$80 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

cementing should improve the implant longevity. Linked elbow prosthesis have employed this philosophy and composite fixation has shown generally satisfactory results for a variety of cases [1,2]. A humeral component that incorporates an anterior flange has the theoretical benefit of transferring stress from the elbow to the humeral bone and relieving stress concentrations at the vulnerable distal humerus cement interface. The purpose of this study was to use finite element analysis to evaluate the biomechanical effects of bone graft beneath the anterior flange for various implant design characteristics A finite element model consisting of the elbow prosthesis implanted in the distal humerus was created. The model included more than 40,000 elements with linear elastic material properties. Various model permutations were evaluated including stem size (4, 6, and 8inch), distal humerus (normal and deficient distal humerus), and graft size (50%, 100% and 150% of flange length). Five loading conditions were simulated (anterior, posterior, axial, 45 degree posterior, and torsion). Von Mises stress and principal strains in the cement region of the distal humerus were compared. Stress and strain in the distal humerus and distal cement mantle are reduced 10-30% with a bone graft between the anterior flange and the bone cortex. For a deficient distal humerus, extending the graft proximally past the flange reduced peak stresses and strains up to 17% compared with a graft just under the flange. Also, up to a 15% reduction in distal cement stress and strain occurs when using a 6 inch versus 4 inch or 8 inch versus 6 inch stem. These findings confirmed the clinical experience that rigid fixation and stress relief due to the implant flange reduce the likelihood of loosening of the humeral implant for primary total elbow arthroplasty [3].

References [1] Hildebrand KA, et al. JBJS 82A: 1379-86, 2000. [2] Ray PS, et al. Injury 31: 687-92, 2000..

4814 Mo, 14:45-15:00 (P10) A "vir tual sensor" tool to simulate accelerometers for upper limb fes t r igger ing S.B. Thies, P.A. Tresadern, L. Kenney, D. Howard, J.Y. Goulermas, C. Smith, J. Rigby. Centre for Rehabilitation and Human Performance Research, University of Salford, Salford, Manchester, UK

Less than 15% of stroke patients with paralysis of the upper limb make a full recovery [1] and the ability to grasp and release objects via finger and wrist extension is often compromised. Few functional electrical stimulation (FES) systems are available that use electrical muscle stimulation to increase patients' upper extremity function [2]. Robust triggering of such systems based on motion sensor output is challenging. A key design problem is the determination of the optimal location/orientation of the sensor on the arm. Tong et al. previously introduced a simulation technique to find optimal sensor positioning for a lower limb FES application [3]. We have developed a virtual sensor tool (VST) that uses Vicon (position) data as the input and simulates accelerometer signals at any given loca- tion/orientation on a rigid body. This paper discusses the validation of the VST for upper limb applications. Simulated signals were compared to real outputs from an inertial measurement unit (XSENS) at a known location. The simulated accelerometer signals closely approximated the real outputs of the XSENS inertial measurement unit. Across 5 trials the average RMS error was 0.55, 0.70, and 0.75m/see 2 for the X, Y, and Z axes, respectively. The Pearson's correlation coefficient was 0.99 for all three axes. The VST is currently being used as part of a system to optimize sensor location and pattern recognition algorithms for the robust triggering of an upper limb FES system [4].

References [1] HT Hendricks et al. Motor recovery after stroke: a systematic review of the

literature. Arch Phys Med Rehabil 2002; 83: 1629-37. [2] Freehand System, NeuroControl Corp., Cleveland, OH, USA. [3] KY Tong et al. Virtual artificial sensor technique for functional electrical stimu-

lation. Med Eng Phys 1998; 20: 458-68. [4] Healthy Aims, EU Framework VI Project: www.healthyaims.org

4801 Mo, 15:00-15:15 (P10) Contact areas and contact pressures in the canine carpal jo in t A. Kaiser, J. Maierl, H.-G. Liebich. Institute of Veterinary Anatomy, Ludwig-Maximilians-University, Munich, Germany

