A scanning electron microscopic study of human normal and osteoarthritic articular cartilage

Ultraviolet light (365 nm) transmission properties of articular cartilage as a function of depth, extracellular matrix, and swelling

Current tissue engineering approaches for treatment of injured or diseased articular cartilage use ultraviolet light (UV) for in situ photopolymerization of biomaterials to fill chondral and osteochondral defects as well as resurfacing, stiffening and bonding the extracellular matrix and tissue interfaces. The most commonly used UV light wavelength is UVA 365 nm, the least cytotoxic and deepest penetrating. However, little information is available on the transmission of UVA 365 nm light through the cartilage matrix. In the present study, 365 nm UV light transmission was measured as a function of depth through 100 μm thick slices of healthy articular cartilage removed from mature bovine knees. Transmission properties were measured in normal (Native) cartilage and after swelling equilibration in phosphate-buffered saline (Swollen). Single-factor and multiple linear regression analyses were performed to determine depth-dependencies between the effective attenuation coefficients and proteoglycan, collagen and water contents. For both cartilages, a significant depth-dependency was found for the effective attenuation coefficients, being highest at the articular surface (superficial zone) and decreasing with depth. The effective attenuation coefficients for full-thickness cartilages were approximately a third lower than the total attenuation coefficients calculated from the individual slices. Analysis of absorption and scattering effects due to the ECM and chondrocytes found that UV light scatter coefficients were ∼10 times greater than absorption coefficients. The greater transmittance of UV light through the thicker cartilage was attributed to the collagen within the ECM causing significant backscatter forward reflectance.

Structural characteristics of the collagen network in human normal, degraded and repair articular cartilages observed in polarized light and scanning electron microscopies

OBJECTIVE

This study characterizes collagen organization (CO) in human normal (n = 6), degraded (n = 6) and repair (n = 22) cartilages, using polarized light (PLM) and scanning electron (SEM) microscopies.

DESIGN

CO was assessed using a recently developed PLM-CO score (Changoor et al. Osteoarthritis Cartilage 2011;19:126-35), and zonal proportions measured. SEM images were captured from locations matched to PLM. Fibre orientations were assessed in SEM and compared to those observed in PLM. CO was also assessed in individual SEM images and combined to generate a SEM-CO score for overall CO analogous to PLM-CO. Fibre diameters were measured in SEM.

RESULTS

PLM-CO and SEM-CO scores were correlated, r = 0.786 (P < 0.00001, n = 32), after excluding two outliers. Orientation observed in PLM was validated by SEM since PLM/SEM correspondence occurred in 91.6% of samples. Proportions of the deep (DZ), transitional (TZ) and superficial (SZ) zones averaged 74.0 ± 9.1%, 18.6 ± 7.0%, and 7.3 ± 1.2% in normal, and 45.6 ± 10.7%, 47.2 ± 10.1% and 9.5 ± 3.4% in degraded cartilage, respectively. Fibre diameters in normal cartilage increased with depth from the articular surface [55.8 ± 9.4 nm (SZ), 87.5 ± 1.8 nm (TZ) and 108.2 ± 1.8 nm (DZ)]. Fibre diameters were smaller in repair biopsies [60.4 ± 0.7 nm (SZ), 63.2 ± 0.6 nm (TZ) and 67.2 ± 0.8 nm (DZ)]. Degraded cartilage had wider fibre diameter ranges and bimodal distributions, possibly reflecting new collagen synthesis and remodelling or collagen fibre unravelling. Repair tissues revealed the potential of microfracture-based repair procedures to produce zonal CO resembling native articular cartilage structure. Values are reported as mean ± 95% confidence interval.

CONCLUSION

This detailed assessment of collagen architecture could benefit the development of cartilage repair strategies intended to recreate functional collagen architecture.

