A dog limping on a back leg can be caused by several possible issues, ranging from injuries to degenerative conditions or infections. Common causes include:

• Torn ligaments, especially a cruciate ligament (ACL/CCL) tear, which is a frequent cause of sudden limping and instability in the knee joint. This injury often requires veterinary diagnosis and sometimes surgical repair due to pain and joint instability.

• Soft tissue injuries such as strains, sprains, or injuries to the paw or nails can cause limping and pain.

• Arthritis or hip dysplasia, which are degenerative joint diseases that cause pain and stiffness, sometimes resulting in limping that can develop gradually or flare up suddenly.

• Conditions like luxating patella, where the kneecap slips out of its normal position, causing intermittent limping or skipping.

• More serious causes such as fractures, dislocations, infections, tumors (including bone cancer), and nerve damage affecting movement or causing pain.

Because limping can arise from a range of causes — some requiring urgent veterinary care — it is advisable to have a veterinarian examine the dog promptly to determine the exact cause and provide appropriate treatment.