Osteoarthritis – modern diagnostics and therapy
Author: Prof. dr Sandra Živanović, specialist in rheumatology
Osteoarthritis is a disease of the locomotor system belonging to one of the most common groups of rheumatic diseases – degenerative joint diseases. It almost never causes death, but its progressive nature causes severe symptoms (pain and restricted movement) in many people, interfering with everyday activities and even self-care. It typically affects the hand joints (Heberden’s and Bouchard’s nodes), the weight-bearing joints of the lower limbs (knees, hips and ankles), and the spine (disc arthrosis and spondylosis).
Osteoarthritis is a slow-progressive disease, which means that symptoms develop gradually over years. The basic pathological process occurs in the cartilage of the joint in which, at the beginning of the disease, there is focal deterioration and abrasion. After that, sclerosis and cystic formations occur below the surface of the bone, and osteophytes form on the joint surface. Thickening of the joint capsule and chronic synovitis usually occur later, so that is why the previously more frequently used name osteoarthrosis is in use. But, given the fact that this is an episodic, inflammatory disease of synovial joints, osteoarthritis is nevertheless a more appropriate name.
Epidemiology of osteoarthritis
With the extension of life expectancy, in the population of older people, the prevalence of osteoarthritis is increasing. Epidemiological studies have shown that in highly developed countries, in 50% of people over 40 years of age, artrotic changes can be demonstrated on the joints, but at that age they are predominantly clinically asymptomatic.
More than 80% of people over 56 years of age have radiographic signs of osteoarthritis. At age 65, prevalence is higher among women (68%) than men (58%).
Aetiology of osteoarthritis
Osteoarthritis is a multifactorial disease that covers above all systemic factors: age, sex, hormones, genetic and dietary factors. In addition to these, causes can also be secondary, such as previous damage to the inside of the joint, weakness of weight-bearing muscles, weakening of ligaments or varus or valgus deformations. Also, external factors that act on the joint can lead to the disease, such as: specific activities, excessive, irregular or insufficient physical activity and obesity.
The causes of biological aging of joint cartilage are constantly being investigated. Theoretically, it concerns poor quality of cartilage and repeated mechanical injuries (traumas). As causes of poor cartilage quality, endocrine reasons (dysfunction of ovaries, disorders in the function of the pituitary, thyroid and parathyroid glands), hereditary component and damage in blood supply (vascularization) of the joint are mentioned.
The question of heredity has been noticed for a long time, but has not yet been examined in detail. It is considered to be a less valuable collagen which is a consequence of changes in the amino acid structure itself. Delicate changes in the structure of type II collagen (change in the order of amino acids in the α-1 chain of the triple helix), genetically determined, change biomechanical properties - reduce the strength of collagen so that in joint cartilages containing this less valuable collagen, under the action of exogenous stimuli, an early dystrophic process occurs. In several female relatives in the same family, polyarticular osteoarthritis occurs, and the marker of this genetic predisposition is the A2B8 antigen in the HLA haplotype.
Pathogenesis of osteoarthritis
Osteoarthritis is defined as damage to and loss of articular cartilage, with proliferation of bone ends at the joint margins, without synovial inflammation, or with mild, secondary inflammation. Thus, the pathogenesis of osteoarthritis involves not only cartilage, but also bone and the synovium.
Microscopic changes in osteoarthritic cartilage
The process of progressive loss of joint cartilage can be divided, for didactic reasons, into three phases: the matrix damage phase, chondrocyte response to matrix lesion, and the phase of reduced chondrocyte response.
1. Matrix damage phase
The first changes of the matrix (fibrillation and/or chondromalacia) are detected by chondrocytes as a change in osmolarity, electric charge or as stress on the cell membrane transmitted via macromolecules of the matrix.
When chondrocytes detect a tissue lesion, they release mediators that stimulate a tissue response, such as nitric oxide. The production of pro-inflammatory cytokines – tumour necrosis factor alpha (TNFα) and interleukin 1 (IL-1) – is triggered, which induce the activity of proteolytic enzymes, metalloproteinases (stromelysin and collagenase enzymes), which further degrade matrix macromolecules. Enzymatic degradation of collagen types IX and XI occurs, which play a role in stabilising the network of dominant type II collagen, maintaining the connection between the collagen network and proteoglycans. In this way, the type II collagen network is destabilised.
Breaking of the collagen network allows expansion of aggrecan and increased water content, which marks entry into the irreversible phase of changes. Synthesis of abnormal matrix components is a sign of chondroblast transformation of chondrocyte metabolism. Dominant collagen becomes type I collagen. Collagen fibrils are thinner, the process of collagen fibrillogenesis is disrupted, the ability of proteoglycan aggregation into supramolecular aggrecan is reduced, and the ratio of keratan to chondroitin sulfate is disrupted to the detriment of chondroitin sulfate.
