Table 1.

Design and sample description of 12 studies assessing preoperative and/or postoperative fractional curve of adult degenerative scoliosis patients

Study	Type	Method of FC measurement (Cobb angle)	Surgery type	Inclusion criteria	Exclusion criteria	Study sample size
Zhang et al. [16] 2021	Single-institution retrospective	Angle between superior endplate of L4 and the line formed by the pedicles of S1	PSIF of > 5 segments ending at L5–S1 with facetectomy and osteotomy (decompression and TLIF were performed if anterior support was needed or to relieve spinal stenosis)	1. Primary spinal deformity correction	1. Fusion levels < 5; 2. history of hip or knee arthroplasty; 3. absolute discrepancy of leg length > 20 mm	N = 101
				2. Instrumented fusion via posterior-only approach
Amara et al. [23] 2020	Single-institution retrospective	The curve below the major curve of thoracolumbar or lumbar scoliosis. Inclusion criterion: Cobb angle between L3–S1 > 10°	PSIF with 1–3 interbody fusions (ALIF/LLIF/TLIF) at the FC	1. FC > 10°	NR	N = 78 (1 level = 19; 2 levels = 36; 3 levels = 23)
				2. Low back or extremity pain ipsilateral to FC concavity
				3. Treatment of FC with interbody fusion
				4. Preop and postop long-standing radiographs
				5. > 1-year follow-up
Amara et al. [20] 2019	Single-institution retrospective	The curve below the major curve of a lumbar or thoracolumbar scoliosis measured via Cobb angle; only Cobb angle > 10° considered FC	PSIF of L4–S1 (FC) versus T10-pelvis (LT) versus T2–4 to pelvis (UT)	1. FC from L4–S1 > 10°	1. Previous lumbar fusion surgery	N = 99 (FC = 27; LT = 46, UT = 26)
				2. Radiculopathy ipsilateral to the concavity of FC
				3. Pre and postop radiography studies
				4. > 1-year follow-up
Chou et al. [22] 2018	Multicenter retrospective study	Coronal Cobb angle of fractional curve	PSIF vs. cMIS	1. > 18 years of age	1. Hybrid open posterior surgery with interbody fusion	N = 118 (open = 79; cMIS = 39)
				2. Minimum of 3 levels fused
				3. Minimum 2-year follow-up
				4. FC > 10°
				5. At least one of the following: SVA ≥ 5 cm, PT ≥ 20°, lumbar Cobb angle ≥ 20°, or a PI-LL ≥ 10°
Brown et al. [17] 2004	Single-institution retrospective	Angle between the line connecting the superior iliac alae and the line formed by the pedicles of l4	PSIF to L5	1. Fusion extending above T12	1. Need for decompression at L5–S1	N = 16
					2. Pre-exisitng L5–S1 deformity (not including isolated degeneration at L5–S1)
Yagi et al. [21] 2014	Single-institution retrospective	Coronal Cobb method	Combined single-rod anterior fusion and short PSIF to sacrum (hybrid) versus long PSIF with anterior release (control)	1. Thoracic and thoracolumbar/lumbar curves (> 80°)	1. Osteoporosis	N = 66 (33 per group)
				2. Nonprogressive thoracic deformity (> 30° flexibility)	2. Revision surgery
				3. Fractional curve (with segmental instability, stenosis or facet arthrosis) or degenerative disc disease
Manwaring et al. [24] 2014	Single-institution retrospective	NR	Staged cMIS with versus without L5–S1 TLIF	1. Treatment of ADS with at least 2-level MIS LLIF procedure	1. Hybrid construct involving posterior osteotomies	N = 15 (TLIF = 11; control = NR)
				2. Delayed second stage procedure with MIS PLIF
Pugely et al. [18] 2017	Single-institution retrospective	Coronal Cobb method	No surgery performed	1. Coronal Cobb angle > 30°	1. Central stenosis	N = 48 (group B = 14°; group F = 16°; group S = 18°)
				2. > 40 years of age	2. Lateral recess stenosis
				3. Standing scoliosis radiographs	3. Disk herniation
				4. Preop CT spine
Buell et al. [25] 2021	Multicenter retrospective study	Coronal L4–S1 Cobb angle	L4–S1 TLIF vs. ALIF	Index operation that involved TLIF or ALIF at L4–5 and/or L5–S1. Minimum 2-year postoperative follow-up	Any patient with active infection, malignancy, diagnosis of scoliosis other than adult degenerative	N = 106 (TLIF = 47, ALIF = 59)
Geddes et al. [26] 2021	Single-institution retrospective	Coronal Cobb method	ALIF+PSF+S2AI screws versus PSF+S2AI screws for thoracolumbar fusion	1. Posterior lumbar fusion to the pelvis using S2AI screws	1. Patients who had posterior 3-column osteotomies	N = 59 (ALIF+PSF = 31, PSF alone = 28)
				2. Presence of fractional curve	2. Those lacking adequate pre- and/or postoperative imaging
Hofler et al. [43] 2022	Single-institution retrospective	Cobb angle method for lumbar fractional curve. The magnitude of the major lumbar coronal curve and fractional lumbar coronal curve caudal to it was measured on preoperative and follow-up anteroposterior imaging	T3-ilum fusion +/- kickstand placement	1. Deformity correction with fusion from upper thoracic spine to pelvis	NR	N = 15 (kickstand = 7, nonkickstand = 8)
				2. Associated coronal deformity
				3. Intraoperative APLCRs performed
Zuckerman et al. [27] 2023	Single-institution retrospective	Cobb angle between the sacrum and most tilted lower lumbar vertebra (either L3/4/5)	Instrumentation to pelvis/fusion to sacrum and TLIF	1. ≥ 6-level fusion	NR	N = 243
				2. At least 1 of the following radiographic criteria (Cobb angle > 30˚, SVA > 5 cm, CVA > 3 cm, PT > 25˚, or TK > 60˚)

PSIF, open posterior spinal instrument fusion; TLIF, transforaminal lumbar interbody fusion; ALIF, anterior lumbar interbody fusion; LLIF, lateral lumbar interbody fusion; FC, fractional curve; LT, lower thoracic; UT, upper thoracic; cMIS, circumferential minimally invasive surgery; SVA, sagittal vertical axis; PT, pelvic tilt; PI-LL, pelvic incidence-lumbar lordosis; NR, not reported; MIS, minimally invasive surgery; CT, computed tomography; PSF, posterior spinal fusion; S2AI, S2 alar iliac screw; APLCR, anteroposterior long cassette radiograph; CVA, coronal vertical axis; TK, thoracic kyphosis.