ObjectiveTo analyse the effectiveness of unicompartmental knee arthroplasty (UKA) for the patients with spontaneous osteonecrosis of the knee (SONK). MethodsBetween January 2012 and December 2016, 31 patients with SONK was admitted and treated with medial UKA. All patients were examined by both plain radiography and magnetic resonance images. The patients were composed of 5 men and 26 women with an average age of 64.3 years (range, 48-79 years), and with 16 left joints and 15 right joints. The average disease duration was 14.7 months (range, 6-26 months). Preoperative visual analogue scale (VAS) was 6.00±1.15, Hospital for Special Surgery (HSS) score was 55.77±11.03, and knee range of motion (ROM) was (114.68±10.40)°. The imaging examinations showed that all the lesions were located in the medial compartment of the knee joint and there were 19 patients with Aglietti stage Ⅳ and 12 patients with Aglietti stage Ⅴ. Preoperative femorotibial angle (FTA) was (177.39±1.63)° and posterior tibial slope (PTS) was (84.05±1.39)°. ResultsAll the incisions healed by first intention. All patients were followed up 14-46 months (mean, 25 months). At last follow-up, VAS score was 2.06±0.72 and HSS score was 86.45±3.67, which both improved significantly when compared with preoperative scores (t=22.73, P=0.00; t=–14.72, P=0.00). ROM was (118.06±3.80)° with no significant difference when compared with preoperative ROM (t=–1.78, P=0.08). The X-ray films showed there was no severe adverse events, such as periprosthetic infection, aseptic loosening, bearing dislocation, and so on. At last follow- up, PTS was (85.30±1.19)° with significant difference compared with preoperative one (t=–4.07, P=0.00); while FTA was (177.51±1.98)° with no significant difference when compared with preoperative FTA (t=–0.38, P=0.71). ConclusionUKA may be an optional management for SONK with minimally invasive, bone-preserving, and rapid recovery.