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Background and purpose -- The main treatments for severe medial compartment knee arthritis are unicompartmental (UKR) and total knee replacement (TKR). UKRs have higher revision rates, particularly for aseptic loosening, therefore the cementless version was introduced. We compared the outcomes of matched cementless UKRs and TKRs. Patients and methods -- The National Joint Registry was linked to the English Hospital Episode Statistics and Patient Reported Outcome Measures (PROMs) databases. 10,552 cementless UKRs and 10,552 TKRs were propensity matched and regression analysis used to compare revision/ reoperation risks. 6-month PROMs were compared. UKR results were stratified by surgeon caseload into low- (< 10 UKRs/year), medium- (10 to < 30 UKRs/year), and highvolume (≥ 30 UKRs/year). Results -- 8-year cementless UKR revision survival for the 3 respective caseloads were 90% (95% CI 87--93), 93% (CI 91--95), and 96% (CI 94--97). 8-year reoperation survivals were 76% (CI 71--80), 81% (CI 78--85), and 84% (CI 82--86) respectively. For TKR the 8-year implant survivals for revision and reoperation were 96% (CI 95--97) and 81% (CI 80--83). The HRs for the 3 caseload groups compared with TKR for revision were 2.0 (CI 1.3--2.9), 2.0 (CI 1.6--2.7), and 1.0 (CI 0.8--1.3) and for reoperation were 1.2 (CI 1.0--1.4), 0.9 (CI 0.8--1.0), and 0.6 (CI 0.5--0.7). 6-month Oxford Knee Score (OKS) (39 vs. 37) and EQ-5D (0.80 vs. 0.77) were higher (p < 0.001) for the cementless UKR. Interpretation -- Cementless UKRs have higher revision and reoperation rates than TKR for low-volume UKR surgeons, similar reoperation but higher revision rates for midvolume surgeons, and lower reoperation and similar revision rates for high-volume surgeons. Cementless UKR also had better PROMs. [ABSTRACT FROM AUTHOR]