Expandable Screws Can Increase Fixation Strength But Can They Be Removed? An in-vivo Sheep Study Demonstrating Removal Of A Novel Expandable Screw

Abstract

Intan Oldakowska, Matt Oldakowski, Martin Cake, Robert Edward Day, Philip Procter, Jeremy Shaw, David Murphy, Anthea Raisis, Gordon Blunn, Allen Goodship and Markus Kuster

Screw fixation failures remain clinically challenging especially hip fracture fixation in osteoporotic bone. Prior studies have shown that expandable orthopaedic screw devices can improve biomechanical fixation in bone, but concerns over their removability remains. A novel expandable screw implant has been developed that optimizes both implant fixation and removability. The present study investigated the implant removability in an animal model.

A specially manufactured expandable screw implant was surgically implanted in 5 sheep for 4 months. The distal femur were extracted with the implants in-situ after euthanasia. The bone samples were scanned using micro-CT and the retraction torque of the expandable screw wings were measured to assess ease of removability.

Analysis of the micro-CT images verified that bone had not grown inside the expandable screw mechanism and all expandable screw samples were removed without complication. The mean peak torque to fully retract the screws ready for removal was 0.86 Nm (0.59-1.30 Nm) and the mean peak removal torque for the retracted screws was 7.98 Nm (1.70-15.40 Nm).

This large animal study result shows the feasibility of an expandable screw concept with a novel gapless design, to improve screw fixation strength mechanically, without compromising on the implant removability.

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