The SP-ET Index is a New Index for Assessing The Vertical Position of Patella

doi:10.21203/rs.3.rs-957545/v1

Background.

There were some parameters in previous studies did not better reflect the vertical position of the patella relative to the femoral trochlear. The purposes of this study were to assess the value of the most superior point of patella - entrance of femoral trochlea distance ratio (SP-ET index) as a newer index in defining the vertical position of patella relative to the trochlea, correlate it with the Insall-Salvati ratio, and investigate the effect of the new index on chondromalacia patella.

Methods.

A total of 99 knees of 77 patients with chondromalacia patella were retrospectively analyzed using magnetic resonance imaging (MRI) data from our database. The Insall–Salvati ratio and SP-ET index were measured on MR images. There were 99 knees just with meniscus rupture were enrolled as the control group. The two parameters of the chondromalacia patella were compared with those of the control group.

Results.

The Insall–Salvati ratio and SP-ET index in the chondromalacia patella group were significantly higher than those in the control group (p < 0.001). The SP-ET index showed a moderate positive correlation with the Insall–Salvati ratio (r=0.307, p<0.001). Receiver operating characteristic (ROC) analysis showed the diagnostic efficiency of the SP-ET index was better than that of the Insall–Salvati ratio in patients with chondromalacia patella.

Conclusion.

The SP-ET index may be a useful complement parameter to define the vertical position of the patella relative to the femoral trochlear. Increased of the SP-ET index may be an important risk factor for chondromalacia patella.

Orthopedics

Orthopedic Surgery

Chondromalacia patella

the SP-ET index

The Insall–Salvati ratio

Patellofemoral joint

vertical position

The primary anatomical factors which were associated with patellofemoral malalignment included: patella alta, trochlear dysplasia, and abnormal lateral patellar tilt [1, 2]. Patellofemoral malalignment resulted in abnormal joint contact and chondral stresses within the patellofemoral compartment, and often associated with chondral damage and chronic anterior knee pain [3]. Chondromalacia patella is characterized by softening, swelling, fissuring or fraying of the patellar cartilage and can lead to a decrease in quality of life, it is one of common causes of chronic anterior knee pain [4–6]. A progressive loss of articular cartilage can lead to the development of patellofemoral osteoarthritis[7, 8]. The most commonly cited hypothesis is patellofemoral malalignment and/or maltracking leads to pathological loadings on the patellofemoral joint, and increases subsequent articular cartilage wear[8, 9], and we found that there have been several studies focusing on the relationship between patellofemoral malalignment and chondromalacia patella.

Duran S et al[10] and Tuna BK et al[5] were reported that there was an association between abnormal trochlear morphology and chondromalacia patella in women. In addition, the study of Tabary M et al showed that the patellar height might be an important factor in the severity of chondromalacia patella[11]. In the study by Deniz Özel, both patella alta and patella baja were found to be related to chondromalacia patella, and the modified Insall-Salvati index ratio was the best indicator to define patella alta[12]. Patella alta may cause excessive lateral motion of the patella during knee flexion[13], and cause the patella may not be able to engage the trochlear groove early in knee flexion[14], all of these could the development of chondromalacia patella. Multiple methods of assessing the vertical position of the patella on imaging have been described in previous literature[14, 15]. Most noteworthy, some studies suggested that the ratio between the articular cartilage of the patella and the trochlear cartilage (parameters Patellotrochlear index[16] and Patellar Articular Overlap[14]) could be better used to assess the vertical position of the patella in patients with patellofemoral instability.

In addition, there were some researches focusing on the role of trochlear dysplasia in chondromalacia patella or patellofemoral cartilage lesions[8, 10, 17], and the study of Ambra LF et al[8] suggested that trochlear dysplasia might be more important than the patellar malposition in the pathogenesis of patellofemoral cartilage lesions. But the measurement parameters defining the vertical position of the patella in some studies did not better reflect the vertical position of the patella relative to the femoral trochlear as Patellotrochlear index[16] and Patellar Articular Overlap[14] do. Therefore, the present study aimed to identify the ratio of the most superior point of patella-entrance of femoral trochlea distance to the length of the patella articular surface on sagittal MRI as a newer index which can define the vertical position of patella relative to the trochlea, and investigate the effect of this parameter on chondromalacia patella. We hypothesized that the new parameter was a good indicator to define the vertical position of the patella, moreover, increased of this parameter might be a potential risk factor for developing chondromalacia patella.

