First, how can you tell if a tear is partial or complete? The short answer is for your veterinarian or rehab therapist to feel the degree of instability in your dog's stifle. Sometimes your dog needs to be sedated for this, to overcome tight and tense muscles as your dog protects and guards an injured area. Veterinarians will usually recommend x-rays too. This is not to see the torn ligament, but to check for any small fractures or signs of bone infections. Joint swelling can be seen on an x-ray too (and usually can be felt in the joint if you know where to look for it.) So if the x-ray shows no other problems, and the stifle joint feels "loose" in a particular manner (called cranial drawer movement) the diagnosis is an ACL tear. A partial tear only shows this particular movement when the stifle is held in certain positions. Partial tears may be quite painful. Partal tears come in 3 varieties too: grade 1,2, and 3 depending on severity. All can benefit from physical therapy to some degree.
So how do you rehab your dog's partially torn ACL? Every patient is different, of course, and so your dog's rehab plan should be tailored to his or her needs and condition. However, there are some basic principles that apply to all. One, activity restriction. Your dog with a partial ACL tear should not be allowed to run, jump, wrestle, or otherwise play for the next 2-3 months. Yes, I said 2-3 months. And yes, I know its hard to restrict it but if you want your dog to get better, not worse (i.e. go from a partial tear to a full tear!) then you must figure out a way to do this. Two, your vet/therapist should use modalities such as laser therapy to increase the blood flow to the ligament to aid healing. Three, the adjacent muscles should be strengthened. Okay guys, this MUST be done in a controlled manner under the guidance of a vet or therapist trained in physical rehab. Doing the wrong exercise, or doing it at the wrong time, or doing it with the wrong form can lead to increased risk of injury. Four, work on balance and coordination. The same warning applies as for strengthening. Five, provide adequate nutrition for healing and strengthening. This is usually done in the form of supplements, including glucosamine, MSM and omega 3 fatty acids. However, again, your dog is an individual and your dog's individual needs should be evaluated.
Every dog is unique. Every rehab program is unique. The results vary for every situation. A partial ACL tear offers a good opportunity to try a conservative approach of physical therapy, though.