Diagnostic ultrasound in the musculoskeletal system

Diagnostic Ultrasound in The Musculoskeletal System

David J. Wilson

Introduction

Diagnostic ultrasound has become an established investigation titr disorders of the heart. abdominal

organs and foetus. The technique is best suited to the study of soft tissues that do not contain gas. It is

therefore a little surprising that there has not been

greater .Ipplication to the musculoskeletal system.

Over the last few years there has been a rapid improvement in image quality combined with reduc- tion in size of the apparatus. The latest machines are easily moved and may be used wherever there is a I? amp power point. Costs have remained low compared to other forms of imaging : an ultrasound room would

be around a quarter of the price of a plain X-Ray. room. The cheapest machines are equivalent to the

price of a larse car although some orthopaedic

applications require higher quality and this figure might be trebled

This paper describes the current state of musculos-

keletal applications of diagnostic ultrasound and leads to the conclusion that there is a strong case for the provision of a clinical service in those centres that deal with these disorders.

Physical Basis

Primydv\

High frequency sound waves are transmitted through soft tissues and reflected by boundaries where there is an abrupt change in density or texture. Fluid provides a medium that transmits sound with little reflection

David J. Wilson MBBS, B!Sc. MRCP, FRCR Department of Radiology. Nuffield Orthopedic Centre, Windmill Road. Heading- ton. Oxford OX? 71 D

and relativ elv little attenuation. Tissue, air and tissue,

bone interfaces reflect such a large proportion of the sound that transmission is effectively blocked.

t’ltrasound pulses applied to soft tissue w-ill generate

echoes that indicate the type of interface by intensity and its depth by the time taken for the echo to occur.

As the direction of the pulse is known an image may

be generated to display the location and character of

the interfaces. A$ the sound IS transmitted there IS a progressive

attenuation of the pulse. The echoes reflected from

identical interfaces will appear weaker at greater depth. To compensate for this the returned signal may

be ampiified progressively depending on the time after

the pulse was produced. I~ltrasound equipment produces the qound by

applying an electrical pulse to a piezoelectric crystal mounted within a hand-held probe. The echoes are

detected by the same crystal during the pause between pulses. The received sound is converted back to an

electrical impulse and this may’ bc digitised for computed .malysis and display. The higher the

frequency the better the re\olution of the system; however the sound is more rapidly attenuated and

useful penetration reduced. In practice this means

high frequency (7 IO MHz) for fine detail in superficial

tissues and lower frequency (7 5 MHz1 fr>r deeper structures.

A stationary probe gives information about a thin

column of tissue below the contact area. If displayed on an oscilloscope this is termed an ‘A’ scan and is used mainly for adjusting the controls 111’ a static scanner. Rapid repetition of the pulses allows the beam of ultrasound to be swept across the patient to

create a cross-sectional image. A hand held sweep paints a static image on the display and is termed a ‘B’ scan This has the advantage of covering either small

42 DIAGNOSTIC ULTRASOUND IN THE MUSCULOSKELETAL SYSTEM

or large areas of tissue. For example a whole limb may

be displayed on one image. If the sweep is mechanised either by oscillating the crystal or by electronically

linking an array of small crystals, a cross-sectional image may be rapidly refreshed giving a ‘real time’ or

apparently moving image. The area covered is limited by the size of the probe and the shape of the displayed

image depends on the type of swept beam. Mechanical

and electronic sector scanners give a ‘pie’ shaped

image whilst electronic-phased array gives a rectan-

gular picture. There are no known side effects or risks involved in the use of ultrasound at diagnostic

frequencies and power.

Application

The main advantage of a real time machine is that by

moving the probe around the operator builds up a three-dimensional concept of the structures below.

This is such an improvement on the single or static

image that the older ‘B’ scans are now rarely used in

general ultrasound departments. Unfortunately the

advantage is only apparent to the operator; even an

experienced ultrasonographer finds the moving image produced by another hand difficult to interpret. Taking hold of the probe adds another dimension and is essential to accurate diagnosis. This means that the

operator must also make the diagnosis. Unlike X-Ray

examinations there is little to be gained by study of

someone else’s ‘films’. Here lies the main disadvantage of diagnostic ultrasound; it is totally dependent on a

skilled operator and is therefore expensive in terms of a radiologist’s time.

