Spain's Jorge Lorenzo has been ruled out of this weekend's German Grand Prix at Sachsenring after a fall during Friday's practice -- just two weeks after he fractured his collarbone.

The reigning world MotoGP champion, who needed surgery after breaking his left collarbone during practice for the Dutch GP two weeks ago, fell again on the second Sachsenring practice session, landing on the damaged area.

According to race organisers, the titanium plate, fixed with eight screws that was fitted after the Assen crash, was bent, causing him intense pain, although the pins remained in place.

As a result, the decision was made to withdraw him from Sunday's race and Lorenzo flew home to Barcelona on Friday to undergo surgery on Saturday.

"I have decided to go home and recover after today's crash at Sachsenring. I prefer to focus on surgery to fix the plate that was unfortunately bent during the crash," said the Spaniard.

"I will try to recover and be back as soon as possible."

Even though he got up quickly after leaving the track on the fourth lap, Lorenzo was seen holding his shoulder and an ice pack before getting in a car bound for the circuit's medical centre.

The incident came as Germany's Stefan Bradl was quickest in the second session in front of his home crowd, clocking 1min 22.030sec, 0.017sec ahead of Lorenzo.

The Spaniard had recorded the quickest time of 1min 22.47sec on Friday morning.

The 26-year-old finished fifth in the Dutch GP on June 26 and earlier on Friday had revealed that he rode the Dutch GP against medical advice.

"In the Netherlands, I had a lot of pain throughout the race. I admit that I was afraid. But I managed to get fifth place and only concede two points (to Pedrosa)," he told reporters. "That was the most important thing."

Doctors had initially prescribed four weeks' rest for Lorenzo, given his injuries after the 200 kilometres (124 miles) per hour crash during practice at Assen.

But they did not count on the defending champion's will to compete, just 36 hours after a two-hour operation in Barcelona under general anaesthetic to repin his shattered left collarbone with a titanium plate and eight screws.

Doctors admitted they had a nervous wait as the race took place, with a bone fragment from the complex fracture near an artery providing particular concern.

Doctor Joaquin Rodriguez said on Friday: "This type of fracture can cause serious damage to the blood vessels under the collarbone.

"We tried to stop him racing but he's stubborn. We then gave him a list of movements that he couldn't do under any circumstances.

"In particular we explained the signs which we would raise to force him to stop racing immediately.

"It (the race) was difficult to watch. My colleagues and I were very worried. Jorge gave us a lesson in physical and mental strength to go through the pain barrier."

Given Rodriguez's medical assessment, many observers have asked how race doctors gave Lorenzo the all-clear to race.

But two race doctors at Assen told AFP that the rider had successfully passed a series of tests that were more demanding than the movements he had to make during the grand prix itself.

At Sachsenring, Lorenzo had been dubbed "Kaizer Jorge" (Emperor Jorge), a reference to Germany football legend Franz Beckenbauer, who finished the 1970 World Cup semi-final against Italy with his arm in a sling and a broken collarbone.