Ligament Mechanics During Three Degree-of-Freedom Motion at the Acromioclavicular Joint

The development of effective treatment and reconstruction procedures for injuries to the soft tissues around the acromioclavicular (AC) joint relies on a comprehensive understanding of overall joint function. The objective of this study was to determine the magnitude and direction of the in situ forces in the AC capsular and coracoclavicular ligaments as well as the resulting joint kinematics during application of three external loading conditions while allowing three degree-of-freedom joint motion. A robotic/universal force-moment sensor testing system was utilized to determine the in situ forces in the soft tissue structures and the resulting joint kinematics. The clavicle translated 5.1±2.0, 5.6±2.2, and 4.2±1.9 mm during application of a 70 N load in the anterior, posterior, and superior directions, respectively, representing almost a 50% increase over previous studies using similar load magnitudes. In response to an anterior load, the magnitude of in situ force in the superior AC ligament (35±18 N) was found to be greater (p<0.05) than the force in the trapezoid and conoid ligaments. In contrast, the magnitude of in situ force in the conoid (49±22 N) was significantly greater (p<0.05) than all other ligaments in response to a superior load. Additionally, the directions of the force vector representing the conoid and trapezoid were different, being located in opposing quadrants of the posterior axis of the scapula with this loading condition. Our data suggest that the kinematic constraints placed on the AC joint during loading affect the resulting joint motion and that the magnitude and direction of force in each ligament are affected by the coupled motions that occur. Based on the differences in magnitude and direction of the in situ force in the coracoclavicular ligaments with each loading condition, surgical procedures should reconstruct these ligaments in a more anatomical manner or treat them separately to prevent joint degeneration. © 2000 Biomedical Engineering Society. PAC00: 8719Rr, 8719Ff, 8719St, 8780Rb     

Effects of Measurement Error and Sampling Resolution on Estimates of Atrial Tissue Recovery Parameters

We studied the effect of sampling resolution and measurement error on estimates of tissue recovery parameters using experimental and simulated data. Action potential duration (APD) was estimated from monophasic action potentials recorded at 250 sites (x=3.5 mm) on the endocardium of the canine right atrium (n=8) during control and acetylcholine perfusion. APD distributions were also simulated using a random number generator, then scaled and filtered to physiological values. The following parameters were estimated at increasing APD sampling interval and measurement error: mean APD, standard deviation of APD, mean APD gradient, standard deviation of APD gradient, APD wavelength, and APD correlation length. We found that large errors can result from APDs collected at inadequate sampling intervals and adequate sampling intervals may be 3–6 times less than the Nyquist interval. Large parameter errors also resulted from data with relatively low levels of measurement error. The effect of measurement error was dependent upon the standard deviation of APD, sampling resolution, and APD wavelength. Inadequate sampling resolution was the largest source of error in experimental parameter estimates. Estimates of mean and standard deviation of APD gradient decreased with spacing as estimates of correlation length and wavelength increased. Careful selection of spacing interval, taking into account the spatial complexity of recovery, as well as considerably low measurement errors will produce accurate estimates of gradients, correlation length, and wavelength. © 2000 Biomedical Engineering Society. PAC00: 8719Nn, 8719Hh, 0620Dk     

Errors Caused by Combination of Di-4 ANEPPS and Fluo3/4 for Simultaneous Measurements of Transmembrane Potentials and Intracellular Calcium

Intracellular calcium concentration and transmembrane potentials are two important measurements used to study the mechanisms of cardiac activity. Fluorescent dyes have been used to measure these separately but not simultaneously in cardiac tissue. Fluo-3 and Fluo-4 (a recently improved version of Fluo-3) have been used to measure changes in intracellular calcium concentration and Di-4 ANEPPS has been used to measure transmembrane potentials. This paper addresses the feasibility of using these fluorescent dyes together in order to measure transmembrane potentials and intracellular calcium concentration simultaneously. For the dyes to be used simultaneously, their respective fluorescence spectra must be sufficiently separated in wavelength in order to allow them to be separated by optical filters or spectrographs. An apparatus was constructed to measure the dyes' spectra in a fluorescence imaging chamber as well as in an isolated perfused rabbit heart. The measured spectra were mathematically modeled in order to assess the spectral overlap error under different conditions. Error graphs were constructed which may help researchers select optical filters and dye concentrations that will result in an acceptable error. © 1999 Biomedical Engineering Society. PAC99: 8716Uv, 8764Ni, 0620Dk, 8716Yc, 8716Dg, 8719Nn     

Respiratory inductive plethysmography accuracy at varying PEEP levels and degrees of acute lung injury

Background and objective: This study was performed to assess the accuracy of respiratory inductive plethysmographic (RIP) estimated lung volume changes at varying positive end-expiratory pressures (PEEP) during different degrees of acute respiratory failure.

