Summary Statement

The mechanism of spinal cord injury varies across the human population and this may be important for the development of effective therapies. Therefore, detailed understanding of how variables such as impact velocity and depth affect cord tissue damage is important.

Summary Statement

Spinal flexibility in bending and axial torque has been shown to exhibit very modest changes with advancing disc degeneration. This study is the first to address the possible relationship in pure anterior shear and no clear relationship was observed.

Purpose: Open reduction internal fixation with a volar plate is a popular surgical option for distal radius fractures. The pronator quadratus (PQ) must be stripped from the distal radius in this procedure. PQ is an important pronator of the forearm and stabilizer of the distal radioulnar joint. The purpose of this study was to investigate pronation torque in healthy volunteers before and after temporary paralysis of the PQ with lidocaine under EMG guidance.

Method: A custom-made apparatus was built to allow isometric testing of pronation torque at 5 positions of rotation: 90° of supination, 45° of supination, neutral, 45° of pronation and 80° of pronation. It was validated using a test-retest design with 10 subjects. For the study, 17 (9 male, 8 female) right hand dominant volunteers were recruited. They were tested at all 5 positions in random order and then had their PQs paralyzed with lidocaine. Repeat testing was performed in the same random order 30 minutes after injection. Three subjects underwent unblinded testing with saline injected instead of lidocaine.

Results: After paralysis of PQ with lidocaine, pronation torque decreased by 23.2% (p=0.0010) at 90° of supination, 16.7% (p=0.0001) at 45° of supination, 22.9% (p=0.0002) in the neutral position, 20.4% (p=0.0066) at 45° of pronation and 22.2% (p=0.0754) at 80° of pronation. All were statistically significant except 80° of pronation. Peak torque values before and after injection were highest in the supinated positions (8.2 Nm at 45° supination) and decreased gradually as the subjects were in more pronated positions (1.8 Nm at 80° pronation). The test-retest trial demonstrated no evidence of fatigue with repeated testing. The subjects who underwent injection of saline demonstrated no evidence of pronation torque loss secondary to pain or a pressure effect of the injectate.

Conclusion: This study demonstrated a significant decrease in pronation torque with controlled elimination of PQ function. Open reduction internal fixation of distal radius fractures damages the PQ. This may result in a pronation torque deficit. Functional significance of this loss should be shown. Pronation torque measurement may add to postoperative outcome analysis of surgical procedures about the wrist.

Purpose: Recent studies have shown differences in short term spinal cord pathology between spinal column injury mechanisms, such as contusion and fracture-dislocation. Such differences may exist at longer time points, and thus survival studies are needed in the dislocation models. A more in-depth characterization of the dislocation model is needed for development of a mild-moderate cervical spine dislocation model in a rat that is suitable for survival studies. Specifically, our objective in this study was to determine the dislocation displacement that produces initial spinal column failure in a Sprague-Dawley rat model and to validate a consistent injury at the desired dislocation in-vitro and in-vivo.

Method: For the dislocation model, the dorsal ligaments and facets at C4–C5 were removed to mimic the type of posterior element fracture and ligament injury commonly seen in a bilateral fracture-dislocation. C3 and C4 were clamped together and held stationary while the clamp holding C5 and C6 was connected to an electromagnetic actuator and displaced dorsally to produce the injury while force and displacement were recorded. Twenty-eight isolated cervical spine specimens of Sprague-Dawley rats were used to determine dislocation displacement at initial spinal column failure. The C4–C5 segment sustained dislocation (> 3mm) injury at 0.05mm/s (n=11), 100mm/s (n=4) and 1000mm/s (n=13). Initial spinal column failure was defined at with maximum force during the dislocation. A dislocation displacement of 1.4mm was applied to 7 isolated specimens and 4 anesthetized rats at 430mm/s. The spinal column failure was inspected up to 3 days after injury, as well as hemorrhage of spinal cord in-situ.

Results: The dislocation displacement at in-vitro spinal column failure was 0.95mm±0.32 and not significantly different among specimens at the three dislocation speeds. Under a dislocation displacement of 1.4mm, rupture of the C4–C5 disc occurred in all in-vitro (0.67mm±0.38) and in-vivo (0.65mm±0.17) cases. SCI hemorrhage at epicenter was observed in 3 of 4 cases.

