## 4 Discussion  
The aim of our study was to investigate the viability of the Orthofix Limb Reconstruction System (LRS)(r) in the dynamic compression mode. Although it is well accepted that the Orthofix ProCallus fixator was designed to incorporate controlled axial compression at the fracture site [1; 25], the LRS external fixator it is also often used in the dynamic compression mode. Actually, the LRS fixator is especially indicated for bone correction through the techniques of bone transport, compression-distraction, partial acute shortening and transport, multifocal surgery, and bifocal lengthening [5; 13; 15; 16]. However, for these clinical situations, patients are encouraged to perform partial weight-bearing exercises, such as walking with crutches, as early as the third or fourth day after the surgery [5; 14] and, afterward, in the consolidation phase, to promote dynamic compression of the callus; releasing one of the clamps from the fixator rail, while the others remains locked, allows the LRS fixator compression mode. There are some published studies reporting comparisons of functional and clinical performance of the LRS system with other external fixators, such as Illizarov and AO fixators [5; 14; 15; 26]. The generality of these works concludes that the LRS fixator offers several advantages, such as ease of application, versatility, stronger fixation, less fixator related complications, early weight bearing, and early bony union. However, in their reports, researchers have not explicitly provided information of fixation strategies, such as the distance between the fixator body and the tibia axe or, even, the average value of bending span of Schanz screws, the number of pins in each bone segment, etc., thereby limiting the applicability of their results to the development of a strategy that optimizes the stress of pin-bone interfaces and the interfragmentary displacement. In fact, fracture healing by callus formation normally happens when the periosteal callus becomes stiffer, reducing interfragmentary movement while the loading amplitude is kept constant [27]. Under external fixation, the amount of interfragmentary motion is dependent not only on the fixation system, but also on other equally important factors, such as the inherent stability of fracture, the accuracy of fracture reduction, the amount of physiologic loading, and the performance of pin-bone interfaces [28]. Hence, the latest three factors were controlled in this study to investigate how the dynamic compression of the fracture site was influenced. The experimental setup was developed using a simplified model of a human tibia, consisting of a nylon bar with 30 mm of diameter. The bone callus was included in the experimental setup by means of a load cell-based system, consisting of two carbon epoxy laminated composite plates with a final stiffness of 220 N/mm. The experimental results have shown that it was possible to quantify the load transferred to the fracture focus, during tests, and register the sensibility of the results to the distance between the external fixator and the bone. The results of the symmetric configurations, shown in the Table 4, indicate that increasing the distance between the bone and fixator body from 50 mm to 70 mm increases the percentage of the load passing through the callus by approximately 6% to 11%. This behaviour could be justified by the global stiffness variation of the external fixator. A smaller free pin span makes the whole structure stiffer than a larger one [29; 30], and for any given regenerate stiffness of callus, a more flexible fixation tends to increase the ratio of the total load supported by the callus [31; 32]. Nevertheless, the results of Figure 5, and Tables 3 and 4 show also that increasing the axial distance from 70 mm to 90 mm does not change load share ratios very much. Moreover, the values of the interfragmentary deformation presented in Table 3 for the AC configuration agree with the previous conclusion.