Han et al.

BMC Musculoskeletal Disorders (2021) 22:565

https://doi.org/10.1186/s12891-021-04369-4

RESEARCH ARTICLE Open Access

A high degree of knee flexion after TKA

promotes the ability to perform high-
flexion activities and patient satisfaction in
Asian population
Hyuk-Soo Han1, Jong Seop Kim1, Bora Lee2, Sungho Won3 and Myung Chul Lee1*

Abstract
Background: This study investigated whether achieving a higher degree of knee flexion after TKA promoted the
ability to perform high-flexion activities, as well as patient satisfaction and quality of life.
Methods: Clinical data on 912 consecutive primary TKA cases involving a single high-flexion posterior stabilized
fixed-bearing prosthesis were retrospectively analyzed. Demographic and clinical data were collected, including
knee flexion angle, the ability to perform high-flexion activities, and patient satisfaction and quality of life.
Results: Of the cases, 619 (68%) achieved > 130° of knee flexion after TKA (high flexion group). Knee flexion angle
and clinical scores showed significant annual changes, with the maximum improvement seen at 5 years and slight
deterioration observed at 10 years postoperatively. In the high flexion group, more than 50% of the patients could
not kneel or squat, and 35% could not stand up from on the floor. Multivariate analysis revealed that > 130° of knee
flexion, the ability to perform high-flexion activities (sitting cross-legged and standing up from the floor), male
gender, and bilateral TKA were significantly associated with patient satisfaction after TKA, while the ability to
perform high-flexion activities (sitting cross-legged and standing up from the floor), male gender, and bilateral TKA
were significantly associated with patient quality of life after TKA.
Conclusions: High knee flexion angle (> 130°) after TKA increased the ease of high-flexion activities and patient
satisfaction. The ease of high-flexion activities also increased quality of life after TKA in our Asian patients, who
frequently engage in these activities in daily life.
Keywords: Knee flexion, High-flexion activity, Patient satisfaction, Quality of life, Total knee arthroplasty

Background improves functional ability [2, 3], and patients with a

The main goals of total knee arthroplasty (TKA) in older range of motion (ROM) of 128–132° achieved the best
patients are pain relief and functional improvement in functional results [4]. Knee flexion < 130° after TKA pre-
common activities of daily living (ADL) [1]. Restoration cluded the performance of high-flexion activities, such
of knee flexion is an important determinant of the func- as squatting, sitting cross-legged, or kneeling in Asian
tional outcome after TKA. Flexion beyond 110° populations [5]. Similarly, TKA failed to meet expect-
ation for high-flexion activities in Western populations
[6]. Crouching and kneeling are the activities most lim-
* Correspondence: leemc@snu.ac.kr
1
Department of Orthopaedic Surgery, Seoul National University Hospital, 101 ited in patients with osteoarthritis of the knee [7]. Fol-
Daehak-ro, Jongno-gu, Seoul 03080, South Korea lowing TKA, kneeling was reported as the second most
Full list of author information is available at the end of the article

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Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 2 of 9

