New sensor options for smart fracture implants and wearable devices: Laser-Doppler and white-light spectroscopy allow monitoring of bone regeneration via perfusion measurement

The diagnostic options for monitoring fracture healing are currently limited to methods that expose patients to ionizing radiation, i.e. X-rays or computed tomography. The development of new methods that ideally allow continuous monitoring via smart implants or wearables is urgently needed. Laser-Doppler and white-light spectroscopy, non-invasive light-based methods, could allow to monitor fracture healing via changes in perfusion, but this has never been investigated. It was hypothesized that 1) blood flow (BF) increases before a linear increase in oxygen saturation (SO₂) and that 2) SO₂ in nonunion cases remains as low as the minimum in union cases. A longitudinal observational cohort study with tibial fracture patients was conducted with additional cross-sectional measurements in nonunion patients and healthy controls. To assess SO₂, relative haemoglobin amount (rHb), and BF in the fracture gap, the 'Oxygen to see' (O2C) device was used. Thirty-five patients (20 longitudinal, 15 nonunion) and 28 controls were included. In the longitudinal group, SO₂ decreased, reaching a minimum (10 mm: 17.96 days, 16 mm: 15.50 days), and subsequently increased. BF increased to a maximum (10 mm: 12.90 days, 16 mm: 33.51 days), followed by a decrease. The SO₂ values in the nonunion group were similar to the minimum values in the longitudinal group. Findings in nonunion patients vs. controls differed only in SO₂ (10 mm: p < 0.001, 16 mm: p = 0.038), not in rHb or BF. Laser-Doppler and white-light spectroscopy provide characteristic SO₂ and BF trajectories that may serve to monitor fracture healing.