脊柱关节病患者骶髂关节细针活检的病理表现及其临床意义

目的了解骶髂关节(SIJ)炎的病理表现,提高强直性脊柱炎(AS)的早期诊断水平。方法对96例脊柱关节病(SpA)患者的SIJ进行CT导引下的细针活检,3例非SpA死亡病例为对照组。记录临床资料进行分析。结果76例SpA的SIJ存在炎症表现,包括骨髓炎、血管翳形成和炎性细胞浸润,软骨下骨板破坏,软骨变性、破坏,滑膜炎,附着点炎等。其中45例0～Ⅰ级CT下SIJ炎均存在骨髓炎、软骨下骨板炎、软骨变性等改变;0级CT下SIJ炎的滑膜无病理改变,而部分Ⅰ级和所有≥Ⅱ级SIJ炎滑膜可见炎症;软骨破坏和骨化发生率以0～Ⅰ级最低,且仅见于软骨下骨板侧,关节面侧未见破坏;骨质硬化也是0～Ⅰ级最少见;附着点炎仅见于部分≥Ⅱ级SIJ炎;炎性细胞浸润程度Ⅳ级最低。45例0～Ⅰ级CT下SIJ炎的SpA经病理检查诊断为AS;其平均病程显著短于≥Ⅱ级者。结论0级CT下SIJ炎时SIJ已可有炎症存在。SIJ炎可能自骨髓开始,继而血管翳形成,软骨下骨板破坏,软骨变性、破坏,最后纤维化、骨化而导致关节融合。滑膜炎和附着点炎不是SIJ炎的最早改变。病理检查有利于AS的早期诊断和鉴别诊断。     

骶髂关节病变病理学与影像学诊断的相关性

目的通过了解骶髂关节炎病理特点,并以病理结果为标准,探讨病理表现与放射性核素单光子发射计算机断层成像术(single-photon emission computed tomography,SPECT)、磁共振显像(magnetic resonance imaging,MRI)、计算机断层扫描(computed tomography,CT)、X线等影像学检查的关系。方法对中轴型脊柱关节炎患者进行CT引导下骶髂关节(sacroiliac joint,SIJ)穿刺,所得组织进行病理检查,并同时行SIJ的SPECT、MRI、CT和X线检查,按组织病理结果分为SIJ炎组和无炎性反应组。结果 36例患者获得组织有软骨、软骨下骨板、关节滑膜、骨髓、肌腱或韧带附着点等。获得率以软骨为最高,继以软骨下骨板、关节滑膜、骨髓和韧带附着点,各占92%、83%、75%、72%、22%;其中28例(2836,78%)患者SIJ炎病理变化包括软骨(93%)和软骨下骨板改变(75%)、滑膜炎(64%)、骨髓炎(46%)、附着点炎(32%),8例(22%)未发现有SIJ炎改变。MRI示29例(81%)患者共56个SIJ可见不同程度的异常信号改变,软骨线改变、软骨下骨板改变、骨髓水肿、脂肪沉积、骨质硬化分别为82%、71%、65%、59%、76%;SPECT示29例(81%)共52个SIJ可见骶髂关节感兴趣区的放射性比值有不同程度的升高,23例(64%)CT阳性,19例(53%)X线阳性。以病理结果为标准,SPECT、MRI、CT、X线的阳性预测值分别为89.6%、93.1%、95.6%、94.7%,敏感性分别为92.8%、96.4%、73.3%、64.2%,特异性分别62.5%、75.0%、87.5%、87.5%。4种诊断方法的受试者工作特征曲线下面积分别为0.79、0.84、0.83、0.76。结论病理检查是诊断Sl J炎和活动性炎性反应的主要方法。MRI和SPECT的相互补充,能提高骶髂关节活动性炎症的敏感性和特异性。MRI和SPECT的诊断价值与CT和X线相当,并可通过对炎症的活动度进行量化,有利于随访和疗效的评价,是诊断SIJ炎重要的辅助手段。     

Methotrexate in the treatment of ankylosing spondylitis

There have been few well-conducted studies into the efficacy of methotrexate in Ankylosing spondylitis. The results of a new prospective study in 51 patients are presented in this issue but the clinical response was poor. A recurring theme, however, is the promising effect noted on peripheral joints compared with that on the axial skeleton. Recent histological and magnetic resonance imaging evidence suggests that synovitis and subchondral bone marrow changes offer a more rational explanation for widespread joint destruction than does enthesitis alone. Furthermore, enthesis lesions close to synovial joints occur frequently and may be intimately linked with peripheral joint synovitis. At the moment there is no hard evidence of efficacy in axial disease, but these observations raise the possibility that suppression of synovitis might help in the spine, and that enthesitis might respond wherever it is anatomically. Thus further long-term, placebo-controlled studies are needed to address specifically the issues of enthesitis. spinal symptom relief and the suppression of long-term ankylosis.     

