- Email: [email protected]
Cervical Center of Rotation Using a Mobile Axis of Rotation Total Disc Arthroplasty
NASS 31st Annual Meeting Proceedings / The Spine Journal 16 (2016) S113–S250 RESULTS: There were 1510 symptomatic patients (550 male, 960 female) and 115 asymptomatic subjects. PI averaged 54.1°±14.4 (range 11–105°) in all patients. Comparison of PI between age subgroups revealed a significant difference (p<.001). In detail, PI was significantly higher in the 4554y age group than the 35-44y age group (55.8° vs 49.7°, p=<0.001). After 45 years, PI remained unchanged. Between genders, PI was significantly higher in women than men (M: 51.8° vs F: 55.5°, p<.001) and there were significant PI differences between genders after age 45. In the asymptomatic subjects, however, a nonsignificant trend of PI increase was observed (p=.548), and females had a higher PI than males (p=.007), especially after 55 years old. In regression analysis of patient samples, age, gender and malalignment were identified as associated factors with R2 of 0.22 (p<.001). CONCLUSIONS: PI is higher in female patients and in older patients, especially those over 45 years old. Spinal malalignment also may have a role in increased PI; however, a causal link cannot yet be identified due to lack of longitudinal data. These findings may benefit both surgical decisionmaking and planning, but should also encourage further investigation. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. http://dx.doi.org/10.1016/j.spinee.2016.07.292
Friday, October 28, 2016 3:50 pm–4:50 pm Biomechanics 259. Quality and Quantity of Motion Using a Mobile Axis of Rotation Cervical Total Disc Arthroplasty Leonard I. Voronov, MD, PhD1, Saeed Khayatzadeh, PhD2, Robert M. Havey, MS1, Gerard Carandang, MS3, Kenneth R. Blank, MS, MHA3, Stephanie Werner, BS4, Avinash G. Patwardhan, PhD5; 1Loyola University Chicago/Edward Hines Jr. VA Hospital, Hines, IL, USA; 2Orthopedic Biomechanics Lab, Hines, IL, USA; 3Edward Hines Jr. VA Hospital, Hines, IL, USA; 4Hines, IL, USA; 5Loyola University Medical Center, Department of Orthopaedic Surgery, Maywood, IL, USA BACKGROUND CONTEXT: Anterior cervical discectomy and fusion has been associated with the development of adjacent segment degeneration (ASD). Cervical total disc arthroplasty (TDA) has been proposed as an alternative to fusion to prevent ASD, as biomechanical studies have demonstrated that TDA can replicate physiologic motion at the affected and adjacent levels. PURPOSE: To assess the effect of an innovative design of cervical TDA on the motion of the human cervical spine in response to moments applied in flexion-extension, lateral bending, and axial rotation after 1- and 2-level arthroplasty. STUDY DESIGN/SETTING: Cadaveric laboratory study. PATIENT SAMPLE: Nine cadaveric cervical spine specimens (C3-T1) (mean age 38.3±5.8 years). OUTCOME MEASURES: Load vs. displacement curves were analyzed to determine range of motion, stiffness, and neutral zone. METHODS: The kinematic testing apparatus allowed continuous cycling between specified maximum moment endpoints in flexion-extension, lateral bending, and axial rotation to ±1.5 Nm. Compressive preload (150 N) was used in flexion-extension. Vertebral motion was measured using an optoelectronic motion measurement system. A six-axis load cell was used to measure applied follower preload and moments. Surgical implantation was performed consistent with company guidelines (Triadyme-C, Dymicron, Orem, UT). The PLL was resected, and uncinate processes were left mostly intact with only the medial portion removed to accommodate the prosthesis endplate. Specimens were tested: Intact, after C5–C6 TDA (n=9), and C6–C7 TDA (n=7). Repeated-measures analysis of variance (ANOVA) with Bonferroni correction was used for statistical analyses. Significance is shown by p<.05.
