Prolonged ethanol replacement by CO2 increases splits on articular cartilage surface after critical point drying

Regional structural and viscoelastic properties of fibrocartilage upon dynamic nanoindentation of the articular condyle

Fibrocartilage,a tissue with macromaterial properties between dense fibrous tissue and hyaline cartilage, is not well understood in its ultrastructure and regional viscoelastic properties. Here nanoindentation with atomic force microscopy was performed on fresh fibrocartilage samples of rabbit jaw joint condyles. Each sample was divided into anteromedial, anterolateral, posteromedial, and posterolateral regions for probing and topographic imaging in 2 x 2 microm and 10 x 10 microm scan sizes. Young's moduli differed significantly among these regions in a descending gradient from the anteromedial (2.34 +/- 0.26 MPa) to the posterolateral (0.95 +/- 0.06 MPa). The Poisson ratio, defined as lateral strain over axial strain, had the same gradient distribution: highest for the anteromedial region (0.46 +/- 0.05) and lowest for the posterolateral region (0.31 +/- 0.05). The same four regions showed a descending gradient of surface roughness: highest for the anteromedial (321.6 +/- 13.8 nm) and lowest for the posterolateral (155.6 +/- 12.6 nm). Thus, the regional ultrastructural and viscoelastic properties of fibrocartilage appear to be coregulated. Based on these region-specific gradient distributions, fibrocartilage is constructed to withstand tissue-borne shear stresses, which likely propagate across its different regions. A model of shear gradient and concentric gradient is proposed to describe the region-specific capacity of fibrocartilage to sustain shear stresses in tendons, ligaments, joints, and the healing bone across species.

Scanning electron microscopy of "fibrillated" and "malacic" human articular cartilage: technical considerations

Specimens of articular cartilage from human knees with gross evidence of malacia (dull appearance and/or softness) or fibrillation (exposed fibrous strands and/or staining with India ink) were prepared for scanning electron microscopy (SEM) and compared to cartilage from apparently intact regions. Vertical cryofractures were made through the center of each specimen, so the matrix collagen structure and its relationship to surface features could be examined. Soft, dull, malacic cartilage was characterized by the presence of numerous clefts among the collagen fibers within the most superficial region of the cartilage. In one form of this condition, these clefts did not extend through the articular surface. In a second form, usually observed where the tangential zone was normally thin or absent, the free ends of radial collagen fibers were exposed, but the deeper layers were intact. Two forms of fibrillation were also identified. The first is created by separation of the superficial lamellae which curl up from the tangential layer and form frondlike projections above the normal plane of the joint surface. In the second, deep radial fibers are exposed by vertical fissures. This second form is associated with advanced damage to the joint. The early stages of cartilage failure are characterized by debonding among the major collagen fiber tracts. This process may initiate in the deep tangential zone where the radial fibers cross into the surface. The patterns of the degenerative changes are dictated by the original architecture of the collagen matrix. The microscopic findings do not correlate adequately with conventional gross grading. SEM provides useful information about injured articular cartilage.

Deformation of loaded articular cartilage prepared for scanning electron microscopy with rapid freezing and freeze-substitution fixation

To investigate the effect of joint loading on collagen fibers in articular cartilage, 45 knees of adult rabbits were examined by scanning electron microscopy. The knees were loaded at the patella with a simulated "quadriceps force" of 0.5-4 times body weight for 0.5 or 25 minutes, plunge-frozen, and fixed by freeze-substitution with aldehydes. Six knees were loaded for 3 hours and then fixed conventionally. Fixed tibial plateaus were examined and then freeze-fractured through the area of tibiofemoral contact, dried, coated, and examined by scanning electron microscopy to assess the overall deformation of the tibial articular surface and matrix collagen fibers. With tissue prepared by conventional fixation used as a standard, the quality of fixation was graded by light and transmission electron microscopy of patellar cartilage taken from half of the freeze-fixed knees. In loaded specimens, an indentation was present where the femur contacted the tibial plateau. The diameter and apparent depth of the dent were proportional to the magnitude and duration of the load; no dent was seen in the controls. The thickness of the cartilage at the center of the indentation was reduced 15-80%. Meniscectomy always produced larger deformations in otherwise equivalent conditions. Icecrystal damage to cells was evident by transmission electron microscopy and scanning electron microscopy, but at magnifications as high as x 30,000 the collagen fibrils prepared by freeze-substitution and conventional aqueous methods were identical. In loaded regions, the collagen matrix of the tibial cartilage was deformed in two ways: (a) radial collagen fibers exhibited a periodic crimp, and (b) in regions where an indentation was created by the femoral condyle, the radial fibers were bent, in effect creating a tangential zone where none had existed before. The radial fibers apparently are loaded axially and buckle under normal loads.

Variation of collagen fiber alignment in a joint surface: a scanning electron microscope study of the tibial plateau in dog, rabbit, and man

To determine if articular cartilage collagen fiber organization differs with location on the tibial plateau, specimens from dogs, humans, and rabbits were studied by scanning electron microscopy. Joint surfaces were fixed, dehydrated, and fractured radially so that the periphery could be compared with the center on single specimens. Generally, fibers were more tightly packed in the lateral side than in the medial and the periphery as compared with the center, where the cartilage was consistently thicker and the radial zone was dominant and composed of straight vertical fibers. In the periphery, the tangential and transitional zones were better developed and contributed up to 50% of the cartilage depth in comparison to only 5% centrally. The soft, dull, malacic appearance of the center results from lack of a true surface layer of tangential collagen fibers.