Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI

The Egyptian Journal of Radiology and Nuclear Medicine xxx (2016) xxx–xxx

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Original Article

Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI Noha Mohamed Osman, M.D. ⇑, Samer M.B. Ebrahim, M.D. Department of Radiodiagnosis, Faculty of Medicine, Ain Shams University, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 22 April 2016 Accepted 28 September 2016 Available online xxxx Keywords: Patellar dislocation Patellofemoral Patellofemoral instability MRI Trochlear dysplasia Patella alta

a b s t r a c t Objective: To determine the contribution of MRI in evaluating patellofemoral instability (PFI) and to compare the underlying predisposing factors between the study and control groups. Materials and methods: We enrolled knee MRI scans of 38 patients with lateral patellar dislocation (LPD) and 38 control subjects. All MRI scans were examined for LPD and patellofemoral joint (PFJ) morphological abnormalities. The lateral trochlear inclination angle, sulcus angle and trochlear depth were the MR measures for trochlear dysplasia (TD), patellar height ratio was used for evaluation of patella alta, the patellar tilt angle, and the tibial tuberosity-trochlear groove (TT-TG) distance were also measured. Results: MRI confirmed PFI in 84.2% of study group and detected abnormal PFJ morphological factors in all cases. The prevalences and mean values of all MR parameters showed significant differences (p < 0.001) between the study and control groups. MR parameters for TD had the highest sensitivity of 57.9%, while the TT-TG distance was the most specific 97.4% for PFI. The prevalence of combined PFJ abnormal morphological factors was 36.8% in the study group. Conclusion: MRI was useful in quantitative measuring of the predisposing factors contributing to PFI resulting in significant difference in all MR parameters between the study and control groups. Ó 2016 The Egyptian Society of Radiology and Nuclear Medicine. Production and hosting by Elsevier. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction The normal physiology of the patellofemoral joint (PFJ) is highly dependent on the harmonious relationship of the patella with the femoral trochlear groove. Alterations in the surface geometry of the PFJ are associated with multiple clinical problems commonly patellofemoral instability (PFI) which leads to recurrent patellar dislocations [1].

Peer review under responsibility of The Egyptian Society of Radiology and Nuclear Medicine. ⇑ Corresponding author at: 81 Mohy Eldeen Abdel Hameed, 8th district, Nasr City, Cairo, Egypt. E-mail addresses: [email protected] (N.M. Osman), [email protected] (S.M.B. Ebrahim).

Patellofemoral instability is a challenging clinical condition for orthopedists, not only because of the diversity and complexity of the causative factors but also because of the large number of therapeutic possibilities [2]. The incidence of primary PFI is estimated at 5.8 cases per 100,000 individuals. Patients from 10 to 17 years of age are at higher risk as the incidence increases to 29 cases per 100,000 individuals. Recurrent dislocations were reported to occur in 17% of all cases following a single episode [3]. After a second patellar dislocation, further recurrence increases to 50%. Casual management in cases of first-time patellar dislocation has typically been directed to non-operative treatment. While operative management has been addressed as being essential for patients who

