A novel assessment of the differences in various measurements in patients with patella alta and baja: a retrospective cross-sectional study

Introduction

The Insall-Salvati index (ISI) is ratio measuring the patella height to the length of the patella tendon and most popular way to assess the position and balance of the patella (1). The patella is an important structure in the center of knee strength. One cause of malalignment or patellar instability occurs when the patella is located above normal (patella alta) or below (Patella baja). Since patellar height affects many angles and vectorial sum of forces related to the knee joint, this leads to malalignment and instability in the knee joint (2,3). Quadriceps patellar tendon angle (QPA) is a newly identified angle which reflect the axis of the quadriceps tendons (4). The force transmitted to the patella changes as the QPA changes. Consequently, the association between the specific measurements and QPA is still unknown, and the potential pathologies may be correlated with abnormal QPA values (5). There is only a limited number of studies on QPA and its association with patellofemoral measurements (4,5).

Patella alta and baja has a clinical aspect, high riding patella is associated with chondromalacia and knee pain (6-8). Low riding patella may lead to painful limitation of knee joint range of motion and anterior knee pain (9). If certain measurements are consistently related to patellar malalignment in patients with patella alta, these specific parameters could be considered clinically relevant and aid in choosing the right interventions.To better assess patients with knee pain or instability and tailor treatment plans accordingly, clinicians can utilize this knowledge. Surgeons may incorporate preoperative assessments of ISI and QPA into their surgical planning process, aiming to enhance postoperative outcomes.

The understanding the relationship between QPA and patellofemoral measurements holds promise for improving diagnostic precision, directing treatment approaches, and guiding surgical interventions for individuals experiencing knee pain, instability, or patellofemoral pathologies. The outcomes of this study have the potential to assist clinicians in formulating more precise and efficient management protocols for these conditions.

In this study, we evaluated whether there was a significant difference between the patients with patella alta and baja in terms of many patellofemoral measurements and pathologies. This is the first and pioneering study investigating the variation of QPA values according to patellar position. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-23-1722/rc).

Methods

Study design and participants

This was a retrospective study including patients with chronic anterior knee pain. We evaluated 446 patients who underwent knee magnetic resonance imaging (MRI) between the year of 2019–2021 in Basaksehir City Hospital. The ISI of these 446 patients were measured, patients with an abnormal ISI and patients with anterior knee pain for more than two years were included. Two hundred and forty-one patients with normal ISI were excluded from the study. Patients with previously known surgical history, formerly known malignancy history, prosthesis or ligament graft, Osgood-Schlatter disease, patients who were younger than 18-year-old were excluded from the study (Figure 1).

Figure 1 Flow diagram of subjects. MRI, magnetic resonance imaging; ISI, Insall-Salvati index.

Patients were divided into two groups according to their ISI (ISI >1.3 and ISI <0.8). The patellar tendon length, patellar height, lateral patellar tilt angle (LPTA), patella-patellar tendon angle (PPTA), QPA, medial and lateral trochlear inclination (MTI and LTI), patella angle, trochlear sulcus angle (TSA), trochlear groove depth (TGD), medial and lateral trochlea length (MT and LT), MT/LT ratio and tibial tubercle-trochlear groove distance (TT–TG), were compared between two groups. The presence of fat-pad edema, patellar chondromalacia and effusion were compared between two groups. It was investigated whether there was a significant difference according to gender and age in terms of patellar tendon length, patellar height, ISI, patella angle, TSA, TGD, MT, LT, LPTA, TT–TG distance, PPTA, QPA, MTI, LTI presence of fat pad edema, presence of chondromalacia and presence of effusion.

This retrospective study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). Ethical approval was obtained from the Local Scientific Research Ethics Committee (decision number: 137, date: 30.06.21). Informed consent for this retrospective analysis was waived. All images were appropriately anonymized. MRI of all subsequent was assessed by a radiologist with 7-year experience in the field of musculoskeletal imaging.

