MUSCULOSKELETAL SYSTEM

SkeletonThe spinal column is normally composed of 24 individualvertebrae divided into three regions, cervical, thoracic andlumbar. These articulate caudally with five fused vertebraeforming the sacrum, which in turn articulate with three tofive fused small vertebrae forming the coccyx. All vertebraedevelop from the same three elements, which form withinthe sclerotome at each level (see page 77). These are thecentra (forming most of the vertebral body), the neural archand the costal process. However, the contribution of each of these elements will differ in the formation of the adultvertebra at each level within the spinal column.

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● Musculoskeletal system◗ Skeleton◗ Articular elements◗ Muscles

● Cardiovascular system◗ Arterial supply◗ Venous drainage

● Lymphatic (immune) system

● Nervous system◗ Central nervous system◗ Peripheral nervous system ◗ Structure of the spinal cord◗ Spinal reflexes

● Learning outcomes● Assessment

Spine

2. The small muscles of her left hand, particularly the thenareminence, were wasted compared to the left.

3. The left radial pulse was more difficult to palpate than the stronger right radial pulse.

4. There was no abnormality when examining her neck.

This combination of symptoms suggested compression ofthe lower roots of the brachial plexus and the main arterialsupply to the limb, the subclavian artery. When x-rayed, herneck clearly demonstrated a cervical rib attached to theseventh cervical vertebra (Fig. 3.1). This rib appears to bearticulating with the first thoracic rib.

The clinical findings are explained by the rib pressing on thelowest trunk of the brachial plexus, carrying the C8 and T1roots, namely decreased sensation on the ulnar border of the forearm and wasting of the small muscles of the hand.Cervical ribs occur in less than 1% of the population.The sub-sequent angiogram confirmed that there was compressionof the subclavian artery as it passed over the cervical rib (Fig. 3.2), especially when the arms were elevated.

A vascular surgeon excised the cervical rib.After some monthsthe nerve function in the T1 root gradually recovered. Theabnormality of the pulse recovered almost immediately.

A 60-year-old woman attended her family doctor becauseshe had persistent pain and pins and needles along the ulnarborder of her left forearm. When examined by her doctor,he noticed:

1. She had some decreased sensation on the medial aspectof her forearm, where she had noticed the discomfort.

Vertebrae

Each vertebra has a body anteriorly (Fig. 3.3). Passing pos-teriorly from the body are two pedicles, one each side of thespinal canal. Posteriorly each pedicle is united to the otherthrough a lamina, passing medially from the pedicle to meetthe lamina from the opposite side in the midline. In additionto the lamina passing medially, there is a transverse processpassing laterally from the posterior end of each pedicle.

Where both laminae meet in the midline, they form a pos-terior projection, the spinous process. The space surroundedby this complete ring of bone is known as the vertebralforamen or canal. The bony ring surrounds and protects keyelements of the central nervous system, the spinal cord andits roots, while acting as key points for the attachment of thespinal musculature.

Those components projecting posteriorly from the ver-tebral body, the pedicles, lamina, transverse and spinousprocesses, are collectively known as the neural arch.Vertebrae articulate with each other through severalarticular surfaces. Anteriorly intervertebral discs (secondarycartilagenous joints) join the vertebral body to the nextvertebral body superiorly and inferiorly. Posteriorly there

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Fig. 3.1 AP radiograph of a cervical spine demonstrating a cervicalrib on the left side.

Cervicalrib

Fig. 3.2 Digital subtraction angiograms demonstrating (a) normalarteries with the arms down, (b) compression of the left subclavianartery when the arms were raised.

Left commoncarotid

Leftsubclavian

Pigtailcatheter

Rightinternal

thoracic

Brachiocephalic

Aorticarch

Vertebral arteries

(a)

(b)

(b)

(c)

AA

S

BB

A

S

C

P

L

A

A

S

B B

AC P

L

A

A

S

B B

A

S

C P

L

V

T

T

S

(a)

V

V

Fig. 3.3 Features of a typical vertebra: (a) cervical, (b) thoracic,(c) lumbar (lateral aspect to the left and transverse view to the right).Key: A, articular facets; B, vertebral body; C, vertebral canal; L, lamina;P, pedicle; T, transverse process; V, intervertebral notch.

