Anterior cervical plating reverses load transfer through multilevel strut-grafts.

STUDY DESIGN: In vitro biomechanical study using a programmable testing apparatus that replicated physiologic flexion/extension cervical spine motion and loading mechanics. OBJECTIVE: To determine the influence of anterior plating on multilevel cervical strut-graft mechanics in vitro. SUMMARY OF BACKGROUND DATA: The addition of anterior instrumentation does not prevent construct failure in multilevel cervical corpectomy. METHODS: Six fresh human cadaveric cervical spines (C2-T1) were tested in the four following sequential conditions: harvested, C4-C6 corpectomy, strut-grafted, and strut-grafted with an anterior cervical plate. A force-sensing strut-graft was used to measure compression/tension, flexion/extension and lateral bending moments, and axial torsion. Parameters of stiffness, vertebral motion, and strut-graft loads were compared to determine differences between the four spine conditions. RESULTS: Application of the anterior plate significantly increased the global stiffness (P < 0.01) and decreased the local motion (P < or = 0.01) of the instrumented levels (C3-C7). Flexion of the strut-grafted spine loaded the strut-graft, whereas extension unloaded the strut-graft. With the anterior plate, flexion of the plated spine unloaded the strut-graft. Extension significantly loaded the strut-graft more than similar degrees of flexion in the strut-grafted condition (P = 0.01). Strut-graft loading end limits of 225 N were reached with a mean 7.5 degrees extension in the plated spines. CONCLUSIONS: Anterior multilevel cervical plating effectively increases stiffness and decreases local cervical motion after corpectomy. However, anterior cervical plating also reverses graft loads and excessively loads the graft in extension, which may promote pistoning and failure of multilevel constructs.

The in vitro effects of instrumentation on multilevel cervical strut-graft mechanics.

STUDY DESIGN: Biomechanical study using a programmable testing apparatus that replicated physiologic flexion-extension cervical spine motion, and loading mechanics. OBJECTIVES: To determine the influence of anterior, posterior, or combined plating on multilevel cervical strut-graft mechanics in vitro. SUMMARY OF BACKGROUND DATA: The addition of instrumentation does not prevent construct failure in multilevel (more than two levels) cervical corpectomy. METHODS: Six fresh human cadaveric cervical spines (C2-T1) were tested in six sequential conditions that included harvested (H), C4-6 corpectomy, strut grafted, strut grafted with an anterior cervical plate (SGAP), strut grafted with posterior plates (SGPP), and strut grafted with combined anterior and posterior plates (SGAPP). A customized force-sensing strut graft (FSSG) was used to measure axial compression-tension, flexion-extension and lateral bending moments, and axial torsion. Parameters of stiffness, segmental vertebral motion, and strut-graft loads were compared, to determine differences among the spine conditions. RESULTS: Flexion of the strut-grafted spine loaded the FSSG, and extension motion unloaded the FSSG. With the anterior plate, flexion of the SGAP spine significantly unloaded the FSSG; extension loaded the FSSG more than flexion of the unplated spine (P = 0.03). The opposite occurred with the posterior plates (SGPP), where flexion of the spine significantly loaded the FSSG (more than the strut grafted spine) and extension unloaded the FSSG (P < 0.03). The combined construct (SGAPP) counteracted the tension band effect of the individual plates and demonstrated significantly less overall FSSG load change than either plate alone (P = 0.03). CONCLUSIONS: Multilevel cervical instrumentation effectively increases stiffness after corpectomy. However, anterior or posterior plating alone excessively loads the graft with small degrees of motion, which may promote pistoning and failure of multilevel constructs.

Ligamentous contributions to pelvic stability.

The purpose of this study was to examine the ligamentous contributions to pelvic stability. Thirteen fresh frozen cadaver pelves were loaded in an MTS materials testing machine, and the supporting ligaments were sequentially cut. After each ligament was cut, measurements of pelvic stability were made. Pelvic stability was maintained most effectively when the pelvic ring remained intact. The sacrotuberous and sacrospinous ligaments contributed little to overall pelvic stability. The posterior sacroiliac ligament and the pubic symphyseal ligaments contributed most to pelvic stability, but overall it was clear that a ligament's contributions to pelvic stability depended not only on the ligament's size, but also on the other ligament remaining intact and the mode in which the pelvis was loaded.

Biomechanical comparison of methods of fixation of isolated osteotomies of the posterior acetabular column.

Ten fresh frozen specimens of a hemipelvis, including the hip joint, capsule and proximal femur, from elderly cadavers were used to evaluate three methods of internal fixation of isolated posterior column osteotomies. Intact and reconstructed specimens were tested at 30 degrees and 60 degrees of hip flexion in a specially designed joint simulator. The three methods of fixation used were a single 3.5 mm reconstruction plate, two such plates, and a 4.5 mm lag screw with a single plate. Motion at the fracture site in three orthogonal directions, and the overall stiffness of the construct, were recorded simultaneously. No significant differences were noted in stiffness for the three procedures and all retained 80% of the intact stiffness. At 60 degrees of flexion, smaller interfragmentary compliances were allowed by fixation with a lag screw and a neutralisation plate (p < 0.05). At 30 degrees, the position of the load plane relative to the fracture plane allowed less interfragmentary motion, so that no significant differences were found between the 3 methods.

Performance assessment of the Terry Fox jogging prosthesis for above-knee amputees.

The Terry Fox jogging (TFJ) prosthesis was developed at Chedoke-McMaster Hospital to alleviate the asymmetric jogging pattern experienced by above-knee amputees when attempting to jog with conventional walking prostheses. This prosthesis features a spring-loaded, telescoping shank designed to eliminate any vaulting action and control the trunk motion during stance. The spring is intended to attenuate the impact forces and release its stored energy at push-off to provide momentum transfer to the jogger. This prosthesis was comprehensively assessed in the gait laboratory, by evaluating the kinematics, energy and power flow patterns of an above-knee amputee jogger wearing the TFJ prosthesis. Included in the assessment is the ability of the prosthesis to satisfy a set of relevant design criteria that have been established from non-amputee jogging patterns. An increased swing phase time for the prosthetic limb and the need to have the knee hyperextended throughout the stance phase contributed to an asymmetric jogging style. The telescoping action did lower the amputee's centre of mass, thereby reducing the vaulting effect. However, the spring only imparted a lifting action to the jogger and the ground reaction forces were double those of a non-amputee jogger. These findings clearly indicate a need to redesign the TFJ prosthesis and are being incorporated in the design of a new physiological jogging prosthesis.