Investigating screw insertion to optimise orthopaedic fixation
: (Alternative Format Thesis)

  • James Fletcher

Student thesis: Doctoral ThesisPhD

Abstract

Most people will break at least one bone in their lifetime. The individual and societal problems caused by broken bones are enormous, with a considerable reduction in quality of life and large healthcare costs. Fractures (broken bones) often require surgery to restore function such as walking, but if operations are performed such that they are at high risk of failure, it increases the risks of revision surgery, worse function and higher financial costs. Active mobile people make fewer demands of health care systems. They have better control of their co-morbidities and so keeping people mobile reduces the burden on the entire UK National Health System (NHS), especially if their function can be restored at the first operation. Furthermore, even small improvements in fracture treatments have tremendous healthcare and financial impacts to the whole population given the number of fractures happening every day.

Most operations to fix broken bones use screws to hold the broken ends together so they can heal. However, despite the incredibly frequent use of screws (tens of thousands every day in the UK alone), no previous research studies have shown how tight these screws should be when put in. There are a multitude of reasons why fixations can fail such as breakage or cutting out of the implants. Poor insertion and incorrect tightening of screws are likely to contribute to fixations failing. Indeed, these factors may explain other failure mechanisms such as screw cut out, where it might actually be overtightening of the screw in the first place that destines the fixation to failure. Failures lead to pain, poor function and increased death rates, often needing further operations to re-fix the bones. To address this and improve patient care, the objectives of this research were to find the correct tightness for screws and to develop a predictive model which is simple enough to be implemented in a surgical theatre to allow a surgeon to insert cortical screws to the optimum tightness, having established the optimum tightness for any screw hole.
By performing tests in a laboratory, a new experimental model was validated for biomechanical testing that uses bovine bones to mimic the behaviour of human bone, including low bone density conditions such as osteoporosis. Experimental simulations on the bovine model determined that the optimum tightness for screws is between 20-30% below the maximum torque that can be applied to a screw (70 to 80% of the maximum is best). Therefore, control is needed to make sure that screws are not as ‘tight as they could be’ but have a targeted amount of tightness. This previously unknown information allows surgeons to consciously tighten screws to an established amount, which is expected to improve bone healing and reducing the risk of fixation failure. The optimum tightness value for a screw changes depending on the depth of the screw hole and the density of the surrounding bone. Methods of calculating what the correct tightness is for any screw hole using adaptations to existing engineering calculations that predict the stripping torque for a homogeneous material were created. These were tested with an augmented screwdriver – one that indicated when a targeted torque had been reached (that torque being 70-80% of the calculated stripping torque) – and found dramatic improvements in screw fixation and inserter confidence. This technique was also tested on multiple biomechanical researchers and surgeons, again finding improvements.

This programme of research has proved to be novel, generating new information about how best to insert screws and has substantial potential impact to patient care given the hundreds of millions of people needing fixation in their lifetime and the billions of screws that are inserted in the UK and around the world each year. Screw insertion had been previously trivialised and thought by some to be easy and performed well. It has been shown how poorly it is often performed by reviewing previous studies into surgeon performance and by undertaking the largest study into surgical insertion techniques. This research has developed simple, clinically deliverable solutions for addressing the variation in achieved screw tightness, and the high rates of over tightening.

By looking in detail at one of the commonest surgical techniques performed – inserting a screw – for one of the commonest conditions sustained – breaking a bone – this project should improve the care for millions of patients. Hopefully surgeon understanding and education of screw insertion will improve, alongside ongoing development of augmented screwdrivers to further aid surgeons and improve patient care.
Date of Award16 Nov 2022
Original languageEnglish
Awarding Institution
  • Department for Health
SupervisorEzio Preatoni (Supervisor), Richie Gill (Supervisor) & Michael Whitehouse (Supervisor)

Keywords

  • screw
  • fracture
  • fixation
  • torque
  • tightness

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