Variability of Measurement of Patellofemoral Indices with Knee Flexion and Quadriceps Contraction: An MRI-Based Anatomical Study

Patella-Posterior Turning Point of the Distal Femur Distance Is a Potential Indicator for Diagnosing Patella Alta in Recurrent Patellar Dislocation Population

PURPOSE

To introduce a simple patellar height measurement method (patella-posterior turning point of the distal femur [P-PTP] Distance) independent of patellar anatomy with standardized patient position, and tested the reliability, validity, and diagnostic accuracy compared with commonly used ratios in knee extension.

METHODS

We retrospectively reviewed 418 computed tomography (CT) images of the knee joint in a group of patients who were diagnosed recurrent patellar dislocation (RPD). With the three-dimensional (3D) CT reconstructed knee, patellar height was qualitatively assessed by the patellar engagement with the femoral trochlea in terminal knee extension to divide RPD population into case (patella alta) and control group. With digitally reconstructed lateral radiographs, patellar height was measured with P-PTP distance (perpendicular distances between the distal edge of patella articular surface and posterior turning point of distal femur), and four commonly used ratios: Caton-Deschamps index, Modified Insall-Salvati index, Blackburne-Peel index, and Insall-Salvati index. An unpaired t-test was conducted to determine significant differences between groups. Correlation coefficient, intra- and inter-observer reliability, receiver operating characteristic (ROC) curves and the area under the ROC curve (AUC) were also calculated.

RESULTS

198 knee images (198 patients) were included for final evaluation. Patella alta was present in 72 knees (36.3%) with RPD. The effect size was relatively large between the case and control group for P-PTP distance (d = -1.619; 95% CI, -1.948 to -1.286). P-PTP distance correlated moderately to strongly with four commonly used ratios (P < .001). Intraobserver and interobserver reliability was good for P-PTP distance. The AUC of the ROC curve was categorized as excellent for P-PTP distance, better than other measurements (P < .001), and the cutoff value was 4.2 mm with the highest sensitivity (86.11%) and specificity (84.92%).

CONCLUSIONS

The measurement method, P-PTP distance, showed good intra-observer and inter-observer reliability, well correlated with commonly used ratios, and presented best diagnostic accuracy among commonly used ratios for predicting RPD. P-PTP distance might be a potential indicator for identifying patella alta in RPD patients when supine and knee extended.

CLINICAL RELEVANCE

The measurement reported in this study may help in advancing clinical evaluation of patella alta, providing an alternative and simple method to measure patellar height. Standing or weight-bearing plain lateral radiographs obtained from the routine practice should be further assessed in the next step to further validate the method.

The Effect of Aberrant Rotation on Radiographic Patellar Height Measurement Using Canton-Deschamps Index: A Cadaveric Analysis

The Caton-Deschamps Index (CDI) is a measurement used to evaluate patella alta based on true lateral radiographs; however, no prior study has investigated how altering the degree of radiograph aberrancy affects CDI measurement. The primary and secondary purpose of this study was to evaluate effects of rotational radiographic changes on patella height measurements and compare these findings to MRI measurements, respectively. Five cadaver knees (n = 5) were utilized in this study. True lateral radiographs were obtained for each specimen by using a fluoroscopic C-arm machine. The C-arm was then altered in two planes (axial and coronal) in both the clockwise and counterclockwise direction and radiographs were taken at 5, 10, and 15 degrees of error from the true lateral position. A CDI measurement of each specimen was performed based on sagittal magnetic resonance imaging (MRI) slices and compared with radiographic CDI measurements. Three orthopedic surgeons measured the CDI for each radiograph and MRI performed. Interrater reliability and changes in CDI were analyzed. Clinically significant difference in CDI was set to 0.1. Mean intraclass correlation coefficient was high (≥0.7) at true lateral and at all varying degrees of error. When performing a pairwise comparison of mean CDI from the true lateral position to increasing degrees of error, statistically significant differences were observed in the axial plane. The largest change in CDI measurements was seen with rotational malposition in the axial plane and counterclockwise direction. No statistically significant differences in mean CDI were observed in the coronal plane. The change in CDI from the true lateral position reached an absolute maximum of at least 0.1 in all four scenarios at each tested degree of error. This study found that aberrant radiographic rotation in the axial plane resulted in a significantly different mean CDI measurement when compared with true lateral radiographs. All degrees of error in both directions and in both planes could have a clinically significant effect on CDI (≥0.1). Our findings confirm the importance of a perfect true lateral radiograph when measuring patella height.