A static or ‘B’ scanner is relatively clumsy and difficult to use. The controls are more complex and the

apparatus rather bulky. Single sections do not give the same ‘look around’ ability as a real time machine. Despite these draw-backs the facility to examine the

full length of a limb and the chance to produce serial

sections at fixed intervals mean that there is still a significant role for the static scanner in musculoskeletal

applications. Those wishing to start a service in an orthopaedic

or rheumatological centre would be best advised to

purchase a modern real time machine with at least two probes. One at a high frequency (7.5 or 10 MHz) for small parts work and one at an intermediate frequency (3.5 or 5 MHz) for deeper structures. Mechanical sector probes would be preferred due to their small contact area with the skin, especially

important in rounded structures like the limbs. Electronic sector scanners are not yet able to provide the high frequency option but would be a viable alternative for lower frequencies. A ‘B’ scanner would be a useful addition and might well be available from a general ultrasound department where they have fallen into disuse.

Techniques

Preparation

Ultrasound equipment may look intimidating at first sight and a word of warning before sending the patient

for examination often allays undue anxiety. Fragile or very sick patients may be examined on

the ward as most of the equipment is transportable.

However this should not be a routine as the constant movement would increase the risk of damaging the

apparatus.

Ultrasound cannot penetrate plaster or wound dressings and these should be removed. Sector scan-

ners only require a small port of contact and a window cut in a plaster is often sufficient to examine an

adequate area of a limb. Thin adhesive plastic dressings are not a barrier to sound and provided that

there are no air bubbles they may be left in place. Indeed their use is recommended if post-operative

ultrasound examination is anticipated.

Traction and splintage equipment may create problems when trying to examine by ‘B’ scanning but

rarely causes any difficulty for real time studies. The

patient should be left on their normal bed for

convenience and comfort.

Methods qf’E.uamination

The type of examination must be tailored to the

individual clinical problem with due account of the limitations of the technique. Comparison with other

imaging modalities is essential. There is a strong case for the routine use of computed tomography with

ultrasound for soft tissue masses and extra-osseous extension of bone tumours.

Methods are largely the same as those used in

general ultrasound. The following additional points

apply to examination of the musculoskeletal system. Throughout all examinations the opposite side

should be examined for comparison. Many potential pitfalls and artefacts mimicking disease may be

avoided by this simple practice. Bone reflects the majority of transmitted sound and

may be seen as a sudden loss of echoes bordered by a bright line. This might be interpreted as a fluid-filled area by the unwary especially as reverberation artefact

often gives ghost images below the bony contour.

Normally a fluid collection would show acoustic enhancement where the beam has traversed an area of fluid with relatively little attenuation compared to adjacent soft tissue. The virtually total block to sound at bone surfaces means that adjacent fluid collections may fail to show such enhancement. This is particularly important in the examination of joints for effusions. Comparison of a suspect area with a known area of fluid at the same depth without changing the gain settings, often resolves the difficult case. Conve- nient fluid areas include adjacent blood vessels or the urinary bladder.

Solid tumouls with a homogeneous texture may

mimic fluid b> giving an area of low echoes and

posterior acoustic enhancement. This phenomenon is particularly striking in tumours of neural or lymphatic tissue’ [Fig. 11. Again comparison with known fluid

areas may resolve the problem. Fat is relatively echogenic compared to muscle but it should be noted that many liposarcomas contain surprisingly little true

fdt and this affects their ultrasound appearance’.

Muscle groups and tendons may be differentiated by

selective patient movements whilst observing the motion on a real time image.

Most ultrasound equipment is capable of measuring

distance and area by cursors displayed on the screen. These may be used to measure size of fluid collections

and masses. Serial sections made with a ‘B’ scanner

have been used to calculate volumes; however this is

time-consuming and simple measurement of the maximum dimension in three planes is usuall>

sufficient ‘. -l.

Hard copies ;lre of value to the individual who

performed the examination but should not be regarded

as the result in the same way as an X-Ray film might

be. The report is the essence of the findings. It is often

helpful to draw diagrams of the abnormalities either in the patients notes or on special report forms

annotated by .my measurements that were made. Close co-operation between clinician and radiologist is essential and discussion considerably enhances the value

Applications

Hurtmtonws. Soft tissue haematomas are most com-

monly observed following injury, although coagulation disorders and anticoagulant treatment are also im-

portant causes. Whatever the aetiology the ultrasound appearances are the same’, 3.

