Dorsal Semirigid Stabilization

Adjacent Segment Pathology

Rigid metal implants augment spinal fusion, imparting a supraphysiological degree of stiffness to a construct that exceeds normal adjacent motion segment stress tolerance or the normal stiffness of a noninstrumented fusion. The stiffness of the instrumented levels directly affects the stress load distribution at the adjacent disc and facet joints. ^, Over time, this additional stress can result in segment dysfunction and degeneration, aggregately labeled ASP. ^,

ASP describes progressive degenerative changes of the functional spinal unit that include hypermobility, sagittal and coronal malalignment, disc and facet spondylosis, facet hypertrophy, disc herniation, and stenosis, in response to lumbar fusion. ASP may be subcategorized as radiographic ASP (RASP) and/or clinical ASP (CASP).

The biomechanics of the degenerative effects with ASP have been studied in both cadaver and animal models. Rigid fusion of a spinal segment increases the stress on the annulus and end plates of adjacent levels, increasing intradiscal pressures and producing higher mobility. In flexion and extension, the loss of mobility across a rigidly fused segment is compensated predominantly at the first rostral adjacent segment. Additionally, fixed sagittal alignment after rigid segmental fusion prevents accommodation of regional alignment changes that normally occur with different body postures. If a segment is fused with suboptimal sagittal alignment, degeneration of adjacent levels may be accelerated further.

Although reported rates of RASP vary widely in the literature, the incidence of CASP appears higher in patients who have undergone an instrumented fusion (12.2%–18.5%) than in patients who have undergone a noninstrumented fusion (5.2%–5.6%). ^{, ,} The supraphysiological biomechanical stress created by rigid fixation seems to be the most influential factor, ^, and many patients subsequently need further surgery with an extension of their fusion.

Park et al. reviewed 22 studies of variable quality reporting highly variable rates and risk factors for RASP and CASP development after lumbar fusion. The rate of RASP ranged between 8% and 100%, and that of CASP ranged between 5.2% and 18.5%. Risk factors included concurrent posterior lumbar interbody fusion (PLIF), adjacent segment facet injury, increased fusion length, sagittal malalignment, preexisting adjacent level disc disease, lumbar stenosis, increased age, osteoporosis, female sex, and postmenopausal state. No effect estimates were obtained, however, making these risk correlations of limited predictive value.

Radcliff et al. reviewed the pertinent literature on ASD and noted an approximate reported rate of 2% to 3% per year following spinal fusion. Adjacent segment laminectomy and sagittal imbalance were the factors most consistently reported to be associated with developing ASD.

Lawrence et al. performed a systematic review of risk factors for CASP in adults following lumbar fusion, and found only five reports that suitably reported CASP over a 5- to 10-year period. The consensus regarding the overall annual incidence of CASP following lumbar fusion was 0.6% to 3.9%. Risk factors for CASP included age greater than 60 years, concurrent adjacent facet degeneration, multilevel fusions, fusion adjacent to L5-S1, and excessive disc space distraction with instrumentation.

Recently Burch at al. performed a metaanalysis to establish rates of reoperation for CASP following instrumented lumbar and thoracic fusion. Fifty-five studies qualified for review and showed a reoperation rate of 8% over 6.4 years (confidence interval 6.5–9.8). Gender, body mass index, proximity to L5‒S1, pelvic incidence, lumbar lordosis, sagittal slope, pelvic tilt, preexisting adjacent degenerative pathology, facet orientation, and initial pathology did not have valid predictive value. The only significant risk factor identified for reoperation was multilevel fusion length.

ASP continues to be an important issue following lumbar fusion using rigid instrumentation techniques and has been a major stimulus for developing semirigid alternatives to fusion. Further longitudinal studies will be needed to clarify important risk categories for developing CASP and needing subsequent revision procedures.

Semirigid System Rationale

As stated, avoiding ASP is one of the principle goals driving semirigid systems development. Reducing the stiffness of a spinal segment will in theory reduce the stress concentration at the adjacent levels. Stress compensation in most semirigid systems instrumentation strategies is usually distributed across the first and second rostral segments and the caudal adjacent segment. Patients with osteoporosis or osteopenia may benefit from semirigid systems, as they are especially prone to construct failure with rigid fixation. If the bone–metal interface exceeds the mechanical tolerance of osteoporotic bone, failure can occur, leading to microfracture and loosening. Less-rigid fixation may be ideal in these patients and reduce the incidence of construct failure. ^,

Semirigid systems are based on principles of flexibility and dynamism, which are fundamental biomechanical characteristics of motion-sparing or motion-altering technologies. They allow controlled yet limited deformation of the spine. An ideal semirigid implant prevents undesirable deformation but allows desirable deformation.

