The spine in Paget’s disease

C. Dell'Atti V. N. Cassar-Pullicino R. K. Lalam B. J. Tins P. N. M. Tyrrell Paget's disease (PD) is a chronic metabolically active bone disease, characterized by a disturbance in bone modelling and remodelling due to an increase in osteoblastic and osteoclastic activity. The vertebra is the second most commonly affected site. This article reviews the various spinal pathomechanisms and osseous dynamics involved in producing the varied imaging appearances and their clinical relevance. Advanced imaging of osseous, articular and bone marrow manifestations of PD in all the vertebral components are presented. Pagetic changes often result in clinical symptoms including back pain, spinal stenosis and neural dysfunction. Various pathological complications due to PD involvement result in these clinical symptoms. Recognition of the imaging manifestations of spinal PD and the potential complications that cause the clinical symptoms enables accurate assessment of patients prior to appropriate management. - Pagets disease (PD) or osteitis deformans is one of the commonest metabolically active bone diseases, second in prevalence only to osteoporosis, characterised by a disturbance in bone modelling and remodelling because of an increase in osteoblastic and osteoclastic activity. It is more common in people of Anglo Saxon origin and is less common in the Far East, India, Middle East and Africa. The overall prevalence of PD is 33.7% and increases with age [13]. By the age of 90 years, the prevalence increases to about 10% [2]. As the aetiology of PD continues to be the subject of debate, it is variously classified as an infection, metabolic disorder and a neoplastic process [1, 415]. However, strictly speaking, as the unaffected skeleton is metabolically normal, it does not fulfil the criteria for a true metabolic bone disorder. The epidemiology of PD shows some significant changing trends in the clinical profilometry. First, recent studies have demonstrated that the incidence and prevalence of PD is gradually declining [1618]. Second, there is a general trend towards reduction in the severity of the disease as measured by serum alkaline phosphatase levels [19, 20]. Third, there is a steady increase in the age at presentation by about 4 years per decade [18] and last, the proportion of patients with monostotic disease is increasing [18]. This in turn means that we are likely to see a higher incidence of monostotic vertebral involvement in the future. The spine is the second most commonly affected site (53%) [6], after the pelvis (70%) [2123]. The disease is polyostotic in 66% of cases and between 35% [24, 25] and 50% [7, 8] have spinal involvement. Using multimodal advanced imaging, this review article focuses on the pathological processes that underlie the varied spinal manifestations and complications of PD. In PD the loss of homeostatic control leads to increased osteoblastic and osteoclastic activity and constitutes the background for the main three phases. The initial lytic phase represents a mainly osteoclastic activity, the late osteoblastic phase is characterised by new bone formation, while the intervening mixed phase is seen when there is a combination of osteoblastic and osteoclastic activities [24, 26]. One more phase, inactive sclerotic phase, characterised by normal or decreased bone activity, has also been described [5], when the stimulation of new osteoblast and osteoclast formation ceases. Although the bone is metabolically inactive, it maintains a sclerotic coarsened architecture [27]. In bones with a low trabecular/cortex ratio like the skull, femur and humerus, the early lytic phase is radiologically depicted by a clear leading edge at the interface with normal bone. The lytic phase is usually not detected in bones with a high trabecular/cortex ratio like the vertebra, sacrum and pelvis [28]. Vertebral body involvement at radiological diagnosis is virtually always complete, and therefore the leading edge present in the other affected bones is not seen in the vertebra [29]. These phases can be evident in the same patient and at the same time in different bones including the vertebral column. Although progression of disease occurs within an affected bone, the sudden appearance of bone involvement at new skeletal sites years after the initial diagnosis is uncommon [30, 31]. The pathomechanisms and the dynamics involved in bone remodelling in PD of the appendicular and axial skeleton at the periosteal and endosteal surfaces have been previously described [3, 5, 32]. The enhanced abnormal osteoblastic activity results in periosteal and endosteal new bone formation (apposition). The abnormal osteoclastic activity on the endosteal surface results in bone resorption (absorption). The various combinations of these mechanisms give rise to four different patterns of bone remodelling at the periosteum/endosteal interface leading to bone enlargement: periosteal and endosteal apposition; periosteal apposition and endosteal absorption; periosteal apposition with normal endosteal surface; and focal periosteal apposition pumice stone appearance (Figs. 1, 2). The mechanisms in the spine commonly responsible for the changes on the periosteal and endosteal surfaces of the vertebral body and posterior neural arches are varied (Figs. 1, 2, 3, 4). These various mechanisms are not exclusive of each other, but can occur in combination in the same vertebra at various borders. Usually, one of the pathomechanisms predominates in the involved vertebra. The most frequent mechanism of vertebral body expansion is periosteal apposition and endosteal absorption. The new bone formation predominates on the periosteal surface and it is responsible for the vertebral body enlargement, while the absorption on the endosteum results in an increased bone marrow space. Periosteal/endosteal apposition and periosteal apposition with normal endosteal surface are two less common remodelling mechanisms seen in the vertebral body. In both, the apposition on the periosteal side results in vertebral body enlargement, but the bone marrow space is decreased or normal in size respectively. The least common mechanism of vertebral body expansion is focal periosteal apposition giving the pumice stone appearance (Figs. 1, 2). Expansion of the vertebral bodies seen radiologically occurs in 63% of cases [25]. The commonest mechanisms in neural arch involvement are a combination involving periosteal and endosteal apposition or periosteal apposition and endosteal absorption (Figs. 3, 4). In both of the mechanisms, the periosteal apposition causes a decrease in the size of the spinal canal resulting in spinal stenosis. Imaging manifestations Pagets disease of the spine can either involve a single level or more than one level. The lumbar spine and more commonly the L4 and L5 levels are the most frequently involved sites (58%) [33], more so than the thoracic (45%) and the cervical vertebrae (14%; Fig. 5) [8, 23]. Involveme (...truncated)