Acquired medial deviation of the right great toe, where the hallux angles toward the midline of the body — the opposite direction from hallux valgus — resulting from post-surgical, traumatic, or inflammatory causes rather than congenital malformation.

Documentation tips

What should appear in the chart to support M20.31.

Source · Editorial brief grounded in 4 cited references ↓

• Explicitly state 'right foot' or 'right great toe' — laterality must appear in the note, not just the order or superbill.
• Document the etiology: post-surgical overcorrection, trauma, or inflammatory arthropathy, since acquired status is required to use M20.31 over a congenital Q-code.
• Include weight-bearing radiograph findings — hallux-to-first-metatarsal angle, joint congruency, and any arthritic changes — to support medical necessity for surgical intervention.
• If the deformity follows a prior hallux valgus correction, note the original procedure and date; this establishes the acquired, post-procedural context.
• Record functional impact: pain with ambulation, shoe-wear problems, or gait alteration — payers use this to evaluate medical necessity for corrective surgery.

Related CPT procedures

Procedure codes commonly billed with M20.31. 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 M20.31 and adjacent codes.

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

• Assigning M20.31 for a congenital toe deformity — M20 has an Excludes1 for congenital deformities of toes (Q66.-, Q68-Q70, Q74.-); those codes are mutually exclusive with M20.31.
• Using M20.30 (unspecified foot) when laterality is documented in the chart — always assign the laterality-specific code when the side is known.
• Confusing hallux varus (M20.31, medial deviation) with hallux valgus (M20.11, lateral deviation) — the direction of the deformity determines the code family; verify with the clinical note and imaging.
• Failing to apply the -RT modifier on procedure codes when the payer requires it alongside the laterality-specific diagnosis code — M20.31 alone does not satisfy all payer laterality requirements at the CPT level.
• Coding bilateral hallux varus with only M20.31 — if both feet are affected, both M20.31 and M20.32 must be reported.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

Sibling codes

Other billable codes under M20.3 (laterality / anatomic variants).