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INTERVERTEBRAL DISC NUCLEUS REPLACEMENTS: COMBINED EXPERIMENTAL AND IN SILICO STUDIES
S612
Presentation 1380 − Topic 40. Spine biomechanics
INTERVERTEBRAL DISC NUCLEUS REPLACEMENTS: COMBINED EXPERIMENTAL AND IN SILICO STUDIES Sandra Reitmaier (1), Aboulfazl Shirazi-Adl (2), Maxim Bashkuev (1), Antonio Gloria (3), Hans-Joachim Wilke (1), Hendrik Schmidt (1)
1. Institute of Orthop Res and Biomech, University of Ulm; 2. École Polytechnique, Montréal; 3. Institute of Comp and Biomed Mat, Nat Res Council, Naples.
Introduction Disc degeneration has been the focus of numerous studies on its aetiology, development process, pain generation and treatment modalities. As a preventive measure in the early stages of disc degeneration, currently there is an increasing number of hydrogels under development for total nucleus replacement. Their clinical success however depends directly on their ability to mimic the near-normal mechanical function of the host structure and hence to preserve the overall function of the entire spinal motion segment. This study aimed to evaluate the extent to which surgical interventions for nucleus replacement influences the mechanical behavior of the disc using a combined in vitro and in silico approach.
Methods Effects of an annulus defect with and without a nucleus replacement and associated destruction at the annulus-nucleus interface on the disc overall response and nucleus pressure (IDP) were measured in vitro using 22 ovine motion segments (14×L2-L3, 8×L4-L5). Following cases were considered: (1) INTACT, (2) DEF-ANN: a small oblique incision into the annulus was made. The access was closed by sealing the incision using cyanoacrylate glue followed by stitching the outer annular layers, (3) REPL-SG: nucleus tissue was removed and subsequently re-implanted. Defect was closed as in DEF-ANN, and (4) REPL-CD: in contrast to REPL-SG, the annulus defect was closed by a hollow mushroom-shaped balloon plug and filled with silicone. To complement measurements, a validated 3D nonlinear osmoporoelastic finite element (FE) model of a human lumbar disc was employed to simulate different implant conditions in order to identify the underlying mechanisms observed in vitro. Both studies considered three loading cycles; each consisting of 15 minutes diurnal load (d) and a 30 minutes night load (n). Applied loads (d/n) were set based on in vivo animal (130 N/58 N) and human (500 N/100 N) studies.
Results A small annulus defect with an intact nucleus (DEF-ANN) did not affect the height loss and IDP Journal of Biomechanics 45(S1)
in vitro. In contrast, removal and re-implantation of the nucleus (REPL-SG) substantially increased the height loss and decreased the IDP (Fig. 1). REPLCD resulted in an IDP closer to INTACT. The FE model showed that alteration in the interface between nucleus and surrounding tissues by itself causes only a small decrease in IDP. Incorporating the likely loss of nucleus tissue into the inner annulus defect with an unrestricted fluid flow, however markedly influenced results in agreement with measurements (REPL-SG). During recovery a rise in IDP was calculated but not seen in vitro (Fig. 1) Reducing the water content strongly increased the disc height loss and decreased the IDP during diurnal load. The IDP remained almost constant during recovery in better agreement with in vitro measurements.
Discussion Measured results did not show any changes in presence of the annulus defect and nearly no alterations in IDP during recovery periods. They were however significantly affected when the nucleus material was also removed and replaced. The success of hydrogels for nucleus replacements is not only dependent on the implant material itself but also on the restoration of the environment perturbed during removal and replacement operations. A model with reduced nucleus water content and osmotic potential can better simulate the disc mechanical condition following a nucleus replacement.
Figure 1: Temporal change in the disc height loss and nucleus pressure.
Acknowledgements This study was funded by the EU project DISC REGENERATION (NMP-2007-CPIP213904) and by the German Research Found. (WI 1352 14-1).
