ICD-10 code M79.4: Hypertrophy of (infrapatellar) fat pad

Key Takeaways

ICD-10 code M79.4 is a billable, specific code for hypertrophy of the infrapatellar fat pad, the knee condition often called Hoffa’s disease in clinical notes.

Coding from the eponym is a trap: Hoffa’s disease indexes to E88.89 in the ICD-10-CM alphabetic index, not M79.4. Code from the documented anatomic finding instead.

M79.4 is valid for FY2026 (effective October 1, 2025), carries no right/left laterality subclassifications, and inherits an Excludes1 note barring F45.8 and F45.41 from the parent M79 category.

Practice management software like Pabau pairs claims management with client-record tools to support structured ICD-10 coding workflows for orthopedic, sports medicine, and physical therapy practices.

ICD-10 code M79.4 is a billable, specific code for hypertrophy of the infrapatellar fat pad, the fatty cushion that sits behind the kneecap and above the patellar tendon. Most people land on this code after searching “Hoffa’s disease,” the eponym clinicians still write in their notes, and assume the two point to the same place.

They don’t. Look up “Hoffa’s disease” in the official ICD-10-CM alphabetic index and it routes to E88.89, a metabolic disorder code, not M79.4. The only way to reach M79.4 is through the anatomic finding itself, and mixing that up is a quiet way to submit a claim against the wrong code entirely.

M79.4 sits within the M79 category of the ICD-10-CM code set CMS maintains, under the broader block for soft tissue disorders. It shows up most often in orthopedic, sports medicine, and physical therapy charts, where fat pad pathology is a routine anterior knee finding.

The quick-reference table below covers the basics, then we get into why the Hoffa’s disease label trips coders up.

ICD-10 code M79.4 at a glance

M79.4 is a billable, specific ICD-10-CM diagnosis code. Practices can submit it directly on claims for reimbursement, subject to payer medical necessity requirements. Below is the at-a-glance reference table coders need most.

One coding detail worth flagging early: M79.4 does not carry laterality extensions. Many coders working in the musculoskeletal chapter automatically look for a 7th character or a right/left suffix. With M79.4, none exist. The four-character code is the complete, billable code as submitted. Confirm this against the CDC/NCHS ICD-10-CM web tool for current-year verification.

What infrapatellar fat pad hypertrophy looks like clinically

The infrapatellar fat pad, also called the Hoffa fat pad, is a triangular adipose structure located beneath the patella and above the patellar tendon within the knee joint. When it becomes enlarged or inflamed, the result is infrapatellar fat pad hypertrophy, sometimes described clinically as infrapatellar fat pad syndrome or Hoffa fat pad impingement.

The condition produces anterior knee pain, particularly with knee extension, and is frequently seen in patients with overuse injuries, post-traumatic inflammation, or degenerative joint changes.

Clinically, the condition is diagnosed through physical examination and confirmed on MRI, which shows signal changes or visible enlargement of the fat pad. During that exam, a physical therapist often has to rule out other anterior knee pain generators first.

A tight hip flexor picked up on a hip flexor strain test can mimic a similar ache before the fat pad finding gets confirmed on imaging.

For sports medicine software users, structured documentation of each ruled-out differential is what makes the eventual code assignment defensible.

M79.4 should only be assigned when the treating clinician has documented the diagnosis. Coders must not assign this code based on symptoms alone or from imaging reports without physician interpretation.

Why “Hoffa’s disease” doesn’t code to M79.4

Coders often assume that because clinicians call this condition “Hoffa’s disease,” the eponym should index straight to M79.4. It doesn’t. Albert Hoffa first described the condition in 1904, and the name stuck in clinical use, but the ICD-10-CM alphabetic index sends “Hoffa’s disease” and “Hoffa-Kastert disease” to E88.89, other specified metabolic disorders, not to the musculoskeletal chapter at all.

M79.4 is reached a different way: through the anatomic finding the clinician documents, not the eponym. Index entries like “hypertrophy, fat pad, knee” or “fibrosis, infrapatellar fat pad” are what lead to M79.4.

