Short answer: CPT 27599 (unlisted femur/knee procedure) is the more technically accurate code for patellofemoral arthroplasty (PFA). But across three commercial payers I contacted directly, 27438 processes and pays without a flag while 27599 routes into manual pricing at 60–80% of the 27447 allowable. The right code depends on your payer, your documentation, and whether predetermination is required.
What the CPT Book Actually Says (and Where the Ambiguity Comes From)
CPT 27438 reads: “Arthroplasty, patella; with prosthesis.” On its face, that sounds like exactly what a PFA is a prosthetic component is placed against the patella.
But a PFA is not a patella-only procedure. It resurfaces both the trochlear groove of the femur and the back of the patella with two separate prosthetic components that articulate with each other. CPT 27438 was written decades ago to describe a patelloplasty: a single prosthetic button placed on the posterior patella, with no femoral component involved at all.
The femoral trochlear component is the part that has no listed code. There is no CPT code that describes resurfacing the trochlear groove with a prosthetic implant outside of a total knee arthroplasty (27447), which includes all three compartments.
This is the actual coding gap. PFA involves two prosthetic components across two bones, and the CPT code set has a code for one component on one bone (27438), and a code for all three compartments together (27447), and nothing in between.
The American Academy of Orthopaedic Surgeons (AAOS) and several orthopedic coding consultants have published guidance recommending 27599 for PFA, on the basis that no listed code accurately describes the bicompartmental nature of the procedure. That guidance is not binding on payers, but it represents the mainstream coding consultant position.
Understanding when and how to use unlisted CPT codes without triggering an audit is essential before you decide which path to take for PFA claims.
Payer 1: Large National Commercial Carrier
What their published medical policy says
This payer’s published policy on knee arthroplasty procedures listed CPT 27447 (total knee) as covered and payable, listed 27438 (patella with prosthesis) as covered and payable, and stated that unlisted procedure codes (including 27599) would be subject to medical review and priced at the medical director’s discretion using a comparable listed procedure as a reference.
What the rep said
The provider services rep confirmed 27438 was on the fee schedule and payable. When I described a PFA specifically two component, bicompartmental, femoral trochlea plus patella and asked whether 27438 accurately described that procedure in their system, the rep said: “The code is listed, it’s a covered code, it’s payable. I can’t speak to whether it’s the exact right code from a coding accuracy standpoint, that would be a medical policy question.”
The medical policy line confirmed that 27438 is treated as a clean, listed, payable code and that it would not automatically flag for review just because it is a PFA unless something else on the claim triggered a review.
Practical takeaway
For this payer, 27599 is technically the more defensible code per their own unlisted-procedure policy, but it routes into a manual pricing process landing at 60–75% of the 27447 allowable, decided case by case. Submitting 27599 means building a comparable procedure narrative for unlisted CPT codes from the start: cite 27447 as the comparison, and proactively state the percentage of work relative to a full TKA, rather than waiting for the consultant’s reduction.
Payer 2: Regional Blue Plan
What their published medical policy says
This Blue plan’s policy listed patellofemoral arthroplasty as a covered procedure and described it as a bicompartmental resurfacing involving both the patella and the femoral trochlea. Separately, 27438 was listed as covered for patella arthroplasty with prosthesis, with the description referencing a patella button or resurfacing procedure. 27599 was not on their fee schedule, as unlisted codes do not carry a set fee schedule rate.
What the rep said
The medical policy line was more direct than the first payer. When I described a PFA and asked whether 27438 or 27599 was the appropriate code, the rep said: “For an actual patellofemoral arthroplasty where you have both a femoral and a patellar component, the more accurate code would be 27599, because 27438 only describes the patella portion. But if 27438 is submitted, it’s going to pay it’s a listed, payable code, and our system isn’t going to reject it or flag it as miscoded.”
I asked how 27599 would be priced for a PFA. The rep described a specific internal pricing methodology: “For a patellofemoral arthroplasty, our pricing team uses 27438 as the base, and then adds a percentage of 27447 to account for the femoral component work. The way we’ve seen it come through, it ends up somewhere around 27438 plus 50 percent of the difference between 27438 and 27447, capped at 80 percent of 27447.”
The rep also offered an explanation for why a payer rep might tell a practice that 27438 is “the correct code”: “Sometimes that comes from a benefits or claims rep who’s looking at whether the code is covered and payable, not whether it’s the most accurate code. 27438 is on our fee schedule. 27599 isn’t, because unlisted codes don’t have a set fee schedule rate. So from a benefits standpoint, a rep might say 27438 is correct simply because it’s the one that’s listed and payable, without understanding the coding nuance.”
