This meta-analysis included eight RCTs published up to November 2011, including a total of 2033 participants who received VSC or SAC. VSC showed improved cecal intubation rate, as well as decreased position change, but no evidence of advantages in cecal intubation time, sedation dose and manual pressure used. Outcomes were also analyzed in two subgroups based on the type of instruments (pediatric VSC and adult VSC) to evaluate cecal intubation rate and time. By contrast, adult VSC shortened the time to achieve cecal intubation compared with SAC, which was the only difference compared with the results considering all groups together.
In the comparison of VSC with SAC, none of the individual studies had shown an advantage in terms of the frequency of cecal intubation, but the pooled data slightly favored VSC. The increased sample size could be the most significant reason explaining the difference in cecal intubation rates. Large numbers of participants reduced the sampling error, which influenced the significance of the difference of the cecal intubation rates between VSC and SAC. This result is meaningful in clinical practice since, as we know, the failure rate for cecal intubation remains high with the use of SAC, so that this part of the anatomy does not receive clear and early diagnosis and treatment. VSC increased the intubation rate, which contributes to an early and accurate diagnosis.
The individual studies included in this meta-analysis had yielded somewhat conflicting data on cecal intubation time. Five trials found no significant difference in the time to cecal intubation, whereas the other 3 reported significantly shortened time to reach the cecum with VSC. The pooled results with all trials showed no significance; however, subgroup analysis reported shortened time with adult VSC.
Compared with the meta-analysis performed by M. O. Othman et al.
, our results showed differences in sedation dose. In the subgroup analysis of our meta-analysis, the sedation dose used during colonoscopy was similar between VSC and SAC, but had been reduced with the use of VSC in the previous analysis
. In addition, we did not pool the data of pain scores for patients due to the differences in the scale.
Several other individual publications have reported discrepant results with the use of VSC and SAC. For example, Odori et al.
 reported a prospective RCT of two prototypes of instruments in 352 consecutive cases and found that the cecal intubation time was significantly shorter with the use of VSC. VSC also reduced the need for abdominal pressure and position changes. Rex et al.
 evaluated the cecal intubation time in a cohort study of 358 consecutive sedated participants amongst VSC, pediatric VSC and SAC. No significant difference was found in the time to reach the cecum. Kaffes et al.
 found no evidence of difference in cecal intubation time between the two colonoscopic instruments in a nonrandomized trial of 803 participants.
In addition, different methods of activating the variable-stiffness function were used in the included studies and might explain part of the variability. In 2003, Ginsberg
 described a ‘standard’ technique for using the VSC: the colonoscopy is started with the ‘minimum’ or ‘soft’ control ring (dial setting 0) until looping occurs or the sigmoid colon is traversed. Then, the users reduce the loop, straighten the colonoscope and increase the stiffness control to the ‘hard’ position (dial setting 3). If a loop forms again, the stiffness dial is turned to the ‘soft’ position and the process above is repeated. Horiuchi et al.
, Sorbi et al.
 and Sola-Vera J et al.
 used the same approach described by Ginsberg. Yoshikawa et al.
 adopted a similar technique to Ginsberg’s, and applied abdominal pressure when activation of the maximum setting (dial setting 3) failed to advance the colonoscope. Al-Shurieki et al.
 made a slight modification, using the stiffening feature intermittently. When significant looping occurred, dial setting 2 was employed initially,and then stiffening to dial setting 3 was applied if the second setting failed. Lee et al.
 began at default dial setting 0 and activated the stiffer modes (dial settings 2 or 3) if looping was encountered. Shumaker et al.
 activated the maximum stiffness mode when the colonoscope was inserted to 30 cm from the anus and left the stiffness setting to full ‘on’ position until the cecum was achieved. The stiffness mechanism was deactivated during withdrawal. In almost all the studies reviewed above, the variable stiffness function was activated when looping was encountered. Furthermore, Shah et al.
 performed an RCT to evaluate the effect of routinely stiffening the straightened VSC after traversing the sigmoid colon, finding that with the stiffening function activated, the time needed to negotiate the proximal colon and splenic flexure shortened and ancillary maneuvers were reduced. These results may reflect another way in which to use the VSC in clinical practice and may explain the differences in cecal intubation time in comparison with SAC.
There were no scope-related complications reported in the studies included in this meta-analysis. To this point, no safety concerns have been raised with the use of VSCs. However, a single case report draws a possible association between the use of pediatric VSC and a sigmoid perforation, and only the distal descending colon was reached in a patient with a fixed and angulated sigmoid colon
. During the procedure, precise judgment and caution must be used, especially when advancing through a narrowed colon or pushing through loops.
A potential limitation of this meta-analysis is that these studies could not be performed to ‘blind’ the endoscopists to the nature of the interventions. Additionally, different models and manufacturers of VSC were used in the studies included. Furthermore, indications for activating the variable stiffness function did not follow the same criteria. There was no universal method for using VSC across the studies and across large tertiary centers, which may limit generalization to other practice settings. Finally, in several studies, specific patient subsets, such as colonic cancer and prior colonic surgery, were excluded.