CPT Code for Partial Knee Arthroplasty: A Complete Billing Guide

If you have been searching for the correct CPT code for partial knee arthroplasty, you probably already know that knee surgery coding can feel like navigating a maze.

You are not alone.

Even experienced coders sometimes pause when distinguishing between a total knee replacement and a partial one. The good news is that once you understand the logic behind the codes, the process becomes much clearer.

In this guide, we will walk through everything you need to know. We will cover the specific codes, the differences between medial, lateral, and patellofemoral compartments, and the common billing mistakes to avoid.

Let us get started.

What Exactly Is a Partial Knee Arthroplasty?

Before we look at the numbers, we need to understand the procedure itself.

A partial knee arthroplasty is also known as a unicompartmental knee replacement. Unlike a total knee replacement, where the surgeon replaces all three compartments of the knee joint, a partial replacement only addresses the damaged compartment.

The knee has three main compartments:

• The medial compartment (inside of the knee)

• The lateral compartment (outside of the knee)

• The patellofemoral compartment (behind the kneecap)

When only one of these areas shows significant arthritis or damage, a partial replacement offers a less invasive option. Patients often recover faster and retain more natural knee motion compared to a total replacement.

But from a coding perspective, the difference matters greatly. You cannot simply use the same code for every partial procedure.

The Primary CPT Code for Partial Knee Arthroplasty

Let us answer the main question directly.

The primary CPT code for a standard partial knee arthroplasty (unicompartmental) is 27446.

The official descriptor for CPT 27446 reads: Arthroplasty, knee, condyle and plateau; medial OR lateral compartment.

In simple English, this code covers the replacement of either the inside part of the knee (medial) or the outside part (lateral). It does not cover both. It also does not cover the kneecap area.

When to Use 27446

You will report 27446 when the surgeon performs a replacement of a single compartment using a unicondylar implant. This is the classic “partial knee” that most orthopedic surgeons refer to in conversation.

What 27446 Includes

The work of this code typically includes:

• Exposure of the knee joint

• Resurfacing of the femoral condyle

• Resurfacing of the tibial plateau

• Placement of the unicompartmental prosthesis

• Closure of the surgical site

It is important to note that this code does not include the replacement of the patella unless otherwise specified.

The Second Key Code: Patellofemoral Arthroplasty

What happens when the damage is only behind the kneecap?

Standard partial knee replacement codes do not cover the patellofemoral joint. For that, we have a separate code: 27447.

Wait a moment.

You might recognize 27447 as the code for total knee arthroplasty. That is correct. However, there is a specific modifier or alternative approach for isolated patellofemoral replacement. Let me clarify.

Actually, the correct code for isolated patellofemoral arthroplasty is 27438.

Yes. CPT 27438 is specifically designated for Arthroplasty, patellofemoral joint.

The official descriptor for CPT 27438 reads: Arthroplasty, patellofemoral joint.

Key Differences Between 27446 and 27438

As you can see, choosing the right code depends entirely on which compartment the surgeon addresses.

Bicompartmental Knee Arthroplasty: A Gray Area

Now things get interesting.

What if the surgeon replaces two compartments but not all three? For example, the medial and patellofemoral compartments, but the lateral compartment remains natural.

Currently, there is no specific CPT code for bicompartmental knee arthroplasty.

This creates a real challenge for coding professionals. Most guidance suggests using the total knee arthroplasty code 27447 when two compartments are replaced. Why? Because the work involved more closely resembles a total replacement than a partial one.

However, some payers may accept 27446 with a modifier if the documentation clearly supports the limited nature of the procedure. You should always check with your specific payer before making this decision.

Important Note: When in doubt, let the operative report guide you. The surgeon’s documentation of which compartments were resurfaced is the final word. Never assume based on the implant name alone.

Complete List of Related CPT Codes

To give you a full picture, here is a table of all relevant codes for partial knee procedures and related services.

Modifiers Commonly Used with Partial Knee Codes

Modifiers change the meaning of a code without changing the code itself. They are essential for accurate billing.

Modifier -LT and -RT

Since the knee has a left and right side, you must specify which knee received the procedure.

• -LT: Left side

• -RT: Right side

Example: *27446-LT* for a left medial unicompartmental knee replacement.

Modifier -50 (Bilateral Procedure)

Partial knee arthroplasty on both knees during the same surgical session requires modifier -50. You report the code once with the modifier, and the payer typically reimburses at 150% of the fee schedule.

Example: *27446-50*

Modifier -22 (Increased Procedural Services)

Sometimes a partial knee replacement becomes more complex than usual. Adhesions, obesity, or unusual anatomy may increase the work. In these cases, you can add modifier -22. However, you must submit supporting documentation. Payers do not automatically approve this modifier.

Modifier -58 (Staged Procedure)

If a patient has a partial knee replacement and later, during the postoperative period, requires a conversion to a total knee replacement, you would use modifier -58. This tells the payer that the second procedure was planned or staged.

How to Differentiate Between 27446 and 27447

This is where many coders lose confidence. Let us break it down with a simple decision tree.

