The abdominal sacrocolpopexy is an effective surgical procedure for the correction of apical vaginal wall prolapse.1–6 The surgical technique, as initially described in 1957 by Arthure and Savage,7 involved attachment of the cervical stump or vaginal vault directly to the ligament overlying the sacral promontory with silk suture.
Initial descriptions of the abdominal sacrocolpopexy recommended suspending the vagina at the L-5–S-1 level.7 However, a study by Nichols et al8 using radiographic colpography suggested that the vagina normally lies in an axis pointing into the hollow of the sacrum. As a result of these findings, Birnbaum9 advocated attaching mesh lower on the sacrum, at the level of S-3–S-4, in an attempt to restore normal vaginal axis.
Vascularity in the presacral space is complex and variable,10,11 and life-threatening hemorrhage occurs in 4.4% of patients who undergo abdominal sacrocolpopexy.1 Significant hemorrhage reported by Sutton et al12 in 1981 prompted recommendations for suture placement in the anterior longitudinal ligament to return to the S-1–S-2 level. By placing sutures at this level, it was hypothesized that the middle sacral vessels would be easier to identify and avoid, thus potentially reducing the risk of massive hemorrhage while, at the same time, not appreciably affecting the axis of the vagina.2,12 Since then, the operation has evolved considerably and currently involves suspending the apex and upper part of the vagina to the sacral portion of the anterior longitudinal ligament using synthetic mesh.
Currently, many variations in surgical technique are described for the abdominal sacrocolpopexy. These include location and orientation of suture placement on the anterior longitudinal ligament, whether or not to place mesh on both the anterior and posterior vaginal walls, how far down to place the mesh on the vagina, and use of different types of suture and mesh materials. Factors that reduce adequate access to the presacral space, such as body habitus, depth of the pelvic cavity, and complexity of the vascular supply in this region may influence how and where sutures are ultimately placed into the anterior longitudinal ligament.
Although the recent evolution of this procedure has sought to determine the most effective graft material for the suspension,13,14 the optimal location and orientation of suture placement for the sacral attachment of the graft material has not been studied. Specifically, there is no consensus on whether location of and/or suture orientation on the anterior longitudinal ligament significantly alters the strength of mesh attachment.
Although others have studied the tensile strength of the anterior longitudinal ligament as a whole,15–17 there is a paucity of data regarding its relative strength at various levels in situ and the effects of modifications to the operation on the strength of support. Thus, the objective of this study was to estimate the strongest location and optimal orientation of suture placement in the anterior longitudinal ligament for abdominal sacrocolpopexy in unembalmed female cadavers.
MATERIALS AND METHODS
Dissection and exposure of the presacral space below the level of the aortic bifurcation was performed in 23 unembalmed female cadavers obtained from The University of Texas Southwestern Willed Body Program. This study was exempt from institutional review board approval, at the University of Texas Southwestern Medical Center, in accordance with the Code of Federal Regulations, Title 45, Part 46, subpart 101 (b)1 as revised in November 13, 2001. Age, race, height, weight, and cause of death were recorded for each cadaver.
A transfemoral amputation was performed to isolate the pelvis, and the cadaver was then secured to the dissection table. The pubic symphysis was transected in the midline and separated to facilitate access to the presacral space and to ensure that pulling forces could be obtained at right angles to the plane of the anterior longitudinal ligament.
After entry into the abdominal cavity, the presacral space was exposed by vertically incising the peritoneum overlying the sacral promontory. Fat, loose areolar, and neural tissue were removed to expose the anterior longitudinal ligament from the level of the fifth lumbar (L-5) to the third sacral (S-3) vertebrae. The ligament was divided into a right and left half along the sagittal plane of the vertebral column, using the midpoint of the sacral promontory as a standard reference point for the division as previously described.10 The midpoint of the sacral promontory was defined as the midpoint between the junctions of the body of the first sacral vertebra (S-1) with the ala of the sacrum.
A one-half taper Mayo needle (Number 216706, Richard-Allan Medical [UK] Limited, Worcestershire, United Kingdom) was used to place a #7 waxed-polyester surgical suture filament (Number 761403, Dodge Company, Cambridge, MA) in a horizontal fashion into the anterior longitudinal ligament 1 cm to the right of the midpoint of the sacral promontory, at the level of the sacral promontory (Fig. 1). To determine the strongest location of suture placement, it was necessary to use a suture material that did not fail before the anterior longitudinal ligament. Waxed-polyester surgical suture filament was used because the pull-out force of the anterior longitudinal ligament at the sacral promontory exceeded that of all other available sutures that were tested during preliminary development of the study protocol. A 5-mm width purchase of tissue was taken such that the suture site incorporated the ligament only and not the underlying intervertebral disk. The suture filament was tied down with six square knots, leaving a 5-cm loop between the surface of the ligament and the first knot to facilitate attachment of and subsequent testing with a tensiometer. Subsequent horizontally oriented sutures were placed on the right side of the ligament, 1 and 2 cm cephalad (sacral promontory+1 and sacral promontory+2) and 1, 2, and 3 cm caudad (sacral promontory–1, sacral promontory–2 and sacral promontory–3) to the sacral promontory, for a total of six sutures (Fig. 1).
