Track 2. Musculoskeletal Mechanics-Joint ISB Track

6093 We-Th, no. 85 (P57) A "closed-loop" forward dynamic model to predict lower extremity kinematics S.M. Zingde 1 , R.D. Komistek 1,2. 1University of Tennessee, Knoxville, TN, USA, 2Oak Ridge National Laboratory, Oak Ridge, TN, USA

Unlike "inverse" solution models where the kinematics are input to predict inter- active and muscle forces, a forward solution model inputs muscle force profiles throughout a sequence of motion to predict the motions. Thus this method can be effectively used to evaluate future implant designs that would generate a specific anticipated motion. Therefore, the objective of this study was to develop a closed-loop, forward solution dynamic model of the lower extremity. The model was based on the principles of rigid body dynamics and calculates the unknowns associated with each rotation and translation of the bones. The model was applied on subjects having an implanted or non implanted knee joint during normal gait and a deep knee bend to maximum flexion. The muscles were added to the system in a sequential manner with the first being the quadriceps muscle which is the main driver in knee motion. Femoro-tibial contact positions, orientation of the tibia and femur with respect to the ground and the patello-femoral articulation predicted by the model were compared to those obtained using fluoroscopy which was used as the reference. If the predicted pattern differed significantly from the experimental, the quadriceps muscle pattern was optimized using a feedback loop until the difference was within a tolerance value. Once the difference is within an acceptable limit, other muscles are added to the system. The inclusion of new muscles leads to further optimization of the system until stability is achieved. Early results have been promising and the kinetics and the kinematics generated by this process have been similar to results derived experimentally. Further development of the process would enable us to have a theoretical system which would perform functions similar to experimental simulators but in a much shorter span of time.

2.8 Tendons and Ligaments - Mechanics of Normal Tissue

7544 We-Th, no. 86 (P57) Noncontact ACL injury through impingement against the intercondylar notch L.-Q. Zhang 1,2,3,4, D.T. Fung 1,4, C. Ahn 1 , H.-S. Park 1 , Y. Ren 1 , J.L. Koh 3, R.W. Hendrix 5, S.Q. Liu 4. 1Rehabilltation Institute of Chicago, Departments of 2 Physical Medicine and Rehabilitation, 3 Orthopaedic Surgery, 4Biomedical Engineering, and 5Radiology, Northwestern University, Chicago, Illlnois, USA

Although tibial internal and external rotations are reported to increase and decrease the direct loading of the ACL respectively, both are associated with non-contact ACL injuries. We hypothesize that tibial external rotation (and abduction) causes ACL impingement against the lateral intercondylar notch wall, which may result in ACL injury. ACL impingement was measured by a paper-thin pressure/force sensor in- serted at the potential impingement site of cadaver knee specimens, together with six degrees of freedom knee kinematics. The knee was moved forcefully throughout its range of motion in 3-D space to evaluate possible ACL im- pingement. Two patterns of ACL impingement were observed. First, for knees with "narrow" (more strictly, certain geometry) intercondylar notch, ACL could impinge the lateral wall of the intercondylar notch during tibial external rotation and abduction in flexed knees. Strong impingement force was recorded when the tibia was externally rotated on top of abduction. Second, for knees with a regular or wide intercondylar notch, ACL did not impinge against the lateral notch wall during various tibiofemoral movements. However, when the knee was at near hyperextension, the ACL impinged the medial corner of the notch roof under extension and tibial internal rotation loading. ACL impingement model was developed based on individual 3-D knee geometry and computer simulation showed ACL impingement that was consistent with the cadaver experimental data. In conclusion, in addition to being over-loaded directly by excessive tibial ante- rior translation, hyperextension, and/or internal rotation, ACL can be injured by impingement with the intercondylar notch in non-contact ACL injuries. Further, different mechanisms may be involved in ACL impingement, depending on the specific 3-D notch and ACL geometries and joint laxity. The commonly used notch width index measure may not be enough to characterize impingement and accurate 3-D characterization of the notch and ACL geometry is needed.

