Debi P. Mukherjee

Biomechanical Study of Atlantoaxial Arthrodesis: Transarticular Screw Fixation Versus Modified Brooks Posterior Wiring

OBJECTIVE The purpose of the present study was to compare the biomechanical stability of C1 and C2 vertebrae after treatment of ligamentous instability by either modified Brooks posterior wiring (MB) or transarticular screw (TAS) techniques. We hypothesized that the TAS technique would be more stable because of direct fixation through the facet joints. STUDY DESIGN We studied the in vitro stability (arthrodesis) of TAS fixation of C1 and C2 versus that of MB. TAS fixation involves placing screws across the facets from posteriorly at C2 to the anterior surface of C1, plus a bone graft and posterior wiring of C1 and C2. METHODS Cervical spines from nine individuals with an average age of sixty-two years (range 51 to 71 years) were harvested from cadavers (six male, three female). C1 and the segment from C2 to C5 were potted to allow motion only at the C1-C2 articulation. The specimens were destabilized by cutting the transverse ligament on both sides of the odontoid and the tectorial membrane between C1 and C2. The MB and TAS techniques were performed by methods similar to those described in the literature. The stiffness of the C1-C2 articulation of each specimen was tested under rotation, lateral bending, flexion, and anterior translation in random order. Intact and destabilized specimens fixed with either MB or TAS were tested in sequence. RESULTS Significantly higher stiffness values in the elastic zone were obtained with the TAS technique than with the MB technique for all modes of testing (p < 0.002, t test). Values for the neutral zone (the region where minimal loads produce displacement) were not significantly different between the MB and TAS techniques (p > 0.1, t test). CONCLUSION We conclude that stability is significantly enhanced by use of the TAS construct for treatment of ligamentous instability at the atlantoaxial joint for all motions tested in the present study.

Anterior Screw Fixation of Type II Odontoid Fractures: A Biomechanical Study

Study Design This study examined the torsional and shear stiffness of Type II odontoid fractures that were stabilized with two 3.5-mm cannulated AO screws or a single 4.5-mm cannulated Herbert screw. Objective To determine the stability of a commonly used two-screw method versus a single-screw fixation method, Insertion of a single screw could make the procedure technically easier. Summary of Background Data Biomechanical studies have shown the 4.5-mm Herbert screw to generate greater compressive forces than 3.5-mm cannulated screws, which are commonly used to anteriorly stabilize Type II odontoid fractures. No previous biomechanical studies have compared the shear or torsional stiffness of Type II odontoid fractures stabilized with these screws. Methods Twelve cadaver C2 vertebrae were harvested and an osteotomy was performed to simulate a Type II fracture pattern. Six were stabilized with two 3.5-mm cannulated screws, and the remaining were stabilized with a single 4.5-mm cannulated Herbert screw. Each specimen then was tested in torsion through ±0.75°, ±1.25°, and ±1.75° for 10 cycles each. Shear forces then were applied from an anterior to posterior direction to a maximum load of 44.48 N. Results The mean torsional stiffness for the Herbert screw was 1196 N-m/deg, which was significantly greater than the mean stiffness of the 3.5-mm screw fixation, which measured 434 N-m/deg. The mean shear stiffness for the Herbert screw fixation was 105.9 kN/m, compared to 86.1 kN/m for the 3.5-mm cannulated screw. This was not statistically significant. Conclusions The biomechanical properties of the 4.5-mm cannulated Herbert screw suggest it may lend itself for use in fixation of Type II odontoid fractures. With only a single screw to insert, this technically demanding procedure may be less daunting.

Mechanical Stability of Thoracolumbar Pedicle Screw Fixation : The Effect of Crosslinks

Study Design. Pedicle screw fixation for unstable thoracolumbar spine injuries is relatively new. The ef fect of one or two crosslinks on rotational and lateral bending stiffness was studied. Objective. To determine the rotational and bending stiffness values of thoracolumbar fractures fixed by the AOs internal fixation system with zero, one, or two crosslinks. Methods. Eight embalmed thoracolumbar spine segments (T12‐L2) were instrumented at T12 and L2 with a pedicle screw‐rod system. Rotational stiffness was determined for 10 cycles to 2.5°, 3.5°, and 5° of rotation, with and without one or two crosslinks, and lateral bending stiffness for 10 cycles to 0.25, 0.40, and 0.50 inch. The results showed a clear trend toward increased stiffness with crosslinks. Results. The stiffness values of the two‐crosslink construct at 2.5° and 3.5° of rotation were significantly higher than those of the zero‐crosslink system. Also, the bending stiffness of the two‐crosslink construct was significantly higher than that of no‐crosslink system at all of the displacements. Conclusions. Rotational stiffness values of the two‐crosslink construct were significantly higher than those of the zero‐crosslink system, at 2.5° and 3.5° of rotation. Lateral bending stiffness of the two‐crosslink system was higher than that of the zero‐crosslink system at all levels of displacement.

