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S M Perren

Publications and source records attributed to S M Perren.

At least 37 records · Page 2Linked to original sources
PubMed2000

Effect of plate position relative to bending direction on the rigidity of a plate osteosynthesis. A theoretical analysis.

Mechanical unloading of the plated bone segment is observed after plate osteosynthesis because the implant takes over a part of the physiological loading. Strain reduction in the bony tissue depends on the rigidity of the plate (cross-sectional area, geometrical form, and modulus of elasticity). The aim of the present study was to calculate theoretically the effect of plate position relative to bending direction on the overall bending stiffness of the composite system plate-bone. To calculate the rigidity, a cylindrical bone model with mechanical characteristics similar to a sheep tibia and a rectangular plate cross-section corresponding to a DC-plate with either a modulus of elasticity of steel or titanium was used. Calculations under different bending directions were performed according to the laws of the linear bending theory and the composite beam theory. The bending stiffness of a plate osteosynthesis reaches a minimum and a maximum respectively, in cases in which the bending moment acts in the direction of the main axis of the area moment of inertia of the plate. The minimum is present with the plate bent vertically, the maximum with the plate bent horizontally, e.g. on the tension side of the composite system--on the assumption that the bone structure opposite the plate is capable of withstanding compressive loading. For steel and titanium plates, factors of 2 and 2.25 respectively were calculated between the minimum and the maximum bending stiffnesses of the osteosynthesis. The bending rigidity of the plate alone has only a minimal effect on the total stiffness of the osteosynthesis. With a plate bent vertically, the difference between steel and titanium plates was 18%, with the plate bent horizontally (situated on the tension side), it was only 7%. The bending stiffness of a plate osteosynthesis depends on the cross-section, the geometrical form, and the modulus of elasticity of the plate, as well as on the plate position relative to the bending direction of the composite system. The modulus of elasticity of the plate is relatively unimportant, while with a given plate the individual plate position relative to the bending direction is of crucial importance. Thus, changing the modulus of elasticity of the plate cannot solve the problem of implant induced unloading of the bone cortex because the bending stiffness of the composite system depends much more on the plate position relative to the bending direction.

Animals↗
PubMed2000

Force transfer between the plate and the bone: relative importance of the bending stiffness of the screws friction between plate and bone.

Stability in plate fixation of fractures relates to the motionless fastening between plate and bone. When the plate is affixed to the bone, high shear force may appear between plate and bone, particularly near the end screws, which can lead to motion under weight-bearing. Two mechanisms might be involved in the prevention of motion: the bending stiffness of the screws or the friction between the screw and the plate. Experiments performed in vivo and in vitro show that with conventional plates, motion is prevented by friction and depends upon the axial force of the screw, pressing the plate onto the bone. The torque applied to the screws is crucial. Motion appears under smooth plates under relatively low physiological loads. With newly developed internal fixators, the motion is prevented by the structural stiffness of the plate-screw system.

Animals↗
PubMed2000

Mechanical analysis of the bone to plate interface of the LC-DCP and of the PC-FIX on human femora.

The scope of this analysis was to evaluate the mechanical behaviour of newly developed plates at the junction between plate and bone (friction between plate and bone) for the limited contact dynamic compression plate (LC-DCP) and the point contact fixator (PC-Fix) under simulated physiological load and using the tension band principle on the human femur. The intact human cadaveric femora were plated on the lateral aspect according to the tension band principle (AO) and subjected to a load which simulated careful physiological load in single stance. Five strain gauges were glued around the bones, parallel to the bone axis, at five levels, whereby three of them had to be covered by a bone plate and the two others were just outside the plate location. The cross-sectional geometry had been obtained at these levels using computed tomography. One side was plated using the conventional compression plate LC-DCP and the contralateral side using the internal fixator PC-Fix. The LC-DCP was affixed using screws tightened at different torque values and the PC-Fix at a standard torque value. Motion (slippage) between the plate and the bone was indicated by a hysteresis of the strain reading during loading and unloading. Slippage was more important for the LC-DCP than for the PC-Fix, particularly at the proximal end of the plate and when the screws were insufficiently tightened on the LC-DCP. As expected, better stability was obtained with the PC-Fix.

