The correlation between preoperative patellar tilt and clinical outcomes following unicompartmental knee arthroplasty

Abstract

Objective

To investigate the association between preoperative patellar tilt angle and clinical outcomes following unicompartmental knee arthroplasty (UKA).

Methods

In this retrospective study, 148 UKA patients were stratified into normal (patellar tilt < 10°, n = 99) and abnormal (≥ 10°, n = 49) groups. Radiographic alignment —including the hip-knee-ankle angle (HKA), mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), and posterior tibial slope were compared. Clinical outcomes were assessed using the Visual Analog Scale (VAS) for pain, the Hospital for Special Surgery (HSS) score, the Feller patellar score, and the incidence of anterior knee pain (AKP). Additional patellofemoral parameters (patellar shift, Caton-Deschamps index, Iwano classification) were also analyzed.

Results

Postoperative mechanical alignment (HKA, mLDFA, MPTA) was successfully corrected to the ideal range (approximately 3° of physiological varus) in both groups, with no significant differences. However, clinical outcomes were significantly worse in the abnormal group compared to the normal group: higher postoperative VAS scores (median 1 vs. 0, p = 0.010), lower HSS scores (median 83 vs. 85, p = 0.000), lower Feller scores (median 19 vs. 20, p = 0.000), and a higher incidence of AKP (12.24% vs. 3.03%, p = 0.027). No significant intergroup differences were found in the additional patellofemoral parameters.

Conclusion

A preoperative patellar tilt angle ≥ 10° is significantly associated with inferior early clinical outcomes and a markedly increased risk of AKP after UKA, despite successful mechanical alignment correction. Preoperative assessment of patellar tilt may provides valuable prognostic information and may help identify patients requiring heightened intraoperative attention to patellofemoral kinematics or tailored rehabilitation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-026-06896-z.

Introduction

Unicompartmental knee arthroplasty (UKA) is a highly effective surgical option for treating anteromedial osteoarthritis (AMOA) of the knee. Compared to total knee arthroplasty (TKA), UKA offers distinct advantages, including being less invasive, preserving more bone stock, facilitating a quicker postoperative recovery, and higher patient satisfaction. Furthermore, UKA better restores knee function closer to physiological conditions, enabling a faster and more efficient return to daily activities and sports [1–3]. Despite these benefits, there is still a portion of patients who express dissatisfaction with the postoperative outcomes of UKA. Numerous factors can influence postoperative satisfaction, including persistent numbness, anterior knee pain (AKP), and joint clicking [4].

Recently, the impact of patellofemoral morphology on clinical outcomes following knee arthroplasty has garnered increasing attention. In a study based on the Wiberg classification, Sun et al. [4] found that patients with Wiberg Type III patellae exhibited poorer patellar function scores, a higher incidence of AKP, and a greater patellar tilt angle following UKA, suggesting that patellar morphology significantly influences the clinical efficacy of UKA. Furthermore, in a multicenter prospective study involving 260 UKA patients, Munk et al. [5] found that preoperative lateral patellar subluxation was an independent predictor of poor early postoperative outcomes, with a mean decrease of 10.3 points in the Oxford Knee Score, and recommended that TKA be prioritized for such patients. The above studies indicate that abnormal patellar morphology (e.g., Wiberg Type III) and abnormal patellar tracking (e.g., patellar subluxation) are both closely associated with poor postoperative outcomes following UKA.

However, most of the aforementioned studies have focused on comprehensive morphological parameters such as Wiberg classification or patellar displacement, while research on the preoperative patellar tilt angle—a specific indicator reflecting abnormalities in the coronal plane patellar trajectory—remains limited in UKA. Previous studies indicate that preoperative AKP or patellofemoral joint degeneration does not significantly impact the clinical outcomes or prosthesis survival rate of UKA [6]. However, AKP persists as a frequent postoperative complication, responsible for roughly 14% of UKA failures [7]. Emerging evidence implicates that the occurrence of AKP may be associated with patellofemoral joint abnormalities, including mechanical malalignment, dynamic knee varus (increased Q angle), and abnormal patellar tracking [7]. Finite element analysis further revealed that excessive or insufficient padding during UKA may lead to residual varus or valgus deformity postoperatively. This may trigger abnormal mechanical conditions in the patellofemoral joint, manifesting as uneven stress distribution and localized stress concentration. These factors accelerate cartilage damage and articular surface wear, ultimately resulting in AKP [7]. However, whether the preoperative patellar tilt angle influences the occurrence of postoperative AKP and overall efficacy after UKA remains unclear.

