In the present study, we investigated the changes in the median nerve strain, application of pressure to the skin, distal latency of motor and sensory conduction studies, and cross-sectional area in CTS patients before and after carpal tunnel release. There was a significant increase in the strain and significant decreases in the applied pressure and the pressure-strain ratio, after carpal tunnel release. In addition, there were significant decreases in the distal latencies and the cross-sectional area. This suggests that carpal tunnel release affects not only functional and morphological recoveries, but also strain and surrounding tissue stiffness. This method and the result of strain/electrophysiological measurements may support understanding of median nerve recovery in relation to the stiffness of the surrounding structures.
Clinical significance of strain measurement has been widely accepted. However, questions remain regarding universality of the evaluation. This is because the ultrasonography is highly dependent on the examiner, and the load on the tissue during strain measurement has not been evaluated in previous studies [26]. These issues limit the reproducibility and universality of the strain measurement. In our previous studies, we tried to develop quantitative methods to measure strain of the tissues. It was found the ultrasonic strain measurements of the median nerve were superior to morphologic assessment in diagnosing CTS [27]. During the process, it was noticed applied pressure had also important factor to measure strain of the tissues. Then, the system was modified to measure strain and applied pressure simultaneously [14]. The method reported in this study may improve the reproducibility of the strain measurement as it measures strain and applied pressure simultaneously, with reproducible stress.
The results suggested that median nerve strain had a higher reliability to identify clinical recovery at three months after carpal tunnel release. Because electrophysiological recovery is associated with histological recovery, once the nerve denervates it may take longer to recover than tissue elasticity. Thus, strain measurement may be more reliable for clinical recovery than electrophysiological test assessment in the early stage after carpal tunnel release. From our previous study, median nerve strain for CTS patients with conservative treatment was 0.21% on average. 14 The strain of the patients for carpal tunnel release in this study was 0.16% on average. The strains were significantly lower in the patients with operative treatment compared with the patients with conservative treatment. This suggests that strain measurement may also be useful in deciding the indications for carpal tunnel release.
The measurement of the pressure reflects the internal pressure around the carpal tunnel. Carpal tunnel pressure in patients with CTS is reported to be higher than in healthy individuals. It was reported that elevated carpal tunnel pressure in CTS patients decreased to a normal level immediately after carpal tunnel release [28,29,30]. In this study, the applied pressure also decreased after carpal tunnel release. This may be due to changes in carpal tunnel compliance after carpal tunnel release. 29 Several reports measured carpal tunnel pressure clinically [15,16,17,18,19,20,21]. Constant infusion flow techniques were the most commonly used. Since this method requires the insertion of a slit catheter into the carpal tunnel, it was difficult to evaluate carpal tunnel pressure over time. The method introduced in this study may be a way to estimate the pressure around the carpal tunnel non-invasively.
The pressure-strain ratio showed the highest accuracy in terms of recognizing clinical recovery compared with other parameters. In our previous study, it was found that the pressure-strain ratio was the highest accuracy for the diagnosis of CTS [14]. The results of this study had the same tendency. Generally, the elasticity of a material is expressed using Young’s modulus, which is defined as the stress (force per unit area) divided by the strain (displacement caused by the force) of a material. The elastography strain is expressed by the subsequent tissue displacement tracked between pairs of echo frames, and the strain calculated from the axial gradient of the displacements. Since the strain is expressed by the rate of tissue displacement, the pressure-strain ratio cannot be an absolute indicator of elasticity. However, it still reflects the relationship between the strain and the applied pressure. This method could be used to evaluate tissue elasticity in the human body. As the pressure-strain ratio reflected the clinical recovery, it may be useful in determining the effectiveness of treatment.
There are several limitations to our study. Firstly, clinical recovery was defined by the qualitative assessment of the patient. Quantitative assessment of clinical recovery based on a questionnaire (i.e., DASH, Boston carpal tunnel syndrome questionnaire, etc.) may be required to compare parameters. In addition, the degree of clinical outcomes were heard by the surgeon. It may be likely that patients will be biased towards presenting more positive outcomes. Secondly, we measured the strain and pressure at the proximal carpal tunnel. Since the median nerve strain differs from proximal to distal, there may be some difference in the results at different levels of the carpal tunnel. In addition, we evaluated these parameter at only three months after the surgery. The results may change during the histological recovery of median nerve. Thirdly, the patient age distribution was heterogeneous. Aging may cause fibrosis and/or atrophy in tissues, so the age distribution may have affected the results. There was one patient relatively older who showed a poor clinical outcome. In the future study, we may need more attention to investigate the poor outcome patients. To evaluate prognostic values of strain and pressure measurements, it needs to measure at multiple time points and from larger groups of patients with various outcomes. Lastly, we did not compare the applied pressure to the internal pressure of the carpal tunnel. Because measurements were performed at outpatient clinic visits, the use of invasive measurements was avoided. To evaluate the relation between applied pressure and carpal tunnel pressure may require simultaneous pressure measurement. In addition, the complexity of the measurements should be simplify in the future study.