What are the possible causes of lower calf and Achilles tendon pain?

Causes of Lower Calf and Achilles Tendon Pain

Lower calf and Achilles tendon pain is most commonly caused by chronic degenerative tendinopathy (tendinosis) from repetitive overuse, not inflammation, though acute Achilles tendon rupture and several other distinct conditions must be ruled out. 1

Primary Causes

Chronic Overuse Tendinopathy (Most Common)

The Achilles tendon is one of the most frequently affected sites for overuse tendinopathy, accounting for a significant portion of injury-related visits. 1 This is a degenerative condition, not inflammatory, and should be labeled "tendinosis" or "tendinopathy" rather than "tendonitis." The natural history involves gradually increasing load-related localized pain that worsens with increased activity. 1

The pathophysiology involves:

• Hypovascular regions proximal to tendon insertion predisposing to hypoxic degeneration
• Repetitive eccentric load-induced microtrauma stressing peritendinous structures 2
• Progression from reactive tendinopathy (tenocyte proliferation, tendon thickening) to tendon disrepair (collagen fiber disruption) to degenerative tendinopathy (cell death, collagen disorganization) 3
• Neovascularization with ingrowth of new nerve fibers may contribute to pain 3

Key risk factors include:

• Training errors: excessive mileage/intensity, hill running, hard/uneven surfaces, poorly designed shoes 2
• Biomechanical abnormalities: gastrocnemius-soleus weakness/inflexibility, hindfoot malalignment with hyperpronation 2
• Female sex, higher BMI, prior tendinopathy, fluoroquinolone antibiotics 3

Acute Achilles Tendon Rupture

This represents a critical diagnosis that must not be missed, as it significantly impacts morbidity and functional outcomes. Diagnosis requires detailed history and physical examination including two or more of the following tests: 4

• Thompson test (Simonds squeeze test)
• Decreased ankle plantar flexion strength
• Palpable gap or defect
• Increased passive ankle dorsiflexion with gentle manipulation

Insertional Tendinopathy

Pain localized to the osteotendinous junction where tendon inserts into bone—the most common site of overuse injury. 1 This may be associated with:

• Bursitis subachillea
• Haglund's exostosis 5
• Retrocalcaneal bursitis 6

Plantaris Tendon Pathology (Often Overlooked)

The plantaris tendon can cause medial Achilles region pain and should be considered when conservative treatments fail. 7, 8 This includes:

• Plantaris tendinopathy
• Plantaris-associated Achilles tendinopathy with medial tenderness
• Plantaris tendon axial instability 7
• Plantaris tendon in rare superficial positions 8

Less Common but Important Differential Diagnoses

The following conditions may present as apparent Achilles pain and require differentiation: 5

• Paratenon irritation (aseptic inflammation)
• Ankle joint pathology: arthritis, chondropathic disease
• Biomechanical issues: hallux rigidus, rotation anomalies, tibia vara
• Os trigonum impingement syndrome
• Flexor tendon tendovaginitis at the retinaculum flexorum
• Stress fractures: calcaneus, fibula, tibia
• Metabolic diseases

Diagnostic Approach

Physical examination should include: 1

• Inspection for swelling, asymmetry, erythema
• Range-of-motion testing
• Palpation for tenderness and gaps
• Examination maneuvers that simulate tendon loading and reproduce pain
• Most useful clinical findings: morning stiffness, pain on palpation, Achilles tendon thickening 3

Imaging considerations:

• Plain radiography, ultrasonography, and MRI can be helpful if diagnosis remains unclear 1
• MRI provides slightly better sensitivity and specificity than ultrasound 3
• For acute rupture, routine imaging is not definitively recommended for confirmation 4
• Ultrasound is essential for visualizing plantaris tendon pathology 7, 8

Clinical Pitfalls

• Do not assume inflammation: Most chronic cases are degenerative, not inflammatory, making the term "tendonitis" inaccurate 1
• Do not overlook plantaris tendon: This can be in unexpected superficial positions and requires careful visualization 8
• Do not miss acute rupture: Always perform Thompson test and assess for palpable gap 4
• Consider biomechanical analysis: Examine running behavior and sports shoes worn 5
• Laboratory examination may be needed to rule out metabolic or systemic causes 5