A comparison of sagittal MRI and lateral radiography in determining the Insall-Salvati ratio and diagnosing patella alta in the pediatric knee

BACKGROUND

The Insall-Salvati ratio is a technique for determining patellar height that relies on bony landmarks. Magnetic resonance imaging (MRI) and plain radiography are used interchangeably to assess the Insall-Salvati ratio in the pediatric population despite the lack of validity in the literature.

OBJECTIVE

The purpose of this study was to investigate if the Insall-Salvati ratio and patella alta as determined on MRI are comparable to those determined on radiography in pediatric patients.

MATERIALS AND METHODS

We conducted a retrospective review of 49 pediatric patients (age range: 7.5-17.0 years) with unfused growth plates who underwent both knee MRI and lateral knee radiography. Measurements for calculating the Insall-Salvati ratio (the ratio of patella tendon length to patella length) were obtained by three observers. Data were analyzed using paired t-tests and Pearson's correlation. A reliability assessment and inter-method agreements were performed. Patella alta was defined as an Insall-Salvati ratio > 1.2. Additional cutoffs of Insall-Salvati ratios > 1.3 and > 1.4 were also analyzed.

RESULTS

There was no statistically significant difference between Insall-Salvati ratio as determined on MRI (mean: 1.20) and radiographs (mean: 1.25; P > 0.05). There was a strong correlation between Insall-Salvati ratio as determined on MRI and radiographs (Pearson's r = 0.6) with moderate consistency (Cronbach's alpha = 0.78). There was a good level of agreement between the diagnosis of patella alta on MRI and radiographs when defined as an Insall-Salvati ratio greater than 1.2 and 1.3 (Cohen's kappa = 0.61).

CONCLUSION

The results demonstrate a strong association between Insall-Salvati ratio and patella alta derived from MRI and radiographs in children ages 7.5 years and older.

Effect of Isolated Medial Patellofemoral Ligament Reconstruction in Patellofemoral Instability Regardless of Predisposing Factors

The purpose of this study was to evaluate the outcomes of isolated medial patellofemoral ligament (MPFL) reconstruction, regardless of the presence of predisposing factors. A total of 21 knees that underwent isolated MPFL reconstruction from March 2014 to August 2017 were included in this retrospective series. Radiographs of the series of the knee at flexion angles of 20, 40, and 60 degrees were acquired. The patellar position was evaluated using the patellar tilt angle, sulcus angle, congruence angle (CA), and Caton-Deschamps and Blackburne-Peel ratios. To evaluate the clinical outcome, the preoperative and postoperative International Knee Documentation Committee (IKDC) and Lysholm knee scoring scales were analyzed. To evaluate the postoperative outcomes based on the predisposing factors, the results were separately analyzed for each group. Regarding radiologic outcomes, 20-degree CA was significantly reduced from 10.37 ± 5.96° preoperatively to -0.94 ± 4.11° postoperatively (p = 0.001). In addition, regardless of the predisposing factors, delta values of pre- and postoperation of 20-degree CA were not significantly different in both groups. The IKDC score improved from 53.71 (range: 18-74) preoperatively to 94.71 (range: 86-100) at the last follow-up (p = 0.004), and the Lysholm score improved from 54.28 (range: 10-81) preoperatively to 94.14 (range: 86-100) at the last follow-up (p = 0.010). Isolated MPFL reconstruction provides a safe and effective treatment for patellofemoral instability, even in the presence of mild predisposing factors, such as trochlear dysplasia, increased patella height, increased TT-TG distance, or valgus alignment. This is a Level 4, case series study.

