---

Anatomic features related to diameter, proximal and distal neck of abdominal aortic aneurysms
(Portuguese PDF version)

Erasmo Simão da Silva¹, Mauro H. Hanaoka², Pedro Puech-Leão¹, Erasmo Magalhães Castro de Tolosa²

1. Professor of Vascular Surgery, School of Medicine, Universidade de São Paulo (USP), São Paulo, SP.
2. Professor of Surgical Technique and Experimental Surgery, School of Medicine, Universidade de São Paulo (USP), São Paulo, SP.

Correspondence:
Erasmo Simão da Silva
Rua Martins, 96
CEP 05511-000 - São Paulo, SP
Brazil
Tel./Fax: +55 (11) 3814.9873
E-mail: ersimao@usp.br

---

ABSTRACT

Objective: To analyze important anatomical/morphological aspects of abdominal aortic aneurysms detected at autopsy.

Method: In this study, 43 specimens were studied. In order to reestablish arterial diameter and aneurysm morphology, a device was introduced in the arterial lumen. This device allows distention of the arterial wall with controlled pressure. The measures done on the specimens consisted of: transverse diameter, aneurysm length, transverse diameter and length of the proximal and distal aneurysm neck.

Results: The mean value of the transverse diameter in this sample was 6.86 cm and the mean length, 10.03 cm. Thirty-nine aneurysms presented proximal neck with mean diameter of 2.33 cm (range 1.9 to 2.8 cm) and length ranging from 0.4 cm to 6.8 cm (mean: 2.41 cm). Twelve specimens displayed a distal neck (mean of diameter: 2.44 cm, mean of length: 1.58 cm). Aneurysms with distal neck present transverse diameter and length significantly smaller than those without distal neck (P < 0.01). Although smaller diameter aneurysms presented longer proximal neck, this correlation was not significant (P > 0.05). The correlation between transverse diameter and length was positive and significant (P < 0.01), whereas the correlation between the length of the aneurysm and the length of the proximal neck was significant and inverse (P < 0.01).

Conclusion: In this sample, the aneurysms with larger diameter and length tend to have shorter proximal and distal neck. This fact shows that as the aneurysm grows, it loses the anatomical neck and advances towards the iliac and renal arteries. The progression is more accentuated towards the distal portion, because in the minority of the specimens the anatomical neck was located before the bifurcation of the aorta.

Key words: aneurysm, autopsy, morphology.
Palavras-chave: aneurisma, necropsia, morfologia..

J Vasc Br 2004;3(2):95-102

---

The anatomical characteristics of abdominal aortic aneurysms became even more important with the advent of endoarterial repair techniques.^1-3 Depending on the aneurysm diameter, changes may occur in the proximal and distal neck, in the degree of kinking, and in the involvement of the iliac vessels.⁴ Thus, it is important to know if the aneurysms become more complex as they grow, which may difficult interventionist treatment.^5-7

The anatomical characterization of aneurysms is obtained in the preoperative period through conventional computed tomography coupled with angiography,⁸ helical computed tomography,^9,10 or magnetic resonance angiography with gadolinium.¹¹ Based on these diagnostic modalities, many studies have been developed on the morphology of aneurysms and the anatomical limitations of treatment.^12,13

Studies on autopsy usually approach diameter and rupture location. This study aims at analyzing various morphological aspects of abdominal aortic aneurysms found in autopsy specimens. The aneurysms have their morphology reestablished by a device that distends the arterial wall by means of controlled intraluminal pressure.^14,15

PATIENTS AND METHODS

A total number of 43 specimens of abdominal aorta were analyzed. From 1998 to 1999, individuals with aneurysms were submitted to autopsy in order to determine the cause of death. The autopsies were performed at the Division of Postdeath Inspection (Serviço de Verificação de Óbitos (SVO)) in the Department of Pathology of the School of Medicine, Universidade de São Paulo.

The average age was 72 years (43 - 94 years). Thirty-five individuals were male (81%). Thirty-one aneurysms were ruptured and 12 nonruptured.

