Surgery is often recommended by vets for ligament injuries when surgery is not the best choice for the dog.  But when the dog cannot begin to re-stabilize the joint with careful restriction of activity, even with the use of a brace, then surgery is appropriate.
With the ligament no longer doing its part in holding the stifle together, the joint is unstable.  After the injury, the dog's body will try to begin to re-stabilize the injured joint.  If the proper low-stress conditions are provided, most dogs will slowly build up new tough, fibrous scar-tissue support at the stifle which will re-stabilize the joint.  But sometimes the dog cannot manage to start to re-stabilize the joint even when carefully restricted.  Then a custom made brace should be considered.  (Please see the FAQ page section on braces.)  If Fido still cannot get a start on re-stabilizing the injured joint, surgery is appropriate. 
People are often told that "Large dogs cannot recover without surgery" or "If the vet says there is a complete rupture rather than a partial tear then surgery is necessary."  Don't believe it!  Such statements are gross oversimplifications of a complex situation.  I hear about many very large dogs who recover well without surgery, including those diagnosed with complete ligament ruptures.  These seriously disabled very large dogs who recover well without surgery disprove surgery-advocates' claims that dogs-over-so-many-pounds always require surgery or dogs-diagnosed-with-complete-ligament-tears always require surgery. 
---- Here's an email about one of many "Impossible" non-surgical recoveries:
  "Max, I want to say thank you for your very informative website. Im happy to say non-surgical recovery worked for my dog Butkus a 2 yr old 170lb Bull mastiff. He was diagnosed with a full tear of his ACL almost 6 months ago. I followed your suggestions for activity restriction and let time do the rest and happy to say we are at 21wks and hes running and playing like a puppy. I think the myth of large breeds always needing surgery has to be blown out of the water by Butkus.
Thanks again! Matt"
Since surgery for these injuries is greatly over-recommended by vets, it is not wise for you to make decisions about your dog's treatment based on trusting in a vet's advice.  The question to ask yourself should not be: "What does the surgeon say Fido needs?" nor "How big is Fido?" nor "Does the vet say the ligament is completely ruptured?"  Some dogs do need surgical intervention to recover, but the answers to these questions will not tell you if your dog is one of those who needs surgery.
---- Many people are told by vets that their dogs require surgery, but the truth is that No vet can examine a dog and know if the dog needs surgery to re-stabilize the injured joint. The information available to vets from x-rays and exams is not sufficient to indicate whether the dog can re-stabilize the joint without surgical intervention. Only careful restriction and time can answer that question.
How Can It Be Determined Whether Surgery Is Necessary?
Many vets recommend surgery be done on all dogs they believe have ligament injuries.  Others suggest a very short period of rest for a few weeks and then surgery if the symptoms have not resolved.  These are not examples of sound medical advice.  Many vets are not well informed on this subject.  Other vets are primarily interested in selling high-profit services like surgery.  You cannot safely assume your vet will give you the best advice. So, if you cannot believe it when a vet tells you "Surgery is required for your dog", how do you decide whether your dog really needs surgery?
---- The way to determine if a dog's stifle is able to recover without surgery is to restrict the dog's activity as described on the 'Non-Surgical Treatment' page here at this website.  Giving non-surgical recovery a chance is almost always appropriate.  Carefully restrict to prevent running, jumping, long walks, and other activities that over-stress the stifle joint.  Be consistent with careful restriction and within 8 weeks the early stages of recovery should be apparent.  If a dog cannot begin to improve in that time, then trying a brace would be appropriate (please see the 'Brace' section on the FAQ page.) If stability improvement still fails to begin, the dog probably needs surgery unless there has been a misdiagnosis. 
