We have previously described a simple and reproducible three-dimensional technique of CT for the measurement of the cover of the femoral head in acetabular dysplasia in adults. We now describe the application of this technique in ten patients with symptomatic dysplasia to assess the degree and direction of dysplasia and to measure the cover obtained at acetabular osteotomy.

The indices obtained gave a useful indication of the degree and direction of the dysplasia and confirmed which components had been used most efficiently to achieve cover. The information is easily presented in graphical form and gives a clearer indication of the cover obtained than the indices derived from plain radiographs.

We analysed 42 weight-bearing lateral radiographs of the ankle, 20 of which were from patients with a clinical and plain radiological diagnosis of talocalcaneal coalition (TCC) who subsequently had CT. The remainder were from 22 healthy volunteers with no clinical findings suggestive of hindfoot pathology. Four observers, blinded to the CT findings, independently evaluated the radiographs on two separate occasions.

With the 95% confidence interval and using the CT findings as the comparison we calculated the sensitivity, specificity, accuracy, and positive and negative predictive values for the C-sign, and for other signs known to be associated with TCC. Similarly, we also calculated the interobserver and intraobserver reliability for these signs using the kappa statistic.

Our results suggest that the C-sign is highly sensitive and specific for TCC. It is an accurate indicator and significantly more reliable than other previously recognised radiological signs of TCC. Features of the C-sign, however, cannot be relied upon to indicate whether the TCC is fibrous or bony.