Oligomeric matrix protein of cartilage - COMP (Cartilage Oligomeric Matrix Protein) is a non-collagenous protein of the articular cartilage matrix. This protein is part of type II collagen, stimulates and regulates fibrillogenesis and stabilises the collagen network in cartilaginous tissue. It is released first upon breaking of the collagen network, resulting in deterioration of the cartilage.
In individuals with healthy cartilage, the level of human cartilage glycoprotein (YKL-40) is low, whereas under conditions of inflammation or extracellular matrix remodelling, there is a significant increase in the serum levels of this glycoprotein.
2. Chondrocyte response to matrix lesion
Enzymatic degradation processes also remove and clear damaged matrix components, and cytokines and growth factors are released from chondrocytes: transforming growth factor (TGFβ) and insulin-like growth factor 1 (IGF1) which antagonise the effects of pro-inflammatory cytokines, stimulate synthesis of matrix components and expression of tissue inhibitors of metalloproteinases (TIMP).
Modulatory cytokines (IL-4, 6, 10, 13) are powerful inhibitors of destructive cytokines and stimulators of IL-1 receptor antagonist (IL-1Ra) expression. Platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) have a mitogenic effect on chondrocytes, and their role in reparative processes in osteoarthritic cartilage is presumed. Many of the aforementioned factors have a synergistic effect on anabolic processes.
Clones of proliferated cells surrounded by newly created matrix are one of the signs of chondrocyte response to cartilage degeneration.
3. Phase of reduced chondrocyte response.
Focal chondrocyte necrosis is a frequent finding in osteoarthritic cartilage. An excess of degradative cytokines and a lack of growth factors can activate apoptosis genes and lead to programmed cell death. The number of apoptotic cells is several times higher in osteoarthritic cartilage than in healthy tissue (54% versus 10%).
The degree of increase in the synthesis of proteoglycans and collagen in various phases of osteoarthritis appears directly proportional to the severity of the joint cartilage damage.
Arthritogenic antigens resulting from the degradation of proteoglycans and/or collagen represent a condition for immune recognition and initiation of the immune cascade, starting from monocytes/macrophages, as antigen-presenting cells, up to the secretion of autoantibodies with the formation of immune complexes and activation of the complement cascade.
Macroscopic changes in osteoarthritic cartilage
Anatomically – osteoarthritic cartilage becomes dry, rough, with altered shine, accompanied by changes in histological structure. Mechanically – cartilage is less elastic, as biologically its ability to utilise necessary substances is reduced, i.e., respiration of cartilage is reduced.
As a complication of osteoarthritis, infarcts in the cartilage or in the tissue beneath the cartilage may occur, thus creating "holes" in the cartilage, often with a part of the cartilage or cartilage and bone, which is separated from its substrate and which moves around the joint causing additional discomfort. Over time, focal fibrillation of the cartilaginous surface occurs and formation of bony prominences (osteophytes). They arise due to vascular invasion of the calcified layer of cartilage from the bone marrow, after which ossification (bone formation) of the affected area occurs.
Changes in the synovial membrane
The synovium, or synovial membrane, lines the inner surface of the joint capsule. It is rich in blood and lymphatic vessels and nerve endings, with type III collagen being dominant.
The function of the synovial membrane is to enable movement in the joint by producing synovial fluid that moistens and smooths the cartilage surface, allowing it to use its elasticity to absorb pressure transmitted through the joint, up to 350 kg/cm².
Synovial fluid is viscous, colourless to yellowish, with a specific gravity of 1010 to 1012, alkaline, i.e., pH value is 8.2 to 8.4. Its role is mechanical (lubricates cartilage surfaces and enables easy movement in the joint) and nutritional (its constituents enable nutrition of the joint cartilage).
Synovial fluid is not a simple ultrafiltrate of plasma; it also contains products of secretion from synovial cells, among which hyaluronic acid is the most important for function, serving as the basis around which proteoglycans aggregate. Such aggregates are essential for maintaining the structural and functional integrity of the articular cartilage and are responsible for the specific visco-elastic properties of synovial fluid. Given that there are no blood vessels and that the fibrous plate between bone and cartilage is practically impermeable, cartilage is nourished by imbibition of nutrients from joint fluid. Therefore, it is sensitive to various metabolic and endocrine disorders.
Changes in the synovium can occur in the early phase of the disease, may progress progressively with increased chondropathy, and consequently pain. Inflammation is usually mild and insufficient to cause an increase in classical laboratory inflammation parameters, but it can be demonstrated by synovial markers which are indicators of synovial activity: COMP, YKL-40, hyaluronic acid and procollagen type III.
Scintigraphic and echosonographic findings show the presence of inflammation in 51% to as much as 79% of osteoarthritic joints.
