Characterization of the mid-coronal plane method for measurement of radiographic change in knee joint space width across different levels of image parallax.

OBJECTIVE: Radiographic measurement of the change in knee joint space width (ΔJSW) is often affected by image parallax, which causes an apparent exaggeration of JSW due to projectional differences. This issue with parallax (quantified by intermargin distance) can in part be addressed with a novel mid-coronal plane (MCP) measurement method. The objectives of the study were to determine 1) accuracy and 2) reproducibility of the MCP method, and 3) compare the MCP method to that used in the Osteoarthritis Initiative (OAI) for different categories of parallax. METHODS: Posteroanterior radiographs (n = 70) with known JSW were digitally reconstructed from CT images of cadaver knees and used to determine the accuracy of ΔJSW using the MCP method for parallax categories of None, Mild/Moderate, and Severe. Reproducibility was determined from pairs of clinical radiographs selected from the OAI (n = 170). The MCP method was also compared to the OAI methodology. Both reproducibility and agreement were characterized by Bland-Altman analysis and intraclass correlation coefficients (ICC). RESULTS: The MCP method was accurate to 0.11 mm in cases with no parallax, and 0.18 mm across all categories of parallax for medial and lateral compartments. Reproducibility of the MCP method was graded "excellent" (ICC 0.98, 95% CI [0.98, 0.99]). The MCP results agreed very well with the OAI (ICC 0.92, 95% CI [0.89, 0.94]), with mean absolute differences between methods increasing with increasing parallax. CONCLUSION: The MCP method is an accurate, reproducible alternative to the OAI method for multi-center clinical trials where subject and X-ray beam positioning may be variable.

The effect of retained intramedullary nails on tibial bone mineral density.

BACKGROUND: Intramedullary nailing has become a standard treatment for adult tibial shaft fractures. Retained intramedullary nails have been associated with stress shielding, although their long-term effect on decreasing tibial bone mineral density is currently unclear. The purpose of this study was to determine if retained tibial intramedullary nails decrease tibial mineral density in patients with successfully treated fractures. METHODS: Patients treated with statically locked intramedullary nails for isolated, unilateral tibia shaft fractures were studied. Inclusion required that fracture had healed radiographically and that the patient returned to the pre-injury activity level. Data on patient demographic, fracture type, surgical technique, implant, and post-operative functional status were tabulated. Dual energy X-ray absorptiometry was used to measure bone mineral density in selected regions of the affected tibia and the contralateral intact tibia. Image reconstruction software was employed to ensure symmetry of the studied regions. FINDINGS: Twenty patients (mean age 43; range 22-77 years) were studied at a mean of 29 months (range 5-60 months) following intramedullary nailing. There was statistically significant reduction of mean bone mineral density in tibiae with retained intramedullary nails (1.02 g/cm(2) versus 1.06 g/cm(2); P=0.04). A significantly greater decrease in bone mineral density was detected in the reamed versus non-reamed tibiae (-7% versus +6%, respectively; P<0.05). INTERPRETATION: The present study demonstrates a small, but statistically significant overall bone mineral density decrease in healed tibiae with retained nails. Intramedullary reaming appears to be a factor potentiating the reduction of tibia bone mineral density in long-term nail retention.

Worksite nutrition supports and sugar-sweetened beverage consumption.

OBJECTIVE: This study examined the link between worksite environmental supports for nutrition behaviours and sugar-sweetened beverage (SSB) consumption and offers insight into potential intervention points for reducing SSB consumption and combatting overweight and obesity. METHODS: Perceived worksite supports for healthy nutrition and self-reported SSB consumption were analysed for 2,015 working adults in the state of Missouri using a subset of questions from the Supports at Home and Work for Maintaining Energy Balance (SHOW-ME) study. RESULTS: Employees' use of vending facilities and the availability of water coolers/water bottles was significantly associated with increased SSB consumption, while use of cafeterias was significantly associated with decreased SSB consumption. Symbols or signs to identify healthy alternatives were significantly associated with sports drink consumption. CONCLUSIONS: This study supports previous work indicating the worksite as a necessary environment for nutrition interventions. When choices (vending and cafeteria) are provided, employees report making healthier decisions. For worksites without cafeterias, alternatives should be explored including mobile food trucks and farmer's markets.

