Abstract
Purpose
To evaluate the relationship between TT–TG distance and lower-limb anatomical parameters in recurrent patellar dislocation.
Methods
Weight-bearing long-leg radiographs and knee CT scans of 99 patients with recurrent patellar dislocation were reviewed. Trochlear dysplasia was classified according to the Dejour version 2 classification. Tibial tubercle–trochlear groove (TT–TG) distance was measured together with coronal alignment (mFTA and related angles) and rotational alignment (tibiofemoral rotation and tibial torsion). Patients were stratified by TT–TG distance and mFTA-based alignment. Pearson correlations examined associations among anatomical parameters. In the TT–TG ≥ 20 mm group, multivariable regression explored factors potentially associated with TT–TG distance.
Results
Most patients had trochlear dysplasia, and a high proportion were classified as Dejour type C or D (71/99, 71.7%). Overall, TT–TG distance was significantly positively correlated with TFR (r = 0.504, P < 0.001). TFR was significantly higher in the TT–TG ≥ 20 mm group than in the TT–TG < 20 mm group (11.05° ± 4.06° vs. 6.68° ± 4.85°, P < 0.001). In the TT–TG < 20 mm group, TT–TG distance was positively correlated with TFR (r = 0.422, P = 0.003) and tibial torsion (TT) (r = 0.320, P = 0.027). In contrast, in the TT–TG ≥ 20 mm group, TT–TG showed no significant correlations with the assessed anatomical parameters; however, TFR was negatively correlated with TGM (r = − 0.328, P = 0.019). Stratified analysis showed significant correlations between TFR and TT–TG distance in valgus (r = 0.676, P = 0.003) and neutral (r = 0.441, P < 0.001) alignment groups, but not in the varus group.
Conclusion
In patients with recurrent patellar dislocation, TT–TG distance was positively associated with tibiofemoral rotation but not with coronal lower-limb alignment. This association was more evident in patients with TT–TG < 20 mm and was attenuated in those with TT–TG ≥ 20 mm. These findings suggest that TT–TG distance should be interpreted together with rotational alignment and trochlear morphology when planning individualized treatment.
Keywords: Recurrent patellar dislocation, Patellar instability, Tibial tubercle-trochlear groove distance, Tibiofemoral rotation, Lower limb alignment
Introduction
Patellar dislocation is a common knee injury, representing approximately 2%-3% of all knee injuries. Recurrent patellar dislocation is particularly prevalent in adolescents [1–3]. In adolescents, patellar instability may become a long-term condition and is associated with chronic functional impairment and arthritis [4]. Multiple anatomical abnormalities contribute to patellar dislocation. Previous studies have defined anatomical risk factors for patellar instability as high patella (Caton-Deschamps Index ≥ 1.2), patellar tilt ≥ 20°, trochlear hypoplasia, and tibial tubercle-trochlear distance (TT–TG) ≥ 20 mm [5–7].
TT–TG distance is an important parameter for the diagnosis and surgical planning of patellar instability. It quantifies the lateral displacement of the tibial tubercle relative to the femoral trochlea and indirectly reflects patellar tracking under patellar tendon traction during knee motion [8]. TT–TG distance has been reported to be more accurate than the Q angle for assessing patellar stability [9]. Previous studies suggest that tibial tubercle lateralization is a major contributor to an increased TT–TG distance. When TT–TG ≥ 20 mm, tibial tubercle transposition is often recommended to normalize patellofemoral contact pressures and improve patellar tracking [10, 11]. However, recent studies have reported less favorable outcomes after tibial tubercle transposition in patients with recurrent patellar dislocation and TT–TG ≥ 20 mm [12]. This suggests that TT–TG distance is influenced by the combined effects of multiple anatomical factors [13]. Previous studies indicate that lower-limb torsional deformities, trochlear groove position, and coronal malalignment can substantially affect TT–TG distance [13, 14]. Recent work has increasingly focused on the anatomical mechanisms underlying an increased TT–TG distance. The tibiofemoral rotation angle (TFR) is defined as the rotational angle between the distal femur and the proximal tibia [15]. In patients whose increased TT–TG distance is primarily driven by excessive TFR, tibial tubercle transfer may leave the abnormal rotation uncorrected postoperatively and thereby limit restoration of normal patellofemoral kinematics [11]. Because TT–TG distance is multifactorial, surgical decision-making for tibial tubercle procedures in recurrent patellar dislocation should not rely solely on TT–TG distance [16, 17]. However, few studies have examined the anatomical factors associated with TT–TG using TT–TG–based stratification. Therefore, identifying the anatomical determinants of an increased TT–TG distance and quantifying their relative contributions are clinically important for individualized diagnosis and treatment planning.
