Lennart Scheys

Personalized MR-based musculoskeletal models compared to rescaled generic models in the presence of increased femoral anteversion: effect on hip moment arm lengths.

Advanced biomechanical analysis of muscle function during gait relies on the use of a musculoskeletal model. In clinical practice, personalization of the model is usually limited to rescaling a generic model to approximate the patients anthropometry, even in the presence of bony deformities, as in the case of cerebral palsy (CP). However, the current state of the art in biomechanics allows highly detailed subject-specific models to be built based on magnetic resonance (MR) images. We hypothesized that moment arm length (MAL) calculations from MR-based models would be more accurate than those from rescaled generic musculoskeletal models. Our study compared hip muscle MAL estimated by (1) a personalized model based on full-leg MR scans and (2) a rescaled generic model of both lower limbs in six children presenting with increased femoral anteversion. Personalized MR-based models were created using a custom-built workflow. Rescaled generic models were created based on three-dimensional positions of anatomical markers measured during a standing trial. For all 12 lower limb models, the hip flexion, adduction and rotation MAL of 13 major muscles were analyzed over a physiological range of hip motion using Software for interactive musculoskeletal modelling (SIMM) (Motion Analysis Corporation, USA). Our results showed that rescaled generic models, which do not take into account the subjects femoral geometry, overestimate MAL for hip flexion, extension, adduction, abduction and external rotation, but underestimate MAL for hip internal rotation. The differences in MAL introduced by taking the aberrant femoral geometry into account in the MR-based model were consistent with major gait characteristics presented in CP patients.

Calculated moment-arm and muscle-tendon lengths during gait differ substantially using MR based versus rescaled generic lower-limb musculoskeletal models

Biomechanical analysis of gait relies on the use of lower-limb musculoskeletal models. Most models are based on a generic model which takes into account the subjects skeletal dimensions by isotropic or anisotropic rescaling. Alternatively, personalized models can be built based on information from magnetic resonance (MR) images. We have studied the effect of these approaches on muscle-tendon lengths (MTLs) and moment-arm lengths (MALs) for 16 major muscles of the lower limb of a normal adult during both normal and pathologic gait. For most muscles, the MTL and MAL calculated using the rescaled generic models showed high correlation values, but large offsets when compared to values calculated using personalized models. MTL and MAL differences with the personalized model are only slightly smaller for an anisotropic than for an isotropic rescaled model. Gait kinematics influenced the observed inter-model differences and correlations due to an altered range of joint angles in both gait patterns. In conclusion, both generic rescaling methods failed to accurately estimate absolute values for MTL and MAL calculated using the personalized model. However, the magnitude of MTL and MAL changes during normal and pathologic gait corresponded between all three models for most muscles. Since rescaling depends strongly on modelling assumptions and cannot fully take into account subject-specific musculoskeletal geometry, interpretation of MTL and MAL even in normal adult subjects requires extreme caution.

Level of subject-specific detail in musculoskeletal models affects hip moment arm length calculation during gait in pediatric subjects with increased femoral anteversion

Biomechanical parameters of gait such as muscles moment arm length (MAL) and muscle-tendon length are known to be sensitive to anatomical variability. Nevertheless, most studies rely on rescaled generic models (RGMo) constructed from averaged data of cadaveric measurements in a healthy adult population. As an alternative, deformable generic models (DGMo) have been proposed. These models integrate a higher level of subject-specific detail by applying characteristic deformations to the musculoskeletal geometry. In contrast, musculoskeletal models based on magnetic resonance (MR) images (MRMo) reflect the involved subjects characteristics in every level of the model. This study investigated the effect of the varying levels of subject-specific detail in these three model types on the calculated hip MAL during gait in a pediatric population of seven cerebral palsy subjects presenting aberrant femoral geometry. Our results show large percentage differences in calculated MAL between RGMo and MRMo. Furthermore, the use of DGMo did not uniformly reduce inter-model differences in calculated MAL. The magnitude of these percentage differences stresses the need to take these effects into account when selecting the level of subject-specific detail one wants to integrate in musculoskeletal. Furthermore, the variability of these differences between subjects and between muscles makes it very difficult to a priori estimate their importance for a biomechanical analysis of a certain muscle in a given subject.

