CT determination of bone mineral density and structural investigations on the axial skeleton for estimating the osteoporosis-related fracture risk by means of a risk score

Clinical practice recommendations for the use of imaging in the diagnosis and management of axial spondyloarthritis in Taiwan

OBJECTIVE

Axial spondyloarthritis (axSpA) is a chronic inflammatory disease bearing challenges in early diagnosis. To improve clinical diagnosis and management of axSpA, recommendations were developed with current axSpA classification criteria and recent advances in medical imaging applications.

METHODS

A systematic literature review was conducted by 10 rheumatologists and radiologists in Taiwan to retrieve research evidence on the utilization of imaging modalities, including conventional radiography (CR), magnetic resonance imaging (MRI), computed tomography (CT), ultrasound (US), quantitative sacroiliac scintigraphy (QSS), and dual-energy X-ray absorptiometry (DXA). The panel of experts proposed six key issues on the role of imaging in early diagnosis of axSpA, monitoring of disease activity and structural changes, predicting treatment effects, and assessing complications such as osteoporosis and spinal fracture. The consensus was established on the basis of research evidence, clinical experiences and expert opinions. For each recommendation statement, the level of evidence was evaluated, the strength of recommendation was graded and the final level of agreement was determined through voting.

RESULTS

In total, four overarching principles and 13 recommendations were formulated. These recommendations outlined different imaging approaches in the diagnosis and management of axSpA disease progression. Considering CT is easy to perform when MRI is less available in Taiwan, the expert panel proposed a concise and practical diagnostic scheme to strengthen the valuable role of MRI and CT in the diagnostic evaluation of axSpA without evident radiographic features.

CONCLUSION

These modified recommendations provide guidance for rheumatologists, radiologists and healthcare professionals on timely diagnosis of axSpA and disease management with appropriate imaging modalities.

Fractal analysis of the computed tomography images of vertebrae on the thoraco-lumbar region in diagnosing osteoporotic bone damage

Fractal analysis was used in the study to determine a set of feature descriptors which could be applied in the process of diagnosing bone damage caused by osteoporosis. The subject of the research involved the computed tomography images of vertebrae on the thoraco-lumbar region. The data set contained the images of healthy patients and patients diagnosed with osteoporosis. On the basis of fractal analysis and feature selection by linear stepwise regression, three descriptors were obtained. They were two fractal dimensions calculated with the variation method (transect - first differences and filter 1 estimators) and one fractal lacunarity calculated by means of the box counting method. The first two descriptors were obtained as a result of the analysis of grey images, and the third was the result of analysis of binary images. The effectiveness of the descriptors was verified using six popular supervised classification methods: linear and quadratic discriminant analysis, naive Bayes classifier, decision tree, K-nearest neighbours and random forests. The best results were obtained using the K-nearest neighbours classifier; they were as follows: overall classification accuracy - 81%, classification sensitivity - 78%, classification specificity - 90%, positive predictive value - 90%, and negative predictive value - 77%. The results of the research showed that fractal analysis can be a useful tool to extract feature vector of spinal computed tomography images in the diagnosis of osteoporotic bone defects.

Biomechanical Comparison of Sacral Fixation Characteristics of Standard S1-Pedicle Screw Fixation versus a Novel Constrained S1-Dual-Screw Anchorage in the S1-Pedicle and S1-Alar Bone

STUDY DESIGN

Biomechanical Laboratory Study.

OBJECTIVE

Analysis of the biomechanical characteristics of a novel sacral constrained dual-screw fixation device (S1-PALA), combining a S1-pedicle screw and a S1-ala screw, compared to a standard bicortical S1-pedicle screw (S1-PS) fixation.

SUMMARY OF BACKGROUND DATA

Instrumented fusions to the sacrum are biomechanically challenging and plagued by a high risk of nonunion when S1-PS is used as the sole means of fixation. Thus, lumbopelvic fixation is increasingly selected instead, although associated with a reasonable number of instrumentation-related complications.