difficult activity to perform, after squatting. Failure to re- years). The main diagnosis for TKA was osteoarthritis
store the ability to kneel and squat, and the importance (903 knees; 99%).
of these movements to ADL, may contribute to lower The primary TKAs were performed by a single sur-
satisfaction with TKA. geon using conventional instruments. The posterior cru-
High-flexion TKA is designed to achieve the > 130° of ciate ligament was sacrificed and fixed-bearing tibia
knee flexion necessary for ADL, including kneeling and inserts were implanted in all patients. The patella was
gardening [8]. However, some patients do not achieve selectively resurfaced and the indications for patellar
satisfactory flexion or performance of high-flexion activ- preservation were a thin patella (< 20 mm thick intraop-
ities after TKA. Although limited ROM is a significant eratively), nearly normal articular cartilage (International
cause of poor functional outcome and patient dissatis- Cartilage Repair Society [ICRS] Grade 0 or 1), no pre-
faction, the associations among high degree of flexion, operative patellar pain, or poor patellar bone quality. All
ability to perform high-flexion activities, and patient sat- prostheses were fixed with cement. All patients followed
isfaction after TKA are rarely studied [2, 9]. The existing the same postoperative rehabilitation protocol, starting
studies involved Western patients, who do not tend to continuous passive motion the day after surgery and be-
use deep flexion frequently compared to Asian patients. ginning full weight-bearing (as tolerated) 2 days after
Therefore, this study investigated whether a higher de- surgery. Passive gentle flexion was performed until the
gree of knee flexion after high-flexion TKA is associated patients achieved nearly 130° of knee flexion.
with the performance of high-flexion activities, increased Patients were clinically assessed preoperatively, post-
patient satisfaction and higher quality of life in an Asian operatively at 6 weeks, 3 months, and 1 year, and annu-
population. We hypothesized that high flexion (> 130°) ally thereafter using the ROM, Knee Society knee score
of TKA knees postoperatively is associated with en- (KSS), Knee Society function score (KSFS), Hospital for
hanced performance of high-flexion activities, and im- Special Surgery (HSS) score, and Western Ontario and
proved patient satisfaction and quality of life. McMaster Universities Osteoarthritis Index (WOMAC).
The non-weight-bearing maximal knee flexion angle was
measured using a standard goniometer in supine pos-
Methods ition by two independent physician assistants, preopera-
We retrospectively reviewed prospectively collected data tively and at each follow-up. While measuring the
for 1069 consecutive primary TKA cases using a single maximal knee flexion angle, patients were asked to bend
high-flexion posterior stabilized fixed-bearing prosthesis their knees as much as they could or until they felt slight
(NexGen®; Zimmer, Warsaw, IN, USA) from July 2001 pain while lying in a supine position. To evaluate the
to July 2012. Of the 1069 knees, 157 were excluded from ability to perform high-flexion activities, the patients
the analysis because the duration of follow-up was less were asked about their ability to kneel, squat, sit cross-
than 2 years (n = 69) or they had revision surgeries (n- legged, and rise after sitting on the floor. The ability to
24) or missing data (n = 64), leaving 912 knees in 610 pa- do high-flexion activities was documented by easiness;
tients eligible for this study (Fig. 1). There were 43 men impossible, hard to do, or easy to do. A questionnaire
and 567 women (mean age, 69 years; range: 41–87 years). scored on a 5-point Likert scale (completely satisfied, 5;
The median follow-up was 5.0 years (range: 2.0–14.3 satisfied, 4; barely acceptable, 3; unsatisfied, 2;

