OA affects any or all three compartments of the knee. However, one third of patients are afflicted in only one of these compartments, many of them having a medial compartment disorder .
The purpose of surgery for unicompartment OA is to reduce pain, restore function, and improve the patient’s quality of life. The most important finding of this study was that both HTO and UKA are satisfactory operative treatment options for symptomatic medial knee OA.
Patient selection is generally stricter for individuals undergoing HTO than for those receiving UKA. However, medial knee arthritis patients selected for HTO experience many benefits. Ideal indications for HTO include (1) young and active patients (age <65 years) [23, 24], (2) normal-range body mass index (BMI) , (3) mild articular destruction (no more than grade 2 Ahlbäck classification), (4) no patellofemoral arthrosis , and (5) good ROM and a stable joint .
Age, BMI, and pre-operative state OA are key factors that optimize clinical outcomes and survival in patients undergoing HTO. Previous studies have reported that a pre-operative BMI higher than 27.5 is a significant risk factor for early failure , and patients with BMI over 30 exhibit significantly lower KSS and WOMAC scores 5 years after HTO . Moreover, HTO is not advisable for patients older than 65 years due to the 7.6% increased risk per year of age and the 1.5-fold relative risk of failure compared to younger patients .
HTO and UKA share similar indications that include the following: age 55–65 years, moderately active, non-obese, presenting with mild varus malalignment and moderate unicompartmental arthrosis, no joint instability, and good ROM .
Indications for UKA are broadening after reports of promising mid- and long-term results, which include isolated medial or lateral compartment OA, osteonecrosis of the knee, age over 60 years, weight under 82 kg, and an ideal ROM of 90 with fewer than 5° flexion contractures. Contraindications include high activity, age under 60 years, and inflammatory arthritis [11, 30, 31].
Our analysis demonstrated a significant difference in outcomes between UKA and HTO patients, with the former showing better functional results (excellent/good results), and the latter better ROM. This discrepancy was correlated with knee score and ROM, indicating the possibility of additional impacts on the functional results.
Earlier publications reported a valgus deformity treated with either procedure. In our opinion, the clinical results for patients with a surgically treated valgus deformity, by either arthroplasty or osteotomy, cannot be compared to results for patients with a varus deformity. Major differences between medial and lateral UKA, as well as between varus and valgus osteotomy have been noted [4, 9, 32–34]. Therefore, our analysis showing excellent/good functional results focused only on studies with a strict inclusion criterion of medial knee OA with a varus deformity; analysis of such cases showed no significant difference between the two procedures. The subgroup analysis yielded similar findings and revealed favorable results in the UKA group relative to closed-wedge HTO (CWHTO) patients; however, these results were not noted when comparing open-wedge HTO (OWHTO) and UKA. CWHTO was the main treatment method for HTO in the past, but OWHTO was recently reported to yield good or excellent results, owing to improvements in surgical techniques and implant stability [35, 36]. However, recent meta-analyses comparing CWHTO and OWHTO did not report superiority of OWHTO over CWHTO [37, 38].
A greater change in ROM was noted in the UKA group relative to the HTO group due to a lower pre-operative score . Takeuchi et al.  reported that OWHTO is a more appropriate treatment method for active patients who require good ROM of the knee. The unsatisfactory results of the HTO group were mostly due to an insufficient deformity correction. Previous studies reported that optimal results can be achieved if the mechanical alignment is adjusted to 7° . Nevertheless, the ultimate post-operative valgus position is technically challenging to achieve.
Free walking speed (velocity) has been proven both a reliable and a valid indicator to evaluate treatment outcomes in knee OA patients [40, 41]. Our meta-analysis found no significant difference between the two procedures in terms of velocity (p = 0.66), although Fu et al.  reported otherwise (p = 0.05). However, given that both studies used the same literature to arrive at this outcome, differing results were not expected. It is also important to note that Jefferson et al.  assessed the velocity outcome of three operative methods (HTO, UKA, and TKA), with post-operative results reported as 1.02 ± 0.19, 0.99 ± 0.21, and 0.81 ± 0.19 m/s, respectively. However, Fu et al.  included the TKA results (0.81 ± 0.19) in their analysis of the HTO group, which may suggest an inaccuracy. Therefore, our results are more accurate and reliable.
