DM 5 - Predictors in durability Flashcards
When studying clinical durability, what is involved in gathering clinical data and analysing it?
retrospective in nature and need to define what is a failure
- having defined failure retrospectively go through notes collecting:
- date of placement
- last date of follow up
- outcome - survival/failure
What is required before gathering and analysing clinical data?
need Caldicott Guardian consent to retrospectively go through notes
What are the 7 principles of Caldicott Guardian consent?
- justify the purpose for using confidential information
- don’t use personal confidential data unless absolutely necessary
- use the minimum necessary personal confidential data
- access to person confidential data should be on a strict need-to-know basis
- everyone with access to personal confidential data should be aware of their responsibilities
- understand and comply with the law
- the duty to share information can be as important as the duty to protect patient confidentiality
How is clinical durability data analysed?
a popular method is to use Survival Analysis
What is the aim of Survival Analysis?
to plot a survival curve
- plot time until failure
- permits comparison of survival of two or more groups
Why is it rare to know true survival times from retrospectively going through notes?
- since the restoration was placed the patient may not have been seen
- the restoration when you look at the notes might not have failed
- such events are termed CENSORED DATA
- don’t know
- can’t judge survival
What is the most popular method of survival analysis?
Kaplan-Meier survival analysis
What data needs to be collected for survival analysis?
- date of placement
- last date of follow up
- outcome - survival/failure
For each record collected during survival analysis what must be stated?
whether there has been an event of certainty which would be a failure (1), or whether the notes fall into censored data i.e. you don’t know if it’s failed or not (0)
summary - event (1) or censored data (0)
Once a 1 or 0 has been allocated to records during survival analysis, what is done with this data?
place in a statistical programme and a survival curve is generated, this is then interpreted
When interpreting a survival curve what is it important to look at?
the level of confidence that the statics give to the estimates
What are the dotted lines?
the 95% confidence intervals
What does the overlap of the red and green dotted lines indicate?
there is no significant difference between the 2 types of restorations as the upper confidence intervals overlap
What is missing from this survival curve?
confidence intervals
What kind of laboratory testing is a predictor of durability?
fatigue testing
What does fatigue testing involve?
restorations are subject to on and off loading many times in function and the number of cycles to failure or survival recorded
What is defined in advance of starting fatigue testing?
define in advance your survival number of cycles and cross head speed
What amount of force is applied during fatigue testing?
apply cyclical force 2 Newtons up to a defined upper load limit
What is the initial test upper load limit during fatigue testing?
60% of applied load equivalent for mean flexural strength of the material
What is done if the specimen survives the survival number of cycles during fatigue testing?
place a new specimen and up the applied load limit by a set increment
What is done if the specimen fails the survival number of cycles during fatigue testing?
new specimen placed and lower the applied load limit by a set increment
What event is used to calculate the flexural fatigue strength?
the least frequent of all rounds of testing i.e. is failure or survival less frequent?
What is the calculation for fatigue limit?
lowest stress + increment * (A/N +/-0.5)
(- if failure data, + if survival data)
What is Weibull statistical distribution function?
a recognised method of assessing dependability world wide, needs at least 30 specimens
According to Weibull analysis, what may low values of strength be explained by?
specimen fault or porosities
What are high strength values explanined as in Weibull analysis?
explained as approaching the true/ideal strength of the material
During Weibull analysis what is it helpful to know?
the Pf (probability of failure) at a particular stress
During Weibull analysis what numbers are given to the different specimens ?
rank based on strength of the specimen, with 1 being the weakest and N being the strongest (N = number of specimens tested)
What does a low Weibull modulus indicate?
a wide distribution of fracture stress values and a long tail at low values of stress - POSSIBLE UNRELIABILITY
What does a high Weibull modulus indicate?
a close grouping of fracture stress values - BETTER DEPENDABILITY
What principle is the strength-probability-time diagram based on?
that structure tend to be of a heterogenous nature
What is meant by ‘structures tend to be of a heterogenous nature’?
- structures are likely to contain defects or flaws in various amount and sizes
- such flaws remain of fixed length unless under load they become unstable and propagate catastrophically
