Strategies to Improve the Function, Kinematic and Implants’ Positioning of a TKA with Minimally Invasive Computer-Assisted Navigation

Strategies to Improve the Function, Kinematic and Implants’ Positioning of a TKA with Minimally Invasive Computer-Assisted Navigation Nicola Biasca1 and Matthias Bungartzに なOrthopedic Clinic Luzern AG, Hirslanden Clinic St. Anna, Luzern にKlinik fur Orthopadie und Unfallchirurgie, Lehrstuhl fur Orthopadie der Friedrich-Schiller-Universitat Jena, Waldkrankenhaus „Rudolf Elle“ Eisenberg 1Switzerland 2Germany


Introduction
Total Knee Arthroplasty (TKA) is a well established, highly successful procedure, with numerous long-term follow-up studies reporting clinical success rates of 72-100% at 10-20 years in terms of pain reduction, functional improvement and overall patient satisfaction [1][2][3][4][5]. Although TKA is generally successful, and despite the advances in surgical techniques, instrumentation and implant designs, between 5 to 8% of all patients still develop complications such as anterior knee pain, loosening, instability, malpositioning, infection or fractures [6][7][8]. Imperfection in the coronar, sagittal and axial alignments of the femoral and/ or of tibial components, improper ligament balancing and incorrect joint line restoration can lead to soft tissue imbalance and inability to re-establish optimal kinematics and the overall biomechanics of the joint, with persistent anterior knee pain, patellar maltracking, instability or limitation of movement [9][10][11][12][13][14]15]. Several investigators have demonstrated on the basis of conventional radiography and computer-tomography that TKA, implanted with computer-assisted navigation and conventional approach, has more accurate component alignment than TKA implanted conventionally [9,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. The introduction of minimally invasive surgery (MIS) has gained in importance in orthopedics and especially in TKA. Patient`s demand for high activity level after TKA, concerns about postoperative pain, fast rehabilitation process, possible reduction in duration of hospitalization and costs in connection with the necessity of health care savings have led to the rapid advancement of less invasive surgical approaches and techniques as well as the development of new instrumentation by the orthopedic implant industry. Regardless of the numerous advantages of minimal-invasive TKA, concern is driven about loss of accuracy for implant placement and increased complications related to skin slough and infection when a minimally invasive approach is used [30,31]. Furthermore reduced operative visualization, a steep learning curve, an increased risk of complications, excessive 358 skin trauma and compromised implant fixation and alignment are topic of objection. In addition few surgeons have expressed concern about minimally invasive surgery and its relevance to TKA as well as the safety of operations performed "through a keyhole" and are convinced that at present there is no credible evidence that smaller incisions significantly benefit the patient receiving MIS TKA. By contrast, proponents of MIS TKA report that MIS patients, compared to patients undergoing conventional TKA, Experience shortened hospital stay, less pain-control medications, faster recovery of knee range of motion and decreased blood loss all without compromise of accuracy or short-term outcome [32][33][34].

Computer-assisted navigated orthopedic surgery
The principle of computer-assisted surgery in orthopedics is based on the creation of a digital map for the different steps during operation. Using this map, the surgeon is guided through the operating process. The development of the digital image is based on three different basic ideas. One system uses anatomical information which is achieved from pre-operatively performed CT-or MRI-scans, the second system is "peri-operatively-imaged" in which anatomical imaging occurs in the operating suite at the time of surgery. This requires a specially modified fluoroscopy unit, which entails the presence of a relatively bulky and expensive apparatus during surgery. These two systems display the "image-based systems". The third group on the contrary is "image-free" and relies on information acquired during surgery. This "image-free" navigation allows the surgeon to quantify data, receive real-time dynamic intra-operative maneuvering feedback and to obtain more reproducible results. A very important feature of this navigation system is its ability to provide instant feedback regarding in vivo kinematics of the joint at different stages of the operation. Alignment and ligament stability can be assessed with the trials in place to ensure proper function. Furthermore, this system allows the surgeon to measure the coronal, sagittal and axial deformities, the alignments and the stability of the joint before, during and after the implantation of a TKA. These characteristics of the navigation system provide the unique opportunity to assess in vivo the kinematics of the knee during surgery and implement beneficial changes of the components or alteration in components selection. We were using at the Orthopedic Clinic at the Hospital Oberengadin in Samedan an "imagefree" navigation system (Stryker® Leibinger Knee Navigation System, Stryker® Leibinger, precisioN Knee Navigation Software V 4.0). This system is available in an active wireless PC-based guidance system, which is based on an image-free navigation method, and thus does not require pre-operative computer tomography or intra-operative fluoroscopy. It comprises a module for analyzing the alignment of the leg, the alignment of the resection planes and thus of the prosthetic components. The system also allows the surgeon to quantify the kinematics of the knee and the balancing of the soft tissue (Further details on the Stryker® Knee Navigation System are available on http://www.europe.stryker.com/).