---- If your dog does improve with careful activity restriction, as most do, continue with the restriction, supplements, and slow increases in activity as described at this website's page 'Recovery Without Surgery'.  Full recovery will take longer than 8 weeks.  Perhaps much longer.  But noticeable, continuing, gradual improvement during the first 8 weeks shows that the injured joint is slowly being re-stabilized by the dog's body.  An inability to improve indicates surgery is probably appropriate.  But remember that what you are looking for is improvement, not complete recovery in the first 8 weeks of restricted activity.  If you can look back at week two from week eight and say "Fido is able to use the injured leg more than he could a week or two after the injury.  He is still limping, but he is better than he was and slowly continuing to improve." then you have a dog who is re-stabilizing the joint.
Surgery Does Not Prevent Arthritic Changes
Many surgically-inclined vets will tell people that immediate surgery will prevent or minimize future arthritis, or that without surgery dogs will be crippled by arthritis.  This Is Not True.  It is sales-talk promoting surgery rather than medical fact.  The claim that surgery is superior in preventing or minimizing future arthritic risk is based on the false premise that dogs who do not have surgery will not have their activity restricted.  That they will run around continually re-injuring the joint so that it can never re-stabilize correctly.  Obviously this is not the non-surgical recovery being advocated here.  Activity must be properly restricted during recovery in order to minimize the risk of future arthritis.  This is true for recovery after surgical procedures as well as for non-surgical recoveries.
--- See this website's page "Arthritis Risk?"
All dogs who have recovered from serious ligament injuries, whether with or without surgery, are more likely to have arthritis in the future than are dogs who have not had a ligament injury.  After recovery, the best way to minimize future arthritic problems is to avoid any activity which results in limping or other symptoms.  After recovery it is usually true that near-normal activity is OK, but any activity which results in a dog having trouble with the leg should be stopped or moderated.  Walks may need to be shorter.  High-stress activities like jumping to catch a Frisbee should be avoided.  Also, you should give your dog the joint-supporting supplements recommended on this website's 'Nutrition & Supplements' page for life, together with top quality food.
Restriction After Surgery
---- It is common for surgeons to make unrealistic claims about how long it will be before a dog can safely return to normal activities after surgery. While many factors enter into how long recovery will take, it is generally true that surgery is not really a faster way to recovery. While a dog may be able to use the leg in a limited way sooner after surgery, this is not full recovery. The length and type of restriction for the fullest possible recovery after surgery will be much the same as with non-surgical recovery.  A too-rapid increase in activity after surgery can be disasterous.  It is easy to do too much too soon.  It is much wiser to be cautious.
 Question: "Max, Your website seems to be strongly against surgery.  Do you think surgery is ever the right choice for a dog with ligament injury?"
---- Some dogs do better with surgery. But which dogs are these? The question "Which is the best way to treat the injury?" in any particular case can be answered much better after a period of careful activity restriction. Nevertheless, it has become commonplace for many vets to recommend immediate surgery for almost all dogs who have ligament injuries. Proceeding immediately to surgery is often based in the business aspects of veterinary treatment.  (To put it bluntly-- There is money to be made with surgery.) Additionally, these same vets often misrepresent the risks involved in surgery and make unrealistic claims about the results that can be expected from surgery.
--- It is also true that there are many cases in which ligament injury diagnosis is mistaken.  Not every limp is caused by a ligament injury, but some vets seem to never see a limping dog without saying "TPLO is needed".
--- Many dogs recover very well from ligament injury without surgery, and of course misdiagnosed dogs should not be subjected to surgery.
---- It is not surgery itself that I am against. I am against surgery that is not in the dog's best interests. It is clear that simply accepting a vet's recommendation for immediate surgery is not wise.  A diagnostic period of 8 weeks of careful restriction will give you a better idea of whether your dog is going to be able to re-stabilize the injured joint without surgical intervention.  If Fido cannot begin to stabilize the joint in 8 weeks, surgery is a reasonable choice.   
Question: "Max, my vet says the non-surgical method you recommend won't work.  He says only surgery will work."
