Thanks for the additional information. It is helpful.
Unfortunately larger dogs really need to have surgery as they seldom heal on their own and usually if they do, the other knee follows the first as it is taking the stress off of the injured knee. As a result, even with surgery the opposite knee may develop the same injury. It is fairly easy to damage the knee. I've personally tore both of my own knees.
Injury can occur from a slip and just catching themselves can twist the knee and result in a tear. They might sustain a hit from another animal while playing rough that causes the injury. One of mine occurred when a dog hit the side of my knee during play. I had a smaller dog sustain an ccl injury from a car accident and another from landing wrong jumping from the back of a pickup truck. The the injury can occur without an owner really knowing exactly what happened.
Given his size surgery is his best bet at regaining the full use of the leg before it is damaged further. I would have cautioned you to keep him as inactive as possible until surgery to prevent further injury. When they have pain killers and anti-inflammatory meds they don't feel they are injured and wll move more like they normally do and can tear the ligament even further.
You will want to ask your vet about post op care as often there is a period of limited activity and physical therapy which often include water therapy to strengthen the leg again. You might ask also about a brace during recovery for the let or one for the opposite leg to help prevent that knee from being injured.
Read more on the various operations here:
Here are some sites that compare knee braces.
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