Summary

Functional alignment technique using robotics would reproducibly achieve the target joint gaps in flexion and extension.

Abstract

Objective

Total knee arthroplasty (TKA) with classical mechanical alignment aims to achieve neutral limb alignment in all patients, whereas TKA with functional alignment aims to restore native, patient-specific anatomy and knee kinematics by manipulating bone resections and fine-tuning implant positioning. With the use of robotic arm assisted TKA, resection thickness, joint gaps, and limb alignment can be assessed during surgery. The additional precision offered by functional alignment technique using robotics means that non-neutral limb alignment targets can achieve more reproducibly, reducing the risk of missing the target and producing significant alignment outliers. In current study, we examined the efficacy of functional alignment technique using Mako robotic-arm system (Stryker) performed at our institution. Our hypothesis was that the functional alignment technique would reproducibly achieve the target joint gaps in flexion and extension.

Materials And Methods

We examined 60 patients, 81 knees, 13 males and 47 females, mean age 76.0 years old (range 63 to 87), all with osteoarthritis of the knee, who underwent robotic arm assisted TKA at our institution from July 2020 to July 2022. The implant was a Stryker Triathlon CR, with cementless fixation of both femur and tibia. A preoperative surgical plan for implant positioning with a goal of near-equal flexion and extension joint gaps was created using the software. Before intraoperative osteotomy, the joint gap was measured in valgus stress at 0 degrees of extension and 90 degrees of flexion, and the osteotomy thickness and angle were adjusted to achieve a medial joint gap of at least 18 mm. Finally, joint gap measurements in valgus stress were performed at the time of implant trial. The final alignment and the joint gaps before and after osteotomy and the effect of adjusting the osteotomy thickness and angle by functional alignment technique were examined using multiple regression analysis. The significance level was less than 5%.

Results

Before osteotomy, the medial joint gap (mm) at valgus stress was 17.0 (range, 12 to 22) in extension and 16.3 (range, 10 to 21) in flexion, and the lateral joint gap was 19.1 (range, 15 to 23) in extension and 18.3 (range, 15 to 25) in flexion. After osteotomy using functional alignment technique, the medial joint gap at valgus stress was 18.8 (range, 17 to 21) in extension and 18.8 (range, 17 to 21) in flexion, and the lateral joint gap was 18.3 (range, 17 to 25) in extension and 18.1 (range, 15 to 25) in flexion. We could achieve the target joint gaps after osteotomy thickness and angle. Before osteotomy, the implant alignment was 10.0°of varus in extension and 5.2°of varus in flexion. After osteotomy, the implant alignment improved to 3.3°of varus in extension and 2.4°of varus in flexion. Significant effective factors for osteotomy adjustment were the distal femoral osteotomy, femoral valgus angle, and proximal tibial osteotomy in extension, and the femoral component size down, tibial posterior slope and proximal tibial osteotomy in flexion.

Discussion

Robotic arm assisted TKA is becoming popular worldwide for its ability to perform accurate osteotomy and gap balancing safely and reproducibly. We could also reproducibly achieve the target joint gaps in flexion and extension using functional alignment technique and have learned some knowledge about gap adjustment that is useful not only for robotic arm assisted TKA but also for conventional TKA.