The use of a virtual three-dimensional model to evaluate the intraosseous space available for percutaneous screw fixation of acetabular fractures.

We created virtual three-dimensional reconstruction models from computed tomography scans obtained from patients with acetabular fractures. Virtual cylindrical implants were placed intraosseously in the anterior column, the posterior column and across the dome of the acetabulum. The maximum diameter which was entirely contained within the bone was determined for each position of the screw. In the same model, the cross-sectional diameters of the columns were measured and compared to the maximum diameter of the corresponding virtual implant. We found that the mean maximum diameter of virtual implant accommodated by the anterior columns was 6.4 mm and that the smallest diameter of the columns was larger than the maximum diameter of the equivalent virtual implant. This study suggests that the size of the screw used for percutaneous fixation of acetabular fractures should not be based solely on the measurement of cross-sectional diameter and that virtual three-dimensional reconstructions might be useful in pre-operative planning.

Method-based differences in the automated analysis of the three-dimensional morphology of trabecular bone.

The three-dimensional (3D) morphology of trabecular bone is frequently quantified using computer programs. However, there are no standardized implementations of morphology programs and many variations are possible. Even though programs may use the same basic method, results can be significantly different because of differences in implementation. Morphology data from different laboratories therefore may not be comparable. The method of directed secants, with parallel plate assumptions, is commonly used to quantify 3D morphology. We examined the effect of several variations in the implementation of this method on measurements of trabecular plate number (Tb.N), trabecular thickness, and trabecular spacing. Three-dimensional micromagnetic resonance images of 10 bovine trabecular bone specimens were analyzed using several variations of the directed secant method. An analysis of covariance with repeated measures suggested that variations in the algorithm used to count test line intersections, variations in the criteria used to classify a test coordinate as bone or marrow, and variations in the number of test grid rotations had significant effects on Tb.N (p < 0.0001). The largest difference in Tb.N (52%) was due to the method used to count test line intersections with the bone-marrow interface. Variations in the classification algorithm and variations in the number of test line grid rotations resulted in a 6% difference in Tb.N. The spacing of the test line grids did not affect Tb.N (p = 0.28), and all differences were independent of volume fraction (p = 0.67). These data show that there can be significant differences in trabecular bone morphology measurements due only to the method used for the measurements. To facilitate comparisons between laboratories, we have made validated computer programs to measure trabecular bone morphology available over the Internet.

Mechanical properties of trabecular bone within and adjacent to osseous metastases.

Despite radiographic and histologic evidence of trabecular bone density changes within and adjacent to osseous metastases, there currently exist no data to demonstrate whether these changes are important in predicting the risk of fracture. To determine if these density changes result in significant reductions in mechanical properties, trabecular bone specimens were prepared from lower thoracic and lumbar vertebrae from two cadavers with radiographic, gross, and histologic evidence of lytic and/or blastic osseous metastases. Each specimen was classified as normal, lytic, or blastic based on appearance in fine-grain radiographs of 8-9 mm thick coronal plane sections. Specimens were tested to failure in uniaxial compression, and tissue and apparent densities were measured. Mean tissue densities were within normal ranges. The mean apparent density for all specimens combined was within the normal range for human vertebrae, and the mean apparent density for radiographically normal (0.131 g/ml) and lytic (0.111 g/ml) specimens was less than the mean apparent density of blastic (0.182 g/ml) specimens (p < 0.02). The moduli of lytic and blastic specimens were less than for normal specimens (p < 0.025). The strength of lytic specimens was less than normal (p = 0.057), but the strength of blastic specimens was not (p > 0.1). Apparent density explained significant fractions of the variations in both modulus (p < 0.001) and strength (p < 0.001). The data suggest that blastic changes associated with osseous metastases to trabecular bone disrupt the normal dependence of trabecular mechanical properties on apparent density, but lytic changes do not.(ABSTRACT TRUNCATED AT 250 WORDS)

Maxillary molar extraction causes increased bone loss in the mandible of ovariectomized rats.

