Comparison of the regeneration of cartilage and the clinical outcomes after the open wedge high tibial osteotomy with or without microfracture: a retrospective case control study

Open wedge (OW) high tibial osteotomy (HTO) is an effective surgical treatment for patients with medial compartmental osteoarthritis combined with varus alignment [1‐4]. The correction of the varus alignment by OW HTO provides change of the load distribution in the knee joint and improved functionality in patients with medial compartment osteoarthritic knees [5, 6]. Additionally, reduced load on the medial compartment due to lateral shift of the axial load has been shown to lead to biological regeneration of the articular cartilage after OW HTO [7‐9]. However, lateral and patellofemoral compartments could be affected and changes of the cartilage status of these compartments are also questionable.

Although excellent short- to mid-term results after OW HTO have been reported in terms of clinical and radiologic results, clinical and radiological deterioration has been observed over long-term follow-up [2, 10, 11]. Therefore, several studies have suggested that OW HTO combined with cartilage repair techniques such as microfracture, subchondral drilling, abrasion arthroplasty, and autologous chondrocyte implantation might enable more effective cartilage regeneration and improved long-term outcomes [12‐15]. Among them, microfracture has been one of the most commonly used procedures for cartilage repair procedures [14, 16‐18]. Mocrofracture creates multiple holes in the subchondral bone in order to stimulate bone marrow. The cartilage defects are filled with precursor cells, resulting in a new cartilage and regenerative tissue [14]. However, it is unclear whether postoperative clinical outcomes are associated with the quality of cartilage regeneration after OW HTO combined with microfracture. Furthermore, little is known about the factors that influence clinical outcomes after OW HTO.

The purpose of this study was to compare the regeneration of the articular cartilage, radiologic, and clinical outcomes after OW HTO with and without microfracture. Our hypotheses were that the articular cartilage in the medial compartment after OW HTO with microfracture would be regenerated better than that without microfracture regardless of the radiologic and clinical outcomes of both groups [19].

Among a total of 87 patients included in this study, 57 patients were included in group 1 (with microfracture) and 30 OWHTOs were included in group 2 (without microfracture), with a mean follow-up of two years. Both groups showed no statistical differences in terms of preoperative demographics. Additionally, no statistical differences between groups were observed with respect to WBL ratio, HKA angle, JLCA, Ahlbäck grade of osteoarthritis, range of motion, WOMAC scores and Knee scores (Table 1).

The preoperative and postoperative assessments of articular cartilage according to the arthroscopic examinations are summarized in Table 2. The improvement of ICRS grade of MFC was observed in 75.4% (43/57) among the patients of group 1, whereas 53.3% (16/30) among the patients of group 2 showed the improvement of ICRS grade of MFC (p = 0.014). The improvement of MTP was observed in 61.4% (35/57) of group 1 and 53.3% (15/30) of group 2 with no statistical difference (p = 0.742). Otherwise, the articular cartilages of the LFC deteriorated in 38.6% (22/57) of group 1 and 26.9% (8/30) of group 2, respectively (p = 0.674), and that of the LTP deteriorated in 36.8% (21/57) of group 1 and 40.0% (12/30) of group 2, respectively (p = 0.936). Additionally, the articular cartilages of the patella and trochlea also showed similar deterioration (group 1 vs. group 2; 47.4% vs. 40.0%, p = 0.480; 45.6% vs. 63.3%, p = 0.496, respectively).

The mean MOCART score of the MFC mean at postoperative 2 years was superior in group 1 compared to that in the group 2 (41.8 ± 18.6 vs. 31.8 ± 19.8, p = 0.023). The number of knees showing complete filling, hypertrophy, > 50% of adjacent cartilage, < 50% of adjacent cartilage, subchondral bone exposure were 14, 16, 9, 15, 3 in group 1 and 2, 1, 5, 6, 16 in group 2, respectively (p < 0.001). In terms of the integration of repaired cartilage to the border zone, the number of knees showing complete integration, visible demarcating border, defect < 50% of the length of repair tissue, defect > 50% of the length of repair tissue were 3, 29, 16, 9 in group 1 and 3, 6, 9, 12 in group 2, respectively (p = 0.035) (Table 3). Although the microfracture of the MFC appeared to be helpful for cartilage healing according to the MOCART score, it was helpful only for the degree of defect repair and the integration to the border zone excluding other variables.

There were no statistically significant differences of the WBL ratio, HKA and JLCA between groups. Moreover, the severity of osteoarthritis indicated by the Ahlbäck grading system showed no statistical difference between groups (p = 0.699). All clinical scores showed no statistical difference between groups at a mean 2-year follow-up (Table 4).

The results of univariate and multivariate regression analysis for evaluating factors affecting clinical scores after OW HTO are shown in Table 5. The univariate analysis showed that the postoperative values of HKA angle, WBL ratio, JLCA, Ahlbäck grade and postoperative cartilage healing of MFC and MTP were significantly related with the postoperative clinical scores at a mean 2-year follow-up. However, in mulivariate analysis, only postoperative WBL ratio was significantly associated with postoperative KS knee score and WOMAC pain score (p = 0.010 and p = 0.045, respectively).

