Tibial Tubercle to Trochlear Groove Distance Measured by Transepicondylar Axis on MRI is a Reliable Parameter in Tibial Tuberosity Osteotomy Decision Making in Patients with Recurrent Patellar Dislocation

doi:10.21203/rs.3.rs-1627581/v1

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Research Article

Tibial Tubercle to Trochlear Groove Distance Measured by Transepicondylar Axis on MRI is a Reliable Parameter in Tibial Tuberosity Osteotomy Decision Making in Patients with Recurrent Patellar Dislocation

https://doi.org/10.21203/rs.3.rs-1627581/v1

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purpose: To (1) verify the reliability of the tibial tubercle-trochlear groove (TT-TG) distance measured by posterior condylar reference line (PCRL), surgical and anatomical transepicondylar axis (SEA and AEA) on MRI (TT-TGp, TT-TGs, and TT-TGa, respectively), (2) validate the diagnostic capacity of each parameter, (3) define the pathological threshold value of each TT-TG distance for tibial tubercle osteotomy (TTO).

Methods: 86 patients with patellar dislocation (PD) and 86 healthy individuals were included. The intraclass correlation coefficient (ICC) was calculated to assess inter-observer reliability of each parameter. Unpaired t-test and Pearson correlation analysis were conducted. Receiver operating characteristic curves (ROC) and the areas under the curves (AUC) were analyzed to evaluate the diagnostic capacity of the parameters. The data of the control group was used to identify the pathological value of the parameters and Binary logistic regression model was established.

Results: The TT-TGp, TT-TGs, and TT-TGa distance were greater in the study group (12.7±4.0, 12.0±4.1, and 11.9±4.2, respectively) than in the control group (8.2±2.9, 8.1±3.0, and 8.0±3.0, respectively) (P<0.001) and were correlated with trochlear dysplasia with a ρ of 0.47, 0.46, and 0.50, respectively (P<0.001). In the study group, the TT-TGp distance was greater than TT-TGs and TT-TGa distance (P<0.001). The ICC of each TT-TG distance was more than 0.85 in any degree of trochlear dysplasia. The AUC of the TT-TGp distance (0.811) was higher than the TT-TGs (0.777) and TT-TGa (0.768) distance. The pathological value of each TT-TG distance was 13.0mm. And the TT-TGp revealed a significant OR of 18.68 (95% CI [6.28, 55.50]) with regard to PD.

Conclusion: Each TT-TG distance can be measured reliably by MRI and has a good diagnostic capacity for PD. TTO should be considered when a pathological TT-TG distance (>13.0 mm) is presented.

Level of Evidence: Retrospective cohort study (diagnosis); Level of evidence, I.

patellar dislocation

TT-TG distance

transepicondylar axis

tibial tubercle osteotomy

Patellar dislocation (PD) is a common disease of sports injuries and often occurs in young individuals ¹. Patients with anatomical variations of the femur and tibia are prone to suffer from PD, including trochlear dysplasia, excessive tibial tubercle to trochlear groove (TT-TG) distance, patella alta, lower extremity malrotation, knee valgus deformity. ^2–4. Among them, the TT-TG distance has been accepted as a significant risk factor, increasing lateral vector applied to the patella ⁵. In addition, a large proportion of patients with PD are accompanied by trochlear dysplasia and/or excessive femoral anteversion ^{6, 7}.

The TT-TG distance was correlated with other anatomical parameters in patients with PD, such as trochlear dysplasia, patella alta, and femoral anteversion ^{8, 9}. The value of the TT-TG distance more than 20mm is considered an indication for the tibial tubercle medialization osteotomy, and if patella alta appears simultaneously, the tibial tubercle distalization should also be considered (TTO) ¹⁰. But the reliability of the TT-TG distance measured on computed tomography (CT) images in patients with PD was questioned. The femoral trochlear morphology distinctly influence the measurement of the TT-TG distance and it is difficult to identify the deepest portion of the trochlea, even the trochlea itself in patients with severe trochlear dysplasia ^{11, 12}. On the other hand, the measurement was verified reliable on axial Magnetic Resonance Imaging (MRI) cuts ^{13, 14}.

