1 The bony pelvis and fetal skull C. Savona-Ventura Trainee Obstetrics Programme - 2007 St. Luke’s Teaching Hospital Introduction Knowledge of the shape and dimensions of the normal female pelvis is essential for a proper understanding of the second stage of labour and its abnormalities since the body pelvis is an important component which determines the birth canal structure. The human female pelvis shows adaptations that are of obstetric advantage and relate also to the relative “big” head of the foetus. These adaptations develop chiefly in childhood and puberty.
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Knowledge of the shape and dimensions of the normal female pelvis is essential for a proper understanding of the second stage of labour and its abnormalities since the body pelvis is an important component which determines the birth canal structure. The human female pelvis shows adaptations that are of obstetric advantage and relate also to the relative “big” head of the foetus. These adaptations develop chiefly in childhood and puberty.
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Comparative anatomy
Human Gynaecoid Orangutan
General anatomy
Pelvic bone is made up of various sections:For obstetrical purposes, the pelvis is divided by the pelvic brim into two parts:
– The False Pelvis– The True Pelvis
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General anatomy
The False Pelvis is that portion above the pelvic brim. It does not take part in the mechanism of delivery and is of no obstetric interest.In the past attempts were made to form a judgement of the size of the true pelvis by measuring the width of the false pelvis. The information thus obtained was often inaccurate
Intercristal diameter [IC ~29 cm]: widest point on lateral aspect of iliac crestInterspinous diameter [IS ~26 cm]: distance between the lateral tips of the anterior superior iliac spinesExternal conjugate [AP] diameter [EC ~20 cm]: distance between apex of spine of 5th lumbar vertebra and centre of the superior border of symphysis pubis.
Martin’s pelvinometer
General anatomy
The True Pelvis is that portion below the pelvic brim. It determines the size and shape of the birth canal.
Brim: formed by the upper margins of pubic bones, the ilio-pectineal lines and the anterior upper margin of the sacrum.Cavity: formed by the pubic bones, ischium, ilium, and sacrumOutlet: diamond-shaped made up of the pubic bones, ischium, ischial tuberosities, sacrotuberous ligament, and 5th segment of sacrum.
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General anatomy
12.513.111.3Anteroposterior
11.813.112.5Oblique
11.812.513.1Transverse
Outlet CavityBrim
Inclination of the Pelvic brim: ~1200
General anatomyFour different types of pelvises, but frequently mixed types.
Gynaecoid Android
Anthrapoid
Platypelloid
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Gynaecoid pelvis
Ideal pelvis favouring a normal delivery; 50.6% of women.
Brim slightly oval transversely but almost roundedSacrum curvedIschial spines not prominentShort-cone pelvisObtuse greater sciatic notchTriangular obturatorforamenSub-pubic arch rounded [Roman arch] angle at least 900
Android pelvis
Male-type pelvis favouring OP positions and apt to cause deep transverse arrest of head; 22.4% of women.
Abnormality of lower limbAbnormality of pelvic girdleAbnormality of vertebral column
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Clinical AssessmentBody build
Gynaecoid Anthrapoid Android
Clinical Assessmentfoetal head as pelvimeter
Fifths palpable above symphysis pubis
Engagement defined as the point when the engaging diameter [BPD = ~10 cm] goes past the pelvic brim. Five fingers = 10 cm.
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Clinical Assessmentfoetal head as pelvimeter
Station of the head in relation to ischial spines
In Gynaecoid & Android pelvis distance between ischial spine to brim is ~5 cm.
In Anthropoid pelvis distance is ~7 cm
In Platypelloid pelvis distance is ~3 cm
5 cm: Station 0
7 cm: Station 0-2
3 cm: Station 0+2
Head 3/5
Clinical Assessmentfoetal head as pelvimeter
Munro Kerr’s method of assessing for engagement
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Clinical Assessmentvaginal examination
Measurement of AP conjugates•Diagonal conjugate ~12.0 cm•True conjugate ~11.0 cm•AP outlet ~12.5 cm
Assess shape of sacrum
Clinical Assessmentvaginal examination
Assess mobility of sacro-coccygeal joint
Assess interspinousdiameter ~12.0 cm
Assess intertuberousdiameter ~11.8 cm
Assess spino-tuberous distance ~4.5 cm
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Clinical Assessmentvaginal examination
Assessment of pubic arch angle
Effect of sub-pubic angle on pelvic depth
Types of pubic arches
Effects of a narrow pubic arch•Thrusts foetal head further posteriorly•Prevents extension of head
Gynaecoid Android
Anthropoid Platypelloid
Clinical Assessmentradiological examination
1. True AP Conjugate2. Obstetric Conjugate3. Mid-cavity AP Conjugate4. Outlet AP conjugate5. Angle Greater Sciatic notch6. Angle of inclination of pelvic brim7. Angle of inclination of sacrum8. Ischial spine9. Ischio-tuberous distance10. Foetal head lie, position, engagement
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Clinical Assessmentradiological examination
Antero-posterior view of pelvis
Thom’s Supero-inferior view of pelvis
Outlet view of pelvis
Relationship of foetal skull to pelvisAxis of birth canal
900 rotation for Occipito-transverse when engaging diameter is at the brim Occiptio-oblique in mid-cavity Occipito-anterior at ischial spines
1. Suboccipitobregamatic: ~9.5 cc Vertex2. Suboccipitofrontal: ~10.0 cm Sinciput3. Occipitofrontal:~11.24 cm persistent OP 4. Mentovertical: ~13.8 cm brow5. Submentobregmatic: ~9.5 cm Face6. Submentovertical: ~11.25 cm incompletely
Bones of base of skull are incompressibleBones of vault are compressible
– Parietal bones override occiptial and frontal– Anterior parietal bone overrides its posterior fellow– Moulding can decrease biparietal diameter by ~1cm
Normal vertex position
Persistent OP position
Brow presentation
Face presentation
Malpositions
Occiptio-posterior position 1 in 5 deliveries [generally left because of dextrorotation and descending colon]
Face presentation 1 in 500 deliveries Brow presentation 1 in 1000 deliveriesBreech presentation 1-2 in 50 deliveriesUnstable lie 1 in 350 deliveries