Exclusion criteria used to select the sample were the following:

- individual without infrarenal abdominal aorta aneurysm;

- corpse in advanced state of autolysis;

- death occurred more than 24 hours;

- damaged specimen at the moment of excision.

The specimen was removed from the corpse and submitted to surgical dissection. A device was placed inside the aneurysm and inflated up to 80 mmHg,^14,15 allowing reestablishment of diameter and morphology.

The following measurements were obtained with the use of a caliper:

1. Transverse aneurysm diameter (latero-lateral: LLD or anteroposterior: APD);

2. Longitudinal aneurysm extent (length);

3. Maximum transverse aneurysm diameter of the proximal neck (between the most distal renal artery and the origin of aneurysm).

4. Longitudinal extent of the proximal neck of aneurysm (length);

5. Maximum transverse diameter of the distal neck (between the end of aneurysm and aorta bifurcation);

6. Angulation between the major axis of the aorta and the major axis of aneurysm (the highest lateral or frontal plane).

According to the morphological characteristics, the aneurysms were divided into five categories:^3,16,17

A) Proximal and distal neck extent greater or equal to 10 mm and diameter smaller than 28 mm;

B) Proximal neck extent greater or equal to 10 mm and diameter smaller than 28 mm, with aneurysm extended to the aorta bifurcation;

C) Proximal neck extent greater or equal to 10 mm and diameter smaller than 28 mm, with aneurysm extended to the common iliac arteries and sparing, at least one side, of the common iliac artery bifurcation;

D) Proximal neck extent greater or equal to 10 mm and diameter smaller than 28 mm, with aneurysm involving two bifurcation of the common iliac arteries or aneurysm of the bilateral internal iliac artery;

E) Proximal neck extent lower than 10 mm or diameter greater or equal to 28 mm.

Classification regarding the intensity of kinking, based on the angle between the major axis of the aorta and the major axis of the aneurysm (the most accentuated angle on the frontal or lateral planes), was the following: degree I (between 180º and 150º), degree II (between 149º and 120º) and degree III (less than 120º).⁴

Regarding the proximal extent of aneurysms in relation to the renal arteries, they were considered suprarenal (above the renal arteries), pararenal (with involvement of the renal arteries, without involvement of the superior mesenteric artery or celiac trunk), juxtarenal (within less than 1.0 cm of the origin of the renal arteries) and infrarenal (aneurysms within 1.0 cm of the origin of the renal arteries).¹¹ In order to analyze the aneurysms according to different maximum diameters, they were divided into type 1 (between 4.0 and 4.9 cm), type 2 (between 5.0 and 5.9 cm), type 3 (between 6.0 and 6.9 cm), and type 4 (> 6.9 cm).¹²

As for the length of the proximal neck, the aneurysms were divided into three groups: extent lower than 1.0 cm, extent between 1.0 cm and 1.9 cm, and extent greater or equal to 2.0 cm.

Statistical method

The statistical analysis was based on the calculation of mean values and standard deviation, paired t-test and Student-Newman-Keuls test, and correlation coefficient. Significance level was set at P < 0.05.

RESULTS

Table 1 shows the values related to transverse diameter of aneurysms (latero-lateral and anteroposterior) and longitudinal extent.

Table 1 - Transverse diameter and longitudinal extent of aneurysms

	Latero-lateral Diameter	Anteroposterior Diameter	Longitudinal extent
Mean	6.8651	6.5186	10.03023
Minimum	3.3	3.4	4.2
Maximum	10.5	10.5	17.7
Standard Deviation	1.818016	1.500993	3.06085

The mean values of latero-lateral diameter and anteroposterior diameter were compared and showed, with statistical significance (paired t = 2.58, P = 0.0134), that the latero-lateral diameter is larger than the anteroposterior diameter (Figure 1). Correlation was found positive with coefficient of 0.87643 and P = 0.0001.