http://dx.doi.org/10.1016/j.ejrnm.2016.09.020 0378-603X/Ó 2016 The Egyptian Society of Radiology and Nuclear Medicine. Production and hosting by Elsevier. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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experience recurrent patellar dislocations, patients’ symptoms often do not spontaneously resolve [4,5]. In the clinical settings, patellar dislocation can be difficult for precise diagnosis as the displaced patella usually recovers its normal alignment spontaneously, with only less than 10% of cases are presented with fixed lateral dislocation. Imaging in PFI is indispensable to diagnose the root causes and hence their adequate treatment. Moreover, in acute cases imaging is sometimes the only proof to provide the diagnosis of patellar dislocation [6]. In the majority of cases of PFI, several anatomic variables involving the PFJ have been shown to predispose to patellar dislocation and MPFL tear. Dejour et al. formerly identified trochlea dysplasia (TD), and increased patellar tilt, patella alta and abnormal tibial tuberosity-tibial groove distance (TT-TG distance) as the commonest predisposing factors [7]. Variable measurements were studied to assess for the different PFJ geometric morphological factors. Each methodology has its various strengths and weaknesses. Different surgical options are available to stabilize the PFJ and correct the predisposing factors. Recognition of these factors is crucial for designating the optimal therapeutic management [8]. For many decades, radiography and computed tomography (CT) were the available imaging tools for evaluating patients of PFI. However, to date magnetic resonance imaging (MRI) has widely been used for investigating PFI after first-time or recurrent patellar dislocation. MRI is crucial for identification of many injuries not seen on radiographs particularly for detecting osteochondral lesions and for evaluating the soft tissue structures supporting the patella [9,10,8]. Moreover, MRI can be useful to assess PFJ morphological variants that may contribute to PFI [11]. The aim of this study was to determine the contribution of MRI in evaluation of patients with PFI and to compare the prevalence of underlying PFJ predisposing anatomic factors among patients affected with PFI and the control group.

2. Materials and methods 2.1. Patients selection This study took place in Ain Shams University Hospitals, in the period from November 2014 to March 2016. We prospectively included 38 consecutive knee MRI scans in 27 patients (18 males and 9 females; age ranged from 17 to 49 years, mean age 32 years) presented by clinically diagnosed single or recurrent episodes of non-traumatic patellar dislocation. Patients with previous knee surgical interventions or history of traumatic knee injuries were excluded. Patients were referred from the outpatient’s orthopedic clinic at our institution by a consultant orthopedist. MRI scans were done to confirm patellar dislocation/ instability and to detect any underlying PFJ morphological abnormalities predisposing to it. We also enrolled 38 consecutive knee MRI scans as control group of mean age 29.1 years (range, 14–56 years); patients of the control group were referred at the same study period. They underwent their knee MRI scans for sake of variable

conditions other than patellar dislocation with the absence of any doubtful history of patellar instability or anterior knee pain. 2.2. MRI knee scans 2.2.1. MRI technique All MRI knee scans were performed using a 1.5 T Achieva; Philips Medical Systems, Eindhoven, The Netherlands, by using knee coil. Patients were lying supine with the knees in the neutral position. The imaging protocol included Proton density fat suppressed images (PD-weighted-SPIR) in sagittal, coronal and axial planes (TR/TE = 3000/30), axial T2-weighted-SENSE (TR/TE = 6500–7500/100), coronal T1-weighted images (TR/TE = 500/17) and sagittal DualTSE SENSE images (TR/TE = 1732/7.1, 110). Section thickness was of 4 mm in all sequences. Field of view, 25 cm and matrix, 300  512. 2.2.2. Analysis of MRI images MRI scans of the study and control groups were interpreted by a consultant radiologist of 12 years experience in musculoskeletal imaging. He was blind to the clinical history and diagnosis of the patients. In all MRI scans, the interpreter reported for any knee MRI abnormalities especially those suggestive of underlying PFI. Patellar dislocation refers to abnormal medio-lateral displacement of the patella in relation to the femoral trochlea which was determined objectively through measuring the bisect offset (the percentage of the patella lateral to the midline of the femur) (Table 1) (13–19), medial patellofemoral ligament (MPFL) injury (sprain, partial or complete tear), characteristic bony contusions typical for patellar dislocation along the lateral femoral condyle and medial portion of patella and patellar chondral injuries. Then, we objectively evaluated our study and control groups for the presence of PFJ morphological abnormalities that could predispose to PFI. We selected the most reproducible MRI parameter/s for measuring each variable. Table 1 analyzes the technique and referenced normal/cut off values of all measurements carried out in our designed study. All measurements for the femoral trochlea were measured at the first craniocaudal axial image showing a complete cartilaginous cover of the trochlea and at the same time corresponding to the deepest part of the intercondylar groove (Roman arch appearance) [19]. For measuring MRI parameters that needed two different axial section images ex: tibial tuberosity-trochlear groove distance and in cases of measuring the patellar tilt or the bisect offset where the maximal transverse diameter of the patella was not at the same level of the plane of reference lines plotted at the determined femoral condyles plane, in such cases, we drew part of the needed reference lines of the measurement in one of either image, then copy it to the other one and complete the targeted measured variable. 2.2.3. Statistical analyses Statistical analyses were performed using SPSS software (version 11.0; Chicago, Ill). Sensitivity, specificity, positive

