Abstract
Objective
The objective of this study is to identify and analyze the risk factors associated with anterior knee pain (AKP) following unicompartmental knee arthroplasty (UKA) and to investigate the underlying mechanisms that contribute to this postoperative complication.
Methods
This case-control study encompassed 30 knees from 24 patients who underwent medial fixed-platform UKA for knee osteoarthritis between 2018 and 2022. Participants were categorized into two groups based on the presence or absence of postoperative AKP. Clinical and imaging data were collected. Logistic regression analysis was utilized to determine independent risk factors for AKP.
Results
Among the 30 knees assessed, 7 (23.3 %) exhibited AKP postoperatively, while 23 (76.7 %) did not report such pain. An analysis of imaging parameters revealed significant differences between the two groups regarding postoperative patellar tilt angle, trochlear groove angle, progression of medial patellofemoral arthritis, and postoperative medial tilt of the patella. Logistic regression analysis identified the progression of medial patellofemoral arthritis as an independent risk factor for the occurrence of AKP. Additionally, an increased patellar tilt angle was identified as a protective factor, whereas postoperative medial tilt of the patella was recognized as a risk factor.
Conclusion
The progression of medial patellofemoral arthritis is identified as an independent risk factor for AKP following fixed-platform UKA. The development of this condition may be influenced by postoperative patellar tilt and trochlear groove angle. These findings underscore the necessity for optimal implant positioning and effective soft tissue management during UKA to mitigate postoperative complications.
Keywords: Unicompartmental knee arthroplasty, Anterior knee pain syndrome, Osteoarthrosis
1. Introduction
Unicompartmental knee arthroplasty (UKA) is an effective intervention for treating unicompartmental osteoarthritis of the knee. For patients with isolated medial or lateral compartment disease, UKA has emerged as a preferable alternative to total knee arthroplasty (TKA). In UKA, only the affected compartment is replaced, while the cruciate ligaments, intact cartilage, and meniscus of the unaffected compartment are preserved. Consequently, UKA offers several advantages over TKA, including reduced surgical trauma1 and enhanced preservation of proprioception.2
With its numerous advantages, the global incidence of UKA has steadily risen, and most patients experience substantial benefits from the procedure, including significant improvements in knee symptoms and function.3 However, a subset of patients develop postoperative anterior knee pain,4,5 which can be a significant source of discomfort and concern. Anterior knee pain (AKP), often referred to as patellofemoral pain syndrome (PFPS), is characterized by discomfort in the anterior knee during weight-bearing activities that involve knee flexion, such as squatting or stair climbing.6 To alleviate the pain, these patients often limit such movements, which can significantly impede their daily activities.
Despite the prevalence of AKP following UKA, its pathophysiology remains poorly understood. In previous studies employing finite element analysis, we identified that improper bearing thickness can result in abnormal stress on the patellar cartilage, leading to cartilage damage and, subsequently AKP.5 However, the precise mechanisms underlying AKP after UKA have yet to be fully elucidated. This study employs a case-control design to investigate the risk factors associated with AKP following UKA and to explore its potential underlying mechanisms. The outcomes of this research are anticipated to enhance orthopedic surgeons' understanding of the etiology of postoperative AKP in UKA patients and provide practical insights for its prevention.
2. Methods
2.1. Study population
This study received ethical approval from the Ethics Committee of Sun Yat-sen Memorial Hospital, Sun Yat-sen University, and utilized previously collected clinical and imaging data while ensuring patient confidentiality. Inclusion criteria encompassed: (1) patients who underwent medial fixed-platform unicompartmental knee arthroplasty (Zimmer Biomet, US) for knee osteoarthritis between 2018 and 2022; (2) availability of complete preoperative imaging data for the affected knee, including MRI, anteroposterior, and Settegast views7,8 (Fig. 1); (3) absence of AKP prior to surgery; and (4) regular postoperative follow-up at our center, with the most recent follow-up including re-evaluation of the affected knee through MRI and the aforementioned X-ray views. Exclusion criteria included: (1) presence of AKP prior to surgery; (2) diagnosis of systemic inflammatory diseases (e.g., rheumatoid arthritis, systemic lupus erythematosus); (3) presence of neurological dysfunction; and (4) incomplete imaging data either preoperatively or postoperatively.
