Articular calcified cartilage canals in the third metacarpal bone of 2-year-old thoroughbred racehorses

Observation of Solute Transport between Articular Cartilage and Subchondral Bone in Live Mice

OBJECTIVE

To establish a method for investigating the permeability of calcified cartilage zone (CCZ) and to observe solute transport between articular cartilage (AC) and subchondral bone (SB) through intact CCZ in vivo.

DESIGN

We developed a novel fixing device combined with un-decalcified fluorescence observation method to address the permeability of CCZ in live mice. Twenty-four Balb/c female mice aged 1 to 8 months were used to observe the development of CCZ. Eighty-four Balb/c female mice (aged 1 or 6 months) with mature or immature CCZ of distal femur were used to investigate the permeability of intact CCZ in vivo. Diffusivity of rhodamine B (476 Da) and tetramethyl-rhodamine isothicyanate-dextran (TRITC-Dextran, 20 kDa) was tested from AC to SB in 0 minutes, 1 minute, 15 minutes, 30 minutes, 1 hour, and 2 hours. None diffused knee joints (0 minutes) served as blank control, while in vitro immersion of distal femurs in rhodamine B or TRITC-Dextran for 72 hours served as positive control.

RESULTS

CCZ was well developed in 6-month mice. Both tracers penetrated immature CCZ down to SB in less than 1 hour in live mice, while the diffusion of both tracers decreased rapidly at tidemark in all testing time points.

CONCLUSION

Current study provided direct evidence of blocking effect of CCZ in solute transportation during short diffusion period in live animal, indicating the important role of CCZ in joint development and microenvironment maintenance.

The Bone Cartilage Interface and Osteoarthritis

This review describes results obtained with tissue from prior studies of equine and human osteoarthritis (OA). The main methods considered are scanning electron microscopy, novel methods in light microscopy and X-ray Micro-tomography. The same samples have been re-utilised in several ways. The tissues described are hyaline articular cartilage (HAC; or substitutes), with its deep layer, articular calcified cartilage (ACC), whose deep surface is resorbed in cutting cone events to allow the deposition of subchondral bone (SCB). Multiple tidemarks are normal. Turnover at the osteochondral (ACC-HAC-SCB) junction is downregulated by overload exercise, conversely, during rest periods. Consequent lack of support predisposes to microfracture of the ACC-SCB plate, in the resorption-related repair phase of which the plate is further undermined to form sink holes. The following characteristics contribute to the OA scenario: penetrating resorption canals and local loss of ACC; cracking of ACC and SCB; sealing of cracks with High-Density Mineral Infill (HDMI); extrusion of HDMI into HAC to form High-Density Mineral Protrusions (HDMP) in HAC which may fragment and contribute to its destruction; SCB marrow space infilling and densification with (at first) woven bone; disruption, fibrillation and loss of HAC; eburnation; repair with abnormal tissues including fibrocartilage and woven bone; attachment of Sharpey fibres to SCB trabeculae and adipocyte-moulded extensions to trabeculae (excrescences).

Influence of Exercise and Intra-articular Site on Canals in Articular Calcified Cartilage of Equine Third Carpal Bones

The third carpal bone (C3) responds to exercise by adaptive modeling of bone and articular calcified cartilage along the dorsal load path. Canals penetrating articular calcified cartilage, thought to contain vascular tissue, are reported in numerous species. Their significance remains unclear. Our objective was to determine if the number of canals was significantly different in strenuously exercised and control young horses and in a site of intermittent high loading compared to sites sustaining lower habitual loads. Volumetric bone mineral density in the radial facet of C3 of strenuously exercised and gently exercised (control) 19-month-old thoroughbred horses ( n = 6/group) was determined by peripheral quantitative computed tomography. The hyaline cartilage was corroded to expose the surface of articular calcified cartilage. The number of canals penetrating the articular calcified cartilage surface in en face scanning electron microscopy images was compared in 4 regions. Volumetric bone mineral density of C3 was significantly greater ( P = .004) in strenuously exercised horses. There were 2 morphologically distinct groups of canals and significantly fewer ( P = .006) large canals in the dorsal than in the palmar aspect of C3 in control but not in exercised horses. Roughly circular depressions in the articular calcified cartilage surface around apparently forming canals were visible in some samples and have not been previously described in the literature. The canals may be evidence of chondroclastic activity reaching the interface of hyaline and calcified cartilage. Further work is needed to elucidate the relationships between presence of canals and the responses to exercise and to joint disease.

