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85. Biomechanical Determination of Distal Level for Fusions Across the Cervicothoracic Junction
44S
Proceedings of the NASS 24th Annual Meeting / The Spine Journal 9 (2009) 1S-205S
PURPOSE: Hence, we conducted an international, multi-center, prospective cohort study to assess the outcomes of operative intervention in this important cause of spinal cord dysfunction. STUDY DESIGN/SETTING: Prospective international, multi-center cohort study. PATIENT SAMPLE: Patients with radiologic and clinical evidence of cervical spondylotic myelopathy. OUTCOME MEASURES: Modified Japanese Orthopedic Association Scale (mJOA), Neck Disability Index (NDI) and SF-36. METHODS: Patients underwent anterior or posterior decompressive/ reconstructive surgery with the surgical techniques chosen by the treating team. RESULTS: There were 40% females with no significant differences in the proportion of females across the regions. Patients in Asia/Pacific region were significantly younger than in other three regions (41.5 and 56.5 years of age, respectively, P ! .01). 32% of subjects were smokers with no differences noted among the regions. There was a significant difference among the regions in the number of spinal levels operated. In Europe and Asia/Pacific patients had fewer number of levels treated operatively (3.2 levels on average) while North and South American subjects had a higher number of levels treated surgically (4.2 on average; P ! .01). The anterior approach was used in 70% of the cases in Europe, 75% in Asia Pacific, 56% in North America and 18% in South America (P !.01). The duration of symptoms at the time of surgery was 20 months in Europe, 18 months Asia Pacific, 32 months North America, and 50 months in South America (P!.01). There were major differences in the average length of hospital stay: Europe 12.7 days; Asia/Pacific 10.7 days; North America 3.2 days; South America 6.4 days (P ! .01). The average Nurick score was highest in Asia/Pacific region (4.7), followed by Europe and North America (4.1 each) and lowest in South America (3.5) (P ! .01). The average NDI was 42, average mJOA was 13, average SF36 PCS was 34 and average SF36 MCS was 38. There were no differences among the regions in these parameters. CONCLUSIONS: This study presents novel data which highlight major variations in the clinical presentation and treatment protocols used to treat cervical spondylotic myelopathy. While some of these variations reflect differences in health care systems, other variations reflect an opportunity to develop more uniform evidence-based treatment protocols. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs.
rods in context of more commonly used rod materials is essential for surgeons to select the optimum rod type. PURPOSE: To compare CoCr rods to steel and titanium rods in 4 point bending mechanical testing with the goal of understanding how these rods differ with regards to bending stiffness, strength (as expressed by yield point), and memory (deformation at yield point). STUDY DESIGN/SETTING: Prospective/Biomechanical Research Laboratory. PATIENT SAMPLE: N/A. OUTCOME MEASURES: Young’s Modulus, Stiffness, Yield Point, Ultimate Load, Memory. METHODS: Five spine rod materials: Standard Strength Steel (SS), High Strength Steel (HSS), Ultra Strength Steel (USS), Titanium Alloy (Ti), and Cobalt Chromium (CoCr) were tested in a four-point rod bending system fixed to an MTS test frame (MTS, Eden Prairie, MN). For each material, nine samples measuring 200 mm in length and 5.5 mm in diameter were bent to varying angles of deformation (3 to 60 degrees). Load (N) and displacement (mm) of the sample’s midpoint were measured directly at 100 Hz utilizing the MTS machine and a three camera motion capture system. Load-deformation curves were extracted and the Young’s modulus, stiffness, yield load, ultimate load and memory values were calculated. The yield point was defined as the load resulting in permanent (plastic) rod deformation while memory referred to the angle of deformation at the yield point. RESULTS: The mechanical properties for each rod are listed in Table 1. The bending stiffness was highest for CoCr (24% greater than SS, 47% greater than Ti) and the Young’s Modulus was greatest for CoCr (20% greater than SS, 55% greater than Ti). The HSS and CoCr rods had similar Young’s Moduli (260 GPa and 282 GPa), with the lowest values for the SS and Ti rods (226 GPa and 155 GPa). CoCr rods displayed a memory lower than that of Ti and USS rods and similar to SS rods. CONCLUSIONS: The elastic and plastic properties of rods used to perform spinal instrumentation vary substantially. Aggressive correction of large curves may result in rod deformations beyond the elastic limit. Choosing the best rod for the individual case and correction strategy requires an appreciation of these differences. CoCr rods offer both a high stiffness and a low yield point option best suited for secure boney fixation and in-situ bending. FDA DEVICE/DRUG STATUS: Cobalt Chromium Spinal Rod: Approved for this indication; Titanium Alloy, Standard Strength Steel, High Strength Steel, Ultra High Strength Steel Spinal Rods: Approved for this indication.
