Teaching Case

A 53-year-old man without any history of trauma, presented with gradually increasing left-shoulder discomfort and swelling for few months. Clinical examination revealed a prominent swelling on the anterior aspect of the shoulder with discolouration of the overlying skin. Pain was felt on shoulder abduction.

Plain radiograph (Fig. 1) demonstrated conglomerate of multiple calcific densities around the greater tuberosity, with further similar densities projected over the scapula. There was no cortical expansion, bony destruction or periosteal reaction. The glenohumeral and acromioclavicular joints looked preserved. The subsequent CT (Fig. 2) revealed extensive hyperostosis appearing as ‘multiple dollops’ on the anterior surface of the scapular blade, with endosteal extension obliterating the medullary cavity of the scapula blade. There were also large masses of peri-articular calcific densities demonstrating mature cortex in the adjacent soft tissues, seen supero-laterally to the greater tuberosity, suggestive of heterotopic ossification. On MRI (Fig. 3), the dollops of cortical hyperostosis were expectedly demonstrated as low-signal abnormalities. The heterotopic ossified soft tissue masses also appeared as signal void on all sequences of the MR (Fig. 4), and were found to be within and beneath the deltoid muscle. There was surrounding soft tissue oedema, but no collection or haemorrhage. Apart from minimal subacromial bursal fluid, there were no other relevant findings like rotator cuff pathology or another lesion to explain patient’s symptoms.

This case was referred and subsequently discussed at the regional tertiary level soft tissue sarcoma centre. On review of all imaging, it was apparent that there were two separate pathologies at play in this case 1) heterotopic ossification (Fig. 1) which was the likely cause of clinical symptoms and 2) melorrheostosis (Figs. 2, 3) causing the incidental changes at the scapula. Our focus in this case report is on melorrheostosis. Melorheostosis, an uncommon non-hereditary sclerosing mesenchymal dysplasia of the bone, was first reported by Léri and Joanny in 1922 [1]. Aetiology is still unknown, but the pathogenesis is hypothetically attributed to genetic mutation involving the spinal sensory nerves of a sclerotome distribution [2]. Clinical symptoms range from being asymptomatic to having pain, stiffness or movement restriction. Although the radiology can look peculiar on first encounter, it is important to recognise this condition due to its mostly indolent nature. It usually affects the long bones on one side of the body, which on plain film has been classically described as “dripping candle wax”. However, it has also been described in flat bones, including the ribs and spine [3]. However, it is worth noting that there have been five different radiological appearances for melorheostosis described in the literature: 1) classic dropping wax appearance 2) osteoma-like lesion 3) myositis ossificans-like lesion 4) osteopathia striata-like lesion and 5) mixed picture. [4] For the changes at the scapula, differentials of osteoma, sessile osteochondromas, parosteal osteosarcoma and osteopathia striata were considered, but all these were thought to be less likely. Osteomas are predominantly found in the paranasal sinuses. Osteochondromas would have demonstrated cartilaginous cap on MRI, lack of a lucent centre went against paraosteal osteosarcoma and osteopathia striata cause vertical striations in the bone, but do not usually cause cortical hyperostosis. Above all, none of these conditions would have explained the concurrent heterotopic ossification found in this case. Due to the combination of myositis ossificans-like heterotopic ossification in the periarticular region and fairly typical radiological findings in the scapula [5], a diagnosis of melorheostosis was reached. The patient was further followed over a six-month period, during which the symptoms settled down and the radiological findings remained static. The overarching histological feature of melorheostosis is excess bone formation [6]. The long tubular bones most typically show the characteristic appearance of a dripping candle wax possibly because of their orientation, physical shape or gravity, allowing the areas of hyperostosis to drip along the length of the bone. However, for flat bones, instead of dripping like candle wax, flat geometry perhaps acts to reduce the effect of gravity, thereby encouraging the hyperostosis to accumulate like dollops along the periosteal surface of the bones [7]. Without taking the liberty of imagination, we are describing this feature on cross sectional imaging (Figs. 2 and 3) as a ‘dumpling on a plate sign’ (Fig. 5). Such an appearance may also be expected in similarly oriented flat bones. The characteristic melting dripping wax sign is not seen in all patients [8] and it may not be feasible for all radiologists to instantly recall the differing radiological patterns [4] seen in such a rare bone dysplasia like melorheostosis. We hope this sign will equip the radiologist’s memory with an another visually striking and relatable sign, along with the “dripping candle wax”, to help them diagnose this condition. Written informed patient consent for publication has been obtained.