Methods: Measurements of inspiratory tidal volume were validated in eight piglets during constant volume ventilation at incremental and decremental PEEP levels and with increasing severity of pulmonary injury. RIP accuracy was assessed with calibration from the healthy state, from the disease state as the measurement error was assessed, and at various PEEP levels.

Results: Best results (bias 3%, precision 7%) were obtained in healthy animals. RIP accuracy decreased with progressing degrees of acute respiratory failure and was PEEP dependent, unless RIP was calibrated again. When calibration was performed in the disease state as the measurement error was assessed, bias was reduced but precision did not improve (bias – 2%, precision 9%).

Conclusions: RIP accuracy is within the accuracy range found in monitoring devices currently in clinical use. Most reliable results with RIP are obtained when measurements are preceded by calibration in pulmonary conditions that are comparable to the measurement period. When RIP calibration is not possible, fixed weighting of the RIP signals with species and subject size adequate factors is an alternative. Measurement errors should be taken into account with interpretation of small volume changes.     

Operating force information on-line acquisition of a novel slave manipulator for vascular interventional surgery

Vascular interventional surgery has its advantages compared to traditional operation. Master-slave robotic technology can further improve the operation accuracy, efficiency and safety of this complicated and high risk surgery. However, on-line acquisition of operating force information of catheter and guidewire remains to be a significant obstacle on the path to enhancing robotic surgery safety. Thus, a novel slave manipulator is proposed in this paper to realize on-line sensing of guidewire torsional operating torque and axial operation force during robotic assisted operations. A strain sensor is specially designed to detect the small scale torsional operation torque with low rotational frequency. Additionally, the axial operating force is detected via a load cell, which is incorporated into a sliding mechanism to eliminate the influence of friction. For validation, calibration and performance evaluation experiments are conducted. The results indicate that the proposed operation torque and force detection device is effective. Thus, it can provide the foundation for enabling accurate haptic feedback to the surgeon to improve surgical safety.     

Genetic homogeneity,high-resolution mapping,and mutation analysis of the urofacial (Ochoa) syndrome and exclusion of the glutamate oxaloacetate transaminase gene (GOT1) in the critical region as the disease gene

The urofacial (Ochoa) syndrome (UFS) is a rare autosomal recessive disorder characterized by abnormal facial expression and urinary abnormalities. Previously, we mapped the gene to a genomic interval of approximately 1 cM on chromosome region 10q23-24, using families from Columbia. Here we demonstrate genetic homogeneity of the syndrome through homozygosity mapping in American patients with Irish heritage. We established a physical map and identified novel polymorphic markers in the UFS critical region. Haplotype analysis using the new markers mapped the UFS gene within one YAC clone of 1,410 kb. We also determined the precise location of the gene encoding for glutamate oxaloacetate transaminase (GOT1) within the new UFS critical region and determined its genomic structure. However, mutation analysis excluded GOT1 as a candidate for the UFS gene. Am. J. Med. Genet. 84:454–459, 1999. © 1999 Wiley-Liss, Inc.     

An Experimentalist's Approach to Accurate Localization of Phase Singularities during Reentry

A phase variable that uniquely represents the time course of the action potential has been used to study the mechanisms of cardiac fibrillation. A spatial phase singularity (PS) occurs during reentrant wave propagation and represents the organizing center of the rotating wave. Here, we present an error analysis to investigate how well PSs can be localized. Computer simulations of rotating spiral waves scaled appropriately for cardiac tissue were studied with various levels of noise added. The accuracy in identifying and localizing singularities depended on three factors: (i) the point chosen as the origin in state space used to calculate the phase variable; (ii) signal to noise ratio; and (iii) discretization (number of levels used to represent data). We found that for both simulation as well as experimental data, there existed a wide range for the choice of origin for which PSs could be identified. Discretization coupled with noise affected this range adversely. However, there always existed a range for choice of the origin that was 20% or more of the action potential amplitude within which the accuracy of localizing PSs was better than 2 mm. Thus, a precise determination of origin was not necessary for accurately identifying PSs. © 2001 Biomedical Engineering Society. PAC01: 8719Nn, 8710+e, 8719Hh     