METHODS

Around 30 fresh-frozen human sacral bones were harvested and embedded after CT scans. Instrumentation was conducted in alternating order with bicortical 7.0 mm S1-PS and with the S1-PALA including a S1-PS screw and a S1-ala screw, of 7.0 and 6.0 mm diameter, respectively. Specimens were subjected to cyclic loading with increasing loads (25-250 N) until a maximum of 2000 cycles or displacement >2 mm occurred. All implant sacral units (ISUs) were subject to coaxial pullout tests. Failure load, number of ISUs surpassing 2000 cycles, number of cycles, and loads at failure were recorded and compared.

RESULTS

Donors' age averaged 77 ± 14.2 years, and BMD was 115 ± 64.8 mgCA-HA/ml. Total working length of screws implanted was 90 ± 8.6 mm in the S1-PALA group and 46 ± 5 mm in the S1-PS group (P = 0.0002). In the S1-PALA group, displacement >2 mm occurred after 845 ± 325 cycles at 149 ± 41 N compared to 512 ± 281 cycles at 106 ± 36 N in the S1-PS group (P = 0.004; P = 0.002). In coaxial pull-out testing, failure load was 2118.1 ± 1166 N at a displacement of 2.5 ± 1 mm in the S1-PALA group compared to 1375.6 ± 750.1 N at a displacement of 1.6 ± 0.5 mm in the S1-PS group (P = 0.0007; P = 0.0003).

CONCLUSION

The novel sacral constrained dual-screw anchorage (S1-PALA) significantly improved holding strength after cyclic loading compared to S1-PS. The S1-PALA demonstrated mechanical potential as a useful adjunct in the armamentarium of lumbosacral fixations indicated in cases that need advanced construct stability, but where instrumentation to the ilium or distal dissection to S2 should be avoided.

LEVEL OF EVIDENCE

N/A.

Impact of constrained dual-screw anchorage on holding strength and the resistance to cyclic loading in anterior spinal deformity surgery: a comparative biomechanical study

STUDY DESIGN

Biomechanical in vitro laboratory study.

OBJECTIVE

To compare the biomechanical performance of 3 fixation concepts used for anterior instrumented scoliosis correction and fusion (AISF).

SUMMARY OF BACKGROUND DATA

AISF is an ideal estimate for selective fusion in adolescent idiopathic scoliosis. Correction is mediated using rods and screws anchored in the vertebral bodies. Application of large correction forces can promote early weakening of the implant-vertebra interfaces, with potential postoperative loss of correction, implant dislodgment, and nonunion. Therefore, improvement of screw-rod anchorage characteristics with AISF is valuable.

METHODS

A total of 111 thoracolumbar vertebrae harvested from 7 human spines completed a testing protocol. Age of specimens was 62.9 ± 8.2 years. Vertebrae were potted in polymethylmethacrylate and instrumented using 3 different devices with identical screw length and unicortical fixation: single constrained screw fixation (SC fixation), nonconstrained dual-screw fixation (DNS fixation), and constrained dual-screw fixation (DC fixation) resembling a novel implant type. Mechanical testing of each implant-vertebra unit using cyclic loading and pullout tests were performed after stress tests were applied mimicking surgical maneuvers during AISF. Test order was as follows: (1) preload test 1 simulating screw-rod locking and cantilever forces; (2) preload test 2 simulating compression/distraction maneuver; (3) cyclic loading tests with implant-vertebra unit subjected to stepwise increased cyclic loading (maximum: 200 N) protocol with 1000 cycles at 2 Hz, tests were aborted if displacement greater than 2 mm occurred before reaching 1000 cycles; and (4) coaxial pullout tests at a pullout rate of 5 mm/min. With each test, the mode of failure, that is, shear versus fracture, was noted as well as the ultimate load to failure (N), number of implant-vertebra units surpassing 1000 cycles, and number of cycles and related loads applied.

