Andrew Perry

Spinal growth modulation with use of a tether in an immature porcine model.

BACKGROUND Spinal growth modulation by tethering the anterolateral aspect of the spine, as previously demonstrated in a nonscoliotic calf model, may be a viable fusionless treatment method for idiopathic scoliosis. The purpose of the present study was to evaluate the radiographic, histologic, and biomechanical results after six and twelve months of spinal growth modulation in a porcine model with a growth rate similar to that of adolescent patients. METHODS Twelve seven-month-old mini-pigs underwent instrumentation with a vertebral staple-screw construct connected by a polyethylene tether over four consecutive thoracic vertebrae. The spines were harvested after six (n = 6) or twelve months (n = 6) of growth. Monthly radiographs, computed tomography and magnetic resonance imaging scans (made after the spines were harvested), histologic findings, and biomechanical findings were evaluated. Analysis of variance was used to compare preoperative, six-month postoperative, and twelve-month postoperative data. RESULTS Radiographs demonstrated 14 degrees +/- 4 degrees of coronal deformity after six months and 30 degrees +/- 13 degrees after twelve months of growth. Coronal vertebral wedging was observed in all four tethered vertebrae and progressed throughout each animals survival period. Disc wedging was also created; however, in contrast to the findings associated with vertebral wedging, the tethered side was taller than the untethered side. Magnetic resonance images revealed no evidence of disc degeneration; however, the nucleus pulposus had shifted toward the side of the tethering. Midcoronal undecalcified histologic sections showed intact bone-screw interfaces with no evidence of implant failure or loosening. With the tether cut, stiffness decreased and range of motion increased in lateral bending away from the tether at both time-points (p < 0.05). CONCLUSIONS In this porcine model, mechanical tethering during growth altered spinal morphology in the coronal and sagittal planes, leading to vertebral and disc wedging proportional to the duration of tethering. The resulting concave thickening of the disc in response to the tether was not anticipated and may suggest a capacity for the nucleus pulposus to respond to the compressive loads created by growth against the tether.

Predictors of Change in Postoperative Pulmonary Function in Adolescent Idiopathic Scoliosis: A Prospective Study of 254 Patients

Study Design. A multicenter study of prospectively collected pulmonary function testing and radiographic measures in patients surgically treated for adolescent idiopathic scoliosis (AIS). Objective. The objectives of this study were 1) to identify the factors that determine pulmonary function more than 2 years after surgery for AIS; and 2) to determine what factors, if any, can predict an increase or decrease in the percent predicted 2-year pulmonary function. Summary of Background Data. Thoracic spinal deformity can lead to significant pulmonary impairment. Studies have shown that patients with AIS experienced a significantly greater improvement in pulmonary function at 2 years after surgery when treated with a posterior approach compared to an anterior approach. Methods. Pulmonary function testing (PFT) and radiographic examination of 254 patients with AIS were completed prospectively. Demographic data, associations between radiographic measurements of spinal deformity, and the results of spirometry underwent correlation analysis and subsequent step-wise multiple regression analysis. Results. The variables found to be significant predictors of 2-year pulmonary function (FVC, FEV1, TLC) include: preop PFT (R2 = 0.20–0.39), having an open thoracotomy (as opposed to thoracoscopic or posterior) (R2 = 0.07–0.09), surgical time (R2 = 0.03–0.07), and thoracoplasty (R2 = 0.02–0.04). These models explain 40 to 51% of the variance in 2-year PFT. For patients undergoing open thoracotomy with a thoracoplasty, approximately 54% had a 15% decrease, or more, in percent predicted PFT. This compared with 11% and 15%, respectively of patients who either had posterior or thoracoscopic procedures with no thoracoplasty that had a 15% decrease or more in percent predicted PFT. Conclusion. Aside from preoperative PFT values, open anterior approaches predict the largest percent of variance in 2-year PFT. Additionally, a clinically significant reduction in the predicted 2-year pulmonary function is more likely when performing a thoracoplasty. The magnitude of the effects for both these variables, however, is modest. This may facilitate the decision-making process as regards to operative intervention.

Biomechanical comparison of a novel percutaneous transfacet device and a traditional posterior system for single level fusion.

