Relationship between the correction angle in medial open wedge high tibial osteotomy and the postoperative state of patellofemoral cartilage

Background: To investigate clinical and radiologic effects of medial open wedge high tibial osteotomy (MOWHTO) on the patellofemoral joint in relation to the correction angle by comparing changes in cartilage from before to after the surgery. Methods: A total of 124 MOWHTO cases were divided into the three groups of small, moderate, and large, depending on the correction angle. Clinical and radiologic outcomes were compared at the mean follow-up time of 38.8 months. Postoperative cartilage changes were assessed during implant removal completed at two years after MOWHTO. Results: There was no signicant difference evident in most clinical outcomes measured, including Lysholm score and the Knee Injury and Osteoarthritis Outcome score subscales, with the exception of postoperative Shelbourne and Trumper score between the groups (P< 0.001). International Knee Documentation Committee scale value on radiologic evaluation did not signicantly differ among the groups. A progression of cartilage degeneration was noted in 40.3% of femoral trochleas and 22.6% of patellas. Cartilage degeneration was signicantly progressed in the large correction angle group versus in the other two groups. Regression analysis showed that the correction angle had a signicant effect on cartilage deterioration (Trochela, P=0.009; Patella, P=0.034). Conclusion: Patellofemoral joint was more adversely affected by MOWHTO in conjunction with the requirement of a larger correction angle. Thus, cases necessitating considerable correction should be forewarned of patellofemoral joint symptoms within a relatively short time after the procedure.

It is only recently that study on the postoperative effect of MOWHTO on the patellofemoral joint has begun to attract attention. Several biomechanical studies have shown that distalization and lateralization of the tibial tuberosity caused by MOWHTO lead to loss in patellar height. [12][13][14][15] Loss in patellar height leads to an increase in patellofemoral contact pressure, which in turn may cause a higher incidence of arthritic change of patellofemoral joint and anterior knee pain. [15,16] On the basis of this biomechanical nding, a few researchers have performed clinical studies with second-look arthroscopy on the knee treated with MOWHTO to examine the postoperative state of patellofemoral joint. [17][18][19] Although these clinical studies have agreed that the MOWHTO cause some degenerative progression in the patellofemoral joint of many patients, the reported postoperative functional assessment showed inconsistent results between the studies, making it di cult to reach a de nite conclusion on the clinical effect of MOWHTO on the patellofemoral joint. Furthermore, there is a lack of study on the in uential  Included patients were divided into three groups depending on the correction angle. The medial proximal tibial angle (MPTA) was measured on knee standing anteroposterior (AP) view before initial MOWHTO and just prior to the second surgery of implant removal. The MTPA was de ned as the angle obtained from an intersection between the articular surface of the proximal tibia and the tibial mechanical axis (Fig. 1). The correction angle was de ned by subtracting MPTA measured before the second surgery from the initial MPTA. [21] The calculated correction angles were divided into quartiles and the patients were subsequently divided into three groups on the basis of the interquartile range of the correction angles. The rst group consisted of patients who were included in the lowest quartile (< 7.8°); these individuals were de ned as the small correction angle group (Group S = 31 patients). The second group of patients, who had correction angles in the middle two quartiles (7.8° to 11.6°), were named as the moderate correction angle group (Group M = 62 patients). Patients with a large correction angle, who were included in upper quartile (> 11.6°), were classi ed as the large correction angle group (Group L = 31 patients).

Surgical Procedure
A single surgeon performed all operations. AP full-length weight-bearing radiograph was taken for preoperative planning, and the target angle of valgus correction was measured with use of Miniaci's method. [22] The realigned weight-bearing line was planned to pass through the Fujisawa point (62.5% from the medial edge of the tibial plateau). [23] Arthroscopic examination was initially done to assess the medial, lateral, and patellofemoral joint. And then, a skin incision was made obliquely on the medial proximal tibia and distal insertion of the super cial medial collateral ligament was stripped off the bone. Osteotomy site was usually determined near the upper border of the pes anserius. Under the guide of an image intensi er, two Kirschner wires were inserted toward just proximal to the tibio bular joint. Along the Kirschner wires osteotomy was performed. The lateral-most 10 mm of the proximal tibia was preserved without damage in order to be used as a hinge during correction. A separate ascending osteotomy for the biplanar cut was performed about 1 cm to 1.5 cm posterior to the tibial tuberosity, and it was cut at an angle of 100° to 110° from the direction of initial osteotomy. Using four chisels, the osteotomy site was gradually opened up. To open and measure the gap of the osteotomy, the TomoFix bone spreader was inserted into the osteotomy gap. The opening process was performed such that the anterior gap of the opening was about two-thirds the size of the posterior opening to maintain the tibial slope. [24] Fixation of the osteotomy was done with the TomoFix® plate (DePuy Synthes, Solothurn, Switzerland) and locking screws.

