Three-dimensional load measurements in an external fixator

On the basis of a six-degree-of-freedom adjustable fracture reduction hexapod external fixator, a system which can be used for measuring axial and shear forces as well as torsion and bending moments in the fixator in vivo was developed. In a pilot study on 9 patients (7 fresh fractures and 2 osteotomies of the tibia), the load in the fixator during the healing process was measured after 2, 4, 8 and 12 weeks and at fixator removal. The measured values enabled both the type of fracture to be determined as well as the monitoring of the healing process. In well-reduced type A3 fractures small axial (direction of the bone axis) forces were found in the fixator. A2, B2 and C3 fractures showed distinct axial forces, which decreased during the healing process, according to an increasing load transfer over the bone. Bending moments in the fixator showed good correspondence with the clinical healing process, except in the case of a C3 fracture. A combination of bending moment and axial force proved to be particularly suitable to assess fracture healing. In transverse fractures, the well-known resorption phenomenon of bone in the fracture gap at approximately 4 weeks was detected by the system. Compared with other external fixator load measurements in vivo, the hexapod offers the advantage of being able to measure all forces and moments in the fixator separately and with a relatively simple mechanical arrangement. In our opinion, it will be possible to control fracture healing using this system, thereby minimizing radiation exposure from radiographs. Furthermore, the measurement system is a step towards the development of external fixator systems that enable automatic adjustments of the callus mechanical situation (“automatic dynamization”) and inform the patients about the optimal weight bearing of their extremity (“intelligent fixator”).