Free-floating medial meniscus implant kinematics do not change after simulation of medial open-wedge high tibial osteotomy and notchplasty.

PURPOSE: The purpose of this in-vitro study was to examine the kinematics of an artificial, free-floating medial meniscus replacement device under dynamic loading situations and different knee joint states. METHODS: A dynamic knee simulator was used to perform dynamic loading exercises on three neutrally aligned and three 10° valgus aligned (simulating a medial openwedge high tibial osteotomy - MOWHTO) left human cadaveric knee joints. The knee joints were tested in three states (intact, conventional notchplasty, extended notchplasty) while 11 randomised exercises were simulated (jump landing, squatting, tibial rotation and axial ground impacts at 10°, 30° and 60° knee joint flexion) to investigate the knee joint and implant kinematics by means of rigidly attached reflective marker sets and an according motion analysis. RESULTS: The maximum implant translation relative to the tibial plateau was < 13 mm and the maximum implant rotation was < 19° for all exercises. Both, the notchplasties and the valgus knee alignment did not affect the device kinematics. CONCLUSIONS: The results of the present in-vitro study showed that the non-anchored free-floating device remains within the medial knee joint gap under challenging dynamic loading situations without indicating any luxation tendencies. This also provides initial benchtop evidence that the device offers suitable stability and kinematic behaviour to be considered a potential alternative to meniscus allograft transplantation in combination with an MOWHTO, potentially expanding the patient collective in the future.

Quantification of in vitro wear of a synthetic meniscus implant using gravimetric and micro-CT measurements.

A synthetic meniscus implant was recently developed for the treatment of patients with mild to moderate osteoarthritis with knee pain associated with medial joint overload. The implant is distinctively different from most orthopedic implants in its pliable construction, and non-anchored design, which enables implantation through a mini-arthrotomy without disruption to the bone, cartilage, and ligaments. Due to these features, it is important to show that the material and design can withstand knee joint conditions. This study evaluated the long-term performance of this device by simulating loading for a total of 5 million gait cycles (Mc), corresponding to approximately five years of service in-vivo. All five implants remained in good condition and did not dislodge from the joint space during the simulation. Mild abrasion was detected by electron microscopy, but µ-CT scans of the implants confirmed that the damage was confined to the superficial surfaces. The average gravimetric wear rate was 14.5 mg/Mc, whereas volumetric changes in reconstructed µ-CT scans point to an average wear rate of 15.76 mm(3)/Mc (18.8 mg/Mc). Particles isolated from the lubricant had average diameter of 15 µm. The wear performance of this polycarbonate-urethane meniscus implant concept under ISO-14243 loading conditions is encouraging.

In-vivo evaluation of the kinematic behavior of an artificial medial meniscus implant: A pilot study using open-MRI.

BACKGROUND: In this pilot study we wanted to evaluate the kinematics of a knee implanted with an artificial polycarbonate-urethane meniscus device, designed for medial meniscus replacement. The static kinematic behavior of the implant was compared to the natural medial meniscus of the non-operated knee. A second goal was to evaluate the motion pattern, the radial displacement and the deformation of the meniscal implant. METHODS: Three patients with a polycarbonate-urethane implant were included in this prospective study. An open-MRI was used to track the location of the implant during static weight-bearing conditions, within a range of motion of 0° to 120° knee flexion. Knee kinematics were evaluated by measuring the tibiofemoral contact points and femoral roll-back. Meniscus measurements (both natural and artificial) included anterior-posterior meniscal movement, radial displacement, and meniscal height. FINDINGS: No difference (P>0.05) was demonstrated in femoral roll-back and tibiofemoral contact points during knee flexion between the implanted and the non-operated knees. Meniscal measurements showed no significant difference in radial displacement and meniscal height (P>0.05) at all flexion angles, in both the implanted and non-operated knees. A significant difference (P ≤ 0.05) in anterior-posterior movement during flexion was observed between the two groups. INTERPRETATION: In this pilot study, the artificial polycarbonate-urethane implant, indicated for medial meniscus replacement, had no influence on femoral roll-back and tibiofemoral contact points, thus suggesting that the joint maintains its static kinematic properties after implantation. Radial displacement and meniscal height were not different, but anterior-posterior movement was slightly different between the implant and the normal meniscus.

