Author: Daniel Ciric
Ciric, Daniel, 2019 The risk of secondary screw perforation in plate osteosynthesis of unstable proximal humerus fractures: prediction and the effect of screw length, Flinders University, College of Science and Engineering
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Proximal humeral fractures (PHF) are the third most common joint fracture and have high rates of mechanical type failures following plating treatment. Secondary screw perforation through the joint is a prominent mechanical type failure requiring reoperation. Identifying what factors influence this, along with pre-operative prediction, could help avoid complications. The work within this thesis investigates the influence that screws tip to joint distance (TJD) has on the biomechanical risk of secondary screw perforation in PHF. Furthermore, a model was established to predict the failure based on bone mineral density and content (BMD) (BMC) and finite element analysis (FEA).
Ten pairs of cadaveric proximal humeri were scanned with high peripheral quantitative computed tomography (HR-pQCT). Specimens were osteotomised and instrumented with the PHILOS plate, simulating a highly unstable 3-part fracture.
Bones were randomised into a long screw group (LSG) with 4mm TJD, or a short screw group (SSG) with 8mm TJD. Instrumented specimens were scanned with a clinical computed tomography (CCT) scanner. A custom biomechanical screw perforation setup tested the samples to failure cyclically with a constant valley load and an increasing peak load. Two key cycle events were identified, the screw loosening and the failure event. The cycles to the initial screw loosening event were significantly higher for the LSG compared to the SSG (p<0.01). The number of cycles to the failure event was not significantly different between groups (p=0.5). Screws with a smaller TJD have better initial stability and resistance against loosening.
Longer screws, within limits dictated by the surgical guide, are expected to decrease the risk produced by secondary perforation failures in unstable PHF.
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Specimen-specific prediction computer models of the plated humeri were generated from co-registered pre- and post-instrumentation computed tomography (CT) scans and meshed with linear tetrahedral elements using ScanIP. Materials were assumed to be elastic and isotropic. The implant was modeled as titanium; the elastic moduli of bone elements were mapped from the HR-pQCT-based BMD. The experimental setup was mimicked, and quasi-static linear elastic simulations were performed
using Abaqus. BMC and average minimum compressive strain were evaluated in cylindrical bone regions around the screw tips. These parameters were correlated with the cycle events to assess their predictive potential. Density-based measures of BMC around the screw tips predicted well the screw loosening and perforation and thus has a high potential for future clinical application and pre-operative planning.
The simple strain-based measures computed with linear FE models predicted well the screw loosening, better than BMC. These measures for pre-operative planning are expected to aid in decreasing the failure rate of complex PHF plate fixations.
Future studies shall investigate if the BMC data available in CCT scans provides similar prediction strengths. Additionally, non-linear modelling approaches will be applied to improve FE-based predictions.
Keywords: Screw Perforation, Proximal Humerus Fractures, Validation Study, Surgical Planning
Subject: Engineering thesis
Thesis type: Masters
Completed: 2019
School: College of Science and Engineering
Supervisor: Associate Professor John Costi