Morphological analysis of the femoral trochlea in patients with recurrent dislocation of the patella using three-dimensional computer models

Three-dimensional magnetic resonance imaging-based statistical shape analysis and machine learning-based prediction of patellofemoral instability

This study performed three-dimensional (3D) magnetic resonance imaging (MRI)-based statistical shape analysis (SSA) by comparing patellofemoral instability (PFI) and normal femur models, and developed a machine learning (ML)-based prediction model. Twenty (19 patients) and 31 MRI scans (30 patients) of femurs with PFI and normal femurs, respectively, were used. Bone and cartilage segmentation of the distal femurs was performed and subsequently converted into 3D reconstructed models. The pointwise distance map showed anterior elevation of the trochlea, particularly at the central floor of the proximal trochlea, in the PFI models compared with the normal models. Principal component analysis examined shape variations in the PFI group, and several principal components exhibited shape variations in the trochlear floor and intercondylar width. Multivariate analysis showed that these shape components were significantly correlated with the PFI/non-PFI distinction after adjusting for age and sex. Our ML-based prediction model for PFI achieved a strong predictive performance with an accuracy of 0.909 ± 0.015, and an area under the curve of 0.939 ± 0.009 when using a support vector machine with a linear kernel. This study demonstrated that 3D MRI-based SSA can realistically visualize statistical results on surface models and may facilitate the understanding of complex shape features.

Open wedge high tibial osteotomy does not decrease patellar height relative to femur: A three-dimensional computer model analysis

BACKGROUND

Patellar height, which decreases after open wedge high tibial osteotomy (OWHTO), has conventionally been assessed by tibial references using lateral radiographs of the knee; however, changes in the proximal tibia shape after OWHTO may affect this method. We aimed to evaluate the changes in patellar height position relative to the transepicondylar axis of the femur after OWHTO using in vivo three-dimensional (3D) computer models.

METHODS

Fourteen patients who underwent 3D magnetic resonance imaging (MRI) at 30° and 50° knee flexion before OWHTO and after hardware removal were included. 3D computer models of the knee were created from the MRI scans and superimposed over the images taken in each position using voxel-based registration. For patellar height evaluation, a patellar reference point was established at each flexion angle and the femoral condylar planes (FCP) were set, including the transepicondylar axis. The patellar center angle was defined as the angle between an FCP that included the top of the intercondylar notch and an FCP that included the patellar reference point. The patellar center angle was evaluated at 30° and 50° knee flexion before and after OWHTO.

RESULTS

The patellar center angle at 30° and 50° knee flexion did not significantly decrease after OWHTO, whereas the Caton-Deschamps index and Blackburne-Peel index based on tibia-referenced measurements significantly decreased postoperatively.

CONCLUSION

Patellar height position relative to the femur in the 3D computer model did not decrease after OWHTO, whereas tibia-referenced conventional radiographic measurements significantly decreased. When evaluating patellar height, characteristics of each parameter should be considered.

Intraoperative analysis of patellofemoral joint morphology before and after total knee arthroplasty

Background

Patellofemoral problems after total knee arthroplast (TKA) are frequent and often associated with a change in the geometry of the trochlear groove.

Objective

The present study aimed to analyze the feasibility of intraoperative examination of the patellofemoral joint geometry before and after the implantation of bicondylar total knee replacements without exposing the patient to radiation.

Material and methods

The patellofemoral joint morphology geometries of 33 patients before and after implantation of a bicondylar total knee replacement was intraoperatively analyzed using a digital scanning method. Femoral surface geometries were extracted from intraoperatively recorded tactile scanning data using an in-house algorithm. The geometries were then characterized by sulcus height, medial femoral condyle height and lateral femoral condyle height.

Results

Our preliminary results show that these key geometric parameters are only partially restored after TKA leading to a distinctly different shaped profile of the anterior distal femur. Maximum and minimum mean differences in sulcus height, medial femoral condyle height, and lateral femoral condyle height before and after surgery were 2.00 mm/−3.06 mm, 2.51 mm/−6.25 mm and 2.74 mm/−3.58 mm, respectively.

Conclusion

A new method for intraoperative analysis of the patellofemoral joint morphology before and after TKA without radiation exposure was developed and utilized. Even with the use of modern total knee designs, the patellofemoral articulation is considerably changed postoperatively as quantified by the key parameters of sulcus height, medial femoral condyle height, and lateral femoral condyle height. This may result in altered knee biomechanics and might explain persistent patellofemoral disorders, which are often reported by patients after TKA.

Decreasing Thickness of Partial Lateral Trochlear Cartilage in Patients with Patellar Instability

Objective

To explore morphological characteristics of patellofemoral joint surface of patients with patellar instability by adopting the MRI‐based method.

