Surgery for Ossification of the Ligamentum Flavum
Yasuhisa Tanaka
1, Tetsuro Sato
2, and Toshimi Aizawa
1Introduction
Ossifi cation of the ligamentum fl avum (OLF) is one of the most common causes of compression myelopathy in degenerative processes of the thoracic spine [1].
When OLF is the sole compressive factor in a patient with thoracic myelopathy, posterior surgery is indi- cated because the myelopathy usually is not alleviated by conservative treatments but, rather, continues to deteriorate. OLF varies in range and shape depending on the patient. Although it has been fairly diffi cult to ascertain OLF correctly on conventional plain radio- graphs, nowadays OLF can be well depicted by com- puted tomography (CT). Posterior surgery for OLF consists basically of two procedures: laminectomy and fenestration. The procedure should be as minimally invasive as possible, but at the same time it is necessary to be safe enough to prevent surgical complications, which are not infrequent with thoracic spine surgery [2]. We describe in this chapter (1) the classifi cation of OLF using CT fi ndings, which is essential for choosing the appropriate procedure; (2) the surgical techniques for each procedure; and (3) the surgical results.
Classification of OLF Using CT Findings
Classification
The ligamentum fl avum is comprised of two parts: the interlaminar portion medially and the capsular portion laterally [3,4]. Ossifi cation usually starts to develop in the capsular portion and then extends gradually to the interlaminar area. The ossifi cation then enlarges ven- trally, compressing the spinal cord. Bilateral ossifi ca- tion then fuses in the middle of the lamina and thickens
to form a central tuberous mass. CT scanning is essen- tial for detecting the range and shape of the OLF and the degree of spinal stenosis due to it, which is the cause of spinal cord compression. CT scanning is performed at the intervertebral level where OLF is present, usually from the middle of the upper adjacent vertebral body to the middle of the lower adjacent vertebral body.
Using the fi ndings on the slice that depicts the most severe narrowing of the spinal canal—generally at the middle level of the zygapophyseal joint—the OLF is classifi ed into one of the following fi ve types (Fig. 1) [5,6].
1 . Lateral type: Ossifi cation is confi ned to the capsular portion of the ligamentum fl avum.
2 . Extended type: Ossifi cation extends into the inter- laminar portion of the ligamentum fl avum but is still thin.
3 . Enlarged type: The width of the ossifi cation is similar to that of the extended type but has greater thick- ness, causing posteromedial narrowing of the spinal canal.
4 . Fused type: The composite size of the ossifi cations is approximately same as that of the enlarged type, but the bilateral masses are fused in the middle of the lamina.
5 . Tuberous type: Bilateral ossifi ed masses are fused in the middle of the lamina and comprise a tuberous mass protruding anteriorly.
Frequency
The frequency of each type described above usually depends on the number of affected intervertebral levels [5]. That is, patients who undergo surgery for single- level OLF usually have a lateral, extended, or enlarged type of OLF. In contrast, those with multilevel OLF tend to have a fused or tuberous type. Among 42 patients we operated on for OLF of a single level (bilateral OLF in 30 patients, unilateral OLF in 12 patients), the inci- dences of the lateral, extended, and enlarged types of OLF were 20%, 12%, and 62%, respectively. Among 21 patients with multilevel OLF, 11 patients (52%) had either the fused or tuberous type at one or more levels.
1Department of Orthopaedic Surgery, Tohoku University Graduate School of Medicine, 1-1 Seiryoumachi, Aoba-ku, Sendai 980-8574, Japan
2Department of Orthopaedic Surgery, Sendai Orthopaedic Hospital, Sendai, Japan
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Surgical Procedures
French-Door Laminectomy
For French-door laminectomy, after removing the spinous process, the outer cortex and the cancellous layer of the two or more laminae that contain OLF are removed dorsally using an air-drill, leaving the inner cortex intact. This fi rst step is necessary for safely making incisions of the laminae during the following steps. Otherwise, the lamina is too thick to be incised using an air-drill under direct inspection. A lazy-V transverse incision is made at each chevron-like portion [7] of the uppermost and lowermost laminae of the inner cortex, where the ligamentum fl avum does not exist and therefore OLF never develops. The lateral ends of the incision are placed at the medial margin of the pedicles. A longitudinal incision is made between two transverse incisions at the midline portion of laminae traversing the interlaminous space(s). A lateral longitudinal incision is made on both sides between the lateral ends of the transverse incisions. This incision is curved slightly more laterally at the level of the zyg- apophyseal joint to avoid the site where the OLF is located deeply anteriorly while preserving the most lateral portion of the joint. The laminae are opened at the midline, and the hemilaminae on both sides are opened outward with the OLF and removed, releasing the adhesion between the OLF and the dura mater, if present (Fig. 2) [5,6].
En Bloc Laminectomy
For the en bloc laminectomy, except for the longitudi- nal incision at the midline portion of the laminae, the same procedures as those for French-door laminec-
tomy are performed through the lateral incisions are made. Then the laminae containing OLF are pulled up at the lateral incision on one or both sides and removed, releasing the adhesion between the OLF and the dura mater, if present (Fig. 3).
Fenestration (Laminotomy)
For fenestration (laminotomy), the outer cortex and cancellous layer of the two laminae that contain the OLF are removed between the chevron-like portions of the upper and lower laminae, but with preservation of the spinous process. A slightly oblique transverse inci- sion is made at each chevron-like portion of the laminae.
