Pseudarthrosis-a pictorial essay

Pseudoarthrosis rarely presents as a diagnostic problem. However, since there are several etiologies, the specific cause may be difficult to identify. Radiologically, five morphological forms exist, although there is some overlap of these. The major form is hypertrophic and the other one is atrophic. The radiological characteristics are described illustratively. Often the etiological factors reflect on radiological presentation. Pseudo joint formation also occurs at transitional vertebrae of the lumbosacral spine, between spinous processes of L4-L5 and in late stages of congenital dislocations of hip.

introduction Pseudoarthrosis is generally defined as a joint formation due to failure of bony union between two segments of a tubular bone away from the joints. It is an abnormal union formed by fibrous tissue between parts of a bone that has fractured. It can be spontaneous due to congenital weakness. The clinical presentation of pseudoarthrosis most often includes pain, swelling and false movement.

Imaging findings and review of the literature
Conventional radiology is adequate most of the times. Rarely the other newer modalities are adopted, particularly when complications occur.
Pseudoarthrosis usually follows a fracture, whether traumatic or otherwise, with no attempt at repair. Non union is permanent failure of healing following a broken bone unless intervention (such as surgery) is performed. A fracture with non union generally forms a structural resemblance to a fibrous joint, and is therefore often called a "false joint" or pseudoarthrosis (the Greek term "pseudo-" means false and "arthrosis" means joint).
The causes for non-union include: avascular necrosis, abnormal alignment of the fractured fragments, infection, particularly osteomyelitis, immobilisation and soft tissue interposition between the two fragments.
It occurs in both adults and children. The tibia and fibula, usually the lower parts are most commonly affected. Less often the upper third of the femur, the upper and middle thirds of the clavicle and humerus and the lower part of the forearm are involved [1].

congenital [1, 2]
Congenital pseudoarthrosis of the tibia is the most common entity in children. The etiology cannot always be established. Congenital pseudoarthrosis is usually apparent shortly after birth and is rarely diagnosed after the age of two. About 50% of cases are associated with neurofibromatosis type 1 (NF1).
Overall, 10% of patients with NF1 are diagnosed with tibial pseudoarthrosis. Fibular, ulnar, and radial pseudoarthrosis can also occur, but much less frequently, and usually only in patients with NF1. Radiological features include: progressive bowing, resorption of a short segment of bone and angulation at the site of resorbed bone.
It is likely to be a primary periosteal disease and secondary bone disease. The pathobiology demonstrates increased osteoclasis by the surrounding fibrous hamartoma and reduced osteogenesis and morphogenic protein production by the bone. This leads to a loss of remodelling potential and gradual bowing and atrophy of the bone with eventual fracture of the tibia and/or fibula ( Figure 1).

Osteogenesis imperfecta [3]
It is a non sclerosing skeletal dysplasia and constitutes four forms. Fragility of bones, thinning of the bones, bidding of the bones and fractures are common ( Figure 2). Pseudoarthrosis occurs when the fractures do not heal properly.

Fibrous dysplasia [4-6]
It is a mixed skeletal dysplasia where both lytic and sclerotic changes occur due to aberrant fibrous tissue formation in skeleton. It can be monostotic, polyostotic and McCune-Albright Syndrome (MAS). Pseudoarthrosis is occasionally encountered in polyostotic fibrous dysplasia and MAS ( Figure 3).
Type I -anterolateral bowing with medullary sclerosis.

2.
Type II -"failure of tubulation," with constriction or narrowing of the cortical diameter.

Osteofibrous dysplasia [5, 6]
The association between congenital pseudoarthrosis and osteofibrous dysplasia of the tibia and fibula is a rare entity that has been recently recognized. Histopathological examination confirmed the diagnosis of congenital pseudoarthrosis of the tibia and fibula with underlying osteofibrous dysplasia involving both bones (Figure 9). Immunohistochemical stains showed cytokeratin positivity.

Bertolotti syndrome [15, 16]
A false joint may be formed at the lumbo sacral junction with transitional vertebrae. The transverse process of L5 articulates with the ala of the sacrum and forms a joint. It could be unilateral or bilateral ( Figure 13). Bertolotti syndrome is where the young patient complains of back pain with transitional vertebrae and is characterised by the presence of a fifth lumbar (L5) vertebra anatomic variation with a large transverse process that forms a pseudoarthrosis or a bony fusion with the sacrum or iliac crest.    ankylosing spondylitis [17,18] A well-known complication in patients with ankylosing spondylitis (AS) is the development of localised vertebral or discovertebral lesions of the spine (Figure 14). This was first described by Andersson in 1937. Since then, many different terms are used in literature to refer to these localised lesions of the spine, including the eponym ' Andersson lesion' (AL). The use of different terms reflects an ongoing debate on the exact aetiology of the AL. AL may result from inflammation or stress fractures of the complete ankylosed spine. There is no evidence for an infectious origin. Regardless of the exact aetiology, a final common pathway exists, in which mechanical stresses prevent the lesion from fusion and provoke the development of pseudoarthrosis. The diagnosis of AL is established on conventional radiography, but computed tomography and magnetic resonance imaging both provide additional information.

Conflicts of interest
Author declares no conflicts of interest.