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RESULTS: One-level TDA, results from C5–C6, ROM (deg) changed from: Flexion-extension: 12.8±2.5 to 10.5±2.1 (p=.03) Lateral bending: 8.5±2.8 to 3.7±1.0 (p<.01) Axial rotation: 10.4±1.1 to 6.2±1.9 (p<.01) Change in segmental stiffness (Nm/deg): Flexion: 0.09±0.03 to 0.21±0.09 (p=.004) Extension: 0.08±0.03 to 0.18±0.07 (p=.003) Change in neutral zone (deg): Flexion-extension 1.8±0.7 to 1.8±0.8 (p=.966). Two-level TDA, results from C6–C7, ROM (deg) changed from: Flexion-extension: 10.0±3.4 to 11.4±3.0 (p=.07) Lateral bending: 7.5±2.8 to 5.1±2.3 (p=.07) Axial rotation: 7.7±1.7 to 5.3±0.9 (p=.02) Change in segmental stiffness (Nm/deg): Flexion: 0.13±0.06 to 0.15±0.08 (p=.424) Extension: 0.12±0.05 to 0.11±0.04 (p=.736) Change in neutral zone (deg): Flexion-extension 1.5±1.0 to 2.1±0.9 (p=.304). CONCLUSIONS: This innovative design of disc prostheses restored ROM in flexion-extension to intact levels. In lateral bending, the TDA maintained 68% of ROM at C6–C7 and 43% at C5–C6. In axial rotation 60% of the ROM was maintained at C5–C6 and 69% at C6–C7. All other biomechanically-tested designs of TDA have shown similar reduction in lateral bending and greater reduction in axial rotation. The decrease in lateral bending and axial rotation after TDA may be a multifactorial phenomenon. Device kinematics, placement and tensioning of the remaining lateral annulus fibers during prosthesis insertion may play a role in maintained motion. Overall, the data suggest that this TDA provides similar cervical spine kinematics as compared to the preoperative condition. FDA DEVICE/DRUG STATUS: Triadyme-C, Dymicron, Inc., UT (Not approved for this indication). http://dx.doi.org/10.1016/j.spinee.2016.07.294
260. Cervical Center of Rotation Using a Mobile Axis of Rotation Total Disc Arthroplasty Saeed Khayatzadeh, PhD1, Leonard I. Voronov, MD, PhD2, Robert M. Havey, MS2, Gerard Carandang, MS3, Stephanie Werner, BS4, Kenneth R. Blank, MS, MHA3, Avinash G. Patwardhan, PhD5; 1 Orthopedic Biomechanics Lab, Hines, IL, USA; 2Loyola University Chicago/Edward Hines Jr. VA Hospital, Hines, IL, USA; 3Edward Hines Jr. VA Hospital, Hines, IL, USA; 4Hines, IL, USA; 5Loyola University Medical Center, Department of Orthopaedic Surgery, Maywood, IL, USA BACKGROUND CONTEXT: Anterior cervical discectomy and fusion (ACDF) has been associated with the development of adjacent segment degeneration (ASD). Cervical total disc arthroplasty (TDA) has been proposed as an alternative to fusion to prevent ASD, as biomechanical studies have demonstrated that TDA can replicate physiologic motion at the affected and adjacent levels. However, some arthroplasty designs have been linked with facet degeneration possibly due to their center of rotation (COR) mismatch with the natural motion segment. Cervical TDA using a disc which has a mobile axis of rotation may better accommodate the unique COR of each implanted motion segment. PURPOSE: To assess the effect that a mobile axis of rotation TDA has on the COR of human cervical motion segments in the high flexibility zone (HFZ) during flexion-extension motion. STUDY DESIGN/SETTING: Cadaveric laboratory study. PATIENT SAMPLE: Eight cadaveric cervical spine specimens (C3-T1) (mean age 38±6 years). OUTCOME MEASURES: Change in the sagittal plane COR location between the intact and implanted motion segments. METHODS: The kinematic testing apparatus allowed continuous cycling between specified maximum moment endpoints in flexion-extension to ±1.5 Nm. Compressive preload (150 N) was applied during the test. Vertebral motion was measured using an optoelectronic motion measurement system. A six-axis load cell was used to measure applied follower preload and flexion-extension moment. Specimen-specific 3D CT modeling was used to locate the segmental COR (projection of the flexion-extension axis of rotation on the sagittal plane) for the implanted motion segments. COR was measured between the start of the HFZ in extension, to its end point in flexion (−0.1 Nm extension to 0.65 Nm flexion). Specimens were tested: Intact, after C5–C6 TDA, and C6–C7 TDA (Triadyme-C, Dymicron, Inc., UT).