Although osteoporosis is a major public health concern, its effect on oral bone has not been determined. More important may be the effect of estrogen depletion on the response of oral bone to dental treatments such as tooth extraction or pathologic processes such as periodontal disease. Our objective was to determine if maxillary molar extraction increases mandibular bone loss in the ovariectomized compared with a sham-operated control. Fifty-three ovariectomized and 53 sham-operated 6-month-old Sprague-Dawley rats were randomly assigned to the following groups: (1) ovariectomized, adult; (2) sham-operated, adult; (3) ovariectomized, adult, extraction; (4) sham-operated, adult, extraction; (5) ovariectomized, old; (6) sham-operated, old; (7) ovariectomized, old, extraction; and (8) sham-operated, old, extraction. Fourteen days following ovariectomy, the extraction groups had their bilateral maxillary molars extracted. The adult and old rats were sacrificed 114 and 200 days postovariectomy, respectively. The right mandible was tested to failure in three point bending. The bone mineral density (BMD) of the left mandible was measured with high resolution dual energy X-ray absorptiometry. The area fraction and area moment of inertia of mandible sections were determined using image processing software. In the ovariectomized rats, maxillary molar extraction resulted in decreases (p < 0.05) in the failure load (21%), stiffness (39%), BMD (3%), and bone area fraction (8%) of the mandible. However, in the sham-operated rats, these decreases following maxillary molar extraction were less (p < 0.05) than those in the ovariectomized rats and only present in the mandibles of the aged rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Trabecular bone morphology from micro-magnetic resonance imaging.

Micro-magnetic resonance imaging (micro-MRI) is potentially a widely available tool to image and quantify the three-dimensional structure of trabecular bone. However, it has not been demonstrated that the same quantitative measurements can be obtained using micro-MRI as would be obtained from conventional light microscope images. Bovine trabecular bone from several anatomic sites was imaged with both optical and micro-MRI methods. The six faces of approximately cubic trabecular bone specimens were examined with the light microscope, and the volume of bone internal to these faces was then imaged using an 8.6 T 25 mm bore magnet. Three-dimensional measures of bone morphology were calculated from both the optical and micro-MR images using the method of directed secants. Quantitative measures from the two imaging methods were compared by paired t-tests. Volume fractions (BV/TV) measured by micro-MRI were linearly related to (r2 = 0.81) and did not differ statistically from (p = 0.96) similar measurements from optical images. The trabecular plate number (Tb.N) measured by micro-MRI also was linearly related to (r2 = 0.53) and did not differ statistically from (p = 0.17) similar measurements from optical images. The orientation of trabeculae predicted from micro-MRI was within 6 degrees of that calculated from optical images in 10 out of 16 specimens. The micro-MRI morphology measurements are relatively easy to perform, and since several hundred small-bore high-field strength MRI systems are available, this technique could be used widely to quantify the morphology of trabecular bone.

Local demineralization as a model for bone strength reductions in lytic transcortical metastatic lesions.

The structural consequences of bone density changes associated with lytic metastatic lesions were investigated using an experimental model of regular, lytic metastatic lesions in bone. Circular holes were drilled in the mid-diaphyseal cortex of paired adult canine femora. The region around the defect was demineralized in one bone of each pair with 0.8 N HCl. Specimens were tested to failure in four-point bending. Defect size was determined from conventional planar radiographs as the maximum apparent defect diameter divided by the periosteal diameter. Demineralization resulted in irregular defect geometries, which increased the maximum defect dimension 33% to 57% with respect to the original drill hole diameter. Demineralization resulted in additional strength reductions beyond those expected from the original drill hole alone. Despite the irregular demineralization patterns observed, strength reductions were in close agreement with those predicted from data for regular, nondemineralized holes (r2 = 0.93). The results demonstrate that irregular diaphyseal defect borders may not require more complex fracture risk predictors than can be determined from analytic and experimental studies of regular defect geometries. Our results also demonstrate that errors of over 100% can occur when measuring diaphyseal defect size from radiographs that are not optimally aligned with respect to the defect.

