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STUDY DESIGN Nondestructive flexibility tests were performed in vitro, comparing multiple conditions of fixation in a single group of specimens. OBJECTIVE To compare the biomechanical behavior of the lumbar spine in the intact condition, after implanting a novel motion stabilizer, and after implanting a rigid fixator. SUMMARY OF BACKGROUND DATA Two specific scenarios that may benefit from dynamic lumbar stabilization are single-level moderate instability, where the stabilizing tissues are relatively incompetent, and juxta-level to fusion, where the last instrumented level requires intermediate stiffness ("topping off") to prevent transfer of high stresses from the stiffer fusion construct to the intact adjacent levels. Both scenarios were evaluated in vitro. METHODS Seven human cadaveric L2-S1 segments were tested (1) intact, (2) after moderate destabilization, (3) after 2-level hybrid posterior fixation, consisting of bilateral dynamic pedicle screws at L4 interconnected with rigid rods to standard pedicle screws at L5 and S1, (4) after 2-level rigid fixation, (5) after 1-level (L4-L5) dynamic fixation, and (6) after 1-level rigid fixation. In each condition, angular range of motion (ROM) and sagittal instantaneous axis of rotation (IAR) were assessed. RESULTS In 1-level constructs, dynamic hardware allowed 104% of intact ROM, whereas rigid hardware allowed 49% of intact ROM. Relative to the intact, the IAR was shifted significantly farther posterior by rigid 1-level instrumentation than by dynamic 1-level instrumentation. In 2-level constructs, the dynamic level allowed significantly greater ROM than the rigid level in all directions but allowed significantly less ROM than the intact level in all directions except axial rotation. CONCLUSION Dynamic instrumentation shifted the IAR less than rigid instrumentation, providing more favorable kinematics. This dynamic stabilizer provided 1-level ROM that was close to intact ROM during all loading modes in vitro. In the topping-off construct, the dynamic segment allowed intermediate ROM to give balanced transitional flexibility. LEVEL OF EVIDENCE N/A.

BACKGROUND: Literature about long implants used together with a minimally invasive spine surgery (MISS) technique is scarce. Our objective is to contribute our surgical experience in this field and to specifically focus on several technical details. PATIENTS AND METHODS: A digitally-dissected canal along the paravertebral muscles was created linking the stab wounds on each side in relation with the pedicles to be cannulated. Screws were inserted following the percutaneous technique. Long rods were modelled, threaded through the extender sleeves along the paravertebral canal and pushed into the screw heads with the reduction forceps. When fusion was needed, the facet complex was decorticated with a drill. To insert a cross-link, a canal between the 2 rods was digitally created and the spinous process was drilled. RESULTS: 8 patients underwent surgery (age range: 25–77 years). Indications were postosteomyelitis kyphosis in 3 patients, bone tumor in 3, and spine fracture in 2. No blood transfusions were necessary during or after surgery. A cross-link was inserted in 2 patients. Posterolateral bone fusion was attempted in 4, but radiologically identifiable in none. In one patient a cantilever manoeuvre was done to correct kyphosis. Mean duration of surgery was 4 h. There were no clinical complications related to the operation or the hardware (mean follow-up of 7.14 months, range: 1–15 months). CONCLUSION: The application of MISS techniques can be broadened to long spinal constructs to assess fractures, tumors or deformity, especially in elderly or debilitated patients. Nevertheless, posterolateral fusion is still a challenge through these limited exposures.