Biomedical subjects

Thomas R Oxland

Publications and source records attributed to Thomas R Oxland.

At least 19 recordsLinked to original sources

PubMed2007

Biomechanical evaluation of the Total Facet Arthroplasty System: 3-dimensional kinematics.

STUDY DESIGN: An in vitro biomechanical study to quantify 3-dimensional kinematics of the lumbar spine following facet arthroplasty. OBJECTIVES: To compare the multidirectional flexibility properties and helical axis of motion of the Total Facet Arthroplasty System (TFAS) (Archus Orthopedics, Redmond, WA) to the intact condition and to posterior pedicle screw fixation. SUMMARY OF BACKGROUND DATA: Facet arthroplasty in the lumbar spine is a new concept in the field of spinal surgery. The kinematic behavior of any complete facet arthroplasty device in the lumbar spine has not been reported previously. METHODS: Flexibility tests were conducted on 13 cadaveric specimens in an intact and injury model, and after stabilization with the TFAS and posterior pedicle screw fixation at the L4-L5 level. A pure moment of +/-10 Nm with a compressive follower preload of 600 N was applied to the specimen in flexion-extension, axial rotation, and lateral bending. Range of motion (ROM), neutral zone, and helical axis of motion were calculated for the L4-L5 segment. RESULTS: ROM with the TFAS was 81% of intact in flexion (P = 0.035), 68% in extension (P = 0.079), 88% in lateral bending (P = 0.042), and 128% in axial rotation (P = 0.013). The only significant change in neutral zone with TFAS compared to the intact was an increase in axial rotation (P = 0.011). The only significant difference in helical axis of motion location or orientation between the TFAS and intact condition was an anterior shift of the helical axis of motion in axial rotation (P = 0.013). CONCLUSIONS: The TFAS allowed considerable motion in all directions tested, with ROM being less than the intact in flexion and lateral bending, and greater than the intact in axial rotation. The helical axis of motion with the TFAS was not different from intact in flexion-extension and lateral bending, but it was shifted anteriorly in axial rotation. The kinematics of the TFAS were more similar to the intact spine than were the kinematics of the posterior fixation when applied to a destabilized lumbar spine.

Adult↗

PubMed2006

Regional trabecular morphology assessed by micro-CT is correlated with failure of aged thoracic vertebrae under a posteroanterior load and may determine the site of fracture.

INTRODUCTION: Spinal mobilization is commonly used in the treatment of patients with back pain, including individuals with osteoporosis. Previous data indicated that traditional predictors of skeletal failure-lateral or anteroposterior bone mineral density (BMD) by dual energy X-ray absorptiometry (DXA) or geometry of the spinous process or vertebral body-do not predict failure load during posteroanterior spinal mobilization. Morphological differences and inhomogeneities in BMD may have important effects on vertebral strength but integral BMD values by DXA cannot reflect these potentially important differences. We investigated the determinants of spinal fracture using muCT. MATERIALS AND METHODS: We measured failure load and failure site in 11 T5-8 cadaveric specimens (mean age 78 years) when a posteroanterior load was applied at the spinous process of T6 using a servohydraulic material testing machine. Radiography and CT scan were used to verify failure site. We observed no damage to the adjacent T7 vertebrae following the T6 posteroanterior failure test. The T7 vertebrae were sectioned to produce regional samples of the spinous process, the lamina and a vertebral body core. Each sample was scanned with muCT to measure bone microarchitectural parameters. We segmented and analysed four trabecular regions (spinous process base and middle, central lamina and central vertebral body). We used one-way repeated measures ANOVA to compare regions and computed Pearson correlations to assess the relation between PA failure load of T6 and the morphological parameters of T7. RESULTS: The BV/TV at the base or middle of the T7 spinous process (fracture sites), Tb.N and Tb.Th at the base were significantly correlated with posteroanterior failure load of T6 (BV/TV base: r=0.74, p=0.01; BV/TV middle: r=0.73, p=0.01; Tb.N base: r=0.64, p=0.03; Tb.Th base: r=0.65, p=0.03). The Tb.Th of the lamina was significantly greater than Tb.Th of the spinous process base (p=0.002). CONCLUSIONS: Whereas previous data indicated that BMD by DXA was not a good predictor of posteroanterior failure load, regional BV/TV of the spinous process base and middle regions, the sites of fracture, are correlated with posteroanterior failure load. Trabecular thickness differed significantly between the base of the spinous process and the lamina, and may have influenced the site of fracture.

