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J C Lotz

Publications and source records attributed to J C Lotz.

At least 37 records · Page 2Linked to original sources
PubMed1998

Analytic model to predict the strength of tendon repairs.

We developed an analytic model to predict suture load-sharing immediately after flexor tendon repair in the hand. Tendon repair was mathematically modeled as two nonlinear springs in parallel, representing separate core and peripheral sutures that were in series with a third nonlinear spring representing the tendon. To serve as a basis for, and validation of, our analytic model, fresh human flexor digitorum profundus tendons were harvested and mechanically tested either intact or after surgical repair in a variety of ways: core suture alone, superficial peripheral suture alone, deep peripheral suture alone, core suture plus superficial peripheral suture, and core suture plus deep peripheral suture. The stiffness and strength of the composite repairs predicted with use of the analytic model were comparable with those determined experimentally. Furthermore, the model predicted inequities in suture load-sharing, with 64% of the applied load carried by the peripheral suture when it was placed superficially, as compared with 77% when the peripheral suture was placed deep. Our results demonstrate a disparity in load-sharing within composite suture systems, the rectification of which may lead to significant improvement in the repair strength. To this end, we expect that our analytic model will serve as a basis for the design of more efficient, and consequently stronger, suture techniques.

Aged↗
PubMed1997

Frozen storage affects the compressive creep behavior of the porcine intervertebral disc.

STUDY DESIGN: A biomechanical study of the compressive creep behavior of the porcine intervertebral disc before and after frozen storage. OBJECTIVE: To determine whether frozen storage alters the creep response, hydration, and nuclear swelling pressure of the intact intervertebral disc. SUMMARY OF BACKGROUND DATA: The mechanical response of the disc is dominated by swelling and fluid flow, whose effects are time-dependent. Because fluid content, which may change during storage, plays a significant role in the disc's time-dependent behavior, changes in mechanical response due to freezing may have been missed in previous studies that focused on time-independent behavior only. METHODS: Porcine intervertebral discs were tested in repeated cycles of compressive creep either immediately postmortem or after 3 weeks of frozen storage. Swelling pressure and nuclear hydration were also measured in fresh and frozen discs. A fluid transport model was used to analyze the creep data. RESULTS: The creep behavior of the intact porcine intervertebral disc is dramatically affected by frozen storage. The apparent permeability of the frozen discs was 82% higher than that of the fresh discs, and the swelling pressure of frozen discs was 25% lower in frozen discs (P < 0.01). The behavior of fresh and frozen discs became more dissimilar with repeated cycles of creep. CONCLUSIONS: In vitro tests of frozen porcine intervertebral discs do not represent fresh behavior. Frozen storage appears to permanently alter disc behavior. The precise nature of any freezing-induced damage, and whether frozen storage similarly affects human discs, remains to be seen.

Animals↗
PubMed1997

Carbonated apatite cement augmentation of pedicle screw fixation in the lumbar spine.

STUDY DESIGN: Biomechanical testing with human cadaveric lumbar vertebral bodies was used to determine the utility of an injectable carbonated apatite cancellous bone cement for improving the structural performance of pedicle screws subjected to axial pull-out or transverse cyclic loading. OBJECTIVES: To ascertain whether augmentation with a carbonated apatite cement can enhance pedicle screw fixation in the lumbar spine. SUMMARY OF BACKGROUND DATA: The beneficial effects of polymethylmethacrylate augmentation on pedicle screw pull-out strength have been demonstrated. Cancellous bone cement, however, may provide an attractive alternative in this application, as it is remodelable, biocompatible, and nonexothermic. METHODS: Forty-three cadaveric lumbar vertebral bodies were instrumented with pedicle screws. In 20 of these specimens, axial pull-out strength was compared between the control screws and those augmented with cancellous bone cement. In the remaining 23 specimens, the screws were loaded in the superior-inferior direction with a peak displacement of +/- 1 mm at a frequency of 3 Hz for 5000 cycles. Three parameters were calculated from the force-versus-time data: 1) the energy dissipated, 2) the peak force at the start of the test, and 3) the peak force at the end of 5000 cycles. RESULTS: The pull-out strength of the augmented pedicles averaged 68% greater than that of the control side. In response to cyclic loading, all measures of bio-mechanical performance improved 30-63%. CONCLUSIONS: The data suggest that augmentation with this carbonated apatite cancellous bone cement can enhance immediate screw fixation.

