One of every 10 patients undergoing total knee arthroplasty (TKA) is dissatisfied with their outcome, according to Bourne et al. and DeFrance et al. One of the major dissatisfiers is mid-flexion instability, defined as medial-lateral instability between 30 degrees to 60 degrees of knee flexion, which can limit patients’ activity following TKA.
Assessing implant design and alignment techniques in TKA
Because multi-radius femoral components can contribute to mid-flexion instability, manufacturers developed single-radius versions to enhance stability through the knee’s range of motion. Yet evidence on their enhanced stability compared with multi-radius components has been mixed.
Beyond implant design, studies have explored various alignment methods, including robotic and computer-assisted techniques, to optimize TKA based on patients’ unique bone morphology and soft tissue envelope. Robotic-assisted TKA allows for precise placement of implant components and incorporates predictive balancing based on implant alignment and ligament laxity before and after bone resections, according to Vasili Karas, MD, MS, an orthopedic surgeon specializing in robotic knee and hip replacement at Rush University Medical Center.
The aim and methodology of the study
To explore the effects of implant design and alignment techniques on mid-flexion laxity, Dr. Karas’ co-authors, including Rush orthopedic surgery resident Enrico Forlenza, MD, conducted a multicenter, retrospective cohort study of 154 consecutive robotic-assisted, primary, elective TKAs. Cases were performed by three experienced surgeons at three centers. Inclusion criteria involved patients having robotic-assisted TKA with the OMNIBotics system and a cruciate-retaining single radius implant, Unity CR. The average age of patients was 67, and nearly three-fourths were female.
Using the same robotic-assisted TKA system and workflow, surgeons performed the standardized tibia-first restricted inverse kinematic alignment (iKA) technique as outlined by Winnock de Grave et al. “Compared with mechanical alignment where we’re looking to hit the exact same parameters for every single patient regardless of what their actual anatomy is, iKA is a much more personalized approach,” Dr. Karas says. “With robotic optimization of the procedure, we have very granular data that we can use to make decisions in the operating room.”
To simulate multi-radius implantation, intraoperative pre-resection laxity data from each single-radius implant case was used. Simulated mechanical alignment (sMA) was also performed using intraoperative data, and the robotic planning software provided the laxity data.
Researchers compared mediolateral balance and medial and lateral laxity between single- and multi-radius implants, and between iKA, sMA and the native, arthritic knee throughout the flexion range.
The results
Researchers found that single- versus multi-radius design had minimal impact on mid-flexion laxity, whereas iKA — but not sMA — significantly reduced laxity and mediolateral balance throughout the arc of motion. Specifically, sMA had greater laxity variability both medially and laterally. sMA had greater mean lateral laxity at 45 degrees, 60 degrees and 90 degrees, and tighter gaps medially from 20 degrees to 60 degrees. Researchers also found that the variation in laxity and balance of the arthritic knee was similar to that of sMA.
When comparing single- versus multi-radius implants, the difference in laxity was less than 0.2 millimeters at 45 degrees and less than 0.1 millimeters at 60 degrees. This study corroborates earlier findings from Jo et al. and Matsumoto et al. that found that implant design had a limited effect on laxity in the mid-flexion range but had no effect throughout the rest of the functional arc of motion.
“It was a poignant finding for us that implant choice matters less than alignment technique in TKA,” Dr. Karas says. “Surgeons can perform a great knee replacement with the right technique and philosophy with almost any modern implant.”
The next steps
Dr. Karas believes the findings will help surgeons improve satisfaction after TKA in a higher percentage of patients. “We want to make sure that every single patient we operate on is not only able to enjoy the traditional activities of daily living after total knee replacement, but can also perform higher-stress activities such as pickleball, golf and tennis,” Dr. Karas says. “All of that requires being as bespoke as possible when we replace knees and having a better grasp on balance in the knee throughout the range of motion, particularly in mid-flexion. Based on these findings, we believe that the techniques that allow for personalization of the joint replacement, rather than implant design, will improve patients’ ability to be active and to have improved outcomes across the board.”
Dr. Karas is also working on research through Rush’s Motion Analysis Lab to further compare the outcomes of traditional versus robotic-assisted knee replacement. “We’re looking specifically at how the knee is functioning in mid-flexion and comparing that to an arthritic knee and a well-functioning native knee,” he says. Another Motion Analysis Lab study on healthy volunteers aims to build a database of how high levels of activity create dynamic forces across the hip and knee, so surgeons can better understand how to optimize TKA using robotic assistance.
“Because Rush is one of the leaders in the country for minimally invasive surgery and rapid recovery after joint replacement, we not only want to get the patient through surgery as quickly and as pain-free as possible, but we also want to get that patient back to a higher level of activity than after a typical knee replacement,” Dr. Karas says. “And there is growing evidence that robotics can help us achieve that.”
