Lessons in Alignment: Bi-Uni vs TKA

What effect does bi-unicompartmental knee arthroplasty have on hip-knee-ankle angle alignment? Does using a robotic arm in this surgery have a different effect on the knee anatomy than a mechanically aligned total knee arthroplasty (TKA)? This is what a team from the Glasgow Royal Infirmary in the UK set out to determine.

The researchers looked at 38 patients who’d undergone a total knee arthroplasty and compared them to 32 patients who’d received bi-unicompartmental knee arthroplasty. They took pre- and post-operative CT (computed tomography) measurements of the coronal, sagittal, and axial alignments of each patient’s knee.

The resulting paper, “Robotic arm-assisted bi-unicompartmental knee arthroplasty maintains natural knee joint anatomy compared with total knee arthroplasty: a prospective randomized controlled trial.” appears in the November 2020 edition of The Bone and Joint Journal.

“Our group’s work at the Glasgow Royal Infirmary is led by Mark Blyth and is exploring the compartmental approach to knee arthroplasty treatment,” said Dr. Matthew Banger, D.Eng. a research associate in the Department of Biomedical Engineering at the University of Strathclyde in Glasgow, UK, and co-author on the study to OTW.

“Bi-unicondylar knee arthroplasty treats medial and lateral tibiofemoral arthritis through separate uni-condylar implants. This approach retains the cruciate ligaments that are fundamental in driving normal knee kinematics. The concept of bi-unicondylar knee arthroplasty is not new, but the technical demands can now be addressed with a robotic-arm assisted approach that allows for accurate positioning of implants in all anatomical planes.”

“On reviewing the X-rays, there was a noticeable difference in the joint line postoperatively between the treatment groups. Hence, this paper explored the differences between the radiological implant alignment of the two procedures.”

“The difference in the joint line alignments might not have been a surprise, but the statistical significance and the 3-dimensional alignment difference wasn’t initially expected. This difference wasn’t just the anatomical alignment; it was the relative change between the diseased and treated alignment across all of the rotational planes. And, 47% of bi-unicondylar knee arthroplasty and 24% TKAs rotational changes were smaller than 2°, which was significantly different between the groups. So robotic-arm assisted bi-unicondylar knee arthroplasty was able to maintain nearly half of the native compartmental orientations.”

Putting a fine point on the findings, Dr. Banger told OTW, “The key differences between these alignments are first that the robotic bi-unicondylar knee arthroplasty maintains coronal, sagittal, and axial knee joint anatomy, compared with a mechanically aligned TKA. Second, this reduced correction in the bi-unicondylar knee arthroplasty alters overall hip-knee-ankle angle alignment towards neutral less than conventional TKA.”

“This study does confirm the variability of natural alignment in the pre-disease knee is not accounted for in all mechanically aligned TKAs. Bi-unicondylar knee arthroplasty surgery can be considered a true kinematic procedure, allowing re-tensioning of the soft tissue envelope back towards to the pre-disease constitutional varus or valgus.”

“TKAs are largely a successful procedure and this robotic approach might be best suited for patients with severe proximal tibia vara, either because a mechanically aligned TKA would necessitate much larger soft tissue releases, or a kinematically aligned TKA may incompletely restore the native joint anatomy or require an increased obliquity with the associated concerns of possible early failure. Currently, robotic-arm assisted bi-unicondylar knee arthroplasty is an off label procedure and remains to be seen whether the changes in alignment will translate into improved long-term outcomes and implant survivorship. This trial will report its 10-year findings in 2028.”