Literature search
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.
Study characteristics
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.
Table 1 Summary of evidence graded using the GRADE approach for the included RCTs Outcome parameters
Operation time
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).
Implant failure
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).
Post-operative infection
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).
Subgroup analysis
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.