Flatback syndrome localized to the lumbosacral region — loss of the normal lumbar lordosis at the L5-S1 junction, producing a straightened or reversed sagittal profile.

Documentation tips

What should appear in the chart to support M40.37.

Source · Editorial brief grounded in 5 cited references ↓

• Provider must name the lumbosacral region explicitly — 'L5-S1,' 'lumbosacral junction,' or 'lumbosacral segment' in the note to justify M40.37 over M40.36 or M40.30.
• Document the etiology: prior spinal fusion, degenerative disc collapse, or idiopathic. If the cause is a prior surgical procedure, evaluate M96.- before assigning M40.37.
• Record sagittal alignment measurements from standing lateral radiographs — lumbar lordosis angle, pelvic incidence, and sagittal vertical axis (SVA) — to support medical necessity for surgical correction.
• Note functional impact: forward trunk lean, compensatory knee flexion, hip extension deficit, and pain with prolonged standing or ambulation strengthen the clinical picture for coverage review.
• If a prior fusion extends to the lumbosacral region and flatback is the presenting complaint, document the surgical history and the number of levels fused to support MS-DRG grouping under 456–458.

Related CPT procedures

Procedure codes commonly billed with M40.37. 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 M40.37 and adjacent codes.

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

• Assigning M40.37 when the deformity is postprocedural — if flatback is a direct complication of prior spine surgery, M96.3 (postlaminectomy kyphosis) or the appropriate M96.- subcategory applies instead.
• Defaulting to M40.30 (unspecified site) when the lumbosacral region is clearly documented — specificity is always required if supported by the record.
• Confusing M40.37 with M40.36 (lumbar region) when documentation says 'lumbar' but the imaging and operative reports indicate L5-S1 involvement; clarify with the provider before coding.
• Missing the Excludes1 note: do not code M40.37 alongside Q76.4 (congenital kyphosis/lordosis) or M41.- (kyphoscoliosis) for the same region in the same encounter.
• Omitting a 'Code first underlying disease' secondary code when the flatback syndrome is a manifestation of a documented systemic or structural condition.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

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