The main findings of this study were that patients with age ≥ 55, BMI ≥ 28, and postoperative LLD ≥ 10mm had a higher probability of developing symptomatic LLD after OWHTO.
For osteoarthritis patients with varus deformity of the lower limbs, due to the influence of space narrowing and varus angulation, the effective length of lower limbs tends to be shortened. After osteotomy or orthodontic surgery, the length of lower limbs is also prone to change due to the opening and closing of the bony structure, among which, medial open osteotomy is more likely to lead to lengthening of the limbs [2, 5, 9]. Mathematical models had predicted that correction of 10° deformity in OWHTO would result in an extension of lower limb length of 17-20.5mm, while in actual studies the change of lower limb length was less than in the theoretical model [10, 5]. Bae DK et al found an average extension of 6.2–7.8 mm in lower limbs after OWHTO on both intraoperative computer-based navigation and radiographic measurements, and there was only one patient with extension of more than 10 mm [5].In another study, Kim Ji et al also demonstrated an average 7.6 mm lengthening of lower limbs after OWHTO, and found that the lengthening was positively correlated with the open height of the wedge gap [6].Our study found a mean extension of 8.6 mm in the lower limbs of postoperative patients, which was similar to the above reports.
Studies have found that only 10% of the normal population has definite bilateral lower limb isometric length, and 90% of individuals present with LLD within 1 cm [8, 11].There is a certain compensatory mechanism of the body, and not all patients with LLD have symptoms [12]. The theoretical model study found that the body could compensate for LLD by the coronal pelvic tilt and the flexion of knee and hip joint on the side of the long leg under static conditions, and the pelvic tilt was about 6.1° when the LLD reached 2-3cm.Nonetheless,Under dynamic conditions, the tilt of the pelvis decreased and the shorter lower limb was extended through the ankle plantarflexion, while the longer side was compensated by hip and knee flexion and ankle dorsal extension [13, 14]. It has been reported that when the chronic LLD is less than 2cm, most people can tolerate it [8, 15].However, even small LLD can bring long-term adverse effects, such as accelerated degeneration of hip and knee joints on the long side and the lumbar spine [16, 17]. It had been documented [8, 18] that acute LLD was more likely to make patients feel subjectively uncomfortable, and even patients with LLD < 5mm may suffer LLD symptoms abidingly. The cases we present showed that, patients with LLD ≥ 10 mm had a 4.72-fold higher risk of developing LLD symptoms than those with LLD < 10 mm, but not all patients with LLD ≥ 10 mm were symptomatic, indicating that the degree of LLD was an important factor in inducing symptoms, but not the only factor.
Another finding of our study was that patients with BMI ≥ 28 were more likely to have symptoms of LLD. Biomechanics of the lumbar spine, pelvis, and lower limbs were altered after acute LLD, which was more significant in patients with greater BMI. The compensation of the body may lead to the stress concentration of a joint, a muscle or ligament, and that may induce uncomfortable symptoms over time, especially for patients with BMI ≥ 28.A study [7] found that lower back pain was more likely to occur among meat cutters and standing service workers than office staff for patients, which indirectly confirmed the inductive effect of body weight on symptomatic LLD. However, in patients aged ≥ 55, who tend to already have some degree of degeneration in the hip, knee and lumbar spine, as well as declining the muscle and ligament function due to their older age, and thus less tolerance to acute LLD after OWHTO, resulting in a higher risk of developing LLD symptoms. A study of predictors of self-perceived LLD after total hip arthroplasty has identified BMI < 26 and an increase in LLD of more than 5 mm were the high risk factors [19].
This study has some limitations. Firstly, this study is a retrospective study with a small sample size and a lack of CWHTO cases as the control group. Secondly, in this study, the length was measured on the full-length radiography of both lower limbs in the weight-bearing position, and the average length was obtained after the independent measurement by two researchers. However, some measurement errors may exist due to the influence of radiation angle and distance. Thirdly, in the current literature, the symptoms of LLD were mostly evaluated by low back pain, abnormal gait, subjective feelings. In this group of cases, the LLD was relatively small, and questionnaire was used to ask whether the patients had the uncomfortableness of LLD which can be used to identify the symptoms, but the degree cannot be evaluated. In the future, it still needs to be confirmed by prospective studies with larger sample size.