Postoperative Rehabilitation and Implant Removal
A splint was applied for initial two weeks, and then knee brace was worn for next four weeks, during which time range of motion (ROM) of the knee joint was allowed within the tolerable range. Partial weight-bearing was initiated beginning at two weeks after the operation. The brace was discarded, and full-weight bearing was allowed at six weeks. Implant removal was performed at two year postoperatively with con rmation of bone union under simple radiography. Second-look arthroscopy to evaluate the cartilage state was also performed during the operation of removal.

Radiologic Evaluation
For radiological assessments, standing true anteroposterior full-length lower extremity radiograph, anteroposterior and lateral view of the both knee weight bearing radiographs, and 45° Merchant view were taken. In addition to MPTA, the weight bearing line ratio, hip-knee-ankle angle, modi ed Blackburn-Peel ratio, and the International Knee Documentation Committee (IKDC) radiographic scale value were evaluated by two orthopedic surgeons. Their evaluations were performed independently of each other, and were completed preoperatively and at the last follow-up for all patients. For both the preoperative and last evaluations, each surgeon measured these parameters twice in all knees, with an interval of two weeks between the measurements. The average of these two measurements was used for nal analysis.
The weight bearing line ratio was obtained by rst drawing a weight bearing line from the center of the femoral head to the midpoint of the talar superior articular surface; subsequently, the horizontal distance from the medial edge of the tibia plateau to the tibial intersection of the weight bearing line was recorded.
With the medial edge at 0% and the lateral edge at 100%, the weight bearing line ratio was set as a ratio of the horizontal distance from the medial edge of the tibia divided by the whole width of the tibia (

Arthroscopic Examination of the Articular Cartilage Status
Changes in patellofemoral articular cartilage between initial and second-look arthroscopy were compared. Grades of the cartilage status were evaluated for both the patellar and femoral trochlea surfaces with ICRS grade, (Fig. 3-A, B). Grading results were rst determined by a single operator, with the nding subsequently needing to be concurred with by two trained fellows. When the cartilage showed improvement with partial or full recovery at the defect area or if there was no interval change between the ndings of initial and second-look arthroscopy, it was categorized into the non-progressed group. Cases with any evidence of progressive change in either the patellar or femoral trochlear cartilage were categorized in the progressed group.

Statistical Analysis
One-way analysis of variance (ANOVA) was used for normally distributed continuous variables. If the result of the ANOVA test had statistical signi cance, subsequent post-hoc analysis (Bonferroni's test) was performed. The Kruskal-Wallis test was rst applied for non-normal distribution and the Mann-Whitney U test was then carried out as the post-hoc analysis for any result that had statistical signi cance. The chi-square test or Fisher's exact test was employed to compare categorical variables. Logistic regression analysis was used to determine any effect of independent variables such as age, sex, body mass index, preoperative hip-knee-ankle angle, postoperative modi ed Blackburn-Peel ratio, and correction angle on the cartilage status progression of patellofemoral joint. The intraclass correlation coe cient (ICC) was calculated to evaluate both intraobserver and interobserver reliabilities of the measured values. Statistical analysis was done using SPSS software (version 23.0; IBM Co., Armonk, NY, USA). A p-value of less than 0.05 was set to be statistically signi cant. The statistical power was calculated with the use of PASS software (version 11, NCSS, LLC, Kaysville, Utah, USA).

