine fusion. PATIENT SAMPLE: Eleven skeletally-mature Suffolk sheep, 80-120 kg at implantation day. OUTCOME MEASURES: Clinical and radiographic data, histology of specimens, deployment and un-deployment, removal of the expanding pedicle screw. METHODS: Lumbar spines of 11 sheep were implanted with a total of 44 expanding pedicle screws Ø5.8 mm and 22 standard pedicle screws Ø5.5 mm (control), using standard spinal fusion techniques and instrumentation. In each spine 6 screws were inserted, one per vertebra, through a standard posterior trans-pedicle approach, in 3 one-level unilateral fusion pairs: one pair of controls and 2 pairs of expanding pedicle screws. Each pair was connected using Ø5 mm rods. No discs were removed and no bone graft was added. Pre and post-operative x-ray films were obtained. The animals were kept up to 180-day follow-up period with no activity restriction. Intermediate time points were at 35, 77 and 90 days postoperation. At the end of each time point, the animals were sacrificed and the spines were harvested for removal of the screws or histological evaluation. RESULTS: Successful deployment of the expanding pedicle screws was achieved in all implanted screws using an active mechanism. The histological analysis demonstrated osteointegration of the expanding pedicle screw in exterior and interior surface of the expandable portion from the 35-day time point and onward. The average amount of bone ingrowth inside the expandable area was elevated with each consecutive time point showing maturation of the bone matrix from woven bone to lamellar osteonal bone over time. Removal of the expanding pedicle screws was achieved for all screws despite the bone ingrowth, due to an active undeployment mechanism allowing collapse of the expandable area and removal through the pedicle. CONCLUSIONS: Implantation and deployment of a novel expanding pedicle screw was demonstrated in the lumbar spine of mature sheep. The increased surface area of the metal-bone interface allowed immediate fixation. The increased osteointegration with time, as well as bone ingrowth inside the expandable area, provided a progressively robust fixation of the screw within the vertebral body. The immediate mechanical fixation followed by the long-term biological fixation was designed to enhance construct stability and subsequent fusion. The ability to undeploy and remove the screws demonstrated the mechanical properties of this novel screw and its instrumentation. FDA DEVICE/DRUG STATUS: Expanding Pedicle Screw: Not approved for this indication. doi: 10.1016/j.spinee.2011.08.239
184. Biomechanical Aspects of Pedicle Screw Loosening in the Lumbar Spine Joel Boerckel, PhD1, Liz Peters2, Lalissie Merga3, Tamkeenat Syed4, Mona Arabshahi3, Rick Chappuis, PA-C3, James Chappuis, MD, FACS5; 1 Georgia Tech, Atlanta, GA, USA; 2Ball Ground, GA, USA; 3Atlanta, GA, USA; 4Crystal Lake, IL, USA; 5Orthopaedic & Spine Surgery of Atlanta, Atlanta, GA, USA BACKGROUND CONTEXT: Previously at NASS, we reported that removal of pedicle screws following fusion resulted in significant reduction in pain and medication use and increased activity and quality of life. As posterior pedicle screw systems increase in strength and rigidity, more loads are present at the bone screw interface and loosening can occur. PURPOSE: The purpose of this study was to quantify pedicle screw loosening using insertion and removal torque and evoked EMG measurement and identify factors which may predict loosening. STUDY DESIGN/SETTING: Patients received either pedicle screw insertion or removal at Emory University Hospital, Midtown, Atlanta, GA.
All referenced figures and tables will be available at the Annual Meeting and will be included with the post-meeting online content.