ESB2012: 18th Congress of the European Society of Biomechanics
Presentation 1380 − Topic 40. Spine biomechanics
INTERVERTEBRAL DISC NUCLEUS REPLACEMENTS: COMBINED EXPERIMENTAL AND IN SILICO STUDIES Sandra Reitmaier (1), Aboulfazl Shirazi-Adl (2), Maxim Bashkuev (1), Antonio Gloria (3), Hans-Joachim Wilke (1), Hendrik Schmidt (1)
1. Institute of Orthop Res and Biomech, University of Ulm; 2. École Polytechnique, Montréal; 3. Institute of Comp and Biomed Mat, Nat Res Council, Naples.
Introduction Disc degeneration has been the focus of numerous studies on its aetiology, development process, pain generation and treatment modalities. As a preventive measure in the early stages of disc degeneration, currently there is an increasing number of hydrogels under development for total nucleus replacement. Their clinical success however depends directly on their ability to mimic the near-normal mechanical function of the host structure and hence to preserve the overall function of the entire spinal motion segment. This study aimed to evaluate the extent to which surgical interventions for nucleus replacement influences the mechanical behavior of the disc using a combined in vitro and in silico approach.
Methods Effects of an annulus defect with and without a nucleus replacement and associated destruction at the annulus-nucleus interface on the disc overall response and nucleus pressure (IDP) were measured in vitro using 22 ovine motion segments (14×L2-L3, 8×L4-L5). Following cases were considered: (1) INTACT, (2) DEF-ANN: a small oblique incision into the annulus was made. The access was closed by sealing the incision using cyanoacrylate glue followed by stitching the outer annular layers, (3) REPL-SG: nucleus tissue was removed and subsequently re-implanted. Defect was closed as in DEF-ANN, and (4) REPL-CD: in contrast to REPL-SG, the annulus defect was closed by a hollow mushroom-shaped balloon plug and filled with silicone. To complement measurements, a validated 3D nonlinear osmoporoelastic finite element (FE) model of a human lumbar disc was employed to simulate different implant conditions in order to identify the underlying mechanisms observed in vitro. Both studies considered three loading cycles; each consisting of 15 minutes diurnal load (d) and a 30 minutes night load (n). Applied loads (d/n) were set based on in vivo animal (130 N/58 N) and human (500 N/100 N) studies.
Results A small annulus defect with an intact nucleus (DEF-ANN) did not affect the height loss and IDP Journal of Biomechanics 45(S1)
in vitro. In contrast, removal and re-implantation of the nucleus (REPL-SG) substantially increased the height loss and decreased the IDP (Fig. 1). REPLCD resulted in an IDP closer to INTACT. The FE model showed that alteration in the interface between nucleus and surrounding tissues by itself causes only a small decrease in IDP. Incorporating the likely loss of nucleus tissue into the inner annulus defect with an unrestricted fluid flow, however markedly influenced results in agreement with measurements (REPL-SG). During recovery a rise in IDP was calculated but not seen in vitro (Fig. 1) Reducing the water content strongly increased the disc height loss and decreased the IDP during diurnal load. The IDP remained almost constant during recovery in better agreement with in vitro measurements.
Discussion Measured results did not show any changes in presence of the annulus defect and nearly no alterations in IDP during recovery periods. They were however significantly affected when the nucleus material was also removed and replaced. The success of hydrogels for nucleus replacements is not only dependent on the implant material itself but also on the restoration of the environment perturbed during removal and replacement operations. A model with reduced nucleus water content and osmotic potential can better simulate the disc mechanical condition following a nucleus replacement.
Figure 1: Temporal change in the disc height loss and nucleus pressure.
Acknowledgements This study was funded by the EU project DISC REGENERATION (NMP-2007-CPIP213904) and by the German Research Found. (WI 1352 14-1).
ESB2012: 18th Congress of the European Society of Biomechanics