Imaging of Patellofemoral Instability

The Morphologic Patella Entry Point Into the Proximal Trochlea Is More Lateral in Recurrent Dislocators Than Controls as Measured by Entry Point-Trochlear Groove Angle

PURPOSE

To create a metric for evaluating the degree of laterality of the patella's entry into the trochlea, the entry point-trochlear groove (EP-TG) angle, and to evaluate if this laterality is associated with recurrent patella instability.

METHODS

The time frame of the study was January 2020 to February 2023. The inclusion criteria were patients treated by the senior author (J.P.F.) (with the exception of 2 patients who were treated by another provider at the institution who was aware of the study) who have been diagnosed with recurrent atraumatic patellar dislocations. Controls without knee pathology were selected from the New Mexico Decedent Imaging Database (NMDID). Simpleware ScanIP was used to create 3-dimensional (3D) models of the distal femurs from computed tomography scans. Anteroposterior images of these 3D models were uploaded to a custom EP-TG angle measuring tool. Three measurers used the tool to measure the EP-TG angle of the distal femurs.

RESULTS

Twenty-eight patients were included for the recurrent dislocator group. Twenty-four decedents from the NMDID were selected for the control group, each with a left or right knee chosen randomly for measurement. A 1-sided Mann-Whitney U test, used to evaluate whether the recurrent dislocators had higher EP-TG angle values, yielded a P value <.001, demonstrating a high level of significance. A Bayesian mixed-effect model, used to determine how different the EP-TG angles are between the 2 groups, gave a posterior predictive interval of [11.93°, 19.12°] for the EP-TG angle shift of dislocators. The intraclass correlation coefficient was 0.648.

CONCLUSIONS

The morphologic entry point of the patella into the proximal trochlea is more lateral in recurrent patella dislocators than in controls. This increased laterality can be measured by the EP-TG angle, which may be useful information for optimizing treatment of recurrent patella instability.

LEVEL OF EVIDENCE

Level III, case control study.

Measurement of TT-TG can change with sequential MRIs due to variations in tibiofemoral rotation in patellofemoral instability patients

PURPOSE

There are various anatomic risk factors for patellofemoral instability (PFI) that help guide surgical treatment, including the tibial tubercle to trochlear groove (TT-TG) distance. However, no study has analysed the temporal changes in TT-TG prior to surgical intervention. This study sought to understand the variations in TT-TG over time for pediatric patients suffering from PFI prior to surgical intervention. The authors hypothesised that the TT-TG would substantially change between time points.

METHODS

Patients undergoing medial patellofemoral ligament (MPFL) reconstruction between 2014 and 2019 by one of two fellowship-trained orthopaedic surgeons were identified. Patients were included if they had two preoperative magnetic resonance imaging (MRI) performed on the same knee within 7.5 months of each other prior to any surgical intervention and had an initial TT-TG greater than 10 mm.

RESULTS

After considering 251 patients for inclusion, 21 patients met the final inclusion criteria. The mean age was 14.5 ± 2.5 years and 61.9% were female. TT-TG was initially noted to be 15.1 ± 1.8 mm. At mean time after sequential MRIs of 5.0 ± 1.9 months, TT-TG was noted to be 16.7 ± 3.2 mm. The differences between initial and subsequent TT-TG ranged from a 21.2% decrease to a 61.1% increase, with a mean difference of an 11.3% increase. Comparison between initial and subsequent TT-TG values demonstrated a significant difference (p = 0.017). Change in tibiofemoral rotation ranged from -9.2° to 7.5°. When comparing the change in TT-TG to change in tibiofemoral rotation, a significant correlation was found (p = 0.019).

CONCLUSION

Despite only a mean time between MRIs of 5 months, variations in TT-TG ranged from a decrease of 21.2% to an increase of 61.1%. The significant relationship between the changes in TT-TG and changes in tibiofemoral rotation between MRIs suggest that TT-TG measurements may vary due to variations in tibiofemoral rotation at the time of individual MRIs.

