Rotational stresses from box-post impingement have been implicated in the loosening of posterior-stabilized total knee prostheses. A bench model was constructed to assess the forces generated by tibiofemoral rotation. Rotational torque under load was measured in two different posteriorstabilized total knee prostheses using an axial-torsion load cell at 0 degrees, 20 degrees, and 40 degrees flexion over 20 degrees internal and external rotation. The Sigma posterior-stabilized prosthesis generated little torque through 5 degrees internal and external rotation. An increase in torque then occurred because of box-post impingement, generating peak torques of 17 to 18 N-m at 12 degrees to 14 degrees rotation. The bench model produced the same deformation of the polyethylene post as seen on retrieved specimens. The Scorpio posterior-stabilized prosthesis had a relatively continuous rise in generated torque from tibiofemoral conformity. Box-post impingement did not occur resulting in 32% lower torque between 12 degrees and 14 degrees rotation. Peak rotational torques of 15 to 16 N-m were reached at 19 degrees to 20 degrees rotation. Tibiofemoral conformity is the primary source of rotational constraint. Box-post impingement can be a source of additional rotational constraint. Depending on specific design features, small changes in relative tibiofemoral component rotation can more than double the generated torque. Axial rotation of the knee in vivo can generate substantial torque. Relative tibiofemoral rotational position is an important factor influencing component function and fixation.

Abstract-A retrospective study of endoprosthetic replacements and total hip prostheses was undertaken to determine factors that have the greatest effect on the success or failure of femoral hip components.

Five fresh nonembalmed elbows were tested for resistance to valgus stress in their anatomic state, after radial head resection, and after insertion of Silastic® and polymethylmethacrylate (PMMA) radial head replacements. The resistance to valgus stress was found to be reduced an average of 28% after radial head resection. The PMMA and Silastic® implants restored valgus stiffness an average of 86% and 78% respectively, as compared to intact elbow values for corresponding flexion angles. Testing in pronation, supination and neutral forearm rotation demonstrated no difference in valgus stiffness. For each elbow, resistance to valgus stress was greatest at full extension and dropped approximately 30% at all other flexion angles as compared to corresponding full extension value. These data support the concept of the radial head as a stabilizer to valgus stress in the in vitro elbow. Further, this data demonstrated the ineffectiveness of current radial head replacement systems in restoring this biomechanical function and suggest that the use of a stiffer implant material may be beneficial in resisting valgus stress. Additional testing is indicated to determine the performance of a stiffer implant at the clinical and biological levels.

No abstract