CPT code 27446: Unicompartmental knee arthroplasty billing guide

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Key takeaways

CPT code 27446 covers unicompartmental knee arthroplasty, meaning the medial or the lateral compartment, never both.

The operative note decides between 27446 and 27447, so it has to name the compartment that was resurfaced.

Every 27446 claim carries LT or RT, and modifier 50 covers both knees treated on the same date.

The diagnosis has to match the knee you billed, which makes the unspecified code M17.9 a fast route to review.

Most rejections trace back to three things: laterality, compartment wording, or a thin conservative treatment history.

CPT code 27446 is the procedure code for arthroplasty of the knee, condyle and plateau, medial OR lateral compartment. In plain terms, it is the partial knee replacement code. One tibiofemoral compartment gets resurfaced, and the rest of the joint is left alone.

That capitalized “OR” is the whole billing story. Resurface both tibiofemoral compartments and the correct code becomes 27447 instead. So the operative note has to name the compartment, and the claim has to carry LT or RT.

What follows is the detail that keeps these claims moving: the rates, the modifiers, the diagnosis pairing, and the notes payers will read.

CPT code 27446 covers one compartment, not the whole knee

CPT code 27446 is the billable code for arthroplasty, knee, condyle and plateau, medial OR lateral compartment.

That wording comes straight from the American Medical Association’s CPT code set, and it sits in the Musculoskeletal System surgery section.

The knee has three compartments: medial tibiofemoral, lateral tibiofemoral, and patellofemoral. Code 27446 covers resurfacing of the condyle and plateau in exactly one of the first two.

Surgeons call it a unicompartmental or unicondylar replacement. Read the descriptor literally, because that “OR” sets an anatomical limit on what the code can cover.

When a partial knee replacement is the right call

Payers approve 27446 when the arthritis sits in one compartment and the rest of the joint still works.

The surgeon removes and resurfaces the cartilage and subchondral bone of one tibiofemoral compartment. A femoral component goes on the condyle, a tibial component on the plateau.

Because the procedure preserves the anterior cruciate ligament and the untouched compartment, that preserved anatomy is what supports the code.

If the ACL is gone, or the opposite tibiofemoral compartment is badly involved, the clinical picture points to a total arthroplasty instead. Reviewers know this, so they read the imaging notes closely.

On medical necessity, payers want documented evidence that the disease is confined to one compartment. Qualifying indications usually include:

• Unicompartmental osteoarthritis, medial or lateral, confirmed on weight-bearing radiographs
• An intact ACL on physical examination and imaging
• Minimal patellofemoral involvement
• Age, weight, and activity profile consistent with partial arthroplasty
• Failed conservative management, covering NSAIDs, physical therapy, and injections

That last point causes the most trouble later. Conservative care is easy to capture while it is happening, and painful to reconstruct months afterward for a prior authorization request.

27446 or 27447? The compartments decide

Compartment involvement decides the code, and nothing else does. Implant brand, surgical approach, and robotic assistance have no bearing on it.

27447 is the total knee arthroplasty code, covering both tibiofemoral compartments, with patella resurfacing at the surgeon’s discretion. The diagram below shows how the two codes split on the operative note alone.

The National Correct Coding Initiative, known as NCCI, treats 27446 and 27447 as mutually exclusive for the same knee on the same date of service.

Submitting both would need supporting documentation that almost never exists in practice. Payers reject the pair as a bundling violation.

What Medicare pays for 27446 in 2026

Medicare pays 27446 from Relative Value Units, not from a fixed national price. The Medicare Physician Fee Schedule holds the current values, and the Geographic Practice Cost Index then adjusts them for your locality.

So two identical claims in different states pay differently.

Here is how that plays out. A medial compartment replacement on the left knee, done in a hospital outpatient department, goes out as 27446-LT with M17.12.

The surgeon is paid on the facility values in the table above. The hospital bills its own facility claim for the same encounter, which is why the physician line looks small on its own.

Commercial contracts usually start from the Medicare rate and negotiate upward. Rather than assume parity, pull your own contracted rate for 27446 before you quote a patient. Pull the Medicare figure fresh each January too, since the values move every year.

