Flexions of the Popliteal Artery: Dynamic Angiography712

Hinge point of the popliteal artery.(A) Stent in the popliteal artery. (B) The hinge point of the popliteal artery impedes the normal outflow through the stent during knee flexion.
Correlation between conventional angiography and dynamic angiography in the same patient. (A) Knee extension during conventional angiography. (B) Knee flexion during dynamic angiography. 1: Hinge point. 2, 3 & 4: Accessory flexions. 5: Horizontal plane at
Relationship of proximity between the hinge point of the popliteal artery, medial supracontylar tubercle and upper border of the patella. (A) Geometrical model. (a) Line with 3 popliteal artery diameters; (b) Radium of the circumference = 3 popliteal arte
Author(s): 

Jose A. Diaz, MD, Miguel Villegas, MD, Gustavo Tamashiro, MD, Marisa H. Miceli, MD, Daniel Enterrios, MD, Aristobulo Balestrini, MD, Alberto Tamashiro, MD

EDITOR’S NOTE: This article by Diaz et al. contains extremely valuable information. The importance of assessing the “dynamic anatomy” of the popliteal artery (and other vessels) was only recently appreciated as a result of developments with endovascular therapy and the increasing use of fracture-prone intraluminal metallic stents. The findings described by the Argentinian group should prove useful to interventionists who are planning to perform a stenting procedure in a given patient. But even more so, they will likely have an impact on current R&D efforts and concepts surrounding stent technology for treatment of infra-inguinal disease — a very significant area in interventional medicine indeed! — Frank J. Criado, MD, Director, Center for Vascular Intervention, Chief, Division of Vascular Surgery, Union Memorial Hospital/MedStar Health, Baltimore, Maryland.

Stenting of the popliteal artery (PA) is a therapeutic option for several diseases affecting this artery, including atherosclerosis, aneurysm, and injury with pseudoaneurysm and/or arteriovenous fistulae formation.1–6 Because the PA is exposed to movements of the knee joint, self-expandable stents and endovascular stent grafts are commonly used owing to their crush-reversible properties. This condition is advantageous in areas where the body is exposed to external force or movements.7 However, fractures of self-expandable stents have been reported, including those used in the PA.8,9 Vessel compression and movement are thought to promote development of hinge points (HPs), which ultimately lead to stent fractures.9 However, it is still not completely clear where HPs occur in the PA.
The purpose of the present retrospective study was to describe the morphologic changes of the PA during knee flexion and their relationship with bone structures using dynamic angiography (DA).

Patients and Methods
In February 2000, a patient who had a stent implanted in her right PA underwent angiography at our institution. Because the patient was unable to extend her right knee due to severe pain, angiography was performed with the knee bent at 100º. Arterial blood flow was severely decreased in the distal end of the stent. The low-flow segment was coincident with the presence of an HP of the PA (Figure 1).
This observation led us to routinely perform conventional angiography (CA) (with the knee in extension) followed by a DA (with the knee in flexion), as an attempt to identify the exact place where the PA has an HP during knee flexion in those patients requiring a therapeutic procedure involving the PA.
Patients. Adult patients who had been referred to our department by their primary physician for angiography of the lower extremities to diagnose arterial disease were included. Only symptomatic legs were studied. DA was not performed in patients with renal failure (serum creatinine >= 1.40 mg/dL), history of allergy or intolerance to the contrast material injected during CA, and/or a segment of the PA that was not visible during CA.

Methods
We retrospectively evaluated 63 DA procedures performed in the Department of Hemodynamics of the Hospital Nacional Alejandro Posadas (Buenos Aires, Argentina) between February 2000 and June 2003. This study was approved by the Institutional Ethics Committee.
Angiography procedure. DA was performed immediately after CA using the same arterial access. An infusion of nonionic, low osmolality contrast in 2 boluses of 10 cc each in 1 second was administered through a pump. Images were captured with 35 mm film at 30 photograms/second. The DA had a static phase and a dynamic phase. During the static phase, the patient laid on the side to be studied, with the knee in flexion at 100° (grade 0° is defined as the axis of the leg that continues to the axis of the thigh).10 Focus was on the knee joint, using lateral and medial projection. Once opacification of the PA had occurred, the leg was passively extended to total extension (dynamic phase). Evaluation of the artery took place during the entire recording process using the same projection and focal point.
Definitions. HP of the PA: First curve in the PA in an acute angle toward the femur that appeared during knee flexion (Figure 2). AF of the PA: Any curve in the PA (other than the HP) identified during knee flexion (Figure 2).
Interpretation of DA. Images were evaluated with a Tagarno projector. Dynamic morphology of the PA was analyzed by running the film in antegrade and retrograde directions and evaluating each one of the photograms.
Statistical analysis. A Wilcoxon rank-sum test and test of binomial difference of proportions were used to analyze the presence of AFs and how they are related to risk factors for arteriosclerosis (ie, age, hypertension, smoking, dyslipidemia, and diabetes mellitus).

