Unicompartmental Knee Arthroplasty Study: High Prevalence of Outliers, Revisions

by Elizabeth Hofheinz, M.P.H., M.Ed.

Researchers from Barnes-Jewish Hospital, Washington University School of Medicine in St. Louis, Missouri have undertaken a retrospective analysis of 253 primary fixed-bearing and mobile-bearing medial unicompartmental knee arthroplasties (UKAs). The study on these surgeries, which were performed by two high-volume fellowship-trained arthroplasty surgeons, was meant to determine implant survival and postop radiographic outcomes along with the impact of component alignment and overhang on implant survival.

Their research, “High Prevalence of Radiographic Outliers and Revisions with Unicompartmental Knee Arthroplasty,” appears in the May 8, 2020 edition of The Journal of Bone and Joint Surgery.

Robert L. Barrack, M.D. is the Charles and Joanne Knight Distinguished Professor and Fellowship Director in the Department of Orthopaedic Surgery at the Washington University School of Medicine. Dr. Barrack told OSN, “There is variability in the reported revision rate of unicompartmental knee arthroplasty (UKA). Some specialty centers report low revision rates of 5% or less in the 5-10-year time frame while national registries and large data bases such as CMS and Market Scan have reported revision rates 3-4 times higher.”

“We wanted to determine the revision rate for UKA at our center which is a specialty center for joint replacement with all procedures done by high volume total joint specialists. There are a number of technologies available to improve accuracy, however we wanted to determine whether our results warranted the time and expense of adopting a new approach.”

The authors wrote, “…UKAs comprised <10% of their knee arthroplasty practices, with an average of 14.2 medial UKAs per surgeon per year. Implant survival was assessed. Femoral coronal (FCA), femoral sagittal (FSA), tibial coronal (TCA), and tibial sagittal (TSA) angles as well as implant overhang were radiographically measured. Outliers were defined for FCA (>±10° deviation from neutral), FSA (>15° of flexion), TCA (>±5° deviation from neutral), and TSA (>±5° deviation from 7°). ‘Far outliers’ were an additional >±2° of deviation. Outliers for overhang were identified as >3 mm for anterior overhang, >2 mm for posterior overhang, and >2 mm for medial overhang.”

Dr. Barrack commented to OSN, “Our revision rate was over 15% in the 5-10-year time frame consistent with the national registry and insurance database reports. Revisions were highly correlated with radiographic accuracy of sizing and orientation.”

“The proportions of UKA revisions and alignment outliers were greater than expected, even among high-volume arthroplasty surgeons performing an average of 14.2 UKAs per year (just below the high-volume UKA threshold of 15),” wrote the authors. “Alignment and overhang outliers were significant risk factors for implant failure.”

Dr. Barrack told OSN, “Based on these results we have adopted more advanced technology (robotics) to insure more accurate sizing and 3-dimensional orientation of the components. Results of our first year utilizing robotic assistance confirmed 5-10X improvement in accuracy which will hopefully translate into much improved implant survival.”

Dr. Barrack notes: “I am a designer and consultant for Stryker which manufactures the Mako robot we now utilize. The two other surgeons involved in this study and the follow-up Robotic UKA study, however, have no affiliation with Stryker. Virtually all major total joint companies, however, either have a robot for use in joint replacement or are in the process of releasing one.”