Adult↗
PubMed1997

Use of electromagnetic fields in a spinal fusion. A rabbit model.

STUDY DESIGN: The biomechanical and histologic characteristics of posterolateral spinal fusion in a rabbit model with and without the application of a pulsed electromagnetic field were analyzed in a prospective, randomized trial. In addition, fusion rate with and without a pulsed electromagnetic field in this model was assessed by biomechanical testing, radiographs, and manual palpation. OBJECTIVES: To evaluate the influence of a pulsed electromagnetic field on the spinal fusion rate and biomechanical characteristics in a rabbit model. SUMMARY OF BACKGROUND DATA: Previous studies performed to assess the benefits of a pulsed electromagnetic field in spinal fusion have been complicated by the use of instrumentation, and the animal models used do not have a pseudarthrosis rate comparable to that seen in humans. In contrast, the posterolateral intertransverse process fusion in the rabbit is uncomplicated by the use of instrumentation and has been shown to have a pseudarthrosis rate similar to that found in humans (5-35%). METHODS: Ten New Zealand white rabbits each were randomly assigned to undergo spinal fusion using either 1) autologous bone with electromagnetic fields, or 2) autologous bone without electromagnetic fields. A specially designed plastic constraint was used to focus the pulsed electromagnetic field over the rabbits' lumbar spine 4 hours per day. Animals were killed at 6 weeks for biomechanical and histologic testing. RESULTS: The rate of pseudarthrosis, as evaluated radiographically and manually in a blinded fashion, decreased from 40% to 20% with the pulsed electromagnetic field, but this decrease in the nonunion rate was not statistically significant given the number of animals per group. Biomechanical analysis of the fusion mass showed that a pulsed electromagnetic field resulted in statistically significant increases in stiffness (35%), area under the load-displacement curve (37%), and load to failure of the fusion mass (42%). Qualitative histologic assessment showed increased bone formation in those fusions exposed to a pulsed electromagnetic field. CONCLUSIONS: This study demonstrates the reproducibility of a rabbit fusion model, and the ability of a pulsed electromagnetic field to induce a statistically significant increase in stiffness, area under the load-displacement curve, and load to failure of the fusion mass. This investigation provides a basis for continued evaluation of biologic enhancement of spinal arthrodesis with the use of a pulsed electromagnetic field.

Animals↗
PubMed1997

Biomechanical analysis of multilevel fixation methods in the lumbar spine.

STUDY DESIGN: The authors measured and compared the stiffness of cadaveric lumbar spines stabilized with several anterior interbody fusion devices. The information obtained provides a foundation for determining how methods of anterior lumbar fixation can maximize rigidity and promote development of bony fusion. OBJECTIVES: To compare the utility of three anterior spinal instrumentation systems for stabilizing the lumbar spine. SUMMARY OF BACKGROUND DATA: Anterior spinal instrumentation is used to prevent progressive spinal deformity and maintain correction after spinal fusion surgery. Newer instrumentation systems developed for anterior interbody fusions can be inserted by minimally invasive procedures. The stability of these systems has not been tested adequately in human cadaveric specimens. METHODS: Fusion constructs were evaluated in 12 human cadaveric specimens sequentially loaded in axial compression and torsion, flexion and extension, and lateral bending. The fusion constructs used were 1) two anterior bilateral threaded interbody fusion devices, 2) lateral hollow interbody screws (Texas Scottish Rite Hospital-B screws), and 3) femoral allograft and conventional anterior Texas Scottish Rite Hospital instrumentation. RESULTS: The construct with Texas Scottish Rite Hospital-B screws connected by a rod produced stiffness comparable with that produced by conventional Texas Scottish Rite Hospital instrumentation with femoral ring allografts. The threaded interbody fusion device stiffness tested in axial rotation was comparable with that achieved with Texas Scottish Rite Hospital instrumentation. CONCLUSIONS: Our data demonstrate the effectiveness of threaded interbody fusion device and the Texas Scottish Rite Hospital-B screw in immobilizing the L3-L4 and L4-L5 disc spaces. Rigidity of fixation in the lumbar spine may aid in the maintenance of lordosis.