Detection of early osteoarthritis in the centrodistal joints of Icelandic horses: Evaluation of radiography and low-field magnetic resonance imaging

REASONS FOR PERFORMING STUDY

Validated noninvasive detection methods for early osteoarthritis (OA) are required for OA prevention and early intervention treatment strategies.

OBJECTIVES

To evaluate radiography and low-field magnetic resonance imaging (MRI) for the detection of early stage OA osteochondral lesions in equine centrodistal joints using microscopy as the reference standard.

STUDY DESIGN

Prospective imaging of live horses and imaging and microscopy of cadaver tarsal joints.

METHODS

Centrodistal (distal intertarsal) joints of 38 Icelandic research horses aged 27-29 months were radiographed. Horses were subjected to euthanasia approximately 2 months later and cadaver joints examined with low-field MRI. Osteochondral joint specimens were classified as negative or positive for OA using light microscopy histology or scanning electron microscopy. Radiographs and MRIs were evaluated for osteochondral lesions and results compared with microscopy.

RESULTS

Forty-two joints were classified OA positive with microscopy. Associations were detected between microscopic OA and the radiography lesion categories; mineralisation front defect (P<0.0001), joint margin lesion (P<0.0001), central osteophyte (P = 0.03) and the low-field MRI lesion categories; mineralisation front defect (P = 0.01), joint margin lesion (P = 0.02) and articular cartilage lesion (P = 0.0003). The most frequent lesion category detected in microscopic OA positive joints was the mineralisation front defect in radiographs (28/42 OA positive joints, specificity 97%, sensitivity 67%). No significant differences were detected between the sensitivity and specificity of radiography and low-field MRI pooled lesion categories, but radiography was often superior when individual lesion categories were compared.

CONCLUSIONS

Early stage centrodistal joint OA changes may be detected with radiography and low-field MRI. Detection of mineralisation front defects in radiographs may be a useful screening method for detection of early OA in centrodistal joints of young Icelandic horses.

Histomorphometric evaluation of the effect of early exercise on subchondral vascularity in the third carpal bone of horses

OBJECTIVE

To investigate histomorphometric changes in the cartilage and subchondral bone of the third carpal bone associated with conditioning exercise in young Thoroughbreds.

ANIMALS

Nine 18-month-old Thoroughbreds. Procedures-Both third carpal bones of 9 horses (4 exercised spontaneously at pasture only and 5 given additional conditioning exercise beginning at a mean age of 3 weeks) were evaluated. Histomorphometric variables (hyaline and calcified cartilage thickness and collagen orientation; vascular channel area, number, and orientation; and osteochondral junction rugosity) of the third carpal bone, sampled at 4 dorsopalmar sites in the radial facet, were compared between the exercised and nonexercised groups.

RESULTS

The vascular channel area measured at the 4 dorsopalmar sites was larger in the exercised group than in the control group, but none of the variables were significantly different between groups. Both groups had significant site-specific variations in all measured variables. Most importantly, the vascular channel area was highest in the most dorsal aspect.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that the mild exercise imposed in both groups during the developmental period appeared to be associated with an increase in the vascular channel area beneath the calcified cartilage layer in the third carpal bone. This increased vascular channel area could also be associated with high stress in the dorsal aspect of the radial facet, a region that is known to be vulnerable to osteochondral fragmentation.

The mineral phase of calcified cartilage: its molecular structure and interface with the organic matrix

We have studied the atomic level structure of mineralized articular cartilage with heteronuclear solid-state NMR, our aims being to identify the inorganic species present at the surfaces of the mineral crystals which may interact with the surrounding organic matrix and to determine which components of the organic matrix are most closely involved with the mineral crystals. One-dimensional (1)H and (31)P and two-dimensional (1)H-(31)P heteronuclear correlation NMR experiments show that the mineral component is very similar to that in bone with regard to its surface structure. (13)C{(31)P} rotational echo double resonance experiments identify the organic molecules at the mineral surface as glycosaminoglycans, which concurs with our recent finding in bone. There is also evidence of gamma-carboxyglutamic acid residues interacting with the mineral. However, other matrix components appear more distant from the mineral compared with bone. This may be due to a larger hydration layer on the mineral crystal surfaces in calcified cartilage.

