Abstract
Background
Interfragmentary compression at the fracture site facilitates healing. Headless compression screws used to treat scaphoid fractures can be grouped as shank screws, conical tapered screws and double component screws. There has been no meta-analysis of biomechanical studies to compare interfragmentary compression produced by the above screws.
Methods
A computerised search of Pubmed, Embase and OVID database was undertaken to identify the studies. We estimated the weighted mean difference of interfragmentary compression (in Newton) with 95% confidence intervals. Random effects model was selected for meta-analysis.
Results
The pooled estimate of nine studies demonstrated that conical tapered screw produced significantly higher interfragmentary compression force compared to the shank screw (WMD 19.96, 95% CI 11.2–28.8, p < 0.0001, I2 = 99%). The pooled estimate of four studies demonstrated that dual component screw produced significantly higher interfragmentary compression force compared to the shank screw (WMD 16.93, 95% CI 12.3–21.6, p < 0.0001, I2 = 97.7%). The pooled estimate of four studies showed that there was no significant difference in the interfragmentary compression force generated by either conical tapered screw or dual component screw (WMD 3.93, 95% CI − 8.3 to 16.2, p = 0.53, I2 = 99.7%). There was evidence of minimal publication bias.
Conclusion
Conical tapered screws and dual component screws produced statistically significant higher interfragmentary compression force at the scaphoid fracture site compared to shank screws. There was no difference in the compression force generated by either conical tapered screw or dual component screw.
Graphic abstract
Keywords: Scaphoid bone, Bone screws, Fracture fixation, Internal, Meta-analysis, Fracture fixation
Introduction
Rationale
The standard fixation of displaced intra-articular fractures is open anatomic reduction and internal fixation with an implant that provides compression and absolute stability. The conventional orthopaedic screw has various parts, namely head, shaft and tip. The head of the screw abuts against the cortical part of the bone and prevents the screw from sinking into the bone. In the wrist joint, the carpal bones are in close proximity and any prominence due to the screw head can impinge and lead to erosion of the articular cartilage of the adjacent carpal bones. Hence, headless compression screws were designed wherein the trailing end of the screw can get buried below the chondral surface of the bone without impinging or eroding surrounding structures. Another benefit of the headless compression screw is due to its subchondral location; removal of the screw is not warranted.
The most dreaded complication of scaphoid fracture is non-union. Untreated nonunion can possibly lead to osteonecrosis [1] and to progressive pattern of wrist arthritis that is termed as scaphoid non-union advanced collapse [2]. It is estimated that around 20% of the fractures of the waist of the scaphoid tend to be displaced fractures [3]. If displaced scaphoid waist fractures are treated in cast then there is a fourfold increase in the risk of non-union compared to cast treatment of undisplaced scaphoid waist fractures and hence operative intervention yields better union in displaced fractures [4]. As the risk of non-union is higher with displaced scaphoid waist fractures, it is advisable to treat displaced fractures with operative open reduction and internal fixation [5]. Internal fixation is indicated for displaced fractures of the scaphoid waist and for fractures with gap of more than 1 mm or translation greater than 1 mm [3, 6].
Compression between bone fragments of an intra-articular fracture provides more stability to fixation and facilitates healing of the fracture. Also since the scaphoid is surrounded by synovial fluid, the synovial fluid can interpose between the fracture ends and could be a contributory factor for non-union in addition to the recognized retrograde precarious blood supply to the waist and the proximal pole of the scaphoid [7]. Hence abolishing the gap between the fracture ends of scaphoid is crucial to avoid union related complications. Higher the compression between bone fragments more is the reduction of gap at the fracture site, more is the stability of fixation and more is the chance for the fracture to unite [8].
There are various types of headless compression screws with different design available for internal fixation of scaphoid fractures.Broad classification of headless compression screws used in scaphoid fracture is depicted in Fig. 1.
Fig. 1.
Shows delineation between various types of headless compression screws. HCS headless compression screw
Objectives
As there are multiple designs of headless compression screws available, it is unclear as to which headless compression screw provides the highest interfragmentary compression and is most ideally suited to fix acute fractures of the scaphoid. As there is paucity of high-level evidence from basic science biomechanical studies pertaining to scaphoid fracture fixation, the objective of the present study was to undertake a meta-analysis to compile, analyze and compare the interfragmentary compression provided by various headless compression screws used for internal fixation of acute scaphoid fractures. The results of this meta-analysis may have a significant bearing on clinical practice. Hence, the aim of the present study was to determine which HCS provided the highest interfragmentary compression across the scaphoid fracture. Our null hypothesis was that there would be no difference in the interfragmentary compression provided by shank screw, conical tapered screw or the dual component screws.
Methods
The present meta-analysis was conducted as per PRISMA guidelines [9].
Study Approval
The present meta-analysis was approved by the institutional ethics committee.
Eligibility Criteria
The eligibility criteria were defined following the PICOS approach [9]. Biomechanical studies on headless compression screw (HCS) in simulated acute scaphoid fracture and fulfilling the following inclusion and exclusion criteria and published between 1st January 2000 and 31st December 2019 were included in the meta-analysis.
Inclusion criteria were biomechanical studies published in English language on cadavers, animal models and polyurethane foams comparing two or more different types of HCS in the same study in acute scaphoid fracture and should have reported mean interfragmentary compression (Newton) and measure of variance such as standard deviation or 95% confidence interval (so that if studies did not report standard deviation, the standard deviation could be calculated based on the 95% confidence interval). Any other type of study was excluded.
