Feature Article 

Correlating Anatomy and Congruence of the Patellofemoral Joint With Cartilage Lesions

Bin Yang, PhD; Hongbo Tan, PhD; Lui Yang, MD; Gang Dai, PhD; Botao Guo, PhD

  • Orthopedics
  • January 2009 - Volume 32 · Issue 1

No abstract available for this article.

Abstract

Cartilage lesion of the patellofemoral joint is a common and challenging disease of the knee and an important cause of anterior knee pain. There are many naturally occurring variations in the anatomy and congruence of the patella and femoral trochlea. The purpose of this study was to identify the variations in patellofemoral anatomy and congruency that predispose to cartilage lesions. Among patients who underwent knee arthroscopy in our center from January 2005 to December 2006, 111 patients with chronic patellofemoral cartilage lesions and anterior knee pain were selected as the lesion group, while 124 patients with isolated meniscus rupture without anterior knee pain were selected as the control group. Twenty-one parameters measured on magnetic resonance images were used to assess the patellofemoral anatomy and congruence. A binary logistic regression model was used to look for possible associations between each of these parameters and the occurrence of patellofemoral cartilage lesions. The Bonferroni correction with a type I error rate of 0.0024 (0.05/21) was adopted to indicate statistical significance. Based on examination of the patellofemoral anatomy, 4 parameters were significantly associated with patellofemoral cartilage lesions. These were the patella lateral facet width, patella lateral facet ratio, sulcus depth and sulcus relative depth (P for linear trend <.0024). for="" patellofemoral="" congruence,="" 3="" parameters="" were="" significantly="" associated="" with="" patellofemoral="" cartilage="" lesions.="" these="" were="" the="" lateral="" patella="" displacement,="" patella="" epicondylar="" axis="" angle="" and="" congruence="" angle="">P for linear trend <.0024). among="" the="" many="" kinds="" of="" patellofemoral="" variations,="" several="" were="" found="" to="" correlate="" with="" the="" development="" of="" patellofemoral="" cartilage="" lesions.="" these="" problems="" could="" be="" important="" risk="" factors="" for="" patellofemoral="" cartilage="">

Cartilage lesion of the patellofemoral joint is a common and challenging disease of the knee and an important cause of anterior knee pain. Davies et al1 demonstrated that 32.7% of men and 36.1% of women older than 60 years suffer from cartilage lesions and osteoarthritis of the patellofemoral joint. Since articular cartilage has low regenerative ability, patients who have patellofemoral cartilage lesions usually experience anterior knee pain from activities that overload the patellofemoral joint, such as stair climbing, squatting, and prolonged sitting with flexed knees.2

There are many naturally occurring variations in the anatomy and congruence of the patella and femoral trochlea. These include variations in patella width, patella thickness, sulcus width, sulcus depth, etc, which involve bony constraints of the patella and the trochlea in addition to its cartilaginous layer.3 The anatomic morphology and the congruence of the patella and femoral trochlea are the biomechanical basis for knee flexion and extension.3 Abnormal anatomy and congruence of the patellofemoral joint may lead to abnormal distribution of patellofemoral stress and predispose patients to developing patellofemoral cartilage lesions.4,5 This suggests that certain variations of anatomy and congruence may cause patellofemoral dysfunction and abnormal distribution of stress, which then greatly increases the risk for cartilage lesions.

The precise relationship of patellofemoral anatomy and congruence with the development of cartilage lesions remains unclear. The lack of this basic knowledge impedes understanding of the functional anatomy of the patellofemoral joint.3 To identify the causes of patellofemoral cartilage lesions, it is necessary to identify the particular variations of patellofemoral anatomy and congruence that are associated with lesions of patellofemoral cartilage.

Compared to radiography and computer tomography (CT), magnetic resonance imaging (MRI) is a more effective method for evaluating the anatomy and congruence of the patellofemoral joint. Since cartilage surface and subchondral bone surface of the patellofemoral joint are not parallel to each other, the articular cartilage greatly improves the congruence of the patellofemoral bone structures6,7 and plays an important biomechanical role in patellofemoral joint function. Radiographs can reveal only bone structure, not the cartilaginous layer; moreover, bone structures overlap on radiographs.

Although the patellofemoral joint can be observed with multiple orientations by CT, chondral morphology and pathological lesions cannot. All of these factors contribute to the accuracy of a given method in characterizing the patellofemoral joint. In contrast, MRI is superior for detecting the morphology of cartilage in vivo. Magnetic resonance imaging not only allows observation of the patellofemoral joint from multiple directions, but also clearly shows the cartilage and surrounding soft tissues of the joint.8 In addition, MRI can accurately reveal the contact area between the patella and the femoral tronchlea.9 Therefore MRI is now increasingly used to examine the patellofemoral joint.

