Biomedical subjects

T H Smit

Publications and source records attributed to T H Smit.

17 recordsLinked to original sources

PubMed2006

Intervertebral disc recovery after dynamic or static loading in vitro: is there a role for the endplate?

In vivo studies on disc mechanics show loss of fluid from the intervertebral disc (IVD) during loading and full recovery during rest. Previous work indicated that in vitro recovery is hampered after static loading. The aim of the present study was to investigate the role of the endplate after dynamic and static loading on mechanical recovery in vitro. Lumbar spines (caprine) were obtained from the local slaughterhouse and stored frozen. Twenty-four intervertebral discs were thawed and subjected to a compression test in a saline bath (37 degrees C). The discs were pre-loaded at 20 N for 15 min. Three 15-min loading cycles (static: 2.0 MPa or dynamic: average load 2.0 MPa at 0.5 Hz) were applied, each followed by a 30-min period of unloading (20 N). After this protocol, the endplates of half of the discs were blocked with silicone paste and the long-term recovery protocol was applied; the discs were subjected to a single loading cycle (15 min of static or dynamic loading) followed by 10h of unloading at 20 N. All specimens showed a net loss of height and a gain in stiffness during the first part of the test. Eventually, height and stiffness were restored during a long-term recovery test. The difference in recovery between blocked and free endplates was marginal. If fluid flow plays a role during recovery in vitro, the role of the endplate appears to be limited. Our findings show no influence of loading type on recovery in vitro.

Animals↗

PubMed2006

Biomechanical comparison of hard and soft hip protectors, and the influence of soft tissue.

INTRODUCTION: Hip protectors appear to be promising in preventing hip fractures. Currently, many different hip protectors exist, and it is not clear which hip protector has the best biomechanical properties. Therefore, the objective of this study was to compare the force attenuation capacity of 10 different hip protectors. Both hard hip protectors, which primarily shunt away energy, and soft hip protectors, which primarily absorb energy, were included. METHODS: Using a drop weight impact testing system and a surrogate femur, a weight of 25 kg was dropped from a height of 8 cm causing a force of almost 7,806 N on the bare femur, which simulates a severe fall. After this calibration test, soft tissue and the different hip protectors in combination with the soft tissue were tested. Each test was repeated six times. To simulate normal-weight elderly people, a 1/2-inch-thick layer of foam was chosen, reducing the force by 18%. To examine the influence of soft tissue thickness, soft tissue was also simulated by a 1-inch-thick layer of foam, reducing the force by 49%. RESULTS: In the 1-inch soft tissue test, all hip protectors were capable in reducing the impact to below the average fracture threshold of elderly people (3,100 N), although the hard types performed significantly better than the soft ones (P < 0.001). In the 1/2-inch soft tissue test, only the hard hip protectors were capable of attenuating the peak force to below the average fracture threshold of 3,100 N (hard vs. soft hip protectors: P < 0.001). CONCLUSIONS: This study showed that the hard, energy-shunting hip protectors were superior to the soft, energy-absorbing ones, especially in a simulation of normal-weight elderly people. With increased soft tissue thickness, soft hip protectors were also capable in reducing the impact to below the average fracture threshold of 3,100 N.

Aged↗

PubMed2006

Application of polylactides in spinal cages: studies in a goat model.

Spinal cages are currently made of non-resorbable materials, but they only have a temporary function: after fusion, resorption is desirable both from a biological and mechanical point of view. We studied different polylactides in stand-alone condition in a goat model. Cages were made of 100% poly(L-lactic acid) (PLLA) or 70/30 poly(L/DL-Lactic acid) (PLDLLA); titanium served as control. After six months, all titanium cages showed non-unions comparable to that observed in a clinical retrieval, thus showing validity of the goat model. PLLA cages maintained their mechanical integrity for six months, enough to allow fusion. After that, the material resorbed within 48 months without adverse tissue reactions. Bone formation was faster in PLDLLA cages, but these already failed within three months, thus losing their stabilising function: 50% ended in pseudo-arthrosis. Additional internal fixation provided enough stability for fusion (83%). Biocompatibility of both PLLA and PLDLLA was excellent. The long-term results show that PLLA cages can be used for stand-alone interbody fusion, and that PLLA is an improvement over titanium in terms of fusion rate. PLDLLA showed enhanced bone formation, but also earlier failure of the implant. Chances for spinal fusion were better with additional internal fixation.

