Glossary · Anatomy

Anterior cruciate ligament (ACL)

The anterior cruciate ligament (ACL) is a primary intra-articular stabilizing ligament of the knee that resists anterior tibial translation and rotational forces. It is one of the most commonly injured knee structures in orthopedic practice, and its repair or reconstruction drives a distinct set of CPT, ICD-10-CM, and modifier decisions.

Verified May 8, 2026 · 9 sources ↓

Drawn from AAOSNIHCMSArthrexAAPC

Definition

Source · Editorial summary grounded in 9 cited references ↓

The ACL runs diagonally through the center of the knee joint, connecting the posterior lateral femoral condyle to the anterior tibial plateau. Its two functional bundles—anteromedial and posterolateral—work together to control anterior drawer, rotational laxity, and combined pivot-shift instability. Because the ligament has limited intrinsic healing capacity, complete tears rarely resolve without surgical intervention in active patients.

From a clinical management perspective, the AAOS 2022 evidence-based Clinical Practice Guideline distinguishes among acute tears, chronic instability, and skeletally immature patients, each carrying different intervention thresholds, timing considerations, and rehabilitation protocols. The guideline acknowledges that optimal timing of surgery, return-to-sport criteria, and injury-prevention strategies remain active research areas.

On the coding side, both primary repair/augmentation and reconstruction—regardless of graft source (autograft or allograft)—are captured under a single arthroscopic CPT code. Open intra-articular reconstruction uses a different code family entirely. Whether a case involves a first-time injury, a chronic instability presentation, or a revision of a prior reconstruction determines which ICD-10-CM codes apply, and conflating these categories is the single most common compliance risk in ACL billing.

Why it matters

Miscoding ACL encounter type has direct financial and compliance consequences. Assigning an acute sprain code (S83.51xA) to a chronic instability case (M23.50–M23.52) can trigger claim denial or payer audit because the documented clinical timeline contradicts the code's 'initial encounter' flag. Conversely, reporting a revision reconstruction without supporting diagnosis codes that document prior hardware, graft failure, or osseous defects leaves reimbursement on the table and exposes the claim to medical-necessity challenges. Add-on arthroscopy codes (e.g., 29877, 29880–29883) are only separately billable when performed in a different knee compartment with explicit operative-note documentation—failing that requirement violates NCCI bundling rules and can result in recoupment.

Common mistakes

Where people most often go wrong with this concept.

Source · Editorial brief grounded in cited references ↓

Coding a chronic ACL tear or chronic instability as an acute injury (S83.51xA) when the operative note describes longstanding laxity or prior failed reconstruction.
Reporting 29888 for an open intra-articular reconstruction instead of 27428, or 27429 when both intra- and extra-articular stabilization are performed.
Billing concomitant meniscal or chondroplasty codes (29877, 29880–29883) without documenting that the work was performed in a separate knee compartment, which is required to unbundle from the ACL global package under NCCI rules.
Omitting external cause codes on acute traumatic ACL claims, leaving the mechanism of injury undocumented and increasing audit vulnerability.
Failing to append graft-failure or prior-hardware diagnosis codes on revision cases, which weakens medical-necessity support and reduces the likelihood of full reimbursement.
Using the same ICD-10-CM code for both primary reconstruction and revision surgery without distinguishing the underlying reason for revision (e.g., graft failure vs. new traumatic re-tear).
Assuming autograft and allograft reconstructions always share identical payer coverage criteria—allograft coverage typically requires documented contraindication to autogenous harvest.

Related codes

Codes commonly involved when this concept appears in practice.

CPT

ICD-10

M23.50 M23.51 M23.52 M76.50

Modifiers

59 Distinct procedural service XS Separate structure RT Right side LT Left side 51 Multiple procedures 22 Increased procedural services

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 9 cited references ↓

01Does CPT 29888 cover both ACL repair and ACL reconstruction?

Yes. CPT 29888 describes arthroscopically aided ACL repair, augmentation, or reconstruction under a single code regardless of whether the surgeon repairs native tissue or places a new graft. The distinction between repair and reconstruction does not change the CPT code, but it should be clearly documented in the operative note to support medical necessity.

02What ICD-10-CM code applies to a chronic ACL tear with knee instability?

