ICD-10 code S42.442K: Displaced medial epicondyle fracture, nonunion

Key takeaways

ICD-10 code S42.442K covers a displaced avulsion fracture of the medial epicondyle of the left humerus at a subsequent encounter with nonunion.

The 7th character K is only correct once the clinician has documented fracture nonunion, not routine or delayed healing.

Laterality sets the fifth digit. S42.441 is the right humerus, S42.442 is the left, and S42.443 is unspecified.

Practice management software like Pabau ties the code to structured note data, so laterality and nonunion status reach the claim intact.

ICD-10 code S42.442K is the billable code for a displaced avulsion fracture of the medial epicondyle of the left humerus. It applies at a subsequent encounter, once imaging has confirmed the fracture has failed to unite. Most medial epicondyle fractures heal without incident, but a subset progress to nonunion.

Using the wrong 7th character on a subsequent encounter claim is one of the most common reasons an S42.44x claim is flagged on audit.

Assign K only when the record states the fracture has failed to unite. If the note says healing is slow but union is still possible, the code is S42.442G.

ICD-10 code S42.442K: Full description and billable status

S42.442K is a billable, specific ICD-10-CM diagnostic code valid for HIPAA-covered transactions from October 1, 2025 through September 30, 2026 (FY2026). Its full official description is: Displaced fracture (avulsion) of medial epicondyle of left humerus, subsequent encounter for fracture with nonunion.

7th character meaning: Understanding the K designator

K signals a subsequent encounter where the fracture has failed to unite. It means more than a follow-up visit, which is why the 7th character is where most S42.44x coding errors happen. Confusing routine healing follow-up (7th character D) with nonunion (K) is a documentation and billing error.

Category S42 uses those seven characters and no others. The same set runs across the whole S42.4 subcategory, so a transcondylar fracture with delayed healing reads as S42.475G. Unlike several lower-limb fracture categories, it carries no separate Gustilo type designators, so any open fracture at the initial encounter takes B.

Key distinction: K (nonunion) and G (delayed healing) describe different clinical states. Delayed healing means union is still possible. Nonunion means imaging has confirmed the fracture line will not close without intervention.

According to CMS ICD-10-CM guidance, the 7th character must reflect the status documented by the treating clinician at that specific encounter.

Clinical overview: displaced avulsion fracture of the medial epicondyle

The medial epicondyle is the bony prominence on the inner side of the elbow where the flexor-pronator muscle group and the ulnar collateral ligament attach. An avulsion fracture occurs when these structures pull a fragment of bone away from the humerus, typically during a valgus stress event.

Sports medicine practices and orthopedic teams see this pattern most often in throwing athletes and young patients with open growth plates.

• Displacement: S42.442K specifies a displaced fracture, meaning the fragment has separated from its anatomic bed rather than staying in position. The nondisplaced equivalent for the same side is the S42.445x series.
• Laterality: The code is left humerus only. Right-sided injuries use S42.441K, and undocumented laterality falls to S42.443K.
• Nonunion mechanism: Nonunion is more likely when displacement is significant or fixation is delayed. It is also more likely when the fragment becomes incarcerated in the joint and cannot appose.
• Age consideration: In skeletally immature patients, this injury can be mistaken for a normal apophysis on imaging. Clinical correlation is required before applying a fracture code.

In physical therapy practices handling rehabilitation after nonunion repair, accurate coding keeps the clinical progression visible in the billing record.

Where S42.442K sits in the ICD-10-CM hierarchy

Reading the parent chain is the fastest way to confirm you have reached the billable level. S42.442 already fills every character position, so it needs no X placeholder. It is still not billable until the 7th character is attached. Neighboring codes in the same subcategory, such as S42.409P and S42.489P, follow the same pattern.

Sibling and related codes

ICD-10 code S42.442K belongs to the S42.44x subsection covering fractures (avulsion) of the medial epicondyle of the humerus. Coders have to get three dimensions right, and those are laterality, displacement status, and the 7th character.

The table below covers the most frequently referenced codes in the family. The same K designator carries over to shaft fractures such as S42.343K.

The fifth digit carries laterality inside each displacement series, and the two series run in the same order. Displaced is S42.441 right, S42.442 left, S42.443 unspecified. Nondisplaced is S42.444 right, S42.445 left, S42.446 unspecified.

Avulsion fractures of the lateral epicondyle sit in a separate S42.43x series, so a code such as S42.436B never applies to a medial injury.

ICD-9-CM crosswalk

Legacy systems and historical claim reviews still reference ICD-9-CM codes. The approximate ICD-9-CM equivalent for S42.442K is 733.82, nonunion of fracture.

That mapping is approximate rather than one to one, because ICD-9-CM never captured this level of anatomical detail or laterality. The WHO ICD-10 browser sets out the structural differences between the two code architectures.

For practices converting historical records, or answering a payer request for legacy equivalents, 733.82 is the standard reference. Do not use ICD-9-CM codes for current FY2026 billing.

Coding guidelines: choosing between A, D, G, K, and P

The choice between S42.442A, D, G, K, and P turns on two things. Those are the phase of care and the status of the fracture. ICD-10-CM coding guidelines require the 7th character to match the status documented at each encounter. A status carried forward from the original injury visit will not do.