Search Strategy and Criteria (Information Sources)
Two researchers undertook independent search of electronic databases such as Pubmed (Medline), Ovid (Embase) and Proquest. The Pubmed (MEDLINE), Ovid (EMBASE) and PROQUEST database were electronically searched using the following MeSH terms and Boolean operators: [“Scaphoid Bone AND (“Fracture fixation” OR “Fracture Fixation, Internal” OR “Fracture Healing” OR “Fractures, Bone”)], [“Scaphoid Bone” AND “Bone Screws”], [“Scaphoid Bone” AND (“Biomechanical Phenomena” OR “Mechanics” OR “Shear Strength” OR “Tensile Strength” OR “Stress, Mechanical”)] and [“Scaphoid Bone” AND “Compression Strength”]. Any disagreement was settled amicably by mutual discussion between the researchers.
Data Items, Collection and Extraction
Two researchers worked independently to extract relevant data from the included studies. The primary outcome was mean interfragmentary compression (measured in Newton). Information pertaining to name of authors, publication year, journal name, type of headless compression screws compared, biomechanical model used in study (human cadaver/polyurethane foam), density of the biomechanical model (mean age in cadaver studies, density of the polyurethane foam to mimic young bone or osteoporotic bone), mean interfragmentary compression ( mean peak interfragmentary compression/mean final interfragmentary compression), timing of measurement of the interfragmentary compression (soon after insertion/30 s after insertion/3 min after insertion/5 min after insertion), position at which the interfragmentary compression force (flush position or chondral position/beyond flush position or subchondral position), simulated fracture pattern (transverse/oblique/comminuted), gap in millimetres at simulated fracture site, number of cases in each group, measure of variance of the interfragmentary compression force (standard deviation either given/standard deviation not reported but standard deviation calculated by the researchers based on the reported 95% confidence interval) and conclusion of the study. The extracted data (Table 1) were stored on a Microsoft excel sheet.
Table 1.
Depicts the structured format of the data abstraction used in the study
| Data abstraction form |
|---|
| 1. Author, year, journal |
| 2. Types of headless compression screw used in study |
| 3. Biomechanical model used in study (human cadaver/polyurethane foam) |
| 4. Density of the biomechanical model (mean age in cadaver studies, density of the polyurethane foam to mimic young bone or osteoporotic bone) |
| 5. Type of interfragmentary compression (peak interfragmentary compression/final interfragmentary compression) |
| 6. Mean interfragmentary compression force (Newton) |
| 7. Timing of measurement of the interfragmentary compression (soon after insertion/30 s after insertion/3 min after insertion/5 min after insertion) |
| 8. Position at which the interfragmentary compression force (flush position or chondral position/beyond flush position or subchondral position) |
| 9. Measure of variance of the interfragmentary compression force (standard deviation either given/standard deviation not reported but standard deviation calculated by the researchers based on the reported 95% confidence interval) |
| 10. Simulated fracture pattern (transverse/oblique/comminuted) |
| 11. Gap in millimetres at simulated fracture site |
| 12. Number of cases in each group |
| 13. Comments on the study |
| 14. Assessment of quality of the study (Newcastle–Ottawa Scale). Total score as well as score for selection, comparability and outcome |
Assessment of Study Quality (Risk of Bias in Individual Studies)
The methodological rigour of each biomechanical study was evaluated using the Newcastle–Ottawa scale. The Newcastle–Ottawa scale consists of evaluation of selection of study population (4 points), comparability of groups (2 points) and assessment of outcome (3 points). The value of the Newcastle–Ottawa scale ranges from 0 point (least methodological rigour) to 9 points or 9 stars (highest methodological rigour) [10]. Two researchers independently rated each included study using the Newcastle–Ottawa scale and the final scoring was agreed after consensus between the researchers. All disagreements if any were resolved through mutual discussion.
Statistical Methodology
We used StatsDirect software (StatsDirect, Cambridge, UK) for the present meta-analysis. The principal outcome was the mean interfragmentary compression measured in Newton. In some studies, the standard deviation (SD) was not mentioned. If the SD is not reported in a study but if 95% confidence interval is described, then firstly the standard error (SE) is estimated using the formula SE = upper value of 95% CI—lower limit of 95% CI divided by 3.92. The SD was then estimated using the formula SD = SE × square root of the sample size [11].
Summary Measures and Synthesis of Results
The weighted difference in means (WMD) for interfragmentary compression along with its 95% confidence interval and p value was calculated and Forrest plotting was done for graphical depiction. I2 statistic was used to evaluate the presence of heterogeneity of the data. For pooling of the data, fixed effect model was used if I2 statistic was < / = 25% and random effect model (DerSimonian-Laird) was used if the I2 statistic was greater than 25% [12].
Risk of Bias Across Studies
Egger’s regression test, Begg-Mazumdar test and Funnel plot were used to evaluate for presence of any publication bias.
Sensitivity Analysis
Sensitivity analysis was done to determine whether the interfragmentary compression force generated by the headless compression screws differed in synthetic bone and in cadavers. Additional advantage of performing the sensitivity analysis would be to evaluate whether separate analysis of compression force in cadaver and synthetic bone will have any bearing on the degree of heterogeneity of the meta-analysis.
Results
Identification of Relevant Studies
Figure 2 portrays the flow chart for identifying the relevant studies. Initial screening of electronic databases led to identification of 3254 citations including 1739 hits from Pubmed, 1513 hits from Ovid (Embase) and 2 hits from Proquest.
Fig. 2.
