simple bone cyst or unicameral bone cyst ( ubc ) are benign cystic lesions commonly found in femur and humerus . however hand is a very rare site of occurrence . a 7 year old right hand dominant girl presented to us with chief complaints of pain and swelling in right 4th metacarpal since 2 month . on imaging , plain radiographs of right hand showed expansile lytic lesion on metaphyseal - diaphyseal region of 4th metacarpal with pathological fracture . mri showed cystic lesions with internal loculations and fluid - fluid levels ( fig 2 ) . we performed aspiration which showed serosanguinous fluid with haemorrhagic tinge . with the diagnosis of unicameral bone cyst in mind we performed and closed intramedullary nail with k wire . however it should be considered as important differential since it warrants simple treatment and extensive procedures should be avoided . simple bone cyst or unicameral bone cyst ( ubc ) are benign cystic lesions seen commonly in skeletally immature persons . ubc are also seen in tibia , calcaneus , cuboid , lumbar spine and pelvis . very few cases of ubc have been described in hand include metacarpals ; phalanx ; hamate   and lunate . treatment described for ubc of hand commonly involves curettage and bone grafting [ 2 , 3 , 4 , 5 , 6 , 7 ] . we describe ubc of metacarpal in a skeletally immature girl with diagnostic dilemma treated simply with closed k wire insertion . a 7 year old right hand dominant girl presented to us with chief complaints of pain and swelling in right 4th metacarpal since 2 month . swelling was of insidious onset and healed gradually progressive . no history of trauma or infection . , it was a solitary , diffuse and bony hard swelling over 4th metacarpal of right hand which was tender on palpation and no local rise in temperature . the range of movement at 4th metacarpophalangeal joint was terminally restricted due to pain in all directions . patient was earlier seen by various orthopaedic surgeons who investigated the lesion and advised excision of cyst with bone grafting . on imaging , plain radiographs of right hand showed expansile lytic lesion on metaphyseal - diaphyseal region of 4th metacarpal with pathological fracture . mri showed cystic lesions with internal loculations and fluid - fluid levels ( fig 2 ) . we repeated the plain radiograph which showed expansile lesion with pathologic fracture on 4th metacarpal ( fig 1 ) . with these investigations available we performed a biopsy which showed serosanguinous fluid with haemorrhagic tinge . with the diagnosis of unicameral bone cyst in mind we performed and closed intramedullary nail with k wire ( fig 3a ) . the cyst showed signs of healing within a month ( fig 3b ) and k wire was removed . there is no recurrence at 18 month follow up and range of motion is full at metacarpo - phalangeal joint . cystic lesion at meta diaphyseal region with pathological fracture t2w coronal section showing fluid fluid levels in cystic lesion of 4th metacarpal a : immediate postoperative radiograph ; 3b : at 1 month follow up cyst showing signs of healing ; 3c : at 2 month follow up the cyst is completely healed the differential diagnosis of lytic cystic lesion in hand in a skeletally immature patient include unicameral bone cyst , aneurysmal bone cyst , osteoblastoma , giant cell reparative granuloma , enchondroma and intraosseous ganglion cyst . intraosseous ganglion cyst is a benign lesion commonly seen in carpus and presents as expansile lesion . giant cell reparative granuloma is reactive benign lesion classically seen in skull and facial bones and also in small tubular bones of hands and feet . osteoblastoma are benign bone forming lesion commonly found in spine and rarely in hand . on radiograph osteoblastoma , enchondroma , intraosseous ganglion cyst and giant cell reparative granuloma were excluded in our case based on typical histopathology findings . aneurysmal bone cyst ( abc ) presents as eccentric expansile lesion with blood filled cavities . on mri it shows fluid - fluid levels , however this finding is not specific for it . radiograph may show classic  fallen leaf sign  representing a piece of bone due to pathologic fracture . the characteristic fluid - fluid level seen in abc is due to sedimentation of erythrocytes within serosanguinous fluid   this finding is observed only in 60% of cortical abc . the other lesions showing fluid - fluid levels are chondroblastoma , telangiectatic osteosarcoma , fibrous dysplasia , unicameral bone cyst , giant cell tumour , intraosseous ganglion cyst , plasmacytoma and osteomyelitis . thus in our case the lesion can still be either ubc or abc . to further reach the diagnosis we performed aspiration of the lesion . thus with all above finding radiograph , mri and aspiration taken together an informed diagnosis of ubc was made in our case of centric expansile lesion with fluid fluid level and serosanguinous aspirate . treatment options for ubc include observation , curettage and bone grafting , intralesional steroid injection   and recently intramedullary nailing . various theories have been proposed in pathogenesis of ubc like dysplasia within the cyst due to trauma , intraosseous synovial cyst and more recently venous occlusion leading to increased intramedullary pressures . thus leading to conclusion that re - establishing these vascular channels may lead to healing of cyst . use of titanium elastic nails to open the connection between the medullary canal and cyst and break the septae within the cyst have been well documented in treatment of long bones ubc . the cyst showed signs of healing within a month and was completely healed in 3 months with no recurrence till date . however it should be considered as important differential since it warrants simple treatment and extensive procedures should be avoided . unicameral bone cyst of metacarpal although rare should be kept in mind while diagnosing cystic lesion since it requires simple treatment .