The biomechanical effect of radial shortening on the radiocapitellar articulation.

Repeated trauma to the radial head may be one of the causative factors in the genesis of osteochondritis dissecans of the capitellum. We measured the force, contact area and pressure across the radiocapitellar articulation of the elbow before and after radial shortening osteotomy in five fresh-frozen cadaver upper limbs with loads of 45, 90 and 135 N, respectively. Measurements were made on pressure-sensitive film placed in the radiocapitellar articulation with the forearm in the supinated, neutral and pronated positions before and after radial shortening. Radial shortening significantly reduced the mean force and contact area across the radiocapitellar articulation in all positions of the forearm.

Effect of controlled release of platelet-derived growth factor from a porous hydroxyapatite implant on bone ingrowth.

Platelet-derived growth factor (PDGF) is one of several osteogenic factors which affect bone growth and fracture healing. This study examined the potential of hydroxyapatite (HA) rods with interconnected pores of mean diameter 200 microns to be used as a matrix for the release of PDGF to enhance bone ingrowth into the implant. In the initial phase of the study the sustained release of PDGF from the HA rods was characterized in vitro for two different PDGF loadings, 10 and 100 micrograms per implant. The second phase of the study examined bone ingrowth in HA implants placed into the medullary canals of rabbit femora. The specimens were dumb-bell shaped, with a reduced central diameter so that bone growth across a gap could also be determined. Bone ingrowth into HA implants was compared with growth into HA implants loaded with 100 micrograms of PDGF. Pushout measurements were made of average shear strength across the bone-implant interface and backscatter scanning electron microscopy of thick sections was used to quantify the amount of bone ingrowth into the implant. Although greater interfacial shear strength and area of ingrowth were observed, especially across gap sites, in specimens loaded with PDGF, no difference was statistically significant.

Mechanical and bone ingrowth properties of a polymer-coated, porous, synthetic, coralline hydroxyapatite bone-graft material.

CHAG, that is, porous hydroxyapatite hydrothermally converted from the calcium carbonate exoskeleton of a coral (genus Goniopora), has been shown to be effective as a scaffold for bone ingrowth. The large pores in the material, however, resulted in low compressive strengths. Compressive testing was performed to assess the changes in mechanical properties by coating the internal surfaces of CHAG with DL-PLA. Plugs of CHAG with thick (3:1 chloroform to DL-PLA by weight), medium (10:1), and thin (30:1) coatings as well as uncoated CHAG were then implanted transcortically in the proximal third of the diaphysis of rabbit tibiae to assess the in vivo response. The mechanical tests demonstrated significantly improved compressive strength, stiffness, and energy absorption for coated specimens compared with uncoated specimens. Coated specimens were not significantly different from canine tibial cancellous bone in strength and stiffness although they achieved only 36% of the energy absorption capacity. Specimens from rabbit tibiae were harvested at 3, 12, and 24 weeks for interface shear strength determination and contralaterally for histological and histomorphometric assessment. At 12 weeks, uncoated CHAG plugs developed an average ultimate interface shear stress of 26.7 MPa compared with 17 MPa for specimens with 30:1 coatings and 8 MPa for specimens with 10:1 and 3:1 coatings. At 24 weeks, there were no significant differences in shear stress between any of the specimens. Histomorphometric assessments showed that the ratio of area fraction of new bone to area fraction of new bone and void space increased from 68-70% for specimens with 3:1 and 10:1 coatings at 3 weeks to 85.5-89.5% at 24 weeks. In comparison, uncoated and 30:1 specimens had area fraction ratios of about 82% at 3 weeks and 93% at 24 weeks. Histologic sections demonstrated direct apposition of new bone to both the coating and the hydroxyapatite as well as degradation of the coating.

Bone ingrowth into polymer coated porous synthetic coralline hydroxyapatite.

