Original ArticlesPrediction of vertebral failure loads from spinal and femoral dual-energy x-ray absorptiometry, and calcaneal ultrasound: an in situ analysis with intact soft tissues
Introduction
Vertebral fractures represent the classic hallmark of osteoporosis2, 10 with about 300,000–500,000 men and women being affected each year in the United States.10, 27 In cases of moderate or minimal trauma, the age-adjusted fracture rate of the vertebral body is generally higher in women, but above the age of 85 years the fracture rate is almost identical in both genders.10 In a recent study of clinically diagnosed vertebral fractures, 14% were found to be associated with severe trauma, 25% with a fall, and 9% with lifting; 33% had occurred spontaneously, and 16% were diagnosed incidentally.10 However, it has been estimated that only about one third of all patients with vertebral deformity seek medical attention; thus, only about 10%–15% of all vertebral fractures in the community are attributable to falls, whereas the vast majority appear to occur during normal everyday activity.10
Between 55% and 95% of all spinal fractures after the age of 45 are attributable to osteoporosis, depending on age, gender, and race.28 The direct health care expenditure involved in these fractures has been estimated to be $746 million in the United States in 1995.35 More importantly, spinal fractures lead to a substantial reduction in the quality of life due to back pain, impairment of well-being and mood, functional physical limitations, and disability.25 Because treatment of osteoporosis and for preventing osteoporotic fractures is available, noninvasive measurement techniques for the in vivo assessment of bone quantity and quality are required to allow for intervention at appropriate stages. However, it is difficult to determine the effectiveness of these techniques in predicting spinal fracture risk clinically, because many vertebral fractures pass unnoticed, because radiographs are required, and because the differentiation between vertebral fracture and deformity remains controversial. On the other hand, mechanical failure loads of skeletal regions can be determined in vitro under controlled loading conditions. Experiments of this nature can guide and focus clinical studies by selecting the most promising parameters and measurement techniques; however, the conditions under which the densitometric measurements are obtained should be similar to those in vivo.
Examining the relationship between the compressive strength of cadaveric lumbar vertebrae and quantitative computed tomography (qCT), McBroom et al.26 and Mosekilde et al.30 reported correlation coefficients between 0.68 and 0.76. When multiplying the bone density by the cross-sectional vertebral area, the coefficients were found to increase to between 0.80 and 0.86.4, 6 Cody et al.9 examined bone density in 18 regional subsets and found maximal correlations with failure load of 0.49 (L4–5) to 0.85 (T-11–L-1). Using multiple regression analyses, coefficients of >0.94 could be found. However, qCT has the disadvantage that it is relatively expensive and that the access is limited.
Dual-photon and dual-energy X-ray absorptiometry (DPA/DXA), on the other hand, are considerably less expensive and are more readily available than CT, and involve a relatively low radiation dose. Hansson et al.21 reported a correlation of 0.86 between bone mineral content (BMC), measured in excised lumbar vertebrae in the anteroposterior (AP) direction, and the load to fracture. Eriksson et al.14 compared qCT and DPA in the same specimens, and showed that the DPA-derived BMC was the better predictor (r = 0.80) than CT density multiplied by surface area (r = 0.74). The ultimate failure stress (load/cross-sectional area), however, was better predicted by qCT (r = 0.71 vs. r = 0.64). Ortoft et al.32 and Myers et al.31 indicated that lateral DXA measurements of excised lumbar vertebral bodies yielded a better association with ultimate failure loads than those taken in the AP direction. These findings were extended by Moro et al.29 who showed that the lateral BMD of L-2 correlated better with the failure load of lumbar (r = 0.89) and thoracic vertebrae (r = 0.94) than the AP BMD (r = 0.72 and 0.83).
In contrast to those studies, Bjarnason et al.5 reported much weaker correlations between the BMD and the in vitro failure loads of chemically dried lumbar vertebrae in 14 cadavers. They found that scans obtained in the lateral decubitus position gave a better correlation than those in the AP direction when measured in excised specimens (r = 0.71 vs. 0.51), but not when performed in situ in intact cadavers (r = 0.45 vs. 0.48). They observed, however, a correlation of r = 0.64 with in situ femoral DXA measurements and concluded that this parameter is more useful for assessing vertebral fracure risk than spinal measurements in any projection.
Ultrasound has more recently been advocated for assessment of fracture risk, its advantage being the low cost involved and the lack of ionizing radiation.17, 20 In a recent study, Cheng et al.7 related ultrasound measurements at the calcaneus to vertebral failure loads in 62 cadavers, and compared the correlation with that obtained for DXA and qCT measurements in excised specimens. They found that the fracture load could be best predicted with DXA-determined BMD (r = 0.80) and qCT (r = 0.78 for trabecular density × cross-sectional area), whereas calcaneal ultrasound (r = 0.37–0.41) was only a poor predictor. The investigators did, however, indicate that these findings contradicted emerging clinical data,3, 34, 36 which suggested that ultrasound has the same predictive ability as DXA. They reasoned that in vitro assessment of bone mineral density with DXA may be inherently more accurate than that performed in vivo, because of improved positioning and the absence of soft tissue variation. It is well known that the variable amount and inhomogeneous distribution of soft tissues and extraosseous calcifications may influence DXA measurements of bone mineral density,1, 15, 16, 18, 19, 37 and this has been a potential shortcoming of previous in vitro investigations.
The objective of the present analysis was therefore to determine and compare the correlation between spinal DXA, femoral DXA, and calcaneal ultrasound—when measured in situ with intact skin and soft tissues—with the in vitro failure loads of lumbar vertebral bodies in a relatively large study sample.
Section snippets
Materials and methods
Forty-nine formalin-fixed cadavers (from a macroscopic anatomy course) with intact skin and soft tissues were examined. Thirty-two cadavers were men, aged 82.1 ± 9.0 years (mean ± SD), and 17 were women, aged 83.1 ± 10.1 years. There was no significant difference in age between genders (p = 0.74), but the men were significantly taller (1.71 vs. 1.59 m; p < 0.001) and heavier (69.2 vs. 53.6 kg; p < 0.001) than the women (Table 1). The donors constituted a random sampling of the elderly
Results
The mechanical failure loads of L-4 varied between 1000 and 7870 N (Figure 2). In the men, the fracture load (3848 ± 1354 N) was significantly higher (p < 0.001) than in the women (2651 ± 1209 N) (Table 1). There was a moderate correlation of failure loads with body weight (r = 0.55, p < 0.001 for all subjects; r = 0.38, p < 0.05 for men; r = 0.57, p < 0.05 for women) and with the body height (r = 0.53, p < 0.001 for all subjects; r = 0.38, p < 0.05 for men, r = 0.43 for women). When
Discussion
In the current study we have determined the correlation of spinal and femoral DXA and calcaneal ultrasound, measured in situ with intact skin and soft tissues, with the in vitro failure load of the fourth lumbar vertebral body. We noted a significant, but only moderate, association of spinal DXA with the failure load of the fourth lumbar vertebral body, and a lower predictive ability of femoral DXA and calcaneal ultrasound. Nevertheless, calcaneal ultrasound was able to add independent
Acknowledgements
The authors thank Sabine Mühlsimer for photography help, and Markus Walz (Lunar, Germany) for his dedicated advice and support.
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