Ethan Frome was a man who had suffered great tragedy in the course of his life, who, overall, had not experienced much joy for many, many years. To start off with, he had to take care of an ailing mother and father for years on end, and right upon their deaths, he married a woman who also turned out to be sickly. He could never leave the town because he had to care for sick people nearly his entire life--he had obligations to tend to through their care. And after the horrific accident, he added one more invalid to the roster, his beloved Mattie, who became immobile and in need of constant doctoring. So, even though he might have wanted to leave, and did indeed attend college for a bit, family back home kept him rooted in his miserable circumstance.
After the accident, his injuries further limit his options. The first time we see him, we get a vivid description of his deformities:
"He was but the ruin of a man..there was something bleak and unapproachable in his face, and he was so stiffened and grizzled that I took him for an old man...[there was a] red gash across Ethan Frome's forehead, had so shortened and warped his right side that it cost him a visible effort to take the few steps."
So, a red gash in his forehead seemed to be the cause of the entire right side of his body hitching itself up; perhaps a brain injury, like a stroke, as a result from collision with the tree, caused severe damage on one side of his body. With his deformities making it hard to even walk, Ethan's freedom was even more limited. Despite his injuries and miserable life, Harmon predicts that Ethan will probably "touch a hundred" years of age, because the Fromes "are tough." It's a very sad life indeed, and if he lives that long, hopefully it will be to see some happiness along the long stretch of difficulties that he has encountered. I hope that helped a bit; good luck!