Refer to onsite annual meeting presentations and postmeeting proceedings for possible referenced figures and tables. Authors are responsible for accurately reporting disclosure and FDA device/drug status at time of abstract submission.
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NASS 31st Annual Meeting Proceedings / The Spine Journal 16 (2016) S113–S250
RESULTS: For C5–C6, the intact ROM (12.2±2.2 deg) was split into three zones: extension high stiffness zone (2.5±2.4 deg), HFZ (6.7±2.9 deg) and flexion high stiffness zone (3.0±1.8 deg). The HFZ covers 23±5% (0.7±0.1 Nm) of the applied FE moment, but contributes 54±17% of the total segmental ROM. At C6–C7, the intact ROM (10.0±2.9 deg) yielded: extension high stiffness zone (1.9±0.4 deg), HFZ (4.6±1.1 deg) and flexion high stiffness zone (3.5±3.2 deg). The HFZ covers 23±4% (0.7±0.1 Nm) of the applied FE moment, but contributes 49±13% of the total segmental ROM. The change in location of the C5–C6 HFZ-COR between intact and TDA averaged 1.0±1.1 mm posteriorly (n=8, p<.05) and 0.6±1.4 mm caudally (n=8, p=.3).At C6–C7 the change in C6–C7 HFZ-COR between intact and TDA averaged 1.4±0.8 mm posteriorly (n=7, p<.01) and 0.3±2.0 mm cranially (n=7, p=.7). CONCLUSIONS: Each individual spinal motion segment has a unique COR depending on its bony and soft tissue anatomy. Fixed center of rotation arthroplasty designs force a nonphysiologic COR upon implanted motion segments. This fixed COR may cause abnormal quantity and quality of motion resulting in altered facet loading and degenerative changes. The facet joints and ligamentous structures have a significant effect on COR location. Activities of daily living occur primarily in the HFZ making traditional measures of COR from full extension to full flexion an inaccurate representation of COR location. A new COR measure has been presented to measure COR in the high flexibility zone where it is less affected by the facets and tensioned soft tissues. The results of this study show that the investigated TDA device allowed individual motion segments to maintain their COR position in the anteroposterior direction within 1.2±1.0 mm (p=.00) and in the cranial-caudal direction to 0.2±1.7 mm (p=.70) of the intact COR location. FDA DEVICE/DRUG STATUS: Triadyme-C, Dymicron, Inc., UT (Not approved for this indication).
sive stiffness, disc height loss, and disc height recovery as a function of set number, cyclic compression rate, and spinal level. RESULTS: Mean compressive stiffness increased during each 10,000cycle set (p<.05), and generally increased from the first to the fifth sets (p<.05). Intervertebral disc height loss and restitution were significantly dependent upon set number and cyclic compression rate. Height loss was also dependent on spinal level. Height loss was greatest during the first set (29%) and decreased dramatically for the second through fifth sets, greater at lower cervical levels (C4/5 and C6/7), and greater for testing conducted at 2 Hz. Disc height recovery was generally greatest during following the first set (23%) and decreased dramatically for later sets, was greater at lower cervical levels (C4/5 and C6/7), and was greater for 2-Hz tests. CONCLUSIONS: Results of this experimental study demonstrated changes in cervical spine intervertebral disc properties under vibrational loading that were dependent on cyclic compression rate and spinal region. Specifically, the loading paradigm had a greater effect on the lower cervical spine and at lower frequencies. These findings have significant real-world implication in that the intervertebral disc had lost approximately 29% of its pretest height over the first 10,000 cycles and had not recovered to pre-test levels after a 15-min relaxation period. However, rate dependence indicates that changes in disc properties during vibrational loading will likely depend on the specific environment and, therefore, occupational exposure limits to prevent chronic changes should be based on the characteristics of the environment. These data can be used to develop and validate predictive models for changes in disc properties under a variety of different loading paradigms. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. http://dx.doi.org/10.1016/j.spinee.2016.07.296
http://dx.doi.org/10.1016/j.spinee.2016.07.295