Results
The mean correction angles were 6.9° ± 0.6°, 10.0° ± 1.1°, and 13.2° ± 1.0° for Groups S, M, and L, respectively. The mean ages at the time of surgery were 56.6 ± 2.4 years in Group S, 55.6 ± 3.4 years in Group M, and 56.3 ± 2.6 years in Group L. There were eight males and 23 females in Group S, 14 males and 48 females in Group M, and nine males and 22 females in Group L. The mean follow-up period was 38.8 ± 4.7 months. The three groups did not differ signi cantly in terms of age, sex, affected side, body mass index, the mean follow-up period, and the elapse time from the initial surgery to the implant removal (Table 1). Preoperative functional evaluations such as Lysholm knee score, KOOS Pain, KOOS Symptoms, KOOS Activities of Daily Living, KOOS Sport/Rec, and KOOS Quality of Life did not differ between groups. Radiological evaluation regarding the MPTA, modi ed Blackburn-Peel ratio, and IKDC radiographic scale did not show signi cant difference between the groups. The only statistically signi cant difference was found in the preoperative hip-knee-ankle angle, in which Groups S, M, and L had mean angles of -6.9° ± 2.5°, -8.2° ± 2.1°, and − 10.0° ± 3.4°, respectively (p < 0.001) ( Table 2). The preoperative proportion of ICRS grade of the femoral trochlea and patella did not differ between the groups. n.s.
a The values are given as mean ± standard deviation b The values are given as a n (%) Comparison of postoperative outcomes showed statistical signi cant differences in terms of Shelbourne and Trumper score: Group S = 89.8 ± 7.1, Group M = 89.3 ± 7.6, and Group L = 84.8 ± 5.7 (p = 0.008) ( Table 3). Subsequent post-hoc multiple comparison analysis revealed signi cant differences between Group L and the other two groups, while no signi cance was evident between Groups S and M (Table 4).
Other postoperative functional evluations such as Lysholm knee score and KOOS subscales did not show statistically signi cant. Postoperative comparisons of IKDC radiographic scales, hip-knee-ankle angle, and modi ed Blackburn-Peel ratio had no statistical signi cant difference between the groups ( Table 3). The interobserver and intraobserver reliabilities measured preoperatively were 0.811 and 0.883 for MPTA, 0.872 and 0.897 for hip-knee-ankle angle, and 0.802 and 0.839 for modi ed Blackburn-Peel ratio, and 0.782 and 0.853 for IKDC radiographic scale, while the postoperative interobserver and intraobserver reliabilities for radiographic parameters including the MPTA (0.791 and 0.871), hip-knee-ankle angle (0.835 and 0.911), modi ed Blackburn-Peel ratio (0.772 and 0.829), and IKDC radiographic scale (0.759 and 0.868), respectively. The values are given as a n (%)   (Table 5).