The biomechanics of clinical semirigid constructs vary among device designs. Typically, pain-generating tissue is unaltered surgically but is mechanically supported by the device, which restricts certain types of pathological motion and modulates load transfer. The design of posterior semirigid devices must account for the normal kinematics of a functional spinal unit. The posterior and anterior elements of a healthy motion segment typically complement each other under normal loading. Significant disruption of one, however, can cause excessive loads to be shifted to the other. Semirigid implants should maintain the range of motion of a healthy spinal segment. Excessive motion may accelerate degeneration, whereas inadequate motion defeats the purpose of using a semirigid device.

Some applications of semirigid stabilization capitalize on the phenomenon of Wolff’s law to promote bone healing. As a biologic tissue, bone remodels and strengthens along stress lines under load to resist those loads. The opposite also holds, in that decreasing the load on bone results in resorption and osteopenia. When applied to spinal fusion, strategies that transmit force to an intervertebral graft promote both the rate and success of arthrodesis. If placed in distraction, rigid posterior instrumentation systems may unload an intervertebral graft, so-called stress shielding, resulting in nonunion. A dynamic device that loads the graft when used as a posterior tension band supplement may increase the likelihood of fusion.

Semirigid devices can be flexible, in which case they are used in conjunction with standard fusion techniques (posterolateral or interbody) to reduce standard construct stiffness and promote fusion via micromotion or graft load-sharing. Alternatively, they may be dynamic and function as nonfusion motion restoration devices that replace the function of the spinal motion segment in part or entirely. Three general groups of implants have been developed: pedicle screw/rod-based systems, interspinous spacers, and total facet replacement systems. ^,

Pedicle screw/rod systems and interspinous spacers may be further subdivided by mechanism of action into neutral semirigid systems, compressive semirigid systems, and distractive semirigid systems.

Neutral Semirigid Systems

Neutral semirigid instrumentations systems are considered “neutral” because they do not preload or transmit force to the spinal segment. They typically limit pathological translation while allowing rotational micromotion owing to the flexibility of the fixator. An early crude example is the Luque plate that used a screw head that pivoted in a concave bed within the plate, allowing a “rocking” motion.

Modern examples of neutral systems typically use mobile screws or semirigid rods. These devices are designed to maintain normal motion within a physiological range, unload degenerated discs and facet joints, and stabilize the abnormal segment. The resultant decrease in load potentially alleviates pain from degenerating joints. Ideally these devices also prevent ASP.

A neutral system may be used to augment standard fusions (posterolateral or interbody), or as a nonfusion device. When used with interbody devices or grafts they optimize load-sharing across the implant and allow micromovement through the end plates. Nonfusion or stand-alone constructs restore motion without fusion and have been used as hybrids to “top off” a standard fusion. Another indication is stabilization after iatrogenic destabilizing procedures, such as wide laminectomies and facetectomies, to avoid fusion.

A variety of implants have been produced by numerous manufacturers that were initially intended to be less-rigid or “soft” fixators used to augment fusion. Frequent long-term failures, however, have led to many devices being removed from the market. Semirigid systems were reclassified as class II devices by the U.S. Food and Drug Administration in 2014 and are gaining academic and clinical interest in both the United States and other parts of the world.

Dynamic Screw Systems

The Cosmic system (Ulrich GmbH & Co. KG, Ulm, Germany), described by Strempel in 1999, was the first example of a dynamic pedicle screw. It is hinged between the head and threaded body of the screw ( Fig. 124.1 ), and the screws are interconnected by a rigid rod. This allows sagittal movement between the screw head and the screw body during flexion-extension. This system shares axial loads with interbody implants when applied across spinal segments, reducing stress shielding. Cosmic and Safinaz dynamic screw (Medikon, Turkey) are two of the most widely used hinged transpedicular screw–rigid rod devices. The Safinaz hinged screw, like the Cosmic screw, allows both limited motion in flexion and extension and 1 degree of rotation.