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are the posterior articular facets (synovial joints), twoprojecting superiorly and another two projecting inferiorly,located at the junction of each lamina and pedicle. When a vertebra is viewed laterally due to the presence of theinferior articular processes, a notch is observed inferior tothe pedicle, the vertebral notch, which is converted into theintervertebral foramen when articulated with the vertebrabelow. The spinal nerves pass from the vertebral canalthrough the intervertebral foramen.

Cervical vertebrae

Each of the seven cervical vertebrae is typified by the presenceof two foramina transversaria, located laterally where thetransverse process joins the pedicle (Figs. 3.3a, 3.5). Thesetwo foramina transmit the vertebral arteries (see below) andare formed developmentally between the neural archelement and the costal element.

The five typical cervical vertebrae each have a small body, a relatively large vertebral canal and a spinous processwith a bifid tip. Posterior articular facets project superiorlyand inferiorly. The spinous process of the seventh cervicalvertebra, known as vertebra prominens, is particularlyprominent and is a very useful surface anatomical landmark

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A 24-year-old man was involved in a road traffic accident. Hecame off his motorbike at high speed. He immediatelycomplained of a painful neck. On examination he had nodefinite neurological abnormality. However, he complainedof an unusual ‘pins and needles’ sensation in both arms andfeet. He had no other injury.

An X-ray was performed (Fig. 3.4) in the casualty department,which demonstrated a fracture through the posterior elementsof the second cervical vertebra. This is called a ‘hangman’sfracture’ and is a highly unstable injury that requires speedyrecognition and treatment. Alignment was satisfactory. Thepatient had an emergency operation to stabilise the pos-terior elements with metal plates and made a full recovery.His unusual sensation resolved before he left the hospital.

Fig. 3.4 Lateral cervical spine radiograph demonstrating ahangman's fracture (→←); note the fracture through the posteriorelements of the C2 vertebra.

Spinousprocessof C1C2

C3

C4

C5

C6

Anteriorarch C1

Den

s

Fig. 3.5 Relationship of the atlas and the axis: (a) posterior view ofvertebral bodies, (b) superior view of atlas sitting superior to axis,(c) AP radiograph through the oral cavity demonstrating the atlas, axis,and clearly demonstrating the dens.

Dens

Pedicle (cut)

Foramentransversarium

Dens

C1 Atlas

C2 Axis

C2 Axis

C1 Atlas

Dens

C2 Axis

C1 Atlas

(a)

(b)

(c)

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from which spinous processes can be counted superiorly andinferiorly.

In the cervical region there are two atypical vertebrae,the first and second. The first cervical vertebra articulatessuperiorly with the occipital condyles on the base of theskull and is easily identified, as it has apparently no vertebralbody (Figs. 3.5, 3.6), just a ‘thin’ ring of bone. An alternativename for this vertebra is the atlas (after the mythical Atlasholding up the world, the skull). Developmentally the cen-trum of C1 fuses with the centrum of the second cervicalvertebra to form the odontoid peg (Fig. 3.5), which sitswithin the ring of bone anteriorly where the vertebral bodyof C1 should be located. Superiorly the laminae have animpression due to the vertebral artery passing across theircranial surface before entering the cranial cavity. The spinousprocess is rather rudimentary and presents more of a swell-ing or tubercle than a process. Movement between the skulland atlas is that of ‘nodding’ and is reflected in the shape ofthe superior articular facets.

The second cervical vertebra, also known as the axis, istypified by the superiorly projecting odontoid process orpeg. This peg provides the axis for the ‘rotational’ movementfacilitated through the shape of the superior articular facetsof C2 and the inferior facets on C1.

Thoracic vertebrae

There are twelve thoracic vertebrae with heart-shapedvertebral bodies. Developmentally the rib element remainsseparate from the vertebra forming the ribs. The second totenth thoracic vertebrae all show the features of the typicalvertebra with very long-drawn-out inferiorly (caudally)projecting spinous processes (Fig. 3.3b). Classically thesespinous processes project inferiorly as far as the lower border

of the next inferior vertebral body (see Fig. 4.13). The pos-terior articular facets are rather flat in the coronal plane, andproject anteriorly and posteriorly. Each thoracic vertebra hasthree additional facets on each side for articulation with theribs. One facet (for the tubercle of the rib) is located on thetransverse process and the other two (hemifacets) are locatedeither side of the pedicle where it arises from the vertebralbody. The head of a rib normally articulates with the lowerfacet and transverse process of its own level and the superiorfacet on the vertebra below.