The tibial tubercle to posterior cruciate ligament (TT-PCL) distance and the tibial tubercle to Roman arch (TT-RA) distance were depicted by researchers because of their independence from trochlear dysplasia ^{15, 16}. The tangent line from the posterior bony or cartilaginous edge of the medial to the lateral condyle (PCRL) was considered as the reference line in most measurements. However, patients with PD are often in the presence of dysplastic posterior condyles: a shorter posterior lateral condyle and a longer posterior medial condyle ^{17, 18}. Theoretically, the posterior condylar dysplasia might affect the measurement of PCRL, making it more internal rotation, influencing the authenticity of the recorded TT-TG distance. Previous studies focused on the influence of the femoral trochlea to the measurement, but ignored the effect of the PCRL as the reference line on the measurement of the TT-TG distance.

The transepicondylar axis has been proven to be a very reliable and constant reference line in total knee arthroplasty ¹⁹, consisting of anatomical and surgical epicondylar axis (AEA and SEA, respectively). Both of them have been poorly studied in patients with PD ^{17, 20–22}. We first propose to measure the TT-TG distance using AEA and SEA as reference lines on axial MRI slices, which were theoretically independent of posterior femoral condylar dysplasia. Furthermore, the diagnostic capacity for PD and pathological threshold value of the TT-TG distances are unclear.

The purpose of this study was to (1) verify the reliability of the TT-TG distance measured by PCRL, SEA, and AEA on MRI (TT-TGp, TT-TGs, and TT-TGa, respectively), (2) validate the diagnostic capacity of each parameter, (3) define the pathological threshold value of each TT-TG distance for TTO. The hypothesis of our study is that each TT-TG distance measured by MRI is reliable and there are deviations between the TT-TTp distance and/or the TT-TGs, TT-TGa distance.

Study Population

This study was approved by the ethics committee of our hospital. Patients diagnosed with PD from 2016 to 2021 in our hospital were selected from the Electronic Medical Record System. The inclusion and exclusion processes were conducted by two experienced orthopedists. 106 patients were considered eligible for inclusion in this study according to the inclusion criteria: patients diagnosed with unilateral recurrent PD; patients with skeletal maturity; patients with MRI scans of the ipsilateral knees. Patients who met at least one of the following criteria were excluded: patients missing standard or clear MRI scans of the knees (n=15); patients with a history of bone fracture or surgery that may influence the reliability of the measurements (n=3); patients with severe epiphysitis of the femur (n=2). Consequently, 86 patients were designated as the study group in this study.

Then, 86 individuals matched by age and sex with knee MRI showing neither pathology, meniscal nor cruciate ligament injury were assigned as control group in this study. The informed consent was waived by the hospital ethics committee. Medical history inquiries of all the individuals in the control group were conducted by a junior orthopedist and the MRI were evaluated by a senior orthopedist to ensure their eligibility for this study. None of these subjects had symptoms or objective pathologic findings related to the patellofemoral joints.

MRI technique

The MRI examinations were performed within one week prior to surgical planning in patients with PD. All examinations were performed with the same 1.5T MRI scanner (Siemens Magnetom Essenza, Germany). All the participants were placed in the supine position with the leg in full extension and the foot in a neutral position. The target knee was secured in the multichannel 89 phased-array knee coil, and the scan range was centered on the tibia and included the entire knee joint. The coronal and sagittal planes were scanned with the T1-weighted turbo spin-echo (TSE) sequence and proton density (PD) TSE with the fat-suppressed (FS) sequence, and the axial plane was scanned with the PD-TSE-FS. The layer thickness was set to 4 mm, the slice gap was 0.5 mm, the field of view (FOV) was 160 mm, and the matrix size was 512×512.