261. Fatigue in the Cervical Spine Intervertebral Disc with Repetitive Axial Compression Brian D. Stemper, PhD1, Narayan Yoganandan, PhD2, Mingxin Zheng, PhD3, Brian Snyder, MD, PhD4; 1Medical College of Wisconsin, Milwaukee, WI, USA; 2Milwaukee VA Medical Center, Milwaukee, WI, USA; 3Beth Israel Deaconess Medical Center, Boston, MA, USA; 4Children’s Hospital/Orthopaedic Surgery, Boston, MA, USA BACKGROUND CONTEXT: Axial vibration is common in workplace environments such as heavy machinery operators, truck drivers, and military personnel. Long duration and repetitive axial vibration have been associated with cervical spine-based pain syndromes, likely associated with progressive changes soft tissue mechanics over time. PURPOSE: Determine changes in cervical spine intervertebral disc properties during repetitive axial loading. STUDY DESIGN/SETTING: An experimental biomechanical study with cervical segments repeatedly exposed to 10,000 cycles of axial loading followed by a rest period to characterize intervertebral disc fatigue-related height loss and unloaded disc height restitution, accounting for differences by spinal level and loading rate. PATIENT SAMPLE: Twenty-one cervical spine intervertebral disc segments (C2–3, C4–5, or C6–7) obtained from cadaveric specimens (55±11 yrs) were exposed to the experimental protocol. OUTCOME MEASURES: Cycle-by-cycle stiffness was calculated based on axial force and compressive displacement during compressive testing. Intervertebral disc height loss was calculated as the mean disc height (across five locations on lateral X-ray) measured before and immediately after each 10,000 cycle set. Disc height recovery was calculated as change in disc height after a 15 min rest period. METHODS: Specimens were exposed to five sets of 10,000 axial compressive cycles (0–150 N) at either 2 or 4 Hz in a physiologic saline bath maintained at 37°C using an electrohydraulic testing device, and allowed a 60-min rest period between sets. Repeated measures ANOVA analysis was used to determine statistically significant differences (p<.05) in compres-
262. Direct Measure of Cervical Interbody Forces in Vivo: Load Reversal after Plating Eric H. Ledet, PhD1, Josh Peterson1, Rebecca A. Wachs, MS1, Mary Beth M. Grabowsky, PhD1, Joseph Glennon2, Darryl J. DiRisio, MD3; 1Rensselaer Polytechnic Institute, Troy, NY, USA; 2 Capital District Veterinary Surgical Associates, Pattersonville, NY, USA; 3 Albany Medical College, Albany, NY, USA BACKGROUND CONTEXT: Biomechanics play an important role in spine fusion, but the in vivo biomechanics of the cervical spine are not well characterized and the in vivo biomechanics after spinal arthrodesis have never been studied. Load sharing facilitates fusion, but overloading of interbody implants can lead to subsidence and failure. In vitro studies have demonstrated that anterior plating significantly alters mechanical loading in the cervical spine. The instantaneous axis of rotation is shifted anteriorly and loading is reversed relative to an uninstrumented spine; the interbody space is compressed during extension and unloaded during flexion. However, this has never been tested in vivo and the magnitude of loads in the instrumented and uninstrumented cervical spine are unknown. PURPOSE: The purpose of this study was to use a novel force-sensing implant to directly measure interbody loading in the cervical spine in real time in vivo in a large animal model following instrumented or uninstrumented arthrodesis. STUDY DESIGN/SETTING: In vivo biomechanical loading following anterior cervical discectomy and fusion (ACDF) in goats. OUTCOME MEASURES: Real time in vivo interbody forces and kinematic motion data during flexion/extension exercises after interbody arthrodesis in the goat cervical spine. METHODS: A custom interbody implant/load cell was developed to measure real time forces in the interbody space of the goat cervical spine. The implants incorporated a wired button load cell (Futek, Irvine, CA) between implant endplates. The implant/load cells were calibrated and sterilized. After IACUC approval, under general anesthesia, and using standard clinical technique, a discectomy was performed at C4–5 in three skeletally mature goats. In each animal, an interbody implant/load cell was placed in the disc space and the lead wires were tunneled submuscularly to exit percutaneously on the dorsal surface of the neck. In two animals, a PEEK washer was at-
Refer to onsite annual meeting presentations and postmeeting proceedings for possible referenced figures and tables. Authors are responsible for accurately reporting disclosure and FDA device/drug status at time of abstract submission.
