9 Non-Invasive Strength Analysis of the Spine Using Clinical CT Scans

Introduction

Osteoporosis is widely recognized as an underdiagnosed and undertreated disease. According to the National Osteoporosis Foundation and the National Institutes of Health, 10 million Americans are estimated to have osteoporosis, and another 34 million are at increased risk due to low bone mass, but only about 20% of those eligible to be screened are actually tested and only a fraction of those are positively diagnosed and treated. Above age 50, the density of vertebral trabecular bone decreases at a rate of about 2.2% to 3.0% per year for women, depending on age, and by about 1.7% to 2.5% per year for men,¹ with about 700,000 osteoporotic spine fractures occurring annually in the united States².

Management of osteoporosis in the over-50 age group is important both to avoid such fractures and to optimize spine surgery outcomes. A recent study from Taiwan ³ estimated that for all major spine surgical cases, not including vertebroplasty or kyphoplasty, 47% of women and 46% of men over age 50 had low bone mass or “osteopenia” — a BMD T-score of between −1.0 and −2.5 — and 44% of women and 12% of men had osteoporosis — a BMD T-score of less than −2.5. As the size of the aging population continues to increase, a huge and growing proportion of spine surgery patients may have compromised bone strength. This presents a challenge to the spine surgeon using any sort of instrumentation or implant for stabilization, since the underlying bone and the bone-implant interface need to be strong enough to sustain the stresses both from daily activities and spurious overloads.

From a patient-management perspective, it would be desirable clinically to be able to identify more patients at high risk of vertebral fracture. These patients can then be placed on an appropriate therapeutic treatment, which typically reduces fracture risk by about 50%. For spine surgery, surgical planning and postoperative patient management might be improved by identifying patients with compromised bone strength. Improved information on vertebral strength on a patient-specific basis might provide an objective basis for evaluation of actual surgical options, including type and size of implant. In addition to the condition being treated surgically, many spine surgery patients have compromised vertebral strength, which, if recognized, could be treated postoperatively with appropriate therapeutic agents.

A number of different types of imaging modalities are now available for noninvasive assessment of bone density, structure, and strength.⁴ The dual-energy x-ray (DXA) scan is the current clinical standard for bone density assessment. However, DXA for the spine has a number of limitations. Being a 2D imaging modality, a DXA scan combines all bone morphology in the anterior-posterior direction. Thus, arthritic changes in the posterior elements, degenerative osteophytic growths around the endplates, and aortic calcification all produce bone mineral density increases in the DXA scan — increases that confound the measurement of bone mineral density in the load-bearing vertebral body. DXA scans also provide very limited information on the morphology, density, or strength of the pedicles. As a result of these limitations, DXA of the spine is less predictive of the risk of osteoporotic fractures than is DXA of the hip, DXA of the spine can be highly misleading in terms of measuring actual bone mineral density of the vertebrae or pedicles, and there remains a need for improved strength and fracture risk assessment of the spine.

Computed tomography (CT), being a 3D imaging modality, provides a powerful alternative to DXA and is preferable to magnetic resonance imaging (MRI) for bone strength assessment since it provides quantitative information on bone mineral density.⁴ One limitation with CT analysis is the difficulty of interpreting the large amount of information in the scan in terms of a clinically relevant outcome such as bone strength. This is because a low value of bone mineral density at a particular location within the bone does not necessarily indicate a problem with overall bone strength. Conversely, such a local decrease in density may not show up in an averaged measure of bone mineral density, but may be problematic if that local decrease in density occurs in such a location as to appreciably compromise strength. To overcome this limitation, a sophisticated engineering structural computational analysis technique known as “finite element analysis” can be applied to CT scans to provide an estimate of vertebral strength,⁵ in much the same way as engineers perform computational strength analysis of such complex 3D structures as bridges, aircraft components, and engine parts (Figure 9-1). The resulting “biomechanical computed tomography” (BCT) technology, which represents a post hoc analysis of a clinical CT exam, is now being used in a variety of clinical research studies that address vertebral strength, aging, osteoporosis and its various therapeutic treatments. Because BCT creates a mechanical model of the patient’s bone, it can also be adapted to include a virtual implant and in that way provide estimates of strength and stability of various bone-implant constructs — all from analysis of a patient’s preoperative CT scan.

FIGURE 9-1 Details of BCT models for two women, showing sectioned view of the finite element model and two cross-sections for each. The colors indicate different values of material strength assigned to the individual finite elements within each model, which are obtained from quantitative analysis of the calibrated gray scale information in the patient’s CT scan.