Bozkus et al. studied the in vitro behavior of the Cosmic screw system and reported similar stability between dynamic and rigid fixation systems, but reduced stress shielding with dynamic screws. Kaner et al. showed greater preservation of disc space height with the Cosmic system than the fusion control in patients with spondylolisthesis undergoing nonfusion stabilization, with similar clinical outcomes. Clinical results using the Safinaz system were reported by Ozer et al., who noted equivalent pain relief and sagittal balance preservation between semirigidly and rigidly fixed patients. Yilmaz et al. reviewed magnetic resonance imaging (MRI) in 59 patients treated with Cosmic or Safinaz systems for painful disc disease, comparing preoperative and 1-year Pfirrmann grades. Significant improvement was noted in visual analog scale (VAS) and Oswestry Disability Index (ODI) scores at 6 and 12 months. Pfirrmann grades improved in 34%, remained unchanged in 53%, and worsened in only 13% of patients. They concluded that motion preservation and load balancing using dynamic screws may mitigate disc degeneration and facilitate regeneration.

Although the permissive motion of hinged screws may minimize the chance of failure at the screw–rod interface, cyclic loading at the screw–bone interface can impact bony purchase through microfracture, loosening, and possible pullout under flexion or axial loads. The highest stress with this type of fixation is at the screw–bone interface, in contrast to rigid techniques where stress is concentrated at the screw–plate or screw–rod interface. Dynamic neutral stabilization in patients with osteoporosis should always be coupled with sufficient axial load resistance with interbody support to be stable in flexion.

Semirigid Rod Systems

Semirigid rods have been tested and used to reduce the stiffness of pedicle screw constructs. Rods can be made more flexible in several ways, including using elastic materials such as polyetheretherketone (PEEK) or silicone, machining rigid materials into springs, or combining a rigid rod with elastic, plastic segments to increase flexibility. A brief review of the principles and challenges of some of these devices follows.

Dorsal rods made of PEEK polymer (e.g., CD Horizon PEEK Optima LT-1 rod, Medtronic, Memphis, TN) were introduced and FDA-approved in 2005 as a semirigid alternative to titanium rods for fusion ( Fig. 124.2 ). PEEK has a modulus of elasticity between cortical and cancellous bone and allows flexibility but resists extreme degrees of flexion, extension, axial loading, or lateral rotation. PEEK rods reduce stress concentration and hypermobility at adjacent levels as compared with rigid titanium constructs. They also reduce stress at the bone–screw interface, reducing the risk of construct failure. Radiolucency also makes them attractive alternatives to standard titanium rods. Several manufacturers now offer PEEK rods for dorsal fixation when performing a fusion.

A PEEK-silicone fusion-nonfusion hybrid rod (CD Horizon BalanC, Medtronic, Memphis, TN) is available in Europe that combines a PEEK semirigid portion for fusion with a flexible, nonfusion hinged portion. The design, which combines flexible and dynamic elements, mitigates stress concentration at adjacent levels.

Gornet et al. studied the biomechanics of PEEK rod systems and noted that they provide stability comparable to titanium lumbar fusion constructs. Ormond et al. reviewed 42 patients with degenerative lumbar disc disease undergoing posterior lumbar fusion using PEEK rods and found that 19% requiring reoperation: 12% for ASP and 7% for nonunion. The authors felt this was consistent with expected rates of fusion and reoperation for rigid fusion techniques. There were no rod failures. A recent systematic review of PEEK rods versus rigid titanium rods identified five studies showing a 95.6% fusion rate, equivalent clinical outcomes, no rod breakage, and only a 2% rate of screw failure. The authors concluded that the PEEK rods and titanium rods had equivalent safety and effectiveness, but could not comment on ASP effects because of the limited quality of the studies cited.

AccuFlex (Globus Medical, Audubon, PA) is a variant of a standard titanium rod with helical cuts that creates spring-like elasticity. This rod has limited flexibility in flexion and extension but creates a posterior tension band that unloads the intervertebral disc. Accuflex was approved by the FDA in 2005 for single-level use with an interbody fusion graft.

Clinical outcomes appeared to be comparable to rigid fixation, with a trend toward achieving a more rapid fusion. As a nonfusion device, Accuflex showed clinical benefit and prevented or delayed degenerative changes in 83% of patients, but had a 22% rate of rod breakage. There is also concern that fibrous tissue and/or bone ingrowth may alter the mechanical properties of the rod. Accuflex is no longer clinically available.

