Purpose or Case Report: Diffusion tensor imaging (DTI) is a technique in which diffusion of water molecules is restricted by cellular membranes. In the physis and adjacent metaphysis, the physeal metaphyseal complex (PMC), the columns of cartilage and newly formed bone restrict the diffusion of water to primarily the longitudinal direction. DTI indirectly measures physeal activity by revealing tissue microarchitecture of the PMC and by measuring the Brownian motion of water along the columns of bone and cartilage, which can be imaged and quantified as tractography. Having conducted DTI-MRI on over 900 knees, our research group has established that DTI can distinguish between a normal and a dysfunctional physis.
A physeal bar or partial physeal arrest is the consequence of injury to an open physis, resulting in a perpendicular bony bridge through the physis which can lead to limb shortening or angular deformity. This poster presents our initial experience in performing DTI on the growth plate in 7 patients (5 females, 2 males) with MR-confirmed physeal bars to characterize tractography patterns associated with bony bridges and compare the tracotgraphy and diffusion metrics of the injured versus the healthy contralateral physis.
Methods & Materials: All patients had a conventional MRI with 3D gradient echo images. We calculated the area of the physeal bar relative to the bony bridge, and classified the bridges as central or peripheral, and greater or smaller than 25% of the physeal area. We characterized the tractography patterns associated with bony bridges based on: abundance, color of the tracts, and degree of fiber tract organization. We compared a) the conventional MR images, and the characteristics of the bridge with the tractographic appearance; b) the tractographic appearance of the affected with the unaffected contralateral physis c) the DTI metrics (tract number, length, volume and fractional anisotropy) of the affected and the normal contralateral physis; and d) the degree of fiber tract organization with bone bar size (<25% or ≥ 25%).
Results: In the areas of bridges, tracts were reduced or absent, and there was a decrease in the number of (blue) tracts in the direction of the bone length. Compared to the unaffected side, tracts were smaller and less blue even in the areas outside the bridge. DTI parameters were decreased overall on the side of the bridge.
Conclusions: Physeal bars result in abnormalities of the number and direction of tracts even in the uninvolved segments of the physis.