Development and validation of a prediction model for knee joint line orientation after high tibial osteotomy

High tibial osteotomy (HTO) is a common realignment operation for the treatment of medial compartment osteoarthritis (OA) with varus malalignment of the knee joint. Medial open-wedge high tibial osteotomy (OWHTO) has gained popularity in recent years because of advantages of better fixation system, better achievement of the target correction angle, and preservation of proximal tibiofibular joint [1‐4].

However, HTO can only alter the proximal tibial geometry irrespective of the localization of the center of rotation and angulation of the lower limb. A previous study have noted that correcting only the proximal tibia with the aim of achieving a normal mechanical axis(MA) with HTO may result in overcorrection of the tibia [5]. An overcorrection in the proximal tibia may induce abnormal knee joint line orientation (KJLO), which has been raised as a concern in previous studies for the negative effect on joint biomechanics [6‐9]. KJLO plays a crucial role in analysing the balance of forces across the knee joint. It represents the shear stress and biomechanical consequences of joint loading [10, 11]. A previous study has shown that KJLO of more than 5° induced excessive shear stress in the tibial articular cartilage [12].

A joint line parallel to the floor is an important objective after HTO [13]. However, no tools are currently available for surgeons that could predict the value of postoperative KJLO prior to the surgery. Postoperative KJLO can only be measured months after surgery when the patient is able to bear full weight and a standing full-limb anteroposterior (AP) radiograph can be performed.

The primary aim of this study was to determine the effects of various preoperative anatomical alignment parameters to the postoperative KJLO. Based upon this analysis, we aimed to devise an equation that predicts the value of postoperative KJLO.

This group included 36 women and 5 men with a mean age of 55 years (standard deviation [SD] 4.8; range 39–64) and a mean body mass index (BMI) of 26.1 kg/m² (SD 3.2; range, 20.3–32.7) (Table 1).

MTFA, TPI, G-KJLO, and G-AJLO significantly changed after HTO. G-KJLO increased after HTO from a mean value of − 0.8° to 2.9° (P < 0.001). G-AJLO decreased after HTO from a mean value of 8.3° to 1.5° (P < 0.001) (Table 2 and Fig. 3).

Preoperative G-KJLO (P = 0.004), preoperative TPI (P = 0.001), preoperative MTFA (P = 0.004), preoperative tibial width (P = 0.011), preoperative FCO (P < 0.001), preoperative FBA, and correction angle (P < 0.001) were the most significant contributors to postoperative G-KJLO univariate analysis (Table 3).

Based on the multiple linear regression analysis by stepwise elimination of parameters influencing postoperative G-KJLO, an equation has been derived that can estimate postoperative G-KJLO after HTO. The equation is as follows: postoperative G-KJLO (°) = 1.029 + 0.560 × preoperative G-KJLO (°) + 0.310 × preoperative TPI (°) + 0.463 × aimed correction angle (°). The R_adj² value for this equation was 0.721.

Internal validation using bootstrapping method showed good calibration performance of the equation (Fig. 4a). External validation also showed satisfactory performance of the equation with predicted squared correlation coefficient of 0.867 (Fig. 4b and c).

The most important finding in this study is the development of an equation that predicts the postoperative knee joint line orientation relative to the ground based on preoperative anatomical parameters measured from a single radiograph of the lower limb. The adjusted coefficients of determination value for this equation was 0.721.

Accurate preoperative planning is mandatory in the success of HTO. Several methods have been proposed by Hernigou et al. [2], Miniaci et al. [20], and Dugdale et al. [21] using a full-limb radiograph. These methods help the surgeons in determining the correction angle, preoperatively. Intraoperatively, Krettek et al. introduced the “cable method,” [22] and Lobenhoffer et al. used the “alignment rod method” to check the mechanical axis [23]. However, unexpected correction errors in HTO have been reported in many studies [24, 25]. The reason for these correction errors are unknown. Soft tissue laxity has been suspected as a factor affecting correction accuracy. Recently, one study has reported that joint JSTA can preoperatively quantify soft tissue laxity in HTO [26]. Our study also included JSTA in our analysis which would account for the soft tissue laxity. The postoperative value of G-KJLO, however, is unknown until a few months after the operation when the patient is strong enough to stand up straight to take a full-limb radiograph. Several cases have been reported where the G-KJLO is non-anatomical after HTO, while the mechanical axis is within normal range. Victor et al. reported that in knees of constitutional varus with advanced arthritis, KJLO is of a positive value because of the bone loss at the level of the medial distal femur [5]. Since HTO is an alignment procedure correcting only the proximal tibia, therefore, the bone loss at distal femur is not corrected. The aim to achieve normal mechanical axis with HTO, when anatomical factors contributing to varus alignment is still present such as in distal femur, results in overcorrection of the tibia which in turn increases the value of G-KJLO [27].

