Scott A. Yerby

Mechanical properties of the human achilles tendon

OBJECTIVE To determine whether the human Achilles tendon has higher material properties than other tendons and to test for strain rate sensitivity of the tendon. DESIGN Mechanical testing of excised tendons. BACKGROUND While the human Achilles tendon appears to experience higher in vivo stresses than other tendons, it is not known how the Achilles tendons material properties compare with the properties of other tendons. METHODS Modulus, failure stress, and failure strain were measured for excised human Achilles tendons loaded at strain rates of 1% s(-1) and 10% s(-1). Paired t-tests were used to examine strain rate effects, and average properties from grouped data were used to compare the Achilles tendons properties with properties reported in the literature for other tendons. RESULTS Failure stress and failure strain were higher at the faster strain rate, but no significant difference in modulus was observed. At the 1% s(-1)rate, the mean modulus and failure stress were 816 MPa (SD, 218) and 71 MPa (SD, 17), respectively. The failure strain was 12.8% (SD, 1.7) for the bone-tendon complex and 7.5% (SD, 1.1) for the tendon substance. At the 10% s(-1) rate, the mean modulus and failure stress were 822 MPa (SD, 211) and 86 MPa (SD, 24), respectively. The mean failure strain was 16.1% (SD, 3.6) for the bone-tendon complex and 9.9% (SD, 1.9) for the tendon substance. These properties fall within the range of properties reported in the literature for other tendons. CONCLUSIONS The material properties of the human Achilles tendon measured in this study are similar to the properties of other tendons reported in the literature despite higher stresses imposed on the Achilles tendon in vivo.

The Treatment Mechanism of an Interspinous Process Implant for Lumbar Neurogenic Intermittent Claudication

Study Design. The spinal canal and neural foramina dimensions of cadaver lumbar spines were quantified during flexion and extension using magnetic resonance imaging before and after placement of an interspinous process implant. Objective. To quantify the effect of the implant on the dimensions of the spinal canal and neural foramina during flexion and extension. Summary of the Background Data. Lumbar neurogenic intermittent claudication symptoms are typically exacerbated during extension and relieved during flexion. It is understood that the dimensions of the spinal canal and neural foramen increase in flexion and decrease in extension. The authors hypothesized that an interspinous process implant would significantly prevent narrowing of the canal and foramina in extension and have no significant effect in flexion. Methods. Eight L2–L5 specimens were positioned to 15° of flexion and 15° of extension using a positioning frame. Each specimen was magnetic resonance imaged with and without an interspinous implant (X STOP) placed between the L3–L4 spinous processes. Canal and foramina dimensions were compared between the intact and implanted specimens using a repeated measures analysis of variance with a level of significance of 0.05. Results. In extension, the implant significantly increased the canal area by 18% (231–273 mm2), the subarticular diameter by 50% (2.5–3.7 mm), the canal diameter by 10% (17.8–19.5 mm), the foraminal area by 25% (106–133 mm2), and the foraminal width by 41% (3.4–4.8 mm). Conclusions. The results of this study show that the X STOP interspinous process implant prevents narrowing of the spinal canal and foramina in extension.

The Effects of an Interspinous Implant on the Kinematics of the Instrumented and Adjacent Levels in the Lumbar Spine

Study Design. Measurement of the kinematics of the lumbar spine after insertion of an interspinous spacer in vitro. Objectives. To understand the kinematics of the instrumented and adjacent levels due to the insertion of this interspinous implant. Summary of Background Data. An interspinous spacer (X Stop, SFMT, Concord, California) has been developed to treat neurogenic intermittent claudication by placing the stenotic segment in slight flexion and preventing extension. This restriction of motion by the interspinous implant may affect the kinematics of levels adjacent to the instrumented level. Methods. Seven lumbar spines (L2–L5) were tested in flexion–extension, lateral bending, and axial rotation. Images were taken during each test to determine the kinematics of each motion segment. The interspinous implant was placed at the L3–L4 level, and the test protocol was repeated. Results. The flexion–extension range of motion was significantly reduced at the instrumented level. Axial rotation and lateral bending ranges of motion were not affected at the instrumented level. The range of motion in flexion–extension, axial rotation, and lateral bending at the adjacent segments was not significantly affected by the implant. Conclusions. The implant does not significantly alter the kinematics of the motion segments adjacent to the instrumented level.

Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs

How evolutionary changes in body size are brought about by variance in developmental timing and/or growth rates (also known as heterochrony) is a topic of considerable interest in evolutionary biology. In particular, extreme size change leading to gigantism occurred within the dinosaurs on multiple occasions. Whether this change was brought about by accelerated growth, delayed maturity or a combination of both processes is unknown. A better understanding of relationships between non-avian dinosaur groups and the newfound capacity to reconstruct their growth curves make it possible to address these questions quantitatively. Here we study growth patterns within the Tyrannosauridae, the best known group of large carnivorous dinosaurs, and determine the developmental means by which Tyrannosaurus rex, weighing 5,000 kg and more, grew to be one of the most enormous terrestrial carnivorous animals ever. T. rex had a maximal growth rate of 2.1 kg d-1, reached skeletal maturity in two decades and lived for up to 28 years. T. rexs great stature was primarily attained by accelerating growth rates beyond that of its closest relatives.

Dinosaurian Growth Patterns and Rapid Avian Growth Rates.

Did dinosaurs grow in a manner similar to extant reptiles, mammals or birds, or were they unique? Are rapid avian growth rates an innovation unique to birds, or were they inherited from dinosaurian precursors? We quantified growth rates for a group of dinosaurs spanning the phylogenetic and size diversity for the clade and used regression analysis to characterize the results. Here we show that dinosaurs exhibited sigmoidal growth curves similar to those of other vertebrates, but had unique growth rates with respect to body mass. All dinosaurs grew at accelerated rates relative to the primitive condition seen in extant reptiles. Small dinosaurs grew at moderately rapid rates, similar to those of marsupials, but large species attained rates comparable to those of eutherian mammals and precocial birds. Growth in giant sauropods was similar to that of whales of comparable size. Non-avian dinosaurs did not attain rates like those of altricial birds. Avian growth rates were attained in a stepwise fashion after birds diverged from theropod ancestors in the Jurassic period.

The effect of an interspinous process implant on facet loading during extension.

Study Design. Facet loading parameters of lumbar cadaver spines were measured during extension before and after placement of an interspinous process implant. Objective. The study was undertaken to quantify the influence of an interspinous implant on facet loading at the implanted and adjacent levels during extension. Summary of Background Data. Facet loading is increased during extension and decreased during flexion. Previous studies have demonstrated that interspinous process decompression relieves disc pressure at the implanted level and does not alter disc pressure at the adjacent levels. Facet joints are believed to play a key role in back pain, especially in patients with collapsed discs and increased motion segment mobility resulting in increased facet loading. Methods. Seven cadaver spines (L2–L5) were loaded to 15 Nm of extension and 700 N compression with and without an interspinous process implant (X STOP) placed between the L3–L4 spinous processes. Pressure-sensitive film was placed in the facet joints of the implanted and adjacent levels. After loading, the film was digitally analyzed for peak pressure, average pressure, contact area, and force. These values were compared between the intact and implanted specimens at the adjacent and implanted levels using a paired t test (P < 0.05). Results. The implant significantly reduced the mean peak pressure, average pressure, contact area, and force at the implanted level. The mean peak pressure, average pressure, contact area, and force at the adjacent levels were not significantly different between the intact and implanted specimens with the exception of contact area at the L2–L3 level. Conclusions. Interspinous process decompression will unlikely cause adjacent level facet pain or accelerated facet joint degeneration. Furthermore, pain induced from pressure originating in the facets and/or posterior anulus of the lumbar spine may be relieved by interspinous pro-cess decompression. Clinical results from patients with a component of lower back pain suggest that this is a valid conclusion.

