Loss of normal lumbar lordosis localized to the thoracolumbar junction (approximately T10–L2), producing a straightened or reversed sagittal contour at that spinal segment.

Documentation tips

What should appear in the chart to support M40.35.

Source · Editorial brief grounded in 7 cited references ↓

• Specify the spinal region by name — 'thoracolumbar' — in the assessment or diagnosis line; 'lower thoracic/upper lumbar flatback' is ambiguous and may prompt M40.30.
• Document the underlying etiology (e.g., prior instrumented fusion, degenerative disc disease, ankylosing spondylitis) so sequencing rules for 'Code first underlying disease' can be applied correctly.
• Include imaging findings that confirm loss of lordosis at the thoracolumbar junction — standing lateral radiograph with Cobb angle or sagittal balance measurements (SVA, pelvic incidence minus lumbar lordosis mismatch) are optimal.
• If the deformity is post-surgical, clarify in the note whether the flat sagittal contour is a direct procedural consequence; this determines whether M96.- should lead instead of M40.35.
• Record functional impact — inability to stand erect, forward trunk lean, compensatory knee flexion — to support medical necessity for surgical correction or neuromonitoring.

Related CPT procedures

Procedure codes commonly billed with M40.35. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

Common coding pitfalls

The recurring mistakes coders make with M40.35 and adjacent codes.

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

• Billing M40.3 (non-billable parent) instead of drilling down to M40.35, M40.36, or M40.37 — payers will reject M40.3 for reimbursement.
• Using M40.35 when the documented region is lumbar (M40.36) or lumbosacral (M40.37); 'thoracolumbar' refers specifically to the T10–L2 junction, not the entire lower spine.
• Failing to apply 'Code first underlying disease' sequencing when flatback syndrome is secondary to a known condition, resulting in incorrect primary diagnosis assignment.
• Applying M40.35 alongside M96.- codes for the same deformity — if postprocedural kyphosis/lordosis (M96.-) fully describes the condition, the Excludes1 note at M40 prohibits simultaneous use.
• Confusing flatback syndrome with kyphosis; flatback is a loss of lordotic curve, not an increase in kyphotic curve — do not substitute M40.0x or M40.2x codes.

Clinical context

Source · Editorial summary grounded in 7 cited references ↓

Sibling codes

Other billable codes under M40.3 (laterality / anatomic variants).