ICD code S99.121K – Salter-Harris Type II right metatarsal

Key takeaways

S99.121K is the billable ICD-10-CM code for a Salter-Harris Type II physeal fracture of the right metatarsal that has not united at follow-up.

The sixth character 1 means right metatarsal, 2 means left and 9 means unspecified side, so laterality must match every document in the record.

The seventh character K separates nonunion from routine healing (D), delayed healing (G), malunion (P) and sequela (S). It needs an explicit nonunion statement from the provider.

Pair the code with metatarsal procedure codes such as 28470, 28475, 28485 or 28322, never the toe phalanx codes 28510 and 28515.

Pabau, the practice software we build, sends US claims through Claim.MD and checks eligibility, so staff follow each nonunion claim in one place.

ICD-10 Code S99.121K: Definition and code hierarchy

ICD-10 Code S99.121K is the billable ICD-10-CM code for a Salter-Harris Type II physeal fracture of the right metatarsal, subsequent encounter for fracture with nonunion. It describes a growth plate fracture of a right metatarsal that has failed to heal. It is reported when the patient returns for care after initial treatment.

The code sits in Chapter 19 (Injury, poisoning and certain other consequences of external causes) of the CMS ICD-10-CM code set. Within that chapter, category S99 holds the physeal fractures of the foot that the S92 fracture codes leave out.

The code hierarchy from broadest to most specific is:

Decoding S99.121K: What each character means

Each character of S99.121K records one clinical fact. Characters 4 through 7 carry the site, the fracture pattern, the side and the encounter, so a misread there changes the code. The table below breaks down all seven positions.

The code does not say which of the five metatarsals is involved, unlike the S92.3- fracture codes. The note should still name the bone, because metatarsal procedure codes are reported per bone treated.

The CDC/NCHS ICD-10-CM lookup tool confirms the current descriptor and billable status. Check it each year, because ICD-10-CM updates take effect every October 1.

Understanding Salter-Harris Type II physeal fractures of the metatarsal

A Salter-Harris Type II fracture is the most common growth plate injury pattern. Pediatric fracture texts such as Rang’s Children’s Fractures put it at roughly three-quarters of physeal fractures. The fracture line runs through the physis and exits through the metaphysis, leaving the epiphysis intact.

Metatarsal physeal fractures occur in children and adolescents whose growth plates are still open. In the lesser metatarsals the physis sits at the distal end, near the head. In the first metatarsal it sits at the base, so imaging reports describe the fracture site differently.

• Type I: The fracture runs through the physis only, and the epiphysis separates without a metaphyseal fragment.
• Type II: The fracture runs through the physis and exits through the metaphysis, leaving a triangular Thurston Holland fragment. This is the most common type.
• Type III: The fracture runs through the physis and the epiphysis into the joint surface.
• Type IV: The fracture crosses the metaphysis, physis and epiphysis, with a higher risk of growth disturbance.
• Type V: A crush injury to the physis, which is rare and often missed on first imaging.

Each Salter-Harris type has its own code group under S99.1, so the type in the record decides the fifth character. A Type I fracture of the right metatarsal codes to S99.111-, and a Type III to S99.131-.

When to use the K (nonunion) seventh character

Use the seventh character K when the patient returns for care and the provider documents that the right metatarsal fracture has not united. Two conditions must both appear in the record. The visit follows the initial treatment, and the provider states that the bone has failed to heal.

The full seventh-character set for S99.121 is:

Section I.C.19 of the ICD-10-CM Official Guidelines for Coding and Reporting governs the seventh character across Chapter 19. ICD-10-CM sets no time threshold for nonunion. The coder relies on the provider’s documented determination, not on how many weeks have passed.

Payers can set their own thresholds for treatment, though. The Medicare bone stimulator policy, for example, defines nonunion by serial imaging that shows healing has stopped for at least 3 months.

The decision path below shows how the encounter and the documented healing status pick the seventh character.

