Axial Penile Buckling Forces Vs Rigiscan TM Radial Rigidity

Axial penile buckling forces vs RigiscanTM radial rigidity as a function of intracavernosal pressure: why Rigiscan does not predict functional erections in individual patients

D Udelson2, K Park1, H Sadeghi-Najed1, P Salimpour1, RJ Krane1 and I Goldstein1*

1Department of Urology, Boston University School of Medicine; 2Department of Aerospace and Mechanical Engineering, Boston University, College of Engineering, Boston, Massachusetts

Aim: An improved understanding of the relationship between radial and axial rigdity values would enable better appreciation of the clinical usefulness of RigiScanTM, the most widely utilized determination of erectile rigidity testing. Previous studies have shown that axial rigidity (measured by buckling forces) correlated well with radial rigidity (measured by RigiScanTM) for radial rigidity values below 60%. For radial rigidity exceeding 60%, there was poor correlation. Heretofore, there has been no physiologic explanation of this phenomenon. Methods: During dynamic pharmacocavernosometry in 36 impotent patients, we investigated the relationship between axial buckling forces and RigiScanTM radial rigidity and, for the ®rst time, how they both vary with pressure, (which we varied over over a wide functional range). In addition, we recorded multiple penile length and diameter values enabling us to relate, also for the ®rst time, axial and radial rigidity to individual mechanical erectile tissue and penile geometric properties. Results: Marked differences were found in the manner RigiScanTM radial rigidity units and axial buckling force magnitudes increased with increases in intracavernosal pressure values in each individual. The former asymptotically approached a maximum ®nite value while the latter increased continuously towards in®nity. Based on data in this study, RigiScanTM radial rigidity values greater than 55% may be considered a necessary criteria for vaginal intromission capability in all partners but it is not a suf®cient one. Conclusions: Axial and radial rigidity share a common dependency upon intracavernosal pressure, however, they are also dependent upon other unique physical determinants. For axial rigidity, additional dependent variables include cavernosal erectile tissue properties and penile geometry, while for radial rigidity, this may include tunical surface wall tension properties. Clinical devices which assess functional penile rigidity should utilize axial and not radial rigidity testing.

Keywords: impotence; erectile dysfunction; erectile physiology; diagnosis; axial penile rigidity; radial rigidity; RigiScan

Introduction The RigiScanTM device does not directly determine axial penile rigidity in a given individual. Axial rigidity, not radial penile deformation, is the In 1985, the RigiScanTM penile rigidity device was physical parameter which best objectively de®nes introduced as a diagnostic tool for patients with the capability of the erect penis to resist deformation erectile dysfunction.1 The RigiScanTM displays from vaginally-mediated compressive forces during radial penile rigidity in arbitrary units by assessing vaginal intromission, and during pelvic thrusting radial penile deformation from a 10 ounce force following penetration, and thus be of suf®cient applied circumferentially around the penile shaft.2 functional quality for satisfactory sexual intercourse.3±5 Axial rigidity is classically measured by the penile buckling force. This is de®ned as the magnitude of the axial compressive force applied to the glans of the penis which results in pronounced curving of the shaft, such that an additional small 4 *Correspondence: Dr I Goldstein, Department of Urology, 720 force would cause collapse (buckling). Harrison Ave., P(606), Boston MA 02118, USA. The RigiScanTM is widely utilized clinically as Received 5 February 1999; accepted 2 May 1999 it provides computerized, portable, continuous Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 328 recordings of the numbers and magnitudes of penile Methods radially-oriented deformation=tumescence events during home nocturnal penile tumescence studies without sleep interference.6 In spite of these A single institution prospective study design was characteristics, the use of the RigiScanTM in im- selected. The study population consisted of patients potence diagnosis has questionable value assessing with erectile dysfunction de®ned as having, for whether erections are of suf®cient quality for greater than six months, the consistent inability to satisfactory sexual intercourse if the clinically obtain and maintain an erection of suf®cient quality relevant variable, axial penile rigidity, is not con- for satisfactory sexual intercourse.8 Each patient sistently predicted by RigiScanTM radial rigidity in a underwent diagnostic dynamic infusion pharmaco- given individual. cavernosometry and pharmacocavernosography to In 1993, Allen et al 7 examined this relationship evaluate hemodynamic integrity as well as axial and between radial and axial rigidity values in 13 radial rigidity over a wide range of intracavernosal impotent patients during nocturnal penile tumes- pressures (0 ± 150 mmHg).3 ± 5,9,10 Patients were ex- cence testing. Based on 25 total data points from the cluded if the pharmacocavernosometry study was penile base, they concluded that for RigiScanTM incomplete, if the patient had penile curvature radial rigidity values lower than 60% (6 data precluding accurate axial buckling determination, points), the two rigidity measurements correlated if complete smooth muscle relaxation was unable to well with each other. However, for RigiScanTM be achieved despite multiple repeat doses (up to 3 radial rigidity values greater than 60% (19 data total doses) of intracavernosal vasoactive agents as points), there was poor correlation. No physiologic determined by non-linear relationships between explanations were provided to explain the discre- ¯ow-to-maintain and intracavernosal pressure9,10 pancy between axial and radial rigidity at high or if erectile rigidity could not be generated or RigiScanTM values. maintained despite saline pump infusion during the Recent advances have been made in the engineer- examination due to severe corporal veno-occlusive ing analysis of axial penile rigidity determinants.3±5 dysfunction. Three factors have been found to directly in¯uence Intracavernosal pressure (mmHg) was determined axial penile rigidity: intracavernosal pressure, pe- by direct cannulation of the corpus cavernosum nile tissue mechanical properties and penile geo- with a 20 gauge angiocatheter. The catheter was metry.4 An analytic expression of theoretic axial ®lled with heparinized saline and connected to a penile rigidity in terms of its three determinants has pressure transducer (Figure 1). The change in been derived and a close correlation between intracavernosal pressure, DP, was de®ned as the theoretic and clinically measured axial buckling increase of intracavernosal pressure above the forces has been reported.4 Based on the above individual's recorded ¯accid state pressure (see the engineering analysis, radial rigidity, the parameter Appendix). measured by RigiScanTM, is not an accurate pre- The RigiScanTM base loop with clean cotton cover dictor of axial rigidity.4 was carefully applied around the lower third of the There is a paucity of research investigating the penile shaft. RigiScanTM radial rigidity values were relationship between axial and radial rigidity in an recorded over multiple intracavernosal pressure individual, a clinically relevant issue since Rigi- values during pharmacologic and saline-infused ScanTM use has been so prevalent as an objective penile erections. A radial rigidity of 100% repre- discriminant of erectile rigidity. In light of recent sented no measurable radial displacement from the advances mentioned above in the understanding of quanti®ed squeezing force (10 ounces) applied axial penile rigidity and its determinants,3±5 it was circumferentially around the penile shaft; for each the aim of this study to re-examine the relationships 0.5 mm of loop shortening that was detected, the between axial and radial rigidity in a large patient radial rigidity measure was reduced by 2.3%.2,6,11 series with a view toward understanding the At 5 ± 10 mmHg intracavernosal pressure intervals, physiologic reasons for the difference between the penile diameter (cm) was measured by paper ruler two readings. The overall objective of this study was placed around the middle third of the penile shaft to compare in each patient, for the ®rst time, and pendulous penile length (cm) was recorded by a multiple RigiScanTM radial rigidity values with plastic ruler placed at the pubic bone to record multiple measured axial penile buckling forces over distance from the pubis to the end of the glans.3±5 wide ranges of functional intracavernosal pressures. Pendulous penile volume was calculated using In addition, an aim was to examine the differing length and diameter measurements assuming a effect that tunical and erectile tissue mechanical cylindrical model.3 properties and penile geometry have on axial and Axial buckling forces (see the Appendix) were radial rigidity.3±5 Our hope was to further the determined in all patients by a technique previously knowledge concerning the role of RigiScanTM radial reported.4 At 5 ± 10 mmHg intracavernosal pressure rigidity testing in individual patients with erectile intervals, the axial compressive force observed to dysfunction. cause curving of the penile shaft, such that a small Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 329