Surgical procedure
The patient is placed supine on a standard operating table. A tourniquet is applied after exsanguination of the limb, and standard skin preparation and draping are undertaken. To obtain best exposition of all structures during the procedure, two distally positioned leg holders are fixed on the operating table to allow full flexion and extension of the leg. Flexing the knee thereby exposes the posterior structures whereas extension facilitates access to the anterior anatomy of the knee.  As standard approach we used a mid-vastus approach [35][36][37][38]. To enable a better exposure and visibility of the lateral compartment the patella was osteotomized freehand to 12-14 mm bone thickness for later resurfacing in order to reconstruct preoperative thickness. All interventions were performed by a single surgeon (NB), who is a high volume arthroplasty surgeon and uses computer-navigation routinely for over six years. Two pin trackers need to be fixed rigidly at the beginning of the operation on the lateral distal femur and on the ventral proximal tibia, both within the surgical access zone (For further details information's please see the references 24 and 25). The digitizing pointer is now used to mark the key anatomical landmarks. After that procedure the surgeon is able to reproduce the correct joint kinematics at any time of the operation in any position of the leg with the Knee Navigation System software. After analyzing the kinematics curves and axis, bone cuts are performed using the information obtained from the navigation system. We usually prefer starting with tibial cut first. The proximal tibial cut is made in a onestep procedure, controlling the desired posterior slope, varus/ valgus and depth. The degree of posterior slope of the tibial cut is aimed to match the original posterior slope of the tibial plateau as measured in the pre-operative lateral x-ray [16]. Using freehand technique, the position of the probe is adjusted according to the image and data shown on the computer. MIS TKA must be performed with accurate instruments that are coordinated with the procedure. The guide is held with a "tripod grip" technique ( Figures  1A-D), and the visual movements of the guide can be monitored in real time on the screen ( Figure 2).     The rotational alignment is subsequently established with the femoral alignment guide and resection plane probe. The femoral component rotation is aimed to be 0° in relation to the special algorithm of surgical transepicondylar axis and the Whiteside line as provided by the computer software. Care is taken to avoid notching the ventral femoral cortex. We finally finish the femur preparation with the femoral resection guide and the femoral trochlea with the corresponding resection guide. Hereby care is taken to release any flexion contracture and to remove all the posterior femoral osteophytes. The femoral component is then inserted and, once correctly seated, the fixation lugholes are drilled. Subsequently the tibial bone resection is finalized: The appropriately sized tibial baseplate is inserted and its rotation is determined through self-adjustment by flexing and extending the knee with the trial femoral and inlay components implanted. The tibial component rotation is marked, and later the component is implanted with the desired rotation [39,40].

Trial component insertion
Before the insertion of the trial components, the tibial and femoral bone cuts are again verified with a small resection plane probe, which is directly applied on the resection planes of the affected bone. Every cut is recorded and stored by the system, and can be used for post-operative evaluation and quality control. After completion and digital check of the tibial, femoral and soft tissue preparation, the trial components are inserted and once again the correct position of the trial components is checked with a small resection plane probe for position. Subsequently, the patella preparation is finished and patellar tracking is checked with the implant trials in place.