Evaluation of acute and chronic MRI features of sacroiliitis in asymptomatic primary hyperparathyroid patients

Asymptomatic primary hyperparathyroidism (PHPT) is characterized with autonomous overproduction of parathyroid hormone without signs or symptoms associated with hyperparathyroidism. Before symptoms become obvious, PHPT may affect structures like sacroiliac joints, which consist of bone. So, in the asymptomatic PHPT patients, structural and inflammatory changes in sacroiliac joints may lead to confusion during diagnosis workup of axial spondyloarthropathy. In this study, we evaluated active and chronic sacroiliac magnetic resonance imaging (MRI) changes relevant to sacroiliitis in the patients with asymptomatic PHPT and interpreted bone marrow edema within the scope of Assessment of SpondyloArthritis International Society–Outcome Measures in Rheumatology Clinical Trials (ASAS-OMERACT) criteria. Forty-nine patients with asymptomatic PHPT, 26 patients with newly diagnosed axial spondyloarthropathy (SpA), and 37 healthy controls were enrolled. All subjects were evaluated by sacroiliac MRI for four active (bone marrow edema, enthesitis, capsulitis, and synovitis) and four chronic (subchondral sclerosis, subchondral/periarticular erosions, periarticular fat deposition, and bony bridges/ankylosis) lesions relevant to sacroiliitis. Bone marrow edema compatible with ASAS-OMERACT active sacroiliitis criteria in sacroiliac MRI was fulfilled by 16.3 % (8/49) of the asymptomatic PHPT patients which was similar with controls but statistically lower than axial SpA. Moreover, asymptomatic PHPT patients and controls were similar for other chronic or active MRI findings. Also, we detected lower frequency of all other MRI findings, except enthesis, in asymptomatic PHPT patients according to axial SpA. Acute inflammatory including bone marrow edema fulfilling ASAS-OMERACT active sacroiliitis criteria and chronic structural sacroiliac lesions relevant to sacroiliitis in MRI were detected in asymptomatic PHPT similar frequency with controls but as expected, lower than axial SpA. But, these findings could not be attributed to excessive secretion of parathyroid hormone.     

Hereditary joint disorder in progressive ankylosis (ank/ank) mice. I. Association of calcium hydroxyapatite deposition with inflammatory arthropathy

Progressive ankylosis (ank), a murine autosomal recessive mutation, produces an inflammatory joint disorder associated with intraarticular calcium hydroxyapatite deposition and culminates in fusion of the joints. Joints in the feet become stiffened and swollen with milky white fluid containing polymorphonuclear leukocytes, large mononuclear macrophage-like cells, and calcium hydroxyapatite. The joints develop a proliferative synovitis, sometimes associated with marginal erosions of the articular cartilage and periosteal bone. Subsequently, cartilaginous osteophytes, extending outward from the subchondral bone, bridge the margins of the joint and undergo ossification. The progressive ankylosis mutation provides a useful system for investigating calcium hydroxyapatite-associated arthropathies.     

Immunohistochemical analysis of hip arthritis in ankylosing spondylitis: evaluation of the bone-cartilage interface and subchondral bone marrow

