Alignment of components in total knee arthroplasty (TKA) influences function and implant longevity [1,2,3]. Although, the ideal alignment principle in TKA is debatable, every surgeon wishes to implant the components in a desired orientation with accuracy and precision. The resultant need has given birth to computer-assisted navigation in TKA. The first computer-navigated TKA was reportedly performed in 1997 [4]. The early system consisted of a computer, optical localizer, and arrays mounted with light emitting diodes (LEDs) which could be attached to bones and surgical instruments to aid in the desired placement of cutting blocks [4]. Although, the basics remain the same, the technology has undergone extensive validation and upgrades over the years to provide the enhanced functionality of current generation of computer navigation systems.
Aseptic loosening (31.2%) and instability (18.7%) are the two commonest modes of failure of TKA [5]. Alignment and soft tissue balance during surgery can influence both of these factors [1, 6, 7]. The number of TKAs performed globally is rising. Kurtz et al [8] estimated a 673% increase in primary TKA from 2005 to 2030 in the United States. Total knee arthroplasties are also being increasingly performed in the younger population. These patients are likely to place higher demands on their prosthesis; revision rates amongst the younger population are projected to be higher [9]. Hence, achieving optimum alignment and soft-tissue balance becomes all the more important, especially in the younger patients. Computer navigation might help address this need.
Adoption of computer navigated TKA
The use of computer navigation for TKA has slowly but steadily increased since its inception. The spread of this technology has not been uniform across the globe. Its adoption has been more rapid in some countries compared to others. In Australia, the proportion of computer-navigated TKA rose from 2.4% in 2003 to 32% in 2019 [10]. In a survey of European Society of Sports Traumatology Knee Surgery and Arthroscopy (ESSKA) and Swiss Orthopaedic Society members, Friederich and Verdonk [11] reported that 51.9% members had a navigation system at their centre and 25.7% surgeons used it for more than 75% of their TKAs. In Germany, more than 30% surgeons use computer technology for TKA [12]. In contrast, Antonios et al [13] reported that the proportion of computer-navigated TKA increased from 1.2% in 2005 to only 6.3% in 2014, in the United States. The proportion of computer-navigated TKA in the UK is estimated to be less than 3% of all TKAs [12].
According to the ‘Diffusion of innovations’ theory by Everett Rogers, adopters of new technology belong to these categories: innovators, early adopters, early majority, late majority, and laggards; and the process of adoption over time is demonstrated by a bell curve of normal distribution [14]. Computer-navigated TKA is in the ‘early adopter’ phase of Roger’s bell curve of technology adoption cycle. Despite clear demonstration that it significantly improves component alignment compared to conventional TKA, computer-navigated TKA has not become mainstream yet. Picard et al [12] looked into the reasons for the same which are discussed herewith. Some of the earlier systems were perceived to be ergonomically unsound and lacked user-friendliness which was an important reason for resistance to adopting this technology [12]. However, most computer navigation systems today are intuitive and easy to use. Another factor is the economic aspects related to it. There is an initial cost of acquiring the system. There may be recurring costs of single use reflective spheres used on the arrays. Depending upon the healthcare model, the associated costs may be passed on to the patient/their insurance provider or absorbed by the state. The increased cost associated with navigated TKA can be offset by savings accrued due to reduction in revision rates. Novak et al [15] looked into the cost-effectiveness of computer-assisted TKA and reported that cost savings can be achieved if the additional cost due to computer navigation was ≤629 $ per operated case. Their findings were based on a decision-analysis model using model inputs obtained from a review of literature. However, this analysis is also influenced by the variability in the cost of the system, the accuracy of the alignment obtained using navigation, and the probability of future revision surgery due to malalignment, as stated by the authors [15]. In reality, the analysis of cost-effectiveness can be very complex as it does not account for factors like increased surgical time and its secondary effects which are difficult to quantify.
Increased surgical time due to the use of computer navigation may also discourage surgeons from switching over from conventional TKA. Computer-navigated TKA may require between 10 and 20 extra minutes as compared to conventional TKAs. Bolognesi et al [16] reported an additional mean tourniquet time of 11 min with computer-navigated TKA as compared to conventional TKA. Surgeons may also be concerned about the learning curve associated with the computer-navigated TKA. The reported learning curve for navigated TKA is between 16 and 20 cases [17, 18]. Also, ‘disruptive innovations’ like minimally invasive surgery and patient-specific instrumentation, which did not clearly demonstrate any significant benefits, but distracted surgeons away from ‘sustaining’ technologies like computer navigation were responsible, to some extent, for navigated TKA not becoming mainstream [12].
