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Original Research |
1 Department of Radiology, University of Texas Medical Branch, 301 University
Blvd., Galveston, TX 77555-0709.
2 Department of Surgery, University of Texas Medical Branch, Galveston, TX
77555-0709.
Received June 24, 2005;
accepted after revision August 26, 2005.
Address correspondence to A. Oto
(ayoto{at}utmb.edu).
Abstract
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MATERIAL AND METHODS. Among 142 adults undergoing consecutive MDCT studies, 100 patients (35 women, 65 men; mean age, 52.1 years) with an identifiable appendix on abdominopelvic MDCT examinations were selected for the study group. The presence of intraabdominal mass or a history of abdominal surgery were the exclusion criteria. Three-dimensional reconstruction of the CT data was performed with a surface shaded display algorithm. The locations of the base of the appendix and McBurney's point were marked on a single 3D image that allowed display of the skin surface markings for each patient. The superoinferior and mediolateral distances from the level of the appendix to the level of McBurney's point were measured, and the radial distance was calculated from these measurements. A surgeon experienced in emergency abdominal surgery reviewed 3D CT images and one axial image showing the appendix, and his choice of incision for each patient based on the CT information was recorded. The influence of the superoinferior and mediolateral distances of the appendix from McBurney's point on the surgeon's decision was analyzed with a multivariate logistic regression model.
RESULTS. The appendix was exactly at McBurney's point in only 4% of the patients. In 36% of the cases, the appendix was within 3 cm, in 28% of cases it was 3-5 cm, and in 36% of the cases it was more than 5 cm away from McBurney's point. Mean ± SD superoinferior, mediolateral, and radial distances between the appendix and McBurney's point were 33.0 ± 24.1, 20.8 ± 19.3, and 42.1 ± 26.7 mm, respectively. After reviewing the images, the surgeon would have altered his incision site in 35% of the cases. The surgeon preferred a higher incision in 28% and a lower incision in 7% of the cases. Both positive and negative superoinferior displacement away from McBurney's point were significant factors regarding the surgeon's decision to alter the incision (p = 0.005), and the superoinferior distance was more than 3 cm in 94% of the cases in which the surgeon would have altered the incision.
CONCLUSION. The location of the appendix varies widely among individuals, and McBurney's point has limitations as an anatomic landmark. Three-dimensional MDCT findings can be useful to surgeons customizing appendectomy incisions. Additional information about the location of the appendix in the CT report (if possible, together with a 3D image showing the location of the appendix) may be beneficial for surgeons performing appendectomy.
Keywords: appendicitis • CT • 3D MDCT • McBurney's point
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Since the late 1980s, the established surgical teaching has been challenged in several studies in which barium enema examinations were used to investigate the location of the appendix [3-5]. The results of these studies suggest that the base of the appendix is rarely even close to McBurney's point but that barium enema cannot reliably show the topographic landmarks such as the umbilicus.
Abdominal CT has been used frequently for the evaluation of acute abdomen and has been shown to decrease the frequency of misdiagnosis in cases of suspected appendicitis [6]. Depending on the amount of intraabdominal fat tissue, MDCT performed with multiplanar reformation display depicts a normal appendix in approximately 97% of patients [7]. In addition to visualization of the appendix, CT accurately shows the location of the ASIS and umbilicus, allowing pinpoint localization of McBurney's point. Three-dimensional reconstruction of CT data with an appropriate algorithm also facilitates visualization of skin and surface markings. The purposes of our study were to use 3D reformation to show the relation between McBurney's point and the appendix in patients undergoing MDCT and to investigate the effect of this information on a surgeon's choice of appendectomy incision.
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All of the MDCT examinations were performed on a 4-MDCT scanner (LightSpeed QX/i, GE Healthcare). All scans were performed after oral administration of contrast medium and 70 seconds after IV administration of 150 mL of contrast material (gadodiamide, Omniscan, Amersham) with a standardized protocol of 4 x 5 mm collimation, 0.8-second rotation time, table feed of 15 mm/s with high-quality mode, 120 kV, and 200-250 mAs. The images were reconstructed every 5 mm. The images were transferred via PACS to a 3D workstation (Advantage Workstation 4.1, GE Healthcare) for processing and analysis.
In every patient, the cecum and terminal ileum were identified. Positive identification of the appendix required visualization of a blind-end tubular structure arising from the cecum. The structure had to be unequivocally separable from the cecum and adjacent small bowel.