Viscoelastic properties of a synthetic meniscus implant.

There are significant potential advantages for restoration of meniscal function using a bio-stable synthetic implant that combines long-term durability with a dependable biomechanical performance resembling that of the natural meniscus. A novel meniscus implant made of a compliant polycarbonate-urethane matrix reinforced with high modulus ultrahigh molecular weight polyethylene fibers was designed as a composite structure that mimics the structural elements of the natural medial meniscus. The overall success of such an implant is linked on its capability to replicate the stress distribution in the knee over the long-term. As this function of the device is directly dependent on its mechanical properties, changes to the material due to exposure to the joint environment and repeated loading could have non-trivial influences on the viscoelastic properties of the implant. Thus, the goal of this study was to measure and characterize the strain-rate response, as well as the viscoelastic properties of the implant as measured by creep, stress relaxation, and hysteresis after simulated use, by subjecting the implant to realistic joint loads up to 2 million cycles in a joint-like setting. The meniscus implant behaved as a non-linear viscoelastic material. The implant underwent minimal plastic deformation after 2 million fatigue loading cycles. Under low compressive loads, the implant was fairly flexible, and able to deform relatively easily (E=120-200 kPa). However as the compressive load applied on the implant was increased, the implant became stiffer (E=3.8-5.2 MPa), to resist deformation. The meniscus implant appears well-matched to the viscoelastic properties of the natural meniscus, and importantly, these properties were found to remain stable and minimally affected by potentially degradative and loading conditions associated with long-term use.

Long-term evaluation of a compliant cushion form acetabular bearing for hip joint replacement: a 20 million cycles wear simulation.

Soft bearing materials that aim to reproduce the tribological function of the natural joint are gaining popularity as an alternative concept to conventional hard bearing materials in the hip and knee. However, it has not been proven so far that an elastic cushion bearing can be sufficiently durable as a long term (∼20 years) articulating joint prosthesis. The use of new bearing materials should be supported by accurate descriptions of the implant following usage and of the number, volume, and type of wear particles generated. We report on a long-term 20 million cycle (Mc) wear study of a commercial hip replacement system composed of a compliant polycarbonate-urethane (PCU) acetabular liner coupled to a cobalt-chromium alloy femoral head. The PCU liner showed excellent wear characteristics in terms of its low and steady volumetric wear rate (5.8-7.7 mm(3)/Mc) and low particle generation rate (2-3 × 10(6) particles/Mc). The latter is 5-6 orders of magnitude lower than that of highly cross-linked polyethylene and 6-8 orders of magnitude lower than that of metal-on-metal bearings. Microscopic analysis of the implants after the simulation demonstrated a low damage level to the implants' articulating surfaces. Thus, the compliant PCU bearing may provide a substantial advantage over traditional bearing materials.

Chondroprotective effects of a polycarbonate-urethane meniscal implant: histopathological results in a sheep model.

PURPOSE: injury or loss of the meniscus generally leads to degenerative osteoarthritic changes in the knee joint. However, few surgical options exist for meniscal replacement. The goal of this study was to examine the ability of a non-degradable, anatomically shaped artificial meniscal implant, composed of Kevlar-reinforced polycarbonate-urethane (PCU), to prevent progressive cartilage degeneration following complete meniscectomy. METHODS: the artificial meniscus was implanted in the knees of mature female sheep following total medial meniscectomy, and the animals were killed at 3- and 6-months post-surgery. Macroscopic analysis and semi-quantitative histological analysis were performed on the cartilage of the operated knee and unoperated contralateral control joint. RESULTS: the PCU implants remained well secured throughout the experimental period and showed no signs of wear or changes in structural or material properties. Histological analysis showed relatively mild cartilage degeneration that was dominated by loss of proteoglycan content and cartilage structure. However, the total osteoarthritis score did not significantly differ between the control and operated knees, and there were no differences in the severity of degenerative changes between 3 and 6 months post-surgery. CONCLUSION: current findings provide preliminary evidence for the ability of an artificial PCU meniscal implant to delay or prevent osteoarthritic changes in knee joint following complete medial meniscectomy.