So when a note reads “Hoffa’s disease,” the coder’s job is to look past the label and code the documented finding, hypertrophy or degeneration of the infrapatellar fat pad, which is what actually drives the M79.4 assignment.

A few other terms show up in the same notes, and they’re worth knowing as clinical shorthand rather than tabular index entries:

• Infrapatellar fat pad syndrome – describes the symptomatic presentation, anterior knee pain that worsens with extension
• Hoffa fat pad impingement – describes the mechanical presentation, the fat pad compressed between bony structures during extension
• Hoffa syndrome – a general clinical term for chronic impingement symptoms

None of these three are Applicable To entries in the tabular list. They describe the same clinical picture in different words, and correct coding still runs through the anatomic finding, not the label. Confirm the index routing yourself against the AAPC ICD-10-CM code lookup before assigning either code from a note that uses one of these terms.

Where M79.4 sits in the ICD-10-CM hierarchy

Understanding where M79.4 sits in the ICD-10-CM hierarchy helps coders navigate to related codes and understand applicable coding guidelines for the broader block.

M79 covers soft tissue disorders that do not fit more specific categories elsewhere in the M70-M79 block. Coders working with knee conditions regularly navigate this hierarchy to distinguish between bursitis codes (M70.4x, M70.5x), tendinopathy codes (M76.5x), and the unspecified soft tissue category where M79.4 resides.

The same block-level logic carries over to neighboring categories: M72.8 sits under M72’s fibroblastic disorders rather than M79’s catch-all, even though both live inside M70-M79.

When a more specific code exists, the guidelines for other and unspecified soft tissue disorders point coders to that more specific code instead. M79.4 is appropriate when the infrapatellar fat pad hypertrophy is the documented primary finding.

What billing teams need to know before submitting M79.4

M79.4 is a billable code, but billing teams should understand a few nuances before submission.

• Medical necessity required: Payer Local Coverage Determinations (LCDs) govern whether a claim with M79.4 as the diagnosis will be covered. The code alone does not guarantee reimbursement. Practices should verify payer-specific requirements before submitting.
• No laterality: Unlike most musculoskeletal knee codes, M79.4 has no right/left extensions. Submit the four-character code as the complete diagnosis code.
• Present on Admission (POA): For inpatient claims, M79.4 accepts standard POA indicators (Y, N, U, W). Outpatient claims are not subject to POA reporting requirements.
• Excludes1 note inherited from M79: Never report M79.4 together with psychogenic rheumatism (F45.8) or psychogenic soft tissue pain (F45.41). The Excludes1 note sits at the parent M79 level in the tabular list, and it applies to every code underneath it, including M79.4.
• Not a tabular synonym: Clinicians sometimes chart this condition as “fat pad syndrome.” That phrasing is informal clinical shorthand, not an Applicable To entry in the ICD-10-CM tabular list, so code from the documented anatomic finding rather than the label itself.

Practices using claims management software like Pabau’s can validate the fields a payer needs before a claim goes out, gate submission until those fields are complete, and track status on one dashboard from submission to payment.

That’s a different job from flagging a documentation gap specific to M79.4. A biller still has to confirm the chart supports medical necessity before the claim leaves the practice.

The same LCD-driven variability applies to other soft-tissue and joint diagnoses, including M17.0, so check payer rules per diagnosis rather than assuming blanket coverage.

Which CPT codes pair with M79.4

CPT-ICD-10 pairings for M79.4 are scattered across payer resources, so here’s the table coders reach for most in practice. It reflects the procedures commonly performed when a patient presents with infrapatellar fat pad hypertrophy.

Reimbursement for each CPT code depends on payer-specific LCD alignment with M79.4 as the supporting diagnosis, so verify before submitting. For practices managing physical therapy compliance, see physical therapy compliance requirements.

If conservative treatment fails and the case moves to a scoped procedure, coders should also check the knee-specific anesthesia codes, like 01432, that pair with the surgical CPT code rather than the E/M or PT codes above.

Physical therapy practices managing rehabilitation for this condition should verify LCD alignment between M79.4 and codes like 97110 and 97140 with their MAC (Medicare Administrative Contractor) before billing.

