Simultaneous Video Assisted Thoracoscopic Costal and Femoral Osteochondromas Resection in a Child with Multiple Exostoses Disease: A Case Report and Literature Review

The case reported concerns a 5-years-old child affected by Hereditary Multiple Exostoses (HME), a genetic disorder with autosomal dominant inheritance, that cause multiple skeletal exostosis. Usually those masses are asymptomatic, but in some cases, they can cause a huge spectrum of problems due to their size and site localization interfering with surrounding structures such as nerves, vessels, or organs. In order to reduce patient distress and hospital stay we resected in the same surgical session, an exostosis on the left knee (that was interfering with the correct bone growth) and, with video-assisted-thoracoscopy (VATS), three rib exostosis (one of them misdiagnosed at the MRI examination).. Temporary femoral hemiepiphysiodesis was performed too


Introduction
Osteochondroma, also called exostosis, is the most common cartilaginous tumour, which is estimated to account for 20 to 25% of all benign bone tumours. The most frequent localization is in the long bones: distal femur, proximal humerus and proximal tibia. It originates from the metaphysis but, with skeletal growth, it tends to move toward the diaphysis. Only about 3% of osteochondromas occur in vertebrae and ribs; among them, 70% are accompanied by hereditary multiple exostoses (HME) [1]. Solitary osteochondromas occur sporadically and arise from a defect in the perichondral ring (en coach of Ranvier) or might be secondary to a traumatic insult.
The exostosis (i.e.) are multiple in the hereditary multiple exostoses (HME). HME is a genetic disorder with an autosomal dominant inheritance and a variable penetrance involving EXT gene family.
Most common mutations involve gene EXT1 on chromosome 8 and gene EXT2 on chromosome 11. Multiple skeletal lesions are usually diffused and relatively symmetrical and typically, osteochondromas involve the bone circumferentially, mostly surrounding the metaphyseal regions, causing swelling and sometimes limiting joint motion. In severe forms, limb shortening, and deformity are associated.
The prevalence of HME in European populations is about 1 case in 50,000 people. The exostoses usually present as painless masses, however sometimes they can cause problems due to interference from surrounding anatomical structures, such as reactive tenosynovitis, nervous or vascular compression, and if located in the ribs even haemothorax or pneumothorax caused by damage to the pleura. Malignant transformation in chondrosarcoma is more common in the setting of multiple hereditary osteochondromato-sis (about 4% risk) [2]. We reported a 5 years old child affected by HME presenting three costal osteochondroma successfully treated by video-assisted thoracoscopic surgery (VATS). In the same surgical session was also removed a large femoral exostosis associated to a routinely emiepiphysiodesys in order to correct secondary valgus of the knee.

Case Presentation
A 5 years old child, affected by HME, came to our observation for two great exostoses respectively localized at the 2nd rib of left hemithorax and at the left distal femur (where a secondary valgus knee was quickly worsening) (Figures 1a-1d). The patient, asymptomatic and free from other illnesses, was candidate for surgery because of the size and the potential risk of malignant transformation of the costal and femoral exostoses and for the correction of the valgus deformity of the knee. The removal of the femoral exostosis was decided also to prevent future major surgery conditioned by the close relationships of the exostosis with the regional great blood vessels.MR Angiography of the lower limbs and thorax MR were performed preoperatively (Figures 1e-1g); these studies showed three major exostosis: one of at the left femur  a.
x-ray chest view of 5 th rib endothoracic exostosis b.
x-ray chest view of external 2 nd rib exostosis and  At first the lateral femoral exostosis was isolated and then resected through a large lateral surgical approach. Subsequently, through a distinct little medial access, a routinely medial emi-epyphisiodesis of the distal femur was performed using Orthofix®'sEight-Plate®.
At the end of the femoral procedure the patient was positioned on left side decubitus to allow the general pediatric surgeon to perform Video-Assisted Thoracoscopic Surgery (VATS). Through a 5mm trocar introduced close the lower margin of the right chest, after insufflation, the osteochondroma localized at the inner side of the right 5th was identified (Figure 1h). At this level the rib was exposed by an open access and then resected en bloc with the exostosis, checking with the thoracoscopy for unintentional pleural damage.
During the procedure we found another misdiagnosed endothoracic osteochondroma, located at the internal side of the 3rd right rib (Figure 1i), that was removed in the same way. Then, we checked with thoracoscopy for abnormal bleeding, without find-