Measurements of Parameters

All the parameters were assessed on MRI slices by a senior orthopedist and a junior orthopedist in a blinded and randomized fashion using the picture archiving and communication system (PACS). To standardize the measurements, the observers met as a group and agreed upon the measuring methods. If a major dispute of the measuring result was presented, especially the degree of trochlear dysplasia, a discussion would be conducted until consensus was reached.

TT-TG Distance

The TT-TG distance using PCRL as the reference line was measured on the MRI scan according to the method described by Friedman et al. ²³. The most proximal axial slice showing the complete femoral trochlea with cartilage and intact posterior femoral condyles was identified to define the following marks: the deepest portion of the cartilaginous trochlear groove, the tangent line from the posterior cartilaginous edge of the medial to the lateral condyle (PCRL), the line through the sulcus of the medial epicondyle and the prominence of the lateral epicondyle (SEA) ²⁴, and the line through the prominence of the medial and lateral epicondyles (AEA) ²⁵ (Figure 1a). The first axial image showing a complete patellar tendon attachment on the tibial tubercle was selected as the reference slice to identify the centre of the tibial tubercle. Then, the TT-TG distances were evaluated: TT-TGp distance (Figure 1b) was defined as the distance between two perpendicular lines of the PCRL (the one across the midpoint of the superficial aspect of the tibial tubercle and the other one passing the deepest point of the trochlear groove); likewise, the TT-TGs distance (Figure 1c) and TT-TGa distance (Figure 1d) was defined was the distance between two vertical lines of the SEA and two vertical lines of the AEA, respectively.

Posterior Femoral Condylar Morphology

To assess the posterior femoral condylar dysplasia, we measured the distances between the SEA and the posterior cartilaginous margin of the medial femoral condyle (MPD) and the lateral femoral condyle (LPD) on the same distal reference axial MRI slice used to define the deepest portion of the trochlear groove ¹⁹. The transepicondylar width (TEW) is defined as the distance between the sulcus of the medial epicondyle and the prominence of the lateral epicondyle. The posterior condylar angle (PCA) is the angle between the SEA and PCRL, which is 3 degrees or so in normal population ²⁴ (Figure 2).

Trochlear Dysplasia

Sharma et al. ²⁶ depicted a new classification method to simplify the evaluation of trochlear dysplasia and reported it as a reliable and superior alternative method to the Dejour classification. Furthermore, he identified that the Oswestry-Bristol Classification (OBC) played an important role in the surgical decision making ²⁷. It is a four-part classification system comprising normal, mild, moderate, and severe trochlear dysplasia. The mild represents to a shallow femoral trochlea; moderate trochlear dysplasia represents to a flat femoral trochlea; the severe means a convex trochlea (Figure 3). The inter-observer reliability of the OBC showed excellent agreement, with the ICC value of 0.869 (95% CI [0.843, 0.889]) via Weighted Kappa analysis.

Statistical Analysis

Another orthopedist conducted all the analysis independently using the average value of the parameters measured by the two observers via the SPSS software (version 21.0; IBM Corp). Those length parameters were normalized by TEW to reduce individual differences. The Shapiro-Wilk normality test was performed to verify the distribution of these parameters. Then independent t-test and Mann-Whitney U test were conducted to assess the differences in the anatomical parameters between the groups. The Pearson and Spearman correlation analysis were used to verify the relationships among these parameters in the study group. Receiver operating characteristic curves (ROC) and the areas under the curves (AUC) were analyzed to evaluate the diagnostic capacity of the parameters for PD via GraphPad Software (version 8.0.2, San Diego, California USA). An AUC greater than 0.7 indicates fair to good capacity ¹⁴. The data in the control group was used to identify the pathological threshold value of each TT-TG distance. We established the Binary logistic regression model based on the pathological value of the TT-TG distances to identify the anatomical risk factors for PD. The intraclass correlation coefficients (ICCs) of the TT-TG distances in different degrees of trochlear dysplasia were calculated to identify the reliabilities, with a value more than 0.75 indicating excellent agreement ¹⁴. We defined α as 0.05.

