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An unusual meniscal finding on magnetic resonance imaging
Radiography (2006) 12, 348e351
CASE REPORT
An unusual meniscal finding on magnetic resonance imaging W.M. Bailey* Leighton Hospital, MRI, Middlewich Road, Crewe, Cheshire CW1 4QJ, United Kingdom Received 2 March 2005; accepted 20 July 2005 Available online 21 June 2006
KEYWORDS Discoid meniscus; Magnetic resonance knee; MR scanning parameters; Knee protocols; Congenital variants of the knee
Abstract A 13-year-old male with a previous one-month history of a ‘locking’ painful knee underwent an MR investigation of his knee. The MR findings showed a discoid meniscus with a meniscal tear, which at arthroscopy proved to be an unusual Wrisberg III variant. The morphology of discoid meniscus is defined including the Wrisberg III variant. The MR factors are discussed including optimum planes and sequences used by MRI to investigate meniscal pathologies including tears. The treatment regimes are investigated including surgical options. ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Magnetic resonance scanning remains the primary imaging modality for the visualisation of meniscal tears within the knee. It is an accessible, cost effective tool that is non-invasive, whilst providing an accurate diagnosis.8
Case report A 13-year-old male presented with a painful ‘locking’ knee that had rapidly progressed over one month. Clinical examination revealed a positive McMurray’s test with a tender lateral joint line suggestive of a torn lateral meniscus. On 15e20
* Tel.: C44 1270 612505. E-mail address: [email protected]
flexion of the knee the joint gave a visible ‘snap’. All other ligaments were stable. This indicated a discoid type meniscus and/or a complex lateral meniscal tear. Magnetic resonance imaging showed an oblique tear of the lateral meniscus involving both the mid-zone and posterior horn, communicating with the inferior joint surface (Fig. 1). The lateral meniscus appeared normal in shape but uncharacteristically was much larger than the medial meniscus (Fig. 2) measuring 20 mm in height compared to the 12 mm of the medial meniscus. The meniscus extended over 60% of the joint surface of the lateral femoral condyle. This was typical for a lateral discoid type meniscus. (MR parameters for sagittal T2* and coronal PD with fat saturation see Table 1). The patient underwent arthroscopic examination, this showed a hyper-mobile lateral meniscus
1078-8174/$ - see front matter ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radi.2005.07.003
An unusual meniscal finding
Figure 1
349
Sagittal T2 gradient echo through the lateral meniscus.
lacking attachment to stabilise the posterior horn to the tibia. This proved to be a rare variant of discoid meniscus known as a Wrisberg type III discoid meniscus, the only attachment being Wrisberg’s ligament. To avoid total meniscectomy the posterior horn was tied down though drill holes and the tear was repaired using Butress sutures. The patient responded well and the knee had settled down on the three monthly review.
Discussion Discoid meniscus occurs in between 1.5% and 5% of the population,1,10 and is much more common in the lateral meniscus.3 There is a much higher rate
Figure 2
of occurrence within the Asian population. A study performed by Ikeuchi at a Tokyo Hospital reported a 16e17% incidence of discoid meniscus of all knees examined arthroscopically.6 Locking is a common presentation in children, but symptoms may not develop until adolescence or young adulthood. The child is often asymptomatic while adults are symptomatic. Bi-lateral discoid meniscus occurs in 20% of patients with a discoid meniscus.7 The normal meniscus is a crescent shaped structure. Both menisci help to stabilise the knee joint, whilst offering smooth articulation of the movement of the femoral condyles over the tibial plateaux. Because the medial femoral condyle is smaller in the knee the medial meniscus is larger
Coronal PD with fat saturation.