The aims of this study were: (1) to examine the associations between TT–TG distance and anatomical parameters, including the mechanical lateral distal femoral angle (mLDFA), mechanical medial proximal tibial angle (mMPTA), mechanical femorotibial angle (mFTA), tibiofemoral rotation (TFR), tibial torsion (TT), and trochlear groove medialization (TGM); (2) to evaluate the association between coronal lower-limb alignment and TT–TG distance; (3) to determine the relative contributions of these anatomical factors to TT–TG distance, thereby informing surgical decision-making. we hypothesized TFR would be more strongly associated with TT–TG than coronal alignment in recurrent patellar dislocation.
Materials and methods
This study was approved by our hospital (No.Ke2025-031-1). The requirement for informed consent was waived because of the retrospective study design.
We retrospectively collected data from patients with recurrent patellar dislocation who underwent surgical treatment at our hospital between January 2021 and October 2024. Inclusion criteria: (1) clinical diagnosis of recurrent patellar dislocation (≥ 2 episodes); (2) positive patellar lateral displacement test; (3) availability of complete clinical records and radiographic evaluation, including X-ray and computed tomography (CT) of the affected knee; (4) no history of prior knee surgery or ligament injury; (5) no concomitant medial or lateral collateral ligament injury. Exclusion criteria: (1) first-time patellar dislocation; (2) history of knee or ankle surgery; (3) concurrent knee or ankle ligament injuries; (4) incomplete or missing clinical records. Ultimately, 99 patients met the eligibility criteria and were included in the final analysis.
Patients were stratified according to predefined criteria to enable within- and between-group analyses. (1) Correlations between TT–TG distance and other anatomical parameters were assessed in the overall cohort. (2) Patients with recurrent patellar dislocation were stratified into TT–TG < 20 mm and TT–TG ≥ 20 mm groups. Correlations between TT–TG distance and anatomical parameters (mLDFA, mMPTA, TFR, TT, and TGM) were assessed within each group and compared between groups. (3) Patients were further classified by coronal alignment as valgus (mFTA > 3°), varus (mFTA < − 3°), or neutral (mFTA ≤ |3°|). Within each alignment group, correlations between TT–TG distance and anatomical parameters (mLDFA, mMPTA, mFTA, TFR, TT, and TGM) were evaluated, and results were compared across groups.
Clinical assessment
Sex and age at the time of surgery were extracted from the medical records. All patients underwent standardized imaging, including weight-bearing long-leg radiographs and computed tomography (CT) of the ipsilateral hip, knee, and ankle. All patients were diagnosed with recurrent patellar dislocation.
Radiologic measurement
Radiographic examination
All patients underwent standardized weight-bearing long-leg radiographs. During image acquisition, patients stood upright with their feet together and the patellae facing forward.
CT examination
All patients underwent CT examination using the same scanner model (SOMATOM Sensation 16; Siemens Medical Solutions, Erlangen, Germany). Images were acquired with a 1-mm slice thickness and a 512 × 512 matrix. All patients were scanned in the supine position with the knee fully extended. Coronal alignment parameters (mLDFA, mMPTA, and mFTA) were measured on standardized weight-bearing long-leg radiographs. All measurements were performed using a dedicated image-analysis software package (iMedPacs DICOM Viewer, desktop version). Two experienced orthopedic surgeons independently reviewed all radiographs and CT images. Both observers were blinded to clinical data during measurements.
Classification of trochlear dysplasia
Trochlear dysplasia was classified as absent or as Dejour type A, B, C, or D according to the Dejour version 2 classification, based on lateral radiographs and axial CT images [18, 19]. Lateral radiographs were used to identify characteristic Dejour signs, including the crossing sign, supratrochlear spur, and double contour, whereas axial CT images were used to assess axial trochlear morphology. Representative lateral radiographic and axial CT examples of Dejour types A-D are shown in Fig. 1, including a shallow trochlea in type A, a flat trochlea in type B, a convex trochlea in type C, and a cliff pattern in type D.
Fig. 1.
Dejour version 2 classification of trochlear dysplasia. A Dejour type A, characterized by a shallow trochlea; B Dejour type B, characterized by a flat trochlea; C Dejour type C, characterized by a convex trochlea; and D Dejour type D, characterized by a cliff pattern. For each type, the true lateral radiograph is shown on the left and the corresponding axial CT image on the right
Tibial tubercle-trochlear groove distance (TT–TG)
The TT–TG distance is defined as the distance between the deepest point of the femoral trochlear groove and the tibial tubercle. Because CT does not directly visualize articular cartilage, the TT–TG distance was measured using osseous landmarks in this study [20]. The deepest point of the osseous trochlear groove was defined as the trochlear reference point. Measurement involves two planes: the first plane is at the deepest point of the femoral trochlear groove. Draw line A through the deepest point of the femoral trochlear groove, perpendicular to the posterior femoral condylar line (fPCL). The second plane is the tibial tubercle plane. At this plane, draw line B through the highest point of the tibial tubercle, perpendicular to the posterior femoral condylar line. The distance between these two lines C is defined as TT–TG distance (Fig. 2).