Absorbable Implants↗

PubMed2005

Bioresorbable polymers: heading for a new generation of spinal cages.

The use of polymer-based bioresorbable materials is now expanding to the realm of spinal interbody fusion. Bioresorbable polymers have important advantages over metals, because they are temporary, much less stiff, and radiolucent. Most promising is a group of alpha-polyesters, in particular polylactide acids (PLAs). Their biocompatibility is excellent, and they have sufficient stiffness and strength to provide initial and intermediate-term stability required for bone healing. However, polylactides have characteristics that make them vulnerable to complications if not properly controlled. Degradation rate strongly depends on polymer type, impurities, manufacturing process, sterilization, device size, and the local environment. The fact that larger implants degrade faster is contra-intuitive, and should be considered in the design process. Also optimal surgical techniques, such as careful bone bed preparation, are required for a successful application of these materials. The purpose of this paper is to highlight the specific properties of these bioresorbable polymers and to discuss their potential and limitations. This is illustrated with early preclinical and clinical data.Bioresorbable cage technology is just emerging: their time-engineered degradation characteristics allow controlled dynamization in interbody applications, facilitating spinal fusion. Their radiolucency improves image assessment of fusion healing. Acceptance and use of bioresorbable implants may increase as further research and clinical studies report on their safety, efficacy, and proper usage.

Absorbable Implants↗

PubMed2005

Transpedicular wedge resection osteotomy for the treatment of a kyphotic Andersson lesion-complicating ankylosing spondylitis.

Two cases with a long-standing thoracolumbar kyphosis due to ankylosing spondylitis are presented with a symptomatic localized destructive kyphotic lesion of the spine. Clinical and radiographic findings demonstrated a progressive vertebral and discovertebral kyphotic pseudarthrosis, known as an Andersson lesion, at the L1 and L1-2 level, respectively. Surgical correction and stabilization was performed by an extending transpedicular wedge resection osteotomy to restore spinal stability, to facilitate fracture healing as well as to restore the sagittal balance of the ankylosed spine. To predict the effect of a surgical correction of the Andersson lesion on the sagittal balance, deformity planning was performed preoperatively. The indication for surgery, the surgical technique and the 2 years' clinical results are described. In addition, the difficulties experienced with preoperative deformity planning are evaluated.

Humans↗

PubMed2005

The effect of the antimicrobial peptide, Dhvar-5, on gentamicin release from a polymethyl methacrylate bone cement.

The objective of this study was to investigate the release mechanism and kinetics of the antimicrobial peptide, Dhvar-5, both alone and in combination with gentamicin, from a standard commercial polymethyl methacrylate (PMMA) bone cement. Different amounts of Dhvar-5 were mixed with the bone cement powders of Osteopal and the gentamicin-containing Osteopal G bone cement and their release kinetics from the polymerized cement were investigated. Additionally, the internal structure of the bone cements were analysed by scanning electron microscopy (SEM) of the fracture surfaces. Secondly, porosity was investigated with the mercury intrusion method and related to the observed release profiles. In order to obtain an insight into the mechanical characteristics of the bone cement mixtures, the compressive strength of Osteopal and Osteopal G with Dhvar-5 was also investigated. The total Dhvar-5 release reached 96% in the 100 mg Dhvar-5/g Osteopal cement, whereas total gentamicin release from Osteopal G reached only 18%. Total gentamicin release increased significantly to 67% with the addition of 50mg Dhvar-5/g, but the Dhvar-5 release was not influenced. SEM showed an increase of dissolved gentamicin crystals with the addition of Dhvar-5. The mercury intrusion results suggested an increase of small pores (< 0.1 microm) with the addition of Dhvar-5. Compressive strength of Osteopal was reduced by the addition of Dhvar-5 and gentamicin, but still remained above the limit prescribed by the ISO standard for clinical bone cements. We therefore conclude that the antimicrobial peptide, Dhvar-5, was released in high amounts from PMMA bone cement. When used together with gentamicin sulphate, Dhvar-5 made the gentamicin crystals accessible for the release medium presumably through increased micro-porosity (< 0.1 microm) resulting in a fourfold increase of gentamicin release.

Antimicrobial Cationic Peptides↗

PubMed2005

A bioresorbable molding mesh for impaction grafting revision hip surgery.

