Summary

Subungual Exostosis is a benign bony outgrowth (tumor) that arises from the dorsal aspect of the distal phalanx, typically of the hallux. It projects upwards, lifting the nail plate and causing intense pain and deformity. It is distinct from a true Osteochondroma (which arises from the metaphysis), as it is largely a reactive lesion caused by chronic micro-trauma. Clinically, it mimics an ingrown toenail, a wart, or even subungual melanoma. Diagnosis is confirmed by a simple lateral X-ray showing a "beak" of bone. Treatment is surgical excision, which provides a permanent cure. [1,2,3]

Key Facts

• The Mimic: Patients often treat this for months as a "wart" or "fungus" or "ingrown nail". The clue is that it is firm (hard as bone) and fixed to the deep structures.
• The Nail Effect: As the bone grows, it acts like a slow-motion car jack, lifting the nail plate off the nail bed (Onycholysis). This separation allows bacteria and debris to enter, causing secondary infection.
• The Difference:
  • Exostosis: Broad-based, reactive, no marrow continuity, fibrous cap. Distal Phalanx.
  • Osteochondroma: Pedunculated, true neoplasm, medullary continuity, hyaline cartilage cap. Metatarsals/Physeal.

Clinical Pearls

"X-ray Everything": Any persistent, painful subungual lesion needs an X-ray. If you don't X-ray it, you will miss the diagnosis 100% of the time.

"Save the Nail": You don't always have to remove the nail. You can just lift it up, remove the bone, and lay the nail back down as a splint.

"Fish Mouth": The classic incision is a curvilinear incision at the tip of the toe ("fish mouth"), allowing you to lift the soft tissue hood and access the bone without cutting through the nail bed.

                 PAINFUL TOE LUMP
                        ↓
                 LATERAL X-RAY
            ┌───────────┴───────────┐
          BONE SEEN             NO BONE
        (Exostosis)           (Soft Tissue)
             ↓                      ↓
          SURGERY            CONSIDER MRI/DERM
         (Excision)         (Glomus/Wart/Fus)

Limitations

• There is no medical treatment.
• Antibiotics only treat secondary infection.
• Soaks/analgesia are temporary.
• Excision is the only cure.

Excision Techniques

The goal is to remove the bone flush with the normal cortex to prevent recurrence.

1. Direct Dorsal Approach (Trans-ungual)

• Indication: Large central lesions.
• Technique:
  • Remove the nail plate (or fenestrate it).
  • Incise the nail bed longitudinally.
  • Rongeur/Burr the bone.
  • Repair the nail bed with 6-0 Vicryl.
• Risk: Nail dystrophy (ridge) if nail bed scarring occurs.

2. Fish-Mouth (Tip) Approach

• Indication: Distal lesions.
• Technique:
  • U-shaped incision at the tip of the toe.
  • Elevate the soft tissue + nail bed off the phalanx (like a hood).
  • Resect the bone.
  • Close the skin.
• Benefit: Avoids cutting the nail bed. Better cosmetic outcome.

3. Lateral/Medial Approach

• Indication: Eccentric lesions.
• Technique: Incision through the nail fold.

Approach Selection

• Suga et al: Compared direct vs lateral approaches. Found that preserving the nail bed (lateral/tip approach) resulted in significantly fewer nail deformities than the direct trans-ungual approach.

Malignant Transformation?

• Extremely rare/non-existent for typical exostosis. However, distinguishing from a subungual melanoma is critical (biopsy the soft tissue if suspicious).

The Lump

A spur of bone is growing out of the tip of your toe bone, pushing your nail up from underneath.

The Fix

We have to go in and shave the bone flat.

1. We usually have to remove the toenail to reach it.
2. We shave the bone.
3. The nail grows back slowly (6-9 months). It might have a small ridge or bump, but the pain will be gone.

Q1: What is the key radiographic difference between Subungual Exostosis and Osteochondroma? A:

• Exostosis: Distal Phalanx. No continuity with medullary canal. Fibrocartilage cap. Reactive.
• Osteochondroma: Metaphysis (proximal phalanx/metatarsal). Continuous with medullary canal. Hyaline cartilage cap. Neoplastic.

Q2: Why is the Lateral X-ray view critical? A: On the AP view, the exostosis is superimposed on the distal phalanx and is easily missed. The Lateral view shows the dorsal projection clearly.

Q3: Does the nail plate need to be removed? A: Not always. In the "Fish-Mouth" approach, the nail plate and bed are lifted as a unit, and the bone is removed from underneath. However, if the nail is severely deformed or infected, removing it facilitates exposure and healing.

Q4: What is the risk of incising the nail bed? A: Onychodystrophy. Cutting the sterile matrix can lead to scar formation, which prevents the nail plate from adhering (onycholysis) or causes a permanent longitudinal ridge/split.

(End of Topic)