Eur Spine J (2015) 24:990–1004 DOI 10.1007/s00586-015-3853-x

REVIEW

Methods to determine pedicle screw placement accuracy in spine surgery: a systematic review Ahmed A. Aoude • Maryse Fortin • Rainer Figueiredo Peter Jarzem • Jean Ouellet • Michael H. Weber

•

Received: 3 August 2014 / Revised: 6 February 2015 / Accepted: 27 February 2015 / Published online: 7 March 2015  Springer-Verlag Berlin Heidelberg 2015

Abstract Study design Systematic review. Objectives The aims of this systematic review were: (1) to determine the most commonly used methods for assessing pedicle screw placement accuracy, and (2) assess the difference in pedicle screw placement accuracy between navigation and free-hand techniques according to the classification method. Background data Pedicle screw fixation and spine surgery have almost become synonymous. However, there is currently no gold standard method to assess pedicle screw placement accuracy. We reviewed the literature to determine current techniques used by spine surgeons for the assessment of pedicle screw accuracy. Methods We systematically reviewed the medical literature (OVID Medline, Embase, PubMed) to identify all articles published between 2010 and 2013 that have assessed pedicle screw placement accuracy in humans. Two independent reviewers with a third independent mediator performed study screening, selection and data extraction using a blinded and objective protocol. Results A total of 68 relevant articles were included in this systematic review, for a total of 3442 patients, 60 cadavers and 43,305 pedicle screws. The most widely used method (37 articles) was based on 2 mm breach increments measured on computer tomography images. The second most widely used method consisted of an ‘‘in’’ or ‘‘out’’ classification system (16 articles). The remaining 15 articles used variable classification systems. Our result

A. A. Aoude (&)
M. Fortin
R. Figueiredo
P. Jarzem
J. Ouellet
M. H. Weber Division of Orthopaedic Surgery, McGill University, 1529 Cedar Avenue, Montreal, QC H3G 1A6, Canada e-mail: [email protected]

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suggests that an average of 91.4 % of pedicle screws placed with free-hand or fluoroscopy technique where within the safe zone (\2 mm breach) in comparison to an average of 97.3 % of pedicle screws using navigation (p \ 0.001) for the 2 mm increment method. Similarly, the in or out classification also showed statistically significant difference between free-hand and navigated techniques (p \ 0.001). Conclusion The grading system based on 2 mm increments seems to be the most widely accepted method for determining pedicle screw placement accuracy. All grading systems were based on imaging alone without taking into account the direction of the breach or patient’s symptoms. Keywords Pedicle screw placement
Accuracy
Computer-assisted surgery
Free-hand technique

Introduction Spinal fusion and pedicle screw fixation have become very common procedures for spine surgeons with proven benefits. However, some drawbacks do exist such as pseudoarthritis, adjacent segmental degeneration, screw loosening and screw misplacement [1, 2]. Screw misplacement can cause a variety of problems to patients with minor to major consequences. Some of these consequences include neurological impairment such as new radicular pain, weakness or sensory loss, but can be as serious as paralysis. Although these risks are real, they usually remain low when surgeries are performed by trained and experienced spine surgeons [2]. Surgical techniques described by Roy-Camille [3, 4], Magerl [5] and Krag et al. [6] are some of the most widely used and help surgeons define pedicle screw entry points and trajectories in the spine. Although

Eur Spine J (2015) 24:990–1004

these three techniques differ in terms of targeted screw trajectory, they all were described to avoid screw misplacement and adequate fixation. Screw placement accuracy is of outmost importance and is critical for a successful surgery. A review by Yahiro [7] showed that in a total of 5756 patients who received pedicle screws, only 146 (2.5 %) had misplaced screws, 65 (1.1 %) had dural tears and 99 (1.7 %) had neurological injuries. A similar prevalence (2.3 %) of permanent nerve root injury due to pedicle screw fixation was also shown in a selective survey of 13 American Back Society surgeons in 617 cases [8]. In some cases, screw misplacement has also been reported to cause major vascular damage such as aortic perforation [9]. Improved instrumentation as well as novel imageguided and navigation techniques has been developed to improve pedicle screw placement. Surgeons typically acquire post-surgery images to assess and ensure accurate pedicle screw placement. However, there is currently no gold standard method for the assessment of pedicle screw placement accuracy. Therefore, the purpose of this study was to systematically review the literature to (1) determine and evaluate the methods currently used for the assessment of pedicle screw placement accuracy and (2) investigate whether there is a difference in screw placement accuracy between navigated and free-hand techniques according to the classification method.