Proceedings of the NASS 26th Annual Meeting / The Spine Journal 11 (2011) 1S–173S PATIENT SAMPLE: 73 patients, age 46610 years, received 2 to 4-level fusion of L2-S1 vertebrae. Thrity-five patients received hardware removal 230630 days post-implantation. OUTCOME MEASURES: Outcome measures included: insertion torque, immediate removal torque, ultimate removal torque, evoked EMG stimulus threshold, screw size, and pedicle-screw clearance (pedicle thickness screw diameter). METHODS: Pedicle screw insertion (n5274) and removal (n5207) torques were measured using a manual torque wrench. Immediately upon insertion, pedicle screws were backed-out to assess immediate removal torque to ensure the insertion torque was not artificially elevated due to initial placement (n510). Paired samples (n526) were measured for both insertion and removal. EMG stimulation threshold currents for screw holes were measured prior to insertion and following removal. Paired and unpaired two-tailed t-tests were used to compare groups, as appropriate, with significance set at p!.05. Linear regressions were performed using GraphPad (Prism). All data reported as mean6SD. RESULTS: There were no differences between insertion and immediate removal torque (p5.56). However, upon ultimate removal, torque measurement yielded a 79.2% reduction in torque compared with insertion for paired measurements (p!.0001). Likewise, the EMG stimulation threshold was 33% lower at removal than at insertion in the paired evaluations (p!.05). Similar results for both torque and EMG threshold were found for unpaired measurements (n$207). Loosening was not affected by sex, age, vertebra level, or screw length, however, the pedicle-screw clearance (p!.05, R50.22) and screw diameter (p!.01, R50.17) correlated significantly with loosening, and for all samples in which the pedicle-screw clearance exceeded 10 mm, the removal torque was extremely low, less than 5 in-lb. CONCLUSIONS: Differences in insertion and removal torque and EMG stimulus threshold suggest that pedicle screws may loosen substantially during the 7-8 months following implantation as a result of degradation of bone matrix quality or quantity. Stimulus thresholds less than 6-8 mA have been shown to be suggestive of cortical perforation by pedicular instrumentation. In this study, despite acceptable threshold levels upon implantation, the average stimulus threshold upon removal was 7.5 mA, suggesting a possible degradation of bone matrix between the hole and nerve root. Also, correlations with pedicle-screw clearance and screw diameter suggest that larger screws which engage cortical bone in the pedicle may reduce screw loosening, and some form of augmentation may be advisable for particularly large pedicles which exceed screw diameter by 10 mm or more. Regardless of screw size, however, pedicle screws experienced significant loosening, likely as a result of matrix degradation and resorption due to gross modulus mismatch at the screw interface resulting in localized stress-shielding. Together these data may serve as a rationale for hardware removal following fusion. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2011.08.240
185. Kinematic Comparison of Lateral Plate Versus Pedicle Screw Construct in Lumbar Spine Instrumented with Lateral Cage at Two Levels Antonio Castellvi, MD1, Aniruddh Nayak, MS2, Brandon Santoni, PhD2, Andres Cabezas2, Sergio Gutierrez, PhD2, James Billys, MD3; 1Florida Orthopaedic Institute, Tampa, FL, USA; 2Foundation for Orthopaedic Research and Education, Tampa, FL, USA; 3Florida Orthopedic Institute, Brandon, FL, USA BACKGROUND CONTEXT: Fusion constructs are designed to provide maximum rigidity at the index level in order to facilitate adequate bone
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growth and fusion. In this study we assessed the biomechanical outcome of two different lumbar lateral cage constructs in stabilizing the instrumented level(s). PURPOSE: This study quantified the overall kinematic stability afforded by two lateral cage (NuVasive XLIFÒ) constructs. Pedicle screw þ rod (SpheRxÔ DBRÔ) Single sided lateral plate (XLP PlateÔ). STUDY DESIGN/SETTING: Cadaveric biomechanical study. PATIENT SAMPLE: 18 Cadaver lumbar spines. METHODS: Eighteen human cadaveric lumbar spines (L1-S1) were cleaned and potted at L1 and S1. Specimens were instrumented at L3L4 and L4-L5 with lateral cages. Nine specimens were instrumented with lateral plate (LP) at L3-L4 and L4-L5 and the remaining nine specimens with the pedicle screwþrod (PSR). Quasi-static pure moment loading was applied in flex/ext (F/E), lateral bending (L/B) and axial rotation (A/R) up to 7.5 Nm for intact and instrumented cases. Range of motion (ROM) at L3-L4 & L4-L5 was recorded using the OptrotrakÔ system and statistically compared. RESULTS: Statistically significant decrease in ROM was documented for both constructs. ROM results for LP group:F/E:At L3-L4 {(intact59.8 61.3; LP55.9 63.3; p5.005); L4-L5 (intact512.1 62.4; LP57.7 63.9; p5.012)}.; A/R:At L3-L4 {(intact54.6 62.5; LP52.4 60.9; p5.035); L4-L5 (intact56.3 