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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N.M. Osman, S.M.B. Ebrahim / The Egyptian Journal of Radiology and Nuclear Medicine xxx (2016) xxx–xxx Table 1 Technique and cutoff values of MRI parameters determining PFI in our study. Measured parameter

Technique

Cutoff value

Patellar dislocation: patellar lateralization (bisect offset)

Line 1 is drawn connecting the posterior femoral condyles. Project a perpendicular line 2 anteriorly to the deepest point of the trochlear groove. This line intersected with the patellar maximum width line 3. Bisect offset represents the extent of the patella lying lateral to the projected perpendicular line and was expressed as a percentage of total patellar width [12]

>65%

The angle (inclination angle) between a line drawn along the subchondral bone of the lateral trochlear facet and a tangential line connecting the posterior aspect of the femoral condyles [13] The angle between the slopes of the medial and lateral trochlea [14] A line drawn parallel to the posterior aspect of the femoral condyles serves as a reference line (line D). The lines drawn perpendicular to the reference line indicate the largest anteroposterior diameters of the lateral (line A) and medial (line C) trochlear facets and the deepest point of the sulcus (line B). Trochlear depth is calculated as follows: (A + C/2) B [15] In a sagittal image that best represented the patellar and patellar tendon lengths measure the patellar tendon–patella ratio [16] The axial images at which the position of the tibial tuberosity at the site of patellar tendon insertion (TT) and the trochlear groove (TG) are determined, and the distance between the TT and the TG is measured [17] Line 1 is drawn across the posterior margins of the medial and lateral condyles, and line 2 is drawn through the maximal transverse axis of the patella [18]

<11°

Trochlear dysplasia: – Lateral trochlear inclination angle

– Sulcus angle – Trochlear depth

Patellar height (Insall-Salvati index) Lateral forces of the patellar tendon acting on the patella (TT-TG distance) Patellar tilt (patellar tilt angle)

predictive values (PPV) and negative predicted values (NPV) were calculated for each MR parameter. The Student t test was used to compare mean values and percentages between the study and control groups. Statistical differences 60.001 were considered statistically significant. 3. Results Table 2 demonstrates the spectrum of MRI findings detected in our control group. In our study group (38 knees), lateral patellar dislocation as detected by the bisect offset was found in 12 (31.5%) knees (Figs. 1 and 2), while typical MRI findings suggestive of recent history of lateral patellar dislocation and confirming the existence of PFI were found in 32 (84.2%), i.e. transient lateral patellar dislocation was approved in 20 (52.6%), where the patella relocated into the femoral trochlea at the time of MRI examination. MRI findings suggestive of PFI are shown in Table 3. In the remaining 6 knees, there were no MRI findings supporting the diagnosis of current or previous patellar instability.

Table 2 Spectrum of findings in MRI scans of the control group (38 knees). MRI findings

No. (%)

Normal Meniscal tear ACL tear PCL tear MCL tear Knee effusion Bone contusions Arthritic changes Osteochondritis dissecans Osteonecrosis

11 (29) 18 (47.3) 10 (26.3) 6 (15.7) 19 (50) 11 (29) 15 (39.4) 20 (52.6) 4 (10.5) 7 (18.4)

>145° <3 mm

>1.2 >20 mm

>15°

However, in all cases of the study group, all knee MRI scans showed existing PFJ morphological abnormality that could preclude for patellar instability (Figs. 1 and 2). All cases in our study group had at least one morphological PFJ abnormality. All measured MR parameters in both groups of the study are demonstrated in Fig. 3 and are meticulously analyzed in Tables 4 and 5 in the study and control groups. Statistically significant differences (p < 0.001) were found between the two groups of the study regarding the prevalences and mean values of all measured MRI parameters (Tables 4 and 5). The prevalences of combinations of PFJ abnormal morphological abnormalities are detailed in Table 6.