OBJECTIVE: Previous histopathologic and magnetic resonance imaging studies suggest that the subchondral bone marrow might be the primary site of inflammation in patients with ankylosing spondylitis (AS) and that this might be reflected by inflammation found in hip joints. The aim of this study was to conduct an immunohistologic assessment of the bone-cartilage interface and subchondral bone marrow in AS patients with hip arthritis. METHODS: We collected femoral heads from patients with AS, osteoarthritis (OA), and rheumatoid arthritis (RA) who were undergoing hip replacement. The subchondral bone marrow and bone-cartilage interface were assessed immunohistochemically by evaluating infiltrating T cells, microvessel density, and osteoclasts. Areas of the femoral head surface with and without cartilage were assessed separately. RESULTS: At sites with surface cartilage, we found subchondral infiltration of CD3+ T cell aggregates at significantly higher numbers in AS patients as compared with OA patients, but not RA patients. At sites of complete cartilage destruction, the frequency of CD3+ T cell aggregates was significantly reduced as compared with sites with cartilage on the surface in AS patients, but not in RA patients. Similar differences were found for CD4+ and CD8+ T cells. Only at sites with surface cartilage, but not those without, angiogenesis and osteoclastic foci in the subchondral bone marrow in AS patients were significantly increased as compared with RA patients and with OA patients. CONCLUSION: These findings suggest that the subchondral bone marrow and bone-cartilage interface are primary sites of inflammation in AS and that cartilage might be necessary for the induction of inflammation.     

Partial chondroprotective effect of zoledronate in a rabbit model of inflammatory arthritis.

OBJECTIVE: To test the hypothesis that bisphosphonate treatment has a chondroprotective effect in the carrageenan model of inflammatory arthritis (IA). METHODS: Experimental IA was induced in rabbits by intraarticular injections of carrageenan. One group also received subcutaneous injections of zoledronate, a new bisphosphonate. After 4 weeks, the joints were harvested. Articular cartilage degradation and the degree of synovitis were assessed by light microscope using qualitative grading scores. Articular cartilage and subchondral and cancellous bone were evaluated histomorphometrically. Bone microhardness was measured. RESULTS: Carrageenan injected knees showed changes of inflammatory arthritis with cartilage erosion. Zoledronate treatment partially protected the articular cartilage from degradation. This effect was unlikely due to an antiinflammatory effect of the drug as the carrageenan induced synovitis was unaffected by zoledronate treatment. The treatment preserved subchondral bone thickness and cancellous bone volume and prevented focal breaks in the osteochondral barrier. The subchondral bone hardness was also maintained. CONCLUSION: Zoledronate had a partial effect in a rabbit model of inflammatory arthritis. The chondroprotection may be due to the prevention of bone resorption. By maintaining an intact subchondral bone, normal joint loading may have been maintained and contact between the bone marrow and the articular cartilage averted.     

Animal models of ankylosing spondylitis

The pathology of ankylosing spondylitis (AS) and related spondyloarthropathies (SpA) characteristically involve a sacroiliitis and inflammation of the intervertebral discs (IVD) in the lumbar spine, and an enthesitis at sites of ligamentous insertions into bone. The proteoglycans aggrecan and versican are large molecules that aggregate with hyaluronic via a globular 1 domain. These domains share significant homology at the level of B and T cell epitope recognition. Both proteoglycans are present in the intervertebral disc and hyaline cartilages of the sacroiliac joint, as well as in entheses. Whereas aggrecan is most concentrated in the nucleus of the IVD and in articular cartilages and endplates, versican is generally absent from these tissues except in the sacroiliac joint, but is concentrated in ligaments and the annulus. Immunity to these molecules in BALB/c mice results in an AS-like pathology, including sacroiliitis, enthesitis, and discitis. The pathology of AS is closely associated with the expression of the class I molecule human leukocyte antigen-B27. Rats bearing this transgene develop an AS-like pathology, as well as other various signs of autoimmunity. Ankylosing spondylitis is characterized by an ankylosing pathology whereby bone formation in the annulus leads to intervertebral fusion. Mice bearing the ank/ank defect gene develop a bony ankylosis of the spine like that seen in advanced AS and related SpA. These three animal models provide insight into the pathogenesis of SpA, and opportunities to investigate their pathology in relationship to human disease where investigation of the pathobiology is very difficult, because of restricted access to involved tissues.     

The role of cartilage minor collagens in inducing arthritis

Arthritogenic properties of native types IX, XI, and II collagen were investigated in female Wistar rats. Immunization with native type-XI or -II collagen led to the arthritic reaction in 60% of investigated rats. After boosting, a distinct increase in anticollagen antibodies was observed followed by temperature rise (swelling and redness of affected joints). At the end of the experiment (after 7 weeks) subchondral bone destruction was detectable upon x-ray examination. Histological observation of the affected knee and ankle joints showed progressive destruction of the articular cartilage and subchondral bone accompanied by proliferative synovitis with extensive formation of fibrous tissue found in joints of rats immunized with native type-XI collagen. Immune response to collagen types (determined by ELISA) reached its peak between the 14th and the 21st day. From the acute stage to the chronic stage a decrease of serum anticollagen antibodies was observed to remain constant. Rats immunized with native type IX and denatured type XI collagen did not develop arthritis.     