The fresh or early haematoma is echogenic and may

(‘URRE NT- OR7 t-tOt’.~\tDI~ S 43

be difficult to diEerentiate from the surrounding

muscle on the basis of its texture &me. The hacmor-

rhage may be diffuse within the hulk ot. :i muscle pi\ing ;t poorly defined loss of the normal herring-

bone pattern ot‘ echoes. Alternatively the bleed may separate muscle groups displacing structure\ by m.Lss

effect and forming ;t lens-shaped collection Within ;I

t‘w days the blood either totally reaorhs or the haematoma appears to become more disttnct as the

internal echo levels fall [Fig. 31 Arcits appear

containing clear tluid and often mobile cxchogcnic

solids. thebe ;lrc probably fibrin clots filthin the liqulfylng hacmatoma. Should the haemorrhage prog-

res\; to this htage it tends to take ;I consider;ihle time to

res(l1\ c: in those with coagulation di\orcler\ this m;ty

be hevcral months. 111 the cx;lmln:ltion of patient\ with an aLute bleed

it I\ important to note the anatomical Ioc,ation, the XI/C in ternla 01 maximum dimensions for use ;I’, a

hahclinc ;tnJ thtt echo pattern as thl mi,ght help to

dart the lesion Most haematomas resolve on conservative trtat-

meIlt. bout-vcr some are aspirated In an LIttempt to

shorten the period of disitbility ;tnd discomfort.

LUtrasound-guided aspiration is CL>\ to perform and

has the adv;mtages of accurate needle plitccment with

the ,tbility to identify loculated or walled k~ll’ areas ot

the collecticrn. II ;I biopsy attachment is used the tip of the needle IS seen as a bright echo whic*h :no\cs on ;L real time di5pla!.

Haemorrhage into joints ih a littlc ditt‘erent In that the cchogenic phase appears to be either very short or

docb. not occur at all. Most joint bleed5 will appear as

;I clear ef‘usi,>n. floating fibrin clot\ are \ometimes visli>le. Thi< 1s unfixtunate in that it kconles difficult

to L:itTerentlatc ;i bleed from rcactibe zll‘usion :<nd inlzition ‘1 he cause ot‘ a joint etrusion m‘fy only be

detcrmlned \\,ith certainty by needle aspir;!tion. Ir IS

often useful IO introduce .I contr,\st ;igent after

aspiration or drv tap. This contirms that the needle

~a> in the joint and allows radiological e\,iminatlon

of the artlcuiar surface. Whilst contrast may be

injected after ultrasound guided aspiration II 15 easier using fluoroscopy.

Fig. 1-A benign neurofibroma WI the posterior thigh of a woman aged 54. Note the acoustic enhancement and the low level echoes within the lesion.

Fig. Z-An echoe free (established) bleed in the gluteal region of a haemophlliac

44 DIAGNOSTIC ULTRASOUND IN THE MUSCULOSKELETAL SYSTEM -

Znjkction. In early infection of soft tissue there is little

to see on ultrasound examination; this includes clinically obvious cellulitis. Occasionally it is possible

to identify oedema by enlargement of affected areas

when compared to the opposite side. Within a few

days, as the infection becomes established, fluid collections appear and these are readily visible.

Occult abscesses are amenable to diagnosis by

ultrasound; large pus or fluid components often make

them easy to identify [Fig. 31. The major problem is that of access. There is an area in the posterior pelvis

that is surrounded by bowel and bone creating a blind area to examination. Fortunately this rarely prevents

the diagnosis of psoas collection as there are sufficient portals through which the upper and lower parts of

this muscle may be studied. If in doubt computed tomography is usually conclusive although this method

cannot differentiate between fluid and poorly vascu- larised solid [Fig. 41.

Joint infection normally causes an effusion that may be accurately identified. It is unwise to try and

differentiate between effusion infection or haema-

toma on ultrasound appearances alone.

Ultrasound guidance is an excellent method for draining most soft tissue infections. The ease of

accurate needle placement and the identification of

loculated areas are the main reasons for selecting this technique. In addition most drainage procedures may

be performed under local anaesthetic, sedation being

required in only a few cases. Psoas collections may be drained successfully by this method and it is rare to

find an abscess that is too deep or inaccessible.