The purpose of this study was to determine whether particular morphologies and congruencies of the patellofemoral joint correlate with the lesions of its cartilage and therefore represent risk factors for developing such lesions. We used MRI to study the anatomy and congruence of the patellofemoral joint in patients who had suffered cartilage lesions and patients who had not. Knee arthroscopy was used as the gold standard for diagnosing cartilage lesions. This study marks the first step in our mission of investigating the functional anatomy of the patellofemoral joint. Understanding which anatomic variations correlate with disease will, therefore, help us identify risk factors for developing patellofemoral cartilage lesions. Such an understanding will be beneficial in preventing and treating patellofemoral disorders.

Materials and Methods

Among patients who underwent knee arthroscopy in our center from January 2005 to December 2006, those with chronic patellofemoral cartilage lesions and anterior knee pain were selected as the lesion group, while those with isolated meniscus rupture without anterior knee pain were selected as the control group. Patients with a history of acute knee trauma, acute patella dislocation, knee joint tumor, cruciate ligament injury, rheumatic arthritis, infective arthritis, crystal arthritis, or pigmented villonodular synovitis were excluded from the study. Patients whose osteoepiphysis had not closed or patients whose patellofemoral joints were not clearly visible by MRI were similarly excluded. A total of 235 patients were included in the study. Informed consent from patients was not required for this retrospective review.

In the lesion group, the lesions were either focal or diffuse defects of the patellofemoral cartilage in each or both of the patellar and femoral trochlear surfaces. Patients with isolated patellofemoral osteoarthritis were included in the study, but patients with concurrent tibiofemoral osteoarthritis were excluded. There were 111 patients in the lesion group. Among them, 56 left and 55 right knees were affected. These patients included 44 men and 67 women whose ages ranged from 21 to 83 (mean, 49.71) years. Their past histories of anterior knee pain were also recorded. In this group, 19 patients suffered from isolated patellofemoral osteoarthritis, and 43 patients showed additional meniscus ruptures.

The control group contained 124 patients, with 47 left and 77 right knees affected. These patients included 57 men and 67 women whose ages ranged from 20 to 71 (mean, 35.71) years. The medical histories of these patients contained no reports of anterior knee pain.

The 0.2 T Artoscan C scan system (Esaote, Genova, Italy), which is specialized for the extremities, was used with the knee joint coil. All of the patients in both groups were examined with MRI preoperatively. During the scanning, every patient lay supine with the affected knee straightened and neutrally rotated. The diseased leg was then immobilized. Sagittal, coronal, and transectional MRI views were obtained. Several types of images were obtained: sagittal turbo spin echo sequence images (T2-weighted: TR/TE 3000/80 ms), coronal gradient echo images (T2*-weighted: TR/TE 560/20 ms; flip angle, 35°), and transectional gradient echo images (T1-weighted: TR/TE 480-560/16-20 ms; flip angle, 90°). The field of view was 12 cm, and the matrix was 192×176. The excitation frequency was 1 or 2. The slice thickness for all sequences was 4 mm with an interval gap of 0.4 mm.

A total of 21 parameters were used to assess patellofemoral anatomy and congruence. Measurements of these parameters were performed with eFilm Workstation software (Version 1.9.6, Merge Healthcare, Milwaukee, Wisconsin). When the chondral surface was involved in any parameter, the contour of the chondral surface was measured as a reference instead of the subchondral bone.

Parameters Used to Assess Patellar Anatomy

To assess patellar anatomic morphology, 8 parameters were used. All 8 parameters were measured on the patellar midtransversal layer of the transectional MRIs, where the patellar maximal transverse diameter was visible. The 8 parameters were the patella angle, patella width, patella thickness, patella lateral facet width, patella facet thickness (Figure 1, Table 1), patella lateral facet ratio, patella relative thickness, and patella facet thickness ratio (Table 1).

To assess femoral trochlear anatomic morphology, 7 parameters were used. All 7 parameters were measured on the initial layer of the transectional MRIs, where the femoral epicondylar axis was clearly visible from the proximal to the distal portions of the patellofemoral joint. The 7 parameters were the sulcus angle, sulcus width, sulcus depth, sulcus lateral facet width, trochlea epicondylar axis angle (Figure 2, Table 2), sulcus lateral facet ratio, and sulcus relative depth (Table 2).

