## 1 Introduction  
Despite much progress in implant design, surgical techniques and early rehabilitation, a significant proportion of patients remain unsatisfied with total knee arthroplasty (TKA). The implant positioning in the three planes of space during total knee arthroplasty (TKA) is crucial to obtain satisfying functional outcomes, few revisions, and a high survival rate [1, 2]. New or \"modern\" concepts about the alignment are a current question in the improvement of TKA outcomes. However, regardless of the type of alignment (mechanical, kinematic, limited kinematic), the accuracy of the tibia and femur cuts is still crucial.  
This is especially important for kinematic alignment. A 3\({}^{\circ}\) error in component alignment has serious consequences if the target alignment is already in varus or valgus.  
A previous study has shown that TKA performed with extramedullary guides has a correct coronal alignment (more or less 3\({}^{\circ}\) around neutral mechanical axis) in only 70 to 80% of cases [3]. Recently, new technologies have been developed to improve the accuracy, reliability, and reproducibility of knee arthroplasty implantation, such as personalized cutting guides [4], computer-assisted navigation [5], or navigation with robotic assistance [6; 7; 8]. Accelerometer-based sensor navigation has also emerged, using an independent mass-spring system that measures linear acceleration in all three space planes. Accelerometer-based navigation is a portable surgical navigation system that does not use a large computer console for TKA (Figure 1). The first validation study about this system was in 2011 (cadaveric study) [9]. Accelerometer-bases navigation is a hand-held, sterile device used within the operative field to determine the resection planes of distal femoral and proximal tibial cut, analyzing the hip rotation center and the femoral mechanical axis. These wireless, imageless systems collect data intraoperatively and display the data directly on pods attached to the femoral and tibial resection guides. These systems guide resection angles in the coronal and sagittal planes, and also can confirm alignment accuracy of cuts after resection. Unlike other assistive technologies, this system does not need intra-bone captors or preoperative 3D imaging. All these navigation technologies aim to make prosthetic positioning more accurate in usual situations by choosing angular values for bone cuts, but also to be able to bypass obstacles to conventional techniques (bone callus, extra-articular deformity, intramedullary material).  
Computer-assisted navigation has already proved its benefits in these particular situations. Still, its usefulness in first-line surgery remains debated because of its questionable clinical benefits and high cost [5; 10]. Accelerometer navigation has the advantage of low cost, small size, and the lack of intraosseous sensors as conventional navigation systems. This navigation system has been described with an accuracy equivalent to or superior to standard mechanical instrumentation [11]. Different models of accelerometers exist with a precision that seems to be demonstrated [12; 13; 14; 15; 16; 17]. However, some systems' accuracy, like Perseus, has not yet been evaluated. The objective of this study was to assess the accuracy of the Perseus System for making the tibial cut on TKA to determine if this technique is safe and accurate. The hypothesis was that the accuracy of accelerometer-based navigation is non-inferior to mechanical conventional instrumentation for tibial cut.