Pelvic Floor Anatomy

Pelvic Floor Anatomy and Dynamics

Ligaments and muscles work synergistically to maintain structure and function of the pelvic floor

Structure and Form

Structure and form in the pelvic floor arise from the interaction of muscles, nerves and ligaments acting on the pelvic organs. The vagina and ligaments must be stretched to their limit of extension to attain the strength required to carry loads. Unequal balance of the forces may stretch the system one way or the other, thereby affecting opening or closure. The ‘suspension bridge’ analogy (figs 1-05 and 1-08) is used to illustrate how structure and form (and thereby function) arise from a system in balanced tension.

Fig 1-05 Form - This diagram illustrates the synergism of the pelvic floor structures. It shows how the organs are suspended by ligaments and stretched by muscle forces acting against these same ligaments to create form and strength.

Fig 1-08 Structure -The Suspension Bridge Analogy The suspension bridge analogy illustrates how the pelvic structures are interdependent. In a suspension bridge strength is maintained through tensioning of suspensory steel wires (arrows). Weakening any one part of the structure may disturb the equilibrium, strength and function of the whole. Relating the analogy to figure 1-05, form (ie shape and strength) is achieved because the vagina and bladder are suspended from the bony pelvis by ligaments (PUL, USL, ATFP) and fascia (F). The structural dimension develops when these are stretched by muscle forces (arrows).

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Function and Dysfunction

The Integral Theory describes normal bladder function as having two stable states: closed (continence) and opening (micturition). These states are deemed to be stable because they are the resultants of a balance of forces.
Three muscle forces interact during closure (fig 2-13) and two muscle forces interact during opening (fig 2-14). If the efficacy or balance of these muscle forces is disrupted through damage to their ligamentous anchoring points, dysfunction may result in both closure (incontinence) or opening (abnormal emptying).

During effort (fig 2-13) the distal vagina is stretched forwards by PCM. The upper vagina and bladder base are stretched down and back by LP and LMA. PCM and LP contract against the pubourethral ligament (PUL). LMA contracts against the uterosacral ligaments (USL). The broken lines represent the resting position of the bladder.

During micturition (fig. 2-14), PCM relaxes.The stretch receptors activate the micturition reflex. The whole system is stretched down and back by LP and LMA, opening out the outflow tract. The detrusor contracts to expel urine. The broken lines represent the closed position of the bladder.

Note that, although the terms ‘opening’ and ‘micturition’ are used interchangeably, they are not exactly the same. ‘Micturition’ is driven by a neurological reflex whereas the ‘open’ state of the urethra may result from entirely mechanical factors, for example, ‘genuine stress incontinence’.

Fig 2-13 Active Closure. Bv = attachment of bladder base to vagina.

Fig 2-14

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Causes of Dysfunction - Delineating Zones of Damage

The damaged suspensory ligaments and associated fascia are sited in three zones: anterior, middle and posterior (fig 1-09). The anterior zone extends from the external urethral meatus to the bladder neck, and contains three structures which may be damaged: the pubourethral ligament (PUL), the suburethral hammock, and the external urethral ligament (EUL).

Fig 1-09 The causes of dysfunction: the zones of damage.

The circles represent the foetal head descending through the birth canal. Damage to one or more ligaments leads to imbalance of the system and hence dysfunction. Congenitally defective collagen may also cause dysfunction.

The middle zone extends from the bladder neck to the cervix or hysterectomy scar. It contains two structures which may be damaged: the pubocervical fascia (PCF) and the arcus tendineus fasciae pelvis (ATFP).

The posterior zone extends from the cervix or hysterectomy scar to the perineal body. The uterosacral ligaments (USL), rectovaginal fascia (RVF) and perineal body (PB) are the key posterior zone structures which may be damaged.

Fig 1-10 The nine main connective tissue structures potentially needing surgical repair

Locating the damaged structures within the zones
Within each zone are key structures which facilitate normal pelvic floor function. If any of these is damaged, dysfunction may result. The Integral Theory delineates nine key structures in which damage may occur. The three zones each contain three main stuctures which may be damaged (fig 1-10). They are:

Anterior zone
1. External urethral ligament (EUL)
2. Suburethral vagina (hammock)
3. Pubourethral ligament (PUL)
Middle zone
4. Arcus tendineus fascia pelvis (ATFP)
5. Pubocervical fascial defect (PCF) (cystocoele)
6. ‘Zone of critical elasticity’ (ZCE) - restoration of elasticity if ‘tethered’ by scarring
Posterior zone
7. Uterosacral ligaments (USL) defect
8. Rectovaginal fascial defect (RVF)
9. Perineal body (PB).

Note that the ZCE is part of the PCF. It is included here as a separate structure to emphasize the importance of maintaining elasticity at the bladder neck area of the vagina (ZCE) during surgery. It is in the ZCE where the ‘tethered vagina syndrome’ originates. The ‘tethered vagina syndrome’ is described fully later in this book. Briefly, it is caused by loss of elasticity at ZCE from vaginal or bladder neck elevation surgery due to scarring or excessive bladder neck elevation. It is entirely iatrogenic and can be included under defect 5 (pubocervical fascia), except that it specifies excessive tightness, not laxity.

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