UKA could be an excellent alternative to TKA for patients with SONK.
Historically, Spontaneous osteonecrosis of the knee (SONK) was thought to occur secondary to vascular insufficiency that disrupts the nutrition supply of the cartilage. Recent investigations have demonstrated that medial meniscus posterior root tear (MMPRT) is associated with the development of cartilage damage of the knee. Total knee arthroplasty (TKA) is the standard of care for those patients who fail conservative management, but considering SONK’s predilection for affecting a single knee compartment, unicompartmental knee arthroplasty (UKA) appears to be a more tailored option.
Radiograph could not identify the subtle SONK lesion, but MRI was helpful to confirm the diagnosis and assist in determining the extent of disease. From January 2013 to December 2017, 42 patients with SONK were diagnosed by MRI and received medial UKA in our department. The pre and post-operative Oxford knee score (OKS) were also compared. Satisfaction of patients was recorded in four categories (very satisfied, satisfied, not satisfied, disappointed).
Average follow-up was 3.2 years, and the average age at operation was 61.4 years. The incidence of MMPRT was 54.7% (23/42) in our study group. The mean pre- and post-operation OKS showed improvement from 35.65±4.28 to 14.91±4.63. No implant was revised, and most patients were satisfied (23.8%) or very satisfied (73.8%) with the outcome of this surgical procedure. One patient was not satisfied (2.3%). The final ROM for patients who underwent UKA for SONK was 125.6o.
MRI is the most useful tool for diagnosis. Although the true mechanism remains unknown, MMPRT may have a key role in the development of SONK. There are many treatment options for SONK, including conservative treatment, arthroscopic debridement, bone grafting, and arthroplasty. Since SONK is a disease that mainly affects the medial femoral condyle, UKA remains a viable and more kinematically friendly option in short- to mid-term, but further investigation is still needed for long-term results.