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

Micro-computed tomography reveals high-density mineralised protrusions and microstructural lesions in equine stifle joint articular cartilage

BACKGROUND

Stifle osteoarthritis (OA) lesions are most common in the medial femorotibial (MFT) compartment. Their characterisation and mapping will inform equine veterinarians towards an accurate diagnosis of OA.

OBJECTIVES

Investigate and map micro-CT (μCT) changes in the hyaline articular cartilage (HAC) in the medial femoral condyle (MFC) and medial tibial plateau (MTP).

STUDY DESIGN

Ex vivo cadaveric.

METHODS

Stifles (n = 7 OA and 17 control [CO]) were retrieved from a tissue bank. The MFC and MFT were imaged with μCT. Regions of interest (ROIs) were cranial (MFCcr; MTPcr) and caudal (MFCca; MTPca) sites. In each ROI, μCT images were scored for HAC fibrillation, surface mineralisation and for the presence of high-density mineralised protrusions (HDMP). The lesions were mapped, and site-matched histology was performed.

RESULTS

The microstructure of healthy and abnormal HAC was discernible on μCT images and confirmed with histology. HAC fibrillation was more prevalent (p = 0.019) in the MFCcr of the OA group (n = 7/7, 100%) when compared with the CO group (n = 7/17, 41%). Score 1 HAC surface mineralisation was more prevalent (p = 0.038) in the OA MFCca (n = 4/7, 57%) when compared with the CO group (n = 2/17, 12%). HDMP were heterogenous and hyperdense mineralised material protruding into the HAC and were more frequent (p = 0.033) in MFCs (n = 12/24, 50%) compared with MTPs (n = 5/24, 20%). Score 3 HDMPs were also more prevalent (p = 0.003) in the MFCcr (n = 7/24, 29%) compared with MFCca (n = 0/24, 0%) and in MFCs (n = 7/24, 29%) compared with MTPs (n = 3/24, 12.5%) (p = 0.046).

MAIN LIMITATIONS

Clinical history was not available for all specimens.

CONCLUSIONS

Equine HDMP and HAC surface mineralisation are imaged for the first time in the MFT joint. HAC fibrillation and erosion and HDMP are more frequent in the cranial aspect of the MFC. μCT images of OA in equine stifle joints provide a novel perspective of lesions and improve understanding of OA.

Comparison of radiography and computed tomography for identification of third metacarpal structural change and associated assessment of condylar stress fracture risk in Thoroughbred racehorses

BACKGROUND

Catastrophic injury has a low incidence but leads to the death of many Thoroughbred racehorses.

OBJECTIVES

To determine sensitivity, specificity, and reliability for third metacarpal condylar stress fracture risk assessment from digital radiographs (DR) and standing computed tomography (sCT).

STUDY DESIGN

Controlled ex vivo experiment.

METHODS

A blinded set of metacarpophalangeal joint DR and sCT images were prepared from 31 Thoroughbreds. Four observers evaluated the condyles and parasagittal grooves (PSG) of the third metacarpal bone for the extent of dense bone and lucency/fissure and assigned a risk assessment grade for condylar stress fracture based on imaging features. Sensitivity and specificity for detection of subchondral structural changes in the condyles and PSG, and for risk assessment for condylar stress fracture were determined by comparison with a reference assessment based on sCT and joint surface examination. Agreement between observers and the reference assessment and reliability between observers were determined. Intra-observer repeatability was also assessed.

RESULTS

Sensitivity for detection of structural change was lower than specificity for both imaging methods and all observers. For agreement with the reference assessment of structural change, correlation coefficients were generally below 0.5 for DR and 0.49-0.82 for sCT. For horses categorised as normal risk on reference assessment, observer assessment often agreed with the reference. Sensitivity for risk assessment was lower than specificity for all observers. For horses with a reference assessment of high risk of injury, observers generally underestimated risk. Diagnostic sensitivity of risk assessment was improved with sCT imaging, particularly for horses categorised as having elevated risk of injury from the reference assessment. Assessment repeatability and reliability was better with sCT than DR.

MAIN LIMITATIONS

The ex vivo study design influenced DR image sets.

CONCLUSIONS

Risk assessment through screening with diagnostic imaging is a promising approach to improve injury prevention in racing Thoroughbreds. Knowledge of sensitivity and specificity of fetlock lesion detection provides the critical guidance needed to improve racehorse screening programs. We found improved detection of MC3 subchondral structural change and risk assessment for condylar stress fracture with sCT ex vivo.