On new bone formation in the pre-osteoarthritic joint

OBJECTIVE

This study investigated the structural alterations in the osteochondral junction, traversing the intact-to-lesion regions, with the aim of elucidating the way in which the pre-osteoarthritic (pre-OA) state progresses to fully developed osteoarthritis (OA).

METHOD

Thirty bovine patellae showing varying degrees of degeneration, with lesions located in the distal-lateral quarter, were used for this study. Cartilage-on-bone blocks were cut along the lateral facet to include both the lesion site in the distal end and the intact site in the proximal end. The blocks were formalin-fixed, mildly decalcified and microtomed to obtain 30 microm - thick osteochondral slices. Using differential interference contrast optics, the tissue microstructure was captured at high resolution in its fully hydrated state.

RESULTS

There were structural changes in the osteochondral junction beneath the still-intact articular cartilage adjacent to the lesion site. The changes observed in traversing from the intact to the lesion site exhibited characteristics that were strikingly similar to those associated with primary bone formation. The evidence suggests that disruption of the cartilage continuum by a lesion has wider mechanobiological consequences at the osteochondral junction.

CONCLUSION

The progression of OA appears to involve new bone formation adjacent to lesion sites. We hypothesise that the new bone spicules that appear in regions beneath intact cartilage adjacent to lesion sites provide a snapshot of the elusive pre-OA state.

High resolution microscopic survey of third metacarpal articular calcified cartilage and subchondral bone in the juvenile horse: possible implications in chondro-osseous disease

The aim was to survey articular calcified cartilage (ACC) and subchondral bone in the palmar and dorsal regions of the condyles of the third metacarpal bone (Mc3) of young horses with minimal or no signs of musculo-skeletal abnormality. Back-scattered electron scanning electron microscopy (BSE SEM) was conducted on polymethyl methacrylate-embedded mediolateral slices and macerated wedges of the right distal Mc3 from seven each of trained and untrained 2-year-old Thoroughbred horses. Furrows or grooves visible to the naked eye in the mineralizing front (MF) of ACC are the commonest "lesion" and are most common in the palmar portions of the medial and lateral condylar grooves. Cracks running predominantly in the parasagittal plane that infill with hypercalcified matrix are found in the same domain. Common to all these defects are deficiencies or absence of the ACC MF. Other anomalies include local excrescences or depressed areas of the MF. More important condylar lesions show displaced fragmented hypermineralized ACC with underlying excess resorption in the bone domain, leaving a thin ACC layer with cavernous space beneath it. The fragments may dislodge and displace to the joint space. Obvious although small lesions are present in horses that have undertaken little or no training. The nature and sites of the lesions indicate that they are possibly the earliest morphological evidence of changes that may lead to specific joint abnormalities. The lesions appear unlikely to be solely due to functional traumatic forces, and developmental influences are likely to be important in their initiation.

Bone bruise in magnetic resonance imaging strongly correlates with the production of joint effusion and with knee osteoarthritis

BACKGROUND

The findings of magnetic resonance imaging (MRI) have not been studied systematically in patients with osteoarthritis (OA). The objective here was to compare MRI findings with radiological findings in patients with knee pain and to identify factors that influence the progression of OA of the knee.

METHODS

Of 212 patients with knee pain and MRI of the knee joint, 161 patients were selected for the study after exclusion of cases of trauma and other arthritides. MRI was used to evaluate the presence and degree of bone bruise, hydrarthrosis, and injuries to the cruciate ligament and meniscus. Bone bruise was classified into four types, and hydrarthrosis into four grades. Radiologically, OA progression in the femorotibial and patellofemoral joints was analyzed according to the Kellgren-Lawrence classification. Age was divided into four groups based on distribution quartiles. Logistic regression analysis and a generalized linear model with Poisson regression were used to analyze correlations among these factors.

RESULTS

Bone bruise was present in 87 cases, hydrarthrosis in 100, cruciate ligament injury in 20, and meniscus injury in 98. The presence of bone bruise was not related to age, cruciate ligament injury, meniscus injury, nor to OA of the patellofemoral joint, but was related to hydrarthrosis and to OA of the femorotibial joint. Femorotibial OA was much more strongly associated with bone bruise than with hydrarthrosis. Furthermore, analyzing the relation between the types of bone bruise and the degree of hydrarthrosis using a generalized linear model with Poisson regression, there was a positive correlation between the grade of bone bruise and the amount of hydrarthrosis.

