Background: The S2 alar-iliac (S2AI) screw is useful as a caudal anchor for lumbosacral fusion. In our clinic, we use MySpine (Medacta International SA, Strada Regina, Switzerland), a patient-specific three-dimensional (3D) guide that allows safe and accurate S2AI screw insertion.

Objective: The objective of this study was to verify the rate of error between the preoperative plan and the actual screw used for insertion with MySpine.

Subjects and Methods: Ten patients underwent S2AI screw insertion using MySpine for treatment of degenerative spinal diseases between September 2020 and August 2021 at our hospital; of these, eight cases were included in the study because we were able to compare them with the superimposing method that uses computed tomography (CT) data taken before and after the surgery. Mimics (Materialise, Leuven, Belgium) and SOLIDWORKS (Dassault Systems, Vélizy-Villacoublay, France) were used to reconstruct images and create 3D models from CT data. The error between the preoperative plan and the trajectory executed along with the final inserted screw was measured by superimposing a model of the sacrum created from pre- and postoperative CT data over the body surface.

Results: All 16 S2AI screws in the eight cases were inserted in the optimal position, and no deviations outside the iliac bone were observed. The mean errors between the insertion points and the superimposed image were 1.65±1.44 mm cephalad and 1.68±1.11 mm medial and lateral, and the mean errors of insertion angles were 2.07±1.19° in the sagittal plane and 2.00±1.26° in the transverse plane. No difference was found between the right and left sides in any of the items (Student’s t-test, P>0.05). Using the above results, we simulated screw insertion using mean + 1 standard deviation from planning in each direction in each case, but none of the screws deviated outside the iliac bone.

Discussions: For the insertion of S2AI screw, several methods are applied. Freehand technique or insertion with fluoroscopy is the simple method based on anatomical features. In these days, navigation systems or robotics are applied for this technique to decrease the risks of malposition, but it needs complex systems and many costs. The rate of S2AI screw deviation outside the iliac bone has been reported to be 2.2 to 20%, and this rate is lower when using a navigation system or with robotics than when using a freehand or fluoroscopy alone. In the present study, accurate insertion of the S2AI screw was achieved by using a patient-specific 3D guide created from preoperative CT data. The insertion error was small with this method, and we believe that safe and accurate insertion of the S2AI screw is possible using patient-specific 3D guides.

Fig.1 preoperative planned screws (purple screws) and actually inserted screws (red screws)