Abstract
Introduction
We evaluated whether the clinical outcomes, including postoperative knee range of motion (ROM), after unicompartmental knee arthroplasty (UKA) were associated with the sagittal spinopelvic parameters and coronal alignment of the full lower extremity.
Methods
Forty-two patients (50 knees: six men, seven knees; 36 women, 43 knees) who underwent medial UKA between April 2015 and December 2022 were included. Preoperative radiographic examinations of the index for sagittal spinopelvic alignment included the sagittal vertical axis (SVA), lumbar lordosis, sacral slope (SS), pelvic tilt (PT), and pelvic incidence. The anteroposterior hip-knee-ankle angle (HKAA) was calculated. The relationship of clinical outcomes and the risk of knee flexion angle ≤125° and knee flexion contracture ≥10° 1-year post-UKA with radiographic parameters were evaluated.
Results
Preoperative HKA angle affected postoperative knee flexion angle ≤125° (p = 0.017, 95% confidence interval [CI]: 0.473–0.930) in logistic regression analysis. Patients with a knee flexion angle ≤125° had a higher preoperative HKAA (9.8 ± 3.0°), higher SVA (83.8 ± 37.0 mm), and lower SS (23.7 ± 9.0°) than those with a flexion angle >125° (preoperative HKAA: 6.6 ± 4.0°, SVA: 40.3 ± 46.5 mm, SS: 32.0 ± 6.3°) (p = 0.029, 0.012, and 0.004, respectively). PT related to postoperative knee flexion contracture ≥10° (p = 0.010, 95% CI: 0.770–0.965) in the logistic regression analysis. Patients with flexion contracture ≥10° had higher PT (35.0 ± 6.6°) and SVA (82.2 ± 40.5 mm) than those with flexion contracture <10° (PT, 19.3 ± 9.0°; SVA, 42.4 ± 46.5 mm) (p = 0.001 and 0.028, respectively). The postoperative clinical outcome was correlated with the postoperative knee flexion angle and SVA (p = 0.036 and 0.020, respectively).
Conclusions
The preoperative HKAA affected postoperative knee flexion angle, and the knee flexion contracture and clinical outcomes post-UKA were associated with PT and SVA, respectively. To predict outcomes for knee ROM and clinical scores after UKA, radiographic examination, including the sagittal spinopelvic parameters and the coronal view of the full lower extremity, is essential.
Keywords: Pelvic tilt, Sagittal vertical axis, Hip-knee-ankle angle, Unicompartmental knee arthroplasty, Knee range of motion
1. Introduction
Total knee arthroplasty (TKA) and unicompartmental knee arthroplasty (UKA) have been developed for treating patients with knee pain due to degenerative diseases and those who have failed conservative treatment, especially older adults. Particularly, UKA is performed for knee osteoarthritis and osteonecrosis isolated in single compartment. In TKA, the knee range of motion (ROM) can affect the postoperative clinical results.1, 2, 3, 4 Patients who undergo UKA have better ROM (≥130° flexion), pre- and postoperatively, than those who undergo TKA.5, 6, 7 Therefore, few reports have revealed the appropriate factors for postoperative knee ROM, the clinical outcomes, and UKA durability due to good knee ROM and outcomes in almost patients undergoing UKA. In contrasting, several studies have shown that postoperative valgus alignment and greater posterior tilt of the tibial component adversely affect longevity in UKAs.8,9 Previous reports have indicated that a maximal flexion angle of >140° after UKA improves clinical outcomes.10 However, the tibial component position might decrease the postoperative knee flexion angle.11 Therefore, definitive factors related to postoperative knee ROM after UKA remain unclear.
The sagittal relationship between spinopelvic alignment and the lower extremity, including the knee flexion angle, has previously been reported,12,13 and compensatory mechanisms for upright posture in spinal deformity patients could affect the alignment of the spine, pelvis, and lower extremity to maintain horizontal gaze.14,15 Obeid et al.16 demonstrated that the deficiency of lumbar lordosis (LL) which was calculated with the subtraction from ideal LL based on the formula defined by Legaye et al.17 and Schwab et al.18 to the actual LL was correlated with the knee flexion angle in a comfortable standing position. Jalai et al.19 showed that the sagittal vertical axis (SVA) and spinopelvic mismatch influenced the knee flexion angle in the standing position among adult patients with spinal deformity. However, no report to date has revealed an association between spinopelvic alignment and knee ROM or clinical outcomes after knee arthroplasty.