MRI-based characterization of bone anatomy in the human knee for size matching of a medial meniscal implant.

Allograft or synthetic menisci have been suggested as a means to restore contact pressures following meniscectomy. However, when the natural meniscus is severely damaged/absent, the necessary size cannot be determined according to the recipient size and there is a need to estimate it from magnetic resonance imaging (MRI) of the contralateral knee or the injured knee bones. The use of the contralateral-knee for size matching is problematic due to economic and practical reasons. Hence, there are significant advantages for a sizing algorithm based only on the candidate knee geometry. The aim of this study is to characterize midrange values and variability of knee dimensions and to develop a set of mathematical relations representing knee dimensions using a minimum of imaging-based bone measurements. Tibia, femur, and meniscus measurements were taken in 118 MRI scans and used to develop a representative parametric knee model in which all dimensions are expressed using tibia plateau width. The model was verified by comparing the predicted values to direct MRI measurements for 20 additional subjects by means of the Pearson correlation and Bland and Altman (1986, "Statistical Methods for Assessing Agreement Between Two Methods of Clinical Measurement," Lancet, 1, pp. 307-310) plot. Anatomical parameters in the male knee were significantly larger (∼17%) compared with corresponding female measurements. However, most relations between tibia, femur, and meniscus measurements (43/56) were not significantly different between male and female populations (p ≥ 0.05), indicating that differences between male and female joints are generally related to scaling and not shape. Dimensions predicted by the knee model were in a good agreement with dimensions measured directly from the MRI (R(2)>0.96) and the Bland and Altman plot indicated that ∼95% of data points were well within the ± 2 standard deviation lines of agreement. The model proposed in this study is advantageous in being able to describe typical knee proportions for a given tibial width and can be used to predict the dimensions of a candidate knee based on a single measurement. The anatomical/anthropometric data presented in the study can be utilized in a sizing algorithm for artificial meniscal implants or in the design of artificial meniscus prostheses.

Design of a free-floating polycarbonate-urethane meniscal implant using finite element modeling and experimental validation.

The development of a synthetic meniscal implant that does not require surgical attachment but still provides the biomechanical function necessary for joint preservation would have important advantages. We present a computational-experimental approach for the design optimization of a free-floating polycarbonate-urethane (PCU) meniscal implant. Validated 3D finite element (FE) models of the knee and PCU-based implant were analyzed under physiological loads. The model was validated by comparing calculated pressures, determined from FE analysis to tibial plateau contact pressures measured in a cadaveric knee in vitro. Several models of the implant, some including embedded reinforcement fibers, were tested. An optimal implant configuration was then selected based on the ability to restore pressure distribution in the knee, manufacturability, and long-term safety. The optimal implant design entailed a PCU meniscus embedded with circumferential reinforcement made of polyethylene fibers. This selected design can be manufactured in various sizes, without risking its integrity under joint loads. Importantly, it produces an optimal pressure distribution, similar in shape and values to that of natural meniscus. We have shown that a fiber-reinforced, free-floating PCU meniscal implant can redistribute joint loads in a similar pattern to natural meniscus, without risking the integrity of the implant materials.

Wear rate evaluation of a novel polycarbonate-urethane cushion form bearing for artificial hip joints.

