Overview

This chapter reviews the clinical outcome, postoperative alignment, and implant survivorship after kinematically aligned (KA) total knee arthroplasty (TKA) based on results of international randomized controlled trials (RCTs), case-control studies, meta-analyses, and single-surgeon case series. The first section describes differences in patient-reported outcome scores and range of motion between patients treated with KA TKA and mechanical alignment (MA) TKA. Featured are a chronologic review of the findings of RCTs, a matched cohort of patients treated with calipered KA and MA performed with manual instruments and a medial pivot implant design published by the author of this chapter, and meta-analyses. Many studies used inclusion restrictions and postoperative correction restrictions in the KA cohort that biased the results against KA TKA and in favor of MA TKA. The second section discusses differences in KA and MA postoperative limb and joint line alignment and the accuracy of the calipered KA technique. The third section reports the mid- and long-term implant survival after KA TKA and discusses the differences in the rate of reoperations other than for infection between the KA and MA TKA. The contrary and unexpected finding of negligible long-term risk of tibial component failure from a varus mechanism after KA TKA is highlighted. The explanation for the negligible risk is the lack of correlation between the degrees of varus alignment and component migration, low knee adduction moment during gait, and restoration of native tibial compartment forces after KA TKA, which MA TKA does not achieve. The educational objective is to provide compelling evidence that the KA target results in a better clinical outcome and that the calipered technique accurately restores the patient’s prearthritic joint lines. Surgeons that perform KA TKA can expect a low risk of mid- and long-term implant failure and a negligible risk of varus failure of the tibial component that is lower than MA TKA.

Differences in Clinical Outcome Between Kinematically Aligned and Mechanically Aligned Total Knee Arthroplasty

Persistent patient dissatisfaction and residual symptoms after mechanically aligned MA TKA contrast with the generally favorable reports of high implant survival. ^, These features of disappointment, unchanged by purported advances in implant design and more consistent neutral limb alignment from the use of intraoperative navigation and robotic technologies, show the intrinsic limitations of the MA technique. Modern views conclude that changing the native joint line, kinematics, and laxities, which is inherent to the MA technique, impede optimal clinical outcomes. For all modern arthroplasties except the knee, reconstruction of the patient’s functional phenotypes, prearthritic joint surfaces, soft tissue envelopes, and joint laxities is recognized as the primary goal. Over the past decade, improvements in the understanding of knee kinematics and the importance of restoring the flexion-extension axis encouraged the evaluation of other alignment techniques and the use of medial pivot or stabilized implant designs. These experiences brought insight and strategies for achieving higher function scores and satisfying even the highly active patient. ^{, ,}

The calipered KA technique reduces the risk of patient disappointment after TKA. KA restores the patient’s prearthritic joint lines regardless of preoperative knee deformity without ligament release and does so by reestablishing the tibial compartment forces, ligament lengths, and laxities of the native knee. ^, KA is now a 15-year-old surgical technique that individualizes component placement, targeting anatomic surface landmarks within the knee, which is substantially different from the MA targets of the center of the hip, knee, and ankle. The international orthopedic community is highly interested in KA because of the generally favorable results of the published RCTs, case-control studies, and meta-analyses, discussed further in this chapter, in which the differences between kinematically aligned and mechanically aligned total knee arthroplasty have been analyzed.

Chronologic Review of the Author’s Experience and International Randomized Controlled Trials that Summarize the Effectiveness of Kinematically Aligned Total Knee Arthroplasty

( Table 13.1 )

In 2012, a US study by Dossett reported results of an RCT of 41 patients treated with KA performed with patient-specific instrumentation (PSI) and 41 patients treated with MA using measured resection and manual instrumentation and the same cruciate ligament–retaining (CR) fixed-bearing implants. There were no inclusion or postoperative correction restrictions. Follow-up was at 6 months. Ligaments were not released in the KA cohort. The KA cohort had significantly higher Knee Society Score (KSS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and Oxford Knee Score (OKS). Radiologically, the hip-knee-ankle angle (HKA) was similar; however, the joint line was more oblique in the KA cohort. A 2-year follow-up article published in 2014 reported 8 degrees more flexion in the KA cohort, and a 3.2 and 4.9 odds ratio of having a pain-free knee based on the Oxford and WOMAC pain subsections, respectively, compared with the MA cohort.