Strong Agreement Between Magnetic Resonance Imaging and Radiographs for Caton-Deschamps Index in Patients With Patellofemoral Instability

PURPOSE

To compare the measurements of the Caton-Deschamps index on preoperative magnetic resonance imaging and radiographs of patients undergoing operative management of patellar instability.

METHODS

Patients who underwent primary medial patellofemoral ligament reconstruction and/or tibial tubercle osteotomy between January 2015 and November 2019 were assessed. Caton-Deschamps indices were measured by 3 independent reviewers on both radiographs and magnetic resonance imaging. Intra- and interclass correlation coefficients and a Bland-Altman analysis were calculated to assess inter-rater reliability and measurement agreement between radiographic and magnetic resonance imaging.

RESULTS

Seventy-two patients (73 knees) were identified. The average Caton-Deschamps index was 1.23 ± 0.18 on radiograph and 1.26 ± 0.18 on magnetic resonance imaging. Strong inter-rater reliability was observed between reviewers for both radiographic and magnetic resonance imaging Caton-Deschamps indices (intraclass correlation coefficients 0.700 and 0.715, respectively). Pooled observer measurements revealed a moderate agreement between radiographic and magnetic resonance imaging for patella to tibia distance, weak agreement for patellar articular cartilage distance, and strong agreement for the Caton-Deschamps index (intraclass correlation coefficients 0.687, 0.485, and 0.749, respectively). Bland-Altman analysis demonstrated a mean difference in Caton-Deschamps index of -0.03 ± 0.15 (95% limits of agreement: -0.29 to 0.23) between radiographic and magnetic resonance imaging, meaning that Caton-Deschamps indices were on average 0.03 lower on radiographic than on magnetic resonance imaging.

CONCLUSIONS

The Caton-Deschamps index has strong agreement between radiographic and magnetic resonance imaging in patients undergoing patellar stabilization surgery. Either modality can be reliably used to preoperatively assess patellar height.

LEVEL OF EVIDENCE

Level IV, diagnostic case series.

A Sagittal Patellar Angle Linear Equation Reflecting Patellofemoral Kinematics: Evaluation of Patellar Height at any Degree of Knee Flexion Angle

Objective

To confirm whether a novel sagittal patellar angle linear equation used for evaluating patellar height by calculating expected sagittal patellar angle (SPA) at any degree of knee flexion angle is suitable for patients older than 17 years and its reliability compared with other commonly used methods.

Methods

From September 2016 to September 2019, a total number of 202 consecutive outpatients' knee lateral X‐ray radiographs were retrospectively measured and evaluated using a recently proposed linear equation Y = 1.94 + 0.74 × knee flexion(KF) angle. Patients were divided by ages into ayounger group, whose ages were between 17–49 years, and an older group, whose ages were older than 49 years, which has not been validated in the original study. Parameters such as KF, SPA, patella and patella tendon length and so on were measured on computer with picture archiving and communication system by two independent observers at an interval of 1 month. Insall‐Salvati (IS) index, Caton‐Deschamps (CD) index and Y value, correlation coefficients were calculated and compared using SPSS 22.0 software.

Results

In the younger group, 143 patients (165 knees) were included, ages were 17–49 (31.62 ± 11.38) years, males/females were 70 (48.95%)/73 (51.05%), left knees/right knees were 83 (50.30%)/82 (49.70%), mean value of Y was 31.50° ± 10.07°, and SPA was 34.38° ± 12.38°, mean value of IS was 1.06 ± 0.17, mean value of CD was 1.04 ± 0.18. While in older group, 59 patients (78 knees) were included, ages were 50–60 (mean 54.61 ± 2.99) years, there were 32 males (54.24%) and 27 females (45.76%), 42 knees were left (53.85%) and 36 knees were right (46.15%), mean values of Y and SPA were 25.90° ± 11.55° and 29.36° ± 14.22°, mean IS index in older group was 1.06 ± 0.18, mean CD index was 1.00 ± 0.16. Intra‐ and inter‐observer reliabilities of Y in younger and older groups were 0.999, 0.999, 1.000 and 0.999, meaning high reliability and reproducibility, but low Pearson's correlation coefficients with IS and CD index were showed as −0.213 and − 0.216 in younger group and − 0.113 and − 0.316 in older group.

