In vivo determination of cam-post engagement in fixed and mobile-bearing TKA

Abstract

Background: Kinematics vary, sometimes in important ways, among the different types of total knee arthroplasty (TKA) designs, yet differences between the in vivo mechanisms of cam-post engagement in rotating-platform posterior-stabilized (PS) TKA, bicruciate-stabilized TKA, and fixed-bearing PS TKA designs remain largely uncharacterized.

Questions/purposes: The objective of this study was to determine the cam-post mechanism interaction for subjects implanted with three different TKA designs.

Methods: In vivo, analysis was conducted for patients implanted with nine rotating-platform PS TKAs, five knees with a fixed-bearing PS TKA, and 10 knees with a bicruciate-stabilized TKA while performing a deep knee bend. Three-dimensional kinematics of the implant components were determined by analysis of fluoroscopic images during flexion. The distances between the interacting surfaces were measured throughout flexion and instances and locations of contact were identified.

Results: Seven of the 10 bicruciate-stabilized knees analyzed had the femoral component engaged with the anterior aspect of the tibial post at full extension. Posterior cam-post engagement occurred at 34° for the bicruciate-stabilized (range, 17°-68°), 93° for the fixed-bearing PS (range, 88°-100°), and at 97° (range, 90°-104°) for rotating-platform PS TKA. In bicruciate-stabilized and fixed-bearing PS knees, the contact initially occurred on the medial aspect of the posterior surface of the tibial post and then moved centrally and superiorly with increasing flexion. For rotating-platform PS TKA, it was located centrally on the posterior surface of the post at all times.

Conclusions: This study suggests that mobility of the polyethylene might play an important role in ensuring central cam-post interaction in PS TKA. The polyethylene insert rotates axially in accordance with the rotating femur, maintaining central cam-post contact. This phenomenon was not observed in the fixed-bearing PS TKAs analyzed in this study.

Clinical relevance: We speculate that this centralized symmetrical contact between the cam and posterior surface of the post could be beneficial clinically in terms of reducing wear of the posterior surface and particularly at the medial extremes of it.

Fig. 1

Fluoroscopy image shown with femoral and tibial 3-D solid models of rotating-platform PS TKA and four visible bead silhouettes that allow for proper positioning of polyethylene bearing when silhouettes match.

Fig. 2

Cam-post contact determination for a sample patient in the rotating-platform PS group exhibits a typical result of the contact analysis conducted to determine cam-post contact.

Fig. 3

Cam-post distance determination for a sample patient in the rotating-platform PS group exhibits a typical result for distance calculation between the femoral cam and the posterior surface of the tibial post.

Fig. 4

Cam-post height of contact determination for a sample patient in the rotating-platform PS group exhibits a typical result for contact height calculation on the posterior surface of the tibial post.

Fig. 5

Anterior cam-post contact was evaluated for a sample patient in the bicruciate-stabilized group depicting loss of contact at early flexion.

Fig. 6

Evaluation of posterior cam-post contact for a sample patient in the bicruciate-stabilized group depicts a medialized contact point, which then moves posteriorly and superiorly with increasing flexion.

Fig. 7

Evaluation of the cam-post interaction for a sample patient in the rotating-platform PS group exhibits central contact on the tibial post through flexion.

Fig. 8

Tibiofemoral and polyfemoral rotation comparison for the patients in the rotating-platform PS group exhibits consistent rotation of the femoral component with polyethylene rotation. DKB = deep knee bend.