Chapter 13 — Diseases of the Musculoskeletal System and Connective Tissue
Current acute injury or chronic condition? Plus site-versus-joint, the pathological fracture 7th characters, and why osteoporosis has no site code.
Chapter 13 answers one question over and over: is this a current acute injury, or a chronic, recurrent or healed condition? Current acute injuries go to Chapter 19. Chronic and recurrent bone, joint and muscle conditions, and conditions resulting from a healed injury, go to Chapter 13. When the documentation will not settle it, the guideline says to query.
Site and laterality#
| Point | Rule |
|---|---|
| Site | Most Chapter 13 codes have site and laterality designations. The site represents the bone, joint or muscle involved |
| Bone versus joint | For certain conditions the bone may be affected at the upper or lower end — avascular necrosis of bone M87, osteoporosis M80, M81. Though the portion of bone affected may be at the joint, the site designation will be the bone, not the joint |
| Multiple sites | Codes describing specified sites are assigned individually by site when documented. When the specified sites are not documented, assign the appropriate code for multiple sites |
Acute traumatic, or chronic and recurrent?#
| Condition | Chapter |
|---|---|
| Any current, acute injury | Chapter 19 — the appropriate injury code |
| Bone, joint or muscle conditions that are the result of a healed injury | Usually Chapter 13 |
| Recurrent bone, joint or muscle conditions | Usually Chapter 13 |
| Difficult to determine from the documentation which code best describes the condition | QUERY Query the provider |
Pathologic fractures — the 7th character#
| 7th character | Use |
|---|---|
A | For as long as the patient is receiving active treatment for the fracture. Assignment is based on whether the patient is undergoing active treatment, not on whether the provider is seeing the patient for the first time — the patient may be seen by a new or different provider over the course of treatment |
D | For encounters after active treatment has been completed and the patient is receiving routine care during the healing or recovery phase |
| Other 7th characters listed under each subcategory | For subsequent encounters for treatment of problems associated with the healing — malunions, nonunions and sequelae |
| Complications of surgical treatment for fracture repairs during the healing or recovery phase | Coded with the appropriate complication codes |
Osteoporosis — a systemic condition#
Osteoporosis is systemic: all bones of the musculoskeletal system are affected, so site is not a component of the M81 codes. The site codes under M80 identify the site of the fracture, not the site of the osteoporosis.
| Category | For | Notes |
|---|---|---|
M81 Osteoporosis without current pathological fracture | Patients with osteoporosis who do not currently have a pathologic fracture due to the osteoporosis — even if they have had a fracture in the past | For a history of osteoporosis fractures, Z87.310 personal history of (healed) osteoporosis fracture follows the M81 code |
M80 Osteoporosis with current pathological fracture | Patients who have a current pathologic fracture at the time of an encounter | ALWAYS A code from M80, not a traumatic fracture code, is used for any patient with known osteoporosis who suffers a fracture — even if the patient had a minor fall or trauma, if that fall or trauma would not usually break a normal, healthy bone |
Chapter 13 also carries a single cross-reference: multisystem inflammatory syndrome M35.81 is governed by the COVID-19 guideline at Section I.C.1.g.1.l, not by a Chapter 13 rule. See Chapter 1.
Practice questions#
Q1An 81-year-old with documented osteoporosis sustains a fracture of the right femoral neck after tripping on a rug at home. What type of fracture code is assigned?
- AA traumatic fracture code from Chapter 19
- BA code from category
M80 - CA code from category
M81with a separate traumatic fracture code - DA code from
M84.4pathological fracture NEC
Show answer & rationale
Correct answer: B. A code from category M80
Rationale. Guideline I.C.13.d.2 states that a code from category M80, not a traumatic fracture code, should be used for any patient with known osteoporosis who suffers a fracture, even if the patient had a minor fall or trauma, if that fall or trauma would not usually break a normal, healthy bone. Guideline I.C.19.c.1 repeats the same rule from the injury chapter's side.
Q2A patient with a pathological fracture is seen by a new orthopaedic surgeon for the first time, but is still receiving active treatment for the fracture. Which 7th character applies?
- A
A, initial encounter - B
D, subsequent encounter - C
S, sequela - D
K, subsequent with nonunion
Show answer & rationale
Correct answer: A. A, initial encounter
Rationale. Guideline I.C.13.c states that 7th character A is for use as long as the patient is receiving active treatment for the fracture, and that while the patient may be seen by a new or different provider over the course of treatment for a pathological fracture, assignment of the 7th character is based on whether the patient is undergoing active treatment and not whether the provider is seeing the patient for the first time. Section I.C.19.a states the identical principle for Chapter 19.
Q3A patient has avascular necrosis of the bone at the upper end of the femur. What is the site designation?
