Topographical Characterization of the Young, Healthy Human Femoral Medial Condyle

Abstract

Objective: The medial femoral condyle of the knee exhibits some of the highest incidences of chondral degeneration. However, a dearth of healthy human tissues has rendered it difficult to ascertain whether cartilage in this compartment possesses properties that predispose it to injuries. Assessment of young, healthy tissue would be most representative of the tissue's intrinsic properties.

Design: This work examined the topographical differences in tribological, tensile, and compressive properties of young (n = 5, 26.2 ± 5.6 years old), healthy, human medial femoral condyles, obtained from viable allograft specimens. Corresponding to clinical incidences of pathology, it was hypothesized that the lowest mechanical properties would be found in the posterior region of the medial condyle, and that tissue composition would correspond to the established structure-function relationships of cartilage.

Results: Young's modulus, ultimate tensile strength, aggregate modulus, and shear modulus in the posterior region were 1.0-, 2.8-, 1.1-, and 1.0-fold less than the values in the anterior region, respectively. Surprisingly, although glycosaminoglycan content is thought to correlate with compressive properties, in this study, the aggregate and shear moduli correlated more robustly to the amount of pyridinoline crosslinks per collagen. Also, the coefficient of friction was anisotropic and ranged 0.22-0.26 throughout the condyle.

Conclusion: This work showed that the posteromedial condyle displays lower tensile and compressive properties, which correlate to collagen crosslinks and may play a role in this region's predisposition to injuries. Furthermore, new structure-function relationships may need to be developed to account for the role of collagen crosslinks in compressive properties.

Keywords: articular cartilage; cartilage mechanical properties; characterization; femoral condyle; human; tribology.

Figure 1.

Experimental design for the characterization of the human medial femoral condyle. (A) Representative diagram of donor grafts used for specimen collection. All grafts were divided into three regions: anterior, central, and posterior. Samples were taken from each of these 3 regions to perform the assessments listed. (B) Detailed information regarding donors sampled. The mean age of the donors was 26.2 ± 5.6 years.

Figure 2.

Histological analysis of the medial condyle. (A) Hematoxylin and eosin, (B) Picrosirius Red, and (C) Safranin-O staining obtained from a single representative donor. Isogenous groups were observed in all regions. Collagen staining was more intense toward the posterior regions, whereas GAG tended to accumulate in the deeper zone on all regions.

Figure 3.

Biochemical properties of the human medial femoral cartilage. (A) GAG/WW and GAG/DW, (B) COL/WW and COL/DW, and (C) PYR/WW and PYR/COL. The posterior region of the medial condyle showed higher collagen content, albeit less crosslinked compared to the collagen in anterior and central regions. Letters depict the connecting letters report. Different uppercase letters over bars indicate statistical difference between groups. Absence of letters indicates no significant differences.

Figure 4.

Coefficient of friction of the articular cartilage surface. Three distinct regions of human femoral cartilage were tested in 2 directions: anteroposterior and mediolateral. The coefficient of friction was isotropic and homogeneous throughout the medial condyle in young, healthy tissues.

Figure 5.

Tensile properties of the human medial femoral condyle. (A) Young’s modulus and ultimate tensile strength (UTS) of articular cartilage in the anteroposterior direction. (B) Young’s modulus and ultimate tensile strength (UTS) of articular cartilage in the mediolateral direction. Letters depict the connecting letters report. Different uppercase or lowercase letters over bars indicate statistical difference between groups for the top 1 mm layer and the full-thickness tissue (~2.5 mm) measurements, correspondingly. Absence of letters indicates no significant differences.

Figure 6.

Compressive properties of 3 distinct regions of human femoral cartilage. (A) Aggregate modulus and shear modulus. (B) Instantaneous modulus and relaxation modulus at 10% strain. (C) Instantaneous modulus and relaxation modulus at 20% strain. Different uppercase letters over bars indicate statistical difference between groups. Absence of letters indicates no significant differences.