Aged↗

PubMed2006

Three-dimensional rotation of the scapula during functional movements: an in vivo study in healthy volunteers.

The goal of this study was to measure 3-dimensional shoulder motion by use of a direct invasive technique during 4 different arm movements in healthy volunteers. Eight subjects with healthy shoulders were recruited. Optoelectronic marker carriers (ie, infrared light-emitting diodes) were mounted on bone pins, which were inserted into the lateral scapular spine. Subjects performed 4 different arm movements while the motion was being recorded by a precision optoelectronic camera. Joint angles were calculated in 3 dimensions. Intraclass correlation coefficients and root-mean-square differences were calculated as measures of reliability. During abduction, the scapula tipped posteriorly (44 degrees +/- 11 degrees), rotated upward (49 degrees +/- 7 degrees), and rotated externally (27 degrees +/- 11 degrees). For reaching, the scapula consistently rotated upward (17 degrees +/- 3 degrees) and rotated internally (18 degrees +/- 6 degrees) whereas tipping was generally less than 10 degrees (5 degrees +/- 2 degrees). Overall, the range of scapular movement for the hand behind the back was small and variable, with most rotations not exceeding 15 degrees. For horizontal adduction, the scapula tipped anteriorly (8 degrees +/- 3 degrees), rotated upward (5 degrees +/- 2 degrees), and rotated internally (27 degrees +/- 6 degrees). These scapular rotations provide normative data that will be useful for diagnosing scapular dysfunction.

Adult↗

PubMed2006

Cement flow during impaction allografting: a finite element analysis.

Cement intrusion into cancellous or impacted bone is not well understood. We adopted an engineering mechanics approach to predict the effect of surgical variables on the cement intrusion into impacted cancellous bone, used for the revision of failed total hip replacement with the impaction allografting technique. Specifically, a three-dimensional finite element model was used to determine the effects of cement viscosity, the magnitude and duration of pressurization, and the distribution of the porosity along the femur on cement intrusion. The overall averaged mean intrusion depth difference between the finite element model prediction and the cadaveric measurements was 1.1mm. The depth of penetration increased with higher pressurization pressure, duration of pressurization, and earlier stem insertion (lower viscosity), but maintained a similar profile. The distribution of the porosity along the femur determined the intrusion profile. Cement viscosity, the applied pressure or the duration of the pressurization can be adjusted to limit the cement volume injected into the medullary canal and therefore prevent the cement from reaching the endosteal surface.

Bone Cements↗

PubMed2006

Accuracy and repeatability of a new method for measuring facet loads in the lumbar spine.

We assessed the repeatability and accuracy of a relatively new, resistance-based sensor (Tekscan 6900) for measuring lumbar spine facet loads, pressures, and contact areas in cadaver specimens. Repeatability of measurements in the natural facet joint was determined for five trials of four specimens loaded in pure moment (+/- 7.5 N m) flexibility tests in axial rotation and flexion-extension. Accuracy of load measurements in four joints was assessed by applying known compressive loads of 25, 50, and 100 N to the natural facet joint in a materials testing machine and comparing the known applied load to the measured load. Measurements of load were obtained using two different calibration approaches: linear and two-point calibrations. Repeatability for force, pressure, and area (average of standard deviation as a percentage of the mean for all trials over all specimens) was 4-6% for axial rotation and 7-10% for extension. Peak resultant force in axial rotation was 30% smaller when calculated using the linear calibration method. The Tekscan sensor overestimated the applied force by 18 +/- 9% (mean+/-standard deviation), 35 +/- 7% and 50 +/- 9% for compressive loads of 100, 50, and 25 N, respectively. The two-point method overestimated the loads by 35 +/- 16%, 45 +/- 7%, and 56 +/- 10% for the same three loads. Our results show that the Tekscan sensor is repeatable. However, the sensor measurement range is not optimal for the small loads transmitted by the facets and measurement accuracy is highly dependent on calibration protocol.

Aged↗

PubMed2005

Biomechanical characterization of the three-dimensional kinematic behaviour of the Dynesys dynamic stabilization system: an in vitro study.