Figure 1 - Relationship between the latero-lateral and anteroposterior diameters of the aneurysms studied.

Correlation between transverse diameter (LLD and APD) and aneurysm extent was positive and statistically significant (P = 0.0001; DLL r = 0.61834,and DAP r = 0.65209), that is, the larger the diameter, the higher the aneurysm extent (Figure2).

Figure 2 - Relationship between transverse diameter and length of aneurysms.

 

Four aneurysms were of pararenal type, six juxtarenal and the remaining aneurysms were of infrarenal type. Among the 43 aneurysms selected, 39 were analyzed with based in the presence of infrarenal proximal neck.

In regard to the anatomical classification, the aneurysms were divided into type A, three specimens (7.0%), type B, 14 specimens (32.6%), type C, 13 specimens (30.22%), type D, three specimens (7.0%), and type E, with 10 specimens (23.2%). In this sample of 43 aneurysms, types A, B, and C represent 69.8% of the total number of specimens whereas types D and E represent 30.2%.

Table 2 shows group distribution of aneurysms according to different intervals of diameter. It also shows how different anatomical shapes were divided into groups.

Table 2 - Anatomical distribution of aneurysms with different diameters

Maximum transverse diameter	n	Anatomical distribution
Group 1 (4.0 a 4.9 cm)	4 (9.3%)	A (2) C (2)
Group 2 (5.0 a 5.9 cm)	6 (14%)	A (1) B (3) D (1) E (1)
Group 3 (6.0 a 6.9 cm)	14 (32.6%)	B (5) C (6) E (3)
Group 4 (> 6.9 cm)	19 (44.1%)	B (5) C (6) D (2) E (6)

Table 3 shows mean diameter and extent of the proximal neck from 39 aneurysms, and mean diameter and extent of the distal neck from12 aneurysms that were suitable to be measured.

Table 3 - Diameter and extent of proximal e distal neck of aneurysms

Diameter	n	Mean (cm)	Minimum (cm)	Maximum (cm)	SD* (cm)
Proximal neck diameter	39	2.3310256	1.9	2.8	0.24038
Proximal neck extent		2.410256	0.4	6.8	1.48674
Distal neck diameter	12	2.441667	2	3.4	0.454189
Distal neck extent		1.58333	0.4	4.5	1.230546

* SD = standard deviation.

In relation with the proximal neck, Table 4 shows the mean transverse diameter values and the mean length values of aneurysms in the three groups with different lengths of the proximal neck.

Table 4 - Extent and transverse diameter from aneurysms with proximal neck of different lengths

Proximal neck extent (cm)	Anteroposterior diameter		Latero-lateral diameter		Longitudinal extent of aneurysm
	Mean	SD*	Mean	SD*	Mean	SD*
< 1.0	6.87	1.3984	7.05	1.8748	12	3.856
1.0 a 1.9	6.622	1.4855	7.3666	2.02608	11.12	3.118
2.0 a 6.8	6.312	1.6211	6.5208	1.7954	8.891	2.308

*SD = standard deviation

Regarding diameter, no significant difference (Student-Newman-Keuls: F = 0.75 and P = 0.48 e F = 0.14, P =0.869) was found in the three groups of aneurysms showed in Table 4, with different extents of the proximal neck. Correlation between the transverse diameter of the 39 aneurysms with proximal neck and the extent of the proximal neck was not statistically significant (P > 0.05). Correlation between aneurysm extent and proximal neck extent was inverse (Figure 3), with statistical significance (F = 4.21, P = 0.023).

Figure 3 - Relationship between aneurysm length and the length of its proximal neck.

The mean transverse diameter and the total extent of 12 aneurysms with distal neck were, respectively, 5.84 cm (standard deviation = 1.53) and 8.88 cm (standard deviation = 2.56). The mean transverse diameter and total extent of 27 aneurysms without distal neck were, respectively, 7.4 cm (standard deviation = 1.69) and 10.69 (standard deviation = 2.9). Both, diameter and extent of the aneurysm sac were higher in aneurysms without distal neck (P < 0.001).

