Introduction

The microstructural effects of aging on the spine may have dramatic consequences on both the individual vertebrae and the vertebra as a constituent within an osteoligamentous structure, that is, a functional spinal unit (FSU). Additionally, the cervical, thoracic, and lumbar regions of the spinal column may be adversely affected by the deleterious effects of senescence. The consequences may cover a spectrum of physical quality-of-life factors ranging from the relatively benign to those that dramatically alter the health of a patient. When clinicians are faced with deteriorating conditions severe enough to warrant surgical intervention, additional considerations must be made for the properties of senescent spines. Therefore, the biomechanical capabilities of the spine should be examined with careful consideration for age along with this caveat: biomechanical changes do not necessarily become symptomatic.

Biomechanical measurements can be affected by numerous indicators, and it is important to distinguish which are related to global measures, for example, body mass index, and which may be relevant specifically to the local spinal elements, e.g., friability of a vertebral body. Two distinct but related indicators should be evaluated with spinal pathologies: the advancement of age and degenerative changes resulting in anatomical transmutation that potentially leads to abnormal loading of the spine. Anatomical changes may be attributed to the primary degenerative conditions associated with age. Miller et al reported an approximate 10% occurrence of severely degenerated intervertebral discs in 50-year-old males, with an increase to 60% in 70-year-olds.¹ The degenerative conditions result in several anatomical changes and, of particular importance to an aging population, is the potential for constriction of the spinal canal diameter. The cause of the constriction may be from a single specific etiology or from a combination of factors, including spinal canal stenosis, disc herniation, osteophyte growth into the canal, hypertrophy of the ligamentum flavum, and calcification of the posterior longitudinal ligament and the ligamentum flavum.

A combination of interrelated mechanobiological conditions and associated kinematic response of the spine due to degenerative diseases is also known to occur with age. Changes in proteoglycan concentration within the intervertebral disc along with matrix disorganization result in a cascade of events over time that affect the anatomical structures within an FSU. The range of motion (RoM) and the ability to absorb and transmit load in the spine are biomechanical capabilities that may be compromised by microstructural changes within the anterior and posterior columns. Under the worst conditions, the degenerative pathology within a FSU results in a significantly different kinematic response to physiologic motion, and abnormal loading may occur.

Aging and Degenerative Changes on the Effects of Biomechanical Range of Motion

The relationship between age, degeneration, and RoM has been studied both in human cadaveric FSU testing and in clinical studies. The instability of the lumbar spine was proposed by Kirkaldy-Willis and Farfan to be categorized into three diskrete stages of degenerative change. In order of progression, the clinical assessment of the lumbar spine categorized pathologic changes as temporary dysfunction, the unstable phase, and finally, stabilization.² Well-defined, controlled, biomechanical testing and clinical studies involving well-documented patient profiles have tested various aspects of this initial hypothesis on spinal instability.

Traditional methods of comparing the effects of age, degeneration, or subsequent treatments have been subjected to biomechanical characterization through the flexibility test method. The methodology of flexibility testing has been well described in the literature, originating with Panjabi’s early description of load input utilizing pure moments.³ Subsequent comparisons, particularly relevant in fixation instrumentation via flexibility testing, have described the performance of these devices relative to the intact spine. often with high mean age donor specimen. Additionally, comparisons between fixation treatments, as well as comparison of fixation treatments from laboratory to laboratory, have been possible. The standardization of the pure moment test protocol by Goel et al has contributed to the repeatability despite biologic variability inherent in cadaveric testing.⁴

It is important to understand the rationale of the test methodology when considering clinically relevant biomechanical studies. The basis of the traditional flexibility test, or pure moment testing, is to apply a uniform moment across all FSUs in a given specimen. Figure 8-1 is an example of a mounted lumbar specimen that will be subjected to flexion-extension bending. The ability to extrapolate the biomechanical effects to clinical outcomes is dependent on study design and successful interpretation of the resulting data. Clinically relevant biomechanical testing in the appropriate form is an important parameter for clinicians to consider in the triage of patients with spinal pathologies.

FIGURE 8-1 Biomechanical test setup subjected to flexibility protocol, with a lumbar specimen mounted in flexion-extension test.

In a cadaveric human lumbar study by Mimura et al, the authors were able to demonstrate a statistically significant difference between RoM in lateral bending, but not in flexion-extension bending, for intervertebral discs with degenerative ratings in whole lumbar specimens under a flexibility protocol.⁵ Biomechanical studies involving age as a variable in the analysis are often shown to be correlated to RoM. Board et al reported on the results of a human cadaveric cervical biomechanical study. Their results suggest that biomechanical flexion-extension in pure moment loading decreases the RoM as a function of the age of the specimen.⁶ These findings agreed with published articles, when extrapolated and compared to equivalent test parameters. In a similar clinical evaluation on bending in the cervical spine involving only males, Sforza et al concluded that young adult males exhibited statistically significant larger flexion-extension RoM compared to their middle-aged counterparts who participated in the study.⁷ Similarly, in a clinical cervical study involving multiple factors including both age and degeneration, Simpson et al determined age to be the most significant factor on RoM.⁸

Confounding these results are clinical considerations in which surgical treatment may be warranted, but subsequent conditions and outcomes related to the specific implant or procedure for the elderly patient may not be clear. For example, symptomatic spine pathology resulting in instability of a FSU and suitable for an instrumented fusion procedure must consider the interaction of the hardware and the patient’s local host tissue. In addition to global metrics of bone quality, the local bone purchase dependent upon the microstructural integrity of bony trabeculation at the index FSU may have undergone severe anatomical changes. These differences affect the load response, exacerbate degenerative pathologies, and require additional considerations for the type of instrumentation suitable for the patient preoperatively. Intraoperatively, additional factors may further alter the structural integrity of the FSU, for example, endplate preparation or pilot hole drilling combined with tapping.

The biomechanical changes inherent to aging are complex in nature. Many steps have been taken toward the understanding the fundamental process of maintaining a healthy spine, including bone healing, the role of the intervertebral disc, and the significance of endplate changes. However, understanding the nature of biomechanical measurement and the clinical relevance of each metric may help further elucidate the suitability of the treatment for the senescent patient and, ultimately, improved treatment options may be developed.

Assessing Anatomical Changes

Accurate measurements of bone strength are essential to the clinical management of a diseased spine. Both the diagnosis of disease, such as osteoporosis, and also its triage, such as the surgical treatment of an unstable spinal motion segment with hardware, would benefit from explicit descriptions of vertebral bone quality. Dual-energy x-ray absorptiometry(DXA)–obtained measures of bone mineral density are widely regarded across many medical diskiplines as the gold standard for assessing fracture risk. The guidelines set by the World Health Organization based on the standard deviation units of bone mineral density (BMD), referred to as T-scores, have limitations that are documented in the literature. Also, BMD has not consistently supported correlations with patient fracture in all risk groups, and additional indicators to further enhance DXA scores would be particularly beneficial to lower-risk patients with higher T-scores.