- Email: [email protected]
Effects of rate of loading on viscoelastic supraspinous ligament inflammation and cumulative lumbar disorder
The Spine Journal 10 (2010) 1086–1088
Commentary
Effects of rate of loading on viscoelastic supraspinous ligament inflammation and cumulative lumbar disorder Vijay K. Goel, PhDa,b,*, Scott C. Molitor, PhDa b
a Department of Bioengineering, College of Engineering, University of Toledo, Toledo, OH 43606, USA Department of Orthopaedic Surgery, College of Medicine, University of Toledo, Toledo, OH 43606, USA
Received 22 September 2010; accepted 30 September 2010
COMMENTARY ON: Pinski SE, King KB, Davidson BS, et al. High-frequency loading of lumbar ligaments increases proinflammatory cytokines expression in a feline model of repetitive musculoskeletal disorder. Spine J 2010:10:1078–85 (in this issue).
Spinal disorders represent a very large segment of medical needs. Overall, the associated economic losses are staggering. The aging of the population in western countries and industrialization of emerging nations is accelerating the demand for improved methods for prevention, diagnosis, and treatment of spinal disease. To achieve these objectives, it is essential to understand the underlying mechanisms that may contribute to degeneration of spinal tissue, such as the vertebral body, disc, and ligaments. For example, fibrinbased products may provide short- and long-term pain relief by reducing inflammation of the disc tissue. Inflammation in the tissue may be assessed by quantifying clinically relevant cytokines [1,2]. The manuscript being commented on, by Pinski et al. [3], deals with the role of rate of loading in inducing inflammation in the viscoelastic supraspinous ligament (SSL). Briefly, the authors state that repetitive loading of the lumbar spine can produce excessive strain of lumbar ligaments, resulting in microdamage and the generation of inflammatory responses. Sequalae of this damage and consequent inflammation includes muscle spasms, chronic inflammation, tissue degeneration, and remodeling. Previous studies have examined the inflammatory responses of muscles to repetitive
DOI of original article: 10.1016/j.spinee.2010.08.030. FDA device/drug status: not applicable. Author disclosures: VKG (royalties, Turning Point, LLC, X-Spine, Inc.; stock ownership, including options and warrants: DMT, Inc.; consulting, DePuy, Orthokinetics Technologies, LLC; scientific advisory board, Turning Point, LLC; grants, DePuy). * Corresponding author. Department of Bioengineering, 5046 NI, MS 303, College of Engineering, University of Toledo, Toledo, OH 43606, USA. Tel.: (419) 530-8035; fax: (419) 530-8076. E-mail address: [email protected] (V.K. Goel) 1529-9430/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2010.09.022
loading. However, the viscoelastic properties of SSLs mean that high strains can also be induced by high-velocity motion without large applied loads. Therefore, it is possible that similar inflammatory sequalae could result from high-frequency cyclical loading. The purpose of this study was to look at cytokines as an assay of inflammatory responses under high-frequency cyclic loading compared with similar loads applied at low frequency. After the 7-hour loading and testing protocol at low or high frequency, the SSLs from L3/L4, L4/L5, and L5/L6 were isolated. Supraspinous ligaments from T10/T11 were isolated as control. Standard methods including RNA extraction, cDNA library creation, and reverse-transcriptase polymerase chain reaction with cytokine probes were used to quantify cytokine mRNA from SSL samples. Cytokine levels in the various lumbar SSL samples were normalized by cytokine levels found in T10/T11 and analyzed using a two-way analysis of variance to investigate effects of lumbar level and loading frequency across 21 animals loaded at 0.25 Hz (N59) or 0.5 Hz (N512). The following cytokines were measured [2]: Interleukin-1 beta (IL-1b) has wide-ranging effects, including adaptive and innate immune responses, inflammation, responses to infection, and tissue remodeling. Interleukin-6 produces various responses related to adaptive immunity, inflammation, and infection. Interleukin-8 serves as a chemoattractant for neutrophils during inflammatory responses. Tumor necrosis factor alpha (TNF-a) is a strong mediator of various immune and inflammatory responses and is associated with cell growth, differentiation, and death during remodeling processes. Transforming growth factor beta (TGF-b) inhibits cell growth and affects tissue remodeling, wound repair, and adaptive immune responses.
