Learn — Musculoskeletal system

17 notes · 32 questions on the topic

Lower limb osteology

The lower limb skeleton is built for weight-bearing and locomotion: 31 bones in each limb — the hip bone, femur, patella, tibia and fibula, 7 tarsals, 5 metatarsals and 14 phalanges — arranged so that body weight passes from the vertebral column through the sacro-iliac joint, the acetabulum, the femur and the tibia into the arches of the foot. Its landmarks are the nurse's working map: the iliac crest and greater trochanter for a ventrogluteal injection, the tibial tuberosity for intraosseous access in a child, and the femoral neck for the fracture that fills geriatric wards.

Each lower limb has 31 bones: 1 hip bone, femur, patella, tibia and fibula, 7 tarsals, 5 metatarsals and 14 phalanges — two in the great toe and three in each of the other four.
The hip bone is the fused ilium, ischium and pubis, which meet at the acetabulum and fuse there around puberty; the ilium ends above at the iliac crest, whose highest point is at the level of the fourth lumbar vertebra and is the surface marker used to place a lumbar puncture below the spinal cord.
The femur is the longest, heaviest and strongest bone in the body; the head is supplied mainly by retinacular branches of the medial circumflex femoral artery running up the neck, so an intracapsular fracture of the neck tears that supply and risks avascular necrosis of the head.
The patella is the largest sesamoid bone in the body, lying in the tendon of quadriceps femoris and inserting through the ligamentum patellae into the tibial tuberosity; it holds the tendon away from the joint and so lengthens the lever arm of knee extension.
The tibia carries the weight of the leg and forms the medial malleolus; the fibula carries almost none and can be harvested for bone grafting, but it forms the lateral malleolus, and the common peroneal nerve winds subcutaneously round its neck, where a tight plaster or a badly placed lithotomy support causes foot drop.
The 7 tarsals are the talus, calcaneus, navicular, cuboid and three cuneiforms; the talus has no muscular attachments and a precarious blood supply, and it transmits body weight from the tibia to the calcaneus behind and the forefoot in front.
The calcaneus is the largest tarsal and the heel-strike bone; it is the commonest site of a heel pressure ulcer in the supine patient, and in the newborn the heel prick is taken from the medial or lateral plantar surface, never the centre of the heel, to avoid injuring the bone.
The knee is the largest synovial joint in the body — a modified hinge with menisci, cruciate and collateral ligaments — while the hip is a ball-and-socket joint that trades range of movement for the stability weight-bearing needs.
Landmarks used at the bedside: the greater trochanter, anterior superior iliac spine and iliac crest bound the ventrogluteal injection site, the femoral pulse lies at the mid-inguinal point, and intraosseous access in a child is taken 1 to 2 cm below and medial to the tibial tuberosity on the flat anteromedial surface.

Plantar flexion

Plantar flexion is the movement at the ankle (talocrural) joint that points the foot and toes downward, away from the shin, through a normal range of about 0 to 50 degrees, against about 0 to 20 degrees of dorsiflexion in the opposite direction. It is produced mainly by gastrocnemius and soleus through the tendo calcaneus and is supplied by the tibial nerve, and it is both the movement that gives push-off in walking and the position a bedbound foot silently adopts on its way to a fixed foot drop.

The main plantar flexors are gastrocnemius and soleus, which together form triceps surae and insert by the tendo calcaneus (Achilles tendon) into the posterior surface of the calcaneus; plantaris, tibialis posterior, flexor hallucis longus, flexor digitorum longus and the two peronei assist.
All the muscles of the posterior compartment of the leg are supplied by the tibial nerve, and plantar flexion is chiefly an S1 and S2 movement; the peronei that assist are supplied by the superficial peroneal nerve.
The ankle jerk tests S1 through the tibial nerve — tapping the tendo calcaneus with the ankle held in slight dorsiflexion produces plantar flexion, and its loss is an early sign of an S1 root lesion or a peripheral neuropathy such as diabetes.
Dorsiflexion is the opposite movement, produced by tibialis anterior, extensor hallucis longus and extensor digitorum longus and supplied by the deep peroneal nerve (L4 and L5); injury to the common peroneal nerve at the neck of the fibula abolishes dorsiflexion and eversion and gives a foot drop with a high-stepping gait.
In gait, plantar flexion supplies the push-off at toe-off and makes standing on tiptoe possible; an inability to stand on tiptoe, a palpable gap in the tendon and a positive Simmonds or Thompson calf-squeeze test mean a ruptured tendo calcaneus.
The ankle is most stable in dorsiflexion, because the wider anterior part of the talus is then gripped in the mortise; in plantar flexion the narrower posterior part sits in the mortise and the joint is least stable, which is why most sprains are inversion injuries sustained with the foot plantar flexed.
Unopposed plantar flexion in a bedbound or unconscious patient shortens the tendo calcaneus into a fixed equinus deformity within a few weeks, and once the tendon has contracted no amount of positioning will reverse it.
Compartment syndrome of the deep posterior compartment shows itself as severe pain on passive dorsiflexion of the toes and ankle — the passive stretch of the muscles that plantar flex — which is why the nurse tests that movement, not the pulse, when it is suspected.

