Micro/nano-mechanics of cartilage with osteoarthritis

Abstract

This study aimed to characterize the in-situ mechanical property and morphology of

individual collagen fibril in osteoarthritic (OA) cartilage using indentation-type

atomic force microscopy (IT-AFM). The specimens with intact articular cartilage

(AC), mild to severe degenerated OA cartilage were collected with informed consent

from the postmenopausal women who underwent hip or knee arthroplasty. The fresh

specimens were cryo-sectioned by layers with 50m thick for each from the articular

surface to calcified cartilage, and then processed for AFM imaging and

nanoindentation test. For each layer, a total of twenty collagen fibrils were randomly

selected for testing. AFM tips with the nominal radius less than 10 nm were employed

for probing the individual collagen fibril, and the obtained cantilever deflection signal

and displacement were recorded for calculating its elastic modulus. Besides AFM

nanoindentation, AFM and scanning electron microscopy (SEM) images,

haematoxylin & eosin (H&E) staining and micro-indentation were performed on AC

to study the changes of ultrastructure and composition between intact AC and OA

cartilage. Results showed that an intact AC exhibited a gradation in elastic modulus of

collagen fibrils from surface region (2.65±0.31GPa) to bottom region (3.70±0.44GPa).

It was noted in the initial stage of OA cartilage that the coefficient of variation for

mechanical properties of collagen fibers, ranging from 25~48%, significantly

increased as compared with intact one (12%). The thickened and stiffened collagen

fibrils initially occurred at either surface region (3.11±0.91GPa) or bottom region

(5.64±1.10GPa) with OA progression. Besides thickens, alteration of D-periodic

banding patterns of collagen fibrils was observed. It was echoed by fibrotic changes

of surface region and tidemark irregularities. On the contrast, the micromechanical

properties of cartilage decreased while AC suffered from OA. This result revealed the

different approachs of nano and micro-mechanical properties changes in AC. In

summary, the alteration of mechanical properties of collagen fibrils started from

calcified cartilage as well as articular surface during OA onset, and the low

compliance of thickened collagen fibrils deteriorated along disease progression. This

study also reveals that the outstanding ability by AFM, in investigating the structure

and mechanical properties of collagen fibrils and AC in nanometer scale, is

impressive and this nanotechnological instrument is worth to be expected in further

development for clinical use.