ICD-10-CM 2027
ICD-10 code for synostosis
From the official ICD-10-CM alphabetic index entry “Synostosis (congenital)”. Page updated September 29, 2026.
About coding synostosis
The ICD-10-CM code for synostosis is Q78.8 (Other specified osteochondrodysplasias). The official index lists 2 more specific codes, so check the documentation for details such as type, cause, site or severity before settling on Q78.8.
Within Q78, choose Q78.8 (Other specified osteochondrodysplasias) only when documentation doesn't support a more specific option: Osteogenesis imperfecta (Q78.0), Polyostotic fibrous dysplasia (Q78.1), Osteopetrosis (Q78.2), progressive diaphyseal dysplasia (Q78.3), Enchondromatosis (Q78.4), Metaphyseal dysplasia (Q78.5), multiple congenital exostoses (Q78.6) and osteochondrodysplasia, unspecified (Q78.9).
It is not a CC or MCC, so as a secondary diagnosis it doesn't change the MS-DRG severity level.
In MS-DRG v44.0, it is part of the grouping logic for DRG 564 (Other Musculoskeletal System and Connective Tissue Diagnoses with MCC, relative weight 1.4908), DRG 565 (Other Musculoskeletal System and Connective Tissue Diagnoses with CC, relative weight 0.9783) and DRG 566 (Other Musculoskeletal System and Connective Tissue Diagnoses without CC/MCC, relative weight 0.7378), in MDC 08 (Diseases and Disorders of the Musculoskeletal System and Connective Tissue), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.
More specific synostosis codes (official index)
Common questions about synostosis ICD-10 codes
What is the ICD-10 code for synostosis?
- Q78.8 — Other specified osteochondrodysplasias. More specific codes apply when the documentation supports them: Q74.2, Q74.0.
What is the ICD-10 code for astragalo-scaphoid synostosis?
- Q74.2 — Other congenital malformations of lower limb(s), including pelvic girdle.
What is the ICD-10 code for radioulnar synostosis?
- Q74.0 — Other congenital malformations of upper limb(s), including shoulder girdle.
Is Q78.8 billable?
- Yes. Q78.8 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).
Can Q78.8 be used as a principal diagnosis?
- Yes. The Medicare Code Editor doesn't restrict Q78.8 as a principal diagnosis.
Is Q78.8 a CC or MCC?
- No. Q78.8 is neither a CC nor an MCC under MS-DRG v44.0.