Collagen architecture and failure processes in bovine patellar cartilage

Cartilage fails by fibrillation and wearing away. This study was designed to identify the microscopic failure processes in the collagen network of bovine cartilage using scanning electron microscopy. Cartilage samples from fibrillated cartilage from the bovine patella were removed from the bone, fixed, digested to remove proteoglycans, freeze-fractured, and processed for SEM. The architecture of the collagen network in the normal cartilage was first defined, and then the failure processes were identified by examining sites of fibrillation and at crack tips. The bovine patellar cartilage was organised with a superficial layer composed of 3-5 lamina, attached to a sub-superficial tissue by angled bridging fibrils. Collagen in the sub-superficial tissue was organised in lamina oriented in the radial direction up to the transition zone. Failure of the system occurred by cracks forming in superficial layer and lamina, creating flaps of lamina that rolled up into the larger 'fronds'. Larger cracks not following the laminar planes occurred in the transition, mid, and deep zones. Failure at the crack tips in the sub-superficial tissue appeared to be by peeling of collagen fibrils, as opposed to breaking of collagen fibrils, suggesting a 'glue' bonding the collagen fibrils in a parallel fashion. Cracks propagated by breaking these bonds. This bond could be a site of disease action, since weakening of the bond would accelerate crack propagation.

The acute structural changes of loaded articular cartilage following meniscectomy or ACL-transection

OBJECTIVE

Meniscectomy and anterior cruciate ligament (ACL) rupture have been identified as precursors of osteoarthrosis (OA) in clinical reviews and animal experiments. In this study, the acute effects of these injuries on articular cartilage matrix deformation, preserved in a loaded state using a cryopreservation technique, were studied by scanning electron microscopy (SEM).

METHOD

Whole knee joints from adult White New Zealand rabbits (N=87) were loaded ex vivo, using a simulated quadriceps pull under static and cyclic loading conditions, following medial meniscectomy or transection of the ACL. Specimens were plunge-frozen while under load, or following a recovery period, and prepared for SEM by cryofixation. Using SEM and photographic images, the medial tibial plateau cartilage was assessed both qualitatively and quantitatively.

RESULTS

After meniscectomy, significantly increased bending and crimping of radial collagen fibers occurred with static loading. Compared to intact knees, the area of tibial cartilage showing an indentation was increased by 80% (P< 0.05), the articular cartilage thickness was significantly more reduced when under load (for high force long duration static loading, intact joints had 53%+/-3 reduction in cartilage thickness compared to 39%+/-4 after meniscectomy, P< 0.05), and it took nearly twice as long for the cartilage thickness to recover following loading. These post-meniscectomy differences were either not present or were minimal when the joint was allowed to extend when loaded. ACL-transection slightly increased collagen deformation in the deeper zones, but only with cyclic loading.

CONCLUSION

The findings indicate that, with static loading, significantly increased deformation of articular cartilage collagen structure can occur following meniscectomy, but is minimized by joint motion. This increased deformation may be relevant to the etiology and progression of joint degeneration.

Composition and dynamics of articular cartilage: structure, function, and maintaining healthy state

Disorders of articular cartilage represent some of the most common and debilitating diseases encountered in orthopaedic practice. Understanding the normal functioning of articular cartilage is a prerequisite to understanding its pathologic processes. The mechanical properties of articular cartilage arise from the complex structure and interactions of its biochemical constituents: mostly water, electrolytes, and a solid matrix composed primarily of collagen and proteoglycan. The viscoelastic properties of cartilage, due primarily to fluid flow through the solid matrix, can explain much of the deformational responses observed under many loading conditions. Degenerative processes can often be explained by a breakdown of the normal load-bearing capacity of cartilage which arises from the mechanics of this fluid flow. Several factors which may lead to such a breakdown include direct trauma to the cartilage, obesity, immobilization, and excessive repetitive loading of the cartilage. Sports activity, without traumatic injury, does not appear to be a risk factor for the development of osteoarthritis in the normal joint; however, such activity may be harmful to an abnormal joint.