63.1; LP53.1 62.2; p5.025)}. L/B:At L3-L4 {(intact511.8 62.7; LP56.2 64.4; p5.006); L4-L5 (intact512.1 63.9; LP54.8 62.9; p!.001)}. ROM results for PSR group:F/E:At L3-L4 {(intact: 10.762.4; PSR: 1.9 60.7; p!.001}; L4-L5 (intact: 10.8 62.9; PSR: 1.7 61.5; p!.001)} levels. A/R: At L3-L4 {(intact: 4.7 61.6; PSR: 2.060.75; p!.001); L4-L5 (intact: 5.8 61.5; PSR: 2.1 61.8; p!.001)}. L/B: At L3-L4 {(intact: 12.9 62.1; PSR: 1.3 60.53; p!.001); L4-L5 (intact: 12.6 61.8; PSR: 1.3 61.4; p!.001)}. ROM comparison of LP vs. PSR @ L3-L4 and L4-L5 for different loading modes: F/E:PSR significantly reduced motion at L3-L4 (p5.003) and L4-L5 (p!.001);L/B: PSR significantly reduced motion at L3-L4 (p5.005) and L4-L5 (p5.006);A/R:No significant difference in reduction of motion was identified between PSR and LP groups at L3-L4 (p5.333) & L4-L5 (0.334) between groups. CONCLUSIONS: Fusion constructs are designed to provide maximum rigidity and assist bone growth and fusion at instrumented levels. In F/E and L/B, up to 60-70% motion was preserved across both levels in the LP group whereas only 10-17% of motion was preserved in the PSR group. The PSR group thus conferred far superior kinematic stability to the instrumented levels than the LP system. Clinically, this may lead to lower incidences of subsidence and pseudoarthrosis relative to the LP construct. However this finding needs to be confirmed by in-vivo clinical data comparing the two lateral cage constructs. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2011.08.241
186. The Biomechanical Consequences of Rod Reduction on Pedicle Screws: Should it be Avoided? Ronald Lehman, MD1, Haines Paik, MD2, Daniel Kang, MD3, Anton Dmitriev, PhD4, Rachel Gaume5, Divya Ambati6; 1Potomac, MD, USA; 2Fairfax, VA, USA; 3Bethesda, MD, USA; 4Clarksville, MD, USA; 5 Washington, DC, USA; 6Walter Reed Army Medical Center, Washington, DC, USA BACKGROUND CONTEXT: Rod contouring is frequently required to allow for appropriate alignment of pedicle screw-rod constructs. When residual mismatch remains after contouring, a rod persuasion device is often utilized to reduce the rod to the pedicle screw head. Despite its popularity and widespread use, the biomechanical consequences of this rod reduction technique have not been evaluated. PURPOSE: The objective of this study is to evaluate the biomechanical effect of the rod reduction technique during longitudinal construct assembly on pedicle screw pull-out strength.
All referenced figures and tables will be available at the Annual Meeting and will be included with the post-meeting online content.
Proceedings of the NASS 26th Annual Meeting / The Spine Journal 11 (2011) 1S–173S
vertebrae were divided into Black line or non-Black line. On lateral view at 6 months X-p, pseudarthrosis and kyphosis were evaluated. Dynamic motion or cleft sign in fractured vertebra was defined as pseudarthrosis. Kyphosis rate was obtained by subtracting the anterior vertebral height in the injured vertebra from the average anterior vertebral height of the normal vertebrae above and below the injured vertebra in the lateral images and dividing the value by the average vertebral height of the normal vertebrae. Kyphosis progression rate was obtained by subtracting the kyphosis rate on initial examination from that at 6 months and dividing the value by the initial examination. All measurements were performed with the investigator blinded concerning the patient’s name, clinical findings, and other imaging data. Visual analogue scale for back pain (VAS) was evaluated at 6 months examination. The chi-square test was used for pseudarthrosis rate, and the Student’s t-test was used for kyphosis progression rate and VAS. p!.05 was taken as a statistically significant difference. RESULTS: Fourteen of 63 vertebrae (22%) resulted in pseudarthrosis. Powder sign (n516) included 0 (0%) pseudarthrosis, and non-Powder sign (n547) included 14 (30%), and Powder sign included pseudarthrosis significantly less than non-Powder sign (p5.01). Sensitivity of bone union by Powder sign was 33%, and specificity of that was 100%. Black line (n524) included 11 (46%) pseudarthrosis, non-Black line (n539) included 4 (10%), and Black line included pseudarthrosis significantly more than non-Black line (p5.004). Sensitivity of pseudarthrosis by Black line was 71%, and specificity of that was 71%. Kyphosis progression rate of Powder sign was 11% (621%) and that of non-Powder sign was 23% (625%), and there was no significant difference (p5.08). Kyphosis progression rate of Black line was 32% (622%) and that of non-Black line was 13% (622%), and there was a significant difference (p5.003). The average value of VAS of powder sign was 26 mm (618 mm) and that of nonPowder sign was 28 mm (623 mm), and there was no significant difference. The average value of VAS of Black line was 35 mm (626 mm) and that of non-Black line was 23 mm (617 mm), and there was a significant difference (p5.04). CONCLUSIONS: On MRI STIR, Powder sign can predict bone union, and Black line was the risk factor of pseudarthrosis, kyphosis progression and back pain on osteoporotic vertebral fractures. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2011.08.237
