ABSTRACT
Increased femoral version is linked to patellofemoral instability, increased lateral loading of the patella and might be treated with femoral derotation osteotomy (FDO) in excessive cases. However, the effects of FDO on patellofemoral stability and loading while walking are not fully understood and not measurable in vivo. This study used personalized in‐silico musculoskeletal modeling to investigate the impact of FDO on medio‐lateral patellofemoral joint loading in individuals with patellofemoral instability. We analyzed retrospective data from 16 participants with recurrent patellofemoral instability and femoral version ≥ 30° using musculoskeletal simulations. We prepared two models per participant based on gait analysis and magnetic resonance data: (i) a model with personalized femoral version and tibial torsion and (ii) a post‐surgery model with femoral version adjusted to 12° to simulate the derotation osteotomy. We assessed joint angles, joint moments, muscle moments, muscle forces, and patellofemoral joint contact forces. Simulated FDO significantly altered superior‐inferior and anterior‐posterior patellofemoral joint contact forces but did not significantly change medio‐lateral patellofemoral joint loading at the group level. Six participants (37.5%) demonstrated a reduction in lateralizing patellofemoral joint contact forces, three (18.8%) showed no relevant change, and seven (43.7%) exhibited increased lateralizing forces. Changes in rectus femoris muscle force correlated with changes in medio‐lateral patellofemoral joint loading. Participants with worsening patellofemoral loading had greater tibial torsion and exhibited distinct gait characteristics. These findings suggest that patient‐specific functional and anatomical characteristics should be considered when selecting candidates for FDO and planning surgical treatment.
Keywords: femoral derotation osteotomy, femoral version, patella dislocation, patellofemoral instability
1. Introduction
Patellofemoral instability is commonly observed in adolescents and children, often leading to a reduction in functional capability, pain and patellar dislocations [1, 2]. Various morphological determinants including the shape of the trochlea, the integrity of the medial patellofemoral ligament, as well as torsional deformities of the lower extremity, influence the stability of the patellofemoral joint [3, 4].
Since joint loadings are not measurable in‐vivo, studies employing both computational models and cadaveric specimens have investigated the role of femoral version in determining the alignment and mechanical load on the patellofemoral joint. Specifically, cadaveric research demonstrated that an internally rotated femur is associated with increased pressure on the lateral facet of the patella [5] and lateralization of the patella [6]. Similarly, in‐silico musculoskeletal simulations confirmed that increased femoral version leads to greater lateral loading of the patellofemoral joint [7, 8], even though this effect could be altered by other factors like tibial torsion [9]. Moreover, studies showed persistent instability after medial patellofemoral ligament reconstruction or tibial tubercle osteotomy in individuals with increased femoral version [10, 11]. Consequently, femoral derotation osteotomy (FDO) is recommended in conjunction with medial patellofemoral ligament reconstruction or other procedures as a treatment for individuals with recurrent patellofemoral instability and excessive femoral version [12].
Currently, there is a lack of research specifically investigating the effects of FDO on patellofemoral loading in individuals with patellofemoral instability. Since individuals with patellofemoral instability tend to walk with an altered gait pattern [13, 14], and femoral version affects knee loads based on joint kinematics [15], it is important to investigate how FDO changes knee loads under dynamic conditions in these individuals.
The objective of this study was to investigate the impact of FDO on medio‐lateral patellofemoral joint loading in individuals with patellofemoral instability. It was hypothesized that FDO results in a reduction of lateralizing forces, thereby increasing medializing forces on the patella. Enhanced understanding of the effects of FDO on patellofemoral joint loading could provide valuable insights to inform clinical decision‐making for the treatment of patellofemoral instability.
2. Methods
Musculoskeletal simulations were conducted using a retrospective dataset derived from our clinical database, which included individuals diagnosed with patellofemoral instability. The analysis entailed a comparative assessment of joint angles, joint moments, muscle forces, and knee joint loadings between two musculoskeletal models for each patient: (i) one personalized model informed by torsional data from medical images and (ii) the same model but incorporating a simulated FDO. The primary outcome measure was the medio‐lateral joint contact force exerted on the patellofemoral joint. Ethical approval for conducting this research was obtained from the Ethics Committee of Medical University of Graz (IRB00002556 EK‐Number 32‐195 ex19/20). Due to the retrospective nature of the study, the requirement for informed consent was waived.