Depending on the anatomy it is sometimes necessary

to perform computed tomography to exclude the

presence of bowel in the planned catheter track. There are a variety of commercial drainage kits available

and with care it should be possible to connect to a sealed collecting bag with minimal contamination of

the room or theatre. Progress should be monitored by

ultrasound or computed tomography-‘. Turnours. Soft tissue tumours and extra-osseous exten-

sion of bone tumours may be visualised and the

anatomy defined by ultrasound ‘. 2. h [Fig 51. The

character of the echoes will help to differentiate cysts

and ganglia from solid tumours and occasionally the high echo levels in fat suggest its presence. Further

than this rather crude tissue characterisation it is not possible to distinguish the histological type of neo-

plasm. Attempts to find echo patterns that tell more about histology have been largely fruitless despite the

application of computed analysis to the digitised image.’

The adjacent vessels may be defined and involve- ment assessed. The definition of the margin of a lesion

gives a rough idea of the degree of local invasion

although computed tomography and magnetic reso-

nance imaging are more precise. In most cases a biopsy is essential to establish the diagnosis. The main

role of ultrasound is in guiding the surgeon to the best site to biopsy and in planning any resection.

There are occasions when neoplasms are not seen on ultrasound but are clear on computed tomography

or magnetic resonance imaging. Equally those two methods may fail to show a lesion well seen by

ultrasound. A combination of methods is strongly

recommended [Fig. 61.

Fig. 3-(A) La teral aspect of the proximal humerus in a man who had suffered several episodes of discharging osteomys

rwing a hay ‘fever injection. There was pain but no discharge. Ultrasound showed fluid (F) adjacent to bone (B). PUS

rated and c ontrast introduced with ultrasound control. (B) Contrast fills the cavity and enters the bone.

rlitis was

Fig. 4- -(A) Flexor spasm of the hip and fever in an alcoholic. Ultrasound showed fluld in the psoas muscle (f) (B) Pus was drained

using a catheter Inserted with computed tomographv control

‘Thcrc IS work to suggest that ultrasound ia more grandatjon tissue. Ultrasound is the best technique

accuratr than computed tomography in the diagnosis for sxcluclll~~ ii lymphocoele. not an uncommon

of recurrent neoplasm f4lowing resection.” T‘hih complication at the resection site [Fig 71.

diagnosis i\ difficult even in the most esperienced lrrjlrr 1’. Muscle snd tendon rupture are ~tsualiy apparent

hands. There arc* ;I number of potential traps including clinlally. Ultrasound has a role in the equivocal case

the masking of arcas by the highly reflective scar tissue or the partlJ rupture. Careful comparison oi’ sides

and the similarity between recurrent mass and may reveal an echogenic area in the tendon or muscle

Fig. 5--(A & B) A homogeneous mass on the flexor aspect of the diglt has low level ethos within its substance (arrow’). BIOPSY showed juxtacortical chondroma.


Fig. 6-(A) A 17 year-old complaining of swelling and pain in the foot. Note the porosis, gracile

metatarsals and calcification (arrow). (B) The mass consisted of fluid filled cavities (arrow) found to contain altered blood on aspiration. There was some solid tissue in the deep plantar space. (C)

Computed tomography helped to plan the site of biopsy. The histology demonstrated

neurofibrosarcoma. The foot was amputated.

Fig. 7-(A) Three weeks after resection of a low grade liposarcoma a new mass developed. CTshows a low attenuation area in the popliteal fossa (arrow). (B) Ultrasound showed a resolving haematoma (H). (C) Ultrasound guided aspiratir

produced altered blood and contrast injected via the same needle confirmed that cavity was as large as the palpable mass and that there was no underlying recurre

on the mce.

C‘URRENT ORTHOP4FI)I(‘S 17

Fig. 8--(A) Plain radlograph of the hip in a man of 55 showed a SUSPICIOUS but subtle line on the femoral neck. (B) Ultrasound of the

hip showed an effusion (arrow), not present on the opposite side. The patle;lt was operated upon and a fracture conflrmed

that represents ;I scar or incomplete tear. Partial

muscle rupture may mimic soft tissue tumours and the obserkat ion of normal muscle bunching up on contrac-

tion is often suficiently reassuring to avold biopsy of

these ‘masses’.’ Ultrasound has been advocated for the diagnosis of

partial tears of the Achilles tendon and tears of the

rotator cuff of the shoulder.” ’ I Our experience of the latter suggests that the diagnosis is far from easy and that, although tears may be identified by ultrasound,

arthrography remains the definitive investigation. Tendon rupture in the hand and forearm is a

particular problem in patients with rheumatoid arthri-

tis. The diagnosis is made clinically but the site of the

rupture determines the surgeons approach to repair.

