Measurement of distance between femoral insertion of fibular collateral ligament and popliteus tendon: A magnetic resonance imaging based study

Abstract

Background

Fibular collateral ligament (FCL), popliteus tendon (PT) and popliteofibular ligament are key components of posterolateral corner (PLC) of the knee. These structures play a vital role in stabilizing the knee against varus and external rotation. The Anatomical location of these structures should be well understood for adequate reconstruction of PLC injuries.

Purpose of our study is to measure mean distance between femoral insertion of FCL and PT; femoral medio-lateral dimension (MLD) and antero-posterior dimension (APD) of lateral femoral condyle on Magnetic Resonance Imaging (MRI) images and its relevance to anatomical reconstruction of PLC injuries of knee.

Methods

The knee MRI data of 68 patients (43 males and 25 females) were studied. Measurement of PT-FCL distance was done in sagittal proton density fat-saturated (PDFS) sequence images. The femoral MLD and APD were measured in axial PDFS sequence images. The mean of each parameter was calculated and analyzed. To identify inter-observer agreement, we calculated Intra-Class Correlation Coefficient (ICC) for each parameter.

Results

The mean PT-FCL distance was 16.85 ± 1.55 mm (range 12.9–21.2 mm). In our observation, PT always inserts anteriorly and distally to FCL insertion on the lateral femoral condyle. The mean MLD was 78.74 ± 5.84 mm (range 65–90 mm). The Mean APD of the lateral femoral condyle was 60.47 ± 4.00 mm (range 52.5–69.5 mm). The difference between the mean values of all the above parameters in both genders was statistically significant (p-value <0.05). Inter-observer agreement was very good for measurement of PT-FCL distance (ICC 0.96), MLD (ICC 0.98) and APD (ICC 0.97).

Conclusion

The mean distance between femoral insertion of FCL and PT in our study was less in comparison to most of the western literature. However, despite this anatomical variation, the method of PLC reconstruction proposed in western literature can be safely advocated in the Indian population as well.

1. Introduction

Fibular collateral ligament (FCL), popliteus tendon (PT), and popliteofibular ligament (PFL) are the main contributors to stabilization of the postero-lateral corner (PLC) of the knee.^{1, 2, 3} These structures play a vital role in stabilizing the knee against varus and external rotation.^{1, 2, 3} In 30° of the knee flexion, FCL and PFL functions as static stabilizers resisting varus and external rotation, whereas, PT works as a dynamic stabilizer of posterior tibial translation and external rotation in increasing knee flexion.^{1,4, 5, 6, 7}

Isolated PLC injuries are rare and commonly occur in combination to posterior cruciate ligament (PCL) or anterior cruciate ligament (ACL) injuries.^8,9 If PLC injuries are not adequately addressed, then the reconstructed cruciate ligaments can have significantly higher rates of failure and in long run osteoarthritis of the knee may inevitable.^{10, 11, 12, 13} Anatomic PLC reconstruction can only be done if the accurate location of various structures with relation to lateral epicondyle and to each other is well known. LaPrade at al⁵ mentioned that the distance between femoral insertion of FCL and PT is 18.5 mm, on the contrary, Takeda et al.¹⁴ concluded that the average distance to be 8.18 mm in Japanese population. There is a huge difference between these values which are largely based on studies done on small number of cadavers. Thus, a study based on larger population is necessary to assess the anatomical relation between femoral insertion of FCL and PT, before standardizing the method of reconstruction.

We feel embalming fluid in cadaver can cause a significant swelling of soft tissues which may lead to enhancement in erroneous measurements. It is also difficult to procure large numbers of cadaver for evaluation due to religious and moral factors. MRI has a superior image quality, easy availability; depth of tissue insertion can be easily seen and can accurately assess the knee ligaments. It is able to demonstrate findings similar to those obtained from anatomical dissection.¹⁵

It is acknowledged Asian population has a smaller distal femur size than that of the Western population.¹⁶ Hence, we feel that the distance between femoral attachment of FCL and PT is smaller in the Indian population than the western population. In our knowledge, there have been no MRI based studies for measurement of distance between femoral attachments of the FCL and PT. We conducted this study to find out the distance between FCL and PT femoral attachment in the Indian population, which would help in anatomical reconstruction of PLC in regional population.

2. Materials and methods

2.1. Study design

This is a retrospective cohort study.

2.2. Patients

We assessed MRI data of 68 patients (43 males and 25 females) between April 2019 to March 2020. The average age at evaluation was 30 years (range 16–52 years). Patients with isolated ACL/PCL/MCL injuries on MRI were included.

2.3. Exclusion criteria

Patients with injured PLC (isolated injuries or associated with ACL, PCL, MCL), fractures or bony pathologies (tumors/infections/condyle hypolasias) of distal femur were excluded.