Trial components soft tissue assessment
After the insertion of the trial components, limb alignment and soft tissue balancing are assessed with the intra-operative kinematics by moving the limb from extension to full flexion under neutral, varus and valgus stress, manually applied through the heel of the foot, carefully keeping the same rotation. Using this information, the surgeon can simulate changes of polyethylene insert and soft tissue release by repeating the assessment. In this way one can obtain a constant dynamic feedback in the process of balancing the knee ( Figures 5A and B).    Any excessive cement is removed under direct vision. In the same way the femoral component is cemented, a polyethylene inlay trial is then inserted and thereafter the patellar component is embedded with cement. After the cement is fully polymerized, the tourniquet is released and subtle hemostasis performed. Before the final choice of the inlay size is made, it is still possible to assess the joint and soft tissue balance using different size of polyethylene inlay trials and to show the resulting kinematics analysis on screen.

Wound closure
The joint is accurately irrigated and an intraarticular drain is inserted. The arthrotomy is closed with interrupted absorbable sutures (Vicryl 2.0) with the knee at 90 o of flexion. After putting a second, subcutaneous drain, the subcutaneous layer is closed with Vicryl 2.0, and the skin is closed with Ethycrin 4.0. Subsequently the final outcome and the kinematics are documented, recorded and compared to the initial data to assess the success of any correction ( Figures 7A and 7B).

Postoperative treatment
For post-operative pain control, our patients first receive a "one-off sciatic nerve block (SB)" and then a continuous femoral nerve block (FB) [41,42], combined with oral analgesics for both groups. Physical therapy is started as early as 4 to 6 hours post-operatively (continuous passive motion) and consequently intensified under supervision of an experienced physiotherapist (i.e. continuous passive motion 3 times a day, early ambulation, walking exercises, active bending and extending exercises, active knee stretching exercises, walking up and down stairs, leg press, ergometer-bike riding, coordination exercises, getting up from a seated position, strengthen exercises, etc.). Patients are allowed to full weight bearing as tolerated. Patients are discharged from the hospital once they are able to flex the knee joint to 120 o , to perform an unassisted straight-leg raise, to walk independently with or without crutches, to rise from a chair to standing and sit from standing without support, and to ascend and descend a full flight of stairs. All patients receive Low Molecular Weight Heparin (LMWH, i.e. Fraxiparine®, Nadroparin) or a direct oral Factor Xa inhibitor (i.e. Xarelto®, Rivaroxaban) for deep venous thrombosis prophylaxis for 6 weeks. Outpatient physical therapy is started immediately after discharge. The Patients are evaluated clinically and radiographically in the office at 6 weeks, 3 months and 6 months.

Radiographic follow-up
All patients received full-length standing antero-posterior radiographs pre-operatively, at 6 weeks as well as 6 and 12 months post-operatively ("Philips® Multidiagnost 3"). Pre-and post-operative mechanical axes (i.e. the coronal mechanical axis of the limb, the Hip-Knee-Angle) were determined from radiographs. A mechanical axis of more than 3 o varus/ valgus was determined as outlier as defined previously [17,18]. Conventional radiographic assessment involved short-leg-length weight-bearing antero-posterior (AP) radiographs, as well as non-rotated short-leg-length lateral radiographs at 30° of knee flexion and patella axial radiographs. The alignment of the prosthetic components was evaluated on the shortlength standard radiographs. Radiographic parameters, including the coronal femoral component angle, the sagittal femoral component angle, the coronal tibial component angle and the sagittal tibial component angle (i.e. tibial slope angle) were evaluated to determine the correct position of the femoral and tibial components [43,44]. The coronal alignment of the femoral component was measured in relation to the anatomical femoral axis (ideal value = 96°) and of the tibial component in relation to the anatomical tibial axis (ideal value = 90°).
To determine the sagittal angle of the femoral and tibial components, a perpendicular line, drawn from the midline of the femoral respectively tibial components, was compared with the midline of the distal segment of the femur and of the proximal segment of the tibia using the Knee Score reference lines [45]. Although little consensus on the ideal reference for defining the slope of the tibia on the lateral radiograph is reached, we used the technique described by Catani et al. and Yoo et al., measuring the slope of the tibial component on conventional short-length sagittal view radiographs with reference to the proximal anatomic axis [16,46]. Pre-and post-operative sagittal tibial component angles (i.e. tibial slope angle) were compared on conventional short-length lateral radiographs in 30° of knee flexion. All patients also received standardized CT-scans of both knees 6 weeks postoperatively to evaluate rotational alignment of the components according to the technique described by Berger et al. for our follow-up study [47].

