M17.4 captures bilateral knee osteoarthritis that is secondary in origin — meaning it developed as a consequence of an identifiable underlying cause other than trauma, such as rheumatoid arthritis, gout, septic arthritis, metabolic disease, or congenital deformity — and affects both knees simultaneously.
Verified May 8, 2026 · 5 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 12
- Region
- Knee
Documentation tips
What should appear in the chart to support M17.4.
Source · Editorial brief grounded in 5 cited references ↓
- Explicitly name the underlying cause of the secondary OA (e.g., 'bilateral knee OA secondary to longstanding rheumatoid arthritis') — payers and auditors need a documented etiology to distinguish M17.4 from primary OA codes.
- Confirm bilateral involvement is stated explicitly in the note; 'bilateral knees' must appear in the assessment or impression, not just inferred from the plan.
- When imaging supports the diagnosis, summarize relevant findings (Kellgren-Lawrence grade, joint space narrowing, osteophytes, subchondral sclerosis) for both knees — left and right findings documented separately strengthen medical necessity.
- Code the causative condition separately (e.g., M06.9 for rheumatoid arthritis, M10.00 for gout) and sequence based on the primary reason for the encounter.
- Document functional limitations (gait deficit, stair-climbing difficulty, ROM restriction) to support both the diagnosis code and any associated PT or injection procedures billed.
Related CPT procedures
Procedure codes commonly billed with M17.4. 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 M17.4 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Using M17.4 when the bilateral secondary OA is post-traumatic in origin — post-traumatic bilateral knee OA belongs under M17.2, not M17.4.
- Assigning M17.4 without a documented underlying secondary cause in the chart — without a named etiology, auditors will flag a downcode to M17.0 (bilateral primary OA) or M17.10 (unilateral primary, unspecified side).
- Splitting bilateral secondary OA into two unilateral codes when both knees share the same secondary etiology and are treated in the same encounter — M17.4 is the correct single code for true bilateral involvement.
- Failing to separately code the underlying condition (e.g., gout, RA) that drove the secondary OA, which can result in incomplete diagnosis capture and missed HCC documentation opportunities.
Clinical context
Source · Editorial summary grounded in 5 cited references ↓
Use M17.4 when the patient's bilateral knee OA is attributable to a non-traumatic secondary cause: prior inflammatory arthropathy, metabolic conditions (e.g., hemochromatosis, gout), congenital or developmental deformity, or other identifiable etiology documented by the treating provider. The 'other' qualifier distinguishes this from post-traumatic bilateral knee OA (M17.2), which has its own dedicated code. If the secondary cause is post-traumatic, do not use M17.4 — use M17.2 instead.
Within the M17 category, the secondary bilateral codes (M17.2 for post-traumatic, M17.4 for other secondary) require documentation of an underlying condition that caused or significantly contributed to the OA. Code the underlying condition separately and sequence it appropriately based on the reason for the encounter. If only one knee is secondary and the other is primary, you cannot use M17.4 — split the encounter to the appropriate unilateral codes.
M17.4 maps to MS-DRG v43.0 groups 553 (Bone diseases and arthropathies with MCC) and 554 (Bone diseases and arthropathies without MCC). It is explicitly listed as a supporting medical necessity code for home health physical therapy under CMS LCD A57311, making accurate diagnosis documentation critical for home health authorizations and therapy reimbursement.
Sibling codes
Other billable codes under M17 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 5 cited references ↓
01What separates M17.4 from M17.2?
02Do I need to code the underlying condition separately when using M17.4?
03Can I use M17.4 if only one knee is secondary and the other is primary?
04Is M17.4 accepted for home health physical therapy authorizations?
05What imaging documentation supports M17.4 for audit purposes?
06Does M17.4 require a 7th-character extension?
07Which MS-DRGs does M17.4 map to under v43.0?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
- 01CDC ICD-10-CM Tabular List 2026 (effective October 1, 2025)
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M15-M19/M17-/M17.4
- 03cms.govhttps://www.cms.gov/medicare-coverage-database/view/article.aspx?articleid=57311&ver=28& (CMS LCD A57311 — Billing and Coding: Physical Therapy - Home Health)
- 04stacks.cdc.govhttp://stacks.cdc.gov/view/cdc/250974 (ICD-10-CM Official Guidelines for Coding and Reporting FY 2026)
- 05aapc.comhttps://www.aapc.com/codes/icd-10-codes/M17.4
Mira AI Scribe
Mira's AI scribe captures the named underlying etiology driving the OA, explicit bilateral knee involvement (both sides documented), relevant imaging findings for each knee, and any prior treatment history (injections, PT, DMARDs). That documentation prevents downcoding to primary OA (M17.0) or an unspecified code, and closes the audit gap created when 'secondary' is asserted without a linked causative condition in the record.
See how Mira captures M17.4 documentation