强直性脊柱炎骶髂关节内皮质激素注射疗效观察

目的了解强直性脊柱炎(AS)骶髂关节(SIJ)内皮质激素注射的疗效.方法患者取俯卧位,在CT导引下穿刺SIJ滑膜部,取组织标本后,在CT导引下原路进入SIJ,每侧注入丙酮缩去炎舒松15mg.记录分析病人术前、术后临床资料.结果28例中22例(78.6%)疼痛明显减轻,晨僵、夜间痛、SIJ压痛、"4”征、ESR升高及需使用NSAIDs的例数均较术前明显减少.所有SIJ标本均有炎症表现.结论SIJ关节内注射长效激素,可作为对NSAIDs不耐受或无效的AS和uSpA病人的一种有效的缓解腰背痛手段,同时uSpA病人SIJ病理标本的获得,有助于早期诊断.     

Immunohistochemical analysis of hip arthritis in ankylosing spondylitis: Evaluation of the bone–cartilage interface and subchondral bone marrow

Objective

Previous histopathologic and magnetic resonance imaging studies suggest that the subchondral bone marrow might be the primary site of inflammation in patients with ankylosing spondylitis (AS) and that this might be reflected by inflammation found in hip joints. The aim of this study was to conduct an immunohistologic assessment of the bone–cartilage interface and subchondral bone marrow in AS patients with hip arthritis.

Methods

We collected femoral heads from patients with AS, osteoarthritis (OA), and rheumatoid arthritis (RA) who were undergoing hip replacement. The subchondral bone marrow and bone–cartilage interface were assessed immunohistochemically by evaluating infiltrating T cells, microvessel density, and osteoclasts. Areas of the femoral head surface with and without cartilage were assessed separately.

Results

At sites with surface cartilage, we found subchondral infiltration of CD3+ T cell aggregates at significantly higher numbers in AS patients as compared with OA patients, but not RA patients. At sites of complete cartilage destruction, the frequency of CD3+ T cell aggregates was significantly reduced as compared with sites with cartilage on the surface in AS patients, but not in RA patients. Similar differences were found for CD4+ and CD8+ T cells. Only at sites with surface cartilage, but not those without, angiogenesis and osteoclastic foci in the subchondral bone marrow in AS patients were significantly increased as compared with RA patients and with OA patients.

Conclusion

These findings suggest that the subchondral bone marrow and bone–cartilage interface are primary sites of inflammation in AS and that cartilage might be necessary for the induction of inflammation.

     

Immunpathologie bei der ankylosierenden Spondylitis und anderen Spondyloarthritiden

Histomorphological analysis shows at least two different patterns in patients with ankylosing spondylitis. Bone marrow edema, lymphocytic infiltrates, increased osteoclast density and increased microvessel density are typical findings in acute inflammation. In areas of new bone formation newly formed trabecular bone shows formation of fibrous tissue in the bone marrow with new cartilage formation and persistently high microvessel density. Morphological changes reminiscent of endochondral ossification can also be observed. Compared to peripheral joints, such as the knee, synovitis is not a predominant finding in the spine, the sacroiliac joints and the hip. Enthesitis is characterised by lymphocytic infiltrates around fibrous cartilage followed by subsequent ankylosis. The molecular mechanisms which promote the transition from inflammation to new bone formation in patients with ankylosing spondylitis are not well understood.     

Vascular mechanisms in osteoarthritis

Superficial injury and fibrillation of articular cartilage as a consequence of ageing, genetic, hormonal or mechanical factors are not necessarily associated with joint pain. However, failure of joint cartilage accompanied by synovitis and abnormalities in subchondral bone and its vasculature generally is, the syndrome being known as osteoarthritis. We suggest that the progression of early cartilage fibrillation to symptomatic OA arises initially as a consequence of the release into synovial fluid of cartilage-derived antigens that activate joint lining macrophages and circulating leukocytes, thereby establishing a synovitis. Pro-inflammatory mediators and pro-coagulant factors etc. not only perpetuate cartilage destruction but also promote a state of hypercoagulation, hypofibrinolysis, thrombosis and ischaemic bone necrosis at compromised sites such as in the subchondral vasculature. These events are augmented by ageing and associated hormonal changes. On the basis of this hypothesis we suggest that anti-thrombotic/anti-lipidaemic agents that also exhibit anti-inflammatory activity could be effective anti-osteoarthritic drugs. Experimental studies are described which support this proposal.     