Figure 1 Schematic view illustrating the methodology utilized during pharmacocavernosometry to measure in each individual the following variables: Rigiscan radial rigidity, axial buckling force, intracavernosal pressure difference, penile circumference (mid-shaft) and pendulous penile length. Such data allowed for penile volume, axial buckling and Rigiscan radial rigidity values to be plotted over a range of intracavernosal pressure values. incremental force would lead to buckling, was of the overall ability of the corpora to expand to determined to the nearest 0.1 kg. A standard kilo- maximum volume at relatively low intracavernosal gram weight scale with a wide protective cap was pressure, was calculated from the volume-pressure applied to the glans penis. Care was taken to apply curve which gave the best least squares ®t to the the compressive force exactly parallel to the long theoretical volume-pressure equation.3 Cavernosal axis of the erect penile shaft. A second observer was expandability is a constant for each patient3 and has used to help con®rm that no buckling force been shown, in an animal model, to be related to the measurements were made at an angle to the erect degree of cavernosal ®brosis.12 Tunical distensibil- penile shaft. The downward force on the weight ity (see the Appendix), the relative volume of the scale was slowly increased and held at the deter- fully erect to the completely ¯accid pendulous mined penile buckling force for 5 ± 10 s. As the penis, was calculated.3 Tunical distensibility is erections were sustained by pharmacologic stimula- believed to be a measure of tunical elastic mechan- tion with or without saline infusion repeated ical properties. Penile aspect ratio (see the Appen- buckling measurements were performed until con- dix), the diameter to length ratio of the pendulous sistency in the value was achieved (Figure 1). penis,3 was recorded as the average ratio over all The penile tissue mechanical properties and the measured intracavernosal pressures. In fact, varia- penile geometric factors which determine axial tions in penile aspect ratio at various intracaverno- penile rigidity were individually calculated. Caver- sal pressures have been found to be minimal, nosal expandability (see the Appendix), a measure thereby leading to the assumption of a constant Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 330 value for each patient.3 Flaccid penile diameter (see radial rigidity magnitudes both varied with intra- the Appendix) was recorded directly at the onset of cavernosal pressure, the magnitudes of these vari- the examination. Theoretical axial buckling forces ables were arbitrarily selected at the same intraca- were calculated using equations previously pub- vernosal pressure difference, DP 50 mmHg. lished from recorded or calculated variables includ- Pressure data are expressed as the intracavernosal ing intracavernosal pressure, cavernosal expand- pressure difference, that is, the pressure value ability, tunical distensibility, penile aspect ratio increase above the ¯accid intracavernosal pressure. and ¯accid penile diameter.3±5 The remaining four variables, cavernosal expandability, tunical distensibility, penile aspect ratio and ¯accid penile diameter were calculated.3 Figure 2 graphically represents data from the six Statistical analysis typical patients. The ®gure includes measured (data points) and theoretical3 (solid line) relative volume, 4 Data are presented as mean Æ standard deviation. measured (data points) and theoretical (solid line) Correlation coef®cients were calculated for the best axial buckling and measured (data points), and TM ®tting curves. calculated curve (solid line) RigiScan radial rigidity values, all plotted against DP. The Rigi- ScanTM (solid line) was calculated by using an exponential type curve similar to that of the Results theoretical volume formula.3 There was high agreement between measured and theoretically derived values for relative volume as A total of 36 impotent patients met the inclusion well as axial buckling force plotted against DP, and exclusion criteria and formed the study popula- (Figure 2), as previously reported.4 Furthermore, tion. Table 1 describes the clinical characteristics of there was high agreement between measured and the study population. Table 2 illustrates the mean calculated RigiScanTM (solid line) using the adapta- and range of hemodynamic and rigidity values tion of the theoretical volume formula. obtained during repeat dosing dynamic pharmacocavernosometry. Such values are typical of a population of men with erectile dysfunction.5,9 Table 3 illustrates data from six typical patients Table 2 Hemodynamic characteristics obtained during dynamic who represented a wide spectrum of axial buckling pharmacocavernosometry in the 36 patients force magnitudes, beginning from the patient with Mean Æ s.d. Range the highest theoretical axial buckling force at DP 50 mmHg (patient SC) to the lowest (patient Equilibrium pressure (mmHg) 58 Æ 20 18 ± 95 TM Flow-to-maintain at: HM). Since axial buckling force and RigiScan 90 mmHg (ml/min) 4 Æ 60±27 120 mmHg (ml/min) 5 Æ 70±30 Table 1 Clinical characteristics of the study population (n 36) 150 mmHg (ml/min) 6 Æ 80±34 Pressure decay in 30 s from 46 Æ 23 15 ± 101 Age (y) 40 Æ 14 (range: 16 ± 76) 150 mmHg (mmHg) Brachial-cavernosal artery systolic Cigarette smoking 28% pressure gradient (mmHg) Hypercholesterolemia 17% Right 39 Æ 17 9 ± 87 Hypertension 8% Left 39 Æ 17 13 ± 72 Diabetes mellitus 8% Axial buckling force at DP 50 mmHg 0.9 Æ 0.5 0.2 ± 2.3 Blunt trauma 19% Rigiscan radial rigidity at DP 50 mmHg 73 Æ 951±88

