Continuous Implant Load Monitoring to Assess Bone Healing Status—Evidence from Animal Testing

Abstract

Background and Objectives: Fracture healing is currently assessed through qualitative evaluation of radiographic images, which is highly subjective in nature. Radiographs can only provide snapshots in time, which are limited due to logistics and radiation exposure. We recently proposed assessing the bone healing status through continuous monitoring of the implant load, utilizing an implanted sensor system, the Fracture Monitor. The device telemetrically transmits statistically derived implant parameters via the patient’s mobile phone to assist physicians in diagnostics and treatment decision-making. This preclinical study aims to systematically investigate the device safety and performance in an animal setting. Materials and Methods: Mid-shaft tibial osteotomies of different sizes (0.6–30 mm) were created in eleven Swiss mountain sheep. The bones were stabilized with either a conventional Titanium or stainless-steel locking plate equipped with a Fracture Monitor. Data were continuously collected over the device’s lifetime. Conventional radiographs and clinical CT scans were taken longitudinally over the study period. The radiographs were systematically scored and CTs were evaluated for normalized bone volume in the defect. The animals were euthanized after 9 months. The sensor output was correlated with the radiologic parameters. Tissue samples from the device location were histologically examined. Results: The sensors functioned autonomously for 6.5–8.4 months until energy depletion. No macroscopic or microscopic adverse effects from device implantation were observed. The relative implant loads at 4 and 8 weeks post-operation correlated significantly with the radiographic scores and with the normalized bone volume metric. Conclusions: Continuous implant load monitoring appears as a relevant approach to support and objectify fracture healing assessments and carries a strong potential to enable patient-tailored rehabilitation in the future.

Continuous Rod Load Monitoring to Assess Spinal Fusion Status–Pilot In Vivo Data in Sheep

“…The principle of continuous implant load monitoring has recently proven its potential as a relevant metric for assessing bone consolidation processes [ 20 ]. After having obtained encouraging results for long-bone healing, the question about transferability of the concept to spinal fusion is apparent.…”

“…The utilized metric to access fusion, relative implant load (RIL), was defined as the average of the highest 50 loading events over one day. This approach, taken from the trauma application [ 20 ], intends to maximize the observable difference between non-fused and fused states. While the vast majority of loading events occur in the low-amplitude range, the higher magnitude events were found to be most predictive for resolving bony consolidation and, thus, are collected in a peak event buffer.…”

“…A direct comparison between the studies is not feasible due to differences in the used model and our single-case data. It is worth noting that, in long-bone trauma applications, the implant load signal of the sensor appears particularly sensitive to early changes in fracture stiffness [ 20 ], while in this first spinal fusion case the signal response seems to be less pronounced early on. Over 16 weeks, RIL dropped to 30% for sensor L34 and only to 67% for sensor L23, indicating that in the least sensitive configuration the fusion process was resolved by ~30% of implant load drop.…”

“…An existing implantable sensor prototype, designed for plate load measurement in long bone fractures [ 20 ], was used in this study in a modified form. The sensor is actively powered by a battery, made for implant applications, to operate continuously and autonomously.…”

“…While Szivek et al [ 16 ] observed a decrease in bone strain during fusion in a single patient, Rohlmann et al [ 17 ] could not demonstrate a distinct response of fixator loading on the fusion process in ten patients with an instrumented screw-rod system. As opposed to these passively powered snapshot measurements, a novel approach involving the continuous measurement of implant loading has been recently developed by our group for trauma applications [ 9 , 18 , 19 , 20 ]. The system comprises an implantable datalogger which can be releasably attached to conventional bone plates.…”