Abstract
An accurate evaluation of the mechanical properties of human tissue is key to understanding and successfully simulating (parts of) human joints. Due to the rapid post-mortem decay, however, the cadavers are usually frozen or embalmed. The main aim of this paper is to quantitatively compare the impact of both techniques on the biomechanical properties. To that extent, the Achilles tendons of seven cadavers have been tested. For each cadaver, one of the Achilles tendons was tested after being frozen for at maximum two weeks, whilst the other tendon was tested following a Thiel embalming process.
All specimens were gripped in custom made clamps and subjected to uniaxial tensile loading. The specimens were scanned using a micro-CT to determine their cross-sectional area, which allowed transferring the applied forces to stresses. During the tensile tests, the specimens’ elongation was measured both using the digital image correlation (DIC) technique and using linear variable displacement transducers (LVDT's) mounted across the grips. The former allowed to assess the severity of slip in the grips. As is well described in literature, the obtained stress-strain relationship is not linear (Figure 1). Accordingly, the following bilinear relationship was fitted through the data points using a least squares fit:
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s = E0 e e <= ê
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s = E0 ê + E (e - ê) e > ê
As a result, the stress-strain response is sub-divided in two regions: a toe-region (e <= ê) with a low slope and stiffness (E0) and a linear elastic region (e > ê) with a higher stiffness (E). Both stiffness values were subsequently compared between the fresh frozen and Thiel embalmed group. Given the non-normal distribution of the test data, the non-parametric Wilcoxon signed rank test was used to assess the statistical significance of the obtained results.
No statistically significant difference was observed between the stiffness of the toe-region (e <= ê) obtained from Thiel embalmed and fresh frozen specimens (p-value = 0.249). In the contrary, the stiffness of linear elastic region (e > ê) was significantly different between both groups (p-value = 0.046 – see Figure 2). An average, the Thiel embalmed specimens displayed a 36% higher stiffness compared to the fresh frozen specimens. The latter contrasts the findings of other studies reported in literature, which report a decrease of the stiffness following Thiel embalming. To the authors’ opinion, this discrepancy could either be attributed to a difference in testing protocol (embalming time, donor factors, …) or tissue perfusion kinetics (Achilles tendon is relatively massive).
In conclusion, this study has demonstrated that Thiel embalming significantly alters the biomechanical properties of tendons. Specimens that underwent Thiel embalming should therefore not be considered for determining input parameters for advanced numerical models.