Currently, patellar reshaping during the UKA procedure is not routinely performed [4]. There is a paucity of research investigating the impact of the preoperative patellar tilt angle on postoperative outcomes following UKA. It remains unclear whether an excessive preoperative patellar tilt angle may lead to residual postoperative patellar tilt of varying degrees, which could consequently compromise the clinical results of UKA. Based on this, the present study proposes the following hypothesis: in patients with a preoperative patellar tilt angle ≥ 10°, although postoperative lower limb alignment can be satisfactorily corrected to a degree comparable to that of patients with normal tilt angles, these patients experience poorer postoperative pain control, less favorable functional recovery, and a higher incidence of AKP. Therefore, this study aims to evaluate the influence of the preoperative patellar tilt angle on postoperative pain and functional recovery following UKA, with the goal of providing a reference for preoperative risk assessment and intraoperative decision-making.

Materials and methods

Patients

This retrospective study analyzed patients who underwent UKA in the orthopedics department between January 2022 and December 2023. Patient data, including age, gender, body mass index (BMI), operative side, and Kellgren-Lawrence (KL) grade, were collected.

The inclusion criteria were as follows: (1) patients diagnosed with anteromedial osteoarthritis (AMOA) of the knee graded as KL III-IV; (2) varus deformity < 15° and flexion contracture < 15°; (3) knee range of motion (ROM) > 90°; (4) Absence of lateral joint space narrowing and passively correctable varus deformity; and (5) agreement to participate in the study and ability to complete telephone follow-ups. Exclusion criteria included inflammatory arthritis, severe bone loss, groove-like changes in the lateral patellofemoral joint, prior high tibial osteotomy (HTO) or anterior cruciate ligament (ACL) reconstruction, lateral osteoarthritis, and spontaneous osteonecrosis of the knee (SONK).

Patellar tilt angle ≥ 10° was defined as abnormal patellar tilt angle [8]. Patients were categorized into a normal patellar tilt angle group (< 10°) and an abnormal patellar tilt angle group (≥ 10°). This study was approved by the ethics committee of our hospital, and informed consent was obtained from all participants. The minimum follow-up period was 12 months.

Surgical technique

All surgical procedures are performed by senior surgeons using anatomically shaped fixed-bearing unicompartmental knee (Tianjin Zhengtian Medical Instrument Co, Ltd). Anesthesia is administered via epidural or general endotracheal anesthesia. The procedure employs a minimally invasive anteromedial approach to assess anterior and posterior cruciate ligament integrity and lateral cartilage condition. A tourniquet is maintained throughout until prosthesis placement is complete, with all components fixed using Palacos bone cement (Heraeus Medical GmbH, Wehrheim, Germany). High-risk patients (those with a history of lower extremity varicose veins, deep vein thrombosis, or pulmonary embolism) receive 30 days of deep vein thrombosis (DVT) prophylaxis starting on the day of surgery. Early rehabilitation training commences within 24 h postoperatively.

Radiographic assessment

Radiographic evaluation was performed by two experienced radiologist blinded to the clinical outcomes. Three measurements of each value were averaged, and the results of each measurement were reconfirmed by two senior orthopedic surgeons. Axial patellar radiographs were acquired with the knee flexed at 45° [9]. The patellar tilt angle was defined as the angle between the transverse axis of the patella and the line tangent to the highest points of the medial and lateral femoral condyles (Fig. 1a) [10]. Patellar Shift was measured using the patella tangential radiographs (Fig. 1b) [11]. Preoperative patellar height was assessed using the Caton–Deschamps index with a normal reference range of 0.6–1.2 (Fig. 1c) [12]. The patellofemoral joint was classified according to the Iwano classification based on preoperative radiographs [13]. Measurements of the patellar tilt angle, patellar shift, mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), posterior tibial slope (PTS), and hip-knee-ankle (HKA) angle were taken preoperatively and at 12 months postoperatively.

