Acquired collapse of the medial longitudinal arch of the left foot, documented as developing after birth due to an identifiable cause such as posterior tibial tendon dysfunction, trauma, or arthritis — not a congenital structural variant.

Documentation tips

What should appear in the chart to support M21.42.

Source · Editorial brief grounded in 5 cited references ↓

• Explicitly state 'left foot' in the assessment — laterality coded as '2' is only supported when the note names the side.
• Document the acquired etiology: posterior tibial tendon dysfunction stage, prior trauma, rheumatologic diagnosis, or obesity-related loading — distinguishing this from a congenital arch variant.
• Include weight-bearing X-ray findings: talar-first metatarsal angle, medial longitudinal arch height loss, or talonavicular coverage angle to substantiate structural collapse.
• If PTT pathology drives the diagnosis, document the MRI or ultrasound findings (tendon degeneration, tear, or attenuation) as clinical validation for both diagnosis and any surgical or orthotic claim.
• Record conservative care history (orthotics, physical therapy, NSAIDs) when the encounter involves DMEPOS authorization or surgical consultation — payers routinely require documented conservative treatment failure.

Related CPT procedures

Procedure codes commonly billed with M21.42. 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 M21.42 and adjacent codes.

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

• Using M21.42 for congenital flat foot: if the patient has lifelong bilateral flat feet with no documented acquired cause, Q66.52 (congenital pes planus, left) applies — M21.4x and Q66.5x are Excludes1 and cannot be reported for the same condition.
• Dropping to M21.40 (unspecified) when the provider clearly examined the left foot but failed to name it in the assessment — query the provider rather than defaulting to the unspecified code.
• Billing M21.42 alongside Z89.411 (acquired absence of left foot): M21.4x and Z89.41x are mutually exclusive per Excludes1 logic; a foot that is absent cannot simultaneously have an acquired flat foot deformity.
• Failing to report M21.41 when both feet are affected: M21.4 has no bilateral code, so both M21.41 and M21.42 must be reported together for bilateral acquired pes planus.
• Coding M21.42 from an ICD-9 crosswalk without confirming acquired vs. congenital status — the ICD-9 code 734 (flat foot) did not require this distinction, but ICD-10 enforces it.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

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