Acetabular screw rings and surface treatment

The Zweymüller truncated self tapered threaded ring is forged in pure titanium with a 3- to 5-mu grit blasted surface roughness. Among 167 consecutive primary total hip arthroplasties prospectively studied, two threaded components failed to achieve initial stability, with one requiring almost immediate revision. One hundred twenty-six hips have been fully documented with a 5-year minimum followup. The mean modified Harris hip score improved from 44.4 points preoperatively to 91 points at 7 years average followup (range, 60-121 months). Bone to implant gap rate decreased from 54.7% after surgery to 2.4% at last review, with only one detectable early migration that had secondary stable osseointegration. Annual linear wear of more than 0.2 mm per year could be detected in only one ceramic on polyethylene bearing surface with one corresponding femoral osteolysis granuloma. At the 9- to 10-year interval, the survivorship with definite loosening as a failure was 98.7%. Delayed loosening by failure of an initially stable reconstruction, progressive lucencies, liner disassociation, or worrisome osteolysis have not been observed. These midterm results are much better than those of former screw rings with smooth surface finishes. The main reason for the general failure and justified abandonment of threaded polished components may not be the screw in mechanism of their primary fixation, but the lack of an appropriate surface for bone ingrowth and osseointegration.