Animals↗
PubMed2000

Strain distribution in plated and unplated sheep tibia an in vivo experiment.

After plate osteosynthesis changes in bone biology and bone mechanics are observed in the plated bone segment. Compromise of the vascular supply in the plate bed leads to a remodelling process and to a temporary porosity in the bone cortex underneath the plate. In addition, the plate takes over some of the physiological loading of the bone, which in turn alters the normal strain distribution of the cortical bone tissue. The aim of the present study is to determine the tissue deformation of the sheep tibia in vivo and the changes in tissue strain due to plating with plates of different rigidities. Measurements were performed on the intact bone at the mid diaphysis using the strain gauge technique. With different connections on the tension bridge (Wheatstone bridge), the strain was measured separately for pure axial loading, bending, and torsion before and after plating with a 4.5 mm stainless steel or titanium DCP. Under physiological load the sheep tibia is mainly deformed in torsion (62%) and bending (33%), and much less in axial loading (5%). Plating with a steel plate reduces the overall tissue strain by 18%, with a titanium plate by 13%. This reduction is mainly due to a reduction in axial tissue strain due to axial loading and bending and less to the reduction of tissue strain under torsion. In our in vivo model, plating with steel or titanium plates leads to a reduction of the physiological tissue strains. The difference between the different plates is small due to the fact that the high tissue strain under torsion is only slightly affected by plating. Thus, from the purely mechanical point of view and with regard to preserving normal tissue strains as much as possible, titanium plates offer little advantage compared with stainless steel plates. But, titanium as an implant material may offer advantages with respect to tissue compatibility and infection resistance.

Animals↗
PubMed2000

Functional load of plates in fracture fixation in vivo and its correlate in bone healing.

In clinical practice efforts are made to apply a fixation plate on the side opposite the strongest muscle pull. This achieves an optimal distribution of compression between the fragment ends (principle of tension band plating). This is however frequently impossible for anatomical or surgical reasons. In an 'in vivo' study lasting 8 weeks a standardized oblique osteotomy was performed on the tibia of 16 sheep in four different models of tension band plating (a contoured and an overbent plate with or without an interfragmentary lag screw) were assessed. Tension on the plate surface was recorded by strain gauges for different gait speeds on the treadmill. These measurements were performed throughout the experiment. Radiographs were taken at regular intervals in order to assess stability and polychrome sequential labelling and microradiographs served to investigate the healing process. Possible relationships and/or interactions between plate tension and bone healing were investigated. Implant loading under bending strain was reduced the most for the combination of plate overbending with a lag screw. The insertion of a lag screw reduces the surface strain on the plate whether it is contoured or overbent. The bending and torsional forces are greatest if a straight plate is used alone and the principle of tension band plating is not applied. Direct bone healing was only observed in the group with contoured plate and lag screw. Overbending combined with a lag screw provided only a relatively unstable fixation. A residual gap immediately beneath the plate permits "dynamic compression" since the screws slide towards the osteotomy when loaded producing bone resorption under the plate and signs of screw loosening. The models with contoured and overbent plates without a lag screw were histologically assessed as very unstable with signs of secondary fragment displacement, obvious callus formation, resorption at the fragment ends and under the plate, delayed and diminished Haversian remodelling and corrosion sites at the screw heads and at the adjacent site on the plate hole. In all groups, stripping of the periosteum under the plate was associated with porosis of the corresponding cortex as a sign of temporarily impaired blood supply. A relationship between implant loading and/or unloading (stress shielding) could not be demonstrated. Callus formation, measured quantitatively on the radiographs, is directly related to the strain on the plate. Direct bone healing is rapid and is seen histologically three weeks postoperatively, particularly for fixations with contoured plate and lag screw. The early appearance of fixation callus in the presence of an intact blood supply indicates a primary instability of the osteosynthesis. Later, it may be an indication of secondary instability. The time at which osteons appear, their number and location provides information on the stability of the osteosynthesis. At a time when indirect fracture reduction and stabilization using minimally invasive techniques and implants is being propagated, additional ways and means must be sought to assess clinically the load on the implants and the risk of implant failure.

Animals↗
PubMed2000

Long-term effects of plate osteosynthesis: comparison of four different plates.