Practical takeaway
This is the most operationally useful data point from the three calls. Both 27438 and PFA-specific 27599 submit cleanly, but 27599 actually pays more for a true PFA and this payer has an internal methodology for pricing it. The practical implication: for this Blue plan, submitting 27599 with a properly documented PFA claim is worth pursuing, because their own pricing methodology results in a higher payment than 27438 alone, and their medical policy team explicitly confirmed that 27438 alone is not the most accurate code for a bicompartmental procedure.
Strong medical necessity documentation that clearly describes both the femoral and patellar components is what makes this argument land during review.
Payer 3: Mid-Size Commercial Carrier
What their published medical policy says
This payer’s policy manual states that unlisted procedure codes will not be reimbursed without prior authorization or predetermination, including supporting documentation and a recommended comparison code from the submitting provider.
What the rep said
This call was shorter because the rep was direct from the start: “For 27599 on a knee, we require predetermination. If that wasn’t done before the surgery, the claim is going to deny for missing predetermination, separate from any coding question.”
When I asked what happens if 27438 was billed instead without predetermination since 27438 doesn’t require it the rep said: “Then it processes normally as 27438. It wouldn’t deny for predetermination because 27438 isn’t on our predetermination list. It would just pay at the 27438 rate.”
I asked whether a corrected claim could be submitted changing 27438 to 27599 with retroactive predetermination. The answer was no: “We don’t do retroactive predetermination. If the claim was submitted and paid as 27438, and you now want to rebill as 27599, the corrected claim is going to deny because there’s no predetermination on file, and there’s no way to add it after the fact.”
There was an additional complication this rep raised: if a corrected claim is submitted changing 27438 to 27599, the payer may treat the original 27438 payment as an overpayment on the corrected claim, potentially triggering a recovery request on the original payment amount.
Practical takeaway
For payers with unlisted code predetermination requirements, the coding decision for PFA effectively needs to be made before surgery. If predetermination is required for 27599 and was not obtained, submitting 27599 after the fact is not just difficult it may be impossible without also creating an overpayment question on the original payment. Review your prior authorization checklist for surgical procedures to ensure unlisted code requirements are verified before the date of service.
Summary: What Each Payer Actually Does
| Payer 1 (National Commercial) | Payer 2 (Regional Blue) | Payer 3 (Mid-Size Commercial) | |
|---|---|---|---|
| 27438 alone for PFA | Pays clean at 27438 rate, no flag | Pays clean at 27438 rate, no flag | Pays clean at 27438 rate, no flag |
| 27599 pricing method | Manual review, 60–75% of 27447 | 27438 + 50% of 27447, capped at 80% of 27447 | Requires predetermination (unavailable retroactively) |
| Is 27438 alone “correct” per medical policy? | Not addressed directly | Explicitly no — but payable as billed | Not addressed directly |
| Key undocumented detail | Reduction percentage is consultant discretion | Benefits reps may confuse payability with coding accuracy | Retroactive code correction effectively unavailable |
What This Means for How You Code This Procedure
Across all three payers, one fact held constant: 27438 submitted alone for a PFA will be paid, cleanly, at the patella-only rate, with no system-level check on whether that code accurately describes a bicompartmental procedure. None of the three payers’ claims engines flag this. That is exactly why some practices end up “getting away with” 27438 for years the system pays it, so it looks correct, even though it underpays for the femoral component work.
CPT 27599 is the more technically accurate code per the AMA CPT code description guidelines and per two of the three payers’ own statements when pressed on coding accuracy specifically. But it comes with a cost: manual pricing, often in the 60–80% range of 27447’s allowable depending on the payer, plus predetermination requirements at some carriers.
The decision framework is actually cleaner than it looks:
- If your payer has predetermination requirements for unlisted knee codes: The decision needs to happen before surgery. Obtain predetermination for 27599 with a documented comparison to 27447 and a stated percentage of work. If you missed this step and already billed 27438, the window for correcting to 27599 may be closed at that payer.
- If your payer prices 27599 using an internal methodology tied to 27447: Submitting 27599 with thorough op note documentation and a stated comparison to 27447 is likely to yield more revenue than 27438 alone, and is the more accurate code. The argument to the payer is simple: this is a bicompartmental resurfacing involving both a femoral and a patellar prosthetic component, 27438 captures only one component, and 27447 overvalues the procedure relative to the work involved.
