Your Patella Dislocated: Will It Happen Again? Key Magnetic Resonance Imaging Differences Between Patients With Multiple Patellar Dislocations and Patients With a One-Time Patellar Dislocation

Abstract

Background:

Identifying anatomic risk factors for recurrent patellar dislocations can help guide clinical decision making and counseling on operative management for patients at the time of the initial dislocation.

Purpose:

To determine if there are significant differences in anatomic factors between patients with a one-time patellar dislocation (OTPD) and those with multiple patellar dislocations (MPDs).

Study Design:

Case-control study; Level of evidence, 3.

Methods:

Patients aged 9 to 21 years who had at least one patellar dislocation between 2012 and 2023 were retrospectively categorized into 1 of 2 groups (verified by telephone communication with at least 2 years’ follow-up): MPDs (≥2 patellar dislocations) and OTPD. Patient characteristics and several magnetic resonance imaging measurements, including proximal tibial tubercle–trochlear groove (pTT-TG) distance, Caton-Deschamps index (CDI), relative tibial external rotation (rTER), and patellar tilt (PT), were collected. Trochlear dysplasia was also assessed using the 2-image lateral trochlear inclination (LTI), sulcus angle (SA), and trochlear depth (TD). Multiple logistic regression was used to determine the association of anatomic risk factors with the likelihood of experiencing MPDs.

Results:

In total, 177 knees (104 female, 73 male) were included in analyses: 108 had MPDs, and 69 had an OTPD. Patients with MPDs had a significantly less TD (P < .001), greater SA (P < .001), greater CDI (P = .03), and greater PT (P = .001). LTI (P = .65), rTER (P = .72), and pTT-TG distance (P = .12) were not strongly associated with having MPDs. A multiple logistic regression model of SA and CDI found an area under the curve of 0.72 for having MPDs; a patient with an SA of 170° and a CDI of 1.5 had a .78 probability of experiencing MPDs.

Conclusion:

Measures of trochlear dysplasia (SA and TD), patella alta (CDI), and PT may be useful to distinguish between patients with MPDs and those with an OTPD. pTT-TG distance, LTI, and rTER were not found to have statistically significant differences between the groups. The combination of SA and CDI distinguished between the groups with reasonable accuracy (area under the curve = 0.72). Applying this model, a patient with an SA of 170° and a CDI of 1.5 would have a .78 probability of experiencing MPDs. This may be helpful in counseling patients after an initial dislocation event on the likelihood of experiencing MPDs.

Keywords

patella knee magnetic resonance imaging pediatric sports medicine patellar instability trochlear dysplasia

Patellar instability (PI) is a common and debilitating condition among pediatric and young adult patients, with an incidence of approximately 42 per 100,000.¹² While there are several anatomic indices that differentiate between knees without PI and those that are susceptible to PI, ongoing discussion remains about the factors that predict the likelihood of experiencing multiple patellar dislocations (MPDs) after a sentinel event. The treatment of a first-time dislocation is typically nonoperative (activity modification, bracing, physical therapy); recurrence rates based on the most recent literature have been reported to be as high as 70%.^{24,29,31,33,42} Experiencing MPDs can disrupt normal participation in activities and lead to poor long-term outcomes such as a predisposition to degenerative changes in the patellofemoral joint. A surgical intervention is typically indicated for patients with MPDs, with greater consideration among high-risk (younger age, bilateral instability, presence of anatomic risk factors) first-time dislocators.^25,28

Multiple bony factors can contribute to patellar maltracking and PI.^1,7,28 These factors are best assessed with magnetic resonance imaging (MRI), which is the standard of care because of its access and ability to evaluate chondral damage.³⁰ It is widely accepted that certain MRI findings, such as trochlear dysplasia, increased tibial tubercle–trochlear groove (TT-TG) distance, and patella alta, are pertinent to maltracking.⁴¹ A systematic review found trochlear dysplasia, increased TT-TG distance, and patella alta to be risk factors for recurrence.¹⁵ However, recent literature suggests that the TT-TG distance may not effectively differentiate knees by PI severity and, importantly, may be confounded by variations in knee rotation.³⁸ Relative tibial external rotation (rTER) is an indicator of knee rotation and has recently been implicated in PI literature, but its applicability for predicting recurrence has not yet been clearly delineated.^7,22 Lateral trochlear inclination (LTI), patellar tilt (PT), and patellar morphology are also considered to have some predictive value for PI.^16,25 However, evidence is mixed regarding the utility of LTI and PT for predicting the recurrence of dislocations.^2,3,20,42

