Disclosed is an implant for treating lumbar spinal canal stenosis, comprising a spacer that is proved with an elongate interior space into which an insert part can be inserted. A first retaining element is fixed in the spacer in the inserted state. Said first retaining part can be folded apart by inserting the insert part while a second rear retaining element that is molded onto the insert part simultaneously comes to rest on the vertebral processes, thus making it possible to insert the implant unilaterally while reducing the risk of having to undergo possible postoperative interventions.