the Bible says that you can come to God with all kinds of prayer requests and petitions. If you have the faith of a mustard seed you can say the mountain to fall into the sea and it will.(Bible).When Jesus died on the cross He took all our sins and our diseases and nailed it to the cross. So child of God, you can ask him freely but never allow doubt, because as Christians we have been saved by grace through FAITH and not of our works.
Sounds like you're overweight. Weight loss as well as a good chiropractor and knee doctor could be a help to you. Work your belly, it is tied to your back, weak abs will tend to cause back pain.
While Jesus healed thousands, Paul admitted that not everyone is healed. Paul left one of his helpers sick at a destination. Also, Paul prayed three times asking to be healed, probably of an eye problem. But God's answer was no, the condition was to help him.
HE COULD but sometimes our own mistakes brought on something and he allows pain and trials...i hit someone in jan 87 and my back has been thru many other injuries since then...one mistake messed up my life..