LEVEL OF EVIDENCE

Level IV.

A modified Delphi consensus statement on patellar instability: part I

Aims

The aim of this study was to establish consensus statements on the diagnosis, nonoperative management, and indications, if any, for medial patellofemoral complex (MPFC) repair in patients with patellar instability, using the modified Delphi approach.

Methods

A total of 60 surgeons from 11 countries were invited to develop consensus statements based on their expertise in this area. They were assigned to one of seven working groups defined by subtopics of interest within patellar instability. Consensus was defined as achieving between 80% and 89% agreement, strong consensus was defined as between 90% and 99% agreement, and 100% agreement was considered to be unanimous.

Results

Of 27 questions and statements on patellar instability, three achieved unanimous consensus, 14 achieved strong consensus, five achieved consensus, and five did not achieve consensus.

Conclusion

The statements that reached unanimous consensus were that an assessment of physeal status is critical for paediatric patients with patellar instability. There was also unanimous consensus on early mobilization and resistance training following nonoperative management once there is no apprehension. The statements that did not achieve consensus were on the importance of immobilization of the knee, the use of orthobiologics in nonoperative management, the indications for MPFC repair, and whether a vastus medialis oblique advancement should be performed.

Skyline view of the patella does not increase fracture detection: A non-inferiority diagnostic study

PURPOSE

Patella fractures are frequent injuries in the adult population. Initial study is made by plain radiographs and the standard set includes the skyline view of patella. Recommendation for use of this projection is variable among the experts, without data that support its performance in the diagnosis of patella fractures. The main purpose of this study was to determine the sensitivity of the antero-posterior and lateral view of the knee, without skyline view, in the diagnosis of patella fracture.

METHODS

A retrospective non-inferiority diagnostic study was designed with all the knee trauma adult patients of an Emergency Department in a single center in five years. A random sample of all the consecutive patella fracture cases were taken to elaborate the case group. The control group was matched by sex and age. Two blinded orthopedic surgeons reviewed the cases and control radiographs and defined the presence of fracture, with or without skyline view, with a wash-out time of three weeks between the two evaluations. Non-inferiority was defined a priori at 90% of minimum sensitivity.

RESULTS

140 patients were evaluated (70 cases and 70 controls). Sensitivity of the set without skyline view was 92.86% (CI 95% 0.90 - 0.96) and the standard set was 97.86% (CI 95% 0.96 - 0.99), without significant differences (p = 0.1) CONCLUSION: Sensitivity of the plain radiograph set without skyline view is non-inferior to the standard set. The routine use of skyline view when suspecting patella fractures is questionable.

Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode

The patellofemoral joint (PFJ) is a complex articulation between the patella and the femur which is involved in the extensor mechanism of the knee. Patellofemoral disorders can be classified into objective patellar instability, potential patellar instability, and patellofemoral pain syndrome. Anatomical factors such as trochlear dysplasia, patella alta, and the tibial tuberosity-trochlear groove (TT-TG) distance contribute to instability. Patellofemoral instability can result in various types of dislocations, and the frequency of dislocation can be categorized as recurrent, habitual, or permanent. Primary patellar dislocation requires diagnostic framing, including physical examination and imaging. Magnetic resonance imaging (MRI) is essential for assessing the extent of damage, such as bone bruises, osteochondral fractures, and medial patellofemoral ligament (MPFL) rupture. Treatment options for primary dislocation include urgent surgery for osteochondral fragments or conservative treatment for cases without lesions. Follow-up after treatment involves imaging screening and assessing principal and secondary factors of instability. Detecting and addressing these factors is crucial for preventing recurrent dislocations and optimizing patient outcomes.

