ICD-10-CM · Knee

M22.91

M22.91 captures an unspecified disorder of the patella localized to the right knee — used when a specific patellar condition cannot be identified or documented with greater precision.

Verified May 8, 2026 · 6 sources ↓

Status: Billable
Chapter: 13
Related CPT: 6
Region: Knee

Drawn from CDCICD10DataAAPCOutsourcestrategiesCMS

Documentation tips

What should appear in the chart to support M22.91.

Source · Editorial brief grounded in 6 cited references ↓

Document 'right knee' explicitly — the laterality in the diagnosis code must match the documented side; a generic 'knee pain' note without side-specificity forces a drop to M22.90.
Record why a more specific patellar diagnosis could not be assigned — e.g., 'imaging inconclusive,' 'further workup pending,' or 'patellar disorder, type not yet determined' — to justify use of the unspecified code on audit.
If imaging was performed, include the findings (or absence of findings) that precluded a specific diagnosis such as chondromalacia, subluxation, or patellofemoral syndrome.
Note any prior treatment history relevant to the patella (bracing, PT, prior surgery) to support medical necessity for the associated procedure or service being billed.
If the visit is a follow-up and diagnosis remains unspecified, document clinical reasoning for continued uncertainty rather than leaving it implied.

Related CPT procedures

Procedure codes commonly billed with M22.91. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

27438 $781.92

Patellar arthroplasty with insertion of a prosthetic implant to resurface the kneecap.

27446 $1,047.45

Arthroplasty of the knee involving resurfacing of the condyle and tibial plateau in a single tibiofemoral compartment — medial OR lateral, not both.

27447 $1,159.35

Knee replacement surgery addressing both the medial and lateral tibiofemoral compartments, with or without resurfacing of the patella.

27486 $1,274.91

Revision of a total knee arthroplasty involving a single component, performed with or without the use of donor bone graft material.

27487 $1,574.52

Revision total knee arthroplasty with replacement of both the femoral and tibial components, with or without the use of allograft tissue.

27488 $1,099.89

Removal of a knee prosthesis — including total knee components and methylmethacrylate cement — with or without insertion of a spacer at the same operative session.

Common coding pitfalls

The recurring mistakes coders make with M22.91 and adjacent codes.

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

Using M22.91 when a more specific code is supported by the documentation — chondromalacia (M22.41), patellofemoral syndrome (M22.2X1), or recurrent subluxation (M22.11) are all more precise right-knee patellar codes that should be used when the clinical record supports them.
Coding M22.90 (unspecified laterality) when the provider has clearly documented 'right knee' in the note — M22.91 is required whenever side is specified.
Applying M22.91 to an acute traumatic patellar dislocation — those belong in the S83.0- range, not M22; M22.91 is for chronic or non-traumatic patellar disorders.
Failing to move off M22.91 at follow-up visits once imaging or clinical evaluation yields a definitive patellar diagnosis — the unspecified code should not persist once specificity is established.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

M22.91 sits at the bottom of the M22 hierarchy as the least-specific billable code for right-knee patellar pathology. Use it only when the clinical record cannot support a more definite patellar diagnosis. If the physician documents chondromalacia patellae, code M22.41 (right knee). If patellofemoral syndrome is documented, code M22.2X1. If recurrent dislocation or subluxation is documented, use M22.01 or M22.11 respectively. M22.91 is a last-resort code — appropriate when evaluation is genuinely inconclusive or when a patellar disorder is noted without further characterization.

The M22 category carries a Type 2 Excludes note for traumatic dislocation of patella (S83.0-). That means an acute traumatic patellar dislocation is coded from the S-code range, not M22. However, if a patient presents with both a traumatic episode and a pre-existing chronic patellar disorder, you may code both simultaneously — the Type 2 Excludes note permits dual coding in that scenario.

Laterality is required: M22.91 = right, M22.92 = left, M22.90 = unspecified. Defaulting to M22.90 when the affected side is documented in the record is a laterality error and an audit risk. If the provider documents 'right knee' anywhere in the encounter note, M22.91 is the correct unspecified-level code.

Sibling codes

Other billable codes under M22.9 (laterality / anatomic variants).

M22.90

Unspecified disorder of patella, unspecified knee

M22.92

Unspecified disorder of patella, left knee

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 6 cited references ↓

01When should I use M22.91 instead of M22.2X1 or M22.41?

Use M22.91 only when the provider cannot identify the specific type of patellar disorder. If the note documents patellofemoral syndrome, use M22.2X1. If it documents chondromalacia patellae, use M22.41. M22.91 is appropriate only when the condition is genuinely uncharacterized.

02Can M22.91 be used for an acute traumatic patellar dislocation?

No. Traumatic patellar dislocation is coded from S83.0- with a 7th-character encounter extension (A, D, or S). The M22 category has a Type 2 Excludes for S83.0-, so traumatic events belong in the injury chapter, not M22.91.

03Is M22.91 valid for both initial and follow-up visits?

It is billable at any encounter type, but continued use of an unspecified code at follow-up visits — after diagnostic workup has been completed — is an audit risk. Once a specific patellar diagnosis is established, migrate to the appropriate specific code.

04What is the difference between M22.90 and M22.91?

M22.90 is unspecified disorder of patella with unspecified laterality. M22.91 specifies the right knee. If the provider documents the affected side as right, M22.91 is required — using M22.90 when laterality is documented is a coding error.

05Can M22.91 and an S83.0- traumatic dislocation code be reported together?

Yes. The Type 2 Excludes note under M22 means the conditions are not mutually exclusive — a patient can have a pre-existing chronic patellar disorder (M22.91) and a superimposed acute traumatic dislocation (S83.0-) at the same time, and both may be coded.

06Which CPT procedures most commonly pair with M22.91?

Patellar arthroplasty (27438), total knee arthroplasty with patellar resurfacing (27447), and TKA revision codes (27486, 27487) are common procedural pairings when patellar pathology is a component of the surgical indication. Ensure the operative note supports the patellar involvement.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

Mira AI Scribe

Mira's AI scribe captures laterality (right knee), the absence of a definitive patellar diagnosis, imaging findings or lack thereof, and any clinical tests performed — preventing a drop to the unspecified-laterality code M22.90 and flagging when a more specific M22 subcategory (chondromalacia, patellofemoral disorder, subluxation) may be more appropriate based on documented findings.

See how Mira captures M22.91 documentation