V.K. Goel and S.C. Molitor / The Spine Journal 10 (2010) 1086–1088
The authors found that the higher loading rate produced a significant increase in all five cytokines measured at L5/ L6 and all cytokines except TGF-b at L4/L5. Only IL-6 and IL-8 were significantly elevated with the higher loading rate at L3/L4. Not surprisingly given the loading protocol, the highest normalized levels of cytokines associated with inflammatory responses (IL-1b, IL-6, and IL-8) were observed at L4/L5 and were lower at L3/L4 and L5/L6. Interestingly, the cytokines associated with tissue remodeling (TNF-a and TGF-b) showed a weaker response to the increased loading rate when compared with the other cytokines measured in this study and did not show a systematic distribution with lumber level as expected, given the L4/L5 loading protocol. The authors suggest that the increase in cytokine expression associated with higher loading rates is because of collagen fiber microdamage produced by the increased strain produced by the viscoelastic effects of higher loading velocity. The authors note that the higher frequency loading doubles the number of cycles in a given time, thereby leaving the possibility that the increased number of cycles is responsible for the increased cytokine expression. To refute this, they cite similar results from a previous study, where inflammatory responses are observed with a higher loading rate when the number of loading cycles is kept constant. The authors then go on to support their assertion that increased loading frequency leads to microdamage of collagen fibers, followed by inflammatory responses and their sequelae. Previous observations include increased neutrophil density, spasms, and delayed hyperexcitability in lumbar multifidi. Loading protocol There are a number of unresolved issues raised by this study, specifically the parameters of the applied load. First, the basis for the selection of 40-N cyclic load is not that clear. Does this load value ensure ligament stretch beyond its laxity zone? The cyclic load may not be able to apply extension movement to the spine/ligament that well. A photograph of the entire setup, instead of a line diagram, along with a slightly better description would have been more helpful to the readership. Although a 0.25- to 0.5-Hz frequency range is appropriate for everyday activities such as walking, past work suggests that the resonant frequency of the human spine lies between 4 and 8 Hz [4,5]. Given the hypothesis that increased inflammatory responses are produced by a higher loading velocity, it may be beneficial to show a correlation between cytokine expression and loading velocity over a larger range of loading velocities. Finally, the authors make a substantial effort in the discussion to demonstrate that these results are because of increased viscoelastic effects, not an increased number of loading cycles applied with a higher frequency over the same loading period. It is not clear why the authors chose
1087
not to repeat this experiment with high-frequency loading at the same number of cycles as low-frequency loading. This would provide a reasonable control experiment to insure that increased loading velocity is responsible for these results, not doubling the number of loading cycles. Interpretation We are intrigued by the results that show that the inflammatory cytokines IL-1b, IL-6, and IL-8 show vigorous responses to the lumbar location and loading frequency, whereas the cytokines TNF-a and TGF-b associated with longer term tissue remodeling responses at best show a mixed response to lumbar location and loading frequency. Although this is a simplistic view of the function of these cytokines, the idea that IL-1b, IL-6, and IL-8 mediate shorter term inflammatory responses, whereas TNF-a and TGF-b mediate longer term tissue remodeling responses, fits well with the total duration of the loading protocol and subsequent test loads in this study. It would be interesting to test this loading protocol as a survival surgery, followed by sacrifice, and SSL harvest up to 2 weeks after loading. It is possible that TNF-a and TGF-b would show a much larger response to loading frequency and a more direct correlation to loads applied at the various lumbar levels if enough time were given for tissue remodeling responses to appear. Otherwise, it is not obvious how to interpret the distribution of cytokine expression in some cases. The expression of cytokines in several cases was more significant at the segments adjacent to the L4/L5 segment, where cyclic loads were applied. The load and consequently the deformation of the ligaments of the adjacent segments were less severe than the L4/L5 segment where hook was placed, and these deformations could potentially have fallen within the laxity zone of these ligaments. A more detailed analysis of increased cytokine expression from these adjacent regions would have been helpful. Finally, it is not clear why the authors chose to focus exclusively on SSLs. In the human spine, the SSL is not considered important from the low back pain and degeneration perspectives, and the clinical rationale for undertaking the studies on SSL is not that clear. A similar analysis on more clinically relevant tissues from the same animals would have been appropriate. For example, similar viscoelastic effects should be observed in capsular ligaments, as these ligaments were also undergoing the same cyclic deformation during the loading protocol. It would be interesting to assay the cytokine expression profile for these ligaments, as they are considered clinically relevant for degeneration and pain in the spine during cumulative lumbar disorder. Summary The authors provide a direct demonstration of increased cytokine expression in response to increased loading
1088
V.K. Goel and S.C. Molitor / The Spine Journal 10 (2010) 1086–1088
velocity. These results suggest that inflammatory responses can be created by viscoelastic effects of loading velocity, creating similar pathology observed during the application of excessive loads. This is an interesting observation and may have potential application in the prevention and treatment of cumulative lumbar disorder.