The purpose of this study was to determine if there are significant differences in anatomic factors between patients with a confirmed one-time patellar dislocation (OTPD) and those with MPDs. The following hypothesis was examined: patients who experience MPDs will have significantly increased proximal TT-TG (pTT-TG) distance, patella alta, rTER, PT, and trochlear dysplasia, represented by sulcus angle (SA), LTI, and trochlear depth (TD) measurements, compared with those with only an OTPD.

Methods

After institutional review board approval (No. 2016-6534), a list of patients aged 9 to 21 years (age at the time of MRI) at our institution who were treated in the Department of Orthopedic Surgery for PI between 2012 and 2023 was compiled. Patients’ charts were reviewed to confirm an episode of a patellar dislocation based on patient history, documentation through emergency room visits, or MRI findings of bone marrow edema on the lateral femoral condyle and medial patellar facet, which is pathognomonic for a patellar dislocation. A chart review was also utilized to categorize patients into either the MPD or OTPD group. Those in the MPD group were confirmed to have had multiple episodes of patellar dislocations by a chart review or follow-up telephone call. Those in the OTPD group were called via telephone at final follow-up to confirm their status and the lack of a subsequent dislocation. The minimum follow-up time for patients was 2 years. Exclusion criteria included previous surgery and poor/absent MRI scans.

Age at first patellar dislocation, age at MRI, sex, and laterality were recorded. The following measurements were collected from MRI: pTT-TG distance, Caton-Deschamps index (CDI), rTER, SA, LTI, PT, and TD. The 3 measures of trochlear dysplasia, SA, LTI, and TD, were evaluated along the cartilaginous surface at the proximal trochlear groove.

MRI Measurements

Measurements were performed independently by 2 raters: a postgraduate fourth-year orthopaedic surgery resident and a third-year medical student, both of whom attended a video-recorded training session with the senior author (M.D.J.) and had regular check-ins. All MRI measurements for 30 randomly selected patients were recorded; the time between 2 sets of measurements, which were completed by both raters, for intraobserver reliability was 2 weeks. All patients’ MRI was performed at the same institution in a standard supine position with the knee scanned in the anteroposterior plane using a standard 16-channel knee coil to obtain the following sequences: coronal T1-weighted, coronal proton density–weighted fat-saturated, sagittal proton density–weighted, sagittal proton density–weighted fat-saturated, and axial proton density–weighted fat-saturated. The proximal trochlear groove was defined on the most proximal axial image for which the cartilaginous portion of the medial and lateral trochlear facets was visible; this cut was cross-referenced with the sagittal plane. This image was used for the pTT-TG distance, proximal SA, proximal LTI, and proximal TD.

There were 4 anatomic risk factors evaluated: CDI, pTT-TG distance, rTER, and PT (Figures 1 -4).^{4,10,13,14,21,39,40} rTER was measured using the 2-image technique as previously described.²¹ In addition, 3 measurements of trochlear dysplasia were analyzed: SA, TD, and LTI (Figures 5 -7).^{1,9,17,23,26,32} The 2-image LTI was utilized rather than the 1-image LTI because it is more reliable and better correlated with TD.¹⁷

Figure 1.

The Caton-Deschamps index (CDI) was defined as the ratio of the distance between the distal aspect of patellar articular cartilage to the anterosuperior corner of the tibial plateau (line A), divided by the length of the cartilaginous articular surface of the patella (line B).⁴

Figure 2.

The proximal tibial tubercle–trochlear groove (pTT-TG) distance was defined as the distance between the deepest portion of the trochlear groove and the midpoint of the tibial tubercle.^10,40

Figure 3.