Ultrasound examination with a high frequency. real

time probe and using movement of the digits to identify the different tendon groups may identify the

ends of the rupture and avoid the need for a tenogram. When plain X-Rays arc difficult to interpret and

there is strong clmical suspicion of a fractured neck of

femur. ultrasound evidence of the absence of blood in the joint is a valuable negative sign [Fig. 81.

f$icsiorr. In joints that are easily palpable, such as the

knee. elbow or imkle, ultrasound has little to add in

the diagnosis ol’ effusion. The major role is in the deeper joints such as the hip or shoulder.’ 2 Effusion may be readil} recognised by an echo-free area distending the capsule. The normal side should be used for comparison as the normal articular cartilage is relatively echo-free and may mimic fluid if only one side is studied. Fairly small collections may be detected with this technique, for example in the hip this ma!

be ;I\ little ;LS 3 ml. Care mu\t he taken to ;ilign the prohc and the limb in ;m Identical I‘ri<hlon hhcn

comparing the opposite side; rotation ot‘~h< bcanning plane or limb creates asymmctr! th;lt rn,:! mimic .~n

eff‘u5ton. A Baker’s cyst may be identified :n the: poplitcal

t’oss:i. Rupture is seen as an irregular ;I nd poorly

outlined extension along the c:\lf [Fig 91.’ -I C‘o77,pv77ru/ Z~i.s/~r~7~io77 c!f’ tl7r Hip. Before ~/IL- fcmot al

head anti ttcctabular cup have ossified it IS n51t easy to

determine the precise alignment ot‘ the hip !>y either

plain radiograph\ or physical ex;amln,ition III, i\ ;t parttcular problem in those Int‘ants lbthosc hips arc

found to click at birth. The unossified cartilage of the femoml head and

acetabulum are seen on ultrasound as echo-free areas

clearly distinguishable from the relatively echogenic

fat and muscle. The ossification centrc in the femoral head is seen as a bright focus long before It appears on

plain films. It is therefore possible to detect subluxa- tion. dislocation and dysplastic acetabulae with some

Fig. 9-A ruptured Baker’scyst. (t) tibia. (f) femur. (r) cyst. (arrow) fiuld ruptured down the calf.


accuracy [Fig lo]. 15. I6 There is considerable experience in this field in Europe where many thousands of infants have been screened. Early British work seems

to confirm this finding although the optimum method

is not obvious. Long term follow up studies are required to determine the overall accuracy before

recommending a nationwide service. Articular Surface. Articular cartilage is a homogeneous

hyper-hydrated substance and produces few echoes on

ultrasound examination. It therefore appears as a

black line applied to the surface of bone. Using high resolution equipment this line may be studied and

measured.5 Normal articular thickness varies with age and the joint. In adolescence the femoral articular

surface measures around 3-4 mm; in middle age this narrows by l-2 mm. By flexion, extension and rotation

it is possible to cover a large proportion of the surface of the distal femur or the proximal humerus. More

tightly confined joints such as the hip, are more limiting. Defects in ostechondritis dissecans may be

observed and the changes of progressive arthropathy

monitored [Fig. 111. Synovium. In diseases that cause synovitis it may be

difficult to determine how much of the palpaple

enlargement is due to the chronic synovitis and how much to effusion. This is particularly important in

haemophilic arthropathy where invasive investigation

is dangerous. Ultrasound examination demonstrates the proportion of echogenic synovium to echo-free

fluid. This may be conveniently measured in the

suprapatellar pouch of the knee and anterior to the

joint in the hip, shoulder or elbow [Fig. 121. Alignment. There are a number of methods to

determine the rotation or torsion of long bones. These

include calculation from plain radiographs and com-

Fig. l&-Ultrasound of an infant hip. (H) femoral head, (L) acetabular labrum, (A) acetabulum.

puted tomography. Ultrasound provides a reasonably accurate alternative.lh It is necessary to scan trans- versely across the end of the bone with the probe

linked to some form of rigid base line. Television monitor techniques and purpose built frames have

been used. However a standard ‘B’ scanner is the

simplest device to use as the scanning head is fixed to a solid frame by the nature of its design.

