Abstract
INTRODUCTION
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.
METHODS
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.
RESULTS
Design A closely matched the tibial morphology and had better size and shape conformity than Designs B-F (select metrics shown in Fig. 1). Design A exhibited higher average coverage (92%) than other designs in all ethnicities (85–87%, Fig. 2A) (p<0.01). Designs D-F had no suitable component fit in 1.6–2.4% of the bones (Fig. 2B). Coverage generally decreased with reduced component size (Fig.2C), with Design A having higher coverage than Designs B-F across all sizes. In the randomly selected 32 tibiae, enforcing rotational accuracy significantly compromises coverage in Designs B-F (Fig.3A) (p<0.01), with up to 15% in individual bones. In contrast, coverage of Design A was not influenced by enforcing rotational accuracy (p=0.52). Designs B-F were found to require downsizing on 41–66% of bones due to >5° rotation, with components internally rotated beyond 10° on 31–59% of the bones (Fig.3B). In contrast, Design A required downsizing on only 6% of the bones, caused by small mal-rotations (<10°). Designs B-D and F required downsizing of ≥2 sizes on 3–16% of bones; while a single downsize was sufficient for Design A (Fig.3C).
DISCUSSION
The anatomic design not only has the closest match to the natural tibia, but also consistently has the highest coverage across bone sizes. It also exhibits fewer incidences of downsizing and reduced propensity for mal-alignment than the non-anatomic designs investigated. In contrast, in the non-anatomic tibial component designs, ensuring rotation accuracy considerably compromised tibial coverage. This result, suggests that many non-anatomic designs do not fully accommodate variations in bone anatomy in the Japanese patients, thus forcing a compromise.
