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Ultrasonic driven tools have been developed to facilitate the removal of bone cement during revision arthroplasty. The effect on whole bone strength of cement removal by ultrasonic tools was examined in a canine femur model. Paired, fresh-frozen canine femora were divided into two groups. In group A, one femur from each pair was subjected to cement extraction with ultrasonic tools. In group B, one femur from each pair was subjected to manual cement extraction. Contralateral femora from each pair served as controls to determine the strength of intact femora. Torsional fractures were produced using a servocontrolled hydraulic testing machine (Minneapolis Testing System, Minneapolis, MN). Maximum torque, maximum angle, and energy capacity to failure were determined. Results were recorded as a reduction in percent value of the tested specimen versus the contralateral control. When comparing femora with cement removal by ultrasonic tools to the contralateral control femur, there were no statistical differences in ultimate torque (P = .83), maximum angle (P = .89), and energy capacity (P = .74) by analysis of variance. In addition, there were no significant differences between the group with ultrasonic tool cement removal and the group with manual tool removal. The authors conclude that in this canine model, removal of cement with ultrasonically driven tools has no adverse effects on whole bone strength.

The effect of extracorporeal shock wave lithotripsy on interfacial strength between prosthesis and bone in cementless arthroplasty was examined using a rabbit model. Paired femora, each implanted with fiber mesh porous coated titanium implants, were harvested from rabbits 15 weeks after implantation. In group I, one femur from each pair was exposed to lithotripsy treatment consisting of 2,000 shocks at 20 kV. In group II, one femur from each pair was exposed to 2,000 shocks at 26 kV. Contralateral femora from each pair served as controls in both groups. Mechanical pushout tests were conducted on the implants using a 1321 Instron testing machine at a constant rate of 1 mm/minute. Shock waves generated at 20 kV were found to have no significant decrease on either the prosthesis/bone interfacial strength or energy to failure of cementless implants. Shock waves generated at 26 kV produced a mean 17.45% decrease in the prosthesis/bone interfacial strength, which approached statistical significance (P = .062), and a 7.84% mean decrease in the energy to failure (P = .268). However, in four of the seven group II specimens, cortical fractures occurred. These findings suggest that lithotripsy will not aid in the removal of uncemented porous coated devices and lithotripsy inadvertently focused at an uncemented device will not disrupt significantly the prosthesis-bone interface.

Sixteen embalmed hemipelves were used to determine the optimal acetabular screw placement to provide maximal screw pull-out strength in unicortical and bicortical screw fixation. The anterior column, superior ilium, posterior column, ischium, and pubis regions of the pelvis were tested using 6.5-mm titanium alloy screws and a hydraulic servo-controlled 1321 Instron testing machine. Force vs displacement data were acquired. Bicortical fixation was stronger than unicortical fixation in the four zones compared. This difference was significant in the superior ilium, posterior column, and ischium. The anterior column could not accept unicortical screws due to inadequate bone depth, which ranged between only 6 mm and 10 mm. Bicortical fixation was significantly greater in the superior ilium, posterior column, and ischium than in the anterior column or pubis. Unicortical fixation was greatest in the superior ilium. This information may aid decisions concerning the positioning of screws to augment acetabular component fixation.