Abundant scientific literature is available about navigated TKA. While the evidence unequivocally shows that it results in significant improvement in limb and component alignment as compared to conventional TKA [19,20,21], conflicting data exist regarding functional improvement and longevity which could be a reason why many surgeons would refrain from adopting this technology [19, 22,23,24,25,26,27,28,29]. Total knee arthroplasty using robotic arm or robotic hand-held devices has been introduced in the last 5 years. While navigation systems help accurate placement of cutting blocks, which is followed by freehand cutting of bones, robotic systems help precise sculpting of bones to place the implants in a desired orientation. Although, robotic systems are significantly more expensive than navigation systems, they provide enhanced functionality and may reduce alignment errors due to inaccurate bone cuts. Most leading orthopedic companies have already launched or are in the process of launching robotic TKA platforms. Understandably, in such a scenario, the industry is more likely to focus more on the propagation of this newer, robotics-based technology rather than computer navigation.
Classification
Jones and Jerabek [30] classified computer navigation systems into large-console navigation systems (Fig. 1) and accelerometer-based hand-held navigation systems. Large-console navigation systems could be image-based or imageless. Image-based systems require a preoperative CT scan to build a frame of reference. They are not popular due to the additional cost and radiation exposure to the patient. Imageless navigation systems use intraoperative data to build the reference frame and are the most commonly used systems today.
Basics of large-console navigation systems
The essential components of a computer navigation system are trackers/arrays (Fig. 2), localizer (Fig. 1), and computer system. During surgery, trackers are attached to the bones, surgical instruments, or a probe. They can be ‘passive’ in the form of reflective spheres (Fig. 2) or ‘active’ with LEDs. The localizer or camera receives reflected (passive tracker) or active (active tracker) signals from the trackers to determine their spatial orientation. The most commonly used localizers are ‘optical’ localizers. The downside of optical localizers is that the trackers must always be within the localizer/optical field to be detected. In contrast, electromagnetic (EM) systems have a receiver which can detect signals from the trackers without requiring them to be in the line of sight. Lastly, the ‘brain’ is the computer system which processes information received by the localizer to provide real-time 3-D graphical data about alignment and knee joint morphology to the surgeon throughout surgery.
Steps of computer-navigated TKA
Most modern navigation systems allow the surgeon to customize the surgical workflow according to their preferences. Options include the choice of ‘measured resection’ or ‘gap-balancing’. With ‘measured resection’, the surgeon may choose the ‘tibia’ or ‘femur first’ technique. Additional options include the choice of anatomical landmarks for setting femoral rotation. Some systems may also offer optional steps to determine medio-lateral coverage of femoral component and setting of tibial rotation.
The navigation console is commonly placed contralateral to the side of the knee to be operated upon. After a standard exposure of the knee joint, the trackers are attached to the lower end of the femur and upper end of tibia using two-pin uni-cortical or a single-screw bi-cortical fixation. The trackers must be so attached that they are within the localizer field throughout the surgery and do not interfere with placement of surgical instruments. This is followed by the registration process which involves acquisition of centers of the hip, knee, and ankle joints and surface mapping of the distal femur and proximal tibia. Registration is a critical step as the computer system uses this data to build the frame of reference and inaccuracies during this process will result in incorrect data being displayed by the system, eventually leading to surgical errors. The popular computing phase ‘garbage in, garbage out’ (GIGO) is apt here. Modern navigation systems have built-in redundancies which can alert the surgeon if there is a significantly large error while acquiring certain landmarks during the registration process. Once the registration is completed, the system provides information about limb alignment (Fig. 3) and joint morphology which the surgeon can use to determine the orientation and depth of bone resections, titrate soft-tissue releases, and customize implant position to obtain desired alignment and soft tissue balance. One can control the orientation and depth of proximal tibial and distal femoral resections, adjust femoral size, and set femoral rotation (Fig. 4). Newer versions of some navigation systems now also allow setting of tibial component rotation. Thus, the system allows 3-D customization of implant position based on surgeon’s preference. After the respective bone resections, one can also verify them and re-cut to the desired level, if required. Most contemporary systems also provide data about joint kinematics and balance throughout the range of motion before and after bone cuts. If ‘gap-balancing’ protocol is chosen, medial and lateral knee joint gaps in extension and 90° flexion are displayed which the surgeon can use to plan soft tissue releases and adjust the femoral implant position to achieve desired alignment and balance.
Computer navigation and pin site complications
Uni- or bi-cortical trans-osseous pins are used to attach the trackers to the lower end of femur and upper end of tibia during computer-navigated TKA. The possibility of pin tract complications is often discussed and may discourage surgeons from adopting this technique. These could be in the form of pin tract infection, pin site pain, heterotopic ossification (rare), and fracture (rare). In a systematic review looking into pin site fractures after computer-navigated or robotic knee replacements, Smith et al [18] reported an incidence from 0.06 to 4.8% [31]. Risk factors for pin site fractures included transcortical pin placement, diaphyseal placement, pin diameter size > 4 mm, non-self-drilling/non-self-tapping pins, and multiple drill holes [31]. In my experience of several hundred navigated TKAs, I have not encountered a single fracture related to pin insertion.