Relative tibial external rotation (rTER) was defined as rotation of the posterior tibial condyle minus rotation of the posterior femoral condyle.²² (A) Tibial rotation was defined as the angle formed between a tangential line on the most well-defined posterior tibial condyle and a horizontal line. (B) Femoral rotation was defined as the angle formed between a tangential line on the most well-defined posterior femoral condyle and a horizontal line. A positive value for femoral rotation, tibial rotation, or rTER (tibial rotation minus femoral rotation) indicated external rotation. A negative value for any of these measurements indicated internal rotation.

Figure 4.

Patellar tilt (PT) was defined as the angle formed by a line connecting the maximum patellar width and a line formed by the best image of the posterior femoral condyle.^13,14

Figure 5.

The sulcus angle (SA) was defined as the angle formed by the lateral and medial trochlear facets. Each line was taken from the most prominent portion of the facet to the deepest point of the trochlear groove.³⁴

Figure 6.

Trochlear depth (TD) was defined as the distance of a line (solid) to a line connecting the most prominent portion of the lateral and medial trochlear facets (dashed) and the deepest portion of the trochlear groove.^9,26

Figure 7.

The 2-image lateral trochlear inclination (LTI) was defined as the angle formed between the lateral trochlear facet and the best image of the posterior femoral condyle.¹⁷

Statistical Analysis

Means, medians, ranges, standard deviations, and interquartile ranges were calculated for the continuous variables, and counts and frequencies were tabulated for categorical variables. The distributions of the continuous variables were evaluated for normality utilizing the Kolmogorov-Smirnov test. Age, pTT-TG distance, TD, PT, and CDI were normally distributed. rTER, LTI, and SA were not normally distributed. Differences in sex between groups were assessed with the chi-square test. Differences in age, pTT-TG distance, TD, PT, and CDI between patients with MPDs and those with an OTPD were assessed by a t test. Differences in rTER, LTI, and SA between patients with MPDs and those with an OTPD were assessed with the Mann-Whitney U test. The proportion of pathological SA (>145°) and TD (<3 mm) values, based on the literature, within each group was calculated and compared utilizing the chi-square test.^34,37 All statistical analyses were performed utilizing SPSS (IBM).

Logistic regression was used to determine the association of anatomic risk factors with the likelihood of experiencing MPDs. Any variable with a P value <.25 on initial analysis was a candidate for multiple logistic regression. The area under the curve (AUC) was used to assess the anatomic risk factor's ability to classify the likelihood of experiencing MPDs. A P value <.05 was considered statistically significant.

Interobserver and intraobserver reliability analyses were performed with SPSS using the intraclass correlation coefficient (ICC). An ICC <0.50 was considered poor reliability, 0.50-0.74 was considered moderate reliability, 0.75-0.90 was considered good reliability, and >0.90 was considered excellent reliability.¹⁹

Results

A total of 226 knees were identified that met inclusion criteria. Of these, 46 were excluded because of substandard MRI scans (poor resolution of relevant images that precluded reliable measurements, resulting in motion artifacts). An additional 3 were excluded because their OTPD status could not be confirmed. Thus, the final number of knees was 177: the MPD group included 108 knees (104 patients; 4 bilateral), and the OTPD group included 69 knees (68 patients; 1 bilateral) (Figure 8). Therefore, 61% of our patients had MPDs (defined as >1 dislocation). The mean follow-up time for the MPD group was 5.40 years (range, 2.10-11.43 years) and for the OTPD group was 4.75 years (range, 2.05-10.40 years). There was not a statistically significant difference in sex or age between the MPD and OTPD groups (Table 1).

Figure 8.

CONSORT flow chart depicting study groups. OTPD, one-time patellar dislocation; MPD, multiple patellar dislocation.