Depicts PRISMA literature search flow chart for identification of suitable studies
2924 citations were excluded based on screening of titles because 2252 citations were irrelevant topics, 40 citations were systematic reviews or meta-analyses, 447 citations were narrative reviews and 185 citations were abstracts presented at conference, Editorial commentaries or letters to the Editor. Screening of abstracts led to exclusion of 275 citations because 264 citations were on irrelevant topics, 10 citations were narrative reviews and one citation was a systematic review/meta-analysis.
Full text review led to identification of 55 possible citations and after exclusion of 36 duplicates, 19 articles were considered for review. Nine articles [13–21] satisfying the inclusion criteria were finally included in the meta-analysis for quantitative and qualitative analysis after exclusion of 10 articles [22–31]. Seven biomechanical studies did not report interfragmentary compression force [23, 24, 26–30]. Two biomechanical studies reported mean interfragmentary compression and standard deviation but they described results of only single type of HCS with no comparison between various types of HCS [22, 31]. In another biomechanical study [25], more than one type of HCS were compared but the measure of variance was not reported and the study was conducted on foot bones such as navicular and medial cuneiform.
There were total 275 cases in the pooled data out of which 98 cases (42%) were from cadaveric studies and 134 cases (58%) were from studies on polyurethane foam (synthetic bone) models.
Assessment of Methodological Quality of Included Studies
All the included studies scored 7–9 points on the Newcastle–Ottawa scale (Table 2) suggesting high-quality studies and hence the risk of bias due to poor methodological quality of study can be assumed to be very low. Three studies achieved maximum score of 9 points [16, 17, 20]. All three studies gained maximum score of 2 points for comparability. Pensy et al. [16] mentioned the bone mineral density of cadaver bones of both cohorts that were used for comparison of interfragmentary compression using different types of headless compression screws. Grewal et al. [17] used matched pair comparison technique in order to ensure age and bone mineral density of the cadaver bones were comparable in both cohorts used for comparison. Gruszka et al. [20] performed random allocation of cadaver bones with similar bone mineral density to all three cohorts. One study [15] was allotted least score of two points for outcome because despite ten recordings made in each cohort, the first three readings were excluded and the final analysis included only seven readings.
Table 2.
Depicts assessment of methodological quality of individual studies using the Newcastle–Ottawa Scale (NOS)
| Study, year | NOS-selection | NOS-comparability | NOS-outcome | Total NOS score |
|---|---|---|---|---|
| Beadel et al. 2004 [13] | 4 | 1 | 3 | 8 |
| Bailey et al. 2006 [14] | 4 | 1 | 3 | 8 |
| Hausmann et al. 2007 [15] | 4 | 1 | 2 | 7 |
| Pensy et al. 2009 [16] | 4 | 2 | 3 | 9 |
| Grewal et al. 2011 [17] | 4 | 2 | 3 | 9 |
| Assari et al. 2012 [18] | 4 | 1 | 3 | 8 |
| Crawford et al. 2012 [19] | 4 | 1 | 3 | 8 |
| Gruszka et al. 2012 [20] | 4 | 2 | 3 | 9 |
| Hart et al. 2013 [21] | 4 | 1 | 3 | 8 |
Salient Characteristics of Included Studies
The salient characteristics of the experimental studies included in the meta-analysis have been summarized in Table 3.
Table 3.
Depicts salient characteristics of the included studies showing variation in assessment of primary outcome, position of estimation of interfragmentary compression, time of estimation of the interfragmentary compression and the size of the gap of the simulated scaphoid waist fracture
| Study, year | Model | Primary outcome studied | Position of estimation of interfragmentary compression | Time of estimation of interfragmentary compression after complete screw insertion | Gap at simulated fracture site (width of the interposed load cell washer) (mm) |
|---|---|---|---|---|---|
| Beadel et al. 2004 [13] |
Human cadaver Fresh frozen Mean age:72 years (range 40–88 years) Osteoporotic bone |
Mean final interfragmentary compression | Beyond flush position | 5 min after screw insertion | 4 |
| Bailey et al. 2006 [14] |
Polyurethane blocks Grade 10 foam (cancellous bone) with rim of grade 40 foam representing subchondral bone |
Mean final interfragmentary compression | Flush position | Soon after insertion | 4 |
| Hausmann et al. 2007 [15] |
Polyurethane blocks Value not mentioned due to production error Said it represented osteoporotic bone |
Mean final interfragmentary compression | Flush position | 30 s after insertion | 1.4 |
| Pensy et al. 2009 [16] |
Human cadaver Mean age 70 years |
Mean final interfragmentary compression | Beyond flush position | 5 min after insertion | 5 |
| Grewal et al. 2011 [17] |
Human cadaver Mean age 75 years (matched pair used for comparison) |
Mean final interfragmentary compression | Not specified but looking at figures in the manuscript seems like beyond flush position | 3 min after insertion | 5 |
| Assari et al. 2012 [18] |
Polyurethane blocks Density = 0.16 gm/cm3 |
Mean maximal/peak interfragmentary compression | Not specified | Soon after insertion | 0.5 |
| Crawford et al. 2012 [19] |
Polyurethane foam Density = 0.16 gm/cm3 |
Mean final interfragmentary compression | Flush position | 30 s after insertion | 4.2 |
| Gruszka et al. 2012 [20] |
Human cadaver Mean age: 75 years (range: 60–90 years) |
Mean final interfragmentary compression | Beyond flush position | Soon after insertion | 4 |
| Hart et al. 2013 [21] |
Polyurethane blocks Density = 0.32 gm/cm3 |
Mean final interfragmentary compression | Beyond flush position | Soon after insertion | < 1 |
Four studies [44.4%] [13, 16, 17, 20] were on human cadavers whereas five studies [55.6%] [14, 15, 18, 19, 21] were on sawbones polyurethane foam synthetic bone material.
