ICD-10 Code S42.309K: Humerus shaft fracture nonunion

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Key takeaways

S42.309K is the billable FY2026 code for a humeral shaft fracture nonunion when the record does not name the arm.

The 7th character K means nonunion, which is distinct from G for delayed healing and P for malunion.

Use S42.301K for the right arm and S42.302K for the left whenever the physician documents laterality.

Payers expect an explicit nonunion diagnosis, supporting imaging, and a clear subsequent-encounter designation before they pay.

Tracking denial patterns on nonunion claims shows which documentation habits are costing the practice money.

ICD-10 Code S42.309K is the billable diagnosis code for a humeral shaft fracture that has failed to heal.

The full descriptor reads: unspecified fracture of shaft of humerus, unspecified arm, subsequent encounter for fracture with nonunion. It is valid for fiscal year 2026, effective October 1, 2025.

Reach for it when a patient returns for care on a humeral shaft fracture that has stopped healing. The note also has to leave the affected arm unnamed. Correct use rests on the encounter type, the healing status the 7th character encodes, and whether laterality is documented.

ICD-10 Code S42.309K: Full description and billable status

ICD-10 Code S42.309K describes an unspecified fracture of the shaft of the humerus, unspecified arm, subsequent encounter for fracture with nonunion. The FY2026 tabular carries that descriptor verbatim. According to the CDC/NCHS ICD-10-CM web tool, the code took effect on October 1, 2025 for the 2026 fiscal year.

The code is billable and specific, so it carries enough clinical detail to be submitted for reimbursement on its own. It applies to outpatient and inpatient claims alike. Orthopedic and musculoskeletal coders meet it most often at follow-up visits, once a humeral shaft fracture has missed its expected healing window.

Clinical description: Humeral shaft fracture and nonunion

A humeral shaft fracture is a break in the diaphysis, the elongated middle portion of the humerus between the shoulder and the elbow. The shaft is distinct from the surgical and anatomical neck regions coded under other S42 subcategories. Nonunion occurs when the fracture site fails to bridge with new bone inside the expected healing window. In practice that usually means six months or more after the injury with no radiographic callus.

The “unspecified arm” designation reflects a note that never records whether the fracture is on the right or the left side. Treat that as a prompt to query the physician rather than a preferred code. Per CMS ICD-10 coding guidance, coders assign the most specific code the clinical record supports.

• Shaft (diaphysis): the long cylindrical portion of the humerus between the proximal and distal metaphyses
• Nonunion: failure of fracture healing, with persistent mobility at the fracture site or absent radiographic bridging
• Unspecified arm: laterality not documented, so right or left cannot be determined from the medical record
• Subsequent encounter: the patient is receiving active care for healing complications, not the initial acute injury

Understanding the 7th character K in ICD-10-CM fracture codes

The 7th character in ICD-10-CM fracture codes encodes the encounter type and the healing status. For S42.309K, the character K signals a subsequent encounter where the fracture has developed nonunion. Payers act on that distinction at adjudication, so submitting K where the note supports G or P invites denials and audits. The ICD-10-CM Official Guidelines require the 7th character to reflect the clinical status documented at the encounter, not the original injury.

Coders commonly confuse G and K when a note describes a “slow-healing” fracture. G applies while healing is progressing but running behind schedule. K applies once healing has stopped, the site remains unstable, and imaging shows no bridging callus. Where the physician’s note does not separate the two, query the provider before assigning the character.

The choice narrows quickly once the encounter type and the imaging findings are both on the page. The chart below walks the same two questions in the order a coder asks them.

Subsequent encounter vs. initial encounter: When to use S42.309K

S42.309K is correct only at a subsequent encounter, never at the initial visit for the acute fracture. A subsequent encounter means the patient has already had active treatment and is back for care related to healing. Applying K at a first presentation is one of the most frequent 7th character errors in orthopedic coding. An emergency department visit for the acute break takes A instead.

Approximate synonyms and clinical terminology for S42.309K

Clinical documentation uses several terms that map to ICD-10 Code S42.309K. Recognizing them helps when the physician’s note does not quote the ICD-10 descriptor. They also point to the right entry in the ICD-10-CM alphabetical index, under “Fracture, traumatic, humerus, shaft”. The AAPC ICD-10-CM lookup is a quick way to check a term against the code before you submit.

• Nonunion of fracture of shaft of humerus
• Nonunion of humeral diaphyseal fracture
• Humeral shaft fracture nonunion, arm unspecified
• Fracture of humerus shaft with nonunion, unspecified laterality
• Failed healing of humerus shaft fracture, unspecified arm
• Diaphyseal humerus fracture, subsequent encounter, nonunion

Related ICD-10 codes: Laterality and healing status variants

S42.309K belongs to a family of codes that share the same clinical scenario but differ by laterality or healing status. Picking the most specific one lowers the risk of denials and requests for extra documentation. The rest of the family is indexed in our diagnostic code library. The CMS ICD code lists confirm that each code below is valid for FY2026 submission.

