Patellar instability: will my patella dislocate again?

Combined Medial Patellofemoral Ligament and Medial Quadriceps Tendon Femoral Ligament Reconstruction With Semitendinosus Allograft for Pediatric Patients With First-Time Patella Dislocation Yields Low Failure Rates and Improved Functional Outcomes Compared With Nonoperative Treatment

PURPOSE

To evaluate the clinical outcomes of nonoperative versus operative treatment, consisting of combined medial patellofemoral ligament and medial quadriceps tendon femoral ligament (MPFL/MQTFL) reconstruction, for first-time patellar dislocation in the skeletally immature population through a standardized, rigorous approach.

METHODS

This consecutive retrospective cohort was composed of all skeletally immature patients who sustained a first-time patellofemoral dislocation. Patients were grouped into nonoperative management (bracing and physical therapy) and operative management (MPFL/MQTFL reconstruction) cohorts. The primary outcome, failure, was defined as subsequent dislocation or subluxation. Additional outcomes included demographics, risk factors for patellofemoral instability, functional outcomes (Kujala and Pediatric International Knee Documentation Committee scores), return to sports, and complications. Minimum follow-up was 2 years. A correlation analysis attempted to identify potential associations between failure and risk factors.

RESULTS

In total, 142 consecutive patients were included with 90 patients in the nonoperative group and 52 patients in the operative group. Failure rates differed by intervention; those receiving operative management had significantly lower failure rates than conservative management (15.4% vs 58.8%, P < .0001). Kujala and Pediatric International Knee Documentation Committee scores were significantly greater in the operative group compared with the nonoperative group (91.5 ± 7.8 vs 82.5 ± 12.1, P < .0001 and 89.0 ± 9.3 vs 78.4 ± 12.9, P < .0001, respectively). Tegner activity level was also greater in the operative group (6.0 ± 1.4 vs 4.0 ± 1.4, P < .0001). Return to sport was significantly greater for those in the operative group (88.5% vs 66.2%, P < .001). Return to same level of activity was achieved in greater percentage of patients treated surgically (91.3% vs 69.6%, P = .0075). Complication rates were more frequent in the operative group compared with the nonoperative group (11.5% vs 1.1%, P < .001).

CONCLUSIONS

MPFL/MQTFL reconstruction in skeletally immature patients with first-time patellofemoral dislocation demonstrated lower failure rates and improved functional outcomes at a minimum 2-year follow-up compared with nonoperative management.

LEVEL OF EVIDENCE

Level III, retrospective cohort study.

Low lateral inclination angle, high sulcus angle, high trochlear height and patella alta are risk factors for first lateral patellar dislocation and complete MPFL rupture, comparative study

Purpose

To identify risk factors for complete medial patello-femoral ligament (MPFL) rupture after first lateral patellar dislocation (LPD) and to develop a model to predict the risk of rupture.

Methods

Patients who presented with first LPD between February 2019 and June 2024 and were diagnosed with complete MPFL rupture on magnetic resonance imaging (MRI) were retrospectively reviewed. Patients with normal MRI findings in a 1:1 ratio were selected as the control group by computer-assisted randomisation.All patients in both groups were asked to perform MRI on, tibial tuberosity-trochlear groove (TT-TG) distance, lateral trochlear inclination (LTI) angle, sulcus angle (SA), medial femoral condyle height (MFCH), lateral femoral condyle height (LFCH), trochlear height (TH), patellotrochlear index (PTI), Koshino-Sugimoto Index (KSI), Caton-Deschamps Index (CDI) and Insall-Salvati Index (ISI) were measured and recorded. All measurements were made by two different orthopaedists and intra-observer reliability was evaluated. The measurements between the groups were compared statistically.

Result

A total of 98 patients, including 49 patients with complete MPFL rupture (study group) and 49 patients in the control group, were included in the study. Thirty of the patients in both groups were males and 19 were females. Mean age was 23.55 years in the study group and 24.29 years in the control group (p = 0.447). Satisfactory ICC scores were obtained in all measurements. LTI was lower in the study group than in the control group (p = 0.002), while SA was higher in the study group than in the control group. Both CDI and ISI were statistically significantly higher in the study group compared to the control group (p = 0.002, p = 0.003). The probability of predicting the risk of complete MPFL rupture of the risk analysis model created with radiological risk factors for complete MPFL rupture was 70.4%.