As shown in our multivariable linear regression analysis, several anatomical factors related to the postoperative G-KJLO are known. This represents that G-KJLO is not simply decided by a single factor but rather by a complex mixture of factors. One notable fact in our equation is that aimed correction angle is the only factor that can be controlled by the surgeon. Other factors are anatomical factors already decided before the surgery. It is a reasonable derivation to conclude that increasing 1° of correction angle in HTO increases 0.463° of G-KJLO. Preoperative G-KJLO, TPI, and aimed angle of correction were the most predictable factors associated with G-KJLO. In this study, 3 variables did not show any multicollinearity problems. Preoperative MTFA, preoperative tibial width, preoperative FCO, and preoperative FBA also showed to be significantly related to G-KJLO in univariable analysis; however, it was not included in the equation due to the correlation with TPI. The preoperative G-AJLO was expected to significantly affect the value of postoperative G-KJLO. Thus, it would have been included in our final equation because Lee et al. previously reported that G-KJLO changed significantly less than did the TPI after OWHTO because of the compensatory changes of G-AJLO [8]. The reason for elimination may be because G-AJLO is a dependent variable of G-KJLO.

With the use of our equation, the surgeon can estimate the postoperative value of G-KJLO, preoperatively. Predicting the G-KJLO prior to the surgery is very helpful to the surgeon because there are cases in which the postoperative G-KJLO is greater than a certain value while the mechanical axis is within normal range after HTO. As seen in our algorithm (Fig. 5), if the estimated postoperative G-KJLO is expected to be greater than a certain value, the surgeon should consider for an additional surgery, such as distal femoral osteotomy in addition to HTO or consider reducing the amount of correction. The most accepted target for the weight bearing line is 62.5% of the width of the tibia plateau. However, this value was based on the empirical results of Fujisawa et al. [28] Up to date, no clear scientific background of the correct target point is present. Birmingham et al. [29] proposed of templating the weight bearing line depending on the articular cartilage status of the lateral compartment. If the cartilage status of lateral compartment is of good quality, weight bearing line is templated towards the Fujisawa point. If the cartilage status of lateral compartment is unsatisfactory, weight bearing line is titrated towards the neutral point. Similarly, our algorithm suggests that in cases where G-KJLO is expected to be more than a certain value, the degree of correction should be reduced. The amount of correction, however, should not be reduced as to induce undercorrection of the malalignment. The weight bearing line should be titrated between neutral point and Fujisawa point. If the estimated postoperative G-KJLO is less than a certain value, the surgeon should undergo just the HTO as planned. Knowing the possibility of a secondary surgery beforehand is very helpful to the surgeon because there are a lot of factors influenced by an additional surgery, such as surgical time, surgical instruments, and tourniquet position. Informing the patient on the possibility of a secondary surgery beforehand is important. In our institution, we chose the certain value to be 5° based on the study of Nakayama et al. in which they stated that G-KJLO of more than 5° induced excessive shear stress in the tibial articular cartilage [12].

Some limitation should be noted in our study. First, this study only included patients with varus alignment who underwent OWHTO. The equation is not assured for the use in patients with valgus alignment who is planned for varus-producing osteotomy. Second, the anatomical alignment factors analysed in this study were measured only from standing full-limb AP radiograph. X-ray only reflects a two-dimensional aspect of a three-dimensional human structure. Therefore, predicting G-KJLO would have been more accurate if it was done with means of a three-dimensional imaging tool or gait analysis, even though it would have been less practical. We tried, however, to make the equation as simple and practical as possible, making it possible to derive G-KJLO with the use of only one AP radiograph of the lower limb.

In this study, we analysed which preoperative anatomical alignment factors contributed to the postoperative G-KJLO. Based on this analysis, we were able to devise an equation that could predict postoperative G-KJLO with preoperative anatomical alignment factors. This equation is expected to help select optimal patients and operative plan for HTO.