Results
We evaluated 63 PAs in 57 patients. Patient demographics are shown in Table 1. All patients were able to bend their knees to 100°. Presence of an HP was identified in 62 out of 63 PAs (98.2%). The pre-hinge-point segment (pre-HP segment) and post-hinge-point segment (post-HP segment) also were defined (Figure 2). We were unable to identify the presence of an HP in only 1 PA, as it was visualized as an extensive curve. HPs were never observed at the level of the joint line of the knee joint. However, the joint line observed in the CA corresponded with the post-HP segment of the PA in the DA in 100% of the cases (Figure 2). To find an anatomic relationship between bone structures and HPs, we identified the medial supracondylar tubercle of the femur (MSCT), the site of insertion of the musculus gastrocnemius medialis,11 as a well defined bone parameter. The MSCT was evident in 52 of 63 DAs reviewed (83%).
We then developed a geometric model to establish a relationship of proximity between the MSCT and the HP. With the film stopped at the moment of maximum flexion, we drew a circle with center on MSCT. The radius of this circle was 3 times the diameter of the PA. We evaluated whether or not the HP was inside of this circle (Figure 3). This relationship was assessed in all 52 DAs in which the MSCT had been identified. An HP was found within an area equivalent to 3 times the diameter of the PA in 50 of 52 DAs (95%) and within an area equivalent to 2 times the diameter of the PA in 42 DAs (80%; Figure 3). In addition, the upper edge of the patella and the MSCT were found to be aligned on the horizontal plane.
The HP was the main and most acute angled curve that appeared during knee flexion. However, we identified additional curves of the PA (AFs) when the knee was bent in 46 of 63 PAs (73%). In those cases in which an artery had more than one AF, the DA allowed us to differentiate the HP from the AFs because the HP was the first to appear in a semiflexion position. There were no complications associated with the DAs reviewed in this study. Presence of an AF was associated with high blood pressure (p < 0.01).

Discussion
The PA is an artery exposed to movement and external compression.7 Self-expandable stents are known to be crush-reversible and are considered to be advantageous in mechanically exposed areas of the human body.7 However, failures of self-expandable stents in the PA have been reported.8,9,12
Babalik and colleagues have mentioned that repeated external compression to self-expandable stents may lead to stent fracture, and that this generally occurs when the stent is placed at flexion points of the vessel.8 In a randomized trial comparing sirolimus-coated with uncoated self-expandable stents, fractures occurred in 6 of 33 patients and were equally distributed in both groups. This complication was thought to be related to the increased length of the stented vessel and the overlap of the stents, resulting in an HP.9
We identified the presence of an HP in the PA as the first and most angled curve moving toward the femur observed during DA. In anatomic terms, the HP was found in the superior or supra-articular PA which extends from the adductor ring to the superior border of the femoral condyle. Avisse and colleagues have described this segment of the PA as the “adaptation zone” during knee flexion with arterial tortuousities.13 Interestingly, these authors did not mention the presence of an HP. Although it is difficult to say precisely where the HP was observed, we did find a relationship between the HP and the MSCT. Additionally, the relationship between the MSCT and the superior border of the patella established in this study allowed us to correlate findings from DAs with those from CAs (Figures 2 and 3).
Classically, the PA has only 3 segments: the superior or supra-articular, the middle or articular, and the lower or infra-articular. This is a static anatomic concept. With the use of DA in our institution, we are able to introduce a dynamic anatomic concept: the presence of an HP and division of the PA into pre- and post-HP segments. In the current study, none of the HPs occurred at the level of the joint line. On the contrary, we found an exact correlation between the post-HP segment and the joint line, leading us to conclude that the PA has a point of main flexion (the HP) that does not occur at the same level where the knee bends.
The constant presence of an HP and the high number of AFs seen in our study led us to conclude that the PA adapts to knee joint flexion in 3 zones: the HP, the pre-HP segment, and the post-HP segment. In a previous study, Avisse and colleagues described the middle and lower segments of the PA as relatively fixed in patients without evidence of atheroma.13 However, our findings suggest that these segments are free and allows for adaptation of the PA to flexion of the knee. This is supported by the presence of AFs observed beyond the HP (Figure 2). Differences in the results from these two studies may be due to different patient populations. All of our patients had arterial disease. Future research studies should include patients with arterial disease, as these patients are potential candidates for endovascular treatment.
Zocholl and colleagues conducted a study in which angiography of flexed knees showed that young patients had fewer PA flexions than old patients.14 They also stated that increasing the angle of flexion of the knee joint increased the number of flexions in the PA. Obesity was mentioned as an impediment to knee flexion. In the current study, we were unable to identify the presence of an HP in a 22-year-old patient whose PA was visualized as an extensive curve. High blood pressure was the only significant factor associated with the presence of AFs (p < 0.01). All of our patients had the same angle of knee flexion (100º), and none of them were obese.
In conclusion, we believe that use of an appropriate stent (the best stent) in addition to having the most complete information about the PA (as acquired from DA) may help to decrease complications of PA stenting. DA appears to be an important diagnostic tool to obtain the most accurate morphologic information about the PA in flexion and to determine the most appropriate site for stent implantation. Further studies are needed to establish the clinical implications of our findings.
Acknowledgments. The authors would like to thank Dr. Ricardo Etcheverry and Adrián Fanelli for their lively interest in this work, Silvia Hentschel, for statistical analysis, and the nursing, secretarial, and technical staff of the catheterization laboratory for their excellent work.


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