Discussion
The purpose of this study was to investigate the relation of bony correction angles achieved after MOWHTO to the state of patellofemoral joint cartilage and clinical outcomes. In this study, we discovered that (1) there was cartilage lesion progression in 40.3% of the femoral trochleas and 22.6% of the patellas included in this study; (2) the cartilages were more signi cantly deteriorated when the bony correction angle to be achieved became larger; and (3) there was no signi cant difference evident in most clinical outcomes measured, including Lysholm score and the KOOS subscales, with the exception of postoperative ST score.
The primary nding of the current study was that there is a correlation between MOWHTO and postoperative arthritic progression in the patellofemoral joint. Agneskirchner et al. [12] noted in their biomechanical cadaveric study, which investigated changes in the patellofemoral joint after the osteotomy, that lateralization and distalization of the tibial tuberosity produces loss in patellar height. This leads to elevation in patellofemoral contact pressure and, in turn, the loss results in more incidence of anterior knee pain with a progression of cartilage degeneration in the joint. Based on their biomechanical study, Goshima et al. [17] and Kim et al.[18] further looked into (or assessed/investigated) the clinical aspects of the articular cartilage and their ndings con rmed progressive degeneration through second arthroscopy performed in patients who had undergone MOWHTO previously. Our study found that about 41.1% of all patients had progression, and we consider this high prevalent rate of cartilage degeneration in the joint to be a direct result of patellofemoral pressure elevation subsequent to the surgery.
We also demonstrated that Group L with a higher bony correction angle showed signi cantly more extensive cartilage progression than did the other two groups with less correction. Each articular cartilage on both the patella and femoral trochlea revealed signi cant differences in progression among the groups [Group S = 22.6%, Group M = 40.3%, and Group L = 58.1% for the femoral trochlea (p = 0.017) and Group S = 12.9%, Group M = 19.4%, and Group L = 38.7% for the patella (p = 0.036)]. Following logistic regression, we also discovered statistical differences in the angle of correction. Javidan et al., [14] through a biomechanical study, noticed that MOWHTO leads to a rise in the patellofemoral joint's contact pressure and also that there was a signi cantly greater increase of contact pressure in 15 mm widening than in 10 mm widening. Our clinical results were comparable to their study's outcomes. The reason for higher prevalence of cartilage degeneration in the group with a larger correction angle is in accordance with the results of the biomechanical studies, in which it was found that a larger correction angle consequently further promotes distalization and lateralization of the patella, resulting in a much higher contact pressure of the patellofemoral joint, as found in Javidan et al.'s study.
Goshima et al. [17] performed second-look arthroscopy in 60 cases at 19 months after the initial MOWHTO procedure on average and found that 45% of the patients showed progressive degeneration of the cartilage with respect to ICRS grade. However, when the mean follow-up time was extended to 58.2 months, there was no de nite correlation of this degeneration with clinical outcomes. There may not be much clinical effect of loss in patellar height on signi cantly undesirable patellofemoral symptoms as they mentioned, but we believe that a larger correction angle would cause a signi cantly higher loss in the height, resulting in a rather dissimilar outcome versus their ndings. In addition, their score system for evaluating outcomes was more oriented to general knee symptoms rather than focusing on the patellofemoral joint, and only some of their scoring items were related with this particular joint. Thus, it can be postulated that, if their patients' main symptoms had been originating from the medial compartment, then the procedure performed considerably relieved these symptoms while discriminating developing symptoms related to the patellofemoral joint wherein postoperative anatomical alteration is relatively small compared to a change in coronal axis alignment of the medial compartment. To avoid this discrimination, our study used ST score to more closely evaluate anterior knee pain. Our results showed that postoperative ST score was lower than the score measured preoperatively in all groups, and, in particular, the score was signi cantly lower in Group L than in the other two groups. Thus, considering these ndings, careful patient selection is imperative, because, in patients who are expected to require a large correction angle during MOWHTO, patellofemoral joint symptoms may be incited and aggravated within a relatively short duration after the surgery.

Limitations
The present study had several limitations. First, there might be a selection bias in our study because it is a retrospective study based on medical records, instead of being a prospective controlled study. However, we conclude that our result is still reliable since there was no difference in preoperative demographic, clinical and radiological data among three groups. Second, the duration from MOWHTO to second look arthroscopy was 24.8 months, which was not enough to conduct long term research including cartilage status and degenerative change. However, any articular changes that occur are evident earlier under arthroscopy than are the changes that would be visible on radiography, and our utilization of second-look arthroscopy to detect any early changes and the application of these ndings in our analysis would help to compensate for our relatively short follow-up time. Lastly, no control group with comparable demographic characteristics who did not undergo MOWHTO was available to make a direct comparison of our ndings to the effects of normal age-dependent joint degeneration, since there is a possibility that age-dependent joint degeneration may be a culprit in our nding of patellofemoral joint's cartilage degeneration. This possibility should be evaluated using the same two-stage arthroscopy protocol without undergoing the correction surgery over a period of several years. However, it is considered unethical and impractical to perform multiple arthroscopies without MOWHTO in patients with known medial compartment osteoarthritis of their knee joint merely to examine changes in the patellofemoral joint.

Conclusion
When the correction angle to be achieved during MOWHTO becomes larger, such leads to more prevalent and substantial cartilage degeneration in the patellofemoral joint with further adverse clinical outcomes. Thus, in cases that require considerable bony angle correction, careful patient selection is necessary since these patients may suffer from subsequent patellofemoral joint symptoms within a relatively short time after the procedure. The medial proximal tibial angle was de ned as the angle obtained from an intersection between the articular surface of the proximal tibia and the tibial mechanical axis. (α°)