Eight studies [88.9%] [13–17, 19–21] reported mean final interfragmentary compression while one study [11.1%] [18] reported mean maximal or peak interfragmentary compression.
One study [11.1%] [18] did not clearly mention the position of estimation of interfragmentary compression. Three studies [33.3%] [14, 15, 19] reported interfragmentary compression at the flush position (chondral surface position) of the HCS, whereas five studies [55.6%] [13, 16, 17, 20, 21] reported interfragmentary compression at beyond flush position (subchondral position).
In four studies [44.4%] [14, 18, 20, 21] the interfragmentary compression was measured soon after complete screw insertion, two studies [22.2%] [15, 19] reported measurement at 30 s after complete screw insertion, one study [11.1%] [17] reported measurement at three minutes after complete screw insertion and two studies [ 22.2%] [13, 16] reported measurement at five minutes after complete screw insertion.
The gap at the simulated fracture also varied between the studies. Two studies [22.2%] [18, 21] had simulated fracture gap less than 1 mm, one study [11.1%] [15] reported on gap of 1.4 mm, three studies [33.3%] [13, 14, 20] reported on gap of 4 mm, one study [11.1%] [19] reported on gap of 4.2 mm and two studies [22.2%] [16, 17] reported on gap of 5 mm.
Salient Results of Included Studies
The type of headless compression screw used, number of cases, mean interfragmentary compression along with the standard deviation and findings from each individual studies are summarized in Table 4. All included studies were published in English language between 2004 and 2013. The standard deviation was not reported in two studies [14, 19] and we estimated the standard deviation based on the 95% confidence interval that was reported in those studies.
Table 4.
Depicts salient observations and results of the included studies
| Study, year | Devices studied | Number of cases | Mean ± SD | Conclusion |
|---|---|---|---|---|
| Beadel et al. 2004 [13] |
Bold screw 3.0 Acutrak standard Acutrak mini |
10 10 12 |
103 ± 46 152 ± 21 93 ± 56 |
Acutrak std screw better than Bold screw and Acutrak mini No difference between Acutrak mini and Bold screw |
| Bailey et al. 2006 [14] |
Herbert Herbert Whipple Acutrak standard |
6 6 6 |
13.8 ± 8.3a 16.7 ± 8.3a 35.2 ± 8.3a |
Acutrak std screw better |
| Hausmann et al. 2007 [15] |
HBS Acutrak standard TwinFix |
7 7 7 |
2 ± 1 7.6 ± 0.4 8 ± 1 |
TwinFix and Acutrak std better |
| Pensy et al. 2009 [16] |
3 mm HCS Acutrak std |
10 10 |
32 ± 30 38 ± 24 |
No difference |
| Grewal et al. 2011 [17] |
3 mm HCS Acutrak 2 std |
8 8 |
37.2 ± 26.8 68.6 ± 36.4 |
Acutrak 2 screw better |
| Assari et al. 2012 [18] |
Herbert Whipple 3 mm HCS Acutrak 2 mini TwinFix Kompressor |
5 5 5 5 5 |
13.4 ± 2.4 17.3 ± 1 45.4 ± 0.9 30.7 ± 0.7 20.8 ± 1.2 |
Acutrak 2 mini screw best. TwinFix was 2nd best and Kompressor was 3rd best Herbert Whipple produced least compression |
| Crawford et al. 2012 [19] |
Herbert Herbert Whipple Acutrak std TwinFix Kompressor |
10 10 10 10 10 |
4.5 ± 4.7a 20.8 ± 3.2a 11.4 ± 4.2a 31 ± 5.6a 51 ± 11.3a |
Dual component screws produced higher compression Herbert Whipple better than Acutrak std |
| Gruszka et al. 2012 [20] |
3 mm HCS Acutrak 2 mini TwinFix |
10 10 10 |
136.9 ± 98.2 190.8 ± 116.6 226.1 ± 112.9 |
Acutrak 2 mini and TwinFix both better |
| Hart et al. 2013 [21] |
Herbert Whipple 3 mm HCS Acutrak std Acutrak mini |
5 5 5 5 |
78 ± 9 67 ± 2 113 ± 18 104 ± 15 |
No difference |
aStandard deviation not reported in the study but we calculated the standard deviation based on the 95% confidence interval of the mean that was reported in the study
Strengths and weaknesses of the included studies have been tabulated in Table 5.
Table 5.
