Use And Limits Of Dual X-Ray Absorptiometry For Diagnosis And Follow-Up Of Patients With Osteogenesis Imperfecta.

G. Chevrel, F. Duboeuf, P.J. Meunier
Department of Rheumatology and Bone Diseases, Edouard Herriot Hospital, 69437 Lyon Cedex 3, France.

Dual energy X ray bone densitometry (DXA) is an established tool for the diagnosis of post-menopausal or secondary osteoporosis in adults, where bone mineral density (BMD) has been shown to be significantly associated with the risk of future fracture. A large number of sites have been used to assess BMD or bone mineral content of the whole skeleton or particular appendicular (radius, calcaneus) and axial sites (lumbar spine, hip). In osteoporosis, the different sites of measurement do not differ considerably in their ability to predict future fracture. In osteogenesis imperfecta (OI), where bone brittleness is responsible for fractures caused by minor traumatisms during infancy but also in adult patients and where mutations of type I collagen genes affect the quantity of bone but not the average mineralization density, no prospective study has confirmed the ability of BMD to predict fractures. The WHO definition threshold proposed for post-menopausal osteoporosis (T score < -2.5) has not been confirmed in OI. The precision of DXA in OI is about 2.2% in lumbar spine and 1,2% in femoral neck. Quantitative ultrasonography is one of latest additions to the diagnostic toolbox of osteoporosis and consists in speed of sound (SOS), as well as broadband ultrasound attenuation (BUA) measurements. It has not yet been used in OI.

In adults with OI, most patients have low or very low BMD values. For example, in 39 adult patients recruited in our center according to Sillence classification (30 OI-type I, 2 OI-type III and 7 OI-type IV) the mean lumbar of BMD was not significantly different between the subgroups. Expressed in T scores these mean lumbar values were for Type I : -3.2 ± 1.4, for Type III : -4.1 ± 0.8 and for Type IV : -3.2 ± 1.5. However, normal values (T score > -1 from WHO grades) were found in 3 cases. In OI patients, DXA also allows to estimate with precision BMD and to follow the bone status of these patients and is of major interest for monitoring once a year the effects of treatments. For example we found that fluoride salts could increase lumbar BMD by 6,1 % after 24 months in 15 adult patients. In children, pamidronate may increase BMD of 41.9%/year in mean (Glorieux F. N Engl J Med 1998;339:947-52). Measurement of BMD may also help to diagnose OI if child abuse is discussed or for an early post-natal detection of the disease.

There are a number of problems and limitations in the pratical application of bone mineral measurement in OI patients. The BMD of the lumbar spine is an area-related measurement that affected by both true BMD and the volume of the vertebral body. In growing children, the area-related BMD increases and this increase must induce caution in individual variation especially during therapy. Possible difficulties are also scoliosis, who is frequent and often severe, and deformations of vertebrae after crush fracture. As in other conditions the presence of osteophytes, aortic calcifications, degenerative facet hypertrophy and intervertebral disc space narrowing in degenerative disc disease, may artificially increased BMD in the posteroanterior measurement of the lumbar spine. Another difficulty is the presence of osteosynthesis materials or postfracture callus in lumbar spine and in femoral neck.

Reference: Proceedings of the 7th International Conference on Osteogenesis Imperfecta. Montreal, Canada, 1999.