Fig. 1.

Measurements of the patellar tilt angle (a), the patellar shift (b) and Caton–Deschamps index (c)

Clinical outcomes

Functional scores for the patella and knee joint were recorded at preoperative and 12-month postoperative. The Visual Analogue Scale (VAS, ranging from 0 to 10, where 0 indicates no pain and 10 indicates the most severe pain) was used to document knee pain at preoperative and 12-month postoperative. Knee function was assessed using the Hospital for Special Surgery (HSS) Knee Score, which ranges from 0 to 100 [14]. Patellar function was evaluated with the Feller score, encompassing four domains: severity of AKP, quadriceps muscle strength, ability to rise from a seated position, and capability to ascend and descend stairs. The total score is 30, with higher scores indicating better patellar function [15]. The incidence of AKP was recorded, defined as significant pain in the anterior knee region during squatting or stair climbing [4]. The number of patients experiencing AKP within the first year postoperatively was statistically analyzed.

Statistical analysis

Differences in demographic characteristics between the two groups were analyzed using the Chi-square test, Student's t-test, or the Mann–Whitney U test. Continuous variables between groups were compared using Student's t-test if normally distributed and are presented as mean ± standard deviation (SD). If not normally distributed, they were analyzed using the Mann–Whitney U test and are presented as medians (interquartile ranges [IQR]). The Count data were compared using the Chi-square test. The Kappa test was used to assess inter-observer reliability. The Power calculation were analyzed using the Power Analysis and Sample Size (PASS, version 2008). In this study, a normal patellar tilt angle group of 99 patients and an abnormal patellar tilt angle group of 49 patients achieve a statistical power of 0.96. All statistical analyses were performed using SPSS software (version 20.0; IBM Corp., Armonk, NY, USA). A p < 0.05 was considered statistically significant.

Result

Demographic characteristics of two groups

A total of 148 patients were ultimately enrolled in the statistical analysis of this study. General demographic characteristics of the normal and abnormal patellar tilt angle groups are presented in Table 1. The normal patellar tilt angle group comprised 99 patients with a mean age of 65.79 ± 7.51 years, including 27 males and 72 females. The abnormal patellar tilt angle group consisted of 49 patients with a mean age of 65.49 ± 6.45 years, including 9 males and 40 females. No significant differences were observed between the two groups in terms of gender, age, BMI, operative side, or KL grade (Table 1).

Table 1.

Demographic characteristics of all patients

Variable	Normal patellar tilt group (n = 99)	Abnormal patellar tilt group (n = 49)	Statistic value	P value
Sex (n, %)
Male	27 (27.3)	9 (18.4)	1.412*	0.235
Female	72 (72.7)	40 (81.6)
Age (year)	65.79 ± 7.51	65.49 ± 6.45	0.238#	0.812
BMI	25.31 (23.00, 28.00)	26.04 (23.94, 28.44)	− 1.587&	0.112
Side (n, %)
Left	55 (55.6)	24 (49.0)	0.570*	0.450
Right	44 (44.4)	25 (51.0)
Kellgren⁃Lawrence (n, %)
Ⅲ	11 (11.1)	9 (18. 4)	1.477*	0.224
Ⅳ	88 (88.9)	40 (81.6)

