Value of Magnetic Resonance Imaging for the lumbar spine and the knee

     Value of Magnetic Resonance Imaging for the lumbar spine and the knee

                                           Dr KS Dhillon

What is magnetic resonance imaging (MRI)?

MRI is a non-invasive imaging technology which produces three dimensional detailed anatomical images of the soft tissues as well as the bone. It is used to detect disease for diagnosis, and for monitoring of treatment. Sophisticated technology is used to obtain these images.
 
Our body tissues are made of water which contains protons. MRI uses powerful magnets which produce a strong magnetic field. This magnetic field forces the protons to align with this field.
A low-energy radiofrequency current is then pulsed through the patient which stimulates the protons to spin out of equilibrium, straining against the pull of the magnetic field. When the radiofrequency field is turned off, the protons realign with the magnetic field, the energy released is picked up the MRI sensors. The time taken by the protons to realign, the energy released and changes in the environment and the chemical nature of the molecules are recorded by the computer and images are generated. Patients are advised to remain very still while they are in the MRI machine, so that the images do not become blurred.

Intravenous contrast containing Gadolinium is sometimes given to the patient before or during the MRI to increase the speed of realignment of protons with the magnetic field. The faster realignment results in brighter MRI images.

Unlike X-rays and CT scans there is ionizing radiation with MRI.

Uses of MRI

MRI scans are useful for imaging soft tissues. Imaging of bone is best done with X rays and CT scans. MRI scans are useful for detecting infections and tumours in bones. MRI scans are usually used for scanning the brain, spinal cord and nerves, joints, muscles, ligaments, and tendons.

Risks associated with MRI scanning

MRI machines do not produce ionizing radiation but they create a very powerful magnetic field which extends beyond the machine. This magnetic field can exert very powerful forces iron, steel, and other magnetizable objects. Prior to an MRI scan the patient should notify the doctor if he/she has any form of metallic implant in or on the body.

The following should be taken into consideration when an MRI scan is to be done:

• Implants---People with the following implants, especially those containing iron, should not enter the MRI machine: prosthetic implants, plates and screws, staples, artificial heart valves, implanted cardioverter, insulin pumps, cochlear implants, deep brain stimulators, vagus nerve stimulators, pacemakers, loop recorders, and capsules from capsule endoscopy. 
• Claustrophobia—Narrow tunnel machine can produce claustrophobia in some people especially when the scan times are long. Such patients may need sedation or and anesthesia for the procedure.
• Pregnancy—MRI scanning is not advisable in the first trimester of pregnancy when the fetus’s organs are forming and the use of contrast is strictly prohibited in this stage of pregnancy.
• Contrast agents—Nephrogenic systemic fibrosis can develop in patients with severe renal failure on dialysis, if contrast is used. 
• Noise—In some MRI machines the sound intensity can reach up to 120 decibels and hence special ear protection may be needed.
• Nerve Stimulation—A rapid switch of fields in the MRI can produce twitching sensation which can be very uncomfortable.

Accuracy and reliability of MRI scans

In orthopaedic surgery MRI scans are frequently done for evaluation of the spine and joints of the upper and lower limb. An MRI is a very sensitive imaging tool but unfortunately it is not very specific. Abnormalities are common on  MRI scans but often many of the abnormalities are of no consequence. It is very rare for an M.R.I. to be reported as ‘normal study’.

Furthermore radiologists who analyze these scans are not error-proof. Varying error rates in MRI reporting have been published.

MRI scans in spinal disorders

Lumbar disc nomenclature for interpretation of MRI scans of the spine has been standardised on the recommendations of the combined task forces of the North American Spine Society, the American Society of Spine Radiology and the American Society of Neuroradiology [1].

A normal lumbar disc is composed of a central nucleus pulposus and peripheral annulus fibrosus and is wholly contained within the boundaries of the disc space.

Degenerative changes in the discs are subdivided into the following subcategories, annular fissure, degeneration, and herniation.

Annular fissures are separations of annular fibres from their vertebral attachment or separation of annular fibers. The fissures can be concentric where the separation of annular fibers is parallel to the peripheral contour of the disc or radial where separation is vertical, horizontal or oblique. The use of the term tear should be avoided because a tear denotes injury. Fissures are due to degeneration and not injury.

Degeneration of the disc would include narrowing of the disc space, disc desiccation, diffuse bulging of the annulus beyond the disc space, fibrosis, fissuring, intradiscal gas, mucinous degeneration of the annulus, osteophytes, inflammatory changes, and end plate sclerosis. [1].