Chronic instability of the knee is reported with M23.50 (unspecified), M23.51 (right knee), or M23.52 (left knee). Do not use the acute sprain codes (S83.51x) for a longstanding or previously unrepaired tear, as the encounter-type indicator on the acute code would conflict with a chronic clinical timeline and risk denial.

03Can concomitant meniscal repair be billed alongside 29888?

It depends on compartment and documentation. NCCI policy allows certain arthroscopic add-on codes (such as 29880–29882) to be unbundled from the ACL global package only when the meniscal work is performed in a different knee compartment and the operative note explicitly states this. Work in the same compartment as the ACL procedure is generally considered bundled. Non-Medicare payers may follow AAOS Global Service Data guidelines, which require additional documentation showing the arthroscopy is unrelated to the ACL work.

04Is allograft ACL reconstruction coded differently from autograft?

The primary CPT code (29888) is the same regardless of graft source. However, allograft use often triggers additional payer scrutiny. Many commercial payers consider allograft medically appropriate only when autogenous tissue is contraindicated—for example, due to prior harvest, multi-ligament involvement, or concurrent tendon pathology. Documenting the clinical rationale for allograft selection is essential to avoid claim denial.

05How should a revision ACL reconstruction be coded differently from a primary procedure?

The surgical CPT code (29888) is the same for revision as for primary reconstruction. The critical difference is in the diagnosis coding. Revision cases require codes that document the reason for revision—such as graft failure, prior hardware complication, or osseous defects—along with any relevant history codes. Coding a revision simply as a new acute injury is inaccurate and may not support the complexity or medical necessity of the procedure.

06What is the 2026 Medicare national average payment for CPT 29888?

Based on 2026 Medicare national average rates (unadjusted for geography), the physician professional fee for 29888 is approximately $893.90. The hospital outpatient facility payment averages approximately $7,413.38 under APC 5114, and the ASC rate averages approximately $4,817.25. These figures are subject to geographic adjustment and annual update.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

Mira AI Scribe

When Mira captures an ACL encounter, it flags three coding decision points in real time. 1. Acute vs. chronic vs. revision: The scribe reads operative and clinical note language for terms like 'longstanding instability,' 'prior reconstruction,' or 'graft failure' and steers toward M23.5x (chronic instability) or the appropriate T84/Z-code revision pathway rather than defaulting to S83.51xA. It also prompts for an external cause code whenever the mechanism of injury is documented. 2. CPT pathway: If the note documents arthroscopic-assisted technique, Mira anchors to 29888. If open intra-articular technique is documented, it surfaces 27428 or 27429. It will not auto-assign 29888 for an open approach. 3. Concomitant procedures: When meniscal work or chondroplasty is noted, Mira checks whether the operative note specifies a separate compartment. If compartment laterality is explicit and distinct from the ACL work, it suggests the relevant add-on code with modifier 59 or XS and queues a documentation reminder to include the compartment detail required by NCCI policy. If compartment is ambiguous or the same, it flags the risk and holds the add-on pending clarification rather than appending it automatically. For allograft cases, Mira surfaces the payer-specific medical-necessity checklist (prior autograft compromise, multi-ligament reconstruction, or documented contraindication) so the surgeon attestation is captured before claim submission.

See Mira's approach

The posterior cruciate ligament (PCL) is a strong intra-articular ligament connecting the medial femoral condyle to the posterior tibial plateau, functioning primarily to prevent the tibia from translating posteriorly on the femur. It is roughly 1.3–2 times thicker and up to twice as strong as the ACL, making isolated PCL tears less common than ACL injuries.

Medial collateral ligament (MCL) Anatomy

The medial collateral ligament (MCL) is a broad, flat band of connective tissue on the inner (medial) side of the knee that resists valgus stress and stabilizes the tibiofemoral joint. It runs from the medial femoral epicondyle to the proximal medial tibia and is the most commonly injured knee ligament.

Meniscus (medial / lateral) Anatomy

The medial and lateral menisci are two C-shaped fibrocartilage discs inside the knee joint that distribute load, absorb shock, and stabilize the articulation between the femur and tibia. They occupy distinct compartments and are treated as separate anatomic structures for coding purposes.

Arthroscopy Clinical

Arthroscopy is a minimally invasive surgical procedure in which a small camera (arthroscope) is inserted into a joint to visualize, diagnose, and treat intra-articular pathology. It serves as both a diagnostic tool and a platform for therapeutic interventions such as debridement, meniscectomy, labral repair, and loose body removal.