Cortical bone thickness measurement

CBTavg from XR: Mean average cortical bone thickness (CBTavg) was determined according to the method described by Tingart et al ¹⁸ and Mather et al ²⁰ for the proximal humerus. The method has been reported to allow an excellent intra- and interobserver reliability. ^18,20 Evaluation of the XRs and CT scans was performed in a random order by two observers (CS and SGY) who were blinded to the BMD results and the specimens. In order to assess the inter- and intraobserver reliability, all measurements were performed by each observer at two different times with an interval of at least two weeks.

The combined CBT of the medial and lateral distal radius was measured at two different levels (Figure 2). The first level (M1) was the point where the outer medial and lateral cortical borders were parallel to each other. The second level was obtained 20 mm distal to the first level (M3). A perpendicular line was drawn from the medial outer cortex to the lateral outer cortex and measured by a digital caliper (Synedra View Personal 3.4.0.2; synedra IT GmbH, Austria) with a precision of ± 0.1 mm. The width of the intramedullary canal was measured at the same levels (M2, M4). Then, the M2 distance was subtracted from the M1 distance to get the combined CBT at level one, and the second level was calculated accordingly. CBTavg was calculated as the mean of combined thickness at the two levels ((M1-M2 + M3-M4)/2).

Cortical bone thickness (CBT) measurement in a 46-year-old male: a) standard anteroposterior radiograph (XR) and b) clinical CT in coronal plane. CBT was determined on two levels (M1/M2 and M3/M4) with a distance of 20 mm (dashed line). The identical height in XR and CT was ensured by transferring the length from the styloid to the first level (scattered lines).

The differences in cortical thickness and the resulting CBTavg index are demonstrated in Figure 3. Direct comparison of two specimens, one with low and one with high BMD, reveals a clear difference in the cortical thickness resulting in a highly different CBTavg index. The cortical affection by the bone substance loss is further underlined by the corresponding HR-pQCT sections, showing a circumferential cortical reduction at the specimen with a low BMD.

Cortical bone thickness (CBT) in standard anteroposterior radiographs of two radius samples with a) low areal bone mineral density (aBMD) (0.37 g/cm²) (female, 89 years old) and b) high aBMD (0.67 g/cm²) (male, 32 years old). Apparent difference in the CBT with a mean average CBT (CBTavg) of 3.95 mm (a) versus 6.65 mm (b). CBT visualized in a high-resolution peripheral quantitative CT (HR-pQCT) (volumetric bone mineral density (vBMD)) section (scattered line/insert) and at the CBT levels (L1 and L2 black bars = cortical thickness) with the corresponding CT sections (inserts).

CBTg from XR: Gauge cortical bone thickness (CBTg) was calculated as a second index and intends to account for any effect of the total bone diameter on the BMD measurements and also for magnification differences between the XRs in the daily routine. CBTg was determined as described by Hepp et al ¹⁴ and Mather et al, ²⁰ dividing the CBTavg by the outer bone diameter of the proximal level (M1) (CBTavg/M1).

CBTavg and CBTg from CT: To assess the validity of the cortical measurements obtained from the XRs, CBTavg and CBTg were again determined from the CT scans. To account for the same level in the XR and CT measurements, a line perpendicular to the first level (M1) to the top of the radial styloid was depicted in the XR. This length was used to define the first measurement level in the CT analysis. The second level was then measured accordingly 20 mm distal of the first level (Figure 2 and Figure 3). Calculation of the CBTavg and CBTg was equally performed as described above.