Fig. 2.
Measurement of the tibial tubercle–trochlear groove (TT–TG) distance. (1) TT–TG distance is defined as the mediolateral distance between the deepest point of the femoral trochlear groove and the tibial tubercle. (2) Line A is drawn through the deepest point of the femoral trochlear groove, perpendicular to the posterior femoral condylar line (fPCL). (3) Line B is drawn through the tibial tubercle, perpendicular to the posterior femoral condylar line. (4) The distance (C) between lines A and B is defined as the TT–TG distance
Medial lateral distal femoral angle (mLDFA)
The mLDFA was defined as the angle between the mechanical axis of the femur and the distal femoral joint line (tangent to the distal femoral condyles) on the coronal plane [2, 21] (Fig. 3a).
Fig. 3.
Measurement of anatomical parameters. a (1) mLDFA is the angle between the femoral mechanical axis and the tangent to the distal lateral femoral articular surface. (2) mMPTA is the angle between the medial articular surface of the proximal tibia and the tibial mechanical axis. (3) mechanical femorotibial angle (mFTA) was defined as the angle between the mechanical axes of the femur and the tibia. b The tibiofemoral rotation angle (TFR) was defined as the angle between the tangent lines to the posterior condyles of the distal femur and the proximal tibia. c The tibial torsion angle (TT) was defined as the angle between the tangent line to the posterior condyle of the proximal tibia and the line connecting the centers of the medial and lateral malleoli. d Trochlear groove medialization (TGM) was defined as d/D, where d is the distance from the deepest trochlear point to the lateral border of the femoral condyle and D is the femoral condylar width
Mechanical medial proximal tibial angle (mMPTA)
On standardized weight-bearing long-leg radiographs, the mMPTA was defined as the angle between the proximal tibial joint line (medial articular surface) and the mechanical axis of the tibia [22] (Fig. 3a).
Mechanical femorotibial angle (mFTA)
On weight-bearing long-leg radiographs, mechanical femorotibial angle (mFTA) was defined as the angle between the mechanical axes of the femur and the tibia. Based on mFTA, lower-limb alignment was categorized as valgus (mFTA > 3°), varus (mFTA < − 3°), or neutral (mFTA ≤ |3°|) [23] (Fig. 3a).
Tibiofemoral rotation angle (TFR)
The tibiofemoral rotation angle (TFR) was defined as the angle between the tangent lines to the posterior condyles of the distal femur and the proximal tibia [3]. Tibial external rotation relative to the femur was assigned a positive value, whereas internal rotation was assigned a negative value (Fig. 3b).
Tibial torsion angle (TT)
The tibial torsion angle (TT) was defined as the angle between the tangent line to the posterior condyle of the proximal tibia and the line connecting the centers of the medial and lateral malleoli [24]. Distal tibial external rotation relative to the proximal tibia was assigned a positive value, whereas internal rotation was assigned a negative value (Fig. 3c).
Trochlear groove medialization (TGM)
Select an axial CT slice that clearly delineates the lateral femoral condyle and the trochlear groove for subsequent measurements. Draw a line parallel to the posterior femoral condylar line from the medial-most border of the medial femoral condyle to the lateral-most border of the lateral femoral condyle to obtain the maximum femoral condylar width (D). Measure the distance (d) from the deepest point of the trochlear groove to the lateral border of the femoral condyle. Trochlear groove medialization was defined as d/D [25] (Fig. 3d).
Statistical analysis
All analyses were performed using SPSS Statistics (version 25.0; IBM Corp). Continuous variables are presented as mean ± standard deviation (SD), and categorical variables as number (percentage). Normality was assessed using the Kolmogorov-Smirnov test. For continuous variables, between-group comparisons were performed using an independent-samples t test or the Mann-Whitney U test, as appropriate; comparisons among three groups were conducted using the Kruskal-Wallis test. Categorical variables were compared using the chi-square test. Pearson correlation coefficients were calculated to assess the relationships among the measured parameters. Intraclass correlation coefficients (ICCs) were calculated to assess interobserver reliability for all continuous radiologic measurements Multivariable linear regression analysis was performed to examine possible associations between rotational parameters and TT–TG distance in patients with TT–TG ≥ 20 mm. A two-sided P value < 0.05 was considered statistically significant. Based on a confidence level of 95% (α = 0.05), power (1–β) of 90%, a total sample size of 38 subjects was required.
Results
This study included 99 patients with recurrent patellar dislocation (22 males and 77 females; mean age, 19.1 ± 7.5 years). All radiographic and CT-based measurements showed good-to-excellent interobserver reliability, with ICC values ranging from 0.81 to 0.94 Demographic characteristics and anatomical measurements are summarized in Table 1.
Table 1.