Preoperative Patella Alta on Caton-Deschamps Index Is a Predictor of Outcome Following Isolated Medial Patellofemoral Ligament Reconstruction

Purpose

The purpose of this study was to determine whether a preoperative Caton-Deschamps index (CDI) ≥ 1.30, as measured by magnetic resonance imaging, is associated with rates of postoperative instability, revision knee surgery, and patient-reported outcomes in patients undergoing isolated medial patellofemoral ligament (MPFL) reconstruction.

Methods

Patients who underwent primary medial patellofemoral ligament reconstruction (MPFLR) between 2015 and 2019 at a single institution were assessed. Only those with at least 2 year follow up were included. Patients who had undergone a previous ipsilateral knee surgery, concomitant tibial tubercle osteotomy and/or ligamentous repair/reconstruction at the time of MPFL reconstruction were excluded from the study. CDIs were evaluated by three investigators based on magnetic resonance imaging measurement. Patients with a CDI ≥ 1.30 were included in the patella alta group, while those with a CDI between 0.70 and 1.29 served as controls. A retrospective review of clinical notes was used to evaluate the number of postoperative instability episodes and revisions. Functional outcomes were measured by the International Knee Documentation Committee (IKDC) and 12-Item Short Form Health Survey (SF-12) physical and mental scores.

Results

Overall, 49 patients (50 knees, 29 males, 59.2%) underwent isolated MPFLR. Nineteen (38.8%) patients had a CDI ≥ 1.30 (mean: 1.41, range: 1.30-1.66). The patella alta group was significantly more likely to experience a postoperative instability episode (36.8% vs 10.0%; P = .023) and was more likely to return to the operating room for any reason (26.3% vs 3.0%; P = .022) compared to those with normal patellar height. Despite this, the patella alta group had significantly greater postoperative IKDC (86.5 vs 72.4; P = .035) and SF-12 physical (54.2 vs 46.5; P = .006) scores. Pearson's correlation showed a significant association between CDI and postoperative IKDC (R ² = 0.157; P = .022) and SF-12P (R ² = .246; P = .002) scores. There was no difference in postoperative Lysholm (87.9 vs 85.1; P = .531). and SF-12M (48.9 vs 52.5; P = .425) scores between the groups.

Conclusion

Patients with preoperative patella alta, as measured by CDI had higher rates of postoperative instability and return to the OR with isolated MPFL reconstruction for patellar instability. Despite this, higher preoperative CDI was associated with greater postoperative IKDC scores and SF-12 physical scores in these patients.

Level of Evidence

Retrospective cohort study, Level IV.

Numerical investigation of patellar instability during knee flexion due to an unbalanced medial retinaculum loading effect

Background and aim

Healthy patellofemoral (PF) joint mechanics are critical to optimal knee joint function. Patella plays a vital role in distributing quadriceps load during the knee extension. Patellar tracking, not physiological tracking, causes an increase of strains in PF ligaments, peaks of localized stress of soft tissues and articular cartilage and bony parts, and knee pain; these problems lead to complications such as bone abnormalities and osteoarthritis. This research aimed to develop a Finite Element (FE) model to evaluate patellar instability due to the medial retinaculum asymmetric loading effect.

Methods

A numerical model of the knee was obtained by matching nuclear magnetic resonance (MRI) for soft tissues and computerized tomography (CT) for bones, carried on a normal adult. Loading setup was chosen by using literature data. The intensity of the muscle forces was calculated by a static optimization taking into account ground reaction and knee flexion/extension during walking. The effect of patellar instability was obtained by gradually unbalancing this symmetry, one side was unloaded till 90 N, and the other loaded till 110 N.

Results

Unbalanced forces of 10 N acting on the retinaculum alone can produce a real difference in displacements of about 7 mm, and an increment of about 44% on patellar contact forces.

Conclusion

This research demonstrated how an unbalanced forces acting on the retinaculum can produce significant patellar instability. Patellar instability starts at 25-30° of the knee flexion angle but tends to appear at 15° when the unbalanced muscular loading conditions are acting.

