Brief Answer:
can be scarring or neuropathic pain.

Detailed Answer:
Hello,
Thanks for posting your query.
The throbbing pain in the knee after a replacement can be related to soft tissue scarring after the surgery. This can be a neuropathic pain also. The treatment usually involves pain killers under guidance of a pain therapist, physiotherapy, occupational therapy, etc.
The complete healing of surgical wounds and internal soft tissues usually take six months with regular pain relief medications and regular physiotherapy sessions.

The recovery is considered when the pain and swelling improves much beyond the arthritic pre-op pain level and dysfunction.

I hope this answers your query.
In case you have additional questions or doubts, you can forward them to me, and I shall be glad to help you out.
Wishing you good health.
Regards.
Dr. Praveen Tayal.
For future query, you can directly approach me through my profile URL http://bit.ly/Dr-Praveen-Tayal

---