Table 2.

Summary of results on the statistic evaluation of the correlation between clinical factors and surgical outcome

Author and Year	Surgical approach	Age	Sex	Pre-operative symptom duration	Pre-operative neurological score	Post- or pre-operative radiographic assessment	OPLL type	Other factors	Statistic test
Koyanagi et al. [20]	A, P	No	NA	Yes	No/JOA	Compression rate—no, Transverse area—yes	NA	NA	Multiple regression analysis
Okada et al. [30]	A, P	NA	NA	Yes	NA	IMHSI, Transverse area—yes	NA	NA	Multiple regression analysis
Goto et al. [10]	A	NA	NA	NA	NA	Cervical alignment—yes	NA	Progression of ossification, progression of kyphotic deformity, incomplete decompression, long-range fusion —yes,	Mann-Whitney U test
Inoue et al. [12]	P	NA	NA	NA	NA	NA	NA	Progression of ossification, progression of kyphotic deformity, incomplete decompression, scar formation—yes	Mann-Whitney U test
Nakamura and Fujimura, 1998 [9, 26]	A, P	Yes	NA	Yes	No/JOA	IMHSI, ROM—yes, spinal canal expansion, changes of the curve indices—no	NA	Trauma history, progression of ossification—yes	Mann-Whitney U test, Multiple regression analysis
Kato et al. [16]	P	Yes	NA	No	Yes/JOA	Occupying rate, SAC—no	No	Trauma, OLF—yes, Surgical level, progression of kyphotic deformity, progression of ossification—no	Multivariate stepwise regression analysis
Onari et al. [32]	A	NA	NA	NA	NA	NA	NA	Progression of kyphotic deformity, progression of ossification—yes	NA
Seichi et al. [33]	P	NA	NA	NA	NA	Change in ROM—yes	NA	Progression of ossification—no, OLF, trauma—yes	NA
Matsuoka et al. [24]	A	No	NA	Yes	Yes/JOA	SAC, Occupying rate, ossification thickness—no, transverse area—yes	No	Involved interspaces—no, progression of ossification—yes	Spearman’s rank correlation, Kruskal-Wallis rank test.
Iwasaki et al. [13]	P	Yes	No	NA	Yes/JOA	Occupying ration, SAC, change of the cervical alignment—no	No	Progression of kyphotic deformity, post-operative spontaneous fusion, progression of ossification—no	Multivariate stepwise analysis
Ogawa et al. [27, 28]	P	Yes	NA	Yes	Yes	Cervical range of motion—yes, Change of cervical alignment, expansion of spinal canal—no	Yes	NA	Mann-Whitney U test,
Uchida et al. [36]	A, P	No	No	No	Yes/JOA	IMHSI—no, Compression rate, occupying ratio, expansion of spinal canal (≥40%), increase area of spinal cord (≥40%)—yes	Yes	Involved interspaces, progression of ossification, Progression of kyphotic deformity—no, SCEP—yes	Multivariate analysis multiple regression analysis
Choi et al. [6]	A	No	No	No	No/Nurick	SEA, occupying ratio (CT), Pavlov ratio, double-layer sign—no	No	DM—yes	Univariate analysis, Multiple logistic regression analysis
Chiba et al. [3]	P	NA	NA	NA	NA	Cervical alignment—yes, decrease of ROM—no	NA	Progression of ossification—no	Unpaired t test
Iwasaki et al. [15]	A	NA	NA	NA	NA	Occupying ratio (≥60%), change in cervical alignment—no	No	OLF, progression of ossification—yes	Mann-Whitney U test.
Iwasaki et al. [14]	P	Yes	NA	NA	Yes/JOA	Cervical alignment, occupying ratio (<60%), SAC—no, change in cervical alignment—yes	Yes	NA	Multiple logistic regression
Masaki et al. [21]	P	Yes	NA	Yes	No/JOA	Occupation ratio (CT)—no, Change in ROM, change in C2–C7 angle—yes	NA	NA	Mann-Whitney U test

no no statistically significant correlation (either positive or negative) exists between factor and outcome, yes statistically significant correlation (either positive or negative) exists between factor and outcome, Transverse area transverse area of spinal cord (MRI), Compression rate ratio of flatten of spinal cord caused by compression (MRI), IMHSI intramedullary high signal intensity in T2-weighted, SAC space available for spinal cord in lateral view (X-ray) at the maximum compression level, Occupying rate thickness of ossification/AP at the maximum compression level, Occupying rate (CT) area of ossification/transverse area of spinal canal at the maximum compression level, Low T1 low intensity signal in T1-weighted (MRI), SEA snake-eye appearance, ROM range of motion, SCEP spinal cord evoked potentials, DM diabetes, OLF ossification of ligament flavum