Conclusion

LTI, SA, TH and patella alta are risk factors for complete MPFL rupture after first LPD. Risk analysis of complete MPFL rupture after first dislocation can be successfully performed with MRI findings. This risk analysis can be used to predict the risk of developing complete MPFL after primary LPD, especially in risky patient groups, and can be used in a simple way to decide which patients will receive a preventive programme without the need for additional examination.

Level of Evidence

Level III, case-control study.

Increased Risk of Patellofemoral Instability Events and Surgical Management in Patients With Joint Hypermobility Syndromes: A Matched Cohort Analysis

Purpose

To compare the odds of patellofemoral instability events requiring subsequent surgery and revision surgical intervention in patients with joint hypermobility syndromes (JHS) to that of a matched cohort.

Methods

This is a retrospective cohort study using the PearlDiver Mariner Database. Records were queried between 2010 and 2021 with a diagnosis of JHS, including Ehlers-Danlos syndrome (EDS) and Marfan syndrome. Propensity matching was performed with a randomly generated control cohort without a diagnosis of JHS to account for age, sex, Charlson comorbidity index, diabetes, and obesity. Multivariable logistic regression was used to compare rates of patellar dislocation over a 1- and 2-year period between the 2 cohorts while controlling for previous knee injury or surgery. Patients who sustained a patellar dislocation over the 2-year period were followed to calculate rates of surgical intervention and subsequent revision.

Results

In a population of 91,747, those with JHS experienced patellofemoral instability at a significantly increased rate at both a 1-year (adjusted odds ratio [aOR] 11.40; 95% confidence interval 9.23-14.25, P < .001) and 2-year (aOR 8.73; 7.36-10.44, P < .001) periods. The greatest risk was observed in patients with EDS at 1 year (aOR 16.32; 12.54-21.67, P < .001). Of those with an instability event, patients with JHS experienced a significantly increased rate of surgery at 1 year (aOR 3.20; 1.61-7.28, P = .002) and 2 years (aOR 3.18; 1.70-6.62, P < .001). Of those treated with surgery, there was no significant difference in the rates of revision surgical intervention between the JHS and control cohorts.

Conclusions

Patients with JHS experienced significantly increased rates of patellofemoral instability and subsequent surgery. However, of those treated with surgery, there was no difference in rates of revision surgical intervention between those with or without joint hypermobility syndromes.

Level of Evidence

Level III, retrospective cohort study.

Decreased femoral trochlea axial orientation corrected by derotational distal femur osteotomy in patients with patellar dislocation yields satisfactory outcomes

PURPOSE

The femoral trochlea axial orientation has been shown to be a better predictor of patellar dislocation than the femoral anteversion angle. However, no study has investigated the importance of the femoral trochlea axial orientation in the surgical treatment of patellar dislocation. It is aimed to explore the pathological threshold of the femoral trochlea axial orientation and its guiding implications for surgical interventions in the study.

METHODS

Sixty-four patients with patellar dislocation and 64 controls were included for measurement of the femoral trochlea axial orientation. The ability to predict the patellar dislocation and the pathologic threshold of the femoral trochlea axial orientation were evaluated using the receiver operating characteristic curve. One hundred patients with medial patellofemoral ligament reconstruction and 25 patients with derotational distal femur osteotomy were divided into two groups based on the femoral trochlea axial orientation cut-off value and their postoperative knee functions, and patellar tilt angles were compared.

RESULTS

There were significant differences in the femoral trochlea axial orientation (60.8 ± 7.9 vs. 67.8 ± 4.6, p < 0.05) between patients with patellar dislocation and the normal population. The sensitivity and specificity of the femoral trochlea axial orientation were 0.641 and 0.813, respectively, at the femoral trochlea axial orientation smaller than 63.8°. Amongst patients having had isolated medial patellofemoral ligament reconstruction with decreased femoral trochlea axial orientation, knee function was poorer after surgery. The prognosis of patients with the femoral trochlea axial orientation correction in derotational distal femur osteotomy was better than that for patients without correction.

CONCLUSIONS

The femoral trochlea axial orientation had good predictive efficiency for patellar dislocation. Isolated medial patellofemoral ligament reconstruction is not sufficiently effective for patients with patellar dislocation and decreased femoral trochlea axial orientation. Patients with a decreased femoral trochlea axial orientation can have better surgical outcomes after correction by derotational distal femur osteotomy.