We conducted the Post hoc analysis via G-Power software (version 3.1.9.4, Heinrich-Heine-Universitat Dusseldorf, Dusseldorf, Germany). For the effect size of 1.08 according to the TT-TGs between the two groups, a power of 1.00 was calculated (n, 86; alpha, 0.05).

This study included 86 patients with a mean age of 21.2 years (range, 13–50 years) in the study group, and 86 healthy individuals (mean age ± standard deviation, 22.4 ± 7.2) in the control group. Both the study group and the control group include 66 females (76%) and 20 males (24%). The demographic data of the two groups was shown in Table 1. Except for PCA and age, other parameters conformed to normal distribution.

Table 1

Demographic Characteristics of the Participants
Variables	Study group (n = 86)	Control group (n = 86)	P value
Sex, n			n.s
Female	66	66
Male	20	20
Age, mean ± SD (range), y	21.2 ± 7.8 (13 to 50)	22.4 ± 7.2 (13 to 50)	n.s
Side of knee, n			n.s
Left	40	40
Right	46	46
SD, standard deviation

The TT-TGp distance was greater in the study group than in the control group (12.7 ± 4.0 mm and 8.2 ± 2.9 mm, respectively), and likewise the TT-TGs and TT-TGa distance. The ratios between the TT-TG distances and the TEW were larger in the study group than in the control group (P < 0.001). The LPD/TEW was smaller in patients with PD (28.5 ± 3.3%) than in healthy subjects (30.9 ± 2.7%), in contrast, the MPD/TEW was larger in the study group (35.2 ± 2.8%) compared with controls (33.5 ± 2.3%). Then the ratio of LPD/MPD was 81.3 ± 6.6% in the study group and 92.3 ± 5.2% in the control group (P < 0.001). The PCA (P = 0.005) were larger in patients with PD compared to that in the control group. Trochlear dysplasia was presented in patients with PD: four (4.6%) patients with normal, 24 (27.9%) patients with mild, 20 (23.2%) patients with moderate, and 38 (44.2%) patients with severe trochlear dysplasia (Table 2). In the study group, the TT-TGp was greater than TT-TGs and TT-TGa distance (P < 0.001), and the TT-TGp/TEW was greater than TT-TGs/TEW and TT-TGa/TEW (P < 0.001) (Fig. 4).

Table 2

Differences in the Measurements Between the Study Group and the Control Group
Variables	Study group	Control group	P value
TT-TG distance, mm
TT-TGp	12.7 ± 4.0	8.2 ± 2.9	< 0.001
TT-TGs	12.0 ± 4.1	8.1 ± 3.0	< 0.001
TT-TGa	11.9 ± 4.2	8.0 ± 3.0	< 0.001
TT-TG/TEW, %
TT-TGp/TEW	17.6 ± 5.5	10.7 ± 3.8	< 0.001
TT-TGs/TEW	16.6 ± 5.6	10.5 ± 3.9	< 0.001
TT-TGa/TEW	16.4 ± 5.6	10.4 ± 3.9	< 0.001
Trochlear dysplasia (Normal/Mild/Moderate/Severe)	4/24/20/38	84/2/0/0	< 0.001^a
PCA, degree	4.0 ± 1.9	3.2 ± 1.6	0.005 ^a
LPD, mm	20.6 ± 2.8	23.6 ± 2.0	< 0.001
MPD, mm	25.4 ± 2.5	25.6 ± 1.7	0.457
TEW, mm	72.3 ± 5.7	76.7 ± 5.3	< 0.001
LPD/TEW, %	28.5 ± 3.3	30.9 ± 2.7	< 0.001
MPD/TEW, %	35.2 ± 2.8	33.5 ± 2.3	< 0.001
LPD/MPD, %	81.3 ± 6.6	92.3 ± 5.2	< 0.001
TT-TG distance, tibial tubercle to trochlear groove distance; LPD, lateral posterior femoral condylar distance; MPD, medial posterior femoral condylar distance; TEW, surgical transepicondylar width; PCA, posterior condylar angle; ^a, Mann–Whitney U test; P < 0.05 indicates statistically significant.

