Alexander J. Ghanayem

Cervical interbody fusion cages. An animal model with and without bone morphogenetic protein

Study Design. The Alpine goat model for multilevel anterior cervical discectomy and fusion was used to analyze the use of an intervertebral fusion device to promote an arthrodesis after anterior cervical discectomy. Comparisons were drawn with biomechanical, histologic, and radiographic data. Objectives. To analyze the use of an intervertebral fusion device, with and without a bone graft substitute, to promote an arthrodesis after anterior cervical discectomy. Summary of Background Data. In previous studies, the goat cervical spine has proven to be an excellent model for examining the healing of fusions using bone grafts, instrumentation, or bone substitutes. Methods. Three‐level anterior cervical discectomies were performed on 21 mature Alpine goats. Three treatment groups of seven goats each were used. Group I used a standard titanium cervical BAK device filled with autogenous bone graft. Group II used a hydroxyapatite‐coated BAK device filled with autogenous bone graft. Group III used a BAK device filled with recombinant human bone morphogenetic protein‐2. Results. Radiographically, no cages became displaced. Lucencies were seen around 3 of the 21 cages in Group I, 4 cages in Group II, and none in Group III. Fluorochrome analysis revealed that the recombinant human bone morphogenetic protein‐2‐filled cages had an accelerated rate of bone growth around and through each cage‐vertebral body interface at 3 weeks. A successful arthrodesis was also more likely with a recombinant human bone morphogenetic protein‐2‐filled cage (95%) than the hydroxyapatite‐coated (62%) or the standard (48%) cage. Biomechanical stiffness testing did not reveal any statistically significant differences between the three groups. There was a tendency for successfully arthrodesed interspaces to be stiffer than those that were not. Conclusions. The use of a threaded intervertebral fusion cage, with or without hydroxyapatite coating, filled with autogenous bone graft provides a fusion rate that is slightly better than those previously reported using autogenous interbody bone grafts with or without plate stabilization. Recombinant human bone morphogenetic protein‐2‐filled cages resulted in a much higher arthrodesis rate and accelerated bone formation compared with either autogenous bone‐filled BAK devices, or autogenous interbody bone grafts with or without plate stabilization.

Effect of compressive follower preload on the flexion–extension response of the human lumbar spine

Traditional experimental methods are unable to study the kinematics of whole lumbar spine specimens under physiologic compressive preloads because the spine without active musculature buckles under just 120 N of vertical load. However, the lumbar spine can support a compressive load of physiologic magnitude (up to 1200 N) without collapsing if the load is applied along a follower load path. This study tested the hypothesis that the load–displacement response of the lumbar spine in flexion–extension is affected by the magnitude of the follower preload and the follower preload path. Twenty‐one fresh human cadaveric lumbar spines were tested in flexion–extension under increasing compressive follower preload applied along two distinctly different optimized preload paths. The first (neutral) preload path was considered optimum if the specimen underwent the least angular change in its lordosis when the full range of preload (0–1200 N) was applied in its neutral posture. The second (flexed) preload path was optimized for an intermediate specimen posture between neutral and full flexion. A twofold increase in flexion stiffness occurred around the neutral posture as the preload was increased from 0 to 1200 N. The preload magnitude (400 N and larger) significantly affected the range of motion (ROM), with a 25% decrease at 1200 N preload applied along the neutral path. When the preload was applied along a path optimized for an intermediate forward‐flexed posture, only a 15% decrease in ROM occurred at 1200 N. The results demonstrate that whole lumbar spine specimens can be subjected to compressive follower preloads of in vivo magnitudes while allowing physiologic mobility under flexion–extension moments. The optimized follower preload provides a method to simulate the resultant vector of the muscles that allow the spine to support physiologic compressive loads induced during flexion–extension activities.

Load-carrying capacity of the human cervical spine in compression is increased under a follower load.

Study Design. An experimental approach was used to test human cadaveric cervical spine specimens. Objective. To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine. Summary of Background Data. The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained. Methods. A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, loading the specimen in nearly pure compression. Results. A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes. The specimen collapsed in extension or flexion at a load of less than 40 N. In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05). Conclusion. The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.

