Outcomes of Allogenic Cages in ALIF and PLIF: Anatomy and Biomechanics of Interbody Fusion
Interbody fusions provide the most logical solution to diseases of the intervertebral disc. The definition of "success" in spine care is controversial and can vary from the perspectives of the patient, family, employer, insurance company, attorney, primary care physician, and the treating surgeon. A fusion is considered "clinically" successful if (a) there is increased or maintained bone density within the cage implant due to the presence of mature bony trabeculae bridging the interbody space, (b) there is an absence of a "halo" around the implant (resorption), (c) there is a sclerotic line between the cage and vertebral endplate, due to bone remodeling and new bone formation, (d) resorption of anterior vertebral traction spurs or anterior progression of the graft within the disc space occurs, and (e) there is lack of movement on flexion/extension views . Pseudoarthrosis, or failure of fusion, is suggested by persistent pain, progression of deformity, loss of disc height, vertebral displacement, hardware failure, haloing, migrations, or resorption of the bone graft and obvious movement on flexion extension views.
The FRA/PLIF biological cages are innovative lumbar interbody allografts, manufactured by the Musculoskeletal Transplant Foundation in conjunction with SYNTHES Spine. They come in a variety of sizes to precisely fit the disc space of each individual patient. Each FRA/PLIF spacer is machined from allograft into a wedge-shaped ring with "teeth". The teeth grip the adjacent vertebrae, thereby increasing the stability of the spacer (Fig. 2). The FRA spacer also has a hollow center that can be filled with autograft, allograft bone filler, a synthetic bone substitute, or bone morphogenic protein (BMP). This "bone void filler" may enhance or accelerate the biological fusion process of the spacer. An ideal implant must be capable of withstanding the axial compressive forces of the body. In addition, it must be able to displace the compressive forces without inducing a great deal of motion in the adjacent segment while also promoting arthrodesis .
Figure 2. The "teeth" on the femoral ring allograft (FRA) increase the resistance to implant pullout.
The anterior column of the spine absorbs 80% of axial compressive forces, while the posterior structures absorb the remaining 20%. A study by Brown and colleagues  of motion segments of the lumbar region with static compressive loads indicated that the first component to fail was the vertebral body. This occurred as a result of the fractured endplates. These findings suggest that the vertebral body's strength is dependent on intact endplates.
Tests were conducted on the PLIF and FRA spacers to ensure that they could withstand the loads on the lumbar spine. The ultimate compressive strength of a vertebral body is 8000 N . Test results show that they PLIF and FRA spacers have a compressive strength of over 25,000 N. A successful interbody fusion will restore every mechanical function of the functional spine unit except motion. The bone graft must bear substantially all of the body's weight above the fusion level(s) while it is being incorporated . The goal of any spinal fusion procedure is to maintain the correction, avoid hardware or graft failure, and obtain a solid fusion.
In addition to compressive strength, resistance to implant expulsion is a major factor in the design of intervertebral spacers. The PLIF spacer is designed with "saw teeth" to increase resistance to pullout. Pullout testing was conducted to ensure that the spacer was able to resist expulsion. The maximum shear force that a human disc can withstand is about 150 N . An axial preload (450 N)  and a shear load were applied to the implant to determine the pullout strength. The results show that the PLIF spacer has a pullout strength of (1053±80 N), more than three times the pullout strength of a comparable design without teeth (234±38 N) three times that of a comparable femoral wedge (405±65 N).