Influence of the flattening of the glenohumeral joint on joint reaction force and humeral head translation during rotation in neutral abduction

Track 3. Musculoskeletal systems and Performance - Joint ISB/ESB Track

Stiffness increased linearly with torque irrespective of the subject with correlation coefficients ranging between 0.63 and 0.97. The stiffness index, which corresponds to the slope of the individual musculotendinous stiffness-torque relationship (Lambertz et al., J. App/. Physiol., 2001), was higher (p < 0.01) in women (4.41±1.6 rad -1) than in men (2.50±1.1 rad-1). The present study reports that the musculotendinous stiffness of the wrist flexors was higher (+76%) in women than in men. The results could be, at least in part, explained by the fact that slow-twitch fibres, which have been observed to be stiffer than fast-twitch one (Toursel et al., Exp. Physiol., 1999), occupied a greater area in women than in men (Anianson et al., C/in. Physio/., 1981). The higher stiffness values found in women group could be at the origin of functional differences, which can be discussed in term of improvement in muscle efficiency and musculoskeletal damage occurrence. 5795 Tu, 11:00-11:15 (P18) A user friendly prosthesis for full arm replacement with more than four active axes M. Camposaragna, E Casolo, M. Cocetta. Dept. Electrical Engineering,

Man-machine System Mechanics Unit, Politecnico di Milano, Italy A new full arm have been designed and produced to fill the gap in the rehabilitative devices commercially available for the upper limb amputees. The main features of this system are the low weight, low noise, wide working area, and a user friendly approach to its command. The artificial limb is characterized by many servo actuated axes: two for the shoulder, one for the elbow, one for the wrist and at least one for the hand. This multi-axes architecture excludes any chance to use the sequential approach to the joints motion usually adopted in current active arm prostheses. In order to reduce the mental load, to the subject is only required to focus on the hand motion, driving it without taking into account any other part of the limb kinematic chain. Knowing the target of the hand, the system automatically manages the joints' motion: a central unit synchronizes the axes motion in order to follow the target as fast as possible and peripheral units control separately each brushless motor law of motion. The motors are controlled by two closed loop, in velocity and position using as references the hall sensors; soft stops are also guaranteed by the software. The system follows the requests of the subject changing the motion target during the movement. At present the hand is driven by the patient by means of the head, only the grasping function is directly obtained by processing electromyographic signals. The head kinematics is detected by means of mems accelerometers and gyroscopes. Recent tests have shown that the distance from the hand to the head can be profitably driven, without increasing the subject pressure, also by means an extra electro-mechanical or electro-miographic sensor. In the new model of the prosthesis, presently under test, the mechanical and electrical component have been fully redesigned to better fulfill the patients requests. 5773 Tu, 11:15-11:30 (P18) Comparison between geometric and functional method for the estimation o f the glenohumeral rotation center A. Levasseur 1,2, P. T6treault 1, J.A. de Guise, N. NuSo 1, N. Hagemeister 1.

1Laboratoire de recherche en imagerie et orthop~die, Ecole de technologie sup6rieure, Montr6al, Canada, 2HOpital Notre-Dame, CHUM, Montr6al, Canada Being a reference landmark for the shoulder joint motion analysis, the glenohumeral rotation center (GHRC) has to be localized precisely. Changes in the position of the GHRC could ultimately affect the estimation of the lever arms of the muscles and therefore modify the interpretation of shoulder kinematics. According to the literature, the GHRC can be estimated using a geometric (based on the bone geometry) or a functional method (based on the bone movement). To our knowledge, there is no in vitro study that compares the different methods to one another in terms of localization and excursion of the GHRC during movement of the arm. Therefore, the aim of this study was to determine the GHRC on cadaveric shoulder specimens using a geometric and a functional method and compare their relative localization, their excursion during the abduction motion of the arm and their localization with respect to the center of the glenoid. The experiment was carried out on 8 cadaveric shoulder specimens. The localization of the GHRC and its relative threedimensional (3D) excursion was analysed using 3D imagery reconstruction and an electromagnetic tracking device. The results revealed that the geometric and the functional GHRC were not localized at the same point and did not behave in the same way during abduction. Both GHRC were also found to be not centre on the glenoid. Moreover, excursion of the geometric and functional GHRC recorded during abduction of the arm tends to demonstrate that the shoulder does not behave only as ball and socket joint. In light of the present study, future kinematics analysis should be aware that differences exist when considering the geometric or functional GHRC. The understanding of the localization and behaviour of each GHRC will be valuable for future work, such as the study of cuff tear arthropathy and eventual prosthetic design.

