Ligamenta flava are the longest of the human ligamentous structures in length, which begins from the second cervical to lumbosacral vertebrae. Its fibers are almost in vertical arrangement and are attached to the ventral surface of the ephalad lamina and to the superior lip of the suprajacent one. Ligamentum flavum consists of elastic fibers (80%) and collagen (20%) and serves as assistance in the maintenance of the erect posture and keeper of the ligament taut during extension5)
. Because ligamentum flavum has abundant in motion, any laxity would permit redundancy and infolding toward the ventrally related structures, as occurs in degenerative lumbar spinal stenosis. Other stenotic lesions which induced by ligamentum flavum are calcifi-cation, ossification and ganglion or synovial cyst14)
. Because ligamentum flavum is poorly vascularized and only a few small vessels pass through it, intraligamentous bleeding is very rare phenomenon17)
The twenty cases of LFH, including the present case have been reported and are described in 1-2, 4, 6-13, 15-19)
. There are some striking features in these cases. The authors wish to emphasize the following aspects : (1) the patients were relatively older age (average age : 62.9, range : 30-83) and the almost all patients except three cases were more than 60 years old, (2) male patients had more higher incidence rate (male : female=14 : 7) (3) the most frequently involved spinal segment was lumbar area(cervical : 2, rigid thoracic : 2, mobile thoraco-lumbar : 2, lumbar : 14), (4) the onset of symptom was insidious and clinical course was progressively worsened, (5) the most patients experienced minor repeated trivial back injury during daily living lives or sports activities before admission, and (6) all the patients underwent surgery and clinical outcomes and their prognoses were very good following surgery.
Reported Cases of Ligamentum Flavum Hematoma
From above mentioned characteristics, several features could be extracted. First, LFH occurs in mainly over middle-aged male patients. These epidemiological factors suggest that LFH is a phenomenon associated to age-related ligamental degeneration rather than hypertrophy which generally seen in spinal degenerative diseases. This fact could be identified from the reported pathologic results. The common findings suggested of ligamental degeneration were dispersed ligamental elastic fibers, increased collagen tissues, granulation of fibrous tissues, lymphocyte infiltration and small capillary proliferation8, 16, 18)
Second, more than half of the patients had precipitating factors. These noticable causative factors were repeated trivial abdominal or back trauma which performed during daily living life or sports activities. Coughing and sneezing which could induce increased intraabdominal pressure can also be included in such factors. Increased abdominal pressure transmitted to epidural space resulted in spinal epidural hypertension, which could induce bleeding of degenerated ligamentum flavum. This bleeding occurs in proliferating small blood vessels which are frequently seen in ligamental degeneration and these vessels are very small, thin-walled and irregularly dispersed in degenerated ligamentum flavum and are easily broken-down result in LFH12)
. Anatomically, spinal epidural veins lack of valves, thus are easily permitting flow in both directions. Thus, blood from inferior vena cava may be shifted into the vertebral plexus when increased intra-abdominal pressure by physical activity, straining or trauma. Also, the spinal epidural veins receive blood from the intracranial sinuses and veins. Because of these reasons, bleeding from degenerated ligamentum flavum may occur1)
Third, LFH takes a long clinical course rather than acute spinal epidural hematoma. Preoperative MR images and operative findings of the present case suggested and confirmed that this ligamental hematoma was repeated bleeding and had multi-staged hematoma. Other reported cases also represented similar findings. While acute spinal epidural hematoma have same staged-hematoma (acute or subacute), rapid clinical course and no capsule surrounding hematoma, LFH have opposite features. These are similar to the phenomenon which occur in intracranial chronic subdural hematoma. Minimal and repeated spinal trauma could induce a partial tear in a degenerated ligament and favor intraligamentary hemorrhage. By means of fibrinolytic/hemolytic changes, the hemorrhage would increase significantly in volume inside the ligamentum flavum, causing neural compression. This can explain the insidious onset and progressive clinical worsening of symptoms in the patients of LFH1)
. Also, most patients might have minor trivial back or abdominal injury before symptom onset, but did not remember or forget the time of injury, because there were time interval between trauma and appearance of symptom. Therefore, the patients did not regard that injury as causating and triggering event3,17,18)
Forth, almost of all cases were seen in mobile lumbar and cervical spine. Only two cases including present case occurred in rigid thoracic spine which is supported by strongly built rib cage12)
. A total of four thoracic cases including present case were reported. Two cases happened in mobile thoracolumbar area and only one case in rigid thoracic spine2,8,16)
. Present case was the second case which occurred in rigid thoracic spine. According to above mentioned mechanisms, cervical and lumbar spine prone to receive pressure transmission to epidural space because they have abundant spinal mobility. But, thoracic spine hardly receives this pressure transmission because it lack of spinal motion. There may also be different mechanisms. In reviewing previous reports, Miyakoshi who was the first reporter of pure rigid thoracic LFH suggested fresh opinion that sagittal imbalance of spine might be involved in this phenomenon12)
. According to his case, thoracic hyperlordosis secondary to type 3 lumbar degenerative kyphosis might be contributed to thoracic LFH. Similarly, our case showed sagittal imbalance as a consequence of the old compression fracture in the first lumbar vertebral body. Kyphosis secondary to compression fracture indicates that greater axial distraction stress or force seems to be loaded to the ligamentum flavum and the facet joint at mid-thoracic area compared with normal spinal alignment12)
. Generally, compression of neural elements by age-related ligamentum flavum hypertrophy or degeneration frequently occurs in mobile lumbar and cervical spine but rarely occurs in pure rigid thoracic spine which is protected by rib cages in keeping the normal spinal alignment and sagittal balance. If sagittal imbalance may happen, thoracic ligamentum flavum could be influenced by axial overstress and degenerative change might be promoted.
Fifth, prognoses following surgery were very good. Authors believe that LFH was formed and compressed in relatively long time, so spinal cord had sufficient time to compensate. Thus, cord restored its original shape immediately after hematoma evacuation backed up this suggestion.
Finally, preoperative MR images are the best choice of diagnostic tools. Common MR findings of reported cases are posterior extradural location, one ligamentum flavum involvement, mainly hyperintensities on T1-weighted images which suggested subacute stage hematoma, continuity between base of hematoma and ligamentum flavum and mixed-stage or multi-stage hematoma signal intensities9)
. If these findings are identified in case of posterior extradural mass coincide with intra-lesional hemorrhage, LFH should be included in differential diagnoses.