Discussion
It is well known by now that malposition of TKA affects implant fixation and leads to an increased risk of loosening, instability and decreased survival of the prosthesis. Computerassisted navigation systems have been designed to increase the precision of implantation of TKA allowing the surgeon to reproduce the mechanical axes measured on full-length standing radiographs of the lower limb and reduces the number of outliers in the alignment of the limb compared to traditional mechanical instrumented TKA [9, 15-19, 21, 22, 48-54]. Two recent meta-analysis comparing alignment outcomes for computer-assisted navigated versus conventional TKA indicate a significant improvement in component orientation and mechanical axis, when computer-assisted navig a t i o n i s u s e d [ 5 5 , 5 6 ] . O u r a n a l y s i s demonstrated that it is possible to achieve straight mechanical axes not only in the conventional but also in the minimally invasive approaches by using a computer-assisted navigation. Additionally intra-operative alignment of the femoral and tibial bone resection was accurate in all three planes not only in the conventional but also in the minimally invasive computer navigated TKA group. Similar intra-operative results have been published and our results showed the same accuracy of the intra-operative bone resections with the navigation system as the above mentioned [9,[16][17][18]22]. However, the solely measurement of mechanical axis alone appears too basic as an indicator of correct limb alignment and long-term outcome. Accurate angles of the individual components in the coronal and sagittal planes, correct axial alignment and proper ligament and soft tissue balancing contribute to the success of knee replacement surgery and should be taken in consideration as well. The results of these data were also accurate in both groups using the computer-assisted navigation technique. Different studies have compared computer-assisted systems with traditional implantation for improvement of component orientation. Most authors showed that the coronal alignment (i.e. the varus/ valgus alignment) of the femoral component was improved with the use of navigation [9,[16][17][18][57][58][59]. Only few studies did not report an improvement in component alignment between patients in who navigation was used [60,61]. Despite the fact that in these studies the senior authors have more experience with conventional than navigated TKA, the reduction of outliers was greater in the navigation group. However, all these studies investigated TKA implanted using conventional approach.

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Strategies to Improve the Function, Kinematic and Implants' Positioning of a TKA with Minimally Invasive Computer-Assisted Navigation

