MOWHTO reportedly achieves significant improvements in pain and function, slows down the progression of OA, and postpones the need for TKA[5, 8, 10, 14–18]. However, this surgical treatment also causes numerous problems during follow-up. First, the treatment effect seems to deteriorate over time, with reported survival rates of 51–97.6% at 10 years postoperatively[5, 10, 20–26]. Second, complications occur after MOWHTO in 10–50% of patients[5, 27]. Although there are few serious adverse events, the incidence of complications is higher after MOWHTO (28%) than that after TKA (7%)[28, 29]. Third, MOWHTO is not the ultimate treatment for OA of the medial knee compartment, and the outcome of TKA performed after MOWHTO may be influenced by factors such as the patellar height, condylar offset, and/or tibial inclination angle[19]. Therefore, clinical treatment must be guided by strict indications and predictors, especially for the novel spacer-type MOWHTO.
Because of the differences between surgical procedures, the prognostic factors of spacer-type MOWHTO may differ from those of conventional MOWHTO with internal fixation[18]. In our study, spacer-type MOWHTO obtained better results in patients with a younger age and lower K-L grade, while age > 70 years and K-L grade IV were identified as risk factors for dissatisfaction following spacer-type MOWHTO. Previous studies have shown a correlation between tibial radiological values and clinical signs, but no study has evaluated the effect of these indexes on the outcome of spacer-type MOWHTO. The HKAA, MPTA, and PTSA are corrected to a specific high tibial osteotomy standard position to ensure the attainment of anatomic postoperative lower limb alignment and tibial plateau retroversion. Some studies have reported that a small HKAA (< 15°) is more suitable for spacer-type MOWHTO[18, 19]. Therefore, we excluded patients with radiographic measurements that were outside the ranges suggested in the literature. This means that more stringent preoperative planning is needed for spacer-type MOWHTO than for conventional MOWHTO to ensure efficacy and minimize complications.
Although the incidence of knee OA was significantly higher in women than in men, our study found that gender was not a significant prognostic factor for spacer-type MOWHTO[5, 10]. Furthermore, the long-term history did not affect the frequency of poor results.
Age was a significant predictive factor of a poor outcome after spacer-type MOWHTO. For patients older than 70 years, it is difficult to achieve ideal knee functional recovery after surgery. Long-term incomplete weight-bearing (approximately 3 months) and osteoporosis may be the main causes of muscular atrophy and decreased physical activity[5, 30, 31]. After surgery and muscle strength recovery were still a challenge for elderly patients, even though muscle training was used in patients. Older patients often have more severe osteoporosis, which increases the risk of intraoperative hinge fractures. Advanced age and osteoporosis were also risk factors that affect the healing of the osteotomy plane, which can delay the complete weight-bearing time. Furthermore, the increased recovery time resulting in activity reduction increases the risk of lower extremity thrombosis and decreases lung function.
In our study, the outcome of spacer-type MOWHTO was much worse for patients with severe knee OA with a K-L grade of IV than for patients with a K-L grade of II-III. This suggests that the outcome of spacer-type MOWHTO was affected by degeneration of knee joint. Severe knee lesions such as patellofemoral arthrosis[32], synovitis, cartilage defects or ligamentous knee instability[32] may affected the surgery outcome even though lower limb alignment was completely corrected. Similarly, previous studies have reported that patients with K-L grade IV OA achieve unsatisfactory results after conventional HTO. Studies have shown that inflammatory factors in the joints play a more important role than dynamics in the late clinical manifestations of knee OA[33–35].
Although BMI was not considered to be an independent risk factor for spacer-type MOWHTO, we thought that patients with a high BMI had a higher risk of surgical complications. The incidence of complications such as loss of valgus correction angle due to loosening and prolapse of the implantation, hinge fracture, persistent postoperative pain and numbness, and reoperation was higher in patients with a high BMI. However, for conventional MOWHTO, the locking compression plate provides reliable stability that enables the cut tibia to bear body weight[17]. Due to the instability of the medial tibia after spacer-type MOWHTO, the placement of too much weight on the wedge cross end can cause implantation prolapse or hinge fracture.
Compared with traditional MOWHTO, this novel method has many advantages, such as a lower cost, avoiding the need for secondary surgery for fixation removal, maintaining the patellar height to avoid degeneration of the patellofemoral joint[10, 29, 36–38] and decreasing the posterior tibial slope. However, the longitudinal stability of the cut tibia cannot be guaranteed. Patients needed to experience long-term incomplete weight-bearing (approximately 3 months) to guarantee a great bone union and stability of the osteotomy plane. This was a shortcoming of this novel method, which may have affected the postoperative results of some patients and caused other complications[39]. It might be expected that long-term incomplete weight-bearing would produce muscular atrophy, loss of proprioception, and decreased physical activity[40]. Therefore, strict surgical indications and appropriate pre- and postoperative rehabilitation plans are essential.
Our study had several limitations that may have influenced the results. First, the follow-up duration was relatively short, with a maximum of 1 year. Thus, the follow-up duration was too short to enable the accurate evaluation of the outcome of the spacer-type MOWHTO but can be used to evaluate the short- or medium-term curative effect. Long-term follow-up studies are needed to confirm the predictive factors affecting the clinical outcomes of spacer-type MOWHTO. Second, fibular osteotomy was performed during spacer-type MOWHTO which may have impacted the clinical outcomes. However, recent studies have reported good outcomes after fibular osteotomy.