The plantar ligaments is canines (dogs) are located in the foot (paw). According to dvm360, there are three ligaments: medial, middle, and lateral. These ligaments connect the leg to the foot. Like all ligaments, they are necessary and vital for the dog's foot movement. When a plantar ligament ruptures (tears), a dog will exhibit lameness, swelling and when standing, will be flat-footed, as opposed to its normal stance on its toes.
A ruptured plantar ligament is going to cause lameness in a dog. When a ligament ruptured, it cannot heal itself, and so the dog is going to have trouble walking, let alone running or jumping. The best you can hope for is to move around slowly. This symptom will be obvious, and should leave no doubt that there is an injury that requires medical attention.
The paw will swell with the rupture and it will be tender to the touch. A dog with this injury will not want anyone touching the paw or anywhere else close to the rupture. According to DVM360, the injury is most common in the middle plantar ligament, is not that painful, but it is still a serious injury that will not correct itself.
The Canadian Veterinary Journal, along with dvm360, mention that a dog with a ruptured plantar ligament will have a plantigrade stance. That means that the dog will be standing flat on its foot, and when walking, will have the heel of its foot touch the ground, like a human. Dogs normally walk on their toes, so this action is a good indicator of a rupture.