Multi-body optimization with subject-specific knee models: performance at high knee flexion angles

Does IMU redundancy improve multi-body optimization results to obtain lower-body kinematics? A preliminary study says no

Inertial Measurement Units (IMUs) have been proposed as an ecological alternative to optoelectronic systems for obtaining human body joint kinematics. Tremendous work has been done to reduce differences between kinematics obtained with IMUs and optoelectronic systems, by improving sensor-to-segment calibration, fusion algorithms, and by using Multibody Kinematics Optimization (MKO). However, these improvements seem to reach a barrier, particularly on transverse and frontal planes. Inspired by marker-based MKO approach performed via OpenSim, this study proposes to test whether IMU redundancy with MKO could improve lower-limb kinematics obtained from IMUs. For this study, five subjects were equipped with 11 IMUs and 30 reflective markers tracked by 18 optoelectronic cameras. They then performed gait, cycling, and running actions. Four different lower-limb kinematics were computed: one kinematics based on markers after MKO, one kinematics based on IMUs without MKO, and two based on IMUs after MKO performed with OpenSense (one with, and one without, sensor redundancy). Kinematics were compared via Root Mean Square Difference and correlation coefficients to kinematics based on markers after MKO. Results showed that redundancy does not reduce differences with the kinematics based on markers after MKO on frontal and transverse planes comparatively to classic IMU MKO. Sensor redundancy does not seem to impact lower-limb kinematics on frontal and transverse planes, due to the likelihood of the "rigid component" of soft-tissue artefact impacting all sensors located on one segment.

In vivo static and dynamic lengthening measurements of the posterior cruciate ligament at high knee flexion angles

PURPOSE

Rehabilitation is an important aspect of both non-operative and operative treatments of knee ligament tear. Posterior cruciate ligament (PCL) non-operative treatment consists of a step-by-step rehabilitation protocol and is well described. It goes from rest (phase I) to strengthening exercises (phase IV). More specific and high-intensity exercises such as cutting, sidestepping or jumps are, however, not described in detail, as no in vivo data exist to tell how these exercises constrain the ligaments and whether they have the same effect on all of them, in particular regarding lengthening. The goal of this study was to measure the ligament lengthening in static knee flexion based on 3D reconstructions from magnetic resonance imaging (MRI) and from motion capture and ligament simulation during dynamic exercises.

METHODS

The knee of nine volunteers was first imaged in a closed-bore MRI scanner at various static knee flexion angles (up to 110°), and the corresponding lengthening of the PCL and the other major knee ligaments was measured. Then, the volunteers underwent motion capture of the knee where dynamic exercises (sitting, jumping, sidestepping, etc.) were recorded. For each exercise, knee ligament elongation was simulated and evaluated.

RESULTS

According to the MRI scans, maximal lengthening occurred at 110° of flexion in the anterior cruciate ligament and 90° of flexion in the PCL. Daily living movements such as sitting were predicted to elongate the cruciate ligaments, whereas they shortened the collateral ligaments. More active movements such as jumping put the most constrain to cruciate ligaments.

CONCLUSION

This study provides interesting insights into a tailored postoperative regimen. In particular, knowing the knee ligament lengthening during dynamic exercises can help better define the last stages of the rehabilitation protocol, and hence provide a safe return to play.

A screening method to analyse the sensitivity of a lower limb multibody kinematic model

The study presents a screening method used to identify the influential parameters of a lower limb model including ligaments, at low numerical cost. Concerning multibody kinematics optimisation, the ligament parameters (isometric length) were found the most influential ones in a previous study. The screening method tested if they remain influential with minimised length variations. The most important parameters for tibiofemoral kinematics were the skin markers, segment lengths and joint parameters, including two ligaments. This was confirmed by a quantitative sensitivity analysis. The screening method has the potential to be used as a stand-alone procedure for a sensitivity analysis.