Traumatic spondylolisthesis of the axis: a biomechanical comparison of clinically relevant anterior and posterior fusion techniques

OBJECT Surgical management of unstable traumatic spondylolisthesis of the axis includes both posterior and anterior fusion methods. The authors performed a biomechanical study to evaluate the relative stability of anterior fixation at C2-3 and posterior fixation of C-1 through C-3 in hangmans fractures. METHODS Fresh-frozen cadaveric spine specimens (occipital level to T-2) were subjected to stepwise destabilization of the C1-2 complex, replicating a Type II hangmans fracture. Intact specimens, fractured specimens, and fractured specimens with either anterior screw and plate or posterior screw and rod fixation were each tested for stability. Each spine was subjected to separate right and left rotation, bending, flexion, and extension testing. RESULTS Anterior fixation restored stiffness in flexion and extension movements to values greater than those for intact specimens. For other movement parameters, the values approximated those for intact specimens. Posterior fixation increased the stiffness to above those values seen for anterior fixation specimens. CONCLUSIONS In cadaveric spine specimens subjected to a Type II hangmans fracture, both anterior fixation at C2-3 and posterior fixation with C-1 lateral mass screws and C-2 and C-3 pedicle screws resulted in a consistent increase in stiffness, and hence in stability, over intact specimens.

Changes in Load Distribution in the Knee After Opening-Wedge or Closing-Wedge High Tibial Osteotomy

The treatment of unicompartmental osteoarthritis of the knee by high tibial osteotomy has been carried out by closing-wedge osteotomy. The advantages for opening-wedge osteotomy are ease of procedure and improved correction with comparable short-term to midterm results. It is not known how the opening-wedge high tibial osteotomy procedure alters the load distribution between the medial and lateral compartments of the knee. The current biomechanical study investigated opening-wedge vs closing-wedge osteotomies in 5 pairs of cadaver knees. The results showed that at 5 degrees osteotomy, the closing-wedge provided superior results of load transfer from medial to lateral compartment than that seen with opening-wedge, but at 10 degrees osteotomy, there was no significant difference in load transfer in the knee compartments between the 2 surgery modes.

Comparison of allograft bone and titanium cages for vertebral body replacement in the thoracolumbar spine: a biomechanical study.

BACKGROUND When an anterior approach to repair a burst fracture is indicated, several devices can be used to restore spinal stability (eg, bone graft, free-standing titanium cage, and expandable titanium cage). OBJECTIVE We compare the biomechanical stability and prices of each of these systems. MATERIALS AND METHODS Eight fresh human cadaver T11 through L3 vertebral specimens were harvested and cleaned of soft tissues. T11-T12 and L2-L3 were fixed by screws. The fixed ends were then set in automotive body filler (Bondo). The prepared specimens were tested in the Biaxial Instron tester (8874, Norwood, MA) after a sequence of the following: intact, after the creation of an anterior corpectomy at L1, and after insertion of both of the 2 different titanium cages and the fibular graft. A titanium screw-and-plate anterolateral system was used to secure the construct (VANTAGE, Medtronic Sofamor Danek, Memphis, TN). The conditions of displacement testing were as follows: rotation (± 3.5°), flexion and extension, and left and right bending (± 3.5 mm). For each mode of testing, the stiffness was calculated. RESULTS The stiffness data, when statistically analyzed by repeated-measures analysis of variance (at P = .05 and power > 0.9), indicated no significant differences among these devices. CONCLUSION On the basis of this biomechanical study, the stiffness of the fibular graft was similar to that of the other metallic devices in this cadaver model.

Effect of 3D-microstructure of bioabsorbable PGA:TMC scaffolds on the growth of chondrogenic cells.

Various biomaterial scaffolds have been investigated for cartilage tissue engineering, although little attention has been paid to the effect of scaffold microstructure on tissue growth. Non-woven, fibrous, bioabsorbable scaffolds constructed from a copolymer of glycolide and trimethylene carbonate with varying levels of porosity and pore size were seeded with mesenchymal stroma cells with a chondrogenic lineage. Scaffolds and media were evaluated for both cell and extracellular matrix organization and content after up to 28 days of culture in a spinner flask. Analysis of DNA and glycosaminoglycan contents showed that the most porous of the three scaffold types, with a porosity of 81% and a porometry determined mean flow pore diameter of 54 microm, supported the most rapid proliferation of cells and accumulation of extracellular matrix. Analysis of the high porosity scaffold system, using Western Blot and immunohistochemistry confirmed the presence of collagen type II and absence of collagen type I, and demonstrated cells with a chondrocyte morphology with aggrecan and collagen II accumulation attached to the scaffolds. It was concluded that the 3D-microstructural characteristics of the scaffold (interconnecting porosity and pore size) play an important role in proliferation and phenotype of chondrogenic cells and accumulation of extracellular matrix molecules.

