REGISTRY REVIEW: Partial Knee Arthroplasty in the US

    Primary total knee arthroplasty (TKA) is one of the most commonly performed orthopaedic procedures in the US. The same cannot be said for partial knee arthroplasty: According 2020 Annual Report from the American Joint Replacement Registry (AJRR), unicompartmental knee arthroplasty (medial and lateral UKA) accounted for just 4.1% of knee replacements in 2019, compared with 8.2% in 2012, and patellofemoral arthroplasty decreased from a high of 1.7% in 2016 to 0.04% in 2019.

    Not only is partial knee arthroplasty uncommonly performed, but it is also concentrated in the hands of a relatively small number of surgeons. In 2019, 27.2% of surgeons who performed TKA also performed UKA, while only 9.2% performed patellofemoral arthroplasty, according to the AJRR data.

    Outside the US, UKA is more common. In Sweden, 8% of knee replacements performed in 2018 were UKAs, according to the Swedish Knee Arthroplasty Register, [1] and in Australia, UKAs increased from 4.2% of all primary knee replacements to 5.1% in 2017. [2] Patellofemoral arthroplasty is just as rare in other parts of the world as it is in the US, accounting for fewer than 0.05% of primary procedures in New Zealand, [3] 1.0% in the UK, [4] and 0.3% in Sweden [1]

    Most surgeons in the US who perform TKA avoid partial knee arthroplasty due, at least in part, to research showing high rates of revision following partial knee procedures. The AJRR data support this, reporting a lower cumulative revision rate for primary TKA than for partial knee replacement in patients age 65 and older. The risk of revision for primary TKA patients peaks at 3 years, according to the AJRR data, then steadily declines. But for UKA, the AJRR data show a fairly consistent risk for up to 6 years after surgery, with a pattern of small increases alternating with small decreases through year 6.

    A variety of tibial and femoral implant components were used for UKA between 2012 and 2019. The Restoris MKC Partial Knee Implant System (Stryker; Kalamazoo, Michigan) and the Oxford Partial Knee System (Zimmer Biomet; Brigend, UK) were the top 2 systems used during that period, with the 2 now virtually tied for number 1.

    The Persona Partial Knee System (Zimmer Biomet; Warsaw, Indiana) took over the number 3 position in 2018 and has shown significant growth since then compared with the number 4 through number 8 implant components.

    Highly cross-linked polyethene has been the liner material of choice since 2012, according to the AJRR data, growing to 56.9% of liners used for UKA in 2019. Conventional polyethylene has fallen out of favor, dropping from nearly half of all liners used for UKA in 2012 to just 28.9% in 2019.

    There seems to be a growing interest in antioxidant polyethene: It was used in only 0.06% of all liners for UKA in 2012 but had increased to 14.2% by 2019. That’s a long way from 56.9% for highly cross-linked polyethene, but the trend is worth watching.

    The 2020 Annual Report from AJRR can be found here.


    1. Swedish Knee Arthroplasty Register. Swedish Knee Arthroplasty Register 2018 Annual Report. Accessed January 28, 2021.
    2. Australian Orthopaedic Association National Joint Replacement Registry. Hip, Knee & Shoulder Annual Report 2018. Accessed January 28, 2021.
    3. The New Zealand Joint Registry. NZJR 19 Year Report. Accessed January 28, 2021.
    4. National Joint Registry. 15th Annual Report National Joint Registry for England, Wales, Northern Ireland and the Isle of Man. Accessed January 28, 2021.