Implanon is used as contraceptive devise which helps for prevention from getting pregnant. And it also have many side effects. You can search for it on internet. And I suggest talk to your doctor regarding the lower back pain also. As well there are many other safer ways are also available to prevent pregnancy so I suggest to think on that option rather then this. Now for your lower back pain since it is only at back level looks like muscular pain and you can take some pain killer along with muscle relaxant tablets. Along with this do take some physiotherapy treatment and follow spinal exercise for better result. For major back pain radiating to legs along with numbness, it need to check with MRI of back to rule out other spinal issue. And based on the MRI report the treatment will be planned. Here it can be conservative with medical management for minor problem or surgical correction for major issues. Talk to your orthopedic doctor in detail for proper treatment planning. You can also use hot and cold pack on your back for some more relief by relaxing your soft tissue. Take care.