The Dome Effect: A Dual-Synergy Hypothesis Integrating Thin-Shell Mechanics with the Self-Interlocking Mechanism in Pelvic Floor Biomechanics—Dome Effect in Pelvic Floor Biomechanics

1. Introduction

Pelvic organ prolapse (POP) and stress urinary incontinence (SUI) affect nearly half of women worldwide, 80% of whom are parous[1]. Despite continuous iterations and improvements in surgical techniques, patients of all three surgical approaches (transvaginal native tissue repair, transvaginal mesh repair, and laparoscopic sacrocolpopexy) were highly satisfied according to FIPS with no significant difference (p = 0.058)[2]. Traditional anatomy focuses on the precise location of pelvic floor muscles, organs, and ligamentous fascia, and conventional surgery emphasizes restoring these structures to their normal anatomical positions[3]. Why does the human pelvis look the way it does? Consider its distinctive features among primates. The sacrum and coccyx are thickened and curve inward. The pubic arch is broad and sturdy. The ischial spines project prominently. And the levator ani—that muscle spans the entire pelvic floor, arching upward like a dome before anchoring to the pelvic brim. These traits are not incidental. They reflect evolutionary pressures. Selection for bipedalism and childbirth has reshaped the human pelvis into a form that diverges markedly from that of apes[4]. Now consider what this morphology implies for function. The pelvis must transmit force. When abdominal pressure spikes—during a cough, a sneeze, a sudden movement—the pelvic floor receives a massive impact load. How does it channel that force to the bony framework? The traditional anatomical approach, for all its descriptive power, has not answered this question. It treats the body as if it were a static structure. It overlooks the dynamic nature of pelvic equilibrium. That oversight is precisely what we aim to address.

These features have evolved over thousands of years and represent the mechanical adaptations necessitated by bipedal locomotion, the human skeletal form underlies bipedalism, and specific genetic variants affect the skeletal form, tying a major evolutionary facet of human anatomical change to pathogenesis[5]. They embody a load-bearing design based on thin-shell principles—a perfectly engineered system for force dispersion and transmission.

The self-interlocking mechanism between pelvic organs and surrounding tissues explains how stability is maintained under abdominal pressure surges, PFSD is a group of diseases caused by pelvic floor stress support function injury and stress imbalance, mainly associated with pelvic floor support structure defects, weaknesses, injuries, pregnancy, transvaginal delivery, age, and related factors[6], but it does not address the destination of impact forces or the overall mechanical principles. Building upon this foundation, the present article proposes the "Dome Effect" hypothesis, introducing thin-shell mechanics to reinterpret pelvic floor function from a mechanical perspective.

Where does the argument begin? With a set of physical principles. These principles govern how thin-shell structures bear loads. Engineering has tested them repeatedly—and confirmed them. Now consider the anatomy. The levator ani curves upward like a dome. It attaches to the pelvic rim. That configuration is not accidental. It aligns, quite remarkably, with the structural logic of a thin shell. What happens to the forces that travel through this dome? They concentrate at the sacrococcygeal region. The bone responds by bending and thickening. That is the morphological endpoint of membrane tension. But here is a crucial distinction. Engineered shells are static. The pelvic floor is not. It adapts. It repairs itself. It incorporates self-locking mechanisms that actively maintain its integrity and transform mechanical inputs. This is a living structure, not a passive one. If we project forward, what do we see? The theoretical prediction is a gradual weakening of the dome effect. That weakening should track closely with the clinical progression of prolapse. The correlation is not incidental—it follows from the logic of the model itself.

So the chain runs from physics to morphology. From passive structures to active biological systems. From theoretical prediction to clinical observation. Together, these elements close the loop. What emerges is not a collection of separate insights but a coherent, self-contained logical cycle.

2. Theoretical Framework and Conceptual Model

2.1. Fundamental Principles of Thin-Shell Mechanics

Thin-shell structures represent one of the most reliable large-span load-bearing forms validated over centuries of engineering practice, with ubiquitous applications ranging from the Pantheon in Rome to modern stadium domes[7]. The core principle repeatedly validated is that an upward-convex curved geometry uniformly converts vertical loads into membrane tension distributed along the shell surface, transmitting it to rigid edge supports[8].