Methods

A retrospective analysis was performed from March 2016 to January 2020 to assess morphological characteristics of the patellofemoral joint surface by Magnetic Resonance Imaging (MRI) scanning knees of 30 patients (24 females, six males) with patellar instability and trochlear dysplasia and knees of 30 subjects from a randomly selected control group (25 females, five males). The control group was matched as per age and sex. All participants had undergone MRI scans in the supine position and keep knees in or near full extension. Six parts were measured in total, including thickness of trochlear cartilage, thickness of patella cartilage, cartilaginous sulcus angle, cartilaginous Wiberg angle, contact range and frequency and distributions of the mean difference measurement of the femoral trochlea, to evaluate the difference of trochlear and patellar morphology between the patient group and the control group. The threshold for statistical significance was set at P < 0.05.

Results

There were significant differences in four values between the two groups (P < 0.05). The cartilage thickness two‐third along the lateral condyle in the patient group was significantly lower than that in the control group (LCT2,1.80 ± 0.37 vs 2.06 ± 0.52, 1.92 ± 0.36 vs 2.17 ± 0.50), but there was no significant difference in other sites. There was no significant difference in patella thickness between the patient group and the control group. The cartilaginous sulcus angle in the patient group was larger than that in the control group (157.90 ± 6.64 vs 142.23 ± 3.95, P < 0.001), but there was no significant difference in cartilaginous Wiberg angle. The patient group had a larger initial contact ratio (59.47 ± 6.13 vs 46.50 ± 3.67, P < 0.001), and a smaller contact range (16.55 ± 4.14 vs 27.55 ± 4.09, P < 0.001). The deepest part of the intercondylar suclus appears more often in the lateral of the deepest part of the osseous concavity of the femoral trochlea. Among the patient group, 18 cases (60%) were found with the deepest part of the intercondylar suclus lateral to the deepest point of the osseous concavity of the femoral trochlea while among the control group only 4 cases (13.33%) were found. The distribution of trochlear dysplasia of Dejour grades was type B (n = 22), type C (n = 5), and type D (n = 3).

Conclusion

Thickness of partial lateral trochlear cartilage decreases in patients with patellar instability and the trochlear cartilage develops abnormal morphological characteristics. Moreover, it also suggests that MRI can be used to further present the morphology of cartilage for the convenience of surgical planning.

Three-dimensional assessment of patellofemoral anatomy: Reliability and reference ranges

BACKGROUND

Three-dimensional (3D) imaging and analysis offer new possibilities in preoperative diagnostics and surgical planning. Simultaneous 3D analysis of the joint angles and the patellofemoral anatomy allow for a realistic assessment of bony pathologies in patients with patellofemoral complaints. This study aims to develop a standardized and validated assessment of the 3D patellofemoral morphology and to establish reference ranges.

METHODS

Thirteen patellofemoral anatomic landmarks were defined on 3D bone models of the lower limbs based on computer tomography data and evaluated regarding inter- and intra-observer variability. Further, 60 3D models of the lower limbs of young subjects without any previous knee operation/injury were assessed and rescaled reference values for relevant patellofemoral indices were obtained.

RESULTS

The mean inter- and intra-observer deviation of all landmarks was below 2.3 mm. The interobserver intraclass correlation coefficient (ICC) was between 0.8 and 1.0 and the intra-observer ICC between 0.68 and 0.99 for all patellofemoral parameters. The calculated reference ranges are: Insall-Salvati index 1.0-1.4; patella tilt 6-18°; patella shift -4 to 3 mm; patella facet angle 118-131°; sulcus angle 141-156°; trochlear depth 3-6 mm; tibial-tuberosity to trochlear groove distance(TT-TG) 2D 14-21 mm; TT-TG 3D 11-18 mm; lateral trochlear inclination 13-23°; trochlear facet angle 43-65°.

CONCLUSION

The demonstrated 3D analysis of the patellofemoral anatomy can be performed with high inter- and intra-observer correlation. Applying the obtained reference ranges and using existing 3D assessment tools for lower limb alignment, a preoperative 3D analysis and planning for complex knee procedures now is possible.

Lateral Trochlear Inclination Angle: Measurement via a 2-Image Technique to Reliably Characterize and Quantify Trochlear Dysplasia

Background:

Trochlear dysplasia (TD) is a risk factor for patellar instability (PI). The Dejour classification categorizes TD but has suboptimal reliability. Lateral trochlear inclination (LTI) is a quantitative measurement of trochlear dysplasia on a single axial magnetic resonance imaging (MRI) scan.

Hypothesis:

A modified LTI measurement technique using 2 different axial MRI scans that reference the most proximal aspect of the trochlear cartilage on 1 image and the fully formed posterior condyles on the second image would be as reliable as and significantly different from the single-image measurement technique for LTI. Further, the 2-image LTI would adequately represent overall proximal trochlear morphologic characteristics.

Study Design:

Cohort study (diagnosis); Level of evidence, 2.

Methods:

Patients aged 9 to 18 years treated for PI between 2014 and 2017 were identified. The Dejour classification was radiographically determined. Single-image LTI was measured on a single axial MRI scan at the most proximal aspect of visible trochlear cartilage. A 2-image LTI was measured from 2 separate MRI scans: 1 at the most proximal aspect of trochlear cartilage and the second at the fully formed posterior condyles. This 2-image LTI was repeated at 3 subsequent levels (the first measurement is referred to as LTI-1; repeated measurements are LTI-2, LTI-3, and LTI-4, moving distally). In total, 65 patients met the inclusion criteria, and 30 were randomly selected for reliability analysis.