LEVEL OF EVIDENCE

Level III.

Predicting the Probability of Recurrence Based on Individualized Risk Factors After Primary Lateral Patellar Dislocation Treated Nonoperatively

PURPOSE

To develop a comprehensive and effective personalized scoring system on the basis of demographic and clinical characteristics for predicting recurrence probability in patients with primary lateral patellar dislocation (LPD).

METHODS

Participants included 261 primary patients with LPD with 2-year minimum follow-up from our hospital across 2013 to 2020. Demographic and clinical characteristics were collected retrospectively. The backward stepwise method was performed to identify independent predictors and construct a nomogram to predict the probability of recurrence. The predictive performance was assessed by receiver operating characteristic curves, calibration plots, and decision curve analysis.

RESULTS

After variables selection, 6 independent predictors of recurrence (skeletal maturity, trochlear dysplasia, tibial tuberosity-trochlear groove distance, mechanical axis deviation, Insall-Salvati index, and patellar tilt) were enrolled in our model. Validation of this nomogram in both training and validation cohort revealed powerful predictive ability, with an area under the curve of 0.962 and 0.977, respectively. The nomogram also showed great calibration and good clinical practicability.

CONCLUSIONS

Our study presented a nomogram that incorporates 6 independent risk factors (skeletal maturity, trochlear dysplasia, tibial tuberosity-trochlear groove distance, mechanical axis deviation, Insall-Salvati index, and patellar tilt), which can be conveniently used to accurately predicts the risk of recurrence after primary LPD in individual cases.

LEVEL OF EVIDENCE

Level III, retrospective comparative prognostic 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.

Adductor Canal Block Combined with General Analgesia for Patients with Recurrent Patellar Dislocation Undergoing "3-in-1" Procedure Surgery: A Prospective Randomized Controlled Trial

OBJECTIVE

Patellar dislocation is a common injury in sports medicine. While surgical treatment is an important option, pain is severe after surgery. This study compared the analgesic effect and early rehabilitation quality between adductor canal block combined with general analgesia (ACB + GA) and single general analgesia (SGA) after recurrent patellar dislocation (RPD) for "3-in-1" procedure surgery.

METHODS

From July 2018 to January 2020, a prospective randomized controlled trial was conducted in analgesia management after RPD for "3-in-1" procedure surgery. The 40 patients in the experimental group received ACB (0.3% ropivacaine 30 mL) + GA, while the 38 patients in the control group received SGA. Patients in both groups received "3-in-1" procedure surgery, standardized anesthesia, and analgesia during hospitalization. The outcomes included the visual analog scale (VAS), quadriceps strength, Inpatient Satisfaction Questionnaire (IPSQ), Lysholm scores, and Kujala scores. Total rescue analgesic consumption and adverse events were also recorded. One-way analysis of variance (ANOVA) was used to compare continuous variables between groups and chi-square or Fisher's exact tests were used to compare count data. Nonparametric Kruskal-Wallis H tests evaluated ranked data.

RESULTS

No significant differences in resting VAS scores were observed at 8, 12, and 24 h postoperatively. However, the flexion and moving VAS scores of the ACB + GA group were significantly lower than those of the SGA group (p < 0.05). Meanwhile, the first triggering of rescue analgesics was advanced in the SGA group (p < 0.0001), and the dose of opioid analgesics was significantly higher (p < 0.0001). The quadriceps strength of the ACB + GA group was higher than that of the SGA group at 8 h postoperatively. The IPSQ of the ACB + GA group was significantly higher 24 h postoperatively. We observed no significant differences in Lysholm and Kujala scores between the two groups at 3 months after surgery.

CONCLUSIONS

Early analgesia management of ACB + GA showed excellent analgesia effectiveness and a positive hospitalization experience for RPD patients undergoing "3-in-1" procedure surgery. Moreover, this management was good for early rehabilitation.

Conservative management following patellar dislocation: a level I systematic review

BACKGROUND

Patellar instability is a common and disabling clinical condition. Treatment of acute primary patellar dislocation aims to reduce the risk of recurrence or painful subluxation and improve function. However, the actual clinical efficacy of any management modality following an acute dislocation has never been demonstrated in prospective or retrospective studies, and the optimal way in which the various management modalities should be used is at best unclear.