Degradation of organic macromolecules and reactive activation of the synovial membrane via immunological and non-immunological pathways are consequences of an imbalanced cytokine network. Pro-inflammatory cytokines IL-1 and TNFα induce and stimulate inflammation. They stimulate their own production via autocrine action on cells that secrete them. Via paracrine action on metalloproteinases of synoviocytes, fibroblasts, chondrocytes and osteocytes, on adhesion molecules of post-capillary venules of the vascular endothelium, as well as on secretory immunocompetent cells (B lymphocytes and plasma cells), they activate several basic mechanisms of synovial destruction. Activation of membrane phospholipase A2 in various cells initiates both the cyclooxygenase and lipoxygenase pathways of arachidonic acid metabolism. The amount of vasoactive mediators of inflammation – prostaglandins, leukotrienes, oxygen radicals and nitric oxide – increases.
During inflammation, activated cells produce cytokines IL-4, IL-10, and TGFβ, which to some extent antagonise the effects of pro-inflammatory cytokines through different mechanisms, such as reducing the number of cellular receptors, stimulating IL-1 Ra expression, stimulating matrix component synthesis, inducing TIMP component expression, etc.
Due to long-term hyperactivity, hypertrophy and hyperplasia of the synovial membrane occur. Synovial proliferation contributes to further damage of the cartilage and subchondral bone on the principle of disturbed balance between pro-inflammatory and anti-inflammatory cytokines. Due to chronic inflammation and traumatic lesions (chronic bleeding proven in the form of hemosiderin pigment) with scar healing, all functions of the synovial membrane are reduced, including the function of cartilage nutrition, which leads to further progression of the degenerative process.
Changes in subchondral bone
Macroscopic changes in osteoarthritic cartilage, in the form of fibrillation, erosions and fissures, lead to areas where cartilage is absent. In exposed bone, osteoblasts proliferate and new bone forms. Examination of biochemical markers of subchondral bone has shown increased bone metabolism, with rises in the enzymatic activity of acid and alkaline phosphatases and in the level of osteocalcin in the synovial fluid of damaged joints. In areas of greatest load, focal necrosis of exposed subchondral bone occurs. Local tissue necrosis results in cystic changes. In late-stage osteoarthritis, the joint surfaces of the damaged joint may be covered by regenerated cartilage, which redistributes pressure more evenly in the joint; cysts often decrease or even disappear.
These cartilage and subchondral-bone changes lead to osteophyte formation. Osteophytes are fibrous, cartilaginous and bony formations that usually develop at the joint periphery (marginal osteophytes), along capsular attachments (capsular osteophytes), or may protrude from degeneratively altered joint surfaces (central osteophytes). In superficial joints, they can be palpable and sensitive; in all joints, they may reduce range of motion and contribute to pain on movement.
The volume of the osteoarthritic joint is significantly altered both due to focal loss of cartilage and subchondral bone and due to newly formed bone in the form of osteophytes via the mechanism of enchondral ossification. Thus altered joint shape leads further to disturbance of joint function and further degenerative process.
Clinical picture of osteoarthritis
Diagnostic recommendations for establishing a diagnosis of osteoarthritis from 2009 are: risk factors, clinical diagnosis and radiography.
Risk factors for knee osteoarthritis are: female sex, age, excessive body weight, joint injury, deformities, premature or improper use at work and recreation, genetic predisposition (family history), and Heberden's nodes on the distal interphalangeal joints (DIP) of the hand. Recommendations for the clinical diagnosis of OA are focused on:
three symptoms (pain on movement, short-term morning stiffness, and functional limitations) and three signs (crepitus, limited movements, and bony outgrowths). It was found that these three symptoms and signs can precisely identify 99% of patients with OA of the knee.
Joint pain, as the main clinical symptom of osteoarthritis, usually begins on the front or inner side of the knee, much less on the outer or back side, and later affects the whole knee. Initially, it occurs during and after use of the joint (while walking, especially on uneven terrain, up/down stairs, after prolonged activity), and subsides during non-use of the joint and rest, except in the presence of synovial inflammation in which the presence of pain may also occur at rest, during sleep. Intensity and duration of pain depend primarily on the degree of expression of pathological changes.
Fine and coarse crepitus, clicking and grating occur in the knees during movement. Limitation of movement, especially bending, occurs several years after the onset of the disease. Initially it is mild, and later it can be extremely serious with subsequent shortening of surrounding muscles in the form of flexion contractures, especially of the quadriceps muscle. The ability to move is limited, especially in the later phase when disorders in the axis of the knee also develop. Bony outgrowths – osteophytes most often first appear on the medial condyles of the tibia and femur (asymmetric narrowing of the joint space) and can lead to deformations of the joint bones in the form of valgus/varus (X or O) deformities of the knee. Swelling of the knee and accumulation of a larger amount of fluid in the knee occur in the later phase, and then pain at night and at rest is also present. Knee locking is rare and are usually a result of other damage to the joint cartilage or meniscus.
