The primary literature search showed 644 potential studies from the searched databases, and 20 additional records were identified through other sources. 620 abstracts were screened, and 77 full-text articles were evaluated after removal of irrelevant abstracts. After applying the exclusion criteria, 27 full-text articles were found to be eligible for inclusion in this systematic review and meta-analysis. See Fig. 1.
The 27 selected studies included a total of 1890 patients, with 940 patients in the 4-screw construct group and 950 patients in the 6-screw construct group. Twenty studies were randomized controlled trials and 7 retrospective cohort studies. In three studies a percutaneous surgical technique was used and in 25 studies an open surgical technique.
Quality of the evidence assessment
A risk of bias summary for the included RCTs is presented in Fig. 2. Quality rating of the evidence of each outcome according to GRADE is presented in Table 1.
Fifteen studies [1, 14, 15, 21,22,23,24,25,26,27,28,29,30,31,32] provide information on the operation duration (928 patients). Combining these results shows that the placement of intermediate screws prolongs the operation time significantly with a mean difference of 5.52 min (95% CI 0.79–10.26, P = 0.02, I2 = 87%) (Fig. 3).
Intraoperative blood loss
Sixteen studies [1, 14, 15, 21,22,23,24,25,26,27,28,29,30,31,32,33] including 988 patients described the amount of blood loss during surgery. The pooled analysis of these results showed a mean difference of 25.30 mL of intraoperative blood loss, favouring the 4-screw construct significantly (95% CI 6.91–44.20, P < 0.01, I2 = 96%) (Fig. 4).
Hospital length of stay
Information on hospital length of stay was provided in four studies [14, 15, 21, 26] (255 patients). The duration of hospital length of stay ranged from 8.7–13 days in the 6-screw group and 9–12.2 days in the 4-screw construct group. The mean difference regarding the hospital LOS was not significant (MD 0.44, 95% CI -0.68 to 1.57, P = 0.44, I2 = 50%) (Fig. 5).
Visual analog scale
Information on the VAS is provided in nine of the included studies [1, 14, 21,22,23, 27, 31, 34, 35] describing 501 patients. The VAS in these studies was rated at follow-up at least three months after surgery. The pooled results showed that the 6-screw construct reduces the VAS significantly with a mean difference of 0.64 points (95% CI − 1.08 to − 0.19, P < 0.01, I2 = 93%) (Fig. 6).
Oswestry disability index
Information about ODI was extracted from five studies including 273 patients [14, 22, 23, 26, 31]. The ODI was determined at follow-up at more than one-year post-operative. The pooled results show no significant difference between the 6-screw and 4-screw construct group (MD − 0.19, 95% CI − 1.52 to 1.14, P = 0.78, I2 = 41%) (Fig. 7).
Short-term post-operative Cobb angle
Fourteen studies [1, 15, 22,23,24, 27,28,29, 31, 32, 36,37,38] compared the short-term post-operative Cobb angle between the 4-screw and 6-screw construct group (713 patients). In these studies, the short-term post-operative Cobb angle was measured between 1 week and 1 month post-operative. Patients in the 6-screw construct group had significantly better short-term post-operative Cobb angles with a mean difference of 1.07° (95% CI −1.82 to −0.32, P < 0.01, I2 = 85%) (Fig. 8).
Long-term post-operative Cobb angle
Post-operative Cobb angle measured at follow-up after one year has been described in thirteen studies (766 patients) [1, 21,22,23, 26,27,28, 30, 31, 34, 36,37,38]. The 6-screw construct showed significantly better results regarding the long-term post-operative Cobb angle with a mean difference of 3.69° (95% CI − 5.20 to − 2,18, P < 0.01, I2 = 90%) (Fig. 9).
Correction loss of Cobb angle
Seven of the included studies [1, 15, 22, 23, 25, 32, 38] provide information on the correction loss of Cobb angle in the 6-screw and 4-screw construct group (351 patients). The 6-screw method leads to a significantly lower correction loss of Cobb angle (MD − 1.95, 95% CI − 3.10 to − 0.79, P < 0.01, I2 = 87%) (Fig. 10).
Correction loss of anterior vertebral body height
Twelve studies [14, 22, 23, 25, 26, 32, 33, 38,39,40,41,42] describe values for the correction loss of anterior vertebral height of the 4- and 6-screw construct groups (862 patients). A meta-analysis of these values shows that the 6-screw construct leads to a significantly lower correction loss of the AVBH than the 4-screw method with a mean difference of 4.36 mm (95% CI − 6.56 to − 2.16, P < 0.01, I2 = 98%) (Fig. 11).
Twenty studies [1, 14, 15, 21,22,23,24,25,26,27,28,29,30, 32,33,34, 39, 40, 42, 43] describe rates of post-operative implant failure (1514 patients). The pooled results show that a significantly lower rate of implant failure was seen in the 6-screw construct group with an OR of 0.26 (95% CI 0.15–0.47, P < 0.01, I2 = 0%) (Fig. 12).
Information on post-operative infections is provided in twelve studies [1, 14, 15, 21,22,23,24,25,26,27, 31, 43] (765 patients). Fixed-effect analysis showed no significant difference between the groups concerning infections post-operatively (OR 0.66, 95% CI 0.21 to 2.07, P = 0.47, I2 = 0%) (Fig. 13).
Three of the included studies used a percutaneous technique for the pedicle screw fixation and 24 studies applied an open surgical technique, including 226 and 1654 patients, respectively. A subgroup analysis was performed to analyse the outcomes for the open and percutaneous surgical technique separately. For the open surgical technique, the pooled data on the VAS, short-term and long-term post-operative Cobb angle, correction loss of Cobb angle and AVBH and implant failure all showed significant results favouring the 6-screw construct group. A significantly higher blood loss, but no significantly longer operation time was seen in the 6-screw construct with the open technique. For the percutaneous technique, a significantly longer operation time and significantly better results of long-term Cobb angle and correction loss of AVBH were seen in the 6-screw group compared to the 4-screw group. For the percutaneous technique, the 6-screw construct did not result in an increase in blood loss compared to the 4-screw construct. For the outcomes ODI and post-operative infection, the pooled analysis showed no significant difference between the 6-screw and 4-screw construct group for both percutaneous and open techniques. No studies describing the hospital length of stay made use of the percutaneous surgical technique.