For a study, researchers sought to assess the mechanical qualities of 3 posterior spinal fusion assemblies widely utilized to cross the cervicothoracic junction. As defined in ASTM F1717, 3 titanium alloy posterior fusion assemblies designed to span the cervicothoracic junction were subjected to static compressive bending, tensile bending, and torsion at 2.5 Nm. Multiaxial screws were used to attach 5 samples of each assembly to ultrahigh molecular weight polyethylene blocks for testing. The stiffness of each build was estimated after recording force and displacement. The 2 assemblies that used a 5.5 mm rod were stiffer and had a smaller range of motion than those that only used 3.5 mm rods. Thoracic screws intended to receive cervical rods can be used with the cervical rod (3.5 mm). Traditional thoracic screws could also be utilized to receive 5.5 mm thoracic rods. This necessitates using a 3.5–5.5 mm transition rod or a second 5.5 mm rod with a connection to connect the 2 rods. The immobility of vertebrae was critical for fusion success, but the mechanics offered by these various assemblages were unknown. The investigation results showed that using a 5.5 mm rod in a fusion assembly increased the rigidity of the construct significantly. When fusion stability was a concern, as proven by the ASTM F1717 vertebrectomy (worst-case scenario) model, using 5.5 mm rods could help improve fusion success rates.