Imaging Diagnosis of Thoracic OPLL and OLF
Ichiro Kikkawa and Yuichi Hoshino
Introduction
Although ossifi cation of the posterior longitudinal liga- ment (OPLL) is often detected on cervical radiographs, OPLL can also develop in the thoracic spine. It is impor- tant to note that the incidence of ossifi cation of the ligamentum fl avum (OLF) of the thoracic spine is high.
Because ossifi cation of the spinal ligaments is a disease involving systemic ossifi cation of the ligaments, it often occurs in multiple locations simultaneously.
The symptoms associated with compressive spinal cord lesions in the neck usually fi rst appear in the upper extremities, and thoracic or lumbar spine lesions should be suspected if gait disorders or sensory disorders of the lower extremities occur. When lower extremity tendon refl exes are exaggerated without any symptoms in the upper extremity, the probability of lumbar myelopathy is low, whereas that of thoracic myelopathy is high.
Plain Radiography
With plain radiography of the thoracic spine, experi- ence and skill are necessary to identify OPLL/OLF owing to the presence of the rib cage. First, the posterior wall of the vertebral body should be identifi ed on a lateral view of the thoracic spine. OPLL is suspected if a radiopaque area extends to the posterior region of the intervertebral disc beyond the posterior corner of the vertebral body. OPLL is also suspected if a radiopaque area protrudes dorsally from the posterior wall of the vertebral body (Fig. 1a). It is important to note that OPLL often develops in the midthoracic spine (T4–6) [1] and that OPLL of the upper thoracic spine cannot be identifi ed by plain radiography owing to the pres- ence of the shoulder girdle.
In contrast, OLF often develops in the lower thoracic spine, and it is occasionally detected on plain radio- graphs of the lumbar spine. Concentrating on the radi- opaque area of the intervertebral foramen between the vertebral body and lamina, a smooth line shaped like an ear can be identifi ed on the dorsal side. OLF is strongly suspected if this smooth line is broken by a radiopaque area protruding like a beak into the ventral side (Fig. 2). OLF is seen in all bleached skeletons of Japanese adults [2], and as a result the probability of detecting OLF on plain radiographs of the thoracic spine is extremely high.
Tomography
Tomography is capable of depicting OPLL/OLF over a broad area on a single fi lm (Fig. 1b) and is useful for observing the cervicothoracic junction, whereas plain radiography is inadequate owing to the presence of the shoulder girdle. However, because of high levels of radiation exposure, computed tomography (CT) is per- formed more often to reconstruct the cervicothoracic junction in three dimensions.
Computed Tomography
OPLL/OLF of the thoracic and lumbar spine is con- fi rmed by CT (Fig. 3a). Ascertaining the location of OPLL/OLF inside the spinal canal on axial images is essential for planning surgery (i.e., determining the extent of decompression). In recent years, sagittal reconstruction CT images rather than X-ray tomogra- phy has been used to ascertain the extent of OPLL in the craniocaudal direction (Fig. 4b). It is easy to assess the three-dimensional structure of ossifi cation on reconstructed 3D images, and this type of information is useful when planning surgery (Fig. 5).
Sato et al. [3] observed and classifi ed OLF morphol- ogy on transverse CT (Fig. 6) Their classifi cation is useful for safe surgical decompression.
Jichi Medical University School of Medicine, 3311-1 Yakushiji, Shimotsuke, Tochigi 329-0498, Japan
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Although OPLL occurs in the lumbar spine, it is unlikely to cause neural compression at this location.
This is because of the large diameter of the spinal canal, greater spatial allowance, and narrowness of the caudal nerves in the dura mater (Fig. 7).
Myelography and CT-Myelography
Because of the increased popularity of magnetic reso- nance imaging (MRI), which can provide images of soft tissue such as the spinal cord, myelography (used to ascertain spinal compression) is performed less fre- quently today. Whereas CT is useful for examining osseous tissue and MRI is preferable for examining soft tissues such as the spinal cord and ligaments, CT- myelography is still the most useful technique for ana-
lyzing osseous and soft tissues simultaneously (Fig. 8).
In some cases, OPLL on the anterior side of the spine fuses with OLF on the posterior side of the spine at the lateral aspects of the spine; thus, thorough morphologi- cal analysis by CT-myelography is essential for surgical planning.
Magnetic Resonance Imaging
Although MRI is not suitable for obtaining detailed images of osseous tissue, it should be performed after plain radiography because spinal compression can easily be ascertained; moreover, patient stress is minimal, as no ionized radiation or contrast medium is administered. OPLL/OLF tends to occur in multiple
Fig. 1. Radiographs of a 45-year-man with thoracic myelopathy due to ossifi - cation of the posterior longitudinal ligament (OPLL). a Posterior border of T6–8 vertebral body cannot be identi- fi ed by a radiopaque OPLL shadow.
b Tomography clearly shows the OPLL.
Arrows, OPLL
a b
Fig. 2. Radiograph of a 49-year-woman with ossifi cation of the liga- mentum fl avum (OLF). Note the beak-like protrusion into the ear- shaped intervertebral foramen. Arrows, OLF
Fig. 3. Computed tomography (CT) of thoracic OLF: axial image (a) and reconstructed sagittal image (b). Arrows, OLF
a b
Fig. 5. Three-dimensional CT image of thoracic OPLL and OLF.
The inner aspect of the spinal canal can be seen with the sagittal slicing technique
Fig. 4. CT of thoracic OPLL: axial image (a) and reconstructed sagittal image (b). Arrows, OPLL
a b
Fig. 7. CT-myelography of lumbar OPLL. Despite the canal stenosis due to OPLL and OLF, the cauda eqina still has some room for cerebrospinal fl uid
Fig. 8. CT-myelography of thoracic OPLL and OLF. Com- pressed cord can be seen with clearly visible OPLL and OLF.
These images are useful for planning decompressive surgery Fig. 6. CT classifi cation by Sato et al: lateral (a), enlarged (b), and tuberous (c) types. The decompression technique suitable for each OLF type should be chosen for safety during laminectomy
a,b c
Fig. 9. T2-weighted magnetic resonance imaging (MRI) of thoracic OPLL. Multiple areas of compression by OPLL and OLF can be seen in a single image. T2 high signal change also can be seen at the site of severe compression
Fig. 10. T2-weighted MRI of thoracic OLF. One severe and several moderate areas of compression by OLF can be seen.
There is also cauda compression by disc degeneration in the lumbar area
locations, and sagittal MRI is an effi cient technique for analyzing the entire length of the spine.
OPLL can easily be detected on sagittal images in which the spinal canal is close to the center (Fig. 9), and OLF can easily be detected on paramedian sagittal images as low-signal lesions (Fig. 10). Broad spinal compression is seen with OPLL; however, the level responsible for spinal symptoms is often seen as a high- intensity lesion on T2-weighted images. MRI is also capable of detecting posterior longitudinal ligament hypertrophy, which is considered the precursor of OPLL.
References
1. Tsuyama N (1984) Ossifi cation of the posterior longitudi- nal ligament of the spine. Clin Orthop 184:71–84
2. Sakou T, Tomiura K, Maehara T, Morimoto T, Yano Y, Ohsako T, Kawamura H, Kouji T, Shibuya E, Morizono Y, Itou T (1977) Pathophysiological study of ossifi cation of the ligamentum fl avum (in Japanese). Rinsho Seikei Geka 12:368–376
3. Sato T, Tanaka Y, Aizawa T, KoizumiY, Kokubun S (1998) Surgical treatment for ossifi cation of ligamentum fl avum in the thoracic spine and its complications (in Japanese).
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