Porous hydroxyapatite, converted hydrothermally from the calcium carbon exoskeleton of the coral genus Goniopora (CHAG), has been shown to be effective as a scaffold for bone ingrowth (2,3,5-7,9). However, the large pores in the material resulted in low compressive strengths. In a previous study, we found that microcoating the internal surfaces of CHAG with dilactic-polyactic acid (DL-PLA) improved significantly its compressive properties. The objective of this study was to determine the effect of PLA microcoating on bone ingrowth into CHAG plugs. Plugs of thick- (3:1 chloroform to DL-PLA by weight), medium- (10:1), and thin- (30:1) coated as well as uncoated CHAG were implanted transcortically in the proximal third of the diaphysis of the rabbit tibia. Specimens were harvested at 3, 12, and 24 weeks for mechanical testing and contralaterally for histological and histomorphometric assessment. At 12 weeks, uncoated CHAG plugs developed an average ultimate interface shear stress of 26.7 MPa, compared with 17 MPa for 30:1 and 8 MPa for 10:1 and 3:1 coated specimens. At 24 weeks, there were no significant differences in shear stress among any of the specimens. Histomorphometric assessments showed that the ratio of area fractions of new bone to area fractions of new bone and void space increased from 68-70% for 3:1 and 10:1 coated specimens at 3 weeks, and to 85.5-89.5% at 24 weeks. In comparison, uncoated and 30:1 specimens had area fraction ratios of about 82% at 3 weeks and 93% at 24 weeks. Histologic sections demonstrated direct apposition of new bone to both the coating and the hydroxyapatite as well as degradation of the coating.

A biomechanical comparison of various methods of stabilization of subtrochanteric fractures of the femur.

Subtrochanteric femoral fractures with and without bony contact were simulated in cadaver specimens, fixed with one of seven different types of intramedullary or plate implants, and tested biomechanically. The implants used were Enders pins, Zickel nail, compression hip screw, AO angled blade plate, and intramedullary locked nail systems of the Klemm-Schellman, Brooker-Wills, and Grosse-Kempf types. Femur-implant constructs using intramedullary devices were a maximum of 5% as stiff in torsion as intact cadaveric femora tested in the same manner, while plate-fixed fractures were nearly 50% as stiff. In bending, all devices except the Enders pins were approximately 80% as stiff as intact femora. Loss of bony contact at the fracture site had little effect on stiffness except in the case of the keyless compression hip screw, where the screw rotated freely in the barrel. In combined bending and compression to failure, a test to simulate forces due to body weight, the intramedullary locked rods were found to support between 300 and 400% of body weight while the plate systems failed at loads between 100 and 200% of body weight.

Effect of controlled local release of sodium fluoride on trabecular bone.

Systemic sodium fluoride has been used in the treatment of osteoporosis. Recent studies have shown that it has a positive risk/benefit ratio for use in increasing spinal trabecular bone density. However, thinning of the cortices of the long bones with a resulting increase in fracture incidence has been observed. This study was designed to determine the response of bone to sodium fluoride released from a biodegradable polymer matrix, a technique which could potentially deliver it locally to a site of need in the skeleton which has a positive response to fluoride. In one group of mature New Zealand white rabbits, cylindrical poly(D,L-lactic acid) (PLA) implants, with or without impregnated sodium fluoride, were implanted into the contralateral femoral trochanters and tibial metaphyses. In a second group, similar implants were placed in adjacent vertebrae. Four weeks postimplantation, the femora, tibiae, and vertebrae were removed, sectioned, cleaned of all but mineralized tissue, and the surfaces of the sections stained. The stained surfaces were imaged and analyzed for morphometric properties of the trabeculae. Comparing contralateral vertebrae, those exposed to sodium fluoride had significantly thickened trabeculae, with decreased spacing between them and a greater bone fraction. A similar increase in trabecular width was found in the subchondral bone of the proximal tibiae exposed to local release fluoride. Femoral sections showed no difference, possibly due to the lack of extensive trabecular bone in the region chosen for study.

Effect of controlled local release of sodium fluoride on bone formation: filling a defect in the proximal femoral cortex.

