metal - on - metal ( mom ) bearings have been used with increasing frequency by orthopaedic surgeons over the past decade due to potential improvements in wear rates compared to more conventional implants , especially in younger patients . recently published research has however revealed a number of significant complications associated with mom bearings culminating in national uk guidelines for follow - up and investigation in symptomatic patients . there is release of metallic debris which is thought to cause adverse local and distant problems . pseudotumours are inflammatory soft tissue lesions well known to be found surrounding failing mom implants . histological analysis of these masses have found them to be sterile and consisting mostly of macrophages aggregating around metal particles . the relationship between mom bearings and the development of other tumours is less well documented . a recent study published on behalf of the national joint registry of england and wales retrospectively analyzed the post - operative outcomes and cancer risk of patients with mom hip implants compared to the general population . although no link between mom implants and cancer was found , further conclusive studies were recommended on the effect of metal debris exposure . to date there is no current literature suggesting or proving any association between mom bearings and benign peri - prosthetic soft tissue lesions apart from pseudotumour . we present 2 cases , one male and one female , of post operative mom patients presenting with groin masses . these were both fully investigated with a high index of suspicion for psuedotumour . however following histological and radiological analysis , they were diagnosed as a benign lipoma and ovarian cysts respectively . they were both found to be unrelated to the implants . to the best of our knowledge , there have been no previous reports of example of either lesion in a patient with mom tha . a 65-year - old man was referred to our tertiary centre with an 18-month history of worsening severe right hip pain and an audible grinding . the patient had previously undergone uncomplicated bilateral mom thas ( corail / pinnacle ultramet , depuy , warsaw , indiana , usa ) for osteoarthritis in 2005 ( right ) and 2006 ( left ) at a local hospital . both total hip replacement 's had a 36  mm femoral head component with articular surfaces consisting of cobalt - chromium - molybdenum ( cocrmo ) . in addition to the pain , he complained of a large ipsilateral soft tissue groin mass which was enlarging , particularly over the past 6 months . the patient was otherwise well with no history of other masses , weight loss or night sweats . examination revealed normal vital signs and an antalgic gait with a well healed posterolateral surgical scar over the right hip . there was a large compressible lobular soft tissue mass in the right groin distal to the femoral pulse . right hip range of movement was significantly limited due to pain throughout and irritability on passive rotation . plain radiographs of the pelvis and hip showed a right mom tha in a satisfactory position with no features of loosening , osteolysis or wear ( fig . the white cell count was 8.4    10/l , erythrocyte sedimentation rate 7  mm / h and c reactive protein serum metal ion levels were elevated : chromium ( cr ) 10  ppb ( normal range 00.5  ppb ) and cobalt ( co ) 12  ppb ( normal range 00.4  ppb ) . metal artifact reduction sequence ( mars ) magnetic resonance imaging ( mri ) identified a 23  cm    13  cm    5.7  cm purely fatty mass extending from the pelvis lying lateral to the distal psoas muscle to the iliopsoas tendon insertion in the proximal thigh ( fig . 2 ) . it surrounded the femoral vessels in the proximal thigh region and also extended between the lesser trochanter and the ischium . based on the investigations the decision to revise the failing mom tha with excision biopsy of the mass was made . the operative procedure was performed under general anaesthesia via two separate incisions : anterior longitudinal groin ( supine position ) for the soft tissue mass and posterolateral hip ( lateral position ) for the revision tha . a large fatty mass approximately 23  cm    13  cm    6  cm in size was found directly over the femoral neurovascular bundle . this was protected and preserved throughout excision ( with a small capsular breach ) with subsequent histological analysis . the revision tha was performed via a standard posterior approach with aspiration of joint fluid sent for microbiological analysis prior to prophylactic antibiotics and capsulectomy . there was evidence of mild metallosis and with no significant local soft tissue reaction or muscle damage . the hip was dislocated and the cocrmo head removed showing evidence of wear on the trunnion and taper . the femoral and acetabular components were well fixed with no obvious signs of infection or osteolysis . the femoral component was found to be in 10 of anteversion and the neck shaft angle was 135. the decision was made to retain the femoral component . the acetabular component was in 25 anteversion and 45 inclination , however it was removed using an explant ( zimmer , warsaw , indiana , usa ) without significant residual bone loss . a pinnacle 52  mm uncemented press - fit porous coated cup with a marathon 52/32  mm 10 lipped polyethylene liner ( depuy , warsaw , indiana , usa ) were inserted . a bioball taper adapter ( wg healthcare , lechworth , hertfordshire , uk ) was applied to the trunnion of the retained femoral stem with a 32  mm biolox delta ceramic head ( zimmer , warsaw , indiana , usa ) a stable reduction was achieved . the post - operative course was uncomplicated and the patient was discharged after 5 days of inpatient stay . he was reviewed in clinic 8 weeks post operatively with only mild hip discomfort and mobilising the aid of one crutch . the surgical wounds had healed well without evidence of local recurrence and hip motion was reasonable . he also showed an improvement in his ucla activity score which improved from 3 to 6 . peri - prosthetic tissues and the soft tissue mass were fixed in 10% buffered formalin and processed into paraffin wax using routine histological methods . haematoxylin and eosin ( h&e )  stained sections were examined using a light microscope . the large mass was identified as a lipomatous tumour composed of lobules of well - differentiated adipose tissue separated by fibrous septa ( fig . these features were most consistent with a benign lipoma with no evidence of metallosis . on the