There is growing interest in the use of compliant materials as an alternative to hard bearing materials such as polyethylene, metal and ceramics in artificial joints. Cushion form bearings based on polycarbonate-urethane (PCU) mimic the natural synovial joint more closely by promoting fluid-film lubrication. In the current study, we used a physiological simulator to evaluate the wear characteristics of a compliant PCU acetabular buffer, coupled against a cobalt-chrome femoral head. The wear rate was evaluated over 8 million cycles gravimetrically, as well as by wear particle isolation using filtration and bio-ferrography (BF). The gravimetric and BF methods showed a wear rate of 9.9-12.5mg per million cycles, whereas filtration resulted in a lower wear rate of 5.8mg per million cycles. Bio-ferrography was proven to be an effective method for the determination of wear characteristics of the PCU acetabular buffer. Specifically, it was found to be more sensitive towards the detection of wear particles compared to the conventional filtration method, and less prone to environmental fluctuations than the gravimetric method. PCU demonstrated a low particle generation rate (1-5×106 particles per million cycles), with the majority (96.6%) of wear particle mass lying above the biologically active range, 0.2-10µm. Thus, PCU offers a substantial advantage over traditional bearing materials, not only in its low wear rate, but also in its osteolytic potential.

Patient-specific modeling of deep tissue injury biomechanics in an unconscious patient who developed myonecrosis after prolonged lying.

Deep tissue injury (DTI) is a severe pressure ulcer, characterized by necrotic tissue mass under intact skin. This communication integrates biomechanical analysis tools described in our previous publications, for reconstructing a "real-world" DTI case. A patient-specific finite element (FE) model was developed utilizing MRI of the thigh of a patient who was found unconscious after an estimated time of 3 days. During that time he lay down on the left side while in his left pocket there was a cellular phone. He developed left proximal thigh pain and swelling. MRI of his left thigh revealed deep muscle necrosis. To reconstruct this injury, previously reported injury threshold and damage law for muscle tissue were coupled into the FE-model. This allowed a time-dependent description of the size and shape of the DTI. We also simulated a hypothetical case for the same patient anatomy, where no phone was present. Though peak strains were only mildly (<1.2-fold) higher with the phone, the rigid phone structure "shifted" the sites of localized elevated strains substantially with respect to the simulation case where there was no phone present. Peak muscle stresses also did not differ much with/without the phone (<1.7-fold), but when the phone was present, considerably larger (4-fold) volumes of muscle and fat tissues were exposed to elevated (>10 kPa) stresses. By diverting the flow of internal tissue loads, the phone induced two stress concentrations in soft tissues, adjacent to the bone and adjacent to the phone. Since the phone induced elevated stresses in wider regions, it caused the DTI to develop and progress faster. The simulated DTI appeared in muscle tissue in proximity to the bone after approximately 1.5h, and within less than 10 additional minutes it reached the muscle-fat boundary. In conclusion, our biomechanical modeling tools could successfully reconstruct a "real-world" case of DTI, with reasonable similarity to the actual wound shape seen in the MRI. This case study also indicates that although both strain and stress measures can be considered adequate predictors of DTI, tissue stresses appear to perform better than strains.

Stress analyses coupled with damage laws to determine biomechanical risk factors for deep tissue injury during sitting.