BMI: Body mass index

*Chi-squared test; ^&Mann–Whitney U test; ^#Independent-samples t-test

Patellofemoral joint index

The Kappa statistic for inter-observer reliability was 0.87, indicating good inter-observer reliability. Analysis of all patients' patellar tilt angles revealed that the median preoperative and postoperative values were both 7.00, showing no statistically significant difference (Table 2). Group comparisons demonstrated that the median patellar tilt angle in the normal group showed no significant change postoperatively compared to preoperatively (Z = −1.371, p = 0.170). In contrast, the abnormal patellar tilt angle group (n = 49) showed a significant decrease in the median postoperative patellar tilt angle (Z = −4.979, p = 0.000), indicating that the patellar tilt angle was corrected to a certain extent after surgery (Table 2). Furthermore, the proportion of cases in this group with a postoperative patellar tilt angle exceeding 10° decreased from 100% (49/49) preoperatively to 38.78% (19/49) postoperatively.

Table 2.

Comparison of patellar tilt angle between two groups

Variable	Preoperative	Postoperative	Statistic value	P value
Patellar tilt (n = 148)	7.00 (4.00, 10.00)	7.00 (4.00, 9.00)	− 0.792&	0.428
Normal patellar tilt group (n = 99)	5.00 (3.00, 7.00)	5.00 (3.00, 8.00)	− 1.371&	0.170
Abnormal patellar tilt group (n = 49)	11.00 (10.00, 13.00)	9.00 (7.00, 11.00)	− 4.979&	0.000

^&Mann–Whitney U test

A further analysis of patellofemoral radiographic parameters was performed between the two groups (Table 3). Preoperatively, the median patellar shift was comparable between the normal patellar tilt group and the abnormal patellar tilt group (2.70 vs. 2.80), with no statistically significant difference observed. Similarly, postoperative analysis revealed no significant intergroup difference in patellar shift, with median values of 3.10 in the normal tilt group and 3.90 in the abnormal tilt group. The severity of patellofemoral degeneration, assessed using the Iwano classification, showed no significant difference in the distribution of grades between groups. The proportions of patients classified as grades I, II, and III were largely consistent in both the normal and abnormal patellar tilt groups. Similarly, no statistically significant difference was observed in the Caton–Deschamps index, which was used to evaluate patellar height.

Table 3.

Comparison of additional patellofemoral radiographic parameters between two groups

Variable	Normal patellar tilt group (n = 99)	Abnormal patellar tilt group (n = 49)	Statistic value	P value
Patellar shift
Preoperative	2.70 (1.65, 3.80)	2.80 (1.70, 4.50)	− 0.609&	0.542
Postoperative	3.10 (1.80, 5.10)	3.90 (1.90, 5.30)	− 0.518&	0.605
Iwano classification (n, %)
I	41 (41.4)	15 (30.6)	3.521*	0.172
II	42 (42.4)	20 (40.8)
III	16 (16.2)	14 (28.6)
Caton–Deschamps index	0.90 (0.80, 0.90)	0.90 (0.80, 0.91)	− 0.724&	0.469

^&Mann–Whitney U test; *Chi-squared test

Radiographic change of the knee

To further investigate the changes in lower limb alignment and knee radiographic features between the two groups, a comparative analysis of multiple radiographic parameters was conducted between the normal and abnormal patellar tilt groups (Table 4). Results revealed no statistically significant differences in preoperative and postoperative MPTA, mLDFA, or PTS between the two groups. Postoperatively, the median MPTA was 87° and 86° in the normal and abnormal groups, while the median mLDFA was 87° in both groups, respectively, indicating a comparable alignment correction of the lower limb mechanical axes. Furthermore, the median postoperative HKA angle was 177.00° in both groups. Compared with preoperative values, both groups showed substantial improvement after surgery (Supplementary Table S1, both p < 0.001). No statistically significant difference in postoperative HKA was observed between the two groups. These results indicate that the lower limb alignment after UKA recovered to a mild varus position (approximately 3°), consistent with the anatomical characteristics of physiological knee varus. This suggests that while the surgery effectively corrected varus deformity, it preserved a certain degree of physiological varus angle.

Table 4.