Fig. 1.
The method for obtaining the Settegast view.
2.2. Clinical data
Data collection was conducted by two trained researchers. Baseline characteristics of the included patients were extracted from the electronic medical records system and included age, sex, height, weight, BMI, duration of surgery, and the date of the most recent follow-up. The knee-specific outcome measures utilized were the Knee Society Score (KSS) and the Knee Society Function Score (KSFS), both of which are widely recognized for evaluating the clinical effectiveness of knee arthroplasty.9,10 The Oxford Knee Score (OKS) was employed to assess outcomes both preoperatively and postoperatively, valued for its simplicity and efficiency.11 The Visual Analog Scale (VAS) was used to measure pain intensity,12 specifically focusing on the severity of AKP in this study. All assessments using the KSS, KSFS, OKS, and VAS were completed during the final postoperative follow-up.
2.3. Imaging data
All included patients underwent preoperative MRI of the affected knee, which comprised 3D PDW SPAIR sequences, as well as T1 and T2-weighted imaging. At the final follow-up, to reduce significant artifacts from the metal prosthesis, all patients were scanned using the 3D PDW sequence (rather than fat-suppressed sequences) to enhance visualization of the patellofemoral joint. Additionally, standard anteroposterior and Settegast views of the affected knee were obtained both preoperatively and at follow-up. The Settegast view is an imaging technique that captures an axial perspective of the patella, primarily used to assess the patellofemoral joint's condition7,8 (Fig. 1). In this position, the patient lies prone with the knee flexed at 90°, the thigh resting flat against the examination table, and the lower leg positioned vertically. The X-ray beam is typically directed at a 30-degree angle (slightly adjusted based on individual patient anatomy), entering from the foot side towards the head side, aimed at the joint surface between the patella and the femoral trochlea to ensure clear visualization of the patella. MRI and X-ray images were exported in DICOM format using the Picture Archiving and Communication System (PACS).
2.4. Imaging parameter measurements
-
(1)
Patellar Tilt Angle
In the Settegast view (Fig. 2A–B), a straight line (designated as line a) is drawn along the long axis of the patella, while another line (designated as line b) connects the highest points of the medial and lateral femoral condyles. The angle (α) formed between line a and line b is referred to as the patellar tilt angle.13 Within the context of this study, a positive α indicates a lateral tilt of the patella, whereas a negative α signifies a medial tilt. Medial tilt of the patella postoperatively is defined as an α angle <0 as measured on the postoperative Settegast view.
-
(2)
Trochlear Groove Angle
Fig. 2.
Measure the radiographic parameters on the X-ray film.
In the Settegast view (Fig. 2C–D), two lines are drawn to connect the lowest point of the trochlear groove with the highest points of the medial and lateral condyles, thereby forming an angle (β) referred to as the trochlear groove angle.14
-
(3)
Patellar Congruence Angle
In the Settegast view (Fig. 2C–D), the trochlear groove angle is initially established as previously described. A bisector line (designated as dashed line c) of the trochlear groove angle is subsequently drawn to serve as a zero reference. A straight line (referred to as line d) is drawn to connect the lowest point of the trochlear groove to the vertical ridge of the patella. The angle (γ) formed between line d and dashed line c is termed the patellar congruence angle.13 In the context of this study, if line d is positioned laterally to dashed line c, γ is classified as a positive value; conversely, if line d is located medially to dashed line c, γ is classified as a negative value.
-
(4)
Patellar Height
The assessment of patellar height is conducted utilizing the Insall-Salvati ratio,15 which is derived from measurements taken from lateral X-ray images of the knee (Fig. 2E). A line segment (designated as line e) is constructed from the superior pole to the inferior pole of the patella, thereby representing the length of the patella. Additionally, another line segment (designated as line f) is drawn from the inferior pole of the patella to the tibial tuberosity, which signifies the length of the patellar tendon. The Insall-Salvati ratio is subsequently calculated as the ratio of e to f.