Various phenomena have been observed subsequent to plate osteosynthesis, for example, refracture after plate removal. Experimental research has shown that changes in the cortex occur within the first three months after plating and again several months later. These changes are independent of the fracture and take the form of porosis under the plate and excessive bone growth around the plate. Porosis under the plate was regarded until recently as being due to unloading of the bone by the plate, also known as stress shielding. Investigations of the relationship between bone porosis and the changes in periosteal blood supply due to its compression by the plate, however, have been neglected. In this study, the effect of plate properties such as structural stiffness ('unloading'), implant material, and plate contact surface (altered periosteal blood supply) on bone after osteosynthesis were investigated. This was done by comparative histomorphometry of the altered bone in sheep after application of four plates differing in the above-mentioned properties. After plating the sheep tibia with a trapezoid plate with narrow contact surface, significantly larger bone cross sections were observed one year after the operation and considerable bone growth around the plate. The area of early temporary porosis in the cortex under the plate as observed in the first nine weeks and after one year was not significantly different for any particular plate, all of which were applied subperiosteally. The trapezoid plates were easier to remove, thus causing less damage to the bone lamellae along the sides of the plate. The marked increase in bone cross section after one year and the larger areas of bone growth around the trapezoid plates with smaller contact surface can be attributed to the larger grooves cut along the sides of the plates. Cortical porosis was mainly the result of impaired periosteal blood supply which was of equal size in all groups as a result of careful periosteal stripping and subperiosteal plate insertion. It was assumed that applying the plate onto the periosteum would be associated with effects on periosteal blood supply directly related to the plate and consequently cortical porosis. Plate related stress shielding and the implant material had no significant effect on the extent of cortical porosis.

Animals↗
PubMed2000

Technical innovations in medullary reaming: reamer design and intramedullary pressure increase.

BACKGROUND: Reaming the medullary cavity leads to an increase in intramedullary pressure, which can cause local necrosis and lung malfunction after intramedullary nailing. This investigation concentrates on the effect of reamer design parameters on intramedullary pressure increase. METHODS: Pressure measurements were obtained for three newly designed solid reamers and one hollow reamer. The AO/ASIF reamer was used for reference values. All reamers were connected with small flexible shafts. The pressures were measured in Plexiglas tubes filled with a mixture of Vaseline and paraffin oil with flow properties at 20 degrees C equivalent to those of bovine medullary fat at 36 degrees C. The reaming assemblies were inserted into the tubes using a materials testing machine at a constant speed. In addition, pressure measurements were made using five pairs of human femora. RESULTS: The highest pressures occurred with the AO/ASIF reamer head (258+/-29 mm Hg, p+/-0.05). By creating a conical form and enlarging the flutes, pressures were reduced by up to 37% (164+/-13 mm Hg, p < or = 0.05) compared with the AO/ASIF reamer, depending on the depth of the flutes. With a newly designed hollow reamer, pressure was reduced by 58% (108+/-19 mm Hg, p < or = 0.05) compared with the AO/ASIF reamer. CONCLUSION: The results show that optimizing the design of the reamer head leads to a significant reduction in pressure increase. These results should be taken into consideration when designing new reaming systems in an attempt to minimize the complication rate for intramedullary nailing.

Analysis of Variance↗
PubMed2000

Direction-dependent constriction flow in a poroelastic solid: the intervertebral disc valve.

We hypothesize that a direction-dependent flow resistance exists in the intervertebral disc due to constriction flow in the cartilage endplates. A comparison of the hydrostatic pressure in the nucleus of the healthy intervertebral disc during daily loading with the relatively low osmotic swelling pressure during rest, suggests the necessity of such direction-dependent flow resistance to ensure that all the fluid exuded from the disc during loading is recovered during rest. A physical model demonstrating the direction-dependent resistance of constriction flow in a poroelastic solid is presented. A finite element model was developed and validated against this physical model. The finite element model showed that decrease of the constriction hole area not only increases the resistance to fluid flow, but also causes the direction-dependency of flow resistance to decrease. Through this mechanism, endplate sclerosis could affect normal daily fluid exchange in the intervertebral disc, resulting in decreased mass transport and/or dehydration of the disc.