The Bioflex System (Bio-Spine, Inc, Seoul, South Korea) is a nitinol spring-rod system that illustrates another approach to dynamism. Nitinol is a nickel/titanium alloy memory metal that retains its original shape after deformation. Although it was originally FDA-approved for fusion augmentation in 2008, Bioflex loses range of motion (ROM) at 1 year when applied as a nonfusion device, and is no longer available.

The Stabilimax NZ Dynamic Spine Stabilization System (Rachiotek LLC, Wellesley, MA) is a device that was developed to reproduce the normal kinematics of the functional spinal unit. It is a posterior pedicle screw–based system that features dual concentric springs combined with a ball and socket joint, all to enhance spinal stability around neutral posture. The inner spring limits spinal extension, the outer limits flexion, and a central cable checks hyperflexion. Stabilimax was developed as a nonfusion total facet replacement system, but is more similar to other dynamic spring-based pedicle screw–rod systems. A biomechanical study showed excellent reproduction of functional spinal unit kinematics, fatigue strength, wear resistance, and biocompatibility. The device restores a near-normal neutral zone in destabilized cadaver spines while maintaining substantial ROM. An FDA-approved investigational device exemption (IDE) study began in 2007 but was interrupted because of screw breakage in three patients. The study was stopped, and company assets (Applied Spine Technologies, Inc.) were acquired by Rachiotek LLC for development and distribution in the European Union and Asia. Stabilimax is not available in the United States.

Additional flexible rod designs use composites of elastic sections of polycarbonate interposed between rigid titanium segments to produce a pliable rod. CD Horizon Agile is an example introduced by Medtronic (Medtronic, Inc., Memphis, TN) and approved by the FDA in 2006 as a fusion adjunct. It used a rod design with a polycarbonate flexible bumper surrounding a titanium cable situated between two sections of standard titanium rod. It was intended to improve on the Dynesys device design in flexion and extension; however, high failure rates resulted in its withdrawal from the U.S. market in 2007.

The Axient dynamic stabilization system (Innovative Spinal Technologies, Mansfield, MA) was composed of rigid pedicle screws and cobalt-chromium sliding rods with a polycarbonate-urethane segment that permits segmental movement but limits excessive flexion, extension, and axial rotation. The company was dismantled in 2009, and the Axient system is no longer available.

The NFlex Stabilization System (Depuy Synthes Spine Raynham, MA) is a dynamic rod like Agile and Axient, with an interposed segment of polycarbonate-urethane in the titanium rod. It is designed to create a transition zone between a rigid fusion and adjacent mobile segments to mitigate ASP. In this mode it represents a hybrid technology bridging a rigid fusion and an adjacent, flexible transition zone. It is not available in the United States as a dynamic, nonfusion device, but does carry the CE mark for nonfusion technology in Europe.

Coe et al. reviewed 72 patients with degenerative lumbar disease treated with stand-alone NFlex and reported significantly improved VAS and ODI scores compared with rigid controls. They observed only a 4% rate of minor implant-related complications, and concluded that NFlex may be a viable alternative to rigid fusion methods.

The Isobar TLL Dynamic Rod (Scient’x/Alphatec) is a titanium alloy rod with a midshaft metal motion system conferring flexibility that was FDA approved in 1999 for lumbar fusion. This design allows limited flexibility at the rostral end of a construct, reducing the risk of ASP.

Zhang et al. studied 38 patients treated for lumbar spine degeneration between 2007 and 2011 and showed that Isobar TLL had reliable fixation, no loosening, no breakage, and did not lead to ASP. The authors suggested that Isobar TLL had good short-term effectiveness in treating lumbar degenerative disease when combined with fusion. A biomechanical study using cadaveric human lumbar spines reported that the Isobar TLL device stabilized and maintained disc space height. Ji et al. showed similar clinical and radiographic disc space improvement at adjacent levels using the Isobar TLL versus rigid fusion over two segments.

Compressive Semirigid Systems

Compressive semirigid systems restore a disrupted tension band that occurs with posterior decompression surgery. The effect locks the facet joint in compression, optimizing three-planar stability and limiting supraphysiological motion. Excessive intersegmental movement may inhibit successful bony fusion by physically disrupting the growing, nascent, healing bone. A semirigid dorsal tension band that compresses a ventral interbody fusion graft enhances bone healing by encouraging bone growth along ventral stress lines. When applying a semirigid compression device for axial loading, solid ventral interbody and intact spinal elements that limit excessive flexion are required.