Biomedical subjects

A F Tencer

Publications and source records attributed to A F Tencer.

At least 37 records · Page 2Linked to original sources

PubMed1997

Effects of cyclical mechanical stress on the controlled release of proteins from a biodegradable polymer implant.

The availability of osteogenic proteins for orthopedic applications has led to great interest in developing delivery systems for these substances. Standard release rate models are applicable in most biological settings, but orthopedic implants usually bear mechanical loads. To determine whether a release rate model for load bearing applications must consider mechanical stress, the effects of dynamic mechanical stress on the in vitro release kinetics of two model proteins, bovine albumin (BA) and trypsin inhibitor (TI), from a biodegradable film were evaluated. Biodegradable poly(lacticco-glycolic acid) cylindrical implants with embedded proteins were subjected to cyclic three point bending loading of 720 cycles/day at 0.4 Hz for 2 weeks. Protein release into solution, swelling and mass loss changes, molecular weight degradation, and the presence of microstructural stress cracks and pores in the polymer carrier were evaluated. Cumulative BA and TI releases with time were significantly higher when a cyclic bending load was applied and increased with the magnitude of the load. Mass loss was not significantly greater, nor was swelling or molecular weight change of the polymer carrier in this 2-week interval. Pores on the surface of the polymer in the highest stress region were elongated into cracks, compared with pores in the low-stress region of the same implant, which were roughly circular. This implies that the pores probably act as stress risers to initiate cracks, which then expose more surface area, increasing protein release.

Animals↗

PubMed1997

Stabilizing properties of the halo apparatus.

STUDY DESIGN: A cadaveric cervical spine specimen fixed between a fiberglass torso and a plastic skull was used as a model to determine the effect of halo structural parameters on motion at a lesion simulated at C5-C6. In a second part, nine commercially available halo devices were compared. OBJECTIVES: To define the contributions of the various components of the halo apparatus to reducing motion in an injured cervical spine and to compare the stability offered by a sample of commercially available halo devices. Controversy exists concerning the ability of the halo apparatus to stabilize the injured cervical spine. SUMMARY OF BACKGROUND DATA: The halo apparatus has been shown to be the most effective nonsurgical method for stabilizing the fractured spine. Nonetheless, several clinical studies have demonstrated that unacceptably large motions can occur at the injured spinal segment stabilized with a halo apparatus. METHODS: Each cadaveric cervical spine was mounted onto a fiberglass torso and a rigid plastic skull was attached to the base of the occiput. A posterior ligamentous lesion was created between C5 and C6. The halo ring was fitted to the skull and a vest to the torso. Loads were applied to the skull in flexion, extension, and lateral bending, and relative angulation between C5 and C6 was measured with electroinclinometers. In the first part, the effect of parameters such as vest tightness, vest-thorax friction, vest deformation, and connecting bar rigidity on spinal angulation were measured using one vest. In the second part, the stability offered by each of nine commercially available halo devices was compared. RESULTS: Increasing chest strap tightness and decreasing vest deformation reduced angulation at the spinal lesion. Once connecting bar joints were tightened to 25% of their recommended torque, increased tightening or adding additional bars had no effect on rigidity. Although specific vests permitted significantly greater motion in specific directions, no vest allowed greater angulation consistently in all loading planes. CONCLUSIONS: Increasing vest tightness, decreasing the deformability of the vest, and ensuring a good fit can reduce motion in the fractured spine. Most commercially available halo vests provide similar mechanical stability to the injured cervical spine.

Biomechanical Phenomena↗

PubMed1997

The unstable iliac fracture: a biomechanical evaluation of internal fixation.

Neither plating nor lag screw fixation of a displaced iliac wing fracture as part of an unstable pelvic ring disruption has been studied biomechanically. The purpose of this study was to compare the stability of various combinations of fixation, specifically contrasting lag screws placed between the tables of the ilium with plating in different locations across the fracture line. Various combinations of these fixation implants were evaluated for an unstable iliac fracture. A longitudinal iliac fracture was created in each of six hemi-pelvic specimens prior to testing. Compressive force, up to 500 N or to the magnitude necessary to displace the fracture 2 mm, was applied to the fracture line through the hemi-pelvis for each of the plate and lag screw combinations tested. There was no statistical difference between any of the implants or combinations tested. A single 3.5-mm reconstruction (cephalad) plate placed along the cephalad internal aspect of the iliac crest provided the least stability allowing 2 mm of displacement with a mean load of 80 N. The two combinations of fixation that required the greatest loads for 2 mm of displacement were a single 3.5-mm lag (cephalad) screw inserted into the iliac crest between the tables of the ilium coupled with either a 3.5-mm reconstruction (brim) plate placed along the internal aspect of the inferior iliac fossa at the pelvic brim (239 N) or a 4.5-mm lag (brim) screw between the inner and outer tables at the inferior aspect of the fracture just above the greater sciatic notch (225 N).

