Three-dimensional measurements of the lower extremity in children and adolescents using a low-dose biplanar X-ray device.

OBJECTIVE: To evaluate three-dimensional (3D) measurements of the lower extremity using a biplanar low-dose X-ray device in children and adolescents.

METHODS: Firstly, 3D measurements of eight dried bones were analysed by a biplanar low-dose X-ray device (LDX) using stereoscopic software and compared with 3D computed tomography (CT). Secondly, 47 lower limbs of children and adolescents were studied using LDX two-dimensional (2D) and 3D measurements. Both parts were evaluated for femoral and tibial lengths and mechanical angles, frontal and lateral knee angulations, and the femoral neck-shaft angle.

RESULTS: The 3D specimen comparison between LDX and CT measurements showed no significant differences: femoral length (P = 0.069), tibial length (P = 0.059), femoral mechanical angle (P = 0.475), tibial mechanical angle (P = 0.067), frontal knee angulation (P = 0.198), lateral knee angulation (P = 0.646) and femoral neck-shaft angle (P = 0.068). The comparison between LDX 2D and 3D measurements showed significant differences in tibial length (P = 0.003), femoral mechanical angle (P < 0.001) and femoral neck-shaft angle (P = 0.001); other parameters were unremarkable.

CONCLUSIONS: The 3D LDX system presented reliable measurements compared with 3D CT. Differences between LDX 2D and 3D measurements were noted in the femoral mechanical angle, femoral neck-shaft angle and tibial length. Moderate to good interobserver agreement for the 3D LDX measurements were found.

KEY POINTS: Low radiation dose is essential when assessing potential lower extremity discrepancies. A new biplanar low-dose X-ray device can assess such discrepancies in children/adolescents. This LDX device provides equally reliable 3D measurements as prevalent practice LDX measurements carry good overall interobserver agreement.