Step One: Count the Compartments

Ask yourself: How many compartments did the surgeon resurface?

• One compartment → Look at 27446 or 27438

• Two compartments → Generally 27447 (total knee)

• Three compartments → 27447 (total knee)

Step Two: Identify Which Compartment

If only one compartment:

• Medial OR lateral → 27446

• Patellofemoral → 27438

Step Three: Check the Patella

Did the surgeon replace the patellar surface?

• For 27446: Usually no patellar replacement.

• For 27438: Patellar replacement is part of the procedure.

• For 27447: Patellar replacement is typically included.

Real-World Scenarios and Coding Examples

Let us look at some operative report summaries. These examples will help you apply the rules in practice.

Scenario One: Standard Medial Unicompartmental

Operative Report Summary: “The patient has isolated medial compartment osteoarthritis. We performed a medial parapatellar approach. The medial femoral condyle and medial tibial plateau were resurfaced using a fixed-bearing unicompartmental knee system. The lateral compartment and patellofemoral joint were intact and left untouched.”

Correct CPT Code: 27446-LT or 27446-RT

Why: Only the medial compartment was resurfaced. The lateral and patellofemoral compartments were preserved.

Scenario Two: Isolated Patellofemoral Arthritis

Operative Report Summary: “Arthroscopic evaluation confirmed isolated patellofemoral chondromalacia. The trochlear groove and patellar articular surface were resurfaced with a patellofemoral inlay implant. The medial and lateral compartments were normal.”

Correct CPT Code: 27438-LT or 27438-RT

Why: The surgeon only worked on the patellofemoral joint. The condyles and tibial plateaus were not touched.

Scenario Three: Conversion from Partial to Total

Operative Report Summary: “The patient had a previous medial unicompartmental knee replacement four years ago. Now there is progression of arthritis in the lateral compartment. We removed the existing unicondylar implant and performed a primary total knee arthroplasty with cemented components and patellar resurfacing.”

Correct CPT Code: 27447-58

Why: This is a staged conversion. Modifier -58 indicates a planned second procedure during the postoperative period of the original surgery (even though four years have passed, the -58 may still apply depending on payer rules; otherwise, use 27487 for revision).

Common Billing Mistakes and How to Avoid Them

Even experienced billers make errors. Here are the most frequent mistakes with partial knee arthroplasty coding.

Mistake One: Using 27446 for Patellofemoral Replacement

This is very common. Coders see “partial knee” in the title and automatically think 27446. But patellofemoral replacements have their own code: 27438. Using the wrong code leads to denials or incorrect reimbursement.

Solution: Always verify the specific compartment mentioned in the operative report.

Mistake Two: Reporting Two Separate 27446 Codes for Bicompartmental Work

Some coders try to bill 27446 twice (once for medial and once for lateral) when two compartments are replaced. This is incorrect. You cannot unbundle a procedure that is inherently one surgical session.

Solution: Use 27447 for bicompartmental replacements, or follow your specific payer’s guidance.

Mistake Three: Forgetting Laterality Modifiers

Without -LT or -RT, the claim will likely be rejected. Payers need to know which knee received the service.

Solution: Make laterality a mandatory field in your billing software or double-check every claim before submission.

Mistake Four: Coding from the Implant Label Alone

Surgeons may use a “unicompartmental” implant system but still replace two compartments. The implant name does not dictate the code. The procedure performed does.

Solution: Read the operative report. Look for phrases like “medial and lateral compartments were resurfaced” or “both condyles were addressed.”

Documentation Requirements for Partial Knee Arthroplasty

Good documentation supports good coding. What should a surgeon’s operative report include to justify a partial knee code?

Essential Elements

1. Preoperative diagnosis specifying which compartment has arthritis

2. A statement confirming the other compartments are intact (or at least not severely diseased)

3. Description of the surgical approach (medial parapatellar, lateral, etc.)

4. Exact compartments resurfaced (e.g., “medial femoral condyle and medial tibial plateau”)

5. Implant details (manufacturer, size, fixation method)

6. Intraoperative findings confirming no unexpected damage to other compartments

Without these elements, the medical record may not support the partial knee code. In an audit, this could lead to recoupment of payments.

Reimbursement Landscape for Partial Knee Procedures

Let us talk about money. While we cannot provide specific dollar amounts because rates vary by payer and region, we can discuss the general trends.

Partial knee arthroplasty (27446) typically reimburses less than total knee arthroplasty (27447). This makes sense. The partial procedure requires less operative time, fewer implants, and shorter hospital stays.

However, the reimbursement difference does not always reflect the complexity. Some surgeons argue that partial knees are technically more demanding because the remaining healthy bone and ligaments must be preserved.

What Affects Reimbursement?

• Geographic location (different localities have different fee schedules)

• Payer type (Medicare, Medicaid, commercial)

• Facility setting (hospital outpatient vs. ambulatory surgery center)

• Modifiers used (-LT, -RT, -50, -22)

• Medical necessity documentation

Tips for Maximizing Appropriate Reimbursement

1. Submit complete operative notes when requesting modifier -22

2. Use laterality modifiers on every claim

3. Double-check that diagnosis codes match the procedure (M17.11 for unilateral primary osteoarthritis of right knee, etc.)

