Homology and heterology of cephalometric landmarks: methodological consequences.

Sir, With interest I read the study ‘Effects of maxillary protraction for early correction of class III malocclusion’ by Celikoglu and Oktay (1). The authors should be complimented for careful work. Apparently, much effort was taken to avoid traditional pitfalls in the (prospective) collection of study subjects and controls, in reporting measurement technique and in statistical procedures. But why then do conclusions from this study raise doubts? My criticism is focussed on a fundamental issue in cephalometric methodology. The authors implicitly assume that skeletal landmarks introduced into the radiographic images, when used in longitudinal or serial material in growing individuals are homologous. However, these landmarks are not homologous, but heterologous in the way they are used in the present evaluation. That condition seriously limits the potential for clinically relevant conclusions (2). The reason for the heterology of the landmarks is the dual process of skeletal articular or sutural growth displacement and periosteal remodelling simultaneously taking place during the time interval. The spatial position of all the landmarks used in the present study is directly or indirectly subject to variable displacement and variable periosteal remodelling influences. When skeletal growth remodelling is not taken into account, like in the present study, conclusions must be limited to generalized interpretation of group size/shape changes. The effect is that an explanation of differences and variation is largely impossible. In particular, this holds for the conclusions based on tooth position changes relative to the jaws and the relative contribution of ‘skeletal’ or ‘dentoalveolar’ changes (Figure 5, and conclusions 2 and 3). It is my opinion that the current search for evidence-based treatment methods includes that investigators should select a valid biologically evidence-based method of evaluation. The method available is the structural method of superimposition, based on the principles of bone growth remodelling and a range of implant-marker studies revealing natural reference markers [(3), see (4) for a review]. After structural superimposition landmarks are homologous. The application of structural superimposition in the present study would provide advanced insight in variation and result in clinically more relevant conclusions.