Inflammation & Cytokines in Orthopaedics (TNF-α, IL-1, IL-6)

Cytokines are small secreted signalling proteins that act in autocrine, paracrine and sometimes endocrine fashion to coordinate inflammation, immunity and tissue repair. They include the interleukins (IL), tumour necrosis factors (TNF), interferons, chemokines and growth factors. The acute inflammatory response to injury or infection (the classic rubor, calor, dolor, tumor, functio laesa) is orchestrated by them: tissue macrophages and recruited cells release pro-inflammatory cytokines (TNF-α, IL-1, IL-6) that cause vasodilation, increased vascular permeability, leukocyte recruitment and the systemic acute-phase response (fever, CRP). Resolution is driven by anti-inflammatory mediators (IL-10, TGF-β, IL-1Ra) and pro-resolution lipids.

Osteoimmunology describes the shared signalling between the immune and skeletal systems. The unifying mechanism of inflammatory bone loss is that pro-inflammatory cytokines (TNF-α, IL-1, IL-6, IL-17) up-regulate RANKL (and reduce OPG) on osteoblast-lineage/stromal cells, tipping the RANKL:OPG ratio toward osteoclast formation and activity - producing bone erosion. TNF-α and IL-1 can also act more directly on osteoclast precursors and impair osteoblastic repair, so erosions both enlarge and fail to heal. This is the molecular engine behind the joint destruction of inflammatory arthritis.

The systemic arm of the cytokine response is the acute-phase response, driven chiefly by IL-6 acting on hepatocytes - and it is what gives you the two inflammatory markers used daily in orthopaedics. Knowing their kinetics is the examinable point:

• CRP is a hepatocyte-synthesised acute-phase protein: it rises within about 6 hours, peaks around 48 hours, and has a short half-life (~19 hours), so it falls quickly once inflammation settles. That fast on/off makes it the marker for monitoring an acute process - post-operative course, response to treatment, and suspected periprosthetic joint infection.
• ESR is an indirect marker (it reflects fibrinogen and immunoglobulin-driven red-cell rouleaux): it rises and falls slowly over weeks and is confounded by age, anaemia, pregnancy and paraproteins - so it lags and is less useful for tracking rapid change.
• Orthopaedic use: a normal CRP and ESR together have a high negative predictive value for screening PJI; a persistently rising or non-falling CRP after arthroplasty raises the suspicion of infection. Remember tocilizumab (anti-IL-6R) suppresses CRP, so the marker can be falsely reassuring in patients on that biologic.