The ferris wheel is important because it is located at Ruby Pier, where Eddie works. It symbolizes that Eddie was never able to leave Ruby Pier. After he moved back into his childhood home, the ferris wheel could be seen out of the window and it haunted him of the fact that he never became anything more than a maintenance worker.
Eddie's knee injury is important because it restricted him from being able to walk normally; it constantly pains him and reminds him of the war. The injury symbolizes the many obstacles that he faced during his life. Eddie blames his never leaving Ruby Pier to be an engineer on his shattered knee.