Inflammatory response of intervertebral disc cells is reduced by fibrin sealant scaffold in vitro.

Intervertebral disc (IVD) degeneration is a complex process characterized by elevated concentrations of proinflammatory cytokines and proteolytic enzymes. Because of pro-healing constituents, we hypothesized that fibrin sealant (FS) can reduce inflammation and augment soft tissue healing within the damaged or degenerative IVD. To test this, human and porcine nucleus pulposus (NP) and annulus fibrosus (AF) cells were extracted from tissues and encapsulated into alginate beads (NP cells) and type I collagen sponges (AF cells). Half of the alginate and collagen scaffolds were embedded in FS. To provoke inflammatory behaviours, the constructs were cultured with and without continuous IL-1α (10 ng/ml) for 4, 7 and 14 days. ELISA was used to quantify the cellular synthesis (ng/µg DNA) of clinically relevant cytokines, proteolytic enzymes and growth factors. In NP cell constructs with IL-1α, the syntheses of TNFα, IL-1ß, IL-6, IL-8 was elevated at all culture durations. In the presence of FS, secretion of several pro-inflammatory cytokines were significantly reduced [IL-6 and IL-8 (porcine); and TNFα, IL-1ß, IL-6, IL-8 (human)]. Consistent with these reductions, human NP cultures exposed to FS and FS + IL-1α synthesized significantly reduced amounts of MMP-1 and -3 compared to constructs with IL-1α. For porcine and human AF cells, there were no significant differences in the synthesis of the inflammatory or proteolytic cytokines relative to controls (without IL-1α) at any culture duration. However, the porcine AF cells exposed to FS synthesized elevated amounts of the anti-inflammatory cytokine IL-4. The results suggest that FS may have beneficial effects for patients with degenerated intervertebral discs.

Biomechanical comparison of transosseous versus suture anchor repair of the subscapularis tendon.

PURPOSE: The purpose of this study was to compare the biomechanical properties of transosseous versus suture anchor repair of the subscapularis tendon. We also performed real-time measurement of contact area and pressure of the repair site under rotational loads. METHODS: Six paired human cadaveric shoulders were subjected to rotational loading after repair of the subscapularis tendon. Both shoulders were randomized to transosseous or suture anchor repairs. Real-time pressure sensors were placed between the subscapularis tendon and lesser tuberosity. The repair was subjected to cyclical rotational loading and load-to-failure testing. RESULTS: No significant difference was detected in initial pressurized contact area between transosseous repairs (1.70 +/- 0.99 cm(2), 57.88 +/- 30.02% footprint) and suture anchor repairs (1.08 +/- 0.58 cm(2), 34.26% +/- 17.32% footprint). Under cyclical loading, the conditioning elongation of transosseous repairs (0.64 +/- 0.40 mm) was significantly lower (P < .05) than that of suture anchor repairs (2.38 +/- 1.58 mm). No significant difference was found in mean pressurized contact area between the transosseous repairs (2.72 +/- 1.25 cm(2), 94.2% +/- 37.4% footprint) and suture anchor repairs (2.01 +/- 0.89 cm(2), 65.9% +/- 27.9% footprint). For suture anchor repairs, repair-site contact area was significantly (P < .05) smaller than the area of corresponding native insertional footprints; for transosseous repairs, no significant difference was detected. There were no significant differences in peak pressures between the 2 repairs. In the load-to-failure tensile test, there was no significant difference between transosseous repairs (453.2 +/- 66.1 N) and suture anchor repairs (392.6 +/- 78.0 N). CONCLUSIONS: Transosseous and suture anchor repairs of the subscapularis tendon have comparable biomechanical properties. Despite increased conditioning elongation in suture anchor repairs, we found no significant differences in mean contact area between the 2 repairs under cyclical loading. The suture anchor repairs do have a smaller contact area than the native insertional area. Real-time pressure and contact area measurements enabled mapping of the repair site throughout cyclical loading. CLINICAL RELEVANCE: Rotational loading of the subscapularis tendon may provide a more accurate representation of subscapularis tendon injuries. Both techniques showed adequate repair strength; however, neither surgical technique exhibited normal insertional behavior in this time-zero biomechanical study.