Methods Data sources and searches To identify relevant articles, an electronic database literature search was performed using MEDLINE, EMBASE, PUBMED and Web of science. The following keywords and their respective combinations were used: ‘‘pedicle screw’’, ‘‘placement’’, ‘‘position’’ and ‘‘accuracy’’. To include only the most recent and up to date publications, we limited our search and included only articles that were published between January 2010 and December 2013. Only articles published in English were considered for inclusion. Articles were also sorted from most cited to least cited and then analyzed. Reviewers AA and RF independently screened the titles and abstracts of all retrieved articles for eligibility. In case of disagreement, the reviewers discussed the inclusion criteria until a common consensus was reached. The two independent reviewers obtained and reviewed the full articles using a standardized data extraction form. The bibliographies of eligible studies and relevant systematic reviews were also scrutinized to identify possible articles missed by the electronic search.

991

Study selection Only studies meeting all of the following criteria were considered for possible inclusion: (1) All types of studies (case series, case control, randomized control trials) with the exception of case reports, (2) studies assessing thoracic, lumbar or sacral pedicle screw placement, (3) human studies, (14) studies using conventional methods of vertebral fixation (e.g., pedicle screws and Harington rods), (5) studies with a clear surgical intervention technique (e.g., fluoroscopy, free-hand, fluorobased navigation, CT-based navigation), (6) studies using an objective classification method to assess post-operative pedicle screw accuracy and (7) studies using anatomical location of pedicle screw placement and reporting on complications related to pedicle screw placement. Articles which included cervical pedicle screw placement, biomechanical studies, animal studies or engineering models were excluded. Data extraction Two independent reviewers extracted the data from the selected studies. Final data extraction included: type of study, number of subjects, number of pedicle screws placed in the study, detailed method of screw placement assessment, imaging used and vertebral levels instrumented. Statistical analysis Papers were grouped based on method of determining pedicle screw accuracy, i.e., papers with same methods to determine accuracy were grouped together and analyzed as subgroups. The descriptive statistics for screw placement accuracy according to the classification method and technique (navigation vs free hand) included weighted mean (taking into account the number of screws placed in each study), the median, minimum and maximum, interquartile range and standard deviation. Two-sample t tests using the weighted mean and weighted standard deviation were performed to assess whether there was a difference in screw placement accuracy between navigated and free-hand techniques according to the classification method. Two-sample t tests were also used to assess difference in placement accuracy between the thoracic and lumbar region according to the surgical technique and classification method used. A p value of less than 0.05 was considered to be statistically significant. All statistical analyses were performed using Stata (version 12.0; StataCorp LP, College Station, TX, USA).

Results The initial electronic database search identified a total 316 potential articles. Of these, only 68 met all of our inclusion

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992

Eur Spine J (2015) 24:990–1004

criteria and were included in the final analysis (Fig. 1). Main reasons for exclusions were unclear accuracy measurement method, studies limited to animals and biomechanical studies based on models alone. In total, 19 prospective studies, 40 were retrospective studies, 6 cadaveric studies and 3 case series (of 6–11 patients each) were analyzed. Forty-one out of 68 articles used navigation as part of their operative technique. In total, the review allowed for the analysis of 43,305 pedicle screws placed in the thoracic and lumbar regions in a total of 3442 patients (excluding cadavers). Imaging was used in all 68 articles to assess pedicle screw accuracy. Three articles (4 %) used plain radiographs anterior–posterior and lateral views to assess screw placement [10–12]. However, two of these three articles further imaged patients with Computer Tomography (CT), if a screw was suspected to be breaching the pedicle [10, 12]. The majority of papers (94 % or 64 articles) used CT as the sole modality to determine pedicle screw position post-operatively. While one study (1 %) compared the assessment of pedicle screw accuracy using MRI images and CT scans. Our analysis showed that 37 articles (54 %) had similar methodology and represented the most widely excepted method for pedicle screw placement assessment. In these

316 articles identified from 2010-2013

301 articles included for pedicle screw placement

200 articles included: Human subjects and \ pedicle screws used in the lumbar or thoracic region

15 articles excluded: only cervical screws or biomechanical analysis only

103articles excluded: Case reports and systematic reviews Animal models Engineering models Non convention vertebral fixation No clear method of assessing pedicle screw accuracy in abstract

99 articles with pedicle screw placement accuracy mentioned

68 articles included for data extraction

31articles excluded: Results or methods missing clear indication of pedicle screw accuracy determination, articles include cervical instrumentation

