[Burst suppression detection in pEEG]

Primärstabilität von Press-fit Hüftpfannen – eine in-vitro Studie – / Initial Stability Of Press-fit Acetabular Cups – An In-Vitro Study –

AIM OF THE STUDY

Mechanical lever-out tests were performed in vitro to investigate the initial stability of press fit acetabular cups.

METHOD

Five different uncemented, hemispherical press-fit cups were implanted in a standardized manner into Sawbones, Polyurethane foam blocks. Each cup was levered-out by using a 250 mm stainless steel rod, which was connected to the acetabular cup. Loads were then applied to the rod causing the cup to be diplaced. Lever-out forces were recorded by a computer.

RESULTS

The results of the lever-out forces ranged between 39,2 and 50,8 Nm. The highest initial stability was achieved by two Titanium cups with a Titanium plasmaspray coating, a flattened pole and a sharp equatorial edge. According to our trials the equatorial rim of the polyurethane cavity is the crucial area for the implant's initial stabilty. There the highest amount of attrition was observed.

CONCLUSION

To guarantee a high reproducibilty of the tests it is essential to pay particular attention to the quality of the polyurethane foam blocks, to the exactness of the reaming procedure and to a defined cup insertion. However as our trials were carried out under optimized labaratory conditions one must be careful not to over-interpret its results. Intraoperatively primary stability is also influenced by the quality of the bone.

[Primary stability of press-fit acetabulum cups using a new acetabulum reamer]

The aim of the present study was to assess the initial stability of uncemented press-fit acetabular components using a newly developed reamer designed to optimize the surgical preparation of the acetabulum. Ten synthetic human pelves were used to investigate the stability of 20 uncemented press-fit acetabular components, each of which was tested in a servohydraulic testing machine for 6 cycles under an axial load of 2.4 kN. The results of the micrometric measurements revealed satisfactory stability for a reaming depth of 2 mm, and a press-fit of 2 mm. Micromotion was less than 200 microns in all the anatomical sections of the acetabulum (ischium 63 microns, pubis 150 microns, ilium 85 microns). A press-fit of 4 mm and the smaller reaming depth of 1 mm were associated with a substantial decrease in mechanical stability.

[Primary stability of 2 acetabulum roof cups and an acetabulum reinforcement cup. Effect of osseous defects]

The aim of this experimental study was to analyse the effects of implant design and bone stock defects on the primary stability of three different acetabular components. Fresh frozen human pelves were employed for the investigation. Müller and Ganz rings and a Burch-Schneider cage (Protek, Münsingen, Switzerland) were fixed with screws in 6 normal acetabula using standard techniques, and in 30 acetabula previously prepared with 5 different segmental bone stock defects. A servohydraulic testing machine (Instron, Canton, USA) was used for the investigation. Three electromagnetic displacement transducers (Micro-Epsilon, Ortenburg, Germany) were placed in the three main quadrants of the acetabular rim to detect implant micromotion, which reflects stability. Displacement was recorded during 20 consecutive cycles under loads of up to 2354 N. All implants were stable (< 150 microns) in all quadrants of normal acetabula and also in those with ectatic, protrusive and ventral defects. There was no statistically significant difference in the results between Müller and Ganz rings. Displacement of more than 150 microns was observed in acetabula with cranial or dorsal defects. The cage was stable under all defect conditions. The reinforcement implants showed low displacement rates in most of the acetabular bony defects. Stability is a function of the area of surface contact between prosthesis and bone.