Optimal ilio-sacral screw trajectory in paediatric patients : a computed tomography study

Pelvic fixation during procedures performed to treat spinal deformities in paediatric patients remains challenging. No computed tomography studies in paediatric have assessed the optimal trajectory of ilio- sacral screws to prevent screw malposition. We used pelvic computed tomography from 80 children divided into four groups : females <10 and ≥10 years and males <10 and ≥10 years. A secure triangular corridor parallel to the upper S1 endplate was delineated based on three fixed landmarks. The optimal screw insertion angle was subtended by the horizontal and the line bisecting the secure corridor. Student’s t test was applied to determine whether the optimal screw insertion angle and/or anatomical parameters were associated with age and/or sex. Mean optimal angle was 32.3°±3.6°, 33.8°±4.7°, 30.2°±5.0°, and 30.4°±4.7° in the younger females, younger males, older females, and older males, respectively. The mean optimal angle differed between the two age groups (p=0.004) but not between females and males (p=0.55). Optimal mean screw length was 73.4±9.9 mm. Anatomical spinal canal parameters in the transverse plane varied with age (p=0.02) and with sex in the older children (p=0.008), and those in the sagittal plane varied with sex (p=0.04). Age affected ilio-sacral screw positioning, whereas sex did not. Several anatomical spinal canal parameters varied with age and sex. These results should help to ensure safe and easy ilio-sacral screw placement within a secure corridor.

CORRELATION BETWEEN THE SCHANZ SCREW INSERTION ANGLE AND THE LOSS OF KYPHOSIS CORRECTION IN THORACOLUMBAR FRACTURES

ABSTRACT Objective To compare the Schanz screw insertion angle and the loss of the regional kyphosis correction in thoracolumbar burst fractures following posterior short instrumentation surgery. Methods Patients with a thoracolumbar burst fracture between levels T11-L2 were divided into two groups (parallel and divergent) according to the angle formed between the Schanz screw and the vertebral plateau. Regional kyphosis was evaluated in preoperative, immediate postoperative and last follow-up radiographs. Results Of the 58 patients evaluated, 31 had a parallel assembly and 27 had a divergent assembly. When we analyzed the angle of kyphosis, no statistical difference was observed between the pre- and postoperative radiographs. However, a statistical difference in the last follow-up radiographs and in the final loss of the kyphosis correction was confirmed. Conclusion The insertion of Schanz screws with a divergent assembly presents better radiographic results with less loss of kyphosis correction angle when compared with the parallel assembly technique. Level of Evidence III; Retrospective cohort study.

Biomechanical Analysis of Plate Fixation Compared With Various Screw Configurations for Use in the Latarjet Procedure

Background: The biomechanical properties of coracoid fixation with a miniplate during the Latarjet procedure have not been described. Purpose: To determine the biomechanical properties of miniplate fixation for the Latarjet procedure compared with various screw fixation configurations. Study Design: Controlled laboratory study. Methods: A total of 8 groups (n = 5 specimens per group) were tested at a screw insertion angle of 0°: (1) 3.75-mm single screw, (2) 3.75-mm double screw, (3) 3.75-mm double screw with washers, (4) 3.75-mm double screw with a miniplate, (5) 4.00-mm single screw, (6) 4.00-mm double screw, (7) 4.00-mm double screw with washers, and (8) 4.00-mm double screw with a miniplate. In addition, similar to groups 1 to 3 and 5 to 7, there were 30 additional specimens (n = 5 per group) tested at a screw insertion angle of 15° (groups 9-14). To maintain specimen uniformity, rigid polyurethane foam blocks were used. Testing parameters included a preload of 214 N for 10 seconds, cyclical loading from 184 to 736 N at 1 Hz for 100 cycles, and failure loading at a rate of 15 mm/min until 10 mm of displacement or specimen failure occurred. Results: All single-screw constructs and 77% of 15° screw constructs failed before the completion of cyclical loading. Across all groups, group 8 (4.00-mm double screw with miniplate) demonstrated the highest maximum failure load ( P < .001). There were no differences in failure loads among specimens with single-screw fixation (groups 1, 5, 9, and 12; P > .05). All specimens in groups 9, 10, 11, 12, 13, and 14 (insertion angle of 15°) had significantly lower maximum failure loads compared with specimens in groups 2, 3, 4, 6, 7, and 8 (insertion angle of 0°) ( P < .001 for all). Conclusion: These results indicate significantly superior failure loads with the miniplate compared with all other constructs. Across all fixation techniques and screw sizes, constructs with screws inserted at 0° performed better than constructs with screws inserted at 15°. Clinical Relevance: The use of a miniplate for coracoid fixation during the Latarjet procedure may provide a more durable construct for the high-demand contact athlete.

Screw Orientation and Foam Density Interaction in Pullout of Anterior Lumbar Interbody Fusion Plates

ABSTRACT Introduction Previous studies demonstrated increases in single screw pullout strength with increases in material density. Recent anterior cervical interbody fusion plate pullout studies utilizing a polyurethane foam block model have shown that alterations in screw insertion angle from straight-in are not associated with an increase in pullout strength. The purpose of this study is to characterize the pullout strength of an anterior lumber interbody fusion (ALIF) plate when installed at various screw angles in different simulated bone densities. Materials and methods: Ninety ALIF plate pullout tests were performed using three common screw insertion angles in polyurethane (PU) foam blocks of three densities: 0.08 g/cm3, 0.16 g/cm3 and 0.24 g/cm3, simulating severely, mildly and nonosteoporotic cancellous bone, respectively. Plates were pulled out axially at 1 mm/min and pullout strength and stiffness compared. Results Doubling foam density yielded 2.6-fold and 3.0-fold increases (p < 0.05) in mean pullout strength and stiffness, respectively. Tripling foam density yielded 4.5-fold and 5.3-fold increases (p < 0.05) in mean pullout strength and stiffness, respectively. Screw angle placement contributed relatively less to pullout strength and stiffness compared to PU foam density. Conclusion In our model, ALIF plate pullout strength and stiffness appear to be more associated with increased foam block density than screw trajectory. Vertebral bone density should be a strong consideration in preoperative planning for ALIF with plating. Screw trajectory should be based on vascular anatomy and screw placement safety, rather than the classic lateral-to-medial trajectory. Patacxil WM, Palmer DK, Rios D, Inceoglu S, Williams PA, Cheng WK. Screw Orientation and Foam Density Interaction in Pullout of Anterior Lumbar Interbody Fusion Plates. The Duke Orthop J 2012;2(1):35-39.