Sulcus Angle, Trochlear Depth, and Dejour's Classification Can Be Reliably Applied To Evaluate Trochlear Dysplasia: A Systematic Review of Radiological Measurements

PURPOSE

To summarize data on the reliability of available imaging criteria for the assessment of trochlear dysplasia and to assess the methodological quality of the included studies.

METHODS

This systematic review was conducted following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guideline. Search was performed using major electronic databases from their inception to September 2021. All studies enrolling patients of any age who underwent a radiological exam to rule out features related to trochlear dysplasia were included. After the identification of available imaging criteria, reliability studies were analyzed. Descriptive statistics were used to summarize findings. Methodological quality was assessed using the Quality Appraisal of Reliability studies checklist.

RESULTS

A total of 2391 articles were identified, and 33 articles comprising 3036 patients with a mean age of 28.6 years were included. Thirty different measurements were extracted. Magnetic resonance imaging (MRI) was the most used imaging modalities (21 studies), followed by computed tomography (10 studies), conventional radiology (8 studies) and ultrasonography (US) (1 study). Sulcus angle, trochlear depth, and Dejour's classification were the most explored measurements. Overall, sulcus angle can be reliably assessed on radiography, CT and MRI, whereas trochlear depth can be reliably measured only with CT and MRI. Reliability of Dejour's classification ranged from poor or fair to very good for all imaging modalities. Methodological quality of included studies varied from 2 to 9 positive items out of 11 possible. Twenty-four studies (72.7%) were considered at high risk of bias.

CONCLUSION

Trochlear dysplasia can be reliably evaluated at least with 3 measurements: sulcus angle, trochlear depth and Dejour's classification. Methodological quality assessment showed high risk of bias in most included studies.

LEVEL OF EVIDENCE

Level III, systematic review of Level II-III studies.

Radiographic measurement of the congruence angle according to Merchant: validity, reproducibility, and limits

PURPOSE

The objective of this study was to analyze the intra- and interobserver variability of this measurement according to a strict methodology and on a representative sample of the general population, as well as to identify the possible difficulties of measurement in case of patellar or trochlear dysplasia.

METHODS

This observational study involved radiographic analysis by three independent observers of a total of 50 patients who had a loaded patellofemoral X-ray taken with the knee flexed to 45°. An initial reading was taken to measure the angle of the trochlear sulcus, the Merchant angle, and to classify the knees according to a possible trochlear dysplasia and/or patellar dysplasia according to Wiberg. A second measurement was then performed to analyze intraobserver agreement. Interobserver agreement was measured on all radiographic measurements (n = 100).

RESULTS

The Merchant patellofemoral congruence angle showed good intraobserver concordance ranging from 0.925 (95% CI 0.868-0.957) to 0.942 (95% CI 0.898-0.967), as well as interobserver concordance ranging from 0.795 (95% CI 0.695-0.862) to 0.914 (95% CI 0.872-0.942). Poor results were found in terms of interobserver concordance on the measurement of the Merchant angle in case of stage 3 Wiberg patella ranging from 0.282 (95% CI -0.920 to 0.731) to 0.611 (95% CI 0.226-0.892).

CONCLUSION

Congruence angle is one of most commonly used measurements for patellar tracking. However, the convexity of the patellar surface makes it difficult to identify the patellar apex on its intraarticular facet, making the measurement of the Merchant congruence angle unreliable and not very reproducible in cases of stage 3 Wiberg patella. Registration N°IRB 2021/139.

Individualized tibial tubercle-trochlear groove distance-to-patellar length ratio (TT-TG/PL) is a more reliable measurement than TT-TG alone for evaluating patellar instability

PURPOSE

To evaluate the intra/inter-rater and diagnostic reliability of the sagittal plane adjusted patellar instability ratios (PIRs) compared to tibial tubercle-trochlear groove (TT-TG) distance alone while employing a matched case-control analysis for age and sex to minimize a potential confounding effect.

