Table 1. Characteristics of included studies.

Author (y) Study design (LoE)	Demographics	Diagnosis	Fusion procedure	Definition of fusion outcomea	Follow-up	Funding/conflicts of interest
Anderson (2009) RCT (II)	DC stimulation N = 53 Mean age: 69.3 y (range, 59–80) Male: 37.7% Control (placebo) N = 42 Mean age: 71.5 y (range, 59–84) Male: 31.0%	• Spinal stenosis • Stenosis and degenerative spondylolisthesis • DDD • Stenosis and degenerative scoliosis	• Posterolateral spinal fusion • Allograft • 1–4 levels	• Fusion = continuous bony bridge either between the transverse process or at the lateral side of the facet joints on at least one side or a bilateral fusion of the facet joints (and fusion had to be achieved on all intended levels) • Doubtful fusion = unilateral facet joint fusion, questionable bilateral facet fusion, or possible presence of a cleft in the bony bridge • Nonunion = clearly definable cleft in the bony bridge, question fusion in one facet joint and none in the contralateral, or with resorption of most of the fusion mass	24 mo DC: 90.6% (48/53) Placebo: 85.7% (36/42)	Corporate/industry and federal funds were received in support of this work; no benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this article
Goodwin (1999) RCT (II)	CC stimulation N = 85 Mean age: 45 y (range, 21–76) Male: 56.5% Control (placebo) N = 94 Mean age: 40 y (range, 22–73) Male: 52.1%	• DDD • Herniated disc • Spondylolisthesis • Degenerative arthritis	• Primary PLIF, ALIF, posterolateral fusion • Autograft, allograft, or autograft and allograft • Any type of internal fixation except interbody fusion cages • 1–2 levels	• Posterolateral:  ○ Fusion = presence of mature appearing, uninterrupted bony masses bilaterally at the fusion levels, ideally on both anteroposterior and lateral radiographs. If orthopedic hardware present, there could not be any lucency or motion around the screws  ○ Incomplete fusion = immature-appearing bone mass on either side, lack of a bone mass on one side, or lack on continuity in the bone mass on either side. Any evidence of motion or lucency around internal fixation hardware was also a sign of incomplete fusion  ○ Nonunion (absence of fusion mass) = complete resorption of the graft • Interbody:  ○ Success = > 75% assimilation of graft and vertebrae or 50–75% assimilation of graft and vertebrae  - Failure = 25–50% assimilation of graft and vertebrae or < 25% assimilation of graft and vertebrae	12 mo Overall: 53.1% (179/337)	Biolectron assisted in study design and analysis support
Jenis (2000) RCT (II)	PEMF stimulation  N = 22  Mean age: 53.0 ± 11.1 y  Male: 50.0% DC stimulation  N = 17  Mean age: 51.0 ± 15.1 y  Male: 41.2% Control (no placebo)  N = 22  Mean age: 47.1 ± 13.5 y  Male: 63.6%	• NR	• Primary or revision lumbar or lumbosacral posterolateral fusion • Iliac crest autograft • Pedicle screw-rod instrumentation • 1, 2, or > 2 levels	• Solid fusion = trabecular bridging bone • Possible pseudoarthrosis = lucencies within the fusion mass • Obvious pseudoarthrosis = clefts within the fusion mass and discontinuity between the transverse processes	12 mo (% NR)	NR
Kane (1988) RCT (II)	DC stimulation  N = 31  Mean age: NR  Male: NR Control (no placebo)  N = 28  Mean age: NR  Male: NR	Difficult spinal fusions: • 1 + previous failed spinal fusion(s) • Grade II or worse spondylolisthesis • Extensive bone grafting necessary for a multiple level fusion • Other high risk factors for failure of fusion, including gross obesity	• Posterolateral fusion	• NR	18 mo Overall: 93.7% (59/63)	NR
Linovitz (2002) RCT (II)	PEMF stimulation N = 125 Mean age: 56.8 ± 15.5 y Male: 40.8% Control (placebo) N = 118 Mean age: 56.6 ± 15.0 y Male: 36.4%	• DDD • Instability • Spondylolisthesis • Spinal stenosis • Miscellaneous	• Primary posterolateral fusion • Autograft ± allograft • No instrumentation • 1–2 levels	• Grades:  ○ Three, solid fusion = extensive continuity (≥ 75–100%) of the fusion mass without motion  ○ Two, moderate fusion = continuity (≥ 50 to < 75%) of the fusion mass without motion  ○ One, minimal fusion = a narrow band of continuity (≥ 25 to < 50%) in the fusion mass with motion  ○ Zero, no fusion = discontinuity (0 to < 25%) of the fusion mass with motion • Definitions (when two levels involved, the lowest grade at either level used)  ○ Fusion = grades 2 and 3 (continuity of ≥ 50% of the fusion mass without motion)  ○ Nonunion = grades 0 and 1 (continuity of 0 to < 50% of the fusion mass with motion)	9 mo PEMF: 83.2% (104/125) Placebo: 82.2% (97/118)	Corporate/industry funds were received to support this work. One or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article
Mooney (1990) RCT (II)	PEMF stimulation N = 107 Mean age: 37.9 y Male: 55.1% Control (placebo) N = 99 Mean age: 37.6 y Male: 52.5%	• Internal disc disruption • HNP • DDD • Spondylolisthesis • Stenosis • Failed fusion • Other	• Primary ALIF or PLIF • Autograft, allograft, or autograft and allograft • With or without internal fixation • 1–2 levels	• Fusion = > 50% assimilated (in two-segment fusion, both levels had be graded as solidly fused) • Nonunion = NR	12 mo PEMF: 91.6% (98/107) Placebo: 98.0% (97/99)	NR

Abbreviations: ALIF, anterior lumbar interbody fusion; CC, capacitive coupling; DC, direct current; DDD, degenerative disc disease; HNP, herniated nucleus pulpous; LoE, level of evidence; NR, not reported; PEMF, pulsed electromagnetic field; PLIF, posterior lumbar interbody fusion; RCT, randomized controlled trial.

^aFusion was assessed via radiograph in four studies (Goodwin 1999, Jenis 2000, Kane 1988, and Mooney 1990) and via computed tomography in two studies (Andersen 2009 and Linovitz 2002).