Spinal fusion is a procedure permanently joining two or more vertebrae to eliminate movement between them, reduce pain and stabilize the spine. Today, spinal fusion monitoring relies mainly on CT imaging and patient-reported outcomes. While widely used, CT scans expose patients to radiation during follow up visits, provide only snapshots of the process, and depend on subjective interpretation.

What if the success of a spinal fusion could be monitored continuously, remotely, and without any radiation? What if spine surgeons could detect complications before they became critical – based on objective sensor data rather than relying on subjective image interpretation?

This vision is being explored in the AO Research Institute Davos (ARI), the AO Innovation Translation Center (ITC) and their partners through SmartFusion – a project to translate AO’s sensor technology for fracture healing monitoring (AO Fracture Monitor) into a novel approach to address spinal fusion. (Fig 1).

Figure 1: Fusion monitor sensor (A) attached via clamps to rods of an instrumented spine model (B). X-ray image (C) showing the sensor attached to the instrumentation in an anatomical specimen utilized to monitor the implant load after transforaminal lumbar interbody fusion in a biomechanical study [3].

From Fractures to Fusion: The Spark of an Idea

The AO Fracture Monitor was first developed as a system implementing an implantable sensor to track fracture healing in long bones. By continuously measuring the implant load of orthopedic plates during daily patient activities, the system is designed to allow remote assessment of fracture healing progression at any time – based on quantifiable data. The AO Fracture Monitor is currently part of a clinical safety study with the goal to be certified as a medical device following a successful clinical trial.

As the development advanced, a natural question arose: could this technology be applied to another challenging area of orthopedics – spinal fusion? The possibility of offering objective, real-time and remote feedback on spinal fusion progression without any radiation would open the door to a transformative opportunity.

Testing the Concept

Utilizing the AO Fracture Monitor’s principle, the spinal fusion progression was assessed in ARI by measuring the load on implanted pedicle rod-screw systems. One major milestone was reached in a preclinical study, when an adapted version of the AO Fracture Monitor was successfully implanted in sheep – attached to posterior spinal instrumentation following posterolateral fusion surgery. Despite the in vivo complex loading scenarios, it was demonstrated that the continuously measured implant load decreased with ongoing fusion [1, 2].

The investigation was expanded to lumbar interbody and multilevel fusion procedures (Fig 1). Biomechanical studies were conducted in collaboration with the AO Technical Commission Spine to collect valuable data on both the potential and the limitations of the system [3, 4]. Particular emphasis was placed on the development of concepts to adapt the AO Fracture Monitor for spinal applications. Both preclinical and biomechanical studies supported the hypothesis that continuous implant load monitoring provides a reliable and objective measure of spinal fusion progression.