The results of Vicente Padilla's MRI today came back, and the pitcher was diagnosed with an irritated radial nerve in his forearm, per Dylan Hernandez of the LA Times. What is an irritated radial nerve, you ask? Kyle Boddy of the wonderful SB Nation site Driveline Mechanics wrote about the issue in 2008:
There are two bones in your forearm: The Ulna and the Radius. When pitchers experience "hyperextension" of the elbow joint, or release pitches with a supinated grip, they slam the ulnar olecranon processes into their humeral olecranon fossa (soft tissue), causing inflammation of the hyaline cartilage. In turn, this causes the ulna bone to irreversibly lengthen, which results in loss of flexion about the elbow.
You've probably heard of Ulnar Collateral Ligament replacement surgery, better known as Tommy John surgery. However, you've probably never heard of radial nerve irritation. It is far less common than ulnar nerve irritation, but it does happen to baseball pitchers, and it is often misdiagnosed. As Chris Neault said above, the radial nerve is often irritated as it passes through the forearm or the lateral elbow in the area of the supinator muscle. The supinator muscle does exactly what it sounds like - it turns the palm upwards (or thumbs up).
Boddy has a lot of useful information in that article; I highly recommend reading the whole thing.
Matt Garza had a similar injury in April 2008. He was on the disabled list for a total of 16 days, but came back strong, putting up a 3.46 ERA in 28 starts after returning, and helped the Rays get to the World Series.
What does this mean for Padilla? It is too early to tell. Back in 2008, Garza was able to throw a bullpen session without pain within a week of his final start before landing on the DL. From Brittany Ghiroli of MLB.com in April 2008:
"I fought it all spring," Garza said. "Every bullpen I had to come in and do treatment with the trainers, kind of precautionary stuff, and I didn't want to do it because it hurt too bad."
The good news is the pain appears to have subsided. The righty said he was struck by how great it felt to throw pain-free on Tuesday, and credited the quick diagnosis by Dr. Koco Eaton, as well as a shot of cortisone for his short recovery.
That first part sounds an awful lot like Padilla today. From Molly Knight of ESPN:
"In the past I've been able to throw through this, but I can't even do that. Never had pain this bad before."
Padilla's last start was April 22. If he follows the Garza timetable, he will be able to throw without pain by the end of this week. Ken Gurnick of MLB.com reports that Padilla will begin a treatment plan of "rest and nerve medication" on Tuesday. For now, all we can do is wait.