Polyurethane-based synthetic bones mimic screw migration of intramedullary nails in human long bones

Introduction:

Intramedullary (IM) nails are the gold standard for the treatment of shaft fractures of long bones. The IM nail is mainly fixed to the bone by locking screws. Thus, the screw-bone interface strongly determines the stability of the fixation construct and often constitutes the weak link of the bone-nail construct. Screw loosening results in interfragmentary movements and deteriorates bone healing. To evaluate novel implants or screw configurations, synthetic bones have been frequently used but bone substitutes “off the rack” do not adequately mimic the mechanical properties of human bones. Thus, the aim of this study was to evaluate novel bone surrogates which mimic human bones in screw loosening mechanisms of IM nails.

Materials and methods:

Bone surrogates with a 4 mm cortex, simulating the metaphyseal cortex of human long bones, were developed. Fresh frozen human humeri were used as reference. All specimens were treated with an IM nail dummy, which was implanted using bicortical screws with a distance of 10 mm to simulate the distal part of an IM nail. The nail-bone-construct was loaded in a combined axial-torsional sinusoidal loading protocol to simulate physiological loading. Load was increased for 50N every 2000 cycles until failure. Torsion was kept constant at 2Nm.

Results:

Load of failure of synthetic bone-tubes (2537 ± 54N) was similar to those of human specimens (2712 ± 781N). Typical clinical failure patterns were observed in all specimens. Oval-shaped migration patterns were “cut” by the screws due to the toggling movements.

Discussion:

The novel customized surrogates realistically mimic failure and screw migration behavior in human bones. Thus, they provide a new opportunity to test and design implants and screw configurations.

Key words:

bone surrogate, biomechanical testing, cut out, incision, IM nail, fracture, non-union, movement