Comparison of radiographic change between two groups

Variable	Normal patellar tilt group (n = 99)	Abnormal patellar tilt group (n = 49)	Statistic value	P value
MPTA
Preoperative	86.00 (84.00, 87.00)	86.00 (84.00, 87.00)	− 0.434&	0.665
Postoperative	87.00 (85.00, 89.00)	86.00 (85.00, 88.00)	− 1.491&	0.136
mLDFA
Preoperative	89.00 (87.00, 90.00)	89.00(87.00, 90.00)	− 0.485&	0.628
Postoperative	87.00 (85.00, 89.00)	87.00(85.00, 89.00)	− 0.253&	0.801
PTS
Preoperative	7.00 (4.50, 9.00)	7.00 (6.00, 9.00)	− 1.167&	0.243
Postoperative	7.00 (5.00, 9.00)	7.00 (6.00, 10.00)	− 1.156&	0.248
HKA
Preoperative	172.00 (170.00, 175.00)	173.00(169.00, 175.00)	− 0.254&	0.800
Postoperative	177.00 (174.00, 179.00)	177.00(175.00, 179.00)	− 1.367&	0.172
Patellar tilt angle
Preoperative	5.00 (3.00, 7.00)	11.00(10.00, 13.00)	− 9.921&	0.000
Postoperative	5.00 (3.00, 8.00)	9.00(7.00, 11.00)	− 5.299&	0.000

MPTA: Medial proximal tibial angle; mLDFA: Mechanical lateral distal femoral angle; HKA: Hip-knee-ankle; PTS: Posterior tibial slope

^&Mann–Whitney U test

It is noteworthy that, although significant difference existed in the patellar tilt angle postoperatively between the two groups (p = 0.000), no significant differences were observed in alignment-related parameters, including MPTA, mLDFA, HKA, and PTS. This indicates that the presence or absence of abnormal patellar tilt was not associated with significant differences in the corrective efficacy of UKA on lower limb alignment, as measured by MPTA, mLDFA, HKA, and PTS.

Knee function

A comparative analysis of preoperative and postoperative clinical outcome was performed between the two groups (Table 5). The results demonstrated no significant differences in the VAS, HSS, or Feller scores between the groups preoperatively, indicating comparable baseline levels. Compared with preoperative values, both groups showed marked improvement in VAS, HSS, and Feller scores after surgery (Supplementary Table S1, all p < 0.001). Postoperative assessment revealed that the normal patellar tilt group (n = 99) showed significantly better outcomes in pain, function, and patellar scores compared to the abnormal group (n = 49). Specifically, the median postoperative VAS was 0 in the normal group, significantly lower than the median of 1 in the abnormal group (p = 0.010). The median postoperative HSS was 85 in the normal group, significantly higher than the median of 82 in the abnormal group (p = 0.000). Similarly, the median Feller score was 20 in the normal group, also significantly higher than the median of 19 in the abnormal group (p = 0.000). Furthermore, the incidence of postoperative AKP was significantly higher in the abnormal group, with 6 cases (12.24%), compared to 3 cases (3.03%) in the normal group (p = 0.027). These findings suggest that an abnormal patellar tilt angle is associated with inferior patient-reported outcomes, including higher pain levels (VAS), lower knee function (HSS), and lower patellofemoral satisfaction (Feller score) following UKA.

Table 5.

Comparison of knee function between two groups

Variable	Normal patellar tilt group (n = 99)	Abnormal patellar tilt group (n = 49)	Statistic value	P value
VAS
Preoperative	3.00 (3.00, 3.00)	3.00 (3.00, 3.00)	− 0.671&	0.502
Postoperative	0.00 (0.00, 0.00)	1.00 (0.00, 1.00)	− 2.593&	0.010
HSS
Preoperative	59.64 ± 5.26	58.28 ± 4.07	1.590#	0.114
Postoperative	85.00 (84.00, 86.00)	83.00 (81.00, 84.00)	− 6.268&	0.000
Feller
Preoperative	11.00 (7.00, 11.00)	11.00 (9.00, 13.00)	− 0.812&	0.417
Postoperative	20.00 (19.00, 22.00)	19.00(18.00, 20.00)	− 3.950&	0.000
AKP	3	6	4.873*	0.027

