To The Editor:
We would like to applaud Gill et al. for their attempt to better clarify how to determine ankle stability in the presence of an isolated lateral malleolar fracture with the use of gravity stress radiographs. In their article, "Comparison of Manual and Gravity Stress Radiographs for the Evaluation of Supination-External Rotation Fibular Fractures" (2007;89:994-9), they conclude that the gravity stress radiograph is equivalent to the manual stress radiograph for determining deltoid ligament injury in association with an isolated fibular fracture. However, after closer examination of their statistical results, we believe this conclusion to be uncertain. Using gravity stress radiographs, the authors were able to find only a 0.74-mm average increase in medial clear space (from 4.26 mm to 5.00 mm) when comparing patients who had a supination-external rotation (SER) type-II fracture with patients who had a SER type-IV fracture. It is stated in the article that the p value for this difference is <0.05. This is a small increase in medial clear space, and while the stated p value implies significance we would submit that a 0.74-mm average increase in the medial clear space is not clinically relevant. One would expect a larger increase in medial clear space between an SER type-II injury and an SER type-IV injury if the incompetent deltoid ligament is being adequately evaluated.
The conclusion that the authors propose is further in doubt when one considers the reported standard deviation for the measurement of the medial clear space on the gravity-stress radiographs in the SER type-IV group. This standard deviation was 1 mm, which is larger than the average increase in the medial clear space (0.74 mm) found between the SER type-II and SER type-IV groups with use of gravity stress radiographs. This is a curious finding when one considers the authors' implication that the study holds adequate power. In fact, if a post hoc power analysis for the changes in the medial clear space on the gravity stress radiographs is performed with use of the authors' stated alpha of 0.05 and beta of 0.10 and with an increase from the average of 4.26 mm in the SER type-II group to 5.00 mm in the SER type-IV group, it is evident that the study is underpowered. Therefore, it needs to be stated clearly that there is a substantial chance for a Type-II error in this study, and that the strong conclusions stated are in question.