Objective
The purpose of this study was to describe a new computer-controlled research apparatus for measuring in vivo uterine ligament force–displacement behavior and stiffness and to present pilot data for women with and without prolapse.
Study Design
Seventeen women with varying uterine support underwent testing in the operating room (OR) after anesthetic induction. A tripod-mounted computer-controlled linear servoactuator was used to quantify force-displacement behavior of the cervix and supporting ligaments. The servoactuator applied a caudally directed force to a tenaculum at 4 mm/sec velocity until the traction force reached 17.8 N (4 lbs). Cervix location on Pelvic Organ Prolapse Quantification system (POP-Q) in the clinic, in the OR, at rest, and with minimal force (<1.1 N); maximum force (17.8 N) was recorded. Ligament “stiffness” between minimum and maximum force was calculated.
Results
The mean ± SD subject age was 54.5 ± 12.7 years; parity was 2.9 ± 1.1; body mass index was 29.0 ± 4.3 kg/m 2 , and POP-Q point C was –3.1 ± 3.9 cm. POP-Q point C was correlated most strongly with cervix location at maximum force (r = +0.68; P = .003) and at rest (r = +0.62; P = .009). Associations between cervix location at minimum force (r = +0.46; P = .059) and ligament stiffness (r = –0.44; P = .079) were not statistically significant. Cervix location in the OR with minimal traction lay below the lowest point found on POP-Q for 13 women.
Conclusion
POP-Q point C was correlated strongly with cervix location at rest and at maximum traction force; however, only 19% of the variation in POP-Q point C location was explained by ligament stiffness. The cervix location in the OR at minimal traction lay below POP-Q point C value in three-fourths of the women.
Pelvic organ prolapse is caused by a complex disease process. Among the many pelvic floor structural elements involved, recent data highlight 2 major contributing factors: (1) the loss of apical supports of the uterus by the cardinal/uterosacral ligament complex and (2) birth-related levator ani muscle injury. Although there is information about ex vivo ligament properties that include cellular and molecular changes in the connective tissue, there is a striking lack of data concerning the in vivo ligament properties and prolapse. Specifically, it is not clear whether abnormal ligaments cause apical descent or whether apical descent in some women is the effect of abnormal forces placed on normal ligaments by pressure imbalances because of levator ani muscle damage. The lack of a scientific strategy to measure the in vivo biomechanical aspects of apical support and a structural paradigm based on observed data limits progress in this field. This is especially important because findings in clinic during the Pelvic Organ Prolapse Quantification system (POP-Q) examination and data measured in the operating room (OR) are frequently at variance with one another yet are used in clinical decision-making.
To study this, we developed a system to measure in vivo biomechanical properties of the ligament complex. Our objective was to (1) describe a technique for measurement and display of apical support properties for the cardinal uterosacral ligament complexes and (2) examine early findings in a pilot sample of women with varying degrees of apical support. In addition, we considered the relationship between uterine location that is seen during POP-Q examinations and these observations.
Methods
Seventeen women, representing a full spectrum of uterine support as defined by POP-Q point C from normal to prolapse, were recruited and consented preoperatively to participate in this University of Michigan institutional review board approved study. To obtain this convenience sample for pilot testing, the study team identified potential participants by examining the upcoming gynecology surgery schedule. Inclusion criteria included age >18 years and the plan for an operation during which a tenaculum would be placed. Women were excluded if they were pregnant, had uterine fibroid tumors >12 weeks in size, had a known history of pelvic inflammatory disease, used steroids chronically, had previous pelvic radiation, were undergoing treatment for a malignancy, or had any other factor for which extra time under anesthesia may place them at increased risk of adverse effects.
The study procedure involved the repetition of one step of the routine surgical procedure (tenaculum placement on the cervix and downward uterine traction) for the purpose of recording the research data. Concomitant operations that were performed included robotic-assisted supracervical hysterectomies and cervico-sacrocolpopexies, vaginal hysterectomies with and without sacrospinous and uterosacral ligament suspensions, hysteroscopies, and midurethral slings. Subjects were compensated for their participation.
We developed a tripod-mounted computer-controlled linear servoactuator (model # FA-PO-150-12-8; Firgelli Automation, Inc, Vancouver, Canada) to quantify the force-displacement behavior of the uterine cervix ( Figure 1 ). The servoactuator is a computer-controlled motor that allows displacement to be controlled by information provided by the controlling computer. A load cell (model #TLL-500, capacity 500 lbs, nonlinearity 0.25% of rated output; Transducer Techniques, Temecula, CA) was connected to the end of the servoactuator arm and used to measure the traction force. Location of the cervix before and after tenaculum placement was measured by the study team who used a ruler with millimeter markings of the lateral cervix in relation to the hymeneal ring. The lateral margin of the cervix was chosen rather than the anterior lip as is often used during POP-Q examination because the anterior lip may stretch under traction and because the lateral margin that is closest to the ligament attachment is less affected by this phenomenon. Displacement during traction was tracked to the nearest 0.1 mm by the servoactuator device. During the first 9 trials, the accuracy of these measures was confirmed when we videotaped the traction session with a ruler in view.