RESULTS

Thirty-three percent of vertebrae surpassed 1000 cycles, 38% in the SC group, 19% in the DNS group, and 43% in the DC group. The difference between the DC group and the DNS group yielded significance (P = 0.04). For vertebrae not surpassing 1000 cycles, the number of cycles at implant displacement greater than 2 mm in the SC group was 648.7 ± 280.2 cycles, in the DNS group was 478.8 ± 219.0 cycles, and in the DC group was 699.5 ± 150.6 cycles. Differences between the SC group and the DNS group were significant (P = 0.008) as between the DC group and the DNS group (P = 0.0009). Load to failure in the SC group was 444.3 ± 302 N, in the DNS group was 527.7 ± 273 N, and in the DC group was 664.4 ± 371.5 N. The DC group outperformed the other constructs. The difference between the SC group and the DNS group failed significance (P = 0.25), whereas there was a significant difference between the SC group and the DC group (P = 0.003). The DC group showed a strong trend toward increased load to failure compared with the DNS group but without significance (P = 0.067). Surpassing 1000 cycles had a significant impact on the maximum load to failure in the SC group (P = 0.0001) and in the DNS group (P = 0.01) but not in the DC group (P = 0.2), which had the highest number of vertebrae surpassing 1000 cycles.

CONCLUSION

Constrained dual-screw fixation characteristics in modern AISF implants can improve resistance to cyclic loading and pullout forces. DC constructs bear the potential to reduce the mechanical shortcomings of AISF.

Strain changes on the cortical shell of vertebral bodies due to spine ageing: A parametric study using a finite element model evaluated by strain measurements

The probability of fractures of the cortical shell of vertebral bodies increases as ageing progresses. Ageing involves all the spinal component changes. However, the effect of the spinal component ageing on the fracture risk of the cortical shell remains poorly understood. In this study, the influence of the ageing of the spinal components on cortical shell strain was investigated. A lumbar spinal specimen (L3–L5) was mechanically tested under a quasi-static axial compressive load. Clinical computed tomography images of the same specimen were used to create a corresponding finite element model. The material properties were determined by calibrating the finite element model using the L4 cortical shell strains of the anterior centre measurement site. The remaining experiment data (axial displacement, the intra-discal pressures, L4 cortical shell strain on the lateral measurement site) were used to evaluate the model. The individual ageing process of the six spinal components (cortical shell, cancellous bone, bony endplate, posterior elements, nucleus pulposus and annulus matrix) was simulated by changing their Young’s moduli and Poisson’s ratios, and the effect on cortical shell strain was investigated. Results show that the cortical shell strain is more sensitive to the ageing of the cortical shell and the cancellous bone than to the ageing of the nucleus pulposus, the annulus matrix, and the bony endplates and of the posterior elements. The results can help the clinicians focus on the aspects that mainly influence the vertebral cortex fracture risk factor.

Bone strength measured by peripheral quantitative computed tomography and the risk of nonvertebral fractures: the osteoporotic fractures in men (MrOS) study

Many fractures occur in individuals without osteoporosis defined by areal bone mineral density (aBMD). Inclusion of other aspects of skeletal strength may be useful in identifying at-risk subjects. We used surrogate measures of bone strength at the radius and tibia measured by peripheral quantitative computed tomography (pQCT) to evaluate their relationships with nonvertebral fracture risk. Femoral neck (FN) aBMD, measured by dual-energy X-ray absorptiometry (DXA), also was included. The study population consisted of 1143 white men aged 69+ years with pQCT measures at the radius and tibia from the Minneapolis and Pittsburgh centers of the Osteoporotic Fractures in Men (MrOS) study. Principal-components analysis and Cox proportional-hazards modeling were used to identify 21 of 58 pQCT variables with a major contribution to nonvertebral incident fractures. After a mean 2.9 years of follow-up, 39 fractures occurred. Men without incident fractures had significantly greater bone mineral content, cross-sectional area, and indices of bone strength than those with fractures by pQCT. Every SD decrease in the 18 of 21 pQCT parameters was significantly associated with increased fracture risk (hazard ration ranged from 1.4 to 2.2) independent of age, study site, body mass index (BMI), and FN aBMD. Using area under the receiver operation characteristics curve (AUC), the combination of FN aBMD and three radius strength parameters individually increased fracture prediction over FN aBMD alone (AUC increased from 0.73 to 0.80). Peripheral bone strength measures are associated with fracture risk and may improve our ability to identify older men at high risk of fracture.

