Abstract
Background:
Alignment and rotation of the lower extremities have been suggested to be predisposing pathologic factors for patellar instability.
Purpose:
To elucidate the relationship between the lower limb alignment and lower extremity rotation in patients with patellar instability.
Study Design:
Cross-sectional study; Level of evidence, 3.
Methods:
Included were 83 patients with patellar instability. Computed tomography scans and standing full-leg radiographs were used to measure the tibial tuberosity–trochlear groove (TT-TG) distance, mechanical femorotibial angle (mFTA), mechanical lateral distal femoral angle (mLDFA), mechanical medial proximal tibial angle (mMPTA), femoral torsion, and tibial torsion of the different segments. The relationships between femoral torsion, tibial torsion of the different segments, and the mFTA, mLDFA, and mMPTA were evaluated. The levels of tibial torsion and femoral torsion in patients with varus, normal, or valgus alignment were compared with 1-way analysis of variance and chi-square test.
Results:
The total tibial torsion was significantly associated with total femoral anteversion (r = 0.329; P = .002) and mFTA (r = –0.304; P = .005). There were no significant correlations between mFTA and TT-TG distance or femoral anteversion. Compared with patients with valgus malalignment, patients with varus malalignment tended to have higher tibial torsion.
Conclusion:
Tibial torsion was associated with leg axis alignment and femoral anteversion in patients with patellar instability. Patients with patellar instability, especially those with concurrent leg axis deformities, should undergo further radiological imaging so that tibial torsion can be assessed and a diagnosis of torsion deformity made early in the treatment pathway and the proper surgical plan formulated.
Keywords: patellar instability, lower extremity alignment, femoral anteversion, tibial torsion
Patellar instability is a common knee disorder in adolescents and children. Depending on the underlying pathologic factors, 4 numerous surgical techniques have been described, such as tibial tuberosity transfer, 34 medial patellofemoral ligament reconstruction, 23 and trochleoplasty. In general, an anatomically based approach to correct the main predisposing variants based on each patient’s condition is preferred. 5,7 Of all the anatomic variants, more emphasis has been placed on lower limb alignment 15,24,35 and lower extremity rotation. 3,11 Various complaints of knee discomfort are attributed to lower extremity malrotation 19,44 and malalignment. 15,24,35 In patients with lower limb malrotation, uncorrected femoral anteversion usually leads to patellar maltracking, Q-angle abnormalities, and early cartilage degeneration. In patients with malalignment, excessive abnormal forces exerted on the patella may serve as the basis for further patellofemoral joint problems and patellar redislocation.
The relationship between lower extremity alignment and lower limb rotation, along with their role in patellar instability, has rarely been taken into consideration, even though lower extremity malalignment and torsional deformities often appear simultaneously. It is important to make the diagnosis of torsion deformity or malalignment early in the treatment pathway, as knees that have undergone multiple surgical procedures have significantly poorer outcomes. 1,38
The aims of this study were to elucidate the relationship between lower limb alignment and lower extremity rotation in patients with patellar instability. We hypothesized that (1) femoral anteversion would be associated with leg axis alignment in patients with patellar instability and that (2) tibial torsion would be associated with leg axis alignment in patients with patellar instability.
Methods
Study Population
After approval of the study protocol was granted by the ethics review board of our hospital, 83 consecutive patients diagnosed with patellar instability from May 2020 to March 2021 were included in this study. Patients were included if they had a closed epiphysis and a confirmed diagnosis of patellar instability. Patients were excluded in cases of concomitant cruciate or collateral ligament injuries and prior knee surgery.
Computed Tomography and Radiographic Imaging
The computed tomography (CT) scans were obtained with a Discovery CT750HD (GE Healthcare) scanner. Patients were scanned in the supine position with the knee fully extended and the foot in a neutral position. An ankle brace was employed to minimize motion. The scan ranged from the anterosuperior iliac spine to the toe tips. The following parameters were used: tube current, 125 mAs; tube voltage, 120 kVp; reconstructive slice thickness, 0.625 mm; field of view, ranging from 80 to 110 cm based on the patient.
Standing anteroposterior full-length digital radiographs of lower extremities of all patients were also obtained.