In 2016, a United Kingdom study by Waterson et al. reported results of an RCT of 36 patients treated with KA performed with PSI and 35 patients treated with MA using measured resection and manual instrumentation with a CR fixed-bearing implant. Follow-up was from 6 weeks to 1 year. There were inclusion restrictions, as patients with varus and valgus deformities greater than ±7 degrees were excluded, which biased the results in favor of MA TKA. The authors showed higher KSS outcomes in terms of early functional improvement and peak torque of the quadriceps in the KA cohort with no significant differences at 1-year.

In 2017, a German study by Callies et al. reported results of an RCT of 100 patients treated with KA performed with PSI and 100 patients treated with MA using measured resection and manual instrumentation with a CR fixed-bearing implant. Follow-up was at 1 year. There were inclusion restrictions, as patients were excluded when the preoperative varus and valgus deformities were greater than  ±10 degrees. There were correction restrictions as the postoperative distal lateral femoral angle was limited to 86 to 94 degrees and the medial proximal tibial was limited to 86 to 94 degrees in the KA cohort. Ligaments were not released in the KA cohort. The KA cohort had significantly higher KSS and WOMAC scores. Those few KA TKA patients with poor outcomes had deviations in joint line alignment between the postoperative radiographs and the initial plan, presumably caused by poor-fitting patient-specific guides.

In 2017, a New Zealand study by Young et al. reported results of an RCT of 49 patients treated with KA performed with PSI and 50 with MA using navigation with a CR fixed-bearing implant. Follow-up was at 2 years. The KA cohort showed a trend toward a functional advantage over the MA cohort. There were inclusion and correction restrictions that biased the results in favor of MA TKA. Records of the PSI company showed the surgeon frequently reduced the correction by overriding the preoperative KA plan, which explains the use of ligamentous releases to balance the KA TKA. The mean Forgotten Joint Score (FJS) in the KA cohort of 69 was higher than the MA cohort score of 66, which is lower than other reports of KA and higher than those previously reported for MA. ^, They concluded that functional outcomes were not different.

In 2017, a Japanese study by Matsumoto et al. reported results of an RCT of 30 patients treated with KA and 30 with MA performed with navigation instrumentation with either a mobile-bearing or fixed-bearing CR implant design. There was an inclusion restriction, as only knees with varus deformities were treated. There was a correction restriction, as all tibial components in the KA cohort were placed at 3 degrees of varus to the MA. Follow-up was at 1 year. The postoperative knee flexion and the objective and functional activity components of the Knee Injury and Osteoarthritis Outcome Score (KOOS) were significantly better in the KA cohort.

In 2019, a Canadian study by Laende et al. reported the results of an RCT of 24 patients treated with KA performed with PSI and 23 with MA performed with navigation instrumentation and a fixed-bearing CR implant design. There were no inclusion or correction restrictions. Follow-up was at 2 years. The primary outcome was tibial component migration, measured by radiostereometric analysis (RSA). KA TKA was associated with acceptable tibial component migration, indicating stable fixation. The postoperative alignment of the limb, varus alignment specifically, was not associated with tibial component migration. They concluded these results support continued investigation of implantation of tibial components outside the window of neutral mechanical alignment. There were no differences in the OKS; however the mean score of 31 and 30 at 2 years was uncharacteristically low in both the KA TKA and MA TKA cohorts.

In 2020, an Australian study by McEwan reported results of an RCT of simultaneous bilateral TKA in 41 patients in which navigation performed KA and MA in opposite knees, with a CR implant. They excluded valgus knee deformities. There was a correction restriction, as the postoperative HKA angle limits of the KA cohort was limited from 6-degrees varus to 3-degrees valgus. Follow-up was at 2 years. Significantly more participants preferred their KA joint, although clinical outcomes were equivalent. Fewer releases were required using the KA technique. Participants were visually insensitive to modest HKA asymmetry.