- AThe hip joint
- BThe femur (the bone)
- CEither, at the coder's discretion
- DMultiple sites
Show answer & rationale
Correct answer: B. The femur (the bone)
Rationale. Guideline I.C.13.a.1 states that for certain conditions the bone may be affected at the upper or lower end, giving avascular necrosis of bone M87 and osteoporosis M80, M81 as the examples, and that though the portion of the bone affected may be at the joint, the site designation will be the bone, not the joint.
Q4A patient has a history of a healed osteoporotic vertebral fracture and currently has osteoporosis with no current pathological fracture. What is assigned?
- AA code from
M80only - BA code from
M81, followed byZ87.310 - C
Z87.310only - DA code from
M80followed byZ87.310
Show answer & rationale
Correct answer: B. A code from M81, followed by Z87.310
Rationale. Guideline I.C.13.d.1 states that category M81 is for use for patients with osteoporosis who do not currently have a pathologic fracture due to the osteoporosis, even if they have had a fracture in the past, and that for patients with a history of osteoporosis fractures, status code Z87.310, personal history of (healed) osteoporosis fracture, should follow the code from M81.
Q5The documentation makes it difficult to determine whether a musculoskeletal condition is a current acute injury or a chronic recurrent condition. What does the guideline direct?
- ADefault to the Chapter 13 code
- BDefault to the Chapter 19 injury code
- CQuery the provider
- DAssign both codes
Show answer & rationale
Correct answer: C. Query the provider
Rationale. Guideline I.C.13.b states that any current, acute injury should be coded to the appropriate injury code from Chapter 19, that chronic or recurrent conditions should generally be coded with a code from Chapter 13, and that if it is difficult to determine from the documentation in the record which code is best to describe a condition, the provider should be queried.
Scenarios#
A patient with known osteoporosis sustained a pathological fracture of the left vertebra six weeks ago, treated with kyphoplasty. She now returns for a routine follow-up radiograph during the healing phase. Active treatment is complete.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | M80.08XD | Age-related osteoporosis with current pathological fracture, vertebra(e), subsequent encounter for fracture with routine healing — verify the site and 7th character in the Tabular List |
Rationale. Guideline I.C.13.c states that 7th character D is used for encounters after the patient has completed active treatment for the fracture and is receiving routine care during the healing or recovery phase. Guideline I.C.13.d and I.C.19.c.1 require the M80 category rather than a traumatic fracture code. Section I.C.21.c.7 and I.C.19.a both prohibit using an aftercare Z code for this encounter — the fracture code with 7th character D is the aftercare.
Guideline: Section I.C.13.c, I.C.13.d.2, I.C.19.a
A patient who sustained a severe ankle fracture eight years ago, long since healed, now has post-traumatic osteoarthritis of that ankle. She is seen for management of the arthritis.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | M19.17- | Post-traumatic osteoarthritis, ankle and foot, with laterality — verify in the Tabular List; first-listed |
Rationale. Guideline I.C.13.b states that many musculoskeletal conditions are the result of previous injury or trauma to a site, that bone, joint or muscle conditions that are the result of a healed injury are usually found in Chapter 13, and that chronic or recurrent conditions should generally be coded with a Chapter 13 code. The healed fracture itself is not reported as a current injury; Section I.B.10 would require a sequela pairing only where a Chapter 19 sequela code is the correct classification for the residual, which it is not where Chapter 13 provides a specific post-traumatic condition code.
Guideline: Section I.C.13.b, I.B.10
A rheumatology record documents osteoarthritis affecting the right knee and the left hip specifically, and then adds “generalised osteoarthritis of multiple joints” without naming the others.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | M17.11 | Unilateral primary osteoarthritis, right knee — verify in the Tabular List |
| 2 | M16.12 | Unilateral primary osteoarthritis, left hip — verify in the Tabular List |
| 3 | M15.- | The applicable polyosteoarthritis code for the undocumented sites, if supported |
Rationale. Guideline I.C.13.a.2 states that codes describing specified sites are assigned individually by site when documented, and that when the specified sites are not documented the appropriate code for multiple sites is assigned. Section I.B.20 gives the general version of the same rule: multiple means two or more sites, chapter-specific guidelines are followed first, and the multiple-sites code is used only where the specific sites are not documented. Read the Tabular List notes at M15-M19 before combining codes, because the instructional notes govern.
Guideline: Section I.C.13.a.2, I.B.20
Primary sources#
- ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027CMS — the source document for every page on this site (effective 1 Oct 2026)
- NCHS ICD-10-CM files — Tabular List, Index, POA exempt listOfficial code files, addenda and the list of codes exempt from POA reporting
- ICD-10 Coordination and Maintenance CommitteeWhere code proposals are debated; agendas, summaries and meeting materials
- CMS ICD-10 homeCode sets, transition guidance and Medicare coding policy
- AHA Coding ClinicOfficial coding advice from the AHA Central Office — subscription required
- HHS OIG Work PlanActive audit topics — useful for prioritising internal coding audits
- MEDESUN Medical Coding AcademyTraining, credential preparation and audit services by the author