contrary , the peri - prosthetic tissue was composed of fibrous tissue with a pseudosynovial membrane and diffuse collections of finely pigmented macrophages containing wear debris ( fig . a 57-year - old lady presented back to our tertiary care centre with new onset right groin pain exacerbated by routine exercise . this patient had undergone an uncomplicated right - sided birmingham hip resurfacing operation on our unit 5 years previously . electric shocks  radiating around to her flank and a worsening bloating sensation in her abdomen . comprehensive clinical examination of her hip , spine , knee and peripheral nerves was unremarkable . plain radiographs confirmed the prosthesis was still in a satisfactory position with no features of loosening , osteolysis or wear ( fig . serum metal ions were elevated : serum chromium ( cr ) 79.5  ppb ( normal range 00.5  ppb ) and cobalt ( co ) 103.6  ppb ( normal range 00.4  ppb ) . due to clinical suspicion of a pseudotumour the team requested a mars mri which revealed a large , well defined lobular cystic mass in the recto - uterine pouch within the pelvis measuring 10  cm    8.5  cm    6.8  cm ( fig . the cystic nature of the lesion combined with calcification present in the inferior wall was consistent with a diagnosis of an ovarian cyst . the imaging also demonstrated evidence of a small fluid collection at the medial and superior aspect of the acetabular component measuring 3.7  cm by 2.8  cm . these findings were consistent with development of a pseudotumour . in light of these results the patient was referred to her local gynaecology service who confirmed the diagnosis of ovarian cyst and removed it along with both ovaries and fallopian tubes within 6 months of initial incidental diagnosis . histological analysis demonstrated a hemorrhagic endometriotic ovarian cyst with haemosiderin laden macrophages and focal calcification . no evidence of atypia or malignancy was seen . on her next attendance to our orthopaedic outpatients department rarer causes have been seen in the literature such as synovial cysts , synovial sarcoma , osteosarcoma and malignant fibrous histiocytoma . the increased use of mom hip bearings became popular due to the advantage of reduced wear compared to metal on polyethylene ( mop ) bearings . however mom bearings can generate up to 500 times more nano - meter sized particles compared to mop bearings . there are various adverse tissue reactions to metal wear debris which include lymphocytosis , vasculitis , and necrosis and pseudotumour formation . pseudotumours are sterile inflammatory masses found in the soft tissues surrounding mom implants . they are often associated with elevated chromium and cobalt levels . bosker et al . performed a prospective cohort study evaluating pseudotumour formation following mom tha and patients with a serum cobalt level  > 5  g / l had a four fold increased risk of developing a pseudotumour . in the uk , the medicines and healthcare regulatory agency ( mhra ) has suggested all patients with mom hip arthroplasty should have cross sectional imaging such as mars mri to detect pseudotumours , which can be described radiologically as solid , cystic or a combination of both . characteristic signal intensity when the solid component of the mass or thickened cystic wall returns a low signal on fluid - sensitive sequences relative to muscle . mri is helpful in determining the nature of soft tissue masses around mom bearings however the diagnosis of a pseudotumour can only be made on a histological examination . examined pseudotumour histology following failed mom hip articulations and found diffuse perivascular infiltrates of t-/b- lymphocytes , plasma cells with high endothelial venules , massive fibrin exudation and accumulations of macrophages / eosinophils . the finding of well differentiated adipocytes confirmed the diagnosis of a very large lipoma . to our knowledge , the finding of a very large peri - prosthetic lipoma in a patient with mom tha has never been reported . they suggested that the occurrence was secondary to a subsynovial layer of supporting tissue consisting of fibroareolar tissue . . reported on a painful total hip replacement due to sciatic nerve entrapment in scar tissue and lipoma . the prosthesis that was revised did not have a mom articulation and the size of the lipoma was considerably smaller ( maximum diameter 4  cm ) . the presence of a soft tissue mass near a mom tha with elevated metal ion levels usually suggests a peri - prosthetic pseudotumour . a failing mom tha generates metal debris however little is known about the risk of developing cancer . performed a meta - analysis comparing the incidence of 28 cancers between those receiving hip or knee arthroplasties and found an increased incidence in prostate cancer and malignant melanoma . a recent review of data from the national joint registry of england and wales did not find a link between mom tha and an increased incidence of cancer diagnoses . nevertheless , imaging characteristics of benign and malignant soft tissue masses can overlap . hence , identification and excision of a peri - prosthetic mass following mom bearings is important for diagnosis and specifically exclusion of infection or malignancy . we therefore recommend that these cases are promptly managed at tertiary referral centres with appropriate expertise and possibly the availability of tumour surgeons . this case demonstrates the importance of considering the large number of differential diagnoses when assessing a patient with a painful groin mass following a mom resurfacing . imaging these lesions early and reviewing the cases in a specialist multidisciplinary team meeting is essential so that an appropriate management can be undertaken we had obtained written and signed consent to publish the case report from the patient .
due to their improved wear rates , metal - on - metal bearings have been increasingly used in the past decade by orthopaedic surgeons carrying out total hip arthroplasty . 
 however there is increasing evidence that there are significant complications associated with such implants . 
 one well documented complication is that of metallic debris leading to pseudotumour formation , however there is less known about associations with other tumours within the pelvis . 
 we present two cases where an intra - pelvic mass in patients with metal - on - metal implants were diagnosed as being of a different aetiology . 
 this highlights the need for careful assessment of such patients in order to guide appropriate management .