Lecture 17 - Anatomy (The pelvis) Week 16.pdf

THE PELVIS Thomas Wilson Graphics from Grant’s Method of Anatomy, Netters Atlas of Human Embryology, Larsen’s Anatomy, Human Embryology and Developmental Biology, Moore’s Before We Are Born, Analysis of Vertebrate Structure, McMinn’s & Abrahams Clinical Atlas of Anatomy, Human Anatomy Colour Atlas...

THE PELVIS Thomas Wilson Graphics from Grant’s Method of Anatomy, Netters Atlas of Human Embryology, Larsen’s Anatomy, Human Embryology and Developmental Biology, Moore’s Before We Are Born, Analysis of Vertebrate Structure, McMinn’s & Abrahams Clinical Atlas of Anatomy, Human Anatomy Colour Atlas & Textbook [email protected] Goal: How does the shape of ‘the bowl’ tell us so much about what our pelvis is really good at? Outline: Outcomes: Pelvic parts Identify the features of the pelvic bones, walls & joints. Ligaments of the pelvis Pelvic joints Evaluate the pressures and challenges the structure of the female Walls of the pelvis pelvis faces in relation to parturition and bipedalism, and how this relates to pelvic sexual dimorphism. Male and female pelvis Describe the evolutionary pressures and resulting structural The bigger question adaptations of the human pelvis. Biomechanics of pelvis Relate those structural adaptations to the functional roles, challenges, and mechanics – including joints and stability.  "" " " [ The Human Pelvis bone pelvis / / Sacrum + 2 Os Coxa = Pelvic girdle Os Coxa: 3 bones that fuse during development ↳ Ilium , pubis , ischium → unite in the acetabulum Evolution has dictated its function -4 growth Two main functions: plates ‘Basin’ to hold pelvic viscera (‘The bowl’) ' 1. epkiphyse.at ' lines 2. Force transduction – Girdle for lower limb attachment Transmits weighted loads efficiently between: Vertebrae to Os Coxa Os Coxa to limbs. Limb to limb a@tabigof -mmRicketsweak.no. And back again… Ground reaction force y/☒y reaction n - n.mg#...gg..... forces & stiff bones pelvis is curving inward ground reaction force  True and false pelvis Pelvic brim = Pelvic inlet Sacral promontory Sacral Alae (wings) Iliopectineal line: Arcuate line, Iliopubic eminence, Pecten pubis (pectinate line) Symphysis pubis False: Above ‘pelvic brim’ Mainly Ilium: Iliac blades and crests Abdominal and gluteal aspects " " " "" " " """ " obturator foramen True: Below pelvic brim ( internal) f) Forms the bony walls of pelvic cavity Mainly sacrum, pubis & Ischium Pelvic outlet: Inferior opening of pelvic cavity Ischial tuberosities (meaning: bump-like) Ischiopubic rami (meaning: flat surfaces) Sacrotuberous ligaments Sealed by muscles and fascia  What plugs the holes? What passes through? Obturator fascia covers O. internus m. Forms attachment for muscles of the pelvic diaphragm Obturator foramen (Meaning: To block up, or plug…) Deep to superficial: Obturator muscles (Internal and external) Obturator Ext. m.  Obturator membrane  Obturator Int. m.  Obturator fascia Obturator fascia - Internus tendon passes through lesser sciatic foramen 1. Sacrotuberous lig. (fibrosed tail muscle) Obturator nerve & vessels pass through obturator canal sactotuber-ouslig.sactos-pinouslig.it 2. Sacrospinous lig. (fibrosed perineal muscle) 2 & 3 create greater and lesser sciatic foramina % outside Inside P. greater sciatic I foramina. sacrospinouslig *.. lesser sciatic foramen sa.t.er.su r ⇐¥←. Is.. & '. What passes through? Greater sciatic → foramen: Passes triangular shaped m. Piriformis muscle (Plugs it a bit as well) Sciatic nerve Gluteal nerves & vessels Pudendal nerve & vessels (follow each other) ↳ internal pudendal a. travels out of the pelvis then re - enters it through the lesser sciatic foramen Lesser sciatic foramen: Passes Out: Obturator internus tendon → lateral rotator ofthe leg In: Pudendal nerve & vessels 1 Closed to pelvic cavity by pelvic diaphragm muscles (outside of ‘visceral pelvis’) ‘Gateway’ of the pelvic outlet É ÷ ÷É÷¥÷ ÷ pelvic diaphragm muscles Pubic symphysis → ground - reaction forces pass better through posterior link Anterior mechanical link between limbs, but Weaker than posterior link General structure from one pubic bone to the other: Bone  H. cartilage  F. cartilage  H. cartilage  Bone ↳ fibres connectthe hyaline c. together Reinforced by: superior pubic lig. & inferior (aka Arcuate) lig. Often forms a joint cavity in postpartum women (can see this in resin slides in the lab) L8 micro tears in fibro cartilage components form a capsule (fluid-filled ) I.