VAS: visual analogue score; HSS: Hospital for special surgery; AKP: Anterior knee pain

*Chi-squared test; ^&Mann–Whitney U test; ^#Independent-samples t-test

Discussion

This study compared the radiographic and clinical outcomes of UKA in patients with normal versus abnormal patellar tilt, revealing a key phenomenon: despite achieving equally precise and ideal correction of the lower limb mechanical alignment in both groups, a significant disparity in clinical prognosis was observed. This dissociation phenomenon redirects our focus beyond conventional alignment considerations to a critical role of patellofemoral joint stability in UKA.

The primary finding of this study is that UKA effectively corrects knee varus deformity. As demonstrated by the results, postoperative MPTA, mLDFA, and HKA were restored to a range close to normal physiological anatomy in both groups, preserving approximately 3° of physiological varus, with no significant differences between the groups. This confirms that UKA provides reliable and consistent efficacy in restoring lower limb mechanical alignment, which is consistent with existing literature [3]. However, more importantly, despite similar coronal alignment correction, significant disparities emerged in the clinical functional between the two groups. The abnormal patellar tilt group demonstrated significantly worse outcomes in postoperative VAS, HSS, and Feller patellar scores compared to the normal group, although the median or mean differences were small. Therefore, the differences between the two groups in these scores should be interpreted with caution regarding their clinical significance beyond statistical significance. However, this study also identified an indicator that may have more direct and clear clinical significance—the incidence of postoperative AKP. The incidence of postoperative AKP was significantly higher in the abnormal patellar tilt group (12.24%) than in the normal group (3.03%)—a finding consistent with the results reported by Sun et al. [4]. This disparity may be significant in clinical decision-making, potentially impacting on patient postoperative experience and satisfaction—a symptom highly valued by both surgeons and patients. For patients with abnormal patellar tilt angles, high attention must be paid to the risk of postoperative AKP. The occurrence of AKP may be attributed to the reduced contact area between the patella and the femoral trochlea due to patellar tilt, which leads to stress concentration at the patellofemoral joint, subsequent cartilage damage, and accelerated patellofemoral degeneration [7, 16]. Additionally, some cases of patellar tilt originate from femoral rotational abnormalities that cannot be corrected by UKA. A larger patellar tilt angle may also indicate an underlying lower limb malalignment that UKA is unable to correct, and this may further contribute to the higher incidence of postoperative AKP. Sun’s study also found that patients with Wiberg Type III patellae had larger preoperative patellar tilt angles and exhibited a high incidence of AKP postoperatively [4]. Therefore, preoperative counseling should include informing them of the potential incidence of AKP. These data strongly suggest that achieving optimal lower limb mechanical alignment alone is not the sole determinant of successful clinical outcomes following UKA. Furthermore, contemporary analyses utilizing comprehensive phenotypic classifications such as the coronal plane alignment of the knee (CPAK) and the evaluation of parameters like the joint line convergence angle and Joint line obliquity may provide deeper insight into how inherent individual alignment interacts with UKA and influences the patellofemoral joint. This represents a potential direction for future prospective investigation.

