S13.121A
Dislocation of C1/C2 cervical vertebrae, initial encounter
S13.121A (also written S13121A) is a billable ICD-10-CM diagnosis code for dislocation of C1/C2 cervical vertebrae, initial encounter. It is part of the FY2027 code set, effective October 1, 2026. It is one of the 75 codes in category S13 (Dislocation and sprain of joints and ligaments at neck level). The Alphabetic Index lists it under 3 entries. Its 7th character, A, means initial encounter. Its coding notes include a Use additional code instruction, a Code also instruction and 11 Excludes2 entries.
7th character
Category S13 requires a 7th character.
Guidelines tips
From the FY2027 Official Guidelines. ICD10Ref adds no rules of its own.
The 7th character depends on whether the patient is in active treatment, not on who sees the patient. An aftercare Z code is not used for injury aftercare.
Each injury gets its own code unless a combination code exists, with the most serious injury first. Superficial injuries at the same site as a more severe injury are not coded. Injuries during medical care are coded as complications.
Use additional code
Add the listed code as a secondary code, if known.
Also from chapter/block level (1 note)
- code to identify any retained foreign body, if applicable (Z18.-)
Code also
Two codes may be needed to describe the condition. This note gives no sequencing order.
- any associated open wound
Excludes2
Not included here. Both codes may be reported if the patient has both conditions.
- strain of muscle or tendon at neck level (S16.1)
Also from chapter/block level (9 notes)
- birth trauma (P10-P15)
- obstetric trauma (O70-O71)
Care settings
How CMS programs treat this code. These are CMS claim edits and list entries, not coverage or eligibility rules.
Home Health Primary
Accepted as the primary (principal) diagnosis on a home health claim. The Home Health Grouper assigns it to the Musculoskeletal Rehabilitation clinical group.
Source: CMS Home Health Grouper Software v07.2.26, effective October 1, 2026IRF 60% Rule
On the CMS list of diagnoses used to determine presumptive compliance with the IRF 60 percent rule.
Source: CMS IRF Presumptive Compliance-3, FY 2027 IRF PPS final rule files, effective for discharges on or after October 1, 2026Hospital CC
Counts as a complication or comorbidity (CC) when reported as a secondary diagnosis on an inpatient claim. It does not count when the principal diagnosis is one of the 238 codes in its exclusion list. Some MS-DRGs also ignore a CC or MCC that defines the DRG.
Source: CMS MS-DRG Definitions Manual v44, effective October 1, 2026, Appendix CHAC
On the CMS list of hospital-acquired conditions (HAC 05: Falls and Trauma). When the claim reports it with a present-on-admission (POA) indicator of N (not present) or U (documentation insufficient), CMS does not pay the CC or MCC DRG for it. A POA of Y (present) or W (clinically undetermined) is paid normally.
Source: CMS MS-DRG Definitions Manual v44, effective October 1, 2026, Appendix I; CMS Hospital-Acquired Conditions (HAC) Coding page, POA indicator payment tableIncludes
Further defines or gives examples of the content of the category.
- avulsion of joint or ligament at neck level
- laceration of cartilage, joint or ligament at neck level
- sprain of cartilage, joint or ligament at neck level
- traumatic hemarthrosis of joint or ligament at neck level
- traumatic rupture of joint or ligament at neck level
- traumatic subluxation of joint or ligament at neck level
- traumatic tear of joint or ligament at neck level
Also from chapter/block level (3 notes)
- injuries of nape
- injuries of supraclavicular region
- injuries of throat
Notes
Also from chapter/block level (2 notes)
- Use secondary code(s) from Chapter 20, External causes of morbidity, to indicate cause of injury. Codes within the T section that include the external cause do not require an additional external cause code
- The chapter uses the S-section for coding different types of injuries related to single body regions and the T-section to cover injuries to unspecified body regions as well as poisoning and certain other consequences of external causes.
Alphabetic Index terms
Terms the official Alphabetic Index lists for S13.121. This code adds the 7th character. 3 entries