Biomechanical Rationale

The clinical advantage or utility of new technologies (i.e., motion preservation devices) may initially be unclear particularly in the absence of long-term, prospective, randomized, well-controlled clinical data. As the implants become commercially available, the suitability of specific patients for a particular implant may or may not be clear due to the generally recognized clinical inclusion criteria. For the subset of the population that the spine surgeon deems to consist of viable candidates for surgery, a decision tree should consider at least two factors: the severity of the disease and the surgical approach for optimal postoperative success. With the aid of a methodic, systematic approach in the identification of treatments and devices, the appropriate spinal implant technology suitable for the patient should become more evident. The information garnered from biomechanical studies may play a role in treatment of spinal pathologies but entirely depends on the patient’s symptoms, and triage requires a strategy that includes a multivariate analysis for the most effective clinical outcomes.

Both the spine surgeon and the implant design engineer benefit substantially from a clear understanding of the native function of the spine and the contribution of each of the spinal elements. The motion of the spine can be studied in the most basic form at a single level that begins with the functional spinal unit (FSU). The FSU comprises two vertebral bodies with their articulations, including the intervertebral disc, as well as the two posterior facet joints. The intervertebral disc forms an integral part of the FSU and has a high potential of becoming problematic, especially with age. Anatomically, the disc consists of fibrous layers arranged in alternating lamellar structures, in conjunction with a gelatinous inner core referred to as the nucleus pulposus. The other important articulations within the FSU are the facet joints, which are also susceptible to disease. Facets, in the normal condition, play a role in controlling the motion of the FSU. This three-joint complex within each FSU controls the kinematic response to load. The primary modes of loading taken into consideration are those associated with axial compression, flexion–extension bending, lateral bending, and axial torsion, as shown in Fig. 168-1.

FIGURE 168-1 Demonstration of biomechanical testing parameters.

Over time, as degeneration occurs, the disc becomes fibrotic and less able to dissipate and distribute loads. Consequently, nonphysiologic loads are then distributed to the vertebral endplates and the annulus of the disc, which may lead to morphologic endplate changes and annular fissuring. With the onset of the degenerative cascade, both the intervertebral disc and then facets becomes compromised. Currently, it is unclear whether the onset of anterior column degeneration leads to posterior degeneration, and vice versa. Regardless, the degeneration within the FSU may lead to the inability to withstand even physiologic loads, and eventually, depending on the severity, instability may develop. Both clinically and biomechanically, instability can be defined by the inability of the FSU to control physiologic displacement. With instability, the neurologic structures are prone to impingement and injury. Instability of the intervertebral discs shifts greater-than-normal motion and load to the facet joints and ligamentum flavum. With time, these structures all undergo hypertrophy with narrowing of the central neural canal, as well as of the lateral recesses and neural foramina.

Fusion has been the gold standard in the surgical treatment of certain spinal pathologies, including degenerative disc disease. Fixation instrumentation including screws, rods, plates, and intervertebral cages have served as adjuncts to fusion and positively contributed to radiographic outcomes (i.e., radiologic fusion assessment). However, the techniques have not necessarily improved patient clinical outcomes—hence, the advent of posterior, dynamic stabilization constructs that provide immediate stability but not necessarily rigidity that would allow transmission and propagation of nonphysiologic loads to adjacent segments.

Recent attention has been focused on preventing adjacent-level degeneration, as well as obtaining a positive functional outcome in the treatment of degenerative disc disease. To this end, numerous implants have been developed with the aim of effectively controlling physiologic loading and ensuring the appropriate kinematic motion at the index level. Posterior pedicle-based dynamic stabilization—as well as other motion preservation devices, such as total disc replacement, nucleus augmentation, total facet arthroplasties, and combinations thereof—are at various stages of evaluation and consideration, including as potential alternatives to fusion.

There has been a proliferation of pedicle screw–based designs over the past 20 years. These systems all have some degree of similarity in that they employ pedicle screws connected to a rod, which allows some form and degree of motion. While many of the early products, in theory, provided true motion preservation (i.e., maintenance of a significant proportion of the preoperative segmental motion), this has not been proved clinically. Most available systems appear to reduce preoperative motion. As such, the concept of a true pedicle-based motion preservation device seems faulty. Instead, these devices appear to limit segmental motion but provide varying degrees of “stability” to the spine (biomechanically equivalent to increased stiffness in one or more of the six generally assessed biomechanical ranges of motion). For the purposes of this chapter, devices designed for purposes other than fusion are considered “nonfusion stabilization” devices.

Another design concern that arose during testing of these types of devices was that of robustness. Essentially, these devices are cantilever in design. They transfer loads from the vertebral bodies through the pedicle screws and connecting units. Although this may “offload” the anterior and middle columns of the spine to a certain extent, the devices are theoretically required to bear this load for the life of the patient. Interestingly, traditional pedicle screw devices used to augment fusion are FDA approved as “temporary” fixation devices. They are tested and expected to perform for 2 years, after which they may be removed. Complicating the testing procedures for these devices, the number of cycles that the lumbar spine undergoes in the normal patient over the course is unknown. Estimates vary from 250,000 to 2 million cycles per year.

A robust nonfusion stabilization device may need to withstand millions of cycles without breakage to last for the life of the patient. The device would need to do this without producing harmful wear debris or the patient suffering implant failure. This is a difficult design paradigm to fulfill.