Friday, October 28, 2016 3:50 pm–4:50 pm Biomechanics 259. Quality and Quantity of Motion Using a Mobile Axis of Rotation Cervical Total Disc Arthroplasty Leonard I. Voronov, MD, PhD1, Saeed Khayatzadeh, PhD2, Robert M. Havey, MS1, Gerard Carandang, MS3, Kenneth R. Blank, MS, MHA3, Stephanie Werner, BS4, Avinash G. Patwardhan, PhD5; 1Loyola University Chicago/Edward Hines Jr. VA Hospital, Hines, IL, USA; 2Orthopedic Biomechanics Lab, Hines, IL, USA; 3Edward Hines Jr. VA Hospital, Hines, IL, USA; 4Hines, IL, USA; 5Loyola University Medical Center, Department of Orthopaedic Surgery, Maywood, IL, USA BACKGROUND CONTEXT: Anterior cervical discectomy and fusion has been associated with the development of adjacent segment degeneration (ASD). Cervical total disc arthroplasty (TDA) has been proposed as an alternative to fusion to prevent ASD, as biomechanical studies have demonstrated that TDA can replicate physiologic motion at the affected and adjacent levels. PURPOSE: To assess the effect of an innovative design of cervical TDA on the motion of the human cervical spine in response to moments applied in flexion-extension, lateral bending, and axial rotation after 1- and 2-level arthroplasty. STUDY DESIGN/SETTING: Cadaveric laboratory study. PATIENT SAMPLE: Nine cadaveric cervical spine specimens (C3-T1) (mean age 38.3±5.8 years). OUTCOME MEASURES: Load vs. displacement curves were analyzed to determine range of motion, stiffness, and neutral zone. METHODS: The kinematic testing apparatus allowed continuous cycling between specified maximum moment endpoints in flexion-extension, lateral bending, and axial rotation to ±1.5 Nm. Compressive preload (150 N) was used in flexion-extension. Vertebral motion was measured using an optoelectronic motion measurement system. A six-axis load cell was used to measure applied follower preload and moments. Surgical implantation was performed consistent with company guidelines (Triadyme-C, Dymicron, Orem, UT). The PLL was resected, and uncinate processes were left mostly intact with only the medial portion removed to accommodate the prosthesis endplate. Specimens were tested: Intact, after C5–C6 TDA (n=9), and C6–C7 TDA (n=7). Repeated-measures analysis of variance (ANOVA) with Bonferroni correction was used for statistical analyses. Significance is shown by p<.05.
