The most important finding of the present study was that the lower leg was longer following OWHTO; however, the lower leg length did not change after h-CWHTO. Furthermore, flexion contracture significantly decreased in the h-CWHTO group. Predictive factors of changes in lower limb length were not identified in either group. H-CWHTO may be a feasible surgical procedure for patients who do not require leg length change. Moreover, it is challenging to predict changes in leg length discrepancy before surgery. It may be necessary to explain to patients that leg length might change after surgery.
In the present study, a 6.0-mm average increase in leg length was observed in the OWHTO group, while an average decrease of 0.56 mm—which was not statistically significant—was observed in the h-CWHTO group. According to previous reports [5, 8], our results were similar to those of other OWHTO procedures, which demonstrated increased leg lengths after surgery. In conventional CWHTO, Nerhus et al. [5] reported that the mean decrease in leg length was 5.7 mm, while Magnussen et al. [8] reported that the mean decrease in leg length was 2.7 mm; both leg length changes were significantly different. This indicates that h-CWHTO was able to maintain lower leg length after surgery, unlike conventional CWHTO.
Compared with conventional CWHTO, h-CWHTO requires a small osteotomy gap and hinge point, which forms the center of rotation at the center of the tibia [13]. William et al. [14] reported that the method of osteotomy, as well as the degree of the correction angle, could affect the expected change in leg length according to preoperative planning. In cases requiring a large correction angle to correct a large varus deformity, the leg length is expected to be longer; for example, a 25-degree correction in the lower leg is expected to increase the length from 0.65 cm to 1.05 cm with conventional CWHTO, and 3.35 cm to 3.65 cm with OWHTO [15]. In summary, even with conventional CWHTO—which requires large bony resection of the tibia, as severe varus is corrected to slight valgus alignment—leg length is not always shorter. Our results showed that h-CWHTO was able to maintain leg length, even though the tibial length decreased by a mean of 6.39 mm after surgery; therefore, h-CWHTO is considered to be the preferred surgical technique for patients requiring an unchanged leg length due to the relationship between the leg length discrepancy and the contralateral side.
Our results also indicate that the preoperative opening or closing distances were not associated with changes in the entire leg length. Bae et al. reported that the change in limb length was significantly correlated with the correction angle on the navigational system among 30 cases of OWHTO [7]. Magnussen et al. [8] also reported a significant correlation between the number of opening sites and the entire limb length among 101 cases of OWHTO (R = 0.23, p = 0.003). These discrepancies may be due to the sample numbers and three-dimensional changes, such as rotation of the lower limb. It was recently reported that both CWHTO and OWHTO caused rotational changes (external or internal) associated with the correction angle [15]. Regarding the h-CWHTO group, there was no correlation between the closing angle and entire leg length, as described in cases of conventional CWHTO [7, 8].
Leg length discrepancies are associated with musculoskeletal disorders, such as low back pain, OA of the hip, standing balance, and running injuries [16]. The degree of the leg length discrepancy that introduces musculoskeletal disorders is not clearly defined. Gait analyses have shown that a leg discrepancy of > 1 cm results in gait asymmetry [17]; therefore, even though leg length increased by 6 mm in our study, OWHTO may have no clinical effect on the leg length discrepancy. However, the self-perceived leg length discrepancy was different from the objective leg length discrepancy [18]; thus, it is necessary to inform patients that there is a possibility of this after OWHTO. h-CWHTO may be a better procedure for patients who do not desire a change in leg length.
Since h-CWHTO is a three-dimensional osteotomy in which the tibia is completely separated and fixed, flexion contracture can be improved by fixing the fragment in the extended position when using a locking plate. Flexion contractures of < 30 degrees are thus indicated for surgery [13]. Our results showed that flexion contracture significantly improved from 7.1 degrees to 4.7 degrees in the h-CWHTO group. It was recently reported that OWHTO combined with notchplasty improved flexion contracture [19]. In our study, OWHTO did not improve flexion contracture after surgery; this may be because notchplasty was not performed. We believe that the correction on the sagittal plane also had a positive effect on maintaining the leg length in the h-CWHTO group.
Our study has several limitations. First, anteroposterior full-length lower limb radiographs of the lower limbs were used to evaluate the lower leg length; there is, therefore, a possibility of radiographic measurement errors due to rotation and knee flexion, both of which may influence the measurement results. Nevertheless, when possible, lower limb 3D modeling provides an accurate evaluation, as previously reported [20]. Second, since the indications for OWHTO and h-CWHTO were different, cases with a flexion contracture of > 15 degrees were treated using h-CWHTO. Generally, while OWHTO does not improve flexion contracture, it may still affect the results. Third, there were variations in the % MA after surgery. Over- or under-correction may affect lower leg length despite careful preoperative planning or proper surgical techniques; however, postoperative alignment is affected by soft tissue laxity, and some errors must be tolerated [21, 22]. Fourth, this study did not include clinical results regarding the leg length discrepancy; therefore, further investigation is needed to clarify the clinical significance of leg length discrepancies after OWHTO and h-CWHTO. Fifth, the sample size in this study was relatively small. Further studies with larger sample size or randomized studies are needed to verify the leg length change after HTO.