01OverviewDefinition, clinical context and the essential points that orientate the chapter.
The inflammatory–mechanical distinction is a probability judgement, not a binary test. Establish onset, tempo, distribution, stiffness duration, rest and night symptoms, load response, swelling, systemic illness and functional consequences. Examine the symptomatic region and screen other joints, skin, nails, eyes and entheses. An older patient may have osteoarthritis and new inflammatory arthritis simultaneously; an athlete may have mechanical pain with an inflammatory disease in the background.
Pain location is often misleading. Joint-line pain with restriction of both active and passive movement supports an intra-articular source. Pain on active or resisted movement with relatively preserved passive range suggests tendon or muscle. Focal bursal tenderness, dermatomal sensory change, allodynia or pain reproduced by spinal movement redirects assessment. Hip disease may present at the knee and cervical disease may present around the shoulder or arm.
Investigations should resolve uncertainty that changes care. Persistent clinical synovitis merits early specialist assessment rather than serial screening panels. Serum urate neither confirms nor excludes an acute gout flare, autoantibodies change probability rather than define symptoms, and early radiographs may be normal. Safety-net any provisional mechanical diagnosis with a review point and explicit triggers such as swelling, systemic illness, neurological change or failure to improve.
Key points
- Inflammatory pain is suggested by prolonged morning stiffness, rest or night pain, visible synovitis and improvement after movement; mechanical pain more often tracks load and eases with rest, but no single feature is decisive.
- Separate articular pain from tendon, bursa, muscle, bone, nerve and referred pain by locating tenderness and testing active, passive and resisted movement.
- True synovitis combines soft, often warm swelling with restriction or pain through the joint; bony enlargement and crepitus without soft swelling are more consistent with osteoarthritis.
- Ask about psoriasis, uveitis, inflammatory bowel disease, urethritis, preceding infection, Raynaud phenomenon, sicca symptoms, rash, ulcers and family history because these redirect the phenotype.
- Normal CRP or ESR does not exclude inflammatory arthritis, axial spondyloarthritis, infection or vasculitis; abnormal values are nonspecific and must be interpreted with the clinical pattern.
- Do not delay urgent aspiration of a hot swollen joint to await serum urate, autoantibodies or plain radiographs.
- Persistent synovitis of uncertain cause warrants specialist referral even when rheumatoid factor, anti-CCP antibodies or radiographs are normal.
- Use imaging to answer a defined question: radiography for structural damage, ultrasound for accessible synovitis or guided procedures, and MRI for selected occult inflammation or deep anatomy.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Immune-mediated inflammation
Rheumatoid arthritis, spondyloarthritis, crystal arthritis and connective-tissue disease generate synovial, entheseal or periarticular inflammation through distinct immune and inflammatory pathways.
Mechanical tissue loading
Osteoarthritis, tendinopathy, bursitis and structural injury produce pain when load exceeds the capacity of cartilage, tendon, bone or supporting soft tissue.
Non-musculoskeletal mimics
Infection, malignancy, neuropathy, vascular disease and referred visceral pain may resemble a joint or muscle disorder and alter the urgency of assessment.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Inflamed tissue signalling
Cytokines, synovial vascularity and inflammatory cell recruitment sensitise nociceptors, causing rest pain, prolonged stiffness, warmth, swelling and impaired movement.
- 2Load-dependent nociception
Structural degeneration or focal soft-tissue injury activates pain during movement, compression or resisted loading, usually easing when the provoking demand stops.
- 3Functional deconditioning
Pain, fear and reduced activity weaken muscle and alter movement, so inflammatory and mechanical mechanisms can coexist and reinforce disability over time.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Soft swelling, warmth, prolonged morning stiffness, rest pain and restricted movement across several joints suggests active synovitis, especially with fatigue, weight loss or extra-articular features.
Pain linked to use, brief stiffness, crepitus, bony change or focal pain on resisted movement supports osteoarthritis or regional soft-tissue disease, while acknowledging overlap.
Acutely painful restricted joint with fever, immunosuppression, prosthesis or recent procedure is septic arthritis until urgently assessed and sampled; absence of fever is not reassuring.
Back pain beginning young, improving with movement, waking the patient in the second half of the night, alternating buttock pain, enthesitis or uveitis supports spondyloarthritis.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Focused joint and regional examinationFirst step - Why
- Establish whether pain arises from joint, periarticular tissue, nerve, spine or referred anatomy.
- Interpretation and limitations
- Compare sides, document soft swelling, warmth and range, and use active, passive and resisted movement. A reproducible finding changes probability; a single tender point does not establish pathology.
- 02
FBC, CRP, ESR, renal and liver profile when inflammation is plausible - Why
- Look for inflammation, anaemia, infection and a baseline relevant to treatment or alternative systemic disease.
- Interpretation and limitations
- Normal inflammatory markers do not exclude active disease, while elevation may reflect infection, cancer, obesity, anaemia or age. Interpret the trend and phenotype together.
- 03
Urgent synovial-fluid microscopy and culture for an acute effusion - Why
- Identify infection and crystals before immunosuppression or reassurance.
- Interpretation and limitations
- Send adequate fluid promptly for cell count where available, Gram stain, culture and crystal analysis. Crystals do not exclude concurrent infection and antibiotics should follow sampling only when this is safe.
- 04
Targeted serology - Why
- Refine a clinically defined inflammatory phenotype rather than screen undifferentiated pain.
