# OPF/PMMA Page System as an Alternative Approach for the Treatment of Vertebral Correctomy
## 4 Discussion  
In the current study, we presented six different formulations for OPF hydrogel formulations and found a formulation that can be tailored to a specific size, mimicking human vertebral sizes. The OPF hollow cages expanded more than twice their original size during an acceptable surgical time-frame, becoming malleable and allowing for insertion inside a surgery site in a less invasive manner. Additionally, we performed kinematic testing and showed that the OPF/PMMA cage could provide mechanical stability to the spine while allowing for similar motions as the intact spine during flexion. Spinal cage materials typically range widely, from structural allografts and autografts, pure titanium and its alloys, to ceramics, and plastic such as polyetheretherketone (PEEK). In addition to structural bone grafts, the most commonly used materials are titanium alloys due to their high fracture resistance and biocompatibility. PEEK or PEEK-carbon fiber is also popular as their elastic modulus compares similarly to bone tissue [19]. However, these materials are nondegradable, leading to long-term foreign implant reactions and complications including needing a second intervention for cage removal with pseudarthrosis or cage migration [20]. Our proposed OPF hollow cage, in contrast, is a biocompatible biomaterial that will degrade over time, which makes it an attractive material for spine fusion cages [21; 22; 23; 24]. Currently, minimally invasive procedures are being used effectively to treat a wide range of degenerative conditions of the spine. These spinal procedures have led to less pain, less blood loss, a decrease in hospital stays, as well as improving wound healing and allowing a faster return to activities of daily living [25]. While structural cages can be placed into corpectomy defects via both anterior and posterior approaches, these approaches typically require a large exposure which can be highly morbid. For instance, a posterior approach for a cage placement requires a lateral extracavitary or costotransversectomy approach, in which the medial aspect of the rib is removed and often a thoracic nerve root is sacrificed. While titanium and PEEK expandable cages have been used more recently [7], they still require a fairly substantial exposure and have a significant risk of cage subsidence, especially in patients with metastatic tumors in the spine and osteoporosis. In the current study, a small posterior-only incision could be made to place the OPF hollow cage while letting it expand to occupy the lesion and space created during the corpectomy process, with percutaneous placement of pedicle screws for added stability. Finally, a polymer, similar to the PMMA currently used clinically, can be injected into the lumen of the cage to allow for a more rigid construct that can provide rigidity and stability to the spine. The technique and process presented in this study allows for the use of different sizes of cages based on patients' needs and vertebral levels to be treated. This posterior-only approach has the potential to provide substantial improvements on surgical outcomes. In this technique, pre-expanded cages can be fabricated and stored in advance of surgeries. After incising and accessing a surgical site, the cage can be inserted. As soon as it is exposed to the body's fluid, the cage starts expanding to more-than-twice its original size in a timely manner. To analyze the effects of the corpectomy, pedicle-screw fixation, and the placement of the polymer on the ROMs, we measured the local motion of the treated segment (L2/L4) as well as the global behavior of the spine. Importantly, both analyses showed that the cage restricted motion during flexion to almost similar values as an intact spine. There are several limitations in this study. First, the study was limited to one cadaveric spine; however, the intent of the process was to show a proof-of-concept and describe a technique with the potential for improving surgical outcomes in the treatment of metastasis lesions.