Friday, November 4, 2011 4:45–5:45 PM Focused Paper Presentations: Biomechanics 183. In Vivo Evaluation of a Novel Expanding Pedicle Screw: A Six-Month Study in a Sheep Model Mark Levy, MD, ScD1, Ory Keynan, MD2, Orna Popper3, Elad Sapir3, Yair Spanier3, Michal Ruchelsman3, Jaffar Hleihil3; 1Technion Israel Institute of Technology, Haifa, Isreal; 2Kokhav Yair, Isreal; 3Expanding Orthopedics, Caesarea, Isreal BACKGROUND CONTEXT: Solid fixation of pedicle screws is the cornerstone of successful spinal fusion. It’s particularly important in compromised bone quality such as elderly population, various bone pathologies and revision surgery. Early fixation of pedicle screws as well as long term implant osteointegration, minimize the rate of failure optimizing the chances of fusion. PURPOSE: The objective of the study was to evaluate in-vivo a novel titanium expanding pedicle screw, comprising a screw section in the pedicle and a 4-wing expandable portion in the vertebral body. The study assessed the mechanical performance and biological fixation with time, including implantation and deployment, characterization of bone ingrowth on the
exterior and interior surface of the expandable portion and the ability to remove the screw after bone ingrowth. STUDY DESIGN/SETTING: A GLP-compliant animal study using standard and expanding pedicle screws for lumbar spine fusion. PATIENT SAMPLE: Eleven skeletally-mature Suffolk sheep, 80-120 kg at implantation day. OUTCOME MEASURES: Clinical and radiographic data, histology of specimens, deployment and un-deployment, removal of the expanding pedicle screw. METHODS: Lumbar spines of 11 sheep were implanted with a total of 44 expanding pedicle screws Ø5.8 mm and 22 standard pedicle screws Ø5.5 mm (control), using standard spinal fusion techniques and instrumentation. In each spine 6 screws were inserted, one per vertebra, through a standard posterior trans-pedicle approach, in 3 one-level unilateral fusion pairs: one pair of controls and 2 pairs of expanding pedicle screws. Each pair was connected using Ø5 mm rods. No discs were removed and no bone graft was added. Pre and post-operative x-ray films were obtained. The animals were kept up to 180-day follow-up period with no activity restriction. Intermediate time points were at 35, 77 and 90 days postoperation. At the end of each time point, the animals were sacrificed and the spines were harvested for removal of the screws or histological evaluation. RESULTS: Successful deployment of the expanding pedicle screws was achieved in all implanted screws using an active mechanism. The histological analysis demonstrated osteointegration of the expanding pedicle screw in exterior and interior surface of the expandable portion from the 35-day time point and onward. The average amount of bone ingrowth inside the expandable area was elevated with each consecutive time point showing maturation of the bone matrix from woven bone to lamellar osteonal bone over time. Removal of the expanding pedicle screws was achieved for all screws despite the bone ingrowth, due to an active undeployment mechanism allowing collapse of the expandable area and removal through the pedicle. CONCLUSIONS: Implantation and deployment of a novel expanding pedicle screw was demonstrated in the lumbar spine of mature sheep. The increased surface area of the metal-bone interface allowed immediate fixation. The increased osteointegration with time, as well as bone ingrowth inside the expandable area, provided a progressively robust fixation of the screw within the vertebral body. The immediate mechanical fixation followed by the long-term biological fixation was designed to enhance construct stability and subsequent fusion. The ability to undeploy and remove the screws demonstrated the mechanical properties of this novel screw and its instrumentation. FDA DEVICE/DRUG STATUS: Expanding Pedicle Screw: Not approved for this indication. doi: 10.1016/j.spinee.2011.08.239
184. Biomechanical Aspects of Pedicle Screw Loosening in the Lumbar Spine Joel Boerckel, PhD1, Liz Peters2, Lalissie Merga3, Tamkeenat Syed4, Mona Arabshahi3, Rick Chappuis, PA-C3, James Chappuis, MD, FACS5; 1 Georgia Tech, Atlanta, GA, USA; 2Ball Ground, GA, USA; 3Atlanta, GA, USA; 4Crystal Lake, IL, USA; 5Orthopaedic & Spine Surgery of Atlanta, Atlanta, GA, USA BACKGROUND CONTEXT: Previously at NASS, we reported that removal of pedicle screws following fusion resulted in significant reduction in pain and medication use and increased activity and quality of life. As posterior pedicle screw systems increase in strength and rigidity, more loads are present at the bone screw interface and loosening can occur. PURPOSE: The purpose of this study was to quantify pedicle screw loosening using insertion and removal torque and evoked EMG measurement and identify factors which may predict loosening. STUDY DESIGN/SETTING: Patients received either pedicle screw insertion or removal at Emory University Hospital, Midtown, Atlanta, GA.
