Chapter 22: Diseases of the Musculoskeletal System and Connective Tissue

Introduction to ICD-10-CM Chapter 13: Musculoskeletal Diseases

• Scope and Coding Range: ICD-10-CM Chapter 13 covers "Diseases of the Musculoskeletal System and Connective Tissue," encompassing category codes $M00-M99$.

• Governing Principles: This chapter is governed by the general coding guidelines of the ICD-10-CM handbook.

• Essential Terminology for Coding Professionals:

  • Arthropathy: Defined as a disorder of the joint.

  • Arthritis: Defined as inflammation of the joint.

  • Dorsopathy: Defined as a disorder of the back.

  • Myelopathy: Defined as a disorder of the spinal cord.

  • Radiculopathy: A problem where one or more nerves are affected. This condition results in pain (referred to as radicular pain), weakness, numbness, or difficulty controlling specific muscles.

• General Category Classification:

  • Most arthropathies are classified within categories $M00$ through $M25$.

  • Most dorsopathies are classified within categories $M40$ through $M54$.

Site, Laterality, and Bone vs. Joint Designations

• Specificity in Coding: Most codes in Chapter 13 require designations for both site and laterality.

• Definition of Site: "Site" refers to the specific bone, joint, or muscle involved in the condition.

• Multiple Sites Coding:

  • For conditions that typically involve more than one site (such as osteoarthritis), a specific code for "multiple sites" may be available.

  • If no "multiple sites" code exists for a multi-site condition, the coder should assign separate codes for each individual site involved.

• Bone versus Joint Distinction:

  • Certain conditions affect the bone at its upper or lower end (e.g., $M87$ Avascular necrosis of bone; $M80$, $M81$ Osteoporosis).

  • Even if the affected portion of the bone is at the joint, the site designation must be the bone itself, not the joint.

Acute Traumatic vs. Chronic/Recurrent Conditions

• Current Acute Injuries: These are not coded in Chapter 13. Current, acute traumatic injuries are classified under Chapter 19.

• Chapter 13 Inclusions: This chapter is used for musculoskeletal conditions resulting from a healed injury, as well as chronic or recurrent conditions of the bone, joint, or muscle.

• Coding Guidance: Generally, chronic or recurrent conditions should be assigned a code from Chapter 13. If the documentation is unclear whether a condition is acute traumatic or chronic/recurrent, the provider must be queried.

Back Disorders (Dorsopathies) and Intervertebral Disc Disorders

• Pain Specifications:

  • Low back pain/Lumbago: Coded as $M54.50$ when no further qualification is provided.

  • Dorsalgia (Unspecified back pain): Coded as $M54.9$.

  • Psychogenic back pain: This requires dual coding: $M54.9$ (Dorsalgia) and $F45.41$ (Pain disorder exclusively related to psychological factors).

• Intervertebral Disc Categories:

  • Cervical disc disorders: Category $M50$.

  • Thoracic, thoracolumbar, and lumbosacral disc disorders: Category $M51$.

• Diagnostic Distinctions:

  • Degeneration vs. Displacement: Coder must distinguish between disc degeneration and disc displacement (herniation); they are not the same and require different codes.

  • Myelopathy: This is a functional disorder or pathological change in the spinal cord often caused by compression. Codes for spondylosis and herniated discs must differentiate between conditions with or without myelopathy.

  • Herniated Discs without Myelopathy: These codes include conditions with paresthesia but exclude those with paralysis.

• Site Specificity Examples:

  • Codes in subcategories $M50.0-$ and $M51.0-$ use a $5^{th}$ or $6^{th}$ character to indicate the site (e.g., cervical, thoracic, lumbar, lumbosacral, thoracolumbar).

  • $M50.222$: Herniated intervertebral disc, cervical region, $C5-C6$, without myelopathy.

  • $M51.06$: Herniated intervertebral disc, lumbar region, with myelopathy.

  • $M51.24$: Herniated intervertebral disc, thoracic, without myelopathy.

• Radiculopathy: Subcategories $M50.1$ and $M51.1$ identify radiculopathy. Pain associated with a herniated disc is included in the disc disorder code; no additional code for pain is assigned.

• Spinal Stenosis:

  • Spinal stenosis (compression of the spinal canal) is assigned to subcategory $M48.0$.

  • Stenosis with Radiculopathy (Cervical): Requires two codes: $M48.02$ and $M54.12$.

  • Myelopathy in Spinal Stenosis: Use code $G99.2$ (Myelopathy in diseases classified elsewhere).

  • Example (Lumbar Stenosis with Radiculopathy and Myelopathy): Assign $M48.061$ (Lumbar stenosis without neurogenic claudication), $M54.16$ (Lumbar radiculopathy), and $G99.2$.

Clinical Variations and Coding of Arthritis

• Overview: Arthritis involves joint, muscle, and connective tissue symptoms including inflammation, pain, stiffness, and mobility issues. It can be independent or a manifestation of another condition (requiring dual coding).

• Osteoarthritis (OA):

  • Also known as polyarthritis, degenerative arthritis, or hypertrophic arthritis. In the US, "arthritis" defaults to OA in ICD-10-CM.

  • It is a degenerative joint disease involving chronic degeneration of articular cartilage and bone hypertrophy, typically in older populations.

  • Classification: Categories $M15-M19$. If the spine is involved, it is coded as Spondylosis ($M47$).

• Primary vs. Secondary Osteoarthritis:

  • Primary (Degenerative): Default type; caused by age-related "wear and tear" affecting the spine, knee, hip, hands, and feet.

  • Secondary: Caused by specific factors like injury, obesity, genetics, or inactivity; confined to one area.