Laterality variants: right, left, and unspecified

Once the operative note or the radiology report names the side, the unspecified code drops out of the running. A left-arm nonunion moves to S42.302K, and a right-arm nonunion to S42.301K.

Nonunion vs. malunion: S42.309K vs. S42.309P

Nonunion and malunion are separate clinical outcomes, and they carry different codes and different treatment pathways. Swapping one for the other damages both claim accuracy and the practice’s own data. S42.309K applies when the bone has not healed at all, while S42.309P applies when it healed in the wrong position.

Code hierarchy and parent codes for S42.309K

Reading the ICD-10-CM hierarchy is how coders confirm they have reached the most specific applicable code. S42.309K sits within a six-level hierarchy under the injury and trauma chapter. Category S42 covers fractures of the shoulder and upper arm, from proximal to distal humerus injuries.

• S00-T88: Injury, poisoning and certain other consequences of external causes (chapter)
• S40-S49: Injuries to the shoulder and upper arm (block)
• S42: Fracture of shoulder and upper arm (category)
• S42.3: Fracture of shaft of humerus (subcategory)
• S42.309: Unspecified fracture of shaft of humerus, unspecified arm (6-character code)
• S42.309K: Unspecified fracture of shaft of humerus, unspecified arm, subsequent encounter for fracture with nonunion (7-character billable code)

Billing and reimbursement guidance for S42.309K

S42.309K supports reimbursement at subsequent encounters where nonunion is the documented clinical status. Payers treat nonunion as a complication of the original injury, so they expect the record to carry the diagnosis before the claim is processed. Validating the diagnosis and procedure pairing before submission catches most edits while the claim can still be corrected.

Common CPT codes paired with S42.309K

Nonunion of the humeral shaft often needs surgical intervention. The CPT procedure codes below are commonly paired with S42.309K as the supporting diagnosis. Confirm NCCI bundling edits and payer-specific policies before billing any combination, since compatibility varies between payers.

Documentation requirements to support S42.309K

Payers reviewing a claim billed with S42.309K expect the record to support nonunion as the finding at the time of service. Vague notes describing “ongoing arm pain” or “slow healing” fall short of that threshold. Three documentation pillars carry this code: an explicit nonunion diagnosis, supporting imaging, and a clear encounter type.

• Explicit nonunion diagnosis: the physician note must state “nonunion” or an equivalent term, not simply “fracture follow-up”
• Imaging confirmation: X-ray or CT findings documenting absent callus bridging or persistent fracture mobility support the diagnosis
• Encounter type clarity: the note must show this is a follow-up visit rather than the initial presentation for the acute fracture
• Laterality documentation: where the affected arm is known, record right or left so S42.301K or S42.302K can be used instead
• Treatment plan: document the planned intervention, whether surgical, conservative, or electrical stimulation, to support medical necessity

How Pabau keeps nonunion claims clean from note to payment

In most orthopedic practices, the nonunion diagnosis, the imaging report and the claim live in three separate places. A coder reads the note in one system, checks the X-ray in another, then keys S42.309K into billing. Every hop is a chance for the 7th character to drift away from what the surgeon actually documented.

Practice management software like Pabau keeps the chart, the imaging record and the claim in one patient file. The coder reads the follow-up note and raises the claim from the same screen. The encounter type on the claim then matches the encounter type in the record. Our streamlined claims management then submits electronically and reports what comes back.

Denial tracking is what turns a coding rule into a habit the practice can see. When three nonunion claims come back for missing imaging, the pattern is visible in the same week rather than at the end of the quarter. The surgeon gets one documentation request instead of five, and the rework stops before it becomes a write-off.

Manage fracture-related claims without the back-and-forth

Pabau’s billing tools help orthopedic and musculoskeletal practices submit nonunion claims accurately, track denial patterns, and reduce the administrative load on coding staff.

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Conclusion

S42.309K is the right code in a narrow set of circumstances: a follow-up visit, a documented nonunion, and a note that never names the arm. Two of those three are clinical findings you cannot change. The third one you can, and every laterality query answered moves the claim to a more specific code.

So treat the unspecified code as a working answer rather than a destination. Coders who query early submit fewer claims on S42.309K over time, and that shift is the clearest sign that fracture documentation is improving. Book a demo to see how Pabau keeps fracture coding, documentation and claims in one place for orthopedic teams.

Continue your research

Does the record name the affected arm? ICD-10 code S42.302K covers the same nonunion on the left arm, where laterality is documented.

Want to reduce claim denials on complex diagnoses? Denial codes in medical billing explains the most common denial reason codes and how to address them before resubmission.

Looking for guidance on clean claim submission standards? Claim.MD clearinghouse overview covers how electronic claim validation works before a claim reaches the payer.

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