To assess the effect of sodium fluoride (NaF) in the healing of a defect in cortical bone, an experimental model was created by the drilling of 5.0 mm holes in the proximal ends of both femora of 12 adult male New Zealand White rabbits. An interlocking intramedullary implant constructed of poly(d,l-lactic acid) containing NaF was placed in the right femur and an identical implant without NaF (sham), in the left. The implant in the right femur was designed to release NaF in a controlled manner over the duration of the experiment. Ten weeks after implantation, the specimens were removed and were tested in torsion. The mechanical properties were not significantly different between the groups. The femora exposed to NaF had an 18.6% increase in intact cortex near the defect (p = 0.023), however, the deposition of mineralized bone within the defect was not significantly greater. In fact, healing appeared to be impaired by the presence of NaF. There was complete closure of the defect in all but one of the femora with a sham implant, but the tissue had not yet calcified. In contrast, only one femoral defect exposed to NaF had closed. Examination of the material filling the defects of the femora exposed to NaF showed that it was predominantly uncalcified osteogenic mesenchymal tissue.

A comparison of various types of fixation devices for protection of the posterior cruciate ligament against a posterior drawer force.

We compared four different fixation devices to determine which offers the greatest relative protection to the posterior cruciate ligament (PCL) against elongation with an applied posterior drawer force. We designed a system to monitor and record the change in distance between an approximate origin and insertion of the PCL as known forces were applied posteriorly to the tibia with the femur fixed. Six fresh cadaver knees were used in this investigative series. They were first tested with the PCL intact, then with the PCL transected, and then with the various fixation devices applied. After comparing the results with transarticular pins and quadrilateral and anterior half-frame external fixators, we found that transarticular fixation offers significantly greater protection to the PCL. This study may prove to be helpful to the orthopaedist when selecting ancillary fixation for posterior cruciate repair or reconstruction.

Development of a variable stiffness external fixation system for stabilization of segmental defects of the tibia.

The mechanical properties of a variable stiffness external fixation system were explored. Initial testing of a unilateral fixator configuration demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone. A triangulated system composed of half pin frames mounted anteriorly and medially on the tibial aspects and linked by crossbars was devised. Progressive disassembly of the frame was shown to result in progressive decreases in fixator rigidity in all planes.

Comparison of residual stability in thoracolumbar spine fractures using neutral zone measurements.

Because treatment algorithms for spinal injuries depend largely on the clinical assessment of stability after injury, this study both quantified and compared the mechanical stability after three different patterns of injury in the thoracolumbar spine. We created compression fractures, burst fractures, and flexion-distraction injuries in 26 thoracolumbar specimens from human cadavers in order to compare residual stability as a function of type of injury. Spinal stability was evaluated using measurements of the boundaries of the neutral zone, which provide a measure of spinal laxity in various directions of motion. An increase after injury was indicative of greater spinal laxity and hence reduced residual stability. Geometric characteristics (or parameters) of the neutral zone boundaries were used for statistical comparison between the types of injury. Of the three groups, burst fractures retained the least residual stability and compression fractures, the greatest. The angular ranges of motion in the neutral zone for burst fractures demonstrated increases (compared with average values for intact specimens) of 154% in flexion-extension, 134% in lateral bending, and 108% in torsion after injury. The results for flexion-distraction injuries were similar to those for burst fractures in flexion-extension (126%) and torsion (62%); however, more residual stability was retained in lateral bending than was seen for burst fractures (48%). Compression fractures retained the most residual stability, with increases in motion of 40% in flexion-extension, 56% in lateral bending, and 3% in torsion. These findings may be useful in determining the necessity for surgical stabilization of the spine and selection of the appropriate system of fixation.

Contact characteristics of the subtalar joint: load distribution between the anterior and posterior facets.

