• Zhe-Yu Huang, Shi-Cheng Wang, Hao-Jun Zhang, Long Shao, Zheng-Lin Di, and Kun Tao.
• The Department of Orthopedics Surgery, Ningbo No. 6 Hospital, Ningbo, Zhejiang, People's Republic of China.
• Medicine (Baltimore). 2022 Mar 18; 101 (11).

RationalePseudotumor formation after hip arthroplasty is a rare complication that can occur not only at the head-neck junction but also at the modular neck-stem junction. Dislocation is a challenging and common complication of primary and revision total hip arthroplasty compared with other complications. Similarly, the association between pseudotumors and delayed recurrent dislocation remains unclear.Patient ConcernsWe report the case of a 73-year-old woman with pseudotumor formation after total hip arthroplasty combined with a modular femoral neck. A delayed recurrent dislocation occurred in this case. Approximately 4weeks after the first revision surgery, redislocation occurred.DiagnosisThe patient was eventually diagnosed with delayed recurrent artificial hip dislocation combined with a periprosthetic pseudotumor of the right hip.InterventionsDuring the first revision surgery, a thickened, indurated cyst measuring 8×3×8cm with a red-brown wall containing brown fluid was completely excised. A cemented stem, combined with a BIOLOX Forte ceramic head, was implanted. Approximately 4weeks after surgery, redislocation occurred, and we cemented an elevated rim liner on the acetabular component with a metal head.OutcomesAt the last follow-up, 49 months after revision surgery, the patient was asymptomatic with a Harris hip score of 90. The patient had a satisfactory prognosis after treatment.LessonsThe application of the modular-neck stem should be cautiously performed, particularly for modular prostheses containing different alloys. Pseudotumors and insufficient soft-tissue tension both contribute to hip instability, which may eventually lead to delayed repeated dislocation. In addition, femoral offset must be considered. Cement-liner technology may be used for aging patients who are less active. This case report, focusing on pseudotumors and delayed recurrent dislocations, aimed to identify factors that may support this diagnosis, which is easy to miss. Consequently, it can provide further details on the treatment process and alert orthopedic surgeons to this infrequent but important cause of delayed recurrent dislocation.Copyright © 2022 the Author(s). Published by Wolters Kluwer Health, Inc.

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