From this: "I have not been able to walk for enjoyment or pleasure or exercise in 41 years" - I'd guess the usual way basketball ruins bodies - it's probably fucked up his hips and knees.

Edit: From one of the links:

“I have had 38 orthopedic operations,” Walton told Bicycling. “Both my ankles are fused. I have an artificial knee. I have a new spine. I have broken countless bones. Every bone in both hands. Every bone in both feet; both legs. I have broken my back. I’ve had countless facial and cranial fractures. I spent my life falling down, getting hacked and fouled and getting in fights.”