Deep tissue injury (DTI) is a potentially life-threatening form of pressure ulcer that onsets in muscle tissue overlying bony prominences and progresses unnoticeably to more superficial tissues. To minimize DTI, the efficacy of wheelchair cushions should be evaluated not only based on their performance in redistributing interface pressures but also according to their effects on stress concentrations in deep tissues, particularly muscles. However, a standard bioengineering approach for such analyses is missing in literature. The goals of this study were to develop an algorithm to couple finite element (FE) modeling of the buttocks with an injury threshold for skeletal muscle and with a damage-stiffening law for injured muscle tissue, from previous animal experiments, to predict DTI onset and progression for different patient anatomies and wheelchair cushions. The algorithm was also employed for identifying intrinsic (anatomical) biomechanical risk factors for DTI onset. A set of three-dimensional FE models of seated human buttocks was developed, representing different severities of pathoanatomical changes observed in chronically sitting patients: muscle atrophy and "flattening" of the ischial tuberosity (IT). These models were then tested with cushions of different stiffnesses representing products available on the market and semirigid supports. Outcome measures were the percentage of damaged muscle tissue volumes after 90 min and 110 min of simulated continuous immobilized sitting as well as muscle injury rates post-60 min, -90 min, and -110 min of continuous sitting. Damaged muscle volumes grew exponentially with the level of muscle atrophy. For example, simulation of a subject with 70% muscle atrophy sitting on a soft cushion showed damage to 33% of the muscle volume after 90 min of immobilized sitting, whereas a comparable simulation with a nonatrophied muscle yielded only 0.4% damaged tissue volume. The rates of DTI progression also increased substantially with increasing severities of muscle atrophy, e.g., 70% atrophy resulted in 8.9, 2.7, and 1.6 times greater injury rates compared with the "reference" muscle thickness cases, after 60 min, 90 min, and 110 min of sitting, respectively. Across all simulation cases, muscle injury rate was higher when a "flatter" IT was simulated. Stiffer cushions increased both the extent and rate of DTI at times shorter than 90 min of continuous sitting, but after 110 min, volumes and rates of tissue damage converged to approximately similar values across the different cushion materials. The present methodology is a practical tool for evaluating the performances of cushions in reducing the risk for DTI in a manner that goes far beyond the commonly accepted measurements of sitting pressures.

Real-time finite element monitoring of sub-dermal tissue stresses in individuals with spinal cord injury: toward prevention of pressure ulcers.

Patients with a spinal cord injury (SCI) are susceptible to deep tissue injury (DTI), a necrosis in excessively deformed muscle tissue overlying bony prominences, which, in wheelchair users, typically occurs in the gluteus muscles under the ischial tuberosities. Recently, we developed a generic real-time, subject-specific finite element (FE) modeling method to provide monitoring of mechanical conditions in deep tissues deformed between bony prominences and external surfaces. We previously employed this method to study internal tissue loads in plantar tissues of the foot [Yarnitzky, G., Z. Yizhar, and A. Gefen. J. Biomech. 39:2673-2689, 2006] and in muscle flaps of residual limbs in patients who underwent transtibial amputation (Portnoy, S., G. Yarnitzky, Z. Yizhar, A. Kristal, U. Oppenheim, I. Siev-Ner, and A. Gefen. Ann. Biomed. Eng. 35:120-135, 2007). The goal of the present study was to adapt the method to study the time-dependent mechanical stresses in glutei of patients with SCI during wheelchair sitting, continuously in real-time, and to compare the trends of internal tissue load data with those of controls. Prior to human studies, the real-time FE model-adapted to study the buttocks during sitting-was validated by comparing its predictions to data from a physical phantom of a buttocks and to non-real-time, commercial FE software. Next, real-time, subject-specific, FE models were built for six participating subjects (3 patients with SCI, 3 controls) based on their individual anatomies from MRI scans. Subjects were asked to sit normally in a wheelchair, on a ROHO cushion, and to watch a 90 min movie. Continuous interface pressure measurements from a pressure mat were used as subject-specific boundary conditions for real-time FE analyses of deep muscle stresses. Highest peaks of compression, shear and von Mises stresses throughout the trial period, and averages of peaks of these stresses were recorded over the trial for each individual. These parameters generally had 3-times to 5-times greater values in patients with SCI compared with controls. Likewise, stress doses, defined as the integration of peak compression stress over time, were approximately 35-times and approximately 50-times greater in the subjects with SCI, the values referring to the highest of all peaks recorded throughout the trial, and to average of peaks over the trial, respectively. We believe that by allowing-for the first time-practical and continuous monitoring of internal tissue loads in patients with motosensory deficits, without any risk or interruption to their lifestyle, and either at the clinical setting or at home, the present method can make a substantial contribution to the prevention of severe pressure ulcers and DTI.

Strains and stresses in sub-dermal tissues of the buttocks are greater in paraplegics than in healthy during sitting.