Biological Transport↗
PubMed1999

Susceptibility to local infection in biological internal fixation. Experimental study of open vs minimally invasive plate osteosynthesis in rabbits.

Resistance to local infection after fracture fixation with plate osteosynthesis may be influenced by the implantation technique. It is known that the extent of the surgical approach to the bone can compromise the local defence capacity. We have investigated susceptibility to infection after a local bacterial challenge in rabbit tibiae using either the open surgical approach for 'biological' internal fixation of standard 2.0 dynamic compression plates or the method of minimally invasive plate osteosynthesis (MIPO), a percutaneous, tunnelling insertion technique preserving the integrity of the overlying soft tissue. After the wounds had been closed, various concentrations of Staphylococcus aureus were injected in the direct vicinity of the implants. The infection rate for the open surgical technique was 38.5% and that for the MIPO technique, 25%. This difference is not statistically significant (P > 0.05) suggesting that resistance to local infection associated with the MIPO method is at least equivalent to the open approach for plate osteosynthesis.

Animals↗
PubMed1999

A biomechanical comparison of the antegrade inserted universal femoral nail with the retrograde inserted universal tibial nail for use in femoral shaft fractures.

Femoral shaft fractures with and without bony contact were simulated in cadaver specimens fixed with one of two different types of intramedullary locked nail systems; conventional antegrade nail fixation of the femur with the universal AO femoral nail or retrograde insertion in the femur with the universal tibial nail (a smaller diameter slotted nail) were utilized. Mechanical testing simulated one leg stance, and resultant deformation was measured in bending, torsion, and shortening. In stable fractures, fracture stability was similar to both devices, while in unstable fractures, the larger femoral nail was more stable. Furthermore, the simulation of single leg stance led to a coupled deformation of varus bending, axial shortening, and external rotation, which was dependent on bone geometry.

Adult↗
PubMed1999

Fracture healing of the sheep tibia treated using a unilateral external fixator. Comparison of static and dynamic fixation.

It is generally accepted that when a fracture is treated with external fixation, dynamization of the fixation accelerates formation of the bony callus by transferring part of the functional loads. The aim of the research presented here was to validate this principle using in vivo measurements of callus stiffness. We created a transverse fracture in the mid third of the tibial shaft in 12 sheep and maintained a 3 mm gap between the fragments for 3 weeks. Two types of unilateral external fixators were applied. Axial loading was permitted (dynamization) from the fourth week onwards in 6 animals. In the other 6 animals, fixation remained static for both types of fixation. Weekly measurements of callus stiffness were obtained using a goniometer and load cell to assess bending stiffness. Two slightly different fixators were used. Callus formed in all 12 animals. Callus stiffness increased exponentially to reach the degree of stiffness measured on the contralateral side. There was no clear difference in healing between the two types of fixations nor between dynamic and static fixation. If a unilateral fixator was applied which did not maintain absolutely rigid fixation, the fracture generally healed well even without contact between the fragment ends. If the process of callus formation had begun normally, dynamic fixation offered no further benefit. Measuring the stiffness of the callus with an adequate measurement apparatus in vivo indicated that the fixator could be removed earlier than would have been authorized on the basis of radiological evidence alone.

Animals↗
PubMed1999

Experimental rotator cuff repair. A preliminary study.

