## 2 Materials and Methods  
### Clinical Investigation  
Fifty-five (55) patients who underwent open-wedge HTO with the use of TSF were prospectively followed radiologically and clinically from day one to the day of the frame removal (Figure 1). All patients had symptomatic medial compartment knee osteoarthritis with varus deformity. Fourteen (14) patients also had patellofemoral compartment involvement. One (1) patient was a revision case due to an incomplete medial open-wedge HTO performed with plate and allograft placement. The data regarding the patients' demographics are summarized in Table 1.  
The proximal reference ring was always applied first and placed parallel to the articular surface of the tibia in all patients, while the distal ring was orthogonal to the diaphysis. Frame configuration in all patients consisted of a proximal open ring, 180 mm in diameter, and a distal full ring, 155 mm in diameter. A combination of one or two K-wires and two half-pins was used for the fixation of the proximal ring to the bone, while the distal ring was fixed to the bone with two or three half-pins, according to the surgeon's preference. The osteotomy was performed after the complete fixation of the frame to the bone and the application of all six struts. Due to limited space provided between struts for surgical manipulations, the two anterior struts were removed before starting the osteotomy. After the completion of the osteotomy, these two struts were reapplied at their initial length. HTO was performed through a small anterior incision with low-energy drilling and the use of an osteetome for cracking the tibial cortex (Figure 1). Effort to preserve periosteum was made in all cases. The site of the osteotomy was as proximal as possible, just distal to the tibial tuberosity, due to limited space provided by the proximal ring placement. A biplane osteotomy was performed in nine patients for patellofemoral compartment decompression. The TSF's web application was used in all patients in order to analyze the deformity and calculate the required correction in all planes of the deformity. The configuration setup, including mounting and strut parameters, the number of wires and half-pins and their positioning, the relative angulation of the two rings at the beginning and the end of correction procedure, and the ring to bone offset calculation, was extracted from the device software and is described in more detail in Section 2.2. A latency period to start the correction process was applied to all patients, and an immediate non-weight-bearing mobilization was allowed. Full weight-bearing started by the end of the correction program and according to callus formation progress. The topography of the callus formation and the maturity patterns were analyzed in serial radiographs. Anteroposterior (AP) and lateral standing views of the knee joint were obtained every week during the correction period, and once a month until frame removal. The mechanical axis correction was assessed by measurement of the femorotibial angle (mechanical) (FTAm) in standing long leg alignment AP views. Pre- and post-operative measurements of the tibial slope were also made. The radiographic appearance of the callus was classified into 3 types:  
1. _Circumferential_: homogeneous callus as wide as the original bone;
2. _Eccentric_: callus formation mainly at one corner of the osteotomy gap;
3. _Absent_: only sparse calcification in the osteotomy gap. \begin{table}
\begin{tabular}{c c} \hline
**Patients’ Data** & **Total Number/Range** \\ \hline Number of patients—NoP & 55 \\ \hline Median age (years) & 51 \\ \hline Range of age (years) & 38–58 \\ \hline Gender (M/F) & 25/30 \\ \hline Medial compartment arthritis (NoP) & 41 \\ \hline Medial and patellofemoral compartment & 14 \\ arthritis (NoP) & 1 \\ \hline Revision cases (NoP) & 1 \\ \hline \end{tabular}
\end{table} 
Table 1: Patient demographics. Figure 1: (**A**) Open-wedge high tibial osteotomy performed with a small anterior incision. (**B**) Low-energy osteotomy just distal to the tibial tubercle.