A pressure-related deep tissue injury (DTI) is a severe pressure ulcer, which initiates in muscle tissue overlying a bony prominence (e.g. the ischial tuberosities, IT) and progresses outwards through fat and skin, unnoticed by the paralyzed patient. We recently showed that internal strains and stresses in muscle and fat of individuals at anatomical sites susceptible to DTI can be evaluated by integrating Open-MRI scans with subject-specific finite element (FE) analyzes (Linder-Ganz et al., Journal of Biomechanics, 2007); however, sub-dermal soft tissue strains/stresses from paraplegics are still missing in literature. We hypothesize that the pathoanatomy of the buttocks in paraplegia increases the internal soft tissue loads under the IT, making these patients inherently susceptible to DTI. We hence compared the strain and stress peaks in the gluteus muscle and fat tissues under the IT of six healthy and six paraplegic patients, using the coupled MRI-FE method. Peak principal compression, principal tension, von Mises and shear strains in the gluteus were 1.2-, 3.1-, 1.4- and 1.4-fold higher in paraplegics than in healthy, respectively (p<0.02). Likewise, peak principal compression, principal tension, von Mises and shear stresses in the gluteus were 1.9-, 2.5-, 2.1- and 1.7-fold higher for the paraplegics (p<0.05). Peak gluteal compression and shear stresses decreased by as much as 70% when the paraplegic patients moved from a sitting to a lying posture, indicating on the effectiveness of recommending such patients to lie down after prolonged periods of sitting. This is the first attempt to compare internal soft tissue loads between paraplegic and healthy subjects, using an objective standardized bioengineering method of analysis. The findings support our hypothesis that internal tissue loads are significantly higher in paraplegics, and that postural changes significantly affect these loads. The method of analysis is useful for quantifying the effectiveness of various interventions to alleviate sub-dermal tissue loads at sites susceptible to pressure ulcers and DTI, including cushions, mattresses, recommendations for posture and postural changes, etc.

The effects of pressure and shear on capillary closure in the microstructure of skeletal muscles.

Deep tissue injury (DTI) is a severe pressure ulcer, which initiates in muscle tissue under a bony prominence, and progresses outwards. It is associated with mechanical pressure and shear that may cause capillaries to collapse and thus, induce ischemic conditions. Recently, some investigators stipulated that ischemia alone cannot explain the etiology of DTI, and other mechanisms, particularly excessive cellular deformations may be involved. The goal of this study was to evaluate the functioning of capillaries in loaded muscle tissue, using animal and finite element (FE) models. Pressures of 12, 37, and 78 kPa were applied directly to one gracilis muscle of 11 rats for 2 h. Temperatures of the loaded and contralateral muscles were recorded with time using infrared thermography (IRT) as a measure of the ischemic level. In addition, a non-linear large deformation muscle-fascicle-level FE model was developed and subjected to pressures of 12-120 kPa without and with simultaneous shear strain of up to 8%. For each simulation case, the accumulative percentage of open capillary cross-sectional area and the number of completely closed capillaries were determined. After 2 h, temperature of the loaded muscles was 2.4 +/- 0.3 degrees C (mean +/- standard deviation) lower than that of the unloaded contralateral limbs (mean of plateau temperature values across all pressure groups). Temperature of the loaded muscles dropped within 10 min but then remained stable and significantly higher than room temperature for at least 30 additional minutes in all pressure groups, indicating that limbs were not completely ischemic within the first 40 min of the trials. Our FE model showed that in response to pressures of 12-120 kPa and no shear, the accumulative percentage of open capillary cross-sectional area decreased by up to 71%. When shear strains were added, the open capillary cross-sectional area decreased more rapidly, but even for maximal loading, only 46% of the capillaries were completely closed. Taken together, the animal and FE model results suggest that acute ischemia does not develop in skeletal muscles under physiological load levels within a timeframe of 40 min. Since there is evidence that DTI develops within a shorter time, ischemia is unlikely to be the only factor causing DTI.

A three-dimensional model of the penis for analysis of tissue stresses during normal and abnormal erection.