CONCLUSIONS

A factor associated with the degree of osteoarthritis of the knee is bone bruise observed on MRI. The degree of hydrarthrosis is related to the grade of bone bruise, but is not linked to the degree of osteoarthritis.

On how degeneration influences load-bearing in the cartilage-bone system: a microstructural and micromechanical study

OBJECTIVE

This study investigated the microanatomical response to compression of intact and degenerate cartilage-on-bone samples with the aim of elucidating the functional consequences of articular surface disruption and related matrix changes.

METHOD

Two groups of mature bovine patellae were identified at the time of harvest; those with intact cartilage and those with cartilage exhibiting mild to severe degeneration. Cartilage-on-bone samples were statically compressed (7 MPa) to near-equilibrium using an 8-mm diameter cylindrical indenter, and then formalin-fixed in this deformed state. Following mild decalcification full-depth cartilage-bone sections, incorporating the indentation profile and beyond, were studied in their fully hydrated state using differential interference contrast optical microscopy (DIC).

RESULTS

Differences in matrix texture, degree of disruption of the articular surface layer (or its complete absence), number of tidemarks and absence or presence of vascularization of the calcified cartilage zone were all observable features that provided clear differentiation between the normal and degenerate tissues. Under load a chevron-type shear discontinuity characterized those samples in which the strain-limiting surface layer was still largely intact. The extent to which this shear discontinuity advanced into the adjacent non-directly loaded cartilage continuum was influenced by the integrity of the cartilage general matrix. For those tissues deficient in a strain-limiting articular surface there was no shear discontinuity, the cartilage deformation field was instead shaped primarily by its osteochondral attachment and a laterally-directed compressive collapse of a much weakened matrix. In the degenerate samples the altered matrix textures associated with different regions of the deformation field are interpreted in terms of an intrinsic fibrillar architecture that is weakened by two fundamental processes: (1) a de-structuring resulting from a reduction in connectivity between fibrils and (2) subsequent aggregation of these now disconnected fibrils.

CONCLUSION

DIC microscopy provides a high-resolution description of the integrated osteochondral tissue system across the full continuum of matrices, from normal to severely degenerate. Our study demonstrates the important functional role played by the strain-limiting articular surface, the consequences associated with its disruption, as well as the loss of effective stress transmission associated with a 'de-structured' general matrix. The study also provides new insights into the integration of cartilage with both its subchondral substrate and the wider continuum of non-directly loaded cartilage.

Variations in articular calcified cartilage by site and exercise in the 18-month-old equine distal metacarpal condyle

OBJECTIVES

To interrelate articular calcified cartilage thickness, mineralisation density, tidemark count and tidemark linear accretion rate by site in the equine third metacarpal distal condyle. To determine the effects of exercise during early life on articular calcified cartilage.

METHOD

Six of 12 pasture-raised Thoroughbred horses were exercised from 10 days old. Calcein labels were given 19 and 8 days prior to euthanasia at 18 months old. Osteochondral specimens were cut from the distal third metacarpal condyle and imaged using confocal scanning light microscopy (CSLM) and quantitative backscattered electron scanning electron microscopy (qBSE). Articular calcified cartilage thickness and total thickness mineralisation density were measured on montaged qBSE image sets, and inter-label mineralisation density, tidemark count and linear accretion rate measured on registered CSLM-qBSE image pairs.

RESULTS

Calcified cartilage thickness, mineralisation density, tidemark count and linear accretion rate varied significantly between sites. Regions with thinner calcified cartilage had greater linear accretion rates, hence rapid chondroclastic resorption. Mineralisation density was positively correlated with linear accretion rate. Fewer multiple tidemarks were counted in regions with greater linear accretion rates. Lag time between the tidemark and cement line was estimated (180 days; in the range of 0-648 days). Exercise had little effect on measured parameters.

CONCLUSION

The major determinant of articular calcified cartilage thickness is the rate of chondroclastic resorption, not tidemark linear accretion rate. Our evidence supports coupled, mechanosensitive regulation of chondroclastic resorption and linear accretion rate in articular calcified cartilage. Exercising pasture-reared foals causes little additional adaptation in distal third metacarpal articular calcified cartilage.