On the left portion of the ligament, six sutures were placed in the same fashion, but oriented in the vertical plane (Fig. 1). A random number table was used to determine the sequence of placement and order of traction for each suture to reduce confounding factors. All sutures were placed by a single investigator (C.Y.W.).
To estimate the strongest or “optimal” location of placement (greatest pull-out force), each suture was successively fastened to a hand-held digital tensiometer (Chatillon Digital Force Gauge Model DRC-50, Largo, FL), which was able to record up to a load of 250 N with an accuracy of 0.1 N. Progressive tensile load was applied, perpendicular to the plane of the ligament, until it was dislodged (this was designated as the “pull-out” force or “failure load” of the tissue). It is acknowledged that forces on sutures used to secure mesh in abdominal sacrocolpopexies in vivo may be tangential or at an angle to the ligament. However, application of a perpendicular force was deliberate and designed to ensure a standardized and reproducible vector of force on the ligament, thus minimizing the variability of experimental technique.
Six separate 1×1×1–cm cubed segments of the anterior longitudinal ligament with the underlying bone and intervertebral disk were then sequentially removed from the midline for assessment (Fig. 1). These segments corresponded to each level of suture placement and testing. The tissues were decalcified in RDO rapid decalcifier solution (Apex Engineering Products Corporation, Aurora, IL) for approximately 24–48 hours, until the specimens were soft enough to accommodate a sharp needle to facilitate sectioning with a microtome. Each cubed segment of tissue was placed on its lateral side and was sectioned (4 micrometers), processed, and stained with hematoxylin-eosin. The thickness of the ligament at each interval was estimated by measuring three separate fields using a Nikon Eclipse microscope E600 (Nikon, Inc., Melville, NY) and imaging software equipped with a histologic micrometer (NIS elements, Nikon, Inc.). The measurements were taken by a pathologist who was masked to the level of the ligament.
Because no pilot data were available for which to compute a sample size, the study was powered at 90% to observe a standard deviation change for a paired Student t test. Because a number of multiple comparisons were planned, a significance level of .01 was used. Although a sample size of 18 subjects was required, a population of 23 was targeted in anticipation of technical and instrumentation failure. Comparisons among the different locations on the anterior longitudinal ligament (sacral promontory+2, sacral promontory+1, sacral promontory, sacral promontory–1, sacral promontory–2, sacral promontory–3) were analyzed using repeated measures analysis of variance through a random-effects model using an unstructured covariance structure, and the Tukey-Kramer adjustment for multiple testing. Paired Student t test was used to compare the pull-out strengths between vertically and horizontally placed sutures. Statistical analyses were performed using SAS 9.1 (SAS Institute, Cary NC). Probability values of .05 or less were considered statistically significant.
Limited demographic information was available for the 23 unembalmed female cadavers (Table 1). The majority of the cadavers were white, and the most common causes of death were malignancy and complications of cardiopulmonary diseases.
In general, horizontally placed sutures in the anterior longitudinal ligament had significantly greater pull-out strength at or above the sacral promontory compared with those placed below the level of the promontory (P≤.01). There was no significant difference in the pull-out force of the anterior longitudinal ligament at the level of the promontory or at 1 or 2 cm above the promontory (Fig. 2A). The pull-out forces of the ligament at all 3 tested levels below the sacral promontory (sacral promontory–1, sacral promontory–2, sacral promontory–3) were not significantly different from each other, but were less than those above the promontory (Fig. 2A).
If suture filaments were placed in a vertical orientation, the force required to break the ligament was greatest at the level of the sacral promontory (SP), and at 1 cm superior (SP+1) to it (Fig. 2B). The pull-out force of the ligament at 2 cm above the sacral promontory (SP+2, 20.3±2.9 N) was significantly lower compared with the force at either SP (40.0±5.7 N, P≤.03) or SP+1 (66.4±9.1 N, P≤.03) (Fig. 2B). Similarly, the pull-out strength of the anterior longitudinal ligament at sacral promontory+1 and sacral promontory were both statistically greater than those at 1 or 2 cm below the sacral promontory (sacral promontory–1, sacral promontory–2).
