A Biomechanical Assessment of Shaken Baby Syndrome: What About the Spine?

BACKGROUND: Shaken baby syndrome occurs following inertial loading of the pediatric head, resulting in retinal hemorrhaging, subdural hematoma, and encephalopathy. However, the anatomically vulnerable cervical spine receives little attention. Automotive safety literature is replete with biomechanical data involving forward-facing pediatric surrogates in frontal collisions, an environment analogous to shaking. Publicly available data involving child occupants were utilized to study pediatric neck and head injury potential. We hypothesized that inertial loading provides a greater risk of injury to the cervical spine than to the head. METHODS: Full-scale automotive crash tests (n = 131) and deceleration sled tests (n = 32) utilizing forward-facing 3-year-old surrogates with head accelerometers and cervical force sensors were analyzed. One hundred sixty-seven full-scale vehicle and 33 sled test runs were assessed in the context of published injury assessment reference values (IARVs) for closed head injury (head injury criterion 15 [HIC15]) and cervical tensile strength in the 3-year-old model. RESULTS: One hundred sixty-one (96%) child surrogates in full-scale crash tests exceeded the cervical peak tension IARV, while only 37 (22%) surpassed the HIC15 IARV. Similarly, in sled testing runs, 27 (82%) pediatric surrogates exceeded cervical tension IARVs, while 1 (3%) surpassed the HIC15 IARV. In both full-scale and sled tests, all surrogates surpassing the HIC15 IARV also exceeded the cervical tension IARV. Positive linear correlations were observed between HIC15 and cervical tensile forces in both full-scale vehicle (R2 = 0.15) and sled testing runs (R2 = 0.54). CONCLUSIONS: These data support the hypothesis that inertial loading of the head provides a greater injury risk to the cervical spine than to closed-head injury.

Biomechanical modeling of acetabular component polyethylene stresses, fracture risk, and wear rate following press-fit implantation.

Press-fit implantation may result in acetabular component deformation between the ischial-ilial columns ("pinching"). The biomechanical and clinical consequences of liner pinching due to press-fit implantation have not been well studied. We compared the effects of pinching on the polyethylene fracture risk, potential wear rate, and stresses for two different thickness liners using computational methods. Line-to-line ("no pinch") reaming and 2 mm underreaming press fit ("pinch") conditions were examined for Trident cups with X3 polyethylene liner wall thicknesses of 5.9 mm (36E) and 3.8 mm (40E). Press-fit cup deformations were measured from a foam block configuration. A hybrid material model, calibrated to experimentally determined stress-strain behavior of sequentially annealed polyethylene, was applied to the computational model. Molecular chain stretch did not exceed the fracture threshold in any cases. Nominal shell pinch of 0.28 mm was estimated to increase the volumetric wear rate by 70% for both cups and peak contact stresses by 140 and 170% for the 5.9 and 3.8 mm-thick liners, respectively. Although pinching increases liner stresses, polyethylene fracture is highly unlikely, and the volumetric wear rates are likely to be low compared to conventional polyethylene.

Analysis of three types of fixation of the Weil osteotomy.

This study assessed 3 methods of fixation for the Weil osteotomy. A total of 40 bone models were divided equally into 4 groups: a control group consisting of intact lesser rays; and Weil osteotomies that were fixated with 2 crossed Kirschner wires (0.045-in K-wires), 2.0-mm cortical screws, or cannulated 2.4-mm cortical screws. Each specimen was stressed in a computer-controlled hydraulic tensile testing machine, and maximum load, energy to failure, and stiffness were recorded. The following mean load to failure measurements were found: control, 62.9 Newtons (N); K-wire, 22.9 N; cannulated screw, 31.3 N; and noncannulated screw, 19.9 N. There was no statistical difference among the 3 groups of fixation methods in terms of the maximum load. The mean energy to failure of the control group was 326 joule (J); K-wire, 79 J; cannulated screw, 163 J; and noncannulated screw, 66 J. The cannulated screw generated a statistically greater amount of energy at failure than the noncannulated screw (P < .05). The mean structural stiffness of the control group was 7.3 N/mm; K-wire, 2.8 N/mm; cannulated screw 3.3 N/mm; and noncannulated screw, 3.2 N/mm. There was no statistical difference in structural stiffness among the 3 groups of fixation methods. The results indicated a trend toward better biomechanical stability with the 2.4-mm cannulated screw than the 2.0-mm noncannulated screw for fixation of the Weil osteotomy.