BoneXpert measures the amount of cortical bone in the metacarpals by the Bone Health Index (BHI). BHI is part of the CE-marked medical device, i.e. it is cleared for routing clinical use in Europe.
BHI is based on measurements of cortical thickness, T, in the three middle metacarpals. This is one of the oldest methods to assess bone mass, and it was popular in the 1960’s where T was measured directly on the X-ray films, typically in the middle of the second metacarpal. BoneXpert performs 100 independent measurements of T in three regions of interest indicated below, thereby obtaining a much better precision than the old manual method.
Several measures for bone health have been used in the past. Some have used T directly, and others have preferred the metacarpal index 2T/W, where W is the bone width. Finally Exton-Smith used T/L, where L is the bone length.
BoneXpert’s BHI does something in between, using BHI = T / (LW)0.33. As explained at great length in this publication, this BHI formula was designed to provide the optimal compensation for the highly variable size and proportion of children.
Actually, the exact definition is BHI = π T (1 − T/W) / (LW)0.33
The figure above shows the BHI reference curve, i.e. the average BHI versus bone age, constructed from healthy Dutch children studied in 1997. BoneXpert computes BHI standard deviation scores (SDS) accordingly. Notice the rapid increase of BHI at puberty, illustrating the importance of assessing BHI as a function of bone age rather than age.
As a result of the large number of measurements underlying BHI, its precision (coefficient of variation, or CV) is 1.4% at age 10. This error is due mainly to variations in the way the hand is placed on the imaging detector. Since the standard deviation of BHI at fixed bone age is 7.5%, this CV corresponds to a precision of BHI of 0.19 SDs, or 0.19 units of BHI-SDS.
In growth hormone deficient children it was found that BHI-SDS increases on average 0.74 in the first year of treatment, which means that this change can be readily measured in the individual child (see DD Martin, C Heckmann, MB Ranke, G Binder, HH Thodberg, Differentiation of GH effects on metacarpal bone in children with GHD, ICCBH5, Cambridge, UK. June 23-26, 2009 Bone, Volume 45, Page S60)
BoneXpert automatically rejects the BHI determination if the bones are reconstructed inconsistently, or if the image quality is insufficient, e.g. due to blur of lack of contrast.