The pressure distribution properties of the normal talocalcaneal joint were studied in 13 fresh cadaver specimens using pressure-sensitive film. The film was inserted into the joint through a posterior approach for the posterior facet and an anteromedial approach for the anterior/middle facet. Specimens, comprising the distal half of the tibia and fibula and the intact ankle and foot, were positioned in neutral in the sagittal plane. In the coronal plane, specimens were positioned in neutral, inversion, or eversion, and the contact characteristics were determined in varying positions, with and without loading the fibula, under axial loads of 350 N, 700 N, 1,050 N, and 1,400 N. The transducers were video imaged for quantitative analysis of area and pressure. The contact/joint area ratio increased significantly with applied load in the posterior facet [e.g., in eversion from 0.336 (SD = 0.195) at 350 N to 0.631 (SD = 0.225) at 700 N], as did the proportion of the contact area greater than 6 MPa, indicating an increase in contact pressure. The contact/joint area ratio was significantly lower in inversion than in any other position of the foot; however, high-pressure zones were similar, indicating that higher pressures occur in inversion. In the anterior/middle facets both contact/joint area ratio and high pressure/contact area ratios increased significantly to 700 N, but not with further load increase. At 350 N the anterior/middle facet contact area was 31% that of the posterior facet, yet it carried 63.3% of the load of the posterior facet, so its mean contact pressure was 1.44 MPa compared with 0.93 MPa for the posterior facet.(ABSTRACT TRUNCATED AT 250 WORDS)

Contact characteristics of the subtalar joint: the effect of talar neck misalignment.

In this study we determined the effects of misalignment of the talar neck on the contact characteristics of the subtalar joint. Each of seven fresh cadaver lower extremities was mounted in a loading jig and a vertical load was applied, 90% of which was directed through the tibia and 10% through the fibula. The foot was allowed to displace freely in the horizontal plane so that relative rotations, known to occur in the subtalar joint, would not be prevented. Pressure-sensitive film, inserted into the posterior and anterior/middle articulations, was used to quantify changes in contact characteristics. After testing in the normal condition, the talar neck was osteotomized and stabilized with internal and external skeletal fixation. Contact characteristics were then determined in each of the following stages: anatomic realignment, or with 2-mm displacement of the talar neck either dorsally, medially, laterally, or complex (dorsal and varus) with respect to the body of the talus. Measurements showed no significant changes in overall contact area or high pressure area in the posterior facet, although four of the seven specimens demonstrated increased localization of the contact area into two discrete regions. The combined anterior/middle facet, on the other hand, was significantly unloaded by all but medial displacement of the talar neck. An extraarticular load path and/or increased loading directly on the talonavicular joint was presumed to account for the loss of load transfer in the talocalcaneal joint.

Restoration of function of the thumb flexor apparatus requires repair of the oblique and one adjacent flexor tendon pulley.

Damage to the pulleys of the thumb flexor apparatus may cause bow-stringing of the tendon and affect muscle function. An experiment using the hands and distal forearms of cadavers was designed to determine which damaged pulleys increase excursion length of the flexor tendon with constant tendon and resisting loads. Each specimen was mounted to a loading frame with a dead weight pinned to the tip of the thumb. The thumb flexor tendon was clamped to an actuator that applied a fixed load and measured excursion of the tendon. Ranges of motion of the thumb joint were also measured. The thumb flexor apparatus of each specimen was tested intact first, with the hand in flexed, neutral, and extended positions; then it was tested with progressive sectioning of pulleys from proximal to distal in one group and from distal to proximal in a second group. The length of excursion increased significantly with all pulleys cut but there was no effect on overall range of motion of the thumb. With proximal to distal sectioning, no change in tendon excursion occurred when the flexor retinaculum and the first annular pulleys were cut, until the oblique pulley was sectioned, leaving only the second annular pulley intact (range, 1.17 - 1.31 times that of intact excursion, dependent on position of the hand). With distal to proximal sectioning, tendon excursion was not affected when the second annular and oblique pulleys were cut but did increase when the first annular pulley was sectioned, leaving only the flexor retinaculum intact (range, 1.28 - 1.36 times that of intact excursion). Dependent on the location of damage, therefore, an intact oblique or first annular pulley can maintain normal excursion of the tendon.

Response of cortical bone to local controlled release of sodium fluoride: the effect of implant insertion site.