The main clinical signs of osteoarthritis are painful sensitivity of certain points on the edge of the joint or entheses near the joint, hard thickening of the ends of the bones at the joint, and sometimes enlargement of the soft tissue mass around the joint, crepitus during movement, effusion in the joint cavity with or without elevated joint temperature, limited and painful movements, and joint instability.
Joint innervation originates from branches of peripheral nerves that pass nearby, as well as from muscular branches that penetrate the capsule. Some joints, where surfaces lie close to the skin (the knee and ankle), also receive branches from cutaneous nerves; therefore, the innervation of each joint originates from several sensory roots. Most nerve fibres are thin myelinated fibres (type II) and unmyelinated fibres (type IV), which transmit nociceptive information. The activation threshold of nociceptive endings in the joint is considerably reduced during inflammatory processes. Type IV nerve fibres have been observed to release substance P and a calcitonin-like peptide at their endings through a simple axon reflex, maintaining inflammation. These substances act on inflammatory cells – macrophages, lymphocytes, mast cells and platelets.
Among the various tissues contributing to the source of pain, synovial tissue and subchondral bone, or both, may play an important role. Synovitis is common in painful osteoarthritic knees, and a greater volume of aspirated effusion from a painful knee predicts the progression of osteoarthritis, defined by the narrowing of the joint space.
Joint visualization methods
Methods for visualising a joint may include ultrasound examination, X-rays, magnetic resonance imaging, computed tomography, arthroscopy and arthrography.
1. Ultrasound examination of the joint
Ultrasound examination of joints (artrosonography) is recommended as the gold standard in rheumatology, as it is much more sensitive than clinical examination and provides reliable and rapid data. The examination is non-invasive and much more accessible for use in evaluating a large number of patients compared to other imaging modalities.
Ultrasound is very useful in diagnosing and monitoring joint effusion and synovitis, as well as in investigating cartilage. It has been proven that ultrasound is an indisputable instrument for assessing synovial disease, with results that are comparable to both MRI and arthroscopy.
Arthrosonography began to enter rheumatologists’ clinical practice gradually as far back as the 1970s. Initially, it was used only for the differential diagnosis of Baker’s cysts, and later to visualise cartilage surfaces with irregular and asymmetrical loss.
Arthrosonography is now also used successfully to diagnose traumatic lesions of the medial collateral ligament and the posterior and anterior cruciate ligaments, as well as in cases of patellar instability.
Before knee arthroplasty, determining the position of the popliteal artery is very useful and important; this favours arthrosonography as a method that can precisely depict all periarticular and intra-articular abnormalities.
In early osteoarthritis, artrosonography clearly shows synovitis.
It is recommended for detection and location of joint effusions and synovitis, tenosynovitis and enthesitis, as well as for differentiation of crystalline synovitis from osteoarthritis. Artrosonography can be used for detection of soft tissue abnormalities that cannot be detected using X-rays.
One of the advantages of ultrasound examination is visualisation during movement.
The EULAR report, part 1, from 2005 provides recommendations for arthrosonographic measurements and technique. Ultrasound defines synovial thickness and appearance, as well as the size of the effusion. The report concludes by recommending arthrosonography as the method of choice for recognising and defining synovial inflammation.
Power Doppler sonography is also used today for the detection of vascularisation in synovial tissue, as well as for monitoring the reduction of synovial perfusion after intra-articular corticosteroid injection.
Arthrosonography should become a routine and fundamental diagnostic method in rheumatology, helping with both diagnosis and treatment by guiding intra-articular injections.
The quality of an ultrasound examination always depends on the technical equipment, the patient’s individual tissue characteristics and the doctor’s experience. Standardised techniques and professional knowledge also ensure a high-quality examination. The availability, low cost and non-invasive nature of arthrosonography, its rapid and precise imaging of both normal and pathological soft-tissue structures, and the possibility of frequent repeat examinations are advantages that distinguish it from other imaging methods.
2. Radiological methods
Radiological methods, which, together with clinical signs, still form the basis for diagnosis in many practices, are insufficiently sensitive to depict disease progression. Based on the radiographic finding of the knee, osteoarthritis can be graded according to the Kellgren-Lawrence score from 1-4:
grade 0: no signs of osteoarthritis
grade 1: presence of short osteophytes
grade 2: presence of longer osteophytes, unchanged joint space
grade 3: presence of longer osteophytes, narrowed and altered joint space
grade 4: presence of longer osteophytes, asymmetrical, significantly narrowed joint space with subchondral bone sclerosis
Radiography allows measurement of changes in joint space width, but does not allow detection of early structural changes, nor is it a suitable method for monitoring the progression of osteoarthritis in everyday practice.