350
W.M. Bailey
Table 1 Showing TR, TE and flip angle for sagittal T2* and proton density and fat saturation. (I Tesla General Electric scanner) Parameters
Time for repetition (mS)
Sagittal T2* gradient 500 echo Coronal PD fat 2000 saturated spin echo
Time for echo (mS)
Flip angle ( )
12
35
15
90
than the lateral meniscus. A discoid meniscus is a large, congenitally dysplastic meniscus with loss of semi-lunar shape.10 The etiology of the lateral discoid meniscus has been extensively debated. Two main hypotheses exist. It results from a developmental anomaly before birth and results in a failure of the resorption of the central singular portion of the meniscus4 or, the now generally believed theory that, the lateral discoid meniscus is a result of abnormal development secondary to the deficiency of normal attachments.10 The development of the discoid meniscus is due to the failure of attachment of the meniscotibial ligaments of the posterior horn. The abnormal stresses on the meniscus provoke abnormal growth, causing it to hypertrophy and form a discoid shape.12 There are three types of discoid meniscus defined by the Watanabe classification, these are: Complete e the meniscus extends into the intercondylar notch on the coronal image. Incomplete e the meniscus extends towards the intercondylar notch on the coronal image. Wrisberg type e this meniscus lacks a posterolateral meniscotibial attachment.10 The identification of a discoid meniscus and any meniscal tears is best suited to MRI.9 On MR scanning the discoid meniscus is usually large in the cranio-caudal plane O13 mm with a loss of the normal semi-lunar shape. There is 50% or greater coverage of the tibial plateaux on the sagittal or coronal images. On sagittal images the meniscus exhibits a continuous body/mid-zone that lacks the bowtie appearance and is present on 3 or more consecutive slices (3e4 mm).10 The meniscus can be of normal or the more recognisable ‘pancake’ shape. A discoid meniscus produces an unstable knee joint, thus the chance of meniscal tears caused by trauma is substantially increased.1 The MR sequences used for the imaging of meniscal tears have been the cause of much debate. Runbin and Paletta8 advocate short time for echo (TE) for imaging linear tears,8 with
imaging sequences including spin echo (SE) T1, proton density, or gradient echo. The sensitivity for these sequences ranges between 90% and 95%.5 Turbo/fast spin echo sequences produce sensitivity values at approximately 80%, being significantly inferior to the spin echo sequence.2,5 Meniscal tears become less visible with a TE longer than 16e 20 mS.8 The signal within a tear is not due to free water and unbound protons. The hydrogen nuclei within a meniscal tear are bound to macromolecules, these protons have a much shorter T2 relaxation time than the nuclei in free water. Thus the high signal seen in tears on short TE sequences is due to the local spin density and not T2* decay,8 unless there is a presence of wide clefts in the meniscus communicating with the synovial fluid. The decreased sensitivity of fast spin echo sequences (FSE) is partly due to increased image blurring from the use of echo trains5 and the increased T2 decay from the higher order echoes. Proton density SE fat saturated sequences do not increase the accuracy of diagnosing meniscal tears,5 but can be used to rid the image of distracting high signal originating from fat within marrow and in the soft tissues, thus increasing the dynamic range signal of the menisci, making meniscal tears more conspicuous.5 Gradient T2* images give much better temporal resolution than spin echo proton density images, whilst offering similar sensitivity for meniscal tears,5 efficient time management is one of the concerns in a busy MR department. The gradient sequences limit visualisation of ligament, muscle, bone marrow and articular surfaces.5 A sagittal short TE (w9 mS) T1 weighted sequence in the same location would give further information of the meniscus, ligaments, muscles, etc. as well as further characterisation of other lesions.8 A short TE (9e18 mS) for gradient echo images decreases the signal to noise of the image, but some signal can be recovered by utilising a slightly larger flip angle (30e35 ) adding