Participants

This retrospective study was approved by the Committee for Human Research of our institution (No. 2021-203). We retrospectively reviewed patients who were diagnosed with the modified Noyes classification system grade 1-2B chondromalacia patella based on MRI between January 2020 to April 2021 using picture archiving and communication systems at our department. Inclusion criteria were as follows: (1) patients with typical clinical presentation and MR signs with chondromalacia patella; (2) the flexion angle of the knee in MR images was 0-10°. Exclusion criteria were as follows: (1) participants with a history of traumatism or surgery of the knee; (2) presence of patellar dislocation, meniscus or ligamentous lesions; (3) presence of severe osteoarthritis (≥ KL 3 grade) or inflammatory arthritis; and (4) poor MR image quality (Fig. 1). In order to obtain a control group of participants without anterior knee pain and without any detectable alignment abnormalities or structural lesions on MRI and arthroscopy, patients undergoing meniscus rupture repair were chosen. All the clinical information was acquired by reviewing the clinical records or collected by telephone for medical history if uncertain on records.

MRI protocol

MRI was performed using 1.5-T MRI equipment (Magnetom Essenza; Siemens Healthcare) with an extremity matrix knee coil (a Tim coil). The imaging protocol was as follows: sagittal and coronal T1 turbo spin echo (repetition time/echo time 306/ 12 ms, field of view 16 cm, matrix size 320 × 320, slice thickness 4 mm), sagittal T2 turbo spin echo (repetition time/echo time 3220/99 ms, field of view 16 cm, matrix size 320 × 320, slice thickness 4 mm), sagittal and coronal intermediate-weighted turbo spin echo with fat saturation (repetition time/echo time 2800/38 ms, field of view 16 cm, matrix size 256 × 256, slice thickness 4 mm), and axial intermediate-weighted turbo spin echo with fat saturation (repetition time/echo time 2800/51 ms, field of view 16 cm, matrix size 232 × 256, slice thickness 4 mm).

Image analyses

All MRI images were independently reviewed by two musculoskeletal radiologists (Qiu and Sheng, with 3 and 13 years of clinical experience respectively). The routine MR imaging protocol were used to grade the articular cartilage on the patellofemoral joint according to the modified Noyes classification system: grade 0= normal cartilage, grade 1= increased T2 signal intensity of morphologically normal cartilage, grade 2A = superficial partial-thickness cartilage defect less than 50% of total articular surface thickness, grade 2B = deep partial-thickness cartilage defect greater than 50% of the total articular surface thickness, and grade 3 = full thickness cartilage defect [18, 19] (Fig. 2). The MRI measurements were performed by two other fellowship-trained musculoskeletal radiologists (Li and Yu, with 7 and 18 years of clinical experience respectively) after all the data of the two groups were mixed and blindly and randomly distributed. All measurements were repeated by one of the radiologists (Li), and the interval between the first and second reading was at last 4 weeks.

Two quantitative parameters for evaluating patellar position were assessed on the images of each MRI examination. The Insall–Salvati ratio was calculated as the patellar tendon length divided by the maximum patellar diagonal length on sagittal MRI image (Fig. 3a). The Insall–Salvati ratio was used to reflect the height of the patella, and the threshold was defined as >1.2[20]. The most superior point of patella - entrance of femoral trochlea distance ratio (SP-ET index) was used to reflected the vertical position of the patella relative to the trochlea. The entrance of femoral trochlea was the first craniocaudal plane where the complete cartilaginous trochlea can be seen[21, 22]. The sagittal plain with the greatest longitudinal diameter of the patella was used to obtain the SP-ET index which was calculated as the distance between the most superior point of patella and the entrance of femoral trochlea divided by the patellar articular surface length (Fig. 3b-c).