Measurement of baseline characteristics and anatomical parameters
| Variable | Total patient cohort | ICC |
|---|---|---|
| Number of included patients | 99 | |
| Sex, n (%) | ||
| Female | 77 (77.8%) | - |
| Male | 22 (22.2%) | - |
| Age (years) | 19.11 ± 7.52 (9–45) | - |
| Trochlea dysplasia, n (%) | ||
| None | 1 (1.0%) | - |
| Type A | 2 (2.0%) | - |
| Type B | 25 (25.3%) | - |
| Type C | 41 (41.4%) | - |
| Type D | 30 (30.3%) | - |
| TT–TG (mm) | 19.84 ± 4.02 (10.7–31.7) | 0.94 |
| mLDFA (°) | 85.02 ± 3.23 (76.5–95.4) | 0.88 |
| mMPTA (°) | 87.81 ± 2.78 (75.3–93.9) | 0.89 |
| mFTA (°) | 0.44 ± 3.29 (-7.5-10.4) | 0.87 |
| TT (°) | 27.95 ± 7.35 (10.2–43.6) | 0.86 |
| TFR (°) | 8.93 ± 4.95(-1.5–21.7) | 0.86 |
| TGM (%) | 44.9 ± 3.44 (37–57) | 0.81 |
Categorical variables are presented as number (percentage), and continuous variables as mean ± standard deviation (SD)
ICC Intraclass correlation coefficient, TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion, TFR Tibiofemoral rotation, TGM Trochlear groove medialization
Pearson correlation results are shown in Table 2. In the overall cohort, TT–TG distance was positively correlated with TFR (r = 0.504, P < 0.001; Fig. 4) and was not significantly correlated with mLDFA, mMPTA, mFTA, TT, or TGM. Pearson correlation analyses were performed separately in the TT–TG < 20 mm and TT–TG ≥ 20 mm groups. In the TT–TG < 20 mm group, TT–TG distance was positively correlated with TT (r = 0.320, P = 0.027) and TFR (r = 0.422, P = 0.003) (Table 3). In the TT–TG ≥ 20 mm group, TT–TG distance was not significantly correlated with any parameter in univariate analyses. In this group, TFR was negatively correlated with TGM (r = − 0.328, P = 0.019) (Table 4). Patients with TT–TG ≥ 20 mm had a significantly higher TFR than those with TT–TG < 20 mm (11.05° ± 4.06° vs. 6.68° ± 4.85°, P < 0.001; Fig. 5). No significant between-group differences were observed in sex, age, mLDFA, mMPTA, mFTA, TT, or TGM (Table 5). To further assess the independent contributions of cross-sectional parameters to TT–TG distance, we fitted a multivariable linear regression model including TFR, TGM, and TT in the TT–TG ≥ 20 mm group. All predictors had tolerance values > 0.10 and variance inflation factors (VIFs) < 2, indicating no evidence of multicollinearity. The overall model was not statistically significant (F = 1.850, P = 0.151). The intercept was 14.51, and unstandardized regression coefficients were 0.194 for TFR, 0.111 for TGM, and 0.044 for TT. The model explained 10.6% of the variance in TT–TG (R² = 0.106; adjusted R² = 0.049), indicating limited explanatory power in this subgroup (Table 6). The predicted TT–TG distance can be expressed as: TT–TG = 14.51 + 0.194(TFR) + 0.111(TGM) + 0.044(TT).
Table 2.
Pearson correlation analysis of anatomical parameters
| TT–TG | mLDFA | mMPTA | mFTA | TT | TFR | TGM | |
|---|---|---|---|---|---|---|---|
| TT–TG | - | ||||||
| mLDFA |
r = − 0.083 n.s. |
- | |||||
| mMPTA |
r = − 0.014 n.s. |
0.093 n.s. |
- | ||||
| mFTA |
r = − 0.032 n.s. |
r = − 0.419** P < 0.001 |
r = − 0.356** P < 0.001 |
- | |||
| TT |
r = 0.160 n.s. |
r = 0.111 n.s. |
r = − 0. 025 n.s. |
r = − 0.113 n.s. |
- | ||
| TFR |
r = 0.504** P < 0.001 |
r = 0.053 n.s. |
r = 0.069 n.s. |
r = − 0.069 n.s. |
r = 0.002 n.s. |
- | |
| TGM |
r = 0.044 n.s. |
r = 0.129 n.s. |
r = 0.136 n.s. |
r = − 0.086 n.s. |
r = − 0.040 n.s. |
r = − 0.154 n.s. |
- |
TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion; TFR Tibiofemoral rotation; TGM Trochlear groove medialization
r = Pearson’s correlation coefficient.
**Significant r with P < 0.01
Fig. 4.
TT–TG distance was significantly positively correlated with TFR (r = 0.504, P < 0.001). TT–TG: Tibial tubercle-trochlear groove; TFR: Tibiofemoral rotation
Table 3.