Fig. 1 Flow diagram for the patient enrollment

Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 3 of 9

completely unsatisfied, 1) was used to evaluate patient flexion ≥130°, as a measure of patient satisfaction and
satisfaction, and a questionnaire scored on a 7-point quality of life.
Likert scale was used to evaluate the change in patient A two-sided p < 0.05 was considered statistically sig-
quality of life [10, 11]. The longitudinal data including nificant and all analyses were performed using R soft-
preoperative and postoperative 2-, 5-, and 10-year out- ware (ver. 3.6.1; The R Foundation for Statistical
come were used in the analysis, except the final knee Computing, Vienna, Austria).
flexion angle, satisfaction and quality of life in which the
most recent follow-up data were used. Results
This study complied with the Helsinki Declaration and Table 1 summarizes the demographic and preoperative
was approved by the ethics committee of Seoul National characteristics according to final knee flexion after TKA.
University Hospital (IRB No. 0603–105-170). Knees with ≥130° of flexion after TKA (Group H, n =
619, 68%) had a greater preoperative flexion angle than
Statistical analysis those that had < 130° of flexion after TKA (Group N,
Continuous data are provided as the mean and standard n = 291, 32%). No other demographic or preoperative
deviation, while categorical data are presented as fre- clinical variables differed between the two groups. The
quencies and proportions. The consecutive patients were proportions of the cases with patella resurfacing were
divided into two groups according to whether or not > similar in the two groups (group H, n = 516, 83.4% vs.
130° of knee flexion was achieved. The groups were group N, n = 251, 86.3%). Table 2 summarizes the mean
compared using Student’s t-test or Wilcoxon’s rank-sum annual changes in clinical variables in both groups after
test for continuous data, according to the normality of TKA. Knee flexion angle, KSFS, HSS knee score, and
the data distribution. Pearson’s chi-square test or Fish- WOMAC total and physical function scores after TKA
er’s exact test was used to compare categorical data showed significant annual changes in both groups. These
depending on whether the assumptions for Pearson’s clinical variables showed maximal improvement at 5
chi-square test were met. Within group pre- and postop- years postoperatively, and had deteriorated slightly at 10
erative continuous data were compared with paired t- years postoperatively. Except for knee flexion angle, the
tests. clinical variables did not differ significantly between the
Linear mixed models were generated about the ability two groups. Table 3 summarizes the ability to perform
to perform high-flexion activities with patient as a ran- high-flexion activities after TKA in both groups. A larger
dom effect and assuming a random slope for the follow- proportion of patients in Group H could easily perform
up time. Based on these models, estimated marginal various high-flexion activities compared with Group N
least-square means were calculated at four time points: (p < 0.001). The proportions of patients who could kneel
preoperatively, and at 2, 5, and 10 years postoperatively. and squat easily showed a trend to increase with time in
Group (final knee flexion < 130° vs. ≥ 130°) and follow- both groups, although the changes showed no statistical
up time were included as main effects, in addition to the significances. However, the proportions of those who
interaction term and covariates of age, sex, body mass could sit cross-legged or stand up from the floor easily
index (BMI), bilateral TKA, and patellar resurfacing. The did not change over time.
follow-up time was modeled as a quadratic polynomial. Tables 4 and 5 summarize the results of uni- and
The covariance matrix was selected based on the Akaike multivariate analyses of the association of > 130° of knee
information criterion among an unstructured, com- flexion with patient satisfaction and quality of life after
pound symmetric, or autoregression (1) matrix. Signifi- TKA. Postoperative knee flexion angle, > 130° of knee
cant differences between the groups were tested for at flexion, ability to perform high-flexion activities (sit
each time point. cross-legged and stand up from the floor), male gender,
A generalized estimating equation was fitted to the and bilateral TKA were significant perioperative predic-
performance of high-flexion activities, patient satisfac- tors of patient satisfaction in the univariate analyses.
tion, and quality of life after TKA, considering each pa- Multivariate analysis revealed that > 130° of knee flexion,
tient as a random effect. The group and follow-up time ability to perform high-flexion activities (sit cross-legged
were included as main effects, along with the covariates and stand up from the floor), male gender, and bilateral
age, sex, BMI, bilateral TKA, and patellar resurfacing. A TKA remained as factors significantly associated with
cumulative logits (proportional odds) model was used, patient satisfaction after TKA. Similarly, the postopera-
since the proportional odds assumptions were upheld. tive knee flexion angle, > 130° of knee flexion, ability to
As the postoperative outcome, we estimated the prob- perform high-flexion activities (sit cross-legged and
ability of having the ability to perform high-flexion activ- stand up from the floor), male gender, and bilateral TKA
ities after 2, 5, and 10 years. Odds ratios were calculated were significant perioperative predictors of quality of life
for the performance of high-flexion activities or knee in univariate analyses; the ability to perform high-flexion
Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 4 of 9

Table 1 The demographics and preoperative characteristics according to final knee flexion after TKA
Variable Final knee flexion p-value
≥ 130° (N = 619) < 130° (N = 291)
Age (years) 69.1 ± 6.6 68.4 ± 8.4 n.s.
Sex (female) 575 (92.9%) 274 (94.2%) n.s.
Height (cm) 152.9 ± 6.3 152.3 ± 6.7 n.s.
Weight (kg) 62.5 ± 8.6 62.9 ± 8.9 n.s.
Body mass index (kg/m2) 26.7 ± 3.1 27.1 ± 3.4 n.s.
Diagnosis n.s.
Osteoarthritis 613 (99.0%) 288 (99.0%)
Rheumatoid arthritis 2 (0.3%) 2 (0.7%)
Others 4 (0.6%) 1 (0.3%)
Side (right) 306 (49.4%) 154 (52.9%) n.s.
Bilateral TKA 426 (68.8%) 183 (62.9%) n.s.
Patellar resurfacing 516 (83.4%) 251 (86.3%) n.s.
Additional lateral release 4 (0.7%) 3 (1.0%) n.s.
History of operation on the knee 11 (1.8%) 10 (3.4%) n.s.
Follow-up time (years) 5.1 [2.9, 7.8] 4.6 [2.0, 7.0] 0.010
Preoperative functional evaluation
Knee flexion (°) 129.2 ± 11.4 113.9 ± 19.8 < 0.001
Flexion contracture (°) 12.3 ± 8.2 13.8 ± 8.6 0.012
Range of motion (°) 116.8 ± 15.4 100.1 ± 23.3 < 0.001
Knee Society knee score 47.4 ± 16.8 46.1 ± 18.2 n.s.
Knee Society function score 41.3 ± 18.9 42.0 ± 19.3 n.s.
Hospital for Special Surgery score 56.6 ± 15.2 55.0 ± 15.6 n.s.
WOMAC - pain 9.1 ± 3.5 8.9 ± 3.4 n.s.
WOMAC - stiffness 4.2 ± 1.9 4.1 ± 1.9 n.s.
WOMAC - physical function 38.2 ± 14.6 37.9 ± 14.0 n.s.
WOMAC total 50.4 ± 18.8 49.7 ± 18.3 n.s.
WOMAC Western Ontario and McMaster Universities Osteoarthritis Index, n.s. not significant
Data was reported as mean ± standard deviation or median [interquartile range (IQR)] for continuous variable and frequency (percentage) for categorical variables
P-values were calculated by Student’s t-test or Mann-Whitney U test for continuous variables and chi-square test or Fisher’s exact test for categorical variables
as appropriate

