This meta-analysis verified that knee adduction moment decreased markedly after medial open wedge HTO, reaching about 60% of the preoperative level, whereas knee flexion and extension moments were not altered by surgery.
The mechanical force loaded onto the knee joint in patients with medial osteoarthritis and varus deformity is an important predictor of osteoarthritis progression. Although the mechanisms by which increased mechanical loading adversely affects cartilage are unclear, they may involve chondrocyte death, disruption of the extracellular matrix, and microfracture within the subchondral cortical endplate. Knee adduction moment is a surrogate measure of dynamic mechanical load on the knee joint and increases in proportion to knee varus deformity. The success of medial open wedge HTO depends not only on alignment correction but also on decreasing the adduction moment, thereby reducing the mechanical load on the medial compartment of the knee. Determining the magnitude of adduction moment loss and its clinical relevance after medial open wedge HTO is therefore important. This meta-analysis showed that medial open wedge HTO reduced preoperative adduction moment by about 40%. One recent study reported that a 1% increase in adduction moment increased the risk of progression of knee osteoarthritis 6-fold. These findings suggest that a 40% reduction in adduction moment would delay the progression of medial osteoarthritis. Another recent study found that the load on the medial compartment while walking increases 5% for every 1° change towards varus [16]. Our study found that the magnitude of alignment correction was not significantly related to the reduction in adduction moment. Nevertheless, because the mean amount of correction following medial open wedge HTO in the studies included in this meta-analysis was approximately 8°, the 40% reduction in adduction moment compared with the preoperative level may be explained by the results showing a 5% decrease in adduction moment per 1° change towards varus.
The knee adduction moment is a consequence of the magnitude of the ground reaction force (GRF) passing medial side of the center of the knee joint during gait and the moment arm of the GRF, and defined as the perpendicular distance from the GRF to the center of the knee joint [17]. Therefore, the mechanical axis and knee adduction moment are positively correlated with each other [18]. In contradiction to that knowledge, the results of our meta-analysis revealed an absence of association between the magnitude of alignment correction and/or the degree of valgus and the reduction in adduction moment following medial open wedge HTO. Several causes are possible. First, despite the large amount of the correction, the mechanical axis may not have reached to the ultimate end-point, which was 62.5% of the width of the entire tibial plateau. Second, the adduction moment may have been affected by other factors except the mechanical axis alignment. For instance, in spite of the fact that the mechanical axis was shown as an indicant of the peak adduction moment, it only accounted for roughly 50% of the variation in the peak adduction moment [19]. The foot progression angle, lateral trunk tilt, knee flexion angle, and walking speed may also affect knee adduction moment. Increased lateral trunk tilt and out-toeing gait may decrease the knee adduction moment, whereas bigger knee flexion angle and higher walking speed have been reported as factors that may increase the knee adduction moment [18, 20, 21]. In addition, the strength of extensor and flexor muscles of the knee and the knee joint laxity affect the adduction moment [22,23,24].
Medial open wedge HTO can affect the sagittal plane moments of the knee during gait. Sagittal plane moments may alter the mechanical load borne by the medial compartment of the knee joint. Previous studies showed conflicting results for change in flexion moment. One study showed increased flexion moment after HTO due to increasing gait speed and flexion angle, and improved quadriceps function resulting from reduced pain following HTO [5]. In contrast, another study found that not only peak adduction moment but also flexion moment were reduced 12 months after medial open wedge HTO [12]. Our meta-analysis showed no significant change in either flexion or extension moment. Differences between study results may be due to the small effect on the sagittal plane of the magnitude of correction of mechanical axis alignment, despite the change in mechanical axis alignment being closely associated with change in adduction moment after medial open wedge HTO. Neuromuscular changes, which widely vary among individuals and are unpredictable, may contribute to changes in sagittal plane moments [24]. An unintended posterior slope of the articular surface of the proximal tibia in the sagittal plane may also affect changes in sagittal plane moments. However, a recent meta-analysis showed that the magnitude of increase in the posterior tibial slope following medial open wedge HTO was only 2°, suggesting that this slight change may have little effect on the biomechanical environment of the knee joint [25]. The results of that study may explain, at least in part, the lack of significant change in sagittal plane moments observed in our study.
This study had several limitations. Although adduction moment is a proxy measure of the mediolateral distribution of loads across the knee, it is not a direct measure of the actual force borne by the knee joint. The angular impulse of knee adduction moment was shown to be a more comprehensive indicator of cartilage volume reduction in osteoarthritis than peak knee adduction moment [4], because the former accounts not only for load magnitude but also load duration, which contribute to total knee load exposure [26]. However, angular impulse of adduction moment has only recently been measured, making peak knee adduction moment a more practical measure, as well as being a valid and reliable dynamic indicator of compressive load in the medial compartment during walking and a useful parameter for comparing the results of studies that assess adduction moment. Another limitation was the variation in preoperative mechanical axis and lateral thrust among included studies, which may have resulted in large variations in data, including adduction moment, during gait. This finding may also be a potential cause of heterogeneity in this meta-analysis [22]. Finally, a meta-regression analysis, which performed in this study to investigate the influence of alignment correction amount and postoperative alignment on change of adduction moment, was also prone to instability in the results due to relatively small sample size.