In a previous experiment, sodium fluoride in a biodegradable polymer matrix was introduced into the femoral canal of the rabbit and bone formation was compared with contralateral controls. We noted significant bone formation, but only in the distal third of the periosteal surface of the femur. This experiment was performed to distinguish fluoride-induced periosteal bone formation from that due to the reactive osteogenic changes associated with local injury caused by the process of implantation. A proximal approach on the right leg and a distal approach on the left were used for the insertion of the implants in rabbits. Femurs were removed after 30 days and tested for stiffness and load to failure. The cross-sectional area of mineralized bone was determined at proximal, midshaft, and distal locations. Fluorescent bone tissue growth labels were injected at weekly intervals to measure the rate of new periosteal bone formation. The results were compared with a control group that received sham implants. Results showed no difference between measured properties in right and left femurs in the control group or in those exposed to fluoride. A significant increase was found in the fluoride group in load to failure, along with cross-sectional area of mineralized bone, and periosteal growth rates compared with the control group, but no difference was seen in stiffness. No difference was detected between the response proximally and distally in the fluoride group regardless of the location of insertion. There were no detectable changes in serum fluoride level after implantation of the poly L-lactic acid/sodium fluoride matrix. These results show that fluoride exerts its osteogenic effects equally at proximal, midshaft, and distal regions of diaphyseal bone and is uninfluenced by the site of local injury due to insertion of the implant.

Augmentation of femoral neck fracture fixation with an injectable calcium-phosphate bone mineral cement.

The first goal of this study was to determine if augmentation with an injectable, in situ setting, calcium-phosphate cement that is capable of being remodeled and was designed to mimic bone mineral significantly improved the strength and stiffness of fixation in a cadaveric femoral neck fracture model. The second goal was to determine if greater increases in fixation strength were achieved as the bone density of the specimen decreased. Sixteen pairs of fresh cadaveric human femora with a mean age of 70.9 years (SD = 17.2 years) were utilized. The bone density of the femoral neck was measured with dual-energy x-ray absorptiometry. The femoral head was impacted vertically with the femoral shaft fixed in 12 degrees of adduction using a materials testing machine to create a fully displaced fracture. Following fracture, 30% inferior comminution was created in each specimen. One randomly chosen femur from each pair underwent anatomic reduction and fixation with three cannulated cancellous bone screws, 7 mm in diameter, in an inverted triangle configuration. The contralateral femur underwent the same fixation augmented with calcium-phosphate cement. Specimens were preconditioned followed by 1.000 cycles to one body weight (611.6 N) at 0.5 Hz to simulate single-limb stance loading. The stiffness in the first cycle was observed to be significantly greater in cement-augmented specimens compared with unaugmented controls (p < 0.05). After cycling, each specimen was loaded at 10 mm/min until complete displacement of the fracture surface and failure of fixation occurred. Specimens augmented with bone mineral cement failed at a mean of 4,573 N (SD = 1,243 N); this was significantly greater (p < 0.01) than the mean for controls (3,092 N, SD = 1,258 N). The relative improvement in fixation strength (augmented/control x 100%) was not inversely correlated to femoral neck bone density (p = 0.25, R2 = 0.09), was weakly correlated to the volume of cement injected (p = 0.07, R2 = 0.22), and was inversely related to the fixation failure load of the control specimen (p = 0.001, R2 = 0.54). There was a mean relative improvement in fixation strength of 169.6% (SD = 77.5). These findings suggest that calcium-phosphate cement provides initial beneficial augmentation to fixation of femoral neck fractures.

Pressure distribution in the wrist joint.

