Bone-cartilage crosstalk: a conversation for understanding osteoarthritis

Although cartilage degradation is the characteristic feature of osteoarthritis (OA), it is now recognized that the whole joint is involved in the progression of OA. In particular, the interaction (crosstalk) between cartilage and subchondral bone is thought to be a central feature of this process. The interface between articular cartilage and bone of articulating long bones is a unique zone, which comprises articular cartilage, below which is the calcified cartilage sitting on and intercalated into the subchondral bone plate. Below the subchondral plate is the trabecular bone at the end of the respective long bones. In OA, there are well-described progressive destructive changes in the articular cartilage, which parallel characteristic changes in the underlying bone. This review examines the evidence that biochemical and biomechanical signaling between these tissue compartments is important in OA disease progression and asks whether such signaling might provide possibilities for therapeutic intervention to halt or slow disease development.David M Findlay and Julia S Kuliwab

Raman microspectroscopy demonstrates reduced mineralization of subchondral bone marrow lesions in knee osteoarthritis patients

Introduction: Bone marrow lesions (BMLs) are frequently identified by MRI in the subchondral bone in knee osteoarthritis (KOA). BMLs are known to be closely associated with joint pain, loss of the cartilage and structural changes in the subchondral trabecular bone (SCTB). Despite this, understanding of the nature of BMLs at the trabecular tissue level is incomplete. Thus, we used Raman microspectroscopy to examine the biochemical properties of SCTB from KOA patients with presence or absence of BMLs (OA-BML, OA No-BML; respectively), in comparison with age-matched cadaveric non-symptomatic controls (Non-OA CTL). Methods: Tibial plateau (TP) specimens were collected from 19 KOA arthroplasty patients (6-Male, 13-Female; aged 56–74 years). BMLs were identified ex-vivo by MRI, using PDFS- and T1-weighted sequences. The KOA specimens were then categorized into an OA-BML group (n = 12; containing a BML within the medial condyle only) and an OA No-BML group (n = 7; with no BMLs identified in the TP). The control (CTL) group consisted of Non-OA cadaveric TP samples with no BMLs and no macroscopic or microscopic evidence of OA-related changes (n = 8; 5-Male, 3-Female; aged 44–80 years). Confocal Raman microspectroscopy, with high spatial resolution, was used to quantify the biochemical properties of SCTB tissue of both the medial and the lateral condyle in each group. Results: The ratios of peak intensity and integrated area of bone matrix mineral (Phosphate (v1), Phosphate (v2) and Phosphate (v4)), to surrogates of the organic phase of bone matrix (Amide I, Proline and Amide III), were calculated. Within the medial compartment, the mineral:organic matrix ratios were significantly lower for OABML, compared to Non-OA CTL. These ratios were also significantly lower for the OA-BML medial compartment, compared to the OA-BML lateral compartment. There were no group or compartmental differences for Carbonate:Phosphate (v1, v2 and v4), Amide III (α-helix):Amide III (random-coil), Hydroxyproline:Proline, or Crystallinity. Conclusion: As measured by Raman microspectroscopy, SCTB tissue in BML zones in KOA is significantly less mineralized than the corresponding zones in individuals without OA. These data are consistent with those obtained using other methods (e.g. Fourier transform infrared spectroscopy; FTIR) and with the increased rate of bone remodeling observed in BML zones. Reduced mineralization may change the biomechanical properties of the trabecular bone in BMLs and the mechanical interaction between subchondral bone and its overlying cartilage, with potential implications for the development and progression of OA.Yea-Rin Lee, David M. Findlay, Dzenita Muratovic, Tiffany K. Gill, Julia S. Kuliwab

Bone marrow lesions detected by specific combination of MRI sequences are associated with severity of osteochondral degeneration

