ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

same slice for 2D analysis for repeated measurements reported as very low [8], the out
of plane motion is still unaccounted for in the literature. Load bearing during high
resolution imaging of the knee is also poorly reported in the literature and the results of
our study indicate that load bearing of the in-vivo knee introduces additional posterior
motion of both menisci in comparison to that of the passive in-vivo knee. Load bearing
is therefore an important consideration when investigating patient meniscal
biomechanics in future work in order to better represent the loading conditions of
patients which results in pain and stiffness during daily activities.
The results of this study additionally indicate that the medial meniscus remains
stationary between 25º and 50º knee flexion having initially translated 3.3mm
posteriorly from the neutral position. The medial pivot observed in this flexion range is
consistent with the study of [8] which mapped tibio-femoral contact points during
flexion and found that the lateral condyle rolls posteriorly in a consistent manner
throughout flexion while the medial condyle rests in the centre of the tibial plateau. It
would appear that it is only with increased knee flexion that the medial meniscus
displaces posteriorly to a greater extent. Differences in posterior meniscal motion to
those described in the literature, both in the sagittal and transverse planes, may also be
due to knee kinematics during MR imaging, and geometry of the articulating bones and
menisci. It is unlikely that ‘flexion’ of the knee indeed represents pure flexion without
some combination of transverse or frontal plane rotations. The 6 degrees of freedom
(6DOF) of the knee must therefore be quantified in all future studies of soft tissue and
contact behaviour in the knee in order to more accurately describe the biomechanics of
the meniscus. Although, not presented here, the methodology of this study allows for
the quantification of the 6DOF of the knee simultaneously to the quantification of
meniscal motion using similar methods as those employed to describe 3D movement of
the menisci relative to the tibia. Investigation of 3D bone and menisci geometry for
correlation to meniscal motion observed is also possible from the MR derived 3D
computational models used in this study. The need to report knee 6DOF when
describing meniscal biomechanics is further reinforced by the analysis of meniscal
motion with internal and external rotation of the foot where large differences in
meniscal position were observed. Externally rotating the foot under axial load was
accompanied by a large 6.3mm anterior motion of the medial meniscus and a much
smaller 0.9mm anterior motion of the lateral meniscus. It is interesting to note that this
represents a lateral pivot rather than the expected medial pivot described from previous
in-vitro investigations [10]. Internally rotating the foot under the same conditions causes
the medial meniscus to translate anteriorly 4.5 mm and the lateral translates 3.3mm
posteriorly relative to the loaded knee flexed to 25 degrees and the foot straight. These
motions of the menisci do not appear to be consistent with internal rotation of the tibia
relative to the femur, by medial pivot or by lateral pivot mechanisms. It is possible that
internal rotation of the foot was accommodated by the ankle and resisted in the knee
despite the best efforts to overcome such effects during MR imaging. Quantification of
knee 6DOF is clearly required in future work.
It is important to appreciate the limitations with the current study. Differences in image
contrast, brightness and magnification between MRI scans or between processing of
different operators may lead to changes in the pixels identified as meniscus or the
surrounding tissue and lead to variation in the 3D models created. The registration of
3D computational tibia and meniscus models in the test condition to that of a reference
position (for example registration of the loaded knee models to unloaded knee models)