BACKGROUND: The repair of chronic, massive rotator cuff tears is associated with a high rate of failure. Prospective studies comparing different repair techniques are difficult to design and carry out because of the many factors that influence structural and clinical outcomes. The objective of this study was to develop a suitable animal model for evaluation of the efficacy of different repair techniques for massive rotator cuff tears and to use this model to compare a new repair technique, tested in vitro, with the conventional technique. METHODS: We compared two techniques of rotator cuff repair in vivo using the left shoulders of forty-seven sheep. With the conventional technique, simple stitches were used and both suture ends were passed transosseously and tied over the greater tuberosity of the humerus. With the other technique, the modified Mason-Allen stitch was used and both suture ends were passed transosseously and tied over a cortical-bone-augmentation device. This device consisted of a poly(L/D-lactide) plate that was fifteen millimeters long, ten millimeters wide, and two millimeters thick. Number-3 braided polyester suture material was used in all of the experiments. RESULTS: In pilot studies (without prevention of full weight-bearing), most repairs failed regardless of the technique that was used. The simple stitch always failed by the suture pulling through the tendon or the bone; the suture material did not break or tear. The modified Mason-Allen stitch failed in only two of seventeen shoulders. In ten shoulders, the suture material failed even though the stitches were intact. Thus, we concluded that the modified Mason-Allen stitch is a more secure method of achieving suture purchase in the tendon. In eight of sixteen shoulders, the nonaugmented double transosseous bone-fixation technique failed by the suture pulling through the bone. The cortical-bone-augmentation technique never failed. In definite studies, prevention of full weight-bearing was achieved by fixation of a ten-centimeter-diameter ball under the hoof of the sheep. This led to healing in eight of ten shoulders repaired with the modified Mason-Allen stitch and cortical-bone augmentation. On histological analysis, both the simple-stitch and the modified Mason-Allen technique caused similar degrees of transient localized tissue damage. Mechanical pullout tests of repairs with the new technique showed a failure strength that was approximately 30 percent of that of an intact infraspinatus tendon at six weeks, 52 percent of that of an intact tendon at three months, and 81 percent of that of an intact tendon at six months. CONCLUSIONS: The repair technique with a modified Mason-Allen stitch with number-3 braided polyester suture material and cortical-bone augmentation was superior to the conventional repair technique. Use of the modified Mason-Allen stitch and the cortical-bone-augmentation device transferred the weakest point of the repair to the suture material rather than to the bone or the tendon. Failure to protect the rotator cuff post-operatively was associated with an exceedingly high rate of failure, even if optimum repair technique was used. CLINICAL RELEVANCE: Different techniques for rotator cuff repair substantially influence the rate of failure. A modified Mason-Allen stitch does not cause tendon necrosis, and use of this stitch with cortical-bone augmentation yields a repair that is biologically well tolerated and stronger in vivo than a repair with the conventional technique. Unprotected repairs, however, have an exceedingly high rate of failure even if optimum repair technique is used. Postoperative protection from tension overload, such as with an abduction splint, may be necessary for successful healing of massive rotator cuff tears.

Animals↗
PubMed1998

[Does surgical approach have an effect on the development of local infection? Animal experiment comparison of conventional open vs. minimally invasive bone plate osteosynthesis (MIPO)].

With a standardised model we investigated the influence of two different surgical approaches to the rabbit tibia for plate osteosynthesis on resistance to local infection after postoperative inoculation of graduated concentrations of staphylococcus aureus at the implant. The infection rate for the minimally invasive plate osteosynthesis with insertion of the implant in closed, soft tissue tunneling technique was 25% (3/12 animals; ID50 = 6.2 x 10(6) CFU) and for the conventional open approach 38% (5/13 animals; ID50 = 2 x 10(6) CFU). This difference is statistically not significant (with P < 0.05).

Animals↗
PubMed1998

[Method for measuring the comparative stability of osteosynthesis in the dorsal pelvic ring].

In the past, biomechanical investigations on the dorsal pelvic ring have generally been performed on a small number of cadaveric pelves in various non-standardized procedures. Significant differences in stability between different internal fixation methods of unstable pelvic ring fractures were not found. The experimental design presented here was based as closely as possible on the physiological loading of the pelvis in one-leg stance. This method made it possible to carry out standardized, reproducible tests on different osteosytheses of the sacroiliac joint. Furthermore, the suitability of artificial bones for such investigations can be assessed on the basis of a larger number of similar experiments on artificial and human pelves and the number of human pelves required for such studies could be reduced.

Biomechanical Phenomena↗
PubMed1997

The hybrid ring tubular external fixator: a biomechanical study.

