M20.42 identifies an acquired hammer toe deformity affecting one or more toes of the left foot, distinguished from congenital deformity by its post-birth onset.

Documentation tips

What should appear in the chart to support M20.42.

Source · Editorial brief grounded in 6 cited references ↓

• Specify laterality explicitly — document 'left foot' by name, not just 'affected foot,' to support M20.42 over the unspecified M20.40.
• Document whether the deformity is flexible or rigid; rigidity affects surgical planning and supports medical necessity for operative intervention.
• Record the specific toe(s) involved (e.g., second, third) and any secondary findings such as corns, callosities, or metatarsalgia that warrant additional codes.
• Confirm and document acquired etiology — note footwear history, duration of symptoms, or underlying neuromuscular condition to distinguish from congenital deformity.
• Include imaging results (weight-bearing foot X-ray findings, angular measurements) and conservative treatment history (orthotics, padding, footwear modification) when supporting surgical necessity.

Related CPT procedures

Procedure codes commonly billed with M20.42. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

Common coding pitfalls

The recurring mistakes coders make with M20.42 and adjacent codes.

Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓

• Using the non-billable parent code M20.4 instead of the laterality-specific M20.42 — payers will reject or downcode claims without the fifth character.
• Coding a congenital hammer toe as M20.42 — acquired and congenital deformities map to different code families; verify onset documentation before assigning any M20.4x code.
• Confusing hammer toe (M20.42) with mallet toe or claw toe, which map to M20.5X2 (other deformities of toe[s], left foot) — the distinction matters for both diagnosis accuracy and procedure code pairing.
• Pairing M20.42 with CPT 28308 without verifying payer policy — at least one payer has denied this combination, citing a more appropriate diagnosis requirement for that osteotomy code.
• Omitting secondary codes for concurrent conditions such as corns (L84) or plantar callosities when they are separately documented and treated.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M20.4 (laterality / anatomic variants).