Computer-Assisted Surgery for Total Knee Replacement: Navigating Toward Improved Outcomes

Commentary While total hip replacement has been referred to as the “operation of the century,”1 with almost universally outstanding clinical outcomes, approximately 20% of patients after total knee replacement are somewhat dissatisfied2. The disparity in clinical outcomes between hip and knee replacement, seemingly intransigent over the last 2 decades3, has been the impetus for intense innovation on several fronts. Manufacturers have developed numerous implant designs with improved fidelity of preserving native knee anatomy and kinematics. Individualized, patient-specific implants and instruments that are based on preoperative axial imaging have come to market4. Surgical technique has evolved to minimize soft-tissue disruption, improve osseous as well as soft-tissue balancing, and consider both kinematic and mechanical alignment. Beyond implants and techniques, computer-assisted surgery has emerged as a major opportunity to improve the alignment of implanted components and, hopefully, clinical outcomes as a corollary. Computer-assisted surgery essentially uses specialized instruments and software to help guide the surgeon during component implantation. There are several types of navigation, including image-based navigation (in which preoperative axial images are coupled with intraoperative landmarks) or imageless navigation (in which multiple landmarks are fit to a virtual knee model using surface registration). Recent ameliorations of computer-assisted surgery include the introduction of portable, accelerometer-based navigation5 as well as robotics with haptic assistance to guide bone preparation6,7. No matter the type of computer-assisted surgery used, the prevailing evidence demonstrates superior implant alignment compared with conventionally instrumented knee replacements8. Computer-assisted surgery limits the number of outliers and therefore improves the reproducibility of accurate component placement. The elimination of intramedullary referencing also mitigates fat emboli and is particularly useful in the presence of extra-articular deformity or hardware that precludes the use of intramedullary instrumentation. Despite more reproducible component alignment with computer-assisted surgery, few studies have demonstrated improved clinical outcomes over conventional instrumentation. In a study using follow-up data from the Australian National Joint Replacement Registry, de Steiger et al. reported slightly fewer revisions in computer-assisted knee replacements at the 9-year follow-up in patients who were <65 years old9. Nonetheless, studies clearly demonstrating improved implant survivorship and patient-reported outcomes have not been forthcoming8. The added cost of computer-assisted surgery, increased setup complexity, increased operative time, and potential pin-site complications must also be considered. Muddling the subject even more is the controversy surrounding alignment altogether. Radiographic assessment of healthy, nonarthritic knees has demonstrated variations in native mechanical alignment, with a substantial portion of individuals who have several degrees of “constitutional varus.”10 Furthermore, in a series of 398 total knee replacements performed at the Mayo Clinic, supposedly malaligned knees (a mean mechanical axis [and standard deviation] of >0° ± 3°) did not have an increased risk of aseptic loosening or revision at 15 years11. We may therefore need to further elucidate and define the ideal target alignment of total knee replacement before potentially reaping the benefits of computer-assisted surgery. In the present article, the authors report the short-term clinical outcomes of a well-designed, multicenter, blinded randomized controlled trial comparing computer-assisted with conventionally instrumented total knee replacements. The study randomized 190 patients, and 167 of them completed the final evaluation and were included in the analysis. Comparison of the groups with respect to the 5 subscales of the Knee Injury and Osteoarthritis Outcome Score (KOOS) at 2 years demonstrated significantly larger improvements in the computer-assisted group for 2 subscales (symptoms, and sports and recreation). The authors also used the Outcome Measures in Rheumatology-Osteoarthritis Research Society International (OMERACT-OARSI) criteria to trichotomize Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores into high responders, moderate responders, and nonresponders. The computer-assisted group had significantly more high responders than the conventionally instrumented group. In comparison with previously reported randomized trials12, the present study demonstrated tangible improvements in several functional outcome measures in patients receiving computer-assisted knee replacements. These early clinical results are encouraging and possibly lend credence to the adoption of computer-assisted knee replacement. Nonetheless, it is important to note that significant improvements in the mean difference between the groups were demonstrated for only a subset of the KOOS and WOMAC scores, while the remaining measures and subsets were similar. Furthermore, the segregation of WOMAC scores into 3 binary responses may amplify the true clinical effect of the outcome. The authors engaged 2 pertinent questions in their subgroup analysis. First, they compared patients with postoperative mechanical malalignment (>0° ± 3°) and those with well-aligned knees (<3° of varus or valgus). Interestingly, there was no difference in functional outcomes between these 2 groups, which undermines the concept that neutral anatomic alignment (facilitated by computer-assisted surgery) leads to gains in functional outcomes. Second, the authors looked at patients who had constitutional varus before surgery. Within this subgroup of 82 patients, there were no significant differences in the postoperative KOOS scores between the 60 knees that were left in some varus alignment and the 22 knees that had corrected to neutral alignment. While both analyses may have been underpowered, they certainly contribute to the controversy of what alignment we should be targeting with knee replacement surgery. The present article describes encouraging early functional results with the use of computer-assisted total knee replacement. Nonetheless, it is difficult to attribute these gains solely to improved mechanical alignment. Further studies are needed to better understand the ideal alignment following knee replacement surgery. The use of computer-assisted surgery to achieve such a target might then more easily translate into beneficial clinical outcomes.

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