Structural consequences of endosteal metastatic lesions in long bones.

Lytic metastatic lesions from breast, prostate, and other cancers often develop on the endosteal surface of a long bone without penetrating the cortical wall. Current clinical guidelines for determining the fracture risk associated with these endosteal defects do not account for the structural consequences of the lesion. We undertook a combined experimental and analytical study of the structural consequences of the lesions with the ultimate goal of providing improved fracture risk guidelines. Endosteal defects of variable length and involving a variable amount of the cortical wall were created with an expanding reamer in canine femurs. The contralateral femur served as a control. The femurs were tested to failure in four point bending. The geometry of the experimental defects was determined from radiographs and CT. Finite element models of the canine femurs were then used to examine geometric and material parameters in both four point bending and in torsion. The experimental data demonstrate a linear relation between bone strength and amount of cortical wall remaining: % intact strength = 99.6 x remaining wall thickness - 2.0, R2 = 0.769, standard deviation of regression = 11.57. Four of five data points from the linear finite element models were within the 95% confidence intervals for the experimental data. Experimental and finite element data suggest that the minimum wall thickness is the most critical geometric parameter for predicting the structural consequences of endosteal defects. The length of the defect along the bones' long axis has little effect on bone strength. The anelastic behavior of bone does not need to be represented in finite element models of simple endosteal defects because the defects do not cause significant stress concentrations. However, reduction in the modulus of bone along the border of a defect (due to osteolytic changes) can significantly reduce bone strength. These results indicate that the minimum wall thickness should be determined when clinically evaluating an endosteal defect. The results also suggest that information on bone porosity around metastatic lesions should be considered when making estimates of bone strength.

Maxillary molar extraction decreases stiffness of the mandible in ovariectomized rats.

Although osteoporosis is a major public health concern, the effect of this condition on oral bone has not been determined. Using the ovariectomized rat as a model, we examined whether acute estrogen depletion affects the mandible of the ovariectomized rat with and without maxillary molar extraction. Twenty-two ovariectomized and 20 sham-ovariectomized 90-day-old Sprague-Dawley rats were divided into the following groups: (1) sham-ovariectomized, baseline control; (2) ovariectomized, baseline control; (3) ovariectomized; (4) ovariectomized, tooth extraction; (5) sham-ovariectomized; and (6) sham-ovariectomized, tooth extraction. On day eight, sham-ovariectomized and ovariectomized rats had all their maxillary molars extracted, and the baseline control sham-ovariectomized and ovariectomized rats were killed. On day 35, the remaining animals were killed. After teeth were harvested, the right mandible was tested to failure in three-point bending. The bone mineral density of the left mandible was measured with dual-energy x-ray absorptiometry. The area fraction and area moment of inertia of a section of mandible were determined. Neither ovariectomy nor tooth extraction significantly affected the bone mineral density or structural properties of the rat mandible. However, there was an interaction between ovariectomy and maxillary molar extraction on the stiffness of the rat mandible (p < 0.008). Maxillary molar extraction decreased the stiffness of the mandible by 25% (p < 0.05) in the ovariectomized rat, whereas it did not do so in the sham-ovariectomized rat.

Failure of growth hormone to alter the biomechanics of fracture-healing in a rabbit model.