All referenced figures and tables will be available at the Annual Meeting and will be included with the post-meeting online content.
Proceedings of the NASS 26th Annual Meeting / The Spine Journal 11 (2011) 1S–173S PATIENT SAMPLE: 73 patients, age 46610 years, received 2 to 4-level fusion of L2-S1 vertebrae. Thrity-five patients received hardware removal 230630 days post-implantation. OUTCOME MEASURES: Outcome measures included: insertion torque, immediate removal torque, ultimate removal torque, evoked EMG stimulus threshold, screw size, and pedicle-screw clearance (pedicle thickness screw diameter). METHODS: Pedicle screw insertion (n5274) and removal (n5207) torques were measured using a manual torque wrench. Immediately upon insertion, pedicle screws were backed-out to assess immediate removal torque to ensure the insertion torque was not artificially elevated due to initial placement (n510). Paired samples (n526) were measured for both insertion and removal. EMG stimulation threshold currents for screw holes were measured prior to insertion and following removal. Paired and unpaired two-tailed t-tests were used to compare groups, as appropriate, with significance set at p!.05. Linear regressions were performed using GraphPad (Prism). All data reported as mean6SD. RESULTS: There were no differences between insertion and immediate removal torque (p5.56). However, upon ultimate removal, torque measurement yielded a 79.2% reduction in torque compared with insertion for paired measurements (p!.0001). Likewise, the EMG stimulation threshold was 33% lower at removal than at insertion in the paired evaluations (p!.05). Similar results for both torque and EMG threshold were found for unpaired measurements (n$207). Loosening was not affected by sex, age, vertebra level, or screw length, however, the pedicle-screw clearance (p!.05, R50.22) and screw diameter (p!.01, R50.17) correlated significantly with loosening, and for all samples in which the pedicle-screw clearance exceeded 10 mm, the removal torque was extremely low, less than 5 in-lb. CONCLUSIONS: Differences in insertion and removal torque and EMG stimulus threshold suggest that pedicle screws may loosen substantially during the 7-8 months following implantation as a result of degradation of bone matrix quality or quantity. Stimulus thresholds less than 6-8 mA have been shown to be suggestive of cortical perforation by pedicular instrumentation. In this study, despite acceptable threshold levels upon implantation, the average stimulus threshold upon removal was 7.5 mA, suggesting a possible degradation of bone matrix between the hole and nerve root. Also, correlations with pedicle-screw clearance and screw diameter suggest that larger screws which engage cortical bone in the pedicle may reduce screw loosening, and some form of augmentation may be advisable for particularly large pedicles which exceed screw diameter by 10 mm or more. Regardless of screw size, however, pedicle screws experienced significant loosening, likely as a result of matrix degradation and resorption due to gross modulus mismatch at the screw interface resulting in localized stress-shielding. Together these data may serve as a rationale for hardware removal following fusion. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2011.08.240
185. Kinematic Comparison of Lateral Plate Versus Pedicle Screw Construct in Lumbar Spine Instrumented with Lateral Cage at Two Levels Antonio Castellvi, MD1, Aniruddh Nayak, MS2, Brandon Santoni, PhD2, Andres Cabezas2, Sergio Gutierrez, PhD2, James Billys, MD3; 1Florida Orthopaedic Institute, Tampa, FL, USA; 2Foundation for Orthopaedic Research and Education, Tampa, FL, USA; 3Florida Orthopedic Institute, Brandon, FL, USA BACKGROUND CONTEXT: Fusion constructs are designed to provide maximum rigidity at the index level in order to facilitate adequate bone