Spearman correlation analysis showed that the TT-TGp, TT-TGs, and TT-TGa distance was correlated with trochlear dysplasia in the study group with a ρ of 0.47, 0.46, and 0.50, respectively (P < 0.001). But the correlations between the TT-TG parameters and the measurements of femoral condylar morphologies did not show statistical significance (Table 3). The ROC curves were analyzed to calculate the diagnostic capacity of each TT-TG distance and the ratio of the TT-TG distance/TEW for PD. The TT-TGp had an AUC of 0.811, with a value of > 9.3 mm indicating 80.2% sensitivity and 67.4% specificity. The TT-TGs had an AUC of 0.777. At a value of more than 8.5 mm, the TT-TGs distance had 82.6% sensitivity and 60.0% specificity for predicting PD. The TT-TGa had an AUC of 0.768, with a cutoff value of 9.4 mm (sensitivity 68.6%, and specificity 70.9%) (Fig. 5a). The ROC curves of the TT-TG distances/TEW were displayed in Fig. 5b. The TT-TGp/TEW had an AUC of 0.850, with a value greater than 13.9% indicating 77.9% sensitivity and 77.9% specificity for PD. The TT-TGs/TEW had an AUC of 0.814. At a value greater than 11.9%, The TT-TGs/TEW had 82.6% sensitivity and 65.1% specificity for PD. The TT-TGa/TEW had an AUC of 0.805, with a cutoff value of 14.8% (59.3% sensitivity and 89.5% specificity for PD).

Table 3

The Correlations Between TT-TG distance Parameters and the Measurements of Femoral Condylar Morphology, Showing the r and ρ Value
	PCA	LPD/MPD	Trochlear dysplasia ^a
TT-TGp distance	0.046	0.025	0.474**
TT-TGs distance	0.045	0.019	0.467**
TT-TGa distance	0.037	0.039	0.504**
TT-TGp/TEW	0.037	0.001	0.492**
TT-TGs/TEW	0.037	-0.004	0.479**
TT-TGa/TEW	0.026	0.019	0.509**
TT-TG distance, tibial tubercle to trochlear groove distance; LPD, lateral posterior femoral condylar distance; MPD, medial posterior femoral condylar distance; TEW, surgical transepicondylar width; PCA, posterior condylar angle; ^a, Oswestry-Bristol Classification; **, ρ value of Spearman correlation analysis showing statistical significance, P < 0.001.

The pathological threshold value of the TT-TG distances and the ratio of the TT-TG/TEW was calculated according to the mean and standard deviation in the control group. The pathological value of the TT-TGp, TT-TGs, and TT-TGa distance was 12.9 mm, 13.0 mm, and 12.9 mm, respectively. 47.7% (41/86) patients with PD had a value of the TT-TGp distance greater than 12.9 mm, 40.7% (35/86) patients had a value of the TT-TGs distance more than 13.0 mm, and 39.5% (34/86) patients had a value of the TT-TGa distance greater than 12.9 mm in the study group, but such proportions decreased to 4.6% (4/86) in the healthy individuals. And the pathological value of the TT-TGp/TEW, TT-TGs/TEW, and TT-TGa/TEW was 16.9%, 16.9%, and 16.8%, respectively.