Standardized tibial osteotomies were created and stabilized with external fixation in twenty-seven skeletally mature rabbits. Fourteen animals were treated with a daily injection of human growth hormone (150 micrograms per kilogram of body weight), and thirteen received a daily injection of saline solution. Serial non-destructive biomechanical tests, radiography, and determinations of the levels of serum insulin-like growth-factor I were performed for all of the animals. Destructive strength-testing of the sites of osteotomy was performed at four, six, or eight weeks. Twenty-five of the twenty-seven osteotomies healed uneventfully. There were no significant differences in the serial biomechanical measurements at the sites of the healing osteotomies, in the radiographic measurements, or in the ultimate strength of the sites of the osteotomy between the two groups. The mean level of serum insulin-like growth-factor I increased 33 per cent relative to the preoperative baseline level in the group that received growth hormone and increased 10 per cent in the control group. This difference was not statistically significant. There was no significant correlation between the biomechanical properties at the sites of the osteotomies and the levels of serum insulin-like growth-factor I. Administration of growth hormone had no measurable effect on fracture-healing in this model of normal animals. It remains to be determined whether injection of growth hormone might affect healing when there is a state of deficiency of endogenous growth hormone or when there is a non-union of a fracture.

In vivo forces on dental implants: hard-wiring and telemetry methods.

Methods are presented for measuring vertical force components on bridged titanium dental implants in dog mandibles. These methods have included custom-made strain-gauge transducers, plus hard-wiring and telemetric schemes for data collection. The essential components of the measurement system are described, and typical bite force data are illustrated.

Evaluation of finite element analysis for prediction of the strength reduction due to metastatic lesions in the femoral neck.

Between 30 and 70% of almost one million new cancer patients diagnosed each year will develop osseous metastases. Clinicians are faced with the difficult task of determining which patients require prophylactic stabilization to prevent pathologic fracture. The objective of this study was to test the ability of macroscopic finite element models to predict the fracture strength of the proximal femur with a lesion in the femoral neck. Drill hole defects in human cadaver femora were used to simulate lesions that penetrate one cortex of the femoral neck. Based on the first of two series of in vitro experiments, the fracture strength of a femur with a lesion that penetrates either the inferior-medial or superior-lateral cortex of the neck is approximately 45% less than the fracture strength of the paired intact femur; based on the second series, the fracture strength with the inferior-medial lesion is approximately 20% less than the fracture strength with the superior-lateral lesion. A series of three-dimensional finite element models were used to predict the fracture strength for anterior and posterior lesions, as well as the inferior-medial and posterior-lateral lesions tested in vitro. Based on a direct comparison of the strengths predicted by the finite element models to the measured in vitro fracture strengths, the finite element models performed poorly. In particular, the application of an anisotropic strength criterion to the stresses predicted by the models resulted in a considerable underestimation of the percentage reduction in the in vitro fracture strength. This may reflect a fundamental inability of a linear, macroscopic continuum-based analysis to predict accurately the fracture strength of a bone structure as complex as the proximal femur. However, despite this lack of agreement in absolute fracture strength, the general trends for gait and stair ascent loading for the inferior-medial and superior-lateral lesions were consistent with the in vitro data. The greatest reduction in strength was predicted for the inferior-medial lesion, followed by the anterior lesion and then the superior-lateral lesion, and the least reduction in strength was predicted for the posterior lesion. Most importantly, the predicted strength ratio varied considerably as a function of the applied loads. Any metastatic lesions of the femoral neck may be especially sensitive to some particular activity, making it difficult to determine precisely the risk of fracture.

Structural consequences of transcortical holes in long bones loaded in torsion.

Finite element models were used to predict the structural consequences of transcortical holes through long bones loaded in torsion. Several parameters were investigated including hole size, anelastic behavior of the bone, cortical wall thickness, cortical wall symmetry, curvature along the bone's long axis and the axial length of the defect. Finite element model predictions of percent intact bone strength were compared to experimental data for sheep femora with transcortical drill holes loaded to failure in torsion. Hole size was expressed as hole diameter divided by the outer bone diameter. Linear finite element model predictions were in conservative agreement with the experimental data for large hole sizes. A transcortical hole with a diameter 50% of the outer bone diameter reduced the torsional strength by 60%. However, the linear models predict a 40% drop in strength for small holes whereas in vitro data suggest that small holes have no significant effect on strength. Models which represent non-linear anelastic behavior in bone overpredicted torsional strengths. Asymmetric cortical wall thickness and long bone bowing have minor effects, while the length of an elongated defect strongly influences the torsional strength. Strength reductions are greatest for bones with thin cortical walls.

