Inflammation of the prepatellar bursa — the fluid-filled sac anterior to the kneecap — classified here when the affected side (right or left) is not documented in the medical record.

Documentation tips

What should appear in the chart to support M70.40.

Source · Editorial brief grounded in 5 cited references ↓

• Record laterality explicitly — 'right knee' or 'left knee' — so you can bill M70.41 or M70.42 instead of the unspecified M70.40.
• Note the occupational or activity history (e.g., prolonged kneeling, repetitive pressure) and add a Y93.- external cause code when the disorder is work-related or activity-driven.
• Document the specific bursa involved (prepatellar, not just 'knee bursitis') to distinguish M70.4x from M70.5x (other bursitis of knee) and M71.9- (bursitis NOS).
• If aspiration or injection is performed, document the exact bursal anatomy targeted and the volume/character of fluid removed — this supports both the diagnosis code and the procedure code (20610).
• Capture imaging findings (ultrasound or MRI) that confirm anterior prepatellar fluid collection, especially when conservative care has failed and a higher-intensity procedure is being authorized.

Related CPT procedures

Procedure codes commonly billed with M70.40. 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 M70.40 and adjacent codes.

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

• Using M70.40 when laterality is clearly documented in the note — this triggers specificity edits; always prefer M70.41 (right) or M70.42 (left) when the side is known.
• Confusing M70.40 with M71.9- (bursitis NOS) — M71.9- is excluded from M70 and applies only when the specific bursa cannot be identified; M70.40 requires documented prepatellar involvement.
• Coding M70.40 for infrapatellar or pes anserine bursitis — those fall under M70.5x (other bursitis of knee), not M70.4x.
• Omitting the Y93.- activity code when the bursitis is clearly occupational in origin, which can affect workers' compensation claims and payer adjudication.
• Pairing M70.40 with a shoulder bursitis code from M75.5 — M70 explicitly excludes M75.5, so mixing these on the same claim for different body regions requires careful modifier and diagnosis sequencing.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

Other billable codes under M70.4 (laterality / anatomic variants).