Quote from Scalpel on 5th September 2017, 7:14 pm

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Angus Hotchkies 7/05 07:10PM

Ok great we’ll make a start

Welcome to SCALPEL’s second online journal club of the year! Thanks for coming along, hopefully it will be a useful discussion! We are joined by Daniel Lewis, a neurosurgical registrar who will facilitate and input to the discussion. I’m Angus Hotchkies, a 3rd year based at Salford. Firstly if everyone wants to introduce themselves and include an email (so we can send certificates out later) that would be great!

Here is a link to the paper for quick reference if anyone needs: http://www.bmj.com/content/bmj/347/bmj.f4305.full.pdf

 

Sarah Michael 7/05 07:11PM

Hi I’m sarah, I’m a second year

 

Sashavuc 7/05 07:12PM

Hi guys I’m in third year

 

Sianjuniper 7/05 07:12PM

Siân Juniper, third year at UHSM

 

Danni Wilkinson 7/05 07:12PM

Danielle, third year at Salford

 

Angus Hotchkies 7/05 07:12PM

Cheers guys

 

Danlewis 7/05 07:13PM

What were your thoughts on the paper everyone?

 

Sashavuc 7/05 07:14PM

Are these devices commonly used in practice ?

 

Angus Hotchkies 7/05 07:14PM

I thought it has quite a big impact factor since presumably there is quite a lot of time and money being invested into wound edge protection devices. And this paper seems to show there is no significant benefit to using them.

 

Sianjuniper 7/05 07:14PM

I thought it was interesting when I read it because you don’t often hear about the studies that prove new technologies are NOT useful

 

Danlewis 7/05 07:16PM

Your right Sian I think it goes against the trend in reporting negative results but as you have said the shear size of the study makes its findings more relevant

 

Sianjuniper 7/05 07:17PM

however it is quite an old paper (2013) so i don’t know if there have been newer things introduced now

 

Sarah Michael 7/05 07:17PM

I agree especially as it goes against the two other systematic reviews

 

Sianjuniper 7/05 07:17PM

from a clinical point of view as Angus said, will save a lot of resources if they are deemed unbeneficial

 

Danni Wilkinson 7/05 07:18PM

I was honestly quite surprised at the figures re. wound infections post surgery, had no idea they were so high, especially seeing so much surgery recently and seeing the protocols to avoid this

 

Sianjuniper 7/05 07:19PM

ikr shocking isn’t it! seen so many referred to ID after surgery in recent weeks

 

danlewis 7/05 07:19PM

I think we all agree the clinical question was a valid one. Do you think the study design and randomisation was appropriate?

 

Danni Wilkinson 7/05 07:20PM

I think blind assessment of the site infection was necessary

 

Sianjuniper 7/05 07:20PM

It included so many patients over such a long time which i think is good because it will give clearer results

 

Angus Hotchkies 7/05 07:21PM

designed seemed pretty good: assessors were blinded etc. but they were properly trained

 

danlewis 7/05 07:21PM

The authors do acknowledge the high infection rate and mention in the discussion that their prospective follow up of patients in clinic likely counteracted previous under reporting of infections

 

Danni Wilkinson 7/05 07:23PM

Are there different results from clinicians and the self reported infections? (sorry just trying to find now)

Ignore, found it sorry!

 

Sianjuniper 7/05 07:25PM

there was no difference, have i read that right?

 

Danlewis 7/05 07:26PM

Do we think the study was adequately powered to detect a difference?

 

Sianjuniper 7/05 07:27PM

as in between the surgeon reports and self-reports?

 

Danni Wilkinson 7/05 07:28PM

I think that using data such as if the patient required further antibiotic treatment would be more reliable than self reported infections

 

Sianjuniper 7/05 07:28PM

yeah maybe clinical notes would be more reliable, but then again not everything is documented

 

Danni Wilkinson 7/05 07:29PM

Also, did patients have any information given in aiding them to recognise an infection?

 

Danlewis 7/05 07:29PM

Between device and no device. The authors include a power calculation in the statistical analysis. I highlight it as it’s not something you always see even in studies of this size.

 

Sashavuc 7/05 07:29PM

I presume the questionnaire consisted of questions aimed at uncovering an eventual infection

 

Danni Wilkinson 7/05 07:30PM

Ah, I see apologies I’m still having a read through

 

Sianjuniper 7/05 07:30PM

oh okay! thats interesting – would you mind explaining that please? the stats confuse me slightly

 

Angus Hotchkies 7/05 07:31PM

So does it main that there’s an 80% there is going to be a statistically significant result?