Exceptions to this typical thoracic vertebra occur at thefirst (T1), the eleventh (T11) and twelfth (T12). At T1 there isjust one articular facet where the pedicle and vertebral bodymeet inferiorly for articulation with the head of the first rib.The eleventh and twelfth thoracic vertebrae start to take on features of the lumbar vertebrae. The vertebral bodybecomes more oval and larger; there is a single articular facetat the base of the pedicle for articulation with the head of the11th or 12th rib as appropriate. There is (a) no articular faceton the transverse process as the ribs at this level are missinga tubercle and ‘float’, and (b) the spinous processes becomeshorter and heavier in construct. The posterior articular facetschange their orientation to be more like those of lumbarvertebrae.

In the early embryo starting in week 3, paired andsegmented condensations of the paraxial mesoderm, knownas somites, develop lying parallel to the developing neuraltube. By the end of the fifth week there will be around 42somite pairs. It is at this stage that each somite pair will gaina pair of nerves (one on each side) passing from the develop-ing central nervous system. No matter where the structuresformed by a particular somite migrate to, they will retain this connection to the central nervous system. The cellularelements within a somite then give rise to three subdivisions

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Fig. 3.6 Radiological images of a normal cervical spine: (a) lateral and (b) anteroposterior. Note the shadow of the trachea in the midline.

Dens

Mandible

Epiglottis

Hyoid

Upper aspectof oesophagus

Trachea

Clavicle(a) (b)

Trachea

C3

C4

C5

C6

C7C4

C3

C5

C6

C7

T1

T1

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referred to as the dermatome forming skin structures, themyotome forming muscles and the sclerotome forming thebone and cartilage element. Sclerotome cells migrate initiallyto surround the developing neural tube and will form thevertebral column and associated ribs. Each vertebra as seenin the adult is formed by the fusion of the caudal half of onepair of somites with the cranial half of the subjacent pair ofsomites.

In the fetus, these fused components contribute to theformation of the neural arch and vertebral body, with ossifi-cation centres appearing within each component. Failure offusion of adjacent somites can lead to anomalies such ashemivertebrae, congenital scoliosis and other abnormalities.A common presentation of such anomalies is spina bifida,where the posterior elements of the neural arch fail to formleaving the vertebral canal contents potentially exposed (Fig. 3.7).

Lumbar vertebrae

Normally there are five lumbar vertebrae, each demon-strating all the features of a typical vertebra (Fig. 3.3c). Eachhas a large oval-shaped vertebral body, which increases insize inferiorly, and thin, flat, laterally projecting transverseprocesses. The spinous processes are very sturdy, squat andsquare in shape for muscle attachment (see below). Seenmost clearly on lateral X-rays of the vertebral column (Fig. 3.8), superiorly the posterior articular facets projectanterolaterally and inferiorly they project posteromedially.Also at this point, but passing laterally, are the thin trans-

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Fig. 3.7 AP radiograph of the lumbosacral region. Note the absence ofthe posterior elements of the sacrum and the incomplete posteriorelements of L4/L5 – spina bifida occulta.

Failure of fusionof L4 neural arch

12th rib

Stomach

Missing posteriorelements of

sacral vertebra

Left sacroiliacjoint

T12

L1

L2

L3

A mother, aged 45 years, who had been unable to conceivewas delighted to be pregnant. The pregnancy proceededinitially uneventfully. During her pregnancy her doctoroffered her the possibility of having amniocentesis (with-drawal of fluid from the amniotic sac via percutaneouspuncture under ultrasound guidance) to test the amnioticfluid.The doctor explained that this could detect spina bifida,which is one of the risks of pregnancy in an ‘elderly’ mother.The mother said that, irrespective of whether the baby hadspina bifida or not, she wanted to have the baby and wouldnot entertain a termination under any circumstance. Theprocedure was not performed.

At the time of birth the baby was noted to have another type of abnormality in its lower spine where a large cysticstructure was seen overlying the lumbar vertebrae. The babyunderwent surgery with a neonatal surgeon to obtain coverover the spinal nerve roots. Unfortunately the nerve rootssupplying the lower limbs did not develop adequately andthe patient suffered paralysis of the lower limbs. The patientwas also incontinent of urine and required a permanentcatheter initially and later diversion of the ureters on to theabdominal wall (urostomy).

Fig. 3.8 Radiographs of a normal lumbar spine: (a) lateral,(b) anteroposterior.

