Our findings from this study supported our hypothesis that weight-bearing causes 3D alignment changes of the OA knees, and increased 3D deformities were found in knees with end-stage OA. An important finding of our investigation was the determination of a significantly greater tibial internal rotation with respect to the femur under weight-bearing conditions in knees with K–L grade 2 OA, compared to grade 1 OA. In K-L grade 2, there is subtle joint space narrowing with minimal osteophyte formation on the anterior-posterior radiography [2, 7, 8]. While weight-bearing CT clearly detects early change occurring in the knee joint which is increased internal rotation with weight-bearing.
Consistent with our findings, Hirschmann et al. used cone-beam CT to demonstrate that the tibial internal rotation increased in the knee under weight-bearing conditions. However, that study was significantly limited by the following factors: the subjects without knee OA were involved (17 of 26 were knee OA) ; the relation between the OA grade and tibial internal rotation was not indicated; the axial CT images were subjected to 2D measurements, which should include measurement errors to account for possible differences in the levels of axial CT images; and an unnatural subject standing posture (i.e. standing on the foot of the examined leg with the other knee bent and resting outside the gantry) that placed the knee in a non-physiological position. Matsui et al. used conventional CT to evaluate rotational deformity in patients with knee OA in supine position and observed an increase in the tibial external rotation as the OA severity increased. However, a direct comparison of our results with their results is difficult, as the OA knees in their study were more severely deformed (femorotibial angles > 190º or 200º) than those in our study, and the two studies used different coordinate systems to measure tibiofemoral rotation. Recently, Fujii et al. compared 3D lower limb alignment in OA knees between weight-bearing and non-weight-bearing conditions. They used 2D–3D image-matching with biplanar computed radiography (CR) and 3D bone models of the complete lower extremity rebuilt using computed tomography-based information. Lower limb alignment during standing was evaluated using biplanar CR images obtained while participants stood with complete extension of the knees and toes in the neutral state. The OA knees showed flexion and varus both in the supine and standing positions, and neutral rotation of the tibia to the femur in the supine position and internal rotation of the tibia in the standing position. Fujii et al.’s study used compatible methods and experimental conditions with our study, and their results agreed well with those of our study. Either study indicates that weight-bearing alters the 3D lower limb alignment of OA knees, particularly internal rotation of the tibia occurs in the standing position, although their study did not show the results with each OA grade.
To our knowledge, ours is the first study to demonstrate changes in 3D alignment in patients with knee OA of various grades under natural full weight-bearing conditions.
The classification of knee OA is an essential clinical issue, particularly in terms of the diagnosis of early knee OA. As described above, differentiating early knee OA using 2D radiographs is difficult owing to a very small difference in joint space width between K-L grades 1 and 2. Although cone-beam CT imaging depicted the joint space widths of OA knees under weight-bearing conditions with a high level of repeatability[9–11], even 3D images obtained using this technique could not easily distinguish early changes associated with knee OA. In our analysis, we observed an average difference of 2.6° in the tibial internal rotation under weight-bearing conditions between K–L grades 1 and 2 (Figure 3, Table 1). Given the high level of accuracy of 3D–3D registration based on CT (0.2º about the X-axis and Z-axis), we could clearly visualise early changes associated with knee OA. The weight-bearing conditions led to increased flexion, adduction, and tibial internal rotation as the OA grade increased. Moreover, a significant increase in tibial internal rotation occurred prior to the increases in flexion and adduction, suggesting that internal rotation under weight-bearing conditions is a key pathologic factor in the progression of knee OA.
Several studies have depicted the 3D kinematics of OA knees during weight-bearing activities. Matsuki et al. used fluoroscopy and 2D–3D registration to evaluate the knee kinematics associated with early knee OA (K–L grades 1 and 2) during pivoting and squatting activities. In that study, reduced tibial internal rotation was observed during pivot activity in early OA knees relative to control knees. Another study similarly reported reduced tibial internal rotation during various weight-bearing activities in knees with advanced OA (K–L grades 3 and 4) relative to healthy knees. Both studies demonstrated differences in the tibiofemoral rotation patterns between OA and healthy knees. Till date, only one study has suggested a link between tibial rotation and the onset of knee OA. Andriacchi et al. used a computer simulation to demonstrate the effect of tibial internal rotation during gait on the thinning of cartilage in the knee. The authors found that a 5º increase in tibial internal rotation, which is usually seen in the anterior cruciate ligament deficient knee, was associated with rapid rate cartilage thinning in the medial compartment and speculated that this increase could initiate knee OA. It was suggested that abnormal rotation of the tibia may be the trigger of the OA and may determine the indication for early prevention of progression of the OA and surgeries such as high tibial osteotomy or total knee arthroplasty. However, the relation between abnormal tibial kinematics and the onset and progression of knee OA remains unclear and further investigation is needed.
Several limitations of this study should be noted. First, even when we performed a power analysis prior to the study, to analyse statistical difference in each OA grade, the statistical power of our analysis was limited by the small number of OA patients included in our study. Second, to evaluate accurate tibial internal/external rotation in standing, as shown in the present study and in the study by Fuji et al. , CT images with matching technique are required . As anterior-posterior radiograph is still the gold standard to diagnose knee OA, there is a need to develop method to evaluate tibial internal/external rotation on radiographs. In the future, computer-aided method with deep learning algorithm has a potential to evaluate the tibial rotation and to diagnose early-stage knee OA on 2D radiograph . Third, we evaluated knee kinematics under weight-bearing conditions while the subjects stood on both legs. Under this physiological condition, the entire body weight was divided evenly between the knees. However, the application of greater weight-bearing conditions would likely elucidate the pathology of knee OA. Future studies should aim to examine knee kinematics under various weight-bearing conditions and at different knee flexion angles.