Approximately half of the males between the ages of 40 and 70 suffer erectile dysfunction. Because adequate mechanical interactions in the penis are necessary for functional erection it is important to analyze stresses in the erect penis. Previous penis models were limited to simplified or two-dimensional geometry. Here we developed a three-dimensional model for structural analysis of normal erection as well as erections of a penis with substantial asymmetry of the corporal bodies, and Peyronie's disease. The model was constructed based on anatomical images and included skin, tunica albuginea, corpus cavernosa, and spongiosum. The mechanical behavior of the tunica and skin were assumed to be three-dimensional-orthotropic, and other tissues as well as Peyronie's plaque was taken as linear elastic. Stresses and deformations during erection were analyzed using a commercial finite elements (FE) solver. Erection was simulated by raising blood pressure in the corporal bodies to 100 mmHg. The tunica was found to be the most highly loaded tissue in the erect penis. Peak von Mises stresses in the healthy tunica, tunica of the asymmetric corpora model, and tunica with Peyronie's disease were 114 kPa, 167 kPa, and 830 kPa, respectively. The angles of distortion of the penis with respect to the vertical axis were approximately 4.5 degrees and approximately 2 degrees , for the asymmetric and Peyronie's cases, respectively. The model's ability to determine internal stresses in the erect penis offers a new point of view on the mechanical factors involved with erection, and enables us to relate these data with different penile pathologies.

Assessment of mechanical conditions in sub-dermal tissues during sitting: a combined experimental-MRI and finite element approach.

A common but potentially severe malady afflicting permanent wheelchair users is pressure sores caused by elevated soft tissue strains and stresses over a critical prolonged period of time. Presently, there is paucity of information regarding deep soft tissue strains and stresses in the buttocks of humans during sitting. Strain and stress distributions in deep muscle and fat tissues were therefore calculated in six healthy subjects during sitting, in a double-donut Open-MR system, using a "reverse engineering" approach. Specifically, finite element (FE) models of the undeformed buttock were built for each subject using MR images taken at the coronal plane in a non-weight-bearing sitting posture. Using a second MR image taken from each subject during weight-bearing sitting we characterized the ischial tuberosity sagging toward the sitting surface in weight-bearing, and used these data as displacement boundary conditions for the FE models. These subject-specific FE analyses showed that maximal tissue strains and stresses occur in the gluteal muscles, not in fat or at the skin near the body-seat interface. Peak principal compressive strain and stress in the gluteus muscle were 74+/-7% and 32+/-9 kPa (mean+/-standard deviation), respectively. Peak principal compressive strain and stress in enveloping fat tissue were 46+/-7% and 18+/-4 kPa, respectively. Models were validated by comparing measured peak interface pressures under the ischial tuberosities (17+/-4 kPa) with those calculated by means of FE (18+/-3 kPa), for each subject. This is the first study to quantify sub-dermal tissue strain and stress distributions in sitting humans, in vivo. These data are essential for understanding the aetiology of pressure sores, particularly those that were recently termed "deep tissue injury" at the US National Pressure Ulcer Advisory Panel (NPUAP) 2005 Consensus Conference.

Computational studies of strain exposures in neonate and mature rat brains during closed head impact.

Traumatic brain injury (TBI) is the most common cause of death in childhood, and the majority of fatal cases are due to motor vehicle accidents, falls, sport-related accidents, and child abuse. Rodents and particularly rats became a commonly used animal model of TBI in childhood as well as in adults, and different techniques are described in the literature to induce the brain injury. However, findings reported in the last decade regarding the increased stiffness of brain tissue in young animals, including rats, are not considered in experimental designs of TBI studies, and this may seriously bias the results when TBI effects are compared across different animal ages. In this study, we determined the strain and stress distributions in neonatal (post-natal-day [PND] 13-17) and mature (PND 43 and 90) rat brains during a closed head injury, using age-specific finite element (FE) models. The FE simulations indicated that for identical cortical displacements, the neonatal brain may be exposed to larger peak stress magnitudes compared with a mature brain due to stiffer tissue properties in the neonate, as well as larger strain magnitudes due to its smaller size. The brain volume subjected to a certain strain level was greater in the neonate brain compared with the adult models for all indentation depths greater than 1 mm. In conclusion, our present findings allow better design of closed head impact experiments which involve an age factor. Additionally, the larger peak stresses and larger strain volumetric exposures observed in the neonatal brain support the hypothesis that the smaller size and stiffer tissue of the infant brain makes it more susceptible to TBI.

