Novel polymer networks based on poly(propylene fumarate) (PPF) and the crosslinking agent poly(propylene fumarate)-diacrylate (PPF-DA) were investigated as a material for a biodegradable interbody fusion cage. The aim of this work was to establish the effect of the macromolecular network structure on the physical properties in order to tailor the material to demonstrate high strength, controllable degradation, and suitable biocompatibility for this implant. The PPF/PPF-DA network structure was characterized with a newly developed technique in which the networks were degraded into simpler linear constituents that provided insight to the macromolecular structure. The double bond conversion and crosslinking density of the polymer networks was controlled by the concentrations of PPF and PPF-DA in the network, as dictated by the double bond ratio of fumarate groups in the PPF backbone to acrylate groups in the PPF-DA crosslinker. Lower double bond ratios yielded higher conversions and a more densely crosslinked network. The network structure was further influenced by the free radical initiator system. The mechanical properties of the PPF/PPF-DA networks increased with decreasing double bond ratios as a result of higher crosslinking densities. Photo-crosslinking produced a stronger material and also facilitated processing of PPF/PPF-DA networks because there is greater control over the crosslinking reaction. Examination of the in vitro degradation behavior of PPF/PPF-DA networks in simulated body fluids showed that the degradation rate was faster for networks with lower crosslinking densities. The biocompatibility of the material was also controlled by the macromolecular structure as PPF/PPF-DA networks with higher double bond conversions and crosslinking densities exhibited no adverse cytotoxicity and enabled fibroblast attachment. A prototype PPF/PPF-DA interbody fusion cage was fabricated by photo-crosslinking the polymers in transparent silicone molds. The PPF/PPF-DA implant demonstrated similar mechanical properties as a clinical approved allograft spacer and suggested that the device can provide sufficient support for interbody fusion. This work demonstrated that PPF/PPF-DA networks are a suitable material for a biodegradable interbody fusion device as well as other load bearing orthopaedic implants.