Skeletal Radiol (2006) 35: 679–683 DOI 10.1007/s00256-006-0155-8

Sung Soo Chung Chong Suh Lee Hye Won Chung Chang Seok Kang

Received: 8 September 2005 Revised: 6 April 2006 Accepted: 26 April 2006 Published online: 27 June 2006 # ISS 2006 S. S. Chung . C. S. Lee . C. S. Kang (*) Department of Orthopedic Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, #50 Ilwon-dong, Kangnam-gu, Seoul, 135-710, South Korea e-mail: [email protected] Tel.: +82-2-34103509 Fax: +82-2-34100061 H. W. Chung Department of Radiology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea

SCIENTI FIC A RTICLE

CT analysis of the axis for transarticular screw fixation of rheumatoid atlantoaxial instability

Abstract Objective: To investigate the morphological characteristics of the axis of rheumatoid arthritis (RA) patients with atlantoaxial instability and to determine, by means of sagittal reconstructed computed tomography (CT), the suitability for atlantoaxial transarticular screw fixation. Design and patients: Twenty-seven patients, who had undergone reconstructed cervical spine CT scanning preoperatively and posterior atlantoaxial arthrodesis for atlantoaxial instability, were identified from a database for inclusion in this study. The isthmus height and internal height of the lateral mass of the axis were measured using digital imaging software. Results: The mean isthmus height and internal height of the lateral mass of the axis in RA patients (n=14) were

Introduction Several posterior arthrodesis techniques for atlantoaxial instability have been proposed, including posterior wiring, fixation using Halifax interlaminar clamps, and atlantoaxial transarticular (TA) screw fixation [1–3]. Since Grob and Magerl [3] first introduced TA screw fixation in 1987, it has been considered to be an excellent technique that offers greater biomechanical stability and higher fusion rates than other posterior fusion techniques [4–8]. However, TA screw fixation is technically demanding and carries the risk of lethal injury to the vertebral artery (VA) [9–11]. The VA is more vulnerable to injury on account of the proximity of the screw trajectory to the VA as well as anatomical variations of the VA. Therefore, many authors have recommended preoperative evaluation of the VA in order

significantly lower than in non-RA patients (n=13) (P<0.01). A highriding vertebral artery (VA) was present in 54% (15 joints, 9 patients) of the 28 atlantoaxial joints in the RA group and in 12% (3 joints, 2 patients) of the 26 atlantoaxial joints in the non-RA group (P<0.01). Conclusions: In RA patients, the axis showed more extensive thinning of the isthmus and lateral mass than in non-RA patients. A precise preoperative evaluation of screw trajectory using reconstruction CT imaging may be useful in atlantoaxial transarticular fixation, particularly for RA patients with atlantoaxial instability. Keywords Cervical spine . Axis . Cervical CT . Rheumatoid arthritis . Atlantoaxial instability

to measure the dimension of the isthmus of axis using reconstruction computed tomography (CT) for all patients planned to undergo an atlantoaxial TA screw fixation [12, 13]. The VA makes an acute lateral bend under the superior facet of the axis. The height or the width of the isthmus of the axis reduces when this bending point is too medial, too posterior, and/or too high. This condition is described as a high-riding VA. With this condition, atlantoaxial TA screw fixation carries a significant risk of VA penetration. It has been the experience of the authors that a high-riding VA commonly occurs in rheumatoid arthritis (RA) patients with an atlantoaxial instability. The aim of this study was to investigate the morphological characteristics of the axis in RA patients with an atlantoaxial instability using sagittal reconstructed CT scanning and to determine the suitability of the axis for atlantoaxial TA screw fixation.

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Table 1 Demographic characteristics

