ICD-10 code S52.266R: Nondisplaced ulna fracture with malunion

Key takeaways

ICD-10 code S52.266R covers a nondisplaced segmental fracture of the ulna shaft seen at a subsequent encounter.

The code applies only when the original injury was a Gustilo Type IIIA, IIIB, or IIIC open fracture that healed in malunion.

S52.266R is billable for FY2026, effective October 1, 2025, while the parent code S52.266 is not.

Six clinical facts must appear in the note, including the Gustilo grade recorded in the original treating record.

Practice management software like Pabau captures encounter type and healing status, so fewer complex fracture claims come back denied.

ICD-10 code S52.266R is a billable ICD-10-CM diagnosis code. Its official description is nondisplaced segmental fracture of shaft of ulna, subsequent encounter for open fracture type IIIA, IIIB, or IIIC with malunion. The FY2026 edition took effect on October 1, 2025, per the CDC ICD-10-CM web tool.

In plain terms, the patient broke the ulna in a segmental pattern, and the bone came through the skin with severe soft tissue damage. It then healed in the wrong position, and this visit is a follow-up rather than the first treatment. The R character records all of that in one billable code.

What S52.266R means: Breaking down each code component

Each segment of S52.266R carries a precise clinical meaning. Coders who follow the alphanumeric structure are far less likely to pick the wrong 7th character or bill the non-billable parent code. Here is how the code breaks down.

The 6 in S52.266 stands for the unspecified arm. That base code applies only when the treating record does not name the left or right side. Check the follow-up note for laterality before you default to it.

The 7th character R in ICD-10-CM fracture codes

The 7th character does two jobs at once. It records the encounter type, and on open fractures it also carries the Gustilo wound grade and the healing status. Base code S52.266 accepts 15 different 7th characters, plus S for sequela.

The most expensive error on open fracture follow-up is billing an initial encounter code after active treatment has finished. Reading the full grid below prevents it.

Characters G, H, and J cover delayed healing rather than nonunion or malunion. Character S sits outside the encounter sequence altogether. It codes a late effect treated after the fracture itself has resolved, as with S52.591S.

The ICD-10-CM Official Guidelines define a subsequent encounter as any encounter after the patient has received active or definitive treatment. That covers orthopedic follow-ups, wound care visits, physical therapy referrals, and malunion revision consultations.

Encounter type is set by the stage of care, not by the number of visits. The same rule runs through radius shaft codes like S52.356R. Practices using compliance management software can flag cases where an initial encounter code was used past the active treatment phase.

Gustilo-Anderson classification and ICD-10 coding

The Gustilo-Anderson classification grades open fractures by wound size, soft tissue damage, and vascular compromise. It first appeared in The Journal of Bone and Joint Surgery in 1976 and was refined in 1984. The ICD-10-CM 7th character system maps straight onto it.

For S52.266R, the clinician must have documented a Gustilo Type IIIA, IIIB, or IIIC injury at the original presentation. Without that classification in the record, the R character is unsupported. The AAPC code lookup cross-references coding guidance against the clinical description.

Types I and II share 7th character B for initial encounters and Q for subsequent encounters with malunion. Types IIIA, IIIB, and IIIC share C for initial encounters and R for subsequent encounters with malunion. Reading a Type II wound as a Type IIIA wound produces the wrong code and avoidable audit exposure.

Sports medicine practices treating high-energy trauma should route the Gustilo classification from the ED note into the ongoing treatment record. Follow-up coders then have it in front of them.

Malunion vs. nonunion: Coding differences for subsequent encounters

Malunion and nonunion are different clinical outcomes, and they map to different 7th characters. Malunion means the fracture united in a non-anatomic position, leaving deformity or functional limitation. Nonunion means the fracture failed to heal in the expected timeframe.

For S52.266R, the R character confirms malunion. The bone healed, just in the wrong place. When imaging still shows an unhealed fracture line, the code is S52.266N.

Orthopedic notes should state whether imaging shows malunion at the fracture site. Language such as “healed with angular deformity” or “malunited fracture of the ulnar shaft” does the job. “Healing with some deformity” does not, and it invites a payer query.

Physical therapy EMR platforms tied into the orthopedic record can surface healing status for coders working downstream rehabilitation encounters.

Documentation requirements for S52.266R

Six clinical facts must appear in the documentation to support S52.266R. Missing any one of them exposes the claim to denial or audit. This checklist applies to the treating clinician’s note, not the coder’s abstraction, so the information has to originate with the physician or mid-level provider.

• Fracture type: Segmental. The record must confirm a segmental pattern, meaning at least two fracture lines that isolate a separate bone fragment. Comminuted or transverse patterns do not support the S52.26x codes.
• Displacement status: Nondisplaced. Imaging or the clinical description must confirm the fragments have not shifted out of anatomic alignment. If displacement is documented, a displaced code such as S52.261 or S52.262 applies instead.
• Bone: Shaft of ulna. The fracture must involve the diaphysis of the ulna, not the proximal or distal metaphysis. Radial shaft fractures take their own codes within S52, such as S52.333C.
• Open fracture classification: Gustilo Type IIIA, IIIB, or IIIC. The original treating physician’s record must carry the Gustilo grade. It usually sits in the ED or operative note from the initial encounter.
• Encounter type: Subsequent. The visit must fall after active or definitive treatment of the fracture. A follow-up, wound check, or complication visit qualifies. The first treatment episode does not.
• Healing complication: Malunion. Imaging, usually X-ray or CT, must confirm the fracture united in a non-anatomic position. The radiologist or treating surgeon has to say malunion explicitly.

Practices using structured clinical records can build a fracture template that prompts providers for all six elements at every orthopedic encounter. HIPAA-compliant documentation practices require that the supporting information stays in the record and is available for audit on request.