Pearson correlation results among anatomical parameters in the TT–TG < 20 mm group (n = 48)
| TT–TG | mLDFA | mMPTA | mFTA | TT | TFR | TGM | |
|---|---|---|---|---|---|---|---|
| TT–TG | - | ||||||
| mLDFA |
r = 0.052 n.s. |
- | |||||
| mMPTA |
r = − 0.062 n.s. |
r = 0.068 n.s. |
- | ||||
| mFTA |
r = 0.108 n.s. |
r = − 0.419** P = 0.008 |
r = 0.459** P < 0.001 |
- | |||
| TT |
r = 0.320* P = 0.027 |
r = 0.240 n.s. |
r = − 0.033 n.s. |
r = − 0.098 n.s. |
- | ||
| TFR |
r = 0.422** P = 0.003 |
r = 0.062 n.s. |
r = − 0.100 n.s. |
r = − 0.112 n.s. |
r = 0.121 n.s. |
- | |
| TGM |
r = − 0.113 n.s. |
r = 0.217 n.s. |
r = 0.168 n.s. |
r = − 0.112 n.s. |
r = − 0.028 n.s. |
r = − 0.063 n.s. |
- |
TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion, TFR Tibiofemoral rotation, TFR Trochlear groove medialization
r = Pearson’s correlation coefficient
*Significant r with P < 0.05
**Significant r with P < 0.01
Table 4.
Pearson correlation results among anatomical parameters in the TT–TG ≥ 20 mm group (n = 51)
| TT–TG | mLDFA | mMPTA | mFTA | TT | TFR | TGM | |
|---|---|---|---|---|---|---|---|
| TT–TG | - | ||||||
| mLDFA |
r = − 0.133 n.s. |
- | |||||
| mMPTA |
r = 0.089 n.s. |
r = 0.093 n.s. |
- | ||||
| mFTA |
r = − 0.095 n.s. |
r = − 0.470** P < 0.001 |
r = 0.251 n.s. |
- | |||
| TT |
r = 0.192 n.s. |
r = 0.014 n.s. |
r = − 0.007 n.s. |
r = − 0.105 n.s. |
- | ||
| TFR |
r = 0.159 n.s. |
r = 0.173 n.s. |
r = 0.286* P = 0.042 |
r = 0.018 n.s. |
r = − 0.164 n.s. |
- | |
| TGM |
r = 0.091 n.s. |
r = 0.055 n.s. |
r = 0. 096 n.s. |
r = − 0.053 n.s. |
r = − 0.018 n.s. |
r = − 0.328* P = 0.019 |
- |
TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion, TFR Tibiofemoral rotation, TGM Trochlear groove medialization
r = Pearson’s correlation coefficient
*Significant r with P < 0.05
**Significant r with P < 0.01
Fig. 5.
TFR was significantly higher in the TT–TG ≥ 20 mm group than in the TT–TG < 20 mm group (11.05° ± 4.06° vs. 6.68° ± 4.85°, P < 0.001). TT–TG: Tibial tubercle-trochlear groove; TFR: Tibiofemoral rotation
Table 5.
Subgroup analysis stratified by TT–TG distance
| Variable | TT–TG | P | |
|---|---|---|---|
| < 20 mm | ≥ 20 mm | ||
| Sex | n.s. | ||
| Female | 40 (40.4%) | 37 (37.4%) | |
| Male | 8 (8.1%) | 14 (14.1%) | |
| Age (years) | 20.1 ± 7.4 | 18.3 ± 7.4 | 0.080 |
| mLDFA (°) | 85.47 ± 3.46 | 84.6 ± 2.97 | 0.531 |
| mMPTA (°) | 87.80 ± 3.24 | 87.81 ± 2.31 | 0.801 |
| mFTA (°) | 0.47 ± 3.45 | 0.42 ± 3.16 | 0.750 |
| TT (°) | 27.76 ± 7.25 | 28.12 ± 7.52 | 0.698 |
| TFR (°) | 6.68 ± 4.85 | 11.05 ± 4.06 | < 0.001 |
| TGM(%) | 44.56 ± 2.89 | 45.25 ± 3.88 | 0.647 |
Categorical variables are presented as number (percentage), and continuous variables as mean ± standard deviation (SD)
TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion, TFR Tibiofemoral rotation, TGM Trochlear groove medialization
Table 6.
Multivariable Linear Regression for TT–TG Distance in the TT–TG ≥ 20 mm Group
| Factor | Mean Estimate (95% CI) | β | P |
|---|---|---|---|
| Intercept | 14.51(4.777 to 24.231) | - | 0.004 |
| TFR | 0.194(0.015 to 0.372) | 0.320 | 0.034 |
| TGM | 0.111(-0.076 to 0.298) | 0.174 | 0.237 |
| TT | 0.044(-0.048 to 0.136) | 0.135 | 0.339 |
R2 = 0.106, adjusted R2 = 0.049. Standardized estimates were used to quantify the relative contribution of selected anatomical factors to TT–TG distance. The standardized estimate of TFR (β = 0.320) was larger than those of TGM (β = 0.174) and TT (β = 0.135), TFR may remain clinically relevant in patients with markedly elevated TT–TG distance; however, the overall model was not statistically significant and explained little variance.
