We describe a novel technique for preparing a fibular allograft for use as a transsacral strut with an anterior cruciate ligament (ACL) coring reamer for stabilization of high-grade spondylolisthesis deformities involving the L5--S1 segment in conjunction with instrumented posterolateral fusions. Over 4 years, 6 patients underwent instrumented posterolateral fusions from L3 or L4 to the sacrum or ileum plus posterior implantation of transsacral fibular allografts fashioned with an ACL coring reamer (Arthrex, Naples, Florida). No intraoperative complications were noted. At most recent follow-up (1-6 years), 5 of the patients reported no adverse events; the sixth patient reported prominent hardware, which was removed 5 years after the index surgery. ACL coring reamers seem to facilitate processing of fibular allografts so that they may be used as transsacral dowels for stabilizing high-grade L5--S1 spondylolisthesis deformities.