Is there any difference between tapered titanium stems with similar geometry and hydroxyapatite coating?

Purpose: Several tapered stems with similar geometry and extensive hydroxyapatite coating have recently been introduced. It is not clear, however, whether they share the same design or whether they exhibit any difference that might affect their clinical performances. In this study, we analysed five tapered stems fully coated with hydroxyapatite to establish whether they exhibit similar geometric features and may therefore be used indifferently when a cementless stem is indicated. Methods: The length of the stem, the coronal and sagittal diameters, the length of the stem shoulder and the metadiaphyseal angle were measured. The ratio between the proximal and distal coronal diameters of the stem and that between the proximal and distal cross-sectional areas were calculated as a flare index and tapered index, respectively. Results: The proximal coronal diameter ranged between 24.9 and 28 mm in the smaller size and between 34 and 38.4 mm in the largest sizes. The proximal sagittal diameter ranged between 10.2 and 11.8 in the smallest size and between 14.4 and 17.2 in the largest. A significant difference was found between stems of different brands in the flare index, tapered index, length of stem shoulder and metadiaphyseal angle. Conclusions: Lookalike tapered stems with extensive HA coating actually exhibit significant differences in several geometric features potentially affecting their clinical performances. As a result, these stems should not be used indifferently, but rather they should be selected on the basis of the femoral morphology of the operated patient.