Postfracture instability of vertebrae with simulated defects can be predicted from computed tomography data.

STUDY DESIGN: Structural properties of vertebrae with simulated defects were measured from computed tomography data. Relations between structural properties and postfracture stability were tested using linear regressions. OBJECTIVES: To determine whether the postfracture stability of lumbar and thoracic vertebrae can be predicted from noninvasive, prefracture measurements of structural properties. SUMMARY OF BACKGROUND DATA: Sensitive and specific guidelines are needed that can predict fracture risk and spinal stability after pathologic fractures. Such guidelines may help determine whether treatment is needed to prevent neurologic complications. Simple measurements made from computed tomography data can predict the load-bearing capacity of intact vertebrae and vertebrae with simulated and actual metastatic defects. It is not known whether these same measurements can also predict postfracture stability. METHOD: Simulated metastatic defects were created in human three-vertebrae segments from the lumbar and thoracic spine. Axial rigidity was calculated from quantitative computed tomography data, and failure load and postfracture stability were measured. RESULTS: Postfracture stability was linearly correlated with both failure load (r2 = 0.3-0.6) and axial rigidity (r2 = 0.3-0.6). CONCLUSIONS: The postfracture stability of three-vertebrae segments with simulated defects was modestly related to noninvasively measured, prefracture structural properties.

A motion analysis of the cervical facet joint.

STUDY DESIGN: The stability of motion segments of human cervical spines was sequentially tested as portions of the vertebral anatomy were removed or cut. Isolated, individual facet joints were then similarly studied. OBJECTIVES: To define the laxity of isolated cervical facet joints and the relative contribution of the different components of the vertebral anatomy to the overall stability of the cervical spine. SUMMARY OF BACKGROUND DATA: Facet joints are known to be important in determining cervical stiffness and mobility. This is the first known study in which the biomechanical behavior of isolated cervical facet joints has been documented. METHODS: From five fresh frozen human cervical spines, three C3-C4 and five C5-C6 motion segments were dissected and potted. Rotations and translations in response to 10 bending or twisting moments were recorded by tracking the motion of a testing plate fixed to the superior vertebrae using an articulated arm digitizer. Each motion segment was tested five times, with sequential dissections performed as follows: intact; after removal of the anterior longitudinal ligament intervertebral disc, and posterior longitudinal ligament; after cutting the interspinous ligament; after isolation of the left facet joint; and after isolation of the right facet joint. Each testing sequence involved applying low and high forces 10 cm from the center of the testing plate in each of 10 testing directions. After completion of rotational testing, landmarks on the superior vertebral body and facet joints were digitized to calculate vertebral translations. RESULTS: Isolated facet joints allowed up to 19 degrees of flexion, 14 degrees of extension, 28 degrees of lateral bending, and 17 degrees of rotation. Coupled motions were less in isolated facet joints compared with those in intact vertebral bodies. Isolated facet joints allowed up to 9 mm of translation between superior and inferior surfaces. CONCLUSIONS: Isolated cervical facet joints are highly mobile in comparison with their motions within the constraints of intact motion segments; gliding motions of the isolated facet to near dislocation is possible before the facet capsule constrains motion. Cervical coupled motions are a result of an intact vertebral ring and a combination of the two facet joints. The vertebral ring with facet joints and capsules all intact is necessary for lateral bending stability and rotational stability in the cervical spine.

The internal bony architecture of the sacrum.