Table 3 Associated measurements in six typical patients who represented a wide spectrum of axial buckling force magnitudes (0.32 kg ± 2.02 kg)

Theoretical Measured Rigiscan Penile Flaccid axial axial radial Cavernosal Tunical aspect penile buckling buckling rigidity expandability distensibility ratio diameter a Patient force (kg)* force (kg)* units X (1 mmHg) VE/VF (D/L) Av (cm) SC 2.02 2.3 69 0.082 3.10 0.307 3.09 JK 1.12 0.7 77 0.074 2.50 0.310 2.71 NL 0.87 1.0 77 0.063 3.58 0.326 2.71 RL 0.65 0.4 65 0.065 3.37 0.291 2.55 WC 0.42 0.4 62 0.052 2.66 0.283 2.71 HM 0.32 0.4 74 0.045 3.92 0.294 2.48

aMeasurements performed at DP 50 mmHg. Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 331

Figure 2 Plots of volume ratio (V=VF), axial buckling force (kg) and Rigiscan radial rigidity vs intracavernosal pressure increase DP of six typical individuals are presented in the order of `best' to `worst' theoretical axial buckling force values. Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 332 As depicted in Figure 2, in each of the six expandability (X 0.045 mmHg71). In fact the va- individuals, RigiScanTM radial rigidity and axial lues of cavernosal expandability in Figure 3 buckling forces both increased with pressure. How- decreased continuously from patient SC to HM with ever, the increase with pressure was distinctly the exception of patients NL and RL. As can be seen different for the two measurements. The axial from Table 3, and Figure 3, patient NL, who had a buckling force had positive curvature and increased higher buckling force than patient RL, was found to continuously towards in®nity constrained only by have a lower expandability value. This can be the tunica's ability not to rupture. The RigiScanTM explained by noting that the values of penile aspect radial rigidity had negative curvature and asympto- ratio and ¯accid penile diameter were larger in tically approached a maximum ®nite value in a patient NL than in patient RL, showing that axial manner analagous to the way penile volume asymp- buckling is also sensitive to penile geometric totically approaches its erect value with pressure. factors.3±5 That is, as DP increased in each individual, RigiScanTM radial rigidity values were not ob- RigiScanTM became increasingly insensitive to pres- served to be sensitive either to penile tissue sure while axial buckling force became more mechanical properties, such as cavernosal expand- sensitive to pressure. ability or penile geometric properties, such as penile Figure 3 depicts the bar curves of theoretical axial aspect ratio and ¯accid penile diameter, factors buckling force, cavernosal expandability and Rigi- which affect axial rigidity. ScanTM radial rigidity in the six typical patients. It Figure 4 displays 161 measured penile axial was observed that patient SC, with the best buckling force values (greater than 0.2 kg) with theoretical buckling characteristics, also had the RigiScanTM radial rigidity values in the 36 patients highest cavernosal expandability (X 0.082 over the range of intracavernosal pressures. The mmHg71), whereas patient HM, with the worst correlation coef®cient was 0.58, which is similar to theoretical buckling characteristics had the lowest the result obtained by Allen et al.7 The two

Figure 3 Bar graphs to show six typical patients who represented a wide spectrum of axial buckling forces and their measured determinants compared to Rigiscan radial rigidity. Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 333