doi: 10.1016/j.spinee.2009.08.104
doi: 10.1016/j.spinee.2009.08.106
Thursday, November 12, 2009 11:00 AM–12:00 PM Concurrent Session 2: Biomechanics 84. Cobalt Chromium Rods: How Do They Stack Up? Thomas Nunn1, Eric Varley, DO2, Rattalerk Arunakul, MD3, Arnel Aguinaldo, MA1, Peter Newton, MD1; 1Rady Children’s Hospital and Health Center - San Diego, San Diego, CA, USA; 2University of California, San Diego, San Diego, CA, USA; 3University of California, Irvine, Orange, CA, USA BACKGROUND CONTEXT: Spinal rods have traditionally been available in stainless steel and titanium. While stainless steel offers the advantages of a high stiffness and a high yield point it comes at the cost of producing a large amount of MRI artifact. Titanium on the other hand minimizes MRI artifact but comes at the expense of decreased rod stiffness. Several implant manufacturers have developed cobalt chromium (CoCr) in an effort to reduce MRI artifact while maintaining material properties similar to steel. An understanding of the biomechanical properties of CoCr
85. Biomechanical Determination of Distal Level for Fusions Across the Cervicothoracic Junction Ivan Cheng, MD1, Alexander Iezza, MD1, Eric Sundberg, MD1, Derek Lindsey, MS2, K. Daniel Riew, MD3; 1Stanford University, Stanford, CA, USA; 2Bone and Joint Center of Excellence, VA Palo Alto HCS, Palo Alto, CA, USA; 3Washington University in St. Louis, St. Louis, MO, USA BACKGROUND CONTEXT: When a cervical fusion needs to be extended down to the thoracic spine for adjacent level disease, many surgeons prefer to fuse down to T2 or lower in order to diminish the possibility of further adjacent disease. The optimal distal level, however, remains to be established. PURPOSE: We undertook this study to determine the effect of ending long cervicothoracic fusions at different caudal levels on adjacent-level intradiscal pressure. We hypothesized that stopping a fusion at T1 would be biomechanically less sound than stopping at a more caudal level. STUDY DESIGN/SETTING: This was a biomechanical study. PATIENT SAMPLE: Four cadaveric cervical spine specimens with an intact rib cage and sternum were carefully dissected to preserve costovertebral joints, costosternal joints, facet joints, intercostal muscles, and all ligaments.
Proceedings of the NASS 24th Annual Meeting / The Spine Journal 9 (2009) 1S-205S OUTCOME MEASURES: Changes in intradiscal pressure were compared to baseline and analyzed statistically using a one-way analysis of variance. METHODS: Specimens were stabilized at T12 and a force of 25 N was applied perpendicularly to the long axis of the spine to produce flexion, extension, left and right lateral bending. Intradiscal pressures were recorded first on intact specimens and then after anterior cervical plating at C3-7. Sequential extension of a posterior fusion was simulated with a posterior pedicle screw construct and testing of the intradiscal pressures at the adjacent disc for each level from C7-T1 down to T9-10. RESULTS: While all levels increased their intradiscal pressure after the ‘‘fusion,’’ no level was found to be a more advantageous level to end the construct than the others. There were no significant differences among the levels in the baseline compared to the juxtafusion intradiscal pressure for flexion, extension, left and right bending (see Figure). The largest changes occurred in flexion at T7-8 level (78% increase), in extension at T8-9 (104% increase), left and right bending at the T6-7 level (43% and 32% increases respectively).
45S
Figure 4.
CONCLUSIONS: Based upon juxtafusion intradiscal pressures, no level appeared to have a biomechanical superiority for stopping a long cervicothoracic fusion. While not significant, the largest increases occurred from T6-9. Surprisingly, even stopping the fusion at T1 was no worse than stopping at a more caudal level. This suggests that one may not necessarily need to extend the fusion to a more caudal segment if there are no facet arthrosis, disc degeneration, alignment, fixation point or other concerns. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2009.08.107
Figure 1.
Figure 2.
Figure 3.