TFR Tibiofemoral rotation, TGM Trochlear groove medialization, TT Tibial torsion
Patients were categorized by coronal alignment based on mFTA as valgus (mFTA > 3°), varus (mFTA < − 3°), or neutral (mFTA ≤ |3°|) Anatomical measurements in each alignment group are summarized in Table 7. Within-group Pearson correlation results for the three alignment groups are presented in Table 8. TT–TG distance was positively correlated with TFR in the neutral and valgus groups (r = 0.441, P < 0.001; r = 0.676, P = 0.003). In the varus group, TT–TG distance was not significantly correlated with the other anatomical parameters.
Table 7.
Subgroup analysis based on coronal lower-limb alignment
| Variable | Valgus group (n = 17) |
Neutral group (n = 71) |
Varus group (n = 11) |
P |
|---|---|---|---|---|
| TT–TG (mm) | 19.82 ± 4.44 | 19.90 ± 3.87 | 19.49 ± 4.66 | 0.922 |
| mLDFA (°) | 82.53 ± 2.79 | 85.20 ± 3.07 | 87.70 ± 2.29 | < 0.001 |
| mMPTA (°) | 90.47 ± 2.34 | 87.26 ± 2.67 | 87.21 ± 1.61 | < 0.001 |
| TT (°) | 25.67 ± 5.08 | 27.93 ± 7.68 | 31.57 ± 7.24 | 0.144 |
| TFR (°) | 8.95 ± 5.91 | 8.94 ± 4.84 | 8.84 ± 4.50 | 0.999 |
| TGM (%) | 44.59 ± 2.81 | 44.86 ± 3.35 | 45.82 ± 4.83 | 0.891 |
Categorical variables are presented as number (percentage), and continuous variables as mean ± standard deviation (SD)
TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion, TFR Tibiofemoral rotation, TGM Trochlear groove medialization
Table 8.
Pearson correlation analysis between TT–TG distance and anatomical parameters in the valgus, neutral, and varus groups
| r | P | ||
|---|---|---|---|
| Valgus group (n = 17) | mLDFA (°) | -0.179 | n.s. |
| mMPTA (°) | -0.368 | n.s. | |
| mFTA (°) | -0.058 | n.s. | |
| TT (°) | -0.002 | n.s. | |
| TFR (°) | 0.676** | 0.003 | |
| TGM (%) | 0.190 | n.s. | |
| Neutral group (n = 71) | mLDFA (°) | -0.073 | n.s. |
| mMPTA (°) | 0.071 | n.s. | |
| mFTA (°) | -0.100 | n.s. | |
| TT (°) | 0.162 | n.s. | |
| TFR (°) | 0.441** | < 0.001 | |
| TGM (%) | 0.179 | n.s. | |
| Varus group (n = 11) | mLDFA (°) | -0.245 | n.s. |
| mMPTA (°) | -0.091 | n.s. | |
| mFTA (°) | 0.592 | n.s. | |
| TT (°) | 0.273 | n.s. | |
| TFR (°) | 0.391 | n.s. | |
| TGM (%) | -0.406 | n.s. |
TT–TG Tibial tubercle-trochlear groove, mLDFA Mechanical lateral distal femoral angle, mMPTA Mechanical medial proximal tibial angle, mFTA Mechanical femorotibial angle, TT Tibial torsion, TFR Tibiofemoral rotation, TGM Trochlear groove medialization
r = Pearson’s correlation coefficient
**Significant r with P < 0.01
Discussion
The main finding of this study is that TT–TG distance is positively correlated with tibiofemoral rotation (TFR) in patients with recurrent patellar dislocation, whereas no significant association is observed with coronal lower-limb alignment. These results indicate that rotational deformity is a key anatomical contributor to an increased TT–TG distance. Notably, this relationship is more clearly linear in patients with TT–TG < 20 mm. In contrast, in patients with TT–TG ≥ 20 mm, although the mean TFR is significantly higher, the association between TFR and TT–TG is attenuated, and a negative association between TFR and trochlear groove medialization (TGM) is observed. Collectively, these findings suggest that when TT–TG is markedly increased, its development may reflect more complex, potentially compensatory anatomical interactions.
