Abstract
The torsional strength of cemented implants is likely influenced by stem geometry. Five straight stems with different cross-sectional shapes (circular, oval, triangular, round-rectangular, sharp-rectangular) were custom-machined. The stems were cemented into tubes using bone cement, and subjected to torsion (2.5deg/min)(n=7). At initial failure (crack through the cement mantle or loss of cement-stem adhesion), the sharp-rectangular stem resisted over 33% more torque than the other four stems (p=0.13). At ultimate failure (5° stem rotation), the resistance provided by the circular stem was less than 12% of either rectangular stem (p< 0.05). Additional studies are needed to determine the effects of long-term loading.
To determine the influence of cross-sectional implant stem shape on cement failure under torsional loading.
The sharp-rectangular stem provided the greatest torsional resistance against initial failure. At ultimate failure, the two rectangular stems performed better than the other stems, with the circular stem providing the least torsional resistance.
A stem design that provides increased resistance to torsion will, in all likelihood, improve the longevity of cemented stemmed implants.
Five straight stems with different cross-sectional shapes (circular, oval, triangular, round-rectangular, sharp-rectangular) were custom-machined. Each stem was cemented in a square aluminum tube using Simplex® cement (Stryker, Michigan, USA). A materials testing machine was used to apply torsion to the stem at 2.5 deg/minute until failure. ‘Initial failure’ was defined as the appearance of a crack through the cement mantle and/or the loss of cement-stem adhesion. ‘Ultimate failure’ was defined as 5° of stem rotation. Results (n=7) were compared using one-way ANOVAs with post-hoc Student-Newman-Keuls tests (p=0.05). The sharp-rectangular stem withstood over 33% more torque at initial failure than the other stems (p=0.13). At ultimate failure, the circular stem provided significantly less torsional resistance than the other four stems (p< 0.05), and was able to resist less than 12% of the torque applied to either rectangular stem. These results suggest shape may play a role in the onset of implant loosening due to torsion. Further studies are required to explore other shapes and to examine the effects of cyclic loading and cement soaking.
Funding: Natural Sciences and Engineering Research Council, University of Western Ontario
Correspondence should be addressed to Cynthia Vezina, Communications Manager, COA, 4150-360 Ste. Catherine St. West, Westmount, QC H3Z 2Y5, Canada