Depicts strengths and weakness of individual studies
| Study | Strengths | Weaknesses |
|---|---|---|
| Beadel et al. JHS [Am] 2004 [13] |
Study conducted on human cadaver on scaphoid bone and the compression force was estimated when the trailing end of the screw was in the ideal position (subchondral or beyond flush position) This study evaluated both the intensity and the sustainability of the interfragmentary compression force generated across the simulated fracture |
The mean age of the cadaver specimen was 72 years (Range 40–88 years). The interfragmentary compression force generated by the headless screws might be lower in this cohort due to age related osteoporosis. Scaphoid fractures commonly occur in younger age individuals rather than elderly individuals |
| Bailey et al. JHS Br Eur 2006 [14] |
This study described both mean maximal force and also mean force at flush position Though study was conducted on polyurethane foam, the foam model had grade 10 foam mimicking cancellous bone and a rim of grade 40 foam to represent subchondral bone just like human scaphoid. The density of the foam was not specified |
The study included a bioabsorbable screw called as Little Grafter screw which is not presently in use and there is no clinical data to support the use of the Little Grafter screw. The study also included Asnis III screw which has a screw head and is not usually used to fix scaphoid fracture The Standard deviation of the interfragmentary compression force was not reported by the authors but it was possible to estimate the standard deviation based on the 95% confidence interval of the mean |
| Hausmann et al. Injury 2007 [15] |
The authors mentioned that there was some production error in the polyurethane foam used in the study. The value of the foam density was not specified and hence the foam could be assumed to represent cancellous bone in osteoporosis. Hence the extremely low interfragmentary compression force observed in the study could be due to use of inferior quality polyurethane foam. Had the authors used good quality polyurethane foam, the compression force generated by the headless screws might have been higher Ten screws of each type were used in the study but results of compressive force generated by the first three screws were excluded. The analysis presented in the study is from seven screws of each type |
|
| Pensy et al. J Ortho Surg Adv 2009 [16] | Study conducted on human cadaver on scaphoid bone and the compression force was estimated when the trailing end of the screw was in the ideal position (subchondral or beyond flush position) | The mean age of the cadaver specimen was 70 years. The interfragmentary compression force generated by the headless screws might be lower in this cohort due to age related osteoporosis. Scaphoid fractures commonly occur in younger age individuals rather than elderly individuals |
| Grewal et al. JOSR 2011 [17] | Study conducted on human cadaver. Matched pair of cadaver bones was used in the study to maintain uniformity in bone quality | The mean age of the cadaver specimen was 75 years. The interfragmentary compression force generated by the headless screws might be lower in this cohort due to age related osteoporosis. Scaphoid fractures commonly occur in younger age individuals rather than elderly individuals |
| Assari et al. Injury 2012 [18] | This study compared all the different makes of headless compression screws (shank screws, conical tapered screw and double component screws) | The density of polyurethane foam was 0.16 which represents the density of cancellous bone in elderly patients. It is probable that the interfragmentary compression force generated by the screw might be lower than the force generated in high density cancellous bone of young individuals |
| Crawford et al. JHS Am 2012 [19] | This study compared all the different makes of headless compression screws (shank screws, conical tapered screw and double component screws) |
The density of polyurethane foam was 0.16 which represents the density of cancellous bone in elderly patients. It is probable that the interfragmentary compression force generated by the screw might be lower than the force generated in high density cancellous bone of young individuals The Standard deviation of the interfragmentary compression force was not reported by the authors but it was possible to estimate the standard deviation based on the 95% confidence interval of the mean |
| Gruszka et al. JHS Am 2012 [20] |
Study conducted on human cadaver. Bone densitometry was evaluated and it was ensured that all three groups had bones of similar bone density Study conducted on human cadaver on scaphoid bone and the compression force was estimated when the trailing end of the screw was in the ideal position (subchondral or beyond flush position) |
The mean age of the cadaver specimen was 75 years (Range: 60 to 90 years). The interfragmentary compression force generated by the headless screws might be lower in this cohort due to age related osteoporosis. Scaphoid fractures commonly occur in younger age individuals rather than elderly individuals |
| Hart et al. JHS Am 2013 [21] |
The density of polyurethane foam was 0.32 which represents the density of cancellous bone in young individuals. Scaphoid fracture is most commonly observed in this age group Study conducted on human cadaver on scaphoid bone and the compression force was estimated when the trailing end of the screw was in the ideal position (subchondral or beyond flush position) |
The study was carried out on polyurethane foam instead of human cadaver |
Comparison of Shank Screw and Conical Tapered Screw
All the nine studies [13–21] included in the meta-analysis compared shank screw with conical tapered screw. In each individual study, there were multiple comparisons between various types of headless compression screws. The pooled data consisted of 117 cases of fixation using shank screw and 119 cases of fixation using conical tapered screw.
Two studies [16, 21] concluded that there was no significant difference in the interfragmentary compression produced by Shank screw or conical tapered screw. One study [19] showed that shank screw produced higher interfragmentary compression compared to conical tapered screw whereas five studies [14, 15, 17, 18, 20] showed that conical tapered screw produced higher interfragmentary compression force at the simulated fracture site. Beadel et al. [13] compared two designs of conical tapered screw with a single shank screw and observed that one design of conical tapered screw produced higher interfragmentary compression compared to the shank screw but there was no difference in the compression produced by the second design of conical tapered screw and the shank screw.
The pooled estimate of all nine studies (Fig. 3) demonstrated that conical tapered screw produced significantly higher interfragmentary compression force compared to the shank screw (WMD 19.96, 95% CI: 11.2 to 28.8, p < 0.0001, I2 = 99%).
Fig. 3.
Depicts Forrest plot showing pooled estimate of studies comparing shank screw and conical tapered screw. H Herbert screw, HW Herbert Whipple screw, Bold 3 mm bold screw, HCS 3 mm headless compression screw, HBS headless bone screw, Acutrak std Acutrak standard screw
There was no evidence of publication bias because the funnel plot was symmetrical (Fig. 4), Begg Mazumdar test (p > 0.99) and Eggers test (p = 0.59) showed insignificant p-values for the intercepts.
Fig. 4.
Shows evidence of minimal publication bias because the funnel plot is near symmetrical
Comparison of Shank Screw and Dual Component Screw
The pooled data consisted of 77 cases of fixation using shank screw and 77 cases of fixation using dual component screw. Four studies [15, 18–20] compared interfragmentary compression force generated by shank screws and dual component screws. Three studies [15, 19, 20] concluded that dual component screws produced higher interfragmentary compression force. In the study by Assari et al. [18], TwinFix screw generated higher interfragmentary compression force compared to 3 mm HCS screw and Herbert Whipple screw whereas the Kompressor screw generated higher interfragmentary compression compared to Herbert Whipple screw but was almost equal to that of compression produced by 3 mm HCS screw.