slightly more proton density into the image11 whilst maintaining the overall T2* weighting of the image. Choosing the appropriate orientation and pulse sequence is paramount for an efficient MR investigation. The sagittal plane gives excellent visualisation of patello-femoral compartment, anterior and posterior cruciate ligaments, quadriceps and patella tendons, and is the most useful plane to assess both medial and lateral meniscus.9 The coronal plane gives excellent information on the co-lateral ligaments and a primary view of the mid-zone or body of both menisci.9 Axial planes do not provide higher diagnostic accuracy than sagittal or coronal planes when visualising the menisci,8 but provide primary views of
An unusual meniscal finding patella-femoral joint, proximal anterior cruciate and secondary views of patella-femoral joint, quadriceps and patella tendons and both co-lateral ligaments.9 High resolution imaging of the meniscus is essential to identify meniscal pathologies including discoid type menisci and meniscal tears. A slice thickness of 3 mm gives adequate coverage whilst maintaining high resolution combined with small (16e18 mm) field of views.13 The treatment for a lateral discoid meniscus can vary. An asymptomatic discoid meniscus would need only observation. Symptomatic patients would need interventional treatment. Surgical treatment using arthroscopic techniques can be technically challenging due to the patients being of a younger age with smaller, tighter joints, but this technique can be extremely successful.1 With a complete discoid meniscus the standard therapy is to remove the abnormal central portion while preserving a rim of meniscus to maintain the biomechanical benefits.11 Because of its hyper mobility and risk of tears the treatment for the Wrisberg type lateral discoid meniscus traditionally has been managed with total meniscectomy. This would greatly increase the premature onset of osteoarthritis.12 A relatively new technique of tying down the meniscus through drill holes in the tibia to stabilise the meniscus with repair of the tear has been reported to be very encouraging1 and was successfully performed in this case. Discoid meniscus is not an uncommon finding in the symptomatic paediatric knee with MR proving to be both sensitive and accurate in diagnosing discoid meniscus and associated meniscal tears. The sensitivity of MR is much improved by the use of short TE sequences to dramatically increase the
351 sensitivity of the images in localising meniscal tears. The arrival of newer surgical techniques to treat discoid meniscus/meniscal tears is proving very successful with preservation of the meniscus and the articular surface.
References 1. Di Libero R. eMedicine e discoid meniscus,!http://www. emedicine.com/orthoped/topic76.htmO [Last updated: April 9, 2004]. 2. Blackmon GB, Major NM, Helms CA. Comparison of FSE versus conventional SE MRI for evaluating meniscal tears. AJR Am J Roentgenol 2005;184:1740e3. 3. Choi NH, Kim NM, Kim HJ. Medial and lateral discoid meniscus in the same knee. Arthroscopy 2001;17(2):E9. 4. Clark CR, Ogden JA. Development of the menisci of the human knee joint. Morphological changes and their potential role in childhood meniscal injury. J Bone Joint Surg Am 1983;65(4):538e47. 5. Helms CA. The meniscus: recent advances in MR imaging of the knee. AJR Am J Roentgenol 2002;179:1115e22. 6. Ikeuchi H. Arthroscopic treatment of the discoid lateral meniscus in children. Clin Orthop 1982;167:19e28. 7. Jordan MD. Lateral meniscal variants: evaluation and treatment. J Am Acad Orthop Surg 1996;4:191e200. 8. Rubin DA, Paletta GA. Current concepts and controversies in meniscal imaging. Magn Reson Imaging Clin N Am 2000;8(2): 243e70. 9. Stoller DW. Magnetic resonance imaging in orthopaedics and sports medicine. Philadelphia, USA: JP Lippincott; 1993. 10. Stoller DW, Tirman PFJ, Bredella MA. Diagnostic imaging in orthopaedics. Canada: Amirsys; 2004. 11. Westbrook C, Kaut C. MRI in practice. 2nd ed. Oxford, England: Blackwell publishing; 1998. 12. Youm T, Bosco J. Lateral discoid meniscus. Medscape Orthop Sports Med 2003;7(1). 13. Aiello MR. eMedicine, knee, meniscal tears,!http:// www.emedicine.com/radio/topic852.htmO [Last updated: February 22, 2005].
