Correction of deformities with conventional and hexapod frames--comparison of methods

Abstract

Distraction osteogenesis for the correction of deformities with an external fixator is well established. The hexapod principle of robotic technique was invented for the Ilisarov apparatus (e. g. Taylor-Spatial-Frame/TSF). Treatment with conventional frames needs a patient-customised frame mounting. This demanding procedure is markedly reduced using this technology. The aim of this study was to analyse the value of the hexapod principle in external fixation. The potential of a frame to correct deformities is limited by its work space. The geometry of a conventional frame is different from the geometry of a hexapod frame, which is the reason for their different work spaces. The work space of the hexapod frame is compared to the work space of a conventional frame. Important parameters for this analyses are minimal and maximal frame heights and the potential of correction. The minimal frame height of hexapod fixators is higher compared to conventional Ilisarov fixators. The standard hexapod frame (TSF 155 mm ring diameter) can correct 23 degrees of angulation, 36 mm of shortening, 71 mm of translation and 43 degrees of rotation without changing the telescope rods. The standard conventional frame (160 mm ring diameter) can correct 90 degrees of angulation, 100 mm of shortening, 25 mm of translation and 12.5 degrees of rotation without remounting of the frame. The different work spaces of the different frames result in consequences for their clinical application. The hexapod frame has more power to correct translation and rotational deformities than a conventional frame. Correction of extensive angulation and shortening deformities almost always needs an exchange of telescopic rods. Conventional frames are usually able to correct these deformities with the primary mounting. Because of its increased minimal frame height, the indication for hexapod constructs in child orthopaedics can be limited.