-
(5)
Tibiofemoral Angle
In the anteroposterior X-ray view of the knee, the angle (δ) formed between the anatomical axis of the femur (line g) and the anatomical axis of the tibia (line h) is referred to as the tibiofemoral angle16 (Fig. 2F). In the context of this study, a positive δ signifies that the anatomical axis of the tibia creates an outward angle in relation to the anatomical axis of the femur, whereas a negative δ indicates an inward angle.
-
(6)
Grading of Patellar Cartilage Injury
The evaluation of patellar cartilage injury employed two classification systems to assess the medial and lateral facets of the patella, utilizing magnetic resonance imaging (MRI) of the knee.
The International Cartilage Repair Society (ICRS) classification system categorizes cartilage damage based on its thickness,17,18 with the specific assessment criteria outlined in Table 1.
Table 1.
ICRS classification system of cartilage lesions.
| Grade | MR cartilage morphology |
|---|---|
| 0 | Normal |
| Ⅰ | Abnormal increased signal with intact articular surface |
| Ⅱ | Partial-thickness defect <50 % |
| Ⅲ | Partial-thickness defect 50–99 % |
| Ⅳ | Full-thickness defect with exposure of subchondral bone and marrow edema |
The Whole Organ Magnetic Resonance Imaging Score (WORMS) offers a comprehensive scoring system for assessing cartilage damage and subchondral bone marrow lesions.19,20 This scoring system has exhibited strong reliability, with the specific scoring criteria depicted in Fig. 3A. The patellar cartilage of each patient was three-dimensionally reconstructed using Mimics 21.0 (Materialise, Belgium) to facilitate the WORMS scoring process (Fig. 3B).
Fig. 3.
Assessment of patellar cartilage injury. A: WORMS Cartilage Morphology Scoring: 0 points indicates normal cartilage; 1 point indicates normal cartilage thickness with localized T2-weighted image enhancement; 2.0 points represents partial cartilage loss with a width <1 cm; 2.5 points indicates full-thickness cartilage loss with a width <1 cm; 3 points denotes multiple sites of cartilage loss, mixed with areas of normal thickness, or partial cartilage loss with a width >1 cm but <75 % of the region; 4 points indicates diffuse (≥75 % of the region) partial cartilage loss; 5 points represents multiple sites of complete cartilage loss, or full-thickness cartilage loss with a width >1 cm but <75 % of the region; 6 points indicates diffuse full-thickness cartilage loss, involving greater than 75 % of the region.19,20 B: 3D reconstruction of patella and cartilage, patella in blue and patellar cartilage in white.
In this study, if a patient's postoperative ICRS grade or WORMS score exceeds their preoperative value, it is classified as "progression of patellofemoral arthritis"; otherwise, it is categorized as non-progression.
2.5. Statistical methods
Patients were classified into two distinct groups based on the presence or absence of AKP at the final postoperative follow-up: the AKP group and the non-AKP group. Comparisons between these groups were conducted utilizing GraphPad Prism 8 (GraphPad Software, USA). For continuous variables that followed a normal distribution, data are presented as mean ± standard deviation, and group comparisons were performed using the t-test. In contrast, for continuous variables that did not follow a normal distribution, data are described using the median and interquartile range, with comparisons analyzed using the Mann-Whitney U test. For multi-category variables, the Mann-Whitney U test was used to compare the two groups. For binary variables, the χ2 test was applied, with Fisher's exact test used when the expected frequency was less than 5. Univariate factors showing significant statistical differences in group comparisons were included in a multivariate logistic regression analysis. The logistic regression was performed using R 4.3.0.
3. Results
This study included a total of 30 knees from 24 patients, with AKP occurring in 7 knees (23.3 %) postoperatively, while 23 knees (76.7 %) remained pain-free. In the AKP group, the mean VAS score was 2.1 (95 % CI: 1.5–2.8), indicating mild pain levels. There were no significant differences between the AKP and non-AKP groups regarding age, sex, height, weight, BMI, or follow-up duration (Table 2). However, the OKS and KSS clinical scores were significantly lower in the AKP group than in the non-AKP group (Fig. 4C–D), although the KSFS showed no significant difference between the two groups (Fig. 4D). In terms of imaging parameters, significant differences were observed between the groups in postoperative patellar tilt angle, postoperative trochlear groove angle, progression of medial patellofemoral arthritis, and postoperative medial tilt of the patella.