METHODS

A search was conducted in PubMed, Bireme and Embase databases. Inclusion criteria followed the acronym PICOS, (P) subjects with patellar instability, (I) therapeutic interventions, (C) placebo or control or surgical treatments, (O) rate of dislocations and function, and (S) clinical trials. The Medical Subject Headings (MeSH) terms used were: (("patellar instability") OR ("patellar dislocation")) AND ((physiotherapy) OR (rehabilitation) OR ("conservative treatment") OR (therapy) OR (therapeutic)). The risk of bias was analysed using the PeDRO scale.

RESULTS

Seven randomized controlled trials including 282 patients were considered. The quality of studies detailing the results of conservative treatment was higher than that of surgical procedures, but all studies have relatively low methodological quality. Four studies compared physiotherapeutic interventions with surgical procedures, and three studies compared conservative intervention techniques.

CONCLUSION

An unstructured lower limb physical therapy programme evidences similar outcomes to specific exercises. Surgical management is associated with a lower rate of re-dislocation; however, whether surgery produces greater functional outcomes than conservative management is still unclear. The use of a knee brace with a limited range of motion, stretching and neuromuscular exercises are the most commonly recommended physiotherapy methodologies.

Concomitant anterior medializing osteotomy and MPFL reconstruction improves patellar tilt when compared to MPFL reconstruction alone

PURPOSE

Understanding how surgical procedures influence anatomic factors associated with patellofemoral instability can help guide surgeons when planning treatments for individual patients. This study sought to understand how patellar tilt is affected in adolescent patients with elevated pre-operative tibial tuberosity to trochlear groove (TT-TG) values undergoing medial patellofemoral ligament reconstruction (MPFLR) with or without an anterior medializing osteotomy (AMZ).

METHODS

Utilizing a prospective database of 274 patellofemoral instability patients who underwent MPFLR ± AMZ by one of two orthopedic surgeons at a single institution, those who underwent MPFLR + AMZ were identified. Pre-operative and post-operative magnetic resonance imaging (MRI) were used to measure TT-TG distance, while radiographs were used to measure patellar tilt (tilt). Patients were matched based on age at surgery (within 2 years) and pre-operative TT-TG distance (within 2 mm) to a comparison cohort of patients who underwent isolated MPFLR (iMPFLR) without osseous procedures.

RESULTS

A total of 56 patients were analyzed (28 per group). The mean age of the cohort was 15.5 ± 2.0 years and was similar between both groups (15.9 ± 1.9 versus 15.1 ± 2.0 years [n.s]). When comparing the two cohorts, significant pre- to post-operative decreases in patellar tilt for both MPFLR + AMZ (6.6 degrees, p < 0.001) and iMPFLR (3.9 degrees, p = 0.013) were noted. While there were no differences in pre-operative patellar tilt (21.2 ± 3.5 versus 21.1 ± 3.4 [n.s]), post-operatively, MPFLR + AMZ had significantly less patellar tilt than iMPFLR (13.2 ± 5.5 versus 16.5 ± 4.4, p = 0.017).

CONCLUSIONS

This study found that patellar tilt significantly improved in participants undergoing either MPFLR + AMZ or iMPFLR. In addition, those undergoing MPFLR + AMZ were found to have significantly lower post-operative tilt than those undergoing iMPFLR. If patellar tilt is found pre-operatively to be significantly elevated and a risk for future dislocations, these findings suggest that surgeons might strongly consider MPFLR with AMZ to further address the increased tilt.

LEVEL OF EVIDENCE

III.

Increased lower limb length ratio in patients with patellar instability

PURPOSE

Patellar height is a risk factor for patellar instability, correlated with the tibia length/femur length (T/F) ratio. This study aimed to explore the changes in the T/F ratio in patients with patella instability and the potential correlation with the morphology of the patellofemoral joint and extensor moment arm.

METHOD

A retrospective analysis was performed to assess the ratio of lower limb length morphological characteristics of the patellofemoral by full weight-bearing long-leg standing radiographs, magnetic resonance imaging, and computed tomography in 75 patients with patellar instability and 75 participants from a randomly selected control group from January 2020 to September 2021. A total of eight parts were measured, including mechanical tibia length/femur length (mT/F) ratio, anatomical tibia length/femur length (aT/F) ratio, hip-knee-ankle angle, femoral neck-shaft angle, femoral valgus cut angle, patellar height, Dejour classification, sulcus angle, trochlear angle, medial trochlear inclination, lateral trochlear inclination, patella tilt angle and patellar tendon moment arm to evaluate the difference of morphology between patient group and control groups.