activities (sit cross-legged and stand up from the floor), have reported that greater flexion after TKA is corre-
male gender, and bilateral TKA remained significant in lated with improved clinical outcomes and quality of life
multivariate analysis. [12, 13], whereas another found no correlation between
greater flexion and clinical outcomes [3]. Most of these
Discussion studies evaluated Western patients who had an average
The most important findings of this study were that knee flexion < 120° [14]. Moreover, most patient-based
two-thirds (619/912, 67.9%) of Asian osteoarthritis pa- questionnaires were not designed for use in high-flexion
tients could achieve high flexion (> 130°) after TKA, TKA patients (e.g., no extra points were scored for
which would increase the ease of high-flexion activities ROM > 125°). Therefore, data on whether greater knee
(sitting cross-legged and standing up from the floor) and flexion leads to improved patient satisfaction and quality
patient satisfaction. The performance of high-flexion ac- of life after TKA remain limited. In this study, we com-
tivities also increased the quality of life after TKA, while pared patient satisfaction and quality of life after TKA
postoperative high flexion of TKA knees did not. between groups who did and did not achieve > 130° of
Greater flexion is believed to improve the clinical out- knee flexion. The ability to perform several high-flexion
comes of TKA [9]. However, the relationship between activities was also evaluated, to investigate the relation-
ROM and functional outcome is unclear. Some studies ship with patient satisfaction and quality of life.
Table 2 Mean annual change of clinical variables in groups according to final knee flexion after TKA
Variable Estimated marginal mean (95% CI) Mean change/year (95% CI) p for interaction with time
Preoperative 2 years 5 years 10 year linear quadratic linear quadratic
(n = 912) (n = 912) (n = 718) (n = 261)
Knee flexion (°)
Final knee flexion < 130 119.5 (114.6, 124.3) 123.0 (118.2, 127.8) 124.7 (119.8, 129.6) 118.2 (112.5, 123.8) 2.2 (1.6, 2.9) −0.2 (− 0.3, − 0.2)
0.006 0.023
Final knee flexion ≥130 131.4 (126.6, 136.2)*** 134.6 (129.8, 139.3)*** 137.3 (132.5, 142.1)*** 136.6 (131.5, 141.7)*** 1.9 (1.4, 2.3) −0.1 (− 0.2, − 0.1)
Flexion contracture (°)
Final knee flexion < 130 10.4 (8.2, 12.5) 3.3 (1.2, 5.4) −2.1 (−4.2, 0.0) 2.8 (0.4, 5.2) −4.2 (− 4.5, −3.9) 0.4 (0.3, 0.4)
n.s. n.s.
Final knee flexion ≥130 9.7 (7.6, 11.7) 3.2 (1.1, 5.2) −1.9 (−4.0, 0.2) 2.0 (−0.2, 4.2) −3.9 (− 4.1, − 3.7) 0.3 (0.3, 0.3)
Han et al. BMC Musculoskeletal Disorders

Range of motion (°)

Final knee flexion < 130 107.7 (100.2, 115.2) 117.4 (109.9, 124.8) 124.0 (116.5, 131.6) 114.3 (105.6, 123.1) 5.9 (4.8, 7.0) −0.5 (− 0.7, − 0.4)
n.s. 0.028
Final knee flexion ≥130 119.5 (112.1, 126.9) 127.7 (120.4, 135.1) 134.8 (127.4, 142.2) 132.5 (124.6, 140.4)** 4.8 (4.1, 5.5) −0.4 (− 0.4, − 0.3)
Knee Society score
(2021) 22:565

