title: Percutaneous screw fixation of the iliosacral joint: optimal screw pathways are frequently not completely intraosseous.
abstract: BACKGROUND
  In iliosacral screw fixation, the dimensions of solely intraosseous (secure) pathways, perpendicular to the ilio-sacral articulation (optimal) with corresponding entry (EP) and aiming points (AP) on lateral fluoroscopic projections, and the factors (demographic, anatomic) influencing these have not yet been described.
  
  
  METHODS
  In 100 CTs of normal pelvises, the height and width of the secure and optimal pathways were measured on axial and coronal views bilaterally (total measurements: n=200). Corresponding EP and AP were defined as either the location of the screw head or tip at the crossing of lateral innominate bones' cortices (EP) and sacral midlines (AP) within the centre of the pathway, respectively. EP and AP were transferred to the sagittal pelvic view using a coordinate system with the zero-point in the centre of the posterior cortex of the S1 vertebral body (x-axis parallel to upper S1 endplate). Distances are expressed in relation to the anteroposterior distance of the S1 upper endplate (in %). The influence of demographic (age, gender, side) and/or anatomic (PIA=pelvic incidence angle; TCA=transversal curvature angle, PID-Index=pelvic incidence distance-index; USW=unilateral sacral width-index) parameters on pathway dimensions and positions of EP and AP were assessed (multivariate analysis).
  
  
  RESULTS
  The width, height or both factors of the pathways were at least 7mm or more in 32% and 53% or 20%, respectively. The EP was on average 14±24% behind the centre of the posterior S1 cortex and 41±14% below it. The AP was on average 53±7% in the front of the centre of the posterior S1 cortex and 11±7% above it. PIA influenced the width, TCA, PID-Index the height of the pathways. PIA, PID-Index, and USW-Index significantly influenced EP and AP. Age, gender, and TCA significantly influenced EP.
  
  
  CONCLUSIONS
  Secure and optimal placement of screws of at least 7mm in diameter will be unfeasible in the majority of patients. Thoughtful preoperative planning of screw placement on CT scans is advisable to identify secure pathways with an optimal direction. For this purpose, the presented methodology of determining and transferring EPs and APs of corresponding pathways to the sagittal pelvic view using a coordinate system may be useful.
authors.affiliation: Department of Orthopaedic and Trauma Surgery, University of Bern, Inselspital, Freiburgstrasse 3, 3010 Bern, Switzerland. Electronic address: johannes.bastian@insel.ch.
authors.forename: J D
authors.initials: JD
authors.lastname: Bastian
authors.affiliation: Department of Orthopaedic and Trauma Surgery, University of Bern, Inselspital, Freiburgstrasse 3, 3010 Bern, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Jost
authors.affiliation: Institute for Diagnostic, Interventional and Paediatric Radiology, University of Bern, Inselspital, Freiburgstrasse 3, 3010 Bern, Switzerland.
authors.forename: J L
authors.initials: JL
authors.lastname: Cullmann
authors.affiliation: Institute for Evaluative Research in Medicine, University of Bern, Stauffacherstrasse 78, 3014 Bern, Switzerland.
authors.forename: E
authors.initials: E
authors.lastname: Aghayev
authors.affiliation: Department of Orthopaedic and Trauma Surgery, University of Bern, Inselspital, Freiburgstrasse 3, 3010 Bern, Switzerland.
authors.forename: M J B
authors.initials: MJ
authors.lastname: Keel
authors.affiliation: Department of Orthopaedic and Trauma Surgery, University of Bern, Inselspital, Freiburgstrasse 3, 3010 Bern, Switzerland.
authors.forename: L M
authors.initials: LM
authors.lastname: Benneker
mesh_terms: D000328:Adult; D001863:Bone Screws; D001864:Bone Wires; D005471:Fluoroscopy; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods; D050723:Fractures, Bone / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D007085:Ilium / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000601:surgery*; D010384:Pelvic Bones / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000293:injuries / Q000601:surgery*; D017410:Practice Guidelines as Topic; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012447:Sacrum / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000601:surgery*; D014057:Tomography, X-Ray Computed
keywords: Aiming point; Entry point; Iliosacral screw; Lateral projection; Optimal; Pathway; Placement; Secure