Screw loosening can threaten pedicle screw fixation of the spine. Conical screws can improve the bending strength, but studies of their pullout strength as compared with that of cylindrical screws have shown wide variation. In the present study, polyurethane foam with two different densities (0.32 and 0.16gm/cm3) was used to compare the pullout strength and stripping torque among three kinds of pedicle screws with different degrees of core tapering. Three-dimensional finite element models were also developed to compare the structural performance of these screws and to predict their pullout strength. In the mechanical tests, pullout strength was consistently higher in the higher density foam and was closely related to screw insertion torque (r = 0.87 and 0.81 for the high and low density foam, respectively) and stripping torque ( r = 0.92 and 0.78, respectively). Conical core screws with effective foam compaction had significantly higher pullout strength and insertion torque than cylindrical core screws (p < 0.05). The results of finite element analyses were closely related to those of the mechanical tests in both situations with or without foam compaction. This study led to three conclusions: polyurethane foam bone yielded consistent experimental results; screws with a conical core could significantly increase pullout strength and insertion torque over cylindrical; and,finite element models could reliubly reflect the results of mechunicul tests.

Conical screws effectively increased the bending strength and pullout strength simultaneously. The finite element analyses reliably predicted the results of the mechanical tests.

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This study aimed to evaluate retrospectively the clinical outcomes and complications of structural allografts and Burch-Schneider antiprotrusio cages to treat severe acetabular defects in revision total hip arthroplasty (THA). Between July 2003 and December 2010, 29 patients (representing 31 hips) underwent revision THA using structural allografts and Burch-Schneider cages for acetabular reconstruction. The patients had a mean age of 59 years (range, 37-79 years). Seventeen hips had a Paprosky IIIA defect, 12 hips had a IIIB defect, and two hips had a pelvic discontinuity (PD) acetabular defect. After a mean follow up of 5.5 years (range, 3.0-10.5 years), all patients, except for two patients with recurrent infections, significantly improved, based on the mean Harris hip score (30 points vs. 67 points, p < 0.001) and the Western Ontario and McMaster Universities Arthritis index (WOMAC) score (65 points vs. 32 points, p < 0.001). Using re-revision surgery as the endpoint, the cage had a survival rate of 76% [95%, confidence interval (CI), 67.4-84.6%] at 5 years and 57% (95% CI, 39.3-74.7%) at 10 years. Other complications included hip dislocation [3 (9.7%) hips]; deep infection [3 (9.7%) hips]; and sciatic nerve impingement, vesicle-acetabular fistula, and leg lengthening [1 (3.2%) hip]. There was a trend toward a higher failure rate in hips with four revisions, compared to hips with three revisions or fewer (p = 0.055). Three hips with a failed cage underwent re-revision surgery using a standard noncemented acetabular component. In conclusion, a structural allograft with Burch-Schneider antiprotrusio cage for complex acetabular defect is a technique-demanding procedure with a 5-year survival rate of 76%. Even with failed cage reconstruction, re-revision surgery with a noncemented acetabular component may be feasible once the allograft has healed.