A new point for calcaneal pin insertion: A radiographic study measuring bone density on computed tomography

External fixator schanz pins are often sited in the calcaneal mid-portion. This area is shown to have lower bone density than the posterior region. We propose a safer, more rigid site for pin insertion in the posterior fifth of the bone. This is likely to reduce the risk of early pin loosening and is a higher distance from the neurovascular structures. This study aims to evaluate differences in bone density between the traditional point and our proposed posterior point.

We identified a sample of 46 patients who underwent an ankle computed tomography (CT) scan. We excluded skeletally immature patients and metal artefact on CT. A uniform region-of-interest rectangle to measure Hounsfield Units at the traditional point and posterior calcaneal points was created using software. Data was reviewed separately by three raters. Statistical analysis was performed using the Shapiro-Wilk and Wilcoxon Signed Ranks test.

46 patients of mean age 48 were included. The average bone density at the traditional point was lower at 202 HU (95 % Confidence Interval (CI) 183-222) compared to 335 HU (95 % CI 300-371) at the posterior calcaneal point (p < 0.001). Inter-rate agreement was excellent with average measures intraclass correlation coefficient (ICC) of 0.894 for the AO point and 0.877 for the posterior calcaneal point (both p < 0.001).

Our data demonstrates that the bone density in the posterior fifth of the calcaneus is significantly higher than the traditional insertion point and there is an increased distance to the vital neurovascular structures. Previous research has verified that Hounsfield units as a measure of bone density correlates strongly with screw pull-out strength.