OPK, sometimes called spotted bone disease, is a benign sclerosing bone dysplasia. The etiology and pathogenesis of OPK remain undefined. There are several hypotheses, including: (1) loss-of-function mutations in the LEMD3 gene, causing a generalized fibroproliferative or stenosing disease; (2) hereditary failure to form normal trabeculae along lines of stress; (3) dysplasia of endochondral ossification, affecting resorption or remodeling of secondary spongiosa; and (4) altered osteogenesis [2]. In addition, OPK has been reported as having an autosomal dominant pattern of genetic transmission [8].
OPK and DDH are both a form of skeletal dysplasia, a wide-ranging term used to describe pathologic conditions characterized by disordered development, remodeling and reabsorption of bone and cartilage. Patients with skeletal dysplasia may be predisposed to development of severe degenerative joint disease and need joint replacement at a relatively young age [9]. In our patient, OPK was incidentally found after radiographs were ordered to evaluate symptoms related to DDH and hip osteoarthritis. However, little is known about the longevity and functional outcomes of hip arthroplasty in patients with OPK because of the disease’s rarity. Our search of the literature identified only one article in French by Zahar, et al. describing their successful experience of treating femoral neck fracture in a patient with OPK [10]. The sclerotic bone lesions of OPK consist of dense lamellar osseous tissue that fail to be resorbed [1]. Therefore we theorized that such benign, non-active sclerosing bone lesions would not significantly alter the biomechanical strength of the involved bones, and planned for a cementless total hip arthroplasty. Indeed, in this particular patient, we found that OPK lesions did not cause difficulty with standard reaming of the acetabulum and broaching of the femoral canal. Also the acetabular and femoral components could both be press-fitted in the usual fashion. It is worth noting the extent and severity of sclerotic lesions at the site of prostheses placement could have a bearing on the surgical plan. Although we did not obtain a CT scan of the pelvis and femur at the time, we believe that CT would help determine the most affected area(s) and/or the percentage of OPK lesions in the surgical site. If OPK lesions are abundant or located in areas that might hamper prosthesis placement, the surgeon should be alerted to the possibility of using power tools, such as a high-speed burr, to properly prepare the involved bone(s).
Making a correct diagnosis of OPK is important. The characteristic radiographic feature of OPK is multiple round or oval bone lesions scattered through axial and appendicular skeleton, a picture that resembles osteoblastic metastases, mastocytosis, or tuberous sclerosis [3,4,5]. Since OPK lesions are composed of mature dense bone, they appear small and dark on both T1 and T2 weighted images on MRI [11]. Bone scintigraphy is useful in distinguishing OPK form osteoblastic metastases because OPK lesions usually do not have enhanced tracer uptake due to inactive bone remodeling.
In conclusion, OPK is a benign sclerosing bone dysplasia that may or may not coexist with other joint abnormalities. Bone scintigraphy plays a critical role in differentiating OPK from osteoblastic metastasis. Our experience suggested that cementless total hip arthroplasty in patients with OPK could achieve satisfactory mid-term functional and radiographic outcomes. Continued follow-up is needed to determine the long-term results.