CASE REPORT
An unusual meniscal finding on magnetic resonance imaging W.M. Bailey* Leighton Hospital, MRI, Middlewich Road, Crewe, Cheshire CW1 4QJ, United Kingdom Received 2 March 2005; accepted 20 July 2005 Available online 21 June 2006
KEYWORDS Discoid meniscus; Magnetic resonance knee; MR scanning parameters; Knee protocols; Congenital variants of the knee
Abstract A 13-year-old male with a previous one-month history of a ‘locking’ painful knee underwent an MR investigation of his knee. The MR findings showed a discoid meniscus with a meniscal tear, which at arthroscopy proved to be an unusual Wrisberg III variant. The morphology of discoid meniscus is defined including the Wrisberg III variant. The MR factors are discussed including optimum planes and sequences used by MRI to investigate meniscal pathologies including tears. The treatment regimes are investigated including surgical options. ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Magnetic resonance scanning remains the primary imaging modality for the visualisation of meniscal tears within the knee. It is an accessible, cost effective tool that is non-invasive, whilst providing an accurate diagnosis.8
Case report A 13-year-old male presented with a painful ‘locking’ knee that had rapidly progressed over one month. Clinical examination revealed a positive McMurray’s test with a tender lateral joint line suggestive of a torn lateral meniscus. On 15e20
* Tel.: C44 1270 612505. E-mail address: [email protected]
flexion of the knee the joint gave a visible ‘snap’. All other ligaments were stable. This indicated a discoid type meniscus and/or a complex lateral meniscal tear. Magnetic resonance imaging showed an oblique tear of the lateral meniscus involving both the mid-zone and posterior horn, communicating with the inferior joint surface (Fig. 1). The lateral meniscus appeared normal in shape but uncharacteristically was much larger than the medial meniscus (Fig. 2) measuring 20 mm in height compared to the 12 mm of the medial meniscus. The meniscus extended over 60% of the joint surface of the lateral femoral condyle. This was typical for a lateral discoid type meniscus. (MR parameters for sagittal T2* and coronal PD with fat saturation see Table 1). The patient underwent arthroscopic examination, this showed a hyper-mobile lateral meniscus
1078-8174/$ - see front matter ª 2005 The College of Radiographers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radi.2005.07.003
An unusual meniscal finding
Figure 1
349
Sagittal T2 gradient echo through the lateral meniscus.
lacking attachment to stabilise the posterior horn to the tibia. This proved to be a rare variant of discoid meniscus known as a Wrisberg type III discoid meniscus, the only attachment being Wrisberg’s ligament. To avoid total meniscectomy the posterior horn was tied down though drill holes and the tear was repaired using Butress sutures. The patient responded well and the knee had settled down on the three monthly review.
Discussion Discoid meniscus occurs in between 1.5% and 5% of the population,1,10 and is much more common in the lateral meniscus.3 There is a much higher rate
Figure 2
of occurrence within the Asian population. A study performed by Ikeuchi at a Tokyo Hospital reported a 16e17% incidence of discoid meniscus of all knees examined arthroscopically.6 Locking is a common presentation in children, but symptoms may not develop until adolescence or young adulthood. The child is often asymptomatic while adults are symptomatic. Bi-lateral discoid meniscus occurs in 20% of patients with a discoid meniscus.7 The normal meniscus is a crescent shaped structure. Both menisci help to stabilise the knee joint, whilst offering smooth articulation of the movement of the femoral condyles over the tibial plateaux. Because the medial femoral condyle is smaller in the knee the medial meniscus is larger
Coronal PD with fat saturation.