Conclusion: The initial spinal column failure in an innovative SCI model occurs at displacement between 0.65mm and 0.95mm. Dislocation displacement of 1.4mm results in spinal column failure consistently and SCI hemorrhage, and may be suitable for survival studies.

The Dynesys is a flexible posterior stabilization system that is designed to preserve intersegmental kinematics and reduce loading at the facet joints. The purpose of this study was to determine if the length of the Dynesys spacer has an effect on range of motion (ROM) at the implanted level. Spacer length was found to significantly affect ROM in all three loading directions with and without a follower preload. The longer spacer increased ROM and the shorter spacer decreased ROM, largely due to differences in segmental compression between the two.

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 motion with Dynesys is less than the intact spine (Schmoelz, 2003). The purpose of this investigation was to determine if the length of the Dynesys spacer contributes to differences in range of motion (ROM) at the implanted level.

Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of ±7.5Nm in three directions of loading with and without a follower preload of 600N. Test conditions included: intact, injury at L3-L4, Dynesys at L3-L4 (standard spacer), long spacer (+2mm), and short spacer (−2mm). Intervertebral rotations were measured using an optoelectronic camera. Pressure sensors placed inside the joint capsules measured facet loads. Statistical significance was determined using repeated measures ANOVA.

Spacer length had a significant effect on ROM in all three loading directions with and without a follower preload. Initial contact loads within the facet joints were 150% and 64% that of the standard spacer for the short and long spacer, respectively.

The magnitude of distraction of the segment affects ROM. Shorter spacers increased segmental compression of the intervertebral disc and facet joints and therefore reduced ROM. With a follower preload, the segment is further compressed and ROM is further reduced.

The results contribute to an understanding of the design of such implants and could help guide future research.

Funding: Synos Foundation, Switzerland, National Science and Engineering Research Council of Canada (NSERC)

Please contact author for table or diagram.

Impaction allografting is increasingly used for the treatment of failed total hip replacements. In six human cadaveric femurs the impaction allografting procedure was performed to comprehensively describe the postoperative morphology of impaction allografting. After the procedure, the specimens were sectioned and prepared for histomorphometric analysis. The graft porosity was lowest in Gruen zone four (52%) and highest in Gruen zone one (76%). At the level of Gruen zone six and two, virtually the entire cross-section was filled with bone cement. The presented data will serve as a baseline for future investigations of the impaction allografting.

Impaction allografting is an attractive procedure for the treatment of failed total hip replacements. The purpose of this study was to comprehensively describe the morphology of impaction allografting post operatively to form a baseline for further investigations.

Three experienced surgeons performed the impaction allografting procedure on six cadaveric femurs. After the procedure, the femurs were cut in 6mm thick transverse sections and processed for histomorphometric analysis.

The porosity of the impacted graft was highest proximally in Gruen zone one (76%) and lowest in Gruen zone four (52%). Below the tip of the stem (Gruen zone four), the mean cement penetration was significantly lower compared to the proximal part of the femur. The averaged residual impacted graft layer in Gruen zone six and two was (0.5mm SD 0.4mm) significantly thinner compared with Gruen zone’s one, 7/1, and four.

In the region of Gruen zone six and two the entire cross-section was penetrated with bone cement with almost no residual graft layer (Figure). Even the simulated lytic defects in this region were filled with the graft cement composite which may not be remodelled by the host bone.

The graft porosity was found to be highest proximally and lowest distally. In the region of Gruen zone six and two the entire cross-section was penetrated with bone cement with almost no residual graft layer.

This investigation will serve as a baseline for future studies of the mechanical and biological processes that make the impaction allografting a successful procedure.

Funding: Stryker Howmedica and DePuy for provided implants and instruments.

Please contact author for figures and/or diagrams.

In impaction allografting, the host bone interface consists of morsellized allograft alone or as a composite with bone cement. The objective of this study was to investigate the host bone temporal changes in the interface for these two materials in a rat bone chamber model. The composite-host bone interface strength was significantly higher at three weeks and was higher than the allograft construct. Limited allograft, but extensive periosteal remodelling, was observed at three weeks. At six weeks a new medullary canal was formed and the endosteal cortex was partially absorbed. Endosteal absorption resulting in medullary canal widening may be responsible for clinically unstable stems after impaction allografting.

