DM 5 - Predictors in durability

Question 1

When studying clinical durability, what is involved in gathering clinical data and analysing it?

Answer 1

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

Question 2

What is required before gathering and analysing clinical data?

Answer 2

need Caldicott Guardian consent to retrospectively go through notes

Question 3

What are the 7 principles of Caldicott Guardian consent?

Answer 3

1. justify the purpose for using confidential information
2. don’t use personal confidential data unless absolutely necessary
3. use the minimum necessary personal confidential data
4. access to person confidential data should be on a strict need-to-know basis
5. everyone with access to personal confidential data should be aware of their responsibilities
6. understand and comply with the law
7. the duty to share information can be as important as the duty to protect patient confidentiality

Question 4

How is clinical durability data analysed?

Answer 4

a popular method is to use Survival Analysis

Question 5

What is the aim of Survival Analysis?

Answer 5

to plot a survival curve
- plot time until failure
- permits comparison of survival of two or more groups

Question 6

Why is it rare to know true survival times from retrospectively going through notes?

Answer 6

• 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

Question 7

What is the most popular method of survival analysis?

Answer 7

Kaplan-Meier survival analysis

Question 8

What data needs to be collected for survival analysis?

Answer 8

• date of placement
• last date of follow up
• outcome - survival/failure

Question 9

For each record collected during survival analysis what must be stated?

Answer 9

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)

Question 10

Once a 1 or 0 has been allocated to records during survival analysis, what is done with this data?

Answer 10

place in a statistical programme and a survival curve is generated, this is then interpreted

Question 11

When interpreting a survival curve what is it important to look at?

Answer 11

the level of confidence that the statics give to the estimates

Question 12

What are the dotted lines?

Answer 12

the 95% confidence intervals

Question 13

What does the overlap of the red and green dotted lines indicate?

Answer 13

there is no significant difference between the 2 types of restorations as the upper confidence intervals overlap

Question 14

What is missing from this survival curve?

Answer 14

confidence intervals

Question 15

What kind of laboratory testing is a predictor of durability?

Answer 15

fatigue testing

Question 16

What does fatigue testing involve?

Answer 16

restorations are subject to on and off loading many times in function and the number of cycles to failure or survival recorded

Question 17

What is defined in advance of starting fatigue testing?

Answer 17

define in advance your survival number of cycles and cross head speed

Question 18

What amount of force is applied during fatigue testing?

Answer 18

apply cyclical force 2 Newtons up to a defined upper load limit

Question 19

What is the initial test upper load limit during fatigue testing?

Answer 19

60% of applied load equivalent for mean flexural strength of the material

Question 20

What is done if the specimen survives the survival number of cycles during fatigue testing?

Answer 20

place a new specimen and up the applied load limit by a set increment

Question 21

What is done if the specimen fails the survival number of cycles during fatigue testing?

Answer 21

new specimen placed and lower the applied load limit by a set increment

Question 22

What event is used to calculate the flexural fatigue strength?

Answer 22

the least frequent of all rounds of testing i.e. is failure or survival less frequent?

Question 23

What is the calculation for fatigue limit?

Answer 23

lowest stress + increment * (A/N +/-0.5)

(- if failure data, + if survival data)

Question 24

What is Weibull statistical distribution function?

Answer 24

a recognised method of assessing dependability world wide, needs at least 30 specimens

Question 25

According to Weibull analysis, what may low values of strength be explained by?

Answer 25

specimen fault or porosities

Question 26

What are high strength values explanined as in Weibull analysis?

Answer 26

explained as approaching the true/ideal strength of the material

Question 27

During Weibull analysis what is it helpful to know?

Answer 27

the Pf (probability of failure) at a particular stress

Question 28

During Weibull analysis what numbers are given to the different specimens ?

Answer 28

rank based on strength of the specimen, with 1 being the weakest and N being the strongest (N = number of specimens tested)

Question 29

What does a low Weibull modulus indicate?

Answer 29

a wide distribution of fracture stress values and a long tail at low values of stress - POSSIBLE UNRELIABILITY

Question 30

What does a high Weibull modulus indicate?

Answer 30

a close grouping of fracture stress values - BETTER DEPENDABILITY

Question 31

What principle is the strength-probability-time diagram based on?

Answer 31

that structure tend to be of a heterogenous nature

Question 32

What is meant by ‘structures tend to be of a heterogenous nature’?

Answer 32

• 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