Degenerative softening and breakdown of the articular cartilage on the underside of the left kneecap (patella), classified under patellar disorders in ICD-10-CM Chapter 13.

Documentation tips

What should appear in the chart to support M22.42.

Source · Editorial brief grounded in 6 cited references ↓

• Specify laterality explicitly as 'left' — without it, the only defensible code is M22.40 (unspecified knee), which will downcode the claim.
• Document the structure involved: patellar cartilage (maps to M22.42), not femoral or tibial cartilage (maps to M94.262). This distinction determines the correct code family.
• Record the grade or severity of cartilage degeneration when available — e.g., Outerbridge Grade II or III — to support medical necessity for surgical or advanced conservative intervention.
• Capture clinical exam findings that support the diagnosis: patellar grind test result, crepitus on range of motion, peripatellar tenderness, or apprehension sign.
• Document MRI findings (patellar cartilage thinning, signal change, fissuring) or arthroscopic findings (fibrillation, softening) when imaging was obtained, as payers routinely request this for higher-cost procedures.
• If bilateral disease is present, document both knees explicitly so M22.41 and M22.42 can be coded together — there is no bilateral code in the M22.4 subcategory.

Related CPT procedures

Procedure codes commonly billed with M22.42. 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 M22.42 and adjacent codes.

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

• Assigning M94.262 (chondromalacia, left knee joint) instead of M22.42 when the pathology is specifically patellar — these two codes are separated by a Type 1 Excludes relationship; use M22.42 for patella, M94.262 for femoral or tibial surfaces.
• Using M22.42 for an acute traumatic patellar cartilage injury — M-codes classify disease states, not acute injuries. Traumatic patellar chondral damage requires an S-code (S83.0- for traumatic dislocation with associated chondral damage).
• Defaulting to M22.40 (unspecified knee) when laterality is documented in the note — always assign the laterality-specific code when the record supports it.
• Coding M22.42 as a secondary diagnosis when it is the primary reason for the encounter (e.g., presenting for lateral retinacular release or chondroplasty) — sequence it as the principal diagnosis in that context.
• Conflating chondromalacia patellae with patellofemoral pain syndrome (PFPS) — PFPS is coded M22.2X2. Use M22.42 only when the provider specifically diagnoses cartilage degeneration or softening, not generalized patellofemoral pain.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M22.4 (laterality / anatomic variants).