Results:

Inter- and intrarater reliability trended toward more variability for single-image LTI (intraclass correlation coefficient [ICC], 0.86 and 0.88, respectively) than for 2-image LTI (ICC, 0.97 and 0.96, respectively). The Dejour classification had lower intra- and interrater reliability (ICC, 0.31 and 0.73, respectively). Average single-image LTI (9.2° ± 12.6°) was greater than average 2-image LTI-1 (4.2° SD ± 11.9°) (P = .0125). Single-image LTI classified 60% of patients with PI as having TD, whereas the 2-image LTI classified 71% as having TD. The 2-image LTI was able to capture 91% of overall proximal trochlear morphologic characteristics.

Conclusion:

LTI has higher reliability when performed using a 2-image measurement technique compared with single-image LTI and Dejour classification. The strong correlation between 2-image LTI and average LTI shows that 91% of TD is represented on the most proximal axial image. Because the single-image measurement appears to underestimate dysplasia, previously described thresholds should be reexamined using this 2-image technique to appropriately characterize TD.

The dysplastic trochlear sulcus due to the insufficient patellar stress in growing rats

BACKGROUND

Developmental factors were assumed to be the key factors that influenced the morphology of femoral trochlea. This study investigated the effects of insufficient patellar stress after birth on the morphological development of the femoral trochlea. Effects of insufficient patellar stress on femoral trochlea were investigated using surgical induced patellectomy and patellar dislocation in growing rat model.

METHODS

In this study, two experimental groups and one sham group (SG) were established. Thirty-six Wistar rats (female, 28 days of age) were randomly assigned to three groups. The patellectomy group (PG), rats underwent the patellectomy in this group. The dislocation group (DG), rats underwent the surgery-induced patellar dislocation. Histological staining (Safranin-O and fast green), Micro-computed tomographic (Micro-CT) analysis in two experimental endpoints (3, 12 weeks postoperatively) were selected to evaluate morphological changes of the femoral trochlea.

RESULTS

Articular cartilage on the trochlear sulcus was remodeled at 3 weeks after the surgery, and degenerated at 12 weeks through the histological staining. The femoral trochlear angle (FTA) did not show a significant difference at 3 week between the experimental groups and the sham group (PG vs SG P = 0.38, DG vs SG p = 0.05), but the FTA was significantly increased in experimental groups at 12 weeks(PG vs SG P = 0.001, DG vs SG p = 0.005). The Bone volume density (BV/TV), Trabecular number (Tb.N) under the trochlea groove were significantly reduced at 3 weeks postoperatively in the experimental groups (PG vs SG p = 0.001, DG vs SG p = 0.002). No significant difference was found in BV/TV and Tb. N among the three groups at 12 weeks postoperatively.

CONCLUSION

Surgical induced patellectomy and patellar dislocation leads to the dysplastic trochlear sulcus in growing rats. Besides the bone morphology of trochlear sulcus, the articular cartilage and subchondral trabecula under the trochlear sulcus were remodeled early stage after the surgery.

Patellar instability can be classified into four types based on patellar movement with knee flexion: a three-dimensional computer model analysis

Objective

Patellar instability (PI) represents various underlying pathologies, including patellar malalignment. Continuous patellar alignment develops to patellar tracking and is regarded as the end product of combined predisposing factors. We quantitatively investigated the inhomogeneity of patellar tracking in PI.

Methods

Sixty knees of 56 patients with PI and 15 knees of 10 healthy volunteers (HVs) were studied. Three-dimensional (3D) computer models were created based on MRIs at 10° intervals over 0°–50° of flexion, and patellar tracking was quantitatively analysed using patellar 3D shift. Classification was performed according to the maximum 3D shift (max-shift), indicating the extent of lateral deviation, and the change of 3D shift from 0° to 50° (change0–50), indicating movement direction. First, the cut-off value (COV) of the max-shift was defined based on the data from HVs. When a value was greater than the COV, it was defined as a major subluxation, and when the value was smaller it was defined as a minor subluxation. Next, the two COVs of change0–50 were similarly defined. When a value was greater than the upper COV, it was defined as a major-lateral type, laterally moving the patella with flexion, and when smaller than the lower COV it was defined as a major-medial type, medially moving the patella with flexion. When a value fell between the two COVs, it was defined as a major-straight type.

Results

Fifty-three patellae (88%) with values larger than the COV of the max-shift (mean +1 SD of HV) were defined as major subluxations and seven (12%) showing smaller values as minor subluxations. Among the major subluxations, 25 (42%) showing a smaller value than the lower COV of change0–50 (mean –2 SD of HV) were defined as major-medial type, while 7 (12%) showing a larger value than the upper COV of change0–50 (mean +2 SD) were defined as major-lateral type. Twenty-one (35%) were defined as major-straight type. No further analysis was performed on the seven minor subluxations (the minor type).