86. Biomechanical Comparison of Translaminar vs. Pedicle Screws at T1 and T2 in Long Subaxial Cervical Constructs Matthew McGirt, MD, Edward Sutter, BASC, Risheng Xu, MA, Daniel Sciubba, MD, Timothy Witham, MD, Jean-Paul Wolinsky, MD, Ziya Gokaslan, MD, Ali Bydon, MD; Johns Hopkins University, Baltimore, MD, USA BACKGROUND CONTEXT: Thoracic pedicle screws have become the gold standard for posterior spinal fixation and are frequently utilized in long subaxial cervical constructs crossing the cervico-thoracic junction. Recently, thoracic trans-laminar screws have been reported to be a feasible option for posterior fixation of the upper thoracic spine with less technical demand. PURPOSE: In an ex vivo biomechanical study, we compared the immediate and post-cyclical loading rigidity of thoracic trans-laminar (TL) versus pedicle (PD) screw fixation in sub-axial cervical constructs. STUDY DESIGN/SETTING: Ex vivo biomechanical study. PATIENT SAMPLE: Ten fresh frozen non-osteoporotic human cadaveric spines. OUTCOME MEASURES: Rotational motion after cyclical flexion-extension and lateral bending. METHODS: Ten fresh frozen non-osteoporotic human cadaveric spines underwent C4-6 lateral mass screw and T1-2 trans-laminar screw (n55) versus pedicle screw (n55) fixation, Figure 1. Spines were then potted in polymethylmethacrylate bone cement and placed in a test fixture on a materials testing machine that applied a 4 Nm bending moment, Figure 2. Rotation about the axis of bending was measured using passive retroreflective markers and infrared motion capture cameras. Rotation of C6 relative toT2 in flexion-extension and lateral bending was assessed uninstrumented, immediately after instrumentation, and following 40,000 cycles of 4 Nm flexion-extension moment and lateral bending at 1 Hz. The effect of fixation type (PD versus TL) and state (uninstrumented, instrumented, post cycling) on rotation for flexion tests and lateral bending tests was analyzed for significance via a generalized linear mixed model. RESULTS: Compared to pre-instrumented spines, PD and TL screw constructs significantly (p!0.001) reduced rotational motion during flexionextension and lateral bending. After cyclical loading, rotational motion at the cervico-thoracic junction was significantly increased (p!0.05) during flexion-extension and lateral bending in both groups, Table 1 & 2. With
Proceedings of the NASS 24th Annual Meeting / The Spine Journal 9 (2009) 1S-205S
PURPOSE: Hence, we conducted an international, multi-center, prospective cohort study to assess the outcomes of operative intervention in this important cause of spinal cord dysfunction. STUDY DESIGN/SETTING: Prospective international, multi-center cohort study. PATIENT SAMPLE: Patients with radiologic and clinical evidence of cervical spondylotic myelopathy. OUTCOME MEASURES: Modified Japanese Orthopedic Association Scale (mJOA), Neck Disability Index (NDI) and SF-36. METHODS: Patients underwent anterior or posterior decompressive/ reconstructive surgery with the surgical techniques chosen by the treating team. RESULTS: There were 40% females with no significant differences in the proportion of females across the regions. Patients in Asia/Pacific region were significantly younger than in other three regions (41.5 and 56.5 years of age, respectively, P ! .01). 32% of subjects were smokers with no differences noted among the regions. There was a significant difference among the regions in the number of spinal levels operated. In Europe and Asia/Pacific patients had fewer number of levels treated operatively (3.2 levels on average) while North and South American subjects had a higher number of levels treated surgically (4.2 on average; P ! .01). The anterior approach was used in 70% of the cases in Europe, 75% in Asia Pacific, 56% in North America and 18% in South America (P !.01). The duration of symptoms at the time of surgery was 20 months in Europe, 18 months Asia Pacific, 32 months North America, and 50 months in South America (P!.01). There were major differences in the average length of hospital stay: Europe 12.7 days; Asia/Pacific 10.7 days; North America 3.2 days; South America 6.4 days (P ! .01). The average Nurick score was highest in Asia/Pacific region (4.7), followed by Europe and North America (4.1 each) and lowest in South America (3.5) (P ! .01). The average NDI was 42, average mJOA was 13, average SF36 PCS was 34 and average SF36 MCS was 38. There were no differences among the regions in these parameters. CONCLUSIONS: This study presents novel data which highlight major variations in the clinical presentation and treatment protocols used to treat cervical spondylotic myelopathy. While some of these variations reflect differences in health care systems, other variations reflect an opportunity to develop more uniform evidence-based treatment protocols. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs.