This study focused on UKA due to its ability to alleviate pain without causing substantial alterations in lower extremity alignment and knee kinematics post-surgery. This study aimed to evaluate the impact of coronal full lower extremity and sagittal spinopelvic alignments on postoperative knee ROM and clinical results after UKA.
2. Materials and methods
The institutional review board of our hospital approved this cross-sectional retrospective study and waived the requirement for informed consent because of the retrospective nature.
2.1. Patient selection
Patients who were performed medial UKA for osteoarthritis or osteonecrosis in the isolated compartment of the knee, from April 2015 to December 2022, were enrolled. Patients underwent medial UKA for knee osteoarthritis or osteonecrosis isolated in the medial compartment. The indications of UKA in our hospital include, 1) no pain in the patellofemoral joint,20 2) no symptoms in the lateral compartment of the femorotibial joint, 3) reducible varus alignment to the neutral position during valgus stress, 4) preoperative flexion contractures <15°, and 5) preoperative knee flexion angle >90°. In addition, magnetic resonance imaging was performed preoperatively in all patients for confirmation of intact anterior cruciate and medial collateral ligaments. Patients with hip osteoarthritis, ankle osteoarthritis, or ≥2 vertebral fractures were excluded.
2.2. UKA surgical procedures and postoperative care
We performed all UKAs using a navigation system. The procedure of a CT/Image-free navigation (Stryker Knee Navigation System version 4.0; Stryker Leibinger, Freiburg, Germany) was conducted similar to those described in a previous report.11 Briefly, the tibial medial plateau osteotomy was initially performed with 3° of varus and 5° of posterior flexion alignments under guide by the navigation system. Subsequently, femoral medial condyle osteotomy was performed to ensure equal extension and flexion gaps using spacer blocks. Finally, cementation of fixed-bearing implants was done.
The suction drain was either not used, or it was removed on postoperative day 1. Thereafter, the patients could begin rehabilitation during hospitalization for knee ROM and ambulation, which continued for 2 weeks. All patients underwent outpatient follow-up visits for >1 year after discharge, and their knee ROMs were documented. The passive knee flexion angles and flexion contracture were measured in the supine position on a bed using a goniometer.
2.3. Radiographic examinations and comparison with clinical evaluations
Preoperatively and 1 year after surgery, radiographic evaluations of the full lower extremity in standing position were carried out. The hip-knee-ankle angle (HKAA) was defined as the angle between the straight line connecting the femoral head center to the knee center and the straight line connecting the knee center to ankle center in the coronal plane. Varus alignment of HKAA in the full lower extremity was shown as a positive value. Sagittal whole spinal and pelvic radiography in a standing position was carried out preoperatively. Five spinopelvic parameters, including the SVA, LL, sacral slope (SS), pelvic tilt (PT), and pelvic incidence (PI), were measured (Fig. 1). Fig. 1 was quoted and modified from Lafage et al.‘s study.21 A single observe measured all the parameters twice, and the results were averaged.
Fig. 1.
Measurements of the spinopelvic parameters
Procedures for measuring spinopelvic parameters, including lumbar lordosis (LL) (A), sagittal vertical axis (SVA) (B), sacral slope (SS) (C), pelvic tilt (PT) (D), and pelvic incidence (PI) (E), using sagittal radiographs of the entire spine in the standing position.
The figure was quoted and modified from Lafage et al.21
Femoral and tibial component positions were determined using the procedures outlined in a previous report.11 Preoperative and postoperative CT data were used, and the femoral and tibial component positions were calculated by matching the preoperative planning image and postoperative implanting image using the three-dimensional (3D) image software (ZedView, LEXI, Tokyo, Japan).