This study examined correlations among TT–TG distance and anatomical parameters, including mLDFA, mMPTA, mFTA, TT, TFR, and TGM. Subgroup analyses used a TT–TG threshold of ≥ 20 mm to define an abnormal TT–TG distance, as proposed by D’Amore et al. [11]. Patients were stratified into TT–TG < 20 mm and TT–TG ≥ 20 mm groups for further analyses. In the TT–TG < 20 mm group, TT–TG distance was positively correlated with TFR (r = 0.422, P = 0.003). In the TT–TG ≥ 20 mm group, TFR was significantly higher than in the TT–TG < 20 mm group (11.05° ± 4.06° vs. 6.68° ± 4.85°, P < 0.001), but the linear association between TFR and TT–TG was attenuated. In the TT–TG ≥ 20 mm group, a multivariable linear regression model suggested that TFR had the largest standardized coefficient (β = 0.320), compared with TGM (β = 0.174) and TT (β = 0.135). This finding suggests that tibiofemoral rotational deformity may remain relevant even when TT–TG is markedly increased. However, the overall model was not statistically significant (F = 1.850, P = 0.151) and explained little variance in TT–TG (R² = 0.106; adjusted R² = 0.049). This is consistent with the significant correlation between TT–TG distance and TFR in the overall cohort (r = 0.504, P < 0.001) and suggests that this relationship may be modified by other anatomical factors. Previous studies have identified TFR as a determinant of TT–TG distance [17]. Studies by Tensho and Chen et al. [25, 26] reported that TFR is significantly associated with TT–TG distance. Anley et al. [10] suggested that an increased TT–TG distance may be related to increased TFR. This is partly consistent with our findings. In this study, TGM was defined as the ratio of the distance from the deepest trochlear point to the lateral border of the femoral condyle to the femoral condylar width [22]. A higher ratio indicates a relatively more medial position of the trochlear groove. In the TT–TG ≥ 20 mm group, TFR was negatively correlated with TGM (r = − 0.328, P = 0.019). This may partly explain why the association between TFR and TT–TG was clearer in the TT–TG < 20 mm group but became weaker in the TT–TG ≥ 20 mm group. Therefore, we propose that the attenuated association between TFR and TT–TG in patients with TT–TG ≥ 20 mm may be related to variation in TGM. Xu et al. [17] reported a negative correlation between TT–TG distance and the distance from the deepest trochlear point to the medial border of the femoral condyle when comparing healthy individuals with patients with recurrent patellar dislocation. Further studies indicate that TFR is closely related to tibial torsion, suggesting that increased TFR may reflect excessive tibial external rotation [27, 28]. Concurrently, prior work suggests that femoral internal rotation may shift the trochlear groove toward the femoral center, whereas tibial external rotation may lateralize the tibial tubercle; both mechanisms may increase TT–TG distance [29–31]. In patients with TT–TG ≥ 20 mm, a higher TFR indicates greater tibial external rotation relative to the femur. Lin and Guan et al. [32, 33] reported an association between TFR severity and patellar dislocation severity across different populations. A cadaveric study by Keshmiri et al. [34] suggested that TFR can alter the distribution of medial and lateral patellar contact pressures. TFR is also a key component of lower-limb rotational alignment [35]. We therefore hypothesize that the negative association between TFR and TGM may reflect a compensatory mechanism: when TT–TG is markedly increased and TFR is high, relative lateralization of the trochlear groove (i.e., lower TGM) may partially offset the lateralization of the tibial tubercle and thereby modulate TT–TG. However, this hypothesis requires validation in larger cohorts and mechanistic studies.
Measurement-related factors should also be considered when interpreting these findings. In this cohort, most patients had trochlear dysplasia, and a high proportion were classified as Dejour type C or D (71/99, 71.7%). With increasing severity of trochlear dysplasia, the trochlear groove may become flatter, more asymmetric, or less clearly defined, making reliable identification of the deepest trochlear point more difficult and potentially affecting the precision of TT–TG measurements [36, 37]. Therefore, the attenuated association between TFR and TT–TG in the TT–TG ≥ 20 mm group should be interpreted with caution, as it may reflect both anatomical complexity and measurement-related variability.After stratifying patients into valgus, neutral, and varus groups based on mechanical femorotibial angle (mFTA), within-group correlation analyses indicated that the anatomical correlates of TT–TG differed across coronal alignment patterns. TT–TG distance was positively correlated with TFR in the valgus and neutral groups (r = 0.676, P = 0.003; r = 0.441, P < 0.001), whereas no significant correlations between TT–TG and other anatomical parameters were observed in the varus group. These findings suggest that transverse-plane rotational deformity may be a key contributor to TT–TG variation under neutral and valgus alignment. Mechanistically, tibial external rotation can lateralize the tibial tubercle, thereby increasing TT–TG distance. Therefore, when the coronal alignment shows no significant inward deviation, individual variations in TT–TG are more likely associated with rotational factors. Notably, although TT–TG, TFR, and TGM did not differ significantly across alignment groups, the correlation patterns differed by subgroup. This suggests that coronal alignment may modify the relationships among anatomical factors rather than directly determining the absolute TT–TG distance. Based on these findings, we hypothesize that coronal varus may not directly affect TT–TG distance but may alter the association between rotational deformity and TT–TG.