Figure 4 Correlation between axial buckling force and Rigiscan radial rigidity. horizontal dotted lines represent demarcations rigidities were related. The Bradley et al article based on Karacan's criteria for erectile potency.13 consisted of data from 11 normal and 11 impotent That is, 0 ± 0.5 kg axial buckling is unlikely to be individuals who were examined during three con- associated with vaginal intromission capability in cecutive nights of sleep laboratory examination.1 It any partner, 1.5 kg or greater axial buckling is likely was claimed that `the validity of radial loading as a to be associated with vaginal intromission capability methodology for assessment of penile rigidity has in all partners, while 0.5 ± 1.5 kg axial buckling may been demonstrated by plotting axial buckling force or may not be associated with vaginal penetration (grams) measurements against maximum values of capability depending on various characteristics of rigidity (radial, expressed as %) in the same the partner. The two vertical dotted lines represent patients. The plot of this data is linear and demarcations based on criteria in the literature for documents the validity of radial loading as a RigiScanTM radial rigidity values predicting func- technique for assessing penile rigidity.' The refer- tional erectile potency, ranging from 55 ± enced plot `validating' radial rigidity consisted only 70%.2,7,14,15 However, for the data points relating of eight data points with no explanation provided to the likelihood of successful vaginal penetration, for methodology, subjects examined or missing there were numerous RigiScanTM radial rigidity rigidity measurement information.1 values greater than 55% or 70% in all three The Frohrib et al 11 article consisted of seven categories, whereas, there were numerous data impotent patients who were examined during points of RigiScanTM radial rigidity values less than dynamic pharmacocavernosometry. They did not 55% only in the `unlikely' category. Therefore report a linear relationship between axial and radial having RigiScanTM radial rigidity values greater rigidity. Rather, the RigiScanTM rigidity function than 55% may be considered a necessary criteria was found to be non-linear for axial buckling forces for vaginal intromission capability in all partners below 0.7 kg. It was reported that: `Circumferential but it is not a suf®cient one. It is common to have a (radial) rigidity and axial rigidity associate with false positive RigiScanTM radial rigidity reading corporal body pressure through a distinct functional although false negative RigiScanTM radial rigidity relationship.' In the above publication, examination values are unlikely to occur. of the 29 simultaneous rigidity measurements, however, revealed wide variability in the individual patients. Patient #3, had 50% RigiScanTM radial Discussion rigidity at an axial buckling force of 0.6 kg. Increas- ing the axial buckling 2.5 times to 1.5 kg resulted in a lowered radial RigiScanTM value to 40% on one While the RigiScan device records radial rigidity in recording and the same RigiScanTM reading on the effort to determine the quality of penile another. Patient #4 had 3 RigiScanTM readings erections, there have been limited examinations of which also bore minimal relationship to the axial the relationship between axial and radial rigidity. rigidity value. At axial buckling force magnitudes of More than a decade ago, Bradley et al 1 and Frohrib 0.25 kg, 0.46 kg (almost double the baseline value) et al 11 both claimed that axial and radial penile and 0.71 kg (2.8 times the original value) the Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 334 RigiScanTM values were variable as 25, 35 and 30%, intromission, according to Dr Karacan's criteria13). respectively. Patient #7 revealed that for a RigiS- In these six subjects, the values of cavernosal canTM reading of 42%, the axial buckling force was expandability, penile aspect ratio and ¯accid penile 0.76 kg. Increasing the axial buckling 2.4 times to diameter varied (Table 3, Figure 3). 1.8 kg (one of the highest reported in the above study In the six typical subjects, under the same and one consistent with universal capability for conditions at an intracavernosal pressure 50 mmHg, vaginal penetration based on Karacan's criteria) there was no correlation between axial buckling resulted in a RigiScanTM value of 58%.11 In forces and RigiScanTM radial rigidity (in larger summary, the above two frequently referenced populations of patients, a correlation does exist) publications1,11 examining the relationship between (Figures 3 and 4). In particular, for the patient HM axial and radial rigidity exhibit, on one hand, with the lowest buckling force at 50 mmHg, the methodological ¯aws and, on the other hand, small RigiScanTM radial rigidity was 74 units. For the patient numbers. patient SC with the highest buckling force at More modern publications examined the relation- 50 mmHg, RigiScanTM radial rigidity was 69 units. ship between axial and radial rigidity values. In To demonstrate an example of the inability of axial particular, Allen et al 7 compared axial buckling rigidity to be predicted by radial rigidity, we chose with RigiScanTM radial rigidity in 13 patients. They to further analyze patient HM. This individual, as reported that when RigiScanTM radial rigidity units described above, was found to have both the lowest were in excess of 60%, RigiScanTM could not measured axial rigidity (0.4 kg at DP 50) and the differentiate axial buckling forces between 0.45 lowest theoretical axial rigidity (0.32 kg at and 0.90 kg. Therefore they concluded that a DP 50 Ð poorest chance of vaginal penetration positive RigiScanTM needed to be interpreted cau- according to Karacan13). Patient HM, at the same tiously and that the RigiScanTM might not be able to intracavernosal pressure difference DP 50, was detect mild degrees of abnormal erectile dysfunction found to have one of the highest RigiScanTM radial and may catagorize such patients as normal. No rigidity values (74 units). This radial rigidity value physiologic explanations were provided to explain was consistent