LT or RT goes on every 27446 claim

A laterality modifier is mandatory on 27446. Submitting the code without LT or RT is one of the most common clean-claim failures across Medicare and commercial payers.

Several other modifiers apply to specific billing scenarios, and the table sorts them out.

Bilateral billing needs a second look before you send it. Medicare pays roughly 150% of the single-procedure rate for bilateral procedures on the same date.

Many commercial payers apply their own reduction rules instead, and some want LT and RT on separate lines rather than modifier 50.

The diagnosis has to match the knee you billed

The ICD-10 code on a 27446 claim has to document the compartment and the side. Put M17.11 for the right knee on a claim carrying modifier LT and the edit fires automatically, before a human ever reads the claim.

The laterality has to agree on every line. Our ICD-10-CM codes index is the quickest way to check a neighboring code.

Keep M17.9 off 27446 claims altogether. Reviewers running a medical necessity check expect a laterality-specific diagnosis. The unspecified code reads as incomplete documentation, and it invites an information request or a flat denial.

What the operative note has to say

The record has to prove both the diagnosis and the surgical approach before a payer will process the claim. Thin operative notes are the main driver of post-payment audits on this code.

Six items carry most of the weight:

• Pre-operative imaging: weight-bearing anteroposterior and lateral radiographs showing unicompartmental joint space narrowing. A Rosenberg or Schuss view helps. MRI is not required, but it strengthens the record.
• Compartment specification: the note must say which compartment was resurfaced, medial or lateral. “Knee replacement performed” is not enough, and it will trigger a medical necessity request.
• Implant documentation: implant name, manufacturer, and lot or serial numbers for the femoral and tibial components, matching the implant stickers in the record.
• Conservative treatment history: prior NSAIDs, corticosteroid or hyaluronic acid injections, and physical therapy, with dates and outcomes. Most LCDs want at least three months.
• ACL integrity: the operative note or pre-operative MRI confirming the ACL is intact, which most payer policies treat as a prerequisite.
• Laterality alignment: the operative note, the claim, and the ICD-10 code all naming the same knee. Any disagreement triggers an automatic edit.

Check prior authorization before you schedule

Traditional Medicare does not universally require prior authorization for 27446, but many Medicare Advantage and commercial plans do.

Requirements also change often, plan by plan. Verify them before the surgery is booked, never at the point of billing, because an unmet authorization after the fact is rarely recoverable.

CMS sets medical necessity criteria through Local Coverage Determinations, or LCDs, written by each Medicare Administrative Contractor. Criteria vary by jurisdiction, but most contractors look for the same things:

• Radiographic evidence of isolated medial or lateral compartment arthritis
• An intact ACL, confirmed clinically or by imaging
• No significant inflammatory arthritis, since rheumatoid disease usually points to a total knee
• Body weight inside the implant manufacturer’s specified range
• Failed non-surgical treatment over at least three months

You can read the active LCDs for knee arthroplasty in the CMS Medicare Coverage Database. Before you add any adjunct code to a 27446 claim, check the NCCI edits for the pair.

Orthopedic surgery codes are among the most heavily audited in outpatient and ASC settings.

How a 27446 claim moves from surgery to remittance

Knowing the route helps, because most 27446 denials happen at a handoff rather than in the billing software. A clean claim usually travels like this:

1. Authorization: the practice confirms the plan’s requirement and files the conservative care history, weeks before the surgery date.
2. Surgery and dictation: the surgeon dictates the note, naming the compartment, the ACL finding, and the implant components.
3. Coding: a coder reads the note, selects 27446, and attaches LT or RT plus the matching M17 code.
4. Scrubbing: the claim runs through edits that compare diagnosis laterality against the modifier, and flag NCCI conflicts.
5. Submission: the claim leaves as an 837P file through a clearinghouse, which routes it to the payer.
6. Adjudication and posting: the payer returns an 835 remittance, and the practice posts the payment or works the denial.

Steps two and three are where the money is won or lost. If the dictation and the code disagree, no amount of clean submission fixes it. The appeal then costs more staff time than the correction would have.

