Segmental pedicle screw fixation or cross-links in multilevel lumbar constructs. a biomechanical analysis.

BACKGROUND CONTEXT: The placement of segmental pedicle screws and cross-links in short segment posterior pedicle screw constructs has been shown to increase the construct stiffness in some planes. To date, no studies have looked at the contributions of segmental pedicle screw and cross-link placement in longer constructs. PURPOSE: To evaluate the influence of segmental pedicle screw and/or cross-link placement on flexion/extension, lateral bending and axial torsion stiffness in two- and three-level posterior pedicle screw fixation constructs. STUDY DESIGN/SETTING: An in vitro biomechanical analysis of two- and three-level posterior pedicle screw constructs with and without segmental fixation and/or cross-links was performed using calf lumbar spines. Stiffness of the constructs was compared. METHODS: Six calf lumbar specimens were used to test stiffness in one-, two- and three-level posterior pedicle screw fixation constructs in 12 configurations. A custom-made, four-axis spine simulator applied pure cyclical (+/-5 Nm) flexion/extension, lateral bending and axial torsion moments at 0.1 Hz under a constant 50-N axial compressive load. The stiffness of each construct was calculated about each axis of rotation. Data were analyzed using nonparametric techniques with statistical significance determined at alpha less than .05. RESULTS: The stiffness of the instrumented spines were significantly greater than the noninstrumented intact spines in all loading conditions for one-, two- and three-level constructs. There were no significant changes in flexion/extension stiffness with the addition of either the cross-links or the segmental pedicle screws. In lateral bending, the addition of segmental pedicle screws significantly increased the stiffness in the two- and three-level constructs. The addition of two cross-links increased lateral bending stiffness in the longer three-level constructs, with little change in the two-level constructs. In axial torsion, the progressive addition of cross-links showed a tendency toward increased stiffness in both the two- and three-level constructs. Segmental pedicle screws further increased torsional stiffness of the longer, three-level constructs. CONCLUSIONS: As the use of segmental spinal instrumentation progresses from one to two and three levels, the contribution of cross-links and segmental pedicle screws to the overall construct stiffness increases.

A causal modeling approach to institutionalization.

A causal model of the institutionalization process is presented. Path analysis and logistic regression are performed on NHIS Longitudinal Study of Aging data. Based on initial analysis, the model is revised and re-evaluated. Convergent results support significance for four of five proposed predictors. Results are similar for men and women. Findings suggest unidentified factors are important to nursing home admission and that attention should be given to the process of decision-making that leads to admission.

Primary total knee arthroplasty in the valgus knee: creating a balanced soft tissue envelope.

Lateral tissue releases in valgus total knee arthroplasty frequently produce asymmetric flexion-extension gaps and ligamentous instability. This study compared 2 lateral-release sequences and quantified the effects of sequential lateral capsular ligamentous structure release. One knee from 7 paired specimens was released according to a 4-step sequence: posterior cruciate ligament (PCL), ibiotibial tract (IT band), popliteus tendon/lateral collateral ligament (PT/LCL), and biceps femoris tendon. The contralateral knees were released according to a 5-step sequence: PCL, posterolateral capsule, IT band, PT, and LCL. After each release step, flexion and extension gaps were measured and recorded for the medial and lateral aspects. The 5-step sequence produced more symmetric flexion-extension gaps, whereas the absolute magnitudes of correction were lower than with the 4-step sequence. LCL sacrifice in both sequences produced marked lateral flexion-extension gap asymmetry.