The pooled estimate of four studies (Fig. 5) demonstrated that dual component screw produced significantly higher interfragmentary compression force compared to the shank screw (WMD 16.93, 95% CI 12.3–21.6, p < 0.0001, I2 = 97.7%).
Fig. 5.
Shows Forrest plot showing pooled estimate of studies comparing shank screw and dual component screw. H Herbert screw, HW Herbert Whipple screw, HCS 3 mm headless compression screw, HBS headless bone screw
There was no evidence of publication bias because the funnel plot was symmetrical (Fig. 6), Begg Mazumdar test (p = 0.29) and Eggers test (p = 0.10) showed insignificant p-values for the intercepts.
Fig. 6.
Shows Funnel plot for assessment of publication bias in studies comparing shank screw and dual component screw. There is almost symmetrical distribution around the funnel plot
Comparison of Conical Tapered Screw and Dual Component Screw
The pooled data consisted of 47 cases of fixation using conical tapered screw and 47 cases of fixation using dual component screw. Four studies [15, 18–20] compared interfragmentary compression force generated by conical tapered screw and dual component screws. Two studies [15, 20] demonstrated that no difference in the interfragmentary compression force generated by either conical tapered screw or dual component screws. One study [18] showed that conical tapered screw produced significantly higher interfragmentary compression compared to dual component screw whereas another study [19] showed that dual component screw produced higher compression force across simulated fracture site.
The pooled estimate of four studies (Fig. 7) showed that there was no significant difference in the interfragmentary compression force generated by either conical tapered screw or dual component screw (WMD 3.93, 95% CI − 8.3 to 16.2, p = 0.53, I2 = 99.7%).
Fig. 7.
Shows the pooled estimate of studies comparing interfragmentary compression force generated by conical tapered screw and dual component screw. TF TwinFix screw, K Kompressor screw, Acutrak std Acutrak standard screw
There was no evidence of publication bias because the funnel plot was symmetrical (Fig. 8), Begg Mazumdar test (p = 0.47) and Eggers test (p = 0.75) showed insignificant p-values for the intercepts.
Fig. 8.
Shows minimal publication bias as evidenced by a near symmetrical funnel plot
Sensitivity Analysis
The overall heterogeneity observed while comparing shank screw and the conical tapered screw in the combined cohort of cadaver bone and synthetic bone was very high (I2 = 99%). Four studies [13, 16, 17, 20] compared the interfragmentary compression of shank screw and conical tapered screw in cadavers. Performing sensitivity analysis in the cadaver group led to significant reduction of the heterogeneity (I2 = 46.7%). The pooled estimate of four studies showed that there was no significant difference in the interfragmentary compression force generated by either shank screw or conical tapered screw when the evaluation was done on cadavers (WMD 22.04, 95% CI − 0.23 to 44.3, p = 0.05). Five studies [14, 15, 18, 19, 21] compared the interfragmentary compression of shank screw and conical tapered screw in synthetic bones. Performing sensitivity analysis in the polyurethane foam group led to slight increase in the heterogeneity (I2 = 99.3%). The pooled estimate of five studies showed that conical tapered screw produced significantly higher interfragmentary compression force in simulated scaphoid fracture when the evaluation was done on synthetic bones (WMD 19.55, 95% CI 9.9–29.2, p < 0.0001).
The overall heterogeneity observed while comparing shank screw and the double component screw in the combined cohort of cadaver bone and synthetic bone was very high (I2 = 97.7%). One study [20] was on cadaver whereas three studies [15, 18, 19] were conducted on synthetic bone. As there was a single study in the cadaver cohort, the statistical software was unable to calculate heterogeneity and statistical significance of the cadaver cohort. Performing sensitivity analysis in the polyurethane foam group led to slight increase in the heterogeneity (I2 = 98%). The pooled estimate of three studies showed that double component screw produced significantly higher interfragmentary compression force compared to shank screw in simulated scaphoid fracture when the evaluation was done on synthetic bones (WMD 16.75, 95% CI 12.1–21.4, p < 0.0001).
The overall heterogeneity observed while comparing conical tapered screw and the double component screw in the combined cohort of cadaver bone and synthetic bone was very high (I2 = 99.7%). One study [20] was on cadaver whereas three studies [15, 18, 19] were conducted on synthetic bone. As there was a single study in the cadaver cohort, the statistical software was unable to calculate heterogeneity and statistical significance of the cadaver cohort. Performing sensitivity analysis in the polyurethane foam group led to no change in the heterogeneity (I2 = 99.7%). The pooled estimate of three studies showed that there was no difference in the interfragmentary compression force generated by either conical tapered screw or the double component screw when the evaluation was done on synthetic bones (WMD 3.49, 95% CI − 8.8 to 15.8, p = 0.58).
Discussion
Summary of Findings of Study
The notable findings of the present study were that both conical tapered screws and double component screws produced statistically significant higher interfragmentary compression force at the scaphoid fracture site compared to shank screws. Hence, our null hypothesis was disproved for comparison between conical tapered screw and shank screw and also for comparison between double component screw and shank screw. There was no difference in the compression force generated by either conical tapered screw or dual component screw thereby suggesting that this result was in accordance with the null hypothesis. The clinical significance of the difference of 19.96 N between the interfragmentary compression exerted by conical tapered screw and the shank screw is unknown. Similarly the clinical significance of the difference of 16.93 N between the interfragmentary compression exerted by double component screw and the shank screw is unknown.