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).

Microenvironment in subchondral bone: predominant regulator for the treatment of osteoarthritis

Osteoarthritis (OA) is a degenerative joint disease in the elderly. Although OA has been considered as primarily a disease of the articular cartilage, the participation of subchondral bone in the pathogenesis of OA has attracted increasing attention. This review summarises the microstructural and histopathological changes in subchondral bone during OA progression that are due, at the cellular level, to changes in the interactions among osteocytes, osteoblasts, osteoclasts (OCs), endothelial cells and sensory neurons. Therefore, we focus on how pathological cellular interactions in the subchondral bone microenvironment promote subchondral bone destruction at different stages of OA progression. In addition, the limited amount of research on the communication between OCs in subchondral bone and chondrocytes (CCs) in articular cartilage during OA progression is reviewed. We propose the concept of 'OC-CC crosstalk' and describe the various pathways by which the two cell types might interact. Based on the 'OC-CC crosstalk', we elaborate potential therapeutic strategies for the treatment of OA, including restoring abnormal subchondral bone remodelling and blocking the bridge-subchondral type H vessels. Finally, the review summarises the current understanding of how the subchondral bone microenvironment is related to OA pain and describes potential interventions to reduce OA pain by targeting the subchondral bone microenvironment.

High-Density Mineralized Protrusions and Central Osteophytes: Associated Osteochondral Junction Abnormalities in Osteoarthritis

The aim of this study was to determine the association between high-density mineralized protrusions (HDMPs) and central osteophytes (COs), and describe the varying appearance of these lesions using advanced clinical imaging and a novel histological protocol. Seventeen consecutive patients with clinically advanced knee osteoarthritis undergoing knee arthroplasty were included. Surgical tissues containing the osteochondral region were investigated using computed tomography (CT); a subset was evaluated using confocal microscopy with fluorescence. Tissues from seven subjects (41.2%) contained HDMPs, and tissues from seven subjects (41.2%) contained COs. A significant association between HDMPs and COs was present (p = 0.003), with 6 subjects (35.2%) demonstrating both lesions. In total, 30 HDMPs were found, most commonly at the posterior medial femoral condyle (13/30, 43%), and 19 COs were found, most commonly at the trochlea (5/19, 26.3%). The HDMPs had high vascularity at their bases in cartilaginous areas (14/20, 70%), while the surrounding areas had elevated levels of long vascular channels penetrating beyond the zone of calcified cartilage (p = 0.012) compared to HDMP-free areas. Both COs and HDMPs had noticeable bone-resorbing osteoclasts amassing at the osteochondral junction and in vascular channels entering cartilage. In conclusion, HDMPs and COs are associated lesions in patients with advanced knee osteoarthritis, sharing similar histologic features, including increased vascularization and metabolic bone activity at the osteochondral junction. Future studies are needed to determine the relationship of these lesions with osteoarthritis progression and symptomatology.

Subchondral bone morphology in the metacarpus of racehorses in training changes with distance from the articular surface but not with age

The repetitive large loads generated during high-speed training and racing commonly cause subchondral bone injuries in the metacarpal condyles of racehorses. Adaptive bone modelling leads to focal sclerosis at the site of highest loading in the palmar aspect of the metacarpal condyles. Information on whether and how adaptive modelling of subchondral bone changes during the career of a racehorse is sparse. The aim of this cross-sectional study was to describe the changes in subchondral bone micromorphology in the area of highest loading in the palmar aspect of the metacarpal condyle in thoroughbred racehorses as a function of age and training. Bone morphology parameters derived from micro-CT images were evaluated using principal component analysis and mixed-effects linear regression models. The largest differences in micromorphology were observed in untrained horses between the age of 16 and 20 months. Age and duration of a training period had no influence on tissue mineral density, bone volume fraction or number and area of closed pores to a depth of 5.1 mm from the articular surface in 2- to 4-year-old racehorses in training. Horses with subchondral bone injuries had more pores in cross-section compared with horses without subchondral bone injuries. Differences in bone volume fraction were due to the volume of less mineralised bone. Tissue mineral density increased and bone volume fraction decreased with increasing distance from the articular surface up to 5.1 mm from the articular surface. Further research is required to elucidate the biomechanical and pathophysiological consequences of these gradients of micromorphological parameters in the subchondral bone.