Background: Bone marrow lesions (BMLs) are useful diagnostic and prognostic markers in knee osteoarthritis (OA), but what they represent at the tissue level remains unclear. The aim of this study was to provide comprehensive tissue characterization of BMLs detected using two specific MRI sequences. Methods: Tibial plateaus were obtained from 60 patients (29 females, 31 males), undergoing knee arthroplasty for OA. To identify BMLs, MRI was performed ex vivo using T1 and PDFS-weighted sequences. Multi-modal tissue level analyses of the osteochondral unit (OCU) were performed, including cartilage volume measurement, OARSI grading, micro-CT analysis of bone microstructure, routine histopathological assessment and quantitation of bone turnover indices.Results: BMLs were detected in 74 % of tibial plateaus, the remainder comprising a No BML group. Of all BMLs, 59 % were designated BML 1 (detected only by PDFS) and 41 % were designated BML 2 (detected by both PDFS + T1). The presence of a BML was related to degeneration of the OCU, particularly within BML 2. When compared to No BML, BML 2 showed reduced cartilage volume (p = 0.008), higher OARSI scores (p = 0.004), thicker subchondral plate (p = 0.002), increased trabecular bone volume and plate-like structure (p = 0.0004), increased osteoid volume (p = 0.002) and thickness (p = 0.003), more bone marrow oedema (p = 0.03), fibrosis (p = 0.002), necrosis (p = 0.01) and fibrovascular cysts (p = 0.04). For most measures, BML 1 was intermediate between No BML and BML 2. Conclusions: BMLs detected by specific MRI sequences identify different degrees of degeneration in the OCU. This suggests that MRI characteristics of BMLs may enable identification of different BML phenotypes and help target novel approaches to treatment and prevention of OA.Dzenita Muratovic, Flavia Cicuttini, Anita Wluka, David Findlay, Yuanyuan Wang, Sophia Otto, David Taylor, Julia Humphries, Yearin Lee, Agatha Labrinidis, Ruth Williams and Julia Kuliwab

Trabecular bone modeling and subcapital femoral fracture

Fragility fractures, including neck of femur fractures, result from reductions in the amount, quality and architecture of bone. The aim of this study was to compare the cancellous bone structure, and static indices of bone turnover, in female patients who had sustained fragility fracture at the femoral neck, with age-matched females without fragility fracture. Bone samples were taken from the intertrochanteric region of the proximal femur of female patients undergoing hip arthroplasty surgery for a subcapital fragility fracture of the femoral neck (#NOF) or from age-matched female control individuals at routine autopsy. The histomorphometric data, which were normally distributed, indicated no difference between the mean values for any of the structural parameters in control and fracture samples. In particular, the BV/TV values were not different and did not change significantly with age in these cohorts of individuals aged >65 years. The static indices of bone turnover, eroded surface (ES/BS) and osteoid surface (OS/BS), were positively correlated with age in the >65-year-old control group (p65-year-old controls compared with a group of younger females aged <65 years, suggesting an increase in bone formation in older females in the proximal femur after 65 years of age. When the data were further interrogated, a reduction in the percentage osteoid surface to eroded surface quotient (OS/ES) was found for the fracture group compared with the age-matched control group. These data indicate that perturbations in bone formation and/or resorption surface are potentially important in producing bone in the proximal femur with increased propensity to fracture. These data also support the concept that trabecular bone modeling may be a factor influencing bone strength in addition to bone mass.H. Tsangari, J.S. Kuliwaba, N.L. Fazzalar

A study of cortical bone microdamage and crack morphology utilising confocal microscopy and sequential labelling

The formation and accumulation of microdamage in bone plays an important role in the occurrence of stress and fragility fractures as well as in the initiation of bone remodelling. In this study a novel technique is presented for the investigation of bone microdamage and crack morphology using laser scanning confocal microscopy and sequential labelling with chelating fluorochromes. Compact tension fracture specimens machined from bovine tibial cortical bone, were mechanically tested in a wedge loaded crack-propagating tool. Sequential labelling with xylenol orange and calcein allowed for the crack propagation and microdamage progression to be assessed at each stage using confocal microscopy. Both twodimensional confocal images and three-dimensional z-series reconstructions displayed the formation of a microdamage process zone and wake surrounding the main crack. Further imaging demonstrated the significance of the bone microstructure, such as the vasculature and osteocytes, in the distribution of the microdamage.J. Codrington, J. Kuliwaba, K. Zarrinkalam and N. Fazzalarihttp://www.cp2009.unipr.it/http://www.gruppofrattura.it/index.php?option=com_jombib&task=showbib&id=105

Cancellous bone microdamage in the proximal femur: Influence of age and osteoarthritis on damage morphology and regional distribution

