Electrical Conductivity Method to Determine Sexual Dimorphisms in Human Temporomandibular Disc Fixed Charge Density

Abstract

To investigate potential mechanisms associated with the increased prevalence of temporomandibular joint (TMJ) disorders among women, the study objective was to determine sex-dependent and region-dependent differences in fixed charge density (FCD) using an electrical conductivity method. Seventeen TMJ discs were harvested from nine males (77 ± 4 years) and eight females (86 ± 4 years). Specimens were prepared from the anterior band, posterior band, intermediate zone, medial disc and lateral disc. FCD was determined using an electrical conductivity method, assessing differences among disc regions and between sexes. Statistical modeling showed significant effects for donor sex (p = 0.002), with cross-region FCD for male discs 0.051 ± 0.018 milliequivalent moles per gram (mEq/g) wet tissue and 0.043 ± 0.020 mEq/g wet tissue for female discs. FCD was significantly higher for male discs compared to female discs in the posterior band, with FCD 0.063 ± 0.015 mEq/g wet tissue for male discs and 0.032 ± 0.020 mEq/g wet tissue for female discs (p = 0.050). These results indicate FCD contributes approximately 20% towards TMJ disc compressive modulus, through osmotic swelling pressure regulation. Additionally, FCD regulates critical extracellular ionic/osmotic and nutrient environments. Sexual dimorphisms in TMJ disc FCD, and resulting differences in extracellular ionic/osmotic and nutrient environments, could result in altered mechano-electro-chemical environments between males and females and requires further study.

Keywords: Extracellular matrix; Glycosaminoglycan content; Mechanobiology; Porosity; Temporomandibular joint (TMJ).

Figure 1

Illustration of the collagen extracellular matrix demonstrating fixed negative charges, which attract mobile free ions, with glycosaminoglycans making up proteoglycans attached to hylaruonic acid cores. Fixed charge density measures the negative charges associated with carboxyl and sulfate groups on extracellular matrix glycosaminoglycans. Mobile ions (e.g., Na⁺) neutralize the matrix fixed negative charges.

Figure 2

Intact human temporomandibular joints were dissected from males and females, and articular discs were isolated (Top Left). Cylindrical specimens were taken from each disc from five physiologic regions: the anterior band, lateral disc aspect, intermediate zone, medial disc aspect, and posterior band (Bottom Left). Human discs were sectioned as size allowed; not all discs were able to provide all five specimens. Fixed charge density was determined using a custom electrical conductivity apparatus, consisting of two stainless steel current electrodes, two Ag/AgCL voltage sensing electrodes, a non-conductive Plexiglass chamber, a current-sensing micrometer, and sourcemeter (Right).

Figure 3

Fixed charge density of human temporomandibular articular discs at each region reported for males and females. A significant difference (p=0.050) between males and females was determined in FCD at the posterior band of the articular disc.

Figure 4

GAG content of human temporomandibular articular discs at each region for males and females reported as both % dry weight (Left) and % wet weight (Right).

Figure 5

Porosity (water volume fraction) of human temporomandibular articular discs at each region averaged for males and females.