## 4 Discussion  
The goal of the current investigation was to evaluate the clinical application and the mechanical characteristics of TSF in terms of callus formation and maturation pattern in an HTO environment. The comparison of TSF with other fixation methods (e.g., plates) used in HTO was beyond the scope of this study. The appropriate time for frame removal is a complex decision process based on the surgeon's experience upon the TSF external fixator as well as other external fixator systems used for performing HTO in patients with medial compartment arthritis [7; 33; 34]. General convention suggests that when three of four cortices demonstrate radiographic healing, the bone has enough intrinsic stability and the fixator can be removed [35]. Our study results have demonstrated that the median time of frame removal in HTO patients was 87 days (range: 82-120 days). This removal time is smaller than the times reported in other studies using TSF [23; 36]. [PERSON] et al. [36] report a median time in the frame of 18 weeks (126 days), whereas [PERSON] et al. [23] mention a longer time, reaching 23 weeks (161 days). The reported differences are probably due to the different frame configurations used and their impact in callus formation and maturity time. Although proper bone regeneration and time for frame removal depend primarily on specific patient characteristics, several important parameters may be considered that assist callus development in fractured bone surfaces, and were investigated here using a suitable experimental method of the TSF. From the clinical point of view, the literature findings suggest that a suitable interfragmentary strain is required, which allows the micro-movement between fractured bone parts and assists the development of local pressure in  
Figure 7: Load displacement behavior of TSF with different configurations [32]. the fractured bone surfaces, creating a callus formation pattern according to the applied pressure. The magnitude of applied local pressure, which stimulates bone growth, depends on the interfragmentary strain as well as the stiffness of the TSF. It has been reported that an interfragmentary strain of less than 10% between bone ends was necessary for desirable fracture union [37]. [PERSON], in his experiments on canines, reported a direct correlation between frame stiffness and bone regeneration [13]. The local strains at the wedge-shaped space created at the osteotomy site are asymmetrically distributed and induce both mechanical and biological stimuli that lead to a specific pattern of callus formation [38]. The combination of half-pins with tensioned wires in circular external fixators has an additive effect on frame stiffness [25; 39]. In the present study, we showed that the individual contribution of wire and pin elements on TSF fixator stiffness was similar. Hence, the use of pretensioned wires or half-pins to achieve a specific stiffness value still depends on clinical considerations for the adequate number and geometric characteristics of the transfixing components for each type of deformity. Additionally, removal of a wire or half-pin results in a respectable change in stiffness. This change was quantified, and may be used to assist clinical practice. In terms of callus formation, there is no objective method that can be suggested to determine that a callus is biomechanically solid enough to withstand physiological loads. In our study, the callus formation pattern developed in all HTO patients was eccentric. The clinical findings revealed that the consolidation process was advancing from the point of maximum bone to bone contact, at the posterolateral corner of the osteotomy, leaving a small anteromedial gap by the time of frame removal in most cases. The results obtained from the experimental model indicate that vertical deflection of the upper bar during weight-bearing in the TSF fixator is accompanied by a rotation of the bar axis.