## 1 Introduction  
Achieving the correct alignment in total knee arthroplasty (TKA) is an important factor to restore biomechanical functions of the knee (such as soft tissue balancing and joint line) and prevent early implant failure, thus guaranteeing long-term survival of the implant [1; 2; 3]. In detail, the alignment of the prosthesis components on the frontal plane strongly influences the distribution of tibo-femoral contact forces between the two tibial plateau compartments: biomechanical studies report that varus misalignment causes overload on the medial side while valgus misalignment leads to increased tibo-femoral force in the lateral side [4]. As a result of these deviations, the non-physiological loading of the knee enhances degeneration of the joint and causes overloads on the bone-implant interface and in the bone itself. Alterations of the natural alignment, therefore, may lead to early aseptic loosening, polyethylene wear, erroneous patelltofemoral tracking, instability and infection [2; 5]; all of these factors are usually associated with TKA failure, which consequently leads to the need of a revision total knee replacement. Together with alignment, the choice of an adequate level of constraint is essential to ensure successful clinical outcomes and long-term TKA survival [6]; this is even more important when dealing with revision surgeries. The rotating hinge (RH) prosthesis represents a common solution among the different models available for revision procedures, as thanks to this degree of freedom around the tibial axis, this model is able to achieve reduced shear stress at the bone-cement interface, compared to other devices [7; 8; 9; 10; 11]. It is important to mention that this kind of implant can also be used as an alternative to primary TKA, in case of complex situations, such as extreme joint deformities [7; 12]. The main issues addressed with the use of a RH TKA are therefore large ligamentous instability, severe bone loss, distal femoral or proximal tibial defects (resulting from injury or tumor) and severe varus or valgus deformities [13; 14; 15]. The design of a rotating hinge prosthesis usually consists of a femoral component and a tibial one (constrained among each other with a rotating hinge mechanism) and of a polyethylene insert placed between these two elements, in order to prevent luxation without reducing the range of motion [16]. The current designs are available in multiple sizes and present modularity, with sets of different features/stem lengths to allow for the different possible fixation techniques; this variety of possibilities represents thus a worthwhile opportunity, especially in revision patients where deficiencies of the bone cannot be totally predicted [17]. Stem length, among the other parameters, covers a crucial role in the overall success of the reconstruction. Usually, the choice of which length to use is largely determined by the stem fixation technique selected: in modern revision TKA, the options are usually fully cemented and press-fit fixation [18; 19; 20; 21]. Despite its advantages, this kind of implant also implies the relative risks of mechanical failure and infection [9; 22; 23; 24], which can furthermore increase in case of eventual patient joint deformities. The level of constraint provided by the rotating hinge, paired with the use of different stems and/or fixation approached, may therefore lead to eventual over-constraining issues affecting the tibo-femoral and/or the bone-implant interactions; a more comprehensive guideline on the suggested use and optimal configuration of these implants (when mainly aimed to address high deformities) is however still missing. The present study intends to help and support clinicians and specialists, assisting the decision process concerning the length and type of stems to use in case varus-valgus deformity is present in a patient.