with vaginal penetration by most the discrpancy between axial and radial rigidity at authors.7,14,15 Therefore, for patient HM's erection at high RigiScan rigidity values. DP 50, he would be predicted to have both poor It was the goal of this study to further examine the erection quality by axial buckling and excellent relationship between axial and radial rigidity, erection quality by RigiScanTM. The explanation for especially to record how each variable related to patient HM having low axial penile rigidity was intracavernosal pressure. In contrast to other stu- consistent with patient HM having one of the lower dies, our investigation, for the ®rst time, provided values of the six typical patients in cavernosal multiple data measurements in individual patients expandability, penile aspect ratio, and ¯accid penile at numerous intervals of intracavernosal pressure diameter.3±5 In this study we hypothesized that during pharmacocavernosometric controlled condi- patient HM had a high RigiScanTM radial rigidity tions in a large patient series. The data included value at an intracavernosal pressure DP 50 in part RigiScanTM radial rigidity determinations and axial because the penile volume increase was relatively buckling force with analytic information concerning large at DP 50 (HM had the highest tunical penile tissue mechanical properties and penile distensibility of the six patients) resulting in a geometry. relatively large tunical stretching and wall tension In our study, RigiScanTM radial rigidity failed to (tunical distensibility has a negligible effect on axial consistently predict axial buckling forces in the 36 buckling force4). RigiScanTM units appear to be patients. It was our observation that RigiScanTM dependent, in part, upon intracavernosal volume radial rigidity and axial rigidity measured different and, in some fashion, re¯ective of the surface wall physical parameters in a given individual. Axial tension forces of the tunica albuginea. Such surface rigidity values were shown to be dependent on the wall tension forces may provide the resistance to values of intracavernosal pressure, DP, cavernosal radial deformation by the squeezing force. Rigi- expandability, penile aspect ratio and ¯accid penile ScanTM units, in a given individual, thus may be diameter. The analytic expression of axial buckling in¯uenced by factors which provide stretching force in terms of these parameters has been forces to the tunical wall. These factors may include published.4 intracavernosal volume and tunica albuginea me- To illustrate how axial rigidity is dependent upon chanical properties. such three factors,4 we selected six subjects from the To further illustrate the physical differences overall population who were found to have broad between RigiScanTM radial rigidity measurements variation in axial rigidity, and therefore functional and axial buckling force measurements, we plotted erectile quality, at an intracavernosal pressure each parameter compared to intracavernosal pres- difference, DP 50 (Figure 2). Axial rigidity values sure differences (Figures 1 and 2). The plot of ranged from 0.4 kg (not capable of achieving intro- RigiScanTM units with intracavernosal pressure mission) to 2.3 kg (most capable of achieving difference, in a given individual, had negative Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 335 curvature and RigiScanTM units values ¯attened out nosal pressure elevates during erection, radial with increasing intracavernosal pressure difference, rigidity (and volume) begins to maximize, while thus assymptotically reaching a maximum value. axial penile rigidity values are just beginning to Such a shape would occur if the dependent variable develop. This fact makes the RigiScanTM device was volume and during erection, a maximum ®nite relatively insensitive at intracavernosal pressure volume value was reached as the tunica approached values where axial loading values are most sensitive. maximum stretching. On the other hand, the plot of In our study (Figure 4), there were no patients in axial rigidity values with intracavernosal pressure the group with axial buckling values greater than difference, in a given individual, had positive 1.5 kg who had RigiScanTM values less than 55%. curvature; axial rigidity values increased with However, in the group where axial buckling ex- increasing intracavernosal pressure difference to- ceeded 1.5 kg, the RigiScanTM values varied widely wards in®nity. from 55 ± 98% and in the group where axial One of our conclusions is that in an individual buckling was less than 0.5 kg, the RigiScanTM values impotent patient, the RigiScanTM device is not a again varied widely from 36 ± 84%. Therefore, in an valid means of recording erection quality capable of individual, a RigiScanTM value of 55% appears to be discriminating erectile rigidity suf®cient for achiev- a necessary condition for axial rigidity capable for ing vaginal penetration and repeated pelvic thrust- vaginal intromission in all partners, but it is not a ing. The major explanation for this inability is that suf®cient one. False positive RigiScanTM data were RigiScanTM radial rigidity and axial buckling record also found in our study for radial rigidity units of different physical parameters in an erection. Rigi- 70%, a value commonly held as consistent with ScanTM radial rigidity appears to re¯ect intracaver- erectile potency.14 In our study, no false positive nosal pressure and tunical mechanical properties; radial rigidity data were found for RigiScanTM radial axial rigidity re¯ects intracavernosal pressure, rigidity exceeding 88%. erectile tissue mechanical properties and penile geometry.3±5 It is also our conclusion that in a large population Conclusions of impotent men, a correlation exists between axial and radial rigidity. In other published studies reporting similar data, the correlation coef®cients In summary, the relationship between axial penile between axial and radial (base) rigidity values were rigidity and RigiScanTM radial rigidity was exam- 0.59,7 0.78,11 0.6615 and 0.7016 with 25, 29, 59 and ined, for the ®rst time, as a function of intracaver- 