4. Discussion In the setting of transient lateral patellar dislocation, at time of MRI examination the patella may correctly reengage into the trochlea. MRI confirmed the clinical diagnosis of patellar dislocation in 32 (84.2%) out of 38 of our study group. All 32 (84.2%) patients with MRI positive for patellar dislocation had bone contusions and MPFL injuries typical for recent event. Existing patellar dislocation as determined by the bisect offset was only evident in 12 (31.5%) cases, while in the remaining 20 (52.6%), patellar dislocation was confirmed only by recent bone contusions and MPFL injuries. Our results agreed with similar studies in which MPFL injuries were more useful in confirming transient patellar dislocation. Balcarek et al. [20] reported that more than 98.6% had MPFL complete or partial tears while results by Sillanpaa et al. [21] and Weber-Spickschen et al. [22] revealed all but 1 case sustained MPFL tear. Patellar chondral injury was detected in 12 (31.6%), femoral chondral injury in 4 (10.5%), intra-articular detached bony fragments in 6 (15.8%) and knee joint effu-

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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Fig. 1. Male patient, 31 years old, presented by acute on top of recurrent episodes of lateral patellar dislocation. MRI showed typical findings of lateral patellar dislocation in the form of lateral patellar displacement as measured by the bisect offset (>65%) (a), lateral femoral condyle and medial patellar bony contusions (arrowheads), stretched medial patellofemoral ligament with high signal at the patellar end denoting sprain (white arrow), intra-articular bony fragments (black arrow) and knee joint effusion (b). Patellar tilt was also noted as measured by the patellar tilt angle (34°) (c). Trochlear measurements revealed the presence of TD as measured by the lateral trochlear inclination (8.9°) (b), sulcus angle (155.2°) (d) and the trochlear depth (2.2 mm) (e). The TTTG distance was increased (23.2 mm) (f). Normal Insall-Salvati index indicating the absence of patella alta (g).

sion in 23 (60.5%). These findings also coincides with Souza et al. [23] who reported chondral injuries in 29%, detached bony fragments in 9% and joint effusion in 49% of his patients presented clinically with acute patellar dislocation. Moreover, MRI has been useful in detecting abnormal PFJ morphological factors in all cases of our study compared to 10 (26.3%) cases in the control group. Souza et al. [23] found abnormal PFJ predisposing factors in 73% of his study group while Steensen et al. [24] reported that abnormal anatomic factors were noted in 91.7% of their study group and 30% of the control group.

In our work, we have generally encountered wide variability in comparing our results to relevant studies. This may have been attributed to the variabilities in the sample sizes when dealing with prevalences/percentages. Also, this variability was evident when comparing numerical values of the measured MR parameters which could be due to racial/population differences in the threedimensional knee morphology as assumed by Mahfouz et al. [25]. We found TD to be the commonest (57.9%) PFJ abnormality, followed by patella alta (52.6%), abnormal patellar tilt (26.3) and abnormal TT-TG distance in 13.1% (Fig. 3).

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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Fig. 1 (continued)

Our results were comparable to those of Steensen et al. [24] who found out an incidence of 68.3% for TD (68.3%), patella alta (60.0%), increased TT-TG distance (42.0%) and abnormal patellar tilt in 26.7%. Souza et al. [23] reported that TD was present in 51% of the cases presented by patellar instability and was easily identified using MRI. The prevalences of all measured factors in our study together with their mean values showed statistically significant differences between our study and control groups (p < 0.001) (Fig. 3) (Tables 4 and 5). Our results agree with several studies [24,26–28], who also found a statistically significant difference between both groups. In this study, MR parameters for detecting TD had the highest sensitivity of 58% and negative predictive value (NPV) of 67.3% for PFI, and they also had specificity of 86.8%. There are considerable variations in the reported values in the literature; Esclaa et al. [11] found 71.2% sen-