The relation of the knee ROM at 1 year after surgery with radiographic parameters, such as the HKAA and spinopelvic parameters, and femoral and tibial component positions were evaluated. The new Knee Society Score (KSS),22,23 comprising pain and function scores, was investigated, and compared with the radiographic parameters and femoral and tibial component positions.
2.4. Statistical analyses
Postoperative values of knee ROM and the KSS were compared with preoperative values using a paired t-test. Logistic regression analysis was conducted with postoperative knee flexion angle ≤125° and flexion contracture ≥10° as dependent variables. Age, BMI, preoperative knee flexion angle, HKAA, spinopelvic parameters, and femoral and tibial component positions were used as the independent variables.
Furthermore, patients were stratified into two groups based on knee flexion angle (≤125° or >125°) and flexion contracture (<10° or ≥10°), respectively. Statistical analysis was performed using a parametric t-test. A power analysis was performed with >80% statistical power and an alpha cut-off of 5% (0.05) as the probability of a type-I error. Correlations of the KSS at 1 year after UKA with radiographic parameters, and femoral and tibial component positions were examined with Pearson's correlation coefficient.
All statistical analyses were performed using SPSS version 26 (IBM Corp., Armonk, NY, USA). The significance level was set at p < 0.05.
3. Results
Forty-two patients (50 knees: six men, seven knees; 36 women, 43 knees) who had completed 1 or more years of follow-up were included in this study. Table 1 shows the patient demographics, radiographic parameters, and implant positions. The postoperative knee extension and flexion angles did not significantly change in the preoperative values, respectively. The postoperative KSS significantly improved from the preoperative value (p < 0.001).
Table 1.
Patient demographics, radiographic parameters, and implant positions.
| Sex | 6 men (7 knees), 36 women (43 knees) |
| Age | 72.0 ± 7.6 (range, 59–89 years) |
| Body mass index (kg/m2) | 26.7 ± 4.0 |
| Preoperative knee extension angle (°) | −5.2 ± 4.3 |
| Postoperative knee extension angle (°) | −3.8 ± 4.2 |
| Preoperative knee flexion angle (°) | 132.5 ± 6.5 |
| Postoperative knee flexion angle (°) | 130.8 ± 9.7 |
| Preoperative KSS (pt) | 53.3 ± 22.8 |
| Postoperative KSS (pt) | 84.0 ± 37.8 |
| Preoperative HKAA (°) | 7.5 ± 4.0 |
| Postoperative HKAA (°) | 2.1 ± 3.3 |
| SVA (mm) | 52.4 ± 47.8 |
| LL (°) | 36.2 ± 14.1 |
| SS (°) | 29.7 ± 8.0 |
| PT (°) | 22.9 ± 10.0 |
| PI (°) | 52.9 ± 9.7 |
| Femoral component position in the coronal plane (°) | 1.5 ± 2.8 |
| Femoral component position in the sagittal plane (°) | −2.5 ± 2.5 |
| Femoral component position in the axial plane (°) | 0.9 ± 3.8 |
| Tibial component position in the coronal plane (°) | 2.9 ± 1.3 |
| Tibial component position in the sagittal plane (°) | 3.9 ± 2.2 |
| Tibial component position in the axial plane (°) | 1.8 ± 6.7 |
The values reveal number distribution in Sex, and the mean and standard deviation in others. Varus alignment of HKA angle is shown as a positive value. In the component positions, varus in the coronal plane, posterior flexion in the sagittal plane, and internal rotation in the axial plane of the implant positions are shown as a positive value.
HKAA, Hip-knee-ankle angle; KSS, Knee Society Score; LL, Lumbar lordosis; PI, Pelvic incidence; PT, Pelvic tilt; SS, Sacral slope; SVA, Sagittal vertical axis.
The postoperative knee flexion angle of ≤125° was significantly related with the preoperative HKAA (p = 0.017, 95% confidence interval (CI): 0.473–0.930) in the logistic regression analysis. The preoperative knee flexion angle, spinopelvic parameters, and the femoral and tibial component positions were not associated with postoperative knee flexion angle. Postoperative knee flexion contracture of ≥10° was significantly related to the PT with logistic regression analysis (p=0.010, 95% CI: 0.770–0.965). The preoperative knee extension angle, spinopelvic parameters except for the PT, and the component positions were not correlated with postoperative knee flexion contracture.