Fig. 1 Graphical depiction of the systematic article selection process used

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37 articles pedicle screw accuracy was assessed using CT scans and pedicle breaches were classified by grades based on 2 mm increments. Slight differences in grading categories (or cutoff grade) were used by different authors as enumerated in Table 1. Most articles also considered screws with breaches up to 2 mm as the ‘‘safe/acceptable’’ zone, while those with breaches more than 2 mm as the ‘‘unsafe’’ zone [13–30]. Of this subgroup of articles, 15 articles had pedicle screws placed in the thoracic and lumbar region (10,418 pedicle screws), 10 articles had pedicle screws placed solely in the thoracic region (2234 pedicle screws) and 12 articles pedicle screws placed solely in the lumbar region (3414 pedicle screws). Only 7 articles (19 %) reported on the reliability of the grading system used. Three articles reported on the intra-rater reliability, which varied from slight agreement (k = 0.35) [26] to substantial agreement (k = 0.66–0.83) [31, 32]. Similarly, the inter-rater reliability varied from moderate agreement (k = 0.45) [26] to substantial agreement (k = 0.65–0.85, ICC = 0.62–0.69) [32–34]. In addition, our review revealed that an average of 91.4 % (weighted average 89.3 %) of pedicle screws placed with free-hand or fluoroscopy technique where within the safe zone in comparison to an average of 97.3 % (weighted average 96.6 %) of pedicle screws placed with navigation or robotassisted surgery (statistically significant difference, p \ 0.001). The second most widely used method (16 articles, 24 %) was an ‘‘in’’ or ‘‘out’’ classification system (Table 2); that is, pedicle screws were classified as being contained in the pedicle or breaching it. Different authors classified a screw to be within the pedicle with slight differences. For instance, certain authors [35–38] classified a screw as ‘‘in’’ if it was entirely within the pedicle while others [39] allowed up to 25 % of the screw diameter to be outside the pedicle while still classifying the screw as ‘‘in’’. Of this second subgroup, 6 articles pedicle screws placed in the thoracic and lumbar region (10,515 pedicle screws), 7 articles had pedicle screws placed solely in the thoracic region (4968 pedicle screws) and 3 articles pedicle screws placed solely in the lumbar region (3149 pedicle screws). The ‘‘in’’ or ‘‘out’’ classification also showed a statistically significant difference of pedicle screw placement between free-hand and navigated techniques (p \ 0.001). Descriptive statistics for screw placement accuracy using the 2 mm increment or ‘‘in’’ or ‘‘out’’ classification method with free-hand or navigated techniques are presented in Table 3. The remainder of the papers (15 articles, 22 %) used variable classification systems (Table 4). To name a few, these included classification based on breach location without quantification of amount of breach, models determining distance differences between optimal trajectory and the one actually obtained intra-operatively and a grading

82

504

Cadavers (4)

102

42

20

100

59

Alhabib et al. [41]

Amato et al. [31]

Bai et al. [32]

Cho et al. [33]

Costa et al. [14]

Cui et al. [15]

1040

694

424

136

503

30

Abe et al. [13]

Number of pedicles

Number of patients

References

Thoracic, lumbar

Lumbar

Lumbar

Thoracic, lumbar

Lumbar

Thoracic, lumbar

Thoracic, lumbar

Spine levels

CT

CT

CT

CT

CT

CT

CT

Image tool used

All screws were evaluated and rated on the basis of the 2 mm increment classification: Grade 0: no pedicle perforation; Grade 1: only the threads outside the pedicle (less than 2 mm); Grade 2: core screw diameter outside the pedicle (2–4 mm); Grade 3: screw entirely outside the pedicle [42]

94.8

n/a

95

Grade 0: no breach; Grade I: breach of \2 mm,; Grade II: breach of 2–4 mm; Grade III: breach completely outside pedicle

The screw position was graded as screw inside the pedicle, or perforation of the pedicle cortex by up to 2 mm, from 2 to 4 mm, from 4 to 6 mm, or by more than 6 mm. The perforation location was classified as lateral, inferior, medial, or superior

87.9

96.5

91.2

90.3

% safe zone \2 mm breach free hand

Grade 0: no apparent violation of the pedicle,; Grade 1: \2 mm perforation of the pedicle, with 1 screw thread out of the pedicle; Grade 2: between 2 and 4 mm of perforation of the pedicle, with half of the diameter of the screw outside of the pedicle; Grade 3: [4 mm or complete perforation of the pedicle

Grade I: violation of the cortex without the screw extending beyond the cortical margin; Grade II: screw extending beyond the cortex 2 mm or less; Grade III: a breach of more than 2 mm Screw position was classified as correct when the screw was completely surrounded by the pedicle cortex, as ‘‘cortical encroachment’’ (questionable violation) if the pedicle cortex could not be visualized, and as ‘‘frank penetration’’ when the screw was outside the pedicular boundaries. Frank penetration was further subdivided as minor (when the edge of the screw thread was up to 2.0 mm outside the pedicle cortex), moderate (2.1–4 mm), and severe ([4 mm)

Grade 0: no breach; Grade 1: breach of 0–2 mm; Grade 2: breach of 2–4 mm; Grade 3: breach of 4–6 mm

Quantitative method used

Table 1 Studies using the 2 mm increment grading system based on CT imaging

98.1

99

n/a

97.8

n/a

94.1

96.3

% safe zone \2 mm breach navigated

Not reported

Not reported

ICC = 0.69 (0.52–0.81) sagittal

Inter-rater ICC = 0.62 (0.43–0.76) axial

Inter-rater k = 0.85

Intra-rater k = 0.83

k = 0.66 sagittal

k = 0.76 coronal

Intra-rater k = 0.78 axial

Not reported

Not reported

Reliability

Eur Spine J (2015) 24:990–1004 993

123

Number of patients

139

43

14

42

94

62

6

110

15

Cadavers (12)

