 The aim of this study is to compare outcomes and cost-effectiveness of anterior versus posterior surgical treatment for lumbar Isthmic Spondylolisthesis. The primary objective is to compare treatment effectiveness in terms of functional outcomes as measured by Oswestry Disability Index ODI v2.1a and to compare safety outcomes measured by treatment-related adverse events. Secondary objectives are to compare treatments in terms of differences in pain, quality of life, health utilities, and need for re-operation. Finally, a cost-effectiveness analysis will be performed. This analysis will take the form of cost-effectiveness if there is a difference in treatment effects or cost-minimization if there is no difference in treatment efficacy. 