Muscle origin and insertion

The origin of a muscle is its attachment to the bone that stays relatively fixed when the muscle contracts, and the insertion is its attachment to the bone that moves; contraction shortens the belly and pulls the insertion towards the origin. Knowing which end moves is what lets a nurse or physiotherapist predict a muscle's action from its attachments, position a limb so that a painful muscle is slack, and understand why a paralysis produces the particular deformity it does.

By convention the origin is the proximal, larger, more fixed and usually tendinous attachment and the insertion is the distal, smaller, more mobile one; in the limbs the origin is proximal and the insertion distal, and the muscle always pulls the insertion towards the origin.
The roles reverse whenever the distal end is fixed: in a pull-up the hands are fixed so the body rises to the arms, and in a press-up the insertion of pectoralis major is fixed so the trunk moves on the arm.
Biceps brachii shows the pattern: the long head arises from the supraglenoid tubercle of the scapula and the short head from the coracoid process, both insert into the radial tuberosity and, by the bicipital aponeurosis, into the deep fascia of the forearm, and the muscle flexes the elbow and is the most powerful supinator of the forearm.
Muscles are named from origin and insertion (sternocleidomastoid, coracobrachialis), from action (extensor digitorum, levator scapulae), from shape or number of heads (deltoid, biceps, quadriceps) or from the direction of their fibres (rectus, obliquus, transversus) — so the name usually contains the answer.
Four roles are played in any movement: the agonist or prime mover produces it, the antagonist relaxes to allow it, synergists steady the intermediate joints and refine it, and fixators hold the origin steady — when biceps flexes the elbow, triceps is the antagonist and the scapular muscles are the fixators.
A muscle that crosses one joint acts on that joint alone, but a muscle crossing two — gastrocnemius, the hamstrings, rectus femoris — acts on both and can be passively insufficient; that is why flexing the hip tightens the hamstrings and limits knee extension in the straight-leg raise.
Tendons join muscle to bone and ligaments join bone to bone; a tendon that avulses often takes a flake of bone with it, so an avulsion fracture on an X-ray marks exactly where a muscle inserts.
When a muscle is paralysed the unopposed antagonist pulls the joint into a deformity along its own line of pull — a dead tibialis anterior gives foot drop, dead hand intrinsics give claw hand, and understanding that is what makes splinting and positioning rational rather than routine.

Mouth muscles

The muscles around the mouth fall into two groups that are separate both in function and in nerve supply: the muscles of facial expression, which lie in the superficial fascia, are attached to skin and are supplied by the facial nerve (VII), and the muscles of mastication, which move the mandible and are supplied by the mandibular division of the trigeminal nerve (V3). That division is the whole point at the bedside — it is why a patient with a facial palsy dribbles his food but chews it perfectly well, and why a trigeminal lesion does the reverse.

Orbicularis oris is the sphincter of the mouth: it closes and purses the lips for speech, whistling, sucking and holding food between the teeth, and it is supplied by the buccal and marginal mandibular branches of the facial nerve.
Buccinator forms the muscular wall of the cheek, keeps the food between the grinding surfaces of the teeth and empties the vestibule; it is pierced by the parotid duct opposite the crown of the upper second molar tooth, and although it assists chewing it is a facial nerve muscle, not a muscle of mastication.
The four muscles of mastication — masseter, temporalis, medial pterygoid and lateral pterygoid — are all supplied by the mandibular division of the trigeminal nerve; masseter, temporalis and medial pterygoid close the jaw, and the lateral pterygoid opens it and protrudes the mandible.
The other perioral muscles are all facial nerve muscles: levator labii superioris, zygomaticus major and minor (which raise the angle of the mouth in a smile), levator anguli oris, risorius, depressor anguli oris, depressor labii inferioris, mentalis and platysma.
The facial nerve leaves the skull at the stylomastoid foramen, enters the parotid gland and divides there into five branches — temporal, zygomatic, buccal, marginal mandibular and cervical — which is why parotid surgery, a parotid abscess and forceps pressure at delivery all threaten the nerve.
An upper motor neurone lesion such as a stroke spares the forehead, because the part of the facial nucleus that supplies frontalis has bilateral cortical input; a lower motor neurone lesion such as Bell's palsy paralyses the whole half of the face, forehead and eyelid included — this is the single most-asked discriminator in the subject.
The bedside consequences of a weak orbicularis oris and buccinator are dribbling of saliva and fluids, food pocketing in the affected cheek, an inability to whistle or blow out the cheek, and failure to seal the lips around a straw, a spoon or a nebuliser mouthpiece.
Trismus — an inability to open the mouth from spasm of the muscles of mastication — is an early sign of tetanus and of deep dental, parapharyngeal or peritonsillar infection, and it is also seen in submucous fibrosis from areca nut chewing, which is common in India.