Statistical tests

In this study, descriptive data were calculated for demographic data. The subjects were categorized, according to the inclusion and exclusion criteria, into the chondromalacia patella group and the control group. Intraclass correlation coefficients (ICCs) were used to assess the intra- and inter-reader reliability of MRI quantitative measures. Continuous variables with a normal distribution were expressed as mean ± standard deviation, and Student’s t test was used to detect the difference. The Spearman correlation was used to analyze the relationship between the two parameters. The above statistical analyses were performed using Windows software SPSS (version 17.0; SPSS, Chicago, IL, USA), and the significance level was set at 0.05. In addition, the diagnostic efficacy of two measurements of the patellar position were determined with receiver operating characteristic (ROC) curve analyses, it was performed using MedCalc statistical software (Med-Calc Software, Mariakerke, Belgium). Optimum cut-off point, sensitivity, specificity, and area under the curve (AUC) of each parameter were calculated, then, comparisons of ROC curves between the two groups, and the significance level was set at 0.05.

Demographic characteristics of the subjects in the two groups

During the study period, there was a total of 99 knees (25 left, 30 right, and 22 bilateral) of 77 patients (45 males and 32 females, age range 23–50 years, mean age 43 ± 7 years) were included in the chondromalacia patella group. There were 99 knees (51 left, 42 right, and 3 bilateral) of 96 participants (62 males and 34 females, age range 17–45 years, mean age 33 ± 8 years) in the control group (Table. 1).

Intra- And Inter-observer Reliability Of Quantitative Mri Measurements

The intra-reader ICCs for the SP-ET index and Insall–Salvati ratio quantitative measurements were 0.930 and 0.875, respectively. and the inter-reader ICCs for the SP-ET index and Insall–Salvati ratio were 0.890 and 0.973, respectively, as shown in Table. 2, indicating good intra and inter-reader reliability.

Comparison between the chondromalacia patella group and control group and the correlation between the two parameters

The two measurement parameters showed significant differences between chondromalacia patella group and control group. The Insall–Salvati ratio and SP-ET index in the chondromalacia patella group were significantly higher than those in the control group (p < 0.001) (Table. 3). The SP-ET index showed a moderate positive correlation with the Insall–Salvati ratio (r=0.307, p<0.001).

Roc Analyses Of Mri Quantitative Measurements

The ROC curves of the Insall–Salvati ratio and SP-ET index were generated (Table 4, Fig. 4). The area under the curve (AUC) of the SP-ET index was 0.803 (95% CI 0.740–0.856; Youden’s index 0.525; cutoff value 0.833; sensitivity and specificity were 0.859 and 0.667, respectively). The AUC of the Insall–Salvati ratio was 0.699 [95% confidence interval (CI) 0.629–0.762; Youden’s index 0.343; cutoff value 1.125; sensitivity and specificity were 0.424 and 0.911, respectively]. The AUC of the SP-ET index was significantly higher than the AUC of Insall–Salvati ratio (p = 0.027).

The main findings of this present study were twofold. Firstly, our data showed that increased of the SP-ET index was relationship with the risk of developing chondromalacia patella. Secondly, the SP-ET index might be a better indicator to define the vertical position of the patella relative to femoral trochlear than the Insall–Salvati ratio in chondromalacia patella patients. These findings supported our hypothesis and indicated that the SP-ET index may be a useful complement parameter to define the vertical position of the patella relative to the femoral trochlear, and a higher position of patella might play a role in the pathogenesis of chondromalacia patella.