For each patient, multiplanar and 3D reconstructions of the abdomen and pelvis were performed by surface shaded display (SSD) technique with a threshold value of -500 H. All anatomic surfaces with attenuation greater than -500 H were visible in the 3D reconstructed SSD. These parameters allowed visualization of skin and topographic landmarks such as the umbilicus. The base of the appendix and the ASIS were localized on three-plane (axial, coronal, sagittal) multiplanar reconstructions and marked on the 3D image.
On the anteroposterior coronal view of the 3D image, a line was drawn between the ASIS and the umbilicus, and McBurney's point was marked. The superoinferior and mediolateral distances between the level of the base of the appendix and McBurney's point were measured and recorded (Figs. 1A and 1B).
For each patient, one 3D image (anteroposterior coronal view) showing the locations of the appendix and McBurney's point and one axial CT image at the level of the base of the appendix were reviewed by an experienced surgeon (> 15 years of emergency abdominal surgery experience). On the basis of the locations of the appendix and McBurney's point on the 3D image, the surgeon decided whether he would make an incision through McBurney's point or at a different site (higher or lower than McBurney's point) if the patient were to undergo appendectomy. The choices made by the surgeon were recorded.
The radial distance between the base of the appendix and McBurney's point
was calculated on the basis of superoinferior and mediolateral distances:
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The appendix was cranial to McBurney's point in 51% of cases and caudal in 43% of cases. In 6% of cases the appendix was at the same superoinferior level as McBurney's point. Mean mediolateral, superoinferior, and radial distances between the appendix and McBurney's point were 20.9 ± 19.4, 33.1 ± 24.1, and 42.1 ± 26.7 mm, respectively. Figure 2 shows the distribution of mediolateral, superoinferior, and radial distances between the appendix and McBurney's point. In 74% of the patients, the mediolateral distance from the base of the appendix to McBurney's point was within 3 cm; in 16%, 3-5 cm; and in 10%, more than 5 cm. The superoinferior distance from the base of the appendix to McBurney's point was within 3 cm, 3-5 cm, and greater than 5 cm in 48%, 30%, and 22% of cases, respectively.
After reviewing the 3D and axial images, the surgeon would have altered his incision site in 35% of cases. The surgeon would have used a higher incision in 28% and a lower incision in 7% of the cases. A multivariate logistic regression model showed that both positive and negative superoinferior displacement away from McBurney's point was a significant factor in the surgeon's decision to alter his incision (p =0.005). On the other hand, the model also showed that neither positive nor negative mediolateral displacement from McBurney's point was significant in the surgeon's decision to alter his incision (p > 0.05). The superoinferior distance was more than 3 cm (either positive or negative) in 94% of the cases (33/35) in which the surgeon would have altered the incision. The appendixes in the other two patients in this group were 2-3 cm superior to the level of McBurney's point.
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The first barium enema study was based on anthropometric measurements of 51 healthy persons. The findings suggested that the base of the appendix is lower and more medial than described in established surgical teaching [3]. Ramsden et al. [4] studied the location of the appendix in 275 patients who underwent double-contrast barium enemas. Their results showed that 75% of appendix bases were below and medial to the line joining the umbilicus with the right ASIS. In that study, only 35% of appendix bases were found to lie within 5 cm of McBurney's point, and 15% were more than 10 cm distant. In a prospective study of 100 postevacuation barium enemas, Naraynsingh et al. [5] found only one case in which the base of the appendix was at McBurney's point. In that study, contrary to the two previous studies, the base of the appendix was more cranial (67%) than caudal (32%) to McBurney's point. Difference in the degree of cecal distention (full of barium versus postevacuation) may be a factor contributing to the contradictory results of these studies. In our study, the numbers of appendixes cranial to and caudal to McBurney's point were similar (51% vs 43%).
Our results showed that McBurney's point is not an accurate topographic landmark for location of the base of appendix. In only 36% of the patients was the appendix within 3 cm of McBurney's point; in another 36% the appendix was more than 5 cm away from McBurney's point. Another important result of our study was the marked individual variability in the location of the appendix reflected by very large SDs in distances between the appendix and McBurney's point (mean radial distance, 42.1 ± 26.7 mm). We believe this individual variability may explain the discrepancy between the results of previous studies (caudal vs cranial location of appendix). Although individual variability was not specifically investigated in the previous studies, the distribution of appendixes in the study groups suggested marked individual variation [3-5].
