A comparison of deformity correction capabilities in hexapod frame systems

Abstract

Context: Hexapod fixators can be divided into two basic design groups. One group consists of frames that use ball and socket joint struts attached to the outer surface of the rings. The other group consists of frames that use cardan type universal joint struts attached to the under surface of the rings. Aims: To compare the ability of different hexapod fixator systems for deformity correction. Settings and Design: Nearly, identical two-ring frame constructs were compared to determine if there was any difference in deformity correction capability between cardan type universal joint struts and ball and socket joint struts. Materials and Methods: Maximal deformity was created using the software for each of the frame constructs in all six planes of deformity (angulation, translation, and rotation in the coronal and sagittal planes). Clinical scenarios were also compared (equinus contracture, moderate Blount disease, and severe Blount disease) and the number of strut changes necessary to correct the deformity were recorded. Results: For the small and medium-sized struts, the angular deformity corrections were similar, but the cardan type universal hinges had a greater capability for correcting translational deformity and rotation than the ball and socket joints. However, the amount of lengthening possible was greater for the ball and socket joints with these strut sizes. In the largest size of struts, the ball and socket joints had greater range in every category except rotation. In patients requiring significant rotational correction, the cardan type universal joints were found to impinge on the soft tissues 13° earlier than the ball and socket joints (39° vs. 52°). A Blount disease case with moderate multiplanar deformity and an equinus correction of 45° required the same amount of strut changes for each design. For the Blount disease case with severe multiplanar deformity, the cardan type universal joint struts required six total changes, whereas the ball and socket joint struts required only one strut change and two strut adjustments to achieve the same correction. Conclusions: Both the cardan type universal joint and the ball and socket joint hexapod frame designs allow substantial multiplanar corrections to occur. In the smaller size struts, the cardan type universal joints allow more translation and rotation, whereas the ball and socket joints allow more length. For large rotational corrections and frames built with 90° of offset, the ball and socket joint design is better at avoiding soft tissue impingement. While both systems are comparable with mild to moderate deformity correction, the ball and socket joint design allows more correction with less strut changes for patients with severe deformity in our experimental construct.