Figure 1: Diagram of several parameters used to assess patella anatomy on transectional MR images

Figure 2: Diagram of several parameters used to assess the femoral trochlear anatomy on the transectional MR images

Figure 1: Diagram of several parameters used to assess patella anatomy on transectional MR images. These include the patella angle (PA), patella width (PW), patella thickness (PT), patella lateral facet width (PLFW), and patella facet thickness (PFT). O is the patellar central point. A is the point of the patellar central ridge. B is the patellar anterior point. Figure 2: Diagram of several parameters used to assess the femoral trochlear anatomy on the transectional MR images. These include the sulcus angle (SA), sulcus width (SW), sulcus lateral facet width (SLFW), sulcus depth (SD), and trochlea epicondylar axis angle (TEAA). O is the sulcus central point. A and B are the most lateral and medial points of the femoral trochlea, respectively. C and D are the lateral and medial epicondyles, respectively. The trochlea epicondylar axis angle (TEAA) is the angle between the line AB and the line CD.


Parameters Used to Assess the Femoral Trochlear Anatomy

To assess patellofemoral congruence, 6 parameters were used. Three parameters were measured on the initial layer of the transectional MRIs, where the femoral epicondylar axis was clearly visible from the proximal to the distal portions of the patellofemoral joint. The 3 parameters were the lateral patella displacement, patella epicondylar axis angle (Figure 3, Table 3), and congruence angle (Figure 4, Table 3). Among the 6 parameters, 1 was measured on the sagittal images where the patellar maximal length was visible. This parameter was the Insall-Salvati index, which is the ratio of patella tendon length to the patella length (Figure 5, Table 3). The other 2 parameters were the width congruence and the depth congruence. These 2 parameters can be derived from the others, so they do not require any direct measurements (Table 3).

Figure 3: Diagram of two parameters used to assess the patellofemoral congruence on the transectional MR image

Figure 4: Diagram of the congruence angle

Figure 3: Diagram of two parameters used to assess the patellofemoral congruence on the transectional MR image. These include the patella epicondylar axis angle (PEAA) and lateral patella displacement (LPD). A and B are the most lateral and medial points of the patella, respectively. C and D are the lateral and medial epicondyles, respectively. Figure 4: Diagram of the congruence angle (CA), which is indicated as COD. O is the sulcus central point. A and B are the most lateral and medial points of the femoral trochlea, respectively. AOB is the sulcus angle (SA). OC is the line connecting the point of patellar central ridge and the sulcus central point. OD is the line bisecting SA.


Figure 5: Diagram of the Insall-Salvati index (ISI), including the patellar length (PL) and patellar tendon length

Figure 5: Diagram of the Insall-Salvati index (ISI), including the patellar length (PL) and patellar tendon length (PTL).

After a training session, 2 of the authors (B.Y., H.T.) measured all the parameters repeatedly and separately to estimate intra- and inter-observer reliability. The intra- and inter-observer intraclass correlation coefficient values for all of these parameters were between 0.72 and 0.92 and between 0.75 and 0.92, respectively. Subsequently, the other MRIs were retrospectively measured by 1 of the authors (B.Y.), who was blinded to the arthroscopic findings and clinical histories of patients in both groups.

Parameters Used to Assess the Patellofemoral Congruence

Differences in age and gender between the two groups were analyzed using the t test and x2 test, respectively.

A binary logistic regression model was used to look for possible associations between each of the 21 parameters and the occurrence of patellofemoral cartilage lesions. First, each of the 21 parameters was divided into quartiles according to the range of that particular parameter in both the lesion and control groups. Then, patients in each of the 2 groups were assigned numerical values from 1 to 4 to denote their designated quartile and were analyzed as ordered categories. Subsequently, potential associations between the quartiles of each parameter and the occurrence of patellofemoral cartilage lesions were analyzed using a binary logistic regression model. Finally, linear trends between each of the parameters and the occurrence of patellofemoral cartilage lesions were tested against each of the parameters as a continuous variable in the model. All models were adjusted according to the age and gender of the patients. Statistical analyses were performed with SAS software (release 8.2; SAS Institute Inc, Cary, North Carolina).

The analysis used absolute values for the trochlea epicondylar axis angle, congruence angle, and patella epicondylar axis angle. The Bonferroni correction with a type I error rate of 0.0024 (0.05/21) was adopted to indicate statistical significance.

Results

The two groups of patients showed a statistically significant difference in age (P=.000) but not in gender (P=.328) (Table 4).