Qualitative and quantitative assessment of bone fragility and fracture healing using conventional radiography and advanced imaging technologies--focus on wrist fracture

Fractures of the distal radius are one of the most common injuries presented to orthopaedic surgeons. A variety of treatment options are available for the vast array of fracture patterns. Research that explores bone fragility and fracture healing has led to new treatment modalities. As new products and methods are derived to aid in fracture healing it is essential to develop noninvasive and/or nondestructive techniques to assess structural information about bone. Quantitative assessment of macro-structural characteristics such as geometry, and microstructural features such as relative trabecular volume, trabecular spacing, and connectivity may improve our ability to estimate bone strength. Methods for quantitatively assessing macrostructure include (besides conventional radiographs) dual x-ray absorptiometry (DXA) and computed tomography (CT), particularly volumetric quantitative computed tomography (vQCT). Methods for assessing microstructure of trabecular bone include high resolution computed tomography (hrCT), micro computed tomography (microCT), high resolution magnetic resonance (hrMR), and micro magnetic resonance microMR. Volumetric QCT, hrCT and hrMR are generally applicable in vivo; microCT and microMR are principally applicable in vitro. Clinically, the challenges for bone imaging include balancing the advantages of simple bone densitometry versus the more complex architectural features of bone, or the deeper research requirements versus the broader clinical needs.

Magnetic field distribution in the presence of paramagnetic plates in magnetic resonance imaging: a combined numerical and experimental study

The amount and geometric distribution of paramagnetic components in tissue is considered as the basis of T2*-weighted magnetic resonance imaging (MRI). Such techniques are routinely applied for assessment of iron in parenchymal organs such as the liver (hemosiderosis). Furthermore, susceptibility sensitive MRI is discussed as an alternative method to x-ray techniques for quantitative assessment of paramagnetic spongy bone components in patients with osteoporosis. The presented work is dedicated to systematically examining the possible influences of macroscopic arrangements of paramagnetic plates on the magnetic field. In a theoretical approach magnetic field distribution was simulated applying decomposition of the plates in single dipoles. Plate size and distances between parallel plates, as well as plate orientation with respect to the static field, were varied for these numerical simulations. Experiments on corresponding plate arrangements were carried out on a 3 T whole body MR scanner using the field-sensitive MR sequence technique for B0 field mapping. Further examinations were carried out on a bone preparation of the femur, where T2* maps were measured and analyzed on a pixel-by-pixel basis at two orientations with respect to the static field. A series of experiments were performed using isotropic and anisotropic volume elements in three-dimensional gradient echo sequences. Resulting magnetic field distributions in the experimentally recorded B0 field maps were in good agreement with the numerical simulations. Field distortions dominated in areas close to the plates and especially near the edges. Those areas showed strong local field gradients, leading to pronounced signal dephasing effects. The examination of the bone preparations revealed different T2* values for identical regions in the bone when the orientation of the bone or the pixel geometry was changed with respect to the magnetic field. Those effects amounted to nearly 70% (22.9 ms versus 13.6 ms in a region of interest in the femur) for 90 degrees rotation of the femur in the magnetic fields. The orientation of anisotropic picture elements with constant size also showed a strong influence on the derived T2* value (up to 80%, increasing with anisotropy of picture elements).