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We were also able to demonstrate that, by using a computer-assisted navigation system, it is possible to implant the femoral and tibial components in the desired coronal and sagittal planes not only with the conventional but also with a minimally invasive approach. The post-operative radiographic analyses of the coronal alignment of the femoral and tibial components showed reliable results in both groups without any outliers in either group [For further detailed information please see the references 48]. Few further studies found the same accurate reconstruction of the sagittal alignment of the femoral component by using the navigation system, however with a standard conventional approach [9,16,19,24,25,49,57,58,62]. Literature documents that the influence of computer-assisted navigation on the alignment of the tibial component remains unclear. Several authors confirmed that the coronal alignment of the tibial component (i.e. the varus/ valgus alignment) is improved with the use of navigation [9,24,62], whereas other authors did not find evidence for improvement in coronal alignment [19,49]. We found the same preciseness for the implantation of the tibial component in the coronal plane in the conventional as well as in the minimally invasive approach. Furthermore, we could demonstrate that the sagittal tibial component angle (i.e. the tibial slope angle) can be accurately and reproducibly reconstructed to match the original value of the tibial plateau in both computer-assisted approaches. Although some studies disagree that the alignment in the sagittal plane of the tibial component can be improved with navigation, our result confirmed, as it has been reported by other authors, that the surgeon can use, in practical terms, computer-assisted navigation to accurately restore the tibial slope during TKA using minimally invasive approaches as well [19,24,25,28,48,49,58,[63][64][65][66][67][68]. Even small abnormalities of rotational alignment of the components have a considerable influence on patellar tracking, varus/ valgus stability and on the overall biomechanics of the joint. The accuracy to adjust the rotational alignment of the femoral component is a prerequisite to avoid malfunctioning TKAs. Debate still exists whether a navigation system does improve the rotational alignment of the femoral component or not [67]. Several reference axes have been proposed to establish proper rotational alignment of the femoral components [63]. Of these axes, the transepicondylar axis approximates the flexion-extension axis of the knee. Furthermore, although there is no consensus about the best landmarks to gauge femoral rotation, alignment according to the surgical epicondylar axis seems to come closest to allowing physiological biomechanics [7,8,10,67]. Debate continues with regard to how accurately and easily the transepicondylar axis can be located intra-operatively. Siston et al. found high variability in rotational alignment of the femoral component in a cadaver study [63]. This variability may be explained by the higher or lower ability of the surgeon to identify intra-operatively the medial epicondyle with its bone ridge and sulcus and the attachment of the deep and superficial fibers of the medial ligament, by the learning curve of the surgeon associated with the use of navigation and finally by the individual surgeon's skills. The algorithm of the Knee Navigation Software to establish the proper femoral rotational alignment by averaging the angle subtended by the Whiteside's line and the transepicondylar axis, gives the surgeon the possibility to improve the accuracy of the femoral rotational alignment without excessively increasing operative time. The analysis of our post-operative rotational alignment of the femoral component by CT-scans revealed a statistically significant reconstruction of the desired rotational alignment of the prosthesis parallel to the transepicondylar axis not only in the conventional but also in the minimally invasive computer-assisted navigated approaches. These results are in agreement with other studies using standard approaches, computer-assisted navigation and an improved computer tomography protocol [16,17,24,68]. Although it has been reported that the rotational mismatch between the femoral and tibial components is decreased with navigation, controversy still exists as to whether navigation systems do improve the rotational alignment of the tibial component in the axial plane [24,68,69]. We used the technique describe by Dalury and Eckhoff et al., whereby the orientation of the tibial tray was determined by allowing it to float into position with respect to the femoral component while the knee was placed through a full arc of flexion and extension [39,40]. We were able to document an accurate alignment of the tibial component in the CT scan postoperatively in both computer-assisted navigated groups as well.
H o w e v e r , w e d o b e l i e v e t h a t a n a v i g a t i o n system that relies only on digitization of landmarks to establish the rotational alignment of the tibial component is not reliable enough. Further research is therefore necessary. In addition to component malpositioning, tibiofemoral instability is another very important factor that might lead to implant failure and chronic pain. Some studies point out that 30% to 35% of the revision TKA were due to an uncorrected joint stability [20][21]. Tibiofemoral instability often represents a failure to correct the soft tissues balancing throughout the full range of motion and to adjust the flexion and extension gaps at the time of the primary arthroplasty. Furthermore, it is important to take the different behavior of involved ligaments on the medial and lateral aspects into consideration as well. This instability can be in extension, midflexion and/ or in flexion. Stability and function of TKA are strictly related to the interplay among the prosthetic component alignment, the articular surface geometry (flat or congruent polyethylene insert), the type and designs of prosthesis (cruciate-retaining versus cruciate-substituting prosthesis), as well as the balancing of the soft tissue and muscle action. Of all these factors, implant component alignments, joint line restoration and soft tissue balancing "can and must be" assessed and restored by the surgeon during the intervention. Calculation of the joint line height both at the femur and tibia is usually performed by measurements on pre-and post-operative radiographs using standard anatomical indices, which are very inaccurate