Related ICD-10 codes for nondisplaced ulna shaft fractures

S52.266R belongs to a family of codes sharing the S52.26 base. The table below shows the laterality variants and the neighboring codes orthopedic and emergency coders reach for most. Laterality is a frequent error source, because S52.266 covers the unspecified arm while S52.261 through S52.265 name the side.

When the treating record names the arm, move from S52.266 to S52.264 for the right and S52.265 for the left. The 7th character options, including R, apply identically across every laterality variant. Other ulna codes such as S52.271A follow the same 7th character rules.

CPT codes associated with ulna shaft fracture treatment

S52.266R describes the diagnosis. The claim also needs a CPT code for whatever procedure the encounter involved. The crosswalk below covers the procedures most often billed alongside open ulna shaft fractures and malunion repair. Confirm the exact code against the operative or encounter note before you submit.

Billing teams should confirm current RVU values and payer coverage policies before submitting. The CMS Physician Fee Schedule gives current Medicare payment rates for each CPT code by locality. Practices running high volumes of surgical fracture cases can pull CPT-to-ICD-10 mismatches out of their reporting tools before the claim goes out.

Common coding errors to avoid

Five errors account for most claim denials and audit findings on S52.266R. Each one is preventable with the right documentation habits and a review step before submission.

• Using initial encounter codes on follow-up visits. Submitting S52.266C after definitive treatment is an encounter-type error and a known audit trigger. Once the repair is complete, the 7th character has to move to a subsequent encounter.
• Misclassifying the Gustilo grade. Coding R when the original documentation supports only Type I or II leaves the code unsupported. R requires Types IIIA, IIIB, or IIIC. Check the initial ED or operative note for the grade used at the time of injury.
• Omitting laterality when it is documented. Defaulting to S52.266 when the note names the left or right arm is a specificity failure. Payers increasingly reject unspecified laterality codes when the record contains the side.
• Confusing malunion with nonunion. Malunion means the bone healed in the wrong position, and nonunion means it never healed. Using S52.266R when imaging shows an unhealed fracture line is the wrong call. That case takes S52.266N.
• Submitting the non-billable parent code S52.266. S52.266 without a 7th character is not billable, and any HIPAA-compliant claims system will reject it. Every claim needs the full seven-character code.

Orthopedic practices using digital assessment forms can build the Gustilo field straight into the fracture template. The classification is then captured at the initial encounter and waiting for whoever codes the follow-up. That one workflow change removes the second error on this list at its source.

Code hierarchy: Where S52.266R fits in ICD-10-CM

Knowing where S52.266R sits in the hierarchy helps you navigate to related codes and apply the parent code notes correctly. The path from the classification root down to the specific code runs like this:

1. ICD-10-CM (full classification system)
2. S00-T88: Injury, poisoning and certain other consequences of external causes
3. S50-S59: Injuries to the elbow and forearm
4. S52: Fracture of forearm
5. S52.2: Fracture of shaft of ulna
6. S52.26: Segmental fracture of shaft of ulna
7. S52.266: Nondisplaced segmental fracture of shaft of ulna, unspecified arm (non-billable)
8. S52.266R: Subsequent encounter for open fracture type IIIA, IIIB, or IIIC with malunion (billable)

Parent code S52 carries Type 1 Excludes notes rather than add-on instructions. Open wound codes from S51 and pathological fracture codes from M80 and M84.4 both sit under those notes. A Type 1 Excludes note means the two conditions never belong on the same claim.

Those exclusions apply to every code in the S52 category, S52.266R included. The open wound is already built into the 7th character on S52.266R, so a separate S51 code is redundant. Check the tabular list before you pair this code with another forearm injury code.

How Pabau keeps fracture coding tied to the documentation

In most orthopedic practices the Gustilo grade lives in an ED note from months earlier, and the malunion finding lives in a radiology report. The coder working the follow-up has to hunt both down. When either one is missing, the claim goes out on a weaker code or gets held.

Practice management software like Pabau keeps all of it on one patient record. Custom treatment forms capture the Gustilo grade at the first encounter. Every follow-up note, image, and letter attaches to the same file, so coders read the injury history and the current healing status in one place.

Billing runs off that same record. Pabau’s claims management software pairs the diagnosis with the procedure at checkout. An initial encounter character cannot quietly ride along on a visit six months after the repair. Fewer complex fracture claims come back for documentation, so your team spends less of the month on rework.

Keep fracture coding tied to the documentation

Pabau stores the Gustilo grade, the imaging, and every follow-up note on one patient record. Coders see the whole injury history, so complex fracture claims go out right the first time.

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Conclusion

S52.266R is narrow by design. It fits only when three facts sit on the record. You need a Gustilo Type III open injury at presentation, a subsequent encounter, and malunion confirmed on imaging. Miss one and a different code applies.

The practical work happens upstream of the coder. The Gustilo grade is recorded once, in the ED or the operating room, and it has to survive months of follow-up to stay usable. Practices that capture it as a structured field at the first encounter stop re-litigating it later.

The trade-off worth remembering is specificity against speed. Chasing laterality and healing status adds a minute to the note and removes a denial cycle from the month. Book a demo to see how Pabau keeps fracture documentation and claims on the same record.

Continue your research

Coding the same malunion on the radius? S52.356R applies the identical 7th character to a radius shaft fracture.

Need an open fracture example outside the forearm? S12.690B codes an open cervical vertebra fracture at the initial encounter.

Looking for another nondisplaced fracture code? S82.016B covers a nondisplaced osteochondral fracture of the patella.

Billing a procedure on the proximal ulna? CPT code 24138 covers sequestrectomy of the olecranon process.

Want the documentation side handled properly? Clinical documentation software explains how structured notes support the codes you submit.

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