3.1. Joints - Upper Extremity

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6883 Tu, 11:30-11:45 (P18) Determining in-vivo carrying angle by anatomical landmarks - reliability of a new method M.L. Zampagni, D. Casino, S. Martelli, A. Visani, M. Marcacci. Biomechanics

Laboratory, Isituti Ortopedici Rizzoli, Bologna, Italy The elbow joint presents an angular offset, called carrying angle [1,2], formed by the long axis of the ulna and the long axis of the humerus. The aim of this work is to present a new non invasive method to in-vivo estimate the carrying angle in a reliable way. Forty-four subjects (aged 61±11.1) were considered in this study. Two operators (skilled and novel) acquired five landmarks [3] on the arm and forearm using an electrogoniometer (Faro Arm) to digitalize the 3D coordinates. A numerical algorithm based on Cardan decomposition angles was used to obtain the carrying angle value. The validity of the method was tested by calculating Pearson's correlation coefficients between the values returned by our algorithm and the values measured by a goniometer adopted as gold standard. To gauge the difference between repeated measures, t-student test was adopted and standard error of measurement (SEM) was calculated. Intraoperator and inter-operator reliability were estimated by interclass correlation coefficients (ICC) [4]. The mean carrying angle calculated by our method was 12.0±3.20 and the correlation with the gold standard measures was good (r=0.72). The mean difference between repeated measures on the same subject both by the same operator and by different operators was not significant (P =0.84 and P =0.7 respectively). The standard error of measurements was 1.620 by the skilled operator and 1.950 by the novel one. The intra-operator repeatability was excellent (ICC =0.85) and good the inter-operator (ICC =0.66). Concluding, the method resulted valid and reliable, but the higher SEM for the novel operator indicated that the technique requires skilled operators in order to obtain a correct identification of anatomical points. References [1] An K.N., et al. J. Orth. IRes. 1984; 1: 369-378. [2] Morrey B.E, et al. JBJS 1976; (58a): 501-508. [3] Wu G., et al. ISB recommendation, J. Biomech. 2005; May 38(5): 981-992. [4] Stokdijk M., et al. J. Biomech. 2000; Sep. 33(9): 1139-1145.

4958 Tu, 11:45-12:00 (P18) Working range of elbow joint in confined wheelchair configuration C.-J. Lin, P.-C. Lin, F.-C. Su. Institute of Biomedical Engineering, National

Cheng Kung University, Tainan, Taiwan The low propulsion efficiency, about 10%, in manual wheelchair relates to the limited workspace of upper extremity (UE). The high prevalence of upper extremity pain or injury for manual wheelchair users may also relate to high joint loads due to the confined workspace of UE. Theoretically, the trajectory of elbow joint will be on a sphere with center at shoulder joint. However, when subjects perform wheelchair propulsion, the possible range of elbow joint working would be limited because of the constraints of anatomical structure and the positions of shoulder and hand. This limited elbow working range will affect the propulsion efficiency and joint loads. Therefore, the purpose of this study was to find out the working range of elbow joint while shoulder and hand positions were fixed during wheelchair propulsion. Six healthy subjects who were inexperienced wheelchair users participated in this study. An eightcamera Eva RT system was used to collect the three-dimensional trajectory data of markers placed on the user-wheelchair system. With the shoulder position fixed, each subject was instructed to move the elbow in space as wide as possible while the hand was positioned on the hand rim at wheel angles, 120, 105, 90, 75 and 60 degrees, respectively. The joint center of the glenohumeral joint was treated as the center of the sphere and the length of upper arm was the radius. It was found that the trajectory of the elbow was an arc on the lateral posterior quadrant of the sphere. In addition, the vector of upper arm was projected on the transverse plane to find the maximal arc range that the elbow can moved when the hand was fixed at different wheel angles. The results showed that the mean arc ranges were 59.0, 56.1, 46.0, 51.5 and 44.3 degrees at 60, 75, 90,105 and 120 degrees of wheel angles, respectively. These results provide information on the modeling of upper extremity during wheelchair propulsion. 5755 Tu, 12:00-12:15 (P18) Influence of the flattening o f the glenohumeral joint on joint reaction force and humeral head translation during rotation in neutral abduction A. Reist 1, A. Terrier 1, A. Farron 2 . 1Biomechanical Orthopaedics Laboratory,

Swiss Federal Institute of Technology, Lausanne, Switzerland, 2 Orthopaedic Hospital, University of Lausanne, Switzerland Introduction: Arthritis of the glenohumeral joint is associated with erosion and flattening of the articular surfaces, which are sometimes difficult to correct during total shoulder arthroplasty. The goal of this study was to evaluate the influence of the articular flattening on the joint reaction force and the humeral head translation.