MRI protocol

MRI’s were performed in standard supine position while using dedicated knee coil. The coil enabled the standardized position in 10-15° of flexion (10). MRI’s were performed using 1.5-Tesla MRI.

Measurements

For the measurement of ISI, PPTA, QPA, patellar tendon length and patellar height the midsagittal slice was chosen, where the quadriceps tendon, patellar tendon and upper and lower ends of the patella could be seen (10). Patellar height was accepted as the distal pole to the proximal pole of the patella. ISI was obtained by dividing patellar tendon length with the longest diagonal diameter of patella, on the slice where the longest diameter of patella was seen (11,12) (Figure 2). On midsagittal images, PPTA was calculated as angle the between line connecting upper-lower pole of the patella and the tuberositas tibia. QPA was measured as the angle between the quadriceps tendon and a line drawn from the patella upper pole. (For description of the other measurement methods, see Figure 3).

Figure 2 Patella position is evaluated with ISI. (A) On sagittal T1 images, a high riding patella is seen, ISI (blue line) is calculated as 1.59. (B) On sagittal proton density images, a low-riding patella is seen, ISI ratio (blue line) is calculated as 0.58. ISI, Insall-Salvati index.

Figure 3 MRI evaluation of 40-year-old man with knee pain is demonstrated. (A) On axial proton density images, LPTA was the angle between a line parallel to the lateral patellar facet and a line drawn across the posterior femoral condyles (yellow angle). (B) On axial proton density images, MT (green line) and LT were measured (yellow line). (C) On axial proton density images, MTI was measured as the angle between the lateral trochlear facet and the transcondylar axis (green angle). LTI was measured as the angle between the lateral trochlear facet and the transcondylar axis (yellow angle). (D) On axial proton density images, patellar angle was the angle between the lines parallel to the medial and lateral patellar facets (yellow angle). TSA was measured as the angle between the lines parallel to the medial and lateral trochlear facets (green angle). On midsagittal images. (E) On sagittal proton density images, QPA was the angle between the lines connecting the long axes of the patellar and quadriceps tendons (4) (grey angle), PPTA was calculated as angle the between line connecting upper-lower pole of the patella and the tuberositas tibia (white angle) (4). (F) On axial proton density images TGD was the distance between the most anterior parts of the femoral trochlear facets and the deepest part of the trochlear groove (white vertical line). (G) On axial proton density images TT–TG distance: the lowest part of the trochlear groove was marked on subsequent slices. The middle point of the patellar tendon where it joins the tibial tubercle level was marked, and the distance (yellow line) separating these points is calculated. MRI, magnetic resonance imaging; LPTA, lateral patellar tilt angle; MT, medial trochlea length; LT, lateral trochlea length; MTI, medial trochlear inclination; LTI, lateral trochlear inclination; TSA, trochlear sulcus angle; QPA, quadriceps patellar tendon angle; PPTA, patella-patellar tendon angle; TGD, trochlear groove depth; TT–TG, tibial tubercle-trochlear groove distance.

Statistical analysis

Statistical analysis of the data was performed with the IBM Statistical Package of Social Science (IBM SPSS V26) and MedCalc (Version 19.3.1) package programs. A value of P less than 0.05 was considered as statistically significant. MRI of all subsequent was assessed by a radiologist with 7-year experience and data were only measured once. An intra- or inter-group difference was not analyzed. Data were stored in a local hospital database system. P value tests were two-sided. The difference between two arithmetic means was analyzed with T-test. The Mann-Whitney U test was used for assessing the difference between two independent groups and to determine difference or equality between groups.

