PD is an insidious condition that has been underdiagnosed and underestimated by both patients and physicians for many years. Various diagnostic techniques have been used to diagnose PD .
Some radiological methods, such as radiography and computed tomography, have the ability to correctly visualize calcified plaques, but they are unable to properly investigate tissues without calcified plaques . Magnetic resonance imaging is an appropriate technique for visualizing soft tissues and studying the CC and noncalcified plaques, and it can visualize areas of active inflammation usually present during the early phase of the disease . However, it is less useful for visualizing calcific plaques and, more importantly, it is expensive and cannot be performed in an office .
The use of B-mode ultrasonography is currently the most preferred method in terms of cost and time, and it is advantageous because of its optimal visualization of calcified plaques. However, it fails to visualize noncalcified plaques located in complex areas such as the base of the penis. Moreover, it is unable to visualize areas of active inflammation and is operator-dependent. Furthermore, it is difficult to use as a follow-up method [21, 22]. Therefore, we are investigating new diagnostic techniques that are simple and easy to use. SWE results are based on the speed of propagation of sound waves generated by the probe through the tissues. Stiffer tissues with less elasticity will produce a higher speed of propagation of the wave, but more elastic tissues will produce a lower speed of propagation of the wave.
SWE has been applied in other areas. For example, it has been used in the field of endocrinology to study the thyroid and discriminate between benign nodules and malignant tumors , and in the field of breast cancer to distinguish between fibroadenomas and invasive tumors .
Additionally, the SWE technique, compared to other elastosonographic techniques, is operator-independent, is useful for monitoring patients after drug therapy, and can obtain values expressed in kPa.
Cavernous body biopsy is the gold standard for the study of histological changes in patients with PD. However, it may be infeasible because of ethical implications and the invasiveness of the procedure. Our study demonstrated that PD patients with elastic tissues of the CC and TA that were replaced by stiffer and less elastic tissues had statistically higher kPa values obtained by SWE than the control group.
Riversi et al.  showed that the use of a real-time elastography technique for PD patients allowed the detection of areas with less elasticity that were not visible with B-mode ultrasound. In fact, they analyzed 75 PD patients and observed that the combination of elastography and B-mode ultrasound was able to detect the lesion in 93% of subjects . However, lesions were only detected in 86% of patients when using the B-mode technique and objective examination alone . Moreover, similar to our data, SWE was able to identify lesions in the absence of palpable plaques. In fact, our results indicated that, regardless of the presence or absence of palpable plaques, PD patients had consistently higher stiffness values than the control group. The most plausible hypothesis is that the stiffness of the tissue increases with PD because of the abnormal production of the extracellular matrix, increased number of myofibroblasts, and production of collagen types I and III with fewer elastic fibers. Iacono et al.  reported that patients who undergo radical prostatectomy, which is associated with a PD incidence of approximately 15.9%, have decreased trabecular elastic fibers and smooth muscle fibers and significantly increased collagen content compared with patients who undergo preoperative biopsies . Moreover, organized collagen and trabecular protocollagen deposits increased. This possibly occurs because of perioperative penile trauma and the release of cytokines that activate the abnormal wound healing process .
Hamidi et al. analyzed subjects who underwent radical prostatectomy with or without preservation of the nerve bundle . They showed that patients with damaged nerve bundles had a statistically significant increase in penile stiffness and a consequent decrease in bundle length .
In PD patients, the normal structures of the TA and CC are essentially lost . Specifically, it has been shown that there is an accumulation of myofibroblasts with subsequent activation and release of growth factors such as transforming growth factor-β1 and oxygen free radicals with the development of fibrosis and accumulation of collagen types I and III, thus leading to plaque formation over the long-term, curvature, and reduced penile length .
Moreover, Zhang et al.  reported that as stiffness measured by SWE increased, smooth muscle cells. Smooth muscle cells comprise a main component of the CC and approximately 50% of the penile microarchitecture; therefore, they have an important role in the erection process.
In another study, Zhang et al.  measured penile elasticity in erectile dysfunction patients and PD patients before and after a pharmacologically induced erection. They reported that there was a significant increase in viscoelasticity in patients with pharmacologically induced erections, demonstrating that SWE could be used to measure dynamic changes in erection .
Illiano et al.  analyzed 270 patients with various degrees of erectile dysfunction and observed a correlation between the worsening of IIEF-15 score and a higher degree of penile stiffness demonstrated by the erection hardness score; they also observed a reduction in elasticity demonstrated by an increase in kPa values diagnosed by SWE.
Qiao et al.  demonstrated an age-related increase in collagen fibers with an increase in penile stiffness. These data are in agreement with the epidemiology of PD, which has an incidence that increases with age.
SWE was able to detect an early increase in tissue stiffness, even in the absence of morphological features and detectable changes during objective examination, such as the presence of palpable plaque. Furthermore, it was found that as the VAS score increased, there was a negative correlation between the stiffness value of the TA, which concurs with the pathophysiology of the disease. During the acute phase of the disease, there are no structural changes in the TA and CC; these changes are usually present during the stable phase of PD, which is characterized by an increase in inelastic fibers compared to the quantity of elastic fibers.
Moreover, we found that with an increase in time from diagnosis to curvature onset and an increase in the degree of curvature, there was an increase in the stiffness (positive correlation) of the TA. This is because there is greater involvement of diseased tissues of the penis and diffuse fibrosis involving the total organ.
Our results are also in agreement with those of Illiano et al. , who studied men affected by PD who underwent surgery to plicate the TA. During that study, worsening of all areas of male sexual function (orgasmic, sexual desire, intercourse satisfaction, and overall satisfaction) was observed, but erectile function did not vary in a statistically significant manner preoperatively or postoperatively .
The limitations of this study include the small sample size and the failure to perform biopsies of the TA and the CC to correlate them with the kPa values obtained. Another limitation was that diagnostic confirmation with SWE was not performed after oral, local,or surgical therapy; therefore, any changes in kPa values could not be compared. In addition, the method is subject to inter/intra observer variability and therefore further studies are necessary to help Clinicians in standardizing the diagnostic procedure. The strengths of the study include the presence of a control group, the possibility of using stiffness as a reference to determine a possible conservative therapy response.