S Wolf

Fujita Health University, Nagoya, Aichi, Japan

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Publications (19)35.27 Total impact

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    ABSTRACT: Comparison of one-plane and two-plane external fixation in terms of successful healing, incidence of complications, and biomechanical stability in a sheep model. Rigid fixation is preferred in open and comminuted fractures with a reduced blood supply, preventing infection and healing delay, but more often a flexible device is recommended even in unfavourable healing conditions. The left tibiae of fifteen sheep were osteotomized and laterally fixed with a four-screw unilateral fixator frame (axial stiffness 183 N/mm) to a 3 mm gap size. In 9 of 15 sheep, an additional four-screw unilateral external fixator was anterolaterally attached (total axial stiffness of both frames 388 N/mm). After sacrificing, quality of osteotomy healing was assessed by mechanical and radiological evaluations. Osteogenesis was measured using fluorescence microscopy. Two distal fractures through the pin-tracks, three non-unions and four deep infections occurred after two-plane fixation. These failures excluded, osteotomy healing showed inferior results after two-plane fixation with reduced callus formation, bone mineral content, and bending stiffness amounts, respectively. Osteogenesis was halved following two-plane fixation in the remaining sheep. Two-plane fixation was not sufficient to reach successful osteotomy healing in our study. While higher rigidity was expected to prevent complications, healing in this group might have been disturbed by a reduced blood supply. The optimal stabilisation for a given fracture depends on many factors, including the biomechanical and biological environment. Considering our results and the literature discussed in this manuscript, good bone healing with minor risks of infections can be achieved using an unilateral one-plane fixator with only four screws, and its application on a muscle free position like the medial and anterior site of the sheep tibia.
    Clinical Biomechanics 08/2002; 17(6):470-6. · 1.87 Impact Factor
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    ABSTRACT: Porous composite materials made of poly(L, DL-lactide) and a ceramic component, alpha-tricalcium phosphate (alpha-TCP) or one of the rapidly resorbable glass ceramics, GB9N or GB14N, respectively, were developed to be used as bone substitutes. The present article describes the mechanical properties and the in vitro degradation characteristic of the different composite materials. The yield strength, the elastic modulus, and the molecular weight were measured after in vitro degradation up to 78 weeks. The initial strengths of the alpha-TCP composite (12.5 +/- 0.7 MPa) was higher than that of the GB9N and GB14N composites (8.3 +/- 0.2 MPa and 10.9 +/- 0.2 MPa, respectively). The initial elastic moduli of the composites were between 450 and 650 MPa. The mechanical properties remained constant until a degradation period of 26 weeks. Then they decreased continuously until they were completely lost at week 52. The molecular weight (M(w)) decreased steadily from 91,000 D in the case of the alpha-TCP composite and 78,000 D and 85,000 D in the case of the GB9N or GB14N composites, respectively, to about 10,000 D at week 78. It was concluded that the composites show adequate mechanical properties in the range of cancellous bone and a suitable degradation characteristic to be used as bone substitute materials.
    Journal of Biomedical Materials Research 11/2001; 57(1):126-31.
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    ABSTRACT: Recent studies have shown osteogenic effects of high-frequency mechanical stimuli. The purpose of this study was whether externally applied, high-frequency, low-magnitude interfragmentary movements affect the process of bone healing. In 12 sheep, a transverse osteotomy with a 3 mm gap was created in the right metatarsus and externally stabilized by a rigid circular fixator. External stimulation was performed in six sheep with the use of ground-based vibration. The sheep were standing with their hind limbs on a platform that produced vertical movements resulting in interfragmentary movements of approximately 0.02 mm magnitude at 20 Hz frequency. The other six sheep remained rigidly stabilized by external fixation during the 8-week study and served as a control group. Healing was assessed postmortem by densitometric and mechanical examinations. No significant differences were found between the two groups, although callus formation was slightly enhanced (11%) in the stimulated group compared with the control group. Mechanical stimuli attributable to weightbearing in the control group were sufficient enough to initiate callus formation even under rigid, external fixation. Thus, external mechanical stimulation with the stimulation design described in the current study might not be indicated for improvement of bone healing.