The host bone interface after impaction allografting consists of morsellized allograft alone or as a composite with cement and it may be important for the clinical success of this procedure. The purpose of this study was to investigate the temporal changes of these interfaces in a rat bone chamber model.

Bone chambers were inserted in both tibiae of thirty-three rats and tightened to the endosteal surface to create a microenvironment. One chamber was filled with allograft bone and the other with an allograft/ cement composite. After zero, three, and six weeks, the rats were euthanized, the interfaces mechanically tested and processed for histomorphometric analysis.

The composite-host bone interface strength was significantly higher at three weeks and was higher than the allograft construct. Extensive periosteal remodelling was observed at three weeks. At six weeks a new medullary canal was formed and the endosteal cortex was partially absorbed.

The increased interface strength of the composite-host bone interface was due to fibrous tissue attachment rather than direct bonding of the bone particles. Cortical porosity and cancellisation is known to be caused by a damaged endosteal circulation resulting in medullary canal widening and may cause clinically unstable implants.

Interface strength of the composite-host bone interface was increased at three weeks through fibrous tissue attachment. A damaged endosteal circulation caused cortical porosis and cancellisation.

With this rat bone chamber model a potential cause of stem subsidence after impacting allografting was identified.

Funding: The George W. Bagby Research Fund.

The Canadian Institutse of Health Research.

The Maurice E. Müller Foundation.

The Swiss Academy of Engineering Science.

The Robert Mathys Foundation.

This study identified imaging parameter(s) which best predict the mechanical properties of distal tibia. Seventeen human cadaver tibiae were assessed by PQCT at four, eight and ten percent site from distal and tested in compression at the twenty-five percent distal portion. Ultimate compressive loads were recorded with a mean of 8276 ± 2915 N. Spearson rank correlation and stepwise regression analysis revealed that CoA, total BMC, SSI and SSI4-TrA4-CoD4 combination had statistically significant correlations with the failure loads. Among all imaging parameters, SSI had the highest relevance due to its account for geometry, density and material distribution, important factors for structural properties.

Musculoskeletal diseases, especially hip fractures, have huge and growing impact on Canadian society. To develop techniques for identification of high risk population, we needed a link between clinical evaluations and laboratory measures of bone health. This study identified imaging parameter(s) which best predict the mechanical properties of distal tibia.

Seventeen human cadaver tibiae were considered in this study (mean age seventy-four, SD six years). PQCT was used to assess the four, eight and ten percent site. It measured the cross-sectional area, bone mineral content and bone mineral density of the cortical bone, trabecular bone and combined. Strength Strain Index (SSI) was calculated from these measurements. Each tibia was cut at twenty-five percent distal. Compressive force was applied uniaxially through a custom-made PMMA indentor onto the distal plateau along the longitudinal axis of the tibia at a rate of 10mm/s. Load and displacement data were recorded. Spearson rank correlation and stepwise regression analysis were used to identify individual and combination of imaging variables that were related to ultimate failure load.

Ultimate failure loads were recorded with a mean of 8276 ± 2915 N. Cortical area (R_0.72), total BMC (R_0.72) and SSI (R_0.86) had statistically significant correlations with the failure load. Stepwise regression revealed that the combination of SSI, TrA, CoD at 4% site explained the greatest amount of variance (R2 = 0.868) and SSI was the major contributor. SSI takes the polar moment of inertia (geometry), density and distribution of material into account. This explains its relevance towards predicting the ultimate failure load.

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A biomechanical study assessing pedicle screw fixation with three different augmentation methods was performed in human cadaveric vertebrae. Precision opto-electronic measurement of screw motion assessed motion magnitude and patterns, ie translation and/or rotation. Physiological cyclic loads were applied as opposed to the simple pull out test. Augmentation with wires, hook or cement decreased overall motion. There were no significant differences in motion magnitude between the three augmentation methods. Motion patterns for screws with cement augmentation were mainly rotational and differed from the other two methods. Rigid body translations were observed with wires or hook augmentation, suggesting a loosening behaviour. Augmentation with cement resulted in better fixation than wires or hook.

