The exact etiology of PD is not known
32. Genetic susceptibility
13,16,28, geographic factors
4,28,32 and viral infections
19,28 have all been suggested as causative mechanisms. Family history has been found positive in 15-30% of the cases with PD and an autosomal dominant inheritance pattern has been described
16,28. PD becomes clinically manifest either as a monostotic form (effecting only one part of one bone) or as a polystotic form (effecting multiple bony structures)
28,32. The most commonly involved bony structures are the pelvis, vertebrae, femur, calvaria, tibia and humerus
5,28,32. The vertebral involvement occurs in approximately 35% of the cases
10,23 and the lumbar spine is the most commonly effected segment
10 although few authors have listed the thoracic vertebrae as the most common target for PD
5.
The most common symptom of spinal PD is pain. Neurological examination is usually normal 28. In addition to pain originating from the disease process itself, one-third of PD patients develope spinal stenosis and suffer from the radicular pain originating from this stenosis 10. Stenosis again commonly occurs at the lumbar spine but typically it is a single-level spinal stenosis 25. Neurological dysfunction may occur as a result of various mechanisms. These mechanisms include hypertrophy of the facet joints causing relative instability 3,7,10,14,15,22, compression of the dural sac by hyperthrophic ligaments 11,17, ischemia secondary to steal phenomenon that occurs when the medullary arteries supplying both vertebrae and spinal cord carry more blood to the vertebrae 3,12,22, ischemia secondary to compression of the vascular supply of the spinal cord 22, compression from spinal epidural hematoma that may occur in association with PD 18,22 and spinal cord compression due to compression fracture of the involved vertebral body (ies) 6,10,22,31.
Biochemical markers as the reflection of bone turnover are of extreme importance in the diagnosis of PD 1. Serum calcium levels are usually normal 28. Serum AP is a relatively specific indicator showing the activity of the disease 32. Yet, in cases of elevated serum AP, liver disease must be be ruled first 26,32. The diagnosis of PD is even more difficult with a normal serum AP 5. This may be the case in the monostotic form of PD or when only a small volume of bone is effected. In such instances bone AP will be more helpful 26,32. Bone AP was lower than normal in our case. Considering the fact that we have a solid histopathological diagnosis in hand despite a low bone AP value, even bone AP may not be all that specific as a screening test for PD. Urinary deoxyprydinoline and pridinoline levels are other sensitive markers and are extremely important in the diagnosis of PD 27,28,33. More recent biomarkers include Type I collagen and N-terminal telopeptides 2,9. Nevertheless reaching or excluding the diagnosis of spinal PD is not foolproof 9.
The histopathological picture of PD is characterized by increased number of multinucleated osteoclasts and increased bone resorption which occurs as a result of this. The reason as to why osteoclasts increase in number is still not known 29. During the initial “lytic phase” the osteoclasts dominate the bone and they cause increased bone resorption. The “intermediate phase” that follows this is characterized by increased absorption and increased new bone formation. The bone that is formed at this stage is not real bone and the resultant picture is the pathognomonic “mosaic pattern” 28,30,32. At this second stage, the bony channels are wider than normal, the bone is relatively fragile but at the same time extremely vascular 32. Then comes the third stage also known as the “sclerotic stage” during which osteoblasts predominate the picture 28,30,32.
The plain radiographic appearance of PD include increased bone density, increase in thickness of the involved bone, deformity of the involved bone, cortical thickening and increased trabecular pattern 4. PD of the spine cause expansion and thickening of the involved vertebrae and this appearance is called ‘’picture frame’’. This typical image is usually seen during the second phase also known as the intermediate phase 28. During the sclerotic phase the plain x-rays show the “ivory vertebrae” image. At this stage the vertebra appears radioisodense 28,31. Using plain x-rays or even CT as the diagnostic tool, cases are usually diagnosed during the third sclerotic stage 20. However one must bear in mind that “ivory vertebra” is not specific for PD. Lymphoma, osteosarcoma, metastatic prostate and metastatic lung cancer may also cause vertebral involvement appearing as “ivory vertebra” 28. On MR scans, different stages of spinal PD appear with different signal characteristics 23. During the lytic phase, the involved vertebra shows a lower signal intensity on T1-, and a higher intensity on T2-weighted MR images than the surrounding normal vertenbra. It is the marrow of the involved vertebra that is replaced by fatty deposits, sclerosis, granulation tissue and/or edema that typically appear abnormal on the MR scans 23,35. During the early sclerotic phase, the vertebra will appear hypointense on both T1- and T2-weighted sequences 34. Overall, MR is sensitive for the metabolic changes within the bone yet is not specific for PD, thereofore we believe that a bone biopsy may sometimes be the only other alternative for an unequivocal diagnosis of PD.
The principal treatment of PD is medical. Agents useful in the medical treatment of PD are inhibitors of bone resorption 32. Calcitonin, etidronate, pamidronate, alendronate, tiludronate, residronate, plicamycin (mitramycin) and gallium nitrate has all been found effective 28,32. Medical treatment with calcitonin ve bifosfonates has even been found beneficial for spinal PD associated with spinal stenosis 14. Some reports even suggest that medical treatment may be equal to if not superior to surgical decompression of the stenosis (8,36). One reason as to why surgery may not be a popular treatment method for spinal PD (except for biopsy) is probably the advanced age and relatively poor physical condition 22 or relatively high mortality (11%) and reoperation (9%) rates associated with surgery 24. Recurrence or exacerbation of PD or even malignant degeneration may occur and that is why these patients may need years of follow-up 5,28,32.
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