Abstract
Introduction
The initial mechanical stability of cementless femoral stems in total hip arthroplasty is an important factor for stable biological fixation. Conversely, insufficient initial stability can lead to stem subsidence, and excessive subsidence can result in periprosthetic femoral fracture due to hoop stress. The surface roughness of stems with a surface coating theoretically contributes to initial mechanical stability by increasing friction against the bone, however, no reports have shown the effect of surface roughness on stability. The purpose of this study was to evaluate the effect of differences in surface roughness due to different surface treatments with the same stem design on the initial stability.
Materials and Methods
Proximally titanium plasma-sprayed femoral stems (PS stem) and proximally grit-blasted stems (GB stem) were compared. The stem design was identical with an anatomic short tapered shape for proximal fixation. The optimum size of PS stem based on 3D templating was implanted in one side of 11 pairs of human cadaveric femora and the same size of GB stems was implanted in the other side. After implantation, the specimens were fixed to the jig of a universal testing machine in 25cm of entire length so that the long axis of the femur was positioned at 15-degrees adduction to the vertical. Vertical load tests were conducted under 1 mm/minute of displacement-controlled conditions. After 200 N of preload to eliminate the variance in the magnitude of press-fit by manual implantation, load was applied until periprosthetic fracture occurred.
Results
The same size of PS or GB stem was successfully implanted in all 11 pairs without fracture. The distances of subsidence until fracture occurred were 2.2±1.2 mm in the PS stem and 2.5±1.1 mm in the GB stem and no significant difference was detected. The load applied for 1 mm of subsidence was 792±478 N in the PS stem and 565±431 N in the GB stem and there was a significant difference between the two groups. The load at fracture was 3037±1563 N in the PS stem and 2614±1484 N in the GB stem and there was a significant difference between the groups.
Discussion
A significantly larger load was applied for 1 mm of subsidence in the PS stem compared to the GB stem. This suggests that the plasma-spray porous-coated surface had a less slippery interface than the grit-blasted surface. Both femora of a pair fractured at the same level of hoop stress that was induced by the same amount of stem subsidence but at significantly different loads. The fact that the load at fracture in the PS stem was significantly larger than that in the GB stem was due to differences in shear stress caused by different levels of friction. The scratching effect against the femoral canal due to the rougher surface of the plasma-spray porous-coating works advantageously for initial mechanical stability.
