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New classification of S1 pedicle morphometry impacting pedicle screw insertion technique

Background: The conventional entry point for the S1 pedicle screw insertion has been described as ‘at the base of and lateral to the superior S1 superior facet’. However, many orthopaedic surgeons complain that this technique is extremely demanding and is faced with many challenges in execution. Therefore, alternative entry points and modifications of existing techniques are explored for a greater convenience.Methods: We conducted a survey amongst 136 spine surgeons on the technical difficulties faced during insertion of the S1 pedicle screw. We also classified the S1 pedicles based by measuring their geometric parameters on 100 random computed tomography (CT) images.Results: The S1 pedicle entry technique was considered to be the most difficult and challenging by the orthopaedic surgeons in our survey due to an inadequate medial angulation due to paraspinal muscle mass tension and an overhanging iliac crest. This could be explained by the hourglass shaped pedicle (type 3) with a desired medial angle more 50 degrees (type III) observed as the most common S1 pedicle in the study population. To overcome this limitation, most of them preferred a tri-corticate approach.Conclusions: A more lateral entry point and a longer screw might be considered as solutions for a better and safer S1 pedicle entry. Also, the use of pre-operative CT can be considered to visualize the type of S1 pedicle and hence, decide on the most appropriate technique of screw insertion. 