Purpose of the study: The design of the NANOS femoral stem defined in 2002 is based on anatomic studies and the analysis of 578 scanner slices. The stem is plasma torch coated with titanium and with calcium phosphate to accelerate the osteointegration process. The NANO stem is indicated for young active patients with BMI < 30 and free of coxa vara/valga.

Material and methods: This was a multisite study conducted in five hospitals and including 205 patients; follow-up was 12 months for 77 patients and 24–36 months for 50. For the patients followed for 12–24 months, indications were: primary degenerative disease (n=110, 51.4%), dysplasia (n=57, 26.6%), necrosis of the femoral head (n=32, 15%), other causes including Perthes disease, epiphysiolysis, posttraumatic osteoarthritis, protrusion. Mean patient age was 57.4 years (33–80). There was no case of stem migration or dislocation.

Results: Mean mobility (Merle-d’Aubigné) was 5.49 in 205 patients preoperatively, 5.97 in 73 patients at 12 months and 6.6 in eight patients after 18 months. Pain and walking were scored 7.51 (Merle-d’Aubigné) in 205 patients preoperatively, 11.7 in 77 at 12 months, then 11.96/12 at 24–36 months. The HSS score improved from 47.8/100 preoperatively to 96.62/100 at 24–36 months (50 patients). Outcome was good in more than 280 implants, mainly inserted via an anterolateral approach.

Discussion: Biomechanical studies have demonstrated the advantages of implanting the NANOS prosthesis via a MIS approach: mini-incision, net diminution of undesirable stress. The partially spared femoral neck can be used as an anchorage zone, favouring primary stability. The fine neck of the NANOS stem avoids impingement and increases joint range of motion.

Conclusion: The early results indicate a high level of patient satisfaction and the clinical and radiographic findings suggest good long-term outcome can be expected.