METHODS

A retrospective case-control study was performed of all knee MRI studies of patients diagnosed with patellar instability, between 2005 and 2020 at a regional tertiary medical centre. Using a 1:1 case-control matching of sex and age at the time of the diagnosis, one control subject was assigned to each case of patellar instability. Measurements of TT-TG distance, sagittal patellar length (PL), sagittal patellar tendon length (PTL), TT-TG/PL ratio, and TT-TG/PTL ratio were conducted. Two orthopaedic surgery residents and a senior musculoskeletal radiologist were assigned to assess the intra- and inter-rater reliability. Inter-class coefficients were calculated (ICC). The receiver operating characteristic (ROC) curve and area under curve (AUC) for each parameter were compared to evaluate for diagnostic reliability. Odds ratios (OR) and their 95% confidence intervals (CI) were calculated and a multivariable logistic regression model was performed to control for possible confounders.

RESULTS

The study included 324 individuals (162 case-control matched pairs). In terms of intra- and inter-rater reliability, TT-TG/PL and TT-TG/PTL ratios showed an excellent correlation within and between readers (TT-TG/PL; intra-rater ICC 0.94 and inter-rater ICC 0.92, TT-TG/PTL; intra-rater ICC 0.91 and inter-rater ICC 0.88). The ROC curve showed a slightly greater AUC of the TT-TG/PL ratio compared to TT-TG distance alone (0.75 vs 0.73, p < 0.001). When applying the pathologic cutoff of TT-TG ≥ 20 mm and TT-TG/PL ≥ 0.5; the calculated odds ratios for the above cutoff were as follows; TT-TG distance alone had an OR of 14 (95% CI 1.8-106.5, p = 0.011) and OR for TT-TG/PL ratio was 23 (95% CI 3.1-170.3, p = 0.002). In the multivariable analysis, while controlling for height and weight, only the association between TT-TG/PL ratio and patellar dislocation remained statistically significant with an adjusted OR of 2.7 (CI 1.3-5.4, p = 0.006), compared to TTTG distance alone (OR = 1.9, n.s.).

CONCLUSIONS

Patellar instability ratios are significantly more reliable compared to TT-TG distance alone for the evaluation of patellar instability. Patellar instability ratios present superior diagnostic reliability, sensitivity and specificity, and intra\inter rater reliability. Thus, patellar instability ratios could function as a valuable diagnostic tool for the evaluation of patellar instability.

LEVEL OF EVIDENCE

III.

Radiological evaluation of patellofemoral instability and possible causes of assessment errors: Letter to the editor

This letter to the editor is a commentary on the study titled "Radiological evaluation of patellofemoral instability and possible causes of assessment errors". There are some pertinent structural changes and radiological findings that should be considered in the setting of traumatic knee injuries, as their recognition is of paramount importance.

Differences in Outcomes between Patellar Dislocations Managed in Emergent versus Non-Emergent Care Settings

Patellar dislocations occur at a much higher rate in military than civilian populations. Past population-level studies have shown that surgical management is as good as or superior to conservative care and may reduce future reoccurrences. Although in acute cases and in civilian clinics, patellar dislocations are usually managed first in an emergent care setting, previous work suggests this can lead to increased costs. This study compared differences in downstream care type and intensity of services based on whether initial care occurred in emergent or non-emergent care settings. In our sample of 1,523 Military Health System (MHS) beneficiaries with patellar dislocation and 2-year follow-up, we found non-significant differences in costs, intensity of services, and rates of surgical repair regardless of whether the patient was initially seen in an emergent versus non-emergent care setting. Although we found significant increases in the use of imaging, patellar dislocation-related medical visits, and frequency of closed treatment approaches in emergent care settings, these values were very small and likely not clinically significant. These findings, which included all the patellar dislocations reported across the entire MHS in a 24-month period, suggest that neither emergent nor non-emergent care settings are likely to influence the long-term care received by the individual.