Swelling and curling behaviors of articular cartilage

A new experimental method was developed to quantify parameters of swelling-induced shape change in articular cartilage. Full-thickness strips of cartilage were studied in free-swelling tests and the swelling-induced stretch, curvature, and areal change were measured. In general, swelling-induced stretch and curvature were found to increase in cartilage with decreasing ion concentration, reflecting an increasing tendency to swell and "curl" at higher swelling pressures. An exception was observed at the articular surface, which was inextensible for all ionic conditions. The swelling-induced residual strain at physiological ionic conditions was estimated from the swelling-induced stretch and found to be tensile and from 3-15 percent. Parameters of swelling were found to vary with sample orientation, reflecting a role for matrix anisotropy in controlling the swelling-induced residual strains. In addition, the surface zone was found to be a structurally important element, which greatly limits swelling of the entire cartilage layer. The findings of this study provide the first quantitative measures of swelling-induced residual strain in cartilage ex situ, and may be readily adapted to studies of cartilage swelling in situ.

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.

Effects of valgus tibial angulation on cartilage degeneration in the rabbit knee

Thirteen adult female rabbits underwent unilateral osteotomy of the proximal tibia. In nine animals, 30 degrees of valgus angulation was created; in four animals, osteotomy without angulation was performed. After a 12-week survival period, the knee joints were processed for histology by staining with hematoxylin and eosin and safranin O. Additionally, the chondroitin sulphate epitopes 3-B-3(-) and 7-D-4 were evaluated immunohistochemically as markers of osteoarthritis. Changes of the articular surface of the tibia were visualized by scanning electron microscopy. Light microscopic evaluation by the Mankin et al. scoring system revealed mild or moderate damage of the cartilage in the lateral compartment of angulated extremities when compared with the control side. Immunohistology with the monoclonal 3-B-3 and 7-D-4 antibodies showed no increased expression of these epitopes in the lateral compartments of the knee. Scanning electron microscopic evaluation of the tibial surfaces revealed slight surface damage localized to the central, weight-bearing portion of the lateral tibial plateau of angulated extremities. Angulation of 30 degrees led to only mild degenerative changes of the cartilage. These results indicate that, in the short term, cartilage has considerable capacity to withstand the effects caused by severe angulation of the limb.

Pathobiology of articular cartilage in trapeziometacarpal osteoarthritis. II. Surface ultrastructure by scanning electron microscopy

Regional variation in the surface ultrastructure of hyaline cartilage in the osteoarthritic trapeziometacarpal joint was studied by means of scanning electron microscopy. Articular surface degeneration on intact osteochondral specimens was analyzed and correlated with known patterns of gross, histologic, and biochemical disease. The earliest changes were observed in the tangential surface lamina and occurred in areas of chondromalacia in the palmar contact area of the joint. Progressive disruption of this superficial fibrillar layer of the articular cartilage was followed by disorganization of the underlying chondrocytes and matrix. In palmar regions with more advanced chondromalacia and loss of eosinophilic staining, free cellular elements and craters 20-30 microns in diameter, resembling empty chondrocyte lacunae, appeared in conjunction with the absence of any tangential surface meshwork. Eburnated areas demonstrated no fibrillar or cellular elements. Disruption of the protective surface lamina of trapeziometacarpal articular cartilage occurs in a geographic pattern corresponding to joint contact areas and regions of selective biochemical decomposition of proteoglycan matrix. These observations support an interdependent relationship between mechanically induced abrasive surface wear and biochemical matrix degradation in the production of the cartilage lesion of trapeziometacarpal osteoarthritis.

