A Comparative Review of Lumbar Bone Density Measurement Techniques and Additional Applications

The evaluation of vertebral bone density prior to spinal instrumentation is a critical component of the preoperative assessment. Instrumentation of osteoporotic bone can result in complications such as pedicle screw pullout, device subsidence, or device migration.[1-5] A growing area of research focuses on screening lumbar bone prior to spinal surgery and may yield great benefits to patients in reducing instrumentation complications.

Dual-energy x-ray absorptiometry (DEXA) has been the gold standard for assessing lumbar bone density and classifies bone as either normal, osteopenic, or osteoporotic.[6,7] In light of potential shortcomings in DEXA’s lumbar composite values, alternative imaging for bone density measurements have been investigated, such as computed tomography (CT) Hounsfield units (HU), quantitative computed tomography (QCT), and magnetic resonance imaging (MRI) vertebral bone quality (VBQ).[8,9]

In a previous issue of Vertebral Columns,[10] we explained how to acquire and interpret these lumbar bone density measurements. In the present article, we discuss the literature comparing lumbar bone density measurement techniques and present additional applications of these measurements for spinal surgery. A considerable number of studies have provided evidence that indicate promising alternatives to DEXA for measuring lumbar bone quality.

CT HU and QCT Lumbar Measurements

Multiple studies have tested CT HU as a measure of bone density in comparison to DEXA bone mineral density (BMD) values. Viswanathan et al reported a significant correlation between CT HU and DEXA T-scores at the upper lumbar levels (L1-L3) in patients who underwent 1-2 level lumbar fusions.[11]

The opportunistic availability of CT measurements may provide reasonable identification of osteoporosis, but not all studies agree. Kohan et al found a low to moderate correlation between CT HU measurements and femoral neck DEXA BMD values in patients with adult spinal deformities.[12,13] In addition, the cutoffs for defining osteoporotic versus osteopenic or normal bone are still debated, with normal bone around 160 and 179 HU and osteoporotic/ osteopenic bone closer to 100 and 110 HU.[14-16]

Other studies have researched correlations between CT HU measurements and incidences of postoperative complications. Murata et al experimented with CT measurements and the incidence of vertebral fractures in an elderly population. They found that CT HU measurements of the anterior one-third of the vertebral body can predict the occurrence of postoperative vertebral fractures.[17]

Li et al calculated CT HU measurements for the planned screw position on preoperative CT and found these measures to better predict screw loosening than any other vertebral CT HU measurement. They concluded that using specific CT measures of bone density will reduce the risk of screw loosening and their complications in future patients.[18] Zaidi et al found that CT HU, when compared to DEXA BMD values, correlated with successful lumbar interbody fusion, cage subsidence, adjacent segment fractures, and pedicle screw loosening.16 As a result, CT HU could be a promising measure of bone density.

St. Jeor et al conducted research regarding CT HU to propose a more comprehensive “expanded spine” criteria to refine the classification of spine surgery patients with potentially low bone density.[19] The expanded spine criteria diagnosed osteoporosis on the basis of a CT HU score less than 110 and/or the National Bone Health Alliance (NBHA) guidelines and/or degraded DEXA trabecular bone scores with osteopenic T-score. Retrospectively applying the expanded spine

criteria to lumbar spine surgery patients, the authors found that 70.4% of patients with normal or osteopenia DEXA BMD scores were reclassified with osteoporosis. The expanded spine criteria may be especially useful for patients of older age, as men and women older than 70 years were more frequently classified with osteoporosis based on the CT HU criteria than via DEXA or NBHA.

