Like us, your dogs are likely more active during the summer; playing ball, hiking, swimming, catching frisbees. All of these fun activities can put your dog at a greater risk of tearing her/his cranial cruciate ligament. (In humans it is equivalent to our anterior cruciate ligament - a very common injury on the Sunday football field.)
The crainal cruciate ligament (CCL) stabilizes the femur to the tibia in the stifle (knee) joint. A dog who tears this ligament is initially in pain and will hold the leg up and not weight bear on it. This will put more strain on the non-injured leg. Unfortunately the dogs who tear one CCL will often tear the other CCL over time.
At High Desert Veterinary we have diagnosed a number of torn CCL's in the last month. Dogs that are overweight or have straighter hind leg conformation will be at a greater risk. If you suspect your dog has injured his/her CCL then schedule an appointment for an exam. The doctor can tell a great deal by manipulating the leg during an exam and radiographs can confirm if a tear has occurred.