Table 1

Patient Characteristics and MRI Measurements ^a

	OTPD (n = 69)	MPD (n = 108)	P
Female sex, n (%)	40 (58)	64 (59)	.87
Age, y			.26
Mean ± SD	14.7 ± 2.4	15.1 ± 2.3
Median (IQR)	15.0 (12.0 to 16.2)	15.1 (13.8 to 16.8)
Range	9.1 to 19.9	9.2 to 20.1
pTT-TG distance, mm			.12
Mean ± SD	16.5 ± 5.0	17.5 ± 4.5
Median (IQR)	16.7 (12.6 to 20.2)	17.4 (14.3 to 20.7)
Range	7.0 to 28.7	8.4 to 31.4
rTER, deg			.72
Mean ± SD	6.2 ± 4.2	6.0 ± 4.4
Median (IQR)	6.1 (2.8 to 8.5)	5.5 (2.6 to 8.3)
Range	0.0 to 18.4	0.0 to 20.3
LTI, deg			.65
Mean ± SD	7.4 ± 12.7	6.1 ± 11.8
Median (IQR)	8.4 (–1.0 to 16.3)	9.3 (0.9 to 13.9)
Range	−18.4 to 28.3	−20.7 to 27.2
SA, deg			<.001
Mean ± SD	157.7 ± 3.6	167.0 ± 11.4
Median (IQR)	156.8 (148.4 to 167.9)	166.9 (160.4 to 171.9)
Range	130.8 to 191.3	139.3 to 213.7
TD, mm			<.001
Mean ± SD	1.9 ± 1.4	1.0 ± 1.3
Median (IQR)	1.8 (0.9 to 2.9)	1.1 (0.5 to 1.6)
Range	−1.4 to 5.3	−1.4 to 4.3
CDI			.03
Mean ± SD	1.3 ± 0.2	1.4 ± 0.2
Median (IQR)	1.3 (1.2 to 1.4)	1.3 (1.2 to 1.5)
Range	0.9 to 1.6	0.9 to 1.9
PT, deg			.001
Mean ± SD	17.8 ± 9.5	22.7 ± 9.7
Median (IQR)	18.1 (10.8 to 24.6)	20.9 (16.4 to 29.3)
Range	2.2 to 40.7	6.5 to 42.3

The chi-square test was utilized to assess differences in sex between groups. The independent-samples t test was utilized to assess differences in age, pTT-TG distance, CDI, TD and PT between groups. The Mann-Whitney U test was utilized to assess differences in rTER, LTI, and SA between groups. CDI, Caton-Deschamps index; IQR, interquartile range; LTI, lateral trochlear inclination; MPD, multiple patellar dislocation; MRI, magnetic resonance imaging; OTPD, one-time patellar dislocation; PT, patellar tilt; pTT-TG, proximal tibial tubercle–trochlear groove; rTER, relative tibial external rotation; SA, sulcus angle; TD, trochlear depth.

Patients with MPDs had a higher median SA (P < .001), lower median TD (P < .001), higher mean PT (P = .001), and higher mean CDI (P = .03) compared with patients with an OTPD (Table 1). However, there was no significant difference in age (P = .26), LTI (P = .65), pTT-TG distance (P = .12), or rTER (P = .72) between the groups (Table 1). In the OTPD group, the mean SA was 157.7° (range, 130.8°-191.3°); 57 of 69 knees (83%) in this group had pathological SA values. In the MPD group, the mean SA was 167.0° (range, 139.3°-213.7°) (P < .001). The mean TD in the OTPD group was 1.9 ± 1.4 mm, whereas the mean TD in the MPD group was 1.0 ± 1.3 mm (P < .001).

On multiple logistic regression, SA (odds ratio, 1.06 [95% CI, 1.03-1.09]; P < .001) and CDI (odds ratio, 5.87 [95% CI, 1.11-30.92]; P = .037) were associated with increased odds of having MPDs (Table 2). TD and PT were not significantly associated with MPDs when SA was included in the model. The logistic regression model yielded an AUC of 0.723 (95% CI, 0.644-0.802), with a sensitivity of 82.4% and a specificity of 56.5% based on a cutoff of 0.55 determined by the Youden index (Figure 9). Based on the multiple logistic regression model, the probability of having MPDs was calculated for several combinations of SA and CDI values (Table 3).

Table 2

Factors Associated With Likelihood of Experiencing MPDs ^a

	Odds Ratio (95% CI)	P
SA	1.06 (1.03-1.09)	<.001
CDI	5.87 (1.11-30.92)	.037

CDI, Caton-Deschamps index; MPD, multiple patellar dislocation; SA, sulcus angle.