Augmentation of loosened pedicle screws in poor quality bone is often necessary. The purpose of this study was to contrast the kinematics of loosened pedicle screws augmented with laminar hooks, sublaminar wires or calcium phosphate cement.

Cyclic tests of pedicle screws with compressive force and bending moment were carried out on forty-eight screws in twenty-four cadaveric vertebrae (L3-L5) augmented with hooks, wires or cement. Motion at the screw tip and screw head were measured using an optoelectronic camera system and the magnitudes compared in a paired manner using non-parametric statistics. Motion patterns of the screws were determined for each augmentation method.

Augmentation with hook, wire or cement decreased screw motion. There was no significant difference between augmentation methods when the magnitudes of motion, described as ranges and offsets, were compared. Augmentation with cement resulted in mainly rotations of the screws while there were rigid body translations with wires or hooks.

Comparing magnitudes of motion at the screw head and screw tip were insufficient. The screw head and screw tip could be moving in synchronous, indicating rigid body translations. Using simple pull out tests would not detect such differences.

The method used in this study contrasted pedicle screws motion with different augmentations. While there was no detected significant difference in motion magnitude of the pedicle screws, the motion pattern of the screws suggested better augmentation with cement.

Motion of pedicle screws in situ had not been described in the literature. Previous work comparing pedicle screws fixation used the pull out test, while the current method applied physiological loads.

Funding: Funding from the Canadian Institutes for Health Research, Funding from Synthes

Spine Please contact author for diagrams and/or graphs.

Purpose: In-vitro mechanical tests are often used to pre-clinically assess the primary stability of hip endopros-theses. There is no standard protocol for these tests and the test conditions used vary greatly. This study examined the effect of the abductor muscle and the anterior-posterior component of the hip contact force (Fap) on the primary stability of cementless stems.

Methods: Cementless stems were implanted in 12 composite femurs which were divided into two groups: group 1 (N=6) was loaded with the hip contact force only, whereas group 2 (N=6) was additionally subjected to an abductor force. The cranial-caudal component of the hip contact force was the same in both groups, i.e. 2.3BW at 13° from the femur long axis. Each specimen was subjected to three Fap levels: 0, 0.3BW (walking), and 0.6BW (stair climbing). The implant translation relative to the femur was measured using a custom-built system comprised of 6 LVDT sensors. The resultant migration and micromotion were analyzed using an ANOVA with the abductor a between-group factor and Fap a within-group factor, followed by SNK post-hoc analysis with a significance level of 95%.

Results: Implant motion was not significantly affected when the Fap was increased from 0 to 0.3BW. However, without abductor, increasing Fap from 0.3 to 0.6BW increased migration and micromotion by an average of 291& #956;m (285% increase), and 15& #956;m (75%) respectively. With abductor, increasing Fap to 0.6BW increased migration by 87& #956;m (79%) but did not affect micromotion. The abductor did not significantly affect stem motion at lower Fap, but at Fap = 0.6BW motion was 50% lower compared with hip contact forces only.

Conclusions: Based on these results, inclusion of either abductor and/or Fap has little effect on implant motion when simulating walking. However, stair climbing (higher Fap) generates greater bone-implant motion compared to walking loads, and this effect is greatest in the absence of an abductor force. Funding: Other Education Grant Funding Parties: The Michael Smith Foundation for Health Research

Testing of cadaver ankle specimens was conducted to investigate the changes in kinematics with lateral ligament reconstructions. Testing included an intact condition, after injury at the ATFL and CFL sites, and separately a Brostrom repair and an anatomical gracil-lis graft reconstruction. Calcaneal range of motion was determined about the axis of applied moment in plantarflexion-dorsiflexion and in inversion-eversion directions. The injury and reconstructions were most sensitive during IE applied moment. Both reconstructions appeared to behave similar to intact motion. Failure of some Brostrom repairs however, suggest that the gracillis-graft reconstruction is initially a stronger repair.

Limited research has biomechanically investigated lateral ankle ligament reconstruction procedures. The objective of this study was to determine the changes in ankle kinematics with a dual ligament Brostrom repair and an anatomical gracillis graft reconstruction.

