Mechanical evaluation of a threaded interference interlocking mechanism for angle-stable intramedullary nailing

Area of Science:

• Orthopedic biomechanics
• Veterinary surgical implants
• Biomaterials engineering

Background:

• Intramedullary nails are crucial for fracture fixation.
• Angle stability is a key mechanical property for implant performance.
• Comparing novel designs to established ones is essential for clinical advancement.

Purpose of the Study:

• To compare the fatigue and load-to-failure characteristics of a threaded interference design (TID) intramedullary nail against a Morse taper bolt design (I-Loc) nail.
• To evaluate the angle stability and mechanical integrity of both implant designs under simulated physiological conditions.

Main Methods:

• Constructs of TID and I-Loc nails were subjected to 50,000 cycles of axial or torsional fatigue loading.
• Mechanical properties including insertion/extraction torque, torsional toggle, and load-to-failure were assessed.
• Synthetic bone analogs were used to evaluate construct performance in a defect model.

Main Results:

• Both TID and I-Loc constructs maintained angle stability throughout fatigue testing.
• TID interlocking screws exhibited significantly lower median insertional torque, extraction-to-insertion torque ratio, and torsional yield load.
• No significant differences were found in torsional peak load, stiffness, or axial mechanical properties between the designs.

Conclusions:

• The TID intramedullary nail design provides angle stability and sustains fatigue loading, comparable to the I-Loc nail.
• While the TID demonstrated a lower torsional yield load, it shows potential as a viable mechanism for achieving angle stability in interlocking nails.