Changes in the osteochondral unit during osteoarthritis: structure, function and cartilage-bone crosstalk

In diarthrodial joints, the articular cartilage, calcified cartilage, and subchondral cortical and trabecular bone form a biocomposite - referred to as the osteochondral unit - that is uniquely adapted to the transfer of load. During the evolution of the osteoarthritic process the compositions, functional properties, and structures of these tissues undergo marked alterations. Although pathological processes might selectively target a single joint tissue, ultimately all of the components of the osteochondral unit will be affected because of their intimate association, and thus the biological and physical crosstalk among them is of great importance. The development of targeted therapies against the osteoarthritic processes in cartilage or bone will, therefore, require an understanding of the state of these joint tissues at the time of the intervention. Importantly, these interventions will not be successful unless they are applied at the early stages of disease before considerable structural and functional alterations occur in the osteochondral unit. This Review describes the changes that occur in bone and cartilage during the osteoarthritic process, and highlights strategies for how this knowledge could be applied to develop new therapeutic interventions for osteoarthritis.

Osteoclasts are recruited to the subchondral bone in naturally occurring post-traumatic equine carpal osteoarthritis and may contribute to cartilage degradation

The role of osteoclasts in osteochondral degeneration in osteoarthritis (OA) has rarely been investigated in spontaneous disease or animal models of OA.

OBJECTIVE

The objectives of the current study were to investigate osteoclast density and location in post-traumatic OA (PTOA) and control specimens from racehorses.

METHOD

Cores were harvested from a site in the equine third carpal bone, that undergoes repetitive, high intensity loading. Histological and immunohistochemical (Cathepsin K and Receptor-activator of Nuclear Factor kappa-β ligand (RANKL)) stained sections were scored (global and subregional) and the osteoclast density calculated. The cartilage histological scores were compared with osteoclast density and RANKL scores.

RESULTS

There was a greater density of osteoclasts in PTOA samples and they were preferentially located in the subchondral bone plate. RANKL scores positively correlated to the scores of cartilage degeneration and the osteoclast density. The relationship between hyaline articular cartilage RANKL score and osteoclast density was stronger than that of the subchondral bone RANKL score suggesting that cartilage RANKL may have a role in recruiting osteoclasts. The RANKL score in the articular calcified cartilage correlated with the number of microcracks also suggesting that osteoclasts recruited by RANKL may contribute to calcified cartilage degeneration in PTOA.

CONCLUSION

Our results support the hypothesis that osteoclasts are recruited during the progression of spontaneous equine carpal PTOA by cartilage RANKL, contributing to calcified cartilage microcracks and focal subchondral bone loss.

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.

High density infill in cracks and protrusions from the articular calcified cartilage in osteoarthritis in standardbred horse carpal bones

We studied changes in articular calcified cartilage (ACC) and subchondral bone (SCB) in the third carpal bones (C3) of Standardbred racehorses with naturally-occurring repetitive loading-induced osteoarthritis (OA). Two osteochondral cores were harvested from dorsal sites from each of 15 post-mortem C3 and classified as control or as showing early or advanced OA changes from visual inspection. We re-examined X-ray micro-computed tomography (µCT) image sets for the presence of high-density mineral infill (HDMI) in ACC cracks and possible high-density mineralized protrusions (HDMP) from the ACC mineralizing (tidemark) front (MF) into hyaline articular cartilage (HAC). We hypothesized and we show that 20-µm µCT resolution in 10-mm diameter samples is sufficient to detect HDMI and HDMP: these are lost upon tissue decalcification for routine paraffin wax histology owing to their predominant mineral content. The findings show that µCT is sufficient to discover HDMI and HDMP, which were seen in 2/10 controls, 6/9 early OA and 8/10 advanced OA cases. This is the first report of HDMI and HDMP in the equine carpus and in the Standardbred breed and the first to rely solely on µCT. HDMP are a candidate cause for mechanical tissue destruction in OA.

