Preliminary assessment of compression achieved by pre-loaded lag screw and pre-loaded positional screw in a lateral humeral condylar fracture model

Dataset contains repeated measures of screw placement torque and compression in synthetic and canine cadaver bone as described below.

Methods:

Pilot holes were drilled using an aiming device (IMEX veterinary, Inc., Longview, TX, USA) with a drill bit corresponding to the shaft diameter of each of the screws to undergo testing (2.0mm hole for 2.7mm screws and 2.5mm hole for 3.5mm screws). Points of entry and exit were centred halfway between the epicondyle and the articular surface of the condyle (18)(17). For the lag screw set-up, the glide hole was over-drilled in the lateral condylar fragment with a drill bit corresponding to the overall thread diameter of the appropriate screw.

Torque testing
Prior to compression testing, each screw was tightened to ‘stopping-torque’ by a single, blinded, board-certified surgical specialist. All screws were self-tapping cortical screws of 40mm length (Depuy synthes Vet, West Chester, PA, USA). Torque was measured by a digital torque screwdriver (0.05Nm-5Nm) (Sealey Tools, Bury, UK) which was used to tighten all screws. Each screw, with a washer, was applied into a newly drilled hole in the synthetic bone material for a total of five repeats (20pcf, 40mm diameter for the positional screw [PS] and 20pcf, 20mm diameter for the lag screw [LS]) and three timesfor three repeats in cadaver humeral condyles (whole condyle for PS, and just the medial component for LS). The mean value of the replicates was used as a standard torque for screw application in mechanical testing.

Part 1: Synthetic bone model mechanical testing protocol
A 25.4mm diameter circular piezoresistive force sensor (Tekscan Flexiforce A401; Tekscan, Boston, MA, USA) and load measurement system (Flexiforce ELF system; Tekscan, Boston, MA, USA) was used to measure compression. The sensors were modified by the manufacturers, with a 5mm central hole to accommodate the screw through the centre of the surface to be compressed. The sensor apparatus was calibrated, according to manufacturer guidelines, with a series of known weights prior to each use. The sensor was placed between the two blocks with the screw passing through the central hole. Compression (Newtons) was measured following application of the pointed reduction forceps, following screw placement and then after removal of the reduction forceps. The forceps were applied by the same surgeon, using points 5mm from the central hole. Each screw, with washer, was inserted and tightened by hand, to the previously specified torque as either a PS or LS. Washers were used to avoid deformation of the cis-cortex.

A new block was used for each individual test, with a total of five 20 PCF models per screw size, and per mode of insertion (2.7mm PS, 2.7mm LS, 3.5mm PS, 3.5mm LS, total 20 models tested).

Part 2: Cadaveric humeral lateral condylar fracture model mechanical testing protocol
Cadaver testing was performed with identical sensors in the same manner as in the bone model. In a clinical scenario, a screw of greater diameter is advantageous due to an increased area moment of inertia (AMI) and thus increased resistance to bending. In pilot testing, compression exerted by the 4.5mm screws exceeded the dynamic range of the sensors. Given that 4.5mm screws could not be tested, and 2.7mm screws are less desirable due to a decreased AMI, 3.5mm screws were selected for use in the cadaver limbs. Paired limbs were randomised; one side for PS placement and one side for LS placement. A new bone was used for each screw application. Reduction forceps were applied by the same surgeon, using the same landmarks (medial and lateral epicondyle). All screws were placed from lateral to medial. The dataset relates to the upcoming publication Winter, J., Clements, D.N., Ryan, J.R. (in submission), "Preliminary assessment of compression achieved by pre-loaded lag screw and pre-loaded positional screw in a lateral humeral condylar fracture model".