The biphasic poroviscoelastic behavior of articular cartilage: role of the surface zone in governing the compressive behavior

Surface fibrillation of articular cartilage is an early sign of degenerative changes in the development of osteoarthritis. To assess the influence of the surface zone on the viscoelastic properties of cartilage under compressive loading, we prepared osteochondral plugs from skeletally mature steers, with and without the surface zone of articular cartilage, for study in the confined compression creep experiment. The relative contributions of two viscoelastic mechanisms, i.e. a flow-independent mechanism [Hayes and Bodine, J. Biomechanics 11, 407-419 (1978)], and a flow-dependent mechanism [Mow et al. J. biomech. Engng 102, 73-84 (1980)], to the compressive creep response of these two types of specimens were determined using the biphasic poroviscoelastic theory proposed by Mak. [J. Biomechanics 20, 703-714 (1986)]. From the experimental results and the biphasic poroviscoelastic theory, we found that frictional drag associated with interstitial fluid flow and fluid pressurization are the dominant mechanisms of load support in the intact specimens, i.e. the flow-dependent mechanisms alone were sufficient to describe normal articular cartilage compressive creep behavior. For specimens with the surface removed, we found an increased creep rate which was derived from an increased tissue permeability, as well as significant changes in the flow-independent parameters of the viscoelastic solid matrix. permeability, as well as significant changes in the flow-independent parameters of the viscoelastic solid matrix. From these tissue properties and the biphasic poroviscoelastic theory, we determined that the flow-dependent mechanisms of load support, i.e. frictional drag and fluid pressurization, were greatly diminished in cartilage without the articular surface. Calculations based upon these material parameters show that for specimens with the surface zone removed, the cartilage solid matrix became more highly loaded during the early stages of creep. This suggests that an important function of the articular surface is to provide for a low fluid permeability, and thereby serve to restrict fluid exudation and increase interstitial fluid pressurization. Thus, it is likely that with increasing severity of damage to the articular surface, load support in cartilage under compression shifts from the flow-dependent modes of fluid drag and pressurization to increased solid matrix stress. This suggests that it is important to maintain the integrity of the articular surface in preserving normal compressive behavior of the tissue and normal load carriage in the joint.

Autologous rib perichondrial grafts in experimentally induced osteochondral lesions in the sheep-knee joint: morphological results

The purpose of the present study was to examine the fate of autologous perichondrial grafts after transplantation into cartilage lesions in weight-bearing joints. Osteochondral lesions were made in the articular surface of knee joints in 36 sheep. The defects were filled with autologous rib perichondrial grafts which were secured by either collagen sponges (12 animals) or fibrin glue (12 animals). Defects without perichondrial grafts served as controls (12 animals). Following 1 week of immobilization of the operated leg, the plaster was removed and the animals were allowed to move freely. Animals were sacrificed after 4, 8, 12 and 16 weeks. The grafts were removed and investigated histologically. In contrast to weight-bearing areas and control defects, hyaline-like cartilage formation was seen in non-weight-bearing areas after 4 weeks. This newly formed cartilage revealed strong metachromasia following staining with acidic toluidine blue and reacted positively with periodic acid-Schiff, indicating de novo synthesis of proteoglycans and glycoproteins. Scanning electron microscopy and examinations with polarized light confirmed a hyaline cartilage-like architecture for the surface area as well as for the fibre orientation of the whole graft. Enzyme histochemistry for alkaline and acid phosphatase activity showed positive reactivity only at the base of the transplants.

