UKA has been proved to be an effective method for the treatment of single compartment knee osteoarthritis, which retains the kinematic characteristics of the normal knee joint. In comparison with total knee arthroplasty, UKA exhibits less surgical trauma, less bone loss, short recovery period, and a good range of motion as its advantages[14, 15]. A large number of successful applications of UKA have been reported recently, and the 10-year survival rates of the prosthesis are up to 99.8%[8]. Foran and colleagues[16] reported that UKA had a 15-year survival rate of 93% and a 20-year survival rate of 90%. In 2015, Pandit[17] followed up 1000 patients who undertook UKA for 15 years and concluded that the 10-year and 15-year prosthesis survival rates were 94% and 91%, respectively. However, there are still few reports on the clinical efficacy of UKA in the treatment of SONK, which may be related to the low incidence of SONK. According to statistics, the cases of joint replacement attributes to SONK account for only 0.05–7% of all knee arthroplasty[18, 19]. The incidence rate of SONK is 3.4% in people over 55 years old and 9.4% in people aged 65[20].
At present, the best treatment for SONK is still controversial, while the treatment selection mainly depends on symptoms, stage of the disease, and size of the lesion[7, 21]. For many patients at an early stage, who have no apparent clinical symptoms, a small range of injury, and no collapse of the articular surface, conservative treatment can usually lead to better clinical outcomes. Juréus et al.[22] reported that, after following up 40 patients for an average of 9 years and evaluating the natural course and long-term effects of SONK, they suggested that the size of osteonecrosis could predict the outcome. Aglietti et al.[4] indicated that conservative treatment is not valid when the necrotic focus area is more significant than 5 cm2 or the width is more than 40% of the femoral condyle involved. Soucacos et al.[21] demonstrated that patients with imaging stage I ~ II can be treated conservatively, while patients with stage III ~ IV should be treated more actively if articular cartilage has collapsed. The typical surgical treatment approaches include arthroscopic debridement of the articular cavity, autogenous cartilage transplantation, core decompression, high tibial osteotomy, UKA, and total knee arthroplasty (TKA).
Although SONK can also occur in the lateral condyle of the femur or tibial plateau, the most common necrotic area is still the weight-bearing surface of the medial condyle of the femur. When SONK develops to late-stages, e.g., Mont stage III-IV, it often accords with the characteristics of single compartment lesions. UKA is a suitable surgical choice, which has been demonstrated to be effective in treating osteoarthritis and SONK in recent literature[23]. Servien et al.[18] prospectively compared 33 cases of SONK and 35 cases of knee osteoarthritis with an average follow-up of 5 years. It was found that the preoperative functional score of the SONK group was significantly lower than that of the osteoarthritis group. Still, the postoperative pain degree, knee score, and functional evaluation of the two groups were similar. The two groups showed identical 10-year survival rates (93% vs. 95%). Langdown et al.[24] retrospectively analyzed the clinical data of 29 knees with spontaneous osteonecrosis of the medial compartment treated with UKA and compared with 28 cases of osteoarthritis in the same period. The patients were followed up for an average of 5 years, ranging from 1 to 13 years. No significant difference was observed between the two groups.
In addition, it has been suggested that TKA may produce better results than UKA in the treatment of SONK. However, the studies were not performed rationally, which might lead to unreliable results[25]. Myers et al.[26] discussed the clinical efficacy of TKA and UKA in the treatment of SONK through literature review in 2006. They said that both surgical methods had achieved sound clinical effects after 1985. On the other hand, poor outcomes of UKA in the treatment of SONK mainly attributed to secondary osteonecrosis of patients who enrolled in the studies before 1985. In recent years, the improvement of surgical techniques, the development of prosthesis design, and the strict selection criteria have led to the significantly improved clinical efficacy and long-term survival rate of UKA[27]. Although TKA was not used as a control group in the current work, our practice has indicated that the late stage of SONK is entirely consistent with single compartment disease, while UKA treatment provides positive outcomes, thereby additional TKA is not required.
Choy et al.[28] retrospectively analyzed the data of 21 cases (22 knees) with SONK. The average follow-up period was 70.3 months, ranging from 48 to 93 months. The average HSS score was increased from 64.3 points before the operation to 92.0 points at the last follow-up. The average flexion angle was increased from 138.6° before the operation to 145.6° at the last follow-up. FTA was changed from 0.98° varus preoperatively to 3.22° valgus postoperatively, with an average correction of 4.2°. On the other hand, 84.2% of patients could complete squat, and 90.5% of patients could complete cross-leg movement. Guo et al.[29] performed UKA on 27 patients with SONK. During the average follow-up period of 27.8 months, there were no severe complications occurred. The VAS score of pain was decreased from (6.9 ± 0.9) points before the operation to (2.0 ± 1.1) points at the last follow-up. Additionally, the HSS score increased from (61.3 ± 9.7) points to (93.0 ± 4.8) points at the last follow-up. The overall satisfaction rate was 96.3%. Improved knee joint function, alleviated pain symptoms, and enhanced limb alignment were observed in this study, which exhibited similar results with other UKA studies.
Few reports have been published on the complications of UKA in the treatment of SONK. Bruni et al.[12] followed up 84 patients with SONK for an average of 98 months. A total of 10 patients underwent revision surgery: four cases were caused by the sinking of the tibial prosthesis; three cases attributed to aseptic loosening of the tibial prosthesis; one case was due to aseptic loosening of the femoral prosthesis; one case was due to medial tibial fracture; one case was due to prosthesis infection. None of these patients underwent revision because of the progression of osteoarthritis to the lateral and patellofemoral compartment of the knee joint. No severe postoperative complications were observed in this study. However, the provided evidence is limited according to the short follow-up time. One patient developed unexplained pain again in the medial knee joint 6.5 months after the operation and was treated with local block treatment. One year after the surgery, the pain symptoms were improved, but there was still occasional mild pain. It was found that after the necrotic bone tissue was removed during the operation, the defect became larger based on the data of this case. We thus consider that the pain may be related to massive necrosis, intra-articular soft tissue adhesion, synovitis, and other factors.
It is worth mentioning that the surgical performing techniques of UKA in the treatment of SONK and knee osteoarthritis are not entirely consistent. First, typical SONK is often accompanied by bone defects on the weight-bearing surface of the medial femoral condyle. If the defect is not correctly identified, it will cause the distal grinding bolt to be inserted too deeply, grind out more bone, and make the flexion and extension gap unbalanced[29]. Secondly, when the necrotic area is large, and collapse is deep, the dead bone should be scraped off rather than being grinded in a wide range to retain sufficient bone mass. Finally, the necrotic bone becomes hard around it, which is difficult to grind and easily causes residual necrotic bone. If the prosthesis is installed on the necrotic bone, it may result in early failure. Therefore, the scraping of necrotic focus is critical. Small residual bone defects can be filled with bone cement, while significant bone defects can be filled with autologous bone graft[30–32]. In the current study, most of the necrotic foci were removed by conventional osteotomy. Two cases of small residual lesions after conventional osteotomy were filled with bone cement after curettage. No loosening or subsidence of prosthesis was observed during the follow-up period.
However, there are still several potential limitations of this study. First, the current study was a retrospective study of a single institution without a control group, which may easily cause selection errors. Secondly, only 18 UKA were identified as late-stage SONK. The low incidence rate of SONK in the population is responsible for the small patient size. Finally, the follow-up time was relatively short, while more long-term studies are required to identify possible complications, such as loosening and revision.