$496 Journal of Biomechanics 2006, Vol. 39 (Suppl 1)

5304 We-Th, no. 89 (P57) The effect of stress deprivation on mechanical properties of regenerated and residual tissues in the central third resected rabbit patellar tendon is age dependent E. Maeda 1 , H. Asanuma 2, H. Noguchi 3, H. Tohyama 4, K. Yasuda 4, K. Hayashi 5 . 1 Department of Engineering, Queen Mary University of London, London, UK, 2Graduate School of Engineering Science, Osaka University, Osaka, Japan, 3Dense Corporation, Nagoya, Japan, 4Department of Advanced Surgery, Hokkaido University School of Medicine, Sapporo, Japan, 5Research Institute of Technology, Okayama University of Science, Okayama, Japan

The central one-third portion of the patellar tendon (PT) is commonly used as an autograft for ACL reconstruction. The resulting stress deprivation forms the focus of the present study which examines the mechanical properties of healing tendons from different aged sources. Skeletally immature 3-month old and mature 6-month old rabbits were used. After surgical exposure of the PT in each right knee, an entire rectangular defect was made in the central third portion of the PT. Thus operated rabbits were divided into two groups; one group (SS) where the tendons were completely stress-shielded [1], and the other group (NH) which were not stress-shielded. Both groups were allowed to resume normal joint activity. For both age groups, between 3 and 6 weeks post operation, tensile tests were performed to determine biomechanical properties of both the residual tissue (medial and lateral portions) and regenerated tissue (central portion). The strength of regenerated tissue in both groups were lower than unoperated normal tissue. The difference was more marked in the 6 months animals. A significant decrease was observed in the strength of residual tissues in both groups. The difference in the ratio to the normal level between NH and SS was greater in the younger animals. The change in elastic modulus for both tissues showed essentially similar trends to those for tensile strength. Therefore, stress deprivation leads to a more pronounced effect in the regener- ated tissues of mature animals, whereas, for residual tissues, it is the younger animals which demonstrated a more marked effect.

References [1] Yamamoto et al. J Biomed Eng 1993; 115: 23-28.

7592 We-Th, no. 90 (P57) Inital failure strength of arthroscopic biceps tenodesis techniques R.S. Costic, E. Lopez-Vidriero, P.J. Smolinski, L.G. Gilbertson, RH. Fu, M .W. Rodosky. Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, PA, USA

Objective: Arthroscopic biceps tenodesis has become a popular alternative to open keyhole fixation to minimize risks associated with bony defects and hardware. Arthroscopic techniques include soft tissue fixation to the transverse humeral ligament and anchor fixation to the bicipital groove; however, there is little biomechanical data on the initial strength of these repairs. The objective of this study was to evaluate the biomechanical properties of these two arthroscopic biceps tenodesis techniques. Methods: Fifteen fresh cadaveric humerus with the isolated biceps tendon intact were attached to a materials testing machine. Two arthroscopic biceps tenodesis repairs were tested: suture anchor attachment to biciptial groove (n = 7) and the percutaneous intra-articular transtendon tenodesis (PITT) with attachment to the transverse humeral ligament (n = 8). A load-to-failure protocol was performed and the failure mode was recorded. Ultimate load and stiffness were determined and compared with a Student's t-test with significance set at p < 0.05. Results: Suture anchor and PITT techniques had ultimate loads of 175 ± 16 N and 1 4 3 ± 3 0 N and stiffness of 15 .9±8N/mm and 13.3±3N/mm, with no statistical difference between techniques, respectively (Figure 1, p > 0.05). Both repairs had typical load-displacement curves followed by a constant increase in displacement till failure occurred with suture pull-out through the substance of the biceps. Conclusions: Suture anchor and PITT techniques exhibited satisfactory initial strength with no statistical difference between the two groups. These findings along with the consistent pullout of the suture through the tendon during failure suggests that the most important factor for initial strength is not dependent on the attachment site but dependent on the quality of the biceps tendon. This may be important when considering post-operative rehabilitation.