Anterior thoracic wall

The anterior thoracic wall is the cage formed by the sternum in the midline, the costal cartilages and ribs on each side, the intercostal muscles between them and the pectoral muscles over them, and it moves with every breath. Its surface landmarks are the most used piece of anatomy on any ward, because they are the reference points for counting ribs, listening to the heart valves, siting a chest drain and placing the hands for chest compressions.

The sternum has three parts — manubrium, body and xiphoid process; the manubriosternal joint forms the palpable ridge called the sternal angle (angle of Louis), which marks the second costal cartilage and lies at the level of the intervertebral disc between T4 and T5.
The sternal angle is the crossroads of the thorax: the level of the second rib for counting spaces, the bifurcation of the trachea, the beginning and end of the arch of the aorta, the entry of the azygos vein into the superior vena cava and the upper limit of the pericardium.
There are 12 pairs of ribs: ribs 1 to 7 are true ribs, each reaching the sternum by its own costal cartilage; ribs 8 to 10 are false ribs whose cartilages join the cartilage above; ribs 11 and 12 are floating, with no anterior attachment at all.
There are three layers of intercostal muscle — external, internal and innermost; the external fibres run downwards and forwards and elevate the ribs in inspiration, and the neurovascular bundle lies in the costal groove at the lower border of each rib in the order vein, artery, nerve from above downwards (VAN).
Because the bundle hugs the lower border of the rib, a needle for a pleural tap or a chest drain is always passed immediately above the upper border of the rib below, never below a rib — that single rule prevents intercostal artery haemorrhage.
The safe triangle for an intercostal chest drain is bounded in front by the lateral border of pectoralis major, behind by the lateral border of latissimus dorsi and below by a horizontal line at the level of the nipple, with the apex below the axilla; it corresponds to about the fifth intercostal space in the mid-axillary line.
Pectoralis major arises by clavicular and sternocostal heads and inserts into the lateral lip of the intertubercular (bicipital) groove of the humerus; pectoralis minor runs from the third to fifth ribs to the coracoid process, and the internal thoracic artery runs vertically about 1 cm lateral to the edge of the sternum, which is the vessel taken as the internal mammary graft in coronary surgery.
The auscultation and procedure landmarks: aortic area in the second right intercostal space at the sternal edge, pulmonary area in the second left space, tricuspid at the lower left sternal border and mitral at the apex in the fifth left intercostal space in the mid-clavicular line, with the lower half of the sternum as the hand position for chest compressions.
A flail segment is three or more consecutive ribs each fractured in two places; that segment moves paradoxically, drawn inwards on inspiration and pushed outwards on expiration, and the underlying lung contusion is usually the greater danger.

Abduction, adduction, eversion

Movements are named from the anatomical position — standing erect, facing forward, arms by the sides with the palms turned forward — and are described against three planes: abduction takes a part away from the midline, adduction brings it back, and eversion turns the sole of the foot outwards. Nurses use these words to write range-of-movement charts, to describe a deformity precisely in the notes and to follow a physiotherapy plan, so the definitions have to be exact rather than approximate.

Abduction moves a part away from the median plane of the body and adduction moves it towards it; in the fingers and toes the reference line is not the body's midline but the axis of the middle finger or the second toe, so spreading the fingers is abduction and closing them is adduction.
Shoulder abduction happens in three stages: supraspinatus initiates the first 15 degrees, deltoid carries it from about 15 to 90 degrees, and beyond 90 degrees the scapula rotates under trapezius and serratus anterior to reach about 180 degrees — which is why a winged scapula stops the arm at shoulder height.
Flexion decreases the angle at a joint and extension increases it; the exceptions worth knowing are the ankle, where the terms become dorsiflexion and plantar flexion, and the thumb, whose movements take place in a plane at right angles to those of the fingers.
Eversion turns the sole of the foot outwards and inversion turns it inwards, and both occur at the subtalar and transverse tarsal joints rather than at the ankle joint itself; the evertors are peroneus longus and brevis (superficial peroneal nerve) and the invertors are tibialis anterior and tibialis posterior.
Most ankle sprains are inversion injuries that tear the lateral ligament, in particular the anterior talofibular band, because the invertors are the stronger group and the lateral ligament is weaker than the medial (deltoid) ligament.
Pronation turns the palm downwards and supination turns it upwards (supination is the position that holds a bowl of soup); supination is the stronger movement, which is why screws and taps are cut for a right-handed supinating pull.
Rotation is described as medial (internal) or lateral (external) around the long axis of a bone, and circumduction is the cone-shaped sequence of flexion, abduction, extension and adduction possible only at multiaxial joints such as the shoulder and the hip.
Special movements to name correctly: protraction and retraction of the scapula and mandible, elevation and depression of the shoulders, opposition of the thumb to the fingertips (the movement lost with a median nerve injury), and inversion and eversion, which belong to the foot alone.
Ranges commonly charted: shoulder abduction 0 to 180 degrees, elbow flexion 0 to 150 degrees, hip abduction 0 to 45 degrees, knee flexion 0 to 135 degrees, ankle dorsiflexion 0 to 20 degrees and plantar flexion 0 to 50 degrees.