Age and Gender Differences Between the Lesion and Control Groups

The relationship between all parameters and the occurrence of patellofemoral cartilage lesions is shown in Tables 5-7. Each table shows the number of measured knees in each quartile, the range of each parameter in each quartile, and the odds ratios and the P for trend of the model.

Based on examination of the patellar anatomy, 2 parameters were significantly associated with patellofemoral cartilage lesions. These were the patella lateral facet width (P for linear trend =.000) and patella lateral facet ratio (P for linear trend =.000).

Based on examination of the femoral trochlear anatomy, 2 parameters were significantly associated with patellofemoral cartilage lesions. These were the sulcus depth (P for linear trend =.000) and sulcus relative depth (P for linear trend =.001).

For patellofemoral congruence, three parameters were significantly associated with patellofemoral cartilage lesions. These were the lateral patella displacement (P for linear trend =.000), patella epicondylar axis angle (P for linear trend =.000) and congruence angle (P for linear trend = 0.000).

The remaining 14 parameters were not significantly associated with patellofemoral cartilage lesions.

Association Between Patellar Anatomy and the Occurrence of Patellofemoral Cartilage Lesions

Association Between Femoral Trochlear Anatomy and the Occurrence of Patellofemoral Cartilage Lesions

Association Between Patellofemoral Congruence and the Occurrence of Patellofemoral Cartilage Lesions

Discussion

Our investigation has identified several patellofemoral anatomic variations that are strongly associated with patellofemoral cartilage lesions. Cartilage lesion of the patellofemoral joint is one of the most important factors contributing to anterior knee pain10 and a common problem in orthopedics and sports medicine. Since cartilage does not regenerate easily, patients with cartilage lesions are predisposed to osteoarthritis. The findings in our study may help to identify the etiology of existing lesions, as well as those patients prone to suffer such lesions, in the future.

The patella is the biggest sesamoid bone, and it acts as a marker for the alignment of the extensor mechanism. In the presence of the Q angle and the tibiofemoral angle, the morphology and mechanics of the patellofemoral joint differ to a certain extent between the medial and lateral facets. Wiberg11 classified the patella anatomy into 3 types, and Baumgartl12 added a fourth type. In moving from type I to type IV, the lateral facet increasingly predominates over the medial. Furthermore, Reider et al13 found a correlation between a patellar shape with lateral predominance and a thicker lateral retinaculum in the lateral patellar tracking. Certain patellar types are thought to be associated with patellar dislocation, but neither Wiberg nor Baumgartl could definitively identify the relationship between patella type and cartilage lesions of the patellofemoral joint.

Our study showed that the patella lateral facet width and patella lateral facet ratio correlate significantly with patellofemoral cartilage lesions. Since patella width was similar in patients of both groups, the patella lateral facet width and the patella lateral facet ratio may reflect the patellar anatomical composition ratio with respect to the lateral and medial facets, ie, when the patella is dominated by a lateral articular facet, patellofemoral cartilage lesions are more likely to occur.

The femoral trochlea consists of the lateral and medial facets of the femoral sulcus, and it provides patellar tracking during knee flexion. The femoral epicondylar axis has been shown to be a reproducible reference for knee flexion and extension,14,15 making it an anatomical marker that indicates femoral condyles. Since the insufficiency of the trochlear depth is particularly significant in the proximal aspect of the femoral trochlea,16 our study measured all parameters relevant to femoral trochlear anatomy on the initial layer of the transectional images, where the femoral epicondylar axis was clearly visible from the proximal to distal portions of the patellofemoral joint.

With regards to femoral trochlear morphology, our study revealed that the sulcus depth and sulcus relative depth were significantly associated with patellofemoral cartilage lesions. Although the results did not show significant association between the sulcus angle and patellofemoral cartilage lesions with the strict Bonferroni correction, its P value for trend (.005) was nearly significant according to the Bonferroni correction. All these indicate that a femoral trochlear with a shallower groove can increase the risk of patellofemoral cartilage lesions.