The Effect of Laparotomy and External Fixator Stabilization on Pelvic Volume in an Unstable Pelvic Injury

OBJECTIVE Determine if laparotomy further destabilizes an unstable pelvic injury and increases pelvic volume, and if reduction and stabilization restores pelvic volume and prevents volume changes secondary to laparotomy. DESIGN Cadaveric pelvic fracture model. MATERIALS AND METHODS Unilateral open-book pelvic ring injuries were created in five fresh cadaveric specimens by directly disrupting the pubic symphysis, left sacroliac joint, and sacrospinous and sacrotuberous ligaments. Pelvic volume was determined using computerized axial tomography for the intact pelvis, disrupted pelvis with both a laparotomy incision opened and closed, and disrupted pelvis stabilized and reduced using an external fixator with the laparotomy incision opened. MEASUREMENTS AND MAIN RESULTS The average volume increase in the entire pelvis (from the top of the iliac crests to the bottom of the ischial tuberosities) between a nonstabilized injury with the abdomen closed and then subsequently opened was 15 +/- 5% (423 cc). The average increase in entire pelvic volume between a stabilized and reduced pelvis and nonstabilized pelvis, both with the abdomen open, was 26 +/- 5% (692 cc). The public diastasis increased from 3.9 to 9.3 cm in a nonstabilized pelvis with the abdomen closed and then subsequently opened. Application of a single-pin anterior-frame external fixator reduced the pubic diastasis anatomically and reduced the average entire and true (from the pelvic brim to the ischeal tuberosities) pelvic volumes to within 3 +/- 4 and 8 +/- 6% of the initial volume, respectively. CONCLUSIONS We believe that the abdominal wall provides stability to an unstable pelvic ring injury via a tension band effect on the iliac wings. Our results demonstrate that a laparotomy further destabilized an open-book pelvic injury and subsequently increased pelvic volume and pubic diastasis. This could potentially increase blood loss from the pelvic injury and delay the tamponade effect of reduction and stabilization. A single-pin external fixator prevents the destabilizing effect of the laparotomy and effectively reduces pelvic volume. These data support reduction and temporary stabilization of unstable pelvic injuries before or concomitantly with laparotomy.

A challenge to integrity in spine publications: years of living dangerously with the promotion of bone growth factors

A challenge to integrity in spine publications: years of living dangerously with the promotion of bone growth factors Eugene J. Carragee, MD*, Alexander J. Ghanayem, MD, Bradley K. Weiner, MD, David J. Rothman, PhD, Christopher M. Bono, MD Department of Orthopedic Surgery, Stanford University School of Medicine, 450 Broadway, Mail Code 6342, Redwood, CA 94063, USA Department of Orthopedic Surgery, Loyola University Medical Center, Maywood, IL, USA Department of Orthopedic Surgery, The Methodist Hospital, Houston, TX, USA Department of Social Medicine and Department of History, Columbia University College of Physicians and Surgeons, New York, NY, USA Department of Orthopedic Surgery, The Spine Journal, Brigham & Women’s Hospital, Boston, MA, USA Received 1 June 2011; accepted 1 June 2011

Biomechanical comparison of posterior lumbar interbody fusion cages.

Study Design. Cadaveric human and bovine lumbar spine models simulating the acute postoperative period were used to compare the biomechanical properties of two designs of intervertebral body threaded fusion cages. The instrumented spines were compared with intact spines and with spines with resected posterior elements, representing a revision case. Objective. To determine the relative biomechanical performance of these competing devices. Summary of Background Data. These cages are currently under clinical investigation, and basic biomechanical data are needed. Methods. Insertion torques and maximum pushout loads were measured for each cage. Intact spines, posteriorly instrumented spines (posterior lumbar interbody fusion), and spines with resected posterior elements were loaded in axial compression, flexion and extension bending, and axial torsion. Stiffness comparisons were made between the different configurations. Results. Insertion torques and pushout loads were similar for the cages. Both cages significantly increased stiffnesses above those of the intact spines and the resected spines. The BAK‐instrumented spines were more stiff in axial compression, while the Threaded Interbody Fusion Device spines were more stiff in extension. Conclusions. This study revealed the two cages to have similar biomechanical characteristics immediately after posterior insertion and warrant further clinical studies.

Effect of Prone Positioning Systems on Hemodynamic and Cardiac Function During Lumbar Spine Surgery: An Echocardiographic Study

Study Design. Prospective randomized study of patients undergoing spine surgery. Objective. To compare changes in hemodynamic and cardiac function after prone positioning using different prone positioners. Summary of Background Data. Prone positioning decreases blood pressure and cardiac function. Several studies have evaluated changes in cardiac function after prone positioning, and linked them to reduced venous return and ventricular compliance. This study compares different prone positioners using transesophageal echocardiography, and determines their effect on cardiac function and hemodynamics. Methods. After correction of fluid deficits with the patient under stable anesthesia, hemodynamic and cardiac performance was measured using transesophageal echocardiography. After prone positioning, repeat measurements were performed, and comparisons were made between prone and supine positions. Results. No intergroup differences in demographics, fluid deficit, baseline hemodynamics, or differences from supine to prone position were noted. Cardiac output decreased with the Wilson (Union City, CA) and Siemens AG (Munich, Germany) frames, while cardiac index and stroke volume decreased with the Andrews (Hollywood, CA), Wilson, and Siemens systems. Cardiac preload decreased using the Andrews frame. The Jackson spine table (Hollywood, CA) and bolsters had the least effect on cardiac performance. Conclusion. Adequate fluid replacement reduced hypotension and hemodynamic instability after prone positioning. The Jackson spine table and longitudinal bolsters had minimal effects on cardiac function, and should be considered in patients with limited cardiac reserve.

