Slaven SE, et al. - Thirty-seven unicompartmental knee arthroplasty (UKA) revised for progression of lateral compartment osteoarthritis (“Progression”) and 61 UKAs revised for revisions for aseptic loosening or subsidence (“Loosening”) were recognized from the prospective institutional database of 3,351 medial fixed-bearing UKAs performed since 2000 in order to contrast the postoperative mechanical alignment of well-functioning UKAs against two groups of failed UKAs, including revisions for Progression and revisions for Loosening. In the setting of fixed-bearing, medial UKA, mechanical alignment in the coronal plane was concluded as a significant factor. At 10 years, individuals with well-functioning UKAs showed mild varus mechanical alignment of nearly 4°, whereas people revised for progression of osteoarthritis equated more valgus (mean 0.3° valgus) and the one revised for loosening or subsidence equalized more varus (mean 6.1° varus). Moreover, for component survival, the optimal mechanical alignment is likely a slight varus.