The canine carpal joint is a widely unexplored joint, although it is quite often the cause of unclear forelimb lameness. In order to understand the stresses and strains within the joint in healthy dogs with physiological gait, contact areas and contact pressures in the joint were measured at different loads. A prepared cadaver limb was mounted in a material testing machine and loaded with 0.25 to 4 times body weight at a joint angle of 2050 hyperextension. Contact areas were determined by a silicon casting method in one limb of the dog, contact pressures were measured using Fuji-pressure sensitive film in

the contralateral limb, with a dorsal approach to the joint surfaces in each of them. In all joints of the carpus the contact areas increased in a nonlinear way with increasing loading steps. They never included more than 50% of the respective convex articular surface. Contact areas in the antebrachiocarpal joint always were greater than those of the midcarpal and antebrachiocarpal joint at the same loading step, both of the latter being nearly identical for each step. Because of the hyperextension the contact areas of the antebrachiocarpal joint were located in the dorsal aspect of the articular surfaces, while in the two distal joints they extended further palmar. The centres of the contact pressures corresponded to the centres of the contact areas. At 0.25 times body weight pressures reached a maximum of 2.7 MPa, at 1.5 times they rose over 10 MPa. High pressures in the antebrachiocarpal joint were located medially, in the midcarpal joint they were equally distributed over the articular surface, while in the carpometacarpal joint they "shifted" laterally. The physiological hyperextension and valgus position of the canine carpal joint leads to a unilateral loading situation in the antebrachiocarpal joint. Because of the limited range of motion in the midcarpal and antebrachiocarpal joint, these two are loaded equally over the whole articular surface. This is one of the reasons for a higher incidence of injuries in the proximal antebrachiocarpal joint.

5721 Mo, 15:15-15:30 (P10) Calculat ing jo int movement o f the shoulder complex using the proposed ISB standardizat ions L. Jones, C.A. Holt, A. Bowers. Cardiff School of Engineering, Cardiff University, Cardiff, Wales, UK

The shoulder is a joint complex that is susceptible to a wide range of pathologies and injuries e.g. instability. Surgeons use a range of observations and physical examinations to decide on the type and extent of a patient's shoulder pathology. However, this decision-making process can be far from straightforward and surgeons would benefit from further understanding of the aetiology of shoulder disorders [1]. For this purpose "a useful methodology for measuring 3D shoulder positions is urgently needed" [1, pp.280]. This paper introduces a protocol for the measurement of shoulder movement that uses a motion analysis based technique and the proposed standards of the International Society of Biomechanics (ISB) [2]. During an initial trial, individual retro-reflective markers were placed directly over 14 bony landmarks and a rigid marker cluster covered with four markers was placed on the upper arm. Marker movement was then recorded using digital infrared cameras whilst the subject performed various activities of daily living. In-house Matlab ® (The MathWorks Inc.) software was developed to calculate the 3-D kinematics of the shoulder complex from the global coordinate system data: (1) The glenohumeral joint centre was estimated using regression equations [3], (2) Body segment coordinate systems were established according to ISB recommendations [2], (3) Joint rotations were obtained from transformation matrices [4], according to the joint coordinate system definitions of the ISB [2]. Active range of motion and clinical observations are presented for a cohort of 10 normal subjects to investigate the validity of the protocol. This protocol will be applied to assess the shoulder movement of pathological subjects with the aim of developing a valuable clinical diagnostic tool to aid surgeons in identifying optimum treatment strategies.

References [1] Meskers C.G.M., et al. Journal of Biomechanics 1998; 31:93-96. [2] Wu G., et al. Journal of Biomechanics 2005; 38(5): 981-992. [3] Meskers C.G.M., et al. Clinical Biomechanics 1998; 13: 280-292. [4] S6derkvist I., Wedin EA. Journal of Biomechanics 1993; 26(12): 1473-1477.

4047 Mo, 16:00-16:15 (P12) Deltoid mechanics in the cuff def ic ient shoulder E.A. Audenaert 1 , L. De Wilde 1 , A. Audenaert 2, R. Verdonk 1 . 1Department of Orthopedic Surgery, Physical Medicine and Rehabilitation, Ghent University Hospital, Ghent, Belgium, 2Department of environment, technology and technology management, UFSIA-UA, Antwerp, Belgium.

In the progression to cuff tear arthropathy, the shoulder function is often impaired, ranging from weakness to frank pseudoparalysis. The stages can be followed radiographically in terms of progressive ascension and medialization of the humeral centre of rotation. We investigated theoretically to what extent both parameters mechanically influence the functional performance of the deltoid muscle and so contribute to the clinically observed functional loss in the cuff deficient shoulder. Normal glenohumeral relationships in the scapular plane were derived from true anteroposterior X-ray views of the dominant shoulder of fifty three healthy medical students. A biomechanical model of the shoulder was then used to simulate ascension and medialization of the humeral centre of rotation and mechanically analyze their influence on deltoid muscle performance in absence of the rotator cuff muscles. The model was build based on the CT