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Motion Preservation in the Anterior and Posterior Spine

Wilke, Hans-Joachim, PhD

doi: 10.1097/BRS.0000000000002570
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Institute of Orthopaedic Research and Biomechanics, University Hospital Ulm Helmholtzstrasse, Ulm, Germany.

Address correspondence and reprint requests to Hans-Joachim Wilke, PhD, Institute of Orthopaedic Research and Biomechanics, University Hospital Ulm Helmholtzstrasse 14, 89081 Ulm, Germany; E-mail: hans-joachim.wilke@uni-ulm.de

Received 2 January, 2018

Accepted 12 January, 2018

The manuscript submitted does not contain information about medical device(s)/drug(s).

No funds were received in support of this work.

Relevant financial activities outside the submitted work: grants.

Nonfusion technologies in spinal surgery have gained more and more popularity in recent times, particularly in the last 2 decades. New ideas are constantly created and turned into new products. Each idea has its own philosophy, with the principle goal being to restore and maintain the disc height and/or original mobility of a healthy segment. Furthermore, these ideas allow at least the partial preservation of some spinal structures that would be sacrificed with spinal fusion.

These motion preservation strategies can be categorized into posterior (dynamic stabilization systems, interspinous implants, total posterior-element replacement system) and anterior devices (total disc prostheses nucleus replacement implants, methods to seal or close a hole in the annulus; Figure 1).

Figure 1

Figure 1

The goal of posterior devices is to stabilize the treated segments but preserve the disc and ideally keep the deformation in a physiological range1 (Figure 2) or to just unload or replace the facet joints. The most important representatives are flexible stabilization systems such as dynamic or semirigid fixators. However, they can stiffen the segments as much as rigid internal fixators if they do not allow lengthening of the semirigid rods, because these work as tension bands.2,3 Facet joint replacements can be designed to mimic the bony posterior structures and control the range of motion (ROM) in all motion planes in a physiological range.4 The different interspinous implants in general have a similar behavior, even if they are just placed between the spinous processes, whereas preserving the supraspinous ligament or when the implants are crimped or tightened with cords to the processes after sacrificing the supraspinous ligament.5 In general they all restrict motion in extension, but do not restrict motion in flexion, axial rotation, and lateral bending (Figure 3). However, they unload the disc in extension.

Figure 2

Figure 2

Figure 3

Figure 3

Unconstrained implants for total disc arthroplasty increase the segmental lordosis more than constrained ones, and they allow larger ROM in lateral bending and axial rotation.6 The location of center of rotation and consequently, the location or precision of the placement of the implant, implant height, and preservation of the posterior longitudinal ligament, influence ROM. Nucleus implants, and tissue-engineered nucleus replacements, can restore the natural biomechanics of an L4-L5 segment after nucleotomy, but they often show problems with migration and also extrusion. Annulus-sealing devices seem to perform differently based on their designs. Some of them may fail and are associated with a high risk of reherniation; others seem to be able to withstand aggressive testing.7

Each motion preservation technology leads to hypotheses on how the different implants may behave. In order to prove their potential advantages, all new implants should be tested not only with mechanical tests that follow ASTM or ISO standards, but also with in vitro experiments on real human specimens before the implants are applied in a patient. Despite the limitations of such biomechanical in vitro tests and complementary finite-element calculations, such investigations may provide interesting results for clinical discussion. Sometimes these tests demonstrate that the implants behave as suggested. Sometime the results help to optimize the technology. Sometimes it can be shown that specific features do not lead to significant differences, and sometimes the results do not support the promised behavior. Therefore, preclinical tests under demanding loading conditions should be considered.

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References

1. Heuer F, Schmidt H, Käfer W, et al. Posterior motion preserving implants evaluated by means of intervertebral disc bulging and annular fiber strains. Clin Biomech (Bristol, Avon) 2012; 27:218–225.
2. Schmoelz W, Huber JF, Nydegger T, et al. Influence of a dynamic stabilisation system on load bearing of a bridged disc: an in vitro study of intradiscal pressure. Eur Spine J 2006; 15:1276–1285.
3. Wilke HJ, Heuer F, Schmidt H. Prospective design delineation and subsequent in vitro evaluation of a new posterior dynamic stabilization system. Spine (Phila Pa 1976) 2009; 34:255–261.
4. Wilke HJ, Schmidt H, Werner K, et al. Biomechanical evaluation of a new total posterior-element replacement system. Spine (Phila Pa 1976) 2006; 31:2790–2796.
5. Wilke HJ, Drumm J, Häussler K, et al. Biomechanical effect of different lumbar interspinous implants on flexibility and intradiscal pressure. Eur Spine J 2008; 17:1049–1056.
6. Wilke HJ, Schmidt R, Richter M, et al. The role of prosthesis design on segmental biomechanics: semi-constrained versus unconstrained prostheses and anterior versus posterior centre of rotation. Eur Spine J 2012; 21 (suppl 5):S577–S584.
7. Wilke HJ, Ressel L, Heuer F, et al. Can prevention of a reherniation be investigated? Establishment of a herniation model and experiments with an anular closure device. Spine (Phila Pa 1976) 2013; 38:E587–E593.
Keywords:

biomechanics; motion preservation; spinal implants

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