4928 We-Th, no. 87 (P57) In vivo sonometry measurement of strain in the human achilles tendon A. Arndt 1,2, L. Tomatis 1 , A. Ryberg 2, D. Kleman 1 , M. Peolsson 3, A. Thorstensson 1 . 1 University College of Physical Education and Sport, Stockholm, 2 Sweden, Karolinska University Hospital, Stockholm, Sweden; c Linkdping University, Linkdping, Sweden

Previously applied methods for determining strain in the distal achilles ten- don have inherent problems and the use of sonometry as a possibility for

2.8 Tendons and Ligaments - Mechanics of Normal Tissue $495

accurately recording tendon strain is presented here. In this first in vivo test, two 1 mm, sonometric transducers (Sonometrics Corp. London, Ont., Canada) were inserted with custom designed insertion tools two and three cm proximal to the calcaneal insertion of the tendon of one person. The subject lay in a prone position with the foot firmly fixed in a plantarflexion device recording plantar force and ankle angle. The plantar plate angle was set to neutral when the ankle angle was 900 . Five-second passive trials were performed at 100 increments from 200 plantarflexion to 200 dorsiflexion and three plantarflexion force ramps were conducted at 200 plantarflexion, neutral and 200 dorsiflexion. Passive neutral measurements were conducted after each treatment. Table 1. Achilles tendon strain measured with the sonometry sensors during passive, static ankle joint positions

Condition Mean strain Standard deviation

Passive neutral 1 0 0.09 Passive neutral 2 4.1 0.10 Passive neutral 3 1.8 0.09 Passive neutral 4 1.4 0.08 Passive neutral 5 1.8 0.08 Passive neutral 6 3.2 0.11 10 ° pf 0.38 0.08 20 ° pf 0.78 0.07 10 ° df 3.94 0.08 20 ° df 4.84 0.08

The trial "passive neutral 1" was defined as 0 strain. The mean value is calculated from all data points in the measurement period, and the standard deviation indicates signal stability. The results showed strain levels corresponding to the literature. The strain in the passive neutral trials was shown to vary depending upon the immediately preceding treatment (e.g. large dorsiflexion or strong triceps surae contraction). Tendon strain in the isometric ramp conditions closely followed the plantar force curves. Sonometrics was found to be applicable to human in vivo measurements and a study with 20 subjects is now planned.

4183 We-Th, no. 88 (P57) A comparison between the classical approach and a Volterra-Wiener constitutive model in evaluating the viscoelastic properties of mouse medial collateral ligaments

E. Rizzuto 1 , Z. Del Prete 2, A. Musar61 . 1Department of Histology and Medical Embryology, University of Rome "La Sapienza", Rome, Italy, 21nteruniversity Institute of Myology, Department of Mechanical Engineering, University of Rome 'La Sapienza ', Rome, Italy

Viscoelastic properties of medial collateral ligaments were measured in control (WT) and in MLC/mlgf-1 transgenic (TG) mice. MLC/mlgf-1 mice are a model of functional muscle hypertrophy, which expresses an increase in muscle mass and strength. Therefore our initial purpose was to investigate if the localized mlgf-1 transgene expression presented side effects on the ligamentous tissue. The experimental protocol, based on uniaxial stretching, led to the calculation of tissue complex compliance (CC) in two different modes: directly from tissue hysteresis loop measurements (classical approach) and using a second-order Volterra-Wiener constitutive model, calculated stimulating the tissue with a PGN stress input signal. The classical approach proved that there was no sig- nificant difference of storage compliance (SC) among the two groups. However, TG mice ligaments showed mean values of loss compliance (LC) significantly higher than WT ones (p<0.001). Nevertheless, both SC and LC were not statistically influenced by the frequency factor. The use of a second-order kernel constitutive model led to different results. No significantly differences were revealed between the two groups for SC and LC, but the rate of LC was now affected by the frequency factor (p < 0.001 ). LC values computed with the two methods came out quite different, especially for low frequencies. At these frequencies (0.1 Hz, 0.2 Hz), close to quasi-static stimulation, low values of LC were expected, as they resulted with the Volterra-Wiener model.

References Derwin K.A., Soslowsky L.J. (1999). A quantitative investigation of structure-

function relationships in a tendon fascicle model. Journal of Biomechanical Engineering 121: 598~04.

Hoffman A.H., Grigg R (2002). Using uniaxial pseudorandom stress stimuli to develop soft tissue constitutive equations. Annals of Biomedical Engineering 30: 44-53.

Musaro A., McCullagh K., Paul A., Houghton L., Dobrowolny G., Molinaro M., Barton E.R., Sweeney H.L., Rosenthal N. (2001). Localized Igf-1 transgene expression sustains hypertrophy and regeneration in senescent skeletal muscle. Nature Genetics 27: 195-200.