Boxer muscle (serratus anterior)

Serratus anterior is the fan-shaped muscle that arises by fleshy digitations from the outer surfaces of the upper eight ribs and inserts along the whole medial border of the costal surface of the scapula, holding the scapula flat against the chest wall. It is called the boxer's muscle because it protracts the scapula and so delivers the punch, and its nerve — the long thoracic nerve of Bell, from roots C5, C6 and C7 — runs superficially down the outside of the muscle where surgery, drains and trauma can reach it, which is why winging of the scapula is both a favourite examination item and a real post-operative finding.

Origin: fleshy slips from the outer surfaces of the upper eight ribs in the mid-axillary line. Insertion: the medial border of the scapula on its costal surface, with the strongest lower four digitations converging on the inferior angle.
Nerve supply is the long thoracic nerve (nerve of Bell) from the roots C5, C6 and C7 — a branch taken directly from the roots, so it is spared in cord and trunk lesions but is injured easily where it lies subcutaneously on the muscle in the mid-axillary line.
Actions: it protracts the scapula in punching and pushing, holds the medial border of the scapula against the chest wall, and with the trapezius rotates the scapula upwards so that the glenoid faces up — the movement that allows the arm to be raised above 90 degrees.
Paralysis produces winging of the scapula: the medial border and inferior angle stand away from the chest wall, most obviously when the patient pushes with both outstretched arms against a wall, and abduction beyond about 90 degrees becomes impossible.
The usual causes are surgical injury during axillary lymph node clearance or mastectomy, chest drain insertion in the mid-axillary line, carrying heavy loads or a rucksack strap on the shoulder, a blow to the side of the chest and viral brachial neuritis.
The examiner's contrast: long thoracic nerve palsy gives medial winging with weak forward elevation, while an injury to the spinal accessory nerve (XI) during a posterior triangle lymph node biopsy paralyses trapezius and gives lateral winging with a weak shoulder shrug and a drooping shoulder.
Recovery from a stretch or compression injury of the long thoracic nerve is usually spontaneous but slow, taking many months and sometimes up to two years; management meanwhile is analgesia, avoiding overhead work and heavy carrying, and scapular stabilising physiotherapy, with tendon transfer reserved for failure.
Practical implication on a ward: a patient who develops winging after a chest drain in the mid-axillary line, an axillary clearance or a long anaesthetic in an awkward position has a nerve injury to be reported and referred, not a stiff shoulder to be reassured.

Cervical vertebrae

There are seven cervical vertebrae, the smallest and most mobile of the presacral vertebrae, and they are the only vertebrae with a foramen transversarium in each transverse process. Three of them are atypical — the atlas (C1), the axis (C2) and the vertebra prominens (C7) — and those special features are exactly what make the neck both the most mobile and the most vulnerable part of the spine.

A typical cervical vertebra (C3 to C6) has a small kidney-shaped body, a large triangular vertebral foramen, a bifid spine and a transverse foramen that transmits the vertebral artery, vein and sympathetic plexus.
The vertebral artery arises from the first part of the subclavian artery, enters the transverse foramen of C6 and ascends to C1; the C7 transverse foramen transmits only the vertebral vein, not the artery.
The atlas (C1) has no body and no spine — only anterior and posterior arches with two lateral masses; the body it should have had is fused to the axis as the dens.
The axis (C2) carries the dens or odontoid process, held against the anterior arch of the atlas by the transverse ligament; the atlanto-occipital joint gives the nodding movement and the atlanto-axial joint gives rotation.
C7 is the vertebra prominens: its spine is long, single and not bifid, and it is the first spinous process felt when the neck is flexed — the landmark used for counting ribs and for placing ECG chest leads.
Surface levels in the neck: the hyoid bone lies at C3, the upper border of the thyroid cartilage at C4, the cricoid cartilage at C6, and the carotid tubercle of Chassaignac is the anterior tubercle of the C6 transverse process against which the carotid artery can be compressed.
The cervical curve is a secondary lordotic curvature that appears at about three months when the infant begins to lift its head; it is lost and shows as straightening on a lateral radiograph when the neck muscles are in spasm.
Named injuries: a Jefferson fracture is a burst of the C1 ring from an axial load such as diving into shallow water, and a hangman's fracture is a bilateral fracture of the C2 pars interarticularis from hyperextension.

Cricoid cartilage

The cricoid is the only complete ring of cartilage in the whole airway, a signet-ring shaped hyaline cartilage with a narrow anterior arch and a broad posterior lamina, and it forms the lower end of the larynx at the level of C6 in the adult. Because it is a full ring it cannot expand, which makes the subglottis the narrowest part of a child's airway and makes the cricoid the landmark for several emergency airway procedures.