The Dynesys, a flexible posterior stabilization system that provides an alternative to fusion, is designed to preserve intersegmental kinematics and alleviate loading at the facet joints. Recent biomechanical evidence suggests that the overall range of motion (ROM) with the Dynesys is less than the intact spine. The purpose of this investigation was to conduct a comprehensive characterization of the three-dimensional kinematic behaviour of the Dynesys and determine if the length of the Dynesys polymer spacer contributes to differences in the kinematic behaviour at the implanted level. Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of +/-7.5 Nm in flexion-extension, lateral bending, and axial rotation, with and without a follower preload of 600 N. Test conditions included: (a) intact; (b) injury; (c) injury stabilized with Dynesys at L3-L4 (standard spacer); (d) long spacer (+2 mm); and (e) short spacer (-2 mm). Intervertebral rotations were measured using an optoelectronic camera system. The intersegmental range of motion (ROM), neutral zone (NZ), and three-dimensional helical axis of motion (HAM) were calculated. Statistical significance of changes in ROM, NZ, and HAM was determined using repeated measures analysis of variance (ANOVA) and Student-Newman-Keuls post-hoc analysis with P<0.05. Implantation of the standard length Dynesys significantly reduced ROM compared to the intact and injured specimens, with the least significant changes seen in axial rotation. Injury typically increased the NZ, but implantation of the Dynesys restored the NZ to a magnitude less that that of the intact spine. The Dynesys produced a significant posterior shift in the HAM in flexion-extension and axial rotation. The spacer length had a significant effect on ROM with the long spacer resulting in the largest ROM in all loading directions without a follower preload. The largest differences were in axial rotation. A 4 mm increase in spacer length led to an average intersegmental motion increase of 30% in axial rotation, 23% in extension, 14% in flexion, and 11% in lateral bending. There were no significant changes in NZ with different spacer lengths. Typically, the short spacer caused a greater shift and a greater change in orientation of the HAM than the long spacer. The long spacer resulted in a ROM and a motion pattern, as represented by the HAM, that was closer to that seen in an intact specimen. The results of this study suggest that the length of the Dynesys spacer altered the segmental position and therefore affected kinematic behaviour.

Aged↗

PubMed2005

The effect of interbody cage positioning on lumbosacral vertebral endplate failure in compression.

STUDY DESIGN: A biomechanical investigation using a human cadaver, multisegmental lumbosacral spine model. OBJECTIVES: To determine if 2 small, posterolaterally positioned titanium mesh interbody cages would provide superior construct strength and stiffness in compression compared to central cage placement. In addition, determine construct stiffness with interbody cages as opposed to an intact spine and assess the effect of bone mineral density (BMD). SUMMARY OF BACKGROUND DATA: Previous work has shown that the posterolateral corners of the lumbosacral endplates are stronger than the anterior and central regions. Information to suggest appropriate interbody cage positioning to avoid subsidence into adjacent vertebrae would be valuable for spine surgeons and implant designers. METHODS: A total of 27 functional spinal units from L3 to S1 were dual x-ray absorptiometry scanned for BMD, instrumented with pedicle screw systems, and tested to failure in compression with titanium mesh interbody cages placed in 1 of 3 positions: 2 small posterolateral, 2 small central, or 1 large central. Analysis of covariance was conducted to compare failure load and stiffness across the different cage configurations. Repeated measures analysis of variance was used to analyze stiffness between functional spinal units with intact disc, discectomy, or interbody cages. Failure load was correlated against BMD. RESULTS: Of the 3 placement patterns, 2 small titanium mesh cages in the posterolateral corners had 20% higher failure loads, although the difference was not significant (P = 0.20). Stiffness in compression for the 3 cage positions was not significantly different (P = 0.82). All intact discs with posterior instrumentation were significantly stiffer than any of the cage patterns (P = 0.0001). BMD of the vertebrae significantly correlated with failure loads (P = 0.007). CONCLUSIONS: The placement of 2 small interbody cages posterolaterally tended to result in higher failure loads than central cage placement, although the results were not statistically significant. It is noteworthy that cage placement in any position resulted in a less stiff construct in compression than with an intact disc.

Aged↗

PubMed2005

The effect of shoulder arthroplasty on humeral strength: an in vitro biomechanical investigation.