Patients with knee flexion angle ≤125° had a lower preoperative knee flexion angle, higher preoperative HKAA, higher SVA, and lower SS than those with a flexion angle >125° (p = 0.003, 0.029, 0.012, and 0.004, respectively) (Table 2). Calculations for 13 patients (total: 26 patients) on preoperative knee flexion angle, 26 patients (total: 52 patients) on HKAA, 19 patients (total: 38 patients) on SVA, and 11 patients (total: 22 patients) on SS in each group were required in the power analysis based on the mean and standard deviation values. Patients with flexion contracture ≥10° had a higher PT and SVA than those with a flexion contracture <10° (p = 0.001 and 0.028, respectively) (Table 3). Although seven patients in each group on PT calculation (total: 14 patients) were required in the power analysis based on the mean and standard deviation values, 23 patients in each group on calculation of the SVA (total: 46 patients) were needed.
Table 2.
Comparison between patients with knee flexion angle >125° and patients with knee flexion angle 125°.
| Postoperative knee flexion angle |
p values | ||
|---|---|---|---|
| >125° (n = 39) | ≤125° (n = 11) | ||
| Age | 70.5 ± 6.5 | 75.9 ± 9.1 | 0.053 |
| Body mass index (kg/m2) | 27.1 ± 4.1 | 25.6 ± 3.6 | 0.317 |
| Preoperative knee flexion angle (°) | 134.4 ± 5.9 | 127.5 ± 5.4 | 0.003* |
| Preoperative HKAA (°) | 6.6 ± 4.0 | 9.8 ± 3.1 | 0.029* |
| Postoperative HKAA (°) | 2.1 ± 3.7 | 1.9 ± 2.2 | 0.865 |
| SVA (mm) | 40.3 ± 46.5 | 83.8 ± 37.0 | 0.012* |
| LL (°) | 38.5 ± 13.5 | 30.3 ± 14.9 | 0.124 |
| SS (°) | 32.0 ± 6.3 | 23.7 ± 9.0 | 0.004* |
| PT (°) | 21.7 ± 9.9 | 26.1 ± 9.9 | 0.241 |
| PI (°) | 53.8 ± 9.6 | 50.6 ± 10.1 | 0.381 |
| Femoral component position in the coronal plane (°) | 1.3 ± 3.0 | 1.7 ± 2.1 | 0.721 |
| Femoral component position in the sagittal plane (°) | −2.5 ± 2.2 | −2.5 ± 2.2 | 0.958 |
| Femoral component position in the axial plane (°) | 0.9 ± 3.1 | 1.0 ± 4.1 | 0.934 |
| Tibial component position in the coronal plane (°) | 2.9 ± 1.3 | 2.6 ± 1.4 | 0.491 |
| Tibial component position in the sagittal plane (°) | 3.8 ± 1.9 | 4.2 ± 1.9 | 0.575 |
| Tibial component position in the axial plane (°) | 1.9 ± 5.8 | 2.3 ± 6.2 | 0.863 |
In the component positions, varus in the coronal plane, posterior flexion in the sagittal plane, and internal rotation in the axial plane of the implant positions are shown as a positive value.
*: p < 0.05.
HKAA, Hip-knee- ankle angle; LL, Lumbar lordosis; PI, Pelvic incidence; PT, Pelvic tilt; SS, Sacral slope; SVA, Sagittal vertical axis.
Table 3.