References

Devito et al. [16]

Dinesh et al. [17]

Fan Chiang et al. [18]

Han et al. [43]

Houten et al. [44]

Iampreechakul et al. [45]

Kakarla et al. [46]

Kim et al. [34]

Kuraishi et al. [19]

Lieberman et al. [47]

Table 1 continued

123

211

215

488

37

363

346

176

102

261

646

Number of pedicles

Lumbar

Thoracic, lumbar

Lumbar

Thoracic

Lumbar

Lumbar

Thoracic

Thoracic, lumbar

Thoracic

Thoracic, lumbar

Spine levels

CT

CT

CT

CT

CT

CT, O-arm

CT

CT

CT

CT

Image tool used

Grade 0: no apparent violation of the pedicle; Grade 1: \2 mm perforation of the pedicle, with 1 screw thread outside of the pedicle; Grade 2: between 2 and 4 mm perforation of the pedicle, with half of the diameter of the screw outside of the pedicle; Grade 3: [4 mm or complete perforation of the pedicle Category A: fully contained within the pedicle; Category B: a breach less than 2 mm; Category C: a breach of 2–4 mm; and Category D: a breach greater than 4 mm

Frank penetration was subdivided as minor (\2.0 mm), moderate (2.0–3.9 mm or \1 screw thread diameter), or severe ([4 mm or [1 screw diameter)

Grade I is screw placement within the pedicle without cortical violation. Grade II is a cortical violation of less than 2 mm. Grade III is a cortical violation beyond 2 mm [42]

Grade A = entirely within the pedicle; B = medial or lateral pedicle wall breach less than 2 mm; C = medial or lateral pedicle wall breach equal to 2–4 mm; D = medial or lateral wall breach more than 4 mm

A pedicle breach was defined according to a published scale, with grade 1 representing extension beyond the external cortical bony surface under 2 mm, grade 2 between 2 and 4 mm, and grade 3 as beyond 4 mm

Grade I: no pedicle perforation (good); Grade II: only the threads outside the pedicle (2 mm); Grade III: core screw diameter outside the pedicle (2 mm)

The perforation of the pedicle cortex was classified as either medial or lateral, and categorized into the following four groups: 0–2.0, 2.1–4.0, 4.1–6.0, or 6.1–8.0 mm

A breach of 2 mm is considered Grade I, a breach of 2–4 mm is considered Grade II and a breach of 4 mm is considered Grade III

Screws were categorized as perfectly within the pedicle (group A); breaching less than 2 mm (group B); breaching between 2 and 4 mm (group C); breaching between 4 and 6 mm (group D); or breaching more than 6 mm (group E)

Quantitative method used

86.5

92.4

98.1

n/a

98.4

n/a

97

98.9

97.5

100

99

98.5

98.3

% safe zone \2 mm breach navigated

n/a

n/a

95.1

96.43

n/a

n/a

n/a

% safe zone \2 mm breach free hand

Not reported

Not reported

Inter-rater k = 0.65

Not reported

Inter-rater k = 0.69

Not reported

Not reported

Not reported

Not reported

Not reported

Reliability

994 Eur Spine J (2015) 24:990–1004

Number of patients

53

16

Cadavers (20)

20

26

88

41

60

19

References

Lotfinia et al. [48]

Lu et al. [20]

Ma et al. [21]

Merc et al. [49]

Modi et al. [22]

Raley et al. [50]

Ravi et al. [23]

Ringel et al. [24]

Rodrigues et al. [25]

Table 1 continued

78

298

161

424

482

108

480

168

247

Number of pedicles

Thoracic

Lumbar

Lumbar

Thoracic, lumbar

Thoracic

Thoracic

Thoracic

Thoracic

Thoracic, lumbar

Spine levels

CT

CT

CT

CT

CT

CT

CT

CT

CT

Image tool used

Screws were also classified according to the severity of their breach of the cortical pedicle in both the medial and lateral directions as follows: breaches of 0–2 mm, breaches between 2 and 4 mm, and breaches of more than 4 mm from the cortical pedicle

No breach, breach \2 mm. Breach 2–4 mm, breach 4–6 mm, breach [6 mm

76.92

93

96.3

I, no breach; II, \2 mm; III, 2–4 mm; IV, [4 mm

Agreement = 90 %

n/a

96

Grade 0: screw within cortex of pedicle; Grade 1: screw thread breach of wall of pedicle \2 mm; Grade 2: significant breach [2 mm with no neurological compromise; Grade 3: complication including pedicle fracture, anterior breach with neuro-vascular compromise and lateral or medial breach with neurological sequelae

n/a

85

n/a

n/a

90.7

Grade 0: fully contained in the pedicle; Grade 1: [2 mm; Grade 2: 2.1–4.0 mm; Grade 3: 4.1–6.0 mm, and Grade 4: 6.1–8.0 mm