Table 2.
The results of the comparison between AKP and non-AKP groups.
| Items | AKP (n = 7) | non-AKP (n = 23) | P-value |
|---|---|---|---|
| Gender | >0.9999c | ||
| Men | 0 | 3 | |
| Women | 7 | 20 | |
| Age (years) | 63.57(6.83) | 65.48(8.38) | 0.5884a |
| Height (m) | 1.58 (1.58, 1.60) | 1.58(1.55, 1.60) | 0.8970b |
| Weight (kg) | 66.00(61.00, 66.50) | 64.00(60.00, 73.00) | 0.6564b |
| BMI (kg/m2) | 25.83(1.00) | 26.81(2.56) | 0.3332a |
| Follow-up duration (days) | 1304(773, 1482) | 1268(856, 1689) | 0.508b |
| OKS | 40(39, 42) | 47(44, 47) | 0.0160b∗ |
| KSS | 93(90, 97) | 100(98, 100) | 0.0032b∗∗ |
| KSFS | 90(80, 90) | 90(80, 100) | 0.2705b |
| Pre-op patellar tilt angle (°) | 6.263(1.446) | 4.305(3.151) | 0.1255a |
| Post-op patellar tilt angle (°) | −2.330(-4.220, −0.450) | 3.100(1.170, 4.680) | <0.0001b∗∗∗∗ |
| Post-op medial tilt of the patella | <0.0001c∗∗∗∗ | ||
| Yes | 6 | 0 | |
| No | 1 | 23 | |
| Pre-op patellar congruence angle (°) | 4.780(0.800, 23.680) | 3.270(-9.290, 14.800) | 0.3859b |
| Post-op patellar congruence angle (°) | −5.500(-17.330, 14.200) | 2.570(-15.590, 8.450) | >0.9999b |
| Pre-op trochlear groove angle (°) | 146.2(5.306) | 146.0(3.797) | 0.9178a |
| Post-op trochlear groove angle (°) | 139.2(4.992) | 143.5(4.753) | 0.0474a∗ |
| Pre-op tibiofemoral angle (°) | 1.789(2.788) | 0.2774(3.486) | 0.3048a |
| Post-op tibiofemoral angle (°) | 5.200(3.368) | 3.341(3.451) | 0.2201a |
| Pre-op patellar height | 1.100(1.040, 1.120) | 1.126(1.030, 1.190) | 0.3924b |
| Post-op patellar height | 1.111(0.124) | 1.153(0.102) | 0.3814a |
| Pre-op ICRS of medial patella | 3(2, 3) | 3(2, 4) | 0.7800b |
| Post-op ICRS of medial patella | 4(3, 4) | 3(2, 4) | 0.0777b |
| Pre-op WORMS of medial patella | 4(3, 4) | 3(2, 5) | 0.8760b |
| Post-op WORMS of medial patella | 5(4, 5) | 3(2, 5) | 0.0842b |
| Progression of medial PFA | 0.0003c∗∗∗ | ||
| Yes | 6 | 2 | |
| No | 1 | 21 | |
| Pre-op ICRS of lateral patella | 3(1, 4) | 1(1, 2) | 0.2499b |
| Post-op ICRS of lateral patella | 4(1, 4) | 1(1, 2) | 0.2149b |
| Pre-op WORMS of lateral patella | 3(1, 5) | 1(1, 2) | 0.3249b |
| Post-op WORMS of lateral patella | 5(1, 5) | 1(1, 2) | 0.2149b |
| Progression of lateral PFA | 0.2333c | ||
| Yes | 1 | 0 | |
| No | 6 | 23 |
t-test.
Mann-Whitney U test.
Fisher's exact test; PFA: Patellofemoral arthritis.
Fig. 4.
Significant indicators in comparison between AKP group and non-AKP group.
