M77.42 classifies metatarsalgia localized to the left foot — pain and inflammation at the metatarsal heads, typically aggravated by weight-bearing and tight footwear.

Documentation tips

What should appear in the chart to support M77.42.

Source · Editorial brief grounded in 5 cited references ↓

• Specify 'left foot' by name — vague terms like 'the affected foot' or 'ipsilateral' force a coder to interpret rather than confirm laterality.
• Document the clinical exam findings that establish metatarsalgia: location of tenderness (which metatarsal head[s]), metatarsal squeeze test result, and whether symptoms are weight-bearing-dependent.
• Note aggravating factors such as footwear type and activity level; these strengthen medical necessity for conservative treatment, orthotics, or injection.
• If imaging was obtained, document findings relevant to the forefoot (e.g., stress reaction, plantar plate injury, intermetatarsal bursitis) to support specificity and justify additional codes if warranted.
• Explicitly rule in or rule out Morton's neuroma in the note — co-existing documentation of both conditions without differentiation creates audit risk given the M77.4 Excludes1 note for G57.6-.

Related CPT procedures

Procedure codes commonly billed with M77.42. 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 M77.42 and adjacent codes.

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

• Coding M77.42 when Morton's metatarsalgia (G57.62) is the actual diagnosis — M77.4 has a Type 1 Excludes for G57.6-, making these codes mutually exclusive; do not report both for the same foot.
• Defaulting to M77.40 (unspecified foot) when the provider documents the left foot — laterality is required for full code specificity and unspecified codes invite payer downcoding or medical necessity denials.
• Using M77.42 for general left foot pain or plantar fasciitis — generalized foot pain maps to M79.67- and plantar fasciitis maps to M72.2; M77.42 requires documented metatarsal head involvement.
• Omitting M77.42 from the claim when a therapeutic injection to the left forefoot is billed — CMS LCD A57079 lists M77.42 as a required supporting diagnosis for soft-tissue injection reimbursement.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

Other billable codes under M77.4 (laterality / anatomic variants).