• Specificity in OA Coding:

  • Always seek the specific site and laterality. Example: $M17.12$ (Unilateral primary OA, left knee) is preferred over $M17.9$ (unspecified).

  • $M19.90$: Unspecified osteoarthritis, unspecified site.

  • $M15.9$: Polyosteoarthritis (involving multiple sites).

• Other Types of Arthritis:

  • Rheumatoid Arthritis: Categories $M05-M06$; an autoimmune disease affecting the whole body.

  • Pyogenic Arthritis: Category $M00.-$; infection-related. Coders must identify the causative organism (e.g., $B95.61-B95.8$ for Staphylococcus, $B95.0-B95.2$ for Streptococcus, or $B96$ for others) and the involved joints.

  • Direct Infections: Category $M01$ is for joint infections in parasitic/infectious diseases (e.g., leprosy, mycoses). The underlying disease is coded first.

• Gouty Arthritis:

  • Recurrent arthritis caused by excessive uric acid deposits. Classified under Category $M10$ (Gout).

  • Subdivisions: Idiopathic ($M10.0-$), due to lead ($M10.1-$), drug-induced ($M10.2-$), renal impairment ($M10.3-$), or other secondary reasons ($M10.4-$).

  • Chronic Gout: Category $M1A$; requires a $7^{th}$ character to specify the presence or absence of tophus (crystallized uric acid under the skin).

Osteoporosis and Pathological Fractures

• Osteoporosis: A systemic condition making all bones thinner, weaker, and more prone to pathological fractures.

  • Category M81: Osteoporosis without current pathological fracture. Note: As it is systemic, "site" is not a component of these codes.

  • Category M80: Osteoporosis with current pathological fracture. These codes identify both the osteoporosis and the specific fracture site.

• Pathological Fractures:

  • Occur in bones weakened by disease (osteoporosis, metastatic tumor, Paget's disease, etc.).

  • These are usually spontaneous but can occur with minor trauma that would not break a healthy bone.

  • Spontaneous Fractures: Any fracture described as "spontaneous" must be coded as pathological.

  • Compression Fractures: If trauma is minor (e.g., an elderly person's slight stumble), it is usually pathological. If trauma is major (e.g., fall from a diving board), it is traumatic. Query if documentation is ambiguous.

• Classification of Pathological Fractures by Cause:

  • $M80$: Osteoporosis with current pathological fracture.

  • $M84.4-$: Pathological fracture, NEC.

  • $M84.5-$: Pathological fracture in neoplastic disease (code neoplasm underlying cause also).

  • $M84.6-$: Pathological fracture in other disease (code underlying condition also).

  • $M84.7$: Nontraumatic fracture, NEC.

Seventh Character Designations for Fractures

Pathological fracture coding requires a mandatory $7^{th}$ character to indicate the stage of treatment:

• A (Initial Encounter): Used when the patient is receiving active treatment (surgery, ER encounter, evaluation by a physician). This is based on whether treatment is active, not whether it is the provider's first time seeing the patient.

• D (Subsequent Encounter): Used for encounters after active treatment is complete and the patient is in the routine healing phase.

• G (Subsequent Encounter, Delayed Healing): For problems associated with healing.

• K (Subsequent Encounter, Nonunion): For problems where the bone fails to heal.

• P (Subsequent Encounter, Malunion): For problems where the bone heals in an abnormal position.

• S (Sequela): For encounters treating residuals after the acute phase has ended.

Sequencing and Constraints in Fracture Coding

• Principal Diagnosis: A pathological fracture is the principal diagnosis only if the patient is admitted solely for its treatment.

• Underlying Conditions: Usually, the underlying condition (e.g., the disease causing the weakness) is sequenced first.

• Neoplasm Sequencing Example:

  • If focus is the fracture: Subcategory $M84.5$ first, then the neoplasm code.

  • If focus is the neoplasm: Neoplasm code first, then subcategory $M84.5$.

• Case Study Example: Subsequent encounter for healing pathological hip fracture due to metastatic carcinoma where the patient had ovarian cancer $5$ years ago: assign $M84.559D$, $C79.51$, and $Z85.43$.

• Exclusion Rule: A traumatic fracture and a pathological fracture of the same bone must never be assigned together. Coder must choose one.

Stress and Periprosthetic Fractures

• Stress Fractures (Subcategory M84.3):

  • Caused by repetitive force before tissues can absorb it (not physiological like cancer).

  • Often invisible on X-rays for days or weeks.

  • Includes fatigue fractures, march fractures, and stress reaction fractures.

  • Requires an External Cause of Morbidity code (e.g., $Y93.01$ for walking/marching/hiking).

• Periprosthetic Fractures:

  • Fractures occurring around a prosthesis (e.g., hip, knee, shoulder). These are not complications of the prosthesis itself but result from trauma or pathology.

  • Codes by Region:

    • Hip: $M97.01-$, $M97.02-$

    • Knee: $M97.11-$, $M97.12-$

    • Ankle: $M97.21-$, $M97.22-$

    • Shoulder: $M97.31-$, $M97.32-$

    • Elbow: $M97.41-$, $M97.42-$

    • Other joint: $M97.8-$; Unspecified joint: $M97.9-$

Soft Tissue Disorders: Necrotizing Fasciitis

• Description: Necrotizing fasciitis ($M72.6$) is a fulminating infection starting with extensive cellulitis that spreads into the fascia, causing thrombosis and gangrene.

• Etiology: Frequently caused by Group A Streptococcus, though any bacteria can be responsible.

• Coding: Assign $M72.6$ plus an additional code for the specific organism if known.