Conclusions

In patients older than 17 years, the linear equation Y = 1.94 + 0.74 × KF is a reliable and practical method to evaluate SPA regardless of age and knee flexion angle, but has weak correlation coefficients with the IS and CD index.

Dynamic changes in the infrapatellar knee structures with quadriceps muscle contraction. An in vivo study

OBJECTIVE

To determine the effect of knee flexion and submaximal isometric quadriceps muscle (Q) contraction on the patellar tendon (PT), the infrapatellar fat pad (IPFP), and the deep infrapatellar bursa (IPB) from extension to full flexion.

METHODS

In Study 1, the dominant knee of seven healthy subjects was studied in full extension and at 60° flexion during relaxation and Q contraction. Each knee was inspected and palpated, the transverse infrapatellar diameter was measured by plicometry, and measurements of the anteroposterior (AP) thickness of the IPFP were made by ultrasound (US). In Study 2, the dominant knee of seven healthy subjects was studied by US in full flexion, and then, at 15° decrements, down to 60° flexion during relaxation and Q contraction. Both studies had IRB approval. Results were analyzed with the Wilcoxon test and descriptive statistics.

RESULTS

In Study 1, Q contraction caused straightening of the patellar tendon (PT), a statistically significant widening of the IPFP by plicometry, and an increased AP thickness of the IPFP by US, in both knee positions. In Study 2, in full knee flexion, the PT contacted the tibial cortex in all seven subjects. Upon increasing extension, the PT-tibial cortex contact was lost in all subjects nearing 90° flexion. The contraction of the Q made the concave PT straight, grew the width of the underlying IPFP, and the apron of the IPFP moved distally within the IPB in all knee positions. A small amount of bursal fluid was present in all seven subjects.

CONCLUSION

Q contraction makes the IPFP bulge anteriorly, on both sides of the PT, and distally into the IPB, with possible biomechanical implications. In full knee flexion, the PT contacted the tibia, confirming a fulcrum at this site.

Patella Height as Defined by Patella-Trochlear Articulation on MRI Scan-A Systemic Review of Current Evidence

The objective of this study is to investigate what are the normal values, cutoff values, and optimal method of magnetic resonance imaging (MRI) scan for patella height focusing on patella-trochlear index (PTI). Electronic searches of MEDLINE (PubMed), Cumulative Index to Nursing and Allied Health Literature, Cochrane, Embase, ScienceDirect, and National Institute for Health and Care Excellence evidence up to April 2018 were completed. Eligibility criteria for selected studies included case-controlled studies, prospective cohort studies, cross-sectional and randomized controlled trials reporting on the use of MRI to evaluate direct patella height. Exclusions included animal or biomechanical/computational studies; interventional surgery such as knee arthroplasty, bracing, or realignment; and hereditary/congenital disease. A quality assessment of included studies was completed using the Methodological Index for Non-Randomized Studies (MINORS) Criteria. Sixty-four articles were identified which met the inclusion criteria. Following exclusions, 11 full-text studies were reviewed which reported direct measure of patella height. Nine studies reported the use of PTI. Two low power studies described PTI in normal healthy asymptomatic knee. Similarly, one study evaluated the effect of flexion and weight bearing on PTI values. The cutoff values varied between studies from <0.125 to <0.28 for patella alta and >0.50 to >0.80 for patella baja. This review found that currently, there are a very few studies on the assessment of PTI on MRI scan, with only two low powered studies on a true normal population. In view of this, there is a need for adequately powered studies to investigate patella height and PTI in asymptomatic healthy knees.

Lateral patellar tilt and its longitudinal association with patellofemoral osteoarthritis-related structural damage: Analysis of the osteoarthritis initiative data

BACKGROUND

Increase in lateral patellar tilt-(LPT) can cause increased pressure on the lateral facet of the knee and can lead to patellar or femoral cartilage damage and further osseous changes. This study aims to test the hypothesis whether there is an association between increased LPT and MRI-based patellofemoral osteoarthritis-(OA) features at baseline and their worsening over a 2-year follow-up in participants of the Osteoarthritis Initiative-(OAI).

