Biomechanical, structural, and biochemical indices of degenerative and osteoarthritic deterioration of adult human articular cartilage of the femoral condyle

Abstract

Objective: To compare the tensile biomechanical properties of age-matched adult human knee articular cartilage exhibiting distinct stages of degenerative or osteoarthritic deterioration and to determine the relationships between tensile properties and biochemical and structural properties hypothesized to underlie functional biomechanical deterioration.

Methods: Age-matched articular cartilage samples, obtained from the lateral and medial femoral condyles (LFC and MFC), exhibited (1) minimal fibrillation, characteristic of normal aging (NLA), (2) overt fibrillation associated with degeneration (DGN), or (3) overt fibrillation associated with osteoarthritis (OA). DGN samples were from knees that exhibited degeneration but not osteophytes while OA samples were from fragments removed during total knee arthroplasty. Cartilage samples were analyzed for tensile properties, cell and matrix composition, and histopathological structure.

Results: Differences in tensile, compositional and surface structural properties were indicative of distinct stages of cartilage degeneration, early (OA) advanced (DGN) and late (OA) with early degenerative changes in NLA samples being more advanced in the MFC than the LFC, including higher surface fibrillation, lower intrinsic fluorescence, and lower mechanical integrity. The transition from early to advanced degeneration involved a diminution in mechanical function, surface integrity, and intrinsic fluorescence. The transition from advanced to late degeneration involved an increase in cartilage water content, an increase in degraded collagen, and loss of collagen.

Conclusions: These results provide evidence of coordinated mechanical dysfunction, collagen network remodeling, and surface fibrillation. Even in the cartilage of knees exhibiting overt fibrillation but not extensive erosions characteristic of clinical osteoarthritis, most features of advanced cartilage degeneration were present.

Figure 1

Structural and surface properties of human articular cartilage from the LFC and MFC. Cartilage thickness (a), mean (b) and variance (c) of the reflectance score assessed after India ink staining, overall histopathological index of cartilage degeneration (d) and surface irregularity assessed by histopathological grading (e) from age-matched donors of cartilage with articular surfaces that were macroscopically normal (NLA), mildly fibrillated (DGN), and mildly fibrillated from joints undergoing total knee replacement (OA). *p<0.05, **p<0.01, ***p<0.005 versus NLA samples.

Figure 2

Tensile biomechanical properties of samples described in Figure 1. For specimens from the superficial, middle, and deep layers, the tensile equilibrium modulus (a), tensile ramp modulus (b), tensile strength (c), and failure strain (d) were determined from equilibrium and then non-equilibrium failure testing of articular cartilage from aged NLA, DGN, and OA donors. *p<0.05, **p<0.01, ***p<0.005 versus NLA samples.

Figure 3

Biochemical properties of human articular cartilage samples described in Figure 1. Cartilage tissue adjacent to the mechanical test specimens was analyzed for water content (a), DNA and calculated cell number (b), COL (c), COL in αCT (d), the fluorescence ratio of pentosidine-associated fluorescence (Ex 355/Em 385 nm) to pyridinoline-associated fluorescence (Ex 295/Em 395 nm) (e), and GAG (f). DNA, COL, and GAG were each normalized to wet weight. *p<0.05, **p<0.01, ***p<0.005 versus NLA samples. †p<0.05, ††p<0.01 versus DGN samples.

Figure 4

Relationships between tensile strength and structural and biochemical properties of the superficial layer of human articular cartilage of the LFC and MFC. The relationship between tensile strength of the superficial layer and (a) the histopathological index was assessed using Spearman’s rank method to determine p and ρ² values. The relationship between tensile strength and (b) the fluorescence ratio of the superficial layer was assessed using univariate linear regression to determine p and R² values. Data are shown for NLA (□), DGN (△), and OA (○) samples. Lines represent the linear regression fits of the data and are shown only to indicate trends.

Figure 5

Summary of properties of human articular cartilage related to biomechanical deterioration with aging, and osteoarthritis. Histological depiction of mechanical integrity (degree of gray shading), articular surface fibrillation, chondrocyte density, collagen network alteration (fragmentation of fibrils and decrease in intrinsic fluorescence), and loss of GAG (▨) are shown. Tabulated are location of changes, superficial tangential zone (STZ), middle zone (MZ) and deep zone (DZ), denoted by • or (•) for variable changes. Absence (○) or full presence (●) of parameters are indicated under stages noted, with changes indicated by ➞.