Saturday, 26 May 2012

Number needed to treat - NNT

When someone asks me about something statistical I always panic, if only internally. It's not my strong point and that's why I'm a dentist and not a statistician.

Somehow on clinic on Friday, a student and I got to looking at the 5 year results on the Hall technique trial taking place in Scotland (link here). I was waxing on, as I do, about how great the results looked - major failures (i.e. irreversible pulpitis, loss of vitality, abscess, or unrestorable tooth) were 3% in teeth treated with the Hall technique and 16.5% for those with conventional treatment. The p value was 0.000488 - highly significant. And then it said "NNT 8". That's when the question came - what is NNT?

Let's say there's a drug that prevents heart attacks and in a trial comparing it to placebo, the results report an NNT of 8 over, say, 5 years. What this means is that 8 patients would have to take the drug for 5 years to prevent one heart attack. 

Now, this is a useful indicator of the effectiveness of the drug. Say the NNT was 100. That would mean 100 patients would need to take the drug for 5 years to prevent a single heart attack. Now you might think that it's still worth taking the drug but if the cost were, say, £1000 a year for the drug, it starts to look like a pretty expensive game. And what if the side effects are significant. For example, the drug causes severe and debilitating muscle cramp in 20% of people. So one person in 100 doesn't get a heart attack but 20 people get laid low by leg cramp. Maybe it's still worth it, but the point is that you begin to be able to get a sense of the benefit of an intervention by using NNT.

What the Hall trial here was saying is that since there was an NNT of 8, this means that - compared to conventional treatment - 8 teeth would need to be treated with the Hall technique instead of that conventional treatment to prevent one major failure. If that NNT had been 100, the technique wouldn't have looked that much better than conventional. If the Hall technique is, say double the cost of conventional treatment, treating 100 teeth with it instead of the conventional treatment could seem extravagant.

If the NNT had been 2 then that would mean for every 2 teeth treated with the Hall technique compared to the conventional treatment there'd be one tooth saved. That would be very impressive.

But cost is only one issue. What are the consequences of a major failure? This is an important issue when deciding what the NNT needs to be to have significance clinically. Say you had a drug that stopped a lethal cancer but the NNT was 100. You might feel that this was good as you save a life for every 100 people given the drug. The consequence of the intervention is massive. But what about saving a tooth? Is an NNT of 100 still significant? An NNT of 8 probably is from a humble dentist's (rather than oncologist's) point of view given that this means fewer children needing extractions and the loss of space maintenance. I'd like to know, though, what the cost of doing this is to every deciduous tooth with caries as this would allow an economic evaluation of the technique too.

There are no right and wrong answers to what the NNT should be, but perhaps you can see how the NNT can help inform us as clinicians, our patients and policy people who have to decide how to allocate scarce NHS resources.

If you'd like to read some more about this there's a very accessible explanation here.

Wednesday, 23 May 2012

Writing up research

Learning how to write an academic article can take a long time and involve a lot of frustration. The BioMed group has put together some guidance when submitting research to journals but glancing through this open resource I felt there was sound advice for anyone writing up a bit of undergraduate or postgraduate research:

BioMed Central | BioMed Central author academy

BioMed is an open access online publishing group.

Tuesday, 22 May 2012

EBD intro prezi

I'm experimenting with a series of simple prezis as part of our new curriculum. The idea is to get stuff up online for students to watch and then to spend face-to-face time discussing it and developing the ideas. Any feedback welcome.

Monday, 21 May 2012

Crowns or not for root-filled teeth?

Some time ago I wrote a paper for the EBD journal looking at whether there was high level evidence (i.e. systematic reviews or randomised controlled trials) for restoring heavily-filled vital posterior teeth with crowns (1). I was unable to find a single RCT let alone a systematic review of RCTs. At the time, though, I came across a study that compared crowns versus no crown on root-filled premolars (2). It was as small study with 117 participants and a fairly low failure rate in both groups (root-filling plus composite versus root-filling plus crown) and no statistical difference between the two. My search strategy would have allowed for other trials involving root-filled teeth but there appeared to be none. 

And so, since that time, I have been discussing with students that the evidence for placing crowns on root-filled posterior teeth is poor, and that there is therefore a reasonable degree of uncertainty over whether we should place them or not. I raise this because there are known negative consequences of placing crowns: cost, time, removal of sound tooth tissue, possibly increased risk of caries due to poor margins and poor OH, and probably some more. Do the positives of preventing tooth fracture and maintaining coronal seal outweigh these?

By coincidence, this morning I have just extracted a root-filled and crowned lower left 2nd molar with the students because it was grossly carious beneath the crown, causing it to fail (only roots retained). And that was in a well-motivated patient with good OH and low sugar intake. 

The Cochrane Library - Independent high-quality evidence for health care decision making

A systematic review has just been published (3) that, funnily enough, identified just one RCT comparing crowns to no crowns on root-filled teeth - the one I described above. What was the conclusion? 

"There is insufficient evidence to support or refute the effectiveness of conventional fillings over crowns for the restoration of root filled teeth. Until more evidence becomes available clinicians should continue to base decisions on how to restore root filled teeth on their own clinical experience, whilst taking into consideration the individual circumstances and preferences of their patients."

I think I might have worded this differently and suggest that, equally, there is insufficient evidence to support or refute the effectiveness of crowns over conventional fillings but the point is still the same - we are left with personal experience and patient values to guide us (2 of the 3 components for evidence-based decision-making) but are left bereft of good research to inform this.

Given the number of crowns placed in practice and the cost of these to individuals and society, plus the cost of root-canal fillings in the first place, it seems ludicrous that those who pay for these services (the NHS, private insurance groups, patients) do not demand an RCT or two to be done. If anyone's got an idea of where to get the funding and if there's anyone in practice who wants to participate, I'm ready to run one!

Happy decision-making ;-)