Assessing the influence of measurement level on the deltoid tuberosity index in proximal humerus fractures

BACKGROUND: The deltoid tuberosity index (DTI) value of 1.4 is the clinically relevant threshold for low local bone mineral density (BMD) of the proximal humerus. Since possible changes across the DTI threshold of 1.4 could influence treatment strategies for proximal humerus fractures, the purpose of this study was to find out whether and how the DTI values are affected by variable measurement positions. METHODS: The correct measurement position for the DTI was determined on standard shoulder x-rays of 40 patients. Afterwards three observers placed lines on the inner and outer cortical bone contours. Analysis was performed with the R Statistical Package, the images were imported and converted to data frames. DTI was calculated at all levels from -25 to +25 mm (a total of 500 values) and plotted as line plots. RESULTS: 40 radiographs (32 female and 8 male patients) of acute proximal humerus fractures were analyzed. The median patient age was 72 years. The DTI decreased by 0.29 within the range of ± 25 mm. Within the range of ± 5 mm the DTI decreased by a mean of 0.07. That is equivalent to 91% accuracy, considering errors by crossing the critical threshold of 1.4. CONCLUSIONS: This study shows that in proximal humerus fractures the DTI can be measured in a safe zone of 5 mm above or below the correct level with a high accuracy of 91%.