Herniation of the disc is defined as displacement of disc material beyond the limits of the intervertebral disc space.

Disc bulges are not disc herniation. In disc bulges the disc tissue extends beyond the edges of the ring apophyses, throughout the circumference of the disc (symmetric bulge), or 25% of the circumference (asymmetric bulge).

Depending on the shape of displaced disc material, the herniated disc is classified as protrusion or extrusion.

In a disc protrusion the disc material protrudes beyond the normal confines of the intervertebral disc, over a segment less than 25% of the circumference of the disc. The width of the base of the protrusion is wider than the largest diameter of the disc material which projects beyond the normal disc margins. The protrusion, however, must not extend above or below the relevant vertebral endplates.

Disc extrusion is present when the distance between the edges of the disc material beyond the disc space is greater than the distance between the edges of the base of the disc material. Extrusion is also present when there is no continuity between the disc material beyond the disc space and that within the disc space, and this also referred to as sequestration of the disc.

Herniation of disc into the vertebral body through the endplate is referred to as intravertebral herniation or Schmorl node.

Lumbar MRI diagnostic error rates

Herzog et al [2] carried out a prospective observational study to compare the interpretive findings reported for one patient scanned at 10 different MRI centers over a period of 3 weeks. A 63-year-old woman with a history of low back pain and right L5 radicular symptoms had an MRI of the lumbar spine at 10 different centre within the span of 3 weeks. Two reference MRI examinations were performed at one of the authors' institutions.

They found marked variability in the reporting of interpretive findings and a high prevalence rate of interpretive errors in radiologists' reports.

They found that the overall agreement on interpretive findings was poor. The  average true-positive rate (sensitivity) was 56.4%±11.7 and the miss rate was high at 43.6%±11.7. Effect of spinal pathology on nerve roots was described in only 5 out of the 10 examination reports [2].

The authors concluded that the centre at which the MRI examination was done and the radiologist who interprets the scans, can have a direct impact on radiological diagnosis, subsequent treatment, and clinical outcome.

Harm from overuse of lumbar MRI scans

Wnuk et al [3] carried out a study to determine the proportion of MRI examinations of the lumbar spine which had a detectable impact on the care of the patient (actional outcome). This study had a retrospective cohort of 5,365 outpatient MRI examinations. Actionable outcomes studied included, MRI findings that led to an intervention such as surgery; a new diagnosis such as cancer, infection, or a fracture; when the scanning was for a follow up of known lumbar pathology. Potential harm was identified when the MRI was done for suspicion of cancer or infection but the examination resulted in no positive diagnosis.

They found that the proportion of actionable MRI scans was only 13%. In  36 suspected cases of cancer or infection, the false positives was 81%. In  59% of suspicious examinations further scans were ordered of which 86%  were false positives [3].

The authors concluded that the proportion of lumbar spine MRI examinations that were useful in patient management was small. The false-positive rates were also high and the proportion of false positives which led to further investigation was high too [3].

Low back pain and MRI findings

Besides infections, inflammation, tumours and fracture/dislocation of the spine, the etiology of low back pain remains unknown despite advances in spine imaging and the advent of MRI scanning. Normal morphological changes in the disc which occurs with aging can be present on MRI scanning without any symptoms.

The precise cause of back pain can only be determined in less than 50% of patients with back pain [4]. There is also no relationship between the severity of the lesion seen in imaging and the back pain [5-7].

Clear morphological changes in the aging disc can be seen on MRI scans of the spine in patients who have no back pain [8-11].  On the other hand it is also not uncommon to see patients with severe back pain whose MRI examination shows minimal disc changes [12].

Anterior segment of the lumbar spine

The intervertebral disc is composed of two components, the nucleus pulposus and annulus fibrosus. Fifty percent of the central volume of the disc is composed of the nucleus pulposus which is composed of a loose network of collagen fibres in a proteoglycan matrix which contains some chondrocytes. The nucleus pulposus has a high intensity on T2-weighted  MRI images. After the age of 30 years, dehydration of the disc with fibrous transformation occurs which presents as low intensity linear image in the center of the disc [13].

