Balancing accurate rotational alignment, minimal overhang, and good coverage during total knee arthroplasty (TKA) often leads to compromises in tibial component fit, especially in smaller-sized Asian knees. This study compared the fit and surgical compromise between contemporary anatomic and non-anatomic tibial designs in Japanese patients. Size and shape of six contemporary tibial component designs (A:anatomic, B:asymmetric, C-F:symmetric) were compared against morphological characteristics measured from 120 Japanese tibiae resected following TKA surgical technique. The designs were then digitally placed on the resected tibiae. Each placement selected the largest possible component size, while ensuring <1mm overhang and proper alignment (within 5° of neutral rotational axis). When a compromise on either alignment or overhang was required (due to smaller-sized component unavailable), the design was flagged as “no suitable component fit” for that bone. Tibial coverage was compared across designs. Next, 32 femora were randomly selected from the dataset onto which each design was evaluated in two placements, the first maximizing coverage without attention to rotation and the second enforcing rotational accuracy. Downsizing was identified if in the second placement, enforcing rotational accuracy, required a smaller component size compared the first placement. The degree of mal-alignment while maximizing coverage, the incidence of downsizing, and difference in coverage between the two placements were compared across designs. Statistical significance was defined at p<0.05.INTRODUCTION
METHODS
Intra-operative parameters including all soft-tissue releases and component sizes were recorded.
Previously, we conducted a single surgeon in vivo kinematic study that revealed subjects having a PCR TKA with asymmetrical condyles experienced a high incidence of posterior femoral rollback. Therefore, the objective of this follow-up study was to determine if posterior femoral rollback from our single surgeon series can be attributed to the implant design, surgical technique, or the presence of a well functioning PCL. Three-dimensional femorotibial contact positions for eighty subjects, implanted by three surgeons, were evaluated using fluoroscopy during a deep knee bend. Twenty subjects had a PCR TKA without a PCL, while the other 60 subjects were deemed to have a functional PCL. All subjects were implanted with a PCR TKA having a larger lateral radius of curvature compared to the medial condyle. Fifty-four of sixty subjects in this study having a functional PCL experienced posterior femoral rollback of the lateral condyle, while 13/20 subjects not having a PCL experienced posterior femoral rollback. Also, 48/60 subjects having a well functional PCL and 10/20 subjects without a PCL experienced a normal axial rotation pattern. The incidence for condylar lift-off was low, and only 6/80 subjects in this study experienced greater than 2.0 mm of condylar lift-off. Forty of sixty subjects having a well-functional PCL experienced greater than 100 degrees of weight-bearing range-of-motion and the maximum weight-bearing range-of-motion was 144o. The subjects in this study experienced excellent kinematic patterns, consistent to the normal knee, although less in magnitude than the normal knee. Surprisingly, on average, subjects in this study without a PCL experienced posterior femoral rollback of the lateral condyle leading to the assumption that the PCL did not play a significant role in the excellent kinematic patterns achieved by subjects in this study. The results from this study do support the hypothesis derived from our single surgeon series that asymmetric condylar geometry may lead to better kinematic patters for subjects implanted with a fixed bearing PCRTKA, as it appears implant design is the most influencing factor that lead to normal kinematic patterns.