We performed a study to determine pressure distribution properties of the normal radio-carpal joint. A system was developed for measurement of the contact pressure within the wrist joint surfaces. The transducer was based on Fuji pressure-sensitive paper, which was inserted into the joint space through a dorsal capsular incision. The hand was then positioned using a jig that permitted free axial loading of the joint. Each of five specimens was tested in 36 positions combining flexion/extension with radio/ulnar deviation and supination/pronation. The transducers were analyzed for contact area, scapho-lunate contact area ratio, pressure, and centroid locations using a microcomputer-based video-imaging system. The scaphoid and lunate contact areas on the radius and triangular fibrocartilage were separate and distinct in all wrist positions. Together these contact areas accounted for a relatively small fraction of the total joint surface area (average contact area/total joint area = 0.206, SD = 0.0495). For an applied 103 Newton compressive load, the high pressure averaged 3.17 MPa (SD = 0.83 MPa). Overall, the scaphoid contact area was 1.47 times that of the lunate, although variations occurred with position, as in flexion, in which the scaphoid/lunate area ratio was 0.83. The high-pressure centroids of both scaphoid and lunate contact areas shifted palmarly from 20 degrees of flexion to 20 degrees of extension and then dorsally with further extension. The scaphoid-lunate intercentroid distance averaged 14.91 mm with a range of 10-20 mm.

The response of human volunteers to rear-end impacts: the effect of head restraint properties.

STUDY DESIGN: Human volunteers were subjected to a rear-end impact while sitting on a standard automobile seat, and sagittal plane kinematic responses were quantified. The effect of changing head restraint properties was determined by use of a repeated measures design. OBJECTIVE: To determine the forces acting, and relative motions resulting, on volunteers in a rear-end impact and the effect of head restraint properties. SUMMARY OF BACKGROUND DATA: In several recent studies of the kinematics of the cervical spine during rear-end impact, a forward thrust to the lower cervical spine was produced, and a transient S shape of the spine resulted while the head remained upright during the initial phase of the impact. This may result in nonphysiologic intervertebral motions and tissue strains. METHODS: Nineteen automobile seats were first tested, and a modified head restraint was designed. Each volunteer sitting on a standard vehicle seat was subjected to an impact pulse of 3g with a 4-kph speed change. Testing was performed first with the modified head restraint, then again after replacement by the head restraint that came with the seat. Kinematic responses were compared for both head restraints by use of a repeated measures analysis of variance. RESULTS: There was a measurable time difference between peak chest and peak head accelerations, which resulted in the chest being thrust forward by the seat back before the head was thrust forward by the head restraint. The modified head restraint significantly reduced the contact time difference and therefore decreased the relative chest-to-head forward motion. CONCLUSIONS: Volunteers seated on a standard automobile seat demonstrated differential sagittal plane motion between the chest and head. It is possible to significantly decrease the relative chest-to-head motion by altering the characteristics of the head restraint.

Caudo-cephalad loading of pedicle screws: mechanisms of loosening and methods of augmentation.

The mechanism of failure and the effect of augmentation of single pedicle screws subjected to caudo-cephalad loads applied at the screw head were investigated. In each of 10 lumbar vertebrae, Steffee pedicle screws of appropriate diameter were placed in one pedicle. The cancellous bone of the contralateral pedicle was removed by curettage, a custom fabricated bushing was pressed by hand into the space, and a Steffee screw was inserted through the bushing and into the vertebral body. This arrangement was designed to transfer load directly from the pedicle screw to cortex, bypassing the cancellous bone of the pedicle. In a second experiment, a custom plate was mounted to the posterior surfaces of the articular facets allowing load transfer from the head of the screw directly to cortex. Caudo-cephalad loads were applied by a materials tester and toggle displacement (defined as the total caudo-cephalad movement of the screw under minimal load after loading through a complete cycle) was measured. Under peak loads of +199 N (caudal) and -224 N (cephalad), mean toggle at the screw head was 4.93 mm (standard deviation [SD] = 3.60 mm) for the screw alone and 4.96 mm (SD = 4.42 mm) for the screw augmented by the bushing. Screws without augmentation showed a characteristic pattern of loosening with the base of the pedicle acting as a fulcrum and a butterfly-shape void occurring in the vertebra and the pedicle where cancellous bone had been crushed. Problems with the bushing included poor fit, back-out, and split pedicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical properties of threaded inserts for lumbar interbody spinal fusion.