Impacted morselized allografts are used to treat bone loss in revision surgery. This technique depends on adequate mechanical support of the graft. Metal support devices function well, but there are disadvantages associated with the use of steel meshes. In this cadaveric, surgical simulation model we investigated the surgical and mechanical suitability of a bioresorbable molding mesh for use in impaction grafting revision surgery. Surgical feasibility was assessed, and mechanical deformation of the mesh during the surgical procedure and postoperative cyclic loading of the specimens were measured with strain gauges. All meshes were surgically usable. The exterior surface deformation of the meshes during the surgical procedure and postoperative mechanical loading did not exceed 4500 microm/m, although the meshes were not damaged in a four-point bending test in which deformations higher than 19,000 microm/m were reached. Therefore, results of this study suggest that this type of bioresorbable mesh seems to have sufficient initial mechanical properties to warrant additional preclinical in vivo study.

Arthroplasty, Replacement, Hip↗

PubMed2005

Tricalcium-phosphate and hydroxyapatite bone-graft extender for use in impaction grafting revision surgery. An in vitro study on human femora.

Impacted morsellised allografts have been used successfully to address the problem of poor bone stock in revision surgery. However, there are concerns about the transmission of pathogens, the high cost and the shortage of supply of donor bone. Bone-graft extenders, such as tricalcium phosphate (TCP) and hydroxyapatite (HA), have been developed to minimise the use of donor bone. In a human cadaver model we have evaluated the surgical and mechanical feasibility of a TCP/HA bone-graft extender during impaction grafting revision surgery. A TCP/HA allograft mix increased the risk of producing a fissure in the femur during the impaction procedure, but provided a higher initial mechanical stability when compared with bone graft alone. The implications of the use of this type of graft extender in impaction grafting revision surgery are discussed.

Arthroplasty, Replacement, Hip↗

PubMed2002

In vitro and in vivo degradation of bioabsorbable PLLA spinal fusion cages.

The in vitro and in vivo degradation of poly-L-lactic acid cages used as an adjunct to spinal arthrodesis was investigated. In the in vitro experiments cages were subjected to aging up to 73 weeks in phosphate-buffered solution (pH 7.4) at 37 degrees C. Inherent viscosity, crystallinity, and mechanical strength were determined at different time points. In the in vivo study, the poly-L-lactic acid cages were packed with bone graft and implanted in the L3-L4 spinal motion segment of 18 Dutch milk goats. At 12, 26, and 52 weeks, the motion segments were isolated and poly-L-lactic acid samples retrieved. On evaluation, the in vivo implanted cages showed an advanced decline in inherent viscosity compared to the cages subjected to in vitro degradation experiments. At 6 months of implantation, the geometrical shape and original height of 10 mm was maintained during 6 months of follow up. This finding fits well with the observation that mechanical strength was maintained for a period of 6 months in vitro. At 12 months, the poly-L-lactic acid cage had been disintegrated into multiple fragments with signs of absorption. Despite the high-load-bearing conditions, the poly-L-lactic acid cage allowed interbody fusion to occur without collapse of the cage.

Absorption↗

PubMed2002

Resorbable cages for spinal fusion: an experimental goat model.

OBJECT: A biomechanical cadaveric study and an in vivo monosegmental spinal fusion study were performed to evaluate a novel bioresorbable poly(L-lactic acid) (PLLA) cage. METHODS: The yield strength of a spinal segment was chosen as the main design parameter for the resorbable cages to be used in a goat model. In a 3-year in vivo study the authors found fusion to be significantly faster and more complete when using PLLA cages compared to titanium cages with the same dimensions. In the PLLA group, the intervertebral grafting height did not change and bone remodeling within the cage was completed 2 years after implantation. In terms of degradation of the PLLA, similar features were observed in vivo and in vitro. CONCLUSIONS: Degradation was almost completed 3 years after implantation. Tissue reaction was mild during the 3-year period.

Absorbable Implants↗

PubMed2002

Resorbable cages for spinal fusion: an experimental goat model.

A biomechanical cadaver study and an in vivo monosegmental spinal fusion study were performed to evaluate a novel bioresorbable poly-L-lactic acid (PLLA) cage. The yield strength of a spinal segment was chosen as the main design parameter for the resorbable cages to be used in a goat model. A 3-year in vivo study revealed a significantly faster and more complete fusion using PLLA cages as compared to titanium cages with the same dimensions. In the PLLA group, the intervertebral grafting height did not change and bone remodeling within the cage was completed 2 years after implantation. In terms of degradation of the PLLA, similar features were observed in vivo and in vitro. Degradation was almost completed 3 years after implantation. Tissue reaction was mild during the 3-year period.

