What causes lameness in a dog?

Dear Dr. Hershey,
My dog, Emma, an 8-year-old yellow lab, is lame on her right rear leg, but her X-rays show that the joints in that leg are normal. What else could it be?  
Thank you, Jenny 

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Dear Jenny,

Lameness in a rear leg can be caused by many different problems. Often times, there is a problem with the joints of the leg. In fact, one of the more common problems that we will see with a rear leg lameness in a dog is that they have torn, or partially torn their anterior cruciate ligament (ACL). If palpation and X-ray of the joints in the leg are normal, then we need to look a little deeper to find the source of the problem.

Sometimes, a muscle injury will cause lameness. Just like humans, dogs can strain or tear a muscle. If the dog is not allowed to rest (or doesn’t rest herself), the muscle can get irritated and re-injured every time they over-do it. Muscle strains are diagnosed one of two ways. Sometimes just putting pressure over the muscle will make the dog react. Other times, the muscle needs to be put in maximum stretch in order for the dog to “tell us” that it hurts. 

One of the more common muscle injuries we will see in dogs is a strained iliopsoas muscle. The iliopsoas is a group of muscles that originates on the underside of the spine and ends on the top of the femur (the long bone in the thigh). This muscle will help flex the hip and pull the leg forward during movement. Because this muscle is right at the underside of the belly, sometimes dogs will “bang it” when jumping over a hurdle. Also, if a dog was to fall and splay out the rear leg, that will sometimes pull and strain this muscle.  

This muscle assessed by directly palpating it on the underside of the belly, or by extending the hip back and slightly rotating the leg inward to put the muscle in maximum stretch. Muscle injuries need to be treated with appropriate rest, gentle stretching, massage, and sometimes laser therapy.

Another reason for a rear leg lameness that doesn’t involve the joints of the rear leg, is if the back is injured. The vertebrae in the back are a series of small bones that interconnect with each other. On the top of the vertebrae are bony processes called facet joints and at the bottom of the vertebrae is a large disc. The spinal cord runs down the center of the vertebrae and nerves extend out from the spine in between the facet joints and disk. 

The arrangement of the flexible facet joints on the top of the spine, and the pliable disk on the bottom of the spine allows the back to have an amazing range of movement. Each of these connection points in the back, however, can be a source of pain if injury occurs. If the bony facets in the top of the spine begin to rub together, they can cause inflammation, which leads to irritation of the nerves coming off of the back.

If the disk of the back gets swollen or herniates, this can cause pressure on the spinal cord causing pain and sometimes paralysis to the back legs.

If Emma’s lameness does not seem to be resolving, she should be reassessed by your veterinarian because there are many different treatment options available depending on the cause of the lameness.  

Dr. Teresa Hershey is a veterinarian at Westgate Pet Clinic in Linden Hills. Email her your pet questions at drhershey@westgatepetclinicmn.com.