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RESULTS: One-level TDA, results from C5–C6, ROM (deg) changed from: Flexion-extension: 12.8±2.5 to 10.5±2.1 (p=.03) Lateral bending: 8.5±2.8 to 3.7±1.0 (p<.01) Axial rotation: 10.4±1.1 to 6.2±1.9 (p<.01) Change in segmental stiffness (Nm/deg): Flexion: 0.09±0.03 to 0.21±0.09 (p=.004) Extension: 0.08±0.03 to 0.18±0.07 (p=.003) Change in neutral zone (deg): Flexion-extension 1.8±0.7 to 1.8±0.8 (p=.966). Two-level TDA, results from C6–C7, ROM (deg) changed from: Flexion-extension: 10.0±3.4 to 11.4±3.0 (p=.07) Lateral bending: 7.5±2.8 to 5.1±2.3 (p=.07) Axial rotation: 7.7±1.7 to 5.3±0.9 (p=.02) Change in segmental stiffness (Nm/deg): Flexion: 0.13±0.06 to 0.15±0.08 (p=.424) Extension: 0.12±0.05 to 0.11±0.04 (p=.736) Change in neutral zone (deg): Flexion-extension 1.5±1.0 to 2.1±0.9 (p=.304). CONCLUSIONS: This innovative design of disc prostheses restored ROM in flexion-extension to intact levels. In lateral bending, the TDA maintained 68% of ROM at C6–C7 and 43% at C5–C6. In axial rotation 60% of the ROM was maintained at C5–C6 and 69% at C6–C7. All other biomechanically-tested designs of TDA have shown similar reduction in lateral bending and greater reduction in axial rotation. The decrease in lateral bending and axial rotation after TDA may be a multifactorial phenomenon. Device kinematics, placement and tensioning of the remaining lateral annulus fibers during prosthesis insertion may play a role in maintained motion. Overall, the data suggest that this TDA provides similar cervical spine kinematics as compared to the preoperative condition. FDA DEVICE/DRUG STATUS: Triadyme-C, Dymicron, Inc., UT (Not approved for this indication). http://dx.doi.org/10.1016/j.spinee.2016.07.294
260. Cervical Center of Rotation Using a Mobile Axis of Rotation Total Disc Arthroplasty Saeed Khayatzadeh, PhD1, Leonard I. Voronov, MD, PhD2, Robert M. Havey, MS2, Gerard Carandang, MS3, Stephanie Werner, BS4, Kenneth R. Blank, MS, MHA3, Avinash G. Patwardhan, PhD5; 1 Orthopedic Biomechanics Lab, Hines, IL, USA; 2Loyola University Chicago/Edward Hines Jr. VA Hospital, Hines, IL, USA; 3Edward Hines Jr. VA Hospital, Hines, IL, USA; 4Hines, IL, USA; 5Loyola University Medical Center, Department of Orthopaedic Surgery, Maywood, IL, USA BACKGROUND CONTEXT: Anterior cervical discectomy and fusion (ACDF) has been associated with the development of adjacent segment degeneration (ASD). Cervical total disc arthroplasty (TDA) has been proposed as an alternative to fusion to prevent ASD, as biomechanical studies have demonstrated that TDA can replicate physiologic motion at the affected and adjacent levels. However, some arthroplasty designs have been linked with facet degeneration possibly due to their center of rotation (COR) mismatch with the natural motion segment. Cervical TDA using a disc which has a mobile axis of rotation may better accommodate the unique COR of each implanted motion segment. PURPOSE: To assess the effect that a mobile axis of rotation TDA has on the COR of human cervical motion segments in the high flexibility zone (HFZ) during flexion-extension motion. STUDY DESIGN/SETTING: Cadaveric laboratory study. PATIENT SAMPLE: Eight cadaveric cervical spine specimens (C3-T1) (mean age 38±6 years). OUTCOME MEASURES: Change in the sagittal plane COR location between the intact and implanted motion segments. METHODS: The kinematic testing apparatus allowed continuous cycling between specified maximum moment endpoints in flexion-extension to ±1.5 Nm. Compressive preload (150 N) was applied during the test. Vertebral motion was measured using an optoelectronic motion measurement system. A six-axis load cell was used to measure applied follower preload and flexion-extension moment. Specimen-specific 3D CT modeling was used to locate the segmental COR (projection of the flexion-extension axis of rotation on the sagittal plane) for the implanted motion segments. COR was measured between the start of the HFZ in extension, to its end point in flexion (−0.1 Nm extension to 0.65 Nm flexion). Specimens were tested: Intact, after C5–C6 TDA, and C6–C7 TDA (Triadyme-C, Dymicron, Inc., UT).
Refer to onsite annual meeting presentations and postmeeting proceedings for possible referenced figures and tables. Authors are responsible for accurately reporting disclosure and FDA device/drug status at time of abstract submission.