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Journal of Biomechanics 2006, Vol. 39 (Suppl 1)

Methods: Analysis was conducted with a 3D finite element model of the shoulder, including scapula, humerus, cartilage and 6 major muscles: middle, anterior and posterior deltoid, supraspinatus, subscapularis, and infraspinatus. Bones were rigid and muscles were transverse isotropic. Contact forces caused by muscles wrapping on bony surfaces were accounted for. A rotation movement was performed in neutral abduction. Muscles forces were controlled by a complex feedback algorithm, which assigned constant force ratio to abductor muscles, while setting active and passive force to rotator muscles. Both the glenoid and humeral head were eroded to artificially reproduce an arthritic joint. Muscles forces, glenohumeral contact pressure, contact point location and humerus translation were calculated for the normal and arthritic joint. Results: For the eroded joint, muscles and joint forces were about two times higher, at any rotation angle. For the normal joint, glenohumeral contact point and humeral head remained centred. Conversely, for the eroded joint, eccentric contact point and large AP humerus translation were clearly related to the rocking-horse phenomenon. Conclusions: This study shows that the flattening of the glenohumeral joint increase muscles forces, which partly explains the reduced range of motion observed clinically. This study also suggest that shoulder arthroplasty should reconstruct the joint anatomy as normal as possible, with a particular attention to the radius of curvature.

6082 Tu, 12:15-12:30 (P18) MRI-based estimation of muscle volume and length following tendon transfer surgery

K.R.S. Holzbaur, G.E. Gold, M.E. Johanson, W.M. Murray. Bone and Joint Center, VA Pale Alto HCS, Pale Alto, CA, USA Tendon transfer surgeries that restore voluntary hand function after cervical spinal cord injury are not always as effective as expected. Clinically, it is accepted that most donor muscles lose at least one grade of muscle strength following tendon transfer. The reasons for the reduction in strength have not been established. We used magnetic resonance imaging (MRI) to determine if architectural parameters, which are measures of a muscle's force-generating capability, differ following surgical transfer of the brachioradialis to the flexor pollicis Iongus compared to nonimpaired controls. Axial MRI images were acquired from shoulder to wrist using a 3D spoiled gradient echo sequence in 3mm sections. We identified the boundaries of the brachioradialis on each image and used these contours to create three-dimensional surfaces, from which we calculated muscle volume and length. The volume of the transferred brachioradialis (49.8 cm 3) of an adult male with C6 level tetraplegia fell outside the 95% confidence interval around the mean brachioradialis volume (93.7±25.1 cm 3) measured in 5 nonimpaired male subjects of varying size. The volume of the transferred brachioradialis was 82% of brachioradialis volume in an age- and height-matched nonimpaired subject (60.7cm3). Muscle lengths were comparable for the two matched subjects (22.2 and 22.3cm, respectively); muscle lengths ranged from 21.2 to 28.6cm among the nonimpaired males. This study supports the hypothesis that reductions in muscle strength observed following tendon transfer are associated with muscle atrophy. Understanding how factors such as muscle atrophy contribute to variable surgical outcomes will lead to better treatment decisions.

3.1.5. Upper Extremity Injury 4297 Tu, 14:00-14:15 (P21) Achievable changes in bone mineral density influence predicted distal radius fracture load K.L. Troy, M.D. Grabiner. University ef Illineis at Chicago, Chicago, IL USA Fall-related upper extremity injuries are common in older adults due to their high incidence of falls and decreased bone quality. Because osteoporotic fractures are common in women, interventions to increase bone mineral density (BMD) have been attempted. Interventions are considered successful if BMD is increased by 2-4% [1]. Here, we addressed the extent to which changes in BMD influence predicted distal radius fracture load during impact. To do this we implemented a three-dimensional FE model that includes contact between the radius, scaphoid, lunate, and ligamentous constraints [2]. An axial load of 3 kN was applied across the scaphoid and lunate. The proximal radius was fixed in space. BMD changes of -4%, -2%, +2%, and +4%, were simulated in cortical, cancellous, and concurrently in both types of bone. In all simulations, failure occurred first in the cortical bone proximal to the radial styloid process. Secondary failure occurred in the cancellous bone below the subchondral surface near the palmar aspect of the radius. Fracture loads were estimated as 1.7-2.6 kN. Concurrent changes in cortical and cancellous bone had the most profound effect on fracture strength. A 4% increase in cancellous BMD increased predicted fracture strength by 3.4%, compared to 6.8% if both types of bone are changed. However, increasing cortical bone BMD decreased fracture strength by 1%. The relationship between changes

