DELTA MOTION MONOBLOCK CUP SYSTEM: WHAT CAN WE EXPECT?

Abstract

In almost all countries performing Total Hip Replacement (THR), dislocation is one of the major reasons for revision. Hence, in the last years the trend to larger bearings has been observed, following an improve in the bearing materials, the operation technique, and fixation techniques of stem and shell. Larger bearings allow for more range of motion and higher stability than conventional 28 mm bearing couples, leading to a better postoperative mobility. On the other hand, size limitations on the acetabular side are given by the anatomy of the human pelvic bone as well as the deformation and fracture behaviour of the used artificial materials. Therefore, the best solution to be achieved provides a maximum physiological outcome along with a minimised risk of intraoperative and in-vivo failures.

Investigating the wall thickness of the metal shell which is press-fitted in the human pelvic bone, the general trend towards a smaller wall thickness yielding an increased compliance can be observed with larger bearing diameters. This may lead to deformations of the metal shell making it difficult for the surgeon to properly introduce the insert. Hence, taking into account that a proper seating of the insert is absolutely necessary when using a ceramic insert in order to avoid point loads, operation time may strongly increase especially when minimal invasive surgery technique is used. With decreasing overall wall thickness of the acetabular components the volumetric stresses increase by definition. Therefore, an optimal component coupling between insert and metal shell is necessary in order to avoid point loads and resulting stress concentrations. With pre-assembled systems, this optimal coupling is reached by the force-controlled insertion of the insert in the metal shell without any prior deformation of the shell. This procedure enables to design acetabular components with a much lower overall wall thickness than conventional systems. As an example, in the case of the DELTA motion system, this overall wall thickness has been decreased to 5 mm allowing e.g. for a usage of a 36 mm bearing couple together with a 46 mm outer diameter of the metal shell. Additionally, the coating of the metal shell allows for direct bone ingrowth. Problems involved with larger bearing diameters may also arise from higher wear rates inducing possibly osteolysis and aseptic loosening. Investigations concerning the wear behaviour of large ceramic bearings have shown that there is no increase in the wear volume with increasing diameter.