The ends of the incision are at the midline and the medial margin of the pedicle. A longitudinal incision is made under the spinous process between the upper and lower transverse incisions at the midline portion of the laminae traversing the interlaminous space. A lateral longitudinal incision is made in the same manner as for the French-door laminectomy. The laminae are opened at the midline, and the hemilaminae on both sides are removed outward with the OLF (Fig. 4).
Hemilaminectomy
For hemilaminectomy, the same procedures as those for the French-door laminectomy or fenestration are performed but unilaterally (Fig. 5).
Choice of Surgical Procedure
French-door laminectomy and fenestration are indi- cated for OLF in which the median portion of the lamina is free from ossifi cation because these procedures
a b c d e
Fig. 1. Classifi cation of ossifi cation of the ligamentum fl avum (OLF) based on computed tomography (CT) fi ndings into fi ve types: a Lateral type. b Extended type. c Enlarged type. d Fused type. e Tuberous type. In OLFs of the lateral, extended, and
enlarged types, even when bilateral there is no ossifi cation in the middle of the laminae. In those of the fused and tuberous types, there are bilateral ossifi ed masses fused in the middle of the laminae
Fig. 2. French-door laminectomy. Lateral incisions are curved slightly more later- ally at the level of the zygapophyseal joint to avoid the site where the OLF (*) is located deep, toward the anterior. (The same applies to lateral incisions used in other procedures, that is, en bloc laminec- tomy, fenestration, and hemilaminec- tomy.) The hemilaminae on both sides are opened outward with the OLF and removed
Fig. 3. En bloc laminectomy. No midline inci- sion is made for en bloc excision. The laminae containing OLF (*) are pulled up at the lateral incision on one or both sides and are removed, with release of the adhesion between the OLF and the dura mater
Fig. 4. Fenestration (laminotomy). A mid- line longitudinal incision is made under the spinous process between the transverse incisions, traversing the interlaminous space. The hemilaminae on both sides are opened outward with the OLF (*) and are removed, with preservation of the spinous process
require a safe longitudinal incision at this portion.
Accordingly, they are indicated for the lateral, extended, and enlarged types of bilateral OLF. Although French- door laminectomy can be employed for both single- level and multilevel OLF, its best indication is for multilevel OLF because fenestration, which is less inva- sive, can be used for single-level OLF.
En bloc laminectomy is indicated when the midline of the lamina is ossifi ed, so placing a longitudinal incision is almost impossible without endangering the dura mater and the spinal cord. Thus, it is indi- cated for the fused or tuberous type of single-level or multilevel OLF. However, in cases of OLF of the fused or tuberous type, ossifi cation of the dura mater sometimes occurs. In such cases, it is usually impos- sible to release the ossifi ed portion of the dura mater from the surrounding, intact dura mater. Attempts to release the ossifi ed portion tend to result in a tear of the arachnoid membrane, leakage of cerebrospinal fl uid, and even injury to the spinal cord. For cases of ossifi cation of the dura mater or a severe adhesion between the dura mater and the OLF, extra steps are necessary to complete the en bloc laminectomy. That is, in the portions above and below the two trans- verse incisions of the laminae, the dura mater is exposed approximately 1.0–1.5 cm longitudinally.
Dural incisions are made and en bloc laminectomy is performed by removing the ossifi ed mass and sur- rounding dura mater simultaneously while leaving the arachnoid intact (Fig. 6). The dural defect is repaired using an artifi cial membrane or a sheet of fascial membrane.
Hemilaminectomy is indicated for the lateral, extended, and enlarged types of unilateral OLF.
Surgical Results
For evaluating the severity of thoracic myelopathy caused by OLF before and after surgery, a modifi ed version of the Japanese Orthopaedic Association (JOA) score for cervical myelopathy (see Appendix) is used.
The modifi ed version, in which a normal score is 11
Fig. 5. Hemilaminectomy. The same procedures as those for the French- door laminectomy or fenestration but on the unilateral side alone are indi- cated for unilateral OLF (*)Fig. 6. En bloc laminectomy for OLF with ossifi cation of the dura mater. Dural incisions are made in the portions above and below the two transverse incisions. En bloc laminectomy is performed by removing the ossifi ed mass (*) and surround- ing dura mater simultaneously while leaving the arachnoid intact
points, consists of the following three categories: lower extremity motor function, sensory function of the lower extremity and trunk, and bladder function. The out- comes of surgery are also represented by the recovery rate (%) calculated as follows: (postoperative JOA score
− preoperative JOA score)/(11 − preoperative JOA score) × 100.
For our 87 patients with follow-up of more than 3 months, assessed by the modifi ed version of the JOA score, the preoperative mean score of 5.2 (range, 1 to 9) points improved to 7.8 (range, 0 to 11) at an average follow-up of 2 years 8 months after surgery. The recov- ery rate averaged 48% (range, −37.5% to 100%). The surgical results generally depend on the preoperative severity of the myelopathy. Therefore, better results can be anticipated when the surgery is performed earlier.
Conclusions
Classifi cation of OLF, which causes thoracic myelopa- thy, is based on CT fi ndings and is necessary for choos- ing the appropriate surgical procedure. When OLF is the lateral, extended, or enlarged type at a single inter- vertebral level, fenestration (laminotomy) is the surgical choice. For multilevel cases, French-door lami- nectomy is indicated. For cases of the fused or tuberous type, en bloc laminectomy is indicated. When fused- or tuberous-type OLF is complicated by ossifi cation of the
dura mater, en bloc laminectomy with simultaneous removal of the ossifi ed mass and the surrounding dura mater is performed, leaving the arachnoid membrane intact.
References
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