Our findings demonstrated that although the two groups exhibited significant differences in preoperative patellar tilt angle, no statistically significant differences were observed in patellar shift, patellar height (Caton–Deschamps index), or the severity of patellofemoral degeneration assessed radiographically using the Iwano classification. This further suggests that preoperative patellar tilt angle may be a specific factor independent of other common static patellofemoral morphological parameters—such as shift and height—as well as the degree of patellofemoral degeneration. The patellar tilt angle, a classic radiographic parameter for assessing the coronal plane patellar trajectory, can be measured using a standard lateral knee radiograph [9]. Unlike the Wiberg classification, which reflects the three-dimensional morphological characteristics of the patella, the patellar tilt angle directly reflects the patellar trajectory within the femoral trochlea and serves as an important indicator of dynamic stability in the patellofemoral joint. Previous studies have shown that an increased patellar tilt angle is closely associated with elevated lateral pressure on the patellofemoral joint, patellar subluxation, and AKP [17, 18]. Therefore, the patellar tilt angle can serve as a simple preoperative method for assessing postoperative clinical function. Moreover, the observation that the abnormal patellar tilt group continued to show higher tilt angles postoperatively, despite some improvement, implies that patellar tilt may reflect a distinct predisposition to patellar tracking instability, separate from both degenerative changes and other baseline morphological features. This inherent instability may become clinically symptomatic—manifesting as AKP, for instance—once the knee is placed in a new mechanical environment following surgery. These results indirectly support the potential value of using patellar tilt angle as a straightforward yet effective preoperative screening parameter, given its easy accessibility on X-rays and its distinctive predictive capacity observed in this study. Nevertheless, the biomechanics of the patellofemoral joint are complex. More detailed evaluation of cartilage status—such as via MRI or intraoperative Outerbridge grading—together with dedicated biomechanical studies, will be required to fully understand patellofemoral joint behavior. Further prospective investigations are needed to explore these relationships in greater depth.

Based on our findings, there may be an association between abnormal patellar tilt and the observed discrepancy. The patellar tilt angle serves as a crucial indicator for evaluating patellofemoral joint congruence and stability [18]. Pre-existing abnormal tilt reflects an inherent patellofemoral biomechanical dysfunction, which may include pathologies such as lateral retinacular tightness and elevated lateral patellofemoral compression [19]. Although a significant postoperative improvement in the patellar tilt angle was observed in the abnormal group (from 11 to 9°), it remained at a relatively high level. This suggests that while surgery may offer benefits through alignment correction or potential soft tissue intervention, it may still fail to fully optimize the patellar trajectory into a biomechanically “safe zone”. The persistence of abnormal patellar tilt may lead to maldistribution of patellofemoral contact pressure, increased cartilage stress, and soft tissue irritation. These persistent biomechanical abnormalities are correlated with residual postoperative pain, functional limitations, and diminished patient satisfaction [17]. Consequently, the preoperative status of patellar tilt is associated with clinical outcomes and may serve as a prognostic indicator, though it could also be a marker of more severe patellofemoral pathology.

Based on our findings, we propose a two-pronged optimization of the current UKA clinical pathway: First, in the preoperative assessment phase, the patellar tilt angle should be incorporated as a routine radiographic evaluation metric. Identifying "high-risk" patients with a preoperative patellar tilt angle greater than 10° facilitates more comprehensive surgical planning and enables improved surgeon–patient communication and expectation management. Second, regarding intraoperative decision-making, for this patient cohort, the surgical goal should be expanded from a singular focus on achieving optimal mechanical alignment to a dual optimization of both alignment and patellar tracking. Our results, which showed significant postoperative reduction in the patellar tilt angle within the abnormal group, suggest that intraoperative maneuvers—both soft-tissue and bone, such as lateral retinacular release, lateral patellar osteophyte resection, or medial retinacular tightening—may indirectly ameliorate patellofemoral congruence. Consequently, we recommend that for patients with preoperative confirmed patellar tilt abnormalities, consciously evaluating and implementing relevant maneuvers to improve patellar tracking and reduce lateral patellar pressure may further enhance surgical efficacy and patient satisfaction. Furthermore, the appropriate selection of insert thickness and intraoperative patellar denervation are additional viable strategies to consider [7, 20]. Finally, systematic postoperative quadriceps muscle strength training plays a crucial supportive role in maintaining patellar dynamic stability and should be prioritized as a core component of rehabilitation programs.