Absorptiometry, Photon↗
PubMed1997

Radial tensile properties of the lumbar annulus fibrosus are site and degeneration dependent.

We conducted an in vitro study of the radial tensile properties of the annulus fibrosus. The stress-strain response was nonlinear, with a mean tangent modulus of 0.19 MPa at zero strain and 0.47 MPa at 70% of the yield strain. We also investigated whether these properties varied as a function of location within the disc and degree of degeneration. Specimens harvested from the middle layers of the annulus were stiffer and failed at smaller strain magnitudes than those from the inner or outer annulus (analysis of covariance, p < 0.05). Differences due to degeneration were evident; degenerated discs had a 30% decrease in yield and ultimate stress compared with normal discs. Similarity between our data and those reported for the annulus in compression suggests that these values reflect the material behavior of the interlaminar matrix and are an order of magnitude smaller than values used in previous analytical representations of this tissue. We expect that awareness of these data will result in improved understanding of the physical behavior and tolerance to injury of the annulus fibrosus.

Aged↗
PubMed1997

Partial lacerations of human digital flexor tendons: a biomechanical analysis.

The biomechanical properties of human flexor tendons with partial lacerations have not been previously studied. To determine the loss of tensile strength with varying degrees of partial laceration, tensile tests were performed on 2 matched groups of human cadaver flexor tendons: One group had 50% while the other had 75% transverse volar lacerations of the anteroposterior diameter. The mean failure load of the 50%-lacerated tendons was 93% higher than that of the 75%-lacerated tendons. The forces tolerated by the lacerated tendons before failure were also compared to those measured in vivo during physiologic loading. The breaking loads of both 50%- and 75%-lacerated tendons far exceeded the in vivo forces measured in human flexor tendons during unresisted active finger movement (up to 34 N). Further, the breaking loads of 50% lacerations was higher than the in vivo forces during resisted active finger movement (up to 117 N). This study demonstrates that the threshold load levels to rupture of 50% and 75% lacerations are higher than physiologic load levels measured during active motion, suggesting that partial flexor tendon lacerations of up to 75% can withstand in vivo forces associated with active unresisted mobilization of the digital flexor tendon.

Biomechanical Phenomena↗
PubMed1996

Biomechanical analysis of lumbosacral fixation.

STUDY DESIGN: The authors sought to measure the stiffness at the lumbosacral junction when it has been immobilized by means of two different posterior fixation systems in conjunction with three different anterior interbody fixation techniques. The information obtained provides a foundation for determining how methods of lumbosacral spinal fixation can maximize rigidity and improve fusion rates at this clinically important anatomic site. OBJECTIVES: To determine which spinal instrumentation systems, alone or in combination, provide the most stability at the lumbosacral junction. SUMMARY OF BACKGROUND DATA: In the clinical setting, several different posterior instrumentation systems are currently used, but the stability of these systems, when used in conjunction with anterior interbody fixation, has not been adequately tested. The authors hypothesized that rigidity of fixation at the lumbosacral junction may be enhanced by use of appropriate anterior interbody fixation techniques. METHODS: Two posterior and three anterior fusion constructs were tested in eight fresh-frozen human cadaveric specimens in axial compression and torsion, flexion, extension, and lateral bending. RESULTS: The isolated posterior instrumentation constructs produced greater intervertebral stiffness than the isolated anterior constructs tested. The Jackson instrumentation was stiffer than the Galveston in axial torsion and flexion. Anteriorly, the femoral ring was stiffer than either the threaded interbody fusion device in flexion or the corticocancellous bone graft in flexion and compression. The threaded interbody fusion device was the stiffest anterior construct in axial torsion. CONCLUSIONS: The authors' data demonstrate that the threaded interbody fusion device may be an effective system for immobilization of the L5-S1 disc space. Rigidity of fixation at the lumbosacral junction may be enhanced by use of appropriate anterior interbody fusion techniques.