Bioabsorbable fixation: scientific, technical, and clinical concepts.

Bioabsorbable internal fixation has been clinically available for nearly 20 years and has been used for hard tissue and/or soft tissue fixation from the head to toe. During this time, there has been a rapid evolution in bioabsorbable polymers as well as in the implants and their clinical applications. There is a plethora of bioabsorbable polymers now available for clinical use, and although all are related chemically, seemingly small changes in formulation can have a dramatic effect on their biomechanical and physiological properties. This article summarizes much of the relevant scientific, technical, and clinical literature in a simple, straightforward manner to help surgeons better understand and appreciate the nuances of bioabsorbable fixation. This up-to-date review should also help researchers establish a baseline of knowledge that can provide a platform for further developments in this exciting field.

Axial load weightbearing radiography in determining lateral malleolus fracture stability: a cadaveric study.

Background: Ankle fractures are the most common fracture treated by orthopaedic surgeons in the United States. Isolated lateral malleolus fractures are treated nonoperatively, while associated deltoid disruption injuries are unstable and usually treated by fixation of the fracture. Various stress radiographs and MRI have been used to determine deltoid competency and the subsequent need for operative stabilization of lateral malleolus fractures. To date, no standardization of stress radiography has been performed. The aim of this study was to evaluate the ability of weightbearing ankle X-rays to determine stability and their reliability. Methods: This study sought to utilize simulated weightbearing radiography as a potential method to determine the need for lateral malleolus fixation. Twelve cadaveric ankle specimens were tested by obtaining ankle mortise radiographs for interpretation of medial clear space. Each ankle was tested with 0, 25, 36, and 50 kg of axially applied weight. The groups of measurements obtained were: intact ankles (Group A), ankles with an isolated oblique fibular osteotomy (Group B), and osteotomized ankles after complete deltoid ligament transaction (Group C). Three authors measured the medial clear space for every ankle using PACSR software. Instability of the ankle mortise was defined as medial clear space widening of greater than 2 mm from the intact ankle. Results: An isolated lateral malleolus fracture (Group B) and a lateral malleolus fracture with an incompetent deltoid ligament (Group C) showed no statistical medial clear space widening with simulated axial weightbearing radiography when compared to intact ankles (Group A). ANOVAs between Group A and B had p < 0.001, and between Group A and C of p < 0.001. Conclusion: Our model did not find instability with an osteotomized fibula and a disrupted deltoid. Clinical Relevance: This study suggests a weightbearing radiograph of an isolated lateral malleolus fracture cannot determine deltoid ligament integrity and thus need for fibular operative fixation.

Comparison of pressure effects on adjacent disk levels after 2-level lumbar constructs: fusion, hybrid, and total disk replacement

BACKGROUND With increasing advocacy for the use of TDR procedure as a surgical alternative to fusion in the management of lumbar DDD, intradiskal pressures at the adjacent levels of spine have generated considerable interest. The common belief is that adjacent-level disk pressures will be lower after a TDR as opposed to conventional fusion. The aim of this study is to present the effect of different constructs on adjacent-level disk pressures in lumbar spine. We hypothesized that the adjacent-segment disk pressures after 1- and 2-level TDR and/or a fusion-TDR hybrid procedure will show significant variance within physiological range of motion. METHODS Six adult spine segments T12-S1 with intact ligaments were harvested from cadavers and held firmly in a specially designed fixture. Intradiskal pressures, in motions of flexion, extension, and lateral bending, at L2-L3 and L3-L4 were measured using needle transducers after 2-level TDR L4 through S1, hybrid procedure, and 2-level fusion L4-S1 with femoral ring allograft and pedicle screws. RESULTS The pressures with lateral bending were not significantly lower than those with flexion and extension at both levels (P = .18). Although TDR and hybrid specimens recorded slightly lower pressures specifically during lateral bending, no statistical difference in pressures could be detected when movements were combined with various procedures. CONCLUSION Contrary to the assumed hypothesis, the pressures at the adjacent-level disks (L3-4 and L2-3) did not depend upon the stabilization procedure (2-level disk replacement, hybrid, or 2-level fusion) performed after 2-level diskectomy in the lumbosacral spine.