The annulus fibrosus surrounds the nucleus pulposus on all sides. It is made up of a network of dense elastic collagen fibers which has a low intensity on MRI images. The most peripheral fibers of the annulus fibrosus are known as Sharpey's fibers and they anchor the annulus to the edges of the vertebral endplates. The vertebral endplates are covered by hyaline cartilage which allows the exchange of several metabolites, such as water, glucose and oxygen, with the disc.

Disc degeneration can result from genetic predisposition, ageing, microtrauma and nutritional factors [14]. Disc degeneration usually does not produce symptoms.

There are 4 MRI signs of disc degeneration which may occur together or separately. This includes loss of intensity with T2-weighting, a reduction in disc height, high intensity on posterior aspect of the disc, disc herniation, and endplate changes [12].

T2-weighted low intensity and decreased disc height 

T2-weighted low intensity and decreased disc height is very common in
 asymptomatic individuals. The prevalence is related to the age and between the ages of 20 to 80 years the prevalence rate is between 36% and 85% [15,16]. The changes are most common at the lower two lumbar regions. Mild disc degenerative changes are present in 26% to 100% of the population studied and moderate to severe changes are seen in 35% to 72% of the population studied [10, 11, 17].

High intensity on posterior aspect of the disc,

Desiccation of the disc with age leads to fissuring. Fissuring allows the nucleus pulposus material to leak into these fissures at the posterior aspect of the disc. This nuclear material shows up as high intensity on T2-weighted images. This fissures also allow the disc material to herniate beyond the disc margin and this occurs most frequently at L4-L5 and L5-S1, occasionally at L2-L3 and rarely at L1-L2 [11,18].

The prevalence of posterior high intensity in asymptomatic individuals varies between 12% and 56% [10,18], and it increases with age [10]. In   Stadnik et al’s study involving 36 asymptomatic volunteers, in those who were less then 30 years of age, 11% had a zone of T2-weighted high intensity and in those who were 61 years and above, 100% of the individuals had high intensity zone [10].

Bulging disc and disc herniation herniation

A bulging disc is said to be present when 50% of the circumference of disc tissue extends beyond the edges of the intervertebral ring apophyses. A focal disc protrusion is present when less than 25% of the disc circumference protrudes and a broad-based herniation is present when between 25 and 50% of the disc circumference protrudes beyond the intervertebral ring apophyses. A disc extrusion exists when the greatest  diameter of the fragment beyond the disc space is greater than the distance between the edges of its base.  A  sequestration is present when the disc fragment has lost attachment with the disc and the fragment is in the spinal canal proximal or distal to the disc space [12].
The incidence of global disc bulging, which is usually associated with a decrease in disc height, is about 15 to 81% in asymptomatic individuals [10,19].

Eighty percent of the disc herniations are posterolateral, 10% median, 10%  foraminal or extraforaminal. Anterior disc herniations are rare [12].

Protrusion disc herniations are common in asymptomatic individuals with a prevalence of between 20% to 63% [8,9,10,17,19]. Extrusion disc herniations are rare in asymptomatic subjects with a prevalence of between 0 and 24% [8,9,10, 16,17]. Sequestration disc herniation are not found in asymptomatic individuals [12].

The prevalence of schmorl's nodes is between 19 to 24% in asymptomatic subjects [9].

Vertebral endplate changes

Modic et al [20] carried out an MRI study in 474 patients with low back pain, and they described endplate signal changes in disc degeneration which were divided into three grades:

• Modic type 1 changes with an edematous appearance, hypointense on T1 images and hyperintense on T2 images, with enhancement following gadolinium injection.
• Modic type 2 changes with fatty changes, hyperintense on T1 images and isointense or slightly increased signal intensity on T2 images. 
• Modic type 3 changes with fibrous/osteosclerotic changes, hypointense on both T1 and T2-weighted images.

The etiology and physiopathology of Modic 1 endplate changes remain unknown. The changes may be due microtrauma and biochemical changes due to subchondral strains in the endplates. Modic 1 changes are also seen in patients with discitis/osteomyelitis with low virulence pathogens [21]. Modic type I changes are seen more frequently in patients with low back pain (46%) compared to asymptomatic general population (6%) [22].

Posterior segment of the lumbar spine

The posterior part of the lumbar spine consists of pedicles on either sides behind the vertebral body from which arise the transverse process on either side, superior and inferior articular processes with facets, laminae and the spinous process. The borders of the spinal canal are formed by the posterior part of the vertebrae anteriorly, the pedicles on the sides and the laminae posteriorly.The inferior vertebral notch above and the superior vertebral notch below form the intervertebral foramen through which the nerve root exits the spinal canal. The inferior articular facet of the vertebra above and the superior articular facet of the vertebra below form the posterior facet joint.