It lies at the level of the sixth cervical vertebra, marking the junction of the larynx with the trachea and of the pharynx with the oesophagus, and the level at which the inferior thyroid artery enters and the middle cervical sympathetic ganglion lies.
It is the only complete cartilaginous ring in the airway; every tracheal ring is C-shaped and open behind, closed by the trachealis muscle so that the oesophagus behind can distend.
The cricothyroid membrane lies between the lower border of the thyroid cartilage and the upper border of the cricoid arch and is the site for an emergency cricothyroidotomy when a patient can neither be intubated nor oxygenated.
In a child under about 8 years the cricoid is classically the narrowest part of the airway at the subglottis, while in an adult the narrowest point is the rima glottidis between the vocal cords — the reason uncuffed tubes were traditionally used in small children and why subglottic swelling in croup produces stridor.
The cricoid is felt as the firm ring immediately below the softer notch of the thyroid cartilage and above the tracheal rings, and it is the surface landmark for confirming that the trachea is midline.
Cricoid pressure, the Sellick manoeuvre, presses the ring backwards to occlude the oesophagus against the vertebral body during rapid sequence induction; it is no longer routinely recommended because it can worsen the laryngeal view and does not reliably prevent aspiration.
The cricothyroid muscle, which tenses the vocal cords, is the only intrinsic laryngeal muscle not supplied by the recurrent laryngeal nerve — it is supplied by the external branch of the superior laryngeal nerve.
The cricoid articulates above with the arytenoid cartilages at the cricoarytenoid joints, which abduct and adduct the cords, and with the inferior horns of the thyroid cartilage at the cricothyroid joints, which tilt it.
The cricoid is hyaline cartilage and calcifies with age, unlike the elastic cartilage of the epiglottis and of the corniculate and cuneiform cartilages.

Joints — types

A joint (articulation) is the site where two or more bones meet, and joints are classified by the tissue that unites them into fibrous, cartilaginous and synovial, with mobility rising across those three groups. The classification matters at the bedside because it predicts what a joint can and cannot do: a fibrous suture is not meant to move at all, a symphysis gives only a few millimetres, and a synovial joint has a cavity, a capsule and synovial fluid, which is also why only synovial joints dislocate, swell with effusion and become septic.

Fibrous joints are united by dense fibrous tissue and have no cavity: sutures of the skull vault (immovable), syndesmosis such as the inferior tibiofibular joint (slightly movable), and gomphosis, the peg-and-socket fixation of a tooth in its alveolus.
Cartilaginous joints are primary (synchondrosis, hyaline cartilage, immovable, seen at the epiphyseal growth plate and the first sternocostal joint) or secondary (symphysis, fibrocartilage, slightly movable, always in the midline: pubic symphysis, intervertebral discs, manubriosternal joint).
A synovial joint has five constant features: articular hyaline cartilage, a fibrous capsule lined by synovial membrane, a joint cavity, synovial fluid and supporting ligaments; the articular cartilage has no periosteum, no nerves and no blood vessels, so it heals badly.
The six synovial subtypes are plane (acromioclavicular, intercarpal), hinge or uniaxial flexion-extension (elbow, ankle, interphalangeal), pivot or rotation (atlanto-axial, proximal radio-ulnar), condyloid or ellipsoid and biaxial (wrist, metacarpophalangeal), saddle (first carpometacarpal joint of the thumb, which allows opposition) and ball-and-socket or multiaxial (shoulder and hip).
The shoulder is the most mobile joint and therefore the most commonly dislocated, usually anteriorly, because its glenoid cavity is shallow; the hip is the same ball-and-socket class but is deep, cupped by the acetabular labrum and strongly ligamented, so it needs high energy to dislocate.
The knee is the largest synovial joint in the body and is a modified hinge (bicondylar): it flexes and extends, but also rotates slightly, and it locks in full extension until the popliteus unlocks it.
The temporomandibular joint is synovial but is covered by fibrocartilage rather than hyaline cartilage and carries an intra-articular disc that divides it into two cavities.

Types and number of bones (206)

The adult human skeleton has 206 named bones, divided into an axial skeleton of 80 bones that forms the central bony axis and an appendicular skeleton of 126 bones that forms the limbs and their girdles. A newborn starts with roughly 270 separate bony pieces and loses the difference by fusion during growth, which is why the count is quoted for the adult and why the same skeleton is described by shape as long, short, flat, irregular, sesamoid and pneumatic.