OBJECTIVE: To measure and compare the mechanical properties in bending of the four-ring, and three-ring/one-tube hybrid external fixation frames. DESIGN. IN VITRO: measurements of the mechanical behaviour of ring and ring-tubular external fixation frames. In the latter, one ring of the full circular frame was replaced by one tube and Schanz screws. BACKGROUND: The mechanical properties of the classical Ilizarov four-ring external fixation frames has been compared to those of other external fixation frames by various authors. However, in clinical practice the hybrid fixation frame is being used with increasing frequency. Therefore the mechanical properties of the latter are of immediate interest and clinical value. METHODS: On explanted sheep tibiae with single and double osteotomies, frame stiffness in the four-point bending mode was measured at different K-wire tensions, comparing the values obtained from four-ring frames, to those of three-ring-tubular hybrid frames. These measurements were made under conditions of (a) bone distraction (BD), and (b) segment transport (ST), both at the initial and final stages of this procedure. RESULTS: In circular frames, frame stiffness in bending for increasing K-wire tension showed a Gaussian distribution both in distraction and post-ST with an optimum at 1000 N. In ring tubular hybrid frames, however, frame stiffness showed a more linear relationship to K-wire tension. CONCLUSIONS: In the four-ring Ilizarov external fixation frame, the exchange of one ring with one tube and one Schanz screw both reduced frame stiffness in bending and converted to linear its relationship to K-wire tension. RELEVANCE: Under clinical conditions, the use of a similar ring tubular hybrid external fixator allows the adjustment of frame stiffness in a simple and practical way. This is not the case with the original ring fixation frame.

Journal Article↗
PubMed1997

Strength recovery in fractured sheep tibia treated with a plate or an internal fixator: an experimental study with a two-year follow-up.

OBJECTIVES: Comparison of fracture healing with two different implants: a conventional Dynamic Compression Plate (DCP) and a new internal Point Contact Fixator (PC-Fix). DESIGN: Randomized, prospective study in experimental animals. Observation times: 12, 24, 48 and 96 weeks, with six sheep per group. SETTING: Following surgery, animals were kept with unrestricted weight-bearing in individual stalls for 12 weeks, thereafter in groups. ANIMALS: 56 adult Swiss mountain sheep. INTERVENTION: A standardized oblique fracture of the sheep tibia was reduced and compressed by a lag screw and "neutralized" with one of the implants. MAIN OUTCOME MEASUREMENTS: Standard radiographs were used for callus size measurements. After sacrifice the implant was removed and both the treated bone and the contralateral bone were tested for static strength in bending with the plate side under tension. Broken bones were processed for histological evaluation. RESULTS: In the DCP group all six bones failed through the original fracture at 12 weeks. At 24 and 48 weeks two out of six, at 96 weeks one out of six bones failed through the original fracture, others through one of the screw holes. In the PC-Fix group there were no failures through the original fracture with a single exception at 96 weeks. The strength values in the PC-Fix groups of 12 and 96 weeks were significantly higher then in the corresponding DCP groups. CONCLUSIONS: Healing of simple diaphysial fractures treated by PC-Fix was superior to that achieved by conventional plating. The histological evaluation suggested that the observed differences can be accounted for by the absence of implant-related cortical necrosis and by the circumferentially uninterrupted (if smaller) callus in the PC-Fix group.

Animals↗
PubMed1996

Infection after intramedullary nailing: an experimental investigation on rabbits.

The purpose of this study was to investigate three relevant aspects of intramedullary nailing in terms of their effect on the occurrence of local infection. In an infection model on the rabbit tibia, the following were compared: a hollow and a solid nail (Experiment I), a reamed with an unreamed technique (Experiment II), and a steel with a titanium nail (Experiment III). In order to minimize the number of animals required, a grouped sequential procedure combined with an "up and down" dosage technique was applied. Microbiological evaluation was both qualitative and quantitative. The results in Experiment 1 (n = 44) indicated an infection rate for the hollow nail (59%) almost double that of the solid nail (27%) (P < or = 0.05). Experiment II (n = 44) produced an infection rate of 50% for the unreamed technique compared to 64% for the reamed technique, a difference which, on the basis of the number of bacteria present, was also statistically significant (P < or = 0.05). In Experiment III (n = 44) an infection rate of 82% was recorded for the steel nail compared to 59% for the titanium nail (P < or = 0.05). The results of the three experiments are only partially comparable with each other because of the grouped sequential procedure and the different inocula used. Nonetheless it would seem that the dead space inherent in the design of the hollow nail represents a considerable risk with regard to the occurrence of local infection. Reaming the medullary cavity with the attendant reduction in local vascularity and necrosis and the lesser biocompatibility of steel compared to titanium may be additional risk factors which should not be overlooked.

Animals↗