The difference in spine specimen dual-energy X-ray absorptiometry bone mineral density between in situ and in vitro scans.

BACKGROUND CONTEXT

Human cadaveric specimens are commonly used to evaluate bone-implant interface strength in osteoporotic spine fixation. Dual-energy X-ray absorptiometry (DXA) scans are usually carried out on explanted spine specimens to measure bone mineral density (BMD) before in vitro biomechanical studies are carried out.

PURPOSE

The purposes of this study were to verify and quantify the difference in DXA BMD between unexplanted (in situ) and explanted (in vitro) scans and to develop and validate a correction factor (CF) between in vitro and in situ DXA BMD.

STUDY DESIGN

This is a retrospective analysis of past DXA scans of explanted specimens and a repeated measure scan rescan study of in situ and in vitro spine specimens.

METHODS

Dual-energy X-ray absorptiometry scans were previously carried out on 106 male and 83 female lumbar specimens. Using multiple regressions, the correlation functions between Z score, BMD, and age were determined for male and female groups. The CF was developed based on difference in BMD between mean in vitro and population data. Next, in situ DXA scans were carried out on the lumbar spine of four full human cadavers, and subsequently, in vitro scans were repeated after explantation. The CF was applied to these in vitro scan data and the resulting corrected BMD compared with in situ scan values.

RESULTS

The specimens had significantly lower Z score than population mean. The mean Z score was -0.7+/-1.4 (p<.001) for male and -0.3+/-1.3 (p=.03) for female specimens. The difference between in situ and in vitro scans was quantified to be 0.06 g/cm(2) for male specimens and to be a function of age (6.80 Age(-0.5)-3.76 Age(-0.365)) for female specimens. In vitro BMD was 96+/-11% of in situ BMD and was significantly different (p=.04). Corrected BMD after application of CF was 97+/-11% of in situ BMD and was not significantly different (p=.13).

CONCLUSIONS

In vitro BMD scan on explanted specimens measured lower DXA values than in situ BMD scans on full cadavers. A CF when used resulted in more accurate measure of the in situ BMD.