Adaptation of conduit artery vascular smooth muscle tone to induced hypertension

We studied the changes in vascular smooth muscle (VSM) cell tone during the adaptation of rat common carotids to induced hypertension. Hypertension was induced in 8 week old male Wistar rats by total ligation of the aorta between the two kidneys. Mean blood pressure increased abruptly from 92±2 mm Hg (mean±SE) to 145±4 mm Hg and remained constant thereafter. Rats were sacrificed 2, 4, 8, and 56 days after surgery and the left common carotid artery was excised for analysis. Pressure-diameter curves were measured in vitro under normal, maximally contracted, and totally relaxed VSM. The VSM tone was analyzed in terms of its basal tone (active stress at low strains) and its myogenic tone (increase in active stress at high strains). Our results show that the capacity of the VSM to develop maximal active stress is not altered in hypertension. Basal tone, however, increases rapidly in the acute hypertension phase (2–8 days postsurgery) and drops to nearly control values at 56 days postsurgery. Also, the onset of myogenic response decreases to lower strains following the step change in pressure, to be restored back to control levels at 56 days postsurgery. We conclude that VSM adaptation is most significant in the acute hypertension phase and acts as a first, rapid defense mechanism for the arterial wall. The VSM tone returns back to normal levels once the slower geometrical and structural remodeling is developed sufficiently to restore the biomechanical environment and function of the arterial wall to control levels. © 2002 Biomedical Engineering Society. PAC2002: 8719Rr, 8719Ff, 8719Uv     

Structure and Function of the Healing Medial Collateral Ligament in a Goat Model

In this study knee joint function with a healing medial collateral ligament (MCL) at six weeks was examined with a robotic/universal force-moment sensor testing system during the application of two loading conditions: (1) 5 N m valgus moment and (2) 67 N anterior load. Additionally the structural properties of the femur–MCL–tibia complex and the mechanical properties of the MCL substance were determined by uniaxial tensile tests. The histological appearance of the healing MCL was also observed. At 30° and 60° of knee flexion, valgus rotation of the healing knee was significantly increased compared to the sham. The in situ force in the healing MCL was significantly lower (34±17 N vs 54±12 N) at the same flexion angles (50±10 N vs 62±7 N). The anterior translation of the knee had returned to normal values at 30° and 60° of knee flexion. However, no differences could be found between the corresponding in situ forces in the healing MCL at all flexion angles examined during application of an anterior load. The stiffness of the healing group (52.5±19.4 N/mm) was significantly lower than the sham group (80.3±26.4 N/mm) (p < 0.04). The modulus of the healing group was also significantly decreased (p < 0.05). The findings suggest that the tensile properties of the healing goat MCL and valgus knee rotation have not returned to normal at six weeks after an isolated MCL rupture, however, anterior translation appeared to return to sham levels. © 2001 Biomedical Engineering Society. PAC01: 8719St     

An in vitro uniaxial stretch model for axonal injury

We have developed a unique uniaxial stretching device to study axonal injury and neural cell death resulting from brain tissue deformations common in traumatic head injuries. Using displacement control rather than force control, this device is capable of achieving strains >70% and strain rates up to 90 s^-1, well above those currently used for studying axonal injury. We have demonstrated that the deformation of the specimen was uniaxial, uniform and highly reproducible; the prespecified displacement profiles could be realized almost precisely; and adequate cell adhesion could be achieved readily. The entire device can fit into a biological safety cabinet to maintain sterility, and the specimens are convenient for cell culture. This device can be used to investigate a wide range of biomechanical issues involved in diffuse axonal injury. © 2003 Biomedical Engineering Society. PAC2003: 8719La, 8710+e, 8719Rr, 8717–d     

Role of Preconditioning and Recovery Time in Repeated Testing of Aortic Valve Tissues: Validation Through Quasilinear Viscoelastic Theory

