The most common type of spacer is called an interbody cage . It is a small metal implant packed with bone graft. The cage is porous, like a grate or a kitchen strainer, so that bone graft may grow through it and fuse with the adjacent vertebral bodies. The surgeon may also fill the area around the spacer with morselized (fragmented) bone, which helps promote fusion between vertebral bodies. A concentrated form of Bone Morphogenic Protein (BMP) may also be used in the area where fusion is desired. BMP is a substance produced naturally by the body to promote bone growth, and it helps encourage fusion.
Between database inception and October 2015, 45 studies meeting inclusion criteria were identified with 3472 subjects undergoing MIS fusion and 5925 having an open procedure. There were no significant differences in operative time between the two groups, whereas patients undergoing MIS fusion consistently demonstrated less blood loss (-%) and shorter hospital stays (-% shorter). There was no difference in variably reported VAS, ODI, SF-36, SF-12, or EQ-5D scores between the two techniques at intermediate to long-term follow-up (12-60 months). Complication rates and fusion rates were also equivalent between the two groups. Economic studies demonstrate cost-savings in favor of MIS fusion ranging from to %.