Binary logistic regression analysis showed that, as eliminating other parameters from the regression model (simple analysis), the TT-TGp, TT-TGs, and TT-TGa distance revealed a significant OR of 18.68 (95% CI [6.28, 55.50]), OR of 11.12 (95% CI [4.08, 30.23]), and OR of 14.76 (95% CI [4.96, 43.95]) with regard to PD, respectively. Likewise, the OR of the TT-TGp/TEW, TT-TGs/TEW, and TT-TGa/TEW was 17.78 (95% CI [6.56, 48.21]), 17.01 (95% CI [5.72, 50.58]), and 11.12 (95% CI [4.09, 30.23]), respectively.

Furthermore, we calculated the ICC value to identify the inter-observer reliability of the TT-TG distances in different types of trochlear dysplasia. The ICC of the three TT-TG distances were more than 0.85 in any type of OBC (Table 4). The value of each TT-TG distance was maximum in patients with severe trochlear dysplasia and was minimum in subjects with normal trochlea. The TT-TG distance in patients with shallow trochlea did not differ from that in patients with flat trochlea. (Fig. 6).

Table 4

Inter-Observer Reliability of the TT-TG Distance in Different Oswestry-Bristol Classification of Trochlear Dysplasia ^a
Degree of trochlear dysplasia	TT-TGp distance, mm	ICC (95% CI)	TT-TGs distance, mm	ICC (95% CI)	TT-TGa distance, mm	ICC (95% CI)
Normal	8.1 ± 2.8	0.987 (0.977,0.993)	8.0 ± 2.9	0.984 (0.976,0.993)	7.9 ± 2.9	0.988 (0.983,0.992)
Mild	11.0 ± 3.7	0.985 (0.964,0.996)	10.4 ± 3.8	0.995 (0.988,0.997)	11.0 ± 3.7	0.987 (0.966,0.995)
Moderate	11.5 ± 3.9	0.926 (0.875,0.969)	10.8 ± 3.8	0.923 (0.889,0.945)	10.6 ± 3.8	0.926 (0.895,0.951)
Severe	14.9 ± 3.5	0.892 (0.835,0.938)	14.2 ± 3.6	0.876 (0.841,0.922)	15.2 ± 3.6	0.883 (0.857,0.906)
^a, Distances are expressed as mean ± standard deviation in millimeters. TT-TG distance, tibial tubercle to trochlear groove distance; ICC, intraclass correlation coefficient, with a value more than 0.75 indicating excellent agreement; CI; confidence interval.

The most inspiring finding of the present study is that the TT-TG distances measured by PCRL and/or transepicondylar axis on MRI are reliable no matter how severe trochlear dysplasia is. In patients with PD, the TT-TG distance measured by PCRL is larger than that measured by transepicondylar axis, indicating that such previously recorded TT-TG distance may be over-evaluated. Each TT-TG distance is correlated with the OBC of trochlear dysplasia and also has fair to good diagnostic capacity for PD. The pathological value of each TT-TG distance has been identified, which helps with TTO decision making.

As an important risk factor affecting patellar stability, the TT-TG distance can reflect the lateralization of the tibial tubercle and has been attached much importance by lots of researchers, with a value greater than 20mm indicating pathological ⁸. However, the TT-TG distance measured on axial CT images was inaccurate, because it was greatly affected by dysplastic femoral trochlea ¹². On the other hand, previous literature showed that the measurement of the TT-TG distance on MRI slices was reliable ^{14, 23}. The results of our study showed that the three TT-TG distances were reliable in different types of the OBC, even in patients with severe trochlear dysplasia.

Previous studies focus on identifying a reference point that can avoid the influence of the femoral trochlea to assess the lateralization of the tibial tubercle, such as the TT-PCL distance and TT-RA distance ^{15, 16}. PCRL was depicted as the reference line in most measurements. Yang et al. ¹⁸ suggested that the dysplastic distal femoral condyle was composed of anterior and posterior femoral condylar dysplasia. In accordance with previous studies on CT images ^{17, 20, 22}, the results of our study by MRI showed that patients with PD was in the presence of posterior femoral condylar dysplasia: longer medial and shorter lateral posterior condyle, and larger PCA. We hypothesized that although the TT-TG distance measured by PCRL on MRI had a good consistency, there could be deviations in the recorded TT-TGp distance due to the PCRL itself was affected by posterior femoral morphology.