Inhibition of estradiol-induced early osteoarthritic changes by tamoxifen

Previous studies on osteoarthritic changes induced by intraarticular injections of estradiol benzoate (EB) suggest estrogen as a risk factor in the development of knee osteoarthritis (OA). The present study observed the anti-arthritic effects of tamoxifen (TMX). Oophorectomized rabbits were subjected to intraarticular injections of vehicle control, EB, TMX, or EB/TMX for 3 weeks. The cartilage changes were assessed by light and scanning electron microscopic examination, enzyme histochemical analysis, and the amount of alcian blue stain binding to glycosaminoglycans. EB injections resulted in cell necrosis, chondrocyte clonings, and pittings, whereas the vehicle control, TMX, and EB/TMX-injected groups showed no histologic abnormalities. Histochemical analysis showed that the numbers of lactate dehydrogenase (LDH)-reactive chondrocytes in the EB-injected group were significantly reduced when compared to other groups (p less than 0.001). The injections of EB/TMX significantly reduced the chondrocyte numbers in the lateral superficial layer (p less than 0.05), compared with the vehicle injection. TMX-injected group revealed slight although insignificant decreases in chondrocyte numbers. The amount of alcian blue stains, representing the relative amount of proteoglycans, significantly decreased only in the superficial layer of the EB- and EB/TMX-injected groups (p less than 0.05). TMX, when concurrently injected with EB, antagonized the chondrodestructive effects of estradiol at the early stage of knee OA in rabbits. The results suggest the potential therapeutic use of TMX at the early stage of OA.

Chondrons in cartilage: ultrastructural analysis of the pericellular microenvironment in adult human articular cartilages

A combination of scanning and transmission electron microscopy was used to investigate the morphology and ultrastructure of normal human articular cartilage sampled from adult amputation specimens. This study confirms our previous observations on canine articular cartilage, which showed middle and deep layer chondrocytes surrounded by a pericellular matrix and enclosed within a pericellular capsule composed of filamentous and fine fibrillar materials. Pores in the "felt-like" organization of the capsular weave progressively decreased in size from the inner to the outer border of the capsule. Matrix vesicles were found embedded within the capsular weave and distributed throughout the territorial matrix. It is suggested that the chondrocyte, its pericellular matrix, and capsule together constitute the "chondron," a primary functional and metabolic unit of cartilage that acts hydrodynamically to protect the integrity of the chondrocyte and its pericellular microenvironment during compressive loading.

The organization of collagen in cryofractured rabbit articular cartilage: a scanning electron microscopic study

Adult rabbit articular cartilage was prepared for scanning electron microscopy using, in order, glutaraldehyde fixation, enzymatic removal of proteoglycan, dehydration in ethanol, cryofracture in liquid nitrogen, and critical-point drying. Enzymes were effective in fixed material. Fixation, cryofracture, alignment of fracture surfaces with "split lines," and retention of subchondral bone were found to be necessary steps for the preservation of collagen detail. The fibrous framework was found to be similar to that proposed by Benninghoff and favored by more recent phase-contrast microscopic studies. Vertical fibers extending from subchondral bone and a network of tangentially oriented superficial fibrils converge in the transitional zone. No random layer is seen. Pericellular capsules interdigitate with the vertical fibers. When cartilage is prepared in a manner that minimizes tissue damage, scanning electron microscopy provides useful, unique information.

Articular cartilage after meniscectomy. Rabbit knees studied with the scanning electron microscope

Ten rabbits had a medial meniscectomy in both knees. The articular surface was studied with a scanning electron microscope 2, 4 or 12 weeks after the operation. Three sham operated and two unoperated rabbits served as controls. Progressive articular cartilage damage was observed on the weight-bearing areas of the medial condyles. The findings are compatible with clinical observations after meniscectomy; they confirm that meniscectomy is not a harmless operation.

Spatial arrangement of collagen fibrils in the articular cartilage of the mandibular condyle: a light microscopic and scanning electron microscopic study

To determine the spatial arrangement of collagen fibrils in articular cartilage of the human mandibular condyle, ten healthy condyles obtained at necropsy were examined by light microscopy and scanning electron microscopy (SEM). Observations using light microscopy showed the existence of four different zones. The organization and alignment of the collagen fibrils were different in every zone and varied from layers to bundles of fibrils running parallel, obliquely, or radially to the articular surface. Observations using scanning electron microscopy revealed a thin, surface layer of disorganized small fibrils with a cotton-wool appearance and a well-organized architecture of collagen fibrils in every zone. It was concluded that the organization of collagen fibrils in articular cartilage shows a three-dimensional network with a special system in every zone.