Seven cadaveric ankle specimens were tested independently in an intact condition, after an ATFL/CFL injury model, and two reconstructions. The anatomical graft reconstruction wove a gracillis tendon through the calcaneus and fibula to dually reconstruct the ATFL and CFL, and anchored to the talus. Moments were applied to the calcaneus for three cycles in plantarflexion-dorsiflexion (PD) and inversion-eversion (IE) while allowing unconstrained motion. Three dimensional motions of the calcaneus and tibia were optoelectronically tracked. Range of motion (ROM) was calculated about the axis of applied moment for the calcaneus with respect to the tibia.

The ROM increase from the intact condition with the injury model was only significant for IE (p=0.001). No significant differences were found between intact and any treatments in the PD configuration. In IE, both the graft reconstruction and the Brostrom repair were significantly different from the injury model (p=0.002 and p=0.015 respectively), where the gracillis reconstruction appears more similar to the intact condition. For two specimens the Brostrom repaired ATFL failed during applied inversion moment.

The injury and reconstructions were most sensitive during IE applied moment. Both reconstructions appeared to behave similar to the intact condition. Failure of some Brostrom repairs however, suggest that the gracillis-graft reconstruction is initially a stronger repair.

Funding: Workers Compensation Board of British Columbia

The purpose of this study was to investigate the strength profile of the thoracolumbar endplate. Indentation testing was performed on the T9, T12, and L2 endplates of six fresh-frozen human cadaver vertebrae. Indentations were performed in a standardized rectangular grid pattern of seven columns and five rows. There was an incremental increase in the strength of each row moving anterior and posterior from the central row. The relative strength of the anterior regions of the endplate increased with rostral ascent into the thoracic spine.

The purpose of this study was to map the strength profile of the thoracolumbar endplates using indentation testing.

Indentation testing was performed on the T9, T12, and L2 endplates of six fresh-frozen human cadaver spines using a materials testing machine (Dynamight, Instrom Corporation, Canton, MA). A minimum of twenty-five indentations was performed in a rectangular grid (seven columns by five rows). A 3mm hemispherical indendor was lowered at 0.2mm/s to a depth of 3mm producing endplate failure.

The failure load significantly varied with the AP and LAT positions (p< .0001). Each row was significantly stronger than the rows anterior to it (p < 0.04), except for the most row. The most lateral columns were stronger than the central (range: p = .04 – .0002). The mean strength of the L2 posterior row was greater than that for the thoracic endplates (p< .01), while no difference existed between levels within the two anterior rows. The ratio of the mean strength for the posterior row compared to that of the anterior row was significantly different across level (P< 0.036). The ratios for L2, T12, and T9 were 1.35, 0.97, and 0.91 respectively.

The periphery of the thoracolumbar endplate is stronger than the centre. The interaction identified between position and level suggests a relative strength increase in the anterior aspect of the endplate with rostral ascent into the thoracic spine.

This knowledge may assist in preventing intervertebral inplant subsidence by influencing implant positioning and design.

Funding: Canadian Institutes of Health Research Please contact author for figures and/or graphs.

Purpose: The objectives of this study were to determine the effect of posterior instrumentation extension and/or cement augmentation on immediate stabilization of the instrumented level and biomechanical changes adjacent to the spinal instrumentation.

Methods: This study was designed for repeated measures comparison, using 12 T9-L3 human cadaveric segments, to test the effects of posterior rod extension and cement augmentation following T11 corpectomy. The spine was stabilized with a vertebral body replacement device and with posterior instrumentation from T10 to T12. The T12 pedicle tracts were over-drilled to simulate loosened screws in an osteoporotic spine. The T10 screws were not over-drilled but cemented so as to keep the superior segments constant. Flexibility tests were first carried out on the intact specimen, followed by 3 randomized surgical conditions without cement and lastly the 3 conditions after cement augmentation. The 3 conditions were: 1) no posterior extension rods to L1, 2) flexible extension rods, and 3) rigid extension rods. A combined testing/analysis protocol that used both the traditional flexibility method and a hybrid technique [Panjabi 2005] was adopted. Flexibility tests with +/−5 Nm pure moments in flexion-extension, axial rotation and lateral bending were carried out and vertebral bodies’ motion in 3-D were collected. Two-way repeated measures ANOVA analyses were carried out on ROM between cement augmentation (factor 1) and the posterior rod extension (factor 2) on each flexibility test direction. An alpha of 0.05 was chosen. Newman-Keuls post-hoc analyses were carried out to compare between surgical techniques.

