Abstract
Introduction
Wolff's Law proposes that trabecular bone adapts in response to mechanical loading and that trabeculae align with the trajectory of predominant loads. The current study is aimed to investigate trabecular orientation in the tibia in patients with osteoarthritis of the knee. Consistent with Wolff's Law, it was hypothesised that orientation would reflect the mechanical loading of the joint and hence that there would be a correlation between the trabecular orientation and the mechanical axis of the lower limb.
Methods
51 anonymised radiographs from patients with osteoarthritis were analysed using ImageJ (National Institute of Health). Each patient had both a standard anteroposterior radiograph of the knee and a long leg view taken while weight bearing.
For each anteroposterior radiograph, the angle of the femoral shaft and tibial shaft were measured. The femoral shaft – tibial shaft (FS -TS) angle was then calculated as the difference between the two, as described by Sheehy et al. (2011). A medial rectangle was selected with the top, bottom, medial and lateral borders being the sclerotic bone, the growth line, the bone edge and the centre of the medial tibial spine. Corresponding measurements were done on the lateral side. Trabecular orientation of both areas was measured using OrientationJ (an ImageJ plugin). In all cases the medial and lateral orientation angles were expressed relative to the angle of the tibial shaft.
The mechanical axis of the lower limb was measured from the full length radiographs by calculating the angle formed by the femoral and tibial axes, as described by Goker and Block. All measurements were done independently by two observers, SAS and SL.
Results
Except where indicated, the results are based on analysis of 51 radiographs. Inter-tester analysis indicated excellent reliability (ICC = 0.99) for the mechanical axis measurement and preliminary inter-tester analysis (based on 25 radiographs) indicated good reliability for the orientation measurements (ICC = 0.76). The FS-TS angle calculated from the anteroposterior radiographs was significantly correlated with the mechanical axis calculated from the full-leg views (r = 0.96, p < 0.01), with an average offset of 5.7°, which is consistent with previous research. There was a significant correlation between the lateral trabecular orientation and both the FS-TS angle measured from the anteroposterior radiographs (r = −0.48, p < 0.01) (Figure) and the mechanical axis measured from the long leg views (r = −0.39, p < 0.01). There was also a significant correlation between the medial trabecular orientation and the FS-TS angle (r = 0.35, p = 0.01).
Discussion
There were significant correlations between leg alignment (both the mechanical axis and the FS − TS angle) and trabecular orientation in the human tibia. These findings were consistent with Wolff's Law, which proposes that trabecular bone adapts in response to mechanical loading. To the best of our knowledge, the current study is the first to investigate in vivo trabecular orientation in the human tibia and to establish a correlation with the mechanical axis of the lower limb. The findings also suggest that inspection of the trabecular orientation might provide valuable information on leg alignment and mechanical loading prior to surgery.