A type I collagen defect leads to rapidly progressive osteoarthritis in a mouse model

OBJECTIVE: This study was undertaken to test the hypothesis that abnormalities of the subchondral bone can result in osteoarthritis (OA). METHODS: We used a knockin model of human osteogenesis imperfecta, the Brittle IV (Brtl) mouse, in which defective type I collagen is expressed in bone. OA in individual mice was documented by micro-magnetic resonance imaging (micro-MRI) and micro-computed tomography (micro-CT). Alterations in the knee joints were confirmed by histopathologic and immunohistochemical analysis. In addition, atomic force microscopy (AFM) was used to assess the ultrastructure of the articular cartilage and subchondral bone matrix. RESULTS: Brtl mice had decreased integrity of bone but initially normal articular cartilage. However, by the second month of life, Brtl mice developed alterations of the cartilage that were characteristic of OA, as documented by micro-CT, micro-MRI, and histologic evaluation. In addition, chondrocyte loss and breakdown of the collagen matrix in the residual cartilage were demonstrated using AFM. CONCLUSION: The Brtl mouse model demonstrates that progressive destruction of articular cartilage characteristic of OA may be secondary to altered architecture of the underlying subchondral bone.     

Antigen-induced experimental septic arthritis in rabbits after intraarticular injection of Staphylococcus aureus

With the Dumonde-Glynn model of antigen-induced arthritis, a rabbit model was developed to examine the histopathologic differences between normal and arthritic joints in the same animal infected by intraarticular injections of Staphylococcus aureus. Microscopic examination of whole joint sections and a quantitative histopathologic scale were used to compare changes in all the articular components of 17 normal and 17 arthritic joints infected for less than two weeks. The histological changes were more severe in infected arthritic joints than in infected normal joints (mean +/- SD total histology score, 13.8 +/- 2.4 and 9.3 +/- 4.0, respectively; P less than .001). In infected arthritic joints, subsynovial abscesses extended into subchondral bone via the pannus of chronic synovitis at articular margins and intraarticular attachments of cruciate ligaments, rather than by initial cartilage destruction and direct extension into subchondral bone.     

ENDOTHELIAL CELL STIMULATING ANGIOGENESIS FACTOR--A NEW BIOLOGICAL MARKER FOR DISEASE ACTIVITY IN ANKYLOSING SPONDYLITIS?

Endothelial cell stimulating angiogenesis factor (ESAF) is raisedin conditions of neovascularization, such as may occur at theendochondral surface of the bone growth plate or in associationwith syndesmophyte formation. Syndesmophytes are formed in responseto chronic enthesitis. Twenty unselected patients with AS werestudied and enthesitis was measured clinically using a previouslyvalidated enthesitis index. Biochemical and haematological markersof disease activity were measured along with ESAF. Radiographswere scored for the degree of sacroiliitis and syndesmophyteformation and all these measurements were correlated to theenthesitis index. ESAF was raised in AS compared with normalcontrols. ESAF correlated positively with the enthesitis indexbut not with any of the biochemical or haematological indices.Higher levels of ESAF were found in those with more advancedsacroiliitis. High ESAF activity may be reflecting the angiogenicprocess involved in new bone formation associated with sacroiliitisand syndesmophyte formation. ESAF may be a biochemical markerfor disease activity in AS. KEY WORDS: Ankylosing spondylitis, Angiogenesis, Enthesitis     

Pathophysiologic aspects of inflammatory and degenerative joint diseases

Authors describe the physiological particularities of components of articular system (cartilage, synovialis, synovia, subchondral bone, ligamentous and neuromuscular apparatus). The essential resultant pathophysiological pattern in rheumatoid arthritis and osteoarthritis are demonstrated. Differences between temporary attendant synovitis in osteoarthritis and rheumatoid arthritis are pointed out.     