Results: Using the flexibility protocol, a reduction in ROMs at the destabilized level was observed with cement augmentation of screws or extension with rigid or flexible posterior rods to adjacent distal level. With the hybrid protocol, ROMs at adjacent level (T12-L1) were reduced with rod extension, but not with cement.

Conclusions: The results of this study suggest that cement augmentation would enhance stabilization, but create possible adjacent level effects due to increased motion and strain, while additional flexible extension rods would reduce biomechanical changes at the level of extension. Funding: 2 Funding Parties: CIHR

Spinal cord damage was compared after an injury was inflicted by three clinically relevant mechanisms (contusion, dislocation, and distraction). A novel SCI multi-mechanism system has been developed. Central hemorrhage was common to all mechanisms. Increased membrane permeability was localized to the injury epicenter in contusion but spread further in distraction. Dislocation showed intermediate characteristics exhibiting both local neuronal losses at the epicenter and extended regions of membrane permeability. These preliminary observations suggest that distinct injury mechanisms result in differences in the primary damage of the spinal cord.

This work compared primary damage after spinal cord injury (SCI) inflicted by three clinically relevant mechanisms.

Different injury mechanisms result in regional differences in damage to the spinal cord.

Differences in acute damage may help guide targeted therapies following SCI.

At greater distances from the lesion, dextran was excluded from neuronal somata and in the white matter only distinct accumulation was seen at the Nodes of Ranvier. At the injury site, hemorrhage was common to all mechanisms although more diffuse in the distraction injuries. Increased membrane permeability was localized to the injury epicenter in contusion but spread further in distraction. Dislocation showed intermediate characteristics exhibiting both local neuronal losses at the epicenter and extended regions of permeability.

A novel SCI multi-mechanism system was developed which uses an electromagnetic actuator to permit the modeling of injuries along any direction. Dextran was infused into the cisterna magna 1.5 to 2 hours prior to injury in order to visualize increases in membrane permeability. Stereotaxic clamps were designed to rigidly hold the lower cervical vertebrae of deeply anaesthetized rats permitting displacements at speeds of 100cm/s. A range of displacements was used in this pilot series: 0.9 to 1.1mm contusion, 2 to 6mm dislocation and 3 to 8mm axial distraction. Animals were sacrificed at five minutes in order to analyse the primary injury. These preliminary observations suggest that distinct injury mechanisms result in regional differences in the primary damage of spinal cord gray and white matter.

A biomechanical study assessing compressive failure load, strength and stiffness with three different interbody device shapes was performed in human cadaveric vertebrae. The custom-made interbody devices had similar cross-sectional areas and specimens were tested with 20% or 40% coverage of indentor to endplate area. Axial compressive load was applied at 0.2mm/s to a depth equivalent to 20% of the vertebral height. The clover-leaf shaped device resulted in significantly higher failure load, strength and stiffness over the elliptical and the kidney shaped devices for both areas of coverage. The clover-leaf shaped devices extended over stronger periphery regions of the endplates and resulted in stronger interface properties.

To determine if two novel interbody cage shapes, the kidney and the clover-leaf, are biomechanically superior to a standard elliptical shape of similar cross sectional area.

Uniaxial compression tests with unrestricted rotations were carried out on the superior endplates of forty-eight thoracolumbar (T9-L2) vertebrae with one of three shaped indentors covering 20% or 40% of the endplate area. Compressive load was applied using a servohydraulic testing machine at 0.2mm/s, to depth equivalent to 20% of the vertebral height. Failure load, strength and stiffness were compared.

The clover-leaf shaped indentors resulted in higher failure load (53% average increase), higher strength (67% average increase) and higher construct stiffness (43% average increase), and these results were significant (p< 0.05). Larger indentor coverage area of 40% also resulted in significantly higher failure loads over 20% coverage (75% average increase).

Current elliptical interbody devices are placed over the central region of the endplate, which is also the weakest. A clover-leaf shaped device extended over the stronger peripheral regions of the endplates and resulted in improved bone-implant interface properties. This implant if implemented in vivo could potentially reduce implant subsidence and lead to better long-term outcomes in osteoporotic patients.

