Elsevier

Journal of Biomechanics

Volume 49, Issue 14, 3 October 2016, Pages 3328-3333
Journal of Biomechanics

The association between mechanical and biochemical/histological characteristics in diabetic and non-diabetic plantar soft tissue

https://doi.org/10.1016/j.jbiomech.2016.08.021Get rights and content

Abstract

Diabetes, and the subsequent complication of lower limb ulcers leading to potential amputation, remains an important health care problem in United States, even with declining amputation rates. It has been well documented that diabetes can alter the mechanical properties (i.e., increased stiffness) of the plantar soft tissue, although this finding is not universal. Similarly, biochemical, and histological changes have been found in the plantar soft tissue, but, as with the mechanical changes, these findings are not consistent across all studies. Our group׳s work has demonstrated that diabetes increases plantar soft tissue modulus and increases elastic septal thickness. The purpose of the current study was to explore the association between mechanical, biochemical and histological properties. Using previously collected data, a linear mixed effects regression was conducted. The correlations were weak; of the 32 that were tested, only 3 (modulus to septal thickness when location was accounted for, energy loss to total collagen, and energy loss to collagen/elastin ratio) were statistically significant, none with an R2 greater than 0.10. The main differences in the means were increased tissue stiffness and increased septal wall thickness, both trends were supported in the literature. However, as the correlations were weak, it is likely that another unexamined biochemical factor (perhaps collagen crosslinking) is associated with the mechanical tissue changes.

Introduction

Plantar ulceration and subsequent lower limb amputation are complications of diabetes mellitus that are important clinical problems in the United States. Over 8% of the population has diabetes, disproportionally leading to nearly two thirds of all non-traumatic amputations (nearly 66,000 in 2006) (CDCP, 2011). Recently, there has been a trend of reduced non-traumatic amputation rates, both in veterans (Tseng et al., 2011) and the general population (Belatti and Phisitkul, 2013, Li et al., 2012), likely a result of improved preventative care, increase revascular interventions, and evolving orthopaedic management (Belatti and Phisitkul, 2013). However, in terms of the sheer number of amputations per year (Belatti and Phisitkul, 2013, Tseng et al., 2011), and by the fact diabetic subjects undergo a disproportionate percentage of all amputations (CDCP, 2011, Li et al., 2012), diabetic foot ulceration remains an issue that requires further study.

Ulcer development is a complex and multi-factorial process, with aspects related to autonomic and peripheral neuropathy, poor circulation, and aberrant mechanical tissue loading (Sumpio, 2000). Many groups have studied the mechanical properties of diabetic plantar soft tissue, often with ultrasound devices on living subjects. The findings have not always been repeatable and are sometimes contradictory. Studies have found that diabetic tissue is thicker than normal tissue (Chao et al., 2011, Gooding et al., 1986) and that diabetic plantar skin is harder (Piaggesi et al., 1999). Diabetic plantar soft tissue was shown to have increased energy loss (Hsu et al., 2000, Hsu et al., 2002, Hsu et al., 2007) but no change in elastic modulus (Hsu et al., 2000, Hsu et al., 2002). Conversely, it has been found that diabetic, elderly tissue is stiffer and thinner than non-diabetic, younger tissue, but age may have confounded these findings (Zheng et al., 2000). Others have found that diabetic plantar soft tissue is stiffer at the metatarsal heads, but not at the heel pad (Klaesner et al., 2002). One recent study contradicted earlier work and found no change in thickness in diabetic tissue, but did confirm that diabetic tissue was stiffer (Sun et al., 2011). More recently, it has been shown that diabetic tissue has increased Young׳s and relaxation moduli (Jan et al., 2013). Changing modalities, one group has used indentors and an MRI scanner to determine that older diabetic subjects had increase shear and compressive modulus compared to younger, healthy non-diabetic subjects (Gefen et al., 2001). Magnetic resonance elastography has also been used to demonstrate increased stiffness in diabetic heel pads (Cheung et al., 2006). Our own research group mechanically tested diabetic plantar tissue isolated from cadaveric specimens and demonstrated increased modulus in compression and shear, but no difference in energy loss and very little change in the relaxation properties in either loading mode (Pai and Ledoux, 2010, 2011, 2012).

In addition to the mechanical changes in the diabetic plantar soft tissue, there is also some evidence that the histomorphometric characteristics are altered, although most work has been associated with the heel pad (Buschmann et al., 1995, Jahss et al., 1992, Waldecker and Lehr, 2009). Some of the original studies indicated thicker, frayed septal walls and decreased adipocyte size (Buschmann et al., 1995, Jahss et al., 1992), but a more recent study (Waldecker and Lehr, 2009) found no change in adipocyte size between healthy and diabetic tissue, findings that agreed with our own group׳s research (Wang et al., 2011). This contradiction might be explained by the fact that the studies from the 1990s used limbs that were amputated due to vascular disease. However, our work has confirmed the finding that thick, damaged elastic septa are found in diabetic plantar soft tissue (Wang et al., 2011, Wang et al., 2016).

Finally, concerning the effect of diabetes on the biochemistry of the plantar soft tissue, it is known that diabetes can induce alterations in the metabolism of the macromolecules present in the body. These biochemical changes are complex and have been found to be dependent on the tissue type and the macromolecule being evaluated (Sternberg et al., 1985), but the evaluation of the biochemistry of diabetic plantar soft tissue is not well understood. Our group has shown that there is no difference in the amount of collagen nor collagen I to III ratios, and minimal differences in the amount of elastin between diabetic and non-diabetic plantar soft tissue (Wang et al., 2016).

In summary, there have been many studies that have explored quantitative differences between diabetic and non-diabetic plantar soft tissue, but the relationship between microstructural characteristics (biochemical and histomorphological properties) and macrostructural characteristics (mechanical properties) is not clear. The purpose of the current study was to explore the direct association between the biochemical/histological characteristics of the plantar soft tissue and the mechanical properties of the plantar soft tissue at six locations beneath the foot. It was our hypothesis that the mechanical characteristics (e.g., increased stiffness or increased energy loss) would be directly associated with biochemical/histological characteristics (e.g., increased elastic septal thickness or increased amounts of collagen).

Section snippets

Methods

All cadaveric foot specimens were obtained from the National Disease Research Interchange (NDRI; Philadelphia, PA) and our protocols were all approved by the University of Washington Institutional Review Board.

Results

Demographically, the diabetic and non-diabetic specimens were similar in age, sex, and height, but the diabetic specimens were statistically significantly heavier (p=0.02) and had a greater body mass index (BMI, p=0.002, Table 2). The diabetic specimens had a duration of diabetes of 20.3±8.1 years.

The mechanical, biochemical and histological properties indicated mean differences between diabetic and non-diabetic tissue (Table 3). In particular, the diabetic tissue had a 94% increased modulus (p

Discussion

Although amputation rates due to diabetic ulceration have decreased, there are still a significant number of amputations each year. The processes that lead to an ulcer and then to a subsequent amputation are complex and multi-factorial. One aspect is related to the alteration in the mechanical, histological, and biochemical properties of diabetic tissue. Previous work has shown that the diabetic tissue is stiffer and has frayed septal walls, but there are minimal differences in the

Conflict of interest statement

The authors have no conflicts to report.

Acknowledgments

This study was supported by the National Institutes of Health grant 1R01 DK75633-03 and the Department of Veterans Affairs, RR&D Service grant A4843C. The authors have no conflicts to report.

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