2.1. Participants
The inclusion criteria involved an age range of 10 to 18 years and a documented history of recurrent patellar instability, defined as a minimum of three patellar dislocations, as well as available clinical gait analysis and rotational magnetic resonance imaging (MRI) data. Additionally, a femoral version greater than 30° was required, identified as a potential threshold for considering FDO in patients with patellofemoral instability [12, 16]. Exclusion criteria encompassed the presence of additional leg injuries or neurological disorders. Based on these criteria, we included 16 individuals diagnosed with patellofemoral instability in this study.
2.2. Data Collection
Three‐dimensional gait analysis data were acquired using a ten‐camera infrared‐based motion capture system (Vicon Motion Systems, Oxford, UK) operating at a sampling rate of 120 Hz. Consistent with established laboratory protocols, the gait data acquisition employed either the plug‐in gait marker set [17, 18] or the modified Cleveland marker set [19]. The hip joint centers were determined based on a modified version of the Harrington equation [20], which used pelvis width as an input [21]. Concurrently, ground reaction forces were recorded alongside the marker data using four force plates (Advanced Mechanical Technology Inc., Watertown, MA, USA) operating at a sampling rate of 1080 Hz. All participants walked barefoot at a self‐selected speed across a level surface.
Rotational MRI conducted prior to any surgical procedures were used to assess femoral version and tibial torsion and to inform the musculoskeletal model. Femoral version was measured according to the methods described by Schneider et al. [22] and Guenther et al. [23], while tibial torsion was measured following the protocol established by Rosskopf et al. [24].
2.3. Musculoskeletal Simulations
For each participant, two personalized models were created based on the MRI and three‐dimensional motion capturing data (Figure 1). The first model (pre‐surgery model) was a scaled version of the Lernagopal model [26], which is a modified version of the Rajagopal model [27] that incorporates the more detailed multi‐compartment knee joint of the Lerner model [28]. For each participant and model, the femoral version and tibial torsion were personalized according to the individual's lower limb torsion, utilizing the Torsion Tool for OpenSim [29, 30]. The Torsion Tool alters the torsion of femur and tibia uniformly distributed along their shafts [29]. Altering femoral torsion with this tool does not change the foot progression angle or any joint kinematics in the model's default pose. It only modifies the femoral torsion‐dependent anatomical geometry, for example, the rotational position of the greater trochanter relative to the femoral coordinate system. The second model (post‐surgery model) was based on the same personalized framework; however FDO was simulated on the side with diagnosed patellofemoral instability by adjusting the patient's pre‐surgical model to a femoral version of 12°. To best mimic the surgical procedure, the Torsion Tool was modified to apply femoral torsion at a single point proximal to the femoral condyles. The chosen 12° of femoral version represented the mean value after FDO surgery in individuals with patellofemoral instability, as shown in a systematic review [12]. Due to a lack of post‐surgical gait data, we performed all simulations based on the pre‐surgical data. Hence, the gait kinematics and thus the foot progression angle were the same pre‐ and post‐simulated surgery. Due to the limited number of markers attached to the foot segment, the metatarsophalangeal and subtalar joints were locked in all models. Furthermore, the maximum isometric muscle forces for each model were scaled based on the square of the body weight [31, 32].
Figure 1.
Workflow of model generation and musculoskeletal simulations. Two models were created for each participant. Each model was personalized for femoral version and tibial torsion using the Torsion Tool. Additionally, the femoral version of the affected side was set to 12° in the post‐surgery model. Subsequently, each model was scaled to the anthropometry of each individual [25] and musculoskeletal simulations were performed using OpenSim. On the lower left side, the blue colored structures represent the proximal femur and muscles (strings) pre‐surgery. The red structures represent the proximal femur and muscles post‐surgery.
For each participant, musculoskeletal simulations were conducted based on the gait analysis data and both (pre‐surgery and post‐surgery) models (Figure 1). Inverse kinematics and inverse dynamics were used to calculate joint angles and moments, respectively. Static optimization, minimizing the sum of squared muscle activations, was employed to estimate muscle forces. Finally, joint reaction analysis was used to calculate joint contact forces [33]. All simulations were performed with OpenSim 4.4 [34].