The posterior facet joint is a synovial joint which is covered by a capsule. The capsule is strongly innervated by the medial rami of the posterior branches of the spinal nerves.  These rami also have muscular sensory collaterals. The facet joints are inferiorly, posteriorly and laterally oblique. The joints transmit load and stabilize the spinal segment during flexion, extension and rotational movements [12].

Degeneration of the facet joints occurs with age. The decrease in disc height leads to subluxation of these joints which can lead to degeneration of the facet joints. An MRI of the spine can show loss of articular cartilage, subchondral sclerosis, osteophyte formation and intra-articular effusion.
Facet joint arthritis occurs most often at L4-L5. CT scan abnormalities of the facet joints can be seen in 64% to 67% of asymptomatic individuals. Facet arthritis is very common after the age of 45 to 50 years [23-28].

Interspinous bursitis (Baastrup disease)

Baastrup disease, also known as “kissing spine” syndrome is a relatively common lumbar spinal which is characterized by low back pain arising from the close approximation of adjacent posterior spinous processes and resultant degenerative changes, most often seen at L4-L5. Adjoining spinous processes hypertrophy and friction between the two leads to formation of synovial neoarticulation. The back pain is relieved with spinal flexion and increases with spinal extension. An MRI will show an increased interspinous signal on T2-weighted images and decreased signal on T1-weighted images. Though fairly common this condition is often underdiagnosed and missed.

Kwong et al [28] reviewed the abdominopelvic CT scans of 1008 patients and they found evidence of Baastrup disease in 413 patients (41.0%). In individuals 80 years and above the prevalence was 81%.

Lumbar Spondylolysis

A stress fracture of the isthmus is called spondylolysis and it may be unilateral or bilateral. It is usually caused by repeated microtrauma and rarely it may be caused by acute trauma. When it is bilateral, it may lead to spondylolisthesis. Most often spondylolysis is seen at L5 (85 to 90%) [12].
According to Hollenberg et al [29], based on MRI findings, spondylolysis can be divided into 4 grades:

• Grade 1 where there is isthmic edema with or without edema of the pedicle or adjacent joint. There is no cortical abnormality.
• Grade 2 where there is isthmic edema with incomplete fracture of the isthmus.
• Grade 3 where there is isthmic edema with complete fracture of the isthmus.
• Grade 4 where there is complete fracture of the isthmus without edema.

The prevalence of isthmic fracture with or without spondylolisthesis is about 7% to 8.5% in asymptomatic patients and it increases with sporting activities [30-32].

Degenerative lumbar spondylolisthesis

Degenerative sagittalization of the articular facets leads to the displacement of the superior vertebral body over the inferior vertebral body and this is referred to as spondylolisthesis. This displacement is usually anterior(anterolisthesis) with some degree of rotation [12]. Sometimes the displacement can be posterior (reterolisthesis).The Meyerding classification describes four grades of listhesis [33].

• grade I: 0-25%
• grade II: 26-50% 
• grade III: 51-75% 
• grade IV: 76-100%  
• grade V (spondyloptosis): >100%

Spondylolisthesis mostly occurs at L4 and is more common in females. Individuals who are overweight and have hyperlordosis are more prone to develop spondylolisthesis.

Value of MRI signs

There is a high prevalence of abnormal findings on MRI scanning of the lumbar spine in asymptomatic individuals. This raises the question of the value of MRI scanning in patients with back pain. Only Modic type 1 changes and extensive zygapophyseal edematous changes appear to  have some correlation with low back pain. Large disc herniation and sequestrated disc have correlation with root symptoms but not back pain.

MRI scans of the knee

MRI scanning for detecting ACL and meniscal injuries

Miller [34] carried out a prospective study comparing the accuracy of the clinical diagnosis of meniscus tear with magnetic resonance imaging. It was a prospective, single-blind study of 57 consecutive knees with initial clinical diagnosis of a torn meniscus who underwent arthroscopy. They found that the overall accuracy for the clinical diagnosis of meniscal tear was 80.7% and the accuracy for MRI was lower at 73.7%. Inappropriate treatment would have resulted in 35.1% of the patients if MRI findings were used to determine surgical treatment. Hence clinical diagnosis is more reliable then MRI scanning for detecting meniscal tears. 