Codes that sit next to 27446 on the fee schedule

Knee arthroplasty cases bring a small cluster of related codes with them. Knowing the boundaries between them prevents both mis-selection and NCCI bundling problems.

Four mistakes that sink 27446 claims

This code denies more often than other major surgical codes, because the clinical detail behind it is narrow and specific. Four errors account for most of the rejections.

1. Billing 27447 when the note says one compartment

This is the most common error on these claims. Intentional or not, it counts as upcoding, and it triggers post-payment recovery. Reviewers compare the compartment language in the note against the billed code. Read the note for “medial compartment” or “lateral compartment” before you choose between the two codes.

2. Laterality that does not line up

Sending 27446 with no laterality modifier, or pairing modifier RT with M17.12, produces an automatic edit. No clinician reviews these. They are system-level rejections, and the fix belongs at the pre-submission scrubbing stage rather than after the remittance posts.

3. Conservative care scattered across the chart

Many practices record conservative treatment across a dozen progress notes and never pull it together. A reviewer wants one clear timeline, not a reading exercise. Filing a consolidated conservative treatment summary as its own document in the record cuts information request turnaround dramatically.

4. Reaching for M17.9 by default

Choosing M17.9 over M17.11 or M17.12 tells the payer the documentation is incomplete. Specificity means matching the code to the detail already in the record. Picking the broadest available option is what invites the review. In a high-volume practice, an audit of six months of 27446 claims will usually surface M17.9 as a recurring pattern.

Run this check before you submit

Two minutes on this list is cheaper than a 30-day appeal. Work through it before the claim leaves the practice:

• The operative note names the compartment, medial or lateral, in words
• The modifier, the diagnosis, and the note all name the same knee
• The diagnosis is laterality-specific, so no M17.9
• Conservative care covers at least three months, with dates
• The ACL finding appears in the note or the pre-operative imaging
• Authorization is on file for this plan, this procedure, and this date
• Implant components and lot numbers are recorded against the encounter

If any line fails, hold the claim and fix the record first. A query to the surgeon takes a day. Denials take a month, and they usually come back asking for the same information.

How Pabau keeps 27446 claims moving from note to payment

In most orthopedic practices the 27446 trail is split across systems. The operative note sits in one place, and the authorization in an email thread. The claim lives in a billing tool, and the remittance in a bank file. Reconstructing all of it for an audit takes hours nobody has.

Practice management software like Pabau keeps that trail on one record, with claims management built in. The procedure code attaches to the encounter, and the supporting documents stay with the patient file.

Claims go out electronically to thousands of US payers through the Claim.MD clearinghouse. Remittances post back against the same record.

So when a 27446 claim is questioned six months later, the whole trail sits in one thread. The note, the code, the submission, and the payer’s response are all there. Your team answers the request instead of assembling it.

Keep orthopedic coding and claims on one record

Pabau attaches procedure codes to the encounter they came from, submits claims electronically, and posts remittances back against the same patient record. Your coders stop chasing paperwork across systems to answer a payer request.

Book a demo

Conclusion

Billing 27446 comes back to one word in the descriptor: “OR.” Medial or lateral, one compartment, not both. When the note, the diagnosis, the modifier, and the code all agree on that single fact, these claims clear on the first pass.

So the work is upstream of the billing office. Fix the dictation template so the compartment is always named, and the denials mostly stop. Keep treating it as a claims problem and you will keep appealing the same rejection every month.

If your 27446 documentation currently lives in four systems, that is what to change first. Book a demo to see how Pabau keeps the operative note, the procedure code, and the claim on one patient record.

Continue your research

Need to understand the clearinghouse submission process? EDI 837 file submission explains how CPT claims reach payers electronically and what the file format requires.

Want to reduce claim denials before they reach the payer? Clean claim requirements covers the pre-submission checklist that orthopedic billing teams use to prevent edits.

Handling ERA reconciliation after 27446 payments post? Electronic remittance advice (ERA) explains how 835 files map back to your CPT claims and what denial codes to watch for.

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