18 data points, respectively. In our study popula- nosal pressure. We found that as intracavernosal tion, with 161 data points representing more than pressures increased during erection, axial and radial 2.5 times the data points seen in the other studies, rigidity values increased at different rates. This is the correlation coef®cient was 0.58. because the two rigidity values are dependent upon How is it that our study concluded, in an different mechanical and geometric factors. Radial individual, axial penile rigidity was not predicted rigidity values vary with tunical surface wall by RigiScanTM radial rigidity, yet, at the same time, tension forces; RigiScanTM radial deformation va- in a large data base, our study (and others) lues reaches a maximum ®nite value and become concluded there was correlation between axial and insensitive with increasing intracavernosal pres- radial rigidity? In a large population of patients, sures. In contrast, axial rigidity values are depen- there is correlation between axial and radial rigidity dent upon erectile tissue mechanical properties and most likely because both physical parameters in- penile geometry. With increasing intracavernosal crease with increasing intracavernosal pressure, pressures, axial rigidity values increase towards albeit at different rates. During penile erection in®nity and become more sensitive. development, there is initial volume increase with We and others have found a correlation between minimal change in intracavernosal pressures (high axial buckling forces and RigiScanTM radial rigidity compliance). Subsequently, the volume increase in a population of impotent men. This is most likely diminishes while intracavernosal pressures increase due to the strong dependency of both axial and markedly (low compliance). In any given individual, radial rigidity values with increasing intracaverno- however, radial and axial rigidity are developing sal pressure. However, in an individual, the axial based on different individual physical determinants, buckling measurement was not predicted by the each sharing intracavernosal pressure as a common RigiScanTM radial deformation measurement. Any factor. Radial rigidity has been shown in our study correct prediction by RigiScanTM of axial rigidity to also re¯ect volume increase and tunical mechan- and thus the ability of the erect penis to achieve ical properties. Axial rigidity, on the other hand, has functional intromission and repeated pelvic thrust- been shown to be relatively independent of tunical ing is considered to be chancy. Thus, in a popula- mechanical properties properties.4 Axial rigidity has tion of patients, based on our data, there would be been shown to also re¯ect erectile tissue mechanical more correct guesses than incorrect guesses that properties and penile geometry.4 When intracaver- radial rigidity could predict functional erectile Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 336 potency. In an individual, however, the ability of 6 Levine LA Lenting EL. Use of nocturnal penile tumescence RigiScanTM to predict axial buckling forces is and rigidity in the evaluation of male erectile dysfunction. consistent with a roll of the dice with perhaps a Urol Clin North Amer 1995; 22: 775. 7 Allen RP, Smolev JK, Engel RM and Brendler CB. Comparison somewhat better than even chance of being correct. of Rigiscan and formal nocturnal penile tumescence testing in Improved diagnostic assessment of erection qual- the evaluation of erectile rigidity. J Urol 1993; 149: 1265. ity should be based on devices that either record or 8 NIH Consensus Conference: Impotence. JAMA 1993; 270: 83. predict axial buckling forces and not radial surface 9 Hatzichristou DG et al. In vivo assessment of trabecular wall tension forces. Urologists and device manufac- smooth muscle tone, It's application in pharmaco-cavernoso- turers need to design user-friendly equipment metry and analysis of intracavernous pressure determinants. which allows penile erection quality to be recorded J Urol 1995; 153: 1126. conveniently in terms of axial penile rigidity since 10 Saenz de Tejada I et al. The trabecular smooth muscle this parameter best predicts successful penile modulates the capacitor function of the penis. Studies on a rabbit model. Am J Physiol 1991; 260 (Heart Circ Physiol 29): penetration and functional intercourse. H1590 ± H1595. 11 Frohrib DA et al. Characterization of penile erectile states using external computer-based monitoring. J Biomech Engng References 1987; 109: 110. 12 Nehra A et al. The role of penile tissue mechanical parameters in the relationship between trabecular histology and hemody- 1 Bradley WE et al. New method for continuous measurement of namic function in vasculogenic impotence. J Urol 1998; 159: nocturnal penile tumescence and rigidity. Urology 1985; 26:4. 2229 ± 2236. 2 Kessler WO. Nocturnal penile tumescence. Urol Clin North 13 Karacan I Moore C, Sahmay S Measurement of pressure Amer 1988; 15: 81. necessary for vaginal penetration. Sleep Research 1985; 14: 3 Udelson D et al. Engineering analysis of penile hemodynamic 269A. and structural-dynamic relationships: Part I Ð Clinical im- 14 Rigiscan ambulatory rigidity and tumescence system. Docu- plications of tissue mechanical properties. Int J Impot Res ment #750-156-0486, Dacomed Corporation, Minneapolis, 1998; 10: 15 ± 24. Minnesota, 1986. 4 Udelson D et al. Engineering analysis of penile hemodynamic and structural-dynamic relationships: Part II Ð Clinical im- 15 Licht, MR, Lewis RW, Wollan, PW, Harris CD. Comparison of plications of penile buckling. Int J Impot Res 1998; 10: 25 ± 35. Rigiscan and sleep laboratory nocturnal penile tumescence in 5 Udelson D et al. Engineering analysis of penile hemodynamic the diagnosis of organic impotence. J Urol 1995; 154: 740. and structural-dynamic relationships: Part III Ð Clinical im- 16 Guay AT, Heatley GJ, Murray FT. Comparison of results of plications of penile hemodynamic and rigidity erectile nocturnal penile tumescence and rigidity in a sleep laboratory responses. Int J Impot Res 1998; 10: 89 ± 99. versus a portable home monitor. Urology 1996; 48: 912.