rods in context of more commonly used rod materials is essential for surgeons to select the optimum rod type. PURPOSE: To compare CoCr rods to steel and titanium rods in 4 point bending mechanical testing with the goal of understanding how these rods differ with regards to bending stiffness, strength (as expressed by yield point), and memory (deformation at yield point). STUDY DESIGN/SETTING: Prospective/Biomechanical Research Laboratory. PATIENT SAMPLE: N/A. OUTCOME MEASURES: Young’s Modulus, Stiffness, Yield Point, Ultimate Load, Memory. METHODS: Five spine rod materials: Standard Strength Steel (SS), High Strength Steel (HSS), Ultra Strength Steel (USS), Titanium Alloy (Ti), and Cobalt Chromium (CoCr) were tested in a four-point rod bending system fixed to an MTS test frame (MTS, Eden Prairie, MN). For each material, nine samples measuring 200 mm in length and 5.5 mm in diameter were bent to varying angles of deformation (3 to 60 degrees). Load (N) and displacement (mm) of the sample’s midpoint were measured directly at 100 Hz utilizing the MTS machine and a three camera motion capture system. Load-deformation curves were extracted and the Young’s modulus, stiffness, yield load, ultimate load and memory values were calculated. The yield point was defined as the load resulting in permanent (plastic) rod deformation while memory referred to the angle of deformation at the yield point. RESULTS: The mechanical properties for each rod are listed in Table 1. The bending stiffness was highest for CoCr (24% greater than SS, 47% greater than Ti) and the Young’s Modulus was greatest for CoCr (20% greater than SS, 55% greater than Ti). The HSS and CoCr rods had similar Young’s Moduli (260 GPa and 282 GPa), with the lowest values for the SS and Ti rods (226 GPa and 155 GPa). CoCr rods displayed a memory lower than that of Ti and USS rods and similar to SS rods. CONCLUSIONS: The elastic and plastic properties of rods used to perform spinal instrumentation vary substantially. Aggressive correction of large curves may result in rod deformations beyond the elastic limit. Choosing the best rod for the individual case and correction strategy requires an appreciation of these differences. CoCr rods offer both a high stiffness and a low yield point option best suited for secure boney fixation and in-situ bending. FDA DEVICE/DRUG STATUS: Cobalt Chromium Spinal Rod: Approved for this indication; Titanium Alloy, Standard Strength Steel, High Strength Steel, Ultra High Strength Steel Spinal Rods: Approved for this indication.
doi: 10.1016/j.spinee.2009.08.104
doi: 10.1016/j.spinee.2009.08.106
Thursday, November 12, 2009 11:00 AM–12:00 PM Concurrent Session 2: Biomechanics 84. Cobalt Chromium Rods: How Do They Stack Up? Thomas Nunn1, Eric Varley, DO2, Rattalerk Arunakul, MD3, Arnel Aguinaldo, MA1, Peter Newton, MD1; 1Rady Children’s Hospital and Health Center - San Diego, San Diego, CA, USA; 2University of California, San Diego, San Diego, CA, USA; 3University of California, Irvine, Orange, CA, USA BACKGROUND CONTEXT: Spinal rods have traditionally been available in stainless steel and titanium. While stainless steel offers the advantages of a high stiffness and a high yield point it comes at the cost of producing a large amount of MRI artifact. Titanium on the other hand minimizes MRI artifact but comes at the expense of decreased rod stiffness. Several implant manufacturers have developed cobalt chromium (CoCr) in an effort to reduce MRI artifact while maintaining material properties similar to steel. An understanding of the biomechanical properties of CoCr
85. Biomechanical Determination of Distal Level for Fusions Across the Cervicothoracic Junction Ivan Cheng, MD1, Alexander Iezza, MD1, Eric Sundberg, MD1, Derek Lindsey, MS2, K. Daniel Riew, MD3; 1Stanford University, Stanford, CA, USA; 2Bone and Joint Center of Excellence, VA Palo Alto HCS, Palo Alto, CA, USA; 3Washington University in St. Louis, St. Louis, MO, USA BACKGROUND CONTEXT: When a cervical fusion needs to be extended down to the thoracic spine for adjacent level disease, many surgeons prefer to fuse down to T2 or lower in order to diminish the possibility of further adjacent disease. The optimal distal level, however, remains to be established. PURPOSE: We undertook this study to determine the effect of ending long cervicothoracic fusions at different caudal levels on adjacent-level intradiscal pressure. We hypothesized that stopping a fusion at T1 would be biomechanically less sound than stopping at a more caudal level. STUDY DESIGN/SETTING: This was a biomechanical study. PATIENT SAMPLE: Four cadaveric cervical spine specimens with an intact rib cage and sternum were carefully dissected to preserve costovertebral joints, costosternal joints, facet joints, intercostal muscles, and all ligaments.