Absorbable Implants↗

PubMed2001

Polyurethane real-size models used in planning complex spinal surgery.

STUDY DESIGN: The application of polyurethane real-size models for planning and performing complex spinal surgery is described. OBJECTIVE: To determine the feasibility of using polyurethane real-size models to plan osteotomies, resections, and designs of custom-made spinal implants in complex spinal surgery. SUMMARY OF BACKGROUND DATA: In selected patients with complex spinal pathology, exact planning of the surgical procedure is not possible using current imaging methods. In these cases, real-size spinal models would be desirable to enhance pre- and perioperative planning by visual and tactile feedback, and to improve the production of custom-made spinal implants. METHODS: A real-size spinal model of six patients was produced from hardened polyurethane foam on the basis of data from contiguous computer tomography slices. In two patients, the models were used to plan correction osteotomies and resections, with the assistance of image-guided surgery in one of the patients. In four patients, the models were used to plan tumor resections and to produce custom-made spinal implants. RESULTS: In all the patients, the surgical procedure could be performed exactly according to the preplanned intervention. The polyurethane real-size models provided essential and additional information by direct visual and tactile feedback. They allowed in vitro testing of custom-made spinal implants with a perfect fit. CONCLUSIONS: Real-size spinal models made from polyurethane foam can be used to provide excellent understanding of the complex spinal pathology in highly selected patients. These models allow complex spinal surgery with a more predictable outcome.

Adolescent↗

PubMed2001

Disorders in trunk rotation during walking in patients with low back pain: a dynamical systems approach.

OBJECTIVE: (1) To introduce an evaluation tool for the assessment of walking disorders in low back pain patients. (2) To investigate whether walking patterns in low back pain patients are different from those of control subjects. DESIGN: Relative phase measures of movement coordination are applied in the assessment of trunk function in a small group of patients with non-specific low back pain and in control subjects. BACKGROUND: Normal subjects change the coordination of pelvic and thoracic rotations from an in-phase to an out-of-phase pattern with increasing walking speed. Low back pain patients may have a reduced ability to counter rotate pelvis and thorax at higher walking speeds (from 1.0 m/s onwards) as a result of hyperstable coordination patterns. METHODS: Six patients with non-specific low back pain and six healthy control subjects walked on a treadmill at comfortable walking speeds and during a systematic variation of the treadmill velocity. Coordination of arm and leg movements as well as of pelvic and thoracic rotations was analyzed using a relative phase algorithm. RESULTS AND CONCLUSIONS: The comfortable walking speed was reduced in the patient group. In contrast to the control subjects, four of the six patients were not able to establish an out-of-phase coordination pattern between thorax and pelvis at higher walking speeds. This coincided with an increased stability of movement coordination, indicating guarded behavior. In addition, an increased asymmetry between the phase-relations of left and right side of the body was found in some of the patients.

Adolescent↗

PubMed2000

Deformity planning for sagittal plane corrective osteotomies of the spine in ankylosing spondylitis.

Ankylosing spondylitis (AS) may lead to a severe fixed thoracolumbar kyphotic deformity (TLKD) of the spine. In a few patients, the TLKD is so extreme that a corrective osteotomy of the spine may be considered. Several authors have reported the results of patients treated by a lumbar osteotomy, but there is no consensus on the level of the osteotomy and on the exact degree of correction required. This can be explained by the lack of quantification of the sagittal plane deformity, since compensation mechanisms of the lower extremities have to be reckoned with for the assessment of spinal sagittal balance in AS. Therefore, there is a need for a method of deformity planning for sagittal plane corrective osteotomies of the spine in AS. In this study, a biomechanical analysis and a newly developed planning procedure are presented and illustrated with two cases of AS. Sagittal balance of the spine was defined in relation to the physiologic sacral end plate angle using trigonometric terms. Nomograms were constructed to show the relationship between the correction angle, horizontal position of the C7 plumb line and the level of the spinal osteotomy. The surgical results of two patients were retrospectively analyzed with our method. It showed that the effect of a spinal osteotomy on the horizontal position of the C7 plumb line depends on the combination of correction angle and the level of osteotomy. In one patient, the achieved correction of the deformity proved to correct the sagittal spinal balance and the pelvic sacral endplate angle. In the other patient, the achieved correction was not sufficient. It is concluded that adequate deformity planning for sagittal plane corrective osteotomies of the spine in AS is essential for reliable prediction of the effect of a lumbar osteotomy on the correction of the spine.