100

100

100

n/a

% safe zone \2 mm breach navigated

98.2

89.2

n/a

81.3

% safe zone \2 mm breach free hand

Grade 0: no misplacement, whole screw is in pedicle; Grade 1: \2 mm and \1/2 diameter of screw is out of pedicle; Grade 2: [2 and\4 mm or \1 screw diameter is out of pedicle; Grade 3: [4 mm or whole screw diameter is out of pedicle

Grade 0: no deviation, the screw was contained in the pedicle; Grade 1: deviation \2 mm or \1/2 of the screw diameter; Grade 2: deviation [2 mm and \4 mm, or 1/2–1 screw diameter; Grade 3: deviation [4 mm, or complete deviation

Grade 0: no pedicle perforation; Grade 1: only the threads outside the pedicle (less than 2 mm); Grade 2: core screw diameter outside the pedicle (2–4 mm); Grade 3: screw entirely outside the pedicle

Screw placement was considered correct when the screw was completely surrounded by the pedicle and no portion of the screw perforated outside the cortex … perforation into minor (B2.0 mm), moderate (2.1–4 mm), and severe ([4 mm)

Quantitative method used

Not reported

Not reported

%

Inter-rater k = 0.73 Standard error = 0.026

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Reliability

Eur Spine J (2015) 24:990–1004 995

123

Number of patients

34

10

69

49

19

40

32

1006

42

62

76

References

Schizas et al. [26]

Shin et al. [27]

Shin et al. [52]

Sugimoto et al. [53]

Takahashi et al. [28]

Watanabe et al. 55]

Wang et al. [56]

Waschke et al. [29]

Wu et al. [58]

Wu et al. [30]

Yang et al. [59]

Table 1 continued

123

362

677

176

4424

268

607

265

128

310

47

128

Number of pedicles

Thoracic, lumbar Lumbar

Thoracic

Thoracic, lumbar

Thoracic

Thoracic, lumbar

Thoracic, lumbar

Lumbar

Thoracic, lumbar

Lumbar

Thoracic, lumbar

Spine levels

CT

CT

CT

CT

CT

CT

CT

CT

CT

CT

CT

Image tool used

94.8 99.3

No breach, breach\2 mm, breach 2–4 mm, breach[4 mm No breach, breach \2 mm. Breach 2–4 mm, breach 4–6 mm, breach [6 mm. Breaches were also qualified as lateral medial or inferior

96.43

84.2

Learch classification [54, 57]. no breach, breach \2 mm. Breach 2–4 mm, breach [4 mm Grade I: good, no cortical perforation; Grade II: screw outside the pedicle \2 mm; Grade III: screw outside the pedicle [2 mm

87.3

74.9

n/a

n/a

Pedicle screws were marked as ‘‘in’’ if they were deemed fully contained within the pedicle walls. Any cortical violation was recorded as a violation. Cortical violations were recorded as either medial, lateral or anterior. Violations were quantified in millimeters and graded at either: no violation, 1–2 mm violation, 2–4 mm violation, or greater than 4 mm violation

Intrapedicular, pedicle breach B2 mm, pedicle breach [2 mm, and intracostovertebral joints

Grade 0: no apparent violation of the pedicle; Grade 1: 2 mm perforation of the pedicle, with 1 screw thread out of the pedicle; Grade 2: between 2 and 4 mm of perforation of the pedicle, with half of the diameter of the screw outside of the pedicle; Grade 3: 4 mm or complete perforation of the pedicle [54]

Grade 1 threads barely out cortex, Grade 2 screw perforation of cortex up to 2 mm, Grade 3 screw perforation of the cortex [2 mm

94.1

n/a

No breach, breach \2 mm. Breach [2 mm, breach completely out No breach, breach\2 mm. Breach 2–4 mm, breach[4 mm

92.2

% safe zone \2 mm breach free hand

Rampersaud A to D classification [51]: no breach, breach \2 mm. Breach 2–4 mm, breach [4 mm in 3 planes

Quantitative method used

100

97.9

100

94.5

n/a

95

98.5

100

97.2

95

95.3

% safe zone \2 mm breach navigated

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Inter-rater k = 0.45

Intra-rater k = 0.35

Reliability

996 Eur Spine J (2015) 24:990–1004

28

964

39

30

45

20

50

353

Neal et al. [64]

Parker et al. [39]

Ploss et al. [65]

Samdani et al. [66]

Samdani et al. [67]

Su et al. [68]

Tohtz et al. [69]

Van de Kelft et al. [70]

97

208

85

Idler et al. [62]

Lee et al. [63]

Cadavers (6)

Erkan et al. [38]

Li et al. [10]

83

39

Chang et al. [61]

Bai et al. [36]

Braga et al. [60]

50

453

Oertel et al. [35]