There are many potential sources of inaccuracy in this type of measurement. In practice the major

problem is that even using a dry bone it is difficult to

mark a reproducible line on the proximal humerus or

the femoral neck. The errors generated by the use of an ultrasound method are much less than those due to

problems of anatomy. In practice it seems doubtful whether the overall accuracy is better than plus or

minus 5”.

Spitze

Canal Diameter. There has been considerable interest

in the use of ultrasound to measure the diameter of the

lumbar spinal canal.” -I” The technique relies on the

window to the canal created by the interlaminar

spaces. Unfortunately this requires that the beam is angled inwards away from the mid-line and the

measurements made cannot be a true AP diameter. Difficulty in identifying what boundaries are being

measured and other technical problems mean that the method is only of value in large population studies of

an epidemiological nature. Congenital Dejkcts. Neural arch defects may be

recognised in the unborn infant by ultrasound exami-

nation. There are a few occasions in which it is of

value to identify an occult meningocoele after birth or

even in the adult.‘O

Intra-Operative usage. Special high resolution probes

have been developed that may be sterilised and used for direct application to the spinal cord during surgery. Their purpose is to accurately locate intra-cord

pathology and thereby limit the extent of surgery. Intrathecal and intramedullary tumours may be

detected and delineated. The decompression of a

syrinx is simplified. During surgery for trauma, an intra-operative probe

may show bone fragments or foreign bodies imbedded

in the cord. determine mal-alignment of the vertebrae below the cord, distinguish between myelomalacia

and intramedullary cysts and exclude cord or thecal

sac compression.”

Vessels

Location. Vessels are echo-free structures. With high resolution real time equipment tiny moving echoes may be identified; these are thought to be rouleaux of red blood cells. Real time apparatus is invaluable to detect arterial pulsation and to trace the vessel’s course. Doppler probes use the change in frequency of reflected sound that results from movement to detect

(‘URRF.NT ORTHOPAE:DI(‘S 49 -__-. .-_ - --__ .~ --. _____.____._~~ _~~

Fig. 1 l-(A) Ostechondritrs dessrcans. (t Ultrasound shows thickened overlying car

(C) Arthrography confirms the ultrasounc

impression of thickened articular cartriage

3) til

1 we

Fig. 12-(A) Soft tissue mass in the thigh was close to muscle attenuation on CT. Possibly fluid or solid. (M) mass. (B) Ultrasound indicated that it was solid. (M) mass. Biopsy showed pigmented villonodular synovitis.

- _ -~-- .__._~ ~ --------


flow. Recent equipment allows the operator to choose an area on an ultrasound image to produce a Doppler

flow graph, which is of particular value in vascular disease. The latest equipment shows a real time

ultrasound image in black and white with superim-

posed red and blue areas that indicate movement. The colour represents the direction of flow.

There may be occasions where the pre-operative study of vascular structures would aid the orthopaedic

surgeon although most would rely on angiography or

contrast-enhanced computed tomography. Vascular tumours and aneurysm are readily identified by

ultrasound with or without Doppler.

Thrombosis. Thromboses of the deep veins may be identified as echogenic areas within the vessel lumen.

This sign does not require the use of the Doppler apparatus. In practice the observation is only of real

use in the femoral vein as the veins of the calf are too

small to be certain that the appearance is not artefact.

Limitations

The major limitations of diagnostic ultrasound are

dependence on a skilled observer and the lack of a

record that may be independently analysed. These criticisms could equally be levelled at clinical exami-

nation.

Great importance should be given to hard diagnostic signs and the temptation to express an impression or

feeling about the diagnosis should be resisted. Corre-

lation with clinical examination and other investiga- tions is particularly valuable and every opportunity of

pathological or surgical follow up should be pursued.

Acknowledgements

The Arthritis and Rheumatism Council and the Haemophilia Society purchased ultrasound equipment used in the development of musculoskeletal ultrasound at the Nuffield Orthopaedic Centre.

References

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Diagnostic ultrasound in the musculoskeletal system