2.4. Data Analysis
For each participant and model, a minimum of three gait cycles were simulated. All waveforms were time‐normalized to the duration of the gait cycle (0–100% gait cycle) and averaged over all trials. Joint moments, muscle forces, muscle moments and joint loads were normalized to body weight (body weight = body mass × g). Joint contact forces were reported in the local coordinate system of the patella segment. Statistical parametric mapping [35] utilizing the SPM1D package for Matlab (http://www.spm1d.org) were used to compare waveforms between pre‐ and post‐surgery models. Shapiro‐Wilk tests were used to check for normal distribution [36]. In case of non‐normally distributed data, non‐parametric versions of the statistical tests were used. Correlation analyzes were based on Pearson correlation coefficients [37]. Alpha level was set to 0.05. The alpha level for joint contact force waveforms, which were investigated in three anatomical directions (superior‐inferior, anterior‐posterior, medio‐lateral) were Bonferroni corrected by dividing the alpha level by a factor of three [38].
In our analysis we investigated changes due to FDO in (i) patellofemoral joint contact forces, (ii) hip muscle moments and (iii) quadriceps force. For patellofemoral joint contact forces, we compared waveforms and peak forces. Waveforms were investigated in superior‐inferior, anterior‐posterior, medio‐lateral direction between pre‐ and post‐surgery models. Peak forces were analyzed for the first single stance phase (5%–20% of the gait cycle). This was done based on the fact that patella dislocations often happen during a low knee flexion (0–30°), single leg loading and quadriceps activity situation [39]. First single stance phase is most similar to this state. Peak medio‐lateral patellofemoral joint contact force was compared pre‐ and post‐surgery using dependent t‐test or Wilcoxon test [40, 41]. Further, to investigate if a larger femoral version has more potential for medializing patellofemoral forces, we correlated initial femoral version to change in peak medio‐lateral patellofemoral joint contact forces pre‐ to post‐surgery.
Previous studies reported changes in muscle moment arms due to internal femoral version, especially in the gluteal muscles [8, 42]. Hence, we compared gluteal muscle moments for flexion and adduction moments between pre‐ and post‐surgery models.
The quadriceps forces have an impact on the loading of the patella [4, 42, 43]. To investigate how FDO alters quadriceps muscle forces, we correlated the pre‐ to post‐surgery change in root mean square (RMS) rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius muscle force to the change in peak medio‐lateral patellofemoral joint force.
Furthermore, we compared joint kinematics, joint moments, and morphological measurements, i.e., distance between tibial tuberosity and trochlea groove (TT‐TG), Caton‐Deschamps index (CDI), tibial torsion, and femoral version, between cases in which the FDO led to an improvement, no change, and worsening of patellofemoral joint contact forces. We defined an improvement as a reduction of at least 10% in the lateralizing patellofemoral joint contact force. No change was defined as a post‐FDO medial‐lateral patellofemoral joint contact force within +/− 10% of the pre‐surgical values. A worsening was defined as an increase of at least 10% in the lateralizing patellofemoral joint contact force.
3. Results
The participants presented a mean (standard deviation) femoral version of 36.1° (4.9°) and tibial torsion of 35.6° (7.2°) (Table 1). The frontal knee axis was correctly aligned in all participants. Joint angles and joint moment waveforms can be found in the supplementary material (supporting information Figures S1 and S2). Inverse kinematics marker tracking errors were below OpenSim's best practice recommendations [44].
Table 1.
Demographic data. Parameters are presented as mean (standard deviation).
| Demographic data | |
|---|---|
| Age (years) | 15.8 (1.9) |
| Sex (male/female) | 1/15 |
| Height (m) | 1.68 (0.08) |
| Body weight (kg) | 60.2 (9.6) |
| Body‐mass‐index | 21.4 (3.2) |
| Tibial torsion (°) | 35.6 (7.2) |
| Femoral version (°) | 36.1 (4.9) |
| TT‐TG (mm) | 15.3 (4.3) |
| CDI | 1.24 (0.15) |
| Dejour (A/B/C/D) | 1/9/2/4 |
Abbreviations: CDI, Caton‐Deschamps index; TT‐TG, distance between tibial tuberosity and trochlea groove.