STUDY DESIGN: Calf and human cadaveric spines were used to determine motion segment stiffness and laxity after implantation of threaded inserts (the Ray Threaded Fusion Cage, Surgical Dynamics, Inc., Concord, CA), comparing direction of placement, number of implants, shape of the device, and integrity of anterior spine structures. Stiffness and laxity of spines with inserts were compared with those with bone grafts, with and without posterior fixation plates. OBJECTIVES: To determine the mechanical stabilizing properties of a threaded insert used for lumbar and lumbosacral fusion and the factors affecting stability. SUMMARY OF BACKGROUND DATA: Limited biomechanical information has shown that implantation of these devices adds stiffness to the lumbar spine, but little information is available concerning stiffness in loading directions other than flexion and extension, the effect on stiffness of position and number of implants, and the effect of this device on motion segment laxity. METHODS: Mechanical properties were determined by testing lumbar vertebral motion segments in flexion, extension, lateral bending, and torsion combined with axial compressive loading. Stiffness (slope of the load/deflection curve) and neutral zone angle or laxity (angular displacement of the vertebra from no load to 1.0 Nm moment) were determined. Initial tests were performed on calf lumbar vertebrae to determine the effects of placement and number of inserts. Comparisons of bone grafts and inserts with and without supplemental plates were made using human lumbar spines. Cylindrical- and conical-shaped inserts, when placed from anterior, were tested in calf spines. The load-bearing capacity of the insert supported in calf vertebral body bone was determined. RESULTS: There was no significant effect of placement of inserts in different orientations (lateral, posterolateral, or posterior) on stiffness, except in torsion where posterior placement damaged facets or lamina, reducing stiffness. Placement of two inserts from posterior decreased flexion and lateral bending laxity compared with the intact motion segment. Compared with intact, bone grafts produced more stiffness only in lateral bending and had no effect on laxity. Supplemental posterior plates fixed by pedicle screws across the fusion segment increased flexion and lateral bending stiffness and reduced laxity in flexion, extension, and lateral bending. Conical-shaped inserts placed from anterior into cylindrical holes distracted soft tissue structures, decreasing laxity. Cutting the anterior structures increased laxity by relieving some tissue tension caused by distraction. The mean maximum compressive load that could be supported by the insert was 2998 N (standard deviation = 980 N). Structural failure occurred in the supporting bone. CONCLUSIONS: Threaded inserts increase vertebral motion segment stiffness and decrease laxity by distracting intervertebral structures. They are not sensitive to placement, except if vertebral structures are injured during insertion and produce constructs with more consistent mechanical properties than bone grafts.

Biomechanical comparisons of spinal fracture models and the stabilizing effects of posterior instrumentations.

In this study, the authors evaluated the stiffness of motion segments in intact spines in two spine fracture models, and with each of five implant systems used for posterior fixation of thoracolumbar spine fractures. The devices represented a cross-section of types, including those employing sublaminar wires with and without laminar hooks, pedicle screws, plates, and rods. Two spine fracture models, one partially and one totally destabilized, were used in the tests of the instrumentation. Stiffness, or the magnitude of load needed to produce a unit displacement of the construct in the direction of the applied load, was measured in flexion, extension, lateral bending, and torsion in combination with a compressive force. Both horizontal plane shear and angular displacements were measured in the two fracture patterns. All evaluations were made by testing the difference in stiffness for statistical significance among groups. The results showed significant differences in stiffness without instrumentation among intact spines, partly destabilized spines (anterior two-thirds of disk and posterior ligaments removed), and totally destabilized spines (only anterior longitudinal ligament intact). The implant/spine constructs were least stiff relative to the intact spine in torsion, followed in increasing order of stiffness with flexion, lateral bending, and extension. In the Roy-Camille plate with six-screw fixation was found to produce the stiffest construct, followed by wired Harrington rods, C-rods and J-rods, and the Vermont internal fixator.(ABSTRACT TRUNCATED AT 250 WORDS)