On fragmenting, densely mineralised acellular protrusions into articular cartilage and their possible role in osteoarthritis

High density mineralised protrusions (HDMP) from the tidemark mineralising front into hyaline articular cartilage (HAC) were first described in Thoroughbred racehorse fetlock joints and later in Icelandic horse hock joints. We now report them in human material. Whole femoral heads removed at operation for joint replacement or from dissection room cadavers were imaged using magnetic resonance imaging (MRI) dual echo steady state at 0.23 mm resolution, then 26-μm resolution high contrast X-ray microtomography, sectioned and embedded in polymethylmethacrylate, blocks cut and polished and re-imaged with 6-μm resolution X-ray microtomography. Tissue mineralisation density was imaged using backscattered electron SEM (BSE SEM) at 20 kV with uncoated samples. HAC histology was studied by BSE SEM after staining block faces with ammonium triiodide solution. HDMP arise via the extrusion of an unknown mineralisable matrix into clefts in HAC, a process of acellular dystrophic calcification. Their formation may be an extension of a crack self-healing mechanism found in bone and articular calcified cartilage. Mineral concentration exceeds that of articular calcified cartilage and is not uniform. It is probable that they have not been reported previously because they are removed by decalcification with standard protocols. Mineral phase morphology frequently shows the agglomeration of many fine particles into larger concretions. HDMP are surrounded by HAC, are brittle, and show fault lines within them. Dense fragments found within damaged HAC could make a significant contribution to joint destruction. At least larger HDMP can be detected with the best MRI imaging ex vivo.

Relationship between cartilage and subchondral bone lesions in repetitive impact trauma-induced equine osteoarthritis

OBJECTIVE

To correlate degenerative changes in cartilage and subchondral bone in the third carpal bone (C3) of Standardbred racehorses with naturally occurring repetitive trauma-induced osteoarthritis.

DESIGN

Fifteen C3, collected from Standardbred horses postmortem, were assessed for cartilage lesions by visual inspection and divided into Control (CO), Early Osteoarthritis (EOA) and Advanced Osteoarthritis (AOA) groups. Two osteochondral cores were harvested from corresponding dorsal sites on each bone and scanned with a micro-computed tomography (CT) instrument. 2D images were assembled into 3D reconstructions that were used to quantify architectural parameters from selected regions of interest, including bone mineral density and bone volume fraction. 2D images, illustrating the most severe lesion per core, were scored for architectural appearance by blinded observers. Thin sections of paraffin-embedded decalcified cores stained with Safranin O-Fast Green, matched to the micro-CT images, were scored using a modified Mankin scoring system.

RESULTS

Subchondral bone pits with deep focal areas of porosity were seen more frequently in AOA than EOA but never in CO. Articular cartilage damage was seen in association with a reduction in bone mineral and loss of bone tissue. Histological analyses revealed significant numbers of microcracks in the calcified cartilage of EOA and AOA groups and a progressive increase in the score compared with CO bones.

CONCLUSION

The data reveal corresponding, progressive degenerative changes in articular cartilage and subchondral bone, including striking focal resorptive lesions, in the third carpal bone of racehorses subjected to repetitive, high impact trauma.

Conventional and ultrashort time-to-echo magnetic resonance imaging of articular cartilage, meniscus, and intervertebral disk

Magnetic resonance imaging (MRI) examination of musculoskeletal tissues is being performed routinely for diagnoses of injury and diseases. Although conventional MRI using spin echo sequences has been effective, a number of important musculoskeletal soft tissues remain "magnetic resonance-invisible" because of their intrinsically short T2 values resulting in a rapid signal decay. This makes visualization and quantitative characterization difficult. With the advent and refinement of ultrashort time-to-echo (UTE) MRI techniques, it is now possible to directly visualize and quantitatively characterize these tissues. This review explores the anatomy, conventional MRI, and UTE MRI of articular cartilage, meniscus of the knee, and intervertebral disks and provides a survey of magnetic resonance studies used to better understand tissue structure, composition, and function, as well as subtle changes in diseases.

Innovative approach to investigating the microstructure of calcified tissues using specular reflectance Fourier transform-infrared microspectroscopy and discriminant analysis

Although bone fracture has become a serious global health issue, current clinical assessments of fracture risk based on bone mineral density are unable to accurately predict whether an individual is likely to suffer a fracture. There is increasing recognition that the chemical structure and composition, or microstructure, of mineralized tissues has an important role to play in determining the fracture resistance of bone. The objective of this preliminary study was to evaluate the use of specular reflectance Fourier transform infrared (SR FT-IR) microspectroscopy in conjunction with discriminant analysis as an innovative technique for providing future insights into the origins of orthopedic abnormalities. The impetus for this approach was that SR FT-IR microspectroscopy would offer several advantages over conventional transmission methods. Bone samples were obtained from young racehorses at known fracture predilection sites and spectra were successfully obtained from calcified cartilage and subchondral bone for the first time. By applying discriminant analysis to the spectral data set in biologically relevant regions, microstructural differences between groups of individuals were found to be related to features associated with both the mineral and organic components of the bone. The preliminary findings also suggest that differences in bone microstructure may exist between healthy individuals of the same age, raising important questions around the normal limits of individual variation and whether individuals may be predisposed to later fracture as a result of detrimental microstructural changes during early growth and development.