Results

Finally, 205 (126 females, mean age 40.34±11.41 years, male mean age 38.32±10.97 years) patients with abnormal ISI values were included in our study. The mean age of women was found to be significantly higher than that of men (P=0.001) (Table 1). One hundred and twenty-five patients had ISI >1.3 and 80 patients had ISI <0.8. Fat pad edema was observed in more than one region in 11 patients and in a single anatomical location in 51 patients. Thirty-four of the patients were suprapatellar, 1 prefemoral, 9 non-SL (superolateral) Hoffa, 7 SLHoffa localized. Chondromalacia was observed in 156 patients. Effusion was present in 194 patients. Effusion was observed in more than one region in 160 of the patients and in a single anatomical location in 34 of them. The effusion observed in a single anatomical area was most frequently observed in the retropatellar bursa (n=27). Effusion was observed in the tibiofemoral joint space in 144, infrapatellar bursa in 19, retropatellar bursa in 173, and suprapatellar bursa in 113 patients.

The mean of LPTA and QPA values of the patients participating in the study were 13.11±7.20 and 134.99 ±8.72 (mean ± SD). PPTA, Q-PA, MTI and LTI, PA, TSA, TGD, MT, LT, MT/LT ratio and TT–TG Table 2 is also shown.

When the groups with ISI >1.3 and <0.8 were compared with each other in terms of measurements, TT–TG distance, LPTA, PPTA, and QPA were found to be significantly higher in the group with ISI >1.3 (P<0.005). Patellar height and TGD were found to be significantly higher in the group with ISI <0.8 (P<0.001, P=0.007) (Table 3). There was no significant difference between ISI groups (<0.8, >1.3) in terms of the presence of fat-pad edema, effusion and chondromalacia (all P>0.05) (Table 4).

There was no correlation between ISI and age (P=0.681, R=0.020). Patellar tendon length, patellar tendon height, TT–TG distance, patella angle, TGD, MT, LT, PPTA, QPA, MTI values and frequency of fat-pad edema were significantly higher in men (P<0.001). The ISI, TSA and frequency of chondromalacia was found to be significantly higher in women (P<0.001). There was no difference between the genders in terms of ML/LT, LPTA, LTI measurements and effusion pathologies. There was a negative low-level significant correlation between patellar tendon length and patellar height and age (P=0.002).

Discussion

The abnormal position of the patella significantly affects the vectorial sum of the biomechanical forces of the knee joint. Placing the patella in a position higher than normal causes low contact area and high contact stress. When the patella is placed in a position lower than normal it causes limitation of knee flexion (13-17). In our study, a large number of patients were included in the study, and no significant difference was found between the ISI groups (<0.8, >1.3) in terms of the presence of fat-pad edema, effusion, and chondromalacia. However, in our study, the findings detected in MRI. Öztürk et al. (18) assessed the associations between patellofemoral joint morphology and patellar chondromalacia. They reported that patients with ISI >1.3 have a significantly increased risk for developing chondromalacia. An abnormal value of TGD, TSA, ISI or MT\LT predispose a risk of severe chondromalacia. They argued that abnormality of one of these measurements would disrupt the normal function of the patella and its compatibility with the trochlear groove (18). The reasons for different results from our study may include the following. While the number of patients in our study was significantly higher compared to the study by Öztürk et al. (18), the mean age and severity score of chondromalacia patellae were lower in our study. This indicates that age may have an indirect effect on ISI values and presence of patella alta. In our study, ISI, TSA values and frequency of chondromalacia was found to be significantly higher in women (P<0.001). As individuals age, a decrease in muscle mass, an increase in adiposity, and hormonal factors such as estrogen in women, which cause ligament relaxation, may lead to a more frequent occurrence of patella alta. Both patella alta and chondromalacia are significant contributors to anterior knee pain.

