Inflammation of the prepatellar bursa — the small fluid-filled sac directly anterior to the patella — specifically affecting the left knee, classified under soft tissue disorders related to use, overuse, and pressure.
Verified May 8, 2026 · 5 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 5
- Region
- Knee
Documentation tips
What should appear in the chart to support M70.42.
Source · Editorial brief grounded in 5 cited references ↓
- Explicitly document 'left knee' laterality — 'knee pain' or 'prepatellar bursitis' without a side forces a drop to M70.40 (unspecified).
- Record the mechanism: repetitive kneeling, direct blow, or underlying inflammatory condition (gout, RA) to satisfy the M70 'use, overuse, pressure' category requirement.
- Note whether the bursitis is septic or aseptic — septic prepatellar bursitis requires a separate infectious bursitis code, not M70.42.
- If imaging (ultrasound or MRI) was obtained, document findings such as bursal fluid volume or sac distension to support medical necessity for aspiration or injection.
- For occupational cases, document the patient's job duties and add an external cause code (Y93.-) identifying the activity.
Related CPT procedures
Procedure codes commonly billed with M70.42. 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 M70.42 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Using M70.42 for septic (infectious) prepatellar bursitis — infectious bursitis has its own code family; M70.42 is for non-infectious inflammation only.
- Coding M70.42 alone for bilateral prepatellar bursitis — ICD-10-CM has no single bilateral code; report M70.41 and M70.42 together when both knees are affected.
- Assigning M70.42 when the bursa involved is infrapatellar or pes anserine rather than prepatellar — confirm anatomic location before coding.
- Omitting the external cause code (Y93.-) when the record documents an occupation or specific activity as the cause, which can weaken medical necessity documentation.
- Confusing M70.42 with M71.42 (calcium deposit in bursa, left knee) — bursitis and calcific bursal disease are distinct entities requiring different codes.
Clinical context
Source · Editorial summary grounded in 5 cited references ↓
M70.42 codes prepatellar bursitis of the left knee. The prepatellar bursa sits between the skin and the anterior surface of the patella; repetitive kneeling, direct trauma, or inflammatory conditions (gout, rheumatoid arthritis) cause the bursa to fill with fluid, producing the classic 'goose egg' swelling over the kneecap. This code belongs to the M70 category — soft tissue disorders related to use, overuse, and pressure — so the clinical record should reflect an occupational, activity-related, or traumatic mechanism whenever possible.
If the documentation specifies infection (septic bursitis), do not use M70.42 — route to the appropriate infectious bursitis code and add the causative organism. If the patient has bilateral involvement, code M70.41 (right) and M70.42 (left) separately; there is no single bilateral prepatellar bursitis code in ICD-10-CM. For unspecified laterality, drop to M70.40.
M70.42 maps to MS-DRG 557 (Tendonitis, myositis and bursitis with MCC) or 558 (without MCC) for inpatient encounters. On the outpatient side, this code commonly pairs with aspiration or injection procedures at the knee joint or bursa. Use an external cause code (Y93.-) when an occupational or activity-related etiology is documented.
Sibling codes
Other billable codes under M70.4 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 5 cited references ↓
01What is the difference between M70.42 and M70.40?
02How do I code bilateral prepatellar bursitis?
03Can I use M70.42 for septic prepatellar bursitis of the left knee?
04Which CPT codes most commonly pair with M70.42?
05Should I add an external cause code when the bursitis is work-related?
06Does M70.42 require a 7th-character extension?
07What MS-DRGs does M70.42 map to for inpatient encounters?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
- 01CDC ICD-10-CM Tabular List 2026 (FY2026, effective Oct 1, 2025)
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M70-M79/M70-/M70.42
- 03aapc.comhttps://www.aapc.com/codes/icd-10-codes/M70.42
- 04orthoinfo.aaos.orghttps://orthoinfo.aaos.org/en/diseases--conditions/prepatellar-kneecap-bursitis
- 05cms.govhttps://www.cms.gov/medicare/coding-billing/icd-10-codes
Mira AI Scribe
Mira captures left-side laterality, mechanism of injury or repetitive-use history, bursal swelling location (anterior patella), and any imaging confirming bursal fluid — preventing a downcode to M70.40 (unspecified) and flagging when infectious bursitis language in the note requires a code switch away from M70.42.
See how Mira captures M70.42 documentation