Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

Achilles Tendinitis vs. Tendinosis: Why the Difference Changes Your Treatment

For decades, any painful, inflamed Achilles tendon was called “Achilles tendinitis” — inflammation of the tendon — and treated with anti-inflammatories and rest. But tendon research over the past 20 years has revealed that most chronic Achilles tendon pain isn’t primarily inflammatory. It’s degenerative — a breakdown of the tendon’s collagen architecture that looks completely different under the microscope and responds to treatment quite differently. Getting the diagnosis right changes what actually works.

Achilles Tendinitis: The Acute Inflammatory Phase

True Achilles tendinitis involves an acute inflammatory response within the tendon tissue — increased blood flow, prostaglandin release, white cell infiltration, and classic signs of inflammation (warmth, swelling, redness, pain). This phase typically occurs in response to a sudden acute overload — dramatically increasing training intensity, changing surface, or starting an activity after a period of inactivity. The tendon itself is structurally intact; the problem is the acute inflammatory reaction. Treatment during this phase appropriately targets inflammation: relative rest (modifying activity to below the symptomatic threshold), ice and elevation, NSAIDs, and gentle movement to maintain circulation without mechanical overload. Most cases of acute Achilles tendinitis resolve within 4-6 weeks with appropriate management.

Achilles Tendinosis: The Chronic Degenerative State

Tendinosis represents the failure of the acute inflammatory phase to progress to proper healing — repeated cycles of microtrauma and inadequate repair lead to disorganized collagen, increased ground substance (glycosaminoglycan), and neovascularization (ingrowth of new, abnormal blood vessels accompanied by nerve fibers). The histological picture shows degenerative “mucoid” changes — the collagen fiber structure is disrupted, with areas of fiber separation and a disorganized, ground-glass-appearing matrix. Crucially, there is minimal or no inflammatory cell infiltrate. This is not inflammation — it’s degeneration, and treating it as inflammation with rest and anti-inflammatories misses the pathology.

How to Tell Them Apart Clinically

The distinction is primarily clinical and imaging-based. Acuity matters: a tendon that has been painful for 2-4 weeks after a known acute event is more likely true tendinitis; a tendon that has been painful for months or years with progressive worsening is much more likely tendinosis. Symptom pattern also helps: tendinitis is relatively uniform in its pain throughout activity; tendinosis classically shows the warm-up phenomenon (worst at the start, partially improving during activity, worsening again with continued use).

Ultrasound is the primary imaging tool for distinguishing the two. In tendinitis, the tendon may show increased blood flow on Doppler but relatively normal tendon architecture. In tendinosis, characteristic hypoechoic (dark) zones indicate areas of degenerated, structurally abnormal collagen. Tendon thickness is increased in tendinosis. MRI similarly distinguishes: tendinosis shows intrinsic signal changes within the tendon substance; pure paratendinitis shows peritendinous signal changes without intrinsic tendon involvement.

Treatment Differences: Why This Matters

For acute Achilles tendinitis, anti-inflammatory approaches are appropriate and effective. For Achilles tendinosis, they are largely irrelevant — you can’t reduce inflammation in tissue that isn’t inflamed. The appropriate treatment targets the degenerative process: loading the tendon through progressive resistance exercise (eccentric and heavy slow resistance protocols) stimulates collagen remodeling and replaces the disorganized tissue with proper collagen architecture. This takes time — typically 3-6 months of consistent progressive loading, not 2-3 weeks of rest and anti-inflammatories.

Corticosteroid injection is the clearest example of treatment divergence. For peritendinous inflammation (paratendinitis, an inflammatory condition of the tendon sheath rather than the tendon itself), judicious corticosteroid injection around the tendon reduces the inflammatory sheath reaction. Corticosteroid injection into the Achilles tendon proper, however, carries meaningful rupture risk — the steroid weakens already degenerated collagen — and should generally be avoided in established tendinosis.

Reversibility of Tendinosis

The good news: tendinosis is reversible with appropriate treatment. The bad news: it takes time and consistent effort. Tendon collagen remodeling is slow — improvements on ultrasound typically lag behind symptomatic improvement by months. Patients who commit to 12+ weeks of progressive loading programs achieve durable resolution of tendinosis symptoms in the large majority of cases. Patients who cycle through brief periods of rest and anti-inflammatory treatment without loading programs rarely resolve their tendinosis — they simply suppress symptoms temporarily until the next overload event.

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Clinical References

1. Maffulli N, et al. Achilles tendinopathy: aetiology and management. Journal of the Royal Society of Medicine. 2004;97(10):472-476.
2. Cook JL, Purdam CR. Is tendon pathology a continuum? A pathology model to explain the clinical presentation of load-induced tendinopathy. British Journal of Sports Medicine. 2009;43(6):409-416.
3. Alfredson H. Chronic midportion Achilles tendinopathy: an update on research and treatment. Clinics in Sports Medicine. 2003;22(4):727-741.

Dr. Tom Biernacki, DPM

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Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.