A logistic regression analysis was then conducted with AKP as the dependent variable (Y) and postoperative patellar tilt angle, postoperative trochlear groove angle, progression of medial patellofemoral arthritis, and postoperative medial tilt of the patella as independent variables (X). Results (Table 3) indicated that only the progression of medial patellofemoral arthritis had a significant impact on AKP, with an OR of 63 (95 % CI: 4.840–820.021), establishing it as an independent risk factor for AKP.
Table 3.
The results of logistic regression analysis with AKP as the dependent variable.
| Independent variables (X) | P-value | OR | OR 95 % CI |
|---|---|---|---|
| Progression of medial PFA | 0.002∗∗ | 63.000 | 4.840–820.021 |
| Post-op trochlear groove angle | 0.059 | 0.805 | 0.642–1.009 |
| Post-op patellar tilt angle | 0.979 | <0.001 | – |
| Post-op medial tilt of the patella | 0.999 | <0.001 | – |
Further analysis examined the factors influencing medial patellofemoral arthritis progression, treating this as the dependent variable (Y) in a univariate logistic regression model. The results (Table 4) showed that the postoperative patellar tilt angle was a protective factor against the progression of medial patellofemoral arthritis, with an OR of 0.671 (95 % CI: 0.474–0.950), meaning that an increased patellar tilt angle (i.e., lateral tilt) reduced the risk of progression. Postoperative medial tilt of the patella, however, was identified as a risk factor, with an OR of 35 (95 % CI: 2.977–411.466).
Table 4.
The results of logistic regression analysis with the progression of medial PFA as the dependent variable.
| Independent variables (X) | P-value | OR | OR 95 % CI |
|---|---|---|---|
| Post-op patellar tilt angle | 0.025∗ | 0.671 | 0.474–0.950 |
| Post-op medial tilt of the patella | 0.005∗∗ | 35.000 | 2.977–411.466 |
Finally, a logistic regression analysis was performed with postoperative medial tilt of the patella designated as the dependent variable (Y) to investigate its influencing factors. The results (Table 5) indicated that the postoperative trochlear groove angle served as a protective factor against postoperative medial tilt of the patella, with an OR of 0.724 (95 % CI: 0.543–0.967). This finding implies that an increase in the postoperative trochlear groove angle may contribute to the prevention of medial tilt of the patella.
Table 5.
The results of logistic regression analysis with the post-op medial tilt of the patella as the dependent variable.
| Independent variables (X) | P-value | OR | OR 95 % CI |
|---|---|---|---|
| Post-op trochlear groove angle | 0.029∗ | 0.724 | 0.543–0.967 |
4. Discussion
The findings of this study are summarized in Fig. 5A. We identified that medial patellofemoral arthritis progression is an independent risk factor for AKP following UKA. To better understand the factors contributing to this progression, we conducted a logistic regression analysis, which showed that an increased postoperative patellar tilt angle is protective, while postoperative medial tilt of the patella is a risk factor. Furthermore, a larger postoperative trochlear groove angle was found to mitigate medial tilt of the patella.
Fig. 5.
Summary and structural diagram of the findings of this study. A: Logical summary of the key findings. B: The prosthesis in its normal position does not impinge on the patellar cartilage. C: If the prosthesis is positioned too medially, it may cause impingement of the patellar cartilage.
Our results indicated that the mean postoperative trochlear groove angle was smaller in the AKP group than in the non-AKP group. We hypothesize that a reduced trochlear groove angle may contribute to AKP, potentially due to surgical placement of the femoral component too close to the intercondylar notch, which could reduce the trochlear groove angle (Fig. 5B–C). A smaller angle limits the patella's movement within the femoral groove, resulting in medial tilt of the patella relative to the femoral condyle. This position brings the medial patellar cartilage into closer proximity with the femoral component. Notably, the interface between the femoral component and the native femoral trochlea is irregular, rather than smooth, creating a step-like edge. During knee flexion and extension, the medial patellar cartilage may impinge this interface (Fig. 5C), leading to chronic damage, progressive medial patellofemoral arthritis, and ultimately AKP.