Changes in femur stress after hip resurfacing arthroplasty: response to physiological loads

BACKGROUND

Hip resurfacing arthroplasty is being increasingly considered as an alternative to total hip arthroplasty in young, active patients. Hip resurfacing arthroplasty is reported to preserve the normal joint mechanics. However, there is concern, in the short term, due to frequent occurrence of femoral neck fractures.

METHODS

We evaluated changes in femoral mechanics after hip resurfacing arthroplasty. We used an experimentally validated, distributed material finite element model of a cadaveric femur before and after hip resurfacing arthroplasty. Bone stiffness and strength values representing normal, elderly and osteoporotic bone were used. For a physiological load case, bone strains were compared with literature values for total hip arthroplasty and a risk of fracture scalar calculated.

FINDINGS

The changes in peak stresses after hip resurfacing arthroplasty were low in relation to the failure strength of bone and the fracture risk was low. The intact and implanted finite element models showed bone strains after hip resurfacing arthroplasty were closer to the intact condition than after total hip arthroplasty.

INTERPRETATION

The bone stresses predicted after resurfacing in both the normal and aged femoral neck were not sufficient to be a potential cause of fracture.

Effect of trabecular bone loss on cortical strain rate during impact in an in vitro model of avian femur

Background

Osteoporotic hip fractures occur due to loss of cortical and trabecular bone mass and consequent degradation in whole bone strength. The direct cause of most fractures is a fall, and hence, characterizing the mechanical behavior of a whole osteopenic bone under impact is important. However, very little is known about the mechanical interactions between cortical and trabecular bone during impact, and it is specifically unclear to what extent epiphyseal trabecular bone contributes to impact resistance of whole bones. We hypothesized that trabecular bone serves as a structural support to the cortex during impact, and hence, loss of a critical mass of trabecular bone reduces internal constraining of the cortex, and, thereby, decreases the impact tolerance of the whole bone.

Methods

To test this hypothesis, we conducted cortical strain rate measurements in adult chicken's proximal femora subjected to a Charpy impact test, after removing different trabecular bone core masses to simulate different osteopenic severities.

Results

We found that removal of core trabecular bone decreased by ~10-fold the cortical strain rate at the side opposite to impact (p < 0.01), i.e. from 359,815 ± 1799 μm/m per second (mean ± standard error) for an intact (control) specimen down to 35,997 ± 180 μm/m per second where 67% of the total trabecular bone mass (~0.7 grams in adult chicken) were removed. After normalizing the strain rate by the initial weight of bone specimens, a sigmoid relation emerged between normalized strain rate and removed mass of trabecular bone, showing very little effect on the cortex strain rate if below 10% of the trabecular mass is removed, but most of the effect was already apparent for less than 30% trabecular bone loss. An analytical model of the experiments supported this behavior.

Conclusion

We conclude that in our in vitro avian model, loss of over 10% of core trabecular bone substantially altered the deformation response of whole bone to impact, which supports the above hypothesis and indicates that integrity of trabecular bone is critical for resisting impact loads.

Evaluation of osteoporosis in X-ray CT examination: A preliminary study for an automatic recognition algorithm for the central part of a vertebral body using abdominal X-ray CT images

We have developed an algorithm that can distinguish the central part of the vertebral body from abdominal X-ray CT images to determine whether it is possible to aid a diagnosis of osteoporosis. We classified three measures for the principal component analysis and linear discriminant function. When we judged whether patients had osteoporosis or not, the ratio usable for diagnosing osteoporosis (sensitivity) was 1.00 (15/15), and for diagnosing as normal (specificity) was 0.64 (7/11). We believe that this algorithm can be used to aid in diagnosing osteoporosis, utilizing the measures obtained from the CT images.