This study describes the in vivo distribution of cancellous bone microdamage in the proximal femur of an autopsy control sample. In addition, in vivo microdamage in the region medial to the greater trochanter of the proximal femur is compared between patients with severe osteoarthritis and controls. Taken at autopsy, the control group comprised normal right proximal femora that were then cut in the coronal plane with an Exakt saw (n = 12; aged 20-83 years). Cancellous bone samples were taken from the subchondral principal compressive region, the medial principal compressive region, and medial to the greater trochanter. A cancellous bone core biopsy was taken of the region medial to the greater trochanter (of the proximal femur) from patients with primary osteoarthritis undergoing total hip replacement surgery (n = 33; aged 37-85 years). Samples were embedded in resin, and in vivo microdamage identified in 70-microm-thick sections using the basic fuchsin en bloc staining technique. Microdamage was similar in all proximal femur sites in controls, except in the subchondral principal compressive region, where a significantly smaller crack length (microm) was identified (p < 0.05). In the region medial to the greater trochanter, osteoarthritic vs. control group comparisons showed that the crack density (#/mm(2)) and crack surface density (mm/mm(2)) were not significantly different, but crack length was significantly less (p < 0.03) and damage volume fraction was significantly increased for osteoarthritics (p < 0.005). The osteoarthritic and control data for crack density, and the osteoarthritic data for damage volume fraction, showed a nonlinear increase with age. Furthermore, crack length was not dependent on damage volume fraction or age for either the osteoarthritic or control group. This study identified differences in microdamage between osteoarthritic and autopsy control cases. We hypothesize that these results are consistent with the reported bone material property differences for osteoarthritis. In addition, the relatively uniform distribution of microdamage in the control group suggests that the principal components of the femoral cancellous bone network are equally exposed to deformations resulting in microdamage. Further study into the factors that influence the accumulation and skeletal distribution of microdamage is fundamental to understanding skeletal health

In situ fatty acid profile of femoral cancellous subchondral bone in osteoarthritic and fragility fracture females: Implications for bone remodelling

We report here differences in the fatty acid profile of cancellous bone matrix, including n-3, n-6, mono- and poly-unsaturated, as well as saturated fats, between femoral heads from female OA (n=8, aged 68-88years), fractured neck of femur (#NOF) (n=19, 67-88years) and autopsy controls (CTRL) (n=4, 85-97years). Femoral heads were collected from individuals undergoing orthopaedic surgery for OA or #NOF; the fatty acid profile of sub-samples from the superior principal compressive and superior principal tensile regions were determined by gas chromatography. A total of 42 individual fatty acids were detected at varying concentrations with significant differences between subchondral bone from OA subjects, subchondral bone from #NOF subjects and subchondral bone from CTRL subjects, as well as between the superior principal compressive and superior principal tensile regions (for saturated fats only). Subchondral bone from OA subjects had higher total n-6 (OA=10.89±3.17, #NOF=11.11±1.83, CTRL=8.32±2.05, p=0.008) and total n-3 (OA=1.34±0.38, #NOF=1.19±0.18, CTRL=1.15±0.48, p=0.011) percentages than subchondral bone from #NOF subjects and subchondral bone from CTRL subjects, and there was no difference in the n-6:n-3 ratio, nor within the percentage of n-9 fatty acids. Arachidonic acid (OA=0.42±0.16, #NOF=0.26±0.06, CTRL=0.28±0.06, p=0.01), and γ-linolenic acid (OA=0.11±0.03, #NOF=0.05±0.02, CTRL=0.04±0.02, p<0.001) were higher in subchondral bone from OA subjects than subchondral bone from #NOF subjects and subchondral bone from CTRL subjects. In conclusion, there is a wide diversity of fatty acids in cancellous bone matrix from the femoral heads of OA and #NOF, suggesting they may have regulatory effects on inflammatory processes, and their metabolites. This article is part of a Special Issue entitled "Osteoarthritis".J.M. Humphries, J.S. Kuliwaba, R.J. Gibson, N.L. Fazzalar

Integrity of the osteocyte bone cell network in osteoporotic fracture: implications for mechanical load adaptation

The human skeleton has the ability to modify its material composition and structure to accommodate loads through adaptive modelling and remodelling. The osteocyte cell network is now considered to be central to the regulation of skeletal homeostasis; however, very little is known of the integrity of the osteocyte cell network in osteoporotic fragility fracture. This study was designed to characterise osteocyte morphology, the extent of osteocyte cell apoptosis and expression of sclerostin protein (a negative regulator of bone formation) in trabecular bone from the intertrochanteric region of the proximal femur, for postmenopausal women with fragility hip fracture compared to age-matched women who had not sustained fragility fracture. Osteocyte morphology (osteocyte, empty lacunar, and total lacunar densities) and the degree of osteocyte apoptosis (percent caspase-3 positive osteocyte lacunae) were similar between the fracture patients and non-fracture women. The fragility hip fracture patients had a lower proportion of sclerostin-positive osteocyte lacunae in comparison to sclerostin-negative osteocyte lacunae, in contrast to similar percent sclerostin-positive/sclerostin-negative lacunae for non-fracture women. The unexpected finding of decreased sclerostin expression in trabecular bone osteocytes from fracture cases may be indicative of elevated bone turnover and under-mineralisation, characteristic of postmenopausal osteoporosis. Further, altered osteocytic expression of sclerostin may be involved in the mechano-responsiveness of bone. Optimal function of the osteocyte cell network is likely to be a critical determinant of bone strength, acting via mechanical load adaptation, and thus contributing to osteoporotic fracture risk.http://www.acam6.org/index.htm