Anterior vertebral screw strain with and without solid interspace support

Area of Science:

• Spine biomechanics
• Orthopedic implant analysis
• Vertebral instrumentation

Background:

• Anterior instrumentation offers improved correction but has higher rates of nonunion and implant failure, often at end levels.
• Understanding strain at the bone-screw interface is crucial for preventing fixation loss.
• Previous studies highlight end-level failure as a common complication.

Purpose of the Study:

• To evaluate bone-screw interface strain at end levels during physiologic loading.
• To determine if solid interspace support, using threaded cortical bone dowels, reduces segmental strain on anterior implants.
• To compare construct stiffness with and without dowel augmentation.

Main Methods:

• In vitro biomechanical testing of a bovine spine model (n=8).
• Sequential testing: intact, anterior solid rod with rib grafts, and anterior solid rod with threaded cortical bone dowels (L2-L5).
• Mechanical loading included axial compression and lateral bending; screw strain and construct stiffness were measured.

Main Results:

• Higher strain occurred at end screws during lateral bending toward the implant, similar for rib and dowel groups.
• Dowel augmentation decreased end-level strain in lateral bending away from the implant compared to rib grafts.
• Both augmentation methods increased construct stiffness, with dowels yielding greater stiffness than the intact condition and rib grafts.

Conclusions:

• Increased strain at end screws during lateral bending correlates with clinical failure patterns.
• Threaded cortical bone dowels enhance anterior construct stiffness and reduce critical end-level strain.
• Further research should focus on improving fixation at end screws, with this model aiding evaluation of new strategies.