(Reproduced from Melton LJ, Riggs BL, Keaveny TM, Achenbach SJ, Hoffmann PF, Camp JJ, Rouleau PA, Bouxsein ML, Amin S, Atkinson EJ, Robb RA, Khosla S: Structural determinants of vertebral fracture risk, J Bone Miner Res 22:1885-1892, 2007, Fig 1.)

Clinical Case

The following analysis of proximal junction kyphosis is a hypothetical case to illustrate how strength estimates from BCT analysis could eventually be used clinically to provide spine surgeons with quantitative information as part of the decision-making process in preoperative surgical planning. This case also illustrates how BCT can currently be used for diagnosis of vertebral osteoporosis using clinical CT scans.

A 68-year-old woman presented with an overtly unstable spine involving circumferential disruption of the spinal column around the level of the thoracolumbar junction, including insults to both the vertebral body and posterior elements. Based on a physical exam and review of x-rays and CT and MRI scans of T10 through L2, the surgeon decided to decompress and fuse the T12-L1 disc and provide support by rigid pedicle screw fixation. Because of the patient’s age, the surgeon was unsure about the possibility of osteoporosis. A review of this patient’s medical record revealed that she had a DXA exam of both the hip and spine two years previously, which showed a T-score at the hip (femoral neck) of −2.2 and of the (total) spine of −1.8. Thus, this patient just missed being diagnosed as having osteoporosis as defined by WHO guidelines (any T-score of less than −2.5), but it was unclear as to the status of her osteoporosis classification at the time of surgery, particularly for her spine which had appeared to have a more normal T-score than the hip. To address these issues, the surgeon ordered a BCT analysis to be performed on the preoperative CT exam, focusing on the undamaged levels in order to assess risk of vertebral fracture for the postoperative situation.

The BCT analysis was used to estimate the vertebral strength for T10 and L2 in order to better assess the osteoporotic status of the vertebrae (Table 9-1). Analysis of the scans showed substantial posterior arthritic changes and that the bone strength was three standard deviations lower than the mean value for a young reference population. The volumetric density scores of the trabecular bone based on the CT data indicated low trabecular bone density — almost in the osteoporosis range — but they did not reflect that this patient had low cortical density and relatively small bones, both of which also contributed to her very low bone strength. The DXA spinal T-scores were therefore misleading because of the substantial posterior calcification, arthritic changes, low cortical density, and small bone size. Calculations of the strength-capacity — which take into consideration the expected magnitude of the in vivo forces acting on the patient’s spine (see later in the chapter for more details) — were in the 60% range, indicating that the strength of this patient’s vertebra was only about 60% of what it should be in order to safely lift a 10-kg object with back bent (a “worst case” strenuous loading condition). Based on these findings, the surgeon instrumented from T12-L1, advised the patient of her elevated risk of vertebral fracture, and referred her for an endocrine consultation.

TABLE 9-1 Output Data from the BCT Analysis for Levels T10 to L2

Basic Science

Aging of the Spine

Substantial changes occur to vertebrae with aging. Cadaver studies have shown that whole vertebral strength decreases by about 12% per decade from ages 25 to 85 (Figure 9-2). Although these changes are due primarily to a loss of bone density, which is offset in part by subtle increases in bone size, the loss of cortical bone is generally not as pronounced as the loss of the trabecular bone.¹ DXA generally is unable to distinguish between cortical and trabecular bone in the spine, due to its projectional nature. Aging of the spine is also accompanied by osteoarthritic changes (formation of osteophytes, etc.) around the disc and endplates. Again, due to projectional limitations, such degenerative changes are manifested as increases in BMD on DXA exams — effectively adding noise to the BMD signal from the more biomechanically relevant vertebral body portion of the spine. There is also substantial heterogeneity in trabecular strength across the population at any age (Figure 9-2). Thus, although advanced age is associated with low bone strength, age, sex, and DXA information are inadequate for clinical assessment of vertebral strength for an individual patient.

FIGURE 9-2 A, Cadaveric biomechanical testing values of L2 vertebral strength (expressed in N), for women and men, plotted versus age. (Adapted from Mosekilde L, Mosekilde L: Sex differences in age-related changes in vertebral body size, density and biomechanical competence in normal individuals, Bone 11:67-73, 1990.)

B, Ultimate compressive stress of human vertebral trabecular bone cores (expressed in MPa), versus age, obtained by biomechanical testing of cadaveric material. Despite the clear trend for decreasing strength with advancing age, age is not a very sensitive indicator of bone strength for any given individual. For example, subject A, although older than subject B, has trabecular strength more typical of a 37-year-old, whereas subject B’s trabecular strength is closer to that of a typical 75-year-old.

(Adapted from Mosekilde L, Mosekilde L; Normal vertebral body size and compressive strength: relations to age and to vertebral and iliac trabecular bone compressive strength, Bone 7:207-212, 1986.)

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