Hello
Stones less than 5 mm in size usually will pass spontaneously, however the majority of stones greater than 6 mm will require some form of intervention, especially so if the stone is stuck causing persistent obstruction and/or infection of the urinary tract. But since you are telling size in cm ,i dont think it will pass spontaneously.This needs some intervention . Horseshoe kidneys often sit in front of the great blood vessels in the abdomen. This plus its unusual blood supply make it a risk when performing surgery involving the abdominal aorta .ESWL (Extra corporeal shock wave lithotripsy) is considered the first therapeutic option for cases of lithiasis with a mean area 4 cm2 or less and pelvic location. In case of great lithiasic areas other therapeutic options should be considered (open surgery or percutaneous nephrolithotomy) either in monotherapy or complementary to ESWL.You can have a look on success rate of this procedure in the following link.
Source- http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12701479&dopt=Abstract
Regarding the damage caused to kidney by stones i would say this depends on this factor
1) Degree of Hydronephrosis- Hydronephrosis is distention (dilation) of the kidney with urine, caused by backward pressure on the kidney when the flow of urine is obstructed.Prolonged hydronephrosis results in the eventual loss of kidney function .
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