Absorptiometry, Photon↗
PubMed1996

Effect of peripheral suture depth on strength of tendon repairs.

The peripheral suture in flexor tendon repairs was originally introduced to "tidy up" the repair site and to improve gliding after core suture placement. More recently, it has been proposed that the peripheral suture can be an important structural component of tendon repair; yet, the effect of peripheral suture technique as an independent factor on the strength of the tendon repair has had little attention. We proceeded to determine whether differing levels of penetration of the tendon by the peripheral suture significantly affect the strength of the repair. We performed tensile tests on two groups of fresh frozen cadaver flexor digitorum superficialis tendons that had lacerations repaired by modified Kessler core sutures: one group had supplemental superficial peripheral sutures while the other group had supplemental deep (half the depth to the center of the tendon) peripheral sutures. The mean failure load of the deep peripheral suture group (38.96 N) was almost 80% greater than that of the superficial suture group (21.68 N). This increased strength may be due to an improved tendon-suture interface, with the deep peripheral suture acting as an auxiliary core suture. Our results with deep peripheral sutures, a simple technique based on a modification of commonly employed suture patterns, demonstrate substantially improved repair strength. This suture has the potential to be used in conjunction with early active finger motion rehabilitation program after flexor tendon repair.

Biomechanical Phenomena↗
PubMed1995

Instrumentation of the cervicothoracic junction after destabilization.

STUDY DESIGN: The biomechanics of three different instrumentation constructs applied at the destabilized cervicothoracic junction were evaluated. OBJECTIVES: To find an efficient way in restoring stability of the cervicothoracic junction in cases with and without laminectomy. SUMMARY OF BACKGROUND DATA: Different instrumentation techniques have been evaluated biomechanically and used clinically for managing instabilities between the fourth and sixth cervical vertebrae. These constructs have not been evaluated at the cervicothoracic junction. METHODS: Six human spines were tested nondestructively in axial torsion, flexion, and extension with the C6-T2 motion segments left unconstrained. The three-dimensional displacements and rotations between C7 and T1 vertebrae were measured using a sonic digitizer. After intact testing, a distractive-flexion Stage 3 cervical spinal injury was simulated surgically between C7 and T1. The specimens underwent sequential instrumentation and mechanical testing with three constructs: posterior Synthes lateral mass plate, posterior pediatric Cotrel-Dubousset rod system with lamina hooks and a crosslink, and anterior Synthes cervical locking plate. RESULTS: Posterior stabilization techniques had statistically more stiffness than anterior plates. The Cotrel-Dubousset system offered the largest stiffness ratio (instrumented/intact) in flexion, extension, and rotation. There was no statistical difference between posterior plates and Cotrel-Dubousset instrumentation. The stiffness of the anterior plate did not differ significantly from the intact spine. CONCLUSION: Our data show that instability of the cervicothoracic junction can be efficiently restored by either anterior plates, posterior plates, or posterior hook-rod constructs (Cotrel-Dubousset). Posterior constructs showed increased stiffness over anterior plates.

Aged↗
PubMed1995

Stress distributions within the proximal femur during gait and falls: implications for osteoporotic fracture.