Other Studies Comparing Conical Tapered and Shank Screws
Previous study has demonstrated that conical tapered screw have superior resistance to pull apart distraction force and superior resistance to bending force compared to shank screws [27]. Evidence from the present meta-analysis and considering the work by Hart et al. it can be inferred that conical tapered screws have better biomechanical properties compared to shank screw.
Assessment of Study Methodology
The Newcastle–Ottawa scale has been designed to evaluate study quality of nonrandomized, observational or retrospectives studies. We used the Newcastle Ottawa scale (NOS) to assess the methodological rigour of the included studies because the NOS has been used previously in different meta-analysis and systematic reviews of biomechanical studies studies [10, 32]. Presently there exists no validated checklist or standardised assessment method to evaluate the methodological rigour of biomechanical studies and hence we used the NOS.
Limitations of the Study
We acknowledge the following limitations of our study.
There was significant heterogeneity in all the three comparative analyses. Possible reasons for this high degree of heterogeneity include the use of different models (cadavers and polyurethane block) for testing interfragmentary compression force, use of models simulating different bone densities (0.16 gm/cm3 and 0.32 gm/cm3), use of different screw diameters in various studies, estimation of different types of interfragmentary compression force (some studies used mean final compression force whereas other studies used mean maximal or peak compression force), estimation of mean compression force at different positions of the trailing edge of the screw (some studies estimated at chondral position whereas other studies estimated at subchondral position and in some studies the exact position was not specified) and different time of estimation of compression force after the screw reached its final position (varying from soon after screw reached its final position, to five minutes after screw reached its final position). The gap at the simulated fracture site also varied (range: 0.5 mm to 5 mm) between the studies with no uniformity.
Difference in the Design and Dimensions of Various Screws
There is significant difference in the design of various headless compression screws that are used for fixation of scaphoid fractures. There is variation in the diameter of the leading and the trailing ends of the screws and the pitch design of the screws. Tables 6, 7 and 8 give a synopsis of design of shank screw, conical tapered screw and double component screws. The interfragmentary compression force also depends to the screw diameter which ranged from 2.4 to 5 mm. None of the papers included in the present meta-analysis have discussed the use of the 2.4 mm screw although it may be a commonly used screw. One probable explanation could be that the 2.4 mm screws were introduced later while the latest biomechanical study included in the present meta-analysis dates to 2013.
Table 6.
| Name of screw | Outer thread Diameter of leading edge of screw (mm) | Outer thread Diameter of trailing edge of screw (mm) | Shaft diameter (mm) | Pitch |
|---|---|---|---|---|
| Herbert screw | 3.0 | 3.9 | 1.75 | Varying pitch |
| Herbert whipple screw | 3.0 | 3.9 | 2.5 | Varying pitch |
| Bold screw 3.0 | 3.0 | 4.0 | 2.0 | Partial varying pitch |
| 3.0 HBS 2 Midi | 3.0 | 3.9 | 2.0 | Varying pitch |
| 2.4 mm HCS | 2.4 | 3.1 | 2.0 | Constant pitch |
| 3 mm HCS | 3.0 | 3.5 | 2.0 | Constant pitch |
Table 7.
Design features of conical tapered, titanium, fully threaded, self tapping and cannulated screws having continuously variable pitch between leading and trailing ends [33, 34]
| Name of screw | Outer thread diameter of leading edge of screw (mm) | Outer thread diameter of trailing edge of screw |
|---|---|---|
| Acutrak standard | 3.3 | 3.8–4.6 mm depending on screw length |
| Acutrak mini | 2.8 | 3.2–3.6 mm depending on screw length |
| Acutrak 2 standard | 4.0 | 4.1 mm |
| Acutrak 2 mini | 3.5 | 3.6 mm |
Table 8.
| Name of screw | Outer thread diameter of leading edge of screw (mm) | Outer thread diameter of trailing edge of screw (mm) |
|---|---|---|
| TwinFix 3.2 | 3.35 | 4.07 |
| Kompressor standard | 4.0 | 5.0 |
| Kompressor mini | 2.8 | 3.6 |
Difference Between Chondral and Subchondral Position
Flush position is considered to be reached when the tip of the trailing end of the headless compression screw is at the same level of the proximal surface of the bone or the polyurethane foam. Beyond flush position is considered to be reached when the tip of the trailing end of the headless compression screw is buried 2 mm below the surface of the bone. It is recommended that the trailing end of the headless compression screw must be 2 mm deeper than the chondral surface [24]. It is possible that the interfragmentary compression force exerted by the headless compression screws might be higher in the subchondral position as compared to the chondral position. A recent study [35] has found that the subchondral portion of the scaphoid has the highest bone density compared to other parts of the scaphoid and that any headless compression screw must engage the 2 mm subchondral shell of the scaphoid instead of remaining in the chondral position where fixation strength of the screw can get compromised.
Difference Between Cadavers and Polyurethane Blocks
Scaphoid fracture most commonly occurs in young adults whereas the mean age and range of the cadavers used was very much older and biomechanically inferior to that of young adult patient. Hence, it is probable that the interfragmentary compression force exerted by the headless compression screws could have been underestimated in cadaveric studies. Polyurethane blocks have uniform density and porosity whereas the human scaphoid has varying density and trabecular bone porosity with density of the proximal pole of the scaphoid being higher than the distal pole of the scaphoid [35]. This variation needs to be considered when future biomechanical studies are planned to compare the interfragmentary compression forces exerted by headless compression screws.
