Assessment of Magnetic Resonance Imaging Artifact Following Cervical Total Disc Arthroplasty.

BACKGROUND: Cervical disc arthroplasty has become a technique for the treatment of cervical degenerative disc disease. Clinically, the need to accurately assess the neural elements at the operative and adjacent levels is critical postoperatively. The purpose of this study was to quantitatively and qualitatively measure the amount of MRI artifact produced by various cervical total disc replacements. METHODS: T1 and T2-weighted turbo spin-echo MRI sequences were collected on the cervical spine (C2-T1) of a 68 year-old unembalmed male cadaver. A discectomy was performed at C5-6, followed by successive implantation of six different total disc replacements. The scans were quantitatively evaluated by three of the authors. The volume of artifact was measured using image analysis software. Qualitative analysis of the adjacent and index neural elements was performed. RESULTS: The artifact in the T2 weighted images was noted to be 58.6±7.3 cm3 for Prestige ST, 14.2±1.3 cm3 for ProDisc-C, 7.5±0.8 cm3 for Discover, 8.0±0.3 cm3 for Prestige LP, 6.6±0.7 cm3 for Bryan, and 7.3±0.6 cm3 for ProDisc-C titanium prototype. Acceptable intraobserver and excellent interobsever correlation was demonstrated using Pearson Correlation and Concordance Correlation Coefficient analysis. The adjacent and implanted level neural elements (spinal cord and neuroforamina) were easily visualized on the T2 weighted images after the implantation of titanium devices (ProDisc-C titanium prototype, Discover, Prestige LP and Bryan). After implantation of a cobalt chrome implant (ProDisc-C), the adjacent level neural elements were easily visualized but the implanted level could not be fully visualized due to distortion of the images. The quality of the distortion was least favorable after the implantation of the stainless steel implant (Prestige ST), where neither the adjacent nor the index level could be fully visualized. CONCLUSION: The volume of the artifact seen following cervical total disc arthroplasty is highly dependent upon the material property of the implant. Quantitative analysis described in this study demonstrated sufficiently low intraobserver and interobserver variability to be considered a reliable technique.

Plate selection for fixation of extra-articular distal humerus fractures: a biomechanical comparison of three different implants.

Operative fixation of extra-articular distal humerus using a single posterolateral column plate has been described but the biomechanical properties or limits of this technique is undefined. The purpose of this study was to evaluate the mechanical properties of distal humerus fracture fixation using three standard fixation constructs. Two equal groups were created from forty sawbones humeri. Osteotomies were created at 80mm or 50mm from the tip of the trochlea. In the proximal osteotomy group, sawbones were fixed with an 8-hole 3.5mm LCP or with a 6-hole posterolateral plate. In the distal group, sawbones were fixed with 9-hole medial and lateral 3.5mm distal humerus plates and ten sawbones were fixed with a 6-hole posterolateral plate. Biomechanical testing was performed using a servohydraulic testing machine. Testing in extension as well as internal and external rotation was performed. Destructive testing was also performed with failure being defined as hardware pullout, sawbone failure or cortical contact at the osteotomy. In the proximal osteotomy group, the average bending stiffness and torsional stiffness was significantly greater with the posterolateral plate than with the 3.5mm LCP. In the distal osteotomy group, the average bending stiffness and torsional stiffness was significantly greater with the posterolateral plate than the 3.5mm LCP. In extension testing, the yield strength was significantly greater with the posterolateral plate in the proximal osteotomy specimens, and the dual plating construct in the distal osteotomy specimens. The yield strength of specimens in axial torsion was significantly greater with the posterolateral plate in the proximal osteotomy specimens, and the dual plating construct in the distal osteotomy specimens. Limited biomechanical data to support the use of a pre-contoured posterolateral distal humerus LCP for fixation of extra-articular distal humerus exists. We have found that this implant provided significantly greater bending stiffness, torsional stiffness, and yield strength than a single 3.5mm LCP plate for osteotomies created 80mm from the trochlea. At the more distal osteotomy, dual plating was biomechanically superior. Our results suggest that single posterolateral column fixation of extra-articular humerus fractures is appropriate for more proximal fractures but that dual plate fixation is superior for more distal fractures.

Determining the strongest orientation for "Lisfranc's screw" in transverse plane tarsometatarsal injuries: a cadaveric study.

