Spinal rigid instrumentations have been used to fuse and stabilize spinal segments as a surgical treatment for various spinal disorders to date. This technology provides immediate stability after surgery until the natural fusion mass develops. At present, rigid fixation is the current gold standard in surgical treatment of chronic back pain spinal disorders. However, such systems have several drawbacks such as higher mechanical stress on the adjacent segment, leading to long-term degenerative changes and hypermobility that often necessitate additional fusion surgery. Dynamic stabilization systems have been suggested to address adjacent segment degeneration, which is considered to be a fusion-associated phenomenon. Dynamic stabilization systems are designed to preserve segmental stability, to keep the treated segment mobile, and to reduce or eliminate degenerative effects on adjacent segments. This paper aimed to describe the biomechanical aspect of dynamic stabilization systems as an alternative treatment to fusion for certain patients.
Lower back pain is one of the major health problems around the world. One of the leading causes of lower back pain is considered to be degeneration of intervertebral disc. Disc herniation, spondylolisthesis, spondylosis, and spinal stenosis may follow intervertebral disc degeneration. Back pain occurs when posterior disc bulges out and impinges the nerve roots due to herniated disc. Another nerve root impingement may be seen in the condition of spinal stenosis, which is a reduction of the diameter of the spinal canal.
The treatment options of lower back pain may vary depending on the severity of the case. They include conservative treatment or surgical techniques. Conservative treatments include exercise, medications, physiotherapy, and rehabilitation. Surgical treatment is considered for the patients when the back pain limits their daily activities and when the condition does not respond to other therapies. Surgical methods include decompression with spinal fusion or nonfusion devices.
Spinal fusion supported by rigid instrumentation is widely used in the treatment of various spinal disorders. Since the procedure was first introduced by Albee and Hibbs in 1911, fusion has played an important role in the lumbar spine employed operations. The ideal result in performing fusion is to gain the necessary therapeutic goals with the minimal disruption of normal structure and function of the spinal column [
In recent years, posterior dynamic stabilization devices have been introduced as a trustworthy alternative to fusion and gained increasing popularity. The comparable advantages of these devices to fusion include retention and protection of the intervertebral disc, earlier surgical intervention, and minimally invasive techniques. Dynamic stabilization technique is aimed at preserving motion at the treated segment. It reduces the risk of accelerated degeneration at adjacent levels, which is a major concern in fusion because of the protective effects of continuing segmental motion [
Segmental biomechanics will be altered by the implantation. Therefore, it is crucial to evaluate biomechanical effects of implants on the treated and nontreated spinal segments before clinical trials. Biomechanical evaluation of posterior dynamic stabilization systems can be accomplished by
FEA plays an important role in biomechanical evaluation of implants. It is helpful to determine the structural analysis of an implant, bone, and interaction in between the two. FE analysis gives full inside of load shearing, stresses, and strain of the interested construct under loading scenarios. It provides prospective outline of needed parameters for a desired spinal implant development. These parameters cannot be determined by
(a) FE model of the lumbar spine (E-CORE, University of Toledo), (b) the lumbar spine specimen with posterior dynamic stabilization system.
The Dynesys (Dynamic Neutralization System for the Spine), known as a dynamic stabilization device, is one of the alternative solutions for the degenerative lumbar disc problems, Figure
Posterior dynamic stabilization systems. (a) Dynesys; (b) Graf system; (c) PercuDyn; (d) Cosmic; (e) AccuFlex; (f) BioFlex.
Schmoelz et al. [
Biomechanical investigations reported that some of the posterior dynamic stabilizations show similar effect on flexion, extension, and lateral bending, compared with rigid instrumentation due to dynamic implants with high stiffness [
Another alternative for the rigid spinal fusion is a soft or flexion stabilization technique introduced by Graf [
Cosmic (Ulrich GmbH & Co. KG, Ulm, Germany) is a posterior dynamic stabilization system using pedicle screws to provide nonrigid stability for the degenerative lumbar spine. The head of the pedicle screws is hinged shaped and it connects the threaded part to the screw. This composition enables the load sharing between the Cosmic and the anterior vertebral column, Figure
Wilke et al. [
The PercuDyn (Interventional Spine Inc., Irvine, CA, USA) is known as an extension-limiting posterior dynamic stabilization implant, which is mainly a bilateral facet augmentation system, Figure
The Accuflex (Globus Medical, Inc.) is a posterior dynamic stabilization system that achieves the flexibility from helical cuts on rod. It is categorized as a pedicle screw-based system including a dynamic rod and 6.5 mm pedicle screws made of titanium alloy, Figure
BioFlex, similar to Dynesys, is a posterior dynamic stabilization system using titanium pedicle screws connected by Nitinol rods with coiling consisted of 1-2 turns, Figure
The lumbar interspinous process decompression (IPD) devices are known as trustable alternatives for the treatment of various spinal disorders. The first IPD device, X-STOP (St Francis Medical Technologies, Alameda, CA, USA), was introduced in the US for the treatment of patients with neurogenic intermittent claudication due to spinal stenosis, Figure
Interspinous spacer: (a) X-STOP, (b) Coflex, (c) DIAM system, and (d) Wallis system.
Coflex device (Paradigm Spine, LCC, New York City, NY, USA), formerly Interspinous “U,” is one of the dynamic interspinous implants that was first introduced by the French orthopaedic surgeon Jacques Samani as an alternative to arthrodesis, Figure
Diam implant (Medtronic, Memphis, TN, USA) system is an interspinous spacer and it has a silicon core with a polyethylene cover [
In mid-1980s, Sénégas introduced an interspinous implant, which was a “floating system” with the purpose of avoiding the risk of loosening. The implant system consisted of a titanium spacer placed between the spinous processes of the lumbar spine. Two Dacron ligaments wrapping around the spinous processes were considered to secure the implant. Despite the favorable results, a second-generation device called Wallis (Spine Next, Bordeaux, France) was developed to improve the device functionality (Figure
Fusion is a gold standard for lower back pain treatment to date. However, there have been several complications reported clinically. These complications are related mainly to adjacent segment degeneration due to high stiffness at the stabilized segment. Alternative treatment, nonfusion stabilization systems, became more and more popular in order to preserve mobility of a motion segment and eliminate adjacent segment phenomena. Current research studies emphasize long-term clinical evaluation of dynamic stabilization system.
On the other hand, the stiffness of the dynamic implants is a big concern due to not providing appropriate motion range. Therefore, it is important to optimize the dynamic implant stiffness for desired spinal range of motion achievement.