    Clinical Orthopaedics and Related Research 05/2001; · 2.79 Impact Factor
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    P Augat, J Merk, S Wolf, L Claes
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    ABSTRACT: To determine whether an externally induced interfragmentary movement enhances the healing process of a fracture under flexible fixation. Randomized, prospective in vivo animal study with control group. Twenty-four skeletally mature Merino sheep were randomly assigned to six groups of four animals, which received cyclic interfragmentary movements of 0.2 and 0.8 millimeters and stimulation frequencies of 1, 5, and 10 Hertz, respectively. Twelve animals did not receive any externally applied stimulation and served as a control group. Unrestricted stall activity with weight bearing reduced by tenotomy of the Achilles tendon. Osteotomy of the tibial diaphysis with three-millimeter gap width fixed with a six-pin, monolateral, double-bar external fixator. Interfragmentary movement of the osteotomy gap was externally induced by a motor-driven actuator unit. Five hundred cycles inducing nonuniform tensile strains within the gap were performed each day. Nine weeks after surgery, the animals were killed, and bone mineral density and callus cross-sectional area were measured with quantitative computed tomography. Callus projectional area was assessed by radiographs, and mechanical stability was determined with a three-point bending test. External stimulation with nonuniform cyclic tensile strains did slightly affect but not significantly enhance the fracture healing process. Varying the stimulation frequency had no influence on the healing process. The stimulation with 0.8 millimeter displacement magnitude resulted in a larger periosteal callus, but a decreased bone mineral density compared with the 0.2-millimeter displacement magnitude. The stimulation had no significant influence on the mechanical properties of the healing bone. Induced cyclic tensile strains did not produce a relevant enhancement of bone healing under flexible fixation.
    Journal of Orthopaedic Trauma 02/2001; 15(1):54-60. · 1.75 Impact Factor
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    ABSTRACT: Micromovement at fracture sites is known to promote callus formation and bridging of the bony fragments. The present study was conducted to identify the suitable amount of micromovement, and to analyze the location and timing of callus proliferation. A standardized transverse osteotomy, in the right metatarsus of 32 sheep, was used as a fracture model. The osteotomy was externally fixed with a special ring fixator, which allowed axial micromovements of defined sizes. The animals were divided into four groups, with gaps of 2 mm and 6 mm, and micromovements of 0.3 mm and 0.7 mm, respectively. The labeling of new bone formation was performed by the intravenous injection of calcein green in the fourth week and tetracycline in the eighth week. Nine weeks postoperatively the sheep were killed. The explanted metatarsals were radiographed for the measurement of the periosteal callus area and were nondestructively loaded in a three-point bending test to determine their flexural rigidity. Histological analysis of undecalcified bone was performed in bone slices in the sagittal plane. Fluorescent green (callus formed in the fourth week) and yellow areas (callus formed in the eighth week) and the area of connective tissue were determined, using fluorescence microscopy. Bone formation was larger in the eighth week than that in the fourth week in all groups. In the fourth week, large micromovements in the small gap resulted in increased bone formation, whereas, for large gaps, the large micromovements diminished new bone formation. With large micromovement, the amount of newly formed bone within the gap decreased with increasing gap size, suggesting a delay of bone healing. Stimulation of new bone formation by micromovement was mainly effective in the early healing phase (4 weeks postoperatively). Large gaps showed the least new bone formation at the fracture site and the lowest flexural rigidity. From the histological analysis, it was found that the flexural rigidity correlated with the new bone area in the periosteal region.
    Journal of Orthopaedic Science 02/2001; 6(6):571-5. · 0.96 Impact Factor
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    ABSTRACT: Metastatic spine lesions frequently require corpectomy in order to achieve decompression of the spinal cord and restoration of spinal stability. A variety of systems have been developed for vertebral body replacement. In patients with prolonged life expectancy due to an improvement of both systemic and local therapy, treatment results can be impaired by a loosening at the implant-bone interface or mechanical failure. Furthermore, early detection of a metastatic recurrence using sensitive imaging modalities like computed tomography (CT) and magnetic resonance imaging (MRI) is possible in these patients without artefact interference. The aim of our pilot study was to evaluate the clinical applicability and results of a new radiolucent system for vertebral body replacement in the lumbar spine. The system consists of bone-integrating biocompatible materials - a polyetherurethane/bioglass composite (PU-C) replacement body and an integrated plate of carbon-fibre reinforced polyetheretherketone (CF-PEEK) - and provides high primary stability with anterior instrumentation alone. In a current prospective study, five patients with metastatic lesions of the lumbar spine were treated by corpectomy and reconstruction using this new system. Good primary stability was achieved in all cases. Follow-up (median 15 months) using CT and MRI revealed progressive osseous integration of the PU-C spacer in four patients surviving more than 6 months. Results obtained from imaging methods were confirmed following autopsy by biomechanical investigation of an explanted device. From these data, it can be concluded that implantation of the new radiolucent system provides sufficient long-term stability for the requirements of selected tumour patients with improved prognosis.