Axial skeleton, 80 bones: skull 22 (8 cranial, 14 facial), hyoid 1, auditory ossicles 6, vertebrae 26 (7 cervical, 12 thoracic, 5 lumbar, 1 sacrum, 1 coccyx), ribs 24 in 12 pairs, and sternum 1.
Appendicular skeleton, 126 bones: upper limb 64 in total (pectoral girdle 4, that is 2 clavicles and 2 scapulae, plus 60 in the free limbs) and lower limb 62 in total (pelvic girdle 2 hip bones plus 60 in the free limbs).
Each free upper limb has 30 bones: humerus, radius, ulna, 8 carpals, 5 metacarpals and 14 phalanges; each free lower limb has 30: femur, patella, tibia, fibula, 7 tarsals, 5 metatarsals and 14 phalanges.
The neonate has about 270 bones because the sacrum is still 5 pieces, the coccyx 4, the hip bone 3 (ilium, ischium, pubis meeting at the triradiate cartilage) and the frontal bone two halves; fusion brings the number down to 206 by early adulthood.
By shape: long (femur, humerus), short (carpals, tarsals), flat (sternum, scapula, ribs, skull vault), irregular (vertebrae, hip bone), sesamoid, which develop in tendons, and pneumatic, which contain air sinuses (maxilla, ethmoid, frontal, sphenoid).
The patella is the largest sesamoid bone in the body; the femur is the longest and heaviest bone and the stapes is the smallest; the hyoid is the only bone that articulates with no other bone.
Sutural or wormian bones are extra bones found within skull sutures, most often the lambdoid suture, and they are not included in the count of 206.

Bone coverings (periosteum)

The periosteum is the tough double-layered membrane that covers the outer surface of every bone except the articular surfaces, which are covered by hyaline cartilage instead. Its outer layer is dense fibrous tissue carrying blood vessels and sensory nerves and its inner osteogenic layer holds osteoprogenitor cells, so the periosteum is at once the bone's nerve supply, part of its blood supply and its capacity to lay down new bone in a fracture.

The periosteum has two layers: an outer fibrous layer with vessels and nerves, and an inner cellular or cambium layer of osteoprogenitor cells that become osteoblasts and produce appositional (circumferential) growth in width and callus after a fracture.
Sharpey's fibres are perforating collagen bundles that run from the periosteum into the outer lamellae of bone, anchoring the periosteum and, at their insertions, anchoring tendons and ligaments to bone.
The endosteum is the thin cellular membrane lining the medullary cavity, the Haversian canals and the trabeculae of spongy bone; it carries osteoblasts and osteoclasts and remodels the inner surface as the periosteum builds up the outer.
The nutrient artery enters through the nutrient foramen and supplies the marrow and the inner two-thirds of the cortex, while periosteal vessels supply the outer third; stripping the periosteum at surgery or in a high-energy injury therefore threatens the blood supply and invites delayed union or non-union.
The periosteum is richly supplied with sensory nerve endings and is the pain-sensitive part of bone, whereas the mineralised matrix itself is relatively insensitive; this is why a periosteal haematoma, a shin knock and an intraosseous infusion hurt so much.
In a child the periosteum is thick, vascular and firmly adherent, so fractures are often incomplete greenstick or buckle injuries, the periosteal sleeve holds the fragments and healing is fast; in an elderly patient it is thin and healing is slower.
Periosteal reaction seen on X-ray, such as the onion-skin or Codman triangle appearance, signals that something is lifting the periosteum from bone: osteomyelitis, Ewing sarcoma or osteosarcoma.

Bone cells — osteocytes, osteoclasts, osteoblasts

Bone is a living tissue kept in balance by four cell types: osteoprogenitor cells that divide and replace the working cells, osteoblasts that build matrix, osteocytes that maintain it and sense load, and osteoclasts that resorb it. Three of the four come from mesenchyme; the osteoclast alone comes from the haematopoietic monocyte-macrophage line, which is the fact that explains why bone-resorbing drugs and bone-marrow disease both act through this one cell.

Osteoprogenitor (osteogenic) cells lie in the inner cambium layer of the periosteum and in the endosteum, are the only bone cells that divide by mitosis, and differentiate into osteoblasts whenever bone must be built or repaired.
Osteoblasts are cuboidal cells that sit in a sheet on the bone surface and secrete osteoid, which is mainly type I collagen with ground substance, then mineralise it with calcium hydroxyapatite; they are rich in alkaline phosphatase, so a rising serum alkaline phosphatase marks osteoblastic activity in healing fracture, rickets and Paget disease.
An osteoblast that walls itself into its own matrix becomes an osteocyte, lying in a lacuna and communicating with its neighbours through cytoplasmic processes in canaliculi; osteocytes are the most numerous bone cell, they sense mechanical strain and they maintain the matrix, which is why immobilised and weightless bone demineralises.
Osteoclasts are large multinucleated cells derived from the monocyte-macrophage lineage; they sit in a scalloped Howship's lacuna, seal onto the surface and use a ruffled border to pump out hydrogen ions and release enzymes including tartrate-resistant acid phosphatase and cathepsin K, dissolving both mineral and matrix.
Osteoclasts carry no parathyroid hormone receptor: parathyroid hormone acts on the osteoblast, which then expresses RANK ligand to switch the osteoclast on, while osteoprotegerin is the decoy that switches it off. Calcitonin is the one hormone that inhibits the osteoclast directly, because the osteoclast does carry a calcitonin receptor.
Oestrogen restrains osteoclastic resorption, so the fall at menopause tips the balance towards resorption and produces postmenopausal osteoporosis; denosumab blocks RANK ligand, and bisphosphonates are taken up by the osteoclast and drive it into apoptosis.
Bone turnover is read from formation markers (alkaline phosphatase, osteocalcin) and resorption markers (serum CTX and urinary NTX telopeptides); the two remain coupled in normal remodelling and uncouple in osteoporosis and metastatic bone disease.