Regarding the angulation between the aorta axis and the aneurysm axis, 21 specimens were included in degree I, between 150º and 180º, 16 in degree II, between 120° and 149° and, finally, six specimens presented angles lower than 120° (degree III)

The distribution of transverse diameter and extent of aneurysms in terms of different angle groups are shown in Table 5.

Table 5 - Correlation between transverse diameter and extent values of aneurysms in different angle groups

Angle	Anteroposterior Diameter		Latero-lateral diameter		Extent
	Mean	SD*	Mean	SD*	Mean	SD*
< 120º (III)	6.683	1.147	7.383	2.131	10.85	3.949
120º a 149º (II)	6.868	1.8578	7.05	1.832	10.2	2.717
150º a 180º (I)	6.047	1.2662	6.576	1.758	9.661	3.233

*SD = standard deviation.

It was not observed, with statistical significance, any difference related to diameter and extent of aneurysms in the three groups of aneurysms divided according to the angle between the aorta axis and the aneurysm axis.

DISCUSSION

Diagnostic methods and anatomical evaluation of aneurysms improve day by day. They provide precise and vital information for an adequate implementation of interventionist treatment.^11,18 The anatomical study in specimens contribute to a better understanding of the morphology of aneurysms. It also highlights the most common anatomic features that will be found during repair, having in mind that an imaging method must be analyzed in the light of anatomic knowledge.

Data presented in Table 1 and Figure 1 shows that aneurysms grow symmetrically in relation to their diameters. The tendency to lateral growth overcomes the anteroposterior growth, probably, because of the restraint action of the vertebral column and the abdominal viscera over the aneurysm wall.

One major concern related to repair is the hypothesis that the greater the transverse diameter, the lower the proximal neck would be. This fact makes the procedure harder and more risk. The results presented in Figure 2 show that the greater the transverse diameter, the larger the aneurysm. Figure 3 shows that the higher the extent of aneurysm, the lower the extent of the proximal neck. Thus, larger aneurysms tend to loose its proximal neck, which may difficult treatment.

Despite the mean transverse diameter values of aneurysms have been higher for aneurysms with lower proximal neck extent (Table 4), there was no statistically significant correlation between transverse diameter and proximal neck extent in this sample. Only a trendy was found. On the other hand, the relation between transverse diameter and distal neck extent was significant, which shows that the distal neck reduces its size when the aneurysm diameter increases.

The information provided in Table 2 shows that 76.7% of aneurysms in this sample are aneurysms with unquestionable surgical indication if only diameter was analyzed (longer than 6.0 cm, therefore, large aneurysms) and, approximately, 70% of aneurysms in this sample would present good anatomical conditions for repairing (they belong to A, B, and C classes), based on the anatomical characteristics of aneurysms and on the aneurysmal involvement of the common iliac arteries.

Although the majority of aneurysms of class E belong to group 4, of higher diameter, the aneurysms belonging to this category are also present in groups of lower diameters (2 and 3 - Table 2). The aneurysms, sometimes, can be very unpredictable, since the heterogeneity of factors involved in their natural evolution has also influence on their anatomy.

The main reason for the inclusion of the specimen in the category E was the involvement of the proximal neck. Four aneurysms were pararenal and six juxtarenal (neck extent lower than 1.0 cm). The reason for inclusion in category D was the bilateral involvement of the iliac arteries bifurcation or aneurysm of the two internal iliac.

Diameter measurement and extent of aneurysms with increased kinking (types II and III) were higher than the diameter and extent of straight aneurysms (type I). In this sample, without statistical significance (Table 5), these results show that larger aneurysms tend to be more kinked, which also imposes more difficulties for repairing.

CONCLUSION

The anatomical and morphological knowledge of aneurysms is fundamental for both open or endovascular aneurysm repair. The anatomical study helps the understanding of important aspects regarding imaging diagnostic methods. Generally, as the aneurysm grows (in diameter and length), it looses its proximal neck and becomes more kinked.