2.4. Imaging technique

All MRI scans were performed using a 3-T MR scanner (Magnetom Trio 3T; Siemens Healthineers) with a dedicated knee coil (Tx/Rx 15-Channel). Slice thickness was set at 0.6–1.5 mm, with no spacing between images. The knees were held in 15° of flexion according to the MRI equipment protocol, with neutral rotation.

Following sequences were taken in all the patients: proton density fat saturated (PDFS), T2W, T1W, and short tau inversion recovery in coronal, axial, and sagittal planes. Measurements were taken by software (Radiant Dicom viewer).

2.5. Assessment

The measurement of MRI parameters in the study was done by radiologist who was not involved in initial reporting.

Femoral insertion of FCL and PT were identified in sagittal PDFS sequence images (Since the anatomy of the knee and its surrounding stabilizing structures is best depicted on PDFS), and midpoint of each was marked as the mid of maximum dimensions in two planes. Distance between the two points was then measured (Fig. 1). The maximum medio-lateral dimension (MLD) was measured as distance between medial and lateral epicondyle in axial PDFS sequence images (Fig. 2). Antero-posterior dimension (APD) of lateral femoral condyle was also measured in similar way (Fig. 3).

Fig. 1.

Sagittal Magnetic Resonance Imaging (MRI) image showing method of measurement of PT-FCL distance.

Fig. 2.

Axial Magnetic Resonance Imaging (MRI) image Showing method of measurement of femoral medio-lateral dimension (MLD).

Fig. 3.

Axial Magnetic Resonance Imaging (MRI) image Showing method of measurement of femoral antero-posterior dimension (APD).

2.6. Statistical analysis

Data were recorded by two observers to minimize observer bias. The mean value, standard deviation (SD) was calculated for each parameter. To identify intra-observer agreement we performed paired t-test with 95% confidence level. To identify difference between males and females an independent sample t-test with 95% confidence level was performed. To identify inter-observer agreement, we calculated Intra-class Correlation Coefficient (ICC). Statistical analyses of results were performed using SPSS software (version 22.0. Armonk, NY: IBM Corp.).

3. Results

We have found that there is no significant difference between two readings that were taken at interval of 15 days by same observer. We found inter-observer agreement was very good for measurement of PT-FCL distance (ICC 0.96), MLD (ICC 0.98) and APD (ICC 0.97) in the study population. The mean PT-FCL distance was 16.85 ± 1.55 mm (range 12.9–21.2 mm). In our observation PT was always found to inserts anteriorly and distally to FCL insertion on the lateral femoral condyle. The mean MLD was 78.74 ± 5.84 mm (range 65–90 mm). The Mean APD of lateral femoral condyle was 60.47 ± 4.00 mm (range 52.5–69.5 mm). The difference between the mean values of all the above parameters in both genders were statistically significant (p-value <0.05) (Table 1).

Table 1.

Mean Measurements of PT-FCL distance, fAPD and fMLD in present study on MRI.

	PT-FCL DISTANCE (MM)		fAPD (MM)		fMLD (MM)
	Mean	SD	Mean	SD	Mean	SD
MALES	17.48	1.36	62.31	3.49	81.84	4.28
FEMALES	15.75	1.36	57.36	3.01	73.55	4.45
p-value	0.04		0.03		0.03

Abbreviations: PT: popliteus tendon; FCL: fibular collateral ligament; MM: millimeter; SD: standard deviation; fAPD: femoral antero-posterior dimension; fMLD: femoral Medio-lateral dimension; MRI: magnetic resonance imaging.

4. Discussion

It is the first MRI based study done on Indian population to measure the distance between femoral insertion of FCL and PT. The mean measurement of this parameter was compared with previous studies in literature and found the difference to be statistically significant (Table 2). A cadaveric study by Takeda et al.¹⁴ was conducted in the Japanese population and concluded that the mean linear distance between FCL and PT femoral insertions was 8.18 ± 1.84 mm. They also observed that positional relationship between insertion of FCL and PT varied by specimen. Two different types of relationship, with the PT inserted parallel or posterior to the FCL insertion, were observed. Another cadaveric study by Brinkman et al.¹⁷ was performed in the Netherlands and observed that the mean distance between PT and FCL femoral insertion was 11 ± 4 mm. They also noted that PT inserts 0.84 mm anterior to FCL insertion with variation between 8.5 mm posterior to 9.5 mm anterior to FCL. In our observation PT always inserts anteriorly and distally to FCL insertion on lateral femoral condyle. Kim et al.¹⁸ after studying seven cadaveric knees in Korean population concluded that distance from the attached site of the FCL to the one of PT was 18.5 mm. A cadaveric study on Norwegian population by Laprade et al.⁵ in 10 cadavers, reported that the average distance between the attachments of the FCL and PT on the femur was 18.5 mm. The PT attachment was at the most anterior fifth of the popliteal sulcus, and its attachment was on the proximal half of the sulcus at this position. The PT attachment on the femur was always anterior to the FCL femoral attachment. Jung et al.¹⁹ studied 18 cadaveric knees in Korean population and identified three different patterns of PT attachment; postero-inferior, just inferior and bifurcate attachment, as compared to the FCL attachment on femur. They also mentioned that in most of the cases PT is attached at the postero-inferior site of the FCL attachment and is very close to it. Qualitatively, LaPrade et al.⁵ described the PT tendon as inserting proximally, at the anterior end of the popliteal sulcus on the lateral femoral condyle. It can be argued that difference in the PT-FCL distance among different studies could be due to the difference in the dimensions of distal femur. However, Table 3 demonstrates that although dimensions of femur in Japanese population are comparable to the present study, the PT-FCL distance is still significantly different.