Measurements of Parameters
All imaging data used RadiAnt DICOM Viewer software for measurement of tibial tuberosity–trochlear groove (TT-TG) distance, femoral anteversion, tibial torsion, mechanical femorotibial angle (mFTA), mechanical lateral distal femoral angle (mLDFA), and mechanical medial proximal tibial angle (mMPTA). All measurements were conducted by 2 sports medicine fellowship–trained orthopaedic surgeons(C.X. and S.Z.).
CT Evaluation
TT-TG Distance
The TT-TG distance was measured according to the method of Schoettle et al. 36 First, a line tangent to the posterior aspects of femoral condyles was drawn. Next, the deepest point of the bony femoral trochlear was localized, and a line vertical to the posterior condylar tangents was drawn through it. The line was then transposed to the slice of the tibial tubercle where the patellar tendon inserted. The TT-TG distance was defined as the distance of parallel lines between the most anterior point of the tibial tubercle and the deepest point of the trochlear groove, vertical to the posterior condylar tangents.
Femoral Anteversion
Femoral anteversion was assessed proximally, at midshaft, distally, and as total femoral anteversion, using lines drawn at 4 different levels (Figure 1). 18,37 Line 1 was drawn through the center of the femoral head and femoral neck, line 2 was tangent to the anterior aspect of the proximal femur at the level of the lesser trochanter, line 3 was tangent to the posterior aspect of the distal femur just over the insertion of the gastrocnemius, and line 4 was tangent to the posterior condyles. The angle between lines 1 and 2 was considered the proximal anteversion, the angle between lines 2 and 3 was the shaft anteversion, the angle between lines 3 and 4 was the distal anteversion, and the angle between lines 1 and 4 was the total femoral anteversion.
Figure 1.
Segmental measurement of femoral anteversion and tibial torsion of the right lower extremity. (A) Line through the center of the femoral head and femoral neck. (B) Line tangent to the anterior aspect of the proximal femur at the level of the lesser trochanter. (C) Line tangent to the posterior border of the distal femur just over the insertion of the gastrocnemius. (D) Line tangent to the posterior femoral condyles. (E) Line tangent to the posterior border of the tibial plateau. (F) Line tangent to the posterior tibial cortex. (G) Line through the centers of the medial and lateral malleoli.
Based on the total anteversion, patients were assigned to 1 of 2 groups: normal femoral anteversion (<30°) or high femoral anteversion (≥30°). The cutoff of 30° was chosen based on available data 12,17,28,37,41,45 and the research of Zhang et al 45 and Nelitz, 28 who set the threshold for excessive torsion and derotational osteotomy.
Tibial Torsion
Tibial torsion was measured segmentally as supratuberositary torsion, infratuberositary torsion, and total tibial torsion using lines drawn at 3 different levels 10,42 (Figure 1). Line 5 was tangent to the posterior border of the tibial plateau, line 6 was tangent to the posterior tibial cortex on the image section where the patellar tendon insertion at the tibial tuberosity could be best visualized, and line 7 was the intermalleolar axis connecting the midpoints of the medial and lateral malleoli. Proximal tibial torsion was defined as the angle between lines 5 and 6, and distal tibial torsion was defined as the angle between lines 6 and 7. The angle between lines 5 and 7 was defined as total tibial torsion.
Depending on the total tibial torsion, patients were allocated to 1 of 2 groups: normal tibial torsion (<30°) or high tibial torsion (≥30°). The cutoff value for high tibial torsion was based on existing data 10,16,22,40 and a previously published study 25 evaluating tibial torsion of 100 intact tibiae in healthy Asian volunteers.
Femoral Trochlear Dysplasia
Femoral trochlear dysplasia was categorized into 4 groups based upon the Dejou classification 5,6 : type A (shallow trochlea), type B (fairly flat or convex trochlea), type C (asymmetry of trochlear facets with medial femoral condylar hypoplasia), and type D (asymmetry of trochlear facets with a supratrochlear spur and cliff sign between the facets). The assessment was performed on an axial cross-sectional CT image 3 cm above the medial tibiofemoral joint space. 26
Radiographic Evaluation
The mFTA, representing mechanical axis knee alignment, was measured as the angle between the mechanical axes of the femur and the tibia. The mLDFA was the lateral angle between the femoral joint line (a line tangent to the most distal points on the convexity of the femoral condyles) and the mechanical axis of the femur, and the mMPTA was the medial angle between the tangential of the proximal tibia and the mechanical axis of the tibia (Figure 2). 30,31
Figure 2.