We investigated how preconditioning history and specimen recovery time affect the accuracy of measurements of mechanical properties obtained from sequential, repeated materials testing of porcine aortic valve (PAV) cusps. Strain history protocols were modeled by quasilinear viscoelastic theory and the results compared with the experimental data. Assuming that the model was predictive, the accuracy of predicting experimental data was related to the suitability of the materials testing protocol. We found that the preconditioned state of the PAV material was not unique but was a function of the deformation history that had occurred before the preconditioning cycles. Preconditioning without an adequate rest period between tests increased predictive errors, whereas allowing the material to rest without preconditioning reduced errors. Modeling more of the strain history reduced errors for specimens briefly rested between tests but had no impact on specimens with long rest periods. The smallest predictive errors were obtained for a loading protocol with a 24 h specimen recovery period followed by material preconditioning. We recommend the use of this protocol for estimating material properties of PAV tissues. © 2000 Biomedical Engineering Society. PAC00: 8719Rr, 8780-y     

The effect of anterior cruciate ligament injury on knee joint function under a simulated muscle load: a three-dimensional computational simulation

Understanding the biomechanical effect of various factors on knee behavior after anterior cruciate ligament (ACL) injury or reconstruction is instrumental for the development of an optimal surgical treatment of ACL injury that can better restore normal knee function. This paper presents the application of a three-dimensional (3D) computational knee model for parametric studies of knee kinematics in response to simulated muscle loads. The knee model was constructed using the magnetic resonance images and biomechanical experimental data of the same cadaveric human knee specimen. The kinematics of the knee predicted by the computational model was compared with that measured from different specimens in a wide range of loading conditions and flexion angles. In general, the model predictions were within the range of experimental data. The model was then used to predict knee motion, ligament forces, and contact pressure in response to a simulated quadriceps force when the knee was ACL deficient. Partial ACL injury was simulated by reducing the stiffness of the ACL in the model. The results demonstrated that even with a reduction of 75% of the ACL stiffness, the ACL still carried a significant amount of the load (more than 58%) carried by an intact ACL. The kinematics (both tibial translation and rotation) varied less than 20% compared to that of the knee with intact ACL. The 3D computational model can be a powerful tool to simulate different variables that would influence knee function after ACL reconstruction, such as the initial tension of the ACL graft, the insertion sites of the graft, multibundle grafts, graft materials, and various physiological loading conditions. © 2002 Biomedical Engineering Society. PAC2002: 8719Ff, 8719Rr, 8719St     

Application of robotic technology in biomechanics to study joint laxity

Primary objective:?To evaluate whether in vitro joint testing using a robot with six degrees of freedom is useful for evaluating changes in joint laxity as a result of chronic osteoarthritis (OA).

Research design:?Repeated measures.

Methods:?Broyden's method of solving nonlinear systems of equations drove a hybrid method of load and position robotic control. Sheep stifles (knee joints) were loaded between 3 Nm of internal load through to 3 Nm of external load in 1 Nm increments. Kinematic and morphologic data from five healthy ovine stifles were compared to the chronic OA effects in four surgically destabilized stifles.

Results:?Stifles with chronic OA showed increases in stiffness while range of motion decreased. Gross morphologic changes included osteophytes and cartilage fibrillation.

Discussion:?Robotic testing proved useful for evaluating changes in joint mechanics as a result of chronic OA. We observed morphological changes and associated increases in joint stiffness and decreased laxity.     

Characterization of robotic system passive path repeatability during specimen removal and reinstallation for in vitro knee joint testing

Robotic testing systems are commonly utilized for the study of orthopaedic biomechanics. Quantification of system error is essential for reliable use of robotic systems. Therefore, the purpose of this study was to quantify a 6-DOF robotic system's repeatability during knee biomechanical testing and characterize the error induced in passive path repeatability by removing and reinstalling the knee. We hypothesized removing and reinstalling the knee would substantially alter passive path repeatability. Testing was performed on four fresh-frozen cadaver knees. To determine repeatability and reproducibility, the passive path was collected three times per knee following the initial setup (intra-setup), and a single time following two subsequent re-setups (inter-setup). Repeatability was calculated as root mean square error. The intra-setup passive path had a position repeatability of 0.23 mm. In contrast, inter-setup passive paths had a position repeatability of 0.89 mm. When a previously collected passive path was replayed following re-setup of the knee, resultant total force repeatability across the passive path increased to 28.2 N (6.4 N medial–lateral, 25.4 N proximal–distal, and 10.5 N anterior–posterior). This study demonstrated that removal and re-setup of a knee can have substantial, clinically significant changes on our system's repeatability and ultimately, accuracy of the reported results.     