Reinforcement of thoracolumbar burst fractures with calcium phosphate cement : a biomechanical study

Study Design. A biomechanical study on the stabilization of thoracolumbar burst fractures. Objective. To demonstrate that the addition of a calcium phosphate cement into the fractured vertebral body through a transpedicular approach is a feasible technique that improves the stiffness of a transpedicular screw construct. Summary of Background Data. Short segment pedicle screw instrumentation is a commonly used method for reduction and stabilization of unstable burst fractures. Recent investigators, however, have reported a high rate of instrumentation failure and sagittal collapse when there is a loss of anterior column support. In this study, the ability of a new hydroxyapatite cement to augment anterior column support was investigated in a burst fracture model. Methods. A cadaveric L1 burst fracture model was stabilized using short segment pedicle screw instrumentation. Specially instrumented pedicle screws recorded screw‐bending moments. The L1 vertebral body was reinforced with the hydroxyapatite cement through a transpedicular approach. Mechanical testing of the instrumented and instrumented‐reinforced constructs were performed in flexion, extension, side bending, and torsion. Construct stiffness and screw‐bending moments were recorded. Results. Transpedicular vertebral body reconstruction with hydroxyapatite cement reduced pedicle screw‐bending moments by 59% in flexion and 38% in extension. Mean initial stiffness in the flexion‐extension plane was increased by 40% (P < 0.05). There were no statistically significant differences in these parameters with lateral bending or torsional movements. Conclusions. This hydroxyapatite cement compound augments anterior column stability in a burst fracture model. This technique may improve outcomes in burst fracture patients without the need for a secondary anterior approach.

Short-segment pedicle instrumentation. Biomechanical analysis of supplemental hook fixation

Study Design This biomechanical study of fractures in cadaver vertebrae used specially designed pedicle screws to determine screw strains during loading of two different fixation constructs. Objectives The authors determined the relative benefit of adding offset sublaminar hooks to standard pedicle screw constructs to reduce screw bending moments and prevent fixation failure and sagittal collapse. Summary of Background Data Clinical studies have demonstrated a high incidence of early screw failure in short-segment pedicle instrumentation constructs used to treat unstable burst fractures. Strategies to prevent early construct failure include longer constructs, anterior strut graft reconstruction, and use of offset sublaminar hooks at the ends of standard short-segment pedicle instrumentation constructs. Methods Human cadaver spines with an L1 burst fracture were instrumented with a standard short-segment pedicle instrumentation construct using specially instrumented pedicle screws. Mechanical testing was carried out in flexion, extension, side bending, and torsion, and stiffness and screw bending moments were recorded. Offset hooks were applied initially, then removed and testing repeated. Stiffness data were compared to intact and postfracture results, and between augmented and standard constructs. Results Addition of offset laminar hooks, supralaminar at T11 and infralaminar at L2, to standard short-segment pedicle instrumentation constructs increased stiffness in flexion by 268%, in extension by 223%, in side bending by 161%, and in torsion by 155% (all were significant except torsion). Sublaminar hooks also reduced pedicle screw bending moments to roughly 50% of standard in both flexion and extension (P < 0.05). Conclusions Supplemental offset hooks significantly increase construct stiffness without sacrificing principles of short-segment pedicle instrumentation, and absorb some part of the construct strain, thereby reducing pedicle screw bending moments and the likelihood of postyield deformation and clinical failure.

Comparative morphometry of L4 vertebrae: comparison of large animal models for the human lumbar spine.