A multi-modal multiphoton investigation of microstructure in the deep zone and calcified cartilage

Multi-modal multiphoton microscopy was used to investigate tissue microstructure in the zone of calcified cartilage, focussing on the collagen fibre organisation at the tidemark and cement line. Thick, unstained and unfixed sagittal sections were prepared from the equine metacarpophalangeal joint. Second harmonic generation (SHG) provided contrast for collagen, two-photon fluorescence (TPF) for endogenous fluorophores, and coherent anti-Stokes Raman scattering (CARS) allowed the cells to be visualised. The structure of radial and calcified cartilage was found to vary with location across the joint, with the palma regions showing a more ordered parallel arrangement of collagen fibres than the cortical ridge and dorsal regions. These patterns may be associated with regional variations in joint loading. In addition, the cell lacunae had a greater diameter in the dorsal region than in the palmar region. At the cement line some collagen fibres were observed crossing between the calcified cartilage and the subchondral bone. At the tidemark the fibres were parallel and continuous between the radial and calcified cartilage. Beneath early superficial lesions the structure of the tidemark and calcified cartilage was disrupted with discontinuities and gaps in the fibrillar organisation. Cartilage microstructure varies in the deep zones between regions of different loading. The variations in collagen structure observed may be significant to the local mechanical properties of the cartilage and therefore may be important to its mechanical interactions with the subchondral bone. The calcified cartilage is altered even below early superficial lesions and therefore is important in the understanding of the aetiology of osteoarthritis.

Ultrashort echo time MR imaging of osteochondral junction of the knee at 3 T: identification of anatomic structures contributing to signal intensity

PURPOSE

To image cartilage-bone interfaces in naturally occurring and experimentally prepared human cartilage-bone specimens at 3 T by using ultrashort echo time (TE) (UTE) and conventional pulse sequences to (a) determine the appearance of the signal intensity patterns and (b) identify the structures contributing to signal intensity on the UTE MR images.

MATERIALS AND METHODS

This study was exempted by the institutional review board, and informed consent was not required. Five cadaveric (mean age, 86 years +/- 4) patellae were imaged by using proton density-weighted fat-suppressed (repetition time msec/TE msec, 2300/34), T1-weighted (700/10), and UTE (300/0.008, 6.6, with or without dual-inversion preparations at inversion time 1 = 135 msec and inversion time 2 = 95 msec) sequences. The UTE images were compared with proton density-weighted fat-suppressed and T1-weighted images and were evaluated by two radiologists. To identify the sources of signal on the UTE images, samples including specific combinations of tissues (uncalcified cartilage [UCC] only, calcified cartilage [CC] and subchondral bone [bone] [CC/bone], bone only; and UCC, CC, and bone [UCC/CC/bone]) were prepared and imaged by using the UTE sequence.

RESULTS

On the UTE MR images, all patellar sections exhibited a high-intensity linear signal near the osteochondral junction, which was not visible on protein density-weighted fat-suppressed or T1-weighted images. In some sections, focal regions of thickened or diminished signal intensity were also found. In the prepared samples, UCC only, CC/bone, and UCC/CC/bone samples exhibited high signal intensity on the UTE images, whereas bone-only samples did not.

CONCLUSION

These results show that the high signal intensity on UTE images of human articular joints originates from the CC and the deepest layer of the UCC, without a definite contribution from subchondral bone. UTE sequences may provide a way of evaluating abnormalities at or near the osteochondral junction. (c) RSNA, 2010.

Comparative rheumatology: what can be learnt from naturally occurring musculoskeletal disorders in domestic animals?

Examples of naturally occurring musculoskeletal disorders are extremely common in veterinary species and provide a valuable comparative research resource, which can provide compelling comparative data on the aetiopathogenesis and treatment of many common human musculoskeletal diseases. In particular, orthopaedic diseases are a common morbidity in both dogs and horses. In this review, we give an overview of the common musculoskeletal diseases encountered in these species: for instance, tendon and ligament injuries, arthropathies and stress fractures, as well as an insight into the basic biology of these conditions. In doing so, we aim to demonstrate the similarities and differences between these disorders and similar conditions in man.