Largest bone (femur)

The femur is the longest, heaviest and strongest bone in the body, forming the skeleton of the thigh and measuring roughly a quarter of a person's height. Its head sits in the acetabulum to make the hip joint and its condyles sit on the tibia to make the knee, so the femur transmits the whole body weight from the pelvis to the leg and any break in it is a major, and often a bleeding, injury.

The upper end has a head with a central fovea for the ligament of the head, a neck, and greater and lesser trochanters; the shaft is bowed forwards and carries the linea aspera behind, and the lower end expands into medial and lateral condyles for the knee.
The angle between the neck and the shaft is about 125 degrees in the adult and is wider in the child; a decrease gives coxa vara and an increase coxa valga, and both change the mechanics of walking and the risk of fracture.
The head of the femur is supplied chiefly by retinacular branches of the medial circumflex femoral artery, with a small contribution through the ligament of the head from the obturator artery that matters mainly in children; an intracapsular fracture of the neck tears the retinacular vessels and threatens avascular necrosis of the head.
A fractured neck of femur presents as a shortened, adducted and externally rotated limb with pain in the groin and inability to lift the heel off the bed, classically in an elderly person after a trivial fall onto the hip.
A closed fracture of the femoral shaft can hide one to one and a half litres of blood in the thigh and is enough on its own to cause hypovolaemic shock; it is also the classic source of fat embolism syndrome 24 to 72 hours later, with hypoxia, confusion and a petechial rash.
The greater trochanter is the landmark for the ventrogluteal injection site: the palm of the opposite hand on the greater trochanter, the index finger on the anterior superior iliac spine and the middle finger spread along the iliac crest, injecting into the V between the fingers, well away from the sciatic nerve.
The distal femoral epiphysis is the first epiphysis to ossify, appearing at about the 36th week of intrauterine life, so its presence on an X-ray is taken as evidence of a term fetus.

Carpal tunnel syndrome

Carpal tunnel syndrome is compression of the median nerve where it passes through the carpal tunnel at the wrist, and it is the commonest entrapment neuropathy in the body. The tunnel is a rigid, unyielding space whose floor and walls are the arched carpal bones and whose roof is the flexor retinaculum, so any swelling of its contents raises pressure on the nerve, which is why the syndrome is so common in pregnancy, hypothyroidism, diabetes and rheumatoid arthritis.

The flexor retinaculum forms the roof of the tunnel and is attached to the tubercle of the scaphoid and the ridge of the trapezium laterally and to the pisiform and the hook of the hamate medially; the carpal bones form the concave floor and walls.
The tunnel contains ten structures: the four tendons of flexor digitorum superficialis, the four of flexor digitorum profundus, the tendon of flexor pollicis longus and the median nerve, which is the most superficial and therefore the most vulnerable.
The ulnar nerve and artery do not pass through the carpal tunnel; they cross superficial to the retinaculum in Guyon's canal, which is why the little finger is spared and why sparing of the little finger is a useful discriminator in an examination question.
Sensation is lost or tingles over the palmar aspect of the lateral three and a half digits, the thumb, index, middle and lateral half of the ring finger; the palmar cutaneous branch leaves the median nerve above the retinaculum and runs over it, so sensation over the thenar eminence and central palm is spared, which distinguishes carpal tunnel syndrome from a median nerve lesion higher in the forearm.
Motor loss affects the thenar muscles supplied by the median nerve, abductor pollicis brevis, opponens pollicis and the superficial head of flexor pollicis brevis, plus the lateral two lumbricals; weakness of thumb abduction and opposition comes first and visible thenar wasting is a late and worrying sign.
The story is characteristic: burning pain and pins and needles that wake the patient in the early hours and are relieved by hanging the hand out of bed or shaking it, the so-called flick sign, with symptoms provoked by driving, holding a phone or reading a newspaper.
Bedside tests are Tinel's sign (tapping over the tunnel reproduces the tingling), Phalen's test (holding the wrists fully flexed for about 60 seconds) and the carpal compression test; nerve conduction studies confirm it by showing prolonged distal median latency.
Risk factors are conveniently grouped as fluid, endocrine and mechanical: pregnancy and the premenstrual state, hypothyroidism, diabetes, acromegaly, obesity, rheumatoid arthritis, amyloidosis, long-term dialysis, a healed Colles fracture and repetitive or vibrating hand work.
Treatment is stepwise: a neutral wrist splint worn at night, activity and workstation modification, analgesia, corticosteroid injection into the tunnel, and surgical division of the flexor retinaculum, open or endoscopic, when symptoms persist or the thenar muscles are wasting.