S99.121K vs. similar codes: Avoiding common coding errors

The codes most often confused with ICD-10 Code S99.121K are its own family members, plus the toe and adult metatarsal codes nearby. The table below shows when each one applies instead.

The S92.3 subcategory carries an Excludes2 note for physeal fracture of metatarsal (S99.1-). A growth plate fracture therefore codes to S99.1-, not to the S92.3- codes used for adults. Once the growth plate has closed, a displaced right second metatarsal nonunion codes to S92.321K instead.

The K-to-P swap needs the same care. Nonunion (K) means the fracture has not healed, while malunion (P) means it healed out of position. The two lead to different procedures, so check the full descriptor in the AAPC Codify ICD-10-CM lookup before submission.

Documentation requirements for S99.121K

Supporting S99.121K on a claim requires documentation for each fact the seven characters encode. Medical billing compliance frameworks recommend a pre-submission checklist for subsequent-encounter fracture codes like this one.

• Confirmed nonunion: The provider or radiologist states nonunion, failed union, or no bridging callus on imaging. The word nonunion should appear in the note, not be left for the coder to infer.
• Encounter type: The note shows a follow-up after the initial treatment. A new provider seeing the patient on referral should record that the initial treatment happened elsewhere.
• Laterality and bone: The record states the right foot and names the metatarsal involved, such as the fifth metatarsal.
• Fracture classification: The provider or radiology report confirms a Salter-Harris Type II pattern, ideally naming the Thurston Holland fragment.
• Episode linkage: The note ties this visit to the original fracture through a prior visit note, a referral letter, or an explicit follow-up statement.

For practices focused on building a clean claim, checking these five elements before submission removes the documentation questions payers raise most on this code.

CPT codes commonly paired with ICD-10 Code S99.121K

The procedure codes reported with ICD-10 Code S99.121K depend on what the provider does at the nonunion visit. Options range from an office visit and imaging to bone stimulation or surgical repair. Every code below is a metatarsal code; toe phalanx codes 28510 and 28515 describe a different bone.

Metatarsal fracture codes are reported per bone, so units must match the metatarsals treated. National Correct Coding Initiative edits can bundle imaging or a visit with a same-day procedure, so check each payer’s pairing rules first.

Claims software built for practices tracks these claims from submission to payment, and the Claim.MD clearinghouse lets staff check eligibility before the visit.

Payer requirements and Medicare considerations for metatarsal nonunion

Medicare covers noninvasive bone stimulators for long bone nonunion under the DME MAC policy for osteogenesis stimulators, LCD L33796. That policy counts the metatarsal as a long bone, so a metatarsal nonunion can qualify.

The LCD requires two sets of radiographs at least 90 days apart, each with multiple views. A treating practitioner must also state in writing that the images show no clinically significant healing between them. The device itself is billed under HCPCS E0747 by the DME supplier.

Key payer requirements to verify before submission:

• Prior authorization: Commercial and Medicaid plans may require approval for bone stimulators and nonunion surgery such as CPT 28322. Confirm the rule for each plan before the procedure.
• Medical necessity: Payers expect imaging evidence of nonunion and a record of the treatment tried so far. Many also set a minimum time since injury, so check the plan’s policy.
• Global surgery period: Metatarsal fracture codes such as 28470, 28475 and 28485 are generally listed with a 90-day global period. Confirm each code’s global days in the CMS Physician Fee Schedule lookup, because follow-up visits inside that window are usually not billed separately.
• Medicaid variation: Because this is a pediatric injury, many claims go to Medicaid or CHIP. Coverage for stimulators and nonunion repair varies by state, so check the state policy directly.

Understanding revenue cycle management helps practices handle these multi-visit claims. A good workflow catches global period conflicts and medical necessity rules before submission.

