Bilateral Knee Osteoarthritis: One Code or Two?
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Editorial explanation — not an official CMS/CDC rule. Official coding instructions and source material are identified below. How to read the labels
Current data releases:ICD-10-CM FY2027 · ICD-10-PCS FY2027 · HCPCS October 2026 · MS-DRG v44 · Medicare Code Editor v44.0 · NCCI PTP Q4 2026 · MUE Q4 2026 · NCD code lists 2026-01 · LCD export September 28, 2026 · All releases and sources
Quick answer · MedCoder editorial
When both knees have primary osteoarthritis, assign the single bilateral code M17.0 — not M17.11 plus M17.12. The two-unilateral pattern is only correct when the classification provides no bilateral code, and for primary knee osteoarthritis it does.
Jump to the decision rule ↓What decides the answer?
- Which knee or knees, and whether the osteoarthritis is primary, post-traumatic or secondary
- A bilateral code (M17.0) for both knees, one code
- For staged replacements, whether the first knee has resolved before the second encounter
Why it matters
The laterality rule, and where the two-code pattern actually belongs
This page exists because the pair M17.11 + M17.12 is the single most frequent conflict real claims hit in this site’s Claim Check tool. The laterality guideline (Section I.B.13) settles it:
Official source“Some ICD-10-CM codes indicate laterality, specifying whether the condition occurs on the left, right or is bilateral. If no bilateral code is provided and the condition is bilateral, assign separate codes for both the left and right side. If the side is not identified in the medical record, assign the code for the unspecified side.”
The two-unilateral pattern is the fallback for categories without a bilateral code — and category M17 is not one of them. M17.0 is Bilateral primary osteoarthritis of knee; M17.2 covers the bilateral post-traumatic form. M17.11 and M17.12 assert unilateral disease of the right and left knee — coding both on one claim states two contradictory unilateral assertions rather than one bilateral condition, which is exactly why the pair trips the laterality edit.
The separate-encounters rule, which surprises people twice
The same guideline goes further than most coders expect. When a bilateral condition is treated one side at a time across separate encounters:
Official source“…assign the ‘bilateral’ code (as the condition still exists on both sides), including for the encounter to treat the first side. For the second encounter for treatment after one side has previously been treated and the condition no longer exists on that side, assign the appropriate unilateral code for the side where the condition still exists…”
So the encounter for the first knee replacement in a bilateral patient still takes M17.0 — the disease still exists on both sides. The unilateral code’s moment comes only after one side has been treated and the condition is gone there: the second encounter takes M17.11 or M17.12 for the knee that still has it. The unilateral codes describe the patient’s current state, not the surgical schedule.
How to apply it
How to apply the rule
- Confirm both knees have primary osteoarthritis.
- Assign the single bilateral code, M17.0 — not M17.11 plus M17.12.
- For one knee only, the unilateral code for that side; post-traumatic and secondary osteoarthritis have their own bilateral and unilateral codes.
- When both knees are treated in separate encounters, keep the bilateral code until one knee is resolved (e.g., replaced), then the unilateral code for the remaining knee.
Examples
Coding examples
Common mistakes
Common mistakes
Documentation matters
Documentation that changes the coding
- Which knee(s), and whether the osteoarthritis is primary, post-traumatic or secondary.
- Whether one knee has been treated and resolved, for the second encounter.
Decision rule
Decision framework
FAQ
Can M17.11 and M17.12 be coded together?
Not to describe bilateral osteoarthritis — the classification provides M17.0 for exactly that, and the two unilateral codes each assert single-sided disease. The guideline’s separate-codes-for-each-side instruction applies only where no bilateral code exists.
Which code applies for the first knee replacement when both knees have osteoarthritis?
M17.0, the bilateral code — the guideline says to assign it “including for the encounter to treat the first side,” because the condition still exists on both sides at that point. The unilateral code belongs to the later encounter, after one side has been treated and no longer has the condition.
What if the record shows knee osteoarthritis but no side?
Assign the unspecified-side code, per the laterality guideline — for unilateral primary knee osteoarthritis that is M17.10. A sided or bilateral statement in the record always beats it, so the unspecified code is also a reasonable prompt for a documentation query.
Does the M17.0 rule cover post-traumatic knee osteoarthritis too?
The same structure does: M17.2 is the bilateral post-traumatic code, with M17.31 and M17.32 as its unilateral siblings. The principle transfers to any category that provides a bilateral code — check for one before reaching for the right-plus-left pair.
Code set release
This guide was verified against ICD-10-CM FY2027, effective October 1, 2026.
Code relationships
| Relationship | At | Names | Note |
|---|---|---|---|
| laterality | M17.0 | bilateral primary — one code for both knees | |
| laterality | M17.1 | M17.11 right / M17.12 left (unilateral primary) | |
| parent-child | M17 | M17.0 bilateral primary / M17.1- unilateral primary / M17.2 bilateral post-traumatic / M17.4 bilateral secondary |
Relationships are quoted from the FY tabular notes and Official Guidelines the guide cites; none are inferred.
Relationships are quoted from the tabular notes and Official Guidelines sections the guide cites; the wording of each row is MedCoder’s.
Compare these pairs
Authoritative sources
Applies to FY2027 ICD-10-CM
Verified against the FY 2027 ICD-10-CM Official Guidelines (Section I.B.13, Laterality) and the FY 2027 tabular entries for category M17.
- ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027 (Section I.B.13, Laterality)
- FY 2027 ICD-10-CM Tabular List (category M17)
Official guidance cited
The Official Guidelines sections this guide quotes or applies, by heading. Passages set as quotations above are the sections’ own words; the surrounding text is MedCoder’s reading of them.
- Section I.B.13 — Laterality
Passages set as quotations are verbatim official text from the sources listed. Surrounding explanation, decision tables and examples are MedCoder’s editorial reading of those sources and are not official statements. Examples are illustrative; they assign no code to any real patient.
Coder Takeaway: Both knees with primary osteoarthritis is the single bilateral code M17.0, not M17.11 plus M17.12; after one knee is resolved, the unilateral code for the remaining knee.
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Applicable code set: FY2027. Published September 3, 2026; last updated October 1, 2026; last reviewed October 1, 2026.