Fatty acid transport in articular cartilage

Articular cartilage extracellular matrix imposes a significant transport barrier to albumin, the principal carrier of fatty acids. It has not been previously established whether it also influences the transport of fatty acids important for chondrocyte metabolism. Albumin was labelled with rhodamine-maleimide and bound to NBD-labelled lauric acid. Plugs of fresh equine metacarpal-phalangeal cartilage and subchondral bone were incubated with the complex at 4 degrees C for 2-160 h. The fluorophore distribution was quantified using quantitative microscopy in histological sections. The fluorescence intensity of both fluorophores fell steeply over 300 microm below the articular surface and remained relatively uniform through the mid zone but the ratio of lauric acid to albumin was higher than in the incubation medium. The effective diffusivity of lauric acid in the mid zone was (2.2+/-0.7) x 10(-12) m2 s(-1) (n = 33), higher than that of the carrier albumin, suggesting dissociation in the surface layer. Lauric acid accumulated reversibly at the tidemark.

The response of bone, articular cartilage and tendon to exercise in the horse

Horses can gallop within hours of birth, and may begin training for athletic competition while still growing. This review cites studies on the effects of exercise on bone, tendon and articular cartilage, as detected by clinical and research imaging techniques, tissue biochemical analysis and microscopy of various kinds. For bone, alterations in bone mineral content, mineral density and the morphology of the mineralized tissue are the most common end-points. Apparent bone density increases slightly after athletic training in the cortex, but substantially in the major load paths of the epiphyses and cuboidal bones, despite the lower material density of the new bone, which is deposited subperiosteally and on internal surfaces without prior osteoclastic resorption. With training of greater intensity, adaptive change is supervened by patho-anatomical change in the form of microdamage and frank lesions. In tendon, collagen fibril diameter distribution changes significantly during growth, but not after early training. The exact amount and type of protracted training that does cause reduction in mass average diameter (an early sign of progressive microdamage) have not been defined. Training is associated with an increase in the cross-sectional area of some tendons, possibly owing to slightly greater water content of non-collagenous or newly synthesized matrix. Early training may be associated with greater thickness of hyaline but not calcified articular cartilage, at least in some sites. The age at which adaptation of cartilage to biomechanical influences can occur may thus extend beyond very early life. However, cartilage appears to be the most susceptible of the three tissues to pathological alteration. The effect of training exercise on the anatomical or patho-anatomical features of connective tissue structures is affected by the timing, type and amount of natural or imposed exercise during growth and development which precedes the training.

Registration of confocal scanning laser microscopy and quantitative backscattered electron images for the temporospatial quantification of mineralization density in 18-month old thoroughbred racehorse articular calcified cartilage

Combined backscattered electron scanning electron microscopy (BSE SEM) and confocal scanning laser microscopy (CSLM) have been used to put tissue mineralization data into the context of soft tissue histology and fluorescent label information. Mineralization density (Dm) and linear accretion rate (LAR) are quantifiable parameters associated with mineralizing fronts within calcified tissues. Quantitative BSE (qBSE) may be used to determine Dm, while CSLM may be used to detect label fluorescence from which LAR is calculated. Eighteen-month old Thoroughbred horses received single calcein injections 19 and 8 days prior to euthanasia, labeling sites of active mineralization with fluorescent bands. Confocal scanning laser microscopy images of articular calcified cartilage (ACC) from distal third metacarpal condyles were registered to qBSE images of the same sites using an in-house program. ImageJ and Sync Windows enabled the simultaneous collection of LAR and Dm data. The repeatability of the registration and measurement protocols was determined. Dm profiles between calcein labels were explored for an association with time. Dm was 119.7 +/- 24.5 (mean +/- standard deviation) gray levels (where 0 = backscattering from monobrominated and 255 from monoiodinated dimethacrylate standards, respectively), while modal and maximum LAR were 0.45 and 3.45 microm/day, respectively. Coefficients of variation (CV) for Dm were 0.70 and 0.77% with and without repeat registration, respectively; CVs for LAR were 1.90 and 2.26% with and without repeat registration, respectively. No relationship was identified between Dm and time in the 11-day interlabel interval. Registration of CSLM to qBSE images is sufficiently repeatable for quantitative studies of equine ACC.