Number of patients

References

1922

338

169

856

553

254

6816

65

1123

1634

326

133

992

415

2758

278

Number of pedicles

Table 2 Studies using the ‘‘in’’ or ‘‘out’’ classification method

Thoracic, lumbar

Thoracic, lumbar

Thoracic, lumbar

Thoracic

Thoracic

Thoracic

Thoracic, lumbar

Lumbar

Thoracic

Thoracic

Lumbar

Thoracic

Thoracic, lumbar

Thoracic

Lumbar

Thoracic, lumbar

Spine levels

O-arm

CT ? MRI

CT

CT

CT

CT

CT

CT

X-ray and CT (for misplaced screws)

CT

CT

CT

CT

CT

CT

O-arm

Image tool used

n/a

n/a

88.2

87.9

88.4

92.5

98.3

95.4

97.9

83.5

n/a

88

93

98.07

94.5

n/a

% in free hand

97.5

n/a

94.1

n/a

n/a

n/a

n/a

n/a

n/a

n/a

98.47

n/a

n/a

n/a

96.6

93.2

% in navigated

Not reported

MRI Inter-rater k = 0.33 CT ? MRI Intra-rater k = 0.29–0.70

CT Inter-rater k = 0.52 (0.31–0.69)

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Not reported

Reliability

Eur Spine J (2015) 24:990–1004 997

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998 Table 3 Descriptive statistics for pedicle screw accuracy comparing the 2 mm increment grading system and ‘‘in’’ and ‘‘out’’ grading system at the lumbar and/or thoracic spine levels, with or without navigation

Eur Spine J (2015) 24:990–1004

Descriptive statistics

In and out grading system

Navigated (%)

Navigated (%)

Free hand (%)

Free hand (%)

Mean

97.4

91.3

96.0

92.3

Weighted mean

96.6

89.3

96.7

94.7

Median accuracy

98.1

93.0

96.6

92.8

Minimum accuracy

85.0

74.9

93.2

85.5

Maximum accuracy

100

99.3

98.5

98.3

Accuracy range

15

24.4

5.3

12.8

Interquartile accuracy range

3.7

8.4

3.4

7.9

Standard deviation

3.2

6.5

2.2

4.6

Total screws

13,066

12,984

5453

15,768

classification based on pedicle breaches in 3 mm increments measured on CT scan. The later system was the third most widely used (in 5 articles). A comparison of thoracic versus lumbar pedicle screws for the 2 mm increment classification and the ‘‘in’’ or ‘‘out’’ classification is summarized in Table 5. With regard to the 2 mm increment grading system, the results showed a significantly higher placement accuracy (p = 0.01) using navigated techniques as compared with free-hand techniques for the thoracic region. Our results also suggest statistically higher placement accuracy (p = 0.04) in the lumbar spine region when using a free-hand technique and the ‘‘in’’ or ‘‘out’’ grading system. Finally, the accuracy of screws placed with navigation and graded using the 2 mm increment classification as compared to the ‘‘in’’ or ‘‘out’’ classification proved to show no significant difference (p = 0.33). However, our results suggest that screw placement accuracy was significantly higher with the ‘‘in’’ or ‘‘out’’ grading system as compared with the 2 mm increment system when using a free-hand technique (p \ 0.001).

Discussion This systematic review evaluated the types and reliability of grading systems used to assess pedicle screws placement and accuracy. Our findings demonstrate that a large number of studies used the 2 mm increments and the in and out grading systems. However, there remains no clinical standard method to assess pedicle screw position post-operatively. Different methods were identified in this review and can lead to different results if applied to the same articles. Thus, we believe that a standardized method to determine pedicle screw accuracy should exist. It is clear that most surgeons will use imaging for pedicle screw position assessment intra-operatively and post-operatively. The review of the literature showed that

123

2 mm increment grading system

the most widely used post-operative imaging technique was CT imaging. However, no gold standard exists for the identification of pedicle screw location based on CT imaging. It is important to note that standard CT imaging parameters such as slice thickness and gantry tilt influence the amount of radiation exposure to the patient and the accuracy of imaging measurements. Although gantry tilt is rarely modified, imaging thickness is more commonly adjusted. Images with thickness of 2 mm are commonly obtained to minimize radiation exposure, while maintaining precision. In addition, methods to reduced artifact due to gantry tilt have been proposed in the literature andshould be considered as they influence measurement precision in peddicle screw accuracy [40]. Our review did demonstrate that the most widely used method was a grading system based on pedicle screw breach measurements in 2 mm increments. Most authors also suggested that any breach less than 2 mm of the pedicle was safe. Although this grading system is acceptable, the location of the breach should also be considered especially in determining a safe versus an unsafe screw. The second most widely used method for pedicle screw placement assessment was in or out classification. Although simple to determine via imaging, the clinical relevance of a small breach should be differentiated from that of a large breach. Thus, this method is less likely to determine if a screw is in a safe zone compared to a nonsafe one. Again, we can raise similar argument to those above; the clinical relevance of the breach is what should be of importance for accuracy and safety. Our analysis showed that screw placement accuracy was significantly better when using a navigated technique as compared to a free-hand technique. Differences in screw placement accuracy were also observed according to the grading system used (2 mm increment vs in or out) and spine region (thoracic vs lumbar). Our results showed that screw placement accuracy was significantly higher with the 2 mm grading system as compared to the ‘‘in’’ or ‘‘out’’