A Deformable Finite Element Derived Finite Difference Method for Cardiac Activation Problems

We present a finite element (FE) derived finite difference (FD) technique for solving cardiac activation problems over deforming geometries using a bidomain framework. The geometry of the solution domain is defined by a FE mesh and over these FEs a high resolution FD mesh is generated. The difference points are located at regular intervals in the normalized material space within each of the FEs. The bidomain equations are then transformed to the embedded FD mesh which provides a solution space that is both regular and orthogonal. The solution points move in physical space with any deformation of the solution domain, but the equations are set up in such a way that the solution is invariant as it is constructed in material space. The derivation of this new solution technique is presented along with a series of examples that demonstrate the accuracy of this bidomain framework. © 2003 Biomedical Engineering Society. PAC2003: 8719Hh, 8710+e, 8719Rr     

Torso coupling techniques for the forward problem of electrocardiography

The calculation of body surface potentials from a known cardiac source is traditionally formulated in terms of a two step process. The first step involves the generation of some form of equivalent cardiac source (typically dipole based) at a resolution significantly lower than that of a continuum cell. The second then places that source into a volume conductor within which the potential fields are calculated. This approach does not properly capture the feedback between the torso and the extracellular potential field. Presented here are the details of two new methods which calculate continuous potential fields throughout the torso that are the direct result of cardiac cellular electrical activity. These new methods are termed the Boundary Iteration Method and the Direct Assembly Method. While these two methods return essentially identical answers, there is a definite tradeoff between computational speed and memory overhead with the direct assembly method proving faster but requiring significantly more memory. Examples are given that demonstrate the convergence and accuracy of these methods in an idealized coupled torso system. These techniques are then applied to an anatomically based model of a slice through a human male torso. © 2002 Biomedical Engineering Society. PAC2002: 8719Nn, 8717-d, 8719Hh     

Arterial Wall Adaptation under Elevated Longitudinal Stretch in Organ Culture

Arteries in vivo are subjected to large longitudinal stretch which may change significantly due to vascular disease and surgery. However, little is known about the effect of longitudinal stretch on vascular function and wall remodeling, although the effects of tensile and shear stress from blood pressure and flow have been well documented. To study the effect of longitudinal stretch on vascular function and wall remodeling, porcine carotid arteries were longitudinally stretched 20% more than in vivo for 5 days while being maintained in an ex vivo organ culture system under conditions of pulsatile flow at physiologic pressure. Vessel viability was demonstrated by strong vasomotor responses to norepinephrine (NE, 10^-6M), carbachol (10^-6M), and sodium nitroprusside (10^-5M), as well as by dense staining for mitochondrial activity and a low occurrence of cell necrosis. Cell proliferation was examined by incorporation of bromodeoxyuridine (BrdU). Results showed that arteries maintain normal structure and viability after 5 days in organ culture. Both the stretched and control arteries demonstrated significant contractile responses. For example, both stretched and control arteries showed approximately 10% diameter contraction in response to NE. Stretched arteries contained 8% BrdU-positive cells compared to 5% in controls (p < 0.05). These results indicate that longitudinal stretch promotes cell proliferation in arteries while maintaining arterial function. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr, 8717Ee, 8719Uv     

Surface strains in the anterior leaflet of the functioning mitral valve

The mitral valve (MV) is a complex anatomical structure whose function involves a delicate force balance and synchronized function of each of its components. Elucidation of the role of each component and their interactions is critical to improving our understanding of MV function, and to form the basis for rational surgical repair. In the present study, we present the first known detailed study of the surface strains in the anterior leaflet in the functioning MV. The three-dimensional spatial positions of markers placed in the central region of the MV anterior leaflet in a left ventricle-simulating flow loop over the cardiac cycle were determined. The resulting two-dimensional in-surface strain tensor was computed from the marker positions using a C ⁰ Lagrangian quadratic finite element. Results demonstrated that during valve closure the anterior leaflet experienced large, anisotropic strains with peak stretch rates of 500%–1000%/s. This rapid stretching was followed by a plateau phase characterized by relatively constant strain state. We hypothesized that the presence of this plateau phase was a result of full straightening of the leaflet collagen fibers upon valve closure. This hypothesis suggests that the MV collagen fibers are designed to allow leaflet coaptation followed by a dramatic increase in stiffness to prevent further leaflet deformation, which would lead to valvular regurgitation. These studies represent a first step in improving our understanding of normal MV function and to help establish the principles for repair and replacement. © 2002 Biomedical Engineering Society. PAC2002: 8719Hh, 8719Uv, 8719Rr