BACKGROUND: Periprosthetic humeral fractures are a serious complication of shoulder arthroplasty. While adequate reaming of the canal and insertion of an oversized implant optimizes fit, such maneuvers also weaken the bone and predispose it to fracture. METHODS: The impact of the humeral arthroplasty was assessed in vitro on human cadaveric specimens. Strain gauges were attached to the distal diaphyses and the specimens were mounted in a torsion-loading fixture throughout the tests. An initial series examined the effect of reaming of the canal to its clinically appropriate diameter using uniaxial strain gauges. A second series utilized strain rosettes to evaluate the cumulative effects of reaming, broaching, and implant insertion. FINDINGS: Reaming of the canal to its clinically appropriate diameter significantly increased (P=0.007) uniaxial strain measurements by a mean of 30% with five of eight specimens showing increases of over 49% on at least one of four diaphyseal locations. In the second series, the surface strain was significantly affected by arthroplasty (P<0.008). Post-hoc analysis showed that the maximum in-plane shear strain following implant insertion was significantly increased relative to strain levels following reaming and broaching (P<0.009). The direction of the principal strain axes did not significantly change (P>0.46). Unexpected decreases in some strain measurements were observed as the arthroplasty procedure progressed perhaps reflecting overt mechanical failure within the humeral shaft. INTERPRETATION: The strain increase following reaming suggests a reduction in torsional strength by over 33% which is further reduced following broaching and implant insertion. For the practicing surgeon, post-operative strength can be adversely affected by both canal preparation and implant insertion.

Adult↗

PubMed2005

Biological and mechanical changes of the bone graft-cement interface after impaction allografting.

In impaction allografting, the host bone interface may consist of morsellized allograft alone or as a composite with bone cement. The objective of this study was to investigate the temporal changes in the interface for these two materials in a rat bone chamber model. To simulate the impaired endosteal circulation after impaction allografting, bone chambers were tightened bilaterally to the endosteal surfaces of proximal tibiae of mature rats and filled with pure allograft or cement/allograft composite. The cement/allograft composite-host bone interface strength was significantly higher at 3 weeks and was higher than the allograft construct failure strength. Limited allograft, but extensive periosteal remodeling, was observed at 3 weeks which resulted in a significantly increased cortical porosity and cortical thickness. The allograft porosity decreased significantly at 6 weeks indicating extensive remodeling of the allograft bone. Little or no remodeling of the allograft particles in the cement was found. At 6 weeks a new medullary canal was formed, and the endosteal cortex was partially absorbed. Endosteal absorption resulting in medullary canal widening in revision THR may be responsible for clinically unstable stems after impaction allografting.

Animals↗

PubMed2005

Interbody device shape and size are important to strengthen the vertebra-implant interface.

STUDY DESIGN: An in vitro cadaveric study to compare compressive failure load, strength, and stiffness of the implant-vertebra interface. OBJECTIVES: To determine the effect of cage shape (kidney, cloverleaf, or oval) and cage surface area on endplate failure strength and secondly to determine the extent and pattern of trabecular failure adjacent to an interbody device. SUMMARY OF BACKGROUND DATA: Recent studies indicate that the posterolateral and peripheral regions of the endplate are stronger than the central. Current implants are not designed to take advantage of these stronger regions of the endplate. The zone of trabecular failure that results from interbody device subsidence has not been reported extensively in the literature. METHODS: Uniaxial compression testing with unrestricted rotation was carried out on the superior endplates of 48 thoracolumbar (T9-L2) vertebrae with 1 of 3 shaped indentors covering 20% or 40% of the endplate area. Failure load, failure strength, and stiffness were compared. Quantitative computed tomography scans were carried out before and following indentation tests to identify areas of trabecular densification that indicate localized failure. RESULTS: The cloverleaf-shaped indentors resulted in significantly higher (P < 0.001) failure loads (by >45%), strength (>49%), and construct stiffness (>35%) for both the 20% and 40% cross-sectional area sizes. Trabecular bone failure occurred in a semielliptical zone underlying the interbody devices, leaving the endplate and underlying cancellous bone intact. CONCLUSIONS: The cloverleaf-shaped indentor displayed an improved strength and stiffness profile when compared to oval or kidney-shaped indentors of similar surface areas.

Aged↗

PubMed2005

Anterior cervical plate fixation: a biomechanical study to evaluate the effects of plate design, endplate preparation, and bone mineral density.