Conclusion

PI was quantitatively classified into four types according to the extent of lateral deviation and the movement direction of the patellae with flexion, showing inhomogeneity of patellar tracking.

Stacked sparse autoencoder networks and statistical shape models for automatic staging of distal femur trochlear dysplasia

BACKGROUND

The quantitative morphological analysis of the trochlear region in the distal femur and the precise staging of the potential dysplastic condition constitute a key point for the use of personalized treatment options for the patella-femoral joint. In this paper, we integrated statistical shape models (SSM), able to represent the individual morphology of the trochlea by means of a set of parameters and stacked sparse autoencoder (SSPA) networks, which exploit the parameters to discriminate among different levels of abnormalities.

METHODS

Two datasets of distal femur reconstructions were obtained from CT scans, including pathologic and physiologic shapes. Both of them were processed to compute SSM of healthy and dysplastic trochlear regions. The parameters obtained by the 3D-3D reconstruction of a femur shape were fed into a trained SSPA classifier to automatically establish the membership to one of three clinical conditions, namely, healthy, mild dysplasia, and severe dysplasia of the trochlea. The validation was performed on a subset of the shapes not used in the construction of the SSM, by verifying the occurrence of a correct classification.

RESULTS

A major finding of the work is that SSM are able to represent anomalies of the trochlear geometry by means of specific eigenmodes of variation and to model the interplay between morphologic features related to dysplasia. Exploiting the patient-specific morphing parameters of SSM, computed by means of a 3D-3D reconstruction, SSPA is demonstrated to outperform traditional discriminant analysis in classifying healthy, mild, and severe trochlear dysplasia providing 99%, 97%, and 98% accuracy for each of the three classes, respectively (discriminant analysis accuracy: 85%, 89%, and 77%).

CONCLUSIONS

From a clinical point of view, this paper contributes to support the increasing role of SSM, integrated with deep learning techniques, in diagnostics and therapy definition as quantitative and advanced visualization tools.

Correlation of 3D Shift and 3D Tilt of the Patella in Patients With Recurrent Dislocation of the Patella and Healthy Volunteers: An In Vivo Analysis Based on 3-Dimensional Computer Models

BACKGROUND

The concepts of lateral deviation and lateral inclination of the patella, characterized as shift and tilt, have been applied in combination to evaluate patellar malalignment in patients with patellar dislocation. It is not reasonable, however, to describe the 3-dimensional (3D) positional relation between the patella and the femur according to measurements made on 2-dimensional (2D) images.

PURPOSE

The current study sought to clarify the relation between lateral deviation and inclination of the patella in patients with recurrent dislocation of the patella (RDP) by redefining them via 3D computer models as 3D shift and 3D tilt.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Altogether, 60 knees from 56 patients with RDP and 15 knees from 10 healthy volunteers were evaluated. 3D shift and tilt of the patella were analyzed with 3D computer models created by magnetic resonance imaging scans obtained at 10° intervals of knee flexion (0°-50°). 3D shift was defined as the spatial distance between the patellar reference point and the midsagittal plane of the femur; it is expressed as a percentage of the interepicondylar width. 3D tilt was defined as the spatial angle between the patellar reference plane and the transepicondylar axis. Correlations between the 2 parameters were assessed with the Pearson correlation coefficient.

RESULTS

The patients' mean Pearson correlation coefficient was 0.895 ± 0.186 (range, -0.073 to 0.997; median, 0.965). In all, 56 knees (93%) had coefficients >0.7 (strong correlation); 1 knee (2%), >0.4 (moderate correlation); 2 knees (3%), >0.2 (weak correlation); and 1 knee (2%), <0.2 (no correlation). The mean correlation coefficient of the healthy volunteers was 0.645 ± 0.448 (range, -0.445 to 0.982; median, 0.834). A statistically significant difference was found in the distribution of the correlation coefficients between the patients and the healthy volunteers ( P = .0034). When distribution of the correlation coefficients obtained by the 3D analyses was compared with that by the 2D (conventional) analyses, based on the bisect offset index and patellar tilt angle, the 3D analyses showed statistically higher correlations between the lateral deviation and inclination of the patella ( P < .01).

CONCLUSION

3D shift and 3D tilt of the patella were moderately or strongly correlated in 95% of patients with RDP at 0° to 50° of knee flexion.

CLINICAL RELEVANCE

It is not always necessary to use both parameters when evaluating patellar alignment, at least for knees with RDP at 0° to 50° of flexion. Such a description may enable surgeons to describe patellar alignment more simply, leading to a better, easier understanding of the characteristics of each patient with RDP.

The effect of trochlear dysplasia on patellofemoral biomechanics: a cadaveric study with simulated trochlear deformities

BACKGROUND

Trochlear dysplasia appears in different geometrical variations. The Dejour classification is widely used to grade the severity of trochlear dysplasia and to decide on treatment.