In this study, we found that the TT-TG distances were larger in patients with PD than in healthy individuals, which was consistent with previous studies ^{15, 28}. The mean difference between the TT-TGp distance and the TT-TGs or TT-TGa distance was 0.7 mm in patients with PD, but such difference in healthy individuals was not significant. This to some extent suggests that the measuring method using PCRL as reference line could contribute to an over-evaluated TT-TG distance preoperatively. The TT-TG distances were not statistically correlated with PCA or the ratio of LPD/MPD in patients with PD based on our data. However, in this study, the fact is that the TT-TGp distance is greater than the TT-TGs or the TT-TGa distance in patients with PD. This may attribute to the influence of the limited parameters included in this study regarding posterior condylar morphologies, or other factors affecting the measurement of the TT-TG distance (potential measurement errors or the inclusion of the study group). Whether this 0.7 mm deviation has an impact on TTO decision making or on postoperative outcomes is still unknown.

Trochlear dysplasia was considered as a significant risk factor for PD, which can change the interaction between the patella and the femoral trochlea and contribute to patellar instability. Researchers used to evaluate trochlear dysplasia via the Dejour classification and some specific parameters, like sulcus angle and lateral trochlear facet inclination (LTI) ⁶. Dejour classification has been performed on MRI, but poor inter- and intra-observer agreements have been reported ²⁹. Sharma et al. ²⁶ depicted the OBC to grade the severity of trochlear dysplasia with the superiority of simplicity and good consistency. Our study also confirmed that the four-part classification system was reliable. He also reported that, based on the OBC, 28% patients were classified as ‘normal’ to ‘mild’, 62% as ‘moderate’ to ‘severe’. In our study, among the patients with PD, 28 (32.5%) patients have normal to shallow femoral trochlea, 58 (67.5%) patients are in the presence of moderate to severe trochlear dysplasia. A large proportion of patients were accompanied by trochlear dysplasia ⁶. In this study we found that each TT-TG distance increased with the severity of trochlear dysplasia, which was in accordance with previous study ⁹. It informed the orthopedic surgeons of the existence of excessive TT-TG distance when severe trochlear dysplasia was presented in patients with PD. After normalizing the TT-TG distances by TEW, the ratio of the TT-TG distances/TEW showed similar results.

We also analyzed the diagnostic efficacy of each TT-TG distance for PD and found that the TT-TGp distance had a good diagnostic capacity, with an AUC of 0.811, which was almost equivalent to the results reported by Xu et al. (AUC = 0.820) ¹⁴. While the TT-TGs and TT-TGa distance had a fair capacity, with an AUC of 0.777 and 0.768, respectively. The discrepancies might attribute to the larger TT-TGp distance or caused by the participants included in this study. Similar to the previous literature that the normalized TT-TG distance had a stronger ability to predict PD ^{30, 31}, the ratios of the TT-TG distances/TEW in our study had stronger diagnostic capacities than the TT-TG distances alone. The TT-TGp distance greater than 20mm measured on CT images was considered the indication for TTO and good postoperative outcomes following TTO had been reported ¹⁰. However, since the measuring method of the TT-TG distance differs from CT images to MRI slices, the recorded value of the TT-TG distance measured on such two different images are not equivalent in the same patient ¹⁴. So, it is worthy of identifying the pathological threshold value of each TT-TG distance measured on MRI cuts.