Disc arthroplasty design influences intervertebral kinematics and facet forces.

BACKGROUND CONTEXT: Total disc replacement is a novel approach for dynamically stabilizing a painful intervertebral segment. While this approach is gaining popularity, and several types of implants are used, the effect of disc arthroplasty on lumbar biomechanics has not been widely reported. Consequently, beneficial or adverse effects of this procedure may not be fully realized, and data for kinematic optimization are unavailable. PURPOSE: To characterize kinematic and load transfer modifications at L5/S1 secondary to joint replacement. STUDY DESIGN: A human cadaveric biomechanical study in which the facet forces and instant axes of rotation (IAR) were measured for different spinal positions under simulated weightbearing conditions before and after total disc replacement at L5/S1 using semiconstrained (3 degrees of freedom [DOF]; Prodisc) and unconstrained (5 DOF; Charité) articulated implants. METHODS: Twelve radiographically normal human cadaveric L5/S1 joints (age range 45-64 years) were tested before and after disc replacement using Prodisc II implants (Spine Solutions, Paoli, PA) in six specimens and SB Charité III (Johnson & Johnson, New Brunswick, NJ) in six other specimens. Semiconstrained fixtures in combination with a servo-hydraulic materials testing system subjected the test specimens to a physiologic combination of compression and anterior shear. Multiple intervertebral positions were studied and included up to 6 degrees of flexion, extension, and lateral bending. The IAR was calculated for every 3-degree intervals, and the force through the facet joints was simultaneously measured using flexible intra-articular sensors. Data were analyzed using repeated-measures analysis of variance. RESULTS: During flexion/extension, the average IAR positions and directions were not significantly modified by implantation with the exception that the IAR was higher relative to S1 end plate with the Charité (p=.028). The IAR had a vertically oriented centrode throughout flexion/extension with the Prodisc (p<.001) and the Charité (p<.016). The centrode tended to be greater with the Prodisc. There was a trend that the facet force was decreased throughout flexion/extension for the Prodisc; however, this was statistically significant only at 6 degrees extension (27%, p=.013). In lateral bending, the IAR was significantly modified by Prodisc replacement, with a decreased inclination relative to S1 end plate, (ie, increased coupled axial rotation). While the IAR moved in the horizontal plane toward the side of bending, this effect was more pronounced with the Prodisc. The ipsilateral facet force was significantly increased in 6 degrees lateral bending with the Charité (85%; p=.001). CONCLUSIONS: The degree of constraint affects post-implantation kinematics and load transfer. With the Prodisc (3 DOF), the facets were partially unloaded, though the IAR did not match the fixed geometrical center of the UHMWPE. The latter observation suggests joint surface incongruence is developed during movement. With the Charité (5 DOF), the IAR was less variable, yet the facet forces tended to increase, particularly during lateral bending. These results highlight the important role the facets play in guiding movement, and that implant constraint influences facet/implant synergy. The long-term consequences of the differing kinematics on clinically important outcomes such as wear and facet arthritis have yet to be determined.

The instant axis of rotation influences facet forces at L5/S1 during flexion/extension and lateral bending.

