Intratendon bioimpedance spectroscopy: a first step exploring reliability and sensitivity to change

Tendon structure evaluation is fundamental for research and clinical practice, with the ultrasonography being the primary macrostructural imaging tool. Nevertheless, microstructural tendon adaptations require highly invasive procedures to be evaluated, i.e., tendon biopsies. To address this, the present study aims to explore the reliability and sensitivity to change of a minimally invasive Intra-Tendon Bioimpedance Spectroscopy (IT-BIS), which measures the tendon's electrical response across frequencies using fine needles inserted longitudinally along the tendon.

Forty healthy volunteers were placed in the supine position with the knee flexed at 30°. Four needles were inserted under ultrasound guidance along the patellar tendon and connected to the IT-BIS device in a tetrapolar configuration for impedance measurement. IT-BIS was assessed twice per session and there were two sessions separated by at least two weeks. To evaluate intra- and inter-session reliability we calculated indexes of relative (Intra-class Correlation Coefficient: ICC_1,1) and absolute reliability (Standard Error of Measurement: SEM. Minimal Detectable Change: MDC). Resistance, reactance, magnitude, and phase angle were measured across frequencies to characterize the tendon architecture. The high and low frecuency cutoff was established by the experimental characteristic frequency of our data at basal conditions. Also, intra-tendon electrolysis was applied in the second session to determine sensitivity to change of the IT-BIS.

IT-BIS showed excellent intrasession reliability across all parameters in both high and low frequency ranges with ICCs of 0.927 to 0.999. However, inter-session reliability was predominant poor (ICCs 0.087 to 0.390), with moderate reliability in phase angle at low frequency (ICC = 0.528). Sensitivity to change by needle insertion was evidenced at low frequency in all parameters, specifically resistance (p = 0.009; SRM = -0.54), reactance (p = 0.004; SRM -0.61), magnitude (p = 0.039; SRM = -0.4), and phase angle (p = 0.02; SRM = -0.67). However, sensitivy to change by percutaneous electrolysis was at high frequency, being resistance (p = 0.002; SRM = 0.68), reactance (p = 0.037; SRM = 0.39), magnitude (p = 0.01; SRM = 0.55), and phase angle (p = 0.006; SRM = 0.57). Minimal adverse effects were reported: 4.63% pain occurrence rate and were consistently mild and transient.

Minimally invasive IT-BIS showed excellent intra-session reliability but poor-to-moderate inter-session reliability, underscoring the need for further methodological standardization. The technique demonstrated sensitivity to acute physiological changes, identifying needle insertion effects at low frequencies and percutaneous electrolysis effects at high frequencies. Adverse effects were minimal and mild. Overall, IT-BIS emerges as a safe and promising tool for tendon assessment.