Can I have a sliding genioplasty after I've had a chin implant?

I had a 1.2mm bottom jaw deficiency but the surgeon said the largest implant he could insert would be a 'large size' 8mm implant aslo any bigger would not fit my face width ways. I initially really liked the results but now a year on, with all the swelling gone, I can barely notice any difference to my jaw. What are the largest chin implants available and could a surgeon perform a sliding genioplasty even though I have a chin implant already?