- Interpretation and limitations
- RF and anti-CCP support suspected rheumatoid arthritis; ANA and other antibodies are selected for compatible systemic features. Negative tests do not override persistent synovitis.
- 05
Question-led radiography, ultrasound or MRI - Why
- Demonstrate structural damage, active synovitis, enthesitis or occult deep pathology when the result will change management.
- Interpretation and limitations
- Early inflammatory disease may have normal radiographs. Imaging abnormalities can be incidental, so correlate site and pattern rather than treating the report alone.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Inflammatory arthritis
Persistent synovitis, prolonged morning stiffness, nocturnal symptoms and systemic or extra-articular features support rheumatoid arthritis or a spondyloarthritis phenotype.
Osteoarthritis and regional pain
Activity-related pain, brief stiffness, bony enlargement, crepitus or reproducible tendon and bursal loading suggests a mechanical or periarticular source.
Infection or crystal arthritis
Abrupt severe monoarthritis, fever, immunosuppression or marked inflammatory response requires aspiration because clinical appearance cannot safely distinguish sepsis from crystals.
Neurological or referred pain
Dermatomal symptoms, weakness, altered reflexes or pain unrelated to joint movement suggests root, peripheral nerve or referred visceral disease.
Additional chapter-specific clues
Night pain unrelated to position, focal bone pain, neurological deficit, vascular change, constitutional symptoms or organ-specific features should expand assessment beyond rheumatology.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Cannot-wait patternExclude infection, fracture and neurological threatFirst stepHot swollen joint, systemic illness, major trauma, severe bone pain or neurological and sphincter symptoms.+
- 1Perform ABCDE where unwell, provide proportionate analgesia, document neurovascular status and seek same-day senior or emergency specialty assessment.
- 2Obtain urgent cultures, aspiration and imaging according to the threatened structure without delaying antibiotics or surgery in an unstable patient.
- 3DefinitiveEscalationReassess physiology, pain and function after intervention; record the working differential and the result that will trigger definitive source control or escalation.
02Inflammatory phenotypeConfirm synovitis and refer earlyPersistent swelling, prolonged stiffness, inflammatory axial features or systemic rheumatic manifestations without immediate instability.+
- 1Map joint and extra-articular distribution, request only phenotype-relevant blood tests and establish pregnancy, infection and medicine factors that affect later treatment.
- 2Refer persistent synovitis promptly through the local rheumatology pathway even if inflammatory markers, antibodies or radiographs are normal.
- 3Use analgesia and functional support while awaiting assessment, avoiding prolonged unreviewed corticosteroids that may mask infection or obscure the baseline phenotype.
03Mechanical phenotypeTreat load and review the diagnosisPain reproducibly linked to use or regional tissue loading without synovitis, red flags or systemic features.+
- 1Explain the likely tissue source, agree graded activity and strengthening, address sleep and occupational load, and use the least harmful analgesic strategy.
- 2Reserve imaging for trauma, atypical progression, persistent major functional loss or a result that will alter intervention rather than ordering it for reassurance alone.
- 3Set a review point and reopen the inflammatory, neurological and referred-pain differential if swelling, night pain, systemic illness or objective weakness develops.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Delayed inflammatory control
Unrecognised persistent synovitis can cause erosive damage, tendon failure, deformity, disability and extra-articular disease before the diagnostic label becomes secure.
Missed infection or malignancy
Attributing red flags to a benign mechanical explanation can delay source control, oncological diagnosis or treatment of threatened neurological structures.
Pain-related disability
Avoidance, sleep loss and reduced conditioning can perpetuate weakness, falls, work loss and dependence even after the initiating tissue injury settles.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Track morning stiffness, visible swelling, night waking, functional tasks and analgesic use rather than relying on a pain score alone.
- Repeat examination when symptoms evolve because synovitis, neurological deficits and systemic features may be absent at the first visit.
- Review NSAID exposure, renal function, gastrointestinal and cardiovascular risk when anti-inflammatory medicines are used beyond a short course.
- Confirm that specialist referral was received and give a route back for worsening while serology or imaging remains pending.
- Document working diagnosis, remaining uncertainty and the specific red flags that should prompt urgent reassessment.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Stiffness needs context
Duration helps, but inability to move from pain, poor sleep and osteoarthritis can also create morning stiffness; swelling and distribution provide stronger context.
Passive movement localises
Restriction of passive as well as active range supports an articular or capsular process, while isolated resisted pain points toward a muscle–tendon unit.
Coexisting mechanisms are common
Inflammatory arthritis can leave secondary osteoarthritis, tendinopathy and central pain amplification, so disease control may not remove every symptom.
A normal test is conditional
Normal markers, serology or radiographs lower selected probabilities but cannot erase objectively documented synovitis or a dangerous clinical trajectory.
Function exposes pattern
Watching a patient rise, grip, reach and walk often distinguishes pain inhibition, weakness, stiffness and instability better than isolated palpation.
11Common pitfallsFrequent interpretation and management errors.
- 01
Calling pain inflammatory solely because an NSAID helped; mechanical pain may also improve with anti-inflammatory analgesia.
- 02
Using age to dismiss new inflammatory disease or to attribute every symptom to radiographic osteoarthritis.
- 03
Requesting broad autoimmune panels before defining the clinical phenotype and then chasing incidental low-specificity results.
- 04
Assuming crystal identification makes bacterial culture unnecessary in a hot swollen joint.
- 05
Failing to examine the joint above, joint below, spine, skin and nails when the painful site alone does not explain symptoms.