Awareness, Prevalence, and Magnetic Resonance Imaging Landmarks of the Aberrant Anterior Tibial Artery of the Knee

Abstract

Background:

Anatomic variations of the popliteal artery are critical to recognize during preoperative planning to avoid complications. The aberrant anterior tibial artery (AATA) arises higher than the usual site of the anterior tibial artery, and it passes between the posterior tibial cortex and anterior to the popliteus muscle.

Purpose:

To assess AATA prevalence, magnetic resonance imaging (MRI) landmarks, intraoperative vascular injuries, and surgeon acknowledgment of the AATA presence.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

This study was conducted in 2 stages. During the first stage, 787 knee MRIs were reviewed to detect the AATA prevalence. In the second stage, 10 board-certified knee and sports surgeons were invited to comment on 10 knee MRIs, 5 of which showed AATA variation. This assessment was performed 3 times: (1) at baseline; (2) immediately after an instructional orientation session; (3) 2 weeks after the orientation session.

Results:

The prevalence of the AATA was found to be 2.03%. MRI measurements revealed that the AATA was, on average, 5.69 mm from the tibial attachment of the posterior cruciate ligament (PCL) and 9.89 mm from the posterior horn of the lateral meniscus. The anatomic variation of the AATA was most consistently visualized at the level of the tibial attachment of the PCL in 81.25% of cases. Before instructional orientation, observer surgeons failed to identify the presence or absence of the AATA on all MRI images. However, after the orientation, all observers accurately identified the AATA in both positive and negative cases immediately and after 2 weeks, achieving 100% intraobserver reproducibility.

Conclusion:

Popliteal artery anatomic variations may often be overlooked in preoperative planning; a detailed understanding of these variations is crucial to prevent unfortunate intraoperative complications. None of the surgeons detected the AATA on the first encounter. This highlights the importance of education in improving the detection and diagnosis of the AATA.

The femoral artery continues as the popliteal artery after it passes through the adductor hiatus. Typically, the popliteal artery travels across the popliteal fossa and bifurcates into the anterior and a trunk, which then subdivides into the posterior tibial and peroneal arteries at the lower border of the popliteus muscle. ²² The popliteal artery is in proximity to the posterior tibial cortex, with a distance of <1 cm in an extended knee. Flexion of the knee increases this distance, which puts the neurovascular structures at a safer distance during orthopaedic surgeries.^9,19

Multiple variations of the popliteal artery anatomy and its branches have been reported. These anatomic variations may often be overlooked in preoperative planning. A detailed understanding of these variations is crucial to prevent unfortunate and catastrophic intraoperative complications. One of these variations is the aberrant anterior tibial artery (AATA), which arises from the popliteal vasculature at a higher level compared with the typical site. It passes anterior to the popliteus muscle or its tendon, placing it in a riskier position. The incidence of popliteal artery injury is around 0.003 in iatrogenic injuries, ⁷ and the reported ¹⁰ prevalence of an AATA is around 2% to 3%.

Several classifications have been described for the branching patterns of the popliteal vasculature. Kim et al ⁹ proposed a system that classifies these variations into 3 main categories, which are then subcategorized. Type 1 describes the typical variance of branching, which is at the level of the knee joint. Type 2 higher level of branching occurs above the knee joint; type 3 is characterized by hypoplastic infrapopliteal vessels with diminished supply to the foot. Type 2 is subdivided into 4 subtypes; the AATA is classified as type 2A2.

This study aimed to (1) determine the presence of the AATA in the Saudi population and its magnetic resonance imaging (MRI) landmarks; (2) investigate the presence of intraoperative vascular injuries related to the presence of this variation; and (3) assess the knowledge of this anatomic variant among orthopaedic knee and sports surgeons.

Methods

This retrospective cohort study was conducted in a tertiary center in Riyadh, Saudi Arabia. First, we conducted a retrospective review of 787 knee MRI studies to assess the prevalence of an AATA among the general population. These MRIs were performed for a variety of clinical indications. Two musculoskeletal board-certified radiologists (S.H.A. and M.S.A.) systematically examined all MRIs for the presence of the AATA. For patients who subsequently underwent knee surgery, both the imaging and surgical records were reviewed to identify any intraoperative complications potentially related to the presence of the AATA. The MRI scanner used was a 3T MR system (MAGNETOM Skyra Siemens Healthcare GmbH), with a 15-channel phased-array knee radiofrequency coil. The Position was a full extension for all the routine knee MRIs in this study. All MRIs had 3 T quality with a slice thickness of 2 mm. Images were obtained for the knee joint while the patients were in the supine position and the knee was extended. The picture archiving and communication system was used for medical technology to securely store and digitally transmit images for accurate measurement and reports in each MRI examination.

In the second phase of the study, we selected 10 knee MRI scans, 5 showing AATA variation and 5 that did not (Figure 1).

Figure 1.

