Introduction: Posterior tibial tendonitis affects 3.3% of women over the age of 40 leading to pain, swelling and a flatfoot deformity which limits ambulation. Initial treatment consists of bracing and therapy for 3-6 months. This treatment fails to relieve symptoms in up to 30 % of patients. There is no available method to predict which patients will do well with conservative treatment and which will ultimately require surgery. Degeneration of the posterior tibial tendon (PTT) is closely associated with irreversible degradation of its collagen structure, leading to lengthening of the tendon and changes in its material properties. If these properties could be accurately monitored in vivo, this information could be used for quantifying the severity of tendonosis, assisting with prognosis, and monitoring the effects of therapy. Ultrasound elasticity imaging (UEI) is a non-invasive technique for quantifying the stiffness of soft tissues in vivo. The goal of the present study was to determine whether UEI can detect differences in the material properties of PTT between the symptomatic and asymptomatic tendons in subjects with PTT dysfunction (PTTD) and the tendons of healthy controls. Methods: Nine healthy volunteers, 21 patients with unilateral PTTD and 4 patients with bilateral PTTD provided informed consent to participate in the IRB approved study. Both the left and right feet of each subject were tested, yielding 18 healthy control, 21 asymptomatic and 29 symptomatic posterior tibial tendons. The subject’s leg and foot were secured into a custom designed inversion exercise board in maximum plantar flexion (figure 1). The subjects were asked to exert maximal inversion force against a dynamometer during a 6 second period. Ultrasound data (phase and amplitude) along the PTT were collected with a commercial ultrasound linear array (Zonare Medical Systems, L14-5, 14MHz hockey stick, 50Hz) during the inversion task (Figure 2a - S=subcutaneous tissue, T= tendon, M=Medial malleolus). The inversion force exerted by the foot (F) was measured with a dynamometer fixed to the exercise board. Speckle tracking was then performed on the raw ultrasound data to estimate the displacement along the length of the tendon and strain (S) at each pixel for each frame (Figure 2b). The cross-sectional area (A) of the PTT was estimated from transverse B mode images. Stress / strain curves were plotted for each trial. The elastic modulus was calculated by a linear fit of the stress / strain curves. The maximum inversion force, displacement, and elastic modulus were compared between the 3 groups using paired t tests.
Results: The maximum inversion force averaged 38± 36N in the healthy tendons, 29±16N in the asymptomatic and 26±13N in the symptomatic tendons, although these differences were not significant (fig 3a). The displacement was significantly greater in the symptomatic and asymptomatic tendons (2.4±1.8mm and 1.8±1.3mm) compared to the healthy controls (1.1±-1.1mm, p< 0.01), but they were not different from each other (fig 3b). Similarly, The elastic modulus for both asymptomatic (89±48 MPa) and symptomatic (96±52 MPa) tendons was significantly less than that of the control tendons (152±68 MPa, P<0.005), but were not different from each other (fig 3c). Discussion: While the inversion forces did not differ between healthy control tendons and either the asymptomatic or the symptomatic tendons of patients with PTTD, the displacements were significantly greater in both the asymptomatic and symptomatic tendons leading to lower elastic moduli compared to the healthy controls. Interestingly, there was no difference between the displacements or moduli of asymptomatic as compared to symptomatic tendons suggesting that the changes in material properties either: (1) precede the development of symptoms, (2) are independent of symptoms, or (3) are an inherent property of the tendons in patients who are prone to develop tendonitis.
One limitation of this study was the use of young healthy controls as opposed to control tendons from subjects who were more similar in age, gender, comorbidities, and foot deformity. The use of a dissimilar control leads to the inability to pinpoint the significance of the differences in displacement and modulus that were identified.
Significance/Clinical Relevance: Differences in tendon displacement under load and modulus of elasticity exist between healthy posterior tibial tendons and those of individuals with tendinosis. The cause of these differences and their significance is the subject for future study.

0531 - In Vivo Ultrasound Elastography of Material Properties of Healthy and Diseased Human Posterior Tibial Tendon