Tissue biomechanics of the human head are altered by Thiel embalming, restricting its use for biomechanical validation

Abstract

Thiel embalming is a well-known method of anatomical fixation giving lifelike optical and haptic tissue properties. Beyond these characteristics, Thiel embalming may also be a promising method to provide lifelike tissues for validation purposes of human head biomechanics. Recent investigations using Thielembalmed human tissues of the upper and lower limb yielded contradicting biomechanical results on fixation-induced changes in the tissues' load-deformation behavior. It is to date unclear if Thiel embalming may have a softening or stiffening effect on human soft tissues or no global effect on biomechanics compared to the fresh state, with the latter being the most desirable outcome. The given study aimed at assessing the effects of Thiel embalming on the uniaxial tensile properties of human head soft tissues. Age-matched fresh and Thiel-embalmed dura mater, temporalis muscle, temporalis muscle fascia, and scalp samples were examined. Dura, fascia, and scalp samples showed significantly different elastic moduli compared to fresh tissues (all P < 0.01). The observed ultimate tensile strength supports the theory of an increased collagen crosslinking of the embalmed tissues when compared to the fresh state. Thiel-embalmed muscles failed any tensile testing approach as a result of the muscles dissolving due to the embalming. Furthermore, collagen integrity seems altered in scanning electron microscopy by the Thiel embalming, limiting their use for ultrastructural failure analyses. Thiel-embalmed soft tissues may consequently not serve to reflect the biomechanical properties of the human head. Consequently, the application of Thiel embalming should be limited to preliminary tests for biomechanical purposes. Clin. Anat. 32:903-913, 2019.

Thiel embalming: Quantifying histological changes in skeletal muscle and tendon and investigating the role of boric acid

“…While changes in tissue characteristics of Thiel-embalmed samples have previously been described in a number of investigations (Benkhadra et al, 2011;Fessel et al, 2011;Wilke et al, 2011;Martyniuk et al, 2014;Hammer et al, 2015;Verstraete et al, 2015;Venne et al, 2018;Zwirner et al, 2019) that other components also play a role in driving these changes.…”

“…the brain, is problematic often resulting in additional fixation being required. Despite Thiel preserved cadavers being used in an increasing number of surgical and anatomical investigations (Benkhadra et al, 2009;Boryor et al, 2010;Eisma et al, 2010;McLeod et al, 2010;Fessel et al, 2011;Prasad Rai et al, 2012;Ernesto Ottone, 2016;Verstraete et al, 2016;Lone et al, 2017;Zwirner et al 2019) a distinct paucity of information remains pertaining to how and why it produces cadavers with more life-like qualities. embalmed skeletal muscle and tendon.…”

What Is Considered a Variation of Biomechanical Parameters in Tensile Tests of Collagen-Rich Human Soft Tissues?—Critical Considerations Using the Human Cranial Dura Mater as a Representative Morpho-Mechanic Model

“…Load-deformation properties of human soft tissues are typically accompanied by large variations [ 7 , 11 , 13 , 21 , 22 ] with standard deviations or interquartile ranges of the E mod and UTS frequently being larger than half of the mean value depending on the normal distribution of the experimental data [ 6 , 11 , 18 , 20 , 23 , 24 , 25 ]. The here tested human cranial dura mater is a thin membranous layer that is softly attached to the inside of the skull bone.…”

“…A chemical treatment of the soft tissues such as the use of fixatives for cadaveric preservation may alter the collagen interaction and hence the mechanical parameters of the respective tissue, which was shown for the human scalp, DM and temporal muscle fascia [ 18 , 30 , 31 ], or iliotibial tract specimens when fixed with ethanol or formaldehyde [ 24 ]. Formaldehyde as a component of the Thiel fixation is known to irreversibly block the amino groups of peptides and to cross-link hydroxymethylene bridges [ 32 , 33 ].…”

“…Generally, baseline datasets to obtain the aforementioned mechanical parameters in tensile tests reveal widespread variation throughout the body [ 5 , 6 , 7 , 13 ]—an accepted “expectable” condition for human tissues. However, several factors such as the clamping quality [ 14 , 15 ], strain rate [ 16 ] and the mechanical and bio-physico-chemical tissue structure [ 5 , 17 , 18 ] impact the mechanical parameters to a varying extent ( Figure 1 ). Consequently, it remains unclear to date what the term “variation” describes in the context of mechanical parameters retrieved from human tissues related to their morphological characteristics.…”