In Vivo Analysis of Atlantoaxial Motion in Individuals Immobilized With the Halo Thoracic Vest or Philadelphia Collar

Study Design. In vivo biomechanical comparison of the halo thoracic vest (HTV) and the Philadelphia collar (PC). Objective. To delineate the capacity of both orthoses for immobilization of the atlantoaxial complex (AAC), e.g., for their use in odontoid fracture care. Summary of Background Data. Stable odontoid fractures can be treated with external immobilization using, e.g., a PC or a HTV. Although the HTV confers higher morbidity, particularly in elderly patients, with a similar union-rate in odontoid fracture care compared with the PC, many surgeons are still prone to use the HTV instead of the PC because the former is thought to accomplish increased rigidity at the AAC. Because application of the HTV using pins is an invasive procedure, there is a lack of biomechanical in vivo data on the “real” rigidity conferred by a HTV in comparison with a PC. Methods. Twenty volunteers were subjected to flexion/extension radiographs immobilized in a modified HTV or a PC. The radiographs were performed in extreme position of flexion in sitting position and extension in standing position. The PC was fitted as usual. The 4 cortical pins of a normal clinically used HTV were replaced by 12 modified distance pins. The halo-ring was fixed to the head by tightening of the 12 pins in an alternating fashion, thus yielding a hexapod-like strong fixation between the head and the HTV. The procedure was uncomfortable but there were no adverse events from the HTV placement. Radiographs were analyzed for the segmental rotation angle of C1–C2 in sagittal plane (SRA C1–C2) and the absolute rotation angle of C2–C7 (ARA C2–C7) using the Harrison tangent method. Separation angles (rSRA C1–C2 and rARA C2–C7) were calculated from flexion/extension views. Two observers measured all angles. The means of the measurements were used for statistical analysis. The interobserver reliability was expressed by calculating intraclass correlation coefficients (ICCs). Results. Mean age of 20 volunteers was 30.9 ± 4.2 years. Calculation of the ICCs showed good to excellent interobserver reliability for all angular measurements (ICC = 0.95–0.98). Concerning restriction of subaxial sagittal plane motion, the HTV was more effective than the PC. The difference for the rARA C2–C7 between the PC (mean 20.7°) and HTV (mean 9.2°) yielded significance (P = 0.01). But, concerning restriction of flexion/extension at the AAC, there was no statistical significant difference for the rSRA C1–C2 between the PC and HTV (P = 0.3). The PC (mean 1.3°) was even superior to the HTV (mean, 3.3°) in restricting sagittal motion at C1–C2. In comparison to normal atlantoaxial motion was restricted by 88.5% with the PC and 70.8% with the HTV. In light of the results and a selected review of literature, a treatment algorithm for the elderly patient with odontoid fracture is presented. Conclusion. Under the extremes of flexion and extension bendings, the current study demonstrated that there was no significant difference in restriction of sagittal motion at C1–C2 when using the PC instead of the HTV in a group of 20 young normal adults. In light of the current biomechanical data and a selected review of literature, it is concluded that the use of a PC is sufficient for the treatment of stable odontoid fractures.