Accuracy of using computed tomography to identify pedicle screw placement in cadaveric human lumbar spine

Study Design. Utility of using computed tomography to predict pedicle screw misplacement. Objective. This study defines the sensitivity and specificity of predicting pedicle screw placement by experienced clinicians using a CT scan image. Summary of Background Data. In clinical and research settings, the method most commonly used to evaluate pedicle screws placement has been computed tomography. However, no current literature describes the accuracy of this method of evaluating screw placement. Method. Cobalt‐chrome and titanium alloy pedicle screws of identical size were placed in six cadaveric human lumbar spine. Wide laminectomy was performed to allow complete visualization of the pedicles. Three consecutive lumbar levels were instrumented in each spine, giving 36 pedicle screw placements to identify. The instrumented spines were imaged, and four orthopaedic spine surgeons and a musculoskeletal radiologist were asked to read the images to identify the accuracy of screw placement within the pedicles. Results. The sensitivity rate of identifying a misplaced screw was 67 ± 6% for cobalt‐chrome screws compared with 86 ± 5% for titanium screws (P < 0.005). The specificity rates of radiographic diagnosis of misplaced pedicle screws were 66 ± 10% for cobalt‐chrome screws and 88 ± 8% for titanium screws (P < 0.005). Similarly, a statistically significant difference was found in the sensitivity rates of identifying screws placed correctly in the pedicle: 70 ± 10% for cobalt‐chrome screws versus 89 ± 8% for titanium screws (P < 0.005). Overall accuracy rates were 68 ± 7% for cobalt chrome screws versus 87 ± 3% for titanium screws (P < 0.002). Conclusion. Reliance on the computed tomography scan data alone in determining accuracy of pedicle screws can lead to inaccuracies in both clinical and research conditions.

Effect of physiological loads on cortical and traditional pedicle screw fixation.

Study Design. Human cadaveric biomechanical study. Objective. To determine the fixation strength of laterally directed, cortical pedicle screws under physiological loads. Summary of Background Data. Lateral trajectory cortical pedicle screws have been described as a means of obtaining improved fixation while minimizing soft-tissue dissection during lumbar instrumentation. Biomechanical data have demonstrated equivalent strength in a quasi-static model; however, no biomechanical information is available comparing the fixation of cortical with traditional pedicle screws under cyclic physiological loads. Methods. Seventeen vertebral levels (T11–L5) underwent quantitative computed tomography. On 1 side, a laterally directed, cortical pedicle screw was inserted with a traditional, medially directed pedicle screw placed on the contralateral side. With the specimen constrained in a testing apparatus, each screw underwent cyclic craniocaudal toggling under incrementally increasing physiological loads until 2 mm of head displacement occurred. Next, uniaxial pullout of each toggled screw was performed. The number of craniocaudal toggle cycles and load (N) required to achieve pedicle screw movement as well as axial pullout resistance (N) were compared between the 2 techniques. Results. The mean trabecular bone mineral density of the specimens was 202 K2HPO4 mg/cm3. Cortical pedicle screws demonstrated significantly improved resistance to toggle testing, requiring 184 cycles to reach 2 mm of displacement compared with 102 cycles for the traditional pedicle screws (P = 0.002). The force necessary to displace the screws was also significantly greater for the cortical versus the traditional screws (398 N vs. 300 N, P = 0.004). There was no statistical difference in axial pullout strength between the previously toggled cortical and traditional pedicle screws (1722 N vs. 1741 N, P = 0.837). Conclusion. Laterally directed cortical pedicle screws have superior resistance to craniocaudal toggling compared with traditional pedicle screws. Level of Evidence: N/A

Anterior cervical graft and plate load sharing.

Anterior discectomy and fusion with an interbody bone graft and anterior plate is a common procedure in cervical spine surgical management. However, the graft may be shielded from some mechanical loading by the plate. Mechanical testing was performed on six cadaveric calf spines that were subjected to a simulated anterior cervical discectomy and fusion with an interbody bone graft alone and with an anterior plate to determine the amount of load sharing between the graft and plate. The load-displacement data were used to compute the amount of load sharing between the graft and the plate as a continuous function of the applied axial compression load. Although the percent load transmitted through the graft decreased (53 to 41%) as the axial load increased (45 to 90 N), the magnitude of load transmitted through the graft increased (24 to 37 N), with corresponding intervertebral strains <6%. In a single-level procedure, an anterior cervical plate serves as a load-sharing device rather than a load-shielding device, enabling graft consolidation as observed in clinical studies.