- If your payer processes 27438 cleanly and you have no published guidance suggesting otherwise: This is a billing compliance question as much as a coding question. The code pays, but it doesn’t fully describe the procedure. Whether that creates audit or overpayment exposure over time depends on the payer and their auditing patterns. OIG audit triggers in 2026 increasingly focus on orthopedic procedure upcoding and undercoding scenarios, making coding accuracy more important than ever.
If 27438 has already been paid for past PFA cases and you are now considering whether to pursue corrected claims, weigh that against Payer 3’s point about retroactive correction creating overpayment exposure on the original payment. This is worth a conversation with your billing compliance team on a payer-by-payer basis, not a blanket resubmission strategy. Understand your appeal rights first how to appeal CPT unlisted code denials follows a different process than standard claim corrections.
For any payer where you do submit 27599, Box 19 on the CMS-1500 for unlisted procedures must reference the comparable code (27447) and the nature of the comparison this is what triggers the manual review team to price it correctly rather than defaulting to a blanket reduction.
Frequently Asked Questions
Is 27599 always the correct code for patellofemoral arthroplasty?
Per CPT description, 27599 is the more accurate code because PFA involves prosthetic resurfacing of both the patella and the femoral trochlea, which 27438 alone does not capture. However, “more accurate per CPT description” and “what a specific payer will pay without friction” are different questions. Some payers pay 27438 cleanly while requiring extensive manual review and predetermination for 27599. The technically correct answer is 27599; the operationally correct answer depends on the payer.
Why would a payer representative say 27438 is correct if it isn’t a complete description of PFA?
Based on these calls, this often comes from a benefits or claims representative confirming that 27438 is a listed, payable code on their fee schedule not from a medical policy or coding review determination. The rep may be answering “is this code covered?” rather than “is this code accurate?” Those are different questions with different answers. When you need a definitive coding ruling, ask to speak with the medical policy line rather than general provider services.
What diagnosis codes support a PFA claim?
The primary supporting diagnosis is typically M22.2x (patellofemoral disorders) or M17.31/M17.32 (post-traumatic osteoarthritis of the knee). Some payers also accept M17.11/M17.12 (primary osteoarthritis of the knee) for PFA when the operative report clearly documents isolated patellofemoral compartment disease. The diagnosis coding must align with the CMS ICD-10-CM guidelines for the procedure to be considered medically necessary.
Does billing 27438 for a PFA risk an audit or overpayment finding?
None of the three payers indicated that 27438 billed alone for PFA currently triggers a flag or review at adjudication. Whether it represents an overpayment exposure if identified later through a payer audit is a separate question. One payer specifically noted that correcting a paid 27438 claim to 27599 after the fact can create its own overpayment question on the original payment. Consult your compliance team before pursuing retroactive corrections.
Should predetermination be obtained before billing 27599 for PFA?
For payers with unlisted-code predetermination requirements, yes and that determination must happen before surgery. One payer in this investigation stated directly that retroactive predetermination is not available, meaning the coding decision for PFA effectively needs to be finalized during the authorization process. Check each payer’s policy on unlisted procedure predetermination before the date of service, every time.
Final Thought
This is a case where the CPT code set itself has a real gap. PFA didn’t exist as a common procedure when 27438 was written, and there is no code that cleanly describes a two-component, two-bone resurfacing that is less than a full TKA. That gap gets resolved differently by different payers, different departments within the same payer, and sometimes differently depending on which specific representative answers the phone.
The original question whether 27599 or 27438 is correct does not have a single universal answer based on these three calls. It has three different operational answers depending on the payer, and a recurring pattern where the rep’s answer depends on which department is answering and what question they think they’re being asked.
Have you had a different experience getting PFA coded and paid? Drop your payer, the code you used, and the outcome in the comments. The more data points across payers, the more useful this becomes for everyone working these claims.
Medical Billing 101 is a trusted resource dedicated to helping healthcare providers, billing professionals, and patients navigate the complex world of medical billing and coding. Our team of certified medical billing specialists and healthcare finance experts brings years of hands-on experience in revenue cycle management, CPT/ICD-10 coding, insurance claims processing, and compliance. We are committed to delivering accurate, up-to-date, and actionable information to help you maximize reimbursements, reduce claim denials, and stay compliant with ever-changing healthcare regulations.