Once the patient was placed in the lithotomy position after anesthesia induction, a short-bladed Sherback posterior-weighted vaginal speculum (Aesculap Inc., Center Valley, PA) was placed in the vagina to open the genital hiatus while not interfering with descent of the posterior vaginal fornix ( Figure 1 ). This retracted the levator ani muscles and reduced pelvic floor support interference. A single-tooth tenaculum was placed deep into the cervical stroma to incorporate both the anterior and posterior cervical lips and to help prevent soft-tissue distortion. The handle of the tenaculum was attached to the load cell and to the servoactuator by a short bead chain. Before any force was applied, the weight of the tenaculum was supported by a long vertically placed string that was suspended from an OR light ( Figure 1 ). Anesthetic data including respiratory rate during testing, use of a paralytic agent, and use of a spinal anesthetic were recorded.
The linear servoactuator was designed to quantify the force-displacement behavior of the uterine cervix and to calculate the biomechanical properties, including stiffness, of the uterosacral and cardinal ligaments. A typical “ramp and hold” testing technique was used whereby the tenaculum handle was pulled at a constant 4 mm/sec velocity in a caudal direction until the traction force reached 17.8 N (4 lbs) at the end of the “ramp” phase of the test. It was then kept at constant force for 60 seconds (the “hold” phase of the test) to measure how much the ligament tension decreases over time to allow calculation of viscoelastic behavior (these engineering calculations are not reported in this clinical article). A maximal traction force of 17.8 N was chosen based on the only previous study of force displacement that is in the literature. Foon et al evaluated the maximal traction that is applied by gynecologists during hysterectomy and found that a greater force of 8 lbs (35.6 N) was used. Because not all of our patients were undergoing hysterectomy and because clinically it was believed that 8 lbs might cause cervical lacerations, we chose the more conservative force. In a subset of our subjects, we compared a maximal traction force of 4 vs 6 lbs and found no significant difference in cervix displacement results (data not shown). Because 4 lbs seems well above the physiologic range for women during their normal activities, we chose to use 4 lbs because this force allowed us to demonstrate our study findings while keeping patient safety in mind.
During testing, we collected data on cervix location for each subject under the following conditions ( Figure 2 ): (1) point C on POP-Q examination in the clinic, (2) at rest in the OR in lithotomy position with a posterior speculum in place and tenaculum on the cervix, but no force applied, (3) with minimal force (1.11 N) in the OR, and (4) and with maximum force (17.9 N) in the OR. From this, the ligament “stiffness” (ie, Δ traction force/Δ cervical displacement between minimal and maximum force) was calculated. After testing was complete, the location of the cervix was also measured while the operating surgeon was asked to pull on the tenaculum with the maximal force he would customarily use during a vaginal hysterectomy (“clinical pull”). This clinical pull was purposely not standardized because it was intended to provide additional information about the relationship between our standardized study displacement measurements and the typical amount of force that is used by clinicians to make surgical decisions.
Demographic data that included age, parity, and body mass index were recorded, and descriptive statistics were performed. All data were checked for the assumption of normality and appropriate bivariate correlation that was used and included Pearson correlation for normally distributed variables and Spearman correlation for others. Data analyses were performed with SPSS statistical software (version 19; SPSS Inc, Chicago, IL). Statistical significance for all analysis was defined at the 5% significance level.
Results
Seventeen women with mean (±SD) age of 54.5 ± 12.7 years, parity of 2.9 ± 1.1, and body mass index of 29.0 ± 4.3 kg/m 2 were recruited to include a full range of uterine support from normal support to significant uterine prolapse based on their in-office POP-Q point C value made by a urogynecology faculty or fellow. Point C ranged from –10 cm to +7 cm, with a mean of –3.1 ± 3.9 cm ( Table 1 ). All of the women received a general anesthetic. Five of the women did not receive a muscle paralytic agent, and none of the women received spinal/epidural anesthesia.
| Demographic | Mean | SD | Range |
|---|---|---|---|
| Age, y | 54.5 | 12.7 | 36–74 |
| Parity, n | 2.9 | 1.1 | 1–6 |
| Body mass index, kg/m 2 | 29.0 | 4.3 | 22.1–36.1 |
| Pelvic organ prolapse quantitative method | |||
| Ba | 1.35 | 2.3 | –2 to 6 |
| C | –3.1 | 3.9 | –10 to 7 |
| Bp | –1.6 | 1.2 | –3 to 1 |
The POP-Q point C location that had been determined during preoperative POP-Q examination in the clinic is displayed in the lower portion of Figure 3 . The graph shows the traction portion of each subject’s force-displacement curve. Under minimal force (1.1 N), the cervix location varies greatly. Under the maximum force (17.9 N), the location relative to the hymenal ring depends on both the starting position at minimum force and the stiffness, which is represented in Figure 3 as the slope of the force–displacement curve. The latter demonstrates a hyperelastic characteristic wherein the suspensory ligament stiffness increases with increasing cervical displacement. Note the similar shape of each subject’s curve.
The cervix location in the OR, even with minimal force, is lower than that seen in the clinic with maximal Valsalva maneuver and the prolapse protruding to its maximal clinical extent in 13 individuals (cervix location at minimal force vs POP-Q point C mean, –0.42 ± 2.2 cm [range, –3.0 to 5.1 cm] vs –3.12 ± 3.9 cm [range, –10 to 7 cm]). In 2 individuals, there was a higher position that was seen in the OR and a similar location in another 2 subjects. The former is explained by the significant elongation of the anterior cervical lip in these women with large prolapses that led to a much lower measurement of the anterior lip in the POP-Q examination and higher measure of the lateral cervix. Average cervix locations at various forces are presented in Table 2 .
