What are the four stages of tendon healing?

Why some count three and some four: one continuous sequence, and a split at the front or the back

What are the four stages of tendon healing is a question that a reader asks after meeting the stages in two places and finding the counts differ, three in a textbook and four in a physiotherapist's handout or a clinic's summary, and the honest page begins with the discrepancy because resolving it is what makes the stages usable rather than memorised. The biology is one continuous sequence with no gaps or signposts, and every count is a way of drawing lines across it. In the first hours after a tendon is torn or cut, the bleeding from the torn ends and the surrounding sheath clots, and platelets caught in the clot release the signalling molecules that call in the cells of inflammation; this is haemostasis, and it lasts hours. Over the first several days, inflammatory cells arrive from the blood and the tendon sheath, clear the damaged collagen and dead cells, and release the growth factors that recruit fibroblasts from the sheath and the tendon's own cell population; this is inflammation, and the tendon in it is at its weakest, held together by clot and the stitches a surgeon may have placed. Over the following weeks, the recruited fibroblasts multiply and lay down a new matrix at speed, a matrix rich in type 3 collagen, the small-fibre, fast-made collagen of early repair, bulky, poorly aligned and weak; this is proliferation, and the tendon in it is a scar in the making, gaining strength week by week but far from its original stiffness. And over months stretching to a year or more, type 1 collagen slowly replaces the type 3, the fibres align along the line of pull in response to the loads placed through them, cross-links form between fibrils and the tissue stiffens and strengthens, regaining a large fraction of its original properties but, in every animal and human study that has measured it, never all; this is remodelling. Three stages is what most textbooks count, folding haemostasis into inflammation as its opening hours, and the three-stage telling is on the tendon healing stages page. The four-stage versions make a fourth stage in one of two ways. Some split at the front, giving haemostasis its own stage because the clot and its platelet signals set the tone for everything that follows, which is also why the wound healing literature for skin uses four stages. Others split at the back, dividing remodelling into a consolidation phase in which the repair tissue shrinks, the cells become fewer and the fibres align, over roughly the second and third months, and a maturation phase in which cross-linking slowly stiffens the aligned fibres over the following year. Both are legitimate bookkeeping, both describe the same tendon, and a reader who finds three in one source and four in another has met two sets of lines across one sequence rather than two biologies.

What each stage permits in rehabilitation, the stage that loading can improve, and where collagen's evidence sits

The reason the stages are worth knowing at all is that each one permits something different, and the honest page gives the permissions rather than the durations, since the durations vary with the tendon, the injury and the person while the permissions follow the biology. During haemostasis and inflammation the tendon is held together by clot and sutures and has almost no tensile strength of its own, so the permission is protection, a splint, a boot or a sling, and gentle motion within the protected range, because even in these days complete immobilisation lets the repair stick to its sheath and lets collagen synthesis in the uninjured tendon fall; the flexor tendon surgeons learned this first, replacing six weeks of immobilisation after repair with early controlled motion and halving the stiff fingers that followed. During proliferation the type 3 scaffold strengthens week by week, and the permission widens to progressive controlled loading, more range, more weight-bearing, light resistance, each step gated by the tissue's response rather than by the calendar, with pain and swelling the signals that a step was too large; in the Achilles rupture trials, early weight-bearing in a boot through this stage produced no more re-ruptures than cast immobilisation and better function. During remodelling the permission is heavy loading, and it is more than a permission, because this is the only stage whose outcome loading improves: the fibroblasts that are turning type 3 into type 1 and aligning the fibres read the direction and magnitude of strain through their cell membranes and lay collagen along the line of pull, so a tendon loaded progressively through remodelling ends stiffer, stronger and better aligned than one that is rested, and the heavy slow resistance programmes that treat tendinopathy work by driving exactly this process in a tendon whose remodelling has stalled. The seventy-two hour synthesis curve belongs here too: a loading session raises tendon collagen synthesis for about three days, with breakdown rising alongside it for the first day, so net gain is made at the frequency of a heavy session every second or third day rather than daily. Collagen's evidence sits in this last stage and nowhere earlier. The marker trial gave 15g gelatin with vitamin C an hour before loading and measured a rise in a blood marker of collagen synthesis in healthy men; the one small outcome trial gave 2.5g beside six months of calf strengthening for Achilles tendinopathy and found more improvement than placebo beside the same exercise. Neither reached into inflammation or proliferation, neither tested a torn or repaired tendon, and both timed the supplement to a loading stimulus, so the adjunct is a remodelling-stage adjunct beside the loading that drives remodelling. The worked example is that adjunct at habit scale, stated on its true terms: two capsules, 1200mg hydrolysed type 1 marine collagen with 12mg vitamin C at 15% of its reference value plus zinc, 120 for £16.99 at 28p a day, lab tested and UK made, below the 15g protocol dose, carrying vitamin C's collagen formation sentence for cartilage and bones and no tendon claim. What are the four stages of tendon healing: haemostasis, inflammation, proliferation and remodelling if the split is at the front, or inflammation, proliferation, consolidation and maturation if it is at the back, one sequence either way, with protection, then progressive loading, then heavy loading as what each stage permits.

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