There are several limitations to this study that should be considered when interpreting the results. First, Given the inherent selection bias of its retrospective design, future studies with a prospective design and rigorous adjustment for potential confounders—such as component rotation and soft tissue procedures—are warranted to further validate the present findings. Second, a key limitation of this study is the relatively small sample size in the abnormal tilt group, which may have compromised the statistical power of certain subgroup analyses—such as comparisons across Iwano grade strata—thereby increasing the risk of a Type II error. Consequently, these findings warrant validation through future studies with larger cohorts that allow for robust stratified analyses. Third, the analysis of patellar tilt angle may be confounded by several unmeasured variables, including the rotational alignment of the tibial and femoral components (a known determinant of patellar tracking), intraoperative soft tissue procedures (the manner and extent of release), and variations in surgical technique, which together preclude an isolated assessment of its contributing factors. The rotational alignment of the femoral or tibial prosthetic components was not assessed in this study, although it is known that this parameter influences patellar tracking and patellofemoral biomechanics. This unmeasured confounding factor may have influenced the results, and future prospective studies should employ postoperative CT to account for this. Fourth, while all surgical procedures were uniformly performed by senior surgeons following the established UKA protocol, residual inter-surgeon variability in technical execution remains a plausible, albeit minimized, source of outcome heterogeneity. This factor should be duly acknowledged when evaluating the generalizability and clinical implications of the findings. Furthermore, the follow-up duration of this study was relatively limited. Whether the residual patellar inclination angle following surgery contributes to accelerated patellofemoral cartilage degeneration, the development of AKP, and reduced prosthesis survival remains uncertain and requires further investigation through longer-term, prospective studies. Finally, although the VAS, HSS, and Feller scores used in this study are commonly performed in the joint replacement field, they lack specific outcome measures reporting on patellofemoral joint function, such as the Kujala score. Although the HSS includes assessments of functional items such as stair climbing, which are related to patellofemoral joint function, incorporating the Kujala score in future prospective studies would enable more sensitive and specific evaluation of patellofemoral joint-related function.

In conclusion, the findings of this study suggest that an abnormal patellar tilt angle is associated with an abnormal patellar tilt angle is associated with poor clinical outcomes and a high incidence of AKP following UKA. Achieving optimal limb alignment is necessary but not always sufficient for a successful UKA outcome. Therefore, we propose that a more comprehensive surgical approach, one which prioritizes the assessment of patellofemoral joint stability alongside the pursuit of precise mechanical alignment, warrants further investigation. Should future research validate our findings, incorporating the patellar tilt angle as a routine preoperative evaluation and adopting proactive intraoperative strategies for identified abnormalities could hold significant potential for further enhancing the overall clinical efficacy of UKA.

Supplementary Information

Below is the link to the electronic supplementary material.

Abbreviations

UKA

Unicompartmental knee arthroplasty

HKA

Hip-knee-ankle angle

mLDFA

Mechanical lateral distal femoral angle

MPTA

Medial proximal tibial angle

PTS

Posterior tibial slope

VAS

Visual analog scale

HSS

Hospital for special surgery

AKP

Anterior knee pain

TKA

Total knee arthroplasty

KL

Kellgren-Lawrence

AMOA

Anteromedial osteoarthritis

ROM

Range of motion

HTO

High tibial osteotomy

ACL

Anterior cruciate ligament

SONK

Spontaneous osteonecrosis of the knee

DVT

Deep vein thrombosis

BMI

Body mass index

Author contributions

GC, ZZ and XW were involved in conception and design, and data interpretation for this article. ML, HQ, XL and JZ were involved in data finding and collection for this article. HH, ML and CH were involved in manuscript drafting. ML, JZ, and HQ were involved in data analysis for this article. HQ, XL and YL were involved in imaging measurements. GC, ZZ and XW were involved critical review in for this article. All authors read and approved the final manuscript.

Funding

This work was supported by Natural Science Foundation of Fujian Province (No. 2023J011702, 2023J011703).

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

The study was conducted in accordance with the Declaration of Helsinki, and approved by the Affiliated Hospital of Putian University Ethics Committee. Written informed consent was obtained from all participants. All procedures were performed in accordance with relevant guidelines.

Consent for publication

Not applicable.

Clinical trial number

Not applicable.

Competing interests

The authors declare no competing interests.