The rates of fracture at sites with different relative amounts of cortical and trabecular bone (hip, spine, distal radius) have been used to make inferences about the pathomechanics of bone loss and the existence of type I and type II osteoporosis. However, fracture risk is directly related to the ratio of tissue stress to tissue strength, which in turn is dependent not only on tissue composition but also tissue geometry and the direction and magnitude of loading. These three elements determine how the load is distributed within the tissue. As a result, assumptions on the relative structural importance of cortical and trabecular bone, and how these tissues are affected by bone loss, can be inaccurate if based on regional tissue composition and bone density alone. To investigate the structural significance of cortical and trabecular bone in the proximal femur, and how it is affected by bone loss, we determined the stress distributions in a normal and osteoporotic femur resulting from loadings representing: (1) gait; and (2) a fall to the side with impact onto the greater trochanter. A three-dimensional finite element model was generated based on a representative femur selected from a large database of femoral geometries. Stresses were analyzed throughout the femoral neck and intertrochanteric regions. We found that the percentage of total load supported by cortical and trabecular bone was approximately constant for all load cases but differed depending on location. Cortical bone carried 30% of the load at the subcapital region, 50% at the mid-neck, 96% at the base of the neck and 80% at the intertrochanteric region. These values differ from the widely held assumption that cortical bone carries 75% of the load in the femoral neck and 50% of the load at the intertrochanteric region. During gait, the principal stresses were concentrated within the primary compressive system of trabeculae and in the cortical bone of the intertrochanteric region. In contrast, during a fall, the trabecular stresses were concentrated within the primary tensile system of trabeculae with a peak magnitude 4.3 times that present during gait. While the distribution of stress for the osteoporotic femur was similar to the normal, the magnitude of peak stress was increased by between 33% and 45%. These data call into question several assumptions which serve as the basis for theories on the pathomechanics of osteoporosis. In addition, we expect that the insight provided by this analysis will result in the improved development and interpretation of non-invasive techniques for the quantification of in vivo hip fracture risk.

Accidental Falls↗
PubMed1995

Comparative mechanical analysis of dorsal versus palmar placement of core suture for flexor tendon repairs.

This study was designed to evaluate the effect of core suture placement in the coronal plane on the tensile strength of flexor tendon repairs. We compared the tensile strength of modified Kessler core sutures placed in the palmar versus the dorsal side of matched-lacerated human cadaver flexor digitorum profundus tendons tested in vitro under static loading. We also compared the relative strengths of the dorsal and palmar portions of the flexor digitorum tendons. The mean failure load of a dorsal-side modified Kessler suture was 26.5% greater than that of a palmar-side modified Kessler suture. The mean strength of the dorsal half of the flexor digitorum tendons was 58.3% greater than that of the palmar half of the flexor digitorum tendons. Our data demonstrate that the dorsal tendon can sustain greater loads-to-failure than the palmar tendon. This suggests that there are biomechanical advantages to dorsal as opposed to palmar placement of the core suture in clinical tendon repair.

Cadaver↗
PubMed1991

Mechanical properties of metaphyseal bone in the proximal femur.

We used a three-point bending test to investigate the structural behavior of 123 rectangular flat plate specimens harvested from the metaphyseal shell of the cervical and intertrochanteric regions of five fresh/frozen human proximal femora. For comparison purposes, 36 specimens of similar geometry were also fabricated from bone of the femoral diaphysis. All specimens were oriented in either the local longitudinal or transverse directions. The mean longitudinal elastic modulus was 9650 +/- 2410 (SD) MPa and demonstrated a 24% decrease from that measured for the diaphysis (12500 +/- 2140 MPa) using the same testing technique. However, the transverse elastic moduli did not differ significantly between the proximal (5470 +/- 1720 MPa) and diaphyseal (5990 +/- 1520 MPa) specimens. The globally averaged values for the ultimate tensile strengths of the metaphyseal shell were 101 +/- 26 MPa in the longitudinal and 50 +/- 12 MPa in the transverse directions. These compared with diaphyseal values of 128 +/- 16 MPa and 47 +/- 12 MPa, respectively. While these differences were largely due to the reduced density of the proximal specimens, a slight decrease in transverse anisotropy for the proximal specimens was also noted by comparing the ratio of longitudinal to transverse moduli (1.76) and tensile strength (2.02) to the diaphyseal values (2.09 and 2.71, respectively). Use of these data should lead to improved performance of analytical models for the proximal femur, and thus help focus increased attention on the structural contribution of trabecular bone to the strength and rigidity of the proximal femur.