STUDY DESIGN: A biomechanical study using multidirectional flexibility testing in a human cadaveric cervical spine model of a flexion-distraction injury. OBJECTIVES: To compare the immediate postoperative stabilizing effect of dynamic and rigid anterior cervical plates and to assess the confounding effects of bone mineral density (BMD) and endplate preparation technique. SUMMARY OF BACKGROUND DATA: Dynamic plate designs presumably increase load sharing between the plate and graft, but their effect on spinal stabilization has not been assessed in a traumatic flexion-distraction model. METHODS: Twenty-four fresh frozen human cervical functional spinal units were dual-energy x-ray absorptiometry scanned for bone mineral density and allocated into 4 groups by the type of plate, dynamic (ABC, Aesculap, Germany) versus rigid (Cervical Spine Locking Plate, Synthes USA, Paoli, PA), and the technique of endplate preparation, intact versus removed. Each functional spinal unit had all posterior ligaments transected and both inferior facets excised, after which anterior discectomy, grafting, and plating was performed. Nondestructive testing applied a 1.5 Nm pure moment, whereas ranges of motion and neutral zones were measured in flexion/extension, lateral bending, and rotation. Ratios of the range of motion and neutral zone of the plated to the intact site were analyzed. The load sharing between the plate and the functional spinal unit was measured via strain gauges mounted on the plate. RESULTS: There were no significant differences in the range of motion or neutral zone ratios between the 2 plate designs, except for the range of motion ratio in extension, where the dynamic plate exhibited better stabilization than the rigid plate (P = 0.02). There was a consistent interaction whereby endplate removal resulted in better stabilization for the dynamic plate, but less stabilization for the rigid plate. Significantly less motion was observed with increasing bone mineral density in all loading directions. In flexion and extension, the dynamic plate measured one-third less strain than the rigid plate. CONCLUSIONS: The dynamic plate appeared to provide better stabilization in extension, and the technique of endplate preparation has some effect on immediate stabilization, dependent on the type of plate employed. Bone mineral density of the specimen was a strong determinant of the degree of stabilization achieved, regardless of the type of plate used.

Adult↗

PubMed2005

Calcium phosphate cement composites in revision hip arthroplasty.

Loosening of the femoral component in a total hip arthroplasty with concomitant bone loss can pose a problem for revision surgery due to inadequate structure in the remaining femur. While impaction allografting has shown promise, it has also shown serious complications, especially with moderate to severe bone loss. It may be possible to stabilize the graft layer with a bioresorbable cement to improve clinical results. This study examines the mechanical properties of a potential morsellized bone-bioresorbable composite. Morsellized bone was mixed with a commercially available bioresorbable cement (alpha-BSM, Etex Corp.) in compositions of 0%, 25%, 50% and 75% bone. Unconfined compression and diametral tensile and confined compression tests were performed to determine the composite mechanical properties. The composition containing 50% bone tended to exhibit the highest uniaxial strengths, as well as the highest confined compression modulus. The uniaxial compressive strength and stiffness of this composition was in the range of cancellous bone. Uniaxial compressive modulus decreased with increasing bone fraction whereas elongation exhibited the opposite trend. Bone fraction had a significant effect on compressive strength (p < 0.0001), compressive modulus (p < 0.0001), elongation (p < 0.01), tensile strength (p < 0.0001) and confined compressive modulus (p = 0.04). The addition of a bioresorbable cement to the allograft layer may improve the properties of the layer, preventing early subsidence seen in some clinical studies of impaction allografting, and therefore improving the clinical results. Further testing is required to evaluate the in vitro mechanical performance, as well as in vivo remodelling characteristics.

Aged↗

PubMed2005

A biomechanical study comparing cortical onlay allograft struts and plates in the treatment of periprosthetic femoral fractures.

