Structures Involved in Pain Behind and Deep Under the Achilles Tendon - A Physiotherapy Perspective

The Achilles Enthesis Organ (The Master Concept)

"The enthesis itself, the sesamoid and periosteal fibrocartilage, the bursa, and the fat pad collectively comprise the Achilles enthesis organ... Together, they all function in one way or another to reduce stress concentration at the enthesis itself."

1. Retrocalcaneal Bursa (Primary Suspect for Deep Pain)

• Location: Between the anteroinferior wall of the Achilles tendon and the posterosuperior surface of the calcaneus - exactly "deep under" the tendon
• Structure: A saddle-shaped, synovial-lined space. Its anterior margin is fibrocartilage, posterior margin is the paratenon of the Achilles tendon, and the superior margin is adipose tissue
• Function: Reduces friction associated with tendon movement. During dorsiflexion, the Achilles tendon is compressed against the calcaneal tuberosity; the bursa allows smooth gliding
• Clinical features when inflamed: Deep posterior heel pain, fullness/swelling palpable medially and laterally to the tendon, pain worsened by ankle dorsiflexion (classic finding), morning stiffness on first rising
• Associations: Often co-occurs with Haglund deformity; linked to RA, seronegative spondyloarthropathies (ankylosing spondylitis, psoriatic arthritis), gout, as well as mechanical overuse in runners

2. Insertional Achilles Tendon (Deep Surface and Insertional Footprint)

• Insertional tendinopathy involves degeneration of tendon fibres at or within 2 cm of the calcaneal insertion
• Intratendinous calcification commonly develops with chronicity, creating focal compressive pain deep within the tendon
• The sesamoid fibrocartilage on the deep surface of the Achilles (adjacent to the retrocalcaneal bursa) and the periosteal fibrocartilage on the posterosuperior calcaneus are compressed against each other during dorsiflexion - the mechanical basis of deep insertional pain
• Key clinical distinction: Insertional tendinopathy pain is reproduced on the tendon itself at the heel bone, often with visible/palpable thickening; pain worsens with dorsiflexion and in closed-back shoes

3. Kager's Fat Pad (Pre-Achilles Fat Pad)

• Location: A triangular space bounded by:
  • Posterior: Achilles tendon
  • Anterior: Flexor hallucis longus muscle and tendon
  • Inferior: Superior cortex of the calcaneus
• Function: Highly vascular and sensory nerve-innervated (contains mechanoreceptors). During movement, the tip of Kager's fat pad slides in and out of the retrocalcaneal bursa to minimise pressure changes and promote boundary lubrication
• Pathology: In Achilles tendinopathy, oedema and fibrosis of Kager's fat pad is a recognised finding on MRI/ultrasound. This causes pain because it is richly innervated. Fat pad impingement can occur independently of tendon pathology
• Clinical note: A hardened or fibrotic Kager's fat pad may impair normal tendon gliding, perpetuating pain cycles even after the primary tendinopathy improves

4. Haglund Deformity (Posterosuperior Calcaneal Prominence)

• A prominent exostosis off the posterosuperior calcaneal tuberosity, located anterior to the Achilles tendon
• During dorsiflexion, the bony prominence repetitively abrades the deep surface of the Achilles tendon and compresses the retrocalcaneal bursa
• This leads to the classic triad: Haglund deformity + retrocalcaneal bursitis + insertional Achilles tendinopathy - often co-existing
• Physiotherapy relevance: Heel lifts reduce dorsiflexion range, taking mechanical stress off the tendon-bursa-bone interface; footwear modification (open-backed shoes) reduces compression

5. Calcaneal Bone / Insertional Footprint (Subtendinous Calcaneus)

• The posterosuperior calcaneal tuberosity where the Achilles inserts can develop:
  • Calcaneal enthesophytes (insertional spurs) - bony outgrowths within the tendon at the insertion
  • Bone marrow oedema (stress reaction) at the calcaneal tuberosity - visible on MRI as a signal change
• Calcaneal stress reactions cause deep, diffuse aching pain precisely "under" the tendon
• May be a primary source of pain independent of the overlying tendon

6. Flexor Hallucis Longus (FHL) Tendon

• FHL runs in a groove under the talus (posterior talar process/os trigonum) and can develop tenosynovitis or stenosis
• Pain is often deep, posteromedial, and can mimic or co-exist with retrocalcaneal pathology
• Key differentiator: FHL pain is most prominent with toe/great toe motion, particularly in athletes (dancers, runners); a "trigger toe" phenomenon may be present
• Associated with posterior ankle impingement and os trigonum syndrome

7. Superficial (Retro-Achilles / Pre-tendinous) Bursa

• An adventitious bursa - not always present anatomically
• Especially prone to friction from shoe heel counters
• Pain when wearing shoes is a hallmark; less painful on first arising than retrocalcaneal bursitis
• Relevant when differentiating purely deep pain (retrocalcaneal) from superficial posterior pain (retro-Achilles)

8. Os Trigonum / Posterior Talar Impingement

• The os trigonum is an accessory ossicle at the posterior lateral tubercle of the talus, present in ~5-14% of the population
• During forced plantarflexion, it becomes pinched between the tibia and calcaneus, causing posterior ankle impingement syndrome
• Pain is deep, posterior, and reproduced on full plantarflexion (contrast with retrocalcaneal bursitis which worsens on dorsiflexion)
• Often confused with Achilles pathology but requires very different physiotherapy management

Summary Table - Key Differentiating Features

Physiotherapy Clinical Pearl

"In prolonged or severe cases, all aspects of insertional tendinopathy (retrocalcaneal bursitis, Achilles tendinopathy, and pretendinous bursitis) can exist concomitantly."