Comparison between patients with knee flexion contracture ≥10° and patients with knee flexion contracture <10°.
| Postoperative knee flexion contracture |
p values | ||
|---|---|---|---|
| <10° (n = 42) | ≥10° (n = 8) | ||
| Age | 71.5 ± 6.9 | 73.3 ± 9.8 | 0.542 |
| Body mass index (kg/m2) | 26.4 ± 4.0 | 27.6 ± 4.0 | 0.427 |
| Preoperative knee flexion contracture angle (°) | 4.8 ± 4.3 | 6.4 ± 4.5 | 0.334 |
| Preoperative HKAA (°) | 7.4 ± 4.4 | 7.7 ± 2.7 | 0.887 |
| Postoperative HKAA (°) | 2.1 ± 3.3 | 2.0 ± 3.5 | 0.955 |
| SVA (mm) | 42.4 ± 46.5 | 82.2 ± 40.5 | 0.028* |
| LL (°) | 38.6 ± 13.3 | 28.9 ± 14.9 | 0.074 |
| SS (°) | 30.9 ± 6.4 | 26.0 ± 11.2 | 0.114 |
| PT (°) | 19.3 ± 9.0 | 35.0 ± 6.6 | 0.001* |
| PI (°) | 51.1 ± 9.1 | 58.2 ± 10.2 | 0.058 |
| Femoral component position in the coronal plane (°) | 1.4 ± 2.8 | 1.5 ± 2.6 | 0.986 |
| Femoral component position in the sagittal plane (°) | −2.8 ± 2.3 | −1.7 ± 1.7 | 0.202 |
| Femoral component position in the axial plane (°) | 1.5 ± 3.4 | −0.9 ± 2.8 | 0.067 |
| Tibial component position in the coronal plane (°) | 2.9 ± 1.3 | 2.7 ± 1.5 | 0.649 |
| Tibial component position in the sagittal plane (°) | 3.9 ± 2.0 | 4.1 ± 1.7 | 0.697 |
| Tibial component position in the axial plane (°) | 2.9 ± 6.0 | −1.4 ± 4.4 | 0.054 |
In the component positions, varus in the coronal plane, posterior flexion in the sagittal plane, and internal rotation in the axial plane of the implant positions are shown as a positive value.
*: p < 0.05.
HKAA, Hip-knee- ankle angle; LL, Lumbar lordosis; PI, Pelvic incidence; PT, Pelvic tilt; SS, Sacral slope; SVA, Sagittal vertical axis.
The postoperative clinical outcome with the KSS had a weak positive correlation with postoperative knee flexion angle and a negative correlation with SVA with respect to the Pearson's correlation coefficient (Table 4).
Table 4.
Correlation with postoperative KSS.
| correlation coefficient | p values | |
|---|---|---|
| Age | −0.155 | 0.365 |
| Body mass index | −0.133 | 0.438 |
| Postoperative knee flexion contracture angle | 0.118 | 0.491 |
| Postoperative knee flexion angle | 0.350 | 0.036* |
| Preoperative HKAA | 0.004 | 0.979 |
| Postoperative HKAA | −0.026 | 0.877 |
| SVA | −0.423 | 0.019* |
| LL | 0.245 | 0.149 |
| SS | 0.247 | 0.144 |
| PT | −0.179 | 0.293 |
| PI | −0.028 | 0.870 |
| Femoral component position in the coronal plane | −0.038 | 0.823 |
| Femoral component position in the sagittal plane | −0.085 | 0.621 |
| Femoral component position in the axial plane | −0.101 | 0.557 |
| Tibial component position in the coronal plane | 0.292 | 0.083 |
| Tibial component position in the sagittal plane | −0.073 | 0.670 |
| Tibial component position in the axial plane | −0.080 | 0.639 |
HKAA, Hip-knee- ankle angle; KSS, Knee Society Score; LL, Lumbar lordosis; PI, Pelvic incidence; PT, Pelvic tilt; SS, Sacral slope; SVA, Sagittal vertical axis.
4. Discussion
The most important finding of the study was that the knee flexion angle after UKA was associated with the preoperative HKAA, and postoperative knee flexion contracture was associated with PT. Moreover, the postoperative new KSS was correlated with SVA. Therefore, our data demonstrated that the preoperative coronal lower extremity alignment and sagittal spinopelvic parameters were associated with the clinical outcomes of UKA 1 year following surgery. Previous studies have reported only the association between implant positions in UKA and the postoperative knee flexion angle,10. and studies on the relationship between postoperative knee ROM and spinopelvic alignment are lacking. This is the first report that, to our knowledge, demonstrates the connection between the knee ROM following UKA and both the sagittal spinopelvic alignment and the radiographic alignment of the lower extremities. The reason for non-association of postoperative KSS with the knee ROM, implant positions, and HKAA was considered the good postoperative KSS after UKA in almost all the cases. Nevertheless, as SVA was correlated with postoperative KSS, sagittal spinopelvic alignment should be considered an important factor for UKA outcomes.
