Dual mobility cup systems in total hip arthroplasty consist of a metal back with a non-constrained liner, in which a constrained standard head articulates. While superior stability of such implants in comparison with standard total hip replacements is assumed, it is the purpose of this study to outline the biomechanical concept of dual mobility cups and to describe implant survival and dislocation rate based on the series published in the English-speaking and Francophone literature. A growing body of evidence indicates reduced dislocation rates in primary and revision total hip arthroplasty and in selected tumour cases. The limited availability of studies evaluating long-term implant survival and existing concerns with regard to increased wear rates and aseptic loosening, leads to the conclusion that such implants have to be used with prudence, particularly in standard primary hip arthroplasty and in young patients.

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A 25-year-old, right-hand dominant man presented with a 4-year history of progressively worsening swelling and pain arising from the proximal phalanx of the left index finger. The pain that was described was throbbing in nature and was characteristically worse at night and relieved by non-steroidal anti-inflammatory analgesia. Examination revealed a fusiform swelling of the index finger with no overlying erythema or discolouration and the finger was tender on palpation. There were no neurovascular abnormality or limitation in the range of movement. Blood tests, including inflammatory markers, were unremarkable. Radiographs revealed a lesion within the proximal phalanx, which demonstrated a rim cortical sclerosis and an area of central lucency ( Fig. 1).A bone scan with single photon emission computed tomography/ computed tomography (SPECT/CT) localization showed intense uptake within the lesion and a CT scan revealed a 5-mm nidus surrounded by a lucent halo with extensive sclerosis and bony remodelling within the phalanx sparing only the base (Fig. 2). A subsequent magnetic resonance imaging (MRI) scan was performed and this showed bone marrow oedema within the proximal phalanx with fluid within the surrounding subcutaneous tissues (Fig. 3). Based on the clinical and imaging findings, a diagnosis of osteoid osteoma was made and the lesion was removed by curettage. Histopathological examination confirmed the diagnosis of osteoid osteoma. Following this, the patient's pain resolved with no return of symptoms at follow-up evaluation at 2 months.Osteoid osteomas are uncommon benign osteoblastic tumours that consist of a small focus of osteoid tissue surrounded by a loose stroma of highly fibrous connective tissue. This benign bone tumour, which was first described by Jaffe in 1935, accounts for less than 3% of all primary bone tumours and characteristically presents with pain that is worse at night and is effectively relieved by aspirin. 1,2 The (a) (b) Fig. 1. Anteroposterior (a) and lateral (b) radiographs showing a lucent lesion with surrounding sclerosis within the proximal phalanx (arrows).Fig. 2. Technetium-99m bone scan showing increased uptake within the proximal phalanx of the index finger (a). With single photon emission computed tomography/computed tomography (CT) co-localization, increased uptake of the radiotracer is demonstrated within the lesion (b).The CT scan shows the central nidus surrounded by a lucent halo within extensive sclerosis and bony remodelling.

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