PURPOSE

To investigate the effect of trochlear dysplasia on patellofemoral biomechanics and to determine if different types of trochlear dysplasia have different effects on patellofemoral biomechanics.

STUDY DESIGN

Controlled laboratory study.

METHODS

Trochlear dysplasia was simulated in 4 cadaveric knees by replacing the native cadaveric trochlea with different types of custom-made trochlear implants, manufactured with 3-dimensional printing. For each knee, 5 trochlear implants were designed: 1 implant simulated the native trochlea (control condition), and 4 implants simulated 4 types of trochlear dysplasia. The knees were subjected to 3 biomechanical tests: a squat simulation, an open chain extension simulation, and a patellar stability test. The patellofemoral kinematics, contact area, contact pressure, and stability were compared between the control condition (replica implants) and the trochlear dysplastic condition and among the subgroups of trochlear dysplasia.

RESULTS

The patellofemoral joint in the trochlear dysplastic group showed increased internal rotation, lateral tilt, and lateral translation; increased contact pressures; decreased contact areas; and decreased stability when compared with the control group. Within the trochlear dysplastic group, the implants graded as Dejour type D showed the largest deviations for the kinematical parameters, and the implants graded as Dejour types B and D showed the largest deviations for the patellofemoral contact areas and pressures.

CONCLUSION

Patellofemoral kinematics, contact area, contact pressure, and stability are significantly affected by trochlear dysplasia. Of all types of trochlear dysplasia, the models characterized with a pronounced trochlear bump showed the largest deviations in patellofemoral biomechanics.

CLINICAL RELEVANCE

Investigating the relationship between the shape of the trochlea and patellofemoral biomechanics can provide insight into the short-term effects (maltracking, increased pressures, and instability) and long-term effects (osteoarthritis) of different types of trochlear dysplasia. Furthermore, this investigation provides an empirical explanation for better treatment outcomes of trochleoplasty for Dejour types B and D dysplasia.

Analysis of displacement and deformation of the medial meniscus with a horizontal tear using a three-dimensional computer model

PURPOSE

The displacement and deformation of the knee meniscus significantly affect its roles; however, little is known about the displacement and deformation patterns of a torn medial meniscus. The objective of this study was to evaluate quantitatively the patterns of displacement and deformation in horizontally torn medial menisci during knee flexion.

METHODS

Twenty patients with horizontally torn medial menisci underwent three-dimensional (3-D) magnetic resonance imaging at varying degrees of knee flexion, and 3-D computer models of the tibia, tibial articular cartilage, and meniscus were generated. Based on these, the size of the horizontal tear (% tear) was evaluated and defined as the circumferential ratio between the length of the horizontal tear and that of the entire meniscus. The 3-D meniscus models were automatically superimposed over images taken at 0, 20, 40, and 60° of knee flexion by the voxel-based registration method. Meniscal motion and deformation during knee flexion were visualized and quantitatively calculated on the mid-sagittal plane. Correlations between the size of horizontal tear, displacement/deformation of torn menisci, and clinical symptoms were evaluated after conservative treatment for 3 months.

RESULTS

The % tear was 35.7 ± 12.5 % (range 13.7-55.5 %). During knee flexion, all torn menisci moved posteriorly, with gradual widening of horizontal and vertical gaps (p < 0.05). A direct correlation was observed between % tear and change in the vertical tear gap during knee flexion (p < 0.05). There was an inverse correlation between Lysholm score and % tear (p < 0.05).

CONCLUSION

Medial meniscal horizontal tears widen and deform during knee flexion, and % tear correlates with the change in the vertical gap. Patients with a lower % tear are more capable of performing activities of daily living after conservative treatment. This method may help clarify the cause of pain in patients with medial meniscus tears as well as facilitate the selection of an appropriate treatment plan.

LEVEL OF EVIDENCE

Case series, Level IV.

Semi-automated landmark-based 3D analysis reveals new morphometric characteristics in the trochlear dysplastic femur

PURPOSE

The authors hypothesise that the trochlear dysplastic distal femur is not only characterised by morphological changes to the trochlea. The purpose of this study is to describe the morphological characteristics of the trochlear dysplastic femur in and outside the trochlear region with a landmark-based 3D analysis.

METHODS

Arthro-CT scans of 20 trochlear dysplastic and 20 normal knees were used to generate 3D models including the cartilage. To rule out size differences, a set of landmarks were defined on the distal femur to isotropically scale the 3D models to a standard size. A predefined series of landmark-based reference planes were applied on the distal femur. With these landmarks and reference planes, a series of previously described characteristics associated with trochlear dysplasia as well as a series of morphometric characteristics were measured.

RESULTS

For the previously described characteristics, the analysis replicated highly significant differences between trochlear dysplastic and normal knees. Furthermore, the analysis showed that, when knee size is taken into account, the cut-off values of the trochlear bump and depth would be 1 mm larger in the largest knees compared to the smallest knees. For the morphometric characteristics, the analysis revealed that the trochlear dysplastic femur is also characterised by a 10% smaller intercondylar notch, 6-8% larger posterior condyles (lateral-medial) in the anteroposterior direction and a 6% larger medial condyle in the proximodistal direction compared to a normal femur.