Aged↗

PubMed1997

The effects of distal radius fracture malalignment on forearm rotation: a cadaveric study.

Seven fresh cadaveric specimens were used to determine the loss of forearm rotation with varying distal radius fracture malalignment patterns. Uniplanar malunion patterns consisting of dorsal tilt, radioulnar translation, or radial shortening were simulated by creating an osteotomy at the distal end of the radius, orienting the distal fragment position using an external fixator, and maintaining the position with wedges and a T-plate. Rotation of the forearm was produced by fixing the elbow in a flexed position and applying a constant torque to the forearm using deadweights. Forearm rotation was measured with a protractor. Dorsal tilt to 30 degrees and radial translation to 10 mm led to no significant restriction in forearm pronation or supination ranges of motion. A 5-mm ulnar translation deformity resulted in a mean 23% loss of pronation range of motion. Radial shortening of 10 mm reduced forearm pronation by 47% and supination by 29%.

Biomechanical Phenomena↗

PubMed1997

The role of fibular fixation in combined fractures of the tibia and fibula: a biomechanical investigation.

OBJECTIVES: To determine whether adjunctive plating of the fibula with tibial fixation enhanced the stability of the construct under combined compressive and bending loads in simulated fractures of both the tibia and fibula. METHODS: Each of twelve fresh cadaveric specimens (six pairs) with an intact knee, lower extremity, and foot was mounted on the table of a materials testing machine. An intramedullary (IM) rod locked in the distal femur allowed combined compression, and flexion, valgus bending, or varus bending loads to be transmitted from the actuator of the testing machine to the knee. Three displacement measurement transducers were mounted on the tibia at anterior, posterolateral, and posteromedial positions. Intact tibial deformations under load were measured. Then, in one specimen of each pair a 2 cm osteotomy was created near the tibial midshaft, which was stabilized with an external fixator. Tibial gap displacements were measured under the following conditions: (a) intact fibula, (b) osteotomized fibula, (c) fibula fixed with a plate, (d) fibula fixed with an Enders IM nail. In the other specimen of the pair, tibial fixation was performed with an interlocked unreamed IM nail, with the same successive stages of fibular fixation. RESULTS: Osteotomy of the fibula significantly increased tibial defect motion when external fixation was used, and plating the fibula in this case significantly decreased motion. Using an Enders rod to stabilize the fibula instead of a plate, with tibial external fixation, produced smaller decreases in tibial defect site motion. With IM rod fixation of the tibia, osteotomizing the fibula had no effect on defect site motion or on its subsequent stabilization using a plate or IM rod. CONCLUSION: Plating the fibula can decrease motion across a tibial defect, but only when less rigid (i.e., external) fixation is used.

Biomechanical Phenomena↗

PubMed1997

Biomechanical comparison of internal fixation techniques for the treatment of unstable basicervical femoral neck fractures.

OBJECTIVES: The optimal method of internal fixation of basicervical femoral neck fractures is controversial. This area represents a transition zone between the intracapsular femoral neck fracture, usually fixed with multiple cancellous screws, and the extracapsular interochanteric fracture, fixed with a sliding screw device [sliding hip screw (SHS)] and derotation screw (DRS) device. The authors' specific aim was to biomechanically compare these two methods of fixation in a cadaveric model of a basicervical femoral neck fracture with posteromedial comminution. DESIGN: The authors compared the average peak force during cyclic loading and the maximum axial force sustained by matched pairs of specimens stabilized with either fixation and subjected to axial and torsional loading while flexing and extending the hip. The average peak force was defined as the mean of the peak force values measured in each loading cycle with the maximum displacement of the materials tester actuator the same for each cycle (displacement control) as opposed to the maximum force being held constant (load control). RESULTS: The cancellous screw group maintained a significantly lower average peak force, 470 Newtons (SD = 145 Newtons), compared with 868 Newtons (SD = 186 Newtons) for the SHS and DRS composite group (p < 0.01). Similarly, the cancellous screw group demonstrated a lower ultimate load to failure, 1,863 Newtons (SD = 475 Newtons) compared with 3,557 Newtons (SD = 215 Newtons) for the SHS and DRS composite group (p < 0.01). CONCLUSION: The results support the use of an SHS and DHS composite compared with three cancellous screws in the treatment of unstable basicervical femoral neck fractures.

Aged↗

PubMed1996

Controlled release of antibiotics from coated orthopedic implants.

