Scholarly Paper: Clinical Perspective
Anatomy in practice: the ischiorectal fossae
Susan Mercer PhD, FNZCP
Senior Lecturer
Department of Anatomy & Structural Biology
University of Otago.
E. Jean Hay-Smith PhD, MNZCP
Lecturer
Rehabilitation Teaching and Research Unit,
Department of Medicine,
Wellington School of Medicine and Health Sciences,
University of Otago.
ABSTRACT
External palpation of the levator ani muscles is based on the premise that these
muscles are readily palpable. However, examination of the clinical anatomy of
the perineum revealed that the deep, fat filled, ischiorectal fossae lie between the
perineal skin and these muscles. The three dimensional anatomy of the perineum
is not commonly included in clinical texts. Therefore, the purpose of this paper is to
present the morphology of the perineum and in particular the ischiorectal fossae
and the implications of this morphology for external palpation of the levator ani
muscles. Mercer S, Hay-Smith EJ (2005). Anatomy in practice: the ischiorectal fossa.
New Zealand Journal of Physiotherapy 33(2) 61-64.
Keywords: palpation, levator ani, perineum
INTRODUCTION
Increasing numbers of clinicians are interested
in the function and dysfunction of the pelvic floor
muscles. To address dysfunction, clinicians have
adapted common assessment and treatment
techniques to make them specific to muscles of
the pelvic floor. Examples of assessment include
perineal, vaginal and rectal palpation, and vaginal
and anal surface electromyography, while treatment
techniques include pelvic floor muscle training,
vaginal and rectal electrical stimulation, and
biofeedback. The biological rationale underlying
these techniques draws on the topography of the
pelvic floor muscles.
Levator ani and coccygeus, considered to be the
primary supporting and sphincteric muscles of
the pelvic floor, are the focus of most anatomical
descriptions included in clinical texts concerned
with assessment and treatment techniques.
Notably, the depictions of these muscles in clinical
texts are usually perineal and pelvic views where
the associated soft tissues have been removed to
reveal the muscles of interest. It is difficult for line
drawings to convey the three-dimensional anatomy
of the muscles in situ, along with the necessary
details of the surrounding soft tissues, including
fat, fascia and neurovascular bundles.
The problem of using one anatomical description,
often only a diagram from a single text, is that
clinicians can develop assessment and treatment
techniques based on an incomplete understanding
of the relevant anatomy. An example of this
predicament is illustrated by the ‘pelvic clock exam’,
where it is suggested that the levator ani muscle is
externally palpable in the anal triangle (Kotarinos,
Herman and Wallace, 1997). This assessment
NZ Journal of Physiotherapy – July 2005. Vol. 33, 2
technique is based on a perineal approach to the
levator ani muscle, with the 4, 5, 7 and 8 o’clock
positions representing the levator ani muscles
(Figure 1). External palpation is used to assess
muscle tone, tenderness or trigger points.
Figure 1. A diagram of the ‘Pelvic Clock Exam’, a
technique for external palpation of the levator ani
(adapted from Kotarinos et al., 1997). Numbers 4, 5, 7
and 8 represent the sites where the levator ani is noted
to be readily accessible for external palpation.
Review of the ‘pelvic clock exam’ revealed an
apparent lack of appreciation of the anatomy of
the ischiorectal fossae, which is found between the
levator ani muscles and the skin of the perineum.
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The purpose of this paper is to describe the anatomy
of the ischiorectal fossae by presenting the levator
ani in situ as they are approached during external
palpation. A review of commonly used anatomical
texts (Boileau et al 1965; Hall-Craggs 1995; Rosse
& Gaddum-Rosse 1997; McMinn 1994; Gardner
et al. 1975; Hollinshead 1956; Last 1963; Snell
1995; Woodburne & Burkel,1988) was undertaken
and a synopsis of the anatomical descriptions is
presented. This is supplemented by images chosen
to depict the three dimensional anatomy of the
ischiorectal fossae.
anal triangles is defined by the posterior edge of
the perineal membrane. The external genitalia
and the urogenital diaphragm are found within
the urogenital triangle, while the anal triangle
is composed of the centrally placed anal canal
encircled by the external anal sphincter and flanked
laterally and posteriorly by the ischiorectal fossae
(Figure 3).
