# A Novel Method for Assessing Regional Tendon Stiffness and Its Significance
## 1 Introduction  
Supersonic shear imaging technology (SSI) is a relatively new technique to measure stiffness of soft tissue in real time [1; 2]. It measures the shear wave velocity to estimate the shear modulus (an index of stiffness) of a selected area [3]. A recent study demonstrated that the tendon shear modulus generated from the SSI is correlated with the Young's modulus, which was computed from a material testing system. The study had good intra- and inter-rater reliability [4]. This new technique thereby enabled a non-invasive, direct and reliable measurement of tendon stiffness to be performed on a selected region of a tendon, providing a quantitative value. Assessment of regional tendon stiffness enabled a better understanding of mechanical demand/stress on selected regions of a tendon. This was particularly important in the patellar tendon. First, proximal patellar tendonopathy, a common knee problem, was found to have localized pathological changes, and the pathological changes were mainly detected in the medial portion of the tendon [5; 6], therefore site-specific evaluation at the different regions of a tendon might have shed light on the regional changes in tissue elasticity.  
The patellar tendon runs obliquely, outwardly and distally from the apex of the patella and inserts into the tibial tuberosity [7]. Patellar tendon strain could be affected by tension in the lateral retinaculum [8] or by the position of the patella [9]. Supporting this theory, a higher patellar tendon shear modulus has been associated with a higher vastus lateralis muscle shear modulus [10]. The vastus lateralis is one of the muscle heads of quadriceps femoris, which is attached to the base and thesuperolateral border of the patella and is connected to the lateral side of the patellar tendon through the lateral retinaculum [11]. Using magnetic resonance imaging, a trend of increased patellar lateral tilt was observed in subjects with patellar tendinopathy when compared with controls [12]. In a recent study, patellar height, as measured by the Install-Salvati ratio, was significantly greater in patients with proximal patellar tendinopathy than in healthy controls [9]. Taken together, patella mal-alignment was observed in subjects with patellar tendinopathy. The mal-alignment of the patella may have contributed to changes in the distribution of strain within the patellar tendon. Such a relationship remains to be explored. Note that increases in proximal patellar tendon stiffness have been reported in athletes with patellar tendinopathy [10; 13]. The present study aimed to determine whether experimental displacement of the patella would alter patellar tendon stiffness. Regional changes in tendon stiffness were estimated using ultrasound shear wave elastography. In consideration of gender effects on patellar tendon orientation [14], both female and male subjects were recruited. We hypothesized that sideway displacement of the patella from its resting position would modulate patellar tendon shear modulus, the modulation would be site specific and greater increases in the medial half would be observed with lateral patellar displacement and in the lateral half with medial patellar displacement.