Sample size considerations in soft tissue biomechanics

Time-dependent effects of ethanol-glycerin embalming on iliotibial band biomechanics

When conducting biomechanical testing or clinical training using embalmed human soft tissues, it is essential to understand their impact on biomechanical properties and their time dependence. Previous studies have investigated this influence, but specific variations over different embalming durations have not been thoroughly addressed to date. Ninety-seven human iliotibial band specimens were obtained from nine donors. All specimens were embalmed in ethanol-glycerin for varying durations: one day, eight days, and fourteen days. Prior to the mechanical trials, the specimens underwent osmotic water adjustment, tapering and standardized clamping. Uniaxial tensile tests were conducted to determine elastic modulus, ultimate tensile strength, and ultimate strain. Surface strain measurements were performed using a digital image correlation system. Ethanol-glycerin embalming of soft tissues significantly affects ultimate strain after one day of submersion time, elastic modulus after eight days, and the ultimate tensile strength after fourteen days. For applications requiring consistent and reliable material properties reflecting a (supra-)vital state, caution is advised against using embalmed tissues even following short submersion durations in ethanol-glycerin.

Crosado embalming related alterations in the morpho-mechanics of collagen rich tissues

Crosado-embalming has been successfully used as embalming technique in research and teaching for over 20 years. It is applied in biomechanical testing experiments if the fresh tissues are unavailable, e.g., for cultural, ethical, logistical or health and safety reasons. However, features of human Crosado-embalmed tissues biomechanical characteristics including its load-deformation properties in comparison to fresh tissues and its controllability through hydration fluids may be insightful and therefore need to be studied further. This study compared the uniaxial load-deformation properties and the cross-sectional area (CSA) measurements of fresh-frozen and Crosado-embalmed collagen-rich tissues, namely the iliotibial band (ITB, 16 unembalmed and 35 embalmed specimens) and cranial dura mater (DM, 60 unembalmed cadavers, and 25 embalmed specimens). The water content of 120 Crosado-embalmed ITB samples (30 cadavers) were analysed considering established rehydration treatments, including polyethylene glycol (PEG). Crosado-embalmed tissues presented an increased elastic modulus (EM) (all p < 0.050; e.g., Crosado ITB PEG only 306 ± 91 MPa vs. fresh-frozen ITB PEG only 108 ± 31 MPa; mean ± standard deviation; p < 0.001) and ultimate tensile strength (UTS) (e.g., Crosado ITB PEG only 46 ± 15 MPa vs. fresh-frozen ITB PEG only 21 ± 8 MPa; p < 0.001) when rehydrated similar to the fresh tissues. The maximum force was different for the dura mater (Crosado 25 ± 13 N vs. fresh 21 ± 20 N; mean ± standard deviation; p = 0.050) but not for the ITB. The CSA following rehydration in PEG only was decreased for Crosado-embalmed samples (3.4 ± 1.2mm2, ITB; 1.1 ± 0.5 mm2, DM) compared to fresh-frozen (5.8 ± 2.1mm2, ITB; 3.1 ± 1.2mm2, DM) (all p ≤ 0.003). Furthermore, rehydration effects were observed following 24 h of PEG treatment (untreated tissues, 49 ± 9% vs. PEG only, 77 ± 4%; p < 0.001), in comparison to fresh samples (69%) tissues were hyperhydrated. In conclusion, Crosado-embalming appears to alter collagen-rich tissues' morphological and mechanical properties. While an increase in material properties of Crosado-embalmed tissues was observed (Emod and UTS), the overall load-bearing capacity and peak structural strength remained unaltered for ITB tissues. This may result from CSA-related, geometric or molecular alterations after the fixative and osmotic water protocols related to changes in the collagen backbone and water-binding capacity.