METHODS

Recorded clinical and imaging data of 600 participants in the FNIH-OA biomarkers consortium was extracted from its database. The LPT-(as the angle betweenthe longest patella diameter and posterior aspect of condyles) was measured using theaxial knee MRI. Associations of LPT (every 5° increase) with MRI OA Knee Scoring-(MOAKS) for OA-related features, including cartilage and bone marrow lesions (BMLs) in addition to knee cartilage volume at baseline and their worsening after 2-year follow-up were assessed using regression models adjusted for several possible confounders.

RESULTS

The mean LPT angle in this sample was 8.84° ± 5.19. In baseline, higher LPT was associated with lower cartilage volumes and higher cartilage lesions and BMLs MOAKS scores in the lateral trochlear and patellar subregions. Over the follow-ups, subjects with higher LPT measures in the baseline showed higher odds of experiencing BML score worsening in the lateral trochlear subregion-(OR:1.25[1.01-1.56]) over the 2-year follow-ups.

CONCLUSIONS

Increase in LPT measures may be associated with OA-related features in the trochlear subregion. Therefore, aside from its use as an indicator of patellofemoral instability syndrome, LPT may be associated with longitudinal progression of patellofemoral OA.

Sagittal patellar flexion angle: a novel clinically validated patellar height measurement reflecting patellofemoral kinematics useful throughout knee flexion

PURPOSE

Patellar height measurements on lateral radiographs are dependent on knee flexion which makes standardisation of measurements difficult. This study described a plain radiographic measurement of patellar sagittal height which reflects patellofemoral joint kinematics and can be used at all degrees of flexion.

METHODS

The study had two parts. Part one involved 44 normal subjects to define equations for expected patellar position based on the knee flexion angles for three new patellar height measurements. A mixed model regression with random effect for individual was used to define linear and polynomial equations for expected patellar position relating to three novel measurements of patella height: (1) patellar progression angle (trochlea), (2) patellar progression angle (condyle) and (3) sagittal patellar flexion. Part two was retrospective and involved applying these measurements to a surgical cohort to identify differences between expected and measured patellar position pre- and post-operatively.

RESULTS

All three measurements provided insight into patellofemoral kinematics. Sagittal patellar flexion was the most useful with the least residual error, was the most reliable, and demonstrated the greatest detection clinically.

CONCLUSIONS

Clinically applied radiographic measurements have been described for patellar height which reflect the sagittal motion of the patella and can be used regardless of the degree of flexion in which the radiograph was taken. The expected sagittal patellar flexion linear equation should be used to calculate expected patellar height.

LEVEL OF EVIDENCE

IV.

Static patella tilt and axial engagement in knee extension are mainly influenced by knee torsion, the tibial tubercle-trochlear groove distance (TTTG), and trochlear dysplasia but not by femoral or tibial torsion

PURPOSE

The aim of this study was to investigate the association of femoral (FT), tibial (TT), and knee torsion (KT) on the patella tilt (PT), the axial engagement index (AEI), and the tibial tuberosity-trochlear groove distance (TTTG).

METHODS

Femoral torsion, tibial torsion, knee torsion, patella tilt, the axial engagement index, the TTTG, and trochlear dysplasia were retrospectively evaluated on 59 patients suffering from recurrent patella instability or anterior knee pain with 118 torsional lower limb magnetic resonance imaging studies.

RESULTS

FT and TT did not show any significant associations with TTTG, PT, and AEI (n.s.). KT was significantly associated with a higher TTTG, higher PT, and lower AEI (all, p < 0.001). Higher grade trochlear dysplasia was associated with a higher PT and lower AEI (both, p < 0.001). The Dejour classification showed no significant association with FT, TT, KT, and TTTG (n.s.). All measurement parameters showed an excellent interrater agreement (ICC 0.89-0.97).