CONCLUSIONS

This study shows that knee size is important in the application of absolute metric cut-off values and that the posterior femur also shows a significantly different morphology.

Anatomical factors influencing patellar tracking in the unstable patellofemoral joint

PURPOSE

The current study was performed to relate anatomical parameters to in vivo patellar tracking for pediatric patients with recurrent patellar instability.

METHODS

Seven pediatric patients with recurrent patellar instability that failed conservative treatment were evaluated using computational reconstruction of in vivo patellofemoral function. Computational models were created from high-resolution MRI scans of the unloaded knee and lower-resolution scans during isometric knee extension at multiple flexion angles. Shape matching techniques were applied to replace the low-resolution models of the loaded knee with the high-resolution models. Patellar tracking was characterized by the bisect offset index (lateral shift) and lateral tilt. Anatomical parameters were characterized by the inclination of the lateral ridge of the trochlear groove, the tibial tuberosity-trochlear groove distance, the Insall-Salvati index and the Caton-Deschamps index. Stepwise multivariable linear regression analysis was used to relate patellar tracking to the anatomical parameters.

RESULTS

The bisect offset index and lateral tilt were significantly correlated with the lateral trochlear inclination (p≤0.002) and TT-TG distance (p<0.05), but not the Insall-Salvati index or the Caton-Deschamps index. For both the bisect offset index and lateral tilt, the standardized beta coefficient, used to identify the best anatomical predictors of tracking, was larger for the lateral trochlear inclination than the TT-TG distance.

CONCLUSION

For this population, the strongest predictor of lateral maltracking that could lead to patellar instability was lateral trochlear inclination.

LEVEL OF EVIDENCE

Diagnostic study, Level II.

A statistical shape model of trochlear dysplasia of the knee

BACKGROUND

Trochlear dysplasia is known as the primary predisposing factor for patellar dislocation. Current methods to describe trochlear dysplasia are mainly qualitative or based on a limited number of discrete measurements. The purpose of this study is to apply statistical shape analysis to take the full geometrical complexity of trochlear dysplasia into account.

METHODS

Statistical shape analysis was applied to 20 normal and 20 trochlear dysplastic distal femur models, including the cartilage.

RESULTS

This study showed that the trochlea was anteriorized, proximalized and lateralized and that the mediolateral width and the notch width were decreased in the trochlear dysplastic femur compared to the normal femur. The first three principal components of the trochlear dysplastic femurs, accounting for 79.7% of the total variation, were size, sulcus angle and notch width. Automated classification of the trochlear dysplastic and normal femora achieved a sensitivity of 85% and a specificity of 95%.

CONCLUSIONS

This study shows that shape analysis is an outstanding method to visualise the location and magnitude of shape abnormalities. Improvement of automated classification and subtyping within the trochlear dysplastic group are expected when larger training sets are used.

CLINICAL RELEVANCE

Classification of trochlear dysplasia, especially borderline cases may be facilitated by automated classification. Furthermore, the identification of a decreased notch width in association with an increased sulcus angle can also contribute to the diagnosis of trochlear dysplasia.

The "sliver sign": a specific radiographic sign of acute lateral patellar dislocation

OBJECTIVE

The objective of this work is to assess the prevalence of the sliver sign, defined as an intraarticular linear or curvilinear ossific density, in association with knee effusion in patients with acute knee trauma, as a predictor of recent lateral patellar dislocation (LPD).

MATERIALS AND METHODS

A retrospective radiology database search for the term 'patellar dislocation' on MRI knee exams performed at our institution over a 7-year period identified 216 studies. Of these, 142 exams met true positive gold standard diagnostic criteria for LPD. Imaging findings of both the retrospectively identified radiographs and subsequent MRI were recorded. Accuracy of radiographic interpretation was also analyzed.

RESULTS

After review by an experienced musculoskeletal radiologist, 27 patients (19%) with LPD had knee radiographs demonstrating intraarticular osseous fragments. The majority of these patients had fragments (22/27, 81%) that were linear or curvilinear in configuration. A smaller subset of patients had fragments (5/27, 19%) that were rounded or oblong, felt to most likely represent sequelae of chronic dislocation. The fragments were identified on the patellar view only in eight of 27 patients (30%). These fragments were often initially misinterpreted, with the diagnosis explicitly stated in the report in 2/27 cases (7%). More often, the fractures were described vaguely (13/27, 48%), interpreted incorrectly (6/27, 22%), or missed (6/27, 22%). All 27 patients had moderate or large knee joint effusions.

CONCLUSIONS

In the setting of acute knee trauma, knee radiographs demonstrating a joint effusion and an intraarticular sliver-like osseous fragment correlate with recent lateral patellar dislocation. The routine trauma knee radiographic series does not include a patellar view but probably should, especially in young patients.