RESULTS

The mT/F (0.840 ± 0.031 vs. 0.812 ± 0.026, p < 0.001) and aT/F (0.841 ± 0.033 vs. 0.808 ± 0.028, p < 0.001) ratios in the patient group were significantly greater than that in the control group. There was a significant correlation between patellar height and increased mT/F and aT/F ratios (p < 0.05).

CONCLUSION

Patients with patellar instability had a larger lower limb length ratio, and the change in lower limb length ratio was correlated with patellar height.

LEVEL OF EVIDENCE IV

Failure of isolated medial patellofemoral ligament reconstruction in children: Risk factors and management

Background

Patellofemoral instability is a frequent cause of referral in pediatric sports medicine. Isolated medial patellofemoral ligament reconstruction is widely used and provides satisfactory outcomes with a low failure rate. Given the success of this surgical technique, the literature on medial patellofemoral ligament reconstruction failures in the pediatric population is limited. Moreover, given the multifactorial nature of patellofemoral instability, the heterogeneity of the current literature, and the paucity of pediatric studies, medial patellofemoral ligament reconstruction failures are often difficult to analyze.

Methods

The purpose of this study was to retrospectively review the associated risk factors, surgical management, and the clinical outcomes at 2-year follow-up of skeletally immature patients that presented to our clinic with a failed medial patellofemoral ligament reconstruction.

Results

Of the 181 cases in 155 patients included in this study, treatment failed in 12 (7%). All 12 patients presented with at least one risk factor for patellofemoral instability, the most common being trochlear dysplasia and a high-grade J sign.

Conclusions

We conclude that isolated medial patellofemoral ligament reconstruction for patellofemoral instability in children has a low failure rate. Clinicians must assess pre-operative risk factors before surgical treatment is considered. A high-grade J sign and high-grade trochlear dysplasia were associated with medial patellofemoral ligament reconstruction failure in this cohort. Tailoring treatment to patients' associated risk factors selection may improve outcomes.

Efficient Recognition and Closed Reduction of Locked Lateral Patella Dislocation

Patellar dislocation is not an uncommon injury in the general population that can often be managed in the emergency room with a closed reduction. However, rarely, the patella can become impacted on the lateral femoral condyle and become resistant to closed reduction techniques, which is known as a locked patellar dislocation. This injury is reported in the literature as requiring advanced imaging with a costly workup, operative intervention, or extensive manipulation under general anesthesia, and, to our knowledge, has never been documented to be closed reduced outside the operating room. In this report, we present a 17-year-old male with a locked lateral patella dislocation and describe a new approach to efficiently diagnose and close reduce this injury in the ER under conscious sedation without advanced pre-treatment studies or urgent treatment in the operating room (OR).

Sulcus depth, congruence angle, Wiberg index, TT-TG distance, and CDI are strong predictors of recurrent patellar dislocation

PURPOSE

To characterize patellofemoral morphology, determine radiological cutoff values based on computed tomography (CT) images, and clarify predominant anatomic risk factors in young patients with recurrent patellar dislocation (RPD).

METHODS

A total of 212 knees (age 23.0 ± 5.3 years, female: male = 110:102) with CT-confirmed RPD were enrolled for analysis. The control group was matched by age and sex (2:1). Patellofemoral and trochlear morphology, patellofemoral alignment, and other parameters were assessed by CT. Standardized methods were used for measurements. Receiver operating characteristic (ROC) curves and the areas under the ROC curve (AUCs) were used to assess accuracy in diagnosing RPD. The index of union (IU) was employed to identify a cutoff value for risk. A binary logistic regression model (Forward, LR) was constructed, and standardized coefficients were calculated to compare the relative strength of different predictors. Factors included in the model were employed to calculate distributions in the studied population.

