Who has been key to tight defences this season?

Branislav Ivanovic has been tremendous all season

The top four teams have the best defences in the Premier League but who are the players that have been crucial to their success?

Southampton have only conceded 16 goals in 22 Premier League matches to date, and their surge to third in the table can be put down in a large part to their superb defensive form.

Nathaniel Clyne and Ryan Bertrand have been excellent all season, with their attacking forays almost as important to Ronald Koeman's side as their defensive attributes.

But the form of goalkeeper Fraser Forster, as well as that of central defender and club captain Jose Fonte, have been crucial to the Saints' defensive solidity.

Despite finishing eighth last season, it was never clear who Southampton's first-choice goalkeeper was.

Artur Boruc, Paulo Gazzaniga and Kelvin Davis all had spells in goal and Koeman quickly identified the position as one area that needed sorting out as soon as he came to the club and wasted little time in snapping up the 26-year-old Forster from Celtic for 10million.

It has proved to be one of many master-strokes by the Saints boss.

Forster has kept more clean sheets in the Premier League this season than any other shot-stopper, with his recent shot-outs against Arsenal and Manchester United bringing his tally to ten.

He has already proved himself to be one of the signings of the season, with his form helping to steady the Southampton ship after their four-match losing run in late November and early December.

Meanwhile, Fonte is the epitome of defensive coolness. He has won more tackles than any other Premier League central defender and, at the age of 31, is playing some of the best football of his career.

The Portuguese defender has been at St Mary's for five years, joining the Saints when they were still in the third tier of English football.

When the likes of Luke Shaw, Adam Lallana and Rickie Lambert were leaving the south coast in the summer, Fonte stayed put and Southampton are reaping the benefits of that decision now.

Following the 5-0 hammering of Swansea City at the Liberty Stadium on Saturday, Chelsea's 5-3 capitulation at Tottenham on New Year's Day now appears to be an aberration rather than the beginning of the end for Jose Mourinho's side.

The ever-present Branislav Ivanovic has been a major cog in the Blues' success.

The 30-year-old Serbia international has been the model of consistency since his 9million transfer to Stamford Bridge from Lokomotiv Moscow seven years ago, but this season could well be his best yet.

His work-rate is phenomenal and, despite his regular forays upfield, he is solid in defence, winning most of his aerial duels as well as being one of the league's best tacklers.

If Ivanovic can continue his form into the last four months of the campaign, Chelsea will surely be strong enough to clinch their fourth Premier League title in the last 11 years.

John Terry had been imperious until lately when his form has dropped off so Nemanja Matic is now vital to Chelsea's defensive dominance.

The 26-year-old was criticised for his tardy display at White Hart Lane but that performance was completely out of character and the Serb has been extremely important to Chelsea's success this term.

He offers the Blues defence a reliable shield and his displays have done much to propel Mourinho's outfit to the top of the table.

There has been much scorn placed on Manchester United's defending this season but, despite their injury problems, Louis van Gaal's side have conceded only 21 goals, the third best record in the Premier League.

The likes of Shaw, Phil Jones, Marcos Rojo, Jonny Evans, Chris Smalling, Paddy McNair and Tyler Blackett have been in and out of the side, and it is interesting to note that no United defender has started more than ten Premier League matches so far this season.

It is remarkable that United have such a fine defensive record and that can be put down in no small part to David de Gea's outstanding form.

Since the shocking 5-3 defeat at Leicester City in September, the 24-year-old Spaniard has been in dominant form, keeping clean sheets against Crystal Palace, Hull City, Liverpool, Tottenham and QPR.

He has helped to maintain United's top-four push with a string of fine displays and it now seems inconceivable that, when he first arrived at Old Trafford in the summer of 2011, there were question-marks about whether he would succeed in the Premier League.

He has proved his doubters wrong and, after being United's best player last season, has taken his game to a new level this term.

Rumours abound that De Gea will be snapped up by Real Madrid in the summer, speculation that intensified following the recent arrival at Old Trafford of former Barcelona shot-stopper Victor Valdes.

However, De Gea has sometimes single-handedly kept United in games during the traumatic last 18 months, and Van Gaal will be only too aware that he could be too valuable an acquisition to let go at the end of this season.

Manchester City may be stuttering in their title defence but they have only conceded 22 goals in the league, the fourth-best record in the Premier League.

Even though Joe Hart has made several mistakes this term, he has upped his game following the arrival of Willy Caballero from Malaga in the summer.

The England international has brought a new level of consistency to his game and there has been no repeat of last season's clamour for him to be dropped.

With Vincent Kompany suffering more injury woe this term and Martin Demichelis still causing City fans palpitations because of his inconsistency, there has been more pressure on Pablo Zabaleta to come up with the goods, and he has not disappointed.

The 30-year-old Argentinian continues to give his all to City's cause and has once again been one of Manuel Pellegrini's best players this season.