[2] [3]
[4]
References [5] [1] Zorica B, Jane L, Thorne EK, et al. Fibrin sealant modulates the inflammatory response in intervertebral disc cells. 56th Annual Meeting
of the Orthopaedic Research Society. March 6–9, 2010; New Orleans, LA. Kindt TJ, Osborne BA, Goldsby RA. Kuby immunology. 6th ed. New York: W.H. Freeman, 2007. Pinski SE, King KB, Davidson BS, et al. High-frequency loading of lumbar ligaments increases proinflammatory cytokines expression in a feline model of repetitive musculoskeletal disorder. Spine J 2010;10:1078–85. Wilder DG, Woodworth BB, Frymoyer JW, Pope MH. Vibration and the human spine. Spine 1982;7:243–54. Kong WZ, Goel VK. Ability of the finite element models to predict response of the human spine to sinusoidal vertical vibration. Spine 2003;28:1961–7.
Commentary
Effects of rate of loading on viscoelastic supraspinous ligament inflammation and cumulative lumbar disorder Vijay K. Goel, PhDa,b,*, Scott C. Molitor, PhDa b
a Department of Bioengineering, College of Engineering, University of Toledo, Toledo, OH 43606, USA Department of Orthopaedic Surgery, College of Medicine, University of Toledo, Toledo, OH 43606, USA
Received 22 September 2010; accepted 30 September 2010
COMMENTARY ON: Pinski SE, King KB, Davidson BS, et al. High-frequency loading of lumbar ligaments increases proinflammatory cytokines expression in a feline model of repetitive musculoskeletal disorder. Spine J 2010:10:1078–85 (in this issue).