Entheses and enthesitis: a histopathologic review and relevance to spondyloarthritides

There are two types of entheses: fibrous, by Sharpey's fibers in membranous bone, and fibrocartilaginous, on endochondral bone, with discontinuous cement lines at the interface between insertion and bone. The connection of hyaline cartilage to subchondral bone is a kind of fibrocartilaginous enthesis. Fibrocartilages are structurally, chemically, and biomechanically intermediate between tendon and cartilage. Enthesitis is not the sole pathologic feature of spondyloarthritides. Synovitis and subchondral bone marrow changes outside the ligamentous insertions, and cartilage proliferation, are important too. In the subentheseal bone marrow and in the synovium, CD8+ T cells play a central role. Imaging of early changes is better achieved by ultrasonography and even better by magnetic resonance imaging than by radiography. No single immunologic target can be identified. The G1 domain of aggrecan is the best candidate, but this does not apply to fibrous entheses. In these complex pathologic conditions, no single abnormality can thus far be designated as a unique hallmark.     

Imaging of psoriatic arthritis

Imaging of psoriatic arthritis (PsA) is important for two reasons: the differential diagnosis from other arthritides and the assessment of structural damage that can be inhibited by the new drugs such as the anti-TNFalpha agents. Plain film radiographic findings of peripheral arthritis have been important in elaborating the concept of PsA as a separate disease entity. Characteristic aspects of psoriatic peripheral arthritis help the differentiation from rheumatoid arthritis. High-resolution ultrasonography (US), US combined with power Doppler (PDUS) and magnetic resonance imaging (MRI) can be used to image joint synovitis of PsA. Radiologic features of spondylitis associated with psoriasis are similar to spondylitis associated with reactive arthritis and differ from those of primary ankylosing spondylitis (AS) and the spondylitis associated with inflammatory bowel disease. MRI is very sensitive for the early diagnosis of sacroiliitis. There have been no MRI studies on the spine of patients with PsA. In primary AS bone oedema in the vertebral bodies is an indicator of active disease and can ameliorate during anti-TNFalpha therapy. Historically, plain film radiography have played a pivotal role in defining enthesitis lesions of SpA. However, entheseal bone changes appear late. US and MRI have proved to be a highly sensitive and non invasive tools. Recent US and MRI studies on both finger and toe dactylitis have established that dactylitis is due to flexor tenosynovitis and marked adjacent soft tissue swelling with a variable degree of small joint synovitis. There is no evidence of enthesitis of the insertion of the flexor digitorum tendons and of the attachment of the capsule of the digit joints.     

Nitrite and nitrotyrosine concentrations in articular cartilage, subchondral bone, and trabecular bone of normal juvenile, normal adult, and osteoarthritic adult equine metacarpophalangeal joints

OBJECTIVE: Osteoarthritis (OA) is a chronic debilitating joint disorder in which the importance of inflammation is increasingly recognized. In advanced cases, both the articular cartilage and the underlying bony layers are affected, but the exact sequence of events and their localization in the initial phase of pathogenesis remain uncertain. We measured nitric oxide (NO) end products in tissue layers that constitute the bearing surface of the joint, as possible indicators of physiological and pathological processes. METHODS: Nitrite as a measure for NO and nitrotyrosine was measured in articular cartilage, subchondral bone, and the underlying trabecular bone of the proximal articular surface of the first phalanx of healthy mature horses (n = 15; age range 5-18 yrs), mature horses affected by OA (n = 15; age range 8-22 yrs), and unaffected juvenile horses (n = 13; age range 6 months-4 yrs). Data were correlated with cartilage damage, as quantified by the Cartilage Degeneration Index. RESULTS: In all 3 layers the nitrite concentration was higher in OA joints (cartilage, p < 0.001; subchondral and trabecular bone, p < 0.05). The concentration of nitrite was significantly higher in cartilage and subchondral bone of juvenile horses compared with mature horses (p < 0.001). Nitrotyrosine concentrations were significantly higher in subchondral bone of OA horses compared with healthy controls (p < 0.001), but significantly lower in trabecular bone of juvenile horses (p < 0.01). CONCLUSION: The similarities observed over the 3 tissue layers support the concept of the bearing surface of the joint as a functional entity. Nitrite concentration seems to be a good indicator of tissue metabolic activity, but cannot discriminate between physiological (juvenile animals) and pathological (OA cases) processes. The increased nitrotyrosine levels in subchondral bone of OA-affected animals suggest that this layer is important in early or moderate OA, and implies a role of oxidative stress in the development of the disease.