References [1] Sekiya JK and Rodosky MW, et al. Arthroscopic biceps tenodesis using the

percutaneous intra-articular transtendon technique. Arthro 2003; 19: 1137-41.

Poster Presentations

5683 We-Th, no. 91 (P57) The use of rope theory to model ligaments

S. Puong 1 , A.M.J. Bull 1 , J.J. Evans 2. 1Department of Bioengineering, Imperial College London, UK, 2 Randafl Division of Ceil and Molecular Biophysics, Kings College London, UK

The aim of this study was to build a generalised model of a ligament based on its structural analogy with a single strand rope. This takes into account the rotation of the ligament insertion areas in three dimensions, in order to explain its mechanical behaviour for physiological movements of joints. Our model was inspired by the theoretical work of Machida and Durelli [J Mech Eng Sci, 1973]. The ligament is modelled with fibres being straight in the unloaded state and with different lengths depending on the configuration of insertion sites of the ligament, based on experimental data of the positions of each fibre tipsis. In order to account for the non-linear stress-strain response of the ligament, considered to be due to progressive recruitment of fibres, different zero-strain lengths for each fibre were applied. The behaviour of the ligament model was analysed from a simple stretch along its axis to the combination of axial twist and rotation followed by stretching. Axial twisting of the ligaments had a minor effect on the ligament stiffness values, although as the angle of twist was increased, more fibres were re- cruited. Rotations about the perpendicular axes significantly affected the value of the stiffness. This evolution of the mechanical properties of the ligament with rotation and twisting was verified experimentally on the lateral collateral ligament of the knee joint. Although experimental results were easily matched to the model, further development should include the ability to describe non- straight ligament fibres in its undeformed state, as is the case in many ligamentous structures. The model was thus able to simulate a non linear structural behaviour with elements assumed to be linear by taking into account geometrical changes during the loading cycle.

5694 We-Th, no. 92 (P57) A resultant force and limiting ligament strains approach to computing glenohumeral joint (GHJ) translations H.O. Amadi 1,2, A.M.J. Bull 1 , U.N. Hansen 2. 1Departments efBieengineering, 2Department of Mechanical Engineering, Imperial College London, UK

The aims of this study were (1) to create a tool to investigate the loading of the intracapsular ligaments of the GHJ during shoulder movement, and (2) to investigate how the humeral head may need to translate to minimise ligament overstraining. Imaging datasets were used to create surface geometries of the GHJ. A joint coordinate system (JCS) was applied for kinematics description [1]. Five liga- ments were modelled with literature-provided properties and insertions [2,3]. An algorithm was developed to relate clinical shoulder position data to the JCS. Ligament paths were modelled as a straight line from glenoid to humeral insertion with spherical wrapping on the humeral head adjusted according to the actual surface contour. These were used to quantify a resultant effect of the ligament loading. Physiological kinematics data during passive clinical examinations of the arm [4] were applied to the model. In order to limit the overstraining of the ligaments, translations were imposed on these kinematics data in the direction of the resultant forces due to the ligaments. These translation were limited when the ligament strains were below an experimentally-derived physiological threshold [2,3]. Individual ligament loads of up to 290 N were predicted during impingement test kinematics when translations were not taken into account. This same move- ment resulted in a computed 3.1 mm anteroinferior translation of the humeral head when limiting ligament strains. There were no predicted translations for pure abduction and forward flexion without humeral axial rotation. There is currently no direct measure of translations of the shoulder that can be applied clinically. This work is a novel method that allows the relative ligament restraints during physiological motion to be calculated taking into account possible GHJ translations. This can be applied in guiding clinical kinematics tests, as well as surgical interventions.

References [1] Wu et al. J Biomech. 2005. [2] Boardman et al. J Should EIb Surg. 1996. [3] Bigliani et al. J Orthop Res. 1992. [4] Fung et al. J Should EIb Surg. 2001.