There are many possible causes for thigh pain. The joint replacement is certainly one. But both systemic and musculoskeletal problems can refer pain to the anterior thigh. For example, throbbing pain can be a sign of a vascular problem. Atherosclerosis in the blood vessels of your legs can limit the blood supply causing pain.
Other systemic causes of anterior thigh pain include kidney stones, tumors, abscess, diabetes, and chronic use of alcohol. Less often, thigh pain may be the first symptom of an inguinal hernia.
On the musculoskeletal side, spinal stenosis (narrowing of the spinal canal) can cause anterior thigh pain. So can fractures or stress reactions from osteoporosis, sacroiliac joint problems, and nerve compression.
A medical doctor will need to examine you to diagnose the cause. An X-ray will be taken to check the status of your implant. The type of pain and its location will help the physician find the cause.