Figure 9.

Receiver operating characteristic curve depicting an area under the curve (AUC) of 0.72 (95% CI, 0.64-0.80) for a multiple logistic regression model of patella alta (Caton-Deschamps index) and sulcus angle.

Table 3

Probability of Having MPDs Based on SA and CDI ^a

SA, deg	CDI	Probability
150	1.4	.45
150	1.5	.49
160	1.4	.61
160	1.5	.65
170	1.4	.74
170	1.5	.78

These probabilities were estimated from the formula 1/(1+exp-(-12.2022 + 0.0635(SA) + 1.770(CDI))), based on the multiple logistic regression model displayed in Table 2. CDI, Caton-Deschamps index; MPD, multiple patellar dislocation; SA, sulcus angle.

Intraobserver ICCs were considered excellent for rTER, PT, TD, and SA; good or excellent for CDI and pTT-TG distance; and good for LTI (Table 4). Interobserver ICCs were as follows: excellent for pTT-TG distance, rTER, PT, and TD; good for SA and LTI; and moderate for CDI (Table 5).

Table 4

Intraobserver Reliability ^a

	Rater 1	Rater 2
CDI	0.94 (0.88-0.97)	0.83 (0.64-0.92)
pTT-TG distance	0.98 (0.97-0.99)	0.86 (0.71-0.93)
rTER	0.96 (0.92-0.98)	0.97 (0.94-0.99)
PT	0.96 (0.92-0.98)	0.96 (0.92-0.98)
LTI	0.83 (0.65-0.92)	0.90 (0.78-0.95)
TD	0.98 (0.95-0.99)	0.94 (0.87-0.97)
SA	0.98 (0.96-0.99)	0.94 (0.88-0.97)

Data are presented as intraclass correlation coefficient (95% CI). CDI, Caton-Deschamps index; LTI, lateral trochlear inclination; PT, patellar tilt; pTT-TG, proximal tibial tubercle–trochlear groove; rTER, relative tibial external rotation; SA, sulcus angle; TD, trochlear depth.

Table 5

Interobserver Reliability ^a

	Raters 1 and 2
CDI	0.73 (0.44 to 0.88)
pTT-TG distance	0.91 (0.48 to 0.97)
rTER	0.96 (0.91 to 0.98)
PT	0.95 (0.90 to 0.98)
LTI	0.75 (–0.09 to 0.92)
TD	0.91 (0.39 to 0.97)
SA	0.87 (0.25 to 0.96)

Discussion

These findings indicate that patients with MPDs had a significantly higher SA, CDI, and PT, and a significantly lower TD, compared with those with an OTPD. Conversely, no statistically significant difference was found in pTT-TG distance, rTER, or LTI between the MPD and OTPD groups. The implications of these findings are that measures of trochlear dysplasia (SA and TD), patella alta (CDI), and PT may be most effective at differentiating between patients who are more or less likely to have a subsequent episode of PI after a sentinel event. The results from Table 3 suggest that more severe trochlear dysplasia (higher SA) was associated with a higher likelihood of experiencing MPDs.

The AUC of 0.72 for a logistic regression model of SA and patella alta (CDI) revealed that these 2 factors, together, could distinguish between patients with MPDs and those with an OTPD with reasonable accuracy, which may be useful for counseling patients at initial presentation. This finding aligns with another study that also described increased SA and patella alta as the 2 greatest factors in a “risk of dislocation” algorithm.¹¹ Köhlitz et al¹⁸ similarly found that patients with trochlear dysplasia and patella alta have a 41 times greater risk of dislocations compared with control patients.

