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. 2012 Aug 7;9(73):1869-79.
doi: 10.1098/rsif.2012.0016. Epub 2012 Feb 15.

In vitro and in silico investigations of disc nucleus replacement

Sandra Reitmaier  1 , Aboulfazl Shirazi-Adl, Maxim Bashkuev, Hans-Joachim Wilke, Antonio Gloria, Hendrik Schmidt
Affiliations

In vitro and in silico investigations of disc nucleus replacement

Sandra Reitmaier et al. J R Soc Interface. .

Abstract

Currently, numerous hydrogels are under examination as potential nucleus replacements. The clinical success, however, depends on how well the mechanical function of the host structure is restored. This study aimed to evaluate the extent to and mechanisms by which surgery for nucleus replacements influence the mechanical behaviour of the disc. The effects of an annulus defect with and without nucleus replacement on disc height and nucleus pressure were measured using 24 ovine motion segments. The following cases were considered: intact; annulus incision repaired by suture and glue; annulus incision with removal and re-implantation of nucleus tissue repaired by suture and glue or plug. To identify the likely mechanisms observed in vitro, a finite-element model of a human disc (L4-L5) was employed. Both studies were subjected to physiological cycles of compression and recovery. A repaired annulus defect did not influence the disc behaviour in vitro, whereas additional nucleus removal and replacement substantially decreased disc stiffness and nucleus pressure. Model predictions demonstrated the substantial effects of reductions in replaced nucleus water content, bulk modulus and osmotic potential on disc height loss and pressure, similar to measurements. In these events, the compression load transfer in the disc markedly altered by substantially increasing the load on the annulus when compared with the nucleus. 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 surgery. The substantial effects on the disc response of disruptions owing to nucleus replacements can be simulated by reduced nucleus water content, elastic modulus and osmotic potential.

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Figures

Figure 1.
Figure 1.
(a) Experimental set-up for the in vitro compression tests on sheep motion segments. (b) Time history of the applied compressive force in vitro/in silico: three loading cycles were performed, each consisting of a loading period of 15 min at 130/500 N and a recovery period of 30 min at 58/100 N (single asterisks denote diurnal load resulting in a creep response; double asterisks denote night load resulting in a recovery response). (c) Symmetric finite-element model of the human lumbar intervertebral disc (IA, inner annulus; OA, outer annulus; CEP, cartilage endplate; BEP, bony endplate).
Figure 2.
Figure 2.
Temporal changes of averaged (a) specimen height loss and (b) intradiscal pressure measured in the centre of the nucleus for INTACT and DEF-ANN for the second and third loading cycles. Curves represent the mean and s.d. of six specimens. Specimen height loss is presented relative to the height loss under 58 N at the end of the first loading cycle. Numbers in the diagrams represent the p-values estimated at the end of each loading and recovery phase (Wilcoxon signed-rank test). DEF-ANN: a small oblique incision through the whole annulus depth was set.
Figure 3.
Figure 3.
Temporal changes of averaged (a) specimen height loss and (b) intradiscal pressure measured in the centre of the nucleus for INTACT and REPL-SG for the second and third loading cycles. Numbers in the diagrams represent the p-values estimated at the end of each loading and recovery phase (Wilcoxon signed-rank test). REPL-SG: nucleus tissue was removed and subsequently completely re-implanted.
Figure 4.
Figure 4.
(a,b) Axial view of the intervertebral disc with the re-implanted nucleus (REPL-SG) after three cycles of compression. Displaced nucleus material was observed underneath the stitched and glued outer fibrous annular layers. (c) Schematic and (d) photographic views of the intervertebral disc cross section depicting the annulus closure device in REPL-CD.
Figure 5.
Figure 5.
Temporal changes of averaged (a) specimen height loss and (b) intradiscal pressure measured in the centre of the nucleus for INTACT and REPL-CD for the second and third loading cycles. Numbers in the diagrams represent the p-values estimated at the end of each loading and recovery phase (Wilcoxon signed-rank test). REPL-CD: the annulus defect was closed by a hollow mushroom-shaped balloon plug.
Figure 6.
Figure 6.
Ninety-eight per cent nucleus replacement (REPL-98FE) versus intact disc model (INTACTFE). Temporal changes of the (a) height loss and (b) intradiscal pressure in the centre of the nucleus as well as the total axial forces transmitted through the (c) nucleus and (d) annulus (ground substance and collagen fibres). The fluid flows freely out of the nucleus into the gap assuming a zero boundary pore pressure at the outer nucleus surface.
Figure 7.
Figure 7.
Reduced water content of 20% (WC-80FE) and reduced elasticity of 25% (EL-75FE) of the replaced nucleus material versus intact disc model (INTACTFE). Temporal changes of the (a) height loss and (b) intradiscal pressure in the centre of the nucleus as well as the total axial forces transmitted through the (c) nucleus and (d) annulus (ground substance and collagen fibres).
Figure 8.
Figure 8.
Reduced osmotic potential of 10% (OP-90FE) versus intact disc model (INTACTFE). Temporal changes of the (a) height loss and (b) intradiscal pressure in the centre of the nucleus as well as the total axial forces transmitted through the (c) nucleus and (d) annulus (ground substance and collagen fibres).
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