Comparison of DXA, QCT and trabecular structure in beta-thalassaemia

Osteopathy, as a major feature of homozygous beta-thalassaemia, is a multifactorial disorder, not fully understood. We studied the lumbar vertebrae of 48 patients using Dual-Energy X-ray Absorptiometry (DXA) and Quantitative Computed Tomography (QCT), and we focused on structural properties, assessed by High Resolution Computed Tomography (HRCT). Bone Mineral Density (BMD) values were expressed as Z-scores and the results were correlated. The effect of age, sex, and type of thalassaemia and hormonal factors on BMD was assessed. We estimated, with HRCT, the cortex integrity and the number and thickness of trabeculae; the latter were classified to a three-grade scale. Our results showed the overall prevalence of osteoporosis to be 44% with DXA and 6% with QCT. Both techniques revealed an inverse correlation between age and BMD, whereas hormonal factors demonstrated associations with QCT and DXA measurements. The correlation coefficient between DXA's BMD and QCT's trabecular BMD was 0.545 (P < 0.001) whereas the corresponding value for Z-scores was r = 0.491 (P < 0.001). The classification of the patients into normal, osteopenic and osteoporotic categories, using QCT's Z, was in better agreement with the assignment based on trabecular number (K = 0.209, P = 0.053) than the classification using DXA's Z (K = 0.145, P = 0.120). Cortex evaluation by HRCT showed discontinuity in 15 patients. Both methods indicate a progression of osteoporosis with age. Hormonal deficiency is associated with thalassaemic osteoporosis whereas the visual estimation of cortex indicates that Thalassaemia Intermedia (TI) patients could be more affected than Thalassaemia Major (TM). Using the trabecular number as an indicator of osteoporosis, it seems that QCT may evaluate osteopathy better than DXA. Since the former has the ability to measure trabecular and cortical BMD separately, it could give early indication of which changes more rapidly and to what degree.

Simulated influence of osteoporosis and disc degeneration on the load transfer in a lumbar functional spinal unit

As life expectancy increases, age-related disorders and the search for related medical care will expand. Osteoporosis is the most frequent skeletal disease in this context with the highest fracture risk existing for vertebrae. The aging process is accompanied by systemic changes, with the earliest degeneration occurring in the intervertebral discs. The influence of various degrees of disc degeneration on the load transfer was examined using the finite element method. The effect of different possible alterations of the bone quality due to osteoporosis was simulated by adjusting the corresponding material properties and their distribution and several loadings were applied. An alteration of the load transfer, characterised by changed compression stiffness and strain distributions as well as magnitudes, due to osteoporotic bone and degenerated discs was found. When osteoporosis was simulated, the stiffness was substantially decreased, larger areas of the cancellous bone were subjected to higher strains and strain maxima were increased. Increasing ratios of transverse isotropy in the osteoporotic bone yielded smaller effects than reduced bone properties. Including a degenerated disc mainly altered the strain distribution. Combining osteoporosis and degenerated discs reduced the areas of cancellous bone subjected to substantial strain. Based on these results, it can be concluded that the definition of a healthy disc in osteoporotic spines might be considered as a worst-case scenario. One attempt to evaluate the progress of osteoporosis can be made by introducing increasing degrees of anisotropy. If several parameters in a model are changed to simulate degeneration, it should be pointed out how each individual definition influences the overall result.

Cadaveric-biomechanical evaluation of bone-implant construct of proximal humerus fractures (Neer type 3)

BACKGROUND

A biomechanical cadaver study was performed to test the stability and strength of screw osteosynthesis of surgical neck fractures of the humerus.

METHODS

After bone density measurement, 64 cadaver proximal humerus bones were bent to create a subcapital fracture. The fracture was then stabilized by means of screw osteosynthesis randomly assigned to subgroups of screw positioning, size of screw, and stress test (torsion/bending).

RESULTS

Two screws applied laterally and parallel were 34.2% more stable than the normal arrangement. Bone density had a dominant role with regard to maximal bending and torsion force, but no significance was found with respect to additional screws through the major tuberculum or diameter of screws.

CONCLUSION

Two of the smaller 4.5-mm cannulated screws should be applied parallel from the lateral direction. Only range-of-motion exercises that produce a bending stress should be considered early after surgery, avoiding axial stress.

