Bilateral total hip replacement: Periprosthetic pseudotumor collections are more prevalent in metal-on-metal implants compared to non-metal-on-metal ones

Background: Metal-on-metal (MoM) hip prostheses were shown to have high failure rates including the formation of periprosthetic cystic masses called periprosthetic pseudotumor collections (PPCs). Purpose: To compare MRI prevalence and size of PPCs in patients after bilateral total-hip-replacement (THR) in which at least one hip was replaced by a MoM prosthesis. Material and Methods: All sequential MRI examinations of patients with bilateral THR in which at least one is MoM (2010-2013) were retrospectively evaluated. MRIs were analyzed separately by two readers for the presence and size of PPCs. These were compared between MoM and non-MoM implants and between patients with unilateral or bilateral-MoM prostheses. Blood metal ion levels were also compared. Results: Seventy hips of 35 patients (male:female ratio, 9:26; mean age, 64 years; age range, 35-82 years) were assessed. Sixteen patients (45%) underwent bilateral MoM-THRs and 19 (55%) had one MoM and the other non-MoM, yielding 51 MoM THRs and 19 non-MoM THRs. Twenty-eight PPCs were detected in 19 patients (54%): 26 in MoM THRs (51%) and two in non-MoM THRs (10.5%, P=0.00009). The mean PPC volume in the MoM implants (107 mm³) was higher than that of the non-MoM implants (18 mm³, P=0.49). Cobalt/chromium blood levels were 78 mg/L/25 mg/L for bilateral MoM THRs and 21 mg/L/10 mg/L for unilateral MoM implants (P=0.1 and 0.16, respectively). Conclusion: PPCs are more prevalent in MoM THRs compared to non-MoM THRs. Larger PPC volumes and higher blood metal ion levels were detected in patients with bilateral MoM THRs compared to unilateral MoM THRs (P>0.05).