Because the disc and facets work together to constrain spinal kinematics, changes in the instant axis of rotation associated with disc degeneration or disc replacement may adversely influence risk for facet overloading and arthritis. The relationships between L5/S1 segmental kinematics and facet forces are not well defined, since previous studies have separated investigations of spinal motion and facet force. The goal of this cadaveric biomechanical study was to report and correlate a measure of intervertebral kinematics (the centrode, or the path of the instant axis of rotation) and the facet forces at the L5/S1 motion segment while under a physiologic combination of compression and anterior shear loading. Twelve fresh-frozen human cadaveric L5/S1 joints (age range 50-64 years) were tested biomechanically under semi-constrained conditions by applying compression plus shear forces in several postures: neutral, and 3 degrees and 6 degrees of flexion, extension and lateral bending. The experimental boundary conditions imposed compression and shear representative of in vivo conditions during upright stance. The 3-D instantaneous axis of rotation (IAR) was calculated between two consecutive postures. The facet joint force was simultaneously measured using thin-film sensors placed between both facet surfaces. Variations of IAR location and facet force during motion were analyzed. During flexion and extension, the IAR was oriented laterally. The IAR intersection with the mid-sagittal plane moved cephalad relative to S1 endplate during flexion (P=0.010), and posterior during extension (P=0.001). The facet force did not correlate with posture (P=0.844). However, changes in the facet force between postures did correlate with IAR position: higher IAR's during flexion correlated with lower facet forces and vice versa (P=0.04). During lateral bending, the IAR was oblique relative to the main plane of motion and translated parallel to S1 endplate, toward the side of the bending. Overall, the facet force was increased on the ipsilateral side of bending (P=0.002). The IAR positions demonstrate that the L5 vertebral body primarily rotates forward during flexion (IAR close to vertebral body center) and rotates/translates backward during extension (IAR at or below the L5/S1 intervertebral disc). In lateral bending, the IAR obliquity demonstrated coupling with axial torsion due to resistance of the ipsilateral facet.

Burst fracture in the metastatically involved spine: development, validation, and parametric analysis of a three-dimensional poroelastic finite-element model.

STUDY DESIGN: A finite-element study and in vitro experimental validation was performed for a parametric investigation of features that contribute to burst fracture risk in the metastatically involved spine. OBJECTIVES: To develop and validate a three-dimensional poroelastic model of a metastatically compromised vertebral segment, to evaluate the effect of lytic lesions on vertebral strains and pressures, and to determine the influence of loading and motion segment status (bone density, pedicle involvement, disc degeneration, and tumor size) on the relative risk of burst fracture initiation. SUMMARY OF BACKGROUND DATA: Finite-element analysis has been used successfully to predict failure loads and fracture patterns for bone. Although models for vertebra affected with tumors have been presented, these have not been thoroughly validated experimentally. Consequently, their predictive capabilities remain uncertain. METHODS: A three-dimensional poroelastic finite-element model of the first lumbar vertebra and adjacent intervertebral discs, including a tumor of variable size, was developed. To validate the model, 12 cadaver spinal motion segments were tested in axial compression, in intact condition, and with simulated osteolytic defects. Features of the validated model were parametrically varied to investigate the effects of tumor size, trabecular bone density, pedicle involvement, applied loads, loading rates, and disc degeneration using outcome variables of vertebral bulge and vertebral axial deformation. RESULTS: Consistent trends between the experimental data and model predictions were observed. Overall, the model results suggest that tumor size contributes most toward the risk of initiating burst fracture, followed by the applied load magnitude and bone density. CONCLUSIONS: The parametric analysis suggests that the principal factors affecting the initiation of burst fracture in metastatically affected vertebrae are tumor size, magnitude of spinal loading, and bone density. Consequently, patient-specific measures of these factors should be factored into decisions regarding clinical prophylaxis. Pedicle involvement or disc degeneration was less important according to the outcome measures in this study.

Biomechanically derived guideline equations for burst fracture risk prediction in the metastatically involved spine.

Methods to quantify burst fracture risk and neurologic deficit for patients with spinal metastases have not been well defined. This study aims to develop objective biomechanically based guidelines to quantify metastatic burst fracture risk. An experimentally validated finite element model of a human lumbar motion segment was used to simulate burst fracture. Through parametric analysis, the behavior of metastatically involved vertebrae was quantified and a formula to relate patient-specific variables to burst fracture risk defined. The equation-based guidelines were able to describe the mechanical behavior of the metastatically involved vertebral model (R2 = 0.97) reflecting the risk and mechanism of fracture. Vertebral density was found to influence the mechanism of burst fracture with respect to endplate failure. These analyses provide clinically feasible equation-based guidelines for burst fracture risk assessment in the metastatically involved spine.