Spinal disorders represent a very large segment of medical needs. Overall, the associated economic losses are staggering. The aging of the population in western countries and industrialization of emerging nations is accelerating the demand for improved methods for prevention, diagnosis, and treatment of spinal disease. To achieve these objectives, it is essential to understand the underlying mechanisms that may contribute to degeneration of spinal tissue, such as the vertebral body, disc, and ligaments. For example, fibrinbased products may provide short- and long-term pain relief by reducing inflammation of the disc tissue. Inflammation in the tissue may be assessed by quantifying clinically relevant cytokines [1,2]. The manuscript being commented on, by Pinski et al. [3], deals with the role of rate of loading in inducing inflammation in the viscoelastic supraspinous ligament (SSL). Briefly, the authors state that repetitive loading of the lumbar spine can produce excessive strain of lumbar ligaments, resulting in microdamage and the generation of inflammatory responses. Sequalae of this damage and consequent inflammation includes muscle spasms, chronic inflammation, tissue degeneration, and remodeling. Previous studies have examined the inflammatory responses of muscles to repetitive
DOI of original article: 10.1016/j.spinee.2010.08.030. FDA device/drug status: not applicable. Author disclosures: VKG (royalties, Turning Point, LLC, X-Spine, Inc.; stock ownership, including options and warrants: DMT, Inc.; consulting, DePuy, Orthokinetics Technologies, LLC; scientific advisory board, Turning Point, LLC; grants, DePuy). * Corresponding author. Department of Bioengineering, 5046 NI, MS 303, College of Engineering, University of Toledo, Toledo, OH 43606, USA. Tel.: (419) 530-8035; fax: (419) 530-8076. E-mail address: [email protected] (V.K. Goel) 1529-9430/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2010.09.022
loading. However, the viscoelastic properties of SSLs mean that high strains can also be induced by high-velocity motion without large applied loads. Therefore, it is possible that similar inflammatory sequalae could result from high-frequency cyclical loading. The purpose of this study was to look at cytokines as an assay of inflammatory responses under high-frequency cyclic loading compared with similar loads applied at low frequency. After the 7-hour loading and testing protocol at low or high frequency, the SSLs from L3/L4, L4/L5, and L5/L6 were isolated. Supraspinous ligaments from T10/T11 were isolated as control. Standard methods including RNA extraction, cDNA library creation, and reverse-transcriptase polymerase chain reaction with cytokine probes were used to quantify cytokine mRNA from SSL samples. Cytokine levels in the various lumbar SSL samples were normalized by cytokine levels found in T10/T11 and analyzed using a two-way analysis of variance to investigate effects of lumbar level and loading frequency across 21 animals loaded at 0.25 Hz (N59) or 0.5 Hz (N512). The following cytokines were measured [2]: Interleukin-1 beta (IL-1b) has wide-ranging effects, including adaptive and innate immune responses, inflammation, responses to infection, and tissue remodeling. Interleukin-6 produces various responses related to adaptive immunity, inflammation, and infection. Interleukin-8 serves as a chemoattractant for neutrophils during inflammatory responses. Tumor necrosis factor alpha (TNF-a) is a strong mediator of various immune and inflammatory responses and is associated with cell growth, differentiation, and death during remodeling processes. Transforming growth factor beta (TGF-b) inhibits cell growth and affects tissue remodeling, wound repair, and adaptive immune responses.
V.K. Goel and S.C. Molitor / The Spine Journal 10 (2010) 1086–1088
The authors found that the higher loading rate produced a significant increase in all five cytokines measured at L5/ L6 and all cytokines except TGF-b at L4/L5. Only IL-6 and IL-8 were significantly elevated with the higher loading rate at L3/L4. Not surprisingly given the loading protocol, the highest normalized levels of cytokines associated with inflammatory responses (IL-1b, IL-6, and IL-8) were observed at L4/L5 and were lower at L3/L4 and L5/L6. Interestingly, the cytokines associated with tissue remodeling (TNF-a and TGF-b) showed a weaker response to the increased loading rate when compared with the other cytokines measured in this study and did not show a systematic distribution with lumber level as expected, given the L4/L5 loading protocol. The authors suggest that the increase in cytokine expression associated with higher loading rates is because of collagen fiber microdamage produced by the increased strain produced by the viscoelastic effects of higher loading velocity. The authors note that the higher frequency loading doubles the number of cycles in a given time, thereby leaving the possibility that the increased number of cycles is responsible for the increased cytokine expression. To refute this, they cite similar results from a previous study, where inflammatory responses are observed with a higher loading rate when the number of loading cycles is kept constant. The authors then go on to support their assertion that increased loading frequency leads to microdamage of collagen fibers, followed by inflammatory