The discrepancy between radiographic findings in osteoarthritis and events in the painful knee in the general population has long been recognised and well documented.
Radiographic osteoarthritis can be shown in asymptomatic members of the general population, in contrast to the experienced painful joint associated with instability in classical disease models.
Despite the discrepancy, patients with severe radiological disease still have a higher prevalence of pain than those with less severe radiological disease. This suggests an association between structural markers of disease and the occurrence of illness.
The change in joint space width occurs slowly, so its assessment using X-rays requires one or two years, during which time joint damage significantly progresses. When a radiological diagnosis is established, significant joint destruction has already occurred.
Conventional radiography and computed tomography are methods without the possibility of direct visualisation of joint cartilage; narrowing of the joint space is only an indirect sign of joint destruction; there is no evaluation of early changes in the cartilage.
A much more objective depiction of inflammatory events in osteoarthritis can be obtained using techniques such as nuclear magnetic resonance, arthroscopy, or ultrasound examination (arthrosonography).
3. Nuclear magnetic resonance
Magnetic resonance imaging (MRI) provides direct information about changes in different joint tissues and is therefore suitable for osteoarthritis. Its superior soft-tissue contrast makes it ideal for depicting normal articular cartilage and its lesions, including fissures, full-thickness cartilage defects and signal changes in remaining cartilage.
Magnetic resonance imaging is an undisputed method for detecting synovitis in osteoarthritis, but its use is limited by the high cost of the examination.
4. Arthroscopy and arthrography
Methods for direct visualisation of articular cartilage include arthrography and arthroscopy; they are less commonly used in clinical practice because they are invasive and have limited indications.
Laboratory findings
In osteoarthritis, inflammatory parameters such as an elevated erythrocyte sedimentation rate and raised CRP levels are never present, but the degree of destructive and inflammatory change can be monitored by measuring biomarkers of metabolic activity in cartilage, synovial and bone tissue in blood and urine.
Biomarkers
Biomarkers (biochemical markers) are molecules or fragments of the connective tissue matrix that are released into biological fluids (blood, urine, synovial fluid) during the process of tissue metabolism and can be measured by immunoassay method.
Potentially specific biochemical markers reflecting quantitative and dynamic changes in the remodelling of joint tissue, both reparative and degenerative, are being developed today. Among the various potential clinical uses of biomarkers, their ability to predict disease progression in osteoarthritis is significant, as clinical and radiological prediction is poor and limited in contemporary available research.
Metabolic changes in joint tissue in osteoarthritis cause changes in both the synthesis and degradation of matrix molecules. These are released as fragments into joint fluid, blood and urine, where they can be detected. Biomarkers can complement imaging techniques in monitoring disease progression.
It is recommended to measure and simultaneously monitor several markers of cartilage, bone and synovial metabolism, because the combination of markers reflects the metabolism of all three tissues giving a better picture of pathophysiological changes in osteoarthritis.
1. Biomarkers of cartilage tissue metabolic activity
Type II collagen and non-collagen proteins represent biomarkers of cartilage tissue metabolism.
Biomarkers of cartilage metabolism are divided into indicators of cartilage formation and indicators of cartilage degradation.
Biomarkers of cartilage formation (synthesis) are products of chondroblasts and are formed during synthesis:
1. cartilage collagen - type II procollagen fragments (type II procollagen carboxy-terminal propeptide and type II procollagen amino-terminal propeptide - PIICP and PIINP) and
2. matrix of cartilage tissue – CS epitopes of aggrecan (846, 3-B-3, 7-D-4).
Markers of cartilage degradation include products of type II collagen degradation, namely:
- helix II
- C and N terminal amino acid parts of the telopeptide structure (CTX, NTX),
- their cross-links pyridinoline (PYD) and deoxypyridinoline (DPD)
- collagen type XI, and collagen type IX.
Non-collagen proteins that indicate cartilage degradation are:
- fragments of aggrecan containing isomerized residues,
- fragments of the protein coat of aggrecan (neoepitopes in the G1–G2 interglobular domain), KS epitopes (5-D-4, AN9P1).
Type II collagen
The basis of cartilage damage is the degradation of type II collagen, the most significant component of cartilage tissue.
Type II collagen is a protein that has a structure highly homologous to type I collagen (it has a triple helical structure with short telopeptides at both ends, so-called N and C telopeptides) and, like it, is synthesised in proform.
It arises from pro-collagen type II, which is a 50% longer molecule than the final protein. Its fragments, terminal amino and carboxy peptides, pro-collagen type II C-terminal propeptide (RIICP) and pro-collagen type II N-terminal propeptide (PIINP) are markers of cartilage formation. Collagen type II fragments, C and N terminal amino acid parts of the telopeptide structure (CTX, NTX), helix-II, as well as pyridinoline (PYD) and deoxypyridinoline (DPD) are markers of cartilage degradation. Helix-II is a degradation fragment of the triple helix, as CTX-II is a degradation fragment of the C telopeptide.
