Acquired collapse of the medial longitudinal arch of the right foot, coded specifically when pes planus developed after birth due to injury, tendon dysfunction, arthritis, or other acquired cause — not a structural anomaly present at birth.

Documentation tips

What should appear in the chart to support M21.41.

Source · Editorial brief grounded in 7 cited references ↓

• Document laterality explicitly — 'right foot' must appear in the note; 'bilateral' alone is insufficient for M21.41.
• Distinguish acquired from congenital: note the onset, any prior normal arch function, and the absence of a birth-related structural defect.
• Record the underlying cause or contributing factors — PTTD, prior trauma, inflammatory arthritis, or obesity — to support medical necessity and justify additional codes.
• Include weight-bearing X-ray findings: arch collapse angle, talo-first metatarsal alignment, or loss of calcaneal pitch supports the diagnosis and satisfies payer documentation requirements.
• If MRI was ordered, document whether posterior tibial tendon pathology (tear, tendinosis) was identified — this links the mechanism to the deformity and may require a separate tendon code.
• Record conservative care history (orthotics, physical therapy, bracing) when coding in the context of surgical pre-authorization to demonstrate medical necessity.

Related CPT procedures

Procedure codes commonly billed with M21.41. 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 M21.41 and adjacent codes.

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

• Using M21.41 for congenital flat foot — the Excludes1 note under M21.4 prohibits combining M21.41 with Q66.5-; if onset is at birth or childhood without acquired cause, use Q66.51 instead.
• Defaulting to M21.40 (unspecified foot) when laterality is documented — providers almost always note the affected side; query before accepting an unspecified code.
• Failing to code the underlying cause separately — PTTD or inflammatory arthritis that drove the arch collapse should be coded in addition to M21.41, not instead of it.
• Reporting M21.41 alone for bilateral acquired flat foot — both M21.41 and M21.42 must be reported when both feet are affected; there is no single acquired bilateral code.
• Applying a 7th-character extension to M21.41 — M-codes in this category do not use 7th-character extensions; adding one creates an invalid code.

Clinical context

Source · Editorial summary grounded in 7 cited references ↓

Sibling codes

Other billable codes under M21.4 (laterality / anatomic variants).