2.1 Eligibility criteria

Only randomized control trials (RCT) and prospective/retrospective studies comparing metal screw fixation to either dynamic and/or bioabsorbable screw fixation were eligible for inclusion. Studies written in languages other than English were excluded, except for two comparison studies written in Chinese. Included studies were limited to publication dates ranging between January 1998 and December 2018. The studies were screened and selected based on the PRISMA 2009 Statement (see Fig. 1).¹⁰ No biomechanical studies were included. Each group outcome measurement included comparisons between time to full weight bearing, functional score, postoperative plain film tibiofibular clear spaces (TFCS), postoperative tibiofibular overlap (TFO), postoperative medial clear spaces (MCS), revisions, and complications. Each study also needed to report the number of patients in each group. Data was extracted and put into Microsoft Excel (Microsoft, Seattle) by the senior author. The US Preventive Task Force Quality Rating Criteria was used to select the 18 included studies.¹¹

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Fig. 1 Systematic Review Flow Chart: Shows the screening process, removing of papers based upon eligibility, and inclusion criteria to get final number of papers.

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2.2 Search strategy

PubMed and Google Scholar were used to search for RCTs and prospective/retrospective studies. Search terms such as “metal screw syndesmosis”, “suture button syndesmosis”, and “absorbable screw syndesmosis” were used initially to find a wide variety of studies ranging from biomechanical tests to RCTs. Further in the search process, terms such as “suture button versus metal screw fixation syndesmosis” and “metal screw fixation versus bioabsorbable fixation syndesmosis” were additionally used to narrow search results to studies with sufficient information on different fixation methods. Only clinical RCTs and prospective/retrospective studies that compared metal screw fixation to the dynamic fixation method or the bioabsorbable screw fixation method were kept for the purposes of this meta-analysis. While systematic reviews and meta-analyses were excluded during the search process, some of the studies in their references were included as well.

2.3 Statistical analysis

For analysis of functional scores, such as AOFAS, Olerud-Molander, and Time to Full Weight Bearing, an ANOVA Single Factor test was used to compare the results from each group. The ANOVA Single Factor test was also used to compare the postoperative TFCSs and TFOs. However, a two-sample t-test with unequal variance was used to compare the postoperative MCSs because none of the included bioabsorbable fixation studies reported postoperative MCS values. For statistics that were percentage based, such as complication rates, revision rates, and Baird scoring system (excellent and good percentages), a Chi-squared test was performed to test if a significant difference existed among the outcomes of the three groups. All statistical analysis was performed in Microsoft Excel.

Data for the forest plots were compiled from various studies and analyzed with RevMan 5.3 (Cochrane, Spokane, WA). For these analyses, several studies were further excluded due to missing statistical metrics such as standard deviation. Due to the methodological variability among the selected articles, a random effects model was employed because of its conservative nature relative to other models.

3.1 General information

A total of 18 studies were ultimately included. Twelve studies compared metal screw to dynamic fixation and 6 compared metal screw to bioabsorbable screw fixation. The level of evidence and the year of the study published was recorded (Table 1). There were a total of 509 patients in the metal screw fixation group, 275 in the dynamic fixation group, and 226 in the bioabsorbable screw fixation group. The mean age was 39.7 years for the metal screw fixation group, 41.0 years for the dynamic fixation group, and 43.0 years for the bioabsorbable screw fixation group (Table 1).

Only 7 out of the 18 included studies reported mechanisms of injury; each mechanism of injury was totaled for each fixation group. During the data collection process, injuries due to sports/dance, falling from height, trip/slip and fall from ground level, slipping on ice, traffic accidents, being at home, being at work, being in social environments, and other mechanisms were reported. Between the three fixation groups a total of 132 injuries were due to sports/dance, 58 due to falling from a height, 41 due to tripping/slipping and falling, 8 due to slipping on ice, 127 due to traffic accidents, 4 due to home related activities, 1 due to work related activities, and 16 due to social activities, and 1 due to other mechanisms.

Several methods have been used to classify the syndesmotic ankle injury. The reported fractures were organized into the Fracture Classifications table and put into one of three groups: AO/OTA fracture classification, AO fracture classification, and other additional fracture classifications (Table 2).

There were 4 studies that used the AO/OTA fracture classification. This classification method contains 6 different types of fractures: 44 B-1, 44-B2, 44-B3, 44-C1, 44-C2, 44-C3. There were a total of 274 fractures in this classification method. There were 43 total 44 B-1 fractures, 52 total 44-B2 fractures, 30 total 44-B3 fractures, 69 total 44-C1 fractures, 39 total 44-C2 fractures, and 41 total 44-C3 fractures.