Appendix

De®nitions ratio is assumed to be constant at all intracavernosal pressures for any given patient.4

Axial buckling force: This is the magnitude of an Tunical distensibility, VE=VF: This is the relative axial compressive load which results in penile volume of the fully erect to completely ¯accid buckling. pendulous penis. It is believed to be a measure of tunical elastic mechanical properties and is as- Penile buckling: This is the curving of an otherwise sumed equal to the ratio of erect to ¯accid corporal straight column when subjected to suf®ciently large volumes. axial compressive force which leads to collapse if the force is increased incrementally. Cavernosal expandability, X: This is a measure of the overall ability of the corpora to expand to Intracavernosal pressure difference DP: This is the maximum volume at relatively low intracavernosal intracavernosal pressure value increase over the pressure. Cavernosal expandability is believed to ¯accid intracavernosal pressure where the ¯accid re¯ect the percentage of corporal tissue which is not pressure is assumed equal to the central venous ®brotic. Cavernosal expandability is de®ned as the pressure. Expression of intracavernosal pressure in negative reciprocal of the corporal bulk modulus in this fashion is based on the intracavernosal pressure the semi-erect state, that is, X 7 1=bSE De®ned in rise, not the absolute magnitude of intracavernosal this way, X is a constant for each patient. pressure. Cavernosal bulk modulus (stiffness) b: This is a Penile aspect ratio (D=L): This is the diameter to measure of the dif®culty of erectile tissue to expand length ratio of the pendulous penis. The assumption with increasing intracavernosal pressure. It is is made that linear expansion is virtually the same de®ned as the rate of pressure change ( 7 dP) with in all directions, and therefore the penile aspect the relative volume (dV=V) and varies with pressure. Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 337 Semi-erect state: This is de®ned as the condition ability because in the vicinity of the erect state, when the cavernosal volume is one-half the erect penile expansion is contrained primarily by the volume. This state is chosen to de®ne the expand- tunica albuginea.

Editorial Comment

This group of investigators have made many individual's achievable rigidity to other individuals important contributions to the ®eld of erectile with and without erectile dif®culties. dysfunction. Yet, the methods and formula relied As it has been pointed out in this article, no upon in this particular study which are used to attempt has been made to make radial rigidity derive many of the conclusions in this article have measurement (obtained with the RigiScan) linear not been validated or evaluated by other investiga- with respect to increasing rigidity or to expand the tors. In fact, the biophysics of thin walled, elastic measurable range of rigidity to higher (supra- and expandable structures such as the penis has not clinical) values. In fact, the primary goal is to be been well developed. able to measure adequate rigidity for intromission The critical issue for urologists is the measure of which the penis needs to accomplish intromission. erectile capacity=rigidity which is suf®cient for In fact, if axial pressures and radial pressures are not intromission. As of yet, it is unclear which surrogate related in a linear fashion beyond the level of measure of erection is the most accurate one. Axial adequate rigidity for intromission, it likely has no rigidity was employed initially, but was by necessity useful clinical signi®cance and would be a scienti- done in an unnatural setting (namely, the sleep ®cally arguable but irrelevant point. It should also laboratory) and would often awaken the subject, be noted that the upper limit of radial compression thereby potentially compromising accurate mea- pressure developed by the RigiScan device was surement. Radial rigidity measurement followed in determined empirically by applying progressively the form of nocturnal penile tumescence and higher cable tug forces until the subject could no rigidity monitoring (NPTR). Studies con®rmed the longer tolerate the compressing force without pain relationship between radial compressibility (percent or possible tissue damage. Therefore, the empiri- rigidity), intracorporal pressure (cm of H2O), and cally derived ten ounce cable tug force currently axial buckling force (Newton's).1 Further modi®ca- used provided the clinically useful operating range tions and investigations of NPTR monitoring have of the RigiScan rigidity system. resulted in improvements in utilizing the data Practically speaking then, we may ultimately be obtained from the RigiScan including the develop- able to determine penile erectile capacity by ment of rigidity activity units (RAU) and tumes- measuring some surrogate parameter of the penis cence activity units (TAU) which can be used with a in the erect state. The goal of this procedure is to `nomogram' to compare the results of one's patients' determine how that individual's erection compares nocturnal activity (with ED) to that of a controlled to the pressure necessary to accomplish intromis- population (without ED).2 Measurements of tunical sion with his partner. Certainly, no accepted wall pressure, possibly indirectly measured by measures of this `intromission pressure' have been intracorporal pressure, may best re¯ect penile accepted. There are clearly many factors which will rigidity and capacity for intromission. Although make intromissibility vary, including vaginal size, sheer forces on the erect penis at intromission and lubrication and partner receptivity.3 For the patient penetration may also need to be considered. Clearly, or research subject the RigiScan device does appear further investigation is needed. to provide an acceptable tool for the physician or As I understand buckling, where there is an investigator to obtain some insight into nocturnal increasing axial force applied to a thin walled, erectile activity and the subject's real-time response pressurized, cylindrical vessel, this ultimately pro- to investigational drugs. duces localized collapse of the vessel wall. This The RigiScan system is a useful and possibly the occurs when the radially directed wall force exceeds best testing tool available today to discriminate the inner vessel pressure on that wall segment. Thus, between organic and psychogenic erectile dysfunc- the method for measuring penile rigidity by determin- tion and should not be discarded. Yet, as we learn ing its ability to resist a radial compressing force is more about the biophysics of the penis, ¯accid and related to the organ's ability to withstand an axial load. erect, we will be better able to measure rigidity and The mathematical relationship between axial load other clinically useful parameters as well. Until that and localized wall collapse is as yet undetermined, time, NPTR monitoring will help the practitioner in but does not detract from the NPTR measurement the evaluation of their patient. methods' usefulness as a clinical indicator of an Laurence A Levine Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 338 References 3 Levine LA. Nocturnal penile tumescence and rigidity monitoring for the evaluation of erectile dysfunction. In: Carson CC, Kirby RS, Goldstein I, (eds). Textbook of Male Erectile 1 Frohrib DA et al. Characterization of penile erection states Dysfunction. Isis Medical Media Ltd: Oxford, England. Chap using external computer-based monitoring. J Biomech Enging 24, 1999, pp 267 ± 277. 1987; 109: 110 ± 104. 2 Levine LA, Carrol RA. Nocturnal penile tumescence and rigidity in men without complaints of erectile dysfunction using a new quantitative analysis software. J Urol 1994; 152: 1103 ± 1107.