ÑITÉ - - Sacroiliac joint (SI) ‘Limited’ synovial plane joint Wedged orientation with curved auricular (ear-shaped) surfaces Surrounded by capsular ligaments Ventral sacroiliac: Thin Dorsal interosseous sacroiliacs: Very thick Éf *¥#É Frontal plane stability ' A very strong joint: ‘Keystone’ principle ↳ helps to hold the centre centered SI joint can ‘slip’ in the frontal plane (up and down) Resisted by dorsal ligaments: Iliolumbar lig. Dorsal interosseous SI lig. More slip = More tension on ligs. Holds ilium to sacrum more securely Limits further movement Continuum of severity in injury The load is shared Any force exerted on the ground is also exerted back through our limbs… Impact injury? ‘Feet on the dash’  Iliolumbar ligament Lumbosacral joint (L5-S1) & Sagittal stability {¥ g¥q%µq "" interosseous /µ I ligament horizontal Joint is steeply sloped plane centre -52 of gravity , Male: ~60 ̊ Female: up to 80 ̊ BUT WHY? sacroiliac joint Force on lumbar to slip anteriorly CSIjoint) ¥ÉÉÉ _£ÉÉ÷÷÷: Resisted by: 260° Lumbosacral IVD & articular processes of lumbar vertebrae " " " Iliolumbar lig. resisting sacrum away from the Tendency of sacrum to rotate forwards relative to ilium ischium Vertical centre of gravity (CoG): ~S2 51  anterior pelvic rotation Resisted by: 1. Dorsal interosseous SI ligs. 2. Sacrotuberous lig. 3. Sacrospinous lig. Accentuates lordosis by rotating pelvis over hip Hip movements are depending on ‘pelvic posture’ (Hip hinge example) Having a curve that is closer to the hip optimising how we use the weight of our trunk during walking Sexual dimorphism Relativity is crucial, try to avoid absolute size comparisons Male pelvis External features related to load bearing & muscle attachment are bigger Female pelvis Internal diameters are larger Predict likelihood of successful vaginal delivery Dimorphic features: Size AND/OR shape? 1. Sacral body & alae ratio 2. Pelvic brim 3. Acetabulum 4. Subpubic angle F-120° - & M( 90° 5. Pubic body 6. Ilia orientation 7. Greater sciatic notch Sexual dimorphisms 11.5cm  Human obstetric dilemma: Pelvises, monkeys, apes, & (not) losing your bowels… Why is the obstetric dilemma a dilemma for us, but not other great apes? Female structural pelvic variation was a strong evolutionary pressure Not really a pressure nowadays Male pelvis never had this pressure Result: Less variable internal morphology So… Question: Why is our ape pelvis so much more similar to monkeys and lesser apes, than it is to other great apes? Bipedalism changes axial loading mechanics Something changed our pelvic morphology that makes giving birth harder… Pregnant lordosis: A compensatory mechanism Anterior pelvic tilt & increased lordosis Caused by BIPEDALISM ↑ tendency to fall forwards and risk of pelvic prolapse The hormone Relaxin makes ligaments stretchier: ↓ joint stability Must have been a stronger pressure acting Lumbosacral & pubic bone angles: adaptations to wide hips & being upright gibbon H Gorilla: ~0-20° Gibbon: 24-60° Also locomote with lumbar lordosis = Infant: ~20°  Adult: 60 – 80° Adaptations made to avoid/reduce pressure of prolapse, due to upright body position Ischial spines: An adaptation that obstructs the birth canal?? Attachment for coccygeus muscle & the sacrospinous lig. (Less fibrotic, or still a muscle in other primates) Fibrous ligament holds ischial spine close to sacrum: Strong & efficient, but less dynamic and pliable Female pelvis also evolved for things that were not driven by parturition Pre-birth work capacity is a greater selection pressure than birth itself Birthing is more of a yes/no pressure (‘healthy’ caesarean section rate 10-15%) Ischial spines increase bipedal efficiency, whilst reducing visceral prolapse during pregnancy > IT Larger body size makes us efficient bipeds…. (??) exam " going to be in " Not Pelvic adaptations facilitate our trunk size being efficient for bipedalism Tendency to fall forward at heel strike & backwards when accelerating the limb forwards Lower limb is very LONG and very HEAVY Forces work on limbs AND trunk: Need a counterweight atop the pelvis Short, light trunk + long heavy legs = trunk would move a lot on top of the pelvis Trunk inertia must be comparable to lower limb inertia Angular momentum must be balanced Trunk sways less during bipedal locomotion = ↓ energy requirement to stay upright  ' not in ' exam Human pelvis is highly adapted to bipedalism Later view chimp horn in human Broader sacrum, wider acetabular space, flared ilia Efficiency in rotational & frontal movements Allows abdominal oblique muscles to store elastic energy during walking Superior view of SI joint Acetabular socket angled more inferiorly and ‘less’ laterally Feet fall within pelvic width Ilial neck is shortened and ‘deepened’ Centre of gravity vertically closer to hip joint, supports larger body sized biped Reduces the ‘bending moment in ilia’ Lower limbs are very long and VERY HEAVY