CT-Based Morphological Analysis of The Distal Femur In Patients With Trochlear Dysplasia

Background: The morphology of the femoral trochlear has been well investigated; however, to date, the distal femur morphological characteristics have not been studied. The research team posited the presence of morphological differences in the distal femur in trochlear dysplasia compared to a control group. They also postulated that the underlying pathology of distal femur dysplasia is a medial hypoplasia. Methods: Computed tomography (CT) scans of 80 knees with trochlear dysplasia and 80 knees with normal anatomy of the femoral trochlear were included. The following measurements were assessed by three observers: the distal femur angle, the percentage of medial distal femur height and the percentage of lateral distal femur height between the groups. The independent Student's t-test were used to evaluate the differences of the qualitative variables. Results: The distal femur angle was relatively larger compared to the control group (p<0.001). Percentage of the medial distal femur showed signifcant lower values in the study group (p<0.001). No signifcant diferences were evaluable for the percentage of lateral distal femur height (p=0.087). The study group have a bigger distal femur angle and a steep medial distal femur than the control group. Conclusion: Patients with trochlear dysplasia have a different distal femur compared with those without trochlear dysplasia. Compared to controls, patients with a dysplastic trochlea have a distal femur dysplasia, and the underlying pathology of a dysplastic distal femur is led by a medial distal femur hypoplasia.


Introduction
Morphological abnormalities of distal femur are thought to be the anatomical basis of patellofemoral disorders, especially in patients with symptomatic patellofemoral instability. Dejour et al. showed that femoral trochlear dysplasia is one of the major risk factors associated with patellofemoral instability [1]. Richmond et al. showed that the morphology of the trochlea and patella changed during skeletal development [2]. The studies by Ferlic PW et al. reported that the shear stress in the distal femur plays an important role in the development of the trochlear [3][4][5]. Fu et al. recently reported that early relocation of the patella can prevent the development of trochlear dysplasia in children [6]. Variation of shear stress on the distal femur theoretically might result in a morphology of the patellofemoral joint changed. The morphology of the femoral trochlea has been thoroughly studied [6][7][8][9][10][11]. However, the morphological characteristics of distal femur have not been described in patients with trochlear dysplasia. It is still unknown whether the distal femur in patients with trochlear dysplasia differs from that in those with normal. Therefore, the aim of the present study was to compare, using Computed tomography (CT), the morphology of the distal femur in patients with trochlear dysplasia with normal patients. Our study hypothesized that in patients with trochlear dysplasia, the distal femur develops hypoplasia.

Materials And Methods
The study had been approved by the Ethics Committee and all patients gave informed consent.
Consecutive CT scans of 160 knees were measured retrospectively to analyze the morphology of distal femur in patients suffering from a dysplastic trochlea and in controls. A total of 80 knees with dysplastic trochlea in the study group, 28 males and 52 females, mean age 36 years (range 18-45) were performed between 2010 and 2019. The clinical diagnosis of dysplastic trochlea in study group was con rmed by CT scans. The exclusion criterion was a history of prior surgery of the lower extremity or traumatic patellar dislocation or Multi-ligament injury or arthritis. In the control group, CT scans were performed in 80 knees, 30 males and 50 females, mean age 36 years (range 19-44), with normal morphology of the patellofemoral joint between 2010 and 2019. All controls consulted the orthopedic surgeon for a complaint, such as soft cartilage injury or avulsion fracture. Patients demographic characteristics were matched between both groups according to sex and age (Table 1). Table 1. Demographic characteristics of the study and control groups.
CT views assessment of morphological characteristics of distal femur were performed in the supine position between the groups. Three orthopaedic surgeons assessed the following measurements: the distal femur angle, the percentage of medial distal femur height and the percentage of lateral distal femur height between the groups. All measurements were made on the axial plane. For measurements, the axial plane with the distal femur (the slice just above the point of attachment of the gastrocnemius head [12]) was chosen to measure the morphology of the distal femur. The distal femur angle was determined by a tangent to the medial facet of the distal femur. The angle between this tangent and the dorsal distal femur line (popliteal surface) was taken as the distal femur angle. The distal femur (popliteal surface) is divided into three equal sections (AE, AB and BF). The lateral distal femur height (AC) is the perpendicular distance from the lateral distal femur (A) to dorsal distal femur line (popliteal surface). The medial distal femur height (BD) is the perpendicular distance from the medial distal femur (B) to dorsal distal femur line. In the anteroposterior direction, the medial and lateral distal femur heights were measured on the distal femur axial images and expressed as a percentage of the distal femur width (EF). The methods applied to the assessment of distal femur are summarized in Table 2 and Fig. 1-3. Table 2. Description of measurements CT scans were performed using a Sante DICOM Viewer Free (64-bit) verson (Santesoft, Inc. Athens, Greece) to 0.01°for angles and 0.01 mm. For minimizing measurement error, all measurements were conducted by three blinded orthopaedic surgeons. After an interval of 3 days, the two surgeons measured the 160 samples again. The inter-and intraobserver reliabilities were then determined by calculating intraclass correlation coe cients (ICCs). Statistical analysis SPSS statistical software (version 22.0; SPSS, IL, USA) was performed for data analyses. The data were presented as the mean and standard deviation. The normality of numerical data was assessed by the Kolmogorov-Smirnov test, the measured data passed the test. All values of the two groups were calculated with 95% con dence intervals. After establishing data normality, student's t-test were used to evaluate the differences between the two groups. The categorical values were analyzed using Chi-squared test. P-value < 0.05 was de ned as the threshold for statistical signi cance.

