David,
It sounds like you have done a lot of the normal treatments without any real success. Let me try to go ahead and answer some of your questions. First, I can't say that I would agree that tennis elbow burns itself out after two years. The only way I can see this being true is if the person with it stops what they are doing due to the pain, and it slowly heals over time. That usually won't work in chronic cases, because it usually needs more specific treatments.
I do definitely see cases where the cervical spine (neck) is the primary cause of this injury as impingement of the nerve root to the wrist extensor muscles can cause weakness and chronic overuse symptoms. If you have undergone all the normal treatments without success, and see any signs of cervical problem, then I would definitely clear the neck as the cause before having surgery if you haven't already. Having numbness into the last 2 digits is indicative of nerve compression or injury of the ulnar nerve. That does not usually coincide with tennis elbow (but not impossible) as the ulnar nerve runs on the opposite side of the elbow in the cubital tunnel, and may suggest the possibility of cervical involvement, but that would have to be specifically looked at. A lack of gripping ability at the index finger and thumb would coorelate with the median nerve, but both could get compressed in the neck. You don't have definitive enough symptoms to say it is your neck, but it sounds like it is worth checking out.
Supraspinatus trigger point therapy seems unlikely to help. While shoulder weakness does have a coorelation to tennis elbow that is thought to be due to overcompensation at the wrist for the inability to lift the shoulder, it would seem less likely to be the cause of such a chronic case.
Whenever I treat tennis elbow (lateral epicondylitis), I always look at neck movement and clear the neck first before moving onto things specific to tennis elbow. I also check for normal shoulder strength to see if overcompensation is a possible cause or contributing factor. If I narrow it down to normal tennis elbow, I start with stretching of the muscles of wrist extension, anti-inflammatory modalities such as ice, ultrasound, or iontophoresis (pushes anti-inflamm meds into skin using a small electric current), deep tissue massage of the wrist extensors, and look for any bony mal-alignement between the radius and ulna, or lack of normal movement between them or the humerus and correct as needed using manual techniques. I also strengthen as tolerated. Without restoring normal strength, it will never tolerate normal activity. Research is clear that eccentric strengthening is the best for the treatment of tennis elbow or most tendinitis based injuries for that matter. Initially I avoid all concentric exercise as that seems to aggravate the injury. I would also advise trying a counterforce brace that decreases the tension that the extnesors place on the lateral epicondyle. It works very well for some, but not all.
If none of that works, I re-check the neck and other areas. If nothing benefits the person, surgery has been a very viable option in the past with good results and a fairly quick recovery with most returning to normal activity within 12 weeks, maybe a little longer if there is a lot weakness and damage from a very chronic case. Some people are simply genetically predisopsed to the problem given the anatomy of their elbow, and nothing else will help it. That being said, I would say that less than 5% of all tennis elbow cases I have seen have ever needed surgery. Given that, I would exhaust all options before doing it.
I can't really say much more without actually seeing the elbow myself. That is a fairly basic view of how I would approach it, but If you have more questions about it that I have not answered, please feel free to ask.
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