**80% chance sorry

 

danlewis 7/05 07:34PM

So if anything there study is overpowered to detect a difference. They initially went on the assumption that they were looking for a 12% difference in infection rates so calculated how many patients they would need to be sure of detecting that difference which was 710 having an 80% power. The 80% means an 80% chance of correctly rejecting the null hypothesis, the null hypothesis being that there is no difference in infection rates between device and no device

 

Sianjuniper 7/05 07:35PM

ohhh i see

thanks

 

danlewis 7/05 07:35PM

Put another way using 80% power the chance of a type II error (false negative) is 20%.

 

Sianjuniper 7/05 07:36PM

thats rather big

 

danlewis 7/05 07:36PM

The alpha value was 5% so they wanted the chance of producing a type I error (false negative) less than 5%

 

Sashavuc 7/05 07:37PM

what are the implications of overpowering it ?

 

danlewis 7/05 07:38PM

It does seem big but an issue with power calculations is that there is always a trade off between achieving an acceptable study power whilst reducing th risk of a type I error (false positives). To achieve a very high power and a low type I error rate you often need very high numbers indeed

 

Sianjuniper 7/05 07:39PM

so even though this study had large numbers its still unusual?

 

Danlewis 7/05 07:39PM

Because the actual infection rate was much higher, they probably needed less patient numbers to be 80% sure of seeing a true effect.

 

Sianjuniper 7/05 07:40PM

okay

 

sashavuc 7/05 07:41PM

okay but wouldn’t a larger population sample make the eventual statistical analysis more significant ?

 

Danni Wilkinson 7/05 07:42PM

That’s what I was thinking

 

Danlewis 7/05 07:42PM

That’s why many drug trials which are looking for very small differences in outcome (<5%) require very large patient numbers. Indeed a larger patient allows you to either increase the power of your study or reduce the type 1 error rate (false positive), it depends what you are after

Here’s a useful website for anyone who wants to know more http://powerandsamplesize.com/Calculators/Article-1

 

Sashavuc 7/05 07:44PM

ahh I see thanks for that

 

Angus Hotchkies 7/05 07:44PM

Ok great thanks

 

Sashavuc 7/05 07:45PM

The graph showing the odds ratios between different factors affecting the outcomes are really interesting especially how it seems to be displaying a significant association between lower patient BMI and eventual infection

 

Angus Hotchkies 7/05 07:47PM

Did they even discuss that result?

 

Danlewis 7/05 07:48PM

That’s a good observation. The device appeaed to be better at preventing infections in patients with a BMI of over 25

They didnt discuss the result nor did they randomise patients on the basis of BMI

Do you think BMI >25 is a helpful cutoff though?

 

Sashavuc 7/05 07:50PM

I don’t think they discussed it but theres a graph showing the odds ratios between different factors. I think it summarises them quite well but none of them seem to stand out individually

 

Angus Hotchkies 7/05 07:50PM

I guess it’s standard practice for defining a patient as overweight – it’s not always an accurate figure though

 

Sianjuniper 7/05 07:50PM

people with an overweight BMI are at an increased risk of infection as it is

 

Angus Hotchkies 7/05 07:51PM

Yeah its a good table!

 

Sashavuc 7/05 07:51PM

yeah and I would presume higher BMI’s as having a signficantly higher post-surgical complication rate

but this study shows that its actually the other way around

 

danlewis 7/05 07:51PM

It is standard practice but if you look at the literature in other areas such as lumbar back surgery, the complicatio and infection rate really goes up once BMI is greater than 30

Does higher BMI have a lower infection rate though

 

Sashavuc 7/05 07:53PM

oh no sorry

interpreted that wrong

just has a lower odds ratio

 

danlewis 7/05 07:54PM

If you look at the numbers the infection rate in under BMI 25 is about 13% whereas its 54/206 in the BMI>25

A slight concern with BMI data is that they have quoted median values. Whilst the median values are comparable it suggests that the BMI distribution is non-normal so they likley had some very obese or very thin people in either group

 

Danni Wilkinson 7/05 07:57PM

In the study you just mentioned, how much did the infection rate go up after BMI of 30?

/was it even more than 50% here?