Osteodensitometry of human heel bones by MR spin-echo imaging: comparison with MR gradient-echo imaging and quantitative computed tomography

The aim of the study was to investigate whether quantitative magnetic resonance (MR) fast spin-echo (FSE) imaging with moderate spatial resolution enables osteodensitometry in peripheral yellow bone marrow. Signal intensities in T1-weighted FSE images from yellow bone marrow indicate the amount of adipose tissue per volume. The signal intensity in marrow regions with spongy bone was assessed and compared to signal intensity of pure fatty marrow (100%). Heel bones of 30 patients with suspected osteoporosis were analyzed and the FSE images were compared with results from parallel MR gradient-echo (GE) imaging and quantitative computed tomography (QCT) examinations. High correlation was found between FSE imaging and QCT [r = 0.91 in the dorsal region of interest (ROI); r = 0.86 in ventral ROI]. Linear correlation coefficients between GE imaging and QCT were slightly lower in the dorsal part (r = -0.86) and considerably lower in the ventral part (r = -0.68). Correlation between the two MR techniques amounted to r = -0.72/-0.61 (dorsal/ventral). The high correlation between FSE imaging and bone mineral density (BMD) allows possible clinical applications of FSE imaging for diagnosis of osteoporosis. Further improvements of the accuracy using reference phantoms might be possible.

Quantitative heel ultrasound in assessment of bone structure in renal transplant recipients

Many patients with advanced renal disease have osteopenia or even osteoporosis by the definition of the World Health Organization based on bone mineral density (BMD). Dual-energy X-ray absorptiometry (DXA), the standard method to assess BMD, is not always available. Quantitative heel ultrasound (QUS) is an inexpensive, mobile, and radiation-free diagnostic alternative, yet few data address this method's usefulness in patients with renal disease. The present study assessed the value of QUS in detecting changes in bone structure in renal transplant recipients compared with DXA. In a cross-sectional analysis, 50 patients (29 women) with a mean age of 50 +/- 13 years, mean time since transplantation of 60 months (range, 1 to 205 months), and stable renal allograft function were studied. BMD was quantified by DXA of the hip and spine. QUS of the left heel measured broadband ultrasound attenuation (BUA) and speed of sound (SOS). Stiffness index (SI) was calculated as SI = (0.67 * BUA + 0.28 * SOS) - 420. DXA measurements established the diagnoses of osteopenia and osteoporosis in 49% and 22% of the patients, respectively. Femoral neck BMD and QUS parameters showed good correlation (r = 0.638; P < 0.001). Sensitivities of BUA, SOS, and SI for diagnosing osteoporosis were 100%, and specificities were 73%, 76%, and 78%, respectively. Positive predictive values were 50%, 53%, and 56%, and negative predictive values were 100%. QUS can be recommended for screening patients who do not have osteoporosis. Those suspected of osteopenic bone structure should be examined by additional DXA measurement for quantification before initiation of therapy.

Issues of threshold selection when determining the fractal dimension in HRCT slices of lumbar vertebrae

The box counting dimension is a frequently applied tool for the classification of trabecular bone structure. The algorithm requires a binarization of the gray value data, for example that acquired by high resolution CT (HRCT). We recently proposed a method to eliminate bone mineral density (BMD) by applying a linear normalization scheme. Further consideration has shown that full BMD independence has not been achieved, and the structural parameter proposed was therefore difficult to interpret. In this study we present an alternative approach to obtain a structural parameter that is independent of BMD. HRCT volume data was acquired on 21 lumbar vertebrae from five cadavers. In the segmented spongiosa, thresholding was based on different quantiles of the gray value histogram, yielding invariance over linear and non-linear transformations. Thresholding at high gray value levels (80% quantile) shows the highest level of significance when discriminating between osteoporotic and non-osteoporotic cases. As an addition to the measurement of BMD alone, the determination of structural properties allows an improvement of the assessment of the individual fracture risk.