    European Spine Journal 11/2000; 9(5):437-44. · 2.47 Impact Factor
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    L Claes, S Wolf, P Augat
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    ABSTRACT: Interfragmentary movement and size of the fracture gap influence fracture healing. Limited movements promote callus formation and may result in increased mechanical stability. Although larger movements still promote callus formation, the bony consolidation of the fracture is hampered. Fracture healing is also hampered if the size of the fracture gap is too large. A combination of large movement and large gap bears the risk of non-union. Therefore, having in mind a minimally invasive surgical approach, one should strive for good reduction of the fracture ends and flexible yet stable osteosynthesis. Dynamization of the fracture by enabling axial movement will close the fracture gap, stimulate tissue differentiation and possibly accelerate the healing process. External mechanical stimulation, however, has not been shown to effectively enhance the healing process under flexible fixation or in load-bearing patients.
    Der Chirurg 10/2000; 71(9):989-94. · 0.52 Impact Factor
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    L. Claes, S. Wolf, P. Augat
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    ABSTRACT: Kleine interfragmentäre Bewegungen stimulieren die Callusheilung während zu große Bewegungen häufig eine knöcherne Überbrückung verhindern. Bei großen Frakturspalten kommt es zu einer Verzögerung von Callusbildung und Frakturüberbrückung. Die Gefahr einer Pseudarthrose besteht bei zu großer interfragmentärer Bewegung (Instabilität) und zu großen Frakturspalten. Trotz aller Bestrebungen zu gewebeschonenden Operationstechniken ist deshalb eine gute Reposition und eine flexible aber stabile Osteosynthese die Voraussetzung für eine komplikationsfreie Callusheilung. Eine Dynamisierung durch zusätzliche axiale Bewegung kann sich positiv auswirken, weil dadurch Frakturspalten geschlossen werden und der Reiz zur Callusdifferenzierung erhöht wird. Eine externe mechanische Stimulation scheint bei flexiblen Osteosynthesen und aktiven Patienten nicht erforderlich. Interfragmentary movement and size of the fracture gap influence fracture healing. Limited movements promote callus formation and may result in increased mechanical stability. Although larger movements still promote callus formation, the bony consolidation of the fracture is hampered. Fracture healing is also hampered if the size of the fracture gap is too large. A combination of large movement and large gap bears the risk of non-union. Therefore, having in mind a minimally invasive surgical approach, one should strive for good reduction of the fracture ends and flexible yet stable osteosynthesis. Dynamization of the fracture by enabling axial movement will close the fracture gap, stimulate tissue differentiation and possibly accelerate the healing process. External mechanical stimulation, however, has not been shown to effectively enhance the healing process under flexible fixation or in load-bearing patients.
    Der Chirurg 08/2000; 71(9):989-994. · 0.52 Impact Factor
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    ABSTRACT: Anterior intervention of metastatic lesions of the spine can accomplish relief of pain, spinal decompression, and restoration of spinal stability. Ventral vertebral body replacements have been developed to provide these conditions but there have been problems with loosening at the implant-bone interface, mechanical failure, and X-ray artifacts from the metal. Intraoperative stability of the vertebral body replacement is especially critical to avoid loosening of the implant and to achieve long-term bony incorporation. This study compared the biomechanical performance in vitro of a new radiolucent system for vertebral body replacement to three currently marketed systems. The new system features a composite bioglass-polyurethane body and a new configuration of polymeric fastening hardware. Range of motion, neutral zone, and several interfacial motion parameters were measured under pure moments of 3.75 Nm in the three anatomical directions. The new system provided the significantly highest restraint of motion for all parameters. Mechanically, the new system is preferable at least initially to a sampling of systems representative of those currently used.
    Journal of Biomedical Materials Research 02/1999; 48(1):82-9.