Analysis of the coronal leg geometry according to Paley et al. 30,31 mFTA, mechanical femorotibial angle; mLDFA, mechanical lateral distal femoral angle; mMPTA, mechanical medial proximal tibial angle.
Varus and valgus alignment were defined as mFTA exceeding 3° of varus or valgus according to the reported values by Lobenhoffer et al 20 (negative values represent varus). Depending on the measurements, patients were categorized as having varus, normal, or valgus alignment.
Statistical Analysis
The interrater reproducibility of the measurements was assessed by the intraclass correlation coefficient (ICC). Normality of data was assessed with the Kolmogorov-Smirnov test. Pearson correlation analysis was conducted to analyze the relationship between the measured parameters. The levels of tibial torsion and femoral torsion in patients with varus, normal, or valgus alignment were compared using 1-way analysis of variance and the chi-square test, with the Fisher exact test used when necessary. The level of significance was set at P < .05. All statistical analyses were performed by SPSS Version 26.0 (IBM).
A minimum sample size of 28 was calculated, with an α of .05 and a statistical power of 0.90 (G*Power software Version 3.1).
Results
Included were 83 patients (median age, 19.5 years). Detailed patient characteristics are provided in Table 1. The interrater reliability of the measured parameters (TT-TG distance, femoral anteversion, tibial torsion, mFTA, mLDFA, and mMPTA) showed good agreement, with all ICC values >0.828. The femoral anteversion and tibial torsion values of the different segments are provided in Table 2. The mean mFTA was 0.2° ± 2.8°, and the mean mLDFA and mMPTA were 86.4° ± 3.1° and 86.7° ± 2.3°, respectively.
Table 1.
Descriptive Data of the Included Patients (N = 83) a
| Variable | Value |
|---|---|
| Age, y, median (range) | 19.5 (12-45) |
| Sex | |
| Female | 68 (81.9) |
| Male | 15 (18.1) |
| Side of patellar dislocation | |
| Left | 43 (51.8) |
| Right | 40 (48.2) |
| Trochlear dysplasia | |
| A | 4 (4.8) |
| B | 4 (4.8) |
| C | 32 (38.6) |
| D | 43 (51.8) |
a Data are presented as number (%) unless otherwise indicated.
Table 2.
Values for TT-TG Distance, Femoral Anteversion, Tibial Torsion, mFTA, mLDFA, and mMPTA a
| Mean ± SD | Range | |
|---|---|---|
| TT-TG distance, mm | 23.2 ± 4.3 | 15.1 to 33.7 |
| Anteversion, deg | ||
| Total | 18.6 ± 10.5 | –4.7 to 45.6 |
| Proximal | 1.9 ± 7.1 | –16.6 to 22.7 |
| Diaphyseal | 8.6 ± 12.5 | –22.9 to 47.6 |
| Distal | 8.2 ± 3.6 | 0.1 to 17.4 |
| Tibial torsion, deg | ||
| Total | 30.4 ± 7.7 | 13.3 to 51.3 |
| Proximal | 19.8 ± 7.3 | 5.5 to 37.7 |
| Distal | 10.6 ± 8.0 | –6.6 to 34.1 |
| mFTA, deg b | 0.2 ± 2.8 | –5.9 to 8.5 |
| mLDFA, deg | 86.4 ± 3.1 | 78.7 to 97.3 |
| mMPTA, deg | 86.7 ± 2.3 | 78.9 to 91.1 |
a mFTA, mechanical femorotibial angle; mLDFA, mechanical lateral distal femoral angle; mMPTA, mechanical medial proximal tibial angle; TT-TG, tibial tuberosity–trochlear groove.
b Negative values represent varus.
Results of Correlation Analysis
Total tibial torsion was significantly correlated with the mFTA (r = –0.304; P = .005) and total femoral anteversion (r = 0.329; P = .002). An increased femoral anteversion and varus mFTA indicated an increased tibial torsion. The other results are displayed in Table 3.
Table 3.
