Temporal blood flow changes measured by diffuse correlation tomography predict murine femoral graft healing

<div><p>Blood flow changes during bone graft healing have the potential to provide important information about graft success, as the nutrients, oxygen, circulating cells and growth factors essential for integration are delivered by blood. However, longitudinal monitoring of blood flow changes during graft healing has been a challenge due to limitations in current techniques. To this end, non-invasive diffuse correlation tomography (DCT) was investigated to enable longitudinal monitoring of three-dimensional blood flow changes in deep tissue. Specific to this study, longitudinal blood flow changes were utilized to predict healing outcomes of common interventions for massive bone defects using a common mouse femoral defect model. Weekly blood flow changes were non-invasively measured using a diffuse correlation tomography system for 9 weeks in three types of grafts: autografts (N = 7), allografts (N = 6) and tissue-engineered allografts (N = 6). Healing outcomes were quantified using an established torsion testing method 9 weeks after transplantation. Analysis of the spatial and temporal blood flow reveals that major differences among the three groups were captured in weeks 1–5 after graft transplantation. A multivariate model to predict maximum torque by relative blood flow changes over 5 weeks after graft transplantation was built using partial least squares regression. The results reveal lower bone strength correlates with greater cumulative blood flow over an extended period of time (i.e., 1–5 weeks). The current research demonstrates that DCT-measured blood flow changes after graft transplantation can be utilized to predict long-term healing outcomes in a mouse femoral graft model.</p></div

Blood flow changes in different femur segments over a 9-week healing.

<p>Average <i>rBF</i> was calculated as the average of all mice (N = 19). Position on the femur is illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197031#pone.0197031.g002" target="_blank">Fig 2</a>, with the center of the graft as the origin.</p

Maximum torque measured by biomechanical testing.

<p>* indicates that the difference between autograft group and allograft group is statistically significant.</p

Average blood flow in the graft region (<i>rBF</i>^g) during the 9 weeks of healing.

<p>The first row shows the individual temporal changes in the different graft groups, and the second row shows the corresponding group average with the standard error of the mean. In order to show the full range of temporal changes for the individual mice in the allograft group, a different <i>rBF</i> range is chosen (top middle plot), whereas other plots are fixed with the same <i>rBF</i> range.</p

Segmentation of a femur into 1 mm segments to calculate the average of relative blood flow (<i>rBF</i>).

<p>Each femur segment was represented by its distance from the center of the graft, with negative values representing segments closer to the proximal end (connecting to the pelvis) and positive values closer to the distal end (connecting to the tibia). The yellow part in the center denotes the inserted graft, and the red parts are the parts of the host femur.</p

Average blood flow in the graft region (<i>rBF</i>^g) during the 9 weeks of healing based on the measured maximum torque.

<p>The range of maximum torque for the high, medium, and low torque groups are >10, 5–10, and < 5 N·mm, respectively. The first row shows individual changes and the second row shows corresponding group averages. Note the y range for the low torque group is much larger than the others.</p

Correlation coefficients of log(<i>rBF</i>^g) at different weeks of all mice (N = 19).

<p>(<b>Bold</b>: correlation coefficient > 0.7).</p

Prediction results on bone maximum torque.

<p>(a) Partial Least Squares (PLS) predicted maximum torque versus measured results. Data from the autograft, allograft, and T.E. allograft group are marked with red, blue and black, respectively. The dashed lines are the 1:1 line. (b) Linear correlation between <i>TrBF</i> and measure maximum torque.</p

BloodFlow

BloodFlo

Instrument and probe placement.

<p>(a) Diffuse correlation spectroscopy system consists of a long coherence source, a contact probe, four photon-counting APDs (avalanche photodiodes) and an autocorrelator. (b) Measurement positions P1—P6 were chosen so that they were 1.5 mm apart along the femur starting from the proximal end. The source (red dot) and detectors (blue dots) in the probe were aligned along the femur.</p