Other investigation techniques

Published on 11/03/2015 by admin

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Last modified 22/04/2025

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3 Other investigation techniques

OTHER INVESTIGATIONS

More often than not the diagnosis can be established from the clinical assessment and imaging studies without the aid of other special investigations. In any case the possibilities should be narrowed down to as few as possible before such investigations are ordered. If doubt then exists, appropriate tests are ordered to support or weaken each possible diagnosis.

Tests commonly employed in orthopaedic diagnosis include haematological studies; biochemical tests upon urine, faeces, plasma, and cerebrospinal fluid; serological and bacteriological tests; electrical tests; arthroscopy; and histological examination of specimens excised at biopsy or at definitive operation.

BLOOD TESTS

Haematology

Full blood count. Assessment of the haemoglobin is of great value after trauma and post-operatively, as it gives an indication as to how much blood has been lost. The white cell count is usually, but not always, raised when an infection is present. The ESR (erythrocyte sedimentation rate) and CRP (C-reactive protein) are more sensitive tests for revealing systemic inflammation. The CRP is particularly useful, as it rises rapidly when there is an infective process. The ESR and CRP are also helpful in distinguishing septic and aseptic loosening of total joint replacements. If these tests are normal on two occasions, it is unlikely that the loosening is as a result of an infection.

Biochemistry

If there are concerns about organ dysfunction, liver function tests and renal function tests are of value and part of the pre-operative work-up. Similarly, routine electrolytes can also be of value, particularly if the patient is on drugs such as diuretics.

The biochemical tests that are of particular value for assessing bone function are the plasma levels of calcium, phosphate and alkaline phosphatase. Derangements in the calcium and phosphate balance can confirm the diagnosis of rickets and osteomalacia.

MICROBIOLOGICAL TESTS

The diagnosis of musculoskeletal infection is often challenging for a number of reasons.

1 Bacteria such as the normal skin flora are grown as contaminants from specimens such as sputum. However, in orthopaedics these same organisms can be pathogens and the cause of musculoskeletal infections. To distinguish between a growth of, for example, Staphylococcus epidermidis, which is a contaminant as opposed to one which is a pathogen, it is vital to obtain multiple samples. At operation of a patient with suspected musculoskeletal infection, at least five samples should be obtained under sterile conditions. If two or more of these multiple specimens grow the same bacteria, it is likely that this is a true pathogen rather than a contaminant.
2 The swabs from sinus tracts are often misleading and do not accurately reflect the true deep organisms that are causing the infection. However, aspiration of fluid from the depth of a sinus is worthwhile and often will give a more accurate picture of the true range of causative organisms.
3 In 10–20% of cases with established musculoskeletal infection, despite optimum culture techniques, it may not be possible to grow an organism. Thus, it is vital to obtain material for histological analysis as well as for microbiological culture where infection is suspected (see below).

ELECTRICAL TESTS (ELECTRODIAGNOSIS)

The two most common electrodiagnostic techniques used in orthopaedic practice are nerve conduction studies and electromyography.

Nerve conduction studies are used to determine whether or not a nerve is able to transmit an electrical impulse. The principle is to apply a stimulating electrode over a point on the nerve trunk distal to the lesion, and to observe whether or not the muscles supplied by the nerve will contract in response to the stimulus. The nerves in the sound limb are examined first, to determine the threshold of current required to cause a muscle contraction. If in the affected limb a current at least twice as great as the threshold fails to produce a muscle contraction, nerve conduction is absent. A nerve conduction test provides a simple method of determining whether or not a clinical paralysis is due to a complete lesion of the nerve, which has resulted in degeneration of its myelin sheath. If nerve conduction is present the lesion cannot be complete and myelin degeneration has not occurred.

Nerve conduction tests may also be used to measure motor conduction velocity in the peripheral nerve, and the principle can also be applied to afferent sensory testing. A slowing in the velocity of conduction may indicate the site of an incomplete lesion in the nerve trunk, such as may occur in compression neuropathy. Using stimulating electrodes, applied both proximal and distal to the suspected lesion, the latent period before the appearance of the muscle action potentials is measured. The difference in conduction time and the distance between the electrodes provides a measurement of velocity, which can be compared with the normal side, or with normal values (40–70 m/s).

The measurement of sensory nerve conduction is technically more difficult, but has a useful clinical application in spinal cord monitoring. Surface electrodes are used to provide repetitive peripheral stimulations during spinal surgery, so that recordings of central cortical responses can be used to detect any interference with spinal cord function.

Electromyography. In this technique the electrical changes occurring in a muscle are picked up by a needle or surface electrode, suitably amplified and studied in the form of sound through a loudspeaker, or as a tracing on an oscillograph. Normal muscle is electrically ‘silent’ at rest, but on voluntary contraction shows increasing electrical discharges in the form of triphasic action potentials, as more motor units are recruited into activity. Partly denervated or totally denervated muscle shows only spontaneous contractions of individual fibres (fibrillation potentials). Repeated testing at intervals can be used to detect evidence of re-innervation, as small polyphasic motor unit action potentials reappear and spontaneous fibrillation disappears. The motor unit action potentials gradually increase in duration and amplitude, but do not return to a normal pattern until remyelination of the nerve is complete. Electromyography may show diagnostic changes in some types of myopathy, as well as in anterior horn cell disease such as poliomyelitis and motor neurone disease.

