The most important findings of this study were that two-thirds of Asian osteoarthritis patients could achieve high flexion (> 130°) after TKA, which would increase the ease of high-flexion activities (sitting cross-legged and standing up from the floor) and patient satisfaction. The performance of high-flexion activities also increased the quality of life after TKA, while postoperative high flexion of TKA knees did not.
Greater flexion is believed to improve the clinical outcomes of TKA. However, the relationship between ROM and functional outcome is unclear. Some studies have reported that greater flexion after TKA is correlated with improved clinical outcomes and quality of life [8, 12, 13], whereas another found no correlation between greater flexion and clinical outcomes . Most of these studies evaluated Western patients who had an average knee flexion < 120° . Moreover, most patient-based questionnaires were not designed for use in high-flexion TKA patients (e.g., no extra points were scored for ROM > 125°). Therefore, data on whether greater knee flexion leads to improved patient satisfaction and quality of life after TKA remain limited. In this study, we compared patient satisfaction and quality of life after TKA between groups who did and did not achieve > 130° of knee flexion. The ability to perform several high-flexion activities was also evaluated, to investigate the relationship with patient satisfaction and quality of life.
Despite the overall favorable results after TKA, studies have estimated that 11–20% of TKA patients are dissatisfied after surgery [6, 15, 16]. However, some differences in satisfaction rates after TKA are seen between Western and Asian populations [17–19], which might arise from differences in patient expectations and living habits. Most ADLs require 90–120° knee flexion, while kneeling, squatting, and sitting cross-legged, which necessitate flexion of the knee joint beyond 120°, are also required for various lifestyle activities, including cultural and religious activities in Asian populations . In one study, TKA failed to meet expectations regarding kneeling, squatting, and stair climbing . In a prospective cohort, the largest proportions of patients with unfulfilled expectations were those unable to kneel (47%) or squat (44%) . To meet patient expectations and ensure satisfaction, it is important to reproduce the preoperative knee flexion angle after TKA. However, high-flexion activities are also affected by the efficiency of the quadriceps, stability, and kinematics during deep knee flexion. In a retrospective study of 1,013 TKAs of 748 Chinese patients, the top six items with respect to dissatisfaction were sitting with the legs crossed, squatting, walking fast or jogging, knee clunking, abnormal feeling in the knee, and climbing stairs . More than half of the patients in their study were not satisfied with their ability to squat. In another survey of an Asian population, high-flexion activity ranked lowest for satisfaction among the study variables, and was one of the highest ranked variables in which improvement was desired, reflecting its importance to patients after TKA . Although we included more than 600 knees with > 130° of knee flexion after TKA, the ability to kneel or squat was not achieved in more than 50% of the patients, and the ability to stand from the floor was not achieved in more than 35%. This poor rate of kneeling and squatting ability is consistent with other studies [5, 21, 23], although those studies did not investigate patient satisfaction.
However, high flexion is not always reported to be correlated with functional outcome. A retrospective review of TKAs performed due to a diagnosis of osteoarthritis reported that obtaining deep flexion conferred no benefit regarding overall knee function . Another study reported no significant difference in satisfaction among three groups classified according to knee flexion: low (≤ 110°), intermediate (111–130°), or high (> 130°) . Two other studies similarly found no significant correlation of flexion with patient satisfaction or pain, although there was a positive correlation between increased postoperative flexion and the ability to perform ADL [3, 12]. However, those studies included relatively few cases, and most examined Western populations, in which the average knee flexion is typically low.
Several other factors have been suggested to influence patient satisfaction, including the diagnosis, deformity, age, gender, surgical technique, postoperative pain control and rehabilitation, and lifestyle . In the present study, male gender and bilateral TKA were significantly associated with patient satisfaction and quality of life after TKA. In a previous study of the factors predicting the Forgotten Joint Score after TKA, the “excellent” cluster included mainly male patients with high flexion and low BMI .
The surgeon should be aware of the potential complications associated with performing high-flexion activities after TKA, including excessive wear, fracture, and dislocation of the cam-post mechanism. In a previous study, the mean internal rotation of the tibial component during kneeling exceeded the manufacturer’s safety range, increasing the risk of edge loading not only in the posterolateral area of the polyethylene insert, but also in the post-cam contact area . Another study revealed that post-cam contact stress doubled at 150º of knee flexion; as the average internal rotation of the tibia was > 10°, at which point edge loading readily occurs in this type of prosthesis . Deep-flexion activities generate 1- to 13-times higher net quadriceps moments than walking. High flexion may also be associated with TKA cam-post instability. An in vivo study reported greater contact stress with increasing flexion, which could potentially lead to greater wear, increased patellar fracture, or loosening and earlier failure of the polyethylene insert . They also observed cam-post disengagement at high flexion angles.
Our study was a retrospective review of a prospectively collected database and had several limitations. First, we focused on clinical outcome including high flexion activities, patient satisfaction and quality of life. Patient expectation, radiological outcomes and implant survival were not analyzed. Second, most of the enrolled patients had a diagnosis of osteoarthritis and were female. However, female predominance is a feature of Asian populations undergoing TKA. Our study was also performed in one center, so the influence of cultural and demographic factors on satisfaction and quality of life could not be considered, thus limiting the generalizability. Lastly, we did not evaluate patient expectations, where patient satisfaction is closely related to their expectations. However, despite these partly unavoidable limitations, this study provides detailed insight into the long-term results of TKA.