 

Danlewis 7/05 08:01PM

Some studies report inceases in infection rate from <5% to over 10% amongst obese patients

 

Danni Wilkinson 7/05 08:01PM

Ah ok

 

Danlewis 7/05 08:02PM

Whilst the spine is a cleaner surgical field, the same mechanisms apply- more fat means more tissue retraction, more tissue necrosis, more pain, less ambulatory etc. Never mind the likley imunosuppresive effects of the metabolic syndrome

Did we all think their conclusions were valid?

 

Angus Hotchkies 7/05 08:03PM

What I thought was interesting was the fact that these protection devices should work in theory and the problem might lie in the fact that the infections are just caused when the device is removed and the surgeons are closing up.

 

Danni Wilkinson 7/05 08:04PM

Or even further after that

 

Angus Hotchkies 7/05 08:04PM

Yeah

 

Danlewis 7/05 08:04PM

Exactly and if you look at their secondary analysis its the one thing they didnt control for

 

Angus Hotchkies 7/05 08:05PM

So that seems to be a major weakness but they acknowledge that

I think the conclusions seem valid as they suggest more large scale studies to evaluate efficacy

 

Sashavuc 7/05 08:07PM

Yeah that’s a really good point. Does anyone know if these devices are used at all in Manchester ?

 

Sianjuniper 7/05 08:07PM

ill google it now

okay mayne not that simple haha

 

Sarah Michael 7/05 08:08PM

they also say in some of the other studies that showed it worked they had double ring devices and they only tested single rings

 

danlewis 7/05 08:12PM

That may have been a factor but personally I was not surprised by the findings of this study. It sis difficult to say how efficacious it is without comparing wound closure. the skin edges can be beautifully protected throughout but if the closure is inadequate then the wound will get infected irrespective

 

Sianjuniper 7/05 08:13PM

very true

it is good they acknowledged that though

 

Angus Hotchkies 7/05 08:14PM

Would a study on wound infection and changing gloves/re-sterilising before closure be useful then?

 

Danlewis 7/05 08:17PM

It is an interesting idea, and in some surgeries routinely done. When inserting VP shunts for example, gloves get changed before handling the shunt tubing

 

Angus Hotchkies 7/05 08:18PM

Ah ok that’s interesting

 

Danlewis 7/05 08:18PM

It may be the start of a slippery slope though as the question then arises and what point do the gloves become ‘dirty’

 

Angus Hotchkies 7/05 08:18PM

very true

 

danlewis 7/05 08:21PM

Further to the point I made before here’s an article on obesity in lumbar spine surgery if anyone is interested https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4868585/

 

Danni Wilkinson 7/05 08:22PM

Thanks!

 

Angus Hotchkies 7/05 08:22PM

Great thanks

 

Sianjuniper 7/05 08:22PM

thank you

 

danlewis 7/05 08:24PM

Does anyone have any further comments or questions on the paper?

 

Sianjuniper 7/05 08:24PM

no but thank you so much for the stats advice

i struggle sometimes when i read papers because the stats confuse me and therefore it makes reading the study so much harder and harder to have an opinion on

 

Angus Hotchkies 7/05 08:25PM

No the big one for me was the fact that the devices likely do have an impact – it’s just the procedure before/after that might be causing infection

 

Danlewis 7/05 08:27PM

No problem, power calculations can be a confusing area at first but once you appreciate the power is essentially 1-false negative rate it becomes much clearer. Its an important thing to consider, so many studies especially pre-clinical animal ones have been done without a power calculation and they were likely underpowered to find a difference

 

Danni Wilkinson 7/05 08:28PM

Thanks for clearing things up

 

Danlewis 7/05 08:28PM

I will need to go but thank you for all your insightful comments and enthusiasm

 

Sianjuniper 7/05 08:29PM

thank you so much for your time!

 

Sarah Michael 7/05 08:29PM

Thank you!

 

Sashavuc 7/05 08:29PM

Thanks for everything Dan

 

Angus Hotchkies 7/05 08:30PM

Thanks very much Dr. Lewis and thank you everyone for coming

 

Danlewis 7/05 08:30PM

No problem, speak to you all at the next one hopefully and thanks to Angus for organising

 

Angus Hotchkies 7/05 08:30PM

Sending certificates now – thanks again guys!

 

Sianjuniper 7/05 08:30PM

amazing – thanks!