In 2020, an Australian study by French reported results of an RCT that determined differences in outcomes between 46 patients with a medial pivot design and 44 with a low-conforming CR design. All the arthroplasties were performed using KA principles with manual instruments. Follow-up was at 1 year. The authors concluded that the medial pivot KA TKA scored significantly better on the FJS and the quality-of-life subscale of the KOOS and KOOS-12 than for those who underwent a low-conforming CR KA TKA, which functions like an anterior cruciate ligament and partially meniscus-deficient knee. ^, The mean FJS of those with the medial pivot KA TKA was 80 points, which was 16 points higher than the low-conforming CR KA TKA and comparable with total hip arthroplasty.

In 2020, a German study by the author of the present chapter reported results of a case-control trial of 24 consecutive KA TKA patients performed with the calipered technique and manual instruments and matched to 24 patients treated with MA-TKA. Both knees received a medial pivot knee design. There were no inclusion or postoperative correction restrictions. Follow-up was at 1 year. The mean FJS of the KA medial pivot TKA of 77 points was similar to the 80-point score reported by French et al., and 26 points higher than the mean FJS of the MA medial pivot TKA.

The use of inclusion and postoperative correction restrictions in the KA cohort can bias the results in favor of MA TKA. The exclusion of treatment to those with severe knee deformities and valgus deformities biases the results because of the different frequency of use of ligament release, which adds morbidity and the risk of instability. Calipered KA does not use ligament releases to restore the prearthritic limb alignment. In contrast, MA uses more ligament releases to change the limb, as the deviation of the prearthritic alignment incrementally increases from a neutral HKA. In MA TKA, an increase in the number of ligament releases to treat the valgus deformity lowers clinical outcome scores. Postoperative restrictions that limit correction from the prearthritic limb and joint line alignment biases the results in favor of MA TKA, because treatment of the so-called “restricted version” of KA TKA (better termed “expanded” MA TKA) requires ligament releases. ^,

International Meta-Analyses Summarizing the Effectiveness of Kinematically Aligned Total Knee Arthroplasty

Multiple meta-analysis used systematic reviews of RCTs and case-control studies to determine differences in a variety of primary and secondary outcomes between KA TKA and MA TKA. A US meta-analysis concluded that functional outcome, as measured by the KSS, favored KA TKA over MA TKA and that KA did not adversely impact overall survivorship or complication rates at short-term follow-up. A United Kingdom meta-analysis concluded that the KA TKA had better clinical outcomes with no increase in patients with poor clinical results because of implant position, especially for those with varus placement of the tibial component. A Chinese meta-analysis showed the KA TKA provided better functional outcomes and better flexion following short-term follow-up. An Australian meta-analysis showed no differences between techniques regarding radiological outcomes and complication rates and that KA TKA might produce better functional outcomes than MA and have shorter operation times. A meta-analysis study consisting of only RCTs that performed KA with PSI found no difference in functional outcomes between KA TKA and MA TKA.

Differences in Postoperative Alignment Between Kinematically Aligned and Mechanically Aligned Total Knee Arthroplasty

The target for calipered KA TKA is to restore the patient’s prearthritic limb and joint line alignment and not the MA target of a neutral HKA ( Fig. 13.1 ). Although the targets are different, the mean postoperative HKA was not significantly different between KA and MA in the RCTs. There are differences in the varus-valgus orientation of the distal lateral femoral joint line and the proximal medial tibial joint line between KA and MA. The RCT by Dossett reported 2.1-degrees more valgus alignment of the femoral component and 2.2-degrees more varus alignment of the tibial component in the KA TKA. The RCT of simultaneous bilateral TKA by McEwan reported 1.5-degrees more valgus alignment of the femoral component and 2.3-degrees more varus alignment of the tibial component in the KA TKA. These differences in joint line obliquity between KA and MA techniques benefit the weight-bearing biomechanics of the KA TKA, as the joint line of the tibial component is oriented more parallel to the floor than in MA TKA. ^{, ,} At the same time, the overall limb alignment is comparable.

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