Common S99.121K claim denials and how to prevent them

Claims for ICD-10 Code S99.121K run into a predictable set of denial patterns. Most trace back to character selection or missing documentation, which a pre-submission review can catch. Solid denial management workflows build these checks into the billing queue.

• Initial-encounter character carried forward: The coder repeats the A from the first visit on a follow-up. If the patient has already been treated for this fracture, the visit takes a subsequent-encounter character.
• Missing nonunion statement: The claim uses K, but the imaging report only says the fracture line is still visible. Ask the provider to state the healing status before billing.
• Laterality mismatch: The claim reports the right metatarsal, but a radiology report or prior claim says left. Cross-check the side on every document, then confirm the sixth character matches it.
• Wrong bone family: The claim pairs S99.121K with toe codes 28510 or 28515, or uses an S99.22- toe code for a metatarsal injury. Match the procedure and diagnosis to the same bone.
• Global period conflict: A follow-up visit is billed separately inside the global period of the original fracture treatment. Record the original procedure date and calculate the end of the global period first.
• Nonunion and malunion swap: An ambiguous note leads the coder to the wrong healing status. Query the provider rather than choose between K and P without explicit documentation.

For practices handling higher volumes of fracture claims, reviewing denial codes in medical billing helps staff respond faster. It also shows which adjustment reason codes payers attach to encounter and laterality errors.

Excludes notes and sequencing rules for S99.121K

The physeal metatarsal codes interact with the rest of Chapter 19 through excludes notes and sequencing rules. These apply whenever S99.121K appears with other codes on the same claim.

• Excludes2 at S92.3: Fracture of metatarsal bone excludes physeal fracture of metatarsal (S99.1-). A growth plate fracture codes here, not to S92.3-.
• S99.121K as first-listed diagnosis: Report it first when the main reason for the visit is the nonunion of the right metatarsal fracture.
• Complications: When the visit is driven by a complication such as osteomyelitis or hardware failure, follow the guideline for that complication code and its sequencing.
• External cause codes: An external cause code, with its own matching seventh character, may be reported for each encounter where the injury is treated. Place of occurrence and activity codes are reported only at the initial encounter.
• Nonunion is not sequela: An unhealed fracture is an active injury that takes K, not S.

How claims management software supports S99.121K billing

A nonunion claim for S99.121K often follows several earlier claims for the same fracture. Billing staff need to know which visits were paid, which were denied, and whether the patient is still covered before surgery.

Pabau includes US claims management through Claim.MD. It submits claims electronically, tracks each claim’s status, and checks patient eligibility before the visit. Staff can follow the whole fracture episode without logging in to separate payer portals.

The clinical notes, imaging reports and invoices for each visit sit in the same patient record. When a payer asks for the nonunion documentation, the team can find it in one place and respond faster.

Keep fracture follow-up claims moving

Pabau’s US claims management, built on Claim.MD, submits claims, tracks their status and checks patient eligibility. Your team can follow every S99.121K claim from submission to payment in one place.

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Conclusion

S99.121K depends on three facts in the record: a right metatarsal, a Type II growth plate fracture, and nonunion at a follow-up visit. Confirm the provider has stated all three before choosing the code. Then pair it with metatarsal procedure codes, never toe codes.

The trade-off is time spent upfront. A query to the provider before billing costs minutes, while a denied nonunion claim can stall payment for a surgery or a stimulator. Pabau’s claims management submits these claims through Claim.MD, tracks their status and checks eligibility. Book a demo to see how it keeps fracture follow-up claims moving.

Continue your research

Need to understand how claim denials are structured? Denial codes in medical billing explains the CARC and RARC codes payers use when rejecting fracture claims, including seventh-character errors.

Want to streamline how your practice handles medical billing? Medical billing fundamentals covers the end-to-end billing process from charge capture through remittance posting.

Looking for guidance on managing the full claims lifecycle? Pabau’s Claim.MD clearinghouse integration explains how electronic claim submission, ERA posting, and real-time eligibility work together to reduce denials.

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