Influence of different artificial disc kinematics on spine biomechanics
Introduction
Total disc arthroplasty is becoming more and more common in regard to the surgical treatment of discogenic spinal pathology. Many implants have been designed for the arthroplasty of lumbar discs in the last years (Mayer, 2005). From the mechanical point of view, disc prostheses are defined mainly by their kinematics, and these are distinctly different from disc to disc. Typically, the artificial discs consist of two or three functional components. There are always two disc plates, the relative movements of which are determined by a core. The core is either separate or it is connected to the lower plate. While the CHARITÉ® Artificial Disc (DePuySpine, Inc., Raynham, MA, USA) has a lentoid core with two articulating surfaces, the ProDisc® (Synthes, Inc., West Chester, PA, USA) has an inlay with a fixed centre of rotation. The disc Activ L (Aesculap, Inc., Center Valley, PA, USA) has a core that can slide in the anterio-posterior direction. The core of the Mobidisc® (LDR Médical, Troyes, France) can additionally slide laterally. Another difference is the core’s radius of curvature. The Maverick™ prosthesis (Medtronic, Inc., Minneapolis, MN, USA) and the Flexicore® (Stryker, Kalamazoo, MI, USA) both have a relatively small radius, but the position of the centre of the core differs. The Maverick’s centre of rotation is located more posteriorly than that of the Flexicore.
Several retrospective and some prospective studies (Blumenthal et al., 2005, Delamarter et al., 2003, Huang et al., 2003, Shim et al., 2007, Siepe et al., 2007, Tournier et al., 2007) show the usefulness of total disc arthroplasty as a surgical procedure which can provide pain relief while maintaining mobility, but only few experimental studies have been performed focussing on the mechanical characteristics of artificial discs. Panjabi et al. performed in vitro tests with the Charité disc (Panjabi et al., 2007b) and the ProDisc (Panjabi et al., 2007a) in order to investigate if effects on adjacent levels can be predicted. They did not find any remarkable changes at the adjacent discs for one level total disc arthroplasty. O’Leary et al. (2005) measured the intervertebral rotations of specimen with and without a Charité disc. They found an increased mobility after implantation but did not compare different implant types. Tournier et al. (2007) studied 105 patients with the Maverick disc, the Charité disc and the ProDisc with X-rays in vivo and measured the range of motion, the mean centre of rotation, and the balance. They found no difference in the ranges of motion between the three prostheses.
An implant is unlikely to provide the exact characteristics of native anatomy and different disc kinematics will probably influence relative motion of the adjacent vertebrae, the facet joint forces and adjacent disc loads. But the multitude of biological factors existing in experimental studies impedes an exploration of the pure biomechanical influence of artificial discs. Investigating the isolated influence of the disc kinematics is possible only in simulation studies. Several studies exist which reveal the biomechanical influence of artificial discs (Denoziere and Ku, 2006, Grauer et al., 2006, Rohlmann et al., 2005, Rohlmann et al., 2008, Zander et al., 2007) but these studies were carried out for a different purpose than for studying inter-implant differences and were build up and loaded differently which prevents comparability.
The relative motion of adjacent vertebrae can be characterized in different ways. The clinical relevance of the centre of rotation was pointed out by Pearcy and Bogduk (1988). For movements that cannot be described in two dimensions like lateral bending or axial torsion, the main motion alone does not provide a complete description of the movement. Therefore Baeyens et al. (2005) proposed to describe the motion patterns with the finite helical axes which also have a communicative meaning: The motion of a body can always be described as a rotation around, and a superimposed translation along this axis.
The objective of the present study is to predict differences in the intervertebral rotation, to describe the vertebral kinematics and to estimate the facet loads and the intradiscal pressures in the adjacent discs for the load cases flexion, extension, lateral bending and axial torsion. Kinematics considered in this study are those of the artificial discs Charité, ProDisc, and Activ L.
Section snippets
Intact model
A validated three-dimensional finite element model for static analyses of the lumbar spine ranging from vertebra L1 to the lumbosacral disc L5/S1 was used (Fig. 1). It is build up by about 60,000 elements the nodes of which have about 200,000 degrees of freedom. It incorporates non-linearities originating from material behaviour, contact conditions in the facet joints and from large deflections. The dimensions and orientations of the intervertebral discs and the vertebrae including the curved
Relative intervertebral rotation
The relative rotation of adjacent vertebrae around their helical axes is changed after the insertion of an intervertebral disc prosthesis. Above and below the treated level, there is a small decrease in rotation (Fig. 4, top and bottom) in most of the cases. The decrease is between 5% and 34% of the value for the intact situation when no scar tissue was simulated at the treated level. For extension, simulated scar tissue leads to a small increase of relative rotation at the adjacent levels of
Discussion
The influence of the different design concepts of artificial discs on intervertebral rotation, helical axes of motion, facet joint forces, and on the intradiscal pressure was studied. The kinematical differences between three concepts realized for clinically-used lumbar intervertebral disc prostheses were investigated for different load cases.
Although emphasis was laid on model validation, each model is limited to what it was created for and has its own shortcomings: The present model does not
Conclusions
Total disc arthroplasty inherently alters the kinematics at implant level for all clinically-used prostheses. Compared to these alterations inter-implant differences are small. This result is supported by the clinical outcome available until now, which shows similar success rates for the available artificial discs (Mayer, 2005). By contrast, facet joint forces are strongly dependent on the implant kinematics and on the load case. Only long term clinical results can show if the increased facet
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft, Bonn, Germany (Ro 581/17-2). We thank Dr. W. Baumann and Dr. R. Ehrig from Konrad-Zuse-Zentrum für Informationstechnik Berlin (ZIB), Germany, for computational assistance. Finite element analyses were performed at the Norddeutscher Verbund für Hoch- und Höchstleistungsrechnen (HLRN).
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2020, Journal of BiomechanicsCitation Excerpt :Intervertebral motion was assumed to be a pure rotation (no translation) about a fixed centre of rotation (Pearcy and Bogduk, 1988). However, the in vivo location of the centre of rotation varies throughout the range of motion and is dependent on the participant and the activity (Zander et al., 2009, Kettler et al., 2004, Wachowski et al., 2009), leading to up to 30% differences in joint load estimates (Ghezelbash et al., 2015). Shifting the joint centre posteriorly increases the net external moment due to gravity and decreases extensor moment arms, thus resulting in larger muscle forces and joint loads, while the opposite results occur when shifting the joint anteriorly (Ghezelbash et al., 2018).