Patients with knee flexion contracture after UKA had greater posterior tilt of the pelvis related to high PT and a greater anteversion of the spine related to high SVA, despite under power for SVA. Jalai et al.19 reported significant correlations between the sagittal offset shown by the SVA and PT and compensatory lower-limb mechanisms. Their study showed that patients with a greater anteversion of the trunk cannot stand while maintaining knee extension but rather stand with a mild knee flexion position and posterior tilt of the pelvis. Patients with a greater PT must have significant knee flexion to maintain a stable standing position. Therefore, a contracture at the posterior side of the knee was considered to have occurred, which resulted in knee flexion contracture. Furthermore, patients' knee extension restriction and spinopelvic problems are unlikely to improve, even if knee-related pain is improved by UKA.
Previous studies described the relationship between postoperative knee ROM and the preoperative ROM for TKA.1, 2, 3, 4 Preoperative knee ROM was also important for postoperative ROM in UKA. Moreover, the postoperative knee flexion angle was affected by the preoperative HKAA in UKA. In the present study, the full lower extremity alignment was corrected from varus (preoperative HKAA: varus 7.5 ± 4.0°) to a nearly neutral alignment (postoperative HKAA: varus 2.1 ± 3.3°) after UKA. Furthermore, a great preoperative varus alignment of the full lower limb negatively affected the postoperative knee flexion angle. Thus, it was considered that the significant changes in medial compartment tightness and lateral compartment clearance depending on the correction angle might have affected the knee flexion angle after UKA. As a larger correction was needed in patients with a greater HKAA, the significant changes in medial compartment tension due to the medial collateral ligament and decreased lateral compartment gap may have affected the knee flexion angle after UKA.
There were several limitations in this study. First, the sample size was relatively small. Nevertheless, our data can still aid researchers in better understanding the association between whole lower extremity and spinopelvic alignments and knee surgery outcomes. Second, the intra-class correlation coefficient for inter-rater errors was not calculated. However, previous reports have indicated that inter-rater errors are significantly small when measuring the radiographic spinopelvic parameters in the lateral plane.24,25 Third, the results of this study were obtained by performing all UKAs using a navigation system. It is considered that the use of navigation system in arthroplasties may lead to less errors in the implant positions than in surgeries performed manually without a navigation system. The outcomes in our study might have differed if there had been several cases with relatively greater errors of implant positions, such as excessive varus or posterior tilt position of the tibial component.
5. Conclusions
Our findings indicate that knee ROM and clinical outcomes after UKA are associated with the coronal full lower extremity alignment and sagittal spinopelvic alignment according to HKAA, PT, and SVA. Post-UKA, patients with a higher preoperative posterior PT may have knee flexion contracture. Patients with a greater preoperative varus alignment of the lower extremity may have a low knee flexion angle, and greater spinal anteversion indicated by a higher SVA may negatively impact clinical outcomes after UKA. Overall, our data confirmed that radiographic examination of the full lower extremity in the coronal view and the entire spine and pelvis in the sagittal view is essential for UKA.
Funding/sponsorship
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Informed consent
This retrospective study was approved by the ethics committee of our institution. The need for informed consent was waived owing to the retrospective nature of the study.
Ethical approval
The study was retrospectively conducted and approved by the Ethics Committee of our institution (No. 19-261). All procedures performed during studies involving human participants were in accordance with the ethical standards of our institution and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The requirement for informed consent was waived due to the retrospective nature of the study.
CRediT authorship contribution statement
Mitsuru Hanada: designed the study, performed the experiments, analyzed the data, and wrote the manuscript. Kensuke Hotta: investigated the subjects and confirmed the radiographic measurement. Yukihiro Matsuyama: supervised the experiments, All authors approved the manuscript.
Declaration of competing interest
All authors declare that they have no conflict of interest.
Acknowledgements
None.
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