Reliability and measurement error of multi-segment trunk kinematics and kinetics during cerebral palsy gait.

Children with cerebral palsy (CP) have been shown to have altered trunk movements during gait resulting in increased loads at the lower lumbar spine. Detailed assessment is possible using 3D gait analysis. However, reliability and quantification of measurement error have not been established. The aim of this study was to evaluate test-retest reliability of thorax and lumbar segment kinematics and L5/S1 kinetics during gait in children with CP. Eight children with CP participated in this study with repeat assessments conducted within 1 week. Reliability was assessed using the one-way random ICC, standard error of measurement and an examination of extrinsic-to-intrinsic variability. Thorax kinematics demonstrated mixed level of reliability with SEM values ranging from 5.94o to 1.15o. Lumbar kinematics demonstrated poor-to-good reliability with the largest SEM values for peak lumbar flexion at 4.14o. L5/S1 moment values demonstrated only poor to good test-retest reliability while L5/S1 reactive forces demonstrated poor to excellent test-retest reliability.This study provides estimates of reliability and change needed to exceed measurement error. While reliability was mixed and some measures for thorax movement were above 5o, stated as a measure of acceptable error, the results of this study support the use of these measures in children with CP.

Anatomy transformed.

This paper arose from exhibitions in Oxford and Dublin and comprises three experiments which look at the relationship between anatomy and art. In the first experiment, a passport photograph, photographic portrait and portrait in oils, all of the same sitter, show how artistic input transforms anatomy from a mere likeness into works of art. In the second, the reverse is true, as computer techniques render idealized old master images anatomically accurate. The third experiment addresses the biomechanical consequences of anatomical variation and shows that vehicular design is based on mean body shapes, and so it is the average, rather than the idealized, form that is safer in a collision.

Optimized wound closure using a biomechanical abdominal model.

BACKGROUND: Suturing techniques for midline abdominal wall incisions vary between surgeons. This study uses a biomechanical abdominal model to assess tissue stretch using different suturing techniques for midline laparotomy closure. METHODS: Deformation tests were performed on the linea alba of 48 porcine abdominal walls. Each pattern was tested three times at pressures ranging from 0 to 20 kPa using different continuous suturing techniques and a control. RESULTS: There was a sevenfold improvement when the best performing bite separation and bite width ([5, 16] mm) was compared with the most poorly performing combination ([15, 4] mm). The traditional bite and width separation ([10, 10] mm) and the recently proposed combination ([5, 5] mm) may not be optimal, and substantial improvements in surgical outcome may be achieved by changing to a [5,16]-mm combination. CONCLUSION: These findings suggest using a small bite separation (5 mm) and large bite width (16 mm) during abdominal wound closure may be optimal. Surgical relevance Suturing techniques for midline abdominal wall incisions vary between surgeons. This experimental study suggests substantial potential for improved tissue apposition by changing the suturing approach from the traditional clinical recommendation of 10 mm for both bite separation and bite width to a bite separation of 5 mm and a bite width of 16 mm. These findings support recent European Hernia Society guidelines and the recent randomized STITCH (Suture Techniques to Reduce the Incidence of The inCisional Hernia) trial, which found that small separations are more effective than large separations, but suggest that they should be combined with large bite depths.

Three-dimensional lumbar segment movement characteristics during paediatric cerebral palsy gait.

Kinematic analysis of the trunk during cerebral palsy (CP) gait has been well described. In contrast, movement of the lumbar spine is generally ignored. This is most likely due to the complex nature of the spine. As an alternative to using complex sensor protocols, this study modelled the lumbar region as a single segment and investigated characteristic patterns of movement during CP gait. In addition, the impact of functional level of impairment and the relationship with lower lumbar spinal loading were examined. Fifty-two children with CP (26 GMFCS I and 26 GMFCS II) and 26 controls were recruited. A full barefoot 3-dimensional kinematic and kinetic analysis were conducted. Lumbar segment movement demonstrated increased forward flexion for CP children. This movement became more pronounced according to GMFCS level with GMFCS II children demonstrating increases of up to 8°. In addition, a moderate correlation was present between lumbar flexion/extension and L5/S1 sagittal moments (r=0.427 in the global frame and r=0.448 with respect to the pelvis, p<0.01). Children with CP demonstrated increased movement of the lumbar region compared to TD, with movement becoming more excessive as GMFCS level increased. Excessive forward flexion and loading at the lumbar spine were linked. However, the moderate correlation suggests other contributors to increased loading were present. In conclusion, this study is a first step at identifying how lumbar segment movement is altered during CP gait.

