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Bone Quality Assessment in Spine Surgery
The prevalence of osteopenia and osteoporosis among spine surgery patients older than 50 years has been estimated to be as high as 46% and 31%, respectively.[1] Identifying osteoporosis prior to spine reconstruction surgery is critical, particularly given the aging US population and the increasing rates of spinal fusion procedures.[2,3] Osteoporosis is associated with worse outcomes and higher rates of complications in spine surgery, including pedicle screw loosening, instrumentation failure, pseudarthrosis, vertebral compression fractures, proximal junctional failure (PJF), and revision surgery.[4,5] Optimizing bone quality can positively affect the clinical outcomes.[6] Therefore, assessing bone quality in spine surgery patients is a crucial step for guiding clinical decision-making and improving postoperative outcomes.[2,3] Enhanced knowledge of differing methods to assess bone quality would be beneficial in the preoperative assessment of potential spinal fusion patients.
Overview of Bone Quality Assessment Methods
Several imaging modalities are available for a noninvasive evaluation of bone quality in the spine. Currently, dual-energy x-ray absorptiometry (DEXA) scans are considered the gold standard for bone mineral density (BMD) assessment, which is a critical component of overall bone quality assessment.[4] DEXA with trabecular bone score can assess bone quality besides BMD. Computed tomography (CT) and magnetic resonance imaging (MRI) are frequently used in the preoperative assessment of spine surgery patients. Recently, quantitative computed tomography (QCT)-Hounsfield units (HUs) and MRI-based vertebral bone quality (VBQ) have gained increasing attention as alternative methods for estimating bone quality.[7] These imaging techniques are valuable for identifying candidates for preoperative intervention for osteopenia/osteoporosis, selecting appropriate surgical techniques such as interbody fusion, and determining the need for augments such as cement augmentation or tethering.[8]
DEXA is a widely used method for preoperative planning due to its strong evidence base.[4] It provides a planar image that can assess BMD. However, DEXA has limitations, such as being influenced by factors like bone size, calcified tissues, and degenerative changes, which can lead to falsely elevated T-scores.[4,9,10] While DEXA-based trabecular bone score offers some additional insights into bone microstructure, it still has the inherent limitations of 2-dimensional imaging.[11]
For this reason, CT, specifically HU value and QCT, are increasingly used for evaluating BMD in spine surgery planning.[12] QCT provides true volumetric BMD measurements and offers a 3-dimensional view, making it less susceptible to the degenerative changes that can affect DEXA results.[4,7] It measures BMD in mg/cm³ and is not influenced by bone size or aortic calcifications.[9] However, like DEXA, CT involves exposure to ionizing radiation, which requires consideration when ordering these tests.[3]
MRI-based bone quality assessment, such as VBQ, is a newer technique that uses T1-weighted MRI to identify osteopenia and osteoporosis with high accuracy (Figure).[13] It is not affected by degenerative cortical changes, it is noninvasive, and it avoids the use of ionizing radiation, making it a safer option for patients.[3,11] However, this method can be limited by the potential for claustrophobia in patients, longer tests times, higher costs, and artifacts from metallic implants.[7]
Predicting Postoperative Spinal Complications
Prediction Ability of DEXA
Published literature indicates that low BMD can negatively impact fusion constructs. In a study that compared T-scores of patients with subsidence to those without subsidence who underwent lumbar fusion, researchers found that the mean T-score in patients with subsidence was -1.65 compared to -0.45 in patients without subsidence, and they con- cluded that patients with DEXA T-scores <-1.0 undergoing lumbar fusion are at a much higher risk of developing cage subsidence.[14] In a propensity-matched comparison study of adult spinal deformity patients to elucidate whether low BMD is a true risk factor for PJF, patients were grouped as having mildly low to normal BMD (T-score ≥ -1.5) or significantly low BMD (T-score <-1.5). They found that the incidence of PJF was significantly higher in the low BMD group.[15] Similar findings were confirmed by another study that found patients with PJF have significantly lower BMD than non-PJF patients (T-scores of -1.4 vs. -0.7, respectively).[16] In light of these findings, it seems reasonable to suggest that a T-score below -1.0 may generally indicate a potential need for preoperative interventions to address concerns about bone quality.