Table 2.

The mean measurements of the PT-FCL distance in various studies.

	COUNTRY	PT- FCL DISTANCE (MM)	Significance to present study (p-value)
Laprade et al.5	Norway	18.5	NA
Kim et al.18	Korea	18.5 ± 1.5	0.026
Brinkman et al.17	Netherlands	11 ± 4	<0.001
Takeda et al.14	Japan	8.18 ± 1.84	<0.001
Present study	India	16.85 ± 1.55

Abbreviations: PT: popliteus tendon; FCL: fibular collateral ligament; MM: millimeter; NA: Not available.

Table 3.

The mean measurements of fMLD and fAPD in different ethnicities across the world as found in the Literature.

	ETHNICITY	fMLD (MM)	Significance to present study (p-value)	fAPD (MM)	Significance to present study (p-value)
Terzidis et al.20	Greek males	78.3 ± 4.2	<0.01	53.2 ± 3.3	<0.01
	Greek females	71.5 ± 3.0	<0.01	51.1 ± 2.1	<0.01
Cheng et al.21	Chinese males	74.4 ± 2.9	<0.01	66.6 ± 2.4	<0.01
	Chinese females	66.8 ± 3.1	<0.01	61.6 ± 2.7	<0.01
Chung et al.22	Korean males	76.1 ± 4.0	<0.01	67.2 ± 3.9	<0.01
	Korean females	67.9 ± 3.3	<0.01	61.1 ± 3.2	<0.01
Ishimaru et al.23	Japanese males	82.6 ± 3.38	0.295	63.4 ± 2.91	0.068
	Japanese females	73.4 ± 3.66	0.850	58.9 ± 3.63	0.011
Present study	Indian males	81.84 ± 4.28		62.31 ± 3.49
	Indian females	73.55 ± 4.45		57.36 ± 3.01

Abbreviations: MM: millimeter; fMLD: femoral medio-lateral dimension; fAPD: femoral antero-posterior dimension.

The measured mean values of femoral APD and MLD in females were smaller than males in our study which was similar to observation in previous literatures in different populations.^{20, 21, 22, 23} It is acknowledged that the Asian population has a smaller distal femur size than that of the Western population and it could also affect the distance between PT-FCL. Therefore we have measured the femoral condyle dimension in our Indian population. In our observation we also found that patients with smaller distal femoral dimensions had shorter PT-FCL distance.

• Serra Cruz et al.²⁴ recommended using a drill bit of 9 mm for drilling femoral tunnels for both FCL and PT. This would not be practical if we consider 9 mm drill bit for 8.18 mm as the mean distance as given by Takeda et al.¹⁴ as it led to merging of the tunnels. In our observations as the average distance between LCL and PT was 16.85 ± 1.55 mm (range 12.9–21.2 mm), we can use graft thickness of 9 mm for both FCL and PT femoral insertion without risk of tunnel confluence in the Indian population. Hence, intra-operatively, we can follow the method of reconstruction given by Serra Cruz et al.²⁴ as the anatomical similarity favors it. Surgeons should be vigilant about the shortest PT-FCL distance of at least 10 mm to prevent tunnel confluence before using 9 mm graft thickness. For tunnel size less than 10 mm one should use smaller graft size and/or make divergent tunnel to avoid confluence.

• There are certain weaknesses in present study. Firstly, this is a single center retrospective cohort study. India is a conglomerate of different races and these finding might not hold true for each one of them. Secondly, a small sample size of this study may not exactly represent the whole Indian population however we have attempted this important anatomic study to open the gateway for further studies to strengthen our findings and find out the differences exists; if any.

5. Conclusion

The mean distance between femoral insertion of FCL and PT in our study was less in comparison to most of the western literature. However, despite this anatomical variation, the method of PLC reconstruction proposed in western literature can be safely advocated in the Indian population as well.

Declaration of competing interest

The authors declare that they have no conflict of interest.