"Children with cerebral palsy experience greater levels of loading at the low back during gait compared to healthy controls".

Excessive trunk motion has been shown to be characteristic of cerebral palsy (CP) gait. However, the associated demands on the lower spine are unknown. This study investigated 3-dimensional reactive forces and moments at the low back in CP children compared to healthy controls. In addition, the impact of functional level of impairment was investigated (GMFCS levels). Fifty-two children with CP (26 GMFCS I and 26 GMFCS II) and 26 controls were recruited to the study. Three-dimensional thorax kinematics and reactive forces and moments at the low back (L5/S1 spine) were examined. Discrete kinematic and kinetic parameters were assessed between groups. Thorax movement demonstrated increased range for CP children in all 3 planes while L5/S1 reactive forces and moments increased with increasing level of functional impairment. Peak reactive force data were increased by up to 57% for GMFCS I and 63% for GMFCS II children compared to controls. Peak moment data were increased by up to 21% for GMFCS II children compared to GMFCS I and up to 90% for GMFCS II compared to control. In addition, a strong correlation was demonstrated between thorax side flexion and L5/S1 lateral bend moment (r=0.519, p<0.01) and medial/lateral force (r=0.352, p<0.01). Children with CP demonstrated increased lower spinal loading compared to TD. Furthermore, GMFCS II children demonstrated significantly more involvement. Intervention should be aimed at reducing excessive thorax movement, especially in the coronal plane, in order to reduce abnormal loading on the spine in this population.

A quantitative comparison of two kinematic protocols for lumbar segment motion during gait.

During gait analysis, motion of the lumbar region is tracked either by means of a 2-dimensional assessment with markers placed along the spine or a 3-dimensional assessment treating the lumbar region as a rigid segment. The rigid segment assumption is necessary for inverse dynamic calculations further up the kinematic chain. In the absence of a reference standard, the choice of model is mostly based on clinical experience. However, the potential exists for large differences in kinematic output if different protocols are used. The aim of this study was to determine the influence of using two 3-dimensional lumbar segment protocols on the resultant kinematic output during gait. The first protocol was a skin surface rigid protocol with markers placed across the lumbar region while the second consisted of a rigid cluster utilizing active markers applied over the 3rd lumbar vertebra. Data from both protocols were compared through simultaneous recording during gait. Overall variability was lower in 4 out of 6 measures for the skin surface protocol. Ensemble average graphs demonstrated similar mean profiles between protocols. However, Functional Limits of Agreement demonstrated only a poor to moderate agreement. This trend was confirmed with a poor to moderate waveform similarity (CMC range 0.29-0.71). This study demonstrates that the protocol used to track lumbar segment kinematics is an important consideration for clinical and research purposes. Greater variability recorded by the rigid cluster during lumbar rotation suggests the skin surface protocol may be more suited to studies where axial rotation is a consideration.

Biomechanical abdominal wall model applied to hernia repair.

BACKGROUND: Most surgical innovations require extensive preclinical testing before employment in the operative environment. There is currently no way to develop and test innovations for abdominal wall surgery that is cheap, repeatable and easy to use. In hernia repair, the required mesh overlap relative to defect size is not established. The aims of this study were to develop a biomechanical model of the abdominal wall based on in vivo pressure measurements, and to apply this to study mesh overlap in hernia repair. METHODS: An observational study of intra-abdominal pressure (IAP) levels throughout abdominal surgery was conducted to identify the peak perioperative IAP in vivo. This was then applied in the development of a surrogate abdominal wall model. An in vitro study of mesh overlap for various defect sizes was then conducted using this clinically relevant surrogate abdomen model. RESULTS: The mean peak perioperative IAP recorded in the clinical study was 1740 Pa, and occurred during awakening from anaesthesia. This was reproduced in the surrogate abdomen model, which was also able to replicate incisional hernia formation. Using this model, the mesh overlap necessary to prevent hernia formation up to 20 kPa was found, independent of anatomical variations, to be 2 × (defect diameter) + 25 mm. CONCLUSION: This study demonstrated that a surgically relevant surrogate abdominal wall model is a useful translational tool in the study of hernia repair. Surgical relevance This study examined the mesh overlap requirements for hernia repair, evaluated in a biomechanical model of the abdomen. Currently, mesh size is selected based on empirical evidence and may underpredict the requirement for large meshes. The study proposes a relationship between the defect size and mesh size to select the appropriate mesh size. Following further trials and investigations, this could be used in clinical practice to reduce the incidence of hernia recurrence.