Accelerometer-based navigation (ABN)
Hand-held ABN is basically a system of sensors which are attached to cutting guides or bones in order to aid in the placement of cutting blocks in a desired position. They were launched with the aim of overcoming some of the drawbacks of large-console systems. Compared to large-console systems, its advantages include no initial set-up cost, no ‘line of sight’ problems associated with optical tracking, avoidance of pin site complications as it does not require trans-osseous femoral and tibial tracker fixation, and reduced surgical time [32]. However, ABN systems do not provide information about soft-tissue tension and cannot aid in setting femoral or tibial rotations. Also, there is an additional cost of single-use components at the time of surgery.
Scientific literature
There is evidence that suggests navigated TKA results in significantly better implant and limb alignment as compared to conventional TKA. In a meta-analysis of 23 randomized controlled trials, Hetaimish et al [21] reported that navigated TKA had significantly lower risk of implant malalignment at both ±3° and ± 2° limits. In a meta-analysis of 47 studies, Moskal et al [20] demonstrated that navigated TKA significantly improved component alignment. In a meta-analysis of six studies comparing navigated and conventional TKA in patients undergoing bilateral surgery, Zhao et al [19] reported that navigated TKA resulted in better correction of lower limb mechanical axis and better prosthesis alignment. Mooney et al [33] showed that navigation-enhanced instrumentation significantly reduced the total outlier rate (±2°/2 mm) as compared to conventional instrumentation. Outliers (±3°) in terms of implant and limb alignment do occur after navigated TKA with a reported incidence of 10.4% [34] and can be attributed to the surgeon or the navigation system. Errors or inaccuracies during the registration process, especially during the registration of femoral epicondyles, will provide erroneous data to the surgeon which may result in malalignment [35].
In conventional TKA, unlike navigated TKA, an intramedullary rod is introduced into the femoral canal for determining the distal femoral cut. This breach of the femoral medullary canal may increase the risk of complications. Navigated TKA results in significantly lower blood loss as compared to conventional TKA [20, 36] and has significantly lesser need for perioperative transfusions [37]. Kalairajah et al [38] found significant reduction in systemic emboli with navigated TKA as compared to conventional TKA. Siu et al [39] reported significantly lower D-dimer levels and only milder surge in D-dimer levels, 24 h after navigated TKA as compared to conventional TKA. In contrast, Kim et al [40] found no significant differences in prevalence of embolic phenomenon between navigated and conventional TKA. Kuo et al [41] showed that navigated TKA results in lesser vascular injury as compared to conventional TKA. It also results in lower local and systemic inflammation [42]. These benefits are attributable to the fact that the femoral medullary canal isn’t breached during navigated TKA.
While certain advantages of navigated TKA are evident, controversy exists regarding other benefits. With the ability to obtain a desired implant alignment and better soft-tissue balance, one would expect that navigated TKA would provide better clinical and functional outcomes which has been reported by many authors [22, 25, 26, 29]. However, others found no significant difference between navigated and conventional TKA in terms of functional outcomes [19, 23, 24]. Implant and limb alignment can influence implant longevity. While individual studies and meta-analyses may have shown that navigated TKA did not result in improved implant survival [19, 23, 24], registry data were contrary to that. Data from the Australian Joint Replacement Registry showed that navigated TKA significantly reduced the overall revision rate and revision rate for loosening/lysis in patients < 65 years [27]. Jorgenssen et al [43]reported a significantly higher rate of major aseptic revision in non-navigated TKAs (hazards ratio: 1.32, P < 0.001) based on data from the Australian Joint Replacement Registry. Dyrthovden et al [28] reported similar survival and revision rates between navigated and conventional groups, but significantly reduced revision for malalignment in the navigated group, based on Norwegian registry data.
Literature regarding outcomes of ABN systems is available but not as extensive as large-console based navigation systems. In a meta-analysis of eight studies, Li et al [44] showed that one ABN system (iAssist) significantly improved lower limb alignment as compared to conventional TKA. However, the surgical time was prolonged with no significant difference in short-term functional outcomes. In a randomized controlled trial, Nam et al [45] showed that the KneeAlign ABN system decreased outliers in tibial component alignment as compared to conventional TKA using an extramedullary guide. A version of the same system was also found to be accurate for distal femoral resection during TKA [46]. The KneeAlign-2 system was found to be as accurate as large-console based navigation systems but resulted in significantly shorter tourniquet time [32].
