Two leading sports surgeons have warned that rugby's attrition rate is fast becoming unsustainable with statistics showing the number of players who have had to withdraw through injury from this year's World Cup has more than doubled compared with the 2011 tournament at the same stage.

Already 34 players have been replaced because of injury from the point when the 31-man squads were finalised to the end of the pool stage. At the same stage in the 2011 Rugby World Cup, there had been 16 withdrawals.

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The toll has been particularly heavy on the home nations with the total now at 13 with the withdrawal of Ireland captain Paul O'Connell. At both the 2011 and 2007 World Cup tournaments, they had lost just two players heading into the quarterfinals.

The scale of that brutality was brought sharply into focus in the last round of games. Against Australia, Liam Williams became the 10th back Wales have lost to injury - going back to Jonathan Davies' ruptured anterior cruciate ligament in May.

A day later Ireland's Peter O'Mahony sustained a knee -ligament injury while Jonathan Sexton and O'Connell were substituted in the first half. The latter is likely to have played his last match for Ireland.

It is a sad inevitability there will be many more casualties before the tournament concludes on October 31, particularly now the stakes are that much higher. As enthralling as the spectacles are to watch, there comes a point at which it be must asked, when is too much being sacrificed in the name of entertainment?

Professors John Fairclough and Gordon Mackay are both renowned orthopaedic surgeons in the UK. Fairclough has a 30-year association with the Welsh Rugby Union and Cardiff RFC while Mackay has also worked extensively with the Scottish Rugby Union. Both are rugby men to the core, but are deeply concerned by both the scale and severity of the injury rate in the sport.

"It is not my job as a surgeon to alter the game of rugby union but any sport that produces that level of injury is akin to Rollerball," Fairclough said.

"That is not sustainable in the long term. From a medical point of view, the most important thing is that no sport should be so dangerous that it is going to be not just career threatening but affect people for the rest of their lives.

"When you have injury rates of that nature then somebody needs to be brought to investigate. It needs to be led by people without vested interests in rugby union, but looking at it purely from a point of view of personal injury."

There are two components to the problem. The first is the increasing size of the players. Since the 1987 tournament, the average size of a forward has increased by 8.9-kilograms and a back by 8.5kg.

"The physics of it are fairly simple," Fairclough said.

"A heavy weight travelling at speed versus another heavy weight travelling at speed will create an impact collision force that is very high. Players have got bigger and faster, but joints have not got stronger. The body is not built to withstand those types of collisions so you will have more injuries."

The second part is that those collisions are now being actively sought rather than avoided and that the nature of tackling has changed beyond all recognition.

"The big hit was never a feature of the game 20 years ago," Mackay said.

"The big hit aims to stop the ball-carrier on the gain-line, and if possible, knock him backwards. You also have the chop tackle which is effective at stopping big men, but it is a recipe for a rapid rise in the number of knee injuries in particular.

"Apart from the fact that these big collisions can lead to whiplash or skeletal or muscular trauma, they also place huge stress on the joints," Mackay said.

"Peter O'Mahony's ACL injury against France was a classic example - Pascal Pape, a lock, knocked him backwards and so much force was channelled through his knee by O'Mahony trying to maintain his forward momentum that his ligament ruptured."

Mackay is also hugely worried by the rise of career-ending knee injuries in younger players because of this style of tackling.

"Our frame isn't fully developed until our early 20s, so when you've got huge 18-year-olds trying to run over the top of each other the results are pretty predictable."

World Rugby chief executive Brett Gosper has confirmed the organisation is prepared to amend certain laws in the interests of safety, but says he is unconcerned by the injury rates at this World Cup, which the organisation measures by loss of playing and training time per 1000 hours.

"Visually it is looking more serious because of the size of two of the games where injuries have been happening, but our monitoring is not showing the number to be extraordinary in any particular way," Gosper said.

"Let's wait until the tournament is over before doing a full analysis. We want to make sure it's evidence based rather than emotionally based because player welfare is our number one priority."

This line is parroted time and time again while the calendar is being filled with additional games by both unions and clubs. Should either Ireland or Wales reach the semifinals then they would have played 18 tests in the past 10 months. English players will head straight into a jam-packed domestic season of Premiership and European competition, which will be capped by a three-test tour to Australia.

Whereas in America's National Football League there are only 16 regular season games in a season, England first five-eighth George Ford played 32 matches for club and country last season. The NFL also strictly limits contact training. Some teams ban tackling altogether which is a stark contrast to the full-on sessions some European clubs engage in.

At the World Rugby Medical Conference, taking place in London, the International Rugby Players' Association is trying to focus attention on the playing and training load this generation has to manage.

Rob Nichol, the IRPA chief executive, said: "The starting point should be how much rest do players need and how much time do they need to condition themselves to play a season of rugby. We should not start with how many games do we currently have in different competitions and then try to fit the rest in around it."

- The Daily Telegraph