Oral Presentations in BMD and fracture load was nonlinear. Because failures occurred near the medial distal radius, localized increases in BMD may contribute to fracture prevention. Because upper extremity forces of about 2.5 kN are associated with falls from standing-height [3], modest changes in BMD, especially in targeted locations, may meaningfully affect the incidence of distal radius fractures. References

[1] Liberman UA, et al. N Engl J Med 1995; 333: 1437. [2] Troy KL, Grabiner MD. ORS, 2006. [3] Chiu J, Robinovitch SN. J Biomech 1998; 31: 1169. 7289 Tu, 14:15-14:30 (P21) Rotational stiffness and damping properties of the elbow extensor muscles under impact in young men and women A. Mathias, J.A. Ashton-Miller. Biemechanics Research Laboratory, University of Michigan, Ann Arbor, USA Whether or not the elbow buckles in a fall to the ground is determined by the wrist impact force, the elbow angle at impact, and the rotational stiffness and damping of the elbow extensor muscles. The goal of this study was to make measurements of the rotational stiffness and damping of the elbow extensor muscles under an impact load causing forced elbow flexion. Eight healthy young adults (3 males, 5 females) were tested. Responses were studied at three co-contraction levels (25, 50 & 75% MVC) and two initial flexion angles (10 & 250 flexion). The forearm kinematic (150 Hz) and impact force data (2kHz) were digitally low-pass filtered. Integrated surface electromyography data were collected at 2 kHz from the biceps brachii and the lateral and long heads of triceps brachii. A second-order rotational spring-damper model was used to approximate the properties of the elbow joint: T =/~ + b6 + kS, where T is applied torque, 8 is the angular displacement of the limb, / is the calculated moment of inertia of the limb, b is the damping coefficient, and k is the stiffness coefficient. An optimization program was used to find k and b, with trials being considered valid when the ratio of root-mean-square model error to maximum torque was less than 10%. The values of k and b were then normalized by subject body weight and height. Mean (SD) male and female normalized k and b values at 75% MVC and 100 flexion were 0.730 (0.167) and 0.349 (0.165) Nmrad -1 kg-1 m -1, and 0.041 (0.011) and 0.032 (0.011 ) Nms rad-1 kg -1 m-1 , respectively. Both elbow stiffness and damping significantly increased with muscle activation. Women had significantly lower normalized stiffness and damping parameters than men (56-83% and 47~0%, respectively), reflecting reports of similar differences in the lower extremities. This gender difference likely affects how the arms are used to arrest a fall. Acknowledgments: NIH P30 AG08808 and SCOR P50 AR 049480 grants 7291 Tu, 14:30-14:45 (P21) Does muscle strength explain the gender difference in upper extremity kinematics while arresting a forward fall? J.-H. Lo, J.A. Ashton-Miller. Biomechanics Research Laboratory, University of Michigan, Ann Arbor, Michigan, USA Case et al. (ISB, 2005) found that healthy young women permit four times less post-impact elbow deflection (termed A~Elbow) while arresting a forward fall initiated from a shoulder-height of 1 m than did healthy young men. We used a computer simulation to test the hypotheses that the smaller A~Elbo w in young and older women is due to their lower arm extensor strength normalized per unit body weight. A 7-1ink, sagittally-symmetric, direct dynamics model was developed to simulate forward falls. During the descent and impact phases, a proportionalderivative joint controller and a pair of agonist and antagonist joint torque actuators with assigned neuromuscular latencies, torque-angle, and torquevelocity properties were incorporated to drive each joint to its target angles. The model was assigned four different arm muscle strength levels representing age and gender-specific normalized shoulder and elbow strengths. For each of the four gender/age models, forward falls from the Case study were simulated using arm joint target angles which maximized A~Elbow without incurring head impact using an optimization method. The resulting A~Elbo w of the four gender/age models were then compared to examine muscle strength effects. The results show that, for same impact velocity and elbow angle at impact, the maximal A~Elbo w of young male (YM), old male (OM), young female (YF), and old female (OF) models were 710, 700, 670, and 350, respectively. The resulting negative work done by the elbow and shoulder joints (YM: -78 J; OM: -72 J; YF: -68 J; and OF: -40 J) and peak wrist impact force (YM: 1.013 kN; OM: 1.099 kN; YF: 1.187 kN; and OF: 1.343 kN) also decreased with decreasing elbow strength. We conclude that smaller A~Elbo w must be used in the presence of lower arm extensor strengths in order to prevent the elbows from buckling under impact and the head from striking the ground. Acknowledgements: NIH P30 AG08808