Surface changes in the articular cartilage of rabbit knee during immobilization. A scanning electron microscopic study of experimental osteoarthritis

Osteoarthritis was produced by the immobilization of rabbit knees in extension for 1--8 weeks (with a subsequent mobilization period of 0 or 8 weeks). The development of articular surface changes in the tibia, the femur and the patella was examined with scanning electron microscopy (SEM). During the development of osteoarthritis the normal undulations and fine regular fibre network disappeared, the number of fibres and the variation in the thickness of the fibre bundles increased, and scaly irregularities appeared. Some degenerative changes in the contralateral, non-immobilized hind limb also appeared. The normal features of the articular surface of the patella differed from that of the tibial and femoral condyles. The changes seen with SEM correlated well with histological and biochemical results of earlier studies, but at 1 week of immobilization, the SEM changes were more readilly apparent than the histological changes. The SEM method seems to be useful for the study of articular surfaces; however, the area studied must be defined.

Scanning electron microscopic and radiographic correlation of articular surface and supporting bone of the mandibular condyle

Specimens of mandibular condyle from human cadavers were employed for scanning electron microscopic and radiographic observations of the articulating surface and underlying bone. Smooth articular surface supported by smooth bone was most frequently observed. Craters and depressions in the articular surface were associated with resorption of underlying bone. Lateral radiographs proved to be of limited value in detecting defects of this type.

[Observations of the surface of the articular cartilage after treatment with hyaluronidase (author's transl)]

1. The articular surfaces of the femoral head of rats have been studied by scanning electron microscopy after in vitro treatment with hyaluronidase. 2. The cartilage surface has been digested mildly by the buffer medium (control). 3. The matrix of the cartilage has selectively been removed by the hyaluronidase. 4. Chondrocytes have been seen on the surface after treatment with hyaluronidase for a longer period. 5. We discuss the result of enzymatic digestion.

The fate of the articular cartilage in intracapsular fracture of the femoral neck (articular cartilage in femoral neck fracture)

The fate of the articular cartilage of the hip joint with intracapsular neck fracture was studied by histological, histochemical and autoradiographic techniques and by using a polarized microscope and a scanning electron microscope. Cartilage specimens from 93 femoral heads and 7 acetabula were obtained from fractured hips 2 days to 4 1/3 years postfracture and from control hips with various disorders. The cartilage degeneration appeared 2 weeks after fracture and advanced steadily with time. The matrix was covered, invaded and ultimately replaced by the fibrous tissue. Chondrocyte viability, though it was lost from the surface, was recognized in the deep matrix even in the oldest fracture examined. It is concluded that the humoral factor directly caused by the injury as well as the biomechanical impairment, i.e. a loss of physical stress, may play an essential role in the pathogenesis of the degeneration. The possibility of regeneration was discussed.

The preparation of human articular cartilage for scanning electron microscopy

This paper compares sixteen preparative techniques thought to be of advantage in the study by scanning electron microscopy (SEM) of human articular cartilage surfaces. The adequacy of surface preservation obtained with the techniques, was judged subjectively, first, by the reproducubility of secondary electron images of normal cartilage, and second, by comparing the results with those obtained by reflected light microscopy of the fresh unfixed cartilage surface over a magnification range of X20- X240. Adequate surface preservation was confirmed when cartilage surfaces had been dehydrated through ethanol to propylene oxide and vacuum dried; dehydrated through amyl acetate and quenched in Freon before freeze-drying; dehydrated and passed through amyl acetate at low temperature before freeze drying. Valuable information can be obtained from different specimens by varying the technique of preparation. At different ages, different surface features are best preserved. In a systematic study it has been found essential to adopt a uniform preparative method and to control the results by reflected light microscopy. Even with the most perfect preparation, the surface appearances cannot be identical with those that function under load in vivo.