(a) (b)

Sacrum

L5 L5

L4L4

L3L3

L2

L2

L1

L1

Sacrum

Sacroiliac joints

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verse processes, which are clearly demonstrated on thenormal AP abdominal X-ray. Developmentally the trans-verse process is formed by the rib element. It is of note thatthe lumbar posterior articular facets normally projectmedially and laterally (compared to those of the thoracicspine, which project in an anterior–posterior plane).

Sacrum

Normally five vertebrae fuse to form the wedge-shaped ‘flat’sacrum with calcification of the intervertebral joints. Broadestsuperiorly, the sacrum articulates with the fifth lumbarvertebra (see Figs. 3.8, 7.1), whilst inferiorly (caudally) itnarrows to articulate with the coccyx. The first sacralvertebra may not fuse with the second, in which case there issaid to be lumbarisation of S1 and the patient appears tohave six lumbar vertebrae. The converse can also occur,known as sacralisation of L5, where the individual appearsto have four lumbar vertebrae.

The anterior sacral surface, when viewed from the side, isin the form of a concave curvature, especially in the male. Inthe female it is considerably flatter, with the curvaturelimited to its inferior tip. Lateral to the vertebral bodies is the ala of the sacrum. Prominent on both the anterior and on the convex posterior surface of the ala are two verticalcolumns of foramina, the sacral foramina. Passing throughthese foramina are the right and left primary rami of thesacral spinal nerves. The smaller posterior primary ramipass through the posterior foramina and the larger anteriorrami through the anterior foramina. Posteriorly and inferiorlythe lamina of the sacral vertebra do not always fuse, leavingthe vertebral canal open posteriorly. This is known as thesacral hiatus. In some individuals this can be quite extensiveover two or more of the lower sacral vertebrae. The moreextensive the hiatus, the greater is the risk that the meningesare exposed posteriorly. At its worse the lack of fusionextends proximally to involve the lumbar vertebrae. Such anexposure of the underlying meninges can result in neuro-logical damage to the pelvic structures and lower limb and isknown as spina bifida (see case above) (see Fig. 3.7).

Coccyx

The coccyx, the most inferior (caudal) point of the vertebralcolumn, is triangular in shape and composed of a variablenumber of fused vertebrae. It articulates with the caudalaspect of the sacrum and projects anteriorly towards the analcanal.

Articular elementsViewed as a whole from the anterior aspect the vertebralcolumn appears to be straight. However, when viewed fromthe side it is obvious that the vertebral column is normallycurved. In the newborn child it has a concave curvatureanteriorly, a kyphosis, as in the fetal position (Fig. 3.9). Whenthe infant learns to lift its head up and look around, the neckeffectively extends and anteriorly the cervical spine takes ona convex curvature, a lordosis. Likewise, as the infant learnsto sit up a similar process occurs in the lumbar spine, whichlike the cervical region becomes convex anteriorly with thedevelopment of the lumbar lordosis. Throughout life thethoracic region maintains a slight kyphosis (see Fig. 4.2).These curvatures are important in maintaining good postureand balance. Muscle action is central to maintaining thenormal lumbar and cervical curvatures. An abnormality thatseems to preferentially afflict the young female is the develop-ment of a lateral curvature, particularly of the thoracic com-ponent of the spine. This is known as a scoliosis (Fig. 3.10).It is interesting to note that in the newborn child the spinousprocesses, especially those in the lumbar region, are veryshort compared with the adult. They continue to elongateposteriorly throughout childhood.

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A 72-year-old man presented with a 3-week history of backpain.The pain was constant in nature and unrelated to move-ment. His temperature and white cell count were normal. Hehad a past medical history of renal cell carcinoma, for whichhe underwent a right nephrectomy 2 years earlier. Clinical

examination was unremarkable, as were plain films; theseshowed only degenerative changes, consistent with his age.The orthopaedic surgeon was concerned given the natureand severity of the back pain and the history of renal cellcarcinoma and requested an MRI of the spine.

The MR images were very abnormal. Several vertebral bodieshad an altered signal within them, indicating the replacementof the normal vertebral fatty marrow with tumour material.The changes were consistent with metastatic disease. Therewas no evidence of the spinal cord being compromised, or ofa vertebral fracture. The patient was referred to the oncologyservice for palliative treatment.

Fig. 3.9 The development of spinal curvature: (a) fetal spine, (b) firstmonth, (c) child/adult.

Dorsal Ventral

Cervical

Sacral

Thoracic

Lumbar

Cervical

Sacral

Thoracic

Lumbar(a)

(b)

(c)

C7/T1

T12/L1

L5/S1

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