Quantitative computed tomography (QCT) is a specialized CT scan that transforms the CT signal in HU based on a calibration phantom using an analytical software.[20] Due to the ability of the QCT to separate cortical from trabecular bone, which is a known shortcoming of DEXA, some physicians have ordered QCT to assess bone quality.[21] Lin et al used a QCT-derived volumetric BMD to compare against DEXA T-score values in an older population, finding that QCT was more helpful than DEXA for detecting osteoporosis and predicting vertebral fractures.[22]

MRI VBQ Lumbar Measurements

A number of studies have tested MRI VBQ as a measure of lumbar bone density. Ehresman et al found that MRI VBQ is an important piece of information in order to assess risk for instrumentation failure.[23] Similarly, Roch et al found that VBQ is significantly correlated with CT bone quality values, although they used a different equation for VBQ measurements. 24 Cutoffs for MRI VBQ are also still debated, but lower scores indicate normal bone (<2.41) and greater scores indicate osteoporotic/osteopenic bone quality (>3.0).[25,26]

Kadri et al identified a VBQ threshold above which patients should undergo screening for osteoporosis,[25] and Hu et al reported an association between VBQ and cage subsidence after transforaminal lumbar interbody fusion.[27] Kale et al found a moderate correlation between their measure of VBQ and DEXA, which is consistent with some previously established literature.[28,29] They concluded that this MRI measure has the potential to be a predictor of decreased bone density, specifically pertaining to higher VBQ values.

In a recent study, Courtois et al compared lumbar MRI VBQ and CT HU scores to DEXA BMD scores and found that MRI VBQ scores had a much weaker correlation with DEXA than CT HU composites scores.[14,30] This study also used receiver operating characteristic area under the curve analyses to discern that CT HU demonstrated a greater ability to differentiate patients between normal and osteopenic/osteoporotic bone than MRI VBQ. Thus, CT HU may be a more reliable measure for lumbar bone mineral density compared to MRI.

Additional Applications: Lumbar Periendplate or Pedicle Measurements

Additional applications using CT HU, QCT, and MRI VBQ have recently been proposed such as at the lumbar endplate or the anticipated pedicle screw trajectory. These other methods may be used to help evaluate complication risks.

Cage Subsidence

Using QCT, Okano et al measured lumbar endplate bone density to assess the risk of subsidence and determined that endplate measurements were a better predictor of subsidence than trabecular bone density measurements after standalone lateral lumbar interbody fusions.[31] Jones et al found this to be congruent with their study, noting that significantly elevated endplate measurements using MRI was predictive of severe cage subsidence after the same lumbar procedure.[3]

Pedicle Screw Loosening/Pullout

Numerous studies have linked low HU values from CT pedicle screw bone density measurements with increased levels of pedicle screw loosening or pullout.[18,32-34] These studies confirmed that low bone density predisposes patients to pedicle screw pullout and also showed that this method of measuring bone density using alternative imaging techniques may aid in additional screening prior to lumbar instrumentation. Ishikawa et al described a method of measuring S1 pedicle screw trajectory bone density on CT and its ability to predict screw loosening six months postoperatively.[35] This method defined the HU region of interest (ROI) as a rectangular box where the screw was to be placed intraoperatively, and the authors concluded that lower CT HU correlated with screw loosening, suggesting that pedicle screws be placed in an area of the bone with high HU values. Another study examining the S1 pedicle screw trajectory bone density using MRI instead of CT and concluded that VBQ scores moderately correlated with DEXA T-scores and could be a beneficial tool to utilize in addition to the DEXA report.[36]

In some cases, especially if there is a concern for artificially increased DEXA values, utilizing these other techniques may provide more bone quality information prior to spinal surgery.

Conclusion

Recently, a number of alternative techniques have emerged to measure spine bone density in an effort to address the shortcomings of DEXA. Despite a number of studies comparing these methods, there is an overall lack of standardization within spine surgery. Some studies would argue that CT seems more closely correlated than MRI with DEXA BMD, while other studies found VBQ to be an adequate tool for assessing osteoporosis.[8,23,25]

Complimenting DEXA with alternative methods for measuring bone density prior to spinal instrumentation surgeries may lead to less hardware-related complications. The continued development of this field of study will aid spine surgeons in their decision-making process.

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Authors: Emily C. Courtois, MS

Mary P. Rogers-LaVanne, PhD

Peter B. Derman, MD, MBA

Alexander M. Satin, MD

From the Texas Back Institute in Plano, Texas.