3.1. Pre‐ and Post‐Surgical Joint Contact Forces
Simulated FDO significantly altered patellofemoral joint contact forces in superior‐inferior and anterior‐posterior directions. Medial‐lateral patellofemoral joint contact forces were not significantly altered during the stance phase of gait (Figure 2). Tibiofemoral joint contact forces are shown in supporting information Figure S3.
Figure 2.
Joint contact forces. Patellofemoral joint contact forces in three anatomical planes before (blue waveforms) and after (red waveforms) the simulated femoral derotation osteotomy (FDO).
3.2. Peak Medio‐Lateral Patellofemoral Joint Force in Single Stance
The impact of the FDO on peak medio‐lateral patellofemoral joint contact forces varied between participants. Six individuals showed a more medial patellofemoral joint contact force after the simulated FDO, whereas in three participants the patellofemoral joint contact forces did not change, and in seven participants the forces became more lateral following the FDO (Figure 3). The change in peak medio‐lateral patellofemoral forces due to the FDO did not correlate with the initial femoral version angle (r = 0.17, p = 0.536, supporting information Figure S5).
Figure 3.
Peak medio‐lateral patellofemoral joint force. Each dot represents the peak medio‐lateral patellofemoral joint force of one individuum pre‐ and post‐surgery. Values of one individual are interconnected by a line. Blue, gray, and red indicate participants whose patellofemoral joint contact forces improved, remained unchanged, or worsened following FDO.
3.3. Muscle Moments and Muscle Forces
Hip extension muscle moment of the gluteus medius decreased due to the simulated FDO, while gluteus minimus flexion moment and gluteus medius and maximus abduction moment increased (Figure 4).
Figure 4.
Muscle moments produced by the gluteal muscles. Muscle moments for gluteus minimus, medius and maximus in the sagittal (top row) and frontal (bottom row) planes before (blue waveforms) and after (red waveforms) the simulated femoral derotation osteotomy (FDO). The colored horizontal bar below each subfigure indicates significant differences based on statistical parametric mapping analysis.
The pre‐ to post‐surgery change in RMS rectus femoris muscle force correlated to the change in peak medio‐lateral patellofemoral forces (r = −0.68, p = 0.004) (supporting information Figure S4). Pre‐to post‐surgery change in RMS vastus lateralis, vastus medialis muscle force, and vastus intermedius muscle forces did not correlate to change in medial‐lateral patellofemoral joint force.
3.4. Comparison of Improvement, No Change, and Worsening Groups
No significant differences were observed in the distance between the tibial tuberosity and the trochlear groove (TT‐TG), the Caton‐Deschamps index (CDI) or the femoral version between cases in which the FDO resulted in an improvement, no change or worsening of patellofemoral joint contact forces. Individuals whose patellofemoral joint contact forces worsened after the FDO had significantly larger tibial torsion values (p < 0.05) than those whose forces improved or remained unchanged (Figure 5). Moreover, these individuals whose patellofemoral joint contact force worsened walked with more knee flexion variation during the stance phase compared to the individuals whose forces improved or remained unchanged (Figure 6). Furthermore, the first peak of the knee flexion moment was elevated and the rectus femoris muscle forces during the stance phase were lower in these individuals compared to those whose forces improved (Figure 6).
Figure 5.
Boxplots showing morphological measurements, i.e., distance between tibial tuberosity and trochlea groove (TTTG), Caton‐Deschamps index (CDI), femoral anteversion (AT), tibial torsion (TT), and walking speed for cases in which the FDO led to an improvement (blue boxes), no change (gray boxes), and worsening (red boxes) of patellofemoral joint contact forces.
Figure 6.
Pre‐surgical knee flexion angle, knee flexion/extension joint moments and rectus femoris forces for the three subgroups. Improvement (blue waveforms): reduction of at least 10% in the lateralizing patellofemoral joint contact force post‐FDO. No change (dashed black waveforms): post‐FDO medial‐lateral patellofemoral joint contact force within +/− 10% of the pre‐surgical values. Worsening (red waveforms): increase of at least 10% in the lateralizing patellofemoral joint contact force post‐FDO.