Adult↗

PubMed2000

Is BMU-coupling a strain-regulated phenomenon? A finite element analysis.

Histologically, two types of bone reconstruction are distinguished: modeling and remodeling. Modeling changes the amount of bone and determines its geometrical form in relation to the prevailing mechanical loads and their resulting deformation (strain). Remodeling renews existing bone in a sequence of resorption and formation. However, in both processes the cells responsible for resorption and formation are the same: osteoclasts and osteoblasts. We studied if there is a relation between the activity of these cells and the deformation of the local bone tissue during remodeling. Two finite element models were built on a microscopic, supracellular level: (1) a secondary osteon in cortical bone and (2) a Howship's lacuna in a trabecula. Both models were loaded in the "natural," that is, longitudinal direction. Equivalent strains were determined as a measure for the deformation of the bone tissue. In the first model, the strain field around the osteon showed a region of decreased deformation in front of the tunnel, just where osteoclasts excavate cortical bone tissue. Behind the cutting cone, elevated strain levels appear in the tunnel wall at locations where osteoblasts are active. The second model showed that a local excavation of a loaded trabecula leads to higher strains at the bottom of the lacuna, where resorption is stopped and osteoblasts are recruited to refill the gap. However, in the direction of loading reduced strain levels appear, just where resorption continues to proceed along the trabecular surface. We conclude that at the tissue level, strain distributions occur during the remodeling process that show a relationship to the activity of osteoblasts and osteoclasts. This suggests that BMU coupling, that is, the subsequent activation of osteoclasts and osteoblasts during remodeling, is a strain-regulated phenomenon.

Animals↗

PubMed1998

Star length distribution: a volume-based concept for the characterization of structural anisotropy.

Determination and quantification of anisotropy is of great interest in research fields dealing with physical structures or surface textures. In this paper, a volume-based method is presented, which essentially determines the mean object length in a certain direction for a typical point within a structure or texture. The mean object lengths for all orientations together form the so-called star length distribution (SLD). The validity and the accuracy of the SLD method are investigated, and illustrated by applying it to trabecular bone. By using a line sampling algorithm, the relation with other anisotropy measures could be studied analytically. Preliminary tests suggest that with SLD a more exact description of the mechanical properties of porous structures may be obtained than with other anisotropy measures. However, due to possible secondary orientations that become apparent with SLD, a fabric tensor must be of rank higher than two in order to properly describe an orthogonal structure mathematically.

Adolescent↗

PubMed1997

Structure and function of vertebral trabecular bone.

STUDY DESIGN: A combined morphologic and finite-element study on vertebral trabecular bone. OBJECTIVE: To relate the form and function of vertebral trabecular bone, in an attempt to better understand the mechanical function of a lumbar vertebra. SUMMARY OF BACKGROUND DATA: The architecture of bone is closely related to its mechanical function (Wolff's Law). In the human spine, vertebrae are subjected to a large variety of loads. Yet, these bones show a typical architecture, which means that they carry typical loads. METHODS: Five trabecular bone cubes from specific sites of a lumbar vertebra were 3D-reconstructed for computerized analysis. The architecture of the specimens was quantified by the bone volume fraction and a measure of anisotropy, the mean bone length. A finite element model was used to calculate internal stresses within a homogeneous vertebral body under basic loads. For each load case, bone volume fraction of the specimens was compared with the equivalent von Mises stress, and mean bone length was compared with the principal stress directions. RESULTS: Bone volume fraction poorly related to the von Mises stress in the physiologic load case of axial compression. However, high bone volume fractions exist at locations where multiple load situations occur (e.g., near the pedicles and endplates). Remarkably, these sites also show finer architectures. Comparison of mean bone length with principal stresses revealed that the vertebral trabecular bone architecture particularly, but not entirely, corresponds to the stress field under axial compression. The horizontal struts near the end-plates were found to be due to the function of the healthy intervertebral disc, and facetal joint loads introduce stress components that relate well with the bone structures near the pedicle bases. CONCLUSIONS: The trabecular bone architecture and the vertical orientation of the facet joints suggest that walking may be the main activity that determines the lumbar vertebral bone architecture.

Anatomy, Cross-Sectional↗