Number of patients

49

116

45

12

n/a

38

88

102

112

References

Abul-Kasim et al. [71]

Abul-Kasim et al. [72]

Allam et al. [73]

Ammirati et al. [74]

Chan et al. [37]

Gang et al. [75]

Harimaya et al. [76]

Hu et al. [12]

Kantelhardt et al. [77]

536

949

948

311

240

82

208

2201

873

Number of pedicles

Thoracic, lumbar

Thoracic, lumbar

Thoracic, lumbar

Thoracic

Thoracic

Thoracic, lumbar

Thoracic

Thoracic, lumbar

Thoracic, lumbar

Spine levels

Table 4 Remainder of studies using a variety of classification systems

CT

X-ray, CT

X-ray

CT

CT

CT

CT

CT

CT

Image tool used

Screws entirely in the bone were Grade 0, Grade 1 described an encroachment of the cortical bone, Grade 2 a deviation of less than 3 mm, Grade 3 deviations from 3 to 6 mm and Grade 4 for deviations of more than 6 mm

(1) Successful accurately placed screw using robotic guidance; (2) screw malpositioned using robot; (3) use of robot aborted and screw placed manually; (4) planned screw not placed as screw deemed non-essential for construct stability

(1) violation of the harmonious segmental change of the tip of the inserted screws with reference to the vertebral rotation using the posterior upper spinolaminar junction in the plain PA radiograph (medial or lateral out); (2) no crossing of the medial pedicle wall by the tip of the pedicle screw inserted with reference to the vertebral rotation using the posterior upper spinolaminar junction in the plain PA radiograph (lateral out); and (3) violation of the imaginary midline of the vertebral body using the posterior upper spinolaminar junction in the plain PA radiograph by the position of the tip of the inserted pedicle screw (medial out)

Grade Ia, optimally placed screws, rigidly anchored within the pedicle and vertebral body; Grade Ib, screws placed with [50 % of the pedicle screw diameter (PSD) lateral outside the pedicle and with [50 % of the PSD within the vertebral body; Grade IIa, screws placed with \50 % of the PSD within the pedicle and [50 % of the PSD outside the lateral cortex of the vertebral body; Grade IIb, screws placed with \50 % of the PSD within the pedicle and the tip of the screw crossing the midline of the vertebral body; Grade IIIa, screws located with [50 % of the PSD outside the pedicle and lateral vertebral cortex; and Grade IIIb, screws located with [50 % of the PSD outside the pedicle medially and the tip of the screw crossing the midline of the vertebral body with spinal canal encroachment

Grade 0: no perforation; Grade 1: only thread of screw outside pedicle; Grade 2: core of screw outside pedicle; Grade 3: screw completely outside pedicle

Cutaneous device to detect accuracy

Learch and Wiesner Classification [57]: 1. Encroachment if the pedicle cortex could not be visualized. 2. Minor penetration when the screw trajectory was \3 mm outside the pedicular boundaries. 3. Moderate penetration when the screw trajectory was 3–6 mm outside the pedicular boundaries. 4. Severe penetration when the screw trajectory was [6 mm outside the pedicular boundaries

The following types of misplacement were included in the analysis: medial cortical perforation (MCP), lateral cortical perforation (LCP), anterior cortical perforation (ACP), endplate perforation (EPP), and foraminal perforation (FP)

The following types of misplacement were included in the analysis: medial cortical perforation (MCP), lateral cortical perforation (LCP), anterior cortical perforation (ACP), endplate perforation (EPP), and foraminal perforation (FP)

Method

Eur Spine J (2015) 24:990–1004 999

123

Number of patients

78

30

67

42

n/a

188

References

Sethi et al. [78]

Scheufler et al. [79]

Silbermann et al. [81]

Ughwanogho et al. [82]

von Jako et al. [83]

Yson et al. [84]

Table 4 continued

123 370

62

485

339

415

588

Number of pedicles

Lumbar

Thoracic, lumbar

Thoracic

Lumbar

Thoracic, lumbar

Lumbar

Spine levels

CT

CT

CT

CT

CT

CT

Image tool used

Grade 0: no impingement, Grade 1: screw in contact with joint, Grade 2: screw clearly invades joint

Grade 0 (Ideal): accurate screw with no perforation through any cortex; Grade 1 (minimally displaced): safe screw with cortical perforation \3 mm; Grade 2 (moderately displaced): displaced by C3 mm but B5 mm; Grade 3 (critical perforation): displaced by [5 mm