Prediction Ability of HU Value
CT is not a replacement for DEXA, but if available, HU values can help in predicting surgical spinal complications. For instance, CT scans are used to screen osteoporosis before spinal fusion to help in planning for the surgical treatment.[4] A recent review of 42 studies that including a measure of spinal BMD showed that patients with HU values >160 demonstrated significant low risk of osteoporosis, whereas HU values <110 were significantly correlated with osteoporosis.[4] HU values were also used to evaluate BMD to lower risks of interbody cage subsidence. Several studies showed that low HU values preoperatively are significantly associated with cage subsidence after fusion.[17-19] One of these studies was a review of current literature in which they created a cut-off for HU values to predict postoperative complications.[18] They found that patients with HU values under 120 are at risk for subsidence, screw loosening, and pseudarthrosis of interbody fusion, while those with HU values under 150 are at risk for posterolateral fusion pseudarthrosis and adjacent segment fractures.[18]
Prediction Ability of VBQ Score
While evidence for using VBQ in predicting construct failure is limited, available studies have suggested usefulness of VBQ values in spinal instrumentation. These studies indicated that VBQ is useful in predicting subsidence of fusion cages, vertebral fractures, PJF, and reoperation after lumbar fusion.[20-24] Higher VBQ values have been correlated with implant subsidence, reoperation rates, and vertebral compression fractures. A study investigating the association between VBQ scores and reoperation after lumbar fusion found that the average VBQ score for patients requiring reoperation was 2.92, compared to 3.29 for those who did not.[23] Among those needing reoperation, 70% had a VBQ greater than 3, while only 38.3% of those not requiring reoperation had a VBQ greater than 3. Preoperative VBQ scores were also significant predictors of PJF in patients undergoing corrective surgery for adult spinal deformity, as a recent study showed that the mean VBQ scores were 3.13 for patients with PJF and 2.46 for patients without, with a predictive accuracy of 94.3%.[24]
Comparison of the 3 Methods
MRI and CT-based assessment are emerging techniques for evaluating BMD, particularly in predicting complications and construct failure following fusion surgery. Although these newer methods show promise, their correlations with traditional DEXA are not well established. Previous studies indicate that while HU and VBQ measurements are useful for BMD assessment, they only moderately correlate with traditional DEXA scores.[7,20,25,26] When comparing these methods for predicting osteoporosis before spinal surgery, VBQ and HU values were more associated with identifying patients with osteoporosis and corresponding fractures than the T score.[25,27] One study suggested that HU scores outperform VBQ scores in screening for osteoporosis.[28]
In terms of prediction of spinal construct complications, HU value and VBQ score have been reported to be superior to DEXA in predicting complications.[25,29] For predicting cage subsidence, both HU and VBQ scores demonstrated superior predictive ability for the amount of cage subsidence than DEXA.[21,25] In patients with lumbar pedicle screw fixation, VBQ was a better predictor of pedicle screw loosening than HU, with a VBQ threshold of 3.05 optimizing sensitivity and specificity for predicting this outcome.[30]
Emerging Technologies and Future Directions
The ability to accurately determine spinal bone quality using alternative imaging modalities instead of DEXA is now a key area of emerging interest to spine surgeons. Dual-energy CT and spectral-detector CT are new methods being evaluated for BMD assessment of lumbar bone. They showed strong correlation with bone strength in human cadaver vertebrae specimens and superior predictive ability for the 2-year risk to sustain an osteoporosis-associated fracture without requiring reference compared to DEXA and HU measurements.[31,32] Given that preoperative CT is routinely performed before spinal fusion, utilization of these novel techniques may be able to improve the screening and preoperative optimization protocols.
Another growing interest is quantitative ultrasound (QUS) measurements for detecting and managing osteoporosis. QUS methods have several potential advantages over the conventional measures (absence of ionizing radiation, portable machines, lower cost), but such methods await broader validation regarding their accuracy in identifying osteoporotic patients in spine surgery. [33,34]
Conclusion
Preoperative radiographic evaluation of bone quality may include DEXA, CT, and MRI. The identification and assessment of poor bone health preoperatively can impact postoperative outcomes with the potential to reduce osteoporosis-related complications. In recent years, new tools and diagnostic techniques have been developed to refine bone quality and overcome the drawbacks of conventional techniques. While DEXA can still represent the reference standard for assessing osteoporosis, advancements in alternative imaging modalities show promise. Moving forward, developing a comprehensive, national-level protocol for preoperative bone quality evaluation and preoperative intervention will be essential to optimize surgical outcomes and patient care.
References
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Contributors:
Sereen Halayqeh, MD[1]
Tomoyuki Asada, MD[1]
Sravisht Iyer, MD[1,2]
From the [1]Hospital for Special Surgery and [2]Weill Cornell Medical College, both in New York, New York.