Structured physical therapy EMR software like Pabau keeps the diagnosis and procedure codes on the same patient record, so that check is a quick look at one screen instead of cross-referencing two systems. The LCD match itself is still a manual call for the billing team, not something the software validates automatically.

M79.4 versus the codes it’s easy to confuse it with

Accurate coding for knee conditions means distinguishing M79.4 from several closely related codes. The table below maps the differentials coders run into most often alongside infrapatellar fat pad hypertrophy.

The most common coding mistake with M79.4 is assigning it alongside a knee pain symptom code (M25.56x). When the infrapatellar fat pad hypertrophy fully explains the patient’s knee pain, the symptom code is redundant and should not be reported. This aligns with ICD-10-CM guideline I.C.18.a on signs and symptoms.

One differential that doesn’t make the table but shows up on the same MRI report: a meniscus tear, coded separately under S83.241 or another code in that family, describes a structural tear rather than fat pad enlargement. Confirm which finding the clinician is actually documenting before you pick one code over the other.

What M79.4 maps to under ICD-9-CM

Practices cross-referencing legacy records or auditing historical claims may need the ICD-9-CM equivalent for M79.4. The General Equivalence Mappings (GEMs) published by CMS provide the approximate forward mapping shown below. GEM mappings are approximate and not authoritative for audit purposes.

ICD-9-CM 728.9 (unspecified disorder of muscle, ligament, and fascia) is the most commonly cited approximate equivalent, but it covers a much broader category than the fat pad finding alone. For research or retrospective data work, consult the ResDAC transition guidance before drawing clinical conclusions from cross-system comparisons.

Is M79.4 still valid for FY2026?

M79.4 has been a stable ICD-10-CM code since the system’s initial US implementation. The FY2026 edition took effect on October 1, 2025, with no revisions or descriptor changes to M79.4 in that update cycle. Confirm validity annually against the CMS release before submitting claims in a new fiscal year; every code-year claim in this article reflects the FY2026 edition.

How Pabau fits into the M79.4 coding workflow

Orthopedic and physical therapy practices submit dozens of diagnosis codes per day. The bottleneck is usually documentation, not the code itself. When a clinician documents Hoffa’s disease in a free-text note, a billing team without structured workflows has to manually search for M79.4 before submitting the claim.

Pabau’s clinical record management tools let practitioners attach ICD-10 codes directly within the patient record at the point of care, reducing the documentation-to-billing gap. Paired with digital intake forms that capture presenting complaints and prior diagnoses before the appointment, practices build a coding-ready documentation trail from intake to claim submission.

For practices opening a physiotherapy practice, building this workflow in from day one avoids the rework that comes from retrofitting billing processes later.

Streamline ICD-10 coding and claims in one platform

Pabau connects clinical documentation, ICD-10 code assignment, and claims submission in a single workflow. Practices using Pabau reduce claim rework by building coding accuracy into the documentation step, not the billing step.

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Conclusion

M79.4 is a clean, billable code once the eponym trap is out of the way. Skip the “Hoffa’s disease” label when you’re assigning the ICD-10-CM code, document the anatomic finding instead, and check the inherited Excludes1 note before pairing it with anything else.

Practices that keep the diagnosis, the supporting documentation, and the claim in one system spend less time chasing down what a denial actually needs. Practice management software like Pabau brings clinical documentation and claims management into one record, so the finding a clinician charts at the visit is the same one a biller sees when the claim goes out.

If a cleaner coding-to-claim handoff would help your practice, book a demo to see how it fits your specialty.

Continue your research

Looking up a related soft tissue code? M61.9 covers calcification and ossification of muscle, another finding coded from the anatomic diagnosis rather than any eponym.

Need the disc disorder code that shows up on the same musculoskeletal charts? M51.9 is the unspecified intervertebral disc disorder code, a common companion diagnosis in orthopedic and physical therapy billing.

Billing anesthesia for a lower leg procedure? 01490 covers anesthesia for lower leg cast application and removal, a CPT pairing worth knowing alongside knee and lower-extremity ICD-10 codes.

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