Femoral version is the amount of twist the proximal femur has undergone in relation to the distal femur. Recently axial oblique CT and MRI measurement of version have been described. Oblique sections are taken along the axis of femoral neck and anteversion is calculated from a single slice. Does this obliquely measured angle “oblique version” represent the true version? The aim of this study is to analyze the geometry of femoral version and to establish the relationship between “oblique version” and “true version”.
Mathematical analysis of the geometry of femoral version was done and the relation between oblique version and true version was established. Formulae have been devised to correct for errors due to rotation. Oblique version and true version of 50 dry femora was measured and analysed.
Results: Significant difference was found between oblique version and true version (p=0.02). Using the formulae we were able to calculate the true version from oblique version and vice-versa with an error of +/- 1o. Graphs have been made to illustrate how oblique version is related to true version at different neck-shaft angles.
Conclusion: “Oblique version” measured in MRI and axial oblique CT doesnot correspond to the “true version”. It is a function of neck shaft angle and true version. Even though accurate and simple they may give spuriously wrong results. Four formulae have been elucidated for conversion of oblique version to true version and to correct for rotation.
Clinical Relevance: Abnormalities in the amount of version have been linked to various orthopaedic disorders. Earlier methods have estimated true anteversion only and pre-operative planning and intra-operative decisions in derotational osteotomy and arthroplasties are based on true version only. Therefore it is recommended that only true version is used and the formulae can be used for conversion.