Calculating gait kinematics using MR-based kinematic models

Rescaling generic models is the most frequently applied approach in generating biomechanical models for inverse kinematics. Nevertheless it is well known that this procedure introduces errors in calculated gait kinematics due to: (1) errors associated with palpation of anatomical landmarks, (2) inaccuracies in the definition of joint coordinate systems. Based on magnetic resonance (MR) images, more accurate, subject-specific kinematic models can be built that are significantly less sensitive to both error types. We studied the difference between the two modelling techniques by quantifying differences in calculated hip and knee joint kinematics during gait. In a clinically relevant patient group of 7 pediatric cerebral palsy (CP) subjects with increased femoral anteversion, gait kinematic were calculated using (1) rescaled generic kinematic models and (2) subject-specific MR-based models. In addition, both sets of kinematics were compared to those obtained using the standard clinical data processing workflow. Inverse kinematics, calculated using rescaled generic models or the standard clinical workflow, differed largely compared to kinematics calculated using subject-specific MR-based kinematic models. The kinematic differences were most pronounced in the sagittal and transverse planes (hip and knee flexion, hip rotation). This study shows that MR-based kinematic models improve the reliability of gait kinematics, compared to generic models based on normal subjects. This is the case especially in CP subjects where bony deformations may alter the relative configuration of joint coordinate systems. Whilst high cost impedes the implementation of this modeling technique, our results demonstrate that efforts should be made to improve the level of subject-specific detail in the joint axes determination.

Atlas-based non-rigid image registration to automatically define line-of-action muscle models: a validation study.

Research has raised a growing concern about the accuracy of rescaled generic musculoskeletal models for estimating a subjects musculoskeletal geometry. Information extracted from magnetic resonance (MR) images can improve the subject-specific detail and accuracy of musculoskeletal models. Nevertheless, methods that allow efficient, automated definition of subject-specific muscular models for use in biomechanical analysis of gait have not yet been published to the best of our knowledge. We report a novel method for automated definition of subject-specific muscle paths using non-rigid image registration between an atlas image and the subjects MR images. We validated this approach quantitatively by measuring the distance between automatically and manually defined coordinates of muscle attachment sites. Data was collected for 34 muscles in each lower limb of 5 paediatric subjects diagnosed with diplegic cerebral palsy and presenting varying degrees of increased femoral anteversion. Distances showed an overall median Euclidean error of 6.1mm: 2.0mm along the medio-lateral direction, 1.8mm along the anterior-posterior direction and 3.8mm along the superior-inferior direction. A qualitative validation between automatically defined muscle points and the muscular geometry observed in the subjects medical image data corroborated the quantitative validation. This automated approach followed by visual inspection and, if needed, correction to the muscle paths, reduced the time required for defining 34 lower-limb muscle paths from around 3.5 to 1h. Furthermore, the method was also applicable to aberrant skeletal geometry. Using the proposed method, defining MR-based musculoskeletal models becomes a time efficient and more accurate alternative to rescaling generic models.

Joint kinematics following bi-compartmental knee replacement during daily life motor tasks

In many cases knee osteoarthritis leads to total knee replacement surgery (TKR) even if the lateral compartment is not involved. More recently, a bicompartmental knee replacement system (BKR) (Journey Deuce, Smith & Nephew Inc., Memphis, TN, USA) has been developed that only replaces the medial tibiofemoral and the patellofemoral compartments, thus preserving both cruciate ligaments with its associated benefits. However information on the effect of BKR on in vivo knee joint kinematics is not widely available in the literature. Therefore, this study analyzed full three-dimensional knee joint kinematics in 10 postoperative BKR-subjects for a broad spectrum of relevant daily life activities: walking, walking followed by a cross-over or sidestep turn, step ascent and descent, mild squatting and chair rise. We analyzed to what extent normal knee motion is regained through comparison with their non-involved limb as well as a group of matched controls. Furthermore, coefficients of multiple correlation were calculated to assess the consistency of knee joint kinematics both within and between subject groups. This analysis demonstrated that, despite the presence of differences indicative for retention of pre-operative motion patterns and/or remaining compensations, knee joint kinematics in BKR limbs replicate, for a large range of daily-life motor tasks, the kinematics of the contra-lateral non-affected limbs and healthy controls to a similar extent as they are replicated within both these control groups.