and not reproducible. The computer assisted navigation system allows the surgeon to measure and restore accurately the alignments of the prosthetic components in all three geometrical planes, femoral joint height and the tibial joint line, as well as the desired soft tissue balancing [54]. An established concept is the preparation of a rectangular joint gap in TKA. With a posterior stabilized TKA, flexion and extension gaps can be different. This has been regarded as an important goal achieving good joint function. However, the lateral tibiofemoral joint is physiologically lax, and as consequence the flexion gap may not be rectangular. Van Damme et al. reported in a cadaveric study on normal non-arthritic knee joint an increased laxity lateral compared to medial in full extension, and an increased lateral laxity from 0° to 90° flexion [70]. Because of technical difficulties, only few data are available on the physiological laxity of the joint. Such analysis can only be performed if the flexed knee is imaged three-dimensionally both in neutral position and under a varus/ valgus stress. Tokuhara et al. analyzed quantitatively the stability of the medial and lateral tibiofemoral joint for normal knees in an open MRI [71]. Their results indicate that the flexion gap in a normal knee is not rectangular and that the lateral joint gap is significantly lax. Recent biomechanical studies have further shown that flexion of the knee is associated with a significant medial-pivot internal rotation of the tibia [72][73][74][75]. Thus, in rotation the medial condyle is immobile and the lateral condyle is mobile on the tibial surface.
Since 1977, several studies have investigated the relationship between soft tissue release and the resulting changes in the tibiofemoral gaps in TKA using optical encoders, pressuresensitive film, fluoroscopy or knee analysis system [76][77][78][79]. Computer-assisted surgical technology enables the surgeon to measure and assess knee behavior during operation, allowing real-time monitoring of knee's behavior from extension to flexion and soft tissue balance. In a previous study, we measured the mechanical axis and the varus/ valgus stability of the joint at different time points with the computer-assisted navigation and we documented a similar increased lateral joint laxity before and after implantation of the components at 45° and 90° of knee flexion. We even found that the overall laxity was decreasing beyond 45°/ 60° of flexion to maximal flexion [80]. Therefore, knee navigation allows the surgeon to objectively quantify and monitor kinematics and stability of the TKA through the full ROM pre-, intra-and post-operatively. Leaving the knee too lax after TKA may theoretically lead to tibiofemoral instability and excessive tightness of the joint in different position may cause stiffness. However, differently from TKA alignment, no data are available to define what is a well-balanced knee intra-and/or post-operatively. We suggested as ideal laxity for TKA a varus/ valgus laxity of an approximately total joint-line opening between 1.5° to 2° to be achieved from maximal extension to 45 o / 60 o flexion and decreasing to 0.5 o to 1.0 o by further increasing flexion [80]. These findings serve as a benchmark for future soft tissue laxity measurements and additiona l w o r k s h o u l d b e performed to validate these proposed values. The computer navigation will help to correlate the collected data and clinical outcomes more objectively than in the past and enable the setting of more accurate limits for soft tissue management. Despite some motivating factors, including a potential reduction in duration of hospitalization and costs, one should not discount that the patient-driven desires include their concerns about postoperative pain, prolonged rehabilitation, and less than-ideal functional outcomes associated with conventional TKA. Various authors have also reported superior clinical results and decreased cost using minimally invasive techniques for TKA [24, 32, 43, 51. 52]. Obtaining these results with standard approaches and conventional instruments seem to cause much more soft tissue damages leading to an arduous recovery period for the patients. Although the length of the skin incision is shorter in the MIS approach, MIS knee surgery should not be defined by the size of the skin incision, but rather by the method of soft tissue handling once the skin is incised. Therefore, we should better substitute the misnomer "minimally invasive surgery" (MIS) for "soft-tissue sparing" surgery. We believe that minimal trauma of the soft tissue and bone results in better postoperative function and accelerated rehabilitation. In a previous study, we were able to demonstrate that minimally invasive computer-assisted navigated TKA is able to achieve these objectives. With the minimally invasive approach patients were mobilized more aggressively reaching full weight bearing and profit by earlier discharge. Postoperative ROM after 3 months was significantly higher in MIS CN-TKA, but after 6 months differences were minimal. Clinical scores were identical for both groups six months after surgery. However, these clinical scores have turned out not to be ideal for the evaluation of patient satisfaction immediately after a computer-assisted navigated TKA with a conventional or MIS approach [81][82][83]. It would have been more appropriate to use patients' self-reported measures of outcome, such as the WOMAC and the SF-36 score systems. In addition, a recent prospective randomized controlled study demonstrated a positive correlation between accurate mechanical alignment after TKA and functional and quality-of-life patients' outcomes [84]. At all post-operative follow-up intervals from 6 weeks to 12 months the total IKSS score were significantly better in patients with a mechanical axis within 3 o of neutral compared to those greater than 3 o . Moreover, the SF-12 physical scores at all intervals from 3 months were also significantly better for patients with a mechanical axis within 3 o of neutral, and at 12 months these patients demonstrated better SF-12 mental-scores as well. Furthermore, another recent study showed, that TKA with good alignment lead to better function with quicker rehabilitation and earlier hospital discharge as well [85]. Therefore, the use of a computer-assisted navigation not only leads to reproducible accuracy of implant positioning in all three planes, but also to better functional outcomes with quicker rehabilitation and earlier hospital discharge due to the advantages of minimally invasive techniques.