UNLABELLED: The treatment of tarsometatarsal joint fracture-dislocations generally consists of realignment followed by stabilization with rigid internal fixation. The purpose of this study was to determine the strongest orientation for the "Lisfranc's screw" for repair of disruption of the articulation between the first and second metatarsals and the medial and intermediate cuneiforms. To this end, Lisfranc's ligament was sectioned in 6 pairs of fresh-frozen, human cadaver feet, after which a 3.5-mm partially threaded, cannulated screw was placed across the Lisfranc joint in 1 of 2 opposing directions. In one group, the screw was oriented in the more traditional medial cuneiform to second metatarsal base direction. In the other group, the screw was oriented from the second metatarsal base to the medial cuneiform. After fixation, each construct was pulled to transverse plane failure at the tarsometatarsal joint with a servohydraulic mechanical testing system. The overall force to failure was 157.04 +/- 54.79 N (range, 96.8-249.2 N). For the traditional medial cuneiform to second metatarsal base screw orientation group, the mean force to failure was 148.97 +/- 54.93 N, whereas for the second metatarsal base to medial cuneiform group the mean force to failure was 165.12 +/- 58.57 N, and this difference was not statistically significant (P = .2475). Although not statistically significantly different in regard to force to failure strength, the authors describe an alternative approach to the orientation of "Lisfranc's screw" for stabilization of the relationship of the medial cuneiform to the second metatarsal. LEVEL OF CLINICAL EVIDENCE: 5.

Biomechanical stability of different fixation constructs for ORIF of radial neck fractures.

Radial head and neck fractures are common and at times require operative fixation. There is no consensus on the ideal fixation construct for unstable radial neck fractures. Using 7 fresh frozen cadaveric radii, fractures of the radial neck were created 2 cm from the articular surface. The fractures were stabilized with 5 different commonly used constructs: crossed K-wires; a 2.4-mm T-plate using screws in the head (T-plate and nonlocked screw construct); a T-plate using a screw and locked buttress pin in the head (T-plate and locked buttress pin construct); a T-plate with an interfragmentary screw from the shaft retrograde, through the plate into the head (retrograde interfragmentary screw construct); and a T-plate with an interfragmentary screw from a nonarticular portion of the head antegrade into the shaft (antegrade interfragmentary screw construct). All constructs were tested for bending and torsional rigidity using an Instron mechanical testing machine (Model 306; MTS Systems, Eden Prairie, Minnesota). The highest rigidity in both bending and torsion was the antegrade interfragmentary screw construct. During bending, the antegrade interfragmentary screw construct was significantly stronger than a T-plate and nonlocked screw construct. In torsion, the retrograde interfragmentary screw construct was significantly stiffer than K-wires and approached significance over a T-plate and locked buttress pin construct. Locking bolts vs screws into the head did not significantly increase rigidity in torsion or bending. In this model, plating showed an increase in stiffness in torsional loading as compared to K-wires. The addition of a lag screw across the neck fracture consistently showed an increase in torsional and bending stiffness of the constructs. These data may assist orthopedic surgeons in determining the best fixation for radial neck fractures.

Biomechanical comparison of flexible stainless steel and titanium nails with external fixation using a femur fracture model.

There are several options available for surgical stabilization of pediatric femoral shaft fractures. The purpose of this study was to compare the stability afforded by Ender stainless steel nails, titanium elastic nails, and one-plane unilateral external fixators for the fixation using a synthetic adolescent midshaft femur fracture model. The anterior-posterior (sagittal plane) bending, lateral (coronal plane) bending, torsional, and axial stiffness values were calculated using 6 different fixation configurations. These included pairs of 3.5-mm-diameter Ender nails with and without distal locking, 3.5- and 4.0-mm-diameter titanium elastic nails as well as single- and double-stacked monolateral external fixators. Eight synthetic femur models, 4 each with simulated transverse and comminuted fracture patterns, were sequentially tested for stability afforded by the various fracture fixation configurations. External fixation exhibited significantly greater control of anterior-posterior angulation compared with all flexible-nailing systems. Although Ender nails were slightly superior to titanium nails in control of sagittal plane angulation, this was not statistically significant. Compared with the external fixation constructs, all 4 flexible nail constructs demonstrated higher torsional stability. For prevention of axial shortening, all fixation methods were similar for the transverse fracture pattern, whereas external fixation was superior to flexible nails in the comminuted fracture model. No significant benefit was demonstrated with double stacking of external fixators. These findings may help guide clinicians choose the optimal fixation method for treatment of pediatric femoral shaft fractures.

Salicylic acid-derived poly(anhydride-esters) inhibit bone resorption and formation in vivo.

The objective of this study was to investigate the effects of a novel polymer that biodegrades into salicylic acid (SA) on the healing of critical sized long bone defects. Microspheres of the homopolymer, or a copolymer containing 50% less of the SA, were packed into 5-mm mid-diaphyseal defects in rat femurs. Control animals received collagen sponge implants. After 4 and 8 weeks of implantation, bone healing was evaluated using microradiography and quantitative histomorphometry. Four weeks postsurgery, significantly less new bone was formed in both of the polymer groups (p<0.038). Reduced bone loss was also noted with the polymers at this time, although it was not statistically significant. However, at 8 weeks postsurgery, a statistically significant reduction in bone loss was observed in both of the polymer groups compared with controls (p<0.0072). Both polymers seemed to elicit identical tissue responses because there were no differences detected between the homopolymer and copolymer materials at either time point. These results indicate that locally released SA can significantly reduce both bone loss and bone formation in this animal model.