4. Appeal denials with specific language referencing CPT guidelines

Diagnosis Codes That Support Partial Knee Arthroplasty

You cannot bill a procedure without a supporting diagnosis. Here are the most common ICD-10-CM codes used with partial knee codes.

Important Note: Osteoarthritis must be localized to one compartment to support a partial knee code. If the diagnosis code suggests generalized knee OA (M17.9), the payer may question why a partial rather than total replacement was performed.

Frequently Asked Questions (FAQ)

Q1: Can I use 27446 for a lateral unicompartmental knee replacement?

Yes. The code 27446 specifically includes both medial and lateral compartments. You do not need a different code for the lateral side. Just use the appropriate laterality modifier.

Q2: What is the CPT code for a partial knee arthroplasty revision?

If only one component of a unicompartmental knee needs revision, use 27486 (Revision of unicompartmental knee arthroplasty). If the entire partial knee is being converted to a total knee, use 27487 (Revision of total knee arthroplasty) or 27447 with modifier -58 depending on the timing.

Q3: Does 27446 include patellar resurfacing?

No. Standard medial or lateral unicompartmental arthroplasty does not include patellar resurfacing. If the surgeon also resurfaces the patella during a partial knee, you may need to check if this is bundled. In most cases, it is not separately billable. Some coders append modifier -22 to indicate increased complexity.

Q4: How do I code a partial knee arthroplasty performed bilaterally?

Use 27446-50 for a bilateral procedure performed during the same operative session. Do not report 27446-LT and 27446-RT on separate lines. Use the single-line bilateral code.

Q5: Is there a separate code for robotic-assisted partial knee arthroplasty?

No. Currently, there is no specific CPT code for robotic assistance in partial knee arthroplasty. The robotic component is considered part of the surgical service. You report the primary procedure code (27446 or 27438) as usual. Some facilities may bill for the robotic equipment separately, but this depends on the payer.

Q6: Can an outpatient surgery center bill 27446?

Yes. Partial knee arthroplasty is commonly performed in ambulatory surgery centers (ASCs) and hospital outpatient departments. The facility fee differs from the professional fee. The same CPT code applies.

Q7: What if the surgeon starts a partial knee but converts to a total knee during the same surgery?

This happens occasionally. When the surgeon opens the knee and finds more damage than expected, a conversion to total knee may be necessary. In this case, you report only the total knee code 27447. Do not report 27446 even if the surgeon began with a partial approach. The final procedure determines the code.

Q8: Is 27438 considered a partial knee arthroplasty?

Yes, technically. However, many payers and providers treat it separately from unicompartmental knee replacement. It is best to think of patellofemoral arthroplasty as its own category.

Additional Resources for Coders

You do not have to memorize everything. Keep these resources handy.

• American Academy of Professional Coders (AAPC) – Offers specialty training in orthopedics coding.

• American Medical Association (AMA) CPT Network – The official source for CPT guidelines.

• American Association of Orthopaedic Surgeons (AAOS) – Provides coding resources and webinars.

• Local Coverage Determinations (LCDs) – Check your Medicare Administrative Contractor (MAC) website for specific partial knee arthroplasty policies.

Suggested external link: For the most current official CPT coding guidelines, visit the AMA CPT Code Database (opens external site).

Final Tips for Accurate Partial Knee Coding

Before you submit any claim, run through this quick checklist.

1. Did I identify the correct compartment? (Medial, lateral, or patellofemoral)

2. Did I count the compartments correctly? (One vs. two vs. three)

3. Did I add the laterality modifier? (-LT, -RT, or -50)

4. Does the diagnosis code match the procedure? (Localized OA, not generalized)

5. Is the operative report complete? (Clear documentation of compartments)

6. Did I check payer-specific guidelines? (Some payers have unique rules)

When in doubt, ask for a coding query. Reach out to the surgeon for clarification. It is always better to delay a claim than to submit an incorrect one.

Conclusion

Finding the correct CPT code for partial knee arthroplasty comes down to three simple questions: Which compartment was replaced? How many compartments were replaced? And is the procedure a primary or revision surgery?

CPT 27446 covers medial or lateral unicompartmental replacement. CPT 27438 covers isolated patellofemoral arthroplasty. And when two compartments are involved, you generally move to the total knee code 27447.

Keep this guide nearby when coding orthopedic cases. With practice, these codes will become second nature. And remember: the operative report is your best friend. Let it guide every decision you make.

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Disclaimer (Repeated): This content is for informational purposes only and does not constitute legal, medical, or billing advice. CPT codes are copyright 2024 American Medical Association. All rights reserved. No warranty of accuracy or completeness is provided. Always verify codes with current official sources and payer policies.

Author: Medical Coding Specialist Team
Date: April 06, 2026
Disclaimer: This article is for educational purposes only. CPT codes are copyright of the American Medical Association. Always verify coding with your latest CPT manual and payer-specific guidelines.