Results
The demographic characteristics showed no signi cant differences in age, sex, and side between the two groups ( Table 1). The inter-and intraobserver correlation coe cients was high between measurements (Table 3). Table 3. Intra-observer and inter-observer agreement of geometric measurements with 95% con dence intervals. Many measurements showed signi cant differences between the two groups (Table 4). Compared with the control group, the study group had a larger distal femur angle and a smaller medial distal femur. The measurements of mean distal femur angle in the axial plane was signi cantly different between the groups (P < 0.001). In the control group, the mean distal femur angle was 11.73°. 80 knees in the study group had a femur angle of 24.92°. The percentage of medial distal femur height showed the greater difference: 56.3% in the study group versus 60.1% in the control group (P < 0.001). No signi cant differences were seen in well-known measurements such as the percentage of lateral distal femur height (the study group, 68.8%; the control group, 69.4%; p = 0.087). Overall, in patients with dysplastic trochlea, the distal femur morphology exhibited a steep-slope (Fig. 4).

Discussion
The key nding of the current study was that patients with dysplastic trochlea had a distal femur dysplasia and the underlying pathology of a dysplastic distal femur was led by a medial distal femur hypoplasia. It is considered that distal femur angle of more than 16 degrees as distal femur dysplasia, which could con dently differentiate between health and distal femur dysplasia. The ndings have important reference value to pathogeny of patellar dislocation. In clinical surgery, it is unknown if these abnormalities in uence the surgical effect after soft-tissue procedures. Physicians should pay more attention to these differences for disease diagnosis or surgical procedure.
The passive stability of the patellofemoral joint depends on the shape of the patella and femur [1]. Osteal constraint formed by the distal femur is an important restraint preventing lateral dislocation. Femoral trochlear dysplasia has long been identi ed as a potential risk factor for patella instability [1,[13][14][15][16][17].  [21]. So we believe that it is signi cative to con rm that the morphological changes of the distal femur in patients with trochlear dysplasia. As with the patellofemoral joint dysplasia, the study suggest that the distal femur also develops hypoplasia in patients with trochlea dysplasia. There are no parameters in current studies that described the contour of the distal femur, especially in trochlea dysplasia patients. Therefore, all measurements in the present study were made to evaluate the distal femur morphology. The distal femur angle and the percentage of lateral distal femur height and the percentage of medial distal femur height on a particular axial image which was the slice just above the point of attachment of the gastrocnemius head [12].
The morphological characteristics of distal femur were a main aspect of this study and yielded interesting results. In the present study, the mean distal femur angle was larger in the study group than in the control group (P < 0.001). In the control group, the mean distal femur angle was 11.73°. 80 knees in the study group had a groove angle of 24.92°. It is considered that distal femur angle of more than 16 degrees as distal femur dysplasia. The percentage of medial distal femur height showed a signifcant diference between the study group and the control group (P < 0.001). For percentage of lateral distal femur height, no signifcant diferences could be found (P = 0.087). The study group had a steep distal femur than the control group. But how to explain this phenomenon?
We know the unique matching relation of the patella and femur, which is the basis of its biomechanical function [22]. Stress stimulation plays an important role in bone development [23]. Wolff law that illustrates that cancellous architecture is altered in response to mechanical stress [24]. In the patellofemoral joint, stress is transmitted from the subchondral bone to the cancellous bone [16]. This transmission of stress during function stimulates the growth and remodelling of the femur [25]. Without the appropriate load, the distal femur fails to fully develop. This phenomenon can also be seen in patients sufering from hip dysplasia [26]. Associated distal femur dysplasia formation was the response to the different local stress.
There were some limitations to the study. First, single axial planes on CT scans can not be used accurately describe the morphology of distal femur. Multilayer planes are required in order of precise evaluation [27]. Second, additional parameters other than those described earlier were not able to analyze. It would have been interesting to contain more clinical data. Third, no surgical parameters were assessed. However, the purpose of this study was to assess the morphology of distal femur. Further studies could validate the changes of surgical parameters.

Conclusion
In conclusion, the present study is the rst one comparing the morphology of distal femur in patients with trochlea dysplasia. The important nding was that patients with trochlea dysplasia have steep distal femur compared to health, and the underlying pathology of a dysplastic distal femur is led by a medial femur hypoplasia. It is considered that distal femur angle of more than 16 degrees as distal femur dysplasia, which could con dently differentiate between health and distal femur dysplasia. More attention should be paid to these morphological changes.

Figure 2
The distal femur angle between the medial facet of the distal femur tangent and the dorsal distal femur line (popliteal surface).