4. Discussion
The objective of this study was to investigate the effect of FDO on medio‐lateral patellofemoral forces in individuals with patellofemoral instability. We found that simulated FDO did not significantly reduce lateralizing patellofemoral forces on a group level. However, in six participants (37.5% of all participants), FDO resulted in a favorable medial shift of the patellofemoral forces, three (18.8%) showed no relevant change, and seven (43.7%) exhibited increased lateralizing forces. The heterogeneous response to the same simulated surgical correction appears to be associated with patient‐specific characteristics, as participants with worsening patellofemoral loading had greater tibial torsion and distinct gait patterns compared with those who improved or showed no change.
In participants who demonstrated an improvement in patellofemoral joint contact forces, a medial shift of 0.02 body weight in the patellofemoral joint force was observed during the single stance phase after the simulated FDO. Considering the mean weight of 63.1 kg and the mean change of peak medio‐lateral patellofemoral joint force of 0.02 body weight, the resulting additional medializing force would be 14 N. When compared to the mean tensile strength of the medial patellofemoral ligament, which is approximately 208 N [45], the effect of the FDO represents less than 10% of this force value. Therefore, from a biomechanical perspective, additional FDO can offer benefits in increasing the medializing force on the patellofemoral joint in individuals with patellofemoral instability and excessive femoral version. When used in conjunction with other treatments, such as medial patellofemoral ligament reconstruction or tibial tuberosity transfer, FDO may enhance outcomes in managing recurrent patellofemoral instability [12].
Our personalized simulations gave us a better understanding of how FDO alters muscle forces. A reduction in rectus femoris force correlated with a more medializing peak patellofemoral joint force post‐surgery. A previous study reported an increased co‐contraction of hip flexor and extensor muscles with excessive femoral version, leading to increased rectus femoris force [42]. Hence, reducing femoral version due to FDO could lead to a decrease in rectus femoris muscle force, as observed in our investigation. This is supported by the observed decrease in gluteus medius hip extension muscle moment after the FDO. As a biarticulated muscle, the rectus femoris directly influences the patellofemoral joint and therefore alters joint contact forces [46]. Consequently, the observed changes in medio‐lateral patellofemoral loading post‐FDO could be attributed to a decrease in rectus femoris force.
In seven participants (43.7% of all participants) FDO reduced the medializing patellofemoral joint contact force, resulting in a worsening of the biomechanical situation. Comparing the gait patterns between successful and unsuccessful cases indicated that the unsuccessful cases were characterized by reduced knee flexion during the stance phase and less rectus femoris force during the majority of the gait cycle. One of the primary positive effects of FDO appears to be the reduction of rectus femoris muscle force. If the initial rectus femoris force is already low, as it was in these participants, only a negligible alteration in rectus femoris forces is possible as a result of the FDO. This explains why the patellofemoral joint contact force remained unchanged in these participants.
Considering the variability in our findings, the walking pattern appears to have a critical influence on the effectiveness of FDO. The significance of the gait pattern is further supported by another study, which showed that the impact of femoral version on knee joint loading is influenced by an individuum's joint kinematics [15]. In our investigation, we solely examined the impact of FDO during gait. Given the importance of the movement pattern, it is conceivable that the effects of FDO might be more substantial if individuals perform activities involving higher knee flexion moments.
In our study, we assumed that the FDO does not alter the participants' gait pattern, which enabled us to investigate the impact of the FDO on joint loads without any con‐founding factors. However, it is important to note that, in reality the gait pattern is likely to undergo changes following the FDO. To the best of the authors' knowledge, no studies have compared gait kinematics before and after an FDO in patients with patellofemoral instability. However, a few studies have compared its impact on gait kinematics and kinetics in individuals with idiopathic torsional deformities. The findings of two studies indicate that the pathological gait pattern in individuals with idiopathic torsional deformities approaches a typical gait pattern following an FDO [47, 48]. Such alterations in gait pattern could potentially lead to changes in medio‐lateral patellofemoral joint loads. It is noteworthy that Alexander et al. [47] conducted a musculoskeletal simulation based on pre‐ and post‐FDO gait data, which, in accordance with our findings, revealed a reduction in rectus femoris forces following the FDO. Nevertheless, further studies should investigate how FDO alter the gait pattern, and consequently joint loads, in individuals with patellofemoral instability.