Correlation Between TT-TG Distance and Different Segments of Femoral Anteversion, Tibial Torsion, mFTA, mLDFA, and mMPTA a
| r Value | ||||
|---|---|---|---|---|
| TT-TG Distance | mFTA b | mLDFA | mMPTA | |
| TT-TG distance | — | 0.163 | 0.027 | –0.064 |
| Anteversion | ||||
| Total | 0.189 | –0.053 | 0.080 | 0.008 |
| Proximal | –0.225 c | –0.295 d | 0.258 c | 0.036 |
| Diaphyseal | 0.205 | 0.120 | –0.060 | 0.008 |
| Distal | 0.257 c | 0.010 | –0.070 | –0.077 |
| Tibial torsion | ||||
| Total | 0.018 | –0.304 d | 0.331 d | 0.173 |
| Proximal | 0.028 | –0.084 | 0.173 | 0.056 |
| Distal | –0.008 | –0.218 c | 0.162 | 0.116 |
a mFTA, mechanical femorotibial angle; mLDFA, mechanical lateral distal femoral angle; mMPTA, mechanical medial proximal tibial angle; TT-TG, tibial tuberosity–trochlear groove. Dash indicates not analyzed.
b Negative values represent varus.
c Significant r value at P < .05.
d Significant r value at P < .01.
Results of Alignment Comparisons
Seventeen patients had valgus alignment, and 12 patients had varus alignment. For tibial torsion, 46 patients were categorized in the high-torsion group with 37 patients in the normal-torsion group. For femoral anteversion, 12 patients were categorized in the high-torsion group and 71 patients in the normal-torsion group. The subgroup analysis showed a significantly higher tibial torsion in the varus alignment group compared with the valgus or normal group, which echoed the result of the chi-square test indicating significantly greater proportions of high tibial torsion in the varus alignment group and lower proportions of high tibial torsion in the valgus alignment group (P < .05) (Figure 3). Total tibial torsion did not show a significant difference in the normal-alignment group. Total femoral anteversion did not indicate a significant difference in the normal, valgus, or varus alignment group (Figure 4).
Figure 3.
Percentage of patients with high or normal tibial torsion according to lower limb alignment group. Chi-square tests showed significantly lower proportions of high tibial torsion in the varus alignment group, whereas significantly higher proportions of high tibial torsion were indicated in the valgus alignment group. *P < .05.
Figure 4.
Percentage of patients with high or normal femoral anteversion according to lower limb alignment group. Chi-square tests showed no significant difference between the groups.
Discussion
The most important findings of the present study were that tibial torsion was associated with femoral anteversion in patients with patellar instability (r = 0.329; P = .002) and that patients with varus malalignment tended to have a higher tibial torsion compared with patients with valgus deformities (P < .05). Because of the importance of making the diagnosis of torsion deformity early in the treatment pathway, as knees that undergo multiple surgical procedures have significantly poorer outcomes, 1,38 it is important to consider tibial torsion in the surgical plan, as it could be corrected simultaneously with lower limb realignment or femoral derotation procedures.
Malalignment of the leg axis and rotational deformity have been proven to influence patellar tracking and stability considerably. 15,24,35 To date, various surgical approaches exist to address excessive femoral anteversion in patients experiencing patellar instability with or without leg malalignment. 9,14,29,44 Little consideration has been given to tibial torsion. In some cases, though, torsional deformity of the tibia leads to patellar instability. 3,11 Tibial tuberosity transfer alone was shown to fail in many patients with patellofemoral imbalance secondary to tibial torsional deformity. 3 Fouilleron et al 13 reported a retrospective series of poor postoperative outcome in 29 patients, among whom 7 patients (24.1%) had a history of previous surgery of tibial tubercle transfer. Tibial tubercle transfer did not provide relief for these patients until the tibial derotational osteotomies corrected the rotational deformity. Additionally, Paulos et al 33 conducted tibial derotational osteotomy in 12 patients with patellar instability and compared them with 13 patients who underwent tibial tuberosity transfer alone. Patients who underwent derotational osteotomy showed significantly better results in all functional outcomes than those receiving tuberosity transfer alone. Drexler et al 11 and Manilov et al 21 both reported similar results for patients receiving tibial derotational osteotomy. Therefore, it is important to consider tibial torsion in the treatment pathway.
