A 46-year-old female with incidental finding of bone lesion of the right humerus on a chest x-ray completed in December 2022. The patient had no history of fractures or injury before presentation. The patient was thought to have osteoarthritic changes in the right shoulder and received a steroid injection in the joint space approximately 5 to 6 years ago. The patient’s pain was thought to be work-related as she was a housekeeper. The multidisciplinary tumor board consensus was a diagnosis of melorheostosis with unusual features warranting monitoring for changes. MRI dated in January 2023: Case was discussed with the radiologist who recommended 3-month and 6-month follow-up with x-rays and MRI without contrast to look for consistency of the changes. A biopsy was recommended with changes in follow-up imaging studies. However, if follow-up imaging studies showed stable changes then it was recommended by the tumor board to continue monitoring without aggressive treatment. The multidisciplinary tumor board consensus was a diagnosis of melorheostosis with unusual features warranting monitoring for changes.

Figure 4: X-ray of right humerus showing classic “Candle wax dripping” representing cortical breakthrough with periosteal reaction.

 Figure 5: MRI of right humerus of 46-year-old patient showing large area of sclerosis extending from humeral head to the proximal humeral diaphysis. Radiologist concluded that the findings were consistent with nonaggressive process representing melorheostosis.

Figure 6: Bone scintigraphy of 46-year-old patient. Findings showed intense focal increased activity along the medial right humeral head.

Both patients presented opted to be treated conservatively and with as-needed analgesia as the symptoms were not severe enough to warrant further intervention. Since the radiographic evidence of the condition was pathognomonic, further histopathology and biopsies were not obtained.

Discussion

More commonly known as Candle Wax Bone Disease is a benign sclerosing bone disease that is essentially characterized by a build-up of cortical bone deposition. This is an extremely rare condition with an approximate prevalence of 0.9 per million [1]. This rare condition was initially identified by French neurologists Andre Leri and J. Joanny in 1922 [2]. A study completed in 2021 analysed various case reports and noted that the average age of presentation was 16.3 ± 14.1 years. However, there have been previous case reports where patients had presented in adulthood, which was consistent with the cases presented in this article [3]. The etiology of melorheostosis is poorly understood and limited secondary to lack of prevalence of the condition. However, there have been two dominating theories to explain etiology and pathophysiology. The first theory was described by two surgeons, Murray and McCredie, who suggested that the conditions result via insults during embryogenesis to neural crest cell segments, which can result in segmental sensory lesions. The theory was formulated by the surgeons in 1979 after observing sclerotome distributions [1,4]. The second theory was presented in 1995 by a French geneticist, JP Fryns, who suggested the condition was secondary to post-zygotic mutations identified in the mesenchyme. Fryns noted that these mutations resulted in the involvement of various skeletal structures with variable and asymmetric vascular and hamartomata’s changes in overlying soft tissue [1,5]. Melorheostosis can occur as an isolated finding or in conjunction with a few conditions such as Buschke-Ollendorff Syndrome and osteopoikilosis. While the condition has been associated with the somatic MAP2K1 variant, there have not been any confirmed germline or somatic mutation variants in all case reports of isolated melorheostosis [6]. Both patients discussed in this article presented as isolated cases of melorheostosis. Cases in this article were not tested for any of the variants previously identified in case reports, limiting correlation with mutations and variants. Melorheostosis can present in various ways. The condition is essentially asymptomatic, however, it is normally identified when patients start to develop symptoms. The main complaint that patients present with is pain secondary to mass effect localized to soft tissues around the disease process [3,7]. Patients can also present with pathologic muscle contractures, joint stiffness, altered bone structure with deformities, and limb discrepancies.7 In our cases, both individuals had presented with localized pain where the disease process was taking place. Case 1 patient had interestingly presented with notable lumps as well along with the pain, which is consistent with previously reported symptoms of this condition. Although the case 2 patient was noted to have melorheostosis as an incidental finding, she was noted to have long-standing pain. The condition can present itself by affecting one bone (monostatic form) or multiple adjacent bones (polyostotic form) with varied distribution. A case series completed in 2016 showed that lower extremity bones were more common with an incidence rate of