BACKGROUND: Periprosthetic femoral fractures with well-fixed femoral components can be difficult to manage and internal fixation is usually necessary. This study was designed to determine the effect of cable plate, strut allograft and combined plate and strut allograft fixations of periprosthetic femoral fractures. METHODS: A transverse fracture at the level of the tip of the femoral stem was simulated in six cadaveric femurs. The fracture was fixed with a plate-strut graft construct with and without proximal screws, a plate only construct with and without proximal screws and with both anterior and lateral struts grafts of 12 cm or 20 cm in length. The intact femur and the six constructs were tested using anteroposterior and axial loads to simulate the forces at the hip during gait. The interfragmentary motions were measured with an optoelectronic camera system. FINDINGS: The highest median interfragmentary translations were observed with the plate only construct without proximal screws. No significant differences in median translations were found between the combined plate-strut and the strut-graft alone constructs. Median interfragmentary rotations were largest with the plate only construct and lowest with the plate-strut constructs. INTERPRETATION: The best fracture fixation was achieved with the combined plate and strut graft constructs particularly in combination with the two unicortical screws above the fracture. The plate only constructs may not provide sufficient fixation stability in rotation for the simulated transverse fracture. The within-construct interfragmentary motions were not significantly different except for the plate only constructs. This suggests that the fixation construct type is the determining factor of interfragmentary motion.

Arthroplasty, Replacement, Hip↗

PubMed2005

Posteroanterior stiffness predicts sagittal plane midthoracic range of motion and three-dimensional flexibility in cadaveric spine segments.

BACKGROUND: Spinal joint mobilization is a mainstay of clinical assessment of individuals with back pain. The clinician manually assesses stiffness and joint motion relative to segments above and below. Although clinical theory suggests that manually performed techniques can predict or detect intervertebral motion, this hypothesis remains untested. METHODS: Using a precision opto-electronic camera system and a custom spine testing machine, we measured intervertebral range of motion, neutral zone motion and three-dimensional flexibility in eight T5-T8 cadaveric specimens (mean age=81 years). We then measured stiffness when a cyclic posteroanterior load was applied at the spinous process of T6 using a servohydraulic material testing machine (Instron 8874), simulating the posteroanterior spinal mobilization technique. FINDINGs. There was a strong significant inverse relationship between stiffness during cyclic posteroanterior loading of T6 and flexion or extension range of motion of T6 relative to T7 (r=-0.88, P<0.01, extension; r=-0.81, P=0.01, flexion), and T6-T7 flexibility in all six directions. INTERPRETATION: Stiffness during simulated central cyclic posteroanterior mobilization in the cadaveric midthoracic spine is inversely correlated with flexion and extension range of motion and three-dimensional flexibility at the level at which the technique is applied. These findings provide biomechanical support for the inclusion of specific joint mobilization in the assessment of older adults with back pain.

Aged↗

PubMed2005

Structural characteristics of impaction allografting for revision total hip arthroplasty.

BACKGROUND: The impaction allografting procedure for treatment of failed hip reconstructions has shown promising but variable results. The objective of this study was to compare the structural characteristics of revision total hip arthroplasty constructs with impaction allografting (cement+morsellized bone) with all-cement and all-morsellized bone constructs. METHODS: Uniaxial cyclic compression was applied to a simplified uniaxial, parallel, aluminum tube model to simulate normal gait. Applied force and axial stem displacement were recorded to determine stem subsidence and construct stiffness. FINDINGS: Introduction of a small amount of cement into the bone graft, as suggested in an impaction allografting procedure previously reported, makes the construct behave structurally more similar to an all-cemented construct than to an all-bone graft construct. INTERPRETATION: The results suggest that the structural properties achieved in an impaction allografting construct are sensitive to the amount of cement in the graft and that care should be taken clinically to achieve consistent constructs.

Arthroplasty, Replacement, Hip↗

PubMed2005

Mechanical characteristics of the bone-graft-cement interface after impaction allografting.

Impaction allografting is an attractive procedure for the treatment of failed total hip replacements. The graft-cement-host bone interface after impaction allografting has not been characterized, although it is a potential site of subsidence for this type of revision total hip reconstruction. In six human cadaveric femurs, the cancellous bone was removed proximally and local diaphyseal lytic defects were simulated. After the impaction grafting procedure, the specimens were sectioned in 6 mm transverse sections and push-out tests were performed. From the adjacent sections the percentage cement contact of the PMMA cement with the endosteal bone surface was determined. The host bone interface mechanical properties varied significantly along the femur largely due to different interface morphologies. The apparent host bone interface shear strength was highest around the lesser trochanter and lowest around the tip of the stem. A significant positive correlation was found between the percentage cement contact and the apparent host bone interface shear strength (r2 = 0.52). The sections failed in 69% of the cases through a pure host bone interface failure without cement or allograft failure, 19% failed with local cement failure, and 12% with a local allograft failure. The apparent host bone interface strength was on average 89% lower than values reported for primary total hip replacements and were similar to cemented revisions proximally and lower distally. This study showed that cement penetration to the endosteal surface enhanced the host bone-graft interface.