The clinical impact of hip joint centre regression equation error on kinematics and kinetics during paediatric gait.

Regression equations based on pelvic anatomy are routinely used to estimate the hip joint centre during gait analysis. While the associated errors have been well documented, the clinical significance of these errors has not been reported. This study investigated the clinical agreement of three commonly used regression equation sets (Bell et al., Davis et al. and Orthotrak software) against the equations of Harrington et al. Full 3-dimensional gait analysis was performed on 18 healthy paediatric subjects. Kinematic and kinetic data were calculated using each set of regression equations and compared to Harrington et al. In addition, the Gait Profile Score and GDI-Kinetic were used to assess clinical significance. Bell et al. was the best performing set with differences in Gait Profile Score (0.13°) and GDI-Kinetic (0.84 points) falling below the clinical significance threshold. Small deviations were present for the Orthotrak set for hip abduction moment (0.1 Nm/kg), however differences in Gait Profile Score (0.27°) and GDI-Kinetic (2.26 points) remained below the clinical threshold. Davis et al. showed least agreement with a clinically significant difference in GDI-Kinetic score (4.36 points). It is proposed that Harrington et al. or Bell et al. regression equation sets are used during gait analysis especially where inverse dynamic data are calculated. Orthotrak is a clinically acceptable alternative however clinicians must be aware of the effects of error on hip abduction moment. The Davis et al. set should be used with caution for inverse dynamic analysis as error could be considered clinically meaningful.

A 3-dimensional rigid cluster thorax model for kinematic measurements during gait.

The trunk has been shown to work as an active segment rather than a passenger unit during gait and it is felt that trunk kinematics should be given more consideration during gait assessment. While 3-dimensional assessment of the thorax with respect to the pelvis and laboratory can provide a comprehensive description of trunk movement, the majority of existing 3-D thorax models demonstrate shortcomings such as the need for multiple skin marker configurations, difficult landmark identification and practical issues for assessment on female subjects. A small number of studies have used rigid cluster models to quantify thorax movement, however the models and points of attachment are not well described and validation rarely considered. The aim of this study was to propose an alternative rigid cluster 3-D thorax model to quantify movement during gait and provide validation of this model. A rigid mount utilising active markers was developed and applied over the 3rd thoracic vertebra, previously reported as an area of least skin movement artefact on the trunk. The model was compared to two reference thorax models through simultaneous recording during gait on 15 healthy subjects. Excellent waveform similarity was demonstrated between the proposed model and the two reference models (CMC range 0.962-0.997). Agreement of discrete parameters was very-good to excellent. In addition, ensemble average graphs demonstrated almost identical curve displacement between models. The results suggest that the proposed model can be confidently used as an alternative to other thorax models in the clinical setting.

The influence of estimated body segment parameters on predicted joint kinetics during diplegic cerebral palsy gait.

