Biomechanical comparison of the pullout strengths of C1 lateral mass screws and C1 posterior arch screws.

BACKGROUND CONTEXT: Conditions of the atlantoaxial complex requiring internal stabilization can result from trauma, malignancy, inflammatory diseases, and congenital malformation. Several techniques have been used for stabilization and fusion. Posterior wiring is biomechanically inferior to screw fixation. C1 lateral mass screws and C1 posterior arch screws are used for instrumentation of the atlas. Previous studies have shown that unicortical C1 lateral mass screws are biomechanically stable for fixation. No study has evaluated the biomechanical stability of C1 posterior arch screws or compared the two techniques. PURPOSE: The purpose of the study was to assess the differences in the pullout strength between C1 lateral mass screws and C1 posterior arch screws. STUDY DESIGN: Biomechanical testing of pullout strengths of the two atlantal screw fixation techniques. METHODS: Thirteen fresh human cadaveric C1 vertebrae were harvested, stripped of soft tissues, evaluated with computed tomography for anomalies, and instrumented with unicortical C1 lateral mass screws on one side and unicortical C1 posterior arch screws on the other. Screw placement was confirmed with postinstrumentation fluoroscopy. Specimens were divided in the sagittal plane and potted in polymethylmethacrylate. Axial load to failure was applied with a material testing device. Load displacement curves were obtained, and the results were compared with Student t test. DePuy Spine, Inc. (Raynham, MA, USA) provided the hardware used in this study. RESULTS: Mean pullout strength of the C1 lateral mass screws was 821 N (range 387-1,645 N ± standard deviation [SD] 364). Mean pullout strength of the posterior arch screws was 1,403 N (range 483-2,200 N ± SD 609 N). The difference was significant (p=.009). Five samples (38%) in the posterior arch group experienced bone failure before screw pullout. CONCLUSIONS: Both unicortical lateral mass screws and unicortical posterior arch screws are viable options for fixation in the atlas. Unicortical posterior arch screws have superior resistance to pullout via axial load compared with unicortical lateral mass screws in the atlas.

C1 lateral mass anatomy: Proper placement of lateral mass screws.

STUDY DESIGN: Cadaveric specimens were measured to determine appropriate placement for C1 lateral mass screws. Instrumentation guidelines were developed and used to instrument a series of cadaveric specimens. Clinical experience with C1 lateral mass fixation was reviewed to evaluate results. Postoperative computed tomographic (CT) scans were reviewed to evaluate screw placement. OBJECTIVES: The cadaveric study measured the dimensions of the atlas and determined ideal trajectory for screw placement. This technique was applied clinically, and 50 cases were retrospectively reviewed for fixation difficulties, neurologic or vascular injuries, and perioperative complications. Postoperative CT scans were reviewed when available. SUMMARY OF BACKGROUND DATA: Halo application, posterior wiring, and C1 to C2 transarticular screws have been used to stabilize the upper cervical spine. Each technique has disadvantages, and C1 lateral mass fixation recently has gained popularity as a potential alternative. Recent anatomic studies have documented the dimensions of the C1 lateral mass and its ability to accommodate screw fixation. Small clinical series have documented early success with this technique. METHODS: Fifteen specimens were stripped of soft tissue and measured by using calipers and CT scans. Guidelines were formulated for C1 lateral mass screw fixation. Additional specimens with intact soft tissue were instrumented without difficulty. A clinical series was reviewed to evaluate for complications related to this technique. Postoperative CT scans were reviewed to evaluate screw placement. RESULTS: The C1 lateral mass safely accommodated screw fixation. Trajectory of 10 degrees medial and 22 degrees cephalad was preferred. The technique was safely applied in a series of 50 patients. Postoperative CT scans showed the ability of the surgeon to achieve the intended goals for starting point and safe trajectory. CONCLUSIONS: C1 lateral mass fixation is a safe alternative for upper cervical fixation with several potential advantages versus other techniques, but further clinical evaluation is warranted.

Decreased platelet derived growth factor expression during fracture healing in diabetic animals.

Animal model experiments have suggested that diabetes inhibits cell proliferation during fracture healing. Immunohistochemical analysis of proliferating cell nuclear antigen revealed significant reductions in cellular proliferation rates in the fracture callus of spontaneously diabetic BB Wistar rats as compared with healthy BB Wistar rats. Because platelet derived growth factor is associated with the early stage of fracture healing, it was hypothesized that diabetes causes decreased platelet derived growth factor expression during the early phase of fracture healing with a concomitant decrease in cell proliferation. Midshaft femur fractures were created in healthy and spontaneously diabetic BB Wistar rats and analyzed at Days 2, 4, and 7 after fracture for expression of platelet derived growth factor. Immunohistochemistry showed decreased localization of platelet derived growth factor in early diabetic fracture callus compared with healthy controls. Platelet derived growth factor messenger ribonucleic acid levels, as determined by reverse transcription and polymerase chain reaction, also were decreased in early diabetic fractures compared with healthy controls. Therefore the decreased cell proliferation rates associated with diabetic fracture healing are consistent with decreased platelet derived growth factor levels and suggest a causal relationship. These results suggest that diabetes is affecting the early phase of fracture healing by inhibiting cell proliferation through decreasing expression of platelet derived growth factor.