One of the important advantages of our study is the ability to use CT to determine the location of the appendix in a more physiologic, nondistended state compared with the distention of the colon by barium in previous studies. Another superiority of CT is the ability to localize the appendix, bone landmarks, and skin landmarks and to describe their relative relations accurately, especially by use of multiplanar and 3D reconstruction algorithms.
With state-of-the-art MDCT scanners, it is possible to cover large volumes with narrow slice collimation. This technique allows high-quality multiplanar and 3D visualization without impaired resolution or artifacts. In the emergency setting, 3D reconstruction of CT data is commonly used in CT angiography and after trauma to the musculoskeletal system, face, and head for comprehensive evaluation of the extent of fracture and amount of dislocation and fragmentation [9]. In our study, we used the SSD technique for 3D reconstruction. SSD provides exquisite 3D representation of anatomic features, relying on gray scale to encode surface reflections from an imaginary source of illumination [10]. This method is a quick way to display basic anatomy. The SSD technique is commonly used to communicate findings to the referring physician and does not require editing. In our study, the raw data for 3D reconstruction were acquired with a standard abdominopelvic protocol instead of a dedicated 3D CT protocol, in which a thinner collimation and overlapping reconstructions would be used. It is possible to scan the entire abdomen and pelvis at submillimeter voxels using 16- and 64-MDCT. The added value of isotropic voxels in the diagnosis of acute appendicitis has been shown [11]. Although a dedicated 3D CT protocol would improve the quality of multiplanar and 3D reconstructions, we believe that for the purposes of our study, standard abdominopelvic protocols can produce adequate images while exposing the patient to a lower radiation dose than thinner collimation images.
Use of laparoscopy in the management of acute appendicitis has increased steadily [12]. The location of the appendix is less important in laparoscopic than in open appendectomy because no incision is made and the sites of the trocars are standardized for the most part [13]. Open appendectomy, however, continues be the preferred technique in most the cases, and most surgeons use either a McBurney (oblique) or Rockey-Davis (transverse) right lower quadrant muscle-splitting incision [1]. During open appendectomy, if the appendix cannot be exposed through the initial incision, extension of the wound is required. The need for extension was reported to be as high as 61.9% in a retrospective series of more than 300 appendectomies [14]. When extension of the incision is needed, muscle cutting is usually the only feasible technique and is associated with increased pain, hematoma formation, increased rate of sepsis, nerve injury, and subsequent hernia formation [14, 15]. Therefore, accurate location of the appendectomy incision is critical. When the appendix and cecum are caudal to the incision, it is possible to pull the cecum superiorly and perform the appendectomy. However, when the appendix is cranial to the incision, muscle-cutting extensions often are necessary to access the cecum and appendix [5]. Our emergency abdominal surgeon would have altered the incision site from McBurney's point in 35% of the cases in this study, and because of this technical difficulty, most (28% vs 7%) of these alterations would be higher incisions.
In our study, superoinferior distance rather than mediolateral distance appeared to be an important factor in the surgeon's decision. In most of the patients for whom the surgeon would have altered the incision, the appendix was more than 3 cm superoinferior to the level of McBurney's point. We cannot, however, suggest an absolute displacement distance for predicting the incision, because the superior-inferior displacement of the appendix has a different effect. In addition, patient body habitus (e.g., tall vs short, obese vs lean) can substantially limit the significance of an absolute threshold value.
The limitations of our study were inherent to its retrospective nature. We acknowledge that our study was performed with subjects who had healthy appendixes and did not undergo appendectomy. Therefore, no actual benefit from alteration of the incision could have been shown in our study. Another limitation was that the decision regarding choice of incision was made by a single surgeon, which may have introduced bias and precluded evaluation of interrater reliability. In addition, our protocol was not optimal for 3D reconstruction. Data sets obtained with 16- or 64-MDCT would provide submillimeter isotropic voxels, which would provide better quality 3D reconstruction.
In conclusion, the location of the appendix has wide individual variability, and the limitations of McBurney's point as an anatomic landmark should be recognized. Three-dimensional MDCT can provide useful information to help surgeons customize appendectomy incisions. Additional information about the location of the appendix in the CT report (if possible, together with a 3D image showing the location) may be beneficial for surgeons performing appendectomy.
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