We set the pathological value of all the three TT-TG distances measured on MRI slices at 13.0 mm in our study. And 0.6 mm discrepancy was identified when compared with previous literature (13.6 mm) ¹⁴, which may had been caused by the inclusion of the control group. And the pathological value of the TT-TG distances/TEW was set at 17.0% based on our data. It does not mean that the TTO is needed when the TT-TG distance or TT-TG distances/TEW manifests pathology. We expected to establish a “data base” regarding the indication for TTO based on MRI slices to help with surgical decision making. Whether the 13.0mm of the TT-TG distance was an appropriate indication for TTO and its postoperative outcomes are still needed to be investigated.

Previous studies have shown that when a pathological TT-TGp distance is presented, the risk of PD would increase 5–14 times ^{11, 14, 30, 31}. In our study, all the three TT-TG distances were verified as risk factors for PD, and the risk was 18.7, 11.1, and 14.8-fold higher in patients with pathological TT-TGp, TT-TGs, and TT-TGa distance, respectively. This discrepancy may have been caused by the different cutoff values and methods used for logistic regression analysis. we set the cutoff value at 13.0 mm and established simple regression model, but previous studies set the pathological value at 13.6 or 20 mm.

We acknowledged some limitations to this study. First, we did not correlate the operation outcomes following TTO with this new pathological threshold value of each TT-TG distance that we established. Second, the subjects in the control group of this study were not completely normal individuals, but they did not have anterior knee pain, patellar instability, or other symptoms in terms of patellofemoral joints. Pathological values calculated from this sample may change in a relatively large healthy population. Third, the parameters of the posterior femoral condyles included in this study were limited.

Each TT-TG distance can be measured reliably by MRI and has a good diagnostic capacity for PD. TTO should be considered when a pathological TT-TG distance (> 13.0 mm) is presented.

AEA: Anatomical transepicondylar axis; AUC: The area under the ROC curve: ICC: Intraclass correlation coefficient; LPD: The distance between the SEA and the posterior cartilaginous margin of the lateral femoral condyle; MPD: The distance between the SEA and the posterior cartilaginous margin of the medial femoral condyle; OBC: Oswestry-Bristol Classification of trochlear dysplasia; PCA: Posterior condylar angle; PCRL: Posterior condylar reference line; PD: Patellar dislocation; ROC: Receiver operating characteristic curve; SEA: Surgical transepicondylar axis; TEW: The width of SEA; TTO: Tibial tubercle osteotomy; TT-TG: Tibial tubercle to trochlear groove distance: TT-Tga: The TT-TG distance with the AEA as reference line: TT-TGp: The TT-TG distance with the PCRL as reference line; TT-TGs: The TT-TG distance with the SEA as reference line.

Ethics approval and consent to participate: The study was approved by the Ethics Committee of the First Affiliated Hospital of Chongqing Medical University (IRB Number: 2022-k36). Research involving human participants, human material, or human data have been performed in accordance with the Declaration of Helsinki. We confirm that all methods were performed in accordance with the relevant guidelines and regulations. The informed consent was waived by the hospital ethics committee.

Consent for publication: Not Applicable

Availability of data and materials: The datasets analysed during the current study are not publicly available due to ethical restrictions, but are available from Aiguo Zhou on reasonable request at [email protected].

Competing Interest: None declared.

Funding: This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Authors' contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Pei Zhao, Yi Feng and Baoshan Yin. The first draft of the manuscript was written by Pei Zhao, and modified by Baoshan Yin, Hao Tan and Aiguo Zhou. Hua Zhang and Aiguo Zhou retrospectively verify the reliability of the tibial tubercle-trochlear groove (TT-TG) distance measured by posterior condylar reference line (PCRL), surgical and anatomical transepicondylar axis (SEA and AEA) on MRI (TT-TGp, TT-TGs, and TT-TGa, respectively). Jian Zhang provided theoretical supports. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Acknowledgements: None.

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No competing interests reported.

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Tibial Tubercle to Trochlear Groove Distance Measured by Transepicondylar Axis on MRI is a Reliable Parameter in Tibial Tuberosity Osteotomy Decision Making in Patients with Recurrent Patellar Dislocation

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