Rib head dislocation (RHD) and invasion into the spinal canal is not a rare characteristic of dystrophic scoliosis secondary to type 1 neurofibromatosis (DS-NF1), with an incidence ranging from 12.1–15.9% in the literatures [1,2,3,4,5,6]. The potential risk of cord injury and the painful rib hump by palpation are two unique characteristics of this phenomenon [5, 7]. Resection of the compressing rib head has been advocated for patients with consequential neurological impairments caused by RHD [2, 4, 6, 8, 9]. Those with an increasing occurrence of a provoked shock-like painful rib hump under direct pressure are also candidates for rib head resection. Contrarily, for majority of such patients who are neurologically intact, current consensus proposes that preservation of the dislocated rib head is not a contraindication to deformity correction [5, 8, 10]. Spontaneous migration of rib head from spinal canal has been well reported following scoliosis correction with modern spinal instrumentation techniques [5, 8, 10]. In this situation, a rib excision is not desirable yet may be applied as preventive strategies for neurological impairments, if necessary.
Having been well documented in previous reports, RHD uniformly occur on the convex side of the scoliosis and was combined with rib penciling [11], enlargement of foramen and significant vertebral rotation to the convexity in the apical region among DS-NF1 patients [5, 12, 13]. Thus, theoretically, corrective vertebral translation and derotation to the concavity are beneficial for retracting the penetrated rib head from spinal canal, which is confirmed in the previous reports [5, 8, 10]. Yalcin et al. reported that, under the direct visualization, the penetrated rib head migrated significantly from the canal, and meanwhile, the apex moved to the concavity during correction maneuvers [8]. Mao et al. found that the extraction degree of rib head from the spinal canal was associated with the correction of vertebral translation and rib-vertebrae angle immediately after the surgery [5]. However, no particular surgical manipulation was mentioned in the literature to instruct the spine surgeons to effectively obtain more pull-out of the embedded rib head without rib head resection. Usually, adequate apical screw/hook placement along with correction maneuvers can achieve significant reduction of vertebral translation and desired vertebral derotation in idiopathic scoliosis [14, 15]. For dystrophic scoliosis secondary to NF-1, this concept is supposed to be applicable as well. However, adequate apical screw placement is a big challenge for NF-1 patients even with the assistance of O-arm-based navigation techniques. Screw misplacement due to pedicle dystrophy may cause devastating neurological injury and increasing risk of screw poor pull-out strength, which renders risk/benefit assessment a dilemma for this situation.
Currently, no quantitative analysis was performed regarding whether screw/hook placement in the vertebrae with RHD could be an optimal strategy for retracting the dislocated rib head from the spinal canal. This study is designed to amend this issue, so as to provide detailed data needed to determine whether or not to the fixation placement in dystrophic pedicle is beneficial and should be tried with utmost effort in case of RHD with impending neurological deficits.
Patients and methods
Patients
This is a retrospective study approved by the Institution Review Board of our hospital. Patients with dystrophic scoliosis secondary to NF-1 who underwent corrective scoliosis surgery between March 1998 to January 2018 were identified from our scoliosis database. The following inclusion criteria were used: (1) thoracic scoliosis with typical dystrophic radiographic features [1]; (2) rib head penetrating into the spinal canal identified on preoperative CT scans (Fig. 1); (3) both pre- and postoperative images of axial CT scans. Exclusion criteria were as follows: (1) preoperative intra-canal neurofibromatosis; (2) combined with preoperative neurological impairments; (3) treated with growing rods; (4) history of spine surgery. The medical records, imaging scans, and operative reports were reviewed. Patient demographic information was recorded including age at surgery, sex, curve pattern, vertebral level of RHD, surgical strategies, fusion levels, postoperative neurological status, and surgical complications. Patients were assigned into two groups according to whether or not there existed apical fixation at the vertebrae with RHD: screw/hook group (Fig. 2) and non-screw/hook group (Fig. 3).
Radiographic assessment
Radiographic analysis included the Cobb angle in both coronal and sagittal planes on long-cassette standing posteroanterior and lateral radiographs obtained preoperatively, early postoperatively (within two weeks). The CT scans were used to analyze and assess the variations of the positional rib-canal interrelationships caused by corrective maneuvers. The parameters for positional assessments were measured pre- and post-operatively as follows: (1) intraspinal rib length [5]; (2) the distance between the rib head tip and the most concave spot of the spinal canal (DRCSSC) [5]; (3) rib-vertebrae angle; (4) vertebral-sternum angle [16]; (5) height of main curve; (6) vertebral translation. The definitions of the standard measuring techniques of the aforementioned parameters were as follows.
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1. Intraspinal rib length (IRL, Fig. 1A): length of the penetrated rib head measured from the rib head tip to the intersection point of rib head with the border of the spinal canal [5].
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2. DRCSSC (Fig. 1B): distance between the rib head tip and the most concave point of the osseous spinal canal, which was the point where the deviated spinal cord rested in a distorted and rotated spine canal with apparent lateral deviation [5].
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3. Rib-vertebrae angle (RVA, Fig. 1C): defined as the angle between the axis of the rib head and the line bisecting the vertebral body longitudinally.
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4. Vertebral-sternum angle (Fig. 1D): defined as the angle between the line bisecting the vertebral body longitudinally and the line drawn from the middle point of the sternum to the dorsal central aspect of the vertebral foramen [16].
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5. Height of main curve (Fig. 1E): distance between the two midpoints of the superior and inferior endplates of the corresponding upper and lower end vertebrae of the main curve.
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6. Vertebral translation (Fig. 1E): vertical distance from the center point of the vertebra with RHD to the line defined as the height of main curve.
Parameters were measured on axial CT scans (parameters 1–4) or the standing posteroanterior radiographs (parameters 5 and 6) using the PACS (Picture Archiving and Communications Systems, PACS) workstation (Easy Vision IDS5, version 11.4, Philips, Hamburg, Germany). Pre- and postoperative CT scans in supine position were obtained in all patients, which were performed preoperatively to provide anatomical information on the severity of the deformed bony vertebral structure to guide the selection and insertion of appropriate pedicle screws, and postoperatively to detect any mispositioned pedicle screws potentially causing neural or vascular injury or impingement. All the patients and their parents were informed about the purpose of CT scans and the potential risks, who then signed informed consent forms. The selection of measurement level on CT scan was identified as the rib head tip with maximal rib penetration and intrusion at the level of intervertebral foramina, both pre- and post-operatively. Two of the well-trained authors (S.L. and Y.Y.M.) completed the measurement individually. In addition, all parameters were selected to determine the inter- and intra- observer variability of the measurement. All the radiographic parameters were measured by the authors and then repeated twice. There were strong inter-observer and intra-observer agreements for all the parameters with all the kappa correlation coefficients exceeding 0.8. Therefore, the measured data were highly reliable, and the mean values of the parameters measured by the two investigators were recorded.
Statistical analysis
Data analysis was conducted using SPSS 19.0 software (IBM). Shapiro–Wilk test was firstly used to check for the normal distribution of data before performing the t test. Normal distribution test revealed that all p values were > 0.05 in preoperative and postoperative data, or data from groups with or without screw/hook insertion, which indicated that all the data included for the t test were within the normal distribution. Paired-sample t test was applied to compare the operative changes in each group. In addition, the data were compared between the groups with the use of independent-sample t. Multiple linear regression analysis was used to explore whether correction of Cobb angle, vertebral translation, and rib-vertebrae angle could contribute to the extraction of intra-canal rib heads. The significance level was < 0.05.