There are several complications associated with lumbar interbody fusion surgery however, pseudarthrosis (non-union) presents a multifaceted challenge in the postoperative management of the patient. Rates of pseudarthrosis range from 3 to 20 % in patients with healthy bone and 20 to 30 % in patients with osteoporosis. The current methods in post-operative follow-up - radiographs and CT, have high false positive rates and poor agreement between them. The aim of this study was to develop and test a proof-of-concept load-sensing interbody cage that may be used to monitor fusion progression. Piezoresistive pressure sensors were calibrated and embedded within a polyether ether ketone (PEEK) interbody cage. Silicone and poly (methyl methacrylate) (PMMA) were inserted in the graft regions to simulate early and solid fusion. The load-sensing cage was subjected to distributed and eccentric compressive loads up to 900 N between synthetic lumbar vertebral bodies. Under maximum load, the anterior sensors recorded a 56–58 % reduction in pressure in the full fusion state compared to early fusion. Lateral regions measured a 36–37 % stress reduction while the central location reduced by 45 %. The two graft states were distinguishable by sensor-recorded pressure at lower loads. The sensors more effectively detected left and right eccentric loads compared to anterior and posterior. Further, the load-sensing cage was able to detect changes in endplate stiffness. The proof-of-concept ‘smart’ cage could detect differences in fusion state, endplate stiffness, and loading conditions in this in vitro experimental setup.