REFERENCES

1. Parodi JC, Palmaz JC, Barone HD. Transfemoral intraluminal graft implantation for abdominal aortic aneurysms. Ann Vasc Surg 1991;5:491-9.

2. Parodi JC. Endovascular repair of abdominal aortic aneurysms and other arterial lesions. J Vasc Surg 1995;21:549-57.

3. Moore WS, Rutherford RB. Transfemoral endovascular repair of abdominal aortic aneurysm: results of the North American EVT phase 1 trial. J Vasc Surgery 1996;23:543-53.

4. Ahn SS, Rutherford RB, Johnston KW, et al. Reporting standards for infrarenal endovascular abdominal aortic aneurysm repair. J Vasc Surg 1997;25:405-10.

5. May J, White GH, Yu W, Waugh R, Stephen MS, Harris JP. Results of endoluminal grafting of abdominal aortic aneurysms are dependent on aneurysm morphology. Ann Vasc Surg 1996;10:254-61.

6. Dorros G, Parodi JC, Schonholz C, et al. Evaluation of endovascular abdominal aortic aneurysm repair: anatomical classification, procedural success, clinical assessment, and data collection. J Endovasc Surg 1997;4:203-25.

7. Parodi JC, Barone A, Piraino R, Schonholz C. Endovascular treatment of abdominal aortic aneurysms: lessons learned. J Endovasc Surg 1997;4:102-10.

8. May J, White GH, Yu W, et al. Concurrent comparison of endoluminal versus open repair in the treatment of abdominal aortic aneurysms: analysis of 303 patients by life table method. J Vasc Surg 1998;27:213-21.

9. Gomes MN, Davros WJ, Zeman RK. Preoperative assessment of abdominal aortic aneurysm: the value of helical and three-dimensional computed tomography. J Vasc Surg 1994;20:367-76.

10. Balm R, Stokking R, Kaatee R, Blankensteijn JD, Eikelboom BC, Leeuwen MS. Computed tomographic angiographic imaging of abdominal aortic aneurysms: implications for transfemoral endovascular aneurysm management. J Vasc Surg 1997;26:231-7.

11. Prince MR, Narasimham DL, Stanley JC, et al. Gadolinium-enhanced magnetic resonance angiography of abdominal aortic aneurysms. J Vasc Surg 1995;21:656-69.

12. Bayle O, Branchereau A, Rosset E, Guillemot E, Beaurain P, Ferdani M, Jausseran JM. Morphologic assessment of abdominal aortic aneurysms by spiral computed tomographic scanning. J Vasc Surg 1997;26:238-46.

13. Veith FJ, Abott WM, Yao JST, et al. Guidelines for development and use of trasluminally placed endovascular prosthetic grafts in the arterial system. J Vasc Surgery 1995,21:670-85.

14. Silva ES, Rodrigues Jr. AJ, Tolosa EMC, Pereira PRB, Zanoto A, Martins J. Variation of infrarenal aortic diameter. A necropsy study. J Vasc Surg 1999;29:920-7.

15. Silva ES, Rodrigues Jr. AJ, Rodrigues CJ, Tolosa EMC, Prado GVB, Nakamoto JC. Morphology and diameter of infrarenal aortic aneurysms: a prospective autopsy study. Cardiovasc Surg 1999;7(S1):71.

16. Blum U, Voshage G, Lammer J, et al. Endoluminal stent-grafts for infra-renal abdominal aortic aneurysms. N Engl J Med 1997;336:13-20.

17. Ristow AV, Bonamigo TP, Dutra CF. Seleção de pacientes para tratamento endovascular do aneurisma da aorta abdominal. In: Bonamigo TP, Ristow AV, editores. Aneurismas. Rio de Janeiro: DI; 2000. p. 112

18. Chuter TAM, Green RM, Ouriel K, DeWeese JA. Infrarenal aortic aneurysm structure: implications for transfemoral repair. J Vasc Surg 1994;20:44-50.