(A) PD fat–saturated MRI sequence, axial cut of “typical” popliteal artery anatomic variant (red dot) at the level of tibial insertion of the PCL. (B) Axial cut of PD fat–saturated MRI sequence of a knee showing the AATA (white arrow). AAT, aberrant anterior tibial artery; MRI, magnetic resonance imaging; PCL, posterior cruciate ligament; PD, proton density.

We invited 10 board-certified orthopaedic knee and sports surgeons to participate and test their ability to accurately identify the presence or absence of the AATA on these MRIs. All orthopaedic surgeons were asked to review the MRIs and comment on any pathology or abnormal anatomy in these images. This assessment was performed at 3 intervals: at baseline; immediately after an instructional orientation session; and as a delayed assessment 2 weeks after the orientation session. The instructional sessions provided for orthopaedic surgeons included popliteal artery variation anatomy, AATA anatomy, and methods to detect it on MRI.

Statistical Analysis

Data analysis was conducted using SPSS statistical software (IBM). Descriptive data were summarized and reported across the study cohorts using descriptive statistics. Categorical variables were summarized and reported in terms of frequency distribution and compared statistically using the chi-square test. P≤ .05 and 95% CIs were used to report the statistical significance and precision of the results.

Results

Among the 787 knee MRIs examined, the presence of the AATA was detected in 16 cases, yielding a prevalence rate of 2.03%. Of the total knee MRIs reviewed, 297 (37.59%) belonged to female patients and 490 (62.02%) to male patients. Four of the knees with an identified AATA underwent surgical procedures, including 3 arthroscopic meniscal repairs and 1 arthroscopic anterior cruciate ligament reconstruction; vascular injuries or any other complications did not complicate any of those procedures.

MRI measurements showed that the AATA was found at a mean distance of 5.69 mm (range, 2-15 mm) from the tibial attachment of the posterior cruciate ligament (PCL) and 9.89 mm (range, 6-15 mm) from the menisci (Table 1). The anatomic variant of the AATA was most clearly visualized at the level of the tibial attachment of the PCL in 81.25% (13/16) of cases. Moreover, 12.5% (2/16) of cases showed the variant at the level of the superior margin of the musculotendinous junction of the popliteus muscle, and in 6.25% (1/16) at the root of the posterior horn of the lateral meniscus.

Table 1.

MRI Findings of All 12 Patients With AATA Variant ^a

Laterality	Sex	AATA→PCL, mm	AATA→Menisci, mm	Best MRI Level
Left knee	Female	8	13	Tibial attachment of PCL
Left knee	Male	7	11	Tibial attachment of PCL
Left knee	Male	7	11	Tibial attachment of PCL
Left knee	Male	7	15	Tibial attachment of PCL
Left knee	Female	4	9	Tibial attachment of PCL
Right knee	Male	4	10	Tibial attachment of PCL
Left knee	Female	4	8	Tibial attachment of PCL
Left knee	Male	3	9	Tibial attachment of PCL
Left knee	Male	3	9	Tibial attachment of PCL
Left knee	Female	2	8	Tibial attachment of PCL
Left knee	Female	2	10	Tibial attachment of PCL
Left knee	Male	2	12	Tibial attachment of PCL
Right knee	Male	15	8	popliteus muscle b
Left knee	Female	11	7	Root of PHLM
Right knee	Female	10	12	popliteus muscle b
Left knee	Female	2	6	Tibial attachment of PCL

^aAATA, aberrant anterior tibial artery; MRI, magnetic resonance imaging; PCL, posterior cruciate ligament; PHLM, posterior horn of lateral meniscus.

^bSuperior margin of the musculotendinous junction of the popliteus muscle.

Using the chi-square test, the best MRI cut to detect the presence of the AATA was found to be the tibial attachment of the PCL level, which was statistically significant (P < .001).

Before the instructional orientation, none of the observer surgeons made any comments on the presence or absence of the AATA when shown both positive and negative MRI cases. However, after the orientation, all observers successfully identified the AATA in the positive MRI cases and correctly noted its absence in the negative cases immediately and 2 weeks after, resulting in intraobserver reproducibility of 100% after orientation and education about the AATA and its common sites (see Appendix Table A1).

Discussion

The AATA is an anterior tibial artery variant. Its development is attributed to the failed formation of the communicating artery connecting the superficial and deep popliteal arteries during the embryonic phase; therefore, the AATA is present as the persistence of the primitive deep popliteal artery.^10,11,12 This variant passes anterior to the popliteus muscle and tendon, bounded anteriorly by the posterior tibial cortex.^12,16

Because of the increased risk of injuring vascular structures in unusual variants, ⁵ multiple anatomic and cadaveric studies have been conducted to study popliteal artery anatomy, its variant branches, and termination.