This principle is governed by three physical determinants. First, stress transformation through curved geometry. When a vertical pressure P acts upon an upward-convex thin-shell surface, the pressure decomposes into two components: a normal component perpendicular to the shell surface (resisted by bending stiffness) and a tangential component along the shell surface—membrane tension. For shells with appropriate curvature and thickness, the normal component is minimal; the vast majority of the load transmits as membrane tension along the shell surface, with the curved geometry virtually eliminating all bending moments. This explains why an eggshell of only 0.3 mm thickness can withstand 50 Newtons of vertical pressure[9]. For a spherical thin shell under uniform distributed load P, the membrane stress N (force per unit length) can be expressed as: N = PR/2, where R is the radius of curvature, indicating that membrane stress N is proportional to both load P and radius of curvature R. This also means that a flatter dome (larger R) generates greater dispersed membrane stress, imposing higher demands on edge support. Therefore, an optimal rise-span ratio (height-to-span ratio) is a critical parameter in thin-shell structural design[10]. Second, edge support must sustain membrane tension. Membrane stress transmits along the shell surface and ultimately converges at the edge arch base or ring beam; consequently, if edge support capacity fails, the dome inevitably collapses. Current Levy type suspen-domes use loop cables which are key elements and carry large tensions. The loop-free suspen-dome was proposed for improving the collapse resistance and reducing cable tensions[11]. Within the pelvic floor, the arcus tendineus fasciae pelvis (ATFP) assumes this "arch base" role. Third, efficacy is determined by form. The optimal dome morphology approximates a spherical cap, achieving the most uniform membrane tension distribution and the lowest degree of stress concentration[12]. Once the pelvic floor surface flattens, it enters a dangerous state—the dispersed membrane stress amplifies dramatically, edge support structures become overloaded, and systemic mechanical collapse ultimately ensues.

2.2. Anatomical Correspondence: The Pelvic Floor as a Thin-Shell Structure

We borrow the language of engineering, but we apply it to living tissue. The levator ani and its fascia, when actively contracted, assume a shape that functions like a thin shell. That shape—its curvature, its load-bearing logic—is what we call the pelvic dome. Is it an anatomical structure in the traditional sense? No. It is a functional one. The body does not name it in textbooks, but the body uses it constantly. It operates dynamically, responding to pressure, transmitting force. And it rests on four anatomical pillars: the surface, the support ring, the central strut, and the load-receiving viscera above.It is a living dynamic thin-shell structure comprising four fundamental elements (Table 1):

1.Shell surface: The levator ani (particularly the iliococcygeus and pubococcygeus muscles), together with its superior and inferior fascia, forms a "sandwich-like" composite structure possessing both stiffness and flexibility. Upon active contraction, the structure assumes an upward-convex dome-shaped surface[13].

2.Edge support (arch base): The arcus tendineus fasciae pelvis (ATFP) (anterolaterally), obturator internus fascia (laterally), ischial spine (posterolaterally), coccyx and anococcygeal ligament (posteriorly), and posterior aspect of the pubic symphysis (anteriorly) together constitute a complete and robust "edge support ring." The ATFP plays the critical role of mechanical force transmission[14].

3.Central supporting pillar: The perineal body. Its normal height of approximately 3–4 cm determines the dome's rise-span ratio and is a critical parameter for maintaining membrane tension conversion efficiency[15]. If the perineal body shortens to 2 cm or less, the dome flattens. Efficiency plummets. Above the dome, the load surface consists of organs. The bladder, the uterus, the rectum. They rest on top. They receive abdominal pressure indirectly. Then they channel it to the shell. This indirect path avoids a problem: direct impact would create focal stress on the shell surface[16].

3. Core Hypothesis: Dome Effect and Dual Synergy

3.1. The Dome Effect: Load-Bearing Mechanism

The central proposition of the Dome Effect is that the dynamic thin-shell dome formed by the levator ani during active pelvic floor contraction uniformly converts vertical abdominal pressure into membrane tension through its surface curvature, transmitting this tension to the pelvic skeleton in accordance with the thin-shell law (N = PR/2)[17]. Therefore, if pathology causes the dome to flatten (increased R), the pressure on ATFP edge support correspondingly increases.

