title: Optimal trajectory for the occipital condylar screw.
abstract: METHODS
  Evaluation by simulation of screw insertion using fine-cut computed tomographic (CT) scans and screw trajectory software.
  
  
  OBJECTIVE
  To evaluate the feasibility and optimal trajectory of the occipital condylar screw.
  
  
  BACKGROUND
  To the best of our knowledge, no large series examining the feasibility and optimal trajectory of occipital condylar screws have been published.
  
  
  METHODS
  We simulated unicortical placement of a 4 × 18-mm screw using 1-mm sliced CT scans and 3-dimensional screw trajectory software in 314 occipital condyles of 157 patients. With the screw tip directed toward a point just below the tip of the basion on lateral fluoroscopy, 3 entry points were compared in view of success rate of screw placement, safe range of medial angulation in an axial plane, and maximum screw length.
  
  
  RESULTS
  The lateral entry point had the highest success rate (93.0%) of screw placement, mean safe range of the medial angulation (10.9°), and maximum screw length (20.7 mm), followed by the middle (92.0%, 10.8°, and 20.6 mm, respectively) and medial (74.2%, 7.7°, and 19.7 mm, respectively) entry points. While the lateral and the middle entry points did not have any statistically significant difference in the 3 parameters, they were significantly better than the medial entry point. All 3 entry points had highly variable ranges of safe medial angulation, making it difficult to provide a single recommended value.
  
  
  CONCLUSIONS
  Screw placement is feasible in up to 93% of the occipital condyles. The lateral and the middle entry points are significantly better than the medial entry point. Selection between the middle and the lateral entry points should be individualized taking into account local anatomic variation. Because the medial angulation of these screws is highly variable, preoperative 3-dimensional CT evaluation and possibly even intraoperative navigation may be required.
authors.affiliation: Department of Orthopaedic Surgery, Seoul National University College of Medicine and Seoul National University Hospital, Seoul, Korea.
authors.forename: Joon O
authors.initials: JO
authors.lastname: Lee
authors.forename: Jacob M
authors.initials: JM
authors.lastname: Buchowski
authors.forename: Kyoung M
authors.initials: KM
authors.lastname: Lee
authors.forename: Kun-Woo
authors.initials: KW
authors.lastname: Park
authors.forename: Bong-Soon
authors.initials: BS
authors.lastname: Chang
authors.forename: Choon-Ki
authors.initials: CK
authors.lastname: Lee
authors.forename: K Daniel
authors.initials: KD
authors.lastname: Riew
authors.forename: Jin S
authors.initials: JS
authors.lastname: Yeom
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001269:Atlanto-Occipital Joint / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D001863:Bone Screws / Q000592:standards*; D005260:Female; D005471:Fluoroscopy / Q000379:methods; D006801:Humans; D007593:Joint Instability / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009777:Occipital Bone / Q000601:surgery*; D011300:Preoperative Care / Q000379:methods*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods; D055815:Young Adult