Enlargement or hypertrophy of the infrapatellar (Hoffa's) fat pad, a soft tissue structure anterior to the knee joint and posterior to the patellar tendon, classified under other soft tissue disorders.

Documentation tips

What should appear in the chart to support M79.4.

Source · Editorial brief grounded in 4 cited references ↓

• Specify the affected side (right, left, or bilateral) in the clinical note — M79.4 has no laterality subcode, but payer audit algorithms and clinical record accuracy both require it.
• Record the imaging finding that confirms fat pad hypertrophy: MRI signal change, enlargement on T2-weighted sequences, or ultrasound-detected impingement; Kellgren-Lawrence grade alone is insufficient for this diagnosis.
• Distinguish the fat pad finding from concomitant diagnoses (e.g., patellofemoral OA, patellar tendinopathy) and sequence codes appropriately — M79.4 is often a secondary or incidental finding, not the primary complaint.
• If conservative care (PT, NSAIDs, corticosteroid injection) has been attempted and failed, document the duration and response; this supports medical necessity for surgical intervention such as arthroscopic fat pad resection.
• Note whether the hypertrophy is symptomatic (Hoffa's impingement syndrome) versus an incidental imaging finding — this distinction affects code sequencing and justifies the encounter.

Related CPT procedures

Procedure codes commonly billed with M79.4. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

Common coding pitfalls

The recurring mistakes coders make with M79.4 and adjacent codes.

Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓

• Applying M79.4 as the primary diagnosis for a trigger point injection claim — CMS LCD A57702 explicitly lists M79.4 among codes that do NOT support TPI medical necessity.
• Assuming M79.4 has laterality subcodes: it does not. There is no M79.41 or M79.42; the single code M79.4 covers all laterality variants, unlike most knee diagnosis codes in Chapter 13.
• Confusing M79.4 with E65 (Localized adiposity/fat pad) — the Alphabetic Index cross-references both, but M79.4 is the correct code for a knee fat pad pathology, not a generalized adiposity code.
• Failing to add a second code for the primary knee complaint (e.g., anterior knee pain M25.561) when fat pad hypertrophy is an incidental MRI finding rather than the chief reason for the visit.
• Overlooking the MS-DRG assignment to 606/607 (minor skin disorders) on inpatient claims — if M79.4 is sequenced as principal diagnosis on an inpatient knee procedure claim, expect a grouper mismatch and potential claim denial.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

Sibling codes

Other billable codes under M79 (laterality / anatomic variants).