Acrylic Resins↗
PubMed1991

Fracture prediction for the proximal femur using finite element models: Part I--Linear analysis.

Over 90 percent of the more than 250,000 hip fractures that occur annually in the United States are the result of falls from standing height. Despite this, the stresses associated with femoral fracture from a fall have not been investigated previously. Our objectives were to use three-dimensional finite element models of the proximal femur (with geometries and material properties based directly on quantitative computed tomography) to compare predicted stress distributions for one-legged stance and for a fall to the lateral greater trochanter. We also wished to test the correspondence between model predictions and in vitro strain gage data and failure loads for cadaveric femora subjected to these loading conditions. An additional goal was to use the model predictions to compare the sensitivity of several imaging sites in the proximal femur which are used for the in vivo prediction of hip fracture risk. In this first of two parts, linear finite element models of two unpaired human cadaveric femora were generated. In Part II, the models were extended to include nonlinear material properties for the cortical and trabecular bone. While there was poor correspondence between strain gage data and model predictions, there was excellent agreement between the in vitro failure data and the linear model, especially using a von Mises effective strain failure criterion. Both the onset of structural yielding (within 22 and 4 percent) and the load at fracture (within 8 and 5 percent) were predicted accurately for the two femora tested. For the simulation of one-legged stance, the peak stresses occurred in the primary compressive trabeculae of the subcapital region.(ABSTRACT TRUNCATED AT 250 WORDS)

Accidental Falls↗
PubMed1991

Fracture prediction for the proximal femur using finite element models: Part II--Nonlinear analysis.

In Part I we reported the results of linear finite element models of the proximal femur generated using geometric and constitutive data collected with quantitative computed tomography. These models demonstrated excellent agreement with in vitro studies when used to predict ultimate failure loads. In Part II, we report our extension of those finite element models to include nonlinear behavior of the trabecular and cortical bone. A highly nonlinear material law, originally designed for representing concrete, was used for trabecular bone, while a bilinear material law was used for cortical bone. We found excellent agreement between the model predictions and in vitro fracture data for both the onset of bone yielding and bone fracture. For bone yielding, the model predictions were within 2 percent for a load which simulated one-legged stance and 1 percent for a load which simulated a fall. For bone fracture, the model predictions were within 1 percent and 17 percent, respectively. The models also demonstrated different fracture mechanisms for the two different loading configurations. For one-legged stance, failure within the primary compressive trabeculae at the subcapital region occurred first, leading to load transfer and, ultimately, failure of the surrounding cortical shell. However, for a fall, failure of the cortical and trabecular bone occurred simultaneously within the intertrochanteric region. These results support our previous findings that the strength of the subcapital region is primarily due to trabecular bone whereas the strength of the intertrochanteric region is primarily due to cortical bone.

Accidental Falls↗
PubMed1990

The use of quantitative computed tomography to estimate risk of fracture of the hip from falls.

We conducted an in vitro investigation of the loads and energies needed to fracture the proximal part of the femur in twelve fresh cadavera under loading conditions simulating one particular type of fall. The fracture loads ranged from 778 to 4,040 newtons and the work to fracture, from five to fifty-one joules. We also investigated the relationship between the fracture loads and several potential indices of bone strength, which were measured non-invasively at the subcapital, basic-cervical, and intertrochanteric regions with quantitative computed tomography. A very high positive correlation with the fracture load resulted from use of an intertrochanteric index--the product of the average trabecular computed-tomography number and the total cross-sectional area of the bone (R2 = 0.93, standard error of estimate = 295 newtons, and p less than 0.00001). We expect the use of this parameter to result in improved assessments of the degree of osteoporosis and of the component of risk of fracture of the hip that is associated with bone strength. However, the measured work to fracture for the isolated femur was an order of magnitude smaller than estimates of the energy available during a typical fall (about 450 joules), suggesting that energy absorbed during falling and impact, rather than bone strength, may be the dominant factors in the biomechanics of fracture of the hip.

Accidental Falls↗