Conclusion
Modern computerized knee navigation systems, appropriately used, aid surgeons to accurately optimize mechanical and axial alignments of the components in all three planes to avoid any malrotation and/ or any errors in coronal, sagittal and axial alignments. These advantages can be achieved not only in the conventional but also in minimally invasive approach without loss of accuracy. There is an increasing statistical evidence of a positive correlation between accurate mechanical alignment after TKA and a better functional as well as quality-of-life patient outcomes. Nevertheless the surgeon has to keep potential pitfalls in association with the computer-assisted procedures in mind. If used correctly, the system is very sophisticated and will improve accuracy. Therefore it will enhance the surgeon`s perspective, but should never replace it. The use of a computer-assisted navigation leads to reproducible accuracy of implant positioning in all three planes not only in the conventional but also in minimally invasive approaches. In contrast to even the most elaborate mechanical instrumentation system, which relies on visual inspection to confirm the accuracy of the alignment and stability of the TKA, computer-assisted navigation allows the surgeon to objectify every operative osteotomy, the position of trials and finally of the implants. It is well known that there is a definite relationship between the accuracy of implant positioning and longevity and therefore it is imperative to reproduce the implant positioning after a TKA. Despite the above mentioned advantages and excellent results that may be achieved with computer-assisted navigation, certain factors still cause concern and need to be optimized. As only the cutting guides are navigated, the surgeons may make less than optimal bone resections by bending the saw blade, especially when attempting to cut through sclerotic areas of bone. Differences in cement thickness may also potentially lead to malalignment, even though bone resection was accurate. These latter two problems, which can occur with conventional instrumentation as well, can be obviated only by using the verification plate of the navigation system to verify the correct level and direction of the performed osteotomy.
Computer-assisted technology assists the surgeon to reliably measure kinematics of TKA alignment and stability of the TKA on a screen. Furthermore, surgeons have the opportunity to improve their surgical performance with a direct intra-operative documentation of alignment and orientation of instruments, trials and implants. Additionally computerassisted navigation allows to verify the final alignment of the implants after component implantation and before the cement hardens to avoid or probably correct considerable error in alignment. Incorrect positioning of the components may only be a co-factor together with tibiofemoral instability and soft tissue trauma with MIS approach, leading to suboptimal implant loading with early loosening and increased wear. The use of computer-assisted navigation alone will not empower the surgeon to accurately and reproducibly implant a TKA. This might be especially true for the minimally invasive technique. Much technical expertise in the conventional TKA, the skill of the surgeon and their familiarity with the instruments may also be necessary to obtain good results.