Abstract
Introduction:
Malrotation of the tibial component is a common error in TKR, and has been frequently cited as the cause of clinical symptoms. Correct rotational orientation of the tibial tray is difficult to achieve because the resected surface of the tibia is internally rotated and is not symmetrical in shape. This suggests that anatomically contoured components may lead to improved rotational positioning.
This study was undertaken to test the hypotheses:
- 1.
Use of an anatomically shaped tibial tray can reduce the prevalence of malrotation and cortical over-hang in TKA while increasing coverage of the resected tibial surface, and
- 2.
Component shape has more influence on the results of surgical trainees compared to experienced surgeons.
Materials and Methods:
A standard symmetric design of tibial tray was developed from the profiles of 3 widely used contemporary trays. Corresponding asymmetric profiles were generated to match the average shape of the resected surface of the tibia based on a detailed morphometric analysis of anatomic data. Both designs were proportionally scaled to generate a set of 7 different sizes. Computer models of eight tibias were selected from a large anatomic collection. The proximal tibia was resected perpendicular to the canal axis with a posterior slope of 5 degrees at a depth of 5 mm (medial). Eleven experienced joint surgeons and twelve trainees individually determined the ideal size and placement of each tray on each of the 8 resected tibias. The rotational alignment, coverage of the resected bony surface, and extent of overhang of the tray beyond the cortical boundary were measured for each implantation. Differences in the parameters defining the implantations of the surgeons and trainees were evaluated statistically.
Results:
Bony coverage was significantly greater with the asymmetric vs. the symmetric design (87.0 ± 4.1% vs. 75.6 ± 4.0%; p < 0.0001). Coverage was less than 75% in 37% of symmetric trays, whereas the worst coverage obtained with the asymmetric design was 77.0%. Clinically significant cortical overhang (>1 mm) was present in 35% of symmetric vs. 11% of asymmetric cases (p < 0.0001). On average, the asymmetric tray was placed in 4.1 ± 3.7° of external rotation vs. 1.6 ± 4.6° for the symmetric tray (p < 0.0001). The tray was implanted in some degree of internal rotation in 24% of cases, 15% for the asymmetric design vs. 33% for the symmetric (p < 0.0001). There was minimal difference between the results of implantations performed by trainees vs. experienced surgeons, in terms of tibial coverage (p = 0.245), cortical overhang (p = 0.735), or the prevalence of internal rotation (p = 0.147). Trainees placed 6.3% of all cases in severe internal rotation (>5°) compared with 12.5% of surgeon cases (p = 0.154).
Discussion
-
1.
The incidence of malrotation was substantially less with anatomical vs symmetrical tibial trays.
-
2.
The asymmetric design was also associated with a large reduction in cortical overhang and increased coverage of the resected tibial surface.
-
3.
There was no overall difference between the performance of trainees and experienced joint surgeons, regardless of the design of the implant. This suggests that current training and surgical guides are inadequate in achieving correct positioning of the tibial component in TKR.
