Correlation of thoracic and lumbar vertebral failure loads with in situ vs. ex situ dual energy X-ray absorptiometry
Introduction
Vertebral fractures constitute a common and serious health problem, with an estimated incidence of approximately 500,000 women and men annually in the United States (Melton et al., 1989; Cooper et al., 1992). Although usually not lethal, these fractures substantially reduce the quality of life of elderly individuals, by causing pain, kyphoscoliotic deformity and functional impairment (Leidig-Bruckner et al., 1997). Vertebral fractures also create an increasing problem for health care systems with the direct and indirect costs in North America estimated at $ 750 Mio.(US) each year (Ray et al., 1997). The majority of vertebral fractures occur during normal day-to-day activity, and are often not associated with trauma or exceptional loading events (Cooper et al., 1992). The most frequent cause of fracture, in contrast, is a reduction of mechanical strength due to osteoporotic bone loss (Melton et al., 1997).
Biomechanical studies have demonstrated a high correlation between bone mineral content (BMC) and areal bone mineral density (BMD [g/cm2]) of the vertebrae (as measured by dual photon or X-ray absorptiometry [DPA/DXA]) and their compressive strength (Hansson et al., 1980; Eriksson et al., 1989), in particular when densitometric measurements of the vertebrae are obtained without posterior elements (Ortoft et al., 1993; Moro et al., 1995; Singer et al., 1995; Cheng et al., 1997). However, these experimental studies were confined to the analysis of excised bones, although the soft tissues have been shown to introduce important artifacts in the determination of the BMC and BMD (Gotfredsen et al., 1988; Laskey et al., 1992; Tothill and Pye, 1992; Svendsen et al., 1995; Lochmüller et al., 2001). These artifacts are caused by the presence extraosseous calcifications (Frohn et al., 1991; Adams, 1997) and/or an inhomogeneous distribution of the fat tissue content throughout the region of interest. Bjanarson et al. (1996) and Lochmüller et al. (1998) reported lower correlations between the BMC measured in situ (including intact soft tissues) with DXA, and the failure loads of lumbar vertebrae in relatively small samples than previous ex situ analyses. Additionally, the highest prevalence of fractures has been reported in the thoracic spine (Härmä et al., 1986; De Smet et al., 1988) and in the thoracolumbar junction (De Smet et al., 1988). This region of the spine, however, is not accessible to clinical DXA measurements, due to presence of the rib cage. Taking into account that bone properties may vary considerably in the same individual from site to site (Amling et al., 1996a; Groll et al., 1999), and even between vertebrae (Amling et al., 1996b), it is an open question as to what extent failure loads of the thoracic vertebrae are correlated with in situ DXA in the lumbar spine.
In the current study we explore the hypothesis that the DXA-based estimates of failure loads in the (clinically more relevant) thoracic spine are compromised (a) by soft tissue artifacts in determining the bone mineral status of the lumbar vertebrae and (b) by skeletal heterogeneity between the lumbar and thoracic spine. We aimed to answer the following specific questions:
- 1.
What is the correlation between mechanical failure loads of thoracic (T6 and T10) and lumbar (L3) vertebral units ?
- 2.
What is the correlation of in situ vs. ex situ lumbar DXA measurements with the mechanical strength of lumbar vertebrae (same site), and with the mechanical strength of thoracic vertebrae (distant site), respectively ?
- 3.
Can gender differences be observed in the correlation among failure loads in different spinal segments, and between DXA and mechanical failure strength?
Section snippets
Study sample
The study sample initially comprised 140 formalin fixed cadavers with intact skin and soft tissue from a course of macroscopic anatomy. The donors constituted a representative selection of the elderly population resident in Bavaria, as the only criteria of inclusion in the course of anatomy was the testamentary decree to the institute several years prior to death. After standard histomorphometry of bone samples taken from the iliac crest (Hahn et al., 1991), 3 specimens with malignancy, and 7
Results
The failure loads for the T6 vs. T10 functional unit displayed a significantly higher (p<0.01) correlation than those of the T10 vs. L3 (Fig. 2) and the T6 and L3 unit (Table 2), respectively. This was also the case for separate analyses of men ( p<0.01) and women ( p<0.05 for T6 vs.T10 compared with T10 vs. L3, and p<0.01 for T6 vs. T10 compared with T6 vs. L3). Although the correlation between the T6 and L3 unit was somewhat lower that between T10 and L3, the difference was not statistically
Discussion
In the current study we explored the hypothesis that DXA-based estimates of failure loads in the thoracic spine are compromised by skeletal heterogeneity between lumbar and thoracic vertebrae and by artifacts in determining the actual bone mineral status of lumbar vertebrae. The current study is the first to compare the correlation of in situ vs. ex situ lumbar spinal DXA with thoracic vertebral failure loads. We found a relatively high correlation between mechanical failure loads of thoracic
Acknowledgements
We would like to thank Claus C. Glüer, (Medizinische Physik, Klinik für Diagnostische Radiologie, Kiel, Germany) for his help in organizing the reading of the spinal X-rays, and Günther Delling and co-workers (Abteilung Osteopathologie, Universitätsklinikum Eppendorf, Hamburg, Germany) for the histomorphometric analyses. Gudrun Goldmann and Nadine Krefting (Forschungsgruppe Muskuloskelettales System, Anatomische Anstalt München) are also acknowledged for their help with radiography, DXA
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