There was significant reduction of heterogeneity in the cadaver comparison cohort during the sensitivity analysis of shank screw and conical tapered screw but there was no reduction of heterogeneity in the synthetic bone comparison cohort. There was no reduction of heterogeneity on sub-analysis and sub-comparison of cadaver comparison cohort and synthetic bone cohort during evaluation of interfragmentary compression in shank screw versus double component screw and conical tapered screw versus double component screw.
Gap at the Fracture Site
Tan et al. [31] observed that the Acutrak 2 mini screw produce compression of fracture gaps less than one mm but if the fracture gap was more than one mm then there was a significant reduction in the interfragmentary compression force generated by the screw. The effect of gap at the fracture site on the interfragmentary compression force produced by other types of headless screws also needs to be investigated as this could be a confounding factor.
Bone Density
The density of polyurethane block of 0.16 gm/cm3, 0.24 gm/cm3 and 0.32 gm/cm3 represents osteoporotic, osteopenic and normal bone of a young adult patient respectively [27, 36]. Scaphoid fractures occur most commonly in young adults whose bone density is around 0.32 gm/cm3. The bone density for polyurethane block that is equivalent to the cancellous bone such as scaphoid is 0.24 gm/cm3 [37]. Most biomechanical studies have investigated in polyurethane blocks with densities representing osteoporotic bones and hence this could probably have underestimated the interfragmentary compression force exerted by the headless compression screws.
Simulated Fracture Pattern
Another limitation of the study is that all the simulated fractures were transverse fractures and hence the generalizability of the results of the present study are limited only to transverse scaphoid fractures. None of the biomechanical studies have evaluated interfragmentary screw compression in oblique fractures of the scaphoid or in comminuted fractures of the scaphoid. The most commonly observed pattern in scaphoid waist fracture is the horizontal oblique fracture [38] and this pattern has not been investigated in any of the studies that are included in the present meta-analysis.
Loss of Compression Over Time
A Study [39] has shown that in polyurethane saw bone models, the interfragmentary compression force generated by the screws tends to reduce over a period of time due to the biomechanical property of viscoelasticity. There is no consensus regarding the ideal time to evaluate the interfragmentary compression force after insertion of the headless screw.
Maximal or Peak Interfragmentary Compression Force or Final Compression Force.
Hart et al. [21] have successfully argued that mean maximal interfragmentary compression force might not be a valid surrogate measure because the maximal or peak interfragmentary compression occurs either when the trailing end of the screw is either above the tip of the cortex of the bone or when the trailing end of the screw is extremely below the tip of the cortex. Neither of the above two positions are clinically acceptable during fixation of scaphoid fractures. Hart et al. hence have justified the use of mean final interfragmentary compression force at the final clinically acceptable position of the screw inside the scaphoid bone. Hence, in the statistical analysis of our meta-analysis we included the mean interfragmentary compression at the simulated fracture site instead of maximal interfragmentary position because during surgery the screw is put till it reaches the beyond the flush position.
Strengths of the Study
To the best of our knowledge, this is the first meta-analysis comparing in vitro interfragmentary compression force exerted by headless compression screws in simulated scaphoid waist fracture.
We chose to omit biomechanical studies on scaphoid non-union because the treatment of scaphoid non-union with an interpositional bone graft cannot be compared with an acute scaphoid fracture because in treatment of non-union with a bone graft the screw has to apply compression on two interfaces (first between proximal part of scaphoid and the bone graft and the second interface between the bone graft and the distal part of the scaphoid) whereas in acute scaphoid fracture there is only a single interface [39]. Hence, results of biomechanical studies of acute scaphoid fracture cannot be applied to biomechanical studies on fixation of scaphoid non-unions and vice versa. Staples are used nowadays for fixation of scaphoid fracture [40] and this device was not included in our meta-analysis because we chose to study only headless compression screws.
Conclusion
The present meta-analysis of biomechanical studies concludes than conical tapered screws and double component screws exert significantly higher interfragmentary compression force compared to shank screws. Conical tapered screws are as effective as double component screws in terms of the ability to generate interfragmentary compression force. But the results should be interpreted with caution as the results are from in vitro studies and the behaviour of these headless compression screws might be different in vivo conditions. Also it is still unknown as to what would constitute an ideal interfragmentary compression force for healing of scaphoid fracture treated with headless compression screws.
Author contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by KV and RP The first draft of the manuscript was written by KV and RP. All authors commented on the previous versions of the manuscript. All authors read and approved the final version of the manuscript.
Compliance with Ethical Standards
Conflict of interest
Karthik Vishwanathan, Ravi Patel and Sumedh Talwalkar declare that they have no conflict of interest.
Ethical standard statement
The present study was granted exempt review by the Institutional Ethics Committee of Charutar Arogya Mandal (HM Patel Centre for Medical care and Education) because the meta-analysis consisted of review of published material that was already in public domain and did not involve any direct or indirect participant involvement. The approval numbers for this study were IEC/HMPCMCE/2017/Ex.21/87/17, IEC/HMPCMCE/2017/Ex.23/90/17 and IEC/HMPCMCE/2017/Ex.24/88/17.
Informed consent
For this type of study informed consent is not required.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Karthik Vishwanathan, Email: karthik.vishwanathan@paruluniversity.ac.in, Email: karthik_vishwanathan@yahoo.com.
Ravi Patel, Email: docrsp23@gmail.com.
Sumedh Talwalkar, Email: sctalwalkar@gmail.com.
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