Onlay round and onlay oval patellar implants show better patient-reported outcomes in TKA

New research has revealed that among the various types of Patellar implants, onlay round and onlay oval patellar implants have shown better patient-reported outcomes, superior bone coverage, and improvements in several intraoperative, radiographic, and scintigraphic outcomes. The trial results were published in The Journal of Bone and Joint Surgery. 

Despite much discussion over the role of patellar resurfacing in total knee arthroplasty, it is preferred by most surgeons as it decreases re-operation rates and anterior pain, and an increasing number are adopting resurfacing. As there is very less evidence on the optimal patellar implant design in total knee arthroplasty (TKA), researchers conducted a study to assess clinical, intraoperative, radiographic, and scintigraphic differences between inlay (IN), onlay round (OR), and onlay oval (OO) patellar implants.

A parallel-group, double-blinded, randomized trial was carried out to compare the IN, OR, and OO patellar implants using the same posterior-stabilized TKA prosthesis for each. Out of 121 participants, there were 40 participants in the IN group, 41 in the OR group, and 40 in the OO group.  Patient-reported outcomes were prospectively followed for a minimum of 2 years, with survivorship outcomes followed for a mean of 5 years. The primary outcome was the between-group differences in the mean Kujala score change from preoperatively to 2 years postoperatively. The secondary outcomes included differences in other knee-specific and general health outcomes, intraoperative characteristics, radiographic parameters, patellar vascularity, and implant survivorship. 

Key findings of the study: 

• No significant differences were observed in Kujala score changes between groups at 2 years postoperatively (p = 0.7; Kruskal-Wallis test).
• Greater improvements were exhibited by the OR group in Knee injury and Osteoarthritis Outcome Score (KOOS) Activities of Daily Living when compared with the IN group and in KOOS Quality of Life compared with the OO group.
• Better bone coverage and lower lateral facetectomy rates were exhibited by the OO design group when compared with the IN and OR designs.
• Though the overall value for lateral contact was not significant (p = 0.09; chi-square test), the IN group had more lateral contact compared with the OO group (p = 0.02; Fisher exact test). 
• No differences were seen in postoperative scintigraphic vascularity (p = 0.8; chi-square test).
• There was 1 revision for infection at 3 years postoperatively in the OO group and no revision in the other groups. 

Thus, onlay designs are much better for TKA as they have superior long-term implant survivorship when compared to other designs. 

Further reading: Gharaibeh MA, Tarassoli P, Chen DB, et al. Does Choice of Patellar Implant in Total Knee Arthroplasty Matter?: A Randomized Comparative Trial of 3 Commonly Used Designs. J Bone Joint Surg Am. 2023;105(2):145-156. doi: 10.2106/JBJS.22.00655