Final knee flexion < 130 58.9 (52.4, 65.5) 85.7 (79.3, 92.1) 105.9 (99.4, 112.4) 86.2 (78.5, 94.0) 16.1 (15.0, 17.1) −1.3 (− 1.5, − 1.2)
n.s. n.s.
Final knee flexion ≥130 59.2 (52.7, 65.6) 85.9 (79.6, 92.3) 106.2 (99.8, 112.6) 86.9 (80.1, 93.8) 16.0 (15.3, 16.7) −1.3 (− 1.4, − 1.3)
Knee Society function score
Final knee flexion < 130 50.3 (40.3, 60.3) 73.0 (63.2, 82.9) 87.2 (77.2, 97.3) 57.7 (45.9, 69.5) 14.0 (12.7, 15.4) −1.3 (− 1.5, − 1.2)
0.007 n.s.
Final knee flexion ≥130 50.7 (40.8, 60.6) 73.5 (63.6, 83.3) 90.1 (80.2, 100.0) 71.0 (60.4, 81.5) 13.7 (12.9, 14.6) −1.2 (− 1.3, − 1.1)
Hospital for Special Surgery score
Final knee flexion < 130 63.7 (57.5, 69.9) 83.8 (77.6, 89.9) 97.9 (91.7, 104.1) 78.8 (71.5, 86.0) 12.2 (11.4, 13.0) −1.1 (− 1.2, − 1.0)
0.029 n.s.
Final knee flexion ≥130 64.1 (58.0, 70.2) 84.7 (78.7, 90.8) 100.2 (94.1, 106.3) 84.8 (78.3, 91.3) 12.4 (11.8, 12.9) −1.0 (− 1.1, − 1.0)
WOMAC (total)
Final knee flexion < 130 39.9 (32.6, 47.1) 19.9 (12.7, 27.0) 4.9 (−2.4, 12.1) 20.0 (11.9, 28.1) −12.0 (− 13.0, − 11.1) 1.0 (0.9, 1.1)
0.003 n.s.
Final knee flexion ≥130 40.7 (33.5, 47.8) 18.8 (11.7, 26.0) 1.8 (−5.4, 8.9) 15.0 (7.5, 22.5) −13.0 (− 13.6, − 12.4) 1.0 (1.0, 1.1)
WOMAC (pain)
Final knee flexion < 130 6.4 (5.1, 7.6) 2.1 (0.9, 3.4) −1.3 (−2.5, −0.0) 1.0 (−0.4, 2.4) − 2.5 (− 2.7, − 2.3) 0.2 (0.2, 0.2)
n.s. n.s.
Final knee flexion ≥130 6.6 (5.4, 7.8) 2.1 (0.9, 3.3) − 1.5 (− 2.7, − 0.3) 1.1 (− 0.2, 2.4) −2.7 (− 2.8, − 2.6) 0.2 (0.2, 0.2)
WOMAC (stiffness)
Final knee flexion < 130 3.2 (2.5, 3.9) 1.4 (0.7, 2.1) 0.0 (− 0.7, 0.7) 1.0 (0.2, 1.8) −1.1 (− 1.2, − 1.0) 0.1 (0.1, 0.1)
n.s. 0.027
Final knee flexion ≥130 3.3 (2.6, 4.1) 1.3 (0.6, 2.0) −0.2 (− 0.9, 0.5) 1.1 (0.4, 1.8) −1.2 (− 1.3, − 1.1) 0.1 (0.1, 0.1)
WOMAC (physical function)
Final knee flexion < 130 30.8 (24.9, 36.6) 16.2 (10.5, 21.9) 5.7 (−0.1, 11.5) 18.6 (12.0, 25.2) −8.8 (−9.6, −8.0) 0.76 (0.67, 0.85)
0.001 n.s.
Final knee flexion ≥130 31.0 (25.3, 36.7) 15.2 (9.5, 20.9) 3.0 (−2.7, 8.7) 13.3 (7.3, 19.3) −9.4 (−9.9, −8.9) 0.77 (0.71, 0.82)
WOMAC Western Ontario and McMaster Universities Osteoarthritis Index, n.s. not significant
***, p < 0.001; **, p < 0.01; *, p < 0.05, compared two group at each point
Page 5 of 9
Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 6 of 9

