• Hip and knee surgery

The objectives of this Ph.D. thesis fall in two main categories. • Evaluation of a newly designed minimally invasive surgery acetabular reamer and a • A randomized controlled study using Radiostereometric analysis (RSA) to compare migration and rotation of two different acetabular cups. The introduction of minimally invasive surgery (MIS) has opened new possibilities in orthopedic surgery. Reported benefits of less invasive hip replacement include less pain, more cosmetic incisions, less muscle damage, and maybe faster rehabilitation. However, there is no standard method available for evaluation of the surgical instruments intended for MIS surgery. A new method was developed to evaluate a MIS reamer in a cadaver model and is described in study I and II. In Study I the acetabular geometry was compared in 9 pairs of cadaver acetabuli. MIS reaming was performed in one acetabulum of each pair, and conventional reaming was performed on the contra-lateral side. A new digitizing technique, optical three-dimensional (3D) scanning, was applied to the reamed acetabuli to determine the performance of the two reamers. Best-fit spheres were calculated for the reamed cavities. The deviation between the diameter of the final reamer and the reamed cavity was small for both the MIS and conventional reamers, and no significant differences could be detected between MIS and conventional reaming. In Study II the focus was on the change of the hip joint center location during preparation of the acetabular cavity for the acetabular component. The two 3D images were merged into a single 3D image and displacements in all 3 dimensions were calculated. The results showed no significant difference between MIS and conventional reaming with regard to transition vector length. CE (Conformité Européene) marking of hip prosthesis is legally required in Denmark. However, the Danish Orthopedic Society recommends further clinical evaluation of new implant materials and designs before implementation in the daily clinic. Prospective studies require long observation periods, if the effect parameter is prosthesis replacement since the average prosthesis survival is 90% at 10 years. To obtain knowledge of a new orthopedic implant in a short observation period, pseudo endpoints such as implant migration has to be utilized. Study III In a phantom study, conventional RSA utilizing tantalum markers was compared with an RSA system utilizing a hemispherical cup algorithm and a novel model based RSA system. The precision of the migration was calculated based on double examinations of migration results of a hemispherical and a non hemispherical acetabular component. Conventional RSA (hemispherical component) and model based RSA (hemispherical and non-hemispherical component) were significantly moreprecise than the system based on the hemispherical cup algorithm. No significant difference in precision between the conventional marker system and model based RSA could be detected. Study IV The results of study IV were based on a RCT where a newly designed non hemispherical acetabular cup made of trabecular metal was compared with a hemispherical titanium fiber mesh cup. Both cup types underwent migration analysis using model based RSA. At 3 month follow up no significant difference between the two cup types could be revealed, neither in terms of migration nor in rotation of the cups.

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