What is the safe corridor for anterior column screw placement?

Safe Corridor for Anterior Column Screw Placement

The safe corridor for anterior column screw placement requires a trajectory with a mean posterior angle of 28° to the coronal plane and a mean medial angle of 21.6° to the sagittal plane, with screw insertion beginning lateral to the tuberculum pubicum at the superior-medial margin of the obturator foramen. 1

Anatomical Considerations

Entry Point and Trajectory

• Entry point location:

  • In males: Lateral to tuberculum pubicum at the level of superior-medial margin of foramen obturatum
  • In females: Located significantly more lateral to symphysis and closer to cranial margin of superior pubic ramus 2

• Optimal screw trajectory angles:

  • 31.6° ± 5.5° between screw trajectory and anterior pelvic plane in sagittal section
  • 55.9° ± 4.6° between screw trajectory and midsagittal plane in axial section
  • 42.1° ± 3.9° between screw trajectory and midsagittal plane in coronal section 2

Corridor Dimensions

• Maximum screw length: Average 106.3 mm (range 82.1-135.0 mm) 1
• Maximum screw diameter:
  • Males: 6.5 mm screws generally safe
  • Females: Individual planning essential as 15.4% require screws with diameter less than 6.5 mm 2, 3

Critical Anatomical Constrictions

• Two key constriction points exist along the anterior column corridor:
  • First constriction: After 16-18% of the distance from entry point
  • Second constriction: After 55% of the distance from entry point 2
• These constrictions represent areas where careful navigation is essential to avoid breaching the cortex

Imaging Guidance Techniques

Fluoroscopic Guidance

• Two-dimensional fluoroscopic control is essential for safe placement
• Recommended views:
  • Anterior-posterior (AP) view: For identifying starting point
  • Lateral-oblique view: For advancing the guidewire 1
  • Obturator oblique and pubic ramus inlet views: Significantly improve screw placement accuracy compared to traditional obturator-outlet and iliac-inlet views 4

Preoperative Planning

• CT scan with multiplanar reconstruction is recommended for preoperative planning
• Virtual 3D modeling can be particularly helpful for:
  • Determining optimal screw trajectory
  • Measuring maximum safe screw diameter
  • Identifying individual anatomical variations 1, 3

Alternative Technique: "In-Out-In" Method

For cases with challenging anatomy, the "in-out-in technique" offers a safer alternative:

• Insert 4.5 mm malleolar or 6.5 mm partially threaded cancellous screw from outer iliac wing
• Position entry point 0.5-1 cm posterior and inferior to anteroinferior iliac spine
• Screw exits bone surface then re-enters anterior part of iliopectineal eminence
• Final purchase in lateral part of superior pubic ramus
• Advantage: Direct visualization without fluoroscopy, minimizing radiation exposure 5

Safety Considerations and Pitfalls

• Common complications to avoid:

  • Hip joint penetration
  • Vascular injury
  • Cortical perforation at transition between posterior column and sciatic tuber 1

• Safety measures:

  • Postoperative CT scan recommended to assess screw position and reduction quality
  • For females, individual preoperative planning is essential due to narrower corridors 2, 3
  • Maintain awareness of anatomical variations between males and females in entry point location 2

By carefully adhering to these anatomical parameters and using appropriate imaging guidance, safe placement of anterior column screws can be achieved with minimal risk of complications.