Table 3 Ability to do high-flexion activities in groups according to final knee flexion after TKA at follow-up
High-flexion Impossible (Estimated Hard to do (Estimated Easy to do (Estimated Odd ratio for p- p for
activity proportion, %)a proportion, %)a proportion, %)a easy to do value interaction
(95% CI) with time
2 years 5 years 10 years 2 years 5 years 10 years 2 years 5 years 10 years
Kneel
Final knee flexion 80.0 68.8 45.1 14.1 21.0 31.6 5.9 10.2 23.3 1 (Reference)
< 130°
< 0.001 n.s.
Final knee flexion 59.2 50.2 35.5 26.1 29.9 33.3 14.7 19.9 31.3 3.25 (1.88–5.64)
≥130°
Squat
Final knee flexion 72.9 60.5 37.5 18.6 25.5 33.1 8.5 14.0 29.4 1 (Reference)
< 130°
< 0.001 n.s.
Final knee flexion 47.1 39.4 27.8 31.0 32.8 32.8 22.0 27.8 39.4 3.56 (2.08–6.12)
≥130°
Sit cross-legged
Final knee flexion 41.5 36.5 28.7 32.4 33.2 33.0 26.0 30.3 38.3 1 (Reference)
< 130°
< 0.001 n.s.
Final knee flexion 13.9 13.9 13.9 25.4 25.3 25.3 60.7 60.8 60.9 5.04 (2.95–8.63)
≥130°
Stand from the floor
Final knee flexion 14.5 15.1 16.1 59.5 58.4 56.6 25.9 26.5 27.3 1 (Reference)
< 130°
< 0.001 n.s.
Final knee flexion 35.8 29.5 20.6 31.0 37.4 49.0 33.3 33.1 30.4 4.09 (2.21–7.58)
≥130°
n.s. not significant
a
Estimated after adjustment for age, sex, body mass index, bilateral TKA, and patellar resurfacing

Despite the overall favorable results after TKA, studies among the study variables, and was one of the highest
have estimated that 11–20% of TKA patients are dissat- ranked variables in which improvement was desired,
isfied after surgery [7, 15, 16]. However, significant dif- reflecting its importance to patients after TKA [24]. Al-
ferences in satisfaction rates and the kinds of limited though we included more than 600 knees with > 130° of
activities after TKA are seen between Western and knee flexion after TKA, the ability to kneel or squat was
Asian populations [17–19], which might arise from dif- not achieved in more than 50% of the patients, and the
ferences in patient expectations and living habits. Most ability to stand from the floor was not achieved in more
ADLs require 90–120° knee flexion, while kneeling, than 35%. This poor rate of kneeling and squatting abil-
squatting, and sitting cross-legged, which necessitate ity is consistent with other studies [6, 21, 25], although
flexion of the knee joint beyond 120°, are also required those studies did not investigate patient satisfaction.
for various lifestyle activities, including cultural and reli- However, high flexion is not always reported to be corre-
gious activities in Asian populations [20]. In one study, lated with functional outcome. A retrospective review of
TKA failed to meet expectations regarding kneeling, TKAs performed due to a diagnosis of osteoarthritis re-
squatting, and stair climbing [6]. In a prospective cohort, ported that obtaining deep flexion conferred no benefit
the largest proportions of patients with unfulfilled ex- regarding overall knee function [3]. Another study re-
pectations were those unable to kneel (47%) or squat ported no significant difference in satisfaction among
(44%) [21]. To meet patient expectations and ensure sat- three groups classified according to knee flexion: low (≤
isfaction, it is important to reproduce the pre-arthritic 110°), intermediate (111–130°), or high (> 130°) [2]. Two
knee flexion angle after TKA [9]. However, high-flexion other studies similarly found no significant correlation
activities are also affected by the efficiency of the quadri- of flexion with patient satisfaction or pain, although
ceps, stability, and kinematics during deep knee flexion there was a positive correlation between increased post-
[22, 23]. In a retrospective study of 1013 TKAs of 748 operative flexion and the ability to perform ADL [4, 12].
Chinese patients, the top six items with respect to dissat- However, those studies included relatively few cases, and
isfaction were sitting with the legs crossed, squatting, most examined Western populations, in which the aver-
walking fast or jogging, knee clunking, abnormal feeling age knee flexion is typically low.
in the knee, and climbing stairs [17]. More than half of Several other factors have been suggested to influence
the patients in their study were not satisfied with their patient satisfaction, including the diagnosis, deformity,
ability to squat. In another survey of an Asian popula- age, gender, surgical technique, postoperative pain con-
tion, high-flexion activity ranked lowest for satisfaction trol and rehabilitation, and lifestyle [26]. In the present
Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 7 of 9