Effects of medial patellofemoral ligament reconstruction on patellar tracking

PURPOSE

Medial patellofemoral ligament (MPFL) reconstruction has been performed to treat recurrent patellar dislocation. However, the effects on patellar tracking have not been well documented, particularly in patients. The purpose of this study is to compare patellar tracking pattern and chondral status at MPFL reconstruction with those at second-look arthroscopy.

METHODS

Between 1999 and 2008, 71 patients with recurrent patellar dislocation underwent MPFL reconstruction using a double-looped semitendinosus tendon. Of these, 25 knees in 24 patients underwent second-look arthroscopy (at 6-26 months after initial surgery), forming the subject for the present study. No other surgical procedures such as tibial tuberosity transfer, lateral release, or osteotomy were performed in any patients. To assess the patellar tracking pattern, the position of the patella on femoral groove was evaluated arthroscopically during passive knee motion through lateral suprapatellar portal.

RESULTS

Before MPFL reconstruction, the patella in all patients was shifted laterally throughout the entire range of knee motion. Immediately after MPFL reconstruction, patellar malalignment was corrected in all cases. On second-look arthroscopy, two different patellar tracking patterns were observed. In 9 knees, the patella was located on the center of the femoral groove throughout the range of motion. Meanwhile, in the remaining 16 knees, the patella was shifted laterally at knee extension and migrated to the center of femoral groove with increased knee flexion. No significant deteriorations in chondral status were seen on second-look arthroscopy.

CONCLUSION

The present study revealed that not all improved patellar trackings after MPFL reconstruction remained intact at follow-up. Chondral status in patellofemoral joint was not aggravated by MPFL reconstruction.

LEVEL OF EVIDENCE

Therapeutic studies, Level IV.

Outcomes after patellar realignment surgery for recurrent patellar instability dislocations: a minimum 3-year follow-up study of children and adolescents

BACKGROUND

The purpose of this study was to report early outcomes after patellar realignment surgery (PRS) was performed for patellar instability in children and adolescents.

METHODS

The study cohort consisted of 23 patients (27 knees) who had PRS between March 1, 2000 and July 30, 2004, by a single surgeon, with a minimum 3-year follow-up and validated outcome measures. Preoperative diagnoses were dislocations in 22 knees and subluxations in 5 knees. The mean age was 14 years and 1 month.

RESULTS

Postoperatively, 2 knees had possible recurrent dislocation/subluxation episodes; neither required surgery. Persistent patellofemoral mechanical symptoms have been reported in 10 knees. At a mean follow-up of 5 years and 1 month, the mean Lysholm score was 69.3 and the mean International Knee Documentation Committee (IKDC) score was 65.6. The mean Tegner activity level was 5.4. Patients reported improvement in knee function and pain in 26 of 28 knees (93%). Significant group differences were identified for age at surgery, preoperative diagnosis, and length of preoperative knee symptoms. The group with scores ≥ 70 were younger (13.1 vs. 14.9 y), were all patellar dislocators (vs. 66% in the < 70 group), and with shorter length of symptoms (13 mo vs. 24 mo). In the ≥ 70 groups the Tegner activity levels were higher at 6.8 versus the < 70 group at 4.3.

CONCLUSIONS

At a mean of 5 years after PRS, 93% of patients reported persistent improvement in knee function and pain, and recurrent patellar dislocation/subluxation episodes were infrequent (7%). Despite the low rate of postoperative patellar instability the patients' subjective opinion of knee function was less than expected (mean IKDC 65.5). Further investigation is warranted to identify causes for this suboptimal outcome at a mean of 5 years postoperatively.

CLINICAL RELEVANCE

These data reinforce the recurring theme documented in other areas of orthopaedics of a disconnection between the "surgical" success (93% improved) and patient's subjective outcome (mean IKDC of 65.6).

LEVEL OF EVIDENCE

Level III.

Relationship between three-dimensional geometry of the trochlear groove and in vivo patellar tracking during weight-bearing knee flexion

It is widely recognized that the tracking of patella is strongly influenced by the geometry of the trochlear groove. Nonetheless, quantitative baseline data regarding correlation between the three-dimensional geometry of the trochlear groove and patellar tracking under in vivo weight-bearing conditions are not available. A combined magnetic resonance and dual fluoroscopic imaging technique, coupled with multivariate regression analysis, was used to quantify the relationship between trochlear groove geometry (sulcus location, bisector angle, and coronal plane angle) and in vivo patellar tracking (shift, tilt, and rotation) during weight-bearing knee flexion. The results showed that in the transverse plane, patellar shift was strongly correlated (correlation coefficient R=0.86, p<0.001) to mediolateral location of the trochlear sulcus (raw regression coefficient β(raw)=0.62) and the trochlear bisector angle (β(raw)=0.31). Similarly, patellar tilt showed a significant association with the trochlear bisector angle (R=0.45, p<0.001, and β(raw)=0.60). However, in the coronal plane patellar rotation was poorly correlated with its matching geometric parameter, namely, the coronal plane angle of the trochlea (R=0.26, p=0.01, β(raw)=0.08). The geometry of the trochlear groove in the transverse plane of the femur had significant effect on the transverse plane motion of the patella (patellar shift and tilt) under in vivo weight-bearing conditions. However, patellar rotation in the coronal plane was weakly correlated with the trochlear geometry.

