How many teeth do we need?

There are 32 teeth in the human mouth, including the wisdom teeth at the back that most people have removed. But how many of them do we seriously need?

This question has exercised dentists since the dawn of the NHS with its cash-limited budget for treatment. Now it has surfaced again as ministers plan the introduction of a new payment system for dentists that is designed to transform the way the nation looks after its teeth.

Dental experts are asking how much it is reasonable to expect the NHS to do for us – and how we are going to pay for it as a nation. The question is also relevant for patients who pay privately for treatment – do you really need that crown, bridge or implant costing hundreds of pounds, or could you manage by leaving a gap?

The problem, in a sentence, is that life expectancy is growing, our teeth are decaying, and aspirations are rising. Where once we accepted granny's gap-toothed smile – or gleaming denture – now everyone wants to go to their graves with a natural set of pearly whites. Is it possible?

Teeth evolved to last 45 or 50 years, which has been more than adequate for most of human history. We died before our teeth fell out.

But in the last century or so life expectancy has doubled to almost 90. Teeth are subject to damage and decay from childhood and have very minimal capacity to repair. If they become decayed or lose gum support you don't get it back.

About 20 years ago, when implants – the titanium screws fitted directly into the jaw to replace missing teeth – first came on the dental scene, they prompted a new question: for people with no teeth, how many implants could you get away with?

As the core function of teeth is to allow the individual to eat, researchers considered what constituted the minimum functional dentition. The answer the experts came up with was 20 of the total of 32 teeth in the mouth. In the UK population, among people with more than 20 teeth, the use of dentures falls sharply.

But which 20? Dentists argued that in a system with limited funds such as the NHS, it made sense to concentrate resources on the front teeth and work steadily backwards until the money ran out. Front teeth were more important, cosmetically, than the molars, which were more difficult (and expensive) to treat.

Could there be a way round this? The ultimate treatment for a failing tooth – front or back – is replacement with an implant. Implants are not generally available on the NHS because of their high cost, which ranges from £2,000 to £4,000 per tooth in the private sector. But researchers have calculated that for someone with no teeth, two implants to support a bridge (top and bottom), which would help them eat and substantially improve their quality of life, could be provided for £500 using the bulk-buying power of the NHS.

As the population ages, the pressure to provide implants on the NHS is likely to grow. Yet even at this price it is difficult to see how they will be afforded. The generation over 40 are already heavy users of crowns, root treatments and bridges – and the maintenance costs of this elaborate dental work are bound to grow.

Professor Jimmy Steele, the head of the dental school at Newcastle University and author of the dental review published last year, believes these developments are pushing in one direction – towards higher patient charges.

"Our high-disease, high-treatment past – courtesy of the NHS – is catching us up. Our maintenance costs rise every year and we would quite happily consume everything that the taxpayer could throw at us to save our progressively damaged dentitions from failure and our collective horror at the prospect of dentures. So that leaves us with the rather awkward prospect of higher patient charges within the NHS as a way of keeping a broad dental healthcare system viable. It would be reasonable to ask patients to pay a bit more in order to obtain a reasonably comprehensive service."

His view will not be popular with the residents of Saltash, Devon, where the last NHS practice in the town announced it was going private in September. A new NHS practice is promised by the spring but in the meantime residents of the coastal resort are obliged to travel to neighbouring towns to get NHS treatment.

It was to deal with setbacks like this, caused by the drift of dentists to the private sector, that Professor Steele was called in by the Department of Health to review the 2006 dental contract. Despite increasing spending on NHS dentistry, now standing at £2.8bn annually (including £500m raised in patient charges), in the two years following its introduction the number of patients receiving NHS treatment slumped by more than a million.

To stop the rot, Professor Steele proposed – again – a switch of focus from the drill-and-fill philosophy of the past and its replacement by a new emphasis on maintaining oral health. He recommended a return to patient registration backed by a capitation payment and a new payment for quality of care. To cover the extra costs, payments to dentists for activity such as fillings would be reduced.

Discussions are now under way to select practices to pilot the proposals, which are designed to incentivise dentists to spend more time and effort preventing problems from developing – while still treating those that do. But the bigger problem, which Professor Steele was not asked to address in his report, remains: how to provide a satisfactory dental service to a population with high expectations, from a budget too thinly spread.

NHS dentistry is still cheap by comparison with the private sector, and arguably has helped curb private dental charges, which are lower in the UK than in many comparable countries. But if Professor Steele is right, and there is no reason to doubt his analysis, the pressure to raise patient charges for NHS dentistry looks certain to grow.