Table 4 Multivariable analysis results for the association between more than 130 degrees of knee flexion and patients’ satisfaction
after TKA
Variable Univariable Multivariable Multivariable
OR (95% CI) p-value OR (95% CI) p-value OR (95% CI) p-value
Knee flexion after TKA (°) 1.02 (1.01–1.03) 0.001 1.008 (0.997–1.019) n.s.
≥130° of Knee flexion after TKA (°)
No 1 (Reference) 1 (Reference)
Yes 1.82 (1.34–2.47) < 0.001 1.38 (1.007–1.889) 0.045
Ability to do high-flexion activities
Kneel (reference: impossible) Hard to do 0.78 (0.57–1.06) n.s.
Easy to do 1.07 (0.61–1.88) n.s.
Squat (reference: impossible) Hard to do 0.76 (0.55–1.03) n.s.
Easy to do 0.95 (0.55–1.65) n.s.
Sit cross-legged (reference: impossible) Hard to do 1.71 (1.23–2.39) 0.001 1.426 (1.015–2.003) 0.041 1.414 (1.008–1.985) 0.045
Easy to do 3.89 (2.58–5.87) < 0.001 2.557 (1.64–3.988) < 0.001 2.458 (1.573–3.842) < 0.001
Stand from the floor (reference: impossible) Hard to do 3.89 (1.81–8.34) < 0.001 3.104 (1.403–6.87) 0.005 3.107 (1.405–6.867) 0.005
Easy to do 8.15 (3.63–18.28) < 0.001 4.429 (1.886–10.4) 0.001 4.45 (1.897–10.438) 0.001
Age (years) (reference: < 60) 60–74 0.86 (0.5–1.48) n.s.
≥ 75 0.69 (0.36–1.31) n.s.
Male (vs. female) 2.51 (1.32–4.8) 0.005 2.282 (1.217–4.278) 0.01 2.312 (1.234–4.331) 0.009
Body mass index (kg/m2) (reference: < 25) 25 - < 30 1.27 (0.88–1.84) n.s.
≥ 30 1.07 (0.66–1.73) n.s.
Bilateral TKA (vs. unilateral TKA) 1.41 (1.01–1.98) 0.046 1.443 (1.035–2.013) 0.031 1.456 (1.045–2.028) 0.026
Patellar resurfacing (vs. un-resurfacing) 0.68 (0.45–1.04) n.s.
OR odds ratio, CI confidence interval, n.s. not significant

study, male gender and bilateral TKA were significantly contact stress with increasing flexion, which could po-
associated with patient satisfaction and quality of life tentially lead to greater wear, increased patellar fracture,
after TKA. In a previous study of the factors predict- or loosening and earlier failure of the polyethylene insert
ing the Forgotten Joint Score after TKA, the “excel- [31]. They also observed cam-post disengagement at
lent” cluster included mainly male patients with high high flexion angles.
flexion and low BMI [27]. Another study reported Our study was a retrospective review of a prospectively
that bilateral TKA was found to be more common in collected database and had several limitations. First, this
the satisfied group (77.8%) than in the dissatisfied study was not a prospective controlled one. We grouped
group (66.3%), although the difference was not statis- the cases according to the postoperative knee flexion
tically significant [28]. angle without matching related factors, which might
The surgeon should be aware of the potential compli- cause insufficient statistical power. Second, we focused
cations associated with performing high-flexion activities on clinical outcome including high flexion activities, pa-
after TKA, including excessive wear, fracture, and dis- tient satisfaction and quality of life. Radiological out-
location of the cam-post mechanism. In a previous comes and implant survival were not analyzed. Third,
study, the mean internal rotation of the tibial compo- most of the enrolled patients had a diagnosis of osteo-
nent during kneeling exceeded the manufacturer’s safety arthritis and were female. However, female predomin-
range, increasing the risk of edge loading not only in the ance is a feature of Asian populations undergoing TKA.
posterolateral area of the polyethylene insert, but also in Our study was also performed in one center, so the in-
the post-cam contact area [29]. Another study revealed fluence of cultural and demographic factors on satisfac-
that post-cam contact stress doubled at 150° of knee tion and quality of life could not be considered, thus
flexion; as the average internal rotation of the tibia was limiting the generalizability. Fourth, we did not evaluate
> 10°, at which point edge loading readily occurs in this the ability for our patients to perform the high flexion
type of prosthesis [30]. Deep-flexion activities generate activities preoperatively and patient expectations, where
1- to 13-times higher net quadriceps moments than patient satisfaction is closely related to their expecta-
walking. High flexion may also be associated with TKA tions. Lastly, due to the complexity of our data on differ-
cam-post instability. An in vivo study reported greater ent time points, we could not analyze the relationship
Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 8 of 9