66.6% with upper extremity bones having an incidence rate of 33.3% [4]. Interestingly enough, case 1 patient had presented with involvement of multiple lower extremity bones with polyostotic form. On the contrary, the case 2 patient presented in a monostatic form with involvement of upper extremity bones. Historically, the diagnosis of melorheostosis is achieved only through imaging studies. Since melorheostosis has many polymorphic features similar to other diseases, oftentimes additional imaging studies are needed [3,7]. The condition has a very pathognomonic finding of a “dripping candle wax” appearance on CT scans and X-rays. This appearance is secondary to the disease process extending beyond the cortex, more typically in a craniocaudal pattern. CT scans normally show hyperostotic lesions along with scattered areas of sclerosis and cortical thickening [7]. Although the “dripping candle wax” appearance is pathognomonic, melorheostosis has previously presented differently in imaging studies. Atypical cases have been noted to present with osteopathic striate-like, myositis ossificans-like, osteoma-like, and/or with mixed patterns [3]. A study from 2021 analyzing 15 case reports showed 53.3% of patients had cortical thickening with 40% showing endosteal radiographic patterns [3]. Figures 1 and 4 are x-ray imaging that showed the typical appearance of “dripping candle wax”. X-rays were suspicious of melorheostosis and further imaging studies were obtained. MRI is not routinely utilized in formulating a diagnosis of this condition. However, MRI can show the extension of the disease process into bone marrow, especially in forms that present with endosteal findings.7 MRIs can show cortical hyperostosis of low signal intensity [1]. Figures 2 and 5 represent MRI finding impressions by the radiologist. Images were consistent with sclerosis findings with low signal intensity. For more radiographic evidence, bone scintigraphy (nuclear medicine bone scan) can be a useful tool as well. A nuclear bone scan is helpful to identify metabolic activity in skeletal lesions. Bone scintigraphy would show increased radiotracer uptake and helps dictate whether surgical intervention is needed [7]. Figures 3 and 6 represent the bone scintigraphy scans in each patient showing increased radiotracer uptake. Case 1 patient was noted to have classic cortical thickening of the femur, knee, and right foot on radiographs which is consistent with the pathognomonic finding. Further studies with bone scintigraphy evaluating case 1 patients also showed increased focal radiotracer activity which is also reportedly a typical finding. Case 2 patient’s initial imaging with x-rays showed unusual and atypical features. Hence, an MRI was obtained which showed and confirmed the diagnosis of melorheostosis with unusual findings of endosteal sclerosis.

In some cases, radiological images are not enough, and additionally, bone biopsies are necessary to make a definitive diagnosis. There have been several pathognomonic histologic features that have been identified across the previously studied patients. The key identified variants in histopathology are mainly cortical density with woven bone features and fibrotic changes within the bone marrow space [6,7]. There have also been reports of hypervascularity with an increased number of Haversian systems and irregular bone growth into the medullary cavity of the bone [7]. A small 15-patient cohort study reported that cortical density with woven bone features was prominent in 60% of the patients analyzed [3]. Although histopathology reports were not obtained in the patients discussed in this article, it was highly recommended by the radiologists to further confirm the diagnosis of melorheostosis. The treatment is dictated by the extensive spread of the disease along with symptoms. The condition normally presents asymptomatically.4 However when symptoms do present, a consideration has to be made to the level of treatment as there are no current guidelines by medical boards for standard treatment. Essentially the treatment is divided into surgical and non-surgical categories given the depth of melorheostosis. Since pain is the mainstay of complaints in the majority of patients, targeting analgesia is important with the goal of relieving pain and restoring range of motion [1,8]. A step-up in treatment approach is normally preferred starting with basic physical therapy and NSAIDs or acetaminophen [8]. There have been experimental approaches to treatment as well with the use of monoclonal antibodies, and systemic bisphosphonates [8,9]. The theory behind the use of bisphosphonates relates to inhibition of the proinflammatory markers resulting in apoptosis of osteoclasts leading to decreased bone turnover. Reports with the use of zoledronic acid have shown significant improvement in symptoms, however imaging studies remained unchanged [8]. The use of RANK-L monoclonal antibody, denosumab, has also been experimented with as well with a similar mechanism of action to bisphosphonates and the idea to halt bone turnover [9]. A study from 2018 analysed the biochemical markers and imaging studies in individuals with denosumab intervention. The results showed that cortical thickening and multiple biochemical markers, such as osteocalcin and collagen, remained stable without any further progression.9 In our cases, the patients were all approached based on the level of pain experienced by them. Since the patients had minimal pain, through shared decision-making they opted to continue with asneeded analgesia with continued monitoring of symptoms. Other alternative therapies including nerve blocking, serial casting, cortical thickening [7]. The other end of the spectrum of treatment includes surgical intervention to correct the bone remodelling and for cosmetic reasoning as well [1,6]. Unfortunately, as of now there are no curative measures for the condition.

Conclusions

In summary, melorheostosis represents a rare class of benign sclerosing bone disease that can present as an isolated finding of in conjunction with other rare conditions. There needs to be a high threshold when considering melorheostosis as a diagnosis as the disease prevalence is less than 1 in a million. However, when the condition is suspected, imaging studies help guide and confirm the diagnosis. There can be various presentations of the condition on imaging as highlighted in this article. Since imaging studies are the mainstay method of diagnosis, pathology does not play a crucial role; although, it may aid in further solidifying the diagnosis. Treatment of the condition is highly dictated on an individual basis with the goal of minimizing the pain. With further case reports and experimental treatments, the hope is to educate clinicians about this rare condition with the eventual goal of finding a cure as currently none exists.

Conflict of Interests: Authors declare no conflict of interests.

Funding: This publication was made possible through a grant from the Flinn Foundation

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