H Kuhn (Emstek, Allemagne)
7 authors have developed the Nanos neck preserving stem in conjunction with Endoplant GmbH, Marl, Germany in 2002. The design principal based on the human anatomy and on the evaluation of 578 CT slides in order to preserve the bone stock, to achieve primary stability and the reconstruction of the anatomy/biomechanics. Biomechanical results in solid bodies and in cadavers gave the direction to the actual form of 10 stems sizes and these permit exact positioning of 90% of the specimen. Particularly the minimal invasive approach and neck preserving benefit from short designs. The stem is coated with Ti-plasma and CaP in order to accelerate the osseointegration process.
The indication for the Nanos stem has been considered for patients less than 60 years, body weight index under 30, without osteoporosis and without Coxa vara or valga.
The results of the multicenter study of 205 patients from 5 hospitals includes 73 patients with follow up of 12 month, 8 patients with follow up of 18 month and 9 patients with follow up of 24 months. Patients which have been observed in the interval of 12-24 months had following indications: 110 (51,4%) cases showed primary arthritis, 57 (26,6%) dysplasia, 32 (15%) necroses of the femoral head, others had Perthes disease, epiphysiolisis, posttraumatic arthritis or protrusion.
The average height was 172 cm, the average weight 81,6 kg (min. 49, max. 124 kg). The average age of the operated patients was 57,4 years (min. 33, max. 80 years). Migrations or dislocations of the stem have not been stated.
Three patients had ossifications stage one, three patient’s stage 2 according to Brooker, in the others no ossifications have been observed.
The Merle D’Aubigne average mobility was preoperatively in 205 patient’s 5,46, 12 months postoperatively 73 patient’s had 5,97, 18 months postoperatively 8 patient’s had 6,0 of 6 possible points. The Merle D’Aubigne pain and ability to walk was preoperatively in 205 patient’s 7,51 and increased after 12 months postoperatively in 73 patient’s to 11,7, after 18 months postoperatively in 8 patient’s to 11,8 of 12 possible points. The Harris hip score was preoperatively 47,8 and increased to 96,23 postoperatively (12 months 73 patients) and to 96,38 (18 months 8 patient’s) of 100 possible points.
Special cases of stem implantation in patients beyond the considered indication will be presented. Implantation features favorises tissue conserving techniques and showed good individual results.
Biomechanical studies showed that the Nanos neck preserving hip stem is favourable to the MI-implantation because of a minimum entry with low unfavourable stresses. The femoral neck can partly be saved, serves as anchorage area and facilitates primary stability.
The early results with a high satisfaction rate of the patients motivate to promote the use of the Nanos stem and to await long-term results.