In the latest reports, the use of navigation, the different types of inserts (fixed-bearing / rotating platform) and the different surgical approaches (modified subvastus / medial parapatellar ) have no influence the flexion angle after total knee arthroplasty [7
What is the factor that improves flexion angle after the total knee arthroplasty?
Our study investigated the effects of implant posterior design changes on postoperative flexion angle following TKA. While the need for design changes to allow greater flexion has been recognised, this is the first study to actually provide clinical data in CR mobile-bearing TKA.
In past studies investigating the posterior condylar design of femoral components, Anouchi et al. [10
] used Advantim (Wright Medical Technology, Arlington, TN) and reported no significant difference in degree of postoperative improvement in flexion angle between 232 conventional TKAs and 50 TKAs using a larger posterior condyle for the femoral component. In that study, mean flexion angle two years after surgery was 107°
10°, and mean postoperative flexion angle for the Hi-Flex group in our study was about 10° greater, at 117.0°
11.3°, suggesting that the effect of the change in their posterior condyle was insufficient.
In PS mobile-bearing TKA, Gupta et al. [11
] and Ranawatt et al. [12
] used P.F.C.Sigma RP-F and P.F.C.Sigma RP (DePuy Orthopaedics, Inc.,Warsaw, IN) and reported a significant difference in degree of postoperative improvement in flexion angle between 50 Sigma RP-F TKAs and 50 Sigma RP TKAs. The P.F.C.Sigma RP-F changed the radius of curvature of the posterior femoral condyle less than the P.F.C.Sigma RP, suggesting that the effect of the change in their posterior condyle was sufficient.
In our study, by decreasing the curvature of the posterior condyle for the femoral component, the posterior space was enlarged to allow easier rollback mechanism, thus improving postoperative flexion angle. Furthermore, by moving the base of the insert dish in a posterior direction, the height of the posterior lip on the insert was lowered to reduce posterior impingement, thus leading to better results.
In addition, Walker et al. [13
] demonstrated that the factors which contribute to the improvement of postoperative ROM are proximal femoral condyle, large posterior offset, small anterior distance, and large posterior slope. In this study, the small anterior distance due to the posterior shift in the base of the insert dish would be useful.
We examined only the clinical data and previously reported in vivo kinematics of Standard DBK with fluoroscopy [14
]; we are now investigating the difference in the contact points between Standard DBK and Hi-Flex DBK. Our latest investigation of in vivo kinematics demonstrated that the medial and lateral nearest contact points of Hi-Flex DBK moved significantly posterior compared with Standard DBK (unpublished data).
In our study, in order to exclude the effect of the postoperative period, we compared the postoperative flexion angle at 12 months after operation. Because the periods of the operations between Standard DBK and Hi-Flex DBK were different, if we used the latest follow-up data, the average follow-up periods in each group must be different. However, even when we compared the latest follow-up data, postoperative flexion angle, postoperative range of motion, delta flex and delta range were significantly improved for the Hi-Flex group compared to the Standard group.
Ritter et al. reported that postoperative flexion angle correlated to post- and intraoperative flexion angles [1
]. Anouchi et al. assessed delta flex 24 months after surgery by dividing patients into three groups with respect to preoperative flexion angle (<90°, 91–104° and ≥105°) and documented that degree of improvement for the <90° group was 19° greater compared to the 91–104° group and 35° greater when compared to the ≥105° group (p
]. Thus, the preoperative flexion angle correlates to the postoperative flexion angle, because if the preoperative flexion angle is small, the degree of improvement is large, but if the preoperative flexion angle is large, the postoperative flexion angle decreases.
In our study, a significant positive correlation existed between pre- and postoperative flexion angles for the Hi-Flex and Standard groups (Fig. ). Furthermore, a significant negative correlation existed between preoperative flexion angle and delta flex. In this manner, the results in our study are consistent with previous reports [1
In our study, the value of delta flex would have been zero at 115.4° for the Hi-Flex group and 104° for the Standard group (Fig. ). We believe that this difference of 10.7° due to the component design change is important for obtaining better results using the Hi-Flex-design DBK.
Regarding the relationship between delta flex and disease type, Schurman et al. reported that the degree of improvement for RA and OA was −8° and −3°, respectively, thus significantly greater for OA [15
]. However, Ritter et al. documented no significant differences in postoperative range of motion among patients with OA, RA and ON [1
]. In our study, delta flex for the Standard group was −4.9° for RA patients and −12.4° for OA patients, and a significant difference was noted between RA and OA. However, no significant differences were found between RA and OA in the Hi-Flex group, although delta flex tended to be better for RA patients.
For the relationship between delta flex and age, Ritter et al. reported that mean postoperative flexion angle was lower for women than for men [1
]. In our study, delta flex for female patients in the Hi-Flex group was significantly greater than that for male patients in the Hi-Flex group.
Figgie et al. reported that, in CR-type TKA, when JL was set high, PCL was overly tense, hindering the rollback mechanism and impairing knee flexion, and they noted that the JL should not increase by more than 8 mm [5
]. Kim et al. performed fixed- and mobile-bearing CR-type TKAs and reported that postoperative flexion angle for a group with an increase of ≥5 mm in JL was lower than that for a group with an increase of <5 mm [16
]. In our study, JL was significantly increased for both groups, but no significant correlation between delta JL and delta flex was identified. The reason for this was that degree of increase in JL was suppressed for the Hi-Flex and Standard groups at +2.6 and +3.9 mm, respectively.
Bellemans et al. studied 30 patients who underwent CR-type TKA with postoperative flexion of ≥100° by 3-dimensional video fluoroscopy [6
]. They reported that because the posterior surface of the femur and insert impinged during maximum flexion in 72% of patients, maintaining the posterior condylar offset would be important. Furthermore, when PCO decreased by 1 mm postoperatively, postoperative flexion angle decreased by 6.1°. In our study, no significant change in PCO was noted for either group. In addition, delta PCO for the Hi-Flex and Standard groups was −0.5 and −0.6 mm, respectively, and no correlation existed between delta PCO and delta flex. However, preoperative PCO for the Hi-Flex and Standard groups was 26.4 and 24.5 mm, respectively, and postoperatively the PCO was 25.9 and 23.9 mm, respectively. The relationship between flexion angle and PCO was investigated before and after surgery, but no correlation was seen between preoperative flexion angle and preoperative PCO, between postoperative flexion angle and postoperative PCO in the Hi-Flex group or between postoperative flexion angle and postoperative PCO in the Standard group. In other words, these findings suggest that PCO is not related to the flexion angle.
In a clinical report, Ryu et al. divided patients into groups displaying favourable post-TKA flexion (≥120°) and poor post-TKA flexion (≤90°), and documented that patellar thickness for the poor flexion group was increased [17
]. They subsequently recommended that patellar thickness should remain at the preoperative thickness. Barrack et al. resurfaced the patella within 1 mm of original thickness using calipers and reported no significant differences in postoperative range of motion between a group with patellar replacement and another group without patellar replacement [18
]. In our study, no significant difference was noted in patellar thickness before and after surgery in either group. Also, delta PT for the Hi-Flex and Standard groups was −0.1 and +0.3 mm, respectively, and exhibited no correlation to delta flex. Furthermore, no significant difference in delta flex was identified irrespective of whether the patella was replaced.
In conclusion, postoperative flexion angle, postoperative range of motion, delta flex and delta range were significantly improved for the Hi-Flex group compared to the Standard group. In CR mobile-bearing TKA, the results suggest that two design changes improve post-TKA flexion angle of the knee: a smaller radius of curvature for the posterior condyle and a posterior shift in the base of the insert dish.