Table 5 Multivariable analysis results for the association between more than 130 degrees of knee flexion and patients’ quality of life
after TKA
Variable Univariable Multivariable Multivariable
OR (95% CI) p-value OR (95% CI) p-value OR (95% CI) p-value
Knee flexion after TKA (°) 1.01 (1–1.02) 0.025 1.003 (0.993–1.013) 0.569
≥130° of Knee flexion after TKA (°)
No 1 (Reference) 1 (Reference)
Yes 1.5 (1.12–2) 0.006 1.17 (0.872–1.569) n.s.
Ability to do high-flexion activities
Kneel (reference: impossible) Hard to do 0.96 (0.72–1.27) n.s.
Easy to do 0.94 (0.55–1.62) n.s.
Squat (reference: impossible) Hard to do 0.86 (0.65–1.14) n.s.
Easy to do 1.09 (0.65–1.82) n.s.
Sit cross-legged (reference: impossible) Hard to do 1.25 (0.92–1.7) n.s. 1.039 (0.755–1.428) n.s. 1.028 (0.748–1.413) n.s.
Easy to do 2.97 (2.08–4.24) < 0.001 1.787 (1.204–2.653) 0.004 1.74 (1.169–2.589) 0.006
Stand from the floor (reference: impossible) Hard to do 3.08 (1.38–6.87) 0.006 2.639 (1.172–5.942) 0.019 2.619 (1.163–5.898) 0.02
Easy to do 8.25 (3.59–18.98) < 0.001 5.359 (2.267–12.668) < 0.001 5.325 (2.252–12.589) < 0.001
Age (years) (reference: < 60) 60 - < 75 1.25 (0.76–2.04) 0.38
≥ 75 1.14 (0.64–2.04) n.s.
Male (vs. female) 2.28 (1.31–3.97) 0.004 2.379 (1.383–4.092) 0.002 2.388 (1.388–4.11) 0.002
Body mass index (kg/m2) (reference: < 25) 25 - < 30 1.21 (0.87–1.7) n.s.
≥ 30 1.11 (0.71–1.74) n.s.
Bilateral TKA (vs. unilateral TKA) 1.57 (1.15–2.15) 0.005 1.66 (1.22–2.259) 0.001 1.663 (1.222–2.262) 0.001
Patellar resurfacing (vs. un-resurfacing) 0.87 (0.6–1.27) n.s.
OR odds ratio, CI confidence interval, n.s. not significant

with patient satisfaction by time point, and we had to analysis and interpretation of data, drafting/revision of article, as well as to
analyze it based on recent data. However, despite these the final approval of the article. MCL contributed to the acquisition and
interpretation of data, revision of the article, as well as to the final approval
partly unavoidable limitations, this study provides de- of the article. All authors contributed to and approved the final manuscript.
tailed insight into the long-term results of TKA.
Funding
Conclusion Not applicable.
The achievement of high flexion (> 130°) after TKA in-
creased the ease of high-flexion activities (sitting cross- Availability of data and materials
legged and standing up from the floor) and patient satis- The datasets used and/or analyzed during the current study are available
from the corresponding author on reasonable request.
faction. The ease of high-flexion activities also increased
the quality of life after TKA in our Asian population,
where such populations frequently engage in these activ- Declarations
ities during daily life. However, high knee flexion angle Ethical approval and consent to participate
after TKA itself did not affect the patients’ quality of life. This retrospective study from prospectively collected data was approved by
the Institutional Review Board (no. 0603–105-170).
Abbreviations
TKA: Total knee arthroplasty; ADL: Activities of daily living; ROM: Range of
Consent for publication
motion; KSS: Knee Society knee score; KSFS: Knee Society function score;
Not applicable.
HSS: Hospital for Special Surgery; WOMAC: Western Ontario and McMaster
Universities Osteoarthritis Index; BMI: Body mass index
Competing interests
Acknowledgments The authors declare that they have no competing interests.
Not applicable.
Author details
1
Authors’ contributions Department of Orthopaedic Surgery, Seoul National University Hospital, 101
HH contributed to the conception and design of the study, acquisition of Daehak-ro, Jongno-gu, Seoul 03080, South Korea. 2Department of Statistics,
data, analysis and interpretation of data, and drafting/revision of the article. Graduate School of Chung-Ang University, Seoul, South Korea. 3Department
JSK, BL and SW contributed to the conception and design of the study, of Public Health Sciences, Seoul National University, Seoul, South Korea.
Han et al. BMC Musculoskeletal Disorders (2021) 22:565 Page 9 of 9

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