Biomechanical Comparison of Sustained Dynamic Compression Screws vs Static Compression Screws in Subtalar...

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering
• Materials Science (Nitinol)

Background:

• Subtalar arthrodesis is effective for arthritis but faces nonunion challenges.
• Maintaining adequate compression is crucial for fusion.
• Standard static compression (SC) screws lose compression due to bone resorption and settling.

Purpose of the Study:

• To compare the compression-holding capacity of sustained dynamic compression (SDC) nitinol screws versus standard static compression (SC) screws.
• To evaluate screw performance under simulated bone resorption conditions in a cadaveric subtalar arthrodesis model.

Main Methods:

• Ten fresh talo-calcaneal cadaveric specimens were instrumented with either 2 SC or 2 SDC nitinol screws.
• Compression force was measured during insertion and after simulated bone resorption using a custom mechanical fixture.
• Resorption capacity was calculated for each screw construct.

Main Results:

• No significant difference in initial compression between SC and SDC screws was observed.
• SDC screws retained 90.4% of initial compression after 1 minute, compared to 69.6% for SC screws.
• At 2mm simulated resorption, SDC constructs maintained 111.1 N compression versus 8.3 N for SC constructs.

Conclusions:

• SDC nitinol screws demonstrated superior maintenance of interfragmentary compression compared to SC screws.
• SDC constructs showed significantly greater compression retention during simulated bone resorption.
• This sustained compression may help preserve fixation preload during subtalar arthrodesis, potentially improving fusion outcomes.