Hip contact force in presence of aberrant bone geometry during normal and pathological gait

Children with cerebral palsy (CP) often present aberrant hip geometry, specifically increased femoral anteversion and neck‐shaft angle. Furthermore, altered gait patterns are present within this population. We analyzed the effect of aberrant femoral geometry, as present in CP subjects, on hip contact force (HCF) during pathological and normal gait. We ran dynamic simulations of CP‐specific and normal gait using two musculoskeletal models (MSMs), one reflecting normal femoral geometry and one reflecting proximal femoral deformities. The combination of aberrant bone geometry and CP‐specific gait characteristics reduced HCF compared to normal gait on a CP subject‐specific MSM, but drastically changed the orientation of the HCF vector. The HCF was orientated more vertically and anteriorly than compared to HCF orientation during normal gait. Furthermore, subjects with more pronounced bony deformities encountered larger differences in resultant HCF and HCF orientation. When bone deformities were not accounted for in MSMs of pathologic gait, the HCF orientation was more similar to normal children. Thus, our results support a relation between aberrant femoral geometry and joint loading during pathological/normal gait and confirm a compensatory effect of altered gait kinematics on joint loading.

Image based musculoskeletal modeling allows personalized biomechanical analysis of gait

This paper describes a workflow, for building detailed, subject-specific musculoskeletal models from magnetic resonance (MR) images allowing enhanced biomechanical analysis of gait. . Bones are segmented semi-automatically using a hybrid approach while muscles attachments are retrieved automatically by atlas-based non-rigid registration followed by optional interactive correction using a user-friendly interface. Compared to previously proposed methods for MR based musculoskeletal modeling, integration of automated image processing procedures and problem-tailored visualization techniques result in a considerable reduction of the processing time, thus making MR-based musculoskeletal modeling practically feasible and more attractive

Functional knee axis based on isokinetic dynamometry data: comparison of two methods, MRI validation, and effect on knee joint kinematics

This paper compares geometry-based knee axes of rotation (transepicondylar axis and geometric center axis) and motion-based functional knee axes of rotation (fAoR). Two algorithms are evaluated to calculate fAoRs: Gamage and Lasenbys sphere fitting algorithm (GL) and Ehrig et al.s axis transformation algorithm (SARA). Calculations are based on 3D motion data acquired during isokinetic dynamometry. AoRs are validated with the equivalent axis based on static MR-images. We quantified the difference in orientation between two knee axes of rotation as the angle between the projection of the axes in the transversal and frontal planes, and the difference in location as the distance between the intersection points of the axes with the sagittal plane. Maximum differences between fAoRs resulting from GL and SARA were 5.7° and 15.4mm, respectively. Maximum differences between fAoRs resulting from GL or SARA and the equivalent axis were 5.4°/11.5mm and 8.6°/12.8mm, respectively. Differences between geometry-based axes and EA are larger than differences between fAoR and EA both in orientation (maximum 10.6°).and location (maximum 20.8mm). Knee joint angle trajectories and the corresponding accelerations for the different knee axes of rotation were estimated using Kalman smoothing. For the joint angles, the maximum RMS difference with the MRI-based equivalent axis, which was used as a reference, was 3°. For the knee joint accelerations, the maximum RMS difference with the equivalent axis was 20°/s(2). Functional knee axes of rotation describe knee motion better than geometry-based axes. GL performs better than SARA for calculations based on experimental dynamometry.

Biomechanics of medial unicondylar in combination with patellofemoral knee arthroplasty

PURPOSE Modular bicompartmental knee arthroplasty (BKA) for treatment of medio-patellofemoral osteoarthritis (OA) should allow for close to normal kinematics in comparison with unicondylar knee arthroplasty (UKA) and the native knee. There is so far no data to support this. SCOPE Six fresh frozen full leg cadaver specimens were prepared and mounted in a kinematic rig with six degrees of freedom for the knee joint. Three motion patterns were applied with the native knee and after sequential implantation of medial UKA and patellofemoral joint replacement (PFJ): passive flexion-extension, open chain extension, and squatting. During the loaded motions, quadriceps and hamstrings muscle forces were applied. Infrared cameras continuously recorded the trajectories of marker frames rigidly attached to femur, tibia and patella. Prior computer tomography allowed identification of coordinate frames of the bones. Strains in the collateral ligaments were calculated from insertion site distances. RESULTS UKA led to a less adducted and internally rotated tibia and a more strained medial collateral ligament (MCL). Addition of a patellofemoral replacement led to a more posterior position of both femoral condyles, a more dorsally located tibiofemoral contact point and higher MCL strain with squatting. CONCLUSION In comparison to UKA modular BKA leads to a more dorsal tibial contact point, a medial femoral condyle being located more posteriorly, and more MCL strain. Mainly the changes to the trochlear anatomy as introduced by PFJ may account for these differences.