CONCLUSIONS

Static patella tilt and patellofemoral axial engagement in knee extension are mainly influenced by knee torsion, TTTG, and trochlear dysplasia but not by femoral or tibial torsion. These findings help to understand the underlying reasons for the patella position in knee extensions in CT and MRI investigations in patients suffering from patella instability and patellofemoral pain syndrome.

LEVEL OF EVIDENCE

III.

The quadriceps active ratio: a dynamic MRI-based assessment of patellar height

INTRODUCTION

Patella alta (PA) is one of the primary correctable risk factors for patellofemoral instability (PFI). Both an accurate diagnosis of PA and a clinically relevant target for correction are necessary for optimal treatment. An ideal test for PA should relate the position of the patella to the femur rather than tibia, should do so with the quadriceps contracted and the patellar tendon under tension and should have good sensitivity and specificity. None of the currently used radiographic tests PA meet these criteria, most of which are based on the position of the patella relative to the tibia with diagnostic cutoffs based on 2 standard deviations from the mean rather than optimal sensitivity and specificity. The authors describe the quadriceps active ratio (Q+R), an MRI-based assessment of PA based on patellofemoral contact under quadriceps activated with a cutoff based on optimal sensitivity a specificity for PFI.

MATERIAL-METHODS

Ninety-four participants investigated for knee pain or instability with a clinically indicated MRI were recruited. Routine MRI sequences were obtained, with the addition of a quadriceps contracted sagittal T1-weighted sequence. Participants presenting with PFI were identified. Those with trochlear dysplasia were identified and excluded from analysis so that patellar height could be assessed against PFI without being confounded by trochlear dysplasia. Q+R and patellotrochlear index (PTI) were calculated from the remaining 78 scans by 3 consultant orthopaedic surgeons at three time points. In 54 of these cases, a lateral radiograph was available from which the Insall-Salvati, modified Insall-Salvati, Caton-Deschamps and Blackburn-Peel ratios were also calculated. Intra- and inter-observer reliability was assessed for the Q+R. A cutoff value for the Q+R based on optimal sensitivity and specificity for the diagnosis of PFI was calculated from receiver-operator characteristic (ROC) curves and compared to the PTI. The cutoff for the Q+R was compared for sensitivity and specificity for the diagnosis of PFI against the radiographic ratios.

RESULTS

The Q+R had satisfactory or better ICC values across time points and surgeons. The Q+R was superior to the PTI on area under curve ROC analysis (0.76 vs 0.74). A cutoff value of 0.12 for the Q+R gave sensitivity of 79% and specificity of 55% for the diagnosis of PFI. The radiographic indices were generally insensitive for this diagnosis of PFI with sensitivities ranging from 0-66%.

CONCLUSION

The Q+R is a reliable diagnostic test for patellar height assessment, showing good intra- and inter-rater consistency, and greater diagnostic accuracy than the PTI. A Q+R value of 0.12 is a good test for clinically significant PA. Of the radiographic indices, the Insall-Salvati ratio had the best diagnostic accuracy.

What is the chance that a patella dislocation will happen a second time: update on the natural history of a first time patella dislocation in the adolescent

PURPOSE OF REVIEW

Patellar instability occurs mainly in young patients and shows a high incidence of concomitant cartilage injuries. Recently there has been a strong attempt to identify risk factors and enhance imaging techniques to detect patients with an increased risk for recurrent patella dislocation.We describe current findings on factors associated with recurrent patella dislocation in the adolescent.

RECENT FINDINGS

Trochlear dysplasia, patellar height, patellar tilt, tibial tuberosity-trochlear groove distance, skeletal maturity, and history of contralateral patellar dislocation are well known significant risk factors for recurrence in adolescent patients. Predictive models to calculate risk of recurrence have been reported recently. The Patellar Instability Severity Score was the first to include demographic and anatomic factors, which is of major value when counseling patients and relatives.

SUMMARY

Several classification systems to predict the rate of recurrence after primary patella dislocation have been presented over the last years. Anatomic risk factors such as skeletal immaturity, trochlear morphology, patellar height, patellar tilt, and elevated tibial tuberosity-trochlear groove distance have been investigated. However, there is still a lack of knowledge as to how single risk factors or their interaction with each other may contribute.