## 4 Discussion  
The most interesting finding in this study, due to the surgeon's awareness of a cutting error, is that there was a discrepancy (1.89\({}^{\circ}\) in the femur and 1.32\({}^{\circ}\) in the tibia) between what the surgeon planned intra-operatively and the final real measure when conventional bone-cutting guides were used. The deviation in the femur and in the tibia tended to a varus cut. This result leads us to the realization that there is some uncontrollable error in terms of alignment and stability that must be considered when surgeons a plan knee replacement. Nevertheless, the amount of this discrepancy has been described in several articles as a non-affecting factor for clinical outcomes [18; 19]. Additionally, some other authors observed that the different angulation in the femoral distal bone cut does not influence its precision [20]. This mismatch in terms of accuracy lets the surgeon know that the approach must be applied with certain considerations in mind. When presented with a valgus knee in a safe zone in terms of alignment, it may be safer to stay between the limits because the tendency to error leans toward the varus. However, stability may be more difficult to achieve because the lateral compartment tends to be tighter. The varus knee must be analysed in the opposite sense (Figure 1). \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Femoral Cut**} & \multicolumn{4}{c}{**Tibial Cut**} \\ \cline{2-11}  & **Group** & \(n\) & **Mean** & **SD** & \(p\)**-Value** & **Group** & \(n\) & **Mean** & **SD** & \(p\)**-Value** \\ \hline \multirow{4}{*}{BMI} & \(<\)25 & 29 & 1.58 & 1.45 & & \(<\)25 & 29 & 0.96 & 2.11 & \\  & \(>\)25–\(<\)30 & 114 & 1.75 & 1.56 & & \(>\)25–\(<\)30 & 114 & 1.33 & 1.93 & \\  & \(>\)30–\(<\)35 & 111 & 1.92 & 1.49 & & \(>\)30–\(<\)35 & 111 & 1.37 & 2.39 & \\  & \(>\)35 & 86 & 2.15 & 1.65 & & \(>\)35 & 86 & 1.93 & 2.38 & \\ \hline \multirow{4}{*}{Preoperative alignment} & \(<\)160\({}^{\circ}\) & 7 & 1.14 & 0.89 & & \(<\)160\({}^{\circ}\) & 7 & 2 & 1.33 & \\  & 160–165\({}^{\circ}\) & 16 & 2.56 & 2.63 & & 160–165\({}^{\circ}\) & 16 & 0.86 & 2.55 & \\  & 165–170\({}^{\circ}\) & 57 & 2 & 1.30 & 0.22 & 165–170\({}^{\circ}\) & 57 & 1.05 & 2.20 & \\  & 170–175\({}^{\circ}\) & 151 & 1.74 & 1.56 & & 170–175\({}^{\circ}\) & 151 & 1.26 & 2.17 & \\  & 175–180\({}^{\circ}\) & 109 & 2 & 1.48 & & 175–180\({}^{\circ}\) & 109 & 1.48 & 2.33 & \\ \hline \hline \end{tabular} (SD: standard deviation; \(n\): sample size; BMI: body mass index).  
\end{table} 
Table 2: Mean deviation in bone cuts considering the BMI and preoperative alignment variables. Figure 1: The desired cuts (white lines) are not the final measure obtained. Uncontrollable errors (red lines) tend to close the lateral compartment. The varus tendency in both cuts (either tibial or femoral) must be considered to arrive at the optimum intraoperative ligament balance. The difference between the planned measures and the result after surgery can be analysed by taking several parameters into account. [PERSON] et al. [11] suggested that possible explanations for this discrepancy might be found in inaccuracies in the identification of landmarks, the fixation of the cutting blocks or the rigidity of the sawblade. [PERSON] et al. [21] concluded that femoral shaft bowing may have an influence on the final coronal femoral alignment due to the angulation of the femoral intramedullary guide. One question worth considering is whether cement thickness can be a cause, depending on the final compression the surgeon performs after positioning the implants. Inadequate ligament balancing may lead to an irregular prosthesis-surface contact caused by more compression on one compartment than the other [22]. In addition, the use of different conventional cutting-guide systems may be a reason for the discrepancies between them. However, in the present study, the four instruments demonstrated similarities in precision with no significant differences.