During degradation of the extracellular matrix of cartilage, type II collagen fragments can be found in serum and urine, and their measured concentrations can be indicators of catabolic changes in the cartilage. Degradation of type II collagen entails various enzymatic processes including primarily collagenases, which attach and cut the triple helical region of type II at a single site between 778 and 776, creating two fragments.
Immunoassays can now be used to detect type II collagen degradation fragments: the C-telopeptide fragment (CTX-II) and type II collagen helical peptide (Helix-II). Antibodies recognise either of two neoepitopes formed through collagenase action in the triple-helix region. Increased urinary CTX-II and Helix-II levels have been reported in patients with knee osteoarthritis and active rheumatoid arthritis (RA).
It is known that increased levels of urinary CTX-II and hyaluronic acid predict radiological progression in hip osteoarthritis, as well as that patients with early RA after three-month basic therapy have a reduction in urinary CTX-II. A single measurement of hyaluronic acid in serum or changes in urine over a short period in the level of CTX-II can identify patients with a high risk for progression of osteoarthritis. It has also been proven that CTX-II, glucosyl-galactosyl pyridinoline (Glc-Gal-PYD), and type III collagen N-terminal propeptide (PIIINP) can be useful markers of disease severity in patients with osteoarthritis, as their levels significantly correlate with cartilage loss. Research has determined that low serum levels of PIIINP or high urinary CTX-II are associated with rapid joint destruction over a one-year period, compared with radiography and/or arthroscopy of the joint
2. Biomarkers of synovial tissue metabolic activity
Biomarkers of metabolic activity of synovial tissue are type III collagen and non-collagenous proteins.
Markers of synthesis are fragments of type III procollagen: the N-terminal propeptide of type III procollagen (PIIINP) and the C-terminal propeptide of type III procollagen (PIIICP).
Markers of resorption are fragments of type III collagen, N- and C-telopeptides of type III collagen, pyridinoline (PYD), and glucosyl-galactosyl-pyridinoline – a glycosylated PYD analogue (Glc-Gal-PYD).
The non-collagen protein marker of synovitis is hyaluronan (hyaluronic acid), a high-molecular-weight glycosaminoglycan. In complex with other proteins, hyaluronan ensures viscosity of synovial fluid, protects cartilage from loss of proteoglycans, and prevents migration of inflammatory cells into the joint space.
Several markers are recommended for assessing synovitis in osteoarthritis and rheumatoid arthritis (RA). Serum C-reactive protein (CRP), hyaluronic acid (HA) and type III collagen N-terminal propeptide (PIIINP) are not highly specific for synovial tissue; they are widely distributed and their levels rise in other circumstances, including scleroderma. Glucosyl-galactosyl pyridinoline (Glc-Gal-PYD) has been shown to be a collagen cross-link present in human synovial tissue and released during in-vitro destruction. It was not found in bone, cartilage or other soft tissues. Preliminary in-vivo data indicate increased urinary Glc-Gal-PYD in patients with RA, but not in patients with Paget’s disease; this correlates with the extent of bone erosions assessed by X-ray.
Urinary excretion of Glc-Gal-PYD is increased in knee and hip osteoarthritis. Obtained levels show a stronger correlation with symptoms of osteoarthritis than bone and cartilage tissue markers.
3. Biomarkers of cartilage and synovial tissue metabolic activity
Biomarkers of metabolic activity of cartilage and synovial tissue are:
- Cartilage oligomeric matrix protein – COMP (Cartilage Oligomeric Matrix Protein),
- human cartilage glycoprotein (YKL-40),
- matrix metalloproteinases (MMP): collagenases, stromelysins, gelatinases, membrane-type metalloproteinases and tissue inhibitors of metalloproteinases (TIMP).
Oligomeric matrix protein of cartilage - COMP (Cartilage Oligomeric Matrix Protein)
Oligomeric matrix protein of cartilage - COMP (Cartilage Oligomeric Matrix Protein) is a non-collagenous protein of the articular cartilage matrix. The molecule is a pentamer with a molecular mass of 435 kDa containing five identical disulfide-linked subunits. It is synthesised by chondrocytes and synovial cells after activation by pro-inflammatory cytokines. COMP is detected in very small quantities in tendons, menisci, ligaments and synovial membrane, but cannot be detected in other cartilage-rich organs such as the lungs and bronchi.
This protein is part of type II collagen, stimulates and regulates fibrillogenesis, and stabilises the collagen network in cartilaginous tissue. It is useful as a marker of early cartilage destruction, as it is the first to be released upon tearing of the collagen network, resulting in cartilage deterioration.
