Forces required to dynamize sliding screws in gamma nail and selfdynamizable internal fixator

Area of Science:

• Orthopedic biomechanics
• Surgical implant technology

Background:

• Pertrochanteric fractures often involve dynamic implants with sliding screws.
• Transversal loads during the single limb support phase can impede screw dynamization.
• Successful dynamization depends on axial forces overcoming transversal loads.

Purpose of the Study:

• To biomechanically compare dynamization forces in three dynamic hip implants: Gamma Nail, SIF-7, and SIF-10.
• To investigate the effect of implant design, specifically the contact surface area between the stem and sliding screw, on dynamization.
• To determine the relationship between transversal loads, bending moments, and the axial force required for sliding screw dynamization.

Main Methods:

• Biomechanical testing of Gamma Nail, SIF-7 (two 7mm screws), and SIF-10 (one 10mm screw) dynamic implants.
• Utilized a custom testing device for simultaneous application of bending moments and transversal loads (Qt) on sliding screws.
• Measured axial forces (Qa) initiating dynamization using a tensile testing machine across various bending moments.
• Analyzed results using linear regression models.

Main Results:

• A significant positive correlation was found between transversal load (Qt) and the axial force for dynamization (Qa) (p < 0.05).
• Higher bending moments increased the required axial force (Qa) for dynamization in all tested implants.
• The Gamma Nail required the highest Qa, while the SIF-10 required the lowest Qa for dynamization.

Conclusions:

• A larger contact surface area between the sliding screw and implant stem facilitates dynamization by reducing the required axial force.
• Implant design, specifically stem-screw contact, influences the forces needed for dynamization.
• Patient factors like femoral neck length and body weight may necessitate tailored early postoperative rehabilitation programs for pertrochanteric fracture fixation.