Rose et al [35] carried out a study to compare the accuracy of clinical examination and magnetic resonance imaging in the diagnosis of meniscal tears and anterior cruciate ligament (ACL) tears. It was a prospective and retrospective study involving 154 patients who were clinically diagnosed to have a meniscal or ACL tear and underwent arthroscopy. One hundred patients had clinical examination followed by MRI, and 54 had clinical examination alone. In patient who had clinical examination and MRI, the accuracy of MRI was 98% for ACL tears,75% for medial meniscal tears, 69% for lateral meniscal tears. The accuracy of clinical examination for complete ACL tears was 99%, 82% for medial meniscal tears, 76% for lateral meniscal tears. The accuracy of clinical examination for the 54 patients who underwent clinical examination without MRI was not significantly different from the accuracy of clinical examination in the other 100 patients. The MRI contributed to treatment in only 16 of 100 cases. The authors concluded that MRI is useful only in exceptional case and its routine use is not recommended for patients with suspected ACL and meniscal injuries. They described MRI as an expensive and unnecessary diagnostic test in patients with suspected meniscal and ACL tears.

Rayan et al [36] carried out a prospective study to compare and correlate clinical, MRI, and arthroscopic findings in patients with suspected meniscal  and ACL tears. The study involved 131 patients with suspected meniscal or ACL tears. They found that clinical examination had better sensitivity (0.86 vs. 0.76), specificity (0.73 vs. 0.52), predictive values, and diagnostic accuracy when compared to MRI scans in diagnosis for medial meniscal tears. There was only marginal difference in the diagnosis of lateral meniscal and anterior cruciate ligament injuries. The authors concluded that when clinical examination is carefully performed it can provide an equal or better diagnosis of meniscal and ACL tears when compared to MRI scans.

Kocabey et al [37] carried out a prospective longitudinal study to compare the accuracy of clinical examination as compared to MRI  in diagnosis of meniscal and ACL pathology. The study included 50 consecutive patients who had clinical examination, MRI of the knee and arthroscopy. The authors found no statistical difference between clinical examination and MRI in the diagnosis of medial or lateral meniscal tears or ACL tears ( P >.05). The accuracy of the clinical examination and MRI evaluation was the same. The authors concluded that a well-trained surgeon can rely on clinical examination for diagnosing meniscal and ACL tears and that routine  MRI scan of the knee is not recommended.

Mohan et al [38] conducted a retrospective study of 150 patients  to assess the  diagnostic accuracy of clinical examination. They found that the accuracy for diagnosis of medial meniscus tears was 88% and for lateral meniscal tears the accuracy was 92%. They concluded that the clinical diagnosis of meniscal tears is as reliable as the MRI scan.

Felli et al [38] carried out a prospective study to compare the accuracy of clinical examination to that of MRI for the diagnosis of meniscal tears and chronic anterior cruciate ligament tears. They found that there was no difference between clinical and MRI evaluations in the diagnosis of medial meniscus and anterior cruciate ligament injuries. A trained radiologist was able to obtain better sensitivity, specificity and accuracy in the diagnosis of lateral meniscus tears.

Phelan et al [40] carried out a systematic review and meta-analysis to determine the diagnostic accuracy of MRI in the diagnosis of ACL, medial meniscus and lateral meniscus tears. They found that the overall sensitivity and specificity of MRI for ACL tears was 87 % and 93 % respectively, for  medial meniscal tears the sensitivity and specificity was 89% and 88% respectively and for lateral meniscus the sensitivity and specificity was 78% and 95 % respectively. Magnetic field strength did not affect the accuracy of the diagnosis. They found that most of the studies had a high or unclear risk of bias.

Conclusion

The prevalence of abnormal findings on MRI scanning of the lumbar spine in asymptomatic individuals is high which is why there are questions about the value of MRI scanning in patients with back pain. Only Modic type 1 changes and extensive zygapophyseal edematous changes appear to  have some correlation with low back pain.

There is not much value of MRI scanning for diagnosis of ACL and
meniscal tears in the knee. In fact clinical examination is equal to or more superior then MRI in the diagnosis of ACL and meniscal tears.

In orthopaedic clinical practice the spine and the knee are the two parts of the human body that are most commonly subjected to MRI scanning. The judicious use of MRI scanning can result in tremendous monetary saving and lowering the healthcare expenditure.

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