Number Mean age (±SD) M:F a

RA

Non-RA

P value

14 55.5±3.8 3:11

13 45.5±4.7 7:6

0.058a 0.089b

Mann–Whitney U-test Chi-square test

b

Materials and methods Research ethics committee approval was obtained prior to beginning the study. Twenty-seven patients who had undergone reconstructed cervical spine CT scanning preceding posterior atlantoaxial arthrodesis for atlantoaxial instability, and had no previous cervical surgery, were identified from a database for inclusion in this study. Ten of the patients were men and 17 were women. Their mean age at the time of surgery was 50.7 years (range 15–82 years). The diagnosis was RA with atlantoaxial instability in 14 patients, odontoid process fracture or nonunion in 7 patients, and os odontoideum in 6 patients. The patients were divided into two groups, with and without RA. Demographic data were compiled for all patients in this study (Table 1). There were no significant demographic differences between the two groups. The inclusion criteria for the RA group included clinical diagnosis of RA by a rheumatologist, history of RA of at least 2 years, an atlantodental interval in the flexion position >4 mm, and no clinical evidence of major neck trauma. The inclusion criteria for the non-RA group included no clinical history of RA or other inflammatory diseases and an atlantodental interval in flexion position >4 mm. The preoperative CT studies were performed using a helical CT scanner (General Electric, Milwaukee, WI). All cervical spines were scanned axially using a tube current of 300 mA, a voltage of 120 kV, and a volume pitch of 0.875. Using the C2 body as a center of reference, we reconstructed sagittal slices with slice thicknesses of 1.2 mm or 2 mm based on an acquisition Fig. 1 a, b Sagittal reconstruction CT images transecting the mid-portion of the C1–2 facet joint (long arrow) show the vertebral artery groove (*) and the measurement of the isthmus height (arrowhead) and internal height (hollow arrow) of the lateral mass

slice thickness of 1.25 mm. All images were reconstructed by means of the high-resolution kernel. At the images transecting mid-portion of the atlantoaxial facet joint, the height of the isthmus and the internal height of the C2 lateral mass (measured from the roof of the VA groove to the surface of the superior facet) were measured using digital imaging software (Centricity; GE Medical Systems, Milwaukee, WI) (Fig. 1). Two spine surgeons who were not involved in the patients’ care performed all of the CT analyses independently (C.S.L., C.S.K.). In addition, a high-riding VA was defined as an internal height of the axis <2 mm, an isthmus height <5 mm, or both on the CT reconstruction images. Each independent observer performed the CT measurements twice and the average of the four measurements was used as the final value. The Mann– Whitney U-test and the chi-square test were used to compare the difference in the demographic data, including age and gender, between the RA and the non-RA group. The differences in the isthmus height and internal height of the lateral mass between the right and left sides were compared using a paired-sample t-test. The morphological difference in the isthmus height and internal height of the lateral mass and the difference in the rate of high-riding VA between the RA group and the non-RA group were compared using the Student t-test and the chi-square test, respectively. Intra-observer variation was determined from ten randomly selected sets of CT images measured initially and after several weeks by two of the authors (C.S.L., C.S. K.). Interobserver variability was determined using the same ten CT images, assessed independently and analyzed by means of the Spearman correlation coefficients. A P value <0.05 was considered to be significant, with use of an SPSS program (SPSS 10.0, Chicago, IL).

Results The correlation coefficients for intraobserver and interobserver variability of the technique for measuring the C2 isthmus were r=0.98 (P<0.01) and r=0.97 (P<0.01),

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Table 2 Morphologic measurements of C2 lateral mass (mean and standard deviation) RA Isthmus height (mm) Internal height (mm) a

4.0±1.6 4.8±1.6

Non-RA

P valuea

5.7±1.0 6.2±1.2

0.000 0.001

Student’s t-test

respectively, which validated the measurement method. The mean isthmus height of the left and right sides was 5.6±1.5 mm and 5.2±1.6 mm, respectively. The mean left and right internal height was 5.0±1.5 mm and 4.6±5.2 mm, respectively. There were significant differences in the isthmus height and internal height of the lateral mass between the left and the right sides (P=0.014 for isthmus height, P=0.002 for internal height). There were significant quantitative differences in the isthmus height and internal height of the lateral mass between the RA and the non-RA group (Table 2). In addition, the high-riding VA was 54% (15 joints) of the 28 atlantoaxial joints in the RA group and 12% (3 joints) of the 26 atlantoaxial joints in the non-RA group (P<0.01). Fig. 2 a–d A 63-year-old RA patient with unilateral high-riding vertebral artery. a CT sagittal reconstruction image of the left C1–2 facet joint shows sufficient isthmus height and internal height of lateral mass to allow TA screw passage. b CT sagittal reconstruction image of the right C1–2 facet joint shows the high-riding vertebral artery. The vertebral artery groove (*) is situated more medially, posteriorly, and superiorly than on the left side. c, d Follow-up radiographs after surgery show left unilateral TA fixation with posterior wiring of C1–2