Chronic osteomyelitis is one of the most serious complications of orthopedic open fracture treatment. The objective of this study was to develop a biodegradable implant coating with impregnated antibiotics as an adjunct to current therapy. We used a polylactic-co-glycolic acid copolymer (PLGA) as the biodegradable carrier and gentamicin as the antibiotic. Our objectives were to establish elution characteristics of the antibiotic from the polymer, and determine if the coated orthopedic implants would inhibit bacterial growth in vitro. In the elution study, coated implants were incubated in phosphate buffered saline (PBS) at 37 degrees C and sampled daily for gentamicin levels. The in vitro model consisted of test tubes containing Mueller-Hinton culture broth inoculated with 5 x 10(6) cfu of Staphylococcus aureus and incubated at 37 degrees C. The implants were switched to a new set of inoculated tubes each day. Tubes were sampled for colony counting to determine bactericidal effects. Implant coatings consisted of 40 mg of gentamicin as a 20% mixture with PLGA. The elution curve showed an average level of 138 micrograms/mL over 15 days. This local concentration would be more than adequate to kill susceptible organisms. The in vitro study showed a significant reduction in bacterial growth in the test tubes containing coated implants. Control tubes averaged 2.5 x 10(8) cfu/mL of S.aureus over 24 days. Coated implant tubes averaged 0.9 cfu/mL. This was a reduction of greater than 99.999% (p < 0.0001). This study showed that a thin biodegradable implant coating can be developed with bactericidal activity against the organisms frequently associated with osteomyelitis in cases of open fractures.

Anti-Bacterial Agents↗

PubMed1996

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.

Adult↗

PubMed1996

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.

Adult↗

PubMed1996

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.

Albumins↗

PubMed1996

Effects of intra-articular distal radius depression on wrist joint contact characteristics.

Twelve cadaver extremities were used to study the effect of scaphoid and lunate facet depressions on the contact characteristics of the radiocarpal joint. Pressure-sensitive film was inserted into radiocarpal joints with varying degrees of depression, and the specimens were loaded statically in neutral position, radial deviation, and ulnar deviation. The film was removed and analyzed for contact area and pressure. The only statistically significant effect of a lunate fossa depression was an increase in scaphoid fossa pressure with a 3-mm step-off and the hand in neutral position. Scaphoid fossa depression had more significant effects. With a 1-mm scaphoid fossa depression, lunate fossa pressures increased in neutral position and in radial deviation. Lunate fossa contact area increased, compared to intact joints, in ulnar and radial deviation with 1-mm scaphoid fossa depressions and in all loading positions with 3-mm scaphoid fossa depression. Therefore, it appears that the most significant effect on radiocarpal joint contact characteristics occurs with a depression of the scaphoid side of the joint. Even with depressions as small as 1 mm, significant changes on the lunate side of the joint were observed.

Biomechanical Phenomena↗

PubMed1996

Internal fixation for the transforaminal sacral fracture.

The mechanical stability of alternate forms of internal fixation of the transforaminal sacral fracture were compared. A transforaminal sacral fracture was made in each of 6 fresh-frozen cadaveric pelvic specimens. Implants compared for fixation included: a single and 2 fully threaded iliosacral screws inserted through the posterior ilium and anchored into the first sacral vertebral body both with and without the addition of a posterior tension band plate; and 2 transiliac bars inserted through the posterior tubercles. The femora of each specimen were potted and fixed to the table of a materials tester. The pelvis was restrained only from flexing and extending, and a compressive load was applied through the lumbar spine, representing a standing loading condition. Flexion of the sacrum and displacement at the fracture site were measured during loading. Although creation of the injury increased motion considerably, there was no measurable increase in stability provided by any of the implants or combination of implants in this model when an anatomic reduction was obtained.

Aged↗

PubMed1996

Relationship of mechanical factors to the strength of proximal femur fractures fixed with cancellous screws.

The decision of whether to attempt screw fixation of a femoral neck fracture is based partly on the estimated strength of the fixed bone/implant construct in relation to the loads it will be required to bear. The goal of this study was to determine in vitro the relation of the following biomechanical factors to the strength of internally fixed femoral neck fractures subjected to cyclic and failure loading: (a) square of the density of cancellous bone in the femoral head, (b) percent comminution of the inferior fracture surface, (c) moment arm of the joint force, or distance from the axis of the joint force to the fracture surface, and (d) orientation angle of the fracture surface in the medial/lateral plane relative to the axis of the femoral shaft. Femoral neck fractures were created in each of 38 fresh cadaveric proximal femora using a dropweight or with a materials testing machine. After sustaining a displaced fracture, fixation was achieved using three cannulated cancellous bone screws. The fixed femur was then subjected to 10,000 cycles of a sinusoidially varying load acting on the femoral head, parallel to the femoral shaft, with an initial peak magnitude of 2.2 times body weight, while the hip was flexed, extended, and rotated to mimic some motions of gait. Muscle loading was not simulated. The magnitude of the peak load decreased as the femoral head displaced during cycling. The mean of the peak load for each cycle over the duration of the test was defined as the average load. Following cycling, the bone/screw construct was loaded to failure in the same direction, and this measurement was termed the maximum load. Average and maximum load were then correlated to the four biomechanical factors using a multiple regression analysis. These factors correlated to a high degree with average force (R2 = 0.771; p < 0.0001) and to a lesser but still significant degree with maximum force (R2 = 0.458; p = 0.012), demonstrating that they can be used to estimate the strength of fixation under these loading conditions. The strongest correlation for average force was with fracture angle (p = 0.005) and for failure force was with moment arm length (p = 0.072).