With respect to orientation, horizontally placed sutures had significantly greater pull-out strength than vertically placed sutures at sacral promontory+2 (54.9±7.6 N compared with 20.3±3.0 N, P<.001), sacral promontory–1 (26.9±3.0 N compared with 12.7±1.2 N, P<.001) and sacral promontory–2 (27.7±3.4 N compared with 16.5±2.2 N, P<.01). Ligament pull-out strengths at the sacral promontory and 1 cm above that (sacral promontory+1) were not significantly different when placed in either the horizontal or vertical orientation. At a level 1 and 2 cm below the sacral promontory (sacral promontory–1 and sacral promontory–2), vertically placed sutures required significantly less force to pull out the ligament than horizontally placed sutures. After adjusting for body mass index, the level of suture location on the anterior longitudinal ligament remained as the only significant variable associated with tensile strength.
The mean (±standard error of the mean) histologic thickness of the anterior longitudinal ligament significantly decreased from a level 2 cm above (sacral promontory+2, 1.8±0.1 mm) to 3 cm below (sacral promontory–3, 1.3±0.1 mm) the sacral promontory (P=.026) (Fig. 3).
It is difficult to extrapolate the results of a cadaveric study to surgical practice, and advocating clinical recommendations would not be appropriate based on these data alone. However, the strength of this study lies in its estimate of the relative strength of the anterior longitudinal ligament at various locations, which has not been described. In this study of unembalmed female cadavers, sutures are most secure when placed at the level of the sacral promontory or 1 cm above it. Additionally, it seems that suture orientation does not alter the findings at this level, because vertically and horizontally placed sutures resulted in similar pull-out forces.
However, if sutures are placed below the sacral promontory, the amount of force required to dislodge a horizontally placed suture is significantly less when compared with those placed at or above the sacral promontory. This difference in the pull-out force may reflect the attenuation of the anterior longitudinal ligament as it descends along the vertebral column. The fact that vertically placed sutures, below the sacral promontory, are significantly weaker when compared with the horizontally placed sutures may, in part, be explained by the vertical fiber orientation of the ligament. Additionally, the thickness of the anterior longitudinal ligament significantly decreased as it descends along the vertebral column from L-5–S-3. Thus, placement of vertical sutures, along the grain or fiber orientation of the ligament, below the sacral promontory may initiate fracturing of an already attenuated ligament.
Occasionally, certain clinical scenarios prevent surgeons from placing sutures at a specific desired location into the anterior longitudinal ligament. This may be the case in patients with complex vascularity in the presacral space, in laparoscopic sacrocolpopexies where the specific spatial geometry of the pelvis maybe difficult to navigate, or in those with a large body habitus or deep pelvic cavity where placing sutures lower down in the presacral space is difficult. In fact, previous studies corroborate the potential for significant vascular injury in this region, where a substantial number of vascular anastomoses between the middle and lateral sacral veins were frequently found within 3 cm caudal to the midpoint of the sacral promontory.10,18 Although these results estimate the location of greatest pull-out strength, it is acknowledged that “optimal” suture placement must ultimately take into account vascular and neuroanatomy as well as potential deviation of the vaginal axis during abdominal sacrocolpopexy.
Given the findings of this current study, placing sutures more cephalad on the anterior longitudinal ligament, at or above the sacral promontory, in an attempt to minimize vascular complications does not seem to adversely affect the strength of mesh attachment onto the anterior longitudinal ligament. Taken together with other studies involving the vascular anatomy of the presacral space, the location for suture placement that provides a balance between strength of attachment and safety from vascular injury is at the level of the sacral promontory. Needless to say, this does not obviate the need for careful dissection of the presacral space and systematic identification and avoidance of the iliac vessels.
There are a number of limitations to this study. First, even though cadaver specimens were unembalmed, there was no control over nonmodifiable factors such as time and cause of death, whether there was prior freezing and thawing of the specimen, and the rate of autolysis of the tissue. However, by using each cadaver as its own control and comparison, the relative effects of these variables on the findings of this study were reduced. Second, forces determined in cadaver specimens may not be representative of those generated in living tissue, and freezing and thawing can potentially reduce the ultimate failure load of the tissue. Although the evidence is less clear for muscle and tendon, numerous studies have demonstrated that the biomechanical properties of ligaments, specifically the tensile strength and failure load, are not significantly altered by the freeze–thaw process.19–21 More importantly, the intention of the study was to compare the relative strength of the anterior longitudinal ligament at various levels of its lumbosacral attachments and not necessarily to determine the absolute failure load of the tissue. Last, measurements for this study were taken at a particular point in time and may not represent chronic processes or insults that occur over longer periods of time in living subjects.
The utility of this study lies in providing a viable alternative for suture placement in situations where access to the anterior longitudinal ligament is limited. In summary, it seems that the strength of sutures placed at or above the sacral promontory during an abdominal sacrocolpopexy is not significantly different when placed either horizontally or vertically. However, at a level below the sacral promontory, vertical placement of sutures in an already attenuated ligament may be enough to compromise the strength of the attachment and repair. Based on these findings, in complex situations where sutures need to be placed at or above the sacral promontory, the strength of attachment is not significantly compromised.
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