Purpose: Total knee arthroplasty (TKA) replaces the osteoarthritic knee, while High Tibial Osteotomy (HTO) attempts to preserve it. TKA has been traditionally viewed as a treatment option for older patients with end-stage disease, and HTO a treatment option for younger patients with mal-alignment and OA confined to one tibiofemoral compartment. With increasing rates of TKA, and decreasing rates of HTO, it is important to better understand the eligibility criteria for these quite disparate surgeries. Therefore, the purpose of the present study was to investigate the demographic and clinical characteristics of patients with knee OA who received TKA compared to patients who received HTO. Methods: Patients with or at high risk for developing knee OA were selected from the Osteoarthritis Initiative (OAI), a multi-center, publically accessible database. Only those patients with varus alignment that received TKA or partial medial uniarthroplasty since enrollment were included. Patients who received medial opening wedge HTO were included from a separate cohort from a single centre specializing in knee realignment or osteotomy surgery. Baseline demographic and clinical characteristics (age, body mass index [BMI], sex, Knee Injury and Osteoarthritis Outcomes Score [KOOS]) were collected for both groups. Radiographic measures included lower limb frontal plane alignment (mechanical axis angle [MAA]), Kellgren and Lawrence grade of disease severity (KLG), and medial/lateral joint space width (MJSW/LJSW). Patients from the two cohorts were then split into subgroups according to age (<50y, 50-59y, 60-69y, ≥70y). In patients that received bilateral HTO or TKA surgery, only the first surgical limb was included. Independent t-tests and chi-squared analyses were used to compare differences in means and proportions between groups, respectively. A bonferroni correction for multiple comparisons was applied and adjusted the significance level to p<0.002. Results: Data from 834 patients having received HTO (n=565) or TKA (n=269) were included. Overall, patients receiving HTO were younger with more varus alignment (p<0.001). A total of 367 HTO (n=216) and TKA (n=151) patients were between the ages of 50 and 69. In those patients, although lower limb alignment was more varus in patients receiving HTO (-7.9°±3.7°) vs. TKA (-4.4°±3.9°), radiographic disease severity (KLG) was not significantly different between groups. Although JSW could not be directly compared between cohorts, 90-96% of patients that received joint replacement had minimal to no lateral joint space narrowing (grade 0-1), suggesting lateral compartment disease did not make them ineligible for HTO. BMI (HTO: 29.2±4.4 kg/m2; TKA: 31.0±4.7 kg/m2) and proportions of degenerative vs. post-traumatic OA (HTO: 92%vs.8%; TKA: 93%vs.7%) were also similar between groups (p<0.001). Significantly more males received HTO (82%, p<0.001); significantly more females received TKA (60%, p<0.001). Preoperative KOOS 4 subdomains were consistently lower (worse) in patients that received HTO compared to patients that received TKA, with significant differences for pain (p<0.001) and quality of life (p<0.001). Conclusions: These findings suggest substantial overlap in demographic and clinical characteristics between patients receiving TKA vs. HTO, particularly in those between 50 and 69 years of age. Sex is a notable exception, with relatively few females receiving HTO. TKA was not associated with greater radiographic or patient-reported disease severity. Identifying optimal eligibility criteria for both surgical interventions requires re-evaluation.