A very common and distressing condition in all large breed dogs.

Certainly Rottweilers seem to suffer from this condition more than other dogs
The causes of Cruciate ligament Rupture vary but most include the following  - 

*** Major trauma
*** Injuries to legs while running  jumping or very  sudden turns or twisting
*** Degeneration and weakening of the cruciate prior to the rupture in middle or older aged dogs
***Overweight and unfit dogs
*** hormone problems

Dogs of a certain breed most including large rapidly growing breeds do tend to suffer more but smaller breed dogs do also rupture their cruciates.
Some Vets and experts that say it can be an inherited disease some Vets and experts argue there it is not.
To date there is no evidence to support it is inherited but personally we would not breed from a dog that has cruciate problems

There are 5 major ligaments that cross the knee stifle (joint)
The anterior cruciate ligament is one of these 5 ligaments that keep the knee stable
This can sometimes rupture or tear due to the above this happens as a partial rupture or a complete rupture.
This causes pain lameness and progressive arthritis
Lameness may be sudden or gradual in onset. There may be pain swelling and instability in the knee joint. There may also be a noticeable clunk or click  noise when the dog is walking
X-rays and an examination are needed by a vet to establish cruicate damage and treatment.

If your dog is lame or experiencing difficulty walking especailly after excercise or resting you are advised to seek veterinary advice because if it is a Cruciate problem then the sooner your dog is sorted out he better due to the onset of arthritis.

If surgery is required then your dog should make a full recovery and return to an even better quality of fitness than before. 

In the majority of cases in Rottweilers the treatment choice is surgery to stabilise the joint alleviate pain and minimise the onset of arthritis
Regardless what treatment all dogs will develop arthritis in the knees joint to some extent
There are 2 typical surgery treatments for cruciate damage

TPLO AND EXTRACAPSULAR REPAIR

TPLO is the best and most reccommended surgery
dogs can expect to recover to normal or better fitness after the appropiate surgery and aftercare has been met

Extracapsular surgery is not as effective as TPLO but your vet or specialist would be the best person to access and advise depending on the injury