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NASS 31st Annual Meeting Proceedings / The Spine Journal 16 (2016) S113–S250
RESULTS: For C5–C6, the intact ROM (12.2±2.2 deg) was split into three zones: extension high stiffness zone (2.5±2.4 deg), HFZ (6.7±2.9 deg) and flexion high stiffness zone (3.0±1.8 deg). The HFZ covers 23±5% (0.7±0.1 Nm) of the applied FE moment, but contributes 54±17% of the total segmental ROM. At C6–C7, the intact ROM (10.0±2.9 deg) yielded: extension high stiffness zone (1.9±0.4 deg), HFZ (4.6±1.1 deg) and flexion high stiffness zone (3.5±3.2 deg). The HFZ covers 23±4% (0.7±0.1 Nm) of the applied FE moment, but contributes 49±13% of the total segmental ROM. The change in location of the C5–C6 HFZ-COR between intact and TDA averaged 1.0±1.1 mm posteriorly (n=8, p<.05) and 0.6±1.4 mm caudally (n=8, p=.3).At C6–C7 the change in C6–C7 HFZ-COR between intact and TDA averaged 1.4±0.8 mm posteriorly (n=7, p<.01) and 0.3±2.0 mm cranially (n=7, p=.7). CONCLUSIONS: Each individual spinal motion segment has a unique COR depending on its bony and soft tissue anatomy. Fixed center of rotation arthroplasty designs force a nonphysiologic COR upon implanted motion segments. This fixed COR may cause abnormal quantity and quality of motion resulting in altered facet loading and degenerative changes. The facet joints and ligamentous structures have a significant effect on COR location. Activities of daily living occur primarily in the HFZ making traditional measures of COR from full extension to full flexion an inaccurate representation of COR location. A new COR measure has been presented to measure COR in the high flexibility zone where it is less affected by the facets and tensioned soft tissues. The results of this study show that the investigated TDA device allowed individual motion segments to maintain their COR position in the anteroposterior direction within 1.2±1.0 mm (p=.00) and in the cranial-caudal direction to 0.2±1.7 mm (p=.70) of the intact COR location. FDA DEVICE/DRUG STATUS: Triadyme-C, Dymicron, Inc., UT (Not approved for this indication).
sive stiffness, disc height loss, and disc height recovery as a function of set number, cyclic compression rate, and spinal level. RESULTS: Mean compressive stiffness increased during each 10,000cycle set (p<.05), and generally increased from the first to the fifth sets (p<.05). Intervertebral disc height loss and restitution were significantly dependent upon set number and cyclic compression rate. Height loss was also dependent on spinal level. Height loss was greatest during the first set (29%) and decreased dramatically for the second through fifth sets, greater at lower cervical levels (C4/5 and C6/7), and greater for testing conducted at 2 Hz. Disc height recovery was generally greatest during following the first set (23%) and decreased dramatically for later sets, was greater at lower cervical levels (C4/5 and C6/7), and was greater for 2-Hz tests. CONCLUSIONS: Results of this experimental study demonstrated changes in cervical spine intervertebral disc properties under vibrational loading that were dependent on cyclic compression rate and spinal region. Specifically, the loading paradigm had a greater effect on the lower cervical spine and at lower frequencies. These findings have significant real-world implication in that the intervertebral disc had lost approximately 29% of its pretest height over the first 10,000 cycles and had not recovered to pre-test levels after a 15-min relaxation period. However, rate dependence indicates that changes in disc properties during vibrational loading will likely depend on the specific environment and, therefore, occupational exposure limits to prevent chronic changes should be based on the characteristics of the environment. These data can be used to develop and validate predictive models for changes in disc properties under a variety of different loading paradigms. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. http://dx.doi.org/10.1016/j.spinee.2016.07.296
http://dx.doi.org/10.1016/j.spinee.2016.07.295
