16 Traditionally, the study of spinal biomechanics and stability has been heavily influenced by congenital, traumatic, developmental, and degenerative conditions such as scoliotic deformities, degenerative disks, and burst fractures. Although it has been recognized that spinal lesions due to metastases can cause clinical instability, basic biomechanical testing in metastatic disease states has been limited. This may be due to the inherent limitations in acquiring diseased specimens for testing, or there may be other reasons for the paucity of scientific literature in this highly specialized field. From a clinical oncological standpoint, spinal metastases are common. The prevalence of bony metastases involving the spine has been estimated as high as one in three of all cancer patients1 and one in two elderly patients diagnosed with cancer.2 Many patients who harbor metastatic disease will have systemic issues that take treatment precedence over the spinal diseases. However, because of advances in systemic therapies, patients are living longer, and their spine disease may ultimately require treatment. Spinal metastases can directly afflict one or multiple vertebral bodies and, in specific cases, compromise the structural integrity of the spine segment. Subsequent instability and neurologic complications may result. Thus, spinal metastases have the potential to become a biomechanical issue and require surgical intervention. The proliferation of metastatic disease in the bony anatomy of the spine has been shown to be indiscriminate; consequently, if structural components, including the posterior wall, pedicles, facets, and other osteoligamentous elements of a functional spinal unit (FSU; two vertebral bodies and the disk space), are damaged, the potential for instability, including burst fracture, is highly probable.3 Challenges in anticipating such events, including the inability to visualize the early onset of vertebral body involvement on plain film radiograph, have made diagnostic predictions of structural failure more difficult. Researchers have estimated that 30 to 50% of the vertebrae by volumetric measurements must be involved before a spinal tumor may be radiographically identifiable.4 Different methods of classifying spinal metastases have been proposed with the intent of rationalizing surgical indications. Currently, there is little consensus on an objective criterion in the estimation of the risk of burst fracture and neurologic compromise. It has been suggested that with the many treatment options and combinations available, including irradiation, chemotherapy, steroids, diphosphonates, and surgery, the complete management of a patient’s condition would require that both symptomatic and prophylactic issues be taken into consideration.2 Should the patient still warrant surgery, biomechanical aspects of the metastases, along with the treatment, should be considered. Clinically relevant biomechanical studies involving the spine have been available for more than 5 decades. These studies have not addressed metastatic disease per se, but they have covered several topics from anulus fibril orientation to vertebral bone strength to the effects of instrumented procedures. An early study by Virgin described various biomechanical aspects of the intervertebral disk, including the effects of compressive loading and the observation of hysteresis during loading and unloading.5 Likewise, Hirsch and Nachemson reported on the influence of spine motion and the associated disk pressures.6 The study of FSUs under controlled loading provides biomechanical insight into clinically relevant issues, including instability related to the spine. Moreover, the instrumentation and the ability to achieve FSU stability can potentially be important in the clinical decision-making process for the adjuvant treatment of spinal tumors. Instability has various definitions, and as it relates to degenerative biomechanics of the spine, a kinematic response for an FSU under a given physiologic load in excess of the normal healthy condition (e.g., excessive range of motion [ROM] in a lumbar FSU) can be used to characterize the instability of a motion segment. Clinically, multiple factors have been used to describe instability for symptomatic patients. Surgical intervention has the potential to treat certain aspects of metastatic conditions afflicting the spine, including instability. Such outcomes are of interest to both clinicians and biomechanicians. Should the clinical conditions be appropriate for a surgical intervention, a repeatable kinematic response of the FSU should be predictable and well described. Instability in the spine that harbors metastatic disease may more closely mimic osteoporotic disease, as the metastases most commonly affect the vertebral bodies, as opposed to the soft tissue structures (e.g., ligaments or disks). Biomechanical loading and displacement protocols on test frames have been developed for spinal treatment comparisons. In conjunction, precision measuring devices have been necessary to measure the relative motion between vertebral bodies. Subsequent interpretation of the data becomes more significant if there are clinical conclusions that can be derived from in vitro biomechanical testing. For example, in a cadaveric calf corpectomy model, Heller et al determined that restoration of stability is dependent upon instrumentation constructs.7 Results from other studies have since corroborated the fact that the type of instrumentation significantly alters the ROM, as well as the type of surgical procedure performed. Kanayama et al concluded that anteroposterior instrumention is more effective in cases involving total spondylectomies, whereas anterior instrumentation may not be significantly more stable than circumferential fixation in subtotal spondylectomies.8 The in vitro biomechanical description of stability in a human cadaveric FSU provides a guideline for in vivo clinical techniques. The clinical relevance of biomechanical testing is difficult to correlate perfectly in any disease state. The variables associated with testing—the test apparatus, the representative spine segment model, and the difficulty in patient assessment—cause difficulty in repeating studies in either the clinical or laboratory setting. Regardless of biologic variability, patient compliance, and artifacts of biomechanical studies, the fundamental basis of measurement parameters should be understood and taken into consideration to fully appreciate a patient’s condition and the ramifications of surgical intervention. Surgical treatment for the neoplastic spine is designed to decompress neural elements, restore neurologic function, and reduce pain by providing increased support. Because instrumentation has the potential to improve the quality of life through stabilization, surgical intervention can, to some extent, be evaluated based on in vitro biomechanical performance. The surgical techniques may be anterior or posterior, or both. Choices in implant materials include metal and polymer technologies. Polymer technologies, including polymethyl methacrylate (PMMA), fiber-reinforced composites, and poly(etheretherketone) (PEEK), provide solutions to spinal stability not achievable with traditional fixation hardware. However, metal constructs, including posterior pedicle screw fixation, vertebral body replacements, and anterior rods and plates, are still essential to the reduction of motion and the stiffness of the treated FSU. Regardless of approach, the ability to maximize tumor decompression and restore a viable spinal unit construct is the criterion used to compare techniques and devices. In a study by Oda et al, a series of different metal reconstructive devices were biomechanically compared in human cadaveric spines.9 Pedicle screws were found to be most effective when used in combination with anterior instrumentation. Additionally, effectiveness was measured in the three major modes of loading and was determined to have significant mul-tiplanar reduction in motion compared with the intact state.9 In a similar study, Shannon et al were interested in the use of a PMMA cement, in lieu of rigid metal instrumentation.10 One purported advantage is the ability to implant the material in a liquid state and to cure in situ.10 Subsequently, the implant conforms and fills the void volume and provides an excellent implant bone interface. Additionally, the anterior column support provided by the PMMA can be implanted from a posterior technique. In the study by Shannon et al, the results suggest biomechanical stability can be achieved through a polymer-based implant following a spondylectomy. Vahldiek and Panjabi compared the stability of various constructs in stabilizing a spine following a spondylectomy treatment.11
Biomechanical Assessment of Spinal Instability and Stabilization
Introduction to Biomechanics
Biomechanics of Instrumented FSU in the Treatment of Spinal Tumors
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