The principal findings of this study were that microfracture of the MFC during OW HTO improved the regeneration of the articular cartilage of the MFC, whereas those in the lateral and patellofemoral compartments were found to have deteriorated at a mean 2-year follow-up. It was only effective with respect to the degree of defect repair (filling of the defect) and integration to the border zone of the MFC. Additionally, microfracture could not improve the radiologic and clinical outcomes after the OW HTO.

Several authors reported the regeneration of degenerated articular cartilage after HTO in the absence of any cartilage repair procedure [8, 9, 26, 27]. Fujisawa et al. [26] reported that the cartilage lesion was covered with fibrous and membranous tissue about 1.5 to 2 years after osteotomy if ideal correction was obtained. Koshino et al. [9] reported that 133 of 146 cases (91%) achieved partial or total coverage with fibrocartilage after closed wedge HTO. However, cartilage repair procedures such as microfracture, subchondral drilling, and abrasion arthroplasty in conjunction with HTO have reportedly contributed to more favorable clinical and histological outcomes [14, 18, 25]. Among them, microfracture has been commonly performed with OW HTO by many surgeons because of its safety and convenience. However, the effect of microfracture on clinical outcomes has been controversial [14, 16‐18]. In terms of clinical outcomes, Sterett et al. [28] reported excellent survival rates of 91% survivorship at 7 years after microfracture with OW HTO. Pascale et al. [18] reported that patient satisfaction was increased among those who underwent HTO plus microfracture compared with those who underwent HTO alone at the 5-year follow-up. However, Matsunaga et al. [14] reported that microfracture combined with HTO did not lead to superior clinical outcomes compared to HTO alone at 1, 3, and 5 years postoperatively. Moreover, Ferruzzi et al. [16] reported that microfracture associated with HTO provided the worst Hospital for Special Surgery scores and WOMAC scores compared to both HTO alone and HTO with autologous chondrocyte implantation.

In terms of the macroscopic healing of the articular cartilage after OW HTO, second-look arthroscopies have been found to confirm the regeneration of degenerated articular cartilage in the medial compartment after HTO without any cartilage regeneration techniques [8, 9, 29]. Indeed, Jung et al. [8] reported that the regeneration of degenerated cartilage of the MFC was found in 75–92% of patients after OW HTO without any cartilage regeneration strategies. Kim et al. [29] also reported that lesions in the MFC and the MTP of 104 HTO knees were improved in 54 knees (51.9%) and 36 knees (34.6%) without additional cartilage regeneration techniques. Moreover, Matsunaga et al. [14] reported that the second-look arthroscopy revealed better regeneration of the distal femur cartilage in the patients with the abrasion arthroplasty with HTO than those with either HTO alone (p < 0.0005) or microfracture with HTO (p < 0.01), with no difference between the 2 approaches.

Several articles have investigated the effect of simple HTO without any cartilage repair procedures on clinical outcomes [30, 31]. Spahn et al. [30] found that a patient history of more than 24 months, a poor preoperative clinical score, obesity, and smoking were associated with a worse outcome after HTO alone at midterm follow-up. In another study, age > 56 years and postoperative knee flexion less than 120° were significantly related to a poor outcome after simple HTO (p = 0.008 and p < 0.001, respectively), whereas the Ahlbäck grade of medial compartment arthritis and excellent preoperative KS scores were significantly related to a better outcome after simple HTO (p < 0.001 and p < 0.001, respectively) [31]. However, it has remained unclear which preoperative and postoperative factors affect clinical outcomes after HTO combined with a cartilage procedure. In our study, although no statistical results were obtained, a number of patients, whose cartilage regeneration did not agree with the improvement in clinical outcome after OW HTO with microfrature, have been identified (Figs. 2 and 3).

In terms of the MOCART score after HTO, Verdonk et al. [32] reported that complete integration to the border zone was found in 25% of patients at postoperative 2 years. However, Kim et al. [17] reported complete integration in 1 patient out of 14 patients with microfracture alone at 1-year follow-up. Our study also showed very low incidence of integration, which was consistent with the findings of Kim et al. (Table 3). Kim et al. [17] reported no hypertrophy of the cartilage, and subchondral bone marrow edema was found in 78.6% of patients with OW HTO plus microfracture. Conversely, in our study, hypertrophy of the cartilage was observed in 28.1% of patients, whereas subchondral bone marrow edema was found in only 29.8% of patients with OW HTO plus microfracture. These results could be attributed to the fact that the follow-up period of our study was 2 years, which was markedly longer than the 1-year follow-up of the study by Kim et al.

There are several limitations in this study. First, the follow-up period was only 2 years after surgery. A long-term investigation is necessary to determine whether the changes in articular cartilage affect survival after OW HTO. Second, a comparison has not been performed between the under-correction and over-correction groups. Degree of the correction could affect the biological regeneration of the articular cartilage after OW HTO due to the different loading on the medial compartment. However, there were few outliers under 55% or over 70% of the WBL ratio, so we assumed that degree of the correction would have little effect on the results. Third, the postoperative rehabilitation protocol differed between the patients with and without a microfracture. The differences in the timing of postoperative weight-bearing could affect the regeneration of cartilage.

Microfracture of the MFC during OWHTO only helped the filling of the degenerative cartilage defect and the integration of the cartilage with adjacent cartilage. However, the clinical and radiologic outcome could not be improved by mircrofracture in the OWHTO.