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Biomedical subjects

B A Rahn

Publications and source records attributed to B A Rahn.

At least 55 records · Page 3Linked to original sources
PubMed1990

Fixation effects on membranous and endochondral onlay bone graft revascularization and bone deposition.

This paper investigates the relationship between bone resorption, the process of bone revascularization, and graft fixation. Vital staining techniques and microangiography were used to study the extent of graft revascularization of fixed and nonfixed endochondral (rib) and membranous (skull) onlay bone grafts in 20 adult sheep mandibles bilaterally. This assessment was carried out at 2 and 20 weeks postoperatively. Sequential fluorochrome staining was performed to examine the pattern of new bone deposition. Fixation was achieved using the lagscrew technique. At 2 weeks, membranous bone demonstrated a greater area of graft revascularization if fixed than if the graft was not fixed. The opposite result was seen for endochondral grafts, where nonfixed grafts showed a greater area of revascularization than fixed grafts. At 20 weeks, all bone that was present was fully vascularized. The inconsistencies in the results on the relationship between fixation and revascularization for membranous and endochondral grafts in the early stages of healing (2 weeks) suggest that although revascularization is a necessary precondition for bone resorption and deposition, biomechanical and structural factors may be a more satisfactory explanation for the differences observed in the maintenance of bony volume.

Animals↗
PubMed1990

Histology of the alar and transverse ligaments.

Seven sets of human transverse and alar ligaments, after tensile testing, and eight corresponding ligaments without testing, were examined histologically with respect to their fiber composition and fiber orientation. Various staining techniques were supplemented by polarized light microscopy. Both the transverse and the alar ligaments consist of collagen fibers, with very few elastic fibers in the peripheral layer. In the central portion of the transverse ligament, the collagen fibers cross each other at an angle of approximately 30 degrees. Close to the dens, the transverse ligaments show on their ventral side a transition into fibrocartilage. Except for the immediate site of failure, no differences became evident between tested specimens and controls. The collagen, as the almost exclusive constituent, together with the fiber orientation determine the mechanical properties of these ligaments. This supports the hypothesis that the ligaments could be irreversibly overstretched or even ruptured when the head is rotated and, in addition, flexed by impact trauma, especially in unexpected rear-end collisions.

Aged↗
PubMed1989

The effect of radial preload on the implant-bone interface: a cadaveric study.

The most common complication of external fixation is pin loosening. Preloading the implant-bone interface is believed to retard this process. Radial preload, in particular, may be useful, as it allows loading in more than one direction. To investigate the effect of varying degrees of radial preload on the pin-bone interface, 30 freshly thawed human cadaveric tibiae were sectioned into 4-cm segments. Uniform drill holes were produced in the anterior tibial ridge of all segments and custom experimental bolts, oversized in diameter by as much as 1 mm, were pressed into each specimen. Macroscopic surface fractures were noted at the time of bolt insertion for misfits greater than 0.2 mm. Following histologic preparation, the implant-bone interface was evaluated microscopically based on the appearance of osteonal compression, lamellar distortion, and microfractures. Insertion of external fixator pins with misfits of greater than 0.4 mm resulted in significant microscopic structural damage to the bone surrounding the pin. High degrees of radial preload, exceeding the elastic limit of cortical bone, may be produced around pin holes by a small misfit. The use of oversized pins or screws must therefore be questioned.

Biomechanical Phenomena↗
PubMed1989

Comparison of compression and torque measurements of self-tapping and pretapped screws.

The choice of an internal fixation system for maxillofacial surgery is made difficult because of lack of information with respect to functional load. This study attempted to clarify some of the controversy with respect to maxillofacial use of these implants. Maximal compressive force to torque values were measured in standardized bone thicknesses of 1, 2, 3, and 4 mm. The screws tested were pretapped AO 1.5-, 2.0-, 2.7-, and 3.5-mm rescue screws and self-tapping Luhr, Champy, and AO 1.5- and 2.0-mm screws. Ten measurements were made for each screw type/bone thickness combination using a piezoelectric washer and torque screwdriver. It was apparent that for 1- and 2-mm bone thicknesses the use of self-tapping screws resulted in the highest compression values. In 3- and 4-mm bone thicknesses, pretapped screws offered the highest compression values. As expected, self-tapping screws had the highest torque values on insertion owing to torque loss in cutting the screw threads. The 2.7-mm screw offered no advantage over the 2.0-mm screws in 1- and 2-mm bone thicknesses but resulted in higher compression values in 3- and 4-mm bone thicknesses.

Animals↗
PubMed1989

Theoretical considerations in rigid fixation of facial bones.

The classic facial fracture patterns have gradually been changing toward more severe comminutions. This necessitates an early and accurate anatomical reduction and a fixation in this position until the fragments have united. Stable fixation favors early mineralization in the fracture gap. The mechanical function of the implants is thus only needed for a short term. Similar considerations are true for the incorporation of grafts. Mechanics of fixation of facial bones is influenced by the structural properties as well.

Bone Development↗
PubMed1988

Fixation effects on membranous and endochondral onlay bone-graft resorption.

Difficulties arise in the prediction of maintenance of graft volume over time when bone grafts are used for facial contour reconstruction. We hypothesize that graft fixation will decrease movement and lead to decreased resorption. Fixed and nonfixed endochondral (rib) and membranous (skull) onlay bone grafts measuring 30 X 10 X 4 mm were grafted to the mandible bilaterally in 10 adult sheep. Fixation was achieved using the lag-screw technique. Volume measurements using caliper technique were made 20 weeks postoperatively. The volume of graft present at 20 weeks was significantly greater for the fixed bone grafts (p less than 0.001): fixed membranous, 85.9 percent; fixed endochondral, 76.2 percent; nonfixed membranous, 55 percent; and nonfixed endochondral, 16.6 percent. The results are explained using biomechanical theories related to the effects of strain. At present, it is suggested by this study that when onlay bone grafts are stabilized, improved results with respect to graft resorption can be expected.

Animals↗
PubMed1988

Early temporary porosis of bone induced by internal fixation implants. A reaction to necrosis, not to stress protection?

Stabilization of