A comparison of screw insertion torque and pullout strength

Area of Science:

• Biomaterials Engineering
• Orthopedic Surgery
• Bone Mechanics

Background:

• Plate-screw fixation is vital for fracture repair, relying on compression generated by screw tightening.
• Osteoporotic bone poses challenges, potentially limiting screw insertion torque and compromising fixation stability.
• Maximum insertion torque (MIT) and pullout strength (POS) are key parameters for evaluating screw fixation.

Purpose of the Study:

• To investigate the effect of screw thread pitch on MIT and POS in an osteoporotic cancellous bone model.
• To analyze the relationship between MIT and POS in this specific bone condition.
• To determine if POS is a reliable indicator for optimal screw design in osteoporotic bone.

Main Methods:

• Stainless steel screws with varying thread pitches (1-1.75 mm) and constant diameters were tested.
• MIT and POS were measured in a validated osteoporotic surrogate bone model (160 kg/m³).
• Compression between plate and bone surrogate was evaluated using pressure-sensitive film.

Main Results:

• Screw pitch significantly influenced MIT (P < 0.0001), with smaller pitches yielding lower torque.
• POS did not show statistically significant differences across most screw pitches (P = 0.052).
• No significant correlation was found between MIT and POS (P = 0.069), though increased torque correlated with compression (R² = 0.97).

Conclusions:

• Screw designs that generate high insertion torque in osteoporotic bone do not necessarily have higher pullout strength.
• Relying on pullout strength alone may be insufficient for selecting optimal screw designs for osteoporotic bone.
• Future research should focus on screw designs that maximize insertion torque for improved fixation in osteoporotic bone.