FDO was simulated in our study, while tibial torsion remained unchanged. This allowed us to isolate the biomechanical effects of femoral version correction without the confounding influence of simultaneous tibial correction. Clinically, isolated FDO is more commonly performed at our center than tibial derotation osteotomy, which is generally reserved for selected patients and is associated with additional surgical risks, including injury to the common peroneal nerve. It is noteworthy that tibial torsion was the sole morphological parameter that demonstrated a significant difference between participants with a worsening of patellofemoral contact forces and those with an improvement or no alterations in forces following the FDO. This emphasizes the necessity of incorporating tibial torsion into clinical decision‐making processes and underscores its potential impact on treatment outcomes. Future studies should investigate the biomechanical effects of isolated tibial and combined femoral–tibial derotation procedures, with a particular focus on patients exhibiting excessive tibial torsion values.
Our findings support the current state of knowledge, that FDO can positively influence stability of the patellofemoral joint in selected individuals experiencing recurrent patellofemoral instability [12]. However, given the small changes noted in medio‐lateral patellofemoral force, employing FDO as an isolated treatment for patellofemoral instability may not be advisable. In carefully selected patients, combining FDO with other patellar stabilization procedures, such as medial patellofemoral ligament reconstruction, may improve clinical outcomes. Additionally, the efficacy of FDO appears to be significantly influenced by individual movement patterns, which also can affect the patellofemoral forces in individuals with patellofemoral instability [14]. Therefore, it is essential to consider not only femoral version but also functional movements, such as those observed through gait analysis, in the planning and evaluation of FDO.
This work includes several limitations, which have to be considered, when interpreting our results. First, there was no personalization of knee joint geometry or the location of the tibial tubercle in the musculoskeletal models, which potentially could lead to variations in joint contact forces [49]. Furthermore, the Torsion Tool [29, 30] was employed to personalize femoral version and tibial torsion and to simulate the FDO. Although the personalized femoral version and tibial torsion represented the values measured in rotational MRIs, the specific location of the torsional deformity in each individual was neglected. Lastly, our cohort was predominantly female (1 male, 15 female), reflecting the known epidemiology of patellofemoral instability [50, 51]. However, this sex imbalance may limit the generalizability of the findings to male patients, who may present with different anatomical and biomechanical characteristics.
5. Conclusion
In conclusion, simulated femoral derotation osteotomy did not significantly alter medio‐lateral patellofemoral joint contact forces at the group level in individuals with patellofemoral instability and excessive femoral version. However, substantial inter‐individual variability was observed, with only a subset of participants demonstrating a favorable medial shift in patellofemoral loading. Greater tibial torsion and differences in gait mechanics were associated with less favorable biomechanical outcomes, suggesting that the effectiveness of FDO depends on patient‐specific anatomy and movement patterns. These findings highlight the importance of incorporating both morphological and functional assessments into preoperative planning and indicate that FDO should be considered as part of an individualized treatment strategy rather than a uniform solution for patellofemoral instability.
Author Contributions
Initial idea: Bernhard Guggenberger, Hans Kainz, Martin Svehlik. Conceptualization: Bernhard Guggenberger, Hans Kainz, Martin Svehlik. Data acquisition: Bernhard Guggenberger, Tanja Kraus. Analysis: Bernhard Guggenberger, Willi Koller, Hans Kainz. Methodology: Bernhard Guggenberger, Martin Svehlik, Hans Kainz. Interpretation: Bernhard Guggenberger, Hans Kainz, Willi Koller, Martin Svehlik, Tanja Kraus. Writing original draft: Bernhard Guggenberger, Hans Kainz, Martin Svehlik. Critical revision of final draft all authors. All authors approved the final version.
Supporting information
Supporting File
Acknowledgments
Open Access funding provided by Universitat Wien.
Guggenberger B., Kainz H., Kraus T., Koller W., and Svehlik M., “Impact of Femoral Derotation Osteotomy on Patellofemoral Loading in Patients With Patellofemoral Instability: Insights From a Musculoskeletal Simulation Study,” Journal of Orthopaedic Research® 44 (2026): e70278. 10.1002/jor.70278.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
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Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.