Are current total knee arthroplasty implants designed to restore normal trochlear groove anatomy?

Biomechanical studies have shown that external rotation of the femoral TKA component improves patellar tracking but does not restore it to physiologic values. We hypothesized that this could be due to differences in the trochlear groove geometry of TKA and normal knees. This was investigated via a virtual TKA procedure that mounted femoral components on to 3-dimensional models of healthy femurs, followed by measurement of the trochlear geometry before and after the simulated TKA. The results showed that (1) external rotation of the component brought the trochlear groove closer to normal anatomy than no external rotation; (2) however, even with external rotation, the trochlear anatomy was only partially restored to normal. Further work is needed to determine implications for patellofemoral complications observed with current TKA designs.

The role of trochlear dysplasia in patellofemoral instability

Trochlear dysplasia is characterized by abnormal trochlear morphology and a shallow groove. It is associated with recurrent patellar dislocation, but it is unclear whether the dysplasia is congenital, the result of lateral tracking and chronic instability, or caused by a combination of factors. Lateral radiographs elucidate the crossing sign and characteristic trochlear prominence. Recurrent patellofemoral instability is multifactorial, and each component must be considered in determining treatment. Managing other factors associated with recurrent instability may compensate for a deficient trochlea and provide stability. Medial patellofemoral ligament reconstruction is recommended for patellofemoral instability in the presence of trochlear dysplasia in patients without patella alta or increased tibial tubercle-trochlear groove distance. Trochleoplasty should be reserved for severe dysplasia in which patellofemoral stability cannot otherwise be obtained.

[Imaging examinations of the patellofemoral joint]

Lesions in the patellofemoral region can be caused by trauma, chronic overloading, and especially regarding cartilage alterations by normal aging or pathologic processes. Very commonly these lesions lead to early arthrosis. An accurate clinical evaluation in all these patients is recommended. The combination of clinical information and radiological examinations should end up with an exact diagnosis.As part of the radiological evaluation of complaints of the patellofemoral region MR imaging is of special value since this method allows direct visualization of all intra- and extra-articular structures and their alterations, ultimately aiding in planning sufficient therapy. Moreover, is it possible to exclude pathology by MR imaging, which helps to prevent useless treatment and surgical procedures.

The effect of trochleoplasty on patellar stability and kinematics

Objective patellar instability has been correlated with dysplasia of the femoral trochlea. This in vitro study tested the hypothesis that trochleoplasty would increase patellar stability and normalise the kinematics of a knee with a dysplastic trochlea. Six fresh-frozen knees were loaded via the heads of the quadriceps. The patella was displaced 10 mm laterally and the displacing force was measured from 0° to 90° of flexion. Patellar tracking was measured from 0° to 130° of knee flexion using magnetic sensors. These tests were repeated after raising the central anterior trochlea to simulate dysplasia, and repeated again after performing a trochleoplasty on each specimen. The simulated dysplasia significantly reduced stability from that of the normal knee (p < 0.001). Trochleoplasty significantly increased the stability (p < 0.001), so that it did not then differ significantly from the normal knee (p = 0.244). There were small but statistically significant changes in patellar tracking (p< 0.001).

This study has provided objective biomechanical data to support the use of trochleoplasty in the treatment of patellar instability associated with femoral trochlear dysplasia.

Assessment of the "functional length" of the three bundles of the anterior cruciate ligament

The objectives of the present study are to compare "functional length" between the three bundles of the anterior cruciate ligament (ACL) from extension to deep flexion and to perform a sensitivity study on the patterns of "functional length" change due to deviations in insertion site selection. Nine knees of healthy volunteers were examined using a horizontal open magnetic resonance apparatus with the knee at 0 degrees -150 degrees of flexion and created three-dimensional (3D) virtual models of the knee. The femoral and tibial attachment sites of the three ACL bundles were determined, and the distance between the attachment sites was automatically calculated as the "functional length" of each bundle in each position. After changing the attachment sites, computer simulation was performed to investigate in vivo "functional length" changes for the three bundles of the ACL in normal knee kinematics from extension to deep flexion. Three bundles statistically significantly changed in "functional length" during flexion/extension, while they were longest at full extension, and decreased with increasing knee flexion to 100 degrees . They were smallest at flexion of 100 degrees , with increase lengths with flexion deeper than 100 degrees . Deviation of 6 mm of insertion site beyond attachment area caused significant alteration in the pattern of "functional length" change of each bundle, while deviation of 3 mm within attachment area caused no significant difference. The "functional length" of the three ACL bundles was not isometric, not even the antero-medial bundle. The "functional length" might be of major importance in terms of the site of the ACL insertion. The "functional length" of multiple bundles of the ACL and the pattern of their changes are useful for not only graft choice and preparation but also knee angle and initial tension at graft fixation during multiple-bundle ACL reconstruction surgery.