Patellar tilt is a cause of malalignment and anterior knee pain in which altered patella is observed in axial images (14-20). Although normal reference values have not been clearly determined, LPTA >5° are considered to be increased in many publications (20). Increased LPTA generate excessive pressure on the lateral facet and can cause cartilage damage (5,20). However, there are very few studies that comprehensively evaluate the relationship between ISI values and LPTA. Recently, Conry et al. (21) conducted a dynamic study which investigated the impact of patella alta on the relationship between lateral patellar tilt in patients undergoing treatment for patellar instability. They reported that in patients experiencing patellar instability, lateral maltracking of the patella when the knee is extended is often associated with either a shallow trochlear groove or a combination of patella alta and a lateral patellar tilt. These findings coincide with the results in our study. In our study, in the group with ISI >1.3, patellar tendon length, TT–TG distance, LPTA, PPTA and QPA were found to be significantly higher compared to ISI <0.8 group (P<0.001).

Compared to the relationship between the angles involving the patella alta and the knee joint, the relationship between the patella baja and these angles is an area that has been less studied in the literature and can be considered relatively untouched. Jamalipour Soufi et al. (22). evaluated the relationship between osteoarthritis and knee anatomical variants. They found that there were no significant differences between cases and controls regarding patella baja. Unlike this, in our study we found that patellar height and TGD were significantly higher in the group with ISI <0.8 (P<0.001, P=0.007). The finding that the trochlear groove was deeper in patients with low riding patella in our study can be explained as follows. In patients with patella baja, the patellar tendon is relatively shorter, which may lead to a downward pull on the patella. This downward pull could result in increased stress on the trochlear groove, potentially causing it to deepen over time. Additionally, changes in patellar height can alter the biomechanics of the patellofemoral joint, potentially influencing the development of TGD. However, further research is needed to fully elucidate the mechanisms underlying this association.

QPA is an angle newly defined in the study of Cilengir et al. (4), which display the vectorial sum of the axis of the tendons. This first study assessing QPA evaluated the relationship between morphological measurement and QPA (4). They found that QPA is higher in patient with larger TSA and narrowed LTI. They also reported that larger QPA is related to fat pad oedema and quadriceps tendinosis. Oğuzdoğan et al. (5) evaluated the relationship between QPA and meniscal injury and found that QPA are predisposed risk for ACL tear. In our study; we evaluated a different aspect about QPA and assessed the values of patients with patella alta and baja. We found that QPA were increased in the group with patella alta.

Our study had some limitations. The retrospective design introduces inherent biases and constraints linked with collecting and analyzing retrospective data. In addition, during the data collection phase of the study, many complex measurements are operator dependent and some sections were selected and measured on MRI images. This may lead to differences in imaging techniques and interpretations, potentially affecting the accuracy and reliability of measurements. Secondly, limiting the generalizability of the findings to broader patients populations, the study focused solely on patients with chronic anterior knee pain. Large-scale future studies are therefore necessary.

Conclusions

In conclusion, the patients with patella alta had larger LPTA, PPTA, patellar tendon length, TT–TG distance and QPA. The patients with patella baja had higher patellar height and TGD. Variations in the tendon and trochlear sulcus may be the reason for the low riding placement of the patella. This research provide radiologist an better understanding for the differences in knee structures between patients with patella alta and baja. And lays the groundwork for more precise and informed ways to treat knee disorders. Future research could test the utility and reproducibility of this new measurement, QPA, in different groups of people. With this new measurement and detailed findings on existing ones like the ISI and TT–TG distance, physicians can improve how they diagnose patella alta and baja. Knowing the specific changes in the knee for patients with patella alta versus baja can lead to treatments that are more tailored to each patient’s needs. For example, since certain measurements like LPTA, PPTA, and TT–TG are higher in those with patella alta, treatments can focus on adjusting the knee’s alignment to help reduce pain and instability. The study also highlights the need to consider differences in anatomy and gender when preventing knee problems. Clinicians could create programs to screen and prevent knee issues in people who are more likely to get them, based on their knee structure and other related factors.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-23-1722/rc

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-23-1722/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This retrospective study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). Ethical approval was obtained from the Local Scientific Research Ethics Committee (decision number: 137, date: 30.06.21). Informed consent for this retrospective analysis was waived. All images were appropriately anonymized.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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