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    ABSTRACT: To characterize the site-specific mechanical and histological properties in fracture repair and to relate these properties to the initial mechanical situation, an experimental fracture model was used in the metatarsus of 42 sheep. The mechanical situation of a transverse osteotomy was described by three gap sizes (1, 2, or 6 mm) and two amounts of strain (7 or 31%). An external fixator that allowed a defined axial movement provided control of these settings. Nine weeks following surgery, the healing area was dissected and tensile and compressive properties were measured in subregions of the fracture gap and the periosteal callus. The central, sagittal section was used for quantitative histology. We found the quality of the tissue along the osteotomy line to be most important for regaining mechanical stability. Increasing the size of osteotomy gaps resulted in poorer mechanical and histological qualities, and the repair process was less complete. Interfragmentary strain did not significantly influence the repair process. The smaller strain levels had already stimulated the secondary repair process, and this stimulatory effect could not be further enhanced by increasing the amount of strain. Our finding that large gaps between bone segments were not as well healed as were smaller gaps suggests that it is advantageous to avoid large gaps in fracture treatment.
    Journal of Orthopaedic Research 08/1998; 16(4):475-81. · 2.88 Impact Factor
  • Wolf S, Augat P, Wan S, Claes L
    Journal of Biomechanics 06/1998; 31:37-37. · 2.72 Impact Factor
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    ABSTRACT: OBJECTIVE: The purpose of this study was to investigate the effect of an externally applied mechanical stimulus on fracture healing under flexible fixation. DESIGN: Stimulation of fracture healing under various conditions of interfragmentary movement in an in vivo fracture model on 41 sheep. BACKGROUND: It is generally accepted that small interfragmentary movements (IFMs) yield better bone healing results than larger IFMs (> 1 mm). However, the optimal size of IFM within the 1-mm range remains undetermined. METHODS: Standardized transverse osteotomy of 3 mm gap size in the left ovine tibia was fixed with an unilateral external fixator. The sheep were divided into four IFM groups of 0.0, 0.2, 0.4 and 0.8 mm and stimulated with this amplitude for 1200 cycles per day at 1 Hz. After a healing period of 6 weeks, bone mineral density and biomechanical stability were evaluated to determine the quality of healing. RESULTS: The amount of callus formation increased significantly with increasing IFM (P <0.05). However, highest biomechanical stability of the healed bone and mineral density of the gap tissue was achieved with an IFM of 0.4 mm, although the differences were not significant. CONCLUSIONS: These results suggest that the optimal interfragmentary movement for acceleration of delayed fracture healing is in the range of 0.5 mm. However, the enhancement of the healing of flexibly-fixed fractures by external application of interfragmentary movement is limited. RELEVANCE: In this model the external application of a mechanical stimulus in addition to the stimulation caused by normal loading and the flexibility of the fixation did not enhance the healing process significantly. It appears that the external application of interfragmentary motion is promising perhaps only for patients unable to stimulate their fracture healing by weight-bearing.
    Clinical biomechanics (Bristol, Avon) 06/1998; 13(4-5):359-364. · 1.76 Impact Factor
  • Journal of Biomechanics 01/1998; 31. · 2.72 Impact Factor
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    ABSTRACT: Objective. The purpose of this study was to investigate the effect of an externally applied mechanical stimulus on fracture healing under flexible fixation.Design. Stimulation of fracture healing under various conditions of interfragmentary movement in an in vivo fracture model on 41 sheep.Background. It is generally accepted that small interfragmentary movements (IFMs) yield better bone healing results than larger IFMs (> 1 mm). However, the optimal size of IFM within the 1-mm range remains undetermined.Methods. Standardized transverse osteotomy of 3 mm gap size in the left ovine tibia was fixed with an unilateral external fixator. The sheep were divided into four IFM groups of 0.0, 0.2, 0.4 and 0.8 mm and stimulated with this amplitude for 1200 cycles per day at 1 Hz. After a healing period of 6 weeks, bone mineral density and biomechanical stability were evaluated to determine the quality of healing.Results. The amount of callus formation increased significantly with increasing IFM (P < 0.05). However, highest biomechanical stability of the healed bone and mineral density of the gap tissue was achieved with an IFM of 0.4 mm, although the differences were not significant.Conclusions. These results suggest that the optimal interfragmentary movement for acceleration of delayed fracture healing is in the range of 0.5 mm. However, the enhancement of the healing of flexibly-fixed fractures by external application of interfragmentary movement is limited.