350
W.M. Bailey
Table 1 Showing TR, TE and flip angle for sagittal T2* and proton density and fat saturation. (I Tesla General Electric scanner) Parameters
Time for repetition (mS)
Sagittal T2* gradient 500 echo Coronal PD fat 2000 saturated spin echo
Time for echo (mS)
Flip angle ( )
12
35
15
90
than the lateral meniscus. A discoid meniscus is a large, congenitally dysplastic meniscus with loss of semi-lunar shape.10 The etiology of the lateral discoid meniscus has been extensively debated. Two main hypotheses exist. It results from a developmental anomaly before birth and results in a failure of the resorption of the central singular portion of the meniscus4 or, the now generally believed theory that, the lateral discoid meniscus is a result of abnormal development secondary to the deficiency of normal attachments.10 The development of the discoid meniscus is due to the failure of attachment of the meniscotibial ligaments of the posterior horn. The abnormal stresses on the meniscus provoke abnormal growth, causing it to hypertrophy and form a discoid shape.12 There are three types of discoid meniscus defined by the Watanabe classification, these are: Complete e the meniscus extends into the intercondylar notch on the coronal image. Incomplete e the meniscus extends towards the intercondylar notch on the coronal image. Wrisberg type e this meniscus lacks a posterolateral meniscotibial attachment.10 The identification of a discoid meniscus and any meniscal tears is best suited to MRI.9 On MR scanning the discoid meniscus is usually large in the cranio-caudal plane O13 mm with a loss of the normal semi-lunar shape. There is 50% or greater coverage of the tibial plateaux on the sagittal or coronal images. On sagittal images the meniscus exhibits a continuous body/mid-zone that lacks the bowtie appearance and is present on 3 or more consecutive slices (3e4 mm).10 The meniscus can be of normal or the more recognisable ‘pancake’ shape. A discoid meniscus produces an unstable knee joint, thus the chance of meniscal tears caused by trauma is substantially increased.1 The MR sequences used for the imaging of meniscal tears have been the cause of much debate. Runbin and Paletta8 advocate short time for echo (TE) for imaging linear tears,8 with
imaging sequences including spin echo (SE) T1, proton density, or gradient echo. The sensitivity for these sequences ranges between 90% and 95%.5 Turbo/fast spin echo sequences produce sensitivity values at approximately 80%, being significantly inferior to the spin echo sequence.2,5 Meniscal tears become less visible with a TE longer than 16e 20 mS.8 The signal within a tear is not due to free water and unbound protons. The hydrogen nuclei within a meniscal tear are bound to macromolecules, these protons have a much shorter T2 relaxation time than the nuclei in free water. Thus the high signal seen in tears on short TE sequences is due to the local spin density and not T2* decay,8 unless there is a presence of wide clefts in the meniscus communicating with the synovial fluid. The decreased sensitivity of fast spin echo sequences (FSE) is partly due to increased image blurring from the use of echo trains5 and the increased T2 decay from the higher order echoes. Proton density SE fat saturated sequences do not increase the accuracy of diagnosing meniscal tears,5 but can be used to rid the image of distracting high signal originating from fat within marrow and in the soft tissues, thus increasing the dynamic range signal of the menisci, making meniscal tears more conspicuous.5 Gradient T2* images give much better temporal resolution than spin echo proton density images, whilst offering similar sensitivity for meniscal tears,5 efficient time management is one of the concerns in a busy MR department. The gradient sequences limit visualisation of ligament, muscle, bone marrow and articular surfaces.5 A sagittal short TE (w9 mS) T1 weighted sequence in the same location would give further information of the meniscus, ligaments, muscles, etc. as well as further characterisation of other lesions.8 A short TE (9e18 mS) for gradient echo images decreases the signal to noise of the image, but some signal can be recovered by utilising a slightly larger flip angle (30e35 ) adding