sitivity and 76.7% specificity for lateral trochlear inclination, 85.7% and 71.7% for trochlear depth while Köhlitz et al. [28] found that trochlear depth had 64.5% sensitivity and 95.7% specificity while the lateral trochlear inclination angle had 26.9% sensitivity and 98.9% specificity. Pfirrmann et al. [29] found the trochlear depth <3 mm had sensitivity of 100% and was 96% specific. The patellar height ratio (Insall-Salvati index) as a measure for patella alta had the highest positive predictive value (PPV) of 90.9% in our study, 52.6% sensitivity and 94.7% specificity. Köhlitz et al’s [28] results were 68.3% sensitivity and 63.9% specificity for patellar height ratio while other studies [11,30,31] revealed sensitivity of 78% and specificity of 68%. TT-TG distance >20 mm was the most specific for PFI in our work accounting for specificity of 97.4% and 13.1% sensitivity. This coincides with Dejour et al. [7] who declared in their work that TT-TG distance of more

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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Fig. 2. Male patient, 23 years old, presented by recent lateral patellar dislocation. MRI approved lateral patellar displacement as measured by the bisect offset (>65%), focal thickening and altered signal was noted at the patellar articular cartilage (white arrow) (a). MRI revealed existing patella alta as measured by the Insall-Salvati index (1.64) (b) and the presence of abnormal patellar tilt as measured by the patellar tilt angle (41.9°) (c). No MRI evidence for the occurrence of TD as measured by the lateral trochlear inclination angle (d), sulcus angle (e) and trochlear depth (f) as they all yielded normal values, or the presence of abnormal TT-TG distance (g).

than 20 mm is nearly always associated with PFI. Also, Burmann et al. [32] concluded that TT-TG distance above 20 mm is a specific factor for PFI. Köhlitz et al. [28] found specificity of 98.4% and sensitivity of 15.6% for TT-TG distance. In this study patellar tilt angle had 21.0% sensitivity and 94.7% specificity; Escala et al. [11] reported 92.7% sensitivity and 63.3% specificity. Trochlear dysplasia was the commonest PFJ abnormality found in our study, and it was found in 57.9% of the study group and 13.1% of the control group. Our results are comparable to Steensen et al. [24] and Köhlitz et al. [28] who found out an incidence of 68.3% vs 5.8% and 66% vs 4.3% in the study and control groups respectively.

We examined for TD by measuring the lateral trochlear inclination, sulcus angle and trochlear depth. Their mean values showed wide variability between study and control group in several studies provided by the literature, however most studies resulted in significant statistical difference between both groups. We found mean values of 6.9°, 152.7° and 2.4 mm in our study group compared to 18.1°, 134.4° and 3.7 mm in the control group (Table 5). Charles et al. [27]. found mean values of 13.3° vs 21.3°, 148.5° vs 165.7° and 4.0 mm vs 6.5 mm in their study group compared to the control group respectively. Köhlitz et al. [28] found 14.7° vs 21.5° and 2.5 mm vs 4.5 mm for the lateral trochlear inclination and trochlear depth. Van

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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Fig. 2 (continued)

Table 3 MRI findings suggestive of PFI in our study group. MRI findings

No. (%)

Patellar dislocation Bony contusion at the medial patella Bony contusion at the lateral femoral condyle Medial patellar ligament sprain Medial patellar ligament partial tear Medial patellar ligament complete tear Patellar chondral injury Femoral chondral injury Detached bony fragments (rice bodies) Knee joint effusion

12 (31.5) 32 (84.2) 27 (71) 18 (47.4) 12 (31.6) 2 (5.2) 12 (31.6) 4 (10.5) 6 (15.8) 23 (60.5)