Discussion
Osteochondromas are the most common benign bone neoplasms. They occur in 3% of the general population and account for more than 30% of all benign bone tumours and for 10%-15% of all bone tumours. Exostosis can occur as a sporadic solitary lesion with no hereditary link in 85% of cases, otherwise may be multiple in the context of hereditary multiple exostoses (HME) [3,4]. HME is a rare autosomal-dominant paediatric musculoskeletal disorder with an incidence of about 1:50000 characterized by significant number of osteochondromas that form next to the growth plates of many skeletal elements, including long bones, ribs and vertebrae [5]. HME cases are associated with heterozygous loss-of-function mutations in EXT1 and EXT 2 genes which are involved in heparan sulfate synthesis.
Although in most cases exostoses are asymptomatic, in some children they may grow quickly reaching remarkable dimensions and causing mechanical pressure on the adjacent anatomical structures. Moreover, because of their interference with the growing activity of the physis, they may cause growth retardation, deformities and impingement of surrounding anatomical elements (including other bones, spinal cord, muscles, tendons, blood vessels and nerves).For these relevant functional and cosmetic problems surgical removal is recommended in selected cases and localizations [5]. Moreover it should not be underestimated that, in patients with HME, there is the possibility of malignant degeneration to chondrosarcoma (reported to be about 4%) [2] or, more rarely, other malignancies that can be life threatening because of their resistance to chemo-therapy or radiation therapy.
Apart malignant degeneration, chest complications of thoracic osteochondromas are rare, but cases of haemothorax and pneumothorax consequent to costal exostosis with HME have been reported in literature [6,7]. The aetiology of the damage is either the shearing of pleura or diaphragm by the intrathoracic extrusion of costal exostosis, or focal pleural changes induced by the friction and subsequent inflammation between the pleura and the osteochondroma [8]. To our knowledge, this is the first case % of patients with HME) and in our opinion is best to reduce that possibility in such a complex site [2].
In addition, in our 5-year-old patient, the left distal femur osteochondroma was interfering with the growth and with the alignment of the left lower limb so it was necessary to perform the exostosis excision and the emi-epiphysiodesis. Moreover, the benign tumour was close to the femoral artery and by removing it our proposal was to avoid the risk of future vessels compression and potential secondary pseudo-aneurysm due to growth of the exostosis [16,17]. Temporary emi-epiphysiodesis is a wellknown surgical procedure able to correct valgus knee in skeletally immature patients and in particular it isn't the main topic of our case presentation. In literature patients with HME frequently have a high rate of knee deformity, mainly a genu valgum deformity [18].
Few studies reported the results of emi-epiphysiodesis in patients with HME and correlated genuvalgus deformity, but according to Kang [20].

Conclusion
Even if the treatment of young patients affected with HME is not mandatory, is still worth to take into consideration the possibility to perform preventive surgery if the masses are of considerable size and/or in dangerous positions (near vessels, nerves or internal organs). In our experience in case of multiple localizations performing surgery in one stage session should be preferred to reduce the patient distress and hospital stay.
Pre-operatory thoracic MR can play a necessary role in studying potential malignant lesion and in the surgery-planning in case of ribs exostoses, helping to determine their position, size, connection with their surroundings, and most importantly showing undetected masses. Moreover, to assure the best outcome, it's strongly recommended a multidisciplinary approach performing the excision of the exostosis localized at the internal side of the ribs using VATSby the help and collaboration of the pediatric surgeon; this can help to prevent complications such as accidental lesions, and minimize intra-operatory risks linked to thoracic surgery.