Inverse Dynamic calculations are routinely used in joint moment and power estimates during gait with anthropometric data often taken from published sources. Many biomechanical analyses have highlighted the need to obtain subject-specific anthropometric data (e.g. Mass, Centre of Mass, Moments of Inertia) yet the types of imaging techniques required to achieve this are not always available in the clinical setting. Differences in anthropometric sets have been shown to affect the reactive force and moment calculations in normal subjects but the effect on a paediatric diplegic cerebral palsy group has not been investigated. The aim of this study was to investigate the effect of using different anthropometric sets on predicted sagittal plane moments during normal and diplegic cerebral palsy gait. Three published anthropometric sets were applied to the reactive force and moment calculations of 14 Cerebral Palsy and 14 Control subjects. Statistically significant differences were found when comparing the different anthropometric sets but variability in the resulting sagittal plane moment calculations between sets was low (0.01-0.07 Nm/kg). In addition, the GDI-Kinetic, used as an outcome variable to assess whether differences were clinically meaningful, indicated no clinically meaningful difference between sets. The results suggest that the effects of using different anthropometric sets on the kinetic profiles of normal and diplegic cerebral palsy subjects are clinically insignificant.

Torsion of monofilament and polyfilament sutures under tension decreases suture strength and increases risk of suture fracture.

BACKGROUND: A continuous running suture is the preferential method for abdominal closure. In this technique the suture is secured with an initial knot and successive tissue bites are taken. At each tissue bite, the needle is rotated through the tissue; in doing so, the suture can twist around the knot which acts as an anchor. OBJECTIVE: To determine the effect of axial torsional forces on sutures used in abdominal closure. METHODS: The effect of axial twisting on polydioxanone (PDS*II), polyglactin (Vicryl), polypropylene (Prolene) and nylon (Ethilon) sutures was investigated using a uniaxial testing device. RESULTS: The maximum tensile force withstood for untwisted sutures was determined: polydioxanone failed at a tensile force of 116.4±0.84 N, polyglactin failed at 113.9±2.4 N, polypropylene failed at 71.1±1.5 N and nylon failed at 61.8±0.5 N. Twisting decreased the maximum tensile force of all sutures; one complete twist per 10 mm (i.e., 15 twists) decreased the tensile strength of polydioxanone by 21%, polyglactin by 23%, polypropylene by 16% and nylon by 13%, p<0.001. Excessive twisting caused a nonlinear decrease in suture strength, with one twist per 75 mm (i.e., 20 twists) of polydioxanone decreasing strength by 39%, P<0.001. CONCLUSION: The effect of excessive twisting on the mechanical properties of sutures is a previously unrecognised phenomenon. Surgeons should be aware that this can result in a decrease in suture strength and reduce the elasticity of the material, and therefore need to adapt their practice to reduce the torsional force placed on sutures.

Pedestrian head translation, rotation and impact velocity: the influence of vehicle speed, pedestrian speed and pedestrian gait.

In road traffic collisions, pedestrian injuries and fatalities account for approximately 11% and 20% of casualties in the USA and the EU, respectively. In many less motorised countries, the majority of victims are pedestrians. The significant influences of vehicle speed, pedestrian speed and pedestrian gait on pedestrian post-impact kinematics have been qualitatively noted in the literature, but there has been no quantitative approach to this problem. In this paper, the MADYMO MultiBody (MB) pedestrian model is used to analyse the influences of vehicle speed, pedestrian speed and pedestrian gait on the transverse translation of the pedestrian's head, head rotation about the vertical head axis and head impact velocity. Transverse translation has implications for injury severity because of variations in local vehicle stiffness. Head rotation is related to pedestrian stance at impact, which is known to affect the kinematics of a collision. Increased head impact velocity results in greater head injury severity. The results show that transverse translation of the head relative to the primary contact location of the pedestrian on the vehicle decreases with increasing vehicle speed and increases linearly with increasing pedestrian speed. Head rotation decreases with increasing vehicle speed and increases linearly with increasing pedestrian speed, but these variations are small. The range of head rotation values decreases with increasing vehicle speed. Head impact velocity increases linearly with vehicle speed and is largely independent of pedestrian speed. Transverse translation, head rotation and head impact velocity all vary cyclically with gait in clearly definable patterns.

Viscoelastic properties of passive skeletal muscle in compression-cyclic behaviour.