responses and their sequelae. Previous observations include increased neutrophil density, spasms, and delayed hyperexcitability in lumbar multifidi. Loading protocol There are a number of unresolved issues raised by this study, specifically the parameters of the applied load. First, the basis for the selection of 40-N cyclic load is not that clear. Does this load value ensure ligament stretch beyond its laxity zone? The cyclic load may not be able to apply extension movement to the spine/ligament that well. A photograph of the entire setup, instead of a line diagram, along with a slightly better description would have been more helpful to the readership. Although a 0.25- to 0.5-Hz frequency range is appropriate for everyday activities such as walking, past work suggests that the resonant frequency of the human spine lies between 4 and 8 Hz [4,5]. Given the hypothesis that increased inflammatory responses are produced by a higher loading velocity, it may be beneficial to show a correlation between cytokine expression and loading velocity over a larger range of loading velocities. Finally, the authors make a substantial effort in the discussion to demonstrate that these results are because of increased viscoelastic effects, not an increased number of loading cycles applied with a higher frequency over the same loading period. It is not clear why the authors chose
1087
not to repeat this experiment with high-frequency loading at the same number of cycles as low-frequency loading. This would provide a reasonable control experiment to insure that increased loading velocity is responsible for these results, not doubling the number of loading cycles. Interpretation We are intrigued by the results that show that the inflammatory cytokines IL-1b, IL-6, and IL-8 show vigorous responses to the lumbar location and loading frequency, whereas the cytokines TNF-a and TGF-b associated with longer term tissue remodeling responses at best show a mixed response to lumbar location and loading frequency. Although this is a simplistic view of the function of these cytokines, the idea that IL-1b, IL-6, and IL-8 mediate shorter term inflammatory responses, whereas TNF-a and TGF-b mediate longer term tissue remodeling responses, fits well with the total duration of the loading protocol and subsequent test loads in this study. It would be interesting to test this loading protocol as a survival surgery, followed by sacrifice, and SSL harvest up to 2 weeks after loading. It is possible that TNF-a and TGF-b would show a much larger response to loading frequency and a more direct correlation to loads applied at the various lumbar levels if enough time were given for tissue remodeling responses to appear. Otherwise, it is not obvious how to interpret the distribution of cytokine expression in some cases. The expression of cytokines in several cases was more significant at the segments adjacent to the L4/L5 segment, where cyclic loads were applied. The load and consequently the deformation of the ligaments of the adjacent segments were less severe than the L4/L5 segment where hook was placed, and these deformations could potentially have fallen within the laxity zone of these ligaments. A more detailed analysis of increased cytokine expression from these adjacent regions would have been helpful. Finally, it is not clear why the authors chose to focus exclusively on SSLs. In the human spine, the SSL is not considered important from the low back pain and degeneration perspectives, and the clinical rationale for undertaking the studies on SSL is not that clear. A similar analysis on more clinically relevant tissues from the same animals would have been appropriate. For example, similar viscoelastic effects should be observed in capsular ligaments, as these ligaments were also undergoing the same cyclic deformation during the loading protocol. It would be interesting to assay the cytokine expression profile for these ligaments, as they are considered clinically relevant for degeneration and pain in the spine during cumulative lumbar disorder. Summary The authors provide a direct demonstration of increased cytokine expression in response to increased loading
1088
V.K. Goel and S.C. Molitor / The Spine Journal 10 (2010) 1086–1088
velocity. These results suggest that inflammatory responses can be created by viscoelastic effects of loading velocity, creating similar pathology observed during the application of excessive loads. This is an interesting observation and may have potential application in the prevention and treatment of cumulative lumbar disorder.
[2] [3]
[4]
References [5] [1] Zorica B, Jane L, Thorne EK, et al. Fibrin sealant modulates the inflammatory response in intervertebral disc cells. 56th Annual Meeting
of the Orthopaedic Research Society. March 6–9, 2010; New Orleans, LA. Kindt TJ, Osborne BA, Goldsby RA. Kuby immunology. 6th ed. New York: W.H. Freeman, 2007. Pinski SE, King KB, Davidson BS, et al. High-frequency loading of lumbar ligaments increases proinflammatory cytokines expression in a feline model of repetitive musculoskeletal disorder. Spine J 2010;10:1078–85. Wilder DG, Woodworth BB, Frymoyer JW, Pope MH. Vibration and the human spine. Spine 1982;7:243–54. Kong WZ, Goel VK. Ability of the finite element models to predict response of the human spine to sinusoidal vertical vibration. Spine 2003;28:1961–7.