Current facts on para-osteo-arthropathy (POA)

Neurogenic heterotopic ossification in spinal cord injury

“…63 ± 65 The mineralization process of the soft tissues consists of an amorphous calcium phosphate phase, which is gradually replaced by enlarging hydroxyapatite crystals. 48 The centripetal pattern of maturation that is seen in the following weeks is the basis of the zone phenomenon described by Ackerman. 66 The zone phenomenon is characterized by a thin outer zone in the surrounding muscle that encloses a broader intermediate zone.…”

“…34 ± 36,48,62 Muscle ®bres are not primarily involved in the process, but can be incorporated in or compressed by the ®brosing and calcifying soft tissues leading to local muscle necrosis. 8,34,48 NHO begins as an area of oedematous, in¯amma-tory reaction coinciding with an increased blood¯ow in the a ected soft-tissues. 48 First, an exsudative cellular in®ltration is seen, then ®broblastic proliferation occurs followed by osteoid formation, and subsequent deposition of bone matrix.…”

“…8,34,48 NHO begins as an area of oedematous, in¯amma-tory reaction coinciding with an increased blood¯ow in the a ected soft-tissues. 48 First, an exsudative cellular in®ltration is seen, then ®broblastic proliferation occurs followed by osteoid formation, and subsequent deposition of bone matrix. Primitive osteoid is deposited as small masses within an area of ®broblastic mesenchymal reaction early within the ®rst 2 weeks at ®rst in the periphery.…”

“…71,72 and consists of cancellous bone with Haversian canals, cortex, blood vessels and bone marrow, although with a minor amount of hematopoesis. 34,36,39,48,73 According to Chalmers et al 74 three conditions must be met for the formation of ectopic ossi®cation: the presence of osteogenic percursor cells, an inducing agent, and a permissive environment. Although the precise causal mechanism for NHO is still unknown, humoral, neural and local factors probably all play a role in its pathophysiology ( Figure 1).…”

“…1,3,36,46,48,73,86 Indeed, several studies have shown that the initial stage of NHO is characterized by local microvascular alterations such as an increased vascularity, venous haemostasis and arterio-venous shunting in the involved tissues. 36,48,87 These modi®cations in the blood perfusion and oxygen levels of the soft tissues might play a critical role in the formation of NHO, 36,87,88 although it remains unclear whether these changes are secondary to or a causal factor in the NHO. Whether a disruption of the neuro-immunological pathways could result in an abnormal balance between osteoblast and osteoclasts and the subsequent induction of NHO is also still hypothetical.…”