## 1 Introduction  
Open-wedge high tibial osteotomy (HTO) for knee deformity correction can be performed either with open reduction and internal fixation (ORIF) or with the use of an external fixator. Several types of plates (e.g., short or long, locked or unlocked, with or without metal block) are mostly used for knee deformity correction [1; 2; 3; 4]. Although plates are better tolerated by patients and require less-frequent radiographic follow-up, the introduction of circular frames has revolutionized limb lengthening and deformity correction procedures due to the ability of the external fixators for gradual postoperative deformity corrections with soft tissue sparing and no retained hardware [5; 6; 7; 8; 9; 10; 11]. Ring fixators are based on the principle of allowing axial micromotion at the fracture/osteotomy site with weight-bearing, leading to callus formation and subsequent bone healing [12; 13; 14]. The mechanical behavior of external fixators under applied weight loads affects the biomechanical environment at a fracture/osteotomy site, and thus the bone healing process. Most recently, the development of the Taylor Spatial Frame (Smith and Nephew, Memphis, TN, USA) (TSF) provided unique capabilities of bone deformity correction based on a computerized programming of the Stewart platform that TSF incorporates [15]. TSF is based on a hexapod system of six triangulating distractors. This platform allows a wide range of movements in three dimensions by the adjustment of the length of the distractors. The TSF external fixator and its accompanying web application provide the ability for pre-operative deformity planning and ongoing multiplanar corrections post-operatively. The clinical and functional outcomes with the application of TSF are encouraging [15; 16; 17; 18; 19; 20]. When applied in HTO, the TSF external fixator has unique advantages for managing patients with medial compartment knee osteoarthritis. It can accurately adjust the mechanical axis while providing an effective and safe fixation. The TSF ring fixator furnishes a viable treatment option for managing developmental tibial ava in children, tibial malunion and nonunion, and medial compartment arthritis in adults [21; 22; 23; 24]. Although the TSF's clinical value proves to be significant, the contribution of its mechanical characteristics to the bone healing process, which depends on the TSF element configuration, is not yet sufficiently understood. The TSF rings are connected to each other by six triangulating struts instead of vertical rods, which, in certain cases, may affect the stability of the frame [25]. In clinical practice, the length adjustment of these struts causes the TSF rings to be almost never parallel to each other. Furthermore, a combination of pre-tensioned wires and threaded half-pins is used to mount the TSF rings to the bone. The mechanical effects of using either half-pins or transverse wires as mounting elements to the bone in circular frames have been investigated in the literature (e.g., substitution of half-pins for wires is reported to increase overall construct stiffness in circular frames) [26; 27; 28; 29]. The mechanical response of different TSF constructs and configurations under compressive, torsional, and bending loads has been also analyzed in detail [30; 31]. However, to the best of our knowledge, the contribution of the TSF mechanical characteristics on callus formation has not been clinically evaluated nor experimentally studied in the literature. Our hypothesis is that callus formation and its maturity patterns may be substantially affected when using TSF in high tibial osteectomies (HTOs).