1. Confirm this is a subsequent encounter. If this is the first time the patient is seen for the fracture, use 7th character A instead. Multiple visits still count as initial encounters while definitive care is being provided.
2. Determine healing status from imaging. A follow-up X-ray or CT should record whether the fracture is healing normally, healing slowly, or failing to unite. Nonunion is typically diagnosed when bridging callus is absent at 6 to 9 months, though clinical judgment governs.
3. Do not assume K from delayed healing alone. If the note says delayed healing without saying nonunion, use S42.442G rather than S42.442K. Upgrading to K without a documented nonunion finding is an upcoding risk.
4. Distinguish nonunion from malunion. Malunion (S42.442P) means the bone healed in a poor alignment. Nonunion (S42.442K) means the fracture never closed. The two carry different surgical implications, so never conflate them.
5. Keep using S42.442K until the nonunion resolves. Later visits for bone stimulator use, surgical planning, or post-operative review stay on K until the fracture heals or a new status is documented.

Some practices capture fracture healing status at every visit through structured clinical documentation. Those practices rarely apply the wrong 7th character at submission.

Documentation requirements for S42.442K

An S42.442K claim only survives a medical chart audit when the record supports every element of the code. Four documentation elements are non-negotiable.

• Laterality explicitly stated: The clinical note, operative report, or imaging report has to say left humerus or left elbow. A previous encounter’s wording is not enough if the current note is ambiguous.
• Displacement confirmed: The radiology or operative report should describe the fragment as displaced, which separates the code from the nondisplaced S42.445x series. A report describing fragment position is the most defensible source.
• Encounter type established: The note should show this is a follow-up visit for a known fracture rather than a first presentation. Wording such as returning for management of left medial epicondyle nonunion establishes that clearly.
• Nonunion confirmed on imaging or clinical assessment: This is the critical element behind the K designator. The note needs a statement such as persistent fracture line with no bridging callus consistent with nonunion. A bare mention of nonunion is weaker on audit.

HIPAA-compliant documentation workflows that timestamp notes and preserve imaging links support code specificity on fracture complication claims. Good medical forms and documentation processes remove the omissions that lead to claim denials.

External cause codes to report with the fracture

The ICD-10-CM Official Guidelines (Chapter 20) require external cause codes when the injury resulted from an external event. Two categories usually apply to S42.442K, the mechanism of injury and the activity at the time of injury.

These codes do not change reimbursement directly, but they are required for complete claim coding and population health data.

External cause codes in the W series also take a 7th character. Because S42.442K is always a subsequent encounter, the paired W code takes character D instead of the initial-encounter option. The CMS ICD code lists confirm the pairing requirements for injury claims.

ICD-10-CM code history and annual updates

S42.442K has been part of ICD-10-CM since the classification replaced ICD-9-CM in the United States on October 1, 2015. The code has stayed structurally stable across annual updates, with no description changes or validity interruptions in recent fiscal years.

Practices billing fracture complication codes should still check each October, because 7th character applicability can shift with guideline revisions.

Verify against the CDC/NCHS ICD-10-CM web tool at the start of each fiscal year. That check confirms no reclassification has occurred.

How Pabau supports accurate ICD-10 documentation and coding

Coding errors on fracture complication claims like S42.442K usually start in the documentation rather than the coding step. When the clinical note is vague about laterality, encounter type, or imaging-confirmed nonunion, the coder is left guessing.

Pabau’s claims management software works directly from clinical records, so diagnosis codes are applied from structured note data instead of being reconstructed from free text.

Digital intake forms can be set up to capture injury laterality and mechanism at first presentation. That data is then available for coding at every later encounter.

Orthopedic and musculoskeletal practices managing nonunion cases need documentation and billing to line up. Structured notes and practice management software keep what the clinician observed aligned with what reaches the claim.

Reduce ICD-10 coding errors at the point of care

Pabau's clinical documentation tools help practices capture laterality, encounter type, and fracture status accurately. The right code reaches the claim first time.

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Conclusion

The risk on an S42.442K claim sits in the note rather than the code. A coder can only reach for K when the record says the fracture failed to unite. Where the note describes slow healing without saying nonunion, S42.442G is the safer submission.

Correcting this at the coding desk never holds for long. The durable fix is a documentation template that forces laterality and healing status at every follow-up visit. Practices that do that stop rewriting notes after a denial, and stop guessing between G and K.

Pabau gives orthopedic and musculoskeletal practices that structure at the point of care. Book a demo to see how it keeps laterality and fracture status on the claim.

Continue your research

Coding a surgical neck fracture instead? ICD-10 code S42.242K applies the same nonunion logic further up the left humerus.

Did the bone heal in the wrong position? ICD-10 code S42.242P shows how the P designator changes a subsequent encounter claim.

Fracture healing normally at follow-up? ICD-10 code S42.212D covers the routine-healing 7th character on a left humerus fracture.

Coding a late effect rather than active care? ICD-10 code S42.032S walks through the sequela designator on a clavicle fracture.

Want to test your coding before a payer does? Medical chart audit sets out how to review records for documentation that supports the code.

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