Understanding the changes in cartilage after injury is key to tracing the irreversible path to posttraumatic osetoarthritis. DTI is sensitive to cartilage PG and collagen and thus is an optimal tool to monitor cartilage degradation. Our aim is to test the value of DTI with clinical scanners to detect changes in articular cartilage after mechanical injury.

Cartilage-on-bone samples were harvested from two patients who underwent knee replacement surgery at the NYU Hospital for Joint Diseases. From the macroscopically intact areas we drilled nine 4-mm-diameter cylindrical cartilage-on-bone samples.

Histology before and after injury (Week0/Week2): 
Serial histological sectioning was performed with safranin-O. The intensity of safranin-O-staining increases with PG concentration. 8 non-consecutive slices from each sample were graded with an OARSI score (healthy=0 to bone remodelling=6).

Biomechanics before and after injury (Week0/Week2): 
A repetitive stress-relaxation test with a maximum strain of 20% of cartilage thickness was conducted (four 5%-steps). After the test, cartilage overloading/injury was induced at three levels: three at 0N (controls), three at 120N (mild), and three at 190 N (severe), with a strain rate of 0.1s⁻¹ [1]. The stress-relaxation test was repeated two weeks after injury. We use a standard quasilinear-viscoelastic model to fit the stress-relaxation curves [2].

DTI imaging on Week 0/Week 1/Week 2: 
We used a 3T clinical scanner, with an in-house built 4 cm butterfly coil and a RAISED sequence for DTI [3]. We set TR/TE=1500/49 ms, in-plane resolution=0.18x0.18 mm2, slice thickness=1.2mm, BW=300 Hz/pixel, 360 spokes, Δ=19ms and δ=14.45ms. The 2h MRI protocol consisted of two b=0 and two six-direction b=300s/mm² diffusion-weighted shells. We then fit a cylindrical tensor and calculate the MD and FA.

We then evaluate the correlations between the changes in MD/FA and those in histology and biomechanics.

Fig.1-left shows examples of histology safranin-O stained images. The OARSI score for native baseline cartilages was low, at an average of 0.75±0.36. There was no difference in the baseline OARSI score between the severe (0.61±0.30), mild (0.88±0.41) and negative control (0.77±0.16) groups. The score Week-0-2 change for the severely injured samples was 2.53±1.30, for the mildly injured was 0.64±0.62, and for the controls it was 0.37±0.25.

The DTI maps (fig.1-right) at Week 0/Week2 give the average MD values for Controls 1.34/1.31, for Mild Injury 1.28/1.38, and for Severe Injury 1.36/1.54 (μm²/ms units); for FA they were 0.25/0.3, 0.30/0.31 and 0.25/0.19, respectively.

Fig.2 shows the stress relaxation curves. For severe injury, the slopes ratio was 2.99±0.71, for mild injury1.89±0.39, for controls 0.99±0.14.

Fig.3 shows the ability for both (MD/FA) DTI biomarkers to differentiate between the three time points of the severe injury case. As expected, for the severely injured samples, the MD increases in time, whereas FA decreases. The MD achieves a (statistically-significant) group difference for both Week-1-2, while FA only for Week-2. On average, the severe samples group indicate a Week-0-2 change in bulk MD of 0.15±0.08 μm²/ms and in FA of -0.06±0.03.

In figure 4, the table gives the DTI/biomechanics/histology correlations, for bulk cartilage and specific layers. The correlations concern the before-and-after changes in DTI (MD/FA), histology (OARSI) and biomechanics (`relative’), as well as all the pooled measurements (`absolute’). In the case of MD-biomechanics, the only significant correlation is for bulk and deep layer parameters. In relative MD vs. histology, only the surface layer correlation is significant. This is consistent with the known pattern of cartilage degradation, where the surface is the first to degrade and, with time, the deterioration propagates down the cartilage. In absolute MD vs. histology, all except deep layer correlations are significant. These correlations are similar to previous early cartilage damage studies [3] which uses a larger group of samples (n=43).

Figure 5 visualises the most important result arising from table 1, showing that DTI correlates with both biomechanics and histology. The first subplot shows the relation between the two reference metrics, the change in histology against the change in the biomechanics slopes. The next two subplots show the most sensitive of the two DTI indices, the MD, against biomechanics and the histology. As indicated from the result in table 1, only the MD vs. biomechanics results are statistically significant. Though we do not see significance for the decreasing week-1 FA, there is a clear trend towards lower values. This also suggest that two weeks, rather than one, is a more optimal time for the experiment.

DTI biomarkers promise real-time indicators in the diagnosis and the monitoring of the disease. This study will help us to discriminate not just damaged from non-damaged tissue but also capture early signs of damage.