RESULTS

The majority of measurements showed a significant difference between the RPD and control groups when comparing mean values. The AUC value was favorable for sulcus depth and significantly better than that for other trochlear morphology parameters (P < 0.0001). Cutoff values were 0.58 for the Wiberg index, 153.3° for the sulcus angle, 3.6 mm for the sulcus depth, 1.94 for the trochlear width ratio, 16.2° for the lateral trochlear inclination, 17.2° for the patellar lateral tilt, 25.5° for the congruence angle, 1.2 for the patellar height [Caton-Deschamps index (CDI)], 6.1° for the knee rotation, and 16.4 mm for the tibial tubercle-trochlear groove (TT-TG) distance. A binary logistic regression model (χ² = 20.826, P = 0.008) including the Wiberg index, sulcus depth, congruence angle, patellar height (CDI), and TT-TG distance was constructed for RPD (P ≤ 0.015), which showed excellent diagnostic performance, as indicated by an AUC of 0.989 (95% CI, 0.979 to 0.998). Sulcus depth showed the strongest relationship with RPD, followed by the congruence angle and Wiberg index, as indicated by the standardized coefficients - 1.43, 1.02, and 0.97, respectively. Eighty-seven percent of the RPD population had sulcus depth ≤ 3.6 mm.

CONCLUSIONS

This study provides a systematic reference of cutoff values based on CT data. The combination of five anatomic risk factors (sulcus depth, congruence angle, Wiberg index, TT-TG distance, and CDI) may be an excellent predictor of RPD as initial risk assessment. Sulcus depth, as the most frequently observed anatomic risk factor, showed the best diagnostic performance among trochlear morphological parameters and the strongest relationship with RPD, followed by the congruence angle and Wiberg index. In clinical practice, the sulcus depth, congruence angle, Wiberg index, TT-TG distance, and CDI with the proposed cutoff values can be used in combination to evaluate RPD. When assessing trochlear morphology, sulcus depth is recommended as the best radiological parameter.

LEVEL OF EVIDENCE

Level III.

Axial orientation of the femoral trochlea is superior to femoral anteversion for predicting patellar instability

PURPOSE

The femoral anteversion angle is considered to be the same as femoral torsion; however, the femoral anteversion angle is strongly influenced by the femoral posterior condylar morphology. It remains unclear whether the femoral anteversion angle and axial orientation of the femoral trochlea can predict patellar instability. This study aimed to redefine the femoral inherent torsion, verify whether the femoral anteversion angle reflects the femoral inherent torsion, and compare the validity and calculate the cut-off values of the femoral anteversion angle and femoral trochlear axial orientation for predicting patellar instability.

METHODS

Seventy-three patients with patellar instability and 73 matched controls underwent computed tomography to measure the femoral anteversion angle, femoral inherent torsion, and femoral trochlear axial orientation. Pearson's product moment correlation coefficients and linear regression were calculated to determine correlations between measurements. Receiver operating characteristic curves and nomograms were plotted to evaluate the predictive validity of the femoral anteversion angle and femoral trochlear axial orientation for patellar instability.

RESULTS

All measurements showed excellent intra- and inter-observer reliability. Compared with the control group, the patellar instability group had a significantly larger femoral anteversion angle (25.4 ± 6.4° vs. 20.2 ± 4.5°) and femoral inherent torsion (18.3 ± 6.7° vs. 15.8 ± 3.4°), and significantly smaller femoral trochlear axial orientation (58.1 ± 7.3° vs. 66.9 ± 5.1°). The femoral anteversion angle and femoral trochlear axial orientation had area under the receiver operating characteristic curve values of 79 and 84%, respectively, and cut-off values of 24.5° and 62.7°, respectively. The calibration curve and decision curve analysis showed that the femoral trochlear axial orientation performed better than the femoral anteversion angle in predicting patellar instability. There was a strong correlation between the femoral anteversion angle and femoral inherent torsion (r > 0.8). Linear regression analysis of the femoral inherent torsion with the femoral anteversion angle as the prediction variate showed moderate goodness-of-fit (adjusted R² = 0.69).

CONCLUSION

The femoral anteversion angle moderately reflects the femoral inherent torsion. The femoral trochlear axial orientation is better than the femoral anteversion in predicting patellar instability in terms of predictive efficiency, consistency with reality, and net clinical benefit. These findings warn orthopaedists against overstating the role of the femoral anteversion angle in patellar instability, and suggest that the femoral trochlear axial orientation could aid in identifying at-risk patients and developing surgical strategies for patellar instability.

LEVEL OF EVIDENCE

III.

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.