High throughput screening of airway constriction in mouse lung slices

The level of airway constriction in thin slices of lung tissue is highly variable. Owing to the labor-intensive nature of these experiments, determining the number of airways to be analyzed in order to allocate a reliable value of constriction in one mouse is challenging. Herein, a new automated device for physiology and image analysis was used to facilitate high throughput screening of airway constriction in lung slices. Airway constriction was first quantified in slices of lungs from male BALB/c mice with and without experimental asthma that were inflated with agarose through the trachea or trans-parenchymal injections. Random sampling simulations were then conducted to determine the number of airways required per mouse to quantify maximal constriction. The constriction of 45 ± 12 airways per mouse in 32 mice were analyzed. Mean maximal constriction was 37.4 ± 32.0%. The agarose inflating technique did not affect the methacholine response. However, the methacholine constriction was affected by experimental asthma (p = 0.003), shifting the methacholine concentration-response curve to the right, indicating a decreased sensitivity. Simulations then predicted that approximately 35, 16 and 29 airways per mouse are needed to quantify the maximal constriction mean, standard deviation and coefficient of variation, respectively; these numbers varying between mice and with experimental asthma.

Drying irreversibly affects the elastic behavior of pelvic cortical bone

Various studies have shown that the water content affects the elastic behavior of cortical bone. However, there is disagreement regarding the reversibility of the elastic behavior with rewetting. This study investigates this issue using an intrinsic approach, i.e., moisture manipulation and material testing were always carried out on the same specimen. The test results were then evaluated separately for each of several specimens. In total, 24 specimens of human cortical bone from the ischiopubic ramus were examined. The water content was varied in 11 steps, and the corresponding elastic moduli were determined using three-point bending tests within the elastic range. Moisture adjustment was achieved mainly using desiccators, accelerated by forced convection. Reference samples stored in the same manner were evaluated microscopically. The experiments confirmed the known correlation between water content reduction and stiffness increase of cortical bone. Complete drying increased the elastic modulus by about 83 %. By rewetting, the stiffness was significantly reduced again, though not only to the initial state, but even about 24 % below this. Thus, an irreversible alteration of the elastic behavior was observed. Decay of the reference samples was not observed. Therefore, decay is not the main reason for the significant loss of stiffness. In terms of the storage conditions for cortical bone specimens, an environment with 100 % relative humidity yielded the best match with the initial state. This storage method can therefore be recommended for biomechanical specimens used to determine in-vivo-like material parameters.

On a potential morpho-mechanical link between the gluteus maximus muscle and pelvic floor tissues

Stress urinary incontinence presents a condition not only found in female elderlies, but also in young athletes participating in high-impact sports such as volleyball or trampolining. Repeated jumps appear to be a predisposing factor. Yet the pathophysiology remains incompletely elucidated to date; especially with regard to the influence of the surrounding buttock tissues including gluteus maximus. The present study assessed the morpho-mechanical link between gluteus maximus and the pelvic floor female bodies. 25 pelves obtained from Thiel embalmed females were studied in a supine position. Strands of tissues connecting gluteus maximus with the pelvic floor obtained from 20 sides were assessed mechanically. Plastinates were evaluated to verify the dissection findings. In total, 49 hemipelves were included for data acquisition. The fascia of gluteus maximus yielded connections to the subcutaneous tissues, the fascia of the external anal sphincter and that of obturator internus and to the fascia of the urogenital diaphragm. The connection between gluteus maximus and the urogenital diaphragm withstood an average force of 23.6 ± 17.3 N. Cramér φ analyses demonstrated that the connections of the fasciae connecting gluteus maximus with its surroundings were consistent in the horizontal and sagittal planes, respectively. In conclusion, gluteus maximus is morphologically densely linked to the pelvic floor via strands of connective tissues investing the adjacent muscles. Though gluteus maximus has also been reported to facilitate urinary continence, the here presented morpho-mechanical link suggests that it may also have the potential to contribute to urinary stress incontinence. Future research combining clinical imaging with in-situ testing may help substantiate the potential influence from a clinical perspective.