The process of joint cartilage affection can be monitored by measuring matrix proteins or their fragments released into circulation. Biomarkers also represent a potential tool for monitoring treatment effects on tissue. For example, a therapeutic intervention aims to reduce joint damage, e.g. TNF alpha blockade in patients with RA normalises serum COMP levels. In other studies, serum COMP is used as a prognostic indicator and marker for future joint damage, as recommended for osteoarthritis and RA.
Increased serum and urinary levels of COMP have been described in patients with osteoarthritis and can be prognostic markers of disease progression. Using a valid MRI score, a significant correlation was found between the MRI damage score and two biochemical markers, COMP and CTX-II.
The pathogenesis of osteoarthritis involves not only cartilage but also bone and synovium. Elevated values of COMP and hyaluronic acid may suggest that synovial inflammation plays a central role in the pathogenesis of osteoarthritis. The key role of inflammation in osteoarthritis is supported by histological findings of inflammation, elevated levels of synovium-related biomarkers, and high levels of proinflammatory cytokines in osteoarthritic chondrocytes.
It has been proven that serum COMP, PIIINP and HA levels are higher in patients with osteoarthritis and indicate increased synovial and cartilage metabolism in patients with rheumatoid arthritis, osteoarthritis and in some patients with psoriatic arthritis.
Human cartilage glycoprotein (YKL-40)
Human cartilage glycoprotein (YKL-40), known as human cartilage glycoprotein 39 (HC gp-39 or GP-39), is a glycoprotein with a molecular mass of 38–40 kDa. It has a structure homologous to that of chitin, which forms the exoskeleton of some molluscs, crustaceans and insects. It plays a significant role in tissue remodelling, including joint cartilage.
YKL-40 is synthesised and secreted by chondrocytes and synovial cells, but also by activated macrophages, liver fibrocytes and cells of carcinoma of the colon, breast, lung, ovary and prostate, as well as cells of osteosarcoma (MG-63). The biological function of YKL-40 in malignant tumours is unknown, but it has been proven that this glycoprotein stimulates the growth of connective tissue cells and chemotaxis.
In individuals with healthy cartilage, the level of YKL-40 is low, whereas under conditions of inflammation or extracellular matrix remodelling, there is a significant increase in the serum levels of this glycoprotein. For this reason, YKL-40 can be a marker of cartilage and synovial tissue metabolism.
According to literature data, determination of YKL-40 levels is used in research of early rheumatoid arthritis and advanced osteoarthritis, as well as giant cell arthritis, liver fibrosis, carcinoma of the colon, breast, lungs, ovaries and prostate.
Several studies have shown that serum YKL-40 levels significantly increase (1.5-3 times) in patients with RA and with osteoarthritis and correlate with levels in synovial fluid. However, YKL-40 levels are 10-15 times higher in synovial fluid than in serum, which suggests that increased serum levels originate from damaged joints.
YKL-40 levels are elevated in inflammatory arthritides, with the highest levels found in RA. Following treatment with basic drugs in RA, there was a significant drop in YKL-40 levels in patients.
Levels of this biomarker correlate with markers of systemic inflammation, but are much more specific for joint diseases and may reflect cartilage and synovial damage. In patients in the late stage of knee osteoarthritis, there is an elevated level of YKL-40 in serum, as well as in patients with strong synovial inflammation, which may indicate that YKL-40 is a local prognostic marker for joint destruction.
4. Biomarkers of bone tissue metabolism
Type I collagen and non-collagen proteins represent biomarkers of bone tissue metabolism.
Markers of bone metabolism are divided into indicators of bone formation and indicators of bone resorption.
Markers of bone formation or synthesis are products of osteoblasts and arise during synthesis:
1. bone collagen - type I procollagen fragments (type I procollagen carboxy-terminal propeptide and type I procollagen amino-terminal propeptide - PICP and PINP) and
2. matrix of bone – osteocalcin (non-collagenous protein) and specific bone alkaline phosphatase (enzyme of osteoblasts).
Markers of bone degradation include products of type I collagen degradation, namely:
- C and N terminal amino acid parts of the telopeptide structure (CTX, NTX)
- their cross-links pyridinoline (PYD) and deoxypyridinoline (DPD).
Non-collagen proteins that indicate bone degradation are:
- tartrate-resistant acid phosphatase (TRAP),
- bone sialoprotein (BSP),
- hydroxyproline (main amino acid of collagen)
- calcium (Ca)
Type I collagen
Type I collagen is the most prevalent of all tissue proteins and constitutes 90% of total collagen in the body. Together with non-collagen proteins, type I collagen is a biomarker of bone tissue metabolism. Markers of bone resorption include products of type I collagen degradation, namely the C and N terminal amino acid portions of the telopeptide structure, their cross-links pyridinoline (PYD) and deoxypyridinoline (DPD). Markers of bone synthesis are carboxy terminal propeptide of type I procollagen and amino terminal propeptide of type I procollagen (PICP and PINP).