We found a significantly positive correlation between total tibial torsion and total femoral anteversion. Patients with patellar instability may experience both torsional deformities. In such cases, tibial derotation osteotomy should be taken into consideration with concurrent femoral derotation osteotomy. Naqvi et al 27 noted that 2 patients experienced worse pain than preoperation after a single femoral derotational osteotomy. This complication resolved after they underwent a tibial derotational osteotomy to address the tibial rotational deformity. Stevens et al 39 also reported similar results in 8 patients. While femoral anteversion is often assessed in patients with patellar instability, our findings suggest that tibial torsion should not be ignored. Patients with patellar instability, especially those with concurrent leg axis deformities and excessive femoral anteversion, should undergo further radiological examinations so that tibial torsion can be assessed and the surgical plan formulated, as knees that undergo multiple surgical procedures have significantly poorer outcomes, stressing the importance of making the diagnosis of torsion deformity early in the treatment pathway. 1,38
In addition, we found that, with the exception of proximal anteversion, there was no relationship between lower extremity alignment and different segments of femoral anteversion. Femoral torsion has long been recognized to affect the Q angle, 2 and an excessive Q angle is thought to predispose individuals to injuries caused by an abnormal quadriceps force vector acting at the patellofemoral joint in the frontal plane. Femoral torsion also affects pressure distributions around the knee area and contributes to anterior knee pain. 19,32,44 With respect to lower limb alignment, McWalter et al 24 analyzed the biomechanical factors that may significantly influence the patellar tracking. They discovered that varus and valgus alignment, by just a few angular degrees, were associated with a large variation of patellar tilt and slope. Leg axis malalignment could influence 3-dimensional patellar kinematics.
In contrast to a prior study, 8 we found a significant correlation between distal femoral anteversion and TT-TG distance (r = 0.257; P < .05). As the femoral anteversion increases, more axial pressure is applied to the lateral aspect of the femoral condyle. Consistent with the present study, Xu et al 43 noted that patients with high distal femoral anteversion were inclined to have a greater TT-TG distance. Even though femoral derotational osteotomies have been successfully implemented for years to treat patients with patellar instability, there is still a general lack of knowledge on the topic of femoral anteversion and treatment recommendations. 9,39,44 Our findings, together with previous studies, 37,43 suggest that a femoral derotational osteotomy at the distal femur is probably optimal in patients experiencing pathological TT-TG distance and femoral anteversion.
There was no significant correlation regarding tibial torsion and TT-TG distance in the present study. This finding concurs with previous research. Winkler et al 42 performed a segmental analysis of tibial torsion in patients with patellar instability and found that TT-TG distance did not correlate with any segment of the tibia. The authors of that study only analyzed the association between segments of tibial torsion and TT-TG distance, whereas we evaluated the correlation between these and more structural parameters, as mentioned before.
Limitations
Our study has several limitations. First, we only assessed the association between lower extremity rotation and lower limb alignment in patients with patellar instability because of radiation exposure concerns; hence, our findings cannot be generalized to healthy individuals or patients without patellar instability. Second, this study did not investigate the dynamics of patients with torsional deformity. Only static conditions regarding the patellofemoral joint were evaluated. Additionally, some physical signs such as the J sign were not examined in this study. Third, the biomechanics between the leg axis alignment, lower limb rotation, and patellar instability was not investigated in this study and remains to be explored.
Conclusion
In the current study, tibial torsion was associated with leg axis malalignment and femoral anteversion in patients with patellar instability. Patients with patellar instability, especially those with concurrent leg axis deformities, should undergo further radiological examinations so that tibial torsion can be assessed and a diagnosis of torsion deformity made early in the treatment pathway and the proper surgical plan formulated, as knees that undergo multiple surgical procedures have significantly poorer outcomes.
Footnotes
Final revision submitted August 14, 2022; accepted September 13, 2022.
One or more of the authors has declared the following potential conflict of interest or source of funding: This work was supported by the Exploratory Research Program of Shanghai Jiao Tong University Affiliated Sixth People’s Hospital (grant YNTS202001) and the Shanghai Pujiang Program (grant 2020PJD041). AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.
Ethical approval for this study was obtained from Shanghai Sixth People’s Hospital (approval No. 2020-KY-017).
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