Outcomes of Allogenic Cages in ALIF and PLIF: Biological Fusion Cages
Over the years, many variations of fusion cages have been invented to facilitate the fusion process while maintaining stability of the spine. Metal cages are widely utilized for anterior column reconstruction, with the goal of achieving spinal arthrodesis. The disadvantages of these metallic cages include subsidence and the inability to assess the "biology" of the intervertebral segment. Other potential problems include sizing of the implant, loosening, migrations, and theoretical metallic ion absorption . In the last few years, several interbody cages of different designs have been developed for use through an anterior and posterior approach. The aim was to provide mechanical support to the segment being fused with biocompatible implant material and to allow the use of autogenous bone to promote fusion. Theoretically, these new biological implants give more lasting restoration of disc height and better stabilization to the spine.
There are five processes involved in the incorporation of the graft. The first stage of the graft is the inflammatory process, which occurs within hours after implantation. Inflammation is followed by revascularization, osteogenesis, remodeling, and finally mechanical stability . During the inflammatory stage, the body's defenses elicit an immune response, causing inflammatory cells such as neutrophils and fibroblasts to invade the graft . Rejection of the graft often occurs during revascularization where the host is highly sensitive to the graft's antigen . During revascularization, possible complications may occur, including graft necrosis and occlusion of the host vessels. Osteogenesis, the synthesis of new bone by the host, begins shortly after the immediate postoperative period. This process involves the mesenchymal cells proliferating and eventually differentiating into chondrocytes and later into osteoblasts. "Osteoconduction" refers to the graft's ability to induce osteogenesis, which can persist for several months following surgery. Remodeling and mechanical stability follow, producing a functional and efficient graft . Because allografts are capable of eliciting a more aggressive immune response, freeze-drying, cryopreservation, and other preservation techniques are used to delay the inflammatory and revascularization process.
Surgical Technique and Clinical Outcomes
The femoral ring allograft (FRA) spacer instruments are designed for use with this "biological cage" for a straight anterior or anterolateral approach (Fig. 3). A preoperative planner can aid in determining the size of the adjacent Intervertebral discs and allow the implant to be firmly seated with a secure fit between the endplates.
Figure 3. Anterior and anterolateral approach using a femoral ring allograft (FRA) spacer.
For a direct anterior insertion/approach, the midline of the Intervertebral disc is exposed and evacuated with removal of the superficial layers of the cartilaginous endplates to expose the bleeding bone (Fig. 4a,b). Adequate preparation is essential to facilitate the vascular supply to the biological cage. Distractor blades are inserted into the disc space to restore the disc height, open the neural foramen, and stabilize the biological cage (Fig. 4a,b). The implant size is determined using the trial spacers. The implant corresponding to the correct trial spacer (Fig. 4c) is prepared, and bone graft material (either autograft, demineralized bone matrix, bone morphogenic protein, or allograft) can be inserted into and around the biological implant and in contact with the endplates (Fig. 4d) .
Figures 4a-d. Surgical technique using a femoral ring allograft (FRA) spacer.
For an anterolateral insertion/approach, the center of the implant and the distractor sit 30º offset from the anterior vertebral midline. This approach is commonly used at the L2-L5 vertebral segments and requires less soft tissue dissection and mobilization of vascular midline structures. The anterior longitudinal ligament need not be sacrificed in this approach. The trial size and biological cage are inserted at a 30º offset from the midline (Fig. 3).