# Evaluation of Bone Consolidation in External Fixation with an Electromechanical System
## 4 Discussion  
The main purpose of this study was to contribute to the development of a technology able to promote callus stimulus and simultaneously predict bone fracture healing outcomes. The prototype was developed in the Orthofix(r) monoplane external fixation system, but the concept can be extended to another external fixation type where linear movement might occur. This technology is based on callus stiffness measurements that effectively diagnose and predict fracture healing outcomes [9; 34; 35; 36; 37]. Previous researchers have performed this attempt using strain gages on orthopedic plates [9; 38; 39], instrumented external fixators [34; 40; 41], and plates instrumented with telemetric sensors [37; 42]; however, these techniques are, respectively, limited by the potential development of infection, limited hardware application, and impossibility to be used to promote direct callus stimulus. This study demonstrated the possibility of establishing a relationship between energy consumption, CEMotor, by the electromechanical system used to perform callus stimulus, and the degree of osteotomy consolidation; this conclusion supports the idea of using the CEMotor variable to assess the state of callus growth. The electromechanical system can monitor the healing of regenerated bone or implement callus stimulation strategies of prescribing displacements to estimate callus consolidation and predict the removal of the external fixation system without additional diagnostic tests. Several studies that help support these conclusions have shown that the percental mechanical load transferred to the osteotomy changes during the healing process [16; 38; 43] and that during acute healing (<30 days), the osteotomies progress to the proper union will exhibit a progressive improvement of mechanical strength. In contrast, this progression of increased osteotomy stiffness is arrested and/or delayed in those trending towards nonunion [36]. Moreover, in the biomechanical quantification of callus stiffness using indirect measurements, the load is shared between the fixation device and the bone [41]. Hence, if the fixator stiffness remains constant, the biomechanical quality of the callus formation is responsible for the measured data variation. The results of Figure 7b show that the stiffness of osteotomy was the main  
Figure 13: Effective stress in the numerical models S\({}_{20}\)d\({}_{2}\). (**a**) _PA6_; (**b**) _AN_. reason for the CEMotor variable variation, which was high in the initial phase of \"healing\" and stabilized afterward. Another point worth considering is related to callus stimulus. According to indications of the medical team, the introduction of loads with a view to micromovements can be performed between fibrous tissue and bone with immature stiffness [19; 20; 22]. Several researchers [24; 44; 45] suggest that bone callus stimulation of 1 mm (1000 \(\upmu\)m) per day is beneficial to accelerate the healing process. The results of Table 4 show that the variation of the prescribed displacement at the elongator screw did not change the ratio of displacements for the same type of osteotomy material. Nevertheless, the osteotomy displacement was greater for Sawbones(r) materials with less rigidity. Still, when using materials with higher stiffness, the displacement in osteotomy was relatively reduced, which means that as the bone callus consolidated, the prescription of a constant 1 mm (1000 \(\upmu\)m) displacement led to an increasingly small displacement of the osteotomy material. Moreover, in Figure 6 is possible to identify the differences among the prescribed displacements and the displacements measured at the osteotomy, which can be used to estimate the percental load transferred to the callus [13]. The results showed that, because of the fastener distance, the friction, and the increase of the components in the assembly, it was impossible to reach the imposed displacement's totality at the osteotomy.