Discussion Posterior atlantoaxial arthrodesis is used to remedy instabilityof the atlantoaxial complex caused by fracture and nonunion of the odontoid, transverse ligament injury or laxity, RA, or tumor. In particular, in osteoporotic RA patients, the bone quality does not permit optimal fixation using the isolated posterior wiring technique, which is ineffective against rotatory or lateral bending moments and yields a low fusion rate. Grob and Magerl [3] improved the biomechanical stability of atlantoaxial fixation by adding the atlantoaxial TA screw to the Gallie wiring technique. Casey et al. [14] demonstrated that using this technique, high stability could be achieved in rheumatoid patients with atlantoaxial instability. Posterior atlantoaxial TA screw fixation is an excellent technique, but, carries the potential risk of VA injury. Wright et al. [15] reported that the risk of VA injury was 4.1% per patient and 2.2% per screw inserted. Madawi et al. [16] reported that there were five VA injuries in 37 patients treated using the atlantoaxial TA screw fixation technique, and the risk factors for VA injury included incomplete reduction of atlantoaxial complex prior to screw placement, failure to appreciate the size

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of the VA in the C2 pedicle and lateral mass, and previous transoral surgery. The VA course in C2 varies, with consequent anatomical variability of the C2 isthmus which is the trajectory for the TA screw. In approximately 80% of individuals, the VA makes an acute reverse bend in the lateral mass of C2 immediately below the superior articular facet and then runs through the transverse foramen laterally rather than vertically [10]. In 15% of these patients, the VA can occupy almost two thirds of the superior articular facet of C2. Therefore, several authors investigated the morphometric characteristics of the atlantoaxial complex using CT reconstruction imaging or human cadavers [12, 16, 17]. They concluded that variability in the height and width of the C2 isthmus and the associated anomalous course of the VA might prevent proper posterior TA screw fixation and that preoperative CT imaging with reconstruction was therefore essential. Additionally, Mandel et al. [18] demonstrated that CT measurements closely approximated the actual dimensions of the isthmus. RA causes loss of tensile strength and stretching of the transverse and alar ligaments, and bony erosion of the atlantoaxial synovial joint. A recent morphometric study of the atlantoaxial complex demonstrated that this inflammatory reaction not only exacerbates osteoporosis but also limits the use of TA screw fixation because of the risk of VA injury, and recommended unilateral TA screw fixation, modification of screw diameter, or alternative techniques for atlantoaxial arthrodesis for RA patients with an atlantoaxial instability [19, 20]. The current study showed that the mean isthmus height and internal height of the lateral mass in RA patients were significantly lower than in non-RA patients. We have routinely performed the preoperative CT sagittal reconstruction images for surgical candidates of posterior atlantoaxial arthrodesis. We defined a high-riding VA on the basis of sagittal reconstructed CT images, a widely accepted method of evaluation. Bloch et al.’s definition [21] was used in this study for a high-riding VA: internal height< 2 mm, isthmus height <5 mm, or both.

We adopted unilateral TA fixation with posterior wiring, or isolated posterior wiring for patients with a high-riding VA (Fig. 2). In the present study, the high-riding VA according to Bloch et al. was 54% (15 joints) of the 28 atlantoaxial joints in the RA group. In contrast, the highriding VA was 12% (3 joints) of the 26 atlantoaxial joints in the non-RA group, which is consistent with previous studies, where the rates of a high-riding VA based on various methods and criteria ranged from 11% to 23% [17, 18, 22]. According to previous studies, the dimensions of the isthmus were asymmetric, which correlated with the present studies [17–19]. We acknowledge several constraints and limitations of the present study. First, we were restricted to a relatively small number of subjects because we analyzed only patients with an atlantoaxial instability from a select cohort. Second, although, the two groups (RA and nonRA) were statistically matched for age and gender, there was a tendency for the RA group to be recruited from a slightly older population than the non-RA group. Third, the two groups were not matched for patients’ size. Finally, the current CT study did not include the dimension of isthmus width, which is another important measurement for safe screw fixation. The precise CT axial images for showing the isthmus of axis properly could not be obtained, because only CT images of patients with an unstable atlantoaxial complex were analyzed.

Conclusions In RA patients, the axis showed more extensive thinning of the isthmus and the lateral mass than in non-RA patients. A precise preoperative evaluation of the VA using reconstruction CT imaging should be performed in all patients planned to undergo atlantoaxial TA fixation. In particular, the possibility of a high-riding VA always should be considered in RA patients with an atlantoaxial instability.

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