ReviewBiomechanical studies on cervical total disc arthroplasty: A literature review
Introduction
Spinal fusion offers the surgeon an opportunity to remove the pathologic process, eliminate painful motion and obtain decompression of the neural elements (Hilibrand and Robbins, 2004). Fusion is at present the gold standard treatment for herniated cervical discs, and allows very high fusion rates, currently over 95% after application of anterior cervical implants (Mummaneni and Haid, 2004). However, several clinical studies show that fusion can be associated with symptomatic adjacent-segment disease (Goffin et al., 2003, Robertson et al., 2005), although the actual occurrence frequency of this complication remains an open question (Hilibrand and Robbins, 2004). Early adjacent level degeneration appears to be due to the abnormal kinematics and load transfer patterns at adjacent functional spinal units induced by fusion. Total disc arthroplasty (TDA), allowing the preservation of the mobility of the implanted segment, is believed to reduce the stress sustained by the adjacent levels and avoid, or at least slow down, their early degeneration (Albert and Eichenbaum, 2004). Clinical studies on TDA report no evidence of adjacent symptomatic disc degeneration, and no increased motion at adjacent levels after implantation of a disc prosthesis (Bertagnoli and Kumar, 2002, Duggal et al., 2004, McAfee et al., 2003).
Many models of disc prostheses are currently commercially available or under clinical trial, and are targeted to be used both in the cervical or in the lumbar spine. The prostheses generally consists in two or three components, mainly presenting articulating surfaces, and are built in advanced materials to ensure mechanical reliability and limit wear. The choice of the materials and the design of the shape of the device components influence the biomechanics of the implanted spine, thus affecting the clinical outcome. A number of biomechanical studies about TDA have been published, but the relationship between design and biomechanics of the implanted spine has not been clearly figured out yet. This is evidenced by the significant differences in the designs of the currently available devices.
This review summarizes the biomechanical studies concerning cervical TDA. Only single level TDA is considered, since multilevel and hybrid implantations induce significantly different biomechanical conditions which discussion would require a distinct review. The paper is structured along the targets of TDA which are related to biomechanical parameters:
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restoration of a physiological kinematics and mobility, avoiding spinal instability;
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protection of the biological structures, such as the adjacent intervertebral discs, the facet joints and the ligaments, from overloading;
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restoration of a correct spinal alignment;
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device stability and response to wear debris.
Aim of the present review is the understanding of the possible relationships between the aforementioned points and the geometrical, mechanical and material properties of the various currently available disc prostheses.
The papers available through a PUBMED (National Library of Medicine and National Institute of Health, USA) search (revised January 14th, 2008) were reviewed. Search strings employed were “cervical disc arthroplasty”, “cervical artificial disc”, “cervical disc replacement” and “cervical disc prosthesis”. Among the 309 papers found, 31 papers not written in the English language and 34 review papers were discarded. Furthermore, 190 papers were not considered, since they do not include any relevant biomechanical information on single level cervical TDA. Totally, 54 papers were reviewed.
Section snippets
Design concepts
Disc prostheses have been described, with reference to the allowed motion, using different classification systems (LeHuec et al., 2003, Link et al., 2004, Rousseau et al., 2006, Sekhon and Ball, 2005). In this review, the devices will be classified as unconstrained, semi-constrained and constrained, depending on the degrees of freedom allowed by the specific design (Fig. 1). All the three categories include prostheses featuring the widely employed “ball-and-socket” design, which consists in a
Finite axes of rotation
Most studies of the cervical spine have addressed flexion–extension, since these are believed to be the cardinal movements of this segment. The finite axes of rotation (FARs) of the cervical spine in flexion–extension have been quantitatively investigated in some studies (Amevo et al., 1991, Bogduk and Mercer, 2000, Penning and Wilmink, 1987). Generally, the FARs are located below the disc, posteriorly with respect to the endplate center (Fig. 2). From above downwards, the axis of rotation is
Protection of the biological structures
Although the most investigated issue on TDA biomechanics is motion preservation, the study of loads and stresses acting on the anatomical structures is gaining increasing attention. Indeed, unphysiological stresses in the tissues, which may be induced by the implantation of a disc prosthesis, are a direct cause of their early degeneration (Chang et al., 2007a, Hilibrand and Robbins, 2004). Some studies addressing the evaluation of stresses are currently available (Table 3). The two issues which
Spinal alignment
Disc arthroplasty can alter the spine alignment. In particular, resection of the anterior longitudinal ligament and the anterior portion of the annulus fibrosus, combined to an increase of the intervertebral space height, induces an alteration of the segmental lordosis (Cakir et al., 2005), regardless of the mechanics of the prosthesis itself.
In the same way as kinematics, spinal alignment after TDA is related to the design of the specific prosthesis. The use of the Bryan prosthesis can result
Device stability and wear
With reference to the interface between the disc prosthesis and the surrounding biological system, two topics are primarily discussed in the literature: the osseointegration at the bone–implant surface and the biological response to the possible formation of wear debris (Table 4). Besides the kinematics of the disc prosthesis, the materials and the possible presence of keels or screws are crucial with reference to these problems.
Most published animal studies report good or excellent
Conclusions
Based on the present literature review, TDA is generally able to preserve a nearly physiological motion in the cervical spine. However, several alterations in the cervical biomechanics due to TDA have been reported in the literature. Most authors reported RoMs nearly similar to the physiological values; nevertheless, RoM increase at the implanted level has been observed in some ex vivo studies. Loss of motion and heterotopic ossification have been reported in a significant number of patients.
Conflict of interest statement
All the authors (Fabio Galbusera, Chiara M. Bellini, Marco Brayda-Bruno, Maurizio Fornari) have no proprietary, financial, professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the present manuscript entitled “Biomechanical studies on cervical total disc arthroplasty: A literature review”.
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