In posterior flap hemipelvectomy, preservation of the gluteus maximus with the flap guarantees its viability regardless of the level of ligation of the iliac vessels. In anterior flap hemipelvectomy with the quadriceps femoris attached to the flap, the dominant blood supply is through the lateral femoral circumflex branches of the profunda vessels, which is sufficient to maintain the flap.
Kulaylat MN, Froix A, Karakousis CP. Blood Supply of Hemipelvectomy FlapsThe Anterior Flap Hemipelvectomy. Arch Surg. 2001;136(7):828-831. doi:10.1001/archsurg.136.7.828