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growth and fusion. In this study we assessed the biomechanical outcome of two different lumbar lateral cage constructs in stabilizing the instrumented level(s). PURPOSE: This study quantified the overall kinematic stability afforded by two lateral cage (NuVasive XLIFÒ) constructs. Pedicle screw þ rod (SpheRxÔ DBRÔ) Single sided lateral plate (XLP PlateÔ). STUDY DESIGN/SETTING: Cadaveric biomechanical study. PATIENT SAMPLE: 18 Cadaver lumbar spines. METHODS: Eighteen human cadaveric lumbar spines (L1-S1) were cleaned and potted at L1 and S1. Specimens were instrumented at L3L4 and L4-L5 with lateral cages. Nine specimens were instrumented with lateral plate (LP) at L3-L4 and L4-L5 and the remaining nine specimens with the pedicle screwþrod (PSR). Quasi-static pure moment loading was applied in flex/ext (F/E), lateral bending (L/B) and axial rotation (A/R) up to 7.5 Nm for intact and instrumented cases. Range of motion (ROM) at L3-L4 & L4-L5 was recorded using the OptrotrakÔ system and statistically compared. RESULTS: Statistically significant decrease in ROM was documented for both constructs. ROM results for LP group:F/E:At L3-L4 {(intact59.8 61.3; LP55.9 63.3; p5.005); L4-L5 (intact512.1 62.4; LP57.7 63.9; p5.012)}.; A/R:At L3-L4 {(intact54.6 62.5; LP52.4 60.9; p5.035); L4-L5 (intact56.3 63.1; LP53.1 62.2; p5.025)}. L/B:At L3-L4 {(intact511.8 62.7; LP56.2 64.4; p5.006); L4-L5 (intact512.1 63.9; LP54.8 62.9; p!.001)}. ROM results for PSR group:F/E:At L3-L4 {(intact: 10.762.4; PSR: 1.9 60.7; p!.001}; L4-L5 (intact: 10.8 62.9; PSR: 1.7 61.5; p!.001)} levels. A/R: At L3-L4 {(intact: 4.7 61.6; PSR: 2.060.75; p!.001); L4-L5 (intact: 5.8 61.5; PSR: 2.1 61.8; p!.001)}. L/B: At L3-L4 {(intact: 12.9 62.1; PSR: 1.3 60.53; p!.001); L4-L5 (intact: 12.6 61.8; PSR: 1.3 61.4; p!.001)}. ROM comparison of LP vs. PSR @ L3-L4 and L4-L5 for different loading modes: F/E:PSR significantly reduced motion at L3-L4 (p5.003) and L4-L5 (p!.001);L/B: PSR significantly reduced motion at L3-L4 (p5.005) and L4-L5 (p5.006);A/R:No significant difference in reduction of motion was identified between PSR and LP groups at L3-L4 (p5.333) & L4-L5 (0.334) between groups. CONCLUSIONS: Fusion constructs are designed to provide maximum rigidity and assist bone growth and fusion at instrumented levels. In F/E and L/B, up to 60-70% motion was preserved across both levels in the LP group whereas only 10-17% of motion was preserved in the PSR group. The PSR group thus conferred far superior kinematic stability to the instrumented levels than the LP system. Clinically, this may lead to lower incidences of subsidence and pseudoarthrosis relative to the LP construct. However this finding needs to be confirmed by in-vivo clinical data comparing the two lateral cage constructs. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2011.08.241
186. The Biomechanical Consequences of Rod Reduction on Pedicle Screws: Should it be Avoided? Ronald Lehman, MD1, Haines Paik, MD2, Daniel Kang, MD3, Anton Dmitriev, PhD4, Rachel Gaume5, Divya Ambati6; 1Potomac, MD, USA; 2Fairfax, VA, USA; 3Bethesda, MD, USA; 4Clarksville, MD, USA; 5 Washington, DC, USA; 6Walter Reed Army Medical Center, Washington, DC, USA BACKGROUND CONTEXT: Rod contouring is frequently required to allow for appropriate alignment of pedicle screw-rod constructs. When residual mismatch remains after contouring, a rod persuasion device is often utilized to reduce the rod to the pedicle screw head. Despite its popularity and widespread use, the biomechanical consequences of this rod reduction technique have not been evaluated. PURPOSE: The objective of this study is to evaluate the biomechanical effect of the rod reduction technique during longitudinal construct assembly on pedicle screw pull-out strength.
All referenced figures and tables will be available at the Annual Meeting and will be included with the post-meeting online content.