slightly more proton density into the image11 whilst maintaining the overall T2* weighting of the image. Choosing the appropriate orientation and pulse sequence is paramount for an efficient MR investigation. The sagittal plane gives excellent visualisation of patello-femoral compartment, anterior and posterior cruciate ligaments, quadriceps and patella tendons, and is the most useful plane to assess both medial and lateral meniscus.9 The coronal plane gives excellent information on the co-lateral ligaments and a primary view of the mid-zone or body of both menisci.9 Axial planes do not provide higher diagnostic accuracy than sagittal or coronal planes when visualising the menisci,8 but provide primary views of
An unusual meniscal finding patella-femoral joint, proximal anterior cruciate and secondary views of patella-femoral joint, quadriceps and patella tendons and both co-lateral ligaments.9 High resolution imaging of the meniscus is essential to identify meniscal pathologies including discoid type menisci and meniscal tears. A slice thickness of 3 mm gives adequate coverage whilst maintaining high resolution combined with small (16e18 mm) field of views.13 The treatment for a lateral discoid meniscus can vary. An asymptomatic discoid meniscus would need only observation. Symptomatic patients would need interventional treatment. Surgical treatment using arthroscopic techniques can be technically challenging due to the patients being of a younger age with smaller, tighter joints, but this technique can be extremely successful.1 With a complete discoid meniscus the standard therapy is to remove the abnormal central portion while preserving a rim of meniscus to maintain the biomechanical benefits.11 Because of its hyper mobility and risk of tears the treatment for the Wrisberg type lateral discoid meniscus traditionally has been managed with total meniscectomy. This would greatly increase the premature onset of osteoarthritis.12 A relatively new technique of tying down the meniscus through drill holes in the tibia to stabilise the meniscus with repair of the tear has been reported to be very encouraging1 and was successfully performed in this case. Discoid meniscus is not an uncommon finding in the symptomatic paediatric knee with MR proving to be both sensitive and accurate in diagnosing discoid meniscus and associated meniscal tears. The sensitivity of MR is much improved by the use of short TE sequences to dramatically increase the
351 sensitivity of the images in localising meniscal tears. The arrival of newer surgical techniques to treat discoid meniscus/meniscal tears is proving very successful with preservation of the meniscus and the articular surface.
References 1. Di Libero R. eMedicine e discoid meniscus,!http://www. emedicine.com/orthoped/topic76.htmO [Last updated: April 9, 2004]. 2. Blackmon GB, Major NM, Helms CA. Comparison of FSE versus conventional SE MRI for evaluating meniscal tears. AJR Am J Roentgenol 2005;184:1740e3. 3. Choi NH, Kim NM, Kim HJ. Medial and lateral discoid meniscus in the same knee. Arthroscopy 2001;17(2):E9. 4. Clark CR, Ogden JA. Development of the menisci of the human knee joint. Morphological changes and their potential role in childhood meniscal injury. J Bone Joint Surg Am 1983;65(4):538e47. 5. Helms CA. The meniscus: recent advances in MR imaging of the knee. AJR Am J Roentgenol 2002;179:1115e22. 6. Ikeuchi H. Arthroscopic treatment of the discoid lateral meniscus in children. Clin Orthop 1982;167:19e28. 7. Jordan MD. Lateral meniscal variants: evaluation and treatment. J Am Acad Orthop Surg 1996;4:191e200. 8. Rubin DA, Paletta GA. Current concepts and controversies in meniscal imaging. Magn Reson Imaging Clin N Am 2000;8(2): 243e70. 9. Stoller DW. Magnetic resonance imaging in orthopaedics and sports medicine. Philadelphia, USA: JP Lippincott; 1993. 10. Stoller DW, Tirman PFJ, Bredella MA. Diagnostic imaging in orthopaedics. Canada: Amirsys; 2004. 11. Westbrook C, Kaut C. MRI in practice. 2nd ed. Oxford, England: Blackwell publishing; 1998. 12. Youm T, Bosco J. Lateral discoid meniscus. Medscape Orthop Sports Med 2003;7(1). 13. Aiello MR. eMedicine, knee, meniscal tears,!http:// www.emedicine.com/radio/topic852.htmO [Last updated: February 22, 2005].