Axes of the lower limb. Angle A represents knee mechanical physiologic valgus angle of 3°. Angle B represents tibia shaft angle, that is in 3° of varus from knee transverse axis. Angle C corresponds to angle between femoral anatomic and mechanical axis (6° of valgus). Femoral anatomic axis could be easily determined by two points located at the centre of the shaft. Mechanical axis of the lower limb passes near or through knee center and lies from femoral head center to ankle center. 

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... knee arthroplasty (TKA) is commonly considered to be a reliable procedure, with implant survival rates higher than 90% at 10 to 15 years of follow-up. The goal of total knee replacement surgery is to relieve pain and obtain better knee function, those achieved by correct patient selection, pre-operative deformity, implant design, correct surgical technique and patient participation in the rehabilitation protocol (Nizard et al, 2002). Several technical requirements during TKA are important to obtain good results:  correction of deformities;  achievement of functional joint motion and stability;  optimal balancing of soft tissues;  satisfactory alignment in the frontal, sagittal and horizontal planes. From literature data alignment in frontal plane must be into 2o or 3o range around a neutral alignment; this thought is demonstrated by Ritter at al who observed that prostheses implanted in varus position had a lower survival rate than prostheses implanted in a neutral or valgus position (Ritter et al, 1994); moreover Jeffery at al observed that when mechanical axis was in 3o valgus-varus range, the loosening rate was 3%, whereas it’s 24% when the alignment was out of this range (Jeffery et al, 1991). The alignment in the horizontal plane is of particular importance for extensor mechanism stability, patellar wear, tilted patella, prostheses dislocation or loosening. In a study of Berger et al it was observed that patients with extensor mechanism problems have internal rotation of the femoral and tibial components (Berger et al, 1998). Technically, there is a definite relationship between the accuracy of implant positioning and long-term durability (Jeffery et al, 1991; Stulberg et al, 2002): the position of prosthetic components and, consequently the alignment of mechanical axis, could be the cause of polyethylene wear due to overload stresses, ending finally by prosthetic loosening. The postoperative mechanical axis of the lower limb should be a straight line passing through the center of the hip, the center of the knee, and the center of the ankle; so that satisfactory position of a TKA prosthesis is commonly accepted to be an alignment within 3° from this neutral axis (Fig. 1). To improve precision of implant positioning, various mechanical alignment guides are used, both intramedullary and extramedullary, but technical errors with these conventional surgical techniques still occur. Moreover, mechanical alignment and sizing devices presume a standardized bone geometry that may not be applied to all patients. Even the most elaborate mechanical instrumentation systems rely on visual inspection to confirm the accuracy of stability and of limb and implant alignment at the conclusion of the TKA procedure. Computer assisted surgery (CAS) for TKA was firstly introduced to improve the accuracy of alignment of the implanted prosthesis, thinking that it could make an inexperienced or occasional TKA surgeon performing more like an expert TKA surgeon, or to address the limitations inherent in mechanical instrumentation systems used for total knee replacement surgery (Jeffery et al, 1991; Stulberg et al, 2002). During the last Decades CAS instrumentations have been improved in accuracy and various studies have been made to analyse results using this technique in TKA surgery. Advocates of this technique in total knee replacement claim benefits in terms of improving accuracy for alignment of the leg and orientation of the components, as well as a reduction in blood loss and a lower rate of intracranial micro emboli compared with traditional surgery. The survival rate for modern total knee artroplasty is reported between 80% to 95% after 10 years of follow up (Buechel et al, 2002; Robertsson et al, 2001), and the most important factor of failure is malalignment of mechanical axis (Jeffery et al, 1991; Rand et al, 1988). Recently the introduction of CAS have gained up improvement in post operative mechanical alignment (Bathis et al, 2004; Chauhan et al, 2004; Chin et al, 2005; Decking et al, 2005; Haaker et al, 2005). However, no clear published results associated with superior clinical and patients perceived functional results and consequently longer survival rate (Stulberg et al, 2006). The history of CAS for total knee replacement was dated back to the middle nineties (Picard et al, 1997). Intraoperative navigation in total joint replacement began in 1992, when W. Barger, in Sacramento (California) performed the first computer assisted surgery in orthopaedics for total hip replacement, while the first total knee replacement began in France, in January 1997, by F. Picard and D. Saragaglia after a study on cadavers (Picard et al, 1997; Saragaglia et al, 2001; Delp et al; 1998) and then started a prospective randomized study comparing the computer assisted technique to the conventional surgery in 50 patients. The postoperative mechanical axis was 181.2° ± 2.72° in CAS group and 179.04° ± 2.53° in conventional group, with a statistical significant value in favor of CAS group and reduction of outliers. The mechanical axis was in fact between 177° and 183° in 75% of patients in conventional group and in 84% in CAS group (Saragaglia et al, 2001). The Authors concluded their paper saying that computer-assisted surgery for total knee arthroplasty provides remarkably reliable results and that “once the growing pains of this new material have been mastered, all surgeons should be able to expect an improvement in the positioning of prosthetic implantations”. Bathis et al. in a prospective study compared an imageless navigation system to conventional methods using an intramedullary femoral guide and an extra-medullary tibia jig. They reported the postoperative mechanical axis to be within 3° of varus or valgus in 96% of the navigation cases versus 78% of the conventional cases (Bathis et al, 2004). Other study by Chauman et al. in which they compared a computer-assisted knee arthroplasty with the current conventional jig-based technique in 70 patients randomly allocated to receive either of the methods. All the patients were evaluated postoperatively with computer tomography imaging observing a significant improvement in the alignment of the components using computer-assisted surgery with regard to femoral varus/valgus (p=0.032), femoral rotation (p=0.001), tibial varus/valgus (p=0.047) tibial posterior slope (p=0.0001), tibial rotation (p=0.011) and femoral-tibial mismatch (p=0.037). The Authors reported that computer-assisted surgery took longer time with a mean increase of 13 minutes regarding the conventional technique, but the blood loss was significantly lower (Chauchan et al, 2004). A significant number of recent randomized controlled trial studies compared the use of imageless CAS with conventional methods; the results of these studies are shown in Table 1. Existing computer assisted surgery system must allow the accomplishment of the objectives above described: ensure optimal positioning of the prosthesis in the three planes (frontal, sagittal and horizontal); ensure optimization of the ligament balance; maintain joint stability (Nizard et al, 2002). Firstly, computer-assisted ...