Additionally, we found no significant association between TT–TG distance and coronal alignment, which differs from the findings of Chang et al. [8] in patients with severe osteoarthritis. We speculate that this discrepancy may be attributable to differences in underlying biomechanics and disease context. In severe osteoarthritis, changes in femoral morphology may occur secondary to post-adolescent alignment alterations and long-term degenerative remodeling, which may strengthen the apparent association between TT–TG and alignment. In contrast, recurrent patellar dislocation typically affects adolescents or young adults and is often associated with congenital or developmental anatomical variations during skeletal maturation [14, 38].
Moreover, the biomechanical environment during growth differs substantially from that in advanced osteoarthritis. We therefore hypothesize that differences in the timing and mechanisms of morphological change may underlie the discrepant associations between TT–TG distance and coronal alignment across these populations.
This study not only quantified anatomical factors associated with TT–TG distance but also highlights the potential clinical bias of relying on a single metric for surgical decision-making. Our findings underscore that recurrent patellar dislocation is a complex disorder involving three-dimensional lower-limb alignment, thereby supporting a multiplanar approach to surgical planning. For patients with recurrent patellar dislocation who have mechanical-axis deviation or suspected torsional deformity, systematic assessment of lower-limb torsional parameters and trochlear morphology should be performed before considering tibial tubercle transfer to facilitate individualized surgical planning. This interpretation is consistent with the “menu à la carte” treatment concept for patellar instability, which emphasizes individualized surgical planning based on the combined pattern of anatomical risk factors rather than any single parameter [39]. Within this framework, TT–TG distance should be interpreted in conjunction with trochlear morphology, patellar height, soft-tissue integrity, and lower-limb alignment when considering tibial tubercle transfer or combined procedures. This study has several limitations. First, this was a retrospective, single-center study, which may introduce selection bias. In addition, because the TT–TG distance was measured on CT images using osseous landmarks, potential differences between the osseous and cartilaginous trochlear grooves could not be fully evaluated. This issue may be more pronounced in patients with more severe trochlear dysplasia, in whom the trochlear reference point can be difficult to define and may affect TT–TG measurement precision. Second, the varus subgroup was small (n = 11), limiting statistical power. Third, relevant parameters such as tibial tubercle lateralization (TTL) and femoral anteversion angle (FAA) were not assessed. Fourth, MPFL integrity was not systematically assessed in this retrospective cohort, which represents an important limitation because soft-tissue stabilizers also contribute to recurrent patellar instability and may influence individualized surgical planning. Fifth, the lack of a control group (e.g., asymptomatic individuals) limits the generalizability of our findings to populations without recurrent patellar dislocation. Finally, the primary anatomical determinants of TT–TG distance in patients with TT–TG ≥ 20 mm remain to be clarified. This will be a focus of future research.
Conclusion
In patients with recurrent patellar dislocation, TT–TG distance was positively associated with tibiofemoral rotation, whereas no significant association was observed with coronal lower-limb alignment. Stratification by TT–TG distance showed that the association between rotational factors and TT–TG distance was clearer in patients with TT–TG < 20 mm but attenuated in those with TT–TG ≥ 20 mm. In the TT–TG ≥ 20 mm group, TFR was negatively correlated with TGM, suggesting a potential interaction between trochlear groove position and rotational alignment. Although mFTA-based stratification showed no significant differences in TT–TG distance, correlation patterns differed among alignment subgroups. These findings suggest that TT–TG distance should be interpreted in conjunction with rotational alignment and trochlear morphology to support individualized assessment and surgical planning.
Abbreviations
- TT–TG
Tibial tubercle–trochlear groove
- mLDFA
Mechanical lateral distal femoral angle
- mMPTA
Mechanical medial proximal tibial angle
- mFTA
Mechanical femorotibial angle
- TT
Tibial torsion
- TFR
Tibiofemoral rotation
- TGM
Trochlear groove medialization
Authors’ contributions
L.M. and QY.L.: Writing the manuscript, evaluating the radiographs, collecting and analysing the data. JC.W., SY.Z. and ZC.X.: Collecting and analyzing the data. HN.W. and L.L.: Project coordinators, designing the study. CX.S.: Co-corresponding author, project conception, review of writing and analysis. XF.W.: Corresponding author, The primary investigator, project conception, review of writing and analysis.
Funding
The study was supported by the Government-Funded Program for the Training of Exceptional Talents in Clinical Medicine (No. ZF2024079).
Data availability
The datasets used during the current study are available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
This study was approved by the Ethics Committee of the Hebei Medical University Third Hospital (approval No. Ke2025-031-1). All procedures performed in this study were in accordance with the ethical standards of the institutional review board and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. The requirement for informed consent was waived due to the retrospective nature of the study.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Liang Ma and Qingyu Li contributed equally to this work.
Contributor Information
Chenxia Shi, Email: chenxiashi@hebmu.edu.cn.
Xiaofeng Wang, Email: xiaofengwang@hebmu.edu.cn.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used during the current study are available from the corresponding author on reasonable request.