Foot drop

Foot drop is the loss of active dorsiflexion at the ankle, so the forefoot hangs down and the toes catch on the ground during the swing phase of walking; it is a physical sign and not a diagnosis, and the work at the bedside is to place the level of the lesion. The muscles that lift the foot — tibialis anterior, extensor hallucis longus, extensor digitorum longus and peroneus tertius — are supplied by the deep fibular (peroneal) nerve, which is why the commonest cause of all is compression of the common fibular nerve where it winds subcutaneously round the neck of the fibula.

The common fibular (common peroneal) nerve comes from the posterior divisions of L4, L5, S1 and S2 as the smaller terminal branch of the sciatic nerve, and it is the most frequently injured nerve of the lower limb because at the neck of the fibula it lies directly on bone with nothing but skin over it.
At the fibular neck it divides into the superficial fibular nerve, which supplies the evertors (peroneus longus and brevis) and the skin of the lower lateral leg and most of the dorsum of the foot, and the deep fibular nerve, which supplies the dorsiflexors and only a small patch of skin in the first web space between the great and second toes.
The walk is a high-stepping or steppage gait: the hip and knee are lifted higher than normal to clear the dropped foot and the forefoot slaps the floor at heel strike, so the patient trips on rugs, thresholds and the edges of stairs.
Inversion is the discriminator between a common fibular palsy and an L5 root lesion — tibialis posterior inverts the foot and is supplied by the tibial nerve, so inversion stays normal in a fibular nerve palsy but is weak in an L5 radiculopathy, which also weakens hip abduction and produces a Trendelenburg sign.
The ankle jerk runs through S1 and the tibial nerve, so it is preserved in both a common fibular palsy and an L5 root lesion; a lost ankle jerk points instead to a sciatic nerve or S1 root lesion.
Causes to run through in order: pressure at the fibular neck (habitual leg crossing, prolonged squatting, a tight plaster or bandage, lithotomy stirrups, a knee restraint, or lying on one side while unconscious or under an epidural), an L4-L5 disc prolapse, diabetic mononeuropathy, leprosy involving the lateral popliteal nerve, and anterior horn cell disease.
Management is an ankle-foot orthosis holding the ankle at 90 degrees, daily passive dorsiflexion and stretching to prevent an equinus contracture of the tendo Achillis, removal of whatever is pressing on the nerve, treatment of the underlying cause, and nerve conduction studies to localise the block; a simple compression palsy usually recovers over some weeks to a few months.

Winging of scapula

Winging of the scapula is the abnormal lifting of the medial (vertebral) border of the scapula away from the chest wall, most often because serratus anterior has been paralysed by injury to the long thoracic nerve of Bell (C5, C6 and C7). Serratus anterior is the muscle that holds the scapula flat against the ribs and draws it forwards, so when it fails the scapula loses its anchor and the patient can no longer raise the arm fully above shoulder level.

Serratus anterior arises by fleshy slips from the outer surfaces of the upper eight ribs and inserts along the whole length of the medial border of the scapula on its costal surface; the long thoracic nerve (C5, C6, C7) runs down the outer surface of the muscle on the lateral chest wall, where nothing covers or protects it.
Its actions are protraction of the scapula, which makes it the boxer's or puncher's muscle, and upward rotation of the glenoid cavity together with the trapezius, which is what allows the arm to be raised beyond 90 degrees — so paralysis limits overhead abduction and flexion rather than the first part of the range.
The bedside test is to ask the patient to push against a wall with both arms out and the elbows straight: the medial border and inferior angle of the affected scapula lift off the chest wall and stand out.
Medial winging, with the inferior angle turned towards the spine and the sign worst on pushing forwards, means serratus anterior and the long thoracic nerve; lateral winging, with the inferior angle turned away from the spine, a drooping shoulder and the sign worst on abduction, means trapezius and the spinal accessory nerve, classically after surgery in the posterior triangle of the neck.
The long thoracic nerve is injured by axillary clearance during mastectomy, by blunt trauma or a stretch injury to the shoulder, by heavy loads or a rucksack strap carried on the shoulder, by thoracic surgery and first rib resection, and by viral brachial neuritis.
Most stretch and compression lesions are neurapraxias that recover over about 6 to 24 months with physiotherapy for the scapular stabilisers and avoidance of heavy overhead work; muscle transfer or scapulothoracic fusion is considered only after 1 to 2 years of failed conservative treatment.
A winged scapula unbalances the whole shoulder girdle: the patient hitches the shoulder to compensate, which brings pain, and a shoulder that is not moved daily stiffens into adhesive capsulitis long before the nerve has finished recovering.

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