Precision Assessment and Radiation Safety for Dual-Energy X-Ray Absorptiometry: Position Paper of the International Society for Clinical Densitometry

Fractures are a common and serious complication after stroke and the risk of hip fractures among stroke patients is 2 to 4 times greater than among other elderly people. The aim of this study was to investigate prospectively the change in bone mineral density (BMD) after severe stroke and to study the association between motor impairment, disability and the development of hemiosteoporosis. The study comprised 24 stroke patients, with no persistent paresis from previous strokes or previous osteoporotic fractures, included 1 month after stroke onset. BMD, motor function, ambulation and activities of daily living (ADL) were assessed at 1, 4, 7 and 12 months after stroke onset. At inclusion, the patients' BMD was normal for their age. During the study, there was a significant loss of BMD in the total body (-2.0%; p < 0.05), but not in the head or spine. BMD differed significantly between the paretic and the non-paretic arm at inclusion (-4.8%; p < 0.001). Decrease in BMD was most pronounced in the affected humerus (-17.4%; p < 0.001) and proximal femur (-12.2%; p < 0.01). BMD decreased significantly in both lower extremities during follow-up, but the losses were more pronounced on the paretic side (p < 0.01). In the nonaffected ultradistal radius there was a significant increase in BMD from inclusion to the end of the study (+5.8%; p < 0.01). There was no pattern in the bone losses correlating with presumptive risk factors for hemiosteoporosis such as motor function, ability to perform ADL or ambulation. Two patients had fractures at follow-up, both on the paretic side. Loss of bone mineral density in the paretic extremities is thus pronounced and progressive during the first year after stroke, indicating that loss of BMD is probably an important risk factor for post-stroke fractures. Surprisingly, BMD in the nonaffected arm increased significantly during the first year after stroke, most likely due to increased physical activity, and perhaps a redistribution of bone minerals from the paretic extremities.

Progressive hemiosteoporosis on the paretic side and increased bone mineral density in the nonparetic arm the first year after severe stroke.

In vivo short-term precision of hip structure analysis variables in comparison with bone mineral density using paired dual-energy X-ray absorptiometry scans from multi-center clinical trials.

We investigated prospectively the bone mineral density (BMD) of the proximal femur after implantation of a tapered rectangular cementless stem in 100 patients with a mean age of 60 years (16 to 87). It was determined using dual energy x-ray absorptiometry, performed one week after surgery and then every six months until the end-point of five years. The BMD increased significantly in Gruen zones 2, 4 and 5 by 11%, 3% and 11% respectively, and decreased significantly in Gruen zones 1, 6 and 7 by 3%, 6% and 14% respectively, over the five-year period. The net mean BMD did not change over this time period. The changes in the BMD were not confined to the first 12 months after surgery. This investigation revealed no change in the overall periprosthetic BMD, but demonstrated a regional redistribution of bone mass from the proximal to distal zones.

Changes in bone mineral density in the proximal femur after cementless total hip arthroplasty: A FIVE-YEAR LONGITUDINAL STUDY

RATIONALE AND OBJECTIVES: Dual-energy X-ray absorptiometry (DXA)-derived areal bone mineral density (BMD) is an established predictor of osteoporotic fractures and reflects bone strength as well. The goal of this study was to develop and validate a physical model for appropriate interpretation of BMD. METHODS: DXA and peripheral quantitative computed tomography investigations of the distal tibia (n = 45), proximal tibia (n = 12), distal femur (n = 26), and distal radius (n = 34) were carried out. The DXA-derived BMD was analytically modeled as a nonlinear function of volumetric bone mineral apparent density and the cross-sectional area (eCSA) of given bone; ie, BMD(mod) = apparent BMD x square root of eCSA. RESULTS: At every measured skeletal site, the relationship between BMD and BMD(mod) was systematically stronger than that observed separately between BMD and apparent BMD or cross-sectional area. The models (r2) explained 85%, 94%, 87%, and 74% of the variability in BMD at the distal tibia, proximal tibia, distal femur, and distal radius, respectively. CONCLUSIONS: The mutual contributions of bone density and size to BMD can vary to some extent in a site-dependent fashion. This dual nature of BMD on one hand provides a reasonable mechanical explanation for why BMD is a good surrogate of bone strength and a predictor of osteoporotic fractures but on the other hand, complicates its detailed interpretation.

A physical model for dual-energy X-ray absorptiometry--derived bone mineral density.

Precision in femoral neck scans with dual energy X-ray absorptiometry (DXA) is affected by variability in positioning and subsequent repositioning of the femur for repeated scans. To study the in vitro effect of femoral rotation on the bone mineral density (BMD), four fresh-frozen cadaveric femurs were fixed in a specially designed jig which allows for rotation of the femurs. BMD measurements of the femurs were done in neutral position (0°) i.e., with the femoral neck axis parallel to the surface of the couch and at 15°, 30°, and 45° of internal and external rotation. In vivo precision of the femoral neck scan was determined in five normal male subjects. The scans were first done with the legs positioned using the manufacturer's foot block. Five scans were performed, with repositioning, on the left hip of each subject. The procedure was then repeated with the legs positioned using a custom-designed positioning jig to minimize the rotation of the hips during a scan. In the in vitro study, the femoral neck BMD value was minimum at neutral position (0°) and increased when the femur was rotated internally or externally. In vivo precision error of the femoral neck scan was reduced by almost 50% with the use of the positioning jig when compared with the manufacturer's foot block. Femoral rotation was shown to have a significant effect on BMD measurements, and proper positioning of the femur during a scan can improve precision significantly.

J. C. H. Goh

Papers

Effect of femoral rotation on bone mineral density measurements with dual energy X-ray absorptiometry