---- There are vets who have inaccurate ideas about non-surgical treatment.  There are a number of reasons for this:
-- At many university vet schools, vet students are taught canine ligament injury treatment by surgeons and are given little or no training concerning non-surgical treatment.  As a result, many vets who were trained in these institutions have little knowledge of the possibilities of non-surgical treatment.  Vets may not have a clear idea of how restriction should be done.  They may not understand that activity should be slowly and incrementally increased rather than suddenly resumed when symptoms resolve. They may advise simply "Rest the dog for a few weeks." without detailed instructions or a longer-term plan.  I often hear from people who believed, based on their vets' advice, that some insufficient restriction (like leaving the dog free to run in a large yard) was all that was needed. Or that once the dog's limp improved it was fine to immediately resume normal activity.
---- When restriction is not done properly, the dog's chances of having the best possible recovery are greatly reduced.  If normal activity is resumed abruptly when there is improvement, there is likely to be a re-injury of the joint. When clients report results like that to their vet, this reinforces in the vets' minds the idea that non-surgical treatment doesn't work well.  Since these vets have these misunderstandings about what constitutes proper non-surgical treatment, it is no surprise that they see it as a less attractive choice than surgery.
---- Consider this analogy:
If a doc misunderstood the proper method for casting people's broken legs, and put the leg in a cast, then removed the cast after 2 or 3 weeks and told the patients to resume normal use of the leg, what would happen?  Bones would fail to heal or re-break.  The failure rate for casting would be very high.  Would this mean that putting casts on broken legs is an ineffective method of treating broken bones? Or would it mean that this two-weeks-in-cast method of applying the treatment was flawed? In a parallel way, non-surgical recovery for dogs' ligament injuries is often mis-applied then called ineffective by those who do not understand proper use of this treatment method.
---- When properly done, non-surgical treatment is very often effective and provides a low-risk method of obtaining the best possible result. 

To succeed with non-surgical recovery it is necessary to properly restrict the dog to avoid reinjury.
Here are some ways to fail:
     1) "My dog wants soooo much to run and play! He's been held back from doing what he loves to do for weeks & I just can't deny him what he wants so badly without giving in now & then.  Just a few minutes of romping one or two days a week shouldn't hurt."
---- But it does hurt!  Regardless of whether a dog has surgery or not, running & jumping and other activity which puts excessive stress on the joint must be prevented.  Not just reduced.   
     2) "All this restriction & extra trouble.  Month after month of it. It's more trouble than I want.  The surgeon says recovery is quick after TPLO.  It's expensive so it must be good, right?  We'll try the surgery." 
---- TPLO surgery has horrific risks.  The dangers of TPLO are often lied about.  Being expensive doesn't mean TPLO is superior.  It's not.  There is no quick and easy way to recover from ligament ruptures.  Recovery requires careful restriction and patience.        
     3) "Fido got better very quickly with restricted activity.  Three weeks of restriction and then he was able to run & jump just like before.  But then he hurt himself again in the same way after a few days off restriction.   So he went back on restriction.   He got better again & we let him off restriction again after another few weeks, but a few days later he hurt himself again.  This 'conservative treatment' restriction idea isn't working."
---- The dog will stop limping and seem to be back to normal long before the joint is really fully recovered.  Re-injury of the incompletely recovered joint will continue to be possible for many months.  It's important to increase activity very slowly over months, always watching carefully for signs of over-doing the dog's exercise.  Re-injury happens after surgery too.
      4) "Fido will lose muscle mass if he doesn't get exercise.  I feel like he needs to stay active to keep that leg muscle strong.  I make up for him not being able to run by doing lots of long leash walks but he is limping more at the end of the day and doesn't seem to be getting better."
---- This is a mistaken approach often heard from people who are very active and athletic themselves.  Don't worry about the muscle during the early months of recovery.  Too much activity will cause re-injury.  The re-injury will be obvious if it comes from a sudden excessive stress like jumping.  But you can also cause minor repetitive injuries day after day by too much low-stress walking.  That muscle will build back up just fine after the joint is recovered.  Give the joint the first priority.  The joint needs slow short walks which very gradually increase over many weeks.  Once the dog senses that the stifle is stable again, he will use it more and re-build the muscle. These ligament ruptures are not the kind of injury you can deal with by forcing activity.  That joint needs minimal stressing to recover.