STUDY DESIGN: Radiographic and direct quantitative measurements were made of the cortical and the trabecular anatomy of the sacrum. OBJECTIVES: To define the trabecular patterns and the cortical thickness of the sacrum. SUMMARY OF BACKGROUND DATA: The sacrum is a frequent site of internal fixation. In previous anatomic studies, investigators have focused on specific dimensional measurements of the sacrum, whereas others have described the anatomic course of the anterior sacral neurovascular or visceral structures. Computed tomographic imaging also has been used to quantify the sacral trabecular bone density. The internal architecture of the sacrum has yet to be described in detail. METHODS: Seventeen cadaveric sacra were studied by computed tomographic imaging and then were sectioned at 3-mm intervals in the axial or sagittal plane. The cortical thickness of each section was measured under microscopic visualization. The sections were radiographed with high-resolution imaging to delineate their trabecular patterns. RESULTS: The trabecular bone was densest adjacent to the endplates. The sacral body trabeculae were arranged in a cruciate pattern, and bony atrophy occurred in a systematic fashion. An alar void was a consistent finding in all specimens with definable boundaries. The cortical thickness was uniform throughout the surface of each specimen. The computed tomographic images correlated with the anatomy observed in the cadaveric sections. CONCLUSION: The internal bony architecture of the sacrum has several consistent features. The relatively uniform cortical thickness seen in each sacral specimen may have clinical significance in the internal fixation of this region.

Radiographic verification of pedicle screw pilot hole placement using Kirshner wires versus beaded wires.

STUDY DESIGN: Kappa statistics were used to compare the accuracy of two different techniques for verifying pedicle screw pilot hole placement in cadaveric vertebrae. OBJECTIVE: To determine whether clinicians radiographically detect misplaced pedicle screw holes with greater sensitivity and specificity when beaded wires rather than straight Kirschner wires are used. SUMMARY OF BACKGROUND DATA: Pedicle screws commonly are used in orthopedic surgery to obtain and maintain spinal stability. Pedicle screws are reportedly misplaced at a rate of 20% to 40%. Radiographic verification is commonly used to place pedicle screw pilot holes, but this technique is known to be less than 100% accurate. Computer-assisted techniques may allow more accurate screw placement, but these techniques require expensive equipment. METHODS: Pedicle screw pilot holes were drilled into 12 human lumbar and thoracic vertebrae. Some of the holes were misplaced deliberately so that they violated the pedicle walls. Lateral and posteroanterior radiographs of the vertebrae were evaluated by 13 experienced orthopedic spine surgeons and 3 inexperienced observers. At different times, the observers were shown radiographs depicting Kirschner wires or beaded wires placed in the pilot holes. Observers indicated whether they thought the pedicle screw pilot hole violated the pedicle. RESULTS: The sensitivity and specificity of using posteroanterior or lateral radiographs to detect misplaced pedicle screws were increased when beaded wires were placed in the pilot holes. CONCLUSIONS: Radiographic evaluation of beaded wires placed in pedicle screw pilot holes can be both sensitive and specific for misplaced screws. The highest sensitivity and specificity were found using posteroanterior radiographs.

Use of percutaneous transpedicular external fixation pins to measure intervertebral motion.

STUDY DESIGN: Direct measurement of intervertebral motion was compared to motion determined by measuring the position of the exposed ends of the external fixation pins. OBJECTIVES: To verify the accuracy of this technique, so that this protocol can be used to study intervertebral motion in the clinical setting. SUMMARY OF BACKGROUND DATA: The transpedicular external fixation test has been shown to be a test that can predict the outcome of spinal fusion. In patients who are candidates for this test, intervertebral motion can be calculated from motion at the external ends of these pins. METHODS: Six fresh cadaveric spinal segments from L2 to L5 were instrumented with titanium Schanz screws. Reflective markers were placed on the tips of the pins, and intervertebral motion was measured using a noncontacting camera system. Computed tomography data were used to determine the position of the vertebra relative to the reflective markers. Intervertebral distances were calculated and compared with direct measurements obtained using a three-dimensional digitizing arm. RESULTS: There was an excellent correlation (r2 = 0.931) between the directly measured intervertebral motions and those that were indirectly calculated from measurements of motion at the end of the Schanz screws. CONCLUSIONS: Intervertebral motion can be measured by monitoring motion of the ends of transpedicular external fixation pins. Motion of anatomic landmarks on the vertebrae can be calculated from the pin end's motion if computed tomography data are used to determine the geometric relation between the vertebrae and the external fixation pins. This validation study supports the use of this method in clinical investigations of intervertebral motion in patients with low back pain and external fixation.