Our analysis revealed that free walking speed was improved after both HTO and UKA but with an equivalent rise in the UKA group. Borjesson et al.  stated that, compared to HTO patients, UKA patients had a greater increase in free walking speed, with results 5 years after surgery that were highly similar to the walking speed of healthy people of the same age group . Moreover, both procedures resulted in an almost normal gait pattern.
Ivarsson et al.  showed that UKA patients have better muscle strength than do HTO patients 6 months post-operatively, but the 12-month post-operative results were similar. One explanation for this finding is that rehabilitation of UKA patients normally begins earlier, whereas HTO patients usually undergo an immobilization period. Moreover, HTO patients may require a longer time to adapt due to greater changes in post-surgical leg alignment.
Regarding the progression of knee OA, our analysis showed that the OR of the risk of contralateral and patellofemoral deterioration did not differ between groups, although the HTO group tended to exhibit this problem. One logical explanation is that this phenomenon is due to the overcorrection to unleash the medial compartment during the procedure, thus suppressing the lateral compartment and leading to deterioration. Overcorrection of more than 6° was associated with progressive degeneration of the lateral compartment . In addition, OWHTO above the tibial tubercle can have adverse effects on patellofemoral articulation [2, 45, 46]. Yim et al.  compared OWHTO and UKA patients and reported that two cases of UKA showed patellofemoral joint OA compared to three cases of OWHTO.
Compared to UKA, the chance of post-operative complications is greater after an osteotomy . Our analysis revealed a significant difference in such complications between HTO and UKA patients, supporting previous studies and a meta-analysis by Spahn et al. . Among all included studies in this present study, five trials applied OWHTO, seven trials used CWHTO, and one study used hemicallotasis. OWHTO is considered safe and easy [21, 48, 49] based on the assumption that CWHTO may be associated with a higher incidence of complications, especially peroneal nerve paralysis. Despite improved surgical techniques and implant design, previous studies have reported complications after UKA, such as loosening of the tibial or femoral component or osteoarthritic changes in the development of the lateral compartment due to antero–posterior instability of the knee, which leads to rapid wearing of the polyethylene insert [11, 21]. In the HTO group, most complications were associated with an intra-articular fracture, nonunion, infection, and peroneal nerve palsy.
TKA is defined as a clear end-point after both HTO and UKA. Medial UKA patients tend to require revision sooner , with a mean of 8.2 years compared to a mean of 9.7 years for valgus HTO patients . Barrett and Scott  reported 29 unsuccessful UKA revisions to TKA and observed that the mechanism of failure was loosening in 55% of cases and degeneration advancement of the remaining compartments in 31% of patients. Technical errors during the primary UKA and poor selection of patients contributed to 66% of failures.
Cross et al.  examined the operative time and found that revision to TKA in HTO patients required more time compared to that for UKA patients, which could be because the HTO procedure is complicated by difficulties in obtaining an acceptable exposure, removing retained hardware, achieving correct tibial component positioning, scarring, and additional challenges with ligamentous balancing that have been reported to result from a prior HTO. The major technical difficulty in the revision UKA group was handling the bony defects on both the tibial and femoral sides. Significantly thicker polyethylene inlays were required during the revision of UKA to TKA compared to primary TKA , and the UKA group required substantially more osseous reconstruction (77%) compared to the HTO group (20%) .
Consistent with the previous meta-analysis , the present study also failed to identify any significant difference in the revision rate between the two procedures. Although both groups exhibited higher revision rates over time with deteriorated clinical outcomes, the risk of revision of primary UKA declined with age. The 10-year revision rate was nearly 24% in patients aged less than 55 years, threefold higher than that in those aged 55 years and older .
Robertsson et al.  reported that hospitals that perform 23 or more UKAs per year have a 1.6-fold lower revision rate compared to those who perform fewer than 23. Therefore, routine patient selection and good surgical skills are believed to influence the results of the UKA procedure; this principle may also apply to HTO.
Several limitations of this study should be noted. First, a controlled randomized trial is challenging due to ethical concerns. The present meta-analysis included only three randomized controlled trials of the 15 studies and the patients enrolled for HTO tended to be younger than those enrolled for UKA. Although most studies reported good numbers, the use of diverse analyzing systems and methods can lead to difficulties comparing and assembling the outcomes, as well as inability to evaluate essential items such as radiographic changes due to inadequate data. Moreover, the current analysis showed that UKA and HTO are distinct in terms of their techniques and indications for patients with medial unicompartmental OA. Finally, the small patient population made it difficult to compare the two procedures and arrive at a conclusion regarding the clinical outcomes.