Perineum
Before considering the detailed anatomy of
the ischiorectal fossae one needs an appreciation
of the regional anatomy, namely the perineum.
The perineum is the external aspect of the pelvic
outlet from the levator ani muscles through to the
perineal skin, external genitalia and openings of
the genitourinary and gastrointestinal tracts. It is
a diamond shaped space bounded by the angles
formed by the inferior pubic ligament at the pubic
symphysis, the tip of the coccyx and the ischial
tuberosities. The sides are created by the right and
left pubic arch and the sacrotuberous ligaments
(Figure 2). The levator ani muscles form a common
roof over both triangles. Because these muscles
slope down medially the perineum is relatively
deep laterally, becoming shallower as it reaches
the midline.
Figure 3. A photograph of the perineum from below.
Posteriorly the coccyx (C) flanked by gluteus maximus
(GM). In the midline is the external anal sphincter (EAS)
flanked by the perineal membrane (PM). Lateral and
posterior to the anal canal are the ischiorectal fossae
where the fat has been removed to reveal rectal nerves
and vessels. The levator ani (LA) muscles can be seen
sloping downwards towards the midline.
Figure 2. An inferior view of a model of the pelvis
demonstrating the boundaries of the perineum,
urogenital and anal triangles. The borders of the
perineum are the pubic symphysis (PS), pubic arch (PA),
ischial tuberosities (IT), the sacrotuberous ligaments (ST)
and the coccyx (C). The line drawn between the ischial
tuberosities (IT) represents the posterior edge of the
perineal membrane. It is the dividing line between the
anterior, urogenital triangle (UT) and the posterior, anal
triangle (AT).
A line drawn just anterior to the ischial
tuberosities divides the perineum into two areas:
an anterior region called the urogenital triangle and
a posterior region called the anal triangle (Figure
2). The dividing line between the urogenital and
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Ischiorectal Fossae
The ischiorectal fossae are fascia-lined wedge
shaped spaces found on either side of the anal
canal and rectum. They are potential spaces lying
above the skin of the anal region but below the roof
of the perineum, which is formed by the levator ani
muscles. Although two fossae are described they
communicate posteriorly, behind the anal canal.
The anatomy of the fossae relates to their primary
function as distensible space fillers while also
supplying support to the anal canal.
In order to provide structural soundness and
support to the anal canal, the fossae are filled with
fat rather than air. During defaecation the anal
canal, now full of faeces, is able to distend into
the fossae. When the rectum is empty the shape
of the region is maintained and the anal canal is
supported. The ischiorectal fat pad also allows for
NZ Journal of Physiotherapy – July 2005. Vol. 33, 2
Figure 4. A transverse section through a male pelvis at the level of the pubic symphysis (PS). In the midline lie the
bladder (B) and rectum (R). The lateral walls of the fat filled ischiorectal fossa (IF) are formed by the obturator internus
(OI) while the medial walls are formed by the iliococcygeus (IC) and puborectalis (PR) portions of the levator ani. The
posterior aspect of the fossa are overlaid by the gluteus maximus muscles (GM).
dilation of the vagina during parturition when the
foetal head virtually eliminates the space.
The essentially vertical lateral wall of an
ischiorectal fossa is formed by the ischium with
its attached obturator internus and associated
fascia (Figure 4). The oblique medial wall is formed
by the sloping inferior surface of the levator ani
as it descends to surround the anal canal (Figure
3). Because the levator ani arises from the fascia
overlying the obturator internus the lateral and
medial wall meet superiorly at a sharp angle. This
angle is usually considered to comprise the roof of
the fossa. However, posterior to the anal canal the
levator ani is directed towards the anococcygeal
raphe and the coccyx, so here the muscle forms a
roof to the fossa, not a medial wall.
Posteriorly the ischiorectal fossae extends below
the lower edge of gluteus maximus as far as the
sacrotuberous ligament (Figure 4). Anteriorly it
extends forward above the urogenital diaphragm
to fill the triangular space between the urogenital
diaphragm and the levator ani as they approach
each other. Here, the perineal body prevents
communication between the right and left fossae.
The ischiorectal fossae are filled with adipose
tissue that is infiltrated with numerous tough,
fibrous bands and septa, some of which are
derived from the longitudinal muscle of the anal
canal (Figure 4). In fact, Hollinshead (1956) states
that this fat is notable for its tough and stringy
nature.