Huyssteen et al. [14] determined sulcus angle of mean values of 186.5° and 173° in the study and control groups while Koskinen et al. [33] and Lim and Lim [26] reported

mean values of 167° and 160° and 159.3° and 149.9° respectively. Escala et al. [11] and Carrillon et al. [13] found that the mean values of the lateral trochlear inclination angle were 16.5°, 9.6° and 16.9°, 6.2° in the study and control groups respectively. Patella alta was the second commonest PFJ morphological abnormality as detected in our study, and it was seen in 52.6% of our study group and in 5.2% of the control group compared to 60% and 20.8%, 48% and 12%, and 53.2% and 22.6% in the studies done by Steensen et al. [24], Neyret et al. [34] and Köhlitz et al. [28]. Souza et al. [23] reported in his results prevalence of 53% for patella alta among his study group. Mean values for Install Salvati index were 1.34 and 1.18 in our study and control groups respectively, and our results were comparable to several studies whose results were 1.4 and 1.23 [28], 1.35 and 1.11 [24], 1.26 and 1.08

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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N.M. Osman, S.M.B. Ebrahim / The Egyptian Journal of Radiology and Nuclear Medicine xxx (2016) xxx–xxx Table 5 Comparison of mean values of the MRI morphological and geometric parameters between the study and control groups.

Fig. 3. Prevalences of PFJ morphological abnormalities among both study and control groups.

[27], 1.23 and 1.07 [11], 1.17 and 1.02 [34] and 1.36 and 1.13 [26]. Patellar tilt as measured by the patellar tilt angle in our study showed incidence of 21.0% in our study group compared to 5.2% in the control group. Mean values of the patellar tilt angle were 42.7° in our study group and 9.45° in the control group. This coincides with 24.3° and 8.1°, and 21.7° and 9.2° in the studies performed by Charles et al. [27] and Escala et al. [11] respectively. Increased TT-TG distance was found in 13% of the study group and in 5.2% of the control group with mean values of 27.3 mm and 16.6 mm respectively. Köhlitz et al. [28] found an incidence of 15.6% vs 1.6% with mean values of 14.7 mm and 9.9 mm, and Steensen et al. [24] reported that an increased TT-TG distance was present in 41.7% of the knees in the study group, compared to 3.3% in the control group with mean values of 18.5 and 11.8 mm respectively. Also studies carried out by Charles et al. [27], Wittstein et al. [35] and Lim and Lim [26] revealed mean values of TT-TG distance of 18.7 and 11 mm, 12.6 and 9.4 mm, 13.66 and 7.29 mm in the study and control groups respectively.

MRI parameter

Study group (mean ± SD)

Control group (mean ± SD)

p

Lateral trochlear inclination angle (°) Sulcus angle (°) Trochlear depth (mm) Insall-Salvati index Patellar tilt (°) TT-TG distance (mm)

6.9 ± 2.6

18.1 ± 4.2

<0.001

152.7 ± 7.4 2.4 ± 0.4 1.34 ± 0.09 42.7 ± 6.5 27.3 ± 5.1

134.4 ± 4.1 3.7 ± 1.5 1.18 ± 0.04 9.45 ± 3.0 16.6 ± 4.1

<0.001 <0.001 <0.001 <0.001 <0.001

Twenty-four of our cases (63.2%) had one abnormal PFJ morphological factor, 11 (28.9%) had 2 abnormal factors and 3 (7.9%) had 3 abnormal factors (Table 6). Compared to the control group, 10 (26.3%) had single abnormal factor while no control subjects had combined factors (Table 6). In their study, Steensen et al. [24] included larger numbers of patients (60 patients) and control group subjects (120 subjects) compared to our study, they resulted for more combined factors in the study and control groups, 33.3% of the study group had 1 abnormal factor, 26.7% had 2 abnormal factors, 16.7% had 3 abnormal factors, and 15.0% had all 4 factors with abnormal values. On the other hand in their control group, 28.3% had 1 abnormal factor, 0.8% had 2 abnormal factors and 0.8% had 3 abnormal factors. The major strength of our study is that it is encountered among a very few number of studies measured for the sensitivity, specificity and predictive values of MR parameters that quantitatively measure for PFJ morphological abnormalities predisposing to PFI. The weaknesses of our study was the rather small number of the studied sample, single reader study, the subjects in the control group did not have completely normal knees; they were not asymptomatic volunteers but patients evaluated for other knee conditions. Another weakness of the study is the unequal time interval between the incidence of PFI and MRI scanning among patients.