Posterior spinal fusions are indicated for a variety of spinal disorders. Transfacet fixation minimizes soft tissue disruption and preserves the adjacent facet joint. This technique is uncommon due to concerns with biomechanical stability and proper implant placement. For these reasons, a length adjustable implant may obviate the clinical concerns but necessitates biomechanical study. This study evaluated the in vitro biomechanical stability between a novel transfacet fixation device compared with standard pedicle screws during cyclic physiologic loading in a human cadaveric model. Cadaveric L4-L5 lumbar motion segments from 16 human spines were tested in cyclic flexion/extension, lateral bending, and torsion after insertion of either transfacet fixation devices or 5.5 mm pedicle screw instrumentation. A load cell was used to measure the compressive forces on the anterior column during testing. Motion segment stiffness and anterior column compression were analyzed with a 1-way analysis of variance (P<0.05). The transfacet device demonstrated a statistically similar stiffness when compared with the pedicle screw system for each test direction. For anterior column loading during physiologic testing, there were no biomechanical differences between stabilization systems. Percutaneous transfacet fixation is an attractive surgical option for single-level spinal fusions. A biomechanical evaluation of a novel device for this application demonstrated similar stability to a pedicle screw system. The length adjustability of the device may alleviate concerns for precise device placement and the biomechanical stability may produce similar rates and quality of posterior spinal fusions.

Biomechanical evaluation of an injectable radiopaque polypropylene fumarate cement for kyphoplasty in a cadaveric osteoporotic vertebral compression fracture model.

Vertebral compression fractures cause pain, deformity, and disability. Polypropylene fumarate (PPF) has shown promise as an injectable cement for bone defects but little is known about its performance for kyphoplasty. The purpose of this study was to evaluate the biomechanical performance of PPF for kyphoplasty in simulated anterior compression fractures in cadaveric vertebral bodies. Thirty-one vertebral bodies (T9 to L4) from osteoporotic cadaveric spines were disarticulated, stripped of soft tissue and compressed on a materials testing machine to determine pretreatment strength and stiffness. All fractures were repaired with inflatable balloon tamps and either polymethylmethacrylate or PPF-30 (containing 30% barium sulfate by dry weight) cement and then retested. Strength restoration with PMMA and PPF-30 were 120% and 104%, respectively, of the pretreatment strengths. For stiffness, PMMA and PPF-30 restored vertebral bodies to 69% and 53%, respectively, of the initial values. There was no significant difference in treatment with either PMMA or PPF-30. The biopolymer PPF-30 exhibits mechanical properties similar to PMMA in a cadaveric kyphoplasty model. PPF biopolymer may be a suitable alternative for kyphoplasty.

The healing forearm fracture: a matched comparison of forearm refractures.

Background: Forearm fractures in children usually heal rapidly after closed treatment. Recent studies report forearm refracture rates of 5%. The purpose of this study was to identify risk factors for refracture based on radiographic variables. Methods: We performed a retrospective review of patients that sustained a second forearm fracture (refracture) between 1998 and 2005. Refractures were defined as having a second fracture of the same forearm within 18 months of the original fracture. A comparison group of single-fracture patients followed in a capitated insurance plan were included and matched based on age and sex. Radiographic assessment included initial/final angulation, displacement, and fracture-line visibility at latest follow-up. Results: Sixty-three refractures were compared with 132 age- and sex-matched single-fracture patients. Time to refracture averaged 10 months. Thirty-eight percent of the initial fractures in the refracture group occurred in the proximal or middle third of the forearm compared with 15% for the single-fracture patients (P < 0.001). Because location of the fracture was found to be a risk factor for refracture, a secondary analysis was performed with refracture patients matched to single-fracture patients based on age, sex, bone fractured, fracture location, and treatment method. Fracture-line visibility of the radius at latest follow-up was clearly visible in 48% of refractures compared with 21% of controls (P = 0.05). Initial fracture severity and residual deformity were not significantly different. Conclusions: Proximal and middle one third forearm fractures are at greater risk of refracture compared with distal one third forearm fractures. There was a trend toward incomplete healing seen more commonly in those that refractured, emphasizing the importance of longer immobilization in these fractures. Level of Evidence: Prognostic study, level III, case-control study.