Abstract
Introduction
The SAIPH™ (MatOrtho, UK) total knee replacement is a new fixed-bearing prosthesis design having attributes of a mobile bearing and the posterior stabilised categories for knee arthroplasties. The implant design goal is an articulation that provides definitive anteroposterior stability to beneficially control tibiofemoral translation, the ability for the tibia to axially rotate to accommodate various lifestyle activities, and to maintain a relatively posterior femoral position on the tibia to facilitate range of motion. This study aims to analyze knee kinematics of the SAIPH™ total knee arthroplasty (TKA) by videofluroscopy during four different weightbearing activities.
Method
Fourteen consecutive patients operated on by a single surgeon, with a minimum follow up of 24 months were included in this IRB-approved study. A medially conforming knee was implanted in all cases. Participants in the study were asked to perform weightbearing kneeling, lunging, step-up/down and pivoting activities while their knee motions were recorded by videofluoroscopy. Three-dimensional (3D) joint kinematics were determined using model-image registration. The 3D orientation of each TKA component was expressed using standard joint angle conventions, and the anterior/posterior location of each condyle was expressed relative to the deepest part of the tibial sulcus.
Results
Maximum knee flexion during the kneeling activity averaged 127 ° (100°–155°). Condylar contact was posterior on the tibia during kneeling (Figure 1). The medial femoral condyle (MFC) translated an average of 4 mm (SD 3 mm) posteriorly at 127 ° of kneeling flexion. The lateral femoral condyle (LFC) translated posteriorly 8 mm (SD 3 mm). None of these knees demonstrated paradoxical forward slide of the femur during this activity. The tibia rotated internally an average of 5° during flexion. During the lunge activity mean knee flexion was 121°. There was a similar asymmetric posterior translation of the femoral condyles, 5 mm for the MFC, and 8 mm for the LFC, and an average internal rotation of the tibia of 3°.
During the step-up/down activity the MFC translated posteriorly 2 mm, and the LFC 3 mm (Figure 2). The tibia internally rotated 4° from extension to 85° flexion during stepping. During the pivot activity, the MFC remain stable in the tibial sulcus and the LFC translated posteriorly while the tibia rotated externally to internally (Figure 3).
Conclusion
The SAIPH™ knee shows a medial pivot motion with tibial internal rotation of the tibia during active weightbearing flexion and deep knee flexion, as seen in previous studies. The kinematics are similar in pattern to normal knees showing an asymmetric posterior translation of the lateral femoral condyle and tibial internal rotation with knee flexion. A medially conforming implant design provides intrinsic anteroposterior stability to control femoral translation across the entire range of flexion, allows tibial rotation, and provides functional flexion comparable to specialized posterior-stabilised implant designs.