Table 4 Distribution and statistical significance of the studied MR PFJ morphological abnormalities among the study group compared to the control group. Parameter

Trochlear morphology: – Lateral trochlear inclination angle – Sulcus angle – Trochlear depth Patellar height (Insall-Salvati index) Patellar tilt angle TT-TG distance

Study group (n = 38)

Control group (n = 38)

p

Sensitivity (%)

Specificity (%)

PPV (%)

NPV (%)

Abnormal (No. (%))

Normal (No. (%))

Abnormal (No. (%))

Normal (No. (%))

22 (57.9) 22 (57.9) 22 (57.9)

16 (42.1) 16 (42.1) 16 (42.1)

5 (13.1) 5 (13.1) 5 (13.1)

33 (86.7) 33 (86.7) 33 (86.7)

0.001 0.001 0.001

57.9 57.9 57.9

86.8 86.8 86.8

81.5 81.5 81.5

67.3 67.3 67.3

20 (52.6)

18 (47.3)

2 (5.3)

36 (94.7)

0.001

52.6

94.7

90.9

66.7

8 (21.0) 5 (13.1)

30 (79.0) 33 (86.7)

2 (5.3) 1 (2.6)

36 (94.7) 37 (97.4)

0.001 0.001

21.0 13.1

94.7 97.4

80.0 83.3

54.5 52.8

PPV = positive predictive value, NPP = negative predictive value, TT-TG = tibial tuberosity-trochlear groove.

Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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Table 6 Prevalences of combinations of PFJ abnormal morphological abnormalities. PFJ morphological abnormalities

Study group No. (%)

Control group No. (%)

None

0 (0)

28 (73.7)

1. PFJ abnormality:  Trochlear dysplasia  Patella alta  Abnormal patellar tilt  Increased TT-TG distance

24 (63.2) 12 (31.6) 9 (23.6) 1 (2.6) 0

26.3) 5 (13.1) 2 (5.3) 2 (5.3) 1 (2.6)

2. PFJ abnormalities:  Patella alta + increased TT-TG distance  Trochlear dysplasia + abnormal patellar tilt  Trochlear dysplasia + patella alta  Patella alta + abnormal patellar tilt (Fig. 2)

11 (28.9) 4 (10.5) 3 (7.9) 3 (7.9) 1 (2.6)

(0)

3. PFJ abnormalities:  Trochlear dysplasia + patella alta + abnormal  Patellar tilt (Fig. 1)  Trochlear dysplasia + abnormal patellar tilt + increased TT-TG distance

3 (7.9) 2 (5.3)

(0)

5. Conclusion This study signified the contribution of MRI in confirming the clinical diagnosis of PFI. MPFL injuries and typical bone contusions were more helpful than the bisect offset in confirming recent transient patellar dislocation. MRI was useful in quantitative measuring of the predisposing factors contributing to PFI resulting in significant difference in all MR parameters between the study and control groups. MR parameters for TD were the most sensitive 57.9% and TT-TG distance was the most specific 97.4% for PFI, while patellar height ratio (Insall-Salvati index) had the highest PPV accounting for 90.9%. The prevalence of combined PFJ abnormal morphological factors was 36.8%.

[9]

[10]

[11]

[12]

[13]

Conflicts of interest and source of funding [14]

None declared. [15]

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Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020

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Please cite this article in press as: Osman NM, Ebrahim SMB. Patellofemoral instability: Quantitative evaluation of predisposing factors by MRI. Egypt J Radiol Nucl Med (2016), http://dx.doi.org/10.1016/j.ejrnm.2016.09.020