Postoperative medial tilt of the patella may also be influenced by soft tissue tension abnormalities. UKA is commonly performed via a medial parapatellar approach, which involves incising the medial deep fascia, joint capsule, and other soft tissues, followed by suturing after implant placement. Excessively tight sutures may induce medial tilt and shift of the patella. Furthermore, scar contracture during wound healing may increase medial soft tissue tension, promoting medial tilt of the patella and shift. In summary, excessive medial soft tissue tension may contribute to postoperative medial tilt of the patella, increased impingement, and accelerated progression of medial patellofemoral arthritis, ultimately leading to AKP. Verifying the effect of abnormal soft tissue tension on AKP through direct examination or experimental methods presents considerable challenges. However, this concept provides a compelling theoretical foundation for our findings and suggests important surgical implications. Specifically, to reduce the risk of excessive medial patellar tension, careful attention to suture tension during closure of the medial patellar incision is recommended. Suturing with the knee in a flexed position, for instance, may help prevent undue medial tilt and tension.
This study's novelty lies in utilizing MRI to assess the progression of patellofemoral arthritis in detail among postoperative UKA patients, allowing us to identify medial patellofemoral arthritis progression as an independent risk factor for AKP and to explore related factors. However, certain limitations should be noted. First, all patients in this study underwent fixed-platform UKA, which limits the generalizability of these findings to mobile-platform UKA patients. Second, we used the Settegast view for patellar axial imaging, although other imaging techniques exist, raising the question of whether similar results would be obtained with different views. Lastly, the Settegast view captures only a static image, which does not reflect the dynamic motion of the patella under load during knee flexion and extension. Further investigation into dynamic patellar movement may provide additional insights into the causes of AKP following UKA.
In conclusion, this study demonstrates that medial patellofemoral arthritis progression is an independent risk factor for AKP following fixed-platform UKA. This progression is multifactorial; our findings indicate that postoperative medial tilt of the patella is a risk factor, while the postoperative trochlear groove angle may influence medial patellofemoral arthritis progression by affecting medial tilt of the patella.
CRediT authorship contribution statement
Ziyou Yan: Conceptualization, Investigation, Visualization, Writing – original draft. Meiyi Chen: Conceptualization, Methodology, Formal analysis. Deng Li: Investigation, Resources, Data curation. Zhiqing Cai: Investigation, Resources. Hao Sun: Investigation, Resources. Shuqiang Ma: Supervision, Resources. Ruofan Ma: Supervision, Resources. Jie Xu: Writing – review & editing, Supervision.
Guardian/patient's consent
Prior to initiating the study, we obtained informed consent from the patients. They consented to the use of their imaging data for research purposes. Throughout the study, the patients' privacy has been rigorously safeguarded.
Ethics approval
This study received ethical approval from the Ethics Committee of Sun Yat-sen Memorial Hospital, Sun Yat-sen University.
Funding
This study was funded by the National Key R&D Program of China (No.2021YFA1102600), National Natural Science Foundation of China (82272443,82372435), Guangdong Basic and Applied Basic Research Foundation (Grant No. 2022A1515010256, 2023A1515010501), Science and Technology Planning Project of Guangzhou City, China (Grant No. 202201020495, 202201020481).
Declaration of competing interest
The authors declare that they have no conflict of interest regarding the manuscript entitled “Investigation of Risk Factors for Anterior Knee Pain Following Unicompartmental Knee Arthroplasty: A Case-Control Study.”
Acknowledgments
None.
Contributor Information
Ziyou Yan, Email: yanzy3@mail2.sysu.edu.cn.
Meiyi Chen, Email: chenmy97@mail.sysu.edu.cn.
Deng Li, Email: lideng5@mail.sysu.edu.cn.
Zhiqing Cai, Email: caizhiqing@126.com.
Hao Sun, Email: sunh68@mail.sysu.edu.cn.
Shuqiang Ma, Email: mashuqiang369@163.com.
Ruofan Ma, Email: maruofan@mail.sysu.edu.cn.
Jie Xu, Email: xujie2@mail.sysu.edu.cn.
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