Proceedings of the NASS 24th Annual Meeting / The Spine Journal 9 (2009) 1S-205S OUTCOME MEASURES: Changes in intradiscal pressure were compared to baseline and analyzed statistically using a one-way analysis of variance. METHODS: Specimens were stabilized at T12 and a force of 25 N was applied perpendicularly to the long axis of the spine to produce flexion, extension, left and right lateral bending. Intradiscal pressures were recorded first on intact specimens and then after anterior cervical plating at C3-7. Sequential extension of a posterior fusion was simulated with a posterior pedicle screw construct and testing of the intradiscal pressures at the adjacent disc for each level from C7-T1 down to T9-10. RESULTS: While all levels increased their intradiscal pressure after the ‘‘fusion,’’ no level was found to be a more advantageous level to end the construct than the others. There were no significant differences among the levels in the baseline compared to the juxtafusion intradiscal pressure for flexion, extension, left and right bending (see Figure). The largest changes occurred in flexion at T7-8 level (78% increase), in extension at T8-9 (104% increase), left and right bending at the T6-7 level (43% and 32% increases respectively).
45S
Figure 4.
CONCLUSIONS: Based upon juxtafusion intradiscal pressures, no level appeared to have a biomechanical superiority for stopping a long cervicothoracic fusion. While not significant, the largest increases occurred from T6-9. Surprisingly, even stopping the fusion at T1 was no worse than stopping at a more caudal level. This suggests that one may not necessarily need to extend the fusion to a more caudal segment if there are no facet arthrosis, disc degeneration, alignment, fixation point or other concerns. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2009.08.107
Figure 1.
Figure 2.
Figure 3.
86. Biomechanical Comparison of Translaminar vs. Pedicle Screws at T1 and T2 in Long Subaxial Cervical Constructs Matthew McGirt, MD, Edward Sutter, BASC, Risheng Xu, MA, Daniel Sciubba, MD, Timothy Witham, MD, Jean-Paul Wolinsky, MD, Ziya Gokaslan, MD, Ali Bydon, MD; Johns Hopkins University, Baltimore, MD, USA BACKGROUND CONTEXT: Thoracic pedicle screws have become the gold standard for posterior spinal fixation and are frequently utilized in long subaxial cervical constructs crossing the cervico-thoracic junction. Recently, thoracic trans-laminar screws have been reported to be a feasible option for posterior fixation of the upper thoracic spine with less technical demand. PURPOSE: In an ex vivo biomechanical study, we compared the immediate and post-cyclical loading rigidity of thoracic trans-laminar (TL) versus pedicle (PD) screw fixation in sub-axial cervical constructs. STUDY DESIGN/SETTING: Ex vivo biomechanical study. PATIENT SAMPLE: Ten fresh frozen non-osteoporotic human cadaveric spines. OUTCOME MEASURES: Rotational motion after cyclical flexion-extension and lateral bending. METHODS: Ten fresh frozen non-osteoporotic human cadaveric spines underwent C4-6 lateral mass screw and T1-2 trans-laminar screw (n55) versus pedicle screw (n55) fixation, Figure 1. Spines were then potted in polymethylmethacrylate bone cement and placed in a test fixture on a materials testing machine that applied a 4 Nm bending moment, Figure 2. Rotation about the axis of bending was measured using passive retroreflective markers and infrared motion capture cameras. Rotation of C6 relative toT2 in flexion-extension and lateral bending was assessed uninstrumented, immediately after instrumentation, and following 40,000 cycles of 4 Nm flexion-extension moment and lateral bending at 1 Hz. The effect of fixation type (PD versus TL) and state (uninstrumented, instrumented, post cycling) on rotation for flexion tests and lateral bending tests was analyzed for significance via a generalized linear mixed model. RESULTS: Compared to pre-instrumented spines, PD and TL screw constructs significantly (p!0.001) reduced rotational motion during flexionextension and lateral bending. After cyclical loading, rotational motion at the cervico-thoracic junction was significantly increased (p!0.05) during flexion-extension and lateral bending in both groups, Table 1 & 2. With