GAIT ANALYSIS

Gait analysis is the study of locomotion. In its most rudimentary form, it can be the observation of a patient during walking. Simple measures of walking speed and step length can be obtained with a stopwatch and a walkway of known length covered in powder. However, modern gait laboratories use multiple video cameras, which record simultaneously, and floor load transducers (force plates), which measure the ground reaction force in magnitude and direction. In addition, passive reflective marker balls are placed along the length of the limbs to aid in imaging the movement, and surface electrodes are placed over major muscle groups such as the quadriceps, so that electromyograms can be recorded simultaneously. Gait analysis can be used in sports medicine, Parkinson’s disease and other neuromuscular diseases. In orthopaedics, it has been used to help plan surgery in children with cerebral palsy.

ARTHROGRAMS

Arthrograms consist of injecting contrast medium into a joint and then obtaining serial plain X-rays, CT/MR images. Arthrograms are commonly used with plain X-rays, for assessing children’s dysplastic hip joints, and in adults for determining whether the components of an artificial joint replacement are loose. In this latter situation, the arthrogram is often combined with an aspiration of the joint (under sterile conditions), which allows fluid to be sent for microbiological assessment. CT and MRI arthrograms are frequently used in the assessment of rotator cuff injuries of the shoulder.

ARTHROSCOPY

In recent years the technique of direct inspection of the interior of a joint through a fine telescope introduced through a cannula has become highly developed. Indeed it is relied upon almost routinely in the diagnosis of mechanical derangements within the knee, and also in the study of many non-traumatic affections of doubtful nature. The use of arthroscopy has now been extended to joints other than the knee, and is increasingly used for the diagnosis and treatment of intra-articular disorders of the shoulder, hip, wrist, and ankle.

BIOPSY

Biopsy is the operation of taking a specimen of living tissue for histological, electron-microscopic or other examination in order to elucidate the nature of a disease. Very often it is done as a final step in the diagnosis and staging of a tumour. Two techniques of biopsy are available:

1 needle biopsy, in which a core of tissue is extracted by a special hollow needle
2 open biopsy.

Except in certain situations open biopsy is usually to be preferred despite a theoretically greater risk of tumour dissemination, because it is more likely to yield a representative specimen. Nevertheless needle biopsy has an important place, and with improvements in technique its application has been widened, though diagnostic accuracy may not be as good as with open biopsy, except in very specialised centres.

The main essentials in a biopsy operation are:

1 that an adequate and representative piece of tissue be obtained
2 that the incision be so placed that it does not prejudice the success of a subsequent operation for total eradication of the tumour – especially in malignant disease.

The scar must be so placed that it is conveniently included in the block of tissue to be excised at the time of definitive surgical excision.

INTERPRETATION OF THE FINDINGS

When the study of the patient is complete the abnormal findings elicited from the history, clinical examination, diagnostic imaging, and appropriate special investigations should be assembled together to form a composite clinical picture. This can then be matched against the recognised disorders of the region under consideration. It is comforting to remember that the number of disorders that commonly affect a particular region is limited. Often the number is not large. Theoretically, therefore, if all the possibilities are listed and there-after confirmed or eliminated one by one the correct diagnosis must always be revealed.

This is, of course, an over-simplification. In practice diagnosis is not as simple as that. But it is nevertheless true that if the problem is tackled logically, step by step, in the manner described, a correct conclusion can be formed in the great majority of cases. The only essentials are a capacity for painstaking enquiry, with strict attention to detail, accurate observation, and a working knowledge of the salient features of the common disorders.

PSYCHOGENIC OR STRESS DISORDERS

This heading is included to issue a word of warning. When the cause of a patient’s symptoms remains obscure despite a thorough investigation there is a prevalent tendency to discount the genuineness of the symptoms and to ascribe them to ‘functional’ or ‘psychogenic’ factors, or simply to stress. This must be deplored as a dangerous policy that has led on many occasions to the overlooking of a serious organic disease.

Just because we fail to discover the cause of a particular symptom it by no means follows that the symptom is imaginary or psychogenic; it usually means only that we are not sufficiently skilled in diagnosis. Admittedly, true hysterical disorders are encountered from time to time in orthopaedic practice, but they are few and far between. Much more often a long-continued organic pain leads to a distracted state of mind that is wrongly interpreted as a hysterical manifestation. It is widely accepted that physical symptoms may be prolonged or may seem to be worse if there is an associated psychological disorder. Such aggravation is often termed ‘psychological overlay’ or ‘illness behaviour’, especially in legal practice. It is far safer to err on the side of disregarding possible psychogenic factors than to overlook an organic lesion on the supposition that the symptoms are imaginary.

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