It is thought that, in the presence of trochlear dysplasia, the stabilization of the patellofemoral joint constructed by bone and cartilage is weakened. This could easily lead to patellofemoral malalignment, patellar instability, dislocation, and, finally, breakdown of patellofemoral cartilage.17 This correlation is further supported by clinical experience. For patients with trochlear dysplasia, Schöttle et al18 found that patellofemoral stability cannot be completely restored merely by realigning the soft tissues and that only trochleaplasty can improve trochlear morphology and lead to successful clinical results.19

To assess patellofemoral congruence in the transectional images, we used the patella epicondylar axis angle, in addition to the lateral patella displacement and congruence angle. In previous reports, several parameters taking into account the lines connecting the anterior or posterior condyles of the femur, such as the lateral patellofemoral angle and the patella tilt angle, have been used to evaluate patella tilt.20,21 The femoral epicondylar axis, in contrast, has been identified as the rotational axis of flexion and extension of the knee.14,15

Within the range of knee flexion and extension, the curve of the patellar tracking also occurs around the femoral condyles. Recently the patellar axis, which is the line connecting the most lateral and medial points of the patella in the transactional image, has been shown to correlate strongly with the femoral epicondylar axis in healthy individuals.22 Thus, our study investigated the relationship between patella epicondylar axis angle and the occurrence of patellofemoral cartilage lesions.

The patella enters the trochlear groove when the knee flexion angle is 20°to 30°, and is centered within the groove due to the bony constraints and the tension from the retinaculum and other soft tissues.23 It is thought that abnormalities in the patellar tilt and displacement can be detected effectively while the knee is fully extended or minimally flexed, which is when the patella is in its least stable position.24 Thus in our study, all patients were examined in the supine position, with the affected knee straightened and neutrally rotated. In addition, the parameters used to evaluate the patellofemoral congruence were measured in the most proximal transectional image on which the femoral epicondylar axis was clearly visible.

Our study revealed that the lateral patella displacement, congruence angle, and patella epicondylar axis angle were significantly associated with patellofemoral cartilage lesions. These results indicate that patella tilt or lateral patella displacement may increase the risk of patellofemoral cartilage lesions. It is thought that the distribution of stress in the patellofemoral joint changes as a result of patella tilt and lateral patella displacement. Thus, the lateral patellofemoral cartilage is susceptible to injury by excessive compression and shear stress, and the medial patellofemoral cartilage may be destroyed by malnutrition and decreased stress. Nevertheless, a realignment procedure may rebalance the stress distribution and relieve the symptoms of patella maltracking.25

Our study also looked for a correlation between patellofemoral cartilage lesions and additional parameters that we suspected of affecting the biomechanics and distribution of stress on the patellofemoral joint. Although the results did not show a relationship between these remaining parameters and patellofemoral cartilage lesions, there were, nevertheless, interesting differences in the parameters. One example is the Insall-Salvati index, which has been shown to be a reliable and important parameter for evaluating patellar height26 since the patella alta and baja may cause patellar dislocation, patellofemoral damage, and anterior knee pain.27

In our study, patients with patellofemoral cartilage lesions were randomly assigned to the lesion group irrespective of the patella alta and baja. The percentage of patella alta or baja was lower in the lesion group, and the Insall-Salvati index did not differ significantly between the two groups. However, for other parameters, despite their variations, they do not appear to affect the risk of patellofemoral cartilage lesions.

Although our study makes important advances in the understanding of patellofemoral lesion, it is limited in some respects. First, owing to the retrospective nature of this study, there was no standardization on the position of the knee for MRI, and the MRI procedures differed slightly among the patients. Second, a certain number of patients in both the lesion and control groups also displayed meniscus rupture. Although a large cohort study has shown no association between patellofemoral cartilage lesions and meniscus ruptures,28 it is possible that the meniscus ruptures are a confounding variable. Third, as the MRI procedures were not performed in the weight-bearing condition, some patients with patellofemoral malalignment may have been ignored in our study.

Conclusion

Among the many kinds of patellofemoral anatomic variations, several were found to correlate with the development of patellofemoral cartilage lesions. A patella with a larger lateral facet and a femoral trochlea with a shallower sulcus may be important risk factors for developing patellofemoral cartilage lesions. Patellofemoral malalignments such as patella tilt and lateral patella displacement may also cause breakdown of the patellofemoral cartilage. Further studies should be conducted to explore biomechanical differences in the patellofemoral joint for different configurations of anatomy and congruence. Finally, patients with abnormal anatomy and congruence of the patellofemoral joint should have long-term follow-up to confirm the results of our study.

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Authors

Drs Yang (Bin), Tan, Yang (Liu), and Dai are from the Center for Joint Surgery, Southwest Hospital, the Third Military Medical University, and Dr Guo is from the Department of Medical Statistics, the Third Military Medical University, Chongqing, PR China.

Drs Yang (Bin), Tan, Yang (Liu), Dai, and Guo have no relevant financial relationships to disclose.

Correspondence should be addressed to: Liu Yang, PhD, Center for Joint Surgery, Southwest Hospital, The Third Military Medical University, Chongqing 400038, PR China.

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