Skeletal muscle relaxation behaviour in compression has been previously reported, but the anisotropic behaviour at higher loading rates remains poorly understood. In this paper, uniaxial unconfined cyclic compression tests were performed on fresh porcine muscle samples at various fibre orientations to determine muscle viscoelastic behaviour. Mean compression level of 25% was applied and cycles of 2% and 10% amplitude were performed at 0.2-80Hz. Under cycles of low frequency and amplitude, linear viscoelastic cyclic relaxation was observed. Fibre/cross-fibre results were qualitatively similar, but the cross-fibre direction was stiffer (ratio of 1.2). In higher amplitude tests nonlinear viscoelastic behaviour with a frequency dependent increase in the stress cycles amplitude was found (factor of 4.1 from 0.2 to 80Hz). The predictive capability of an anisotropic quasi-linear viscoelastic model previously fitted to stress-relaxation data from similar tissue samples was investigated. Good qualitative results were obtained for low amplitude cycles but differences were observed in the stress cycle amplitudes (errors of 7.5% and 31.8%, respectively, in the fibre/cross-fibre directions). At higher amplitudes significant qualitative and quantitative differences were evident. A nonlinear model formulation was therefore developed which provided a good fit and predictions to high amplitude low frequency cyclic tests performed in the fibre/cross-fibre directions. However, this model gave a poorer fit to high frequency cyclic tests and to relaxation tests. Neither model adequately predicts the stiffness increase observed at frequencies above 5Hz. Together with data previously presented, the experimental data presented here provide a unique dataset for validation of future constitutive models for skeletal muscle in compression.

Viscoelastic properties of passive skeletal muscle in compression: stress-relaxation behaviour and constitutive modelling.

The compressive properties of skeletal muscle are important in impact biomechanics, rehabilitation engineering and surgical simulation. However, the mechanical behaviour of muscle tissue in compression remains poorly characterised. In this paper, the time-dependent properties of passive skeletal muscle were investigated using a combined experimental and theoretical approach. Uniaxial ramp and hold compression tests were performed in vitro on fresh porcine skeletal muscle at various rates and orientations of the tissue fibres. Results show that above a very small compression rate, the viscoelastic component plays a significant role in muscle mechanical properties; it represents approximately 50% of the total stress reached at a compression rate of 0.5% s(-1). A stiffening effect with compression rate is observed especially in directions closer to the muscle fibres. Skeletal muscle viscoelastic behaviour is thus dependent on compression rate and fibre orientation. A model is proposed to represent the observed experimental behaviour, which is based on the quasi-linear viscoelasticity framework. A previously developed strain-dependent Young's Moduli formulation was extended with Prony series to account for the tissue viscoelastic properties. Parameters of the model were obtained by fitting to stress-relaxation data obtained in the muscle fibre, cross-fibre and 45 degrees directions. The model then successfully predicted stress-relaxation behaviour at 60 degrees from the fibre direction (errors <25%). Simultaneous fitting to data obtained at compression rates of 0.5% s(-1), 1%s(-1) and 10% s(-1) was performed and the model provided a good fit to the data as well as good predictions of muscle behaviour at rates of 0.05% s(-1) and 5% s(-1) (errors <25%).

A validated model of passive muscle in compression.

A better characterisation of soft tissues is required to improve the accuracy of human body models used, amongst other applications, for virtual crash modelling. This paper presents a theoretical model and the results of an experimental procedure to characterise the quasi-static, compressive behaviour of skeletal muscle in three dimensions. Uniaxial, unconstrained compression experiments have been conducted on aged and fresh animal muscle samples oriented at various angles from the fibre direction. A transversely isotropic hyperelastic model and a model using the theory of transverse isotropy and strain dependent Young's moduli (SYM) have been fitted to the experimental data. Results show that the hyperelastic model does not adequately fit the data in all directions of testing. In contrast, the SYM gives a good fit to the experimental data in both the fibre and cross-fibre direction, up to 30% strain for aged samples. The model also yields good prediction of muscle behaviour at 45 degrees from the fibre direction. Fresh samples show a different behaviour than aged tissues at 45 degrees from the fibre direction. However, the SYM is able to capture this difference and gives a good fit to the experimental data in the fibre, the cross-fibre and at 45 degrees from the fibre direction. The model also yields good prediction of muscle behaviour when compressed at 30 degrees and 60 degrees from the fibre direction. The effect of the time of test after death has also been investigated. Significant stiffening of muscle behaviour is noted a few hours after death of the subject.