Preliminary Experience with New Dual-Mobility System for Small Japanese Patients

Background/Objectives: The preliminary experience with new dual-mobility system for small Japanese patients was introduced in this paper. Methods: Twenty-nine hips which underwent primary THA were retrospectively reviewed. All cups were inserted via Hardinge lateral approach. The ability to perform formal Japanese sitting in a kneeling position (Seiza in Japanese) and bowing while sitting (Zarei in Japanese) was evaluated. The mean follow-up was 6 months. Results: The mean age at surgery was 70 years, mean height was 156 cm, mean weight was 58 kg, and mean body mass index was 23.6. The acetabular cups utilized were a hemispherical hydroxy-apatite coated cup (25 hips) and a hemispherical trabecular titanium cup (4 hips), with diameters of 46 mm in 5, 48 mm in 15, 50 mm in 3, 52 mm in 1, 54 mm in 3, 56 mm in 1, and 62 mm in 1; mean diameter was 49.4 mm. No postoperative dislocations including intraprosthetic dislocation or metal allergy were observed. The mean Harris hip score improved significantly from 39 points preoperatively to 89 points postoperatively (p < 0.05). Radiographic evaluation demonstrated bone ingrowth stability in all cases according to Engh's criteria and no aseptic loosening of the implants. Mean hip flexion increased from 75° preoperatively to 90° postoperatively (p < 0.05). The ability to perform Seiza increased from 8 patients preoperatively to 23 patients postoperatively (p < 0.05). The ability to perform Zarei (deep bowing) increased from 7 patients preoperatively to 20 patients postoperatively (p < 0.05). Conclusions: This novel dual-mobility system designed for smaller Japanese patients offers three distinct advantages: (1) availability of 42, 44, 46 and 48-66 mm outer diameter cups, (2) 1 mm deeper center of rotation, providing increased jumping distance compared to other designs, and (3) improved assembly instrumentation (cement-gun-type bearing press). Early clinical results suggest that this newly developed dual-mobility THA system is well-suited to the lifestyle and anatomical characteristics of Japanese patients.