The previous researches have analyzed the influence of patellofemoral malalignment on chondromalacia patella or patellar cartilage defects [5, 8, 10, 12, 17, 23]. Duran S et al[10] suggested that abnormal trochlear morphology was increased the risk of chondromalacia patella in women and, in particular, women with a shallow trochlea and/or flattened lateral trochlea, this conclusion was similar to the result of Tuna BK et al[5]. In addition, the study of Tuna BK et al also found that patellar tilt was related to the presence of chondromalacia patella. Some studies found that a higher position of patella was an important influencing factor of chondromalacia patella[8, 12, 17], and in addition to the standard measurements of patellar height, some other measurements were analyzed, such as modified Insall-Salvati index[12, 17, 24], patellotrochlear index and patellophyseal index[17, 24]. Overall, patellofemoral malalignment and structural abnormalities of the patellofemoral joint leaded to pathologic pressure on the patellofemoral joint might be an important pathogenesis of chondromalacia patella[17, 24].

The Insall-Salvati ratio was defined as the length of the patellar tendon divided by the length of the patella, which was a validated and widely used index for evaluating the position of patella[20, 25]. The Insall-Salvati ratio can be measured on both lateral radiographs and sagittal MR image of knee. A higher position of patella might result in failure of the patella to engage the trochlear groove early in knee flexion [14], and decreased the contact area between the patellar articular surface and trochlea[7, 13]. Previous studies have shown that patella alta was associated with patellofemoral joint pain, instability, chondromalacia, and osteoarthritis [1, 7, 26, 27]. In this study, although the Insall–Salvati ratio was significantly higher in the chondromalacia patella group than that in the control group, there were just 24.2% (24/99) of patients in the chondromalacia patella group showed a pathologic Insall–Salvati ratio of >1.2, which coincided with the results of Ambra LF et al[8]. Moreover, Ali SA et al[28] found that the Insall-Salvati ratio did not correlate with the severity of patellotrochlear articular cartilage defected. In our opinion, the Insall–Salvati ratio in most patients with chondromalacia patella were in the normal range, which might reduce the instruction of this parameter in practical clinical work. Therefore, it was of great clinical impact to find more effective and direct evaluation indicators.

In the present study, the SP-ET index was used to defione the position of the patella relative to the femoral trochlear, and SP-ET index showed a moderate correlation with the Insall–Salvati ratio. Ali SA et al[28] and Mehl J et al[17] had used the patellophyseal index to assess patella height in their studies, this parameter was calculated as the distance from the most superior point of the patellar cartilage to the most superior point of the femoral cartilage divided by length of patellar articular cartilage, there were some similarities between the SP-ET index and patellophyseal index. However, the measurement of the SP-ET index only needed to be done at one plain (the sagittal plain with the greatest longitudinal diameter of the patella), and without the effect of axial displacement of patella. Therefore, we thought that the SP-ET index had a good practicability, and the intra and inter-reader reliability of this parameter were also good in our study.

Despite Mehl J et al[17] suggested that no significant difference of the patellophyseal index between the patellar cartilage defect group and control group. But different from the study of Mehl J et al[17], the study of Ali SA et al [28] have found that there was statistically significant difference in the patellophyseal index between normal group and severe patellar cartilage defect group. In the present study, we found that the SP-ET index in chondromalacia patella group was significantly higher than that in control group, and the AUC of the SP-ET index was significantly higher than that of Insall–Salvati ratio. The difference of the results might be related to the different severity of the included participants in these studies, and the strict control of the flexion angle of the knee in our study. Because in our practical measurement process, it was found that the flexion angle of the knee had a great impact on the value of the SP-ET index. According to the results of these studies, we thought that the correlation of the SP-ET index and patellophyseal index with the occurrence and severity of chondromalacia patella/ patellar cartilage defect required prospective and biomechanical studies to explore this further.

Our study has several limitations. First, this was a retrospective study, and the selection of participants in the chondromalacia patella group were based on MR images without histologic or pathologic evidence. Second, because of the measurement errors might be caused by the formation of patella spurs, only patients with grade 1-2B chondromalacia patella (based on the modified Noyes classification system) were included in the patient group, therefore, patients with severe chondromalacia patella were excluded in this study. Third, the flexion angle of the knee has a great influence on the SP-ET index, so the measurement of the SP-ET index needs to strictly control the flexion angle of the knee. In addition, there were no strict age and gender matching between the two groups of participants. Future research efforts should focus on the relationship between the SP-ET index and patellofemoral osteoarthritis using a prospective longitudinal study, and the role of the SP-ET index in different grades of chondromalacia patella.

