Rotational Constraint in Posterior-Stabilized Total Knee Prostheses

Abstract

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.

Computational assessment of constraint in total knee replacement

“…Previous investigators have considered the constraint offered by implants in isolation, without the effects of ligaments or other tissues (e.g., Thatcher et al, 1987;Klein et al, 2003). The different methodologies employed, however, have made direct comparisons between studies and implants difficult.…”

The Planovalgus Foot: A Harbinger of Failure of Posterior Cruciate-Retaining Total Knee Replacement

“…The effect of weight-bearing activities on an ipsilateral total knee replacement in a patient with posterior tibial tendon insufficiency is unknown. The use of a prosthesis with increased conformity and constraint in patients with such altered kinematics may, however, raise concern about increased polyethylene wear, loosening, and osteolysis 7,8 .…”

Rotational Constraint of Posterior-Stabilized Total Knee Prostheses

“…Resistance to rotation in the PFC design was caused mainly by box-post impingement, as previously reported. 10 Impingement was greatest in hyperextension and full extension, where the box and the post usually play a larger role in constraint. Anterior tibial lips can also add to the resistance, as was demonstrated by the Genesis II design.…”