The novel clover-leaf shaped indentor displayed superior bone-implant interface properties. Larger interbody devices should be used when possible to improve interface properties.

Implant subsidence in osteoporotic patients could be significantly reduced with a clover-leaf shaped device, leading to better long-term outcomes.

Funding: Funding from the Canadian Institutes for Health Research.

This study explored the relationship between the initial stability of the femoral component and penetration of cement into the graft bed following impaction allografting.

Impaction allografting was carried out in human cadaveric femurs. In one group the cement was pressurised conventionally but in the other it was not pressurised. Migration and micromotion of the implant were measured under simulated walking loads. The specimens were then cross-sectioned and penetration of the cement measured.

Around the distal half of the implant we found approximately 70% and 40% of contact of the cement with the endosteum in the pressure and no-pressure groups, respectively. The distal migration/micromotion, and valgus/varus migration were significantly higher in the no-pressure group than in that subjected to pressure. These motion components correlated negatively with the mean area of cement and its contact with the endosteum.

The presence of cement at the endosteum appears to play an important role in the initial stability of the implant following impaction allografting.

Aims: To determine whether certain motion parameters could be linked to clinical signs and symptoms of instability in a group of chronic LBP patients. Methods: Thirty-four patients enrolled for an external þxation (ESF) test performed active ßexion-extension, axial rotation, and lateral bending motions, during which the relative motion between marker carriers attached to the Schanz screws was measured with an optoelectronic camera. The rotations of the vertebrae were analysed with special reference to ranges of motion, motion asymmetries, and coupled motions. Studentñs t-test was used to determine whether these parameters were signiþ cantly different between the patient groups that did and did not receive pain relief from the stabilization of the suspected painful segment/s. Results: The improvement of the patientñs functional status during the external þxation as well as after subsequent lumbar fusion surgery was signiþcantly correlated with the extension ROM (p=0.049 and p=0.036), and the ratio of extension to ßexion ROM (p=0.035 and p=0.044) at the index levels before surgery. No signiþcant correlations with the other motion pattern parameters were observed. Conclusions: In case of a positive ESF test, preserved motion at the symptomatic level/s before the surgery seemed to predict a favorable fusion outcome. On the other hand, abnormal patterns of asymmetry and coupled motion did not seem to be associated with pain relief after stabilization of the suspected painful segment/s.

In clinical studies of cemented total hip arthroplasty (THA), polished stems produce less slippage at the bone-cement interface than roughened stems. Our objective is to assess the effect of stem-cement debonding on the bone-cement interface shear behaviour of hip implants using simplified axisymmetric stem-cement-aluminum models.

We emulated the femoral stems using stainless steel tapered plugs with either a rough (i.e. bonded) or smooth (i.e. unbonded) surface finish. Three different taper angles (5°, 7.5°, 10°) were used for the unbonded constructs. Non-tapered and tapered (7.5°) aluminum shells were used to emulate the diaphyseal and metaphyseal segments of the femur. In all cases, the cement-aluminum interface was designed to have the same shear strength as has been reported for bone-cement interfaces (~8 MPa). The test involved applying axial compressive loading at a rate of 0.02 mm/s until failure. Six specimens were tested for each combination of the parameters.

The unbonded stems sustained about twice as much load as the bonded stem, regardless of taper angle, and the metaphyseal model carried 35-50% greater loads than the diaphyseal models before shear failure or slippage. The unbonded constructs reached peak load with excessive displacement due to creep of the cement mantle while the bonded constructs failed in shear at the cement-aluminum interface. This result supports the hypothesis that the wedging forces created in the unbonded construct increase the compression forces across the aluminum-cement interface, thereby increasing its shear resistance. A finite element analysis predicted that the cement could withstand the hoop stress under these loading circumstances and this prediction was confirmed by visual inspection of the cement after each test.

Our results suggest that smooth or unbonded stems should sustain less slippage and shear damage at the bone-cement interface than roughened or bonded stems due to the wedge-induced compressive stress; this increased load capacity will be particularly valuable when the condition of the bone-cement interface is suboptimal.