261. Fatigue in the Cervical Spine Intervertebral Disc with Repetitive Axial Compression Brian D. Stemper, PhD1, Narayan Yoganandan, PhD2, Mingxin Zheng, PhD3, Brian Snyder, MD, PhD4; 1Medical College of Wisconsin, Milwaukee, WI, USA; 2Milwaukee VA Medical Center, Milwaukee, WI, USA; 3Beth Israel Deaconess Medical Center, Boston, MA, USA; 4Children’s Hospital/Orthopaedic Surgery, Boston, MA, USA BACKGROUND CONTEXT: Axial vibration is common in workplace environments such as heavy machinery operators, truck drivers, and military personnel. Long duration and repetitive axial vibration have been associated with cervical spine-based pain syndromes, likely associated with progressive changes soft tissue mechanics over time. PURPOSE: Determine changes in cervical spine intervertebral disc properties during repetitive axial loading. STUDY DESIGN/SETTING: An experimental biomechanical study with cervical segments repeatedly exposed to 10,000 cycles of axial loading followed by a rest period to characterize intervertebral disc fatigue-related height loss and unloaded disc height restitution, accounting for differences by spinal level and loading rate. PATIENT SAMPLE: Twenty-one cervical spine intervertebral disc segments (C2–3, C4–5, or C6–7) obtained from cadaveric specimens (55±11 yrs) were exposed to the experimental protocol. OUTCOME MEASURES: Cycle-by-cycle stiffness was calculated based on axial force and compressive displacement during compressive testing. Intervertebral disc height loss was calculated as the mean disc height (across five locations on lateral X-ray) measured before and immediately after each 10,000 cycle set. Disc height recovery was calculated as change in disc height after a 15 min rest period. METHODS: Specimens were exposed to five sets of 10,000 axial compressive cycles (0–150 N) at either 2 or 4 Hz in a physiologic saline bath maintained at 37°C using an electrohydraulic testing device, and allowed a 60-min rest period between sets. Repeated measures ANOVA analysis was used to determine statistically significant differences (p<.05) in compres-
262. Direct Measure of Cervical Interbody Forces in Vivo: Load Reversal after Plating Eric H. Ledet, PhD1, Josh Peterson1, Rebecca A. Wachs, MS1, Mary Beth M. Grabowsky, PhD1, Joseph Glennon2, Darryl J. DiRisio, MD3; 1Rensselaer Polytechnic Institute, Troy, NY, USA; 2 Capital District Veterinary Surgical Associates, Pattersonville, NY, USA; 3 Albany Medical College, Albany, NY, USA BACKGROUND CONTEXT: Biomechanics play an important role in spine fusion, but the in vivo biomechanics of the cervical spine are not well characterized and the in vivo biomechanics after spinal arthrodesis have never been studied. Load sharing facilitates fusion, but overloading of interbody implants can lead to subsidence and failure. In vitro studies have demonstrated that anterior plating significantly alters mechanical loading in the cervical spine. The instantaneous axis of rotation is shifted anteriorly and loading is reversed relative to an uninstrumented spine; the interbody space is compressed during extension and unloaded during flexion. However, this has never been tested in vivo and the magnitude of loads in the instrumented and uninstrumented cervical spine are unknown. PURPOSE: The purpose of this study was to use a novel force-sensing implant to directly measure interbody loading in the cervical spine in real time in vivo in a large animal model following instrumented or uninstrumented arthrodesis. STUDY DESIGN/SETTING: In vivo biomechanical loading following anterior cervical discectomy and fusion (ACDF) in goats. OUTCOME MEASURES: Real time in vivo interbody forces and kinematic motion data during flexion/extension exercises after interbody arthrodesis in the goat cervical spine. METHODS: A custom interbody implant/load cell was developed to measure real time forces in the interbody space of the goat cervical spine. The implants incorporated a wired button load cell (Futek, Irvine, CA) between implant endplates. The implant/load cells were calibrated and sterilized. After IACUC approval, under general anesthesia, and using standard clinical technique, a discectomy was performed at C4–5 in three skeletally mature goats. In each animal, an interbody implant/load cell was placed in the disc space and the lead wires were tunneled submuscularly to exit percutaneously on the dorsal surface of the neck. In two animals, a PEEK washer was at-
Refer to onsite annual meeting presentations and postmeeting proceedings for possible referenced figures and tables. Authors are responsible for accurately reporting disclosure and FDA device/drug status at time of abstract submission.