Another design parameter that a potential manufacturer needs to address is that of the amount of stiffness required in an individual patient. Many in vitro biomechanical studies have demonstrated that the degree of inherent stiffness in the spine varies among individual specimens. Generally, older cadaveric spine specimens are more rigid than younger spines. Clinically, this has traditionally been assessed by the Kocher Clamp Test, described by Albert Key in 1944 (personal communication). Essentially, the surgeon grasps the posterior spinous processes and pulls. This provides a tactile indication of the spinal stiffness, which Key used to compare to prior experiences. A more objective measure of assessment was described by Brown et al., who utilized an FDA-approved intraoperative spinal stiffness gauge that measured force displacement (Spinal Stiffness Gauge, Mekanika, Boca Raton, FL)² (Fig. 168-2). Their findings of 655 motion segments demonstrated wide variability in degrees of stiffness. Generally, the L5–S1 segment was stiffer than L4-5, which was stiffer than L3–4. Male patients had stiffer spinal segments than did female patients. Older patients had stiffer spinal segments than did younger patients. Finally, stiffness was found to decrease by 20% following decompression. These basic findings raise the issue of patient-specific requirements and the potential need to customize implants based on intraoperative findings.

FIGURE 168-2 In vitro biomechanical testing probe placed between posterior spinous processes to determine spinal stiffness.

Another design concern is the amount of stability that an individual patient may require to achieve the best clinical outcome. That is, all other variables being equal, it remains unknown as to the required stiffness in an individual situation that will lead to pain relief and enough stabilization so as to prevent recurrence of symptoms and clinical (and/or potential radiographic) instability.

Nonfusion pedicle screw–based stabilization systems have the potential clinical advantage of system revision. Should the nonfusion construct fail to provide the expected clinical improvement, these systems can be converted to fusion with the addition of bone and potentially interbody spacers to promote fusion. Another potential advantage to nonfusion systems is that they can be placed using less invasive techniques with concordantly less tissue disruption.

Most available systems provide the surgeon with the ability to a perform fusion, a nonfusion, or a combination hybrid construct. This latter approach allows the surgeon to “top off” a fusion level with a nonfusion construct, with the thought that the nonfusion device may help mitigate the stresses on the adjacent level and reduce adjacent-level facet and disc disease.

Lumbar posterior, dynamic stabilization, motion preservation devices have the potential to address neurologic deficits without arthrodesis. Although the design intent shares similar clinical goals with fusion devices (i.e., relief of patient pain symptoms), an important consideration in the development of any motion preservation device is the need for a preponderance of evidence supporting the safety and efficacy of the device design. Pedicle screw–based, posterior, dynamic motion preservation devices rely on the perpetual integrity of the construct, whereas fixation devices have the luxury of becoming obsolete once the fusion mass forms and subsequent off-loading of the spinal implant occurs.

System Designs with Biomechanical and Clinical Outcomes

Complex biomechanical and clinical needs have been addressed by designers with varying degrees of success. As previously stated, all pedicel screw–based nonfusion stabilization devices are similar in that they anchor an implant that allows for some degree of mobility to pedicle screws. Perhaps the most basic design type was the Graf ligament. This system limited movement in flexion, and had good early clinical results, but ultimately it has not received larger clinical use.³

More complex designs have included multiarticulated metal systems that allow movements in all planes. These systems may include ball-and-socket components and spring assemblies. These systems were designed to meet many of the aforementioned biomechanical goals, but the systems were quite complex in their mechanical design. They also have not been widely applied in clinical settings.

The systems that are intermediate or simpler in their design complexity include the N-Hance spine system (Synthes, West Chester, PA), Scient’x Isobar TTL system (Alphatech Spine, Carlsbad, CA), Dynesys Spinal System (Zimmer Spine, Warsaw, IN), CD Horizon Legacy polyethylethylketone (PEEK) rods attached to titanium screws (Medtronics, Minneapolis, MN), and the Transition system (Globus, Audubon, PA). The Dynesys and Transition systems are similar in their design and are discussed later.

N-Hance Spine System

The N-Hance spine system is a unique system in that it comprises a solid titanium rod with a sliding component. The sliding component, which attaches to a pedicle screw, has polyurethane bumpers immediately above and below it. This system damps flexion and extension (biomechanically represented by an increase and decrease of the interpedicular distance) against the polyurethane stops (Fig. 168-3). The length of the fixed component of the rod can accommodate span multiple segments so that the system can be utilized for both rigid and nonrigid fixation. This combination of rigid stabilization (acting as a traditional pedicle screw fusion augmentation construct) and nonrigid stabilization (potentially utilized as a nonfusion spinal augmentation construct) is commonly described as a “hybrid” construct.⁴

FIGURE 168-3 A, N-Hance spinal system with a solid rod (inferior) and a nonfusion component (superior). B, Demonstration of the N-Hance spine nonfusion system motion component.

The N-Hance system has been studied biomechanically. The salient feature of the system is that it is less rigid than the Dynesys system and allows for changes, per its design, in interpedicular distances. The system has been tested to 10 million cycles in vitro. Although this system has been implanted in a number of patients throughout the world, larger outcome studies are not available.