Biomechanical Phenomena↗

PubMed1996

Evaluation of new plate designs for symphysis pubis internal fixation.

The purpose of this study was to evaluate and compare biplanar and other newly designed plates used for pubic symphysis internal fixation to other standard plates. Our data demonstrate that neither of the newly designed symphyseal plates, curved (Zimmer four-hole plate with either two or four 4.5-mm cortical screws) nor the biplanar (Zimmer six-hole plate with four 4.5-mm cortical screws in one plane and two 4.5-mm cortical screws in another) significantly reduce motion more than the other plates tested (Synthes two-hole 4.5-mm dynamic compression plate with two 6.5-mm cancellous screws and six-hole 3.5-mm reconstruction plate with four 3.5-mm cortical screws). All of the plate constructs were able to restore motion to nearly that of the intact symphysis pubis and sacroiliac joint. In summary, using the anteroposterior compression disruption pattern and model, the disrupted symphysis does gap under load but has no effect on sacroiliac joint gapping. It does permit increase in relative flexion angles between the wings of the pelvis. All of the plate systems tested restore normal gap motion at the symphysis and normal sacroiliac joint flexion.

Bone Density↗

PubMed1996

Factors affecting the pullout strength of cancellous bone screws.

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

Animals↗

PubMed1996

TGF-beta 1 forms functionally normal bone in a segmental sheep tibial diaphyseal defect.

OBJECTIVE: Transforming growth factor-beta 1 (TGF-beta 1) is a polyfunctional regulatory cytokine that has been shown to have roles in extracellular matrix interactions, soft tissue healing, and osteogenesis. This study was undertaken to determine the efficacy of TGF-beta 1 in the formation of functionally normal bone in tibial-diaphyseal defects. METHOD: Seven hundred fifty micrograms of recombinant human TGF-beta 1 was added to a guanidine-extracted demineralised bone matrix (Gu-DBM) carrier and the implants were used to fill a 2.5 cm tibial diaphyseal defect in skeletally mature female sheep. The defects were allowed to heal over a 12-week period. After sacrifice, they were analyzed using four-point bending mechanical testing. RESULTS: Implants with TGF-beta 1 showed complete bony bridging of the defect and stress-strain curves similar to the normal contralateral limb, while implants with the carrier alone failed to bridge the gap. CONCLUSIONS: These results demonstrate the ability of TGF-beta 1 to induce new bone which has structural and functional characteristics similar to normal bone.

Animals↗

PubMed1995

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.

Animals↗

PubMed1995

Mechanism of the burst fracture in the thoracolumbar spine. The effect of loading rate.

STUDY DESIGN: Calf lumbar spine motion segments were randomly assigned to two groups. After insertion of a transducer capable of measuring transient occlusion of the spinal canal during impact, a low rate axial impact was applied in one group and a high rate load in the other. Post-injury computed tomography scans and peak canal occlusions were measured to determine the effect of rate of load application on occlusion of the spinal canal. OBJECTIVES: This study was designed to determine if for the same direction of impact and total energy delivered, occlusion of the spinal canal postvertebral fracture was related to the rate at which the impact was delivered (time from zero to peak load). SUMMARY OF BACKGROUND DATA: Several reports based on clinical observations have hypothesized that axial burst fractures, which displace bone fragments into the canal, occur because of internal pressurization and explosion of the vertebral body. The extent of bursting of the vertebra may depend on the rate of pressurization of the body, which could be related to the rate at which the load is applied. METHOD: Using calf lumbar spines, a transducer was placed within the spinal canal, after removal of the cord, to measure canal occlusion during impact. One group received axial compressive impacts at a mean loading rate of 400 msec (zero to peak load) using a materials-testing machine. The energy of failure was determined and used to select a drop weight and distance for the high loading rate tests, which would yield equivalent impact energy. The second group received impacts at a loading rate of of 20 msec. The post-injury radiographs and canal occlusion measurements were compared. RESULTS: The same mean energy of impact was used in the fractures for both groups. Post-injury radiographs of the low loading rate group showed compressive fractures with a mean canal occlusion of 6.84%, whereas the high loading rate group had burst fractures with mean canal encroachment of 47.6% (P = 0.0007). CONCLUSIONS: For the same energy and direction of impact, a high impact loading rate produces fractures with significant canal encroachment, whereas minimal encroachment is seen for fractures produced at a low loading rate.

Animals↗