The prevalence of the AATA varies widely in the literature. Kim et al ¹⁰ described different variants of the popliteal artery and its terminal branches. The AATA was classified as type 2A2, which was observed in 0.7%. Aragonés et al ¹ conducted a cadaveric study classifying the AATA as pattern 2, which was observed in 0.8% of the limbs studied. Furthermore, some researchers used angiography to assess and measure the prevalence of these variation, both Celtikci et al ³ and Oner et al ¹⁵ found the AATA to be present in 0.6% of the reviewed images, which is similar to what was reported by Calisir et al ² and Kil et al ⁸ who reported an AATA prevalence of 0.5% and 0.4%, respectively.

In our study, after reviewing 787 MRI images, we found the AATA in 2.03% of our population, which is higher than what is reported in previous studies. However, it is similar to findings of Marin-Concha et al, ¹³ who also used MRI, and Day et al, ⁶ who used angiography, reporting a prevalence of 3.2% and 2.4%, respectively.

The popliteal artery and its branches are at risk in almost any knee surgery. High tibial osteotomies and total knee arthroplasties have been reported to risk the popliteal artery. Although it is uncommon, knee arthroscopic procedures can also be complicated with direct injury to the popliteal artery.^{3,15,17,18,21}

Surgeon’s unfamiliarity with different anatomic variations of the popliteal artery increases the risk of injuries. Surgeons should be aware of the AATA variation as it passes anterior to the popliteus muscle and is much closer to the knee joint than the typical anatomic location. Therefore, identifying this variant during preoperative planning is essential.

The distance between the typical variation of the popliteal artery and tibial insertion of the PCL has been reported to be 7.6 mm. ¹⁴ Dangerously, this distance decreases with the AATA variant, as we found a mean distance of 5.69 mm, and it was as close as 2 mm in 4 of our patients. Typically, it is safer to flex the knee during the surgery to increase the distance between the popliteal artery and the knee joint.^19,20 However, as the AATA is trapped between the popliteus muscle and the knee joint, it is unknown whether flexion of the knee would decrease or increase this distance in these specific cases.

To our knowledge, the prevalence and anatomic relationships of the AATA have not been reported in the orthopaedic or radiologic literature.

In our study, we invited 10 board-certified knee and sports surgeons to comment on 10 knee MRIs, with 5 containing the AATA variant. None of them commented on its presence during the first encounter. However, after orientation, all of them were able to identify it immediately, and 2 weeks later.

Preoperative review of the patients’ anterior tibial artery anatomic variation will aid in the operative planning and reduce the risk of intraoperative injury to the AATA if present. Various imaging modes have been suggested for the preoperative assessment of popliteal artery variants. ²² Conventionally, digital subtraction angiography was thought to be the gold standard modality to study the peripheral vasculature.^2,4,23 However, because of its invasive nature, other imaging modes have been proposed, such as computer tomography angiography, MRI, and color Doppler ultrasonography.^23,24 Recently, MRI has been used for evaluation of popliteal artery variation in 2 retrospective studies by Klecker et al ¹¹ and Marin-concha et al. ¹³ The best sequence to accurately visualize the AATA was the proton density (PD) fat–saturated MRI sequence and/or T1 to confirm the anatomy. The best orientation is the axial sequences, especially the Axial PD fat–saturated MRI sequence.

We propose that the routine preoperative review of patients’ MRIs, with a focus on assessing anatomic variations in the vasculature, represents a safe and cost-effective practice for preventing vascular complications. The retrospective nature of this study is a limitation in itself, as we could not determine whether the detection of the AATA actually prevented iatrogenic injury. However, this limitation did not impact our main objective, and it warrants further research and investigation. Moreover, given that our study is a single-center study, with only a few cases identified, it limits our ability to apply these findings to the general public. More studies should be conducted to address our limitations and further evaluate the clinical value of detecting the AATA.

Conclusion

In a Saudi population, the AATA variant was detected in 2.03% of the 787 assessed knees using MRI. The anatomic location of the AATA was most clearly visualized at the level of the tibial attachment of the PCL in 81.25% of cases. Before instructional orientation, observer surgeons failed to identify the presence or absence of the AATA on all MRI images. However, after the orientation, all observers accurately identified AATA in both positive and negative cases immediately and after 2 weeks, achieving 100% intraobserver reproducibility. Anatomic knowledge, along with MRI detection of popliteal artery variation preoperatively, could reduce the risk of iatrogenic injury.

Appendix

Appendix Table A1.

Detailed Responses of 10 Board-Certified Knee and Sports Surgeons Across the 3 Different Stages of the Assessment ^a

		Present AATA, Positive					Absent AATA, Negative
Case Observer		Case 1	Case 2	Case 3	Case 4	Case 5	Case 6	Case 7	Case 8	Case 9	Case 10
Observer 1	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 Weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 2	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 3	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 4	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 5	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 Weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 6	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 7	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 8	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 9	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
Observer 10	Preorientation	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸	⦸
	Post orientation	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗
	After 2 weeks	✓	✓	✓	✓	✓	✗	✗	✗	✗	✗

^a, no comment; ✓, identified AATA; ✗, did not identify AATA. AATA, aberrant anterior tibial artery.