Assessment of tibial fracture healing with dual energy X-ray absorptiometry: influence of metallic implants at the fracture site

Abstract

Evaluation of the progress of fracture healing traditionally combines manual fracture manipulation with a visual radiographic assessment. This approach can lead to satisfactory healing, but trouble can ensue when the type of fixation compromises the healing assessment, and in fractures of long bones such as the tibia, where a high incidence of delayed union is still a problem. Dual-energy X-ray absorptiometry (DXA) offers a safe, accurate, repeatable and non-invasive method of measuring bone mineral density (BMD), a quantity which is directly related to the mechanical properties of bone. DXA has been successfully used to monitor BMD changes around fractures treated by external fixation. Nine patients with fractures stabilized m this way have been scanned at monthly intervals, and two distinct patterns of healing have been identified. Before this technique could be extended to fractures treated by internal fixation, the effect of large areas of radio-dense material m scans had to be established. To judge the effect of steel implants on scanning and image analysis, tests were conducted on steel and aluminium phantoms in three geometries - parallel rectangular blocks, a square rod in a square case, and a circular rod m a circular case. A perspex base was used in all scans. ANOVA (analysis of variance) performed on the BMD data from scans with and without steel indicated that steel had no effect on scanning (5% significance level). These results implied that steel implants would not affect the reliability of DXA scanning, and that this method of monitoring healing could be used with fractures stabilized by internal fixation. On this basis several patients with nailed tibial shaft fractures have been scanned and their scan images analyzed.