Study Design. Anatomic analysis of L4 vertebral morphometry comparing specimens harvested from humans and five common large animal species. Objective. To compare fundamental structural similarities and differences in the vertebral bodies of commonly used experimental animals relative to human vertebrae. Summary of Background Data. Animal models are commonly used for assessment of spine fusion, instrumentation techniques, and vertebral bone biology. Among the animals used, the lumbar vertebrae exhibit considerable anatomic variability. The goal of this study was to determine which of the animals commonly used for spine research is best suited as an anatomic model for the human lumbar spine. Methods. Morphometric features of the L4 vertebrae of five common research animals were compared with those of the human L4 vertebrae. Mature canines, immature pigs, mature micropigs, mature dairy goats, and mature sheep were analyzed. These species were chosen because they are commonly selected research animals, and most research facilities do not need to be modified to use them. The samples included ten L4 vertebrae of each animal species and seven human L4 vertebrae. Each specimen was meticulously cleaned of all soft tissue. The measurements were grouped into vertebral body parameters, neural canal dimensions, and pedicle and facet morphometery. The mean of each anatomic measurement was compared using a single factor analysis of variance and a Scheffe’s post hoc test, with 0.05 denoting significance. Results. The human vertebral body was significantly wider and deeper in the anteroposterior plane than any of the animals studied. However, the mean vertebral body height of the sheep and goat significantly exceeded that of the human specimens. The mean pedicle angle of every animal species was significantly greater than that of the human. The mean pedicle width of the micropig and goat were significantly narrower than the human pedicles, and the dog specimens lacked a definable pedicle altogether. There was no significant difference in mean pedicle width between any of the remaining species and the human specimens. Facet tropism and radius of curvature of the sheep and goat specimens differed significantly from the remaining selections. Conclusions. When posterior pedicle instrumentation is part of a testing protocol, the increased pedicle angle and lack of vertebral body depth found in all animals studied must be kept in mind. In addition, when testing interbody cages designed to stabilize the spine and promote fusion, one must be aware of the decreased vertebral body depth and width in these animals, as compared with humans. Physeal defects in the immature pig may alter specific biomechanical results during failure or fatigue testing, or in basic studies of vertebral bone material properties. In all cases, instrumentation and hardware must be sized appropriately to the selected model to provide meaningful results.

Revision of failed pedicle screws using hydroxyapatite cement. A biomechanical analysis.

Study Design. The biomechanical influence of in situ setting hydroxyapatite cement was examined for use in pedicle screw revision surgery. Pull‐out testing of control and pedicle screws augmented with hydroxyapatite cement was performed in human cadaver vertebrae. Objectives. To determine the immediate effect of using hydroxyapatite cement to augment revision pedicle screws after failure of the primary pedicle screw fixation. Summary of Background Data. The potential problems associated with using polymethylmethacrylate to augment revision pedicular instrumentation have prompted the search for other solutions. The introduction of resorbable hydroxyapatite pastes may have provided new biocompatible solutions for pedicle screw revision. Methods. Ten human cadaver vertebrae were instrumented with 6.0‐mm pedicle screws in each pedicle. The screws were loaded to failure in axial tension (pull‐out). The failed pedicles then were instrumented with 7.0‐mm pedicle screws, either augmented with hydroxyapatite cement or nonaugmented, which also were loaded to failure. Finally, the nonaugmented 7.0‐mm screw hole was reinstrumented with a hydroxyapatite cement‐augmented, 7.0‐mm pedicle screw and loaded to failure. Results. The pull‐out strength of the 7.0‐mm, hydroxyapatite cement‐augmented screws was 325% (P = 2.9 × 10−5) of that of the 6.0‐mm control screws, whereas the strength of the 7.0‐mm nonaugmented screws was only 73% (P = 2.0 × 10−2) of that of the 6.0‐mm control screws. The 7.0‐mm screws augmented with hydroxyapatite cement also were able to salvage 7.0‐mm pull‐out sites to 384% (P = 6.9E‐5) of the pull‐out strength of the 7.0‐mm nonaugmented screws. Conclusions. Hydroxyapatite cement may be a mechanically viable alternative to polymethyl methacrylate for augmenting revision pedicular instrumentation and should be considered for future experimental, animal, and clinical testing.