The most important finding of our study is that although Cartilage Repair Assessment grade III or IV cartilage regeneration could be found in about one third of all cases, there were no functional benefits compared to poor cartilage regeneration. The main focus of this research was to determine the factors associated with cartilage regeneration and we found that cartilage lesions ≥2.5 cm2 and the presence of a kissing lesion were strongly associated with poor cartilage regeneration, compared with age, sex, BMI, pre- and postoperative HKA angle, correction angle, pre- and postoperative tibial slope, meniscectomy, and root tear of the meniscus. Although there are a few studies to examine factors associated with cartilage regeneration after HTO, microfracture under arthroscopy was not performed. Recently, Kumagai et al. reported that the factors that affected cartilage regeneration after HTO, were such as postoperative limb alignment, preoperative cartilage degeneration grade, and BMI, but not by age or gender. Kim et al. found that regeneration of articular cartilage was associated with a smaller mean preoperative varus mechanical tibiofemoral angle and lower BMI for the medial femoral condyle. Bae et al. performed a study of microfracture in an OA knee with a minimum 10-year follow-up and reported that cartilage lesions >2 cm2 and a preoperative mechanical axis which bisected the total width of the tibia more than 25% were associated with poor results. But BMI and partial meniscectomy was not contributing factors.
According to the patient factors, consistent with Bae et al., we could not find any effect of BMI on cartilage regeneration, contrary to Kumagai et al. and Kim et al.. The relatively low BMI of our study sample might have been the reason why there was no association with cartilage regeneration. In a study of arthroscopic abrasion arthroplasty combined with HTO, Akizuki et al. showed that age was the only factor influencing tibial cartilage. Though some studies have demonstrated a better cartilage regeneration in younger individuals, especially < 35 years, our investigation revealed no age effect since we had included patients with high-grade OA, all of them above 40 years of age.
A large varus deformity as determined by the preoperative HKA angle is also one of the contributing factors for the regeneration of the cartilage in most studies. Kumagai et al. and Koshino et al. have also shown that mature regeneration was observed more frequently in the knees with more than 1° of mechanical valgus angulation after osteotomy than in those with less than 1°. However, Tsukada and Wakui recently reported no significant differences in the ratio of cartilage repair between overcorrected (defined as knees with mechanical valgus angulation ≥8°) and moderately corrected (defined as knees with mechanical valgus angulation ≤8°) knees. Our study did not show any association between clinical outcome and cartilage regeneration with pre- and postoperative HKA angles as well as the correction angle. The goal of our corrective osteotomy was to achieve three to five degrees valgus angulation and hence most of our patients had overcorrected knees. It is well known that an open wedge HTO tends to increase, and a closed wedge HTO tends to decrease the postoperative slope[17,18]. Although the tibial slope increased after surgery, neither the preoperative tibial slope nor its change affected cartilage regeneration postoperatively.
Meniscal root tear is commonly associated with high-grade degenerative arthritis. Menisci are important for resisting axial load by generating hoop stresses. Meniscal root tear alters the knee’s biomechanics[19,20,21,22]. A root tear of the meniscus is almost equivalent to total meniscectomy, increasing tibiofemoral contact pressure as demonstrated by Allaire et al.. Takahashi et al. showed that tears in the posterior root of the medial meniscus were independent risk factors for cartilage degeneration as shown on T1p MRI scans. Though statistically not significant, most cases with poor cartilage regeneration in our study had a root tear of the medial meniscus. Krych et al. reported that non-operative management of the medial meniscus root tear is associated with worsening arthritis and poor functional outcomes at a five-year follow-up. Most of the root tears in our study were managed with either partial or subtotal meniscectomy to obtain a stable peripheral rim. Sterett et al. reported that patients with a tear of the medial meniscus at the time of chondral resurfacing were 9.2 times more likely to undergo TKA than those without tear. But similar to Bae et al., our study also showed that meniscectomy in general does not affect cartilage regeneration.
Among all factors, the most significant were the size of the cartilage lesion and the presence of a kissing lesion. In our study, a cartilage defect ≥ 2.5 cm2 was the most consistent factor associated with poor cartilage regeneration. Moreover, when the kissing lesions is present, it can possibly affect cartilage regeneration by disturbing the stable position of marrow stem cells during the regeneration process. Like in our study, most investigations assessing cartilage regeneration have shown that a larger cartilage defect is associated with poor cartilage regeneration[11,13,24].
Similar to Schuster et al., our study revealed that there were no statistically significant differences in functional outcomes between well regenerated and poorly regenerated groups. This can be interpreted that postoperative functional improvement is mainly due to the medial compartment’s unloading after HTO.
This study had several limitations. First, it was a retrospective study with limited cases. Second, tibial cartilage regeneration was not taken into account. Third, cartilage factors were not treated in detail. Fourth, we did not compare with the control group which patients who had undergone HTO without MF.