Biomechanical Phenomena↗

PubMed2004

Accuracy of DXA scanning of the thoracic spine: cadaveric studies comparing BMC, areal BMD and geometric estimates of volumetric BMD against ash weight and CT measures of bone volume.

Biomechanical studies of the thoracic spine often scan cadaveric segments by dual energy X-ray absorptiometry (DXA) to obtain measures of bone mass. Only one study has reported the accuracy of lateral scans of thoracic vertebral bodies. The accuracy of DXA scans of thoracic spine segments and of anterior-posterior (AP) thoracic scans has not been investigated. We have examined the accuracy of AP and lateral thoracic DXA scans by comparison with ash weight, the gold-standard for measuring bone mineral content (BMC). We have also compared three methods of estimating volumetric bone mineral density (vBMD) with a novel standard-ash weight (g)/bone volume (cm3) as measured by computed tomography (CT). Twelve T5-T8 spine segments were scanned with DXA (AP and lateral) and CT. The T6 vertebrae were excised, the posterior elements removed and then the vertebral bodies were ashed in a muffle furnace. We proposed a new method of estimating vBMD and compared it with two previously published methods. BMC values from lateral DXA scans displayed the strongest correlation with ash weight (r=0.99) and were on average 12.8% higher (p<0.001). As expected, BMC (AP or lateral) was more strongly correlated with ash weight than areal bone mineral density (aBMD; AP: r=0.54, or lateral: r=0.71) or estimated vBMD. Estimates of vBMD with either of the three methods were strongly and similarly correlated with volumetric BMD calculated by dividing ash weight by CT-derived volume. These data suggest that readily available DXA scanning is an appropriate surrogate measure for thoracic spine bone mineral and that the lateral scan might be the scan method of choice.

Absorptiometry, Photon↗

PubMed2004

The radiographic failure of single segment anterior cervical plate fixation in traumatic cervical flexion distraction injuries.

STUDY DESIGN: A radiographic review of 87 patients with either unilateral or bilateral facet dislocations or fracture/dislocations treated with anterior cervical discectomy, fusion, and plating. OBJECTIVE: The primary objective of this study was to report the incidence of radiographic failure and factors that would predispose to this loss of alignment. The secondary objective was to report the rate of pseudarthrosis. SUMMARY OF BACKGROUND DATA: Biomechanical and clinical data conflict regarding the appropriate approach and method of fixation of distractive flexion cervical injuries. Unilateral and bilateral facet fracture subluxations may be surgically stabilized by anterior cervical discectomy, fusion, and plating, posterior instrumentation, or both. There are no documented reports of the rate of radiographic failure of this procedure when limited to a single level injury from a distractive flexion mechanism. METHODS: Inclusion criteria were all single-level unilateral and bilateral facet fracture dislocations or subluxations treated with a single-level anterior cervical discectomy, fusion, and plating. Retrospectively, 107 cases were identified (87 with complete radiographs) from January 1994 to December 2001. Radiographic failure was defined as a change in translation of greater than 4 mm and/or change in angulation of greater than 11 degrees between the immediate postoperative films and the most recent follow-up. Fusion was assessed radiographically. RESULTS: A 13% incidence of radiographic loss of alignment is reported in 87 unilateral and bilateral facet fracture subluxations stabilized with anterior cervical discectomy, fusion, and plating. Radiographic failure correlated with the presence of endplate compression fracture and facet fractures on injury radiographs. There was no correlation between radiographic failure and age, gender, surgeon, unilateral or bilateral injury, plate type, level of injury, degree of translation, or sagittal alignment at the time of injury. CONCLUSION: Loss of postoperative alignment occurred in 13% of facet fracture subluxations treated with anterior cervical discectomy, fusion, and plating. Concern regarding mechanical failure of flexion/distraction injuries should be high when they are associated with fractures of either the facets or of the endplate. Endplate fracture was associated with both mechanical failure and pseudarthrosis.

Adolescent↗