C-terminal telopeptide of type I collagen – CTX-I
It has been shown that bone and cartilage metabolism parameters, CTX-I and CTX-II, are significantly higher in patients with erosive osteoarthritis of the hands and rapidly destructive OA.
It has been proven that three-year radiological progression of knee osteoarthritis can be predicted by a one-year increase in osteocalcin levels in synovial fluid of damaged joints or a decrease in hyaluronic acid levels in serum, as well as an increase in PYD and DPD levels, which are found in both bone, synovial and cartilage tissue.
Osteoarthritis therapy
Osteoarthritis therapy is based on pain reduction and maintaining joint mobility. If there is a degenerative disease of the spine (discarthrosis, spondylosis) or weight-bearing joints (hip, knee, ankle joint), correction of poor posture, reduction of body weight and moderate physical activity are necessary to strengthen the musculature in order to reduce the load on the joint itself.
Appropriate physical therapy should also be included in treatment in consultation with a doctor.
In the event of pain and swelling, non-steroidal anti-inflammatory drugs are prescribed at the dose determined by the doctor.
Chondroprotective medicines are substances that contribute to regeneration, preservation of function and maintenance of the health of articular cartilage and surrounding structures. They are preparations containing higher doses of glucosamine sulphate and chondroitin sulphate, normal constituents of healthy cartilage. Many of these preparations also contain hyaluronic acid, type I collagen and methylsulfonylmethane. They are taken as tablets for no less than three months.
Intra-articular therapy
In cases of more substantial joint swelling, the fluid should be aspirated and a long-acting corticosteroid preparation administered intra-articularly. The therapeutic effect in stopping inflammation is very rapid, though not as long-lasting. Applications may be repeated, but no more than three times a year.
Hyaluronic acid, an important component of healthy synovial fluid, is also administered intra-articularly. The network of hyaluronic acid chains represents an ideal cellular matrix. It is administered in different doses, for example as a single 60 mg dose, once weekly for 3 weeks at a dose of 40 mg, or once weekly for 5 weeks at a dose of 20 mg.
Autologous platelet-rich plasma (A-PRP) is an internal reservoir of growth factors from the patient's own blood. After intra-articular administration, there is an anti-inflammatory effect and activation of signal cells that trigger a cascade reaction. It has a positive effect on cell migration and differentiation.
The combination of hyaluronic acid with the biological and anti-inflammatory effects of platelet-rich plasma is the Cellular Matrix (A-PRP–HA) preparation. In this medicine, the two components act synergistically, restoring homeostasis in the joint and, by protecting cartilage, relieving pain. It has a key role in the synthesis of a new matrix for tissue regeneration. After 4 ml of blood is drawn, a special procedure produces autologous platelet-rich plasma, which is mixed in a closed system with 40 mg of hyaluronic acid. The prepared product is administered into the patient’s joint, immediately creating a hyaluronic-acid cellular network in which platelets are dispersed. Compared with A-PRP, this medicine has significantly greater viscosity and therefore a greater effect on increasing mobility and reducing pain.
Orthokin therapeutic method is used today in the treatment of degenerative diseases, as well as injuries of tendons, ligaments and muscles. This is a unique way of treating joint pain based on the application of autologous (own) serum enriched with anti-inflammatory proteins and growth factors, which is applied to diseased or damaged tissue.
This method of treatment uses nothing other than the patient's own endogenous proteins and healing factors. Anti-inflammatory and regenerative results are long-lasting due to blocking the binding of inflammation factors (proinflammatory cytokine - interleukin 1) to cells, thereby reducing damage to cartilage and other damaged tissues. Growth factors stimulate tissue recovery, epithelialisation, spontaneous wound closure, connective tissue formation, and fibroblast activation at the site of injury.
The drug preparation process is specific. At the beginning of Orthokin therapy, the patient's blood is taken in the morning and processed under special conditions, without the addition of any chemical substances. The incubation process, which lasts 7-9h at 37℃, stimulates cells from the patient's blood to produce factors known to block inflammation (interleukin 1 receptor antagonist) and growth factors in high concentration. After the incubation process, the concentration of these factors increases up to 140 times. By centrifuging the blood, serum is separated from all formed blood elements and fibrinogen. Thus processed serum enriched with inflammation blockers and healing factors is returned periodically, every 3-7 days, in the form of injections, which are administered to the patient under ultrasound control to the affected area of the patient.
Modern regenerative orthopaedics applies autologous transplantation, i.e., transfer of stem cells in the treatment of joint cartilage defects. Adult-mature (partially differentiated cells) stem cells taken from the patient themselves from circulating blood, bone marrow, and adipose tissue are used, via automated closed, sterile procedures, after which they are immediately ready for application to the damaged site.