Editorial Comment

Overview of paper the patient had penile curvature precluding accurate axial buckling determination, if complete smooth muscle relaxation was unable to be achieved ...or if The Boston group is to be commended for their erectile rigidity could not be generated or main- continued efforts to describe the biomechanics of tained despite ... No mention is made of the penile erection. Unfortunately, this study fails to numbers of patients who were excluded from the state what constitutes a ``Functional Erection in an study nor is the etiology of any of the included or Individual Patient'' since that would require knowl- excluded patients described. Thus, to what subset of edge not only of an individual's erectile capability patients with erectile dysfunction do these measure- but also the anatomy, physiological condition and ments apply? Additionally, the criteria constituting receptivity of the individual's partner. ``accurate axial buckling measurement'' are not This report focuses on the linearity of radial disclosed. Would the penile curvature have pre- rigidity values in excess of 60% relative to the vented these excluded subjects from having satis- linearity of axial buckling force values in excess of factory intercourse? The RigiScan system can be 1.5 kg. Most clinicians would characterize erections applied to these patients as well as to any of the of this magnitude as unbuckleable, which raises the others excluded from this study. question: what is the clinical relevance of the Dynamic infusion studies to produce penile linearity of a measurement system above the buckle- erections combined with axial buckling force mea- able penile range? surements are unreliable, invasive and operator Furthermore, axial buckling force measurements dependent, greatly limiting their practicality and fail to record the ability of an individual to maintain reproducibility. This was con®rmed in this report by an erection, another key parameter in assessing the number (unreported) of excluded patients and erectile capability. This renders them of dubious with the requirement that a second observer be clinical value to practitioners evaluating or treating present during the measurement to assure that the erectile disorders. load is applied parallel to the long axis of the penile As the authors cited, previous investigators have shaft. RigiScan rigidity and tumescence studies can con®rmed the interpretation guidelines originally be conducted reliably, reproducibly and untended. established for radial stiffness (rigidity) values as Repeated buckling measurements were per- measured by the RigiScan system; namely, (a) a formed in the study until consistency in the radial stiffness less than 30 ± 40% corresponded to measured value was achieved. This further demon- an erection inadequate for vaginal intromission, (b) strates the operator dependence and impractical above 60 ± 70%, the penis becomes unbuckleable, nature of this measurement method. and (c) the middle values provided enough rigidity No report of infusion rate was given. This would so that vaginal intromission could be achieved with seem to be quite important in examining the ability assistance. All clinicians have stressed the impor- of a thin-walled, pressurized, leaky vessel to resist tance of interpreting the rigidity and tumescence buckling, and even more important when using the data, whether acquired nocturnally or in response to pressurized vessel as a controlled, characterizable other stimuli, in the context of the subject's health model to compare two methods of assessing buck- pro®le and not as the sole determinant of that ling resistance. The literature on the biomechanics individual's erectile capability. of non-leaking, thin-walled structures is sparse; on leaky structures, non-existent. Study design and practicality of methods Support for the authors' assertion that the sensitivity of axial rigidity to penile geometric properties such as ¯accid diameter and penile Patients were excluded from the study ``If the aspect ratio were based on observations from two pharmacocavernosometry study was incomplete, if of the subjects. The small data spread in these two Axial penile buckling forces vs Rigiscan radial rigidity D Udelson et al 339 patients makes this assertion questionable. Never- intracavernosal pressure relative to radial rigidity theless, radial rigidity values obtained from the values, particularly at the higher pressure values. RigiScan system were as sensitive as axial rigidity Above a 1.5 kg axial load (Karacan's criterion) the values to these two properties, contrary to the individual has an erection satisfactory for inter- authors' interpretation. course, and increases in rigidity beyond this are This study reports the results of axial buckling cause for male bravado with little or no known force measurements versus RigiScan base loop diagnostic signi®cance. Similarly, for radial rigidity radial rigidity for arti®cially induced erections using above 60 ± 70%, an individual had an unbuckleable saline infusion. Saline infusion hydrostatically penis and the sensitivity to increases in intracaver- delivers an equally distributed pressure throughout nosal pressure is of unknown clinical value. the corporal bodies. Uniform distribution of corporal pressure in penile erections is at variance with clinical case reports that frequently exhibit differing base and tip rigidities, again calling into question Conclusion and recommendation the validity of the experimental model used by this investigative team. Furthermore, buckling of thin- walled, pressurized vessels occurs by localized This paper is very misleading to readers in that it collapse of the vessel wall (assuming the opposite attempts to discredit a very useful clinical tool by wall does not rupture) when the radially directed questioning its linearity (not its accuracy, or relia- wall force exceeds the inner vessel pressure on that bility, or usefulness) in a measurement range of wall segment. The RigiScan system measures this unknown clinical signi®cance. Until a reasonable phenomenon at the base and tip of the penis; axial hypothesis for the signi®cance of intracorporal buckling force measurements, when they can be pressures above those required to achieve an applied, give no indication of location of penile unbuckleable erection is established, subjecting buckling which could be useful in assessing penile patients to the invasive application of supra-normal shaft abnormalities. and potentially harmful infusion pressures is of The authors draw out attention to the extreme questionable merit. sensitivity of axial buckling loads to increases in Gerald Timm