    Journal of Biomechanics 01/1998; · 2.72 Impact Factor
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    ABSTRACT: The purposes of this study were to determine the effect of including muscle forces in the experimental loading of the spine on the intradiscal pressure and to determine whether this effect correlates with previously established in vivo data. We modeled the spine muscles as of five distinct groups and isolated the effect of each group on the intradiscal pressure (L4-L5). Seven human lumbosacral spines were tested in pure flexion/extension, right/left lateral bending, and left/right axial rotation moments. Stimulated muscle activity strongly influenced load-pressure characteristics, especially for the multifidus. Without muscle forces active, pressure increased proportionately with increasing moment. With five pairs of symmetrical constant muscle forces active (80 N per pair) the pressure increased more than 200% in neutral position and did not increase with increasing moment. The pressure without muscle forces and without axial preload was 0.12 MPa, which is about the same found by earlier in vivo studies of anesthetized subjects in prone position. With simulated muscle forces, the pressure was 0.39 MPa and in the range found for non-anesthetized subjects. We conclude that simulating muscle forces substantially affects intradiscal pressure.
    Journal of Biomechanics 05/1996; 29(4):549-55. · 2.72 Impact Factor
  • Journal of Biomechanics 01/1996; 29(4). · 2.72 Impact Factor
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    ABSTRACT: The effect of speech rate reduction on speech and pause characteristics during a reading task was examined for speakers with Parkinson's disease (PD) and a group of control speakers. Duration of utterances and characteristics of pausing (duration, interpause phrase length, and location) were determined. At habitual reading rate, subjects with PD had shorter speech duration and greater time per pause than control subjects. At reduced reading rates, subjects with PD increased speech duration so that it was equivalent to the control speakers at habitual rate. Both groups had the majority of their pause occurring at appropriate syntactic boundaries. Subjects with PD had a greater proportion of pauses occurring at syntactically inappropriate locations than did the controls. When speech rates were reduced, both groups showed a decrease in pauses located at appropriate syntactic boundaries. The implication of these findings on speech intelligibility is discussed.
    Journal of Communication Disorders 01/1996; 29(6). · 1.55 Impact Factor
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    ABSTRACT: This study investigated the influence of five different muscle groups on the monosegmental motion (L4-L5) during pure flexion/extension, lateral bending, and axial rotation moments. The results showed and compared the effect of different muscle groups acting in different directions on the stability of a single motion segment to find loading conditions for in vitro experiments that simulate more physiologically reasonable loads. In spine biomechanics research, most in vitro experiments have been carried out without applying muscle forces, even though these forces stabilize the spinal column in vivo. Seven human lumbosacral spines were tested in a spine tester that allows simulation of up to five symmetrical muscle forces. Changing pure flexion/extension, lateral bending, and axial rotation moments up to +/- 3.75 Nm were applied without muscle forces, with different muscle groups and combinations. The three-dimensional monosegmental motion was determined using an instrumented spatial linkage system. Simulated muscle forces were found to strongly influence load-deformation characteristics. Muscle action generally decreased the range of motion and the neutral zone of the motion segments. This was most evident for flexion and extension. After five pairs of symmetrical, constant muscle forces were applied (80 N per pair), the range of motion decreased about 93% in flexion and 85% in extension. The total neutral zone for flexion and extension was decreased by 83% muscle action. The multifidus muscle group had the strongest influence. This experiment showed the importance of including at least some of the most important muscle groups in in vitro experiments on lumbar spine specimens.
    Spine 02/1995; 20(2):192-8. · 2.16 Impact Factor
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    ABSTRACT: We report a new apparatus to determine the quasistatic, three-dimensional, load-displacement characteristics of spines including muscle forces. The loading frame can be adapted to mono- and polysegmental specimens from the lumbar or cervical spine as well as to entire spines. Three force and three moment components can be applied in either direction individually or in combination with no constraint on the resulting motion; the loads can be applied at user-chosen rates of application and release with continuous recording of displacements, so as to study either creep or relaxation. The loads and displacement-measuring devices are computer-controlled. Thus, this testing device provides a tool for many kinds of stability tests and for basic research of spine biomechanics. A first experiment shows that the application of muscle forces significantly affects the load-deformation characteristics and intradiscal pressure.
    European Spine Journal 02/1994; 3(2):91-7. · 2.47 Impact Factor

Publication Stats

632 Citations
35.27 Total Impact Points

Institutions

  • 2001
    • Fujita Health University
      • Department of Orthopedic Surgery
      Nagoya, Aichi, Japan
  • 1994–2000
    • Universität Ulm
      • • Clinic of Trauma, Hand, Plastic and Reconstructive Surgery
      • • Institute of Orthopaedic Research and Biomechanics
      Ulm, Baden-Wuerttemberg, Germany