SA was the most important determinant of experiencing MPDs in this population. In the literature, values >145° are considered dysplastic.³⁴ Both of our groups were found to have a mean SA >145°. In the OTPD group, the mean SA was 157.7° (range, 130.8°-191.3°); 57 of 69 knees (83%) in this group had pathological SA values. In the MPD group, the mean SA was 167.0° (range, 139.3°-213.7°); 106 of 108 knees (98%) had SA values >145° (P < .001). Additionally, the findings in Table 3 suggest that more severe trochlear dysplasia (higher SA) was associated with a higher likelihood of experiencing MPDs. These findings further verify previous studies that have demonstrated the importance of elevated SA as the single most important risk factor for having MPDs.^6,8

TD is a frequently cited and reliable measure of trochlear dysplasia. Historically, values ≤4 mm were determined to be pathological. However, more recent studies have indicated that a cutoff of 3 mm is more appropriate, especially on measurements taken along the cartilaginous surface.^36,37 Both groups in our study had mean values below the diagnostic cutoff. The mean TD in the OTPD group was 1.9 ± 1.4 mm, whereas the mean TD in the MPD group was 1.0 ± 1.3 mm (P < .001). In the OTPD group, 46 of 69 knees (67%) had values <3 mm, and in the MPD group, 104 of 108 knees (96%) had values below the cutoff; 15 were measured to be 0 mm or had negative values, indicating convex trochlear morphology.

The measurements for trochlear dysplasia were taken from the most proximal identifiable axial image for which these measurements could be accurately recorded, that is, the most proximal image with cartilage completely covering the trochlear groove. Recording trochlear dysplasia at more proximal sites likely accounts for the increased sensitivity observed, as the trochlear groove tends to normalize distally even in patients with trochlear dysplasia.²⁷ The proximal trochlear groove is hypothesized to be especially clinically important, as it is the strength of this initial engagement during knee flexion at the trochlear groove that is most highly correlated with PI.²²

A notable finding from this study is that the pTT-TG distance was not strongly associated with experiencing MPDs. The TT-TG distance is a multifactorial measure; it could be increased with a more lateralized tibial tubercle, a more medialized trochlear groove, and increased knee rotation. Previous research has shown that a medialized trochlear groove and increased knee rotation are more highly correlated with the TT-TG distance.⁵

Limitations

Overall, 46 knees were excluded because their MRI scans could not be sufficiently interpreted. Moreover, 3 knees that had an OTPD based on a chart review were eliminated because the patients could not be reached by telephone to confirm their status. Because of the retrospective nature of this study, the cases and controls were selected after the outcomes had occurred. These factors result in potential selection bias, and thus, the strength of these results may be limited in this context. Additionally, while the present study identified risk factors that differed between patients with MPDs and those with an OTPD, an accurate predictive model to stratify the risk based on radiographic information after a first-time dislocation is necessary to best treat these patients.³⁵

Another limitation is that when selecting the most proximal axial image on which the cartilaginous portions of the medial and lateral trochlear facets are visible, there is a risk that the chosen slice may be too distal, potentially overlooking the proximal extent of trochlear dysplasia. However, because the SA was an angle formed between the medial and lateral facets, the first cut with both the medial and lateral facets was utilized to ensure an accurate SA measurement. It is acknowledged that trochlear dysplasia may be apparent more proximally, but an accurate SA measurement would not be obtained in that region because of the absence of the medial facet.

Conclusion

Measures of trochlear dysplasia (SA and TD), patella alta (CDI), and PT may be useful to distinguish between patients with MPDs and those with an OTPD. pTT-TG distance, LTI, and rTER were not found to have statistically significant differences between groups. The combination of SA and CDI distinguished between the groups with reasonable accuracy (AUC = 0.72). Applying this model, a patient with an SA of 170° and a CDI of 1.5 would have a .78 probability of experiencing MPDs. This may be helpful in counseling patients after an initial dislocation event on the likelihood of experiencing MPDs.

Footnotes

Final revision submitted February 26, 2025; accepted March 31, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: J.S. is a paid consultant for Johnson & Johnson and OrthoPediatrics and has received support for education from Gotham Surgical Solutions & Devices. E.D.F. has received consulting fees from GE HealthCare and support for education from Gotham Surgical Solutions & Devices. B.J.L. has received support for education from Gotham Surgical Solutions & Devices and Prodigy Surgical. M.D. has received hospitality payments from Smith & Nephew and support for education from Gotham Surgical Solutions & Devices. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from Albert Einstein College of Medicine (No. 2016-6534).

ORCID iDs

Jason D. Brenner

Steven M. Henick

Edina Gjonbalaj

Benjamin J. Levy

Mauricio Drummond

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