In the lateral wall of each fossa a pudendal canal
for the pudendal nerve and internal pudendal
vessels is located about 4cm superior to the lower
border of the ischial tuberosity. The fossae are also
NZ Journal of Physiotherapy – July 2005. Vol. 33, 2
crossed by the inferior rectal nerves and vessels.
Posteriorly the perineal branch of the fourth sacral
nerve and the perforating cutaneous nerve passes
through the fossa. Anteriorly the perineal and
posterior scrotal or labial branches of the perineal
nerve and vessels are found (Figure 3).
Implications for External Palpation of the
Levator ani
Review of the anatomy of the perineum reveals that
external palpation of the levator ani muscles must
occur through the fat filled ischiorectal fossa (Figures
Figure 5. A coronal MRI through a male pelvis at the
level of the anus. External palpation of the levator
ani muscles (LA) must occur through the fat filled
ischiorectal fossa (IF).
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4 and 5). For example, a coronal Magnetic Resonance
Image (MRI) in one living adult male revealed that, at
the level of the anal ring, the approximate distance
from perineal skin to the mid-point of the levator
ani was 10cms (Figure 5). Futhermore, this fat is
traversed by numerous tough fibrous bands and
septa, nerves and blood vessels.
It is conceivable that one could detect levator
ani muscle contraction because of its effect on
the ischiorectal fat pad. However accurate, graded
assessment of levator ani muscle contraction, or
an evaluation of muscle tone at rest or during a
voluntary contraction, would appear to be extremely
difficult. External palpation may provoke pain.
However, the source of this pain could be any of the
structures which lie between the palpating finger
and the levator ani muscle, that is. skin, ischiorectal
fat, fascia and neurovascular bundles. Therefore, on
the basis of the underlying anatomy, diagnosis of
a specific trigger point within the posterior levator
ani is questionable.
Key Points
External palpation of the levator ani musculature is
confounded by the overlying fat filled ischiorectal
fossae.
In order to justify a procedure on the basis of
anatomy, a necessary consideration is the
anatomy of the structure in situ.
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REFERENCES
Boileau Grant JC and Basmajian JV (1965): Grant’s Method of
Anatomy. Baltimore: Williams and Wilkins, pp. 299-302.
Hall-Craggs ECB (1995): Anatomy as a basis for clinical
medicine. London: Williams & Wilkins, pp. 334-339.
Gardner E, Gray DJ and O’Rahilly R. (1975): Anatomy. A
regional study of human structure. London: WB Saunders,
pp. 500-501.
Hollinshead WH (1956): Anatomy for Surgeons: Volume 2. The
Thorax, Abdomen, and Pelvis. Hoeber-Harper, pp 849-852.
Kotarinos RK, Herman H, Wallace K (1997): Chapter 3 Assessment.
In E Wilder (ed), The Gynecological Manual, American Physical
Therapy Association Section on Women’s Health. Alexandria:
American Physical Therapy Association, pp 51-89.
Last RJ (1963): Anatomy regional and Applied. London: J & A
Churchill Ltd.
McMinn RMH. (1994): Last’s Anatomy. Regional and applied.
London: Churchill Livingstone, pp 404-405.
Rosse C and Gaddum-Rosse. (1997): Hollinshead’s Textbook of
Anatomy. New York: Lippincott – Raven pp 9.
Snell RS (1995): Clinical Anatomy for Medical Students. Boston:
Little, Brown and Co, pp 348-355.
Woodburne RT and Burkel WE (1988): Essentials of Human
Anatomy. Oxford: Oxford University Press, pp 520-522.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge Mr Brynley Crosado, Mrs
Shannon O’Neill and Mr Russell Barnett of the Department of
Anatomy and Structural Biology, University of Otago, Dunedin
for preparation of the anatomical material photographed.
Joanna Knox, Anthony Chapman and Fiona Duncan dissected
a specimen during a student project under our supervision, and
this initial work stimulated our continued interest in the area.
ADDRESS FOR CORRESPONDENCE
Dr Susan Mercer, School of Biomedical Sciences, The University
of Queensland, Brisbane, Qld 4072. Australia.
NZ Journal of Physiotherapy – July 2005. Vol. 33, 2