Stress relaxation of porcine gluteus muscle subjected to sudden transverse deformation as related to pressure sore modeling.

Computational studies of deep pressure sores (DPS) in skeletal muscles require information on viscoelastic constitutive behavior of muscles, particularly when muscles are loaded transversally as during bone-muscle interaction in sitting and lying immobilized patients. In this study, we measured transient shear moduli G(t) of fresh porcine muscles in vitro using the indentation method. We employed a custom-made pneumatic device that allowed rapid (2000 mms) 4 mm indentations. We tested 8 gluteus muscles, harvested from 5 adult pigs. Each muscle was indented transversally (perpendicularly to the direction of fibers) at 3 different sites, 7 times per site, to obtain nonpreconditioned (NPC) and preconditioned (PC) G(t) data. Short-term (GS) and long-term (GL) shear moduli were obtained directly from experiments. We further fitted measured G(t) data to a biexponential equation G(t) = G1 x exp(-t/tau1)+ G2 x exp(-t/tau2) + Ginfinity, which provided good fit, visually and in terms of the correlation coefficients. Typically, plateau of the stress relaxation curves (defined as 10% difference from final GL) was evident approximately 20 s after indentation. Short-term shear moduli GS (mean NPC: 8509 Pa, PC: 5711 Pa) were greater than long-term moduli GL (NPC: 609 Pa, PC: 807 Pa) by about an order of magnitude. Statistical analysis of parameters showed that only G2 was affected by preconditioning, while GL, GS, Ginfinity, tau1, tau2, and G1 properties were unaffected. Since DPS develop over time scales of minutes to hours, but most stress relaxation occurs within approximately 20 s, the most relevant property for computational modeling is GL (mean approximately 700 Pa), which is, conveniently, unaffected by preconditioning.

Pressure-time cell death threshold for albino rat skeletal muscles as related to pressure sore biomechanics.

Deep pressure sores (DPS) are associated with inadequate soft tissue perfusion and excessive tissue deformation over critical time durations, as well as with ischemia-reperfusion cycles and deficiency of the lymphatic system. Muscle tissue shows the lowest tolerance to pressure injuries, compared with more superficial tissues. In this communication, we present new histopathology data for muscle tissue of albino (Sprague-Dawley) rats exposed to pressures for 15 or 30 min. These data are superimposed with an extensive literature review of all previous histopathology reported for albino rat skeletal muscles subjected to pressure. The pooled data enabled a new mathematical characterization of the pressure-time threshold for cell death in striated muscle of rats, in the form of a sigmoid pressure-time relation, which extends the previous pressure-time relation to the shorter exposure periods. We found that for pressure exposures shorter than 1 h, the magnitude of pressure is the important factor for causing cell death and the exposure time has little or no effect: even relatively short exposures (15 min - 1 h) to pressures greater than 32 kPa (240 mmHg) cause cell death in rat muscle tissue. For exposures of 2 h or over, again the magnitude of pressure is the important factor for causing cell death: pressures greater than 9 kPa (67 mmHg) applied for over 2 h consistently cause muscle cell death. For the intermediate exposures (between 1 and 2 h), the magnitude of cell-death-causing pressure strongly depends on the time of exposure, i.e., critical pressure levels drop from 32 to 9 kPa. The present sigmoidal pressure-time cell death threshold is useful for design of studies in albino rat models of DPS, and may also be helpful in numerical simulations of DPS development, where there is often a need to extrapolate from tissue pressures to biological damage.