3.2. Dual Synergy: Unification of Load-Bearing and Closure

The Dome Effect and the self-interlocking mechanism constitute a dual-synergy system. The two are functionally distinct—the Dome Effect subserves load-bearing while self-interlocking subserves closure—and mechanically coupled, jointly maintaining pelvic floor stability (Table 2)

The two are inseparable and mutually coordinated. The dome provides a mechanical support platform for self-interlocking—the levator plate, as the primary component forming the dome shell surface, provides the fulcrum for organ restraint. The dynamic closure barrier formed by self-interlocking, while buffering abdominal pressure, precisely delivers pressure to the posterior fornix—the region of the dome shell surface with the greatest load-bearing capacity[18]. Failure of either system disrupts the internal balance.

The Dome Effect and the self-interlocking mechanism are not independent systems. They share three structures: the levator ani, the perineal body, and the ATFP. Damage any one of them. Both systems weaken. The result is a cascading mechanical collapse. How do they couple? Through three interfaces. First, load channeling—self-interlocking concentrates pressure at the posterior fornix. Second, platform stabilization—the dome provides the anchor that makes the binding effect possible. Third, force vector alignment—the dome's curvature steers the resultant force toward the ATFP. These interfaces do not operate in isolation. They reinforce each other. The relationship is circular. Stronger self-interlocking means more efficient load channeling. Greater dome stability means a more stable interlocking platform. That is a positive feedback loop.

4. Pathomechanical Cascade: A Four-Stage Hypothesis of Dome Failure

Based on thin-shell theory, we derive the following mechanical cascade for the development of POP:

Stage 1—Loss of membrane tension conversion function. Childbirth-related tearing or degeneration leads to ATFP relaxation, perineal body damage, and dome flattening. Vertical abdominal pressure loads cannot be converted into membrane tension on a flat curved surface, and thus act directly on pelvic floor organs[19].

Stage 2—Load transfer to organs. With the membrane tension pathway weakened or blocked, abdominal pressure is forced to directly compress the bladder, uterus, and rectum. The primary stress-bearing region shifts to the relatively vulnerable bladder base and trigone [20].

Stage 3—Self-interlocking overload. Since pressure can no longer be shared and borne by the original dome structure, it becomes concentrated on the self-interlocking mechanism—which was designed primarily for closing the outlet to prevent organ prolapse—causing severe overload. The levator plate shifts from its horizontal orientation (approximately 45°) to a steeper angle (>60°), and the vaginal axis transforms from its physiological curvature to a straight vertical configuration, resulting in the loss of restraint on the bladder and uterus[21].

Stage 4—Laplace-driven vicious cycle. Having escaped from the pelvic stress protection zone, focal bulging gradually develops. According to Laplace's law (T = P × R), the smaller the radius of curvature of the bulging sac, the higher the wall tension rises, forming a "stress black hole" that ultimately accelerates prolapse progression[22]. This completes the full pathomechanical cascade from initial SUI to global pelvic prolapse[23].

5. Verification and Predictions

This hypothesis is falsifiable. We propose the following predictions:

Finite element analysis predictions: In normal dome models, pressure should be transmitted along the ATFP to the rigid pelvic supports; in POP models, the stress concentration point shifts instead to the vulnerable bladder base and trigone[24].

Imaging predictions: Ultrasound of normal women during contraction should demonstrate the levator ani assuming a high-curvature dome configuration; in POP patients at rest, the dome should appear flattened or concave, with increased ATFP separation. Following successful dome reconstruction, dome morphology should be restored (increased curvature, reduced hiatus area), with the degree of restoration positively correlated with clinical symptom improvement[25].

Comparative anatomy predictions: Among primates with greater degrees of bipedalism (humans > chimpanzees > macaques), levator ani dome curvature should be greater, with correspondingly greater sacrococcygeal thickening[26].

6. Conclusions

The Dome Effect hypothesis reconceptualizes the pelvic floor as a complete dynamic mechanical thin-shell structure evolved in humans, capable of dynamically adjusting curvature to conduct and dissipate pressure. A structure that bends, adjusts, conducts pressure, and dissipates it. When that curvature fails, the result is POP. That failure is not about position alone. It is about the collapse of an entire mechanical system. This hypothesis gives us a unified framework. It also gives us a surgical imperative. Restoration requires dual reconstruction. The Dome Effect and the self-interlocking mechanism must both be addressed. A surgery that ignores the dome's dynamic function cannot fully restore pelvic floor mechanics. The hypothesis is not ready for clinical adoption, but it is ready for testing. Finite element analysis, dynamic imaging, mechanical measurement, comparative anatomy—all can verify it. The questions it asks may reshape how we think about pelvic floor disorders.