The influence of lipid-extraction method on the stiffness of articular cartilage

Abstract

Background
One of the known characteristics of osteoarthritis is the loss of articular cartilage lipids. Therefore, it is important to study how lipids influence the functions of the tissue. This can only be done successfully by indirect analysis involving the extraction of lipids and subsequent assessment of the delipidized matrix. Therefore, for accuracy, the procedure for lipid extraction must not induce any other modification in the samples to be assessed. Hence, we compare three rinsing agents and methods in this study.

Methods
Normal and delipidized articular cartilage samples were tested under compressive loading at 4 loading velocities to obtain and compare their stiffness values.

Findings
Chloroform rinsing resulted in a 45% decrease in the stiffness of cartilage at low strain-rates (10−2/s and 10−1/s) on average with a corresponding increase of 55% at higher strain-rate of 10/s relative to the normal. Ethanol rinsed cartilage exhibited a corresponding decrease of 40% at the low strain-rates while exhibiting an increase of about 20% at the highest loading rates. Propylene glycol rinsing resulted in a decrease of approximately 20% in stiffness, while an increase of up to 5% at high rates of loading.

Interpretation

• The loss of lipids modifies the stiffness of articular cartilage at all loading rates.

• The relatively larger deviation of the stiffness of chloroform rinsed samples relative to the normal is probably a consequence of the drying process involved in rinsing protocol.

• It is probable that the results of milder rinsing agents, used without vacuum drying are more reflective of physiological delipidization effects on the tissue. Consequently, we recommend propylene glycol and its associated protocol for extracting lipids from articular cartilage.