Learch and Wiesner Classification [57]: 1. Encroachment if the pedicle cortex could not be visualized. 2. Minor penetration when the screw trajectory was \3 mm outside the pedicular boundaries. 3. Moderate penetration when the screw trajectory was 3–6 mm outside the pedicular boundaries. 4. Severe penetration when the screw trajectory was [6 mm outside the pedicular boundaries Optimal: central axis of screw in pedicle plus tip in vert. body. Acceptable: majority of shank of screw outside axis of pedicle, but not unsafe. Unsafe: transverse canal or medial or lateral deviation, or require repositioning

Classification by Xu et al. [80]: Grade I: no breach, Grade II: breach less than half of screw diameter out of pedicle wall, Grade III: breach with more than half of screw diameter out of pedicle wall

Learch and Wiesner Classification [57]: 1. Encroachment if the pedicle cortex could not be visualized. 2. Minor penetration when the screw trajectory was \3 mm outside the pedicular boundaries. 3. Moderate penetration when the screw trajectory was 3–6 mm outside the pedicular boundaries. 4. Severe penetration when the screw trajectory was [6 mm outside the pedicular boundaries

Method

1000 Eur Spine J (2015) 24:990–1004

Eur Spine J (2015) 24:990–1004 Table 5 Pedicle screw accuracy based on the 2 mm increment grading system and ‘‘in’’ or ‘‘our’’ grading system, with or without navigation: comparison between lumbar and thoracic regions

1001

2 mm increment grading Lumbar

Comparison thoracic to lumbar Thoracic

Navigated

Free hand

Navigated

Free hand

Navigated

Free hand

% in safe zone

96.22

94.32

97.60

88.14

0.40

0.07

Number of screws

7431

5215

p value

0.32

0.01

In/out grading

Comparison thoracic to lumbar

Lumbar

Bold values indicate statistically significant results

Thoracic

Navigated

Free hand 96.27

Navigated

Free hand

Navigated

Free hand

N/A

90.90

N/A

0.04

% in pedicle

97.54

Number of screws

3410

5699

p value

0.49

N/A

classification system, when a free-hand technique was used. Significantly better placement accuracy was also observed in the thoracic region when using a navigation technique and the 2 mm increment grading system. These results may be partly explained by the higher precision of the 2 mm classification system in detecting misplaced screws. Furthermore, as thoracic screw placement is generally more technically challenging, it is not unexpected that a difference between thoracic and lumbar screws was noted according to the grading system and surgical technique employed. A major weakness of most of these grading systems is the lack of consideration of the direction of given breach. Although some papers commented on the direction of the breach, no grading systems integrated or accounted for these criteria. For example, a grade B breach that is medial and directly in the canal should be more concerning than a similar graded screw, but one that is lateral and outside the canal or facet joint. In addition, some screws are intentionally placed outside the pedicle if bone stock or pedicle compromise is anticipated intra-operatively. Moreover, in real clinical practice, a patient with symptoms and a concordant breach on image is more worrisome than a larger breach on imaging for which the patient is completely asymptomatic. Thus, a standardized grading system is needed and should include both three-dimensional criteria in addition to clinically relevant patient information. The results of our analysis showed that navigated screws were significantly better placed than pedicle screws placed with a standard technique. However, all analyses were based on imaging alone. None of the studies reported on patient’s symptoms in the navigated group as compared to the non-navigated group. It can be assumed that better screw placement based on imaging should concord with better patient outcomes, but it is not clear if this would be clinically significant. Navigation can be considered for

safer screw placement; however, certain drawbacks of navigation should be considered such as longer operative time, higher learning curve, higher costs and more complex instrumentation. Moreover, although statistically significant, the improvement in accuracy obtained with navigation needs to be justified with cost-benefit analyses. It is also worthy to note that different navigation techniques were included in this review. The majority were navigation systems based on real-time imaging, however, some included robotic navigation, mechanical guides such as rapid prototypes and even neuro-monitors. Therefore, all navigation techniques may not have been directly comparable.

Conclusion This is the first review of the literature reporting on the most widely used methods for the assessment of pedicle screw placement. Our findings suggest that currently, the most widely used and accepted grading system for pedicle screw accuracy is the 2 mm increment grading system based on CT imaging. Our results also confirmed that most assessment methods were solely based on imaging. However, there remains no gold standard technique to assess pedicle screw placement accuracy in the field of spine surgery. This clinical limitation deserves further attentions and needs to be addressed. A grading system should be implemented with the help of expert surgeons to allow for a more objective method of identifying pedicle screws that can be deemed safe or unsafe. In our opinion, a grading system should take into account both, imaging data and patient’s clinical information. Hence, a complete grading system should include (1) the amount of breach as measured on imaging, (2) the location of the breach relative to key anatomical sites and (3) the presence or absence of

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corresponding clinical symptoms. In future work, we aim to develop a new, more complete grading system that will be validated and tested in a prospective study. Conflict of interest

16.

The authors report no conflicts of interest.

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60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

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