The SP-ET index can be easily measured to define the vertical position of the patella relative to the femoral trochlear, and it showed a moderate correlation with the Insall–Salvati ratio which was a conventional measurement of patellar height. ROC analysis showed the diagnostic efficiency of the SP-ET index was better than the Insall–Salvati ratio in patients with chondromalacia patella. Therefore, the SP-ET index showed promise as a useful complement parameter to define the vertical position of the patella, and a higher position of patella might be one of the factors of developing chondromalacia patella.

AUC Area under the curve

CI Confidence interval

ICCs Intraclass correlation coefficients

MRI Magnetic resonance imaging

PL Patellar diagonal length

PT Patellar tendon length

SP-ET Most superior point of patella - entrance of femoral trochlea distance

ROC Receiver operating characteristic

Ethics approval and consent to participate: Institutional Review Board approval was obtained by The First Affiliated Hospital of Chongqing Medical University. (No. 2021-203).

Consent for publication: Not applicable (Written informed consent for this retrospective analysis was waived).

Availability of data and materials: The data used and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests: The authors declare that they have no competing interests.

Funding: No funding was received for this study.

Authors' contributions: Wei Huang Conceptualized the study, Carried out the statistical analysis, reviewed the manuscript, Lead author of original trial data. Jia Li collected and analyzed the data, Wrote the manuscript. Lanyu Qiu Carried out the statistical analysis, reviewed the manuscript. Bo Sheng Collected and analyzed the data. The author(s) read and approved the final manuscript. Jia Li and Lanyu Qiu contribute equally to this work.

Acknowledgements: Not applicable.

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Table 1

Descriptive data
	chondromalacia patella group (n=99)	control group (n=99)
Age	43 ± 7	33 ± 8
Gender (n, %)
male	32(41.6%)	62(64.6%)
female	45(58.4%)	34(35.4%)
Side (n, %)
right	30(39.0%)	42(43.8%)
left	25(32.5%)	51(53.1%)
bilateral	22(28.5%)	3 (3.1%)
Patellar alta (n, %)	24(24.2%)	—

Table 2 The intra- and inter-observer agreement between two radiologists of quantitative MRI measurements

Quantitative parameters	Intra-observer			Inter-observer
Quantitative parameters	ICCs (95%CI)	f value	p value	ICCs (95%CI)	f value	p value
SP-ET index	0.930 (0.863-0.964)	14.088	<0.001	0.890 (0.799-0.945)	8.855	<0.001
Insall-Salvati ratio	0.875 (0.754-0.936)	7.846	<0.001	0.973 (0.943-0.987)	38.141	<0.001

SP-ET index: the most superior point of patella - entrance of femoral trochlea distance ratio.

ICCs: intraclass correlation coefficients.

CI: confidence interval.

Table 3

Comparison of MR quantitative parameters between the chondromalacia patella group and control groups
	SP-ET index	Insall-Salvati ratio
chondromalacia patella group	0.933±0.115	1.100±0.145
control group	0.802±0.108	1.004±0.086
t value	8.227	5.709
P value	<0.001	<0.001

Table 4

The results of ROC analysis of the SP-ET index and Insall-Salvati ratio
	AUC (95%CI)	Abnormal MRI cutoff value
SP-ET index	0.803(0.740-0.856)	>0.833(se= 0.859, sp=0.667)
Insall-Salvati ratio	0.699(0.629-0.762)	>1.125 (se=0.424, sp=0.911)
z value	2.216	—
p value	0.027	—

ROC: receiver operating characteristic.

The SP-ET Index is a New Index for Assessing The Vertical Position of Patella

Status:

Version 1

Abstract

Background.

Methods.

Results.

Conclusion.

Figures

Background

Materials And Methods

Results

Discussion

Conclusions

Abbreviations

Declarations

References

Tables

Status:

Version 1