There were 2 studies that classified ankle injuries into unimalleolar, bimalleolar and trimalleolar fracture groups. There were 27 total lateral malleolar fractures, 26 total bimalleolar fractures, 12 lateral malleolar/bimalleolar fractures, and 17 trimalleolar fractures. The separate classification for lateral malleolar/bimalleolar fractures was done because the study by Sinisaari et al.²² reported the fractures as a combined group rather than reporting the fractures separately.

There were 4 studies that used a Weber classification method. There were a total of 116 Wb fractures found in the studies. There were 21 total Weber Type B fractures and 95 total Weber Type C fractures.

There were 3 studies that used a Maisonneuve Fracture or Soft Tissue Injury classification. There were a total of 63 fractures in this category, with 48 total Maisonneuve fractures, 15 total soft tissue injuries.

There were 5 studies that used the Lauge-Hansen classification method. There were a total of 167 injuries that fell under the Lauge-Hansen classification. These subtypes include: SER, SER 3, SER 4, SA 1, SA 2, PER, PER 4, PER 3, PA 3, PA 2, PA. There were 15 total SER fractures, 4 total SER 3 fractures, 18 total SER 4 fractures, 1 total SA 1 fracture, 1 total SA 2 fracture, 31 total PER fractures, 47 total PER 4 fractures, 19 total PER 3 fractures, 3 total PA 3 fractures, 1 total PA 2 fractures, and 27 total PA fractures.

3.2 Functional and radiological outcomes

Several methods have been used to evaluate the outcomes of patients treated after syndesmotic ankle injury. As shown in Functional Outcomes table (Table 3), the mean functional scores, time to weightbearing, and other outcome measures were reported for each fixation method.

3.3 Complications

During the data collection process, the complications that were reported include screw loosening, screw breakage, malreduction, infection, diastasis, syndesmotic ossification, wound complications, and hardware irritation. The complication rate was calculated for each fixation group by dividing the total amount of complications by the total number of patients in each group. For the metal screw fixation group, 94 patients out of 509 patients had a total of 99 complications. For the dynamic fixation group, 18 patients out of 275 patients had a total of 25 complications. For the bioabsorbable fixation group, 41 patients out of 226 patients had a total of 43 complications (Table 4). Patients with syndesmotic ankle injuries treated with a metal (0.19) or bioabsorbable (0.19) screw were found to have a higher complication rate than that of patients treated with the dynamic fixation method (0.09).

In the forest plot meta-analysis, the suture-button group had a significantly lower risk of post-operative complications compared to the metal screw [RR 0.19, 95% CI 0.10–0.35, P < 0.00001; Fig. 2]. The absorbable screw also carries a significantly lower complication rate when compared to metal screw fixation [RR 0.39, 95% CI 0.20–0.74, P < 0.004; Fig. 3], although one study by Kaukonen et al.²³ found a slightly higher complication rate in the absorbable screw group. Overall, the selected studies demonstrate a significantly higher risk of post-operative complications in the metal screw group compared to suture-button or absorbable screw.

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Fig. 2 Forest plot of complication rate between suture-button and metal screw.

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Fig. 3 Forest plot of complication rate between absorbable screw and metal screw.

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In various studies, revisions were cited due to improper fixation, failed implants, and loss of reduction, among many others. The amount of revisions found in each study were totaled according to each fixation group. There were 10 revisions found in patients treated with the metal screw fixation, 1 revision in patients treated with the dynamic fixation, and 0 revisions found in patients treated with the bioabsorbable fixation. There was no significant difference in the number of revisions among the three fixation methods for syndesmotic ankle injury (p = 0.13).

The tibia-fibula diastasis within each fixation group was collected except for the dynamic fixation group as none of the included studies reported diastasis for dynamic fixation. For the metal screw fixation group, there were 9 cases of reported diastasis. For the bioabsorbable fixation group, there were 2 cases of reported diastasis. There was no statistically significant difference between the number of cases with diastasis in each fixation group (p > 0.05).

The number of loose screws and broken screws were reported and compared between each of the three groups. The metal screw group had 18 loose screws and 30 broken screws total, both of which showed a statistically significant difference (both p < 0.05).

Both deep and superficial infection rates were reported. There were 5, 1, and 4 deep infections in the metal screw, dynamic, and bioabsorbable screw fixation groups, respectively. No statistically significant difference was found (p = 0.289). There were 8, 3, and 1, superficial infections in the metal screw, dynamic and bioabsorbable screw fixation groups, respectively, with no statistically significant difference (p > 0.05).