There are many factors that affect the suitability of pars repair. The older the patient (20s-30s) the more difficult it is to get a repair to heal. The type of pars fracture (atrophic vs hypertrophic) makes a big difference. Also, the amount of separation of the ends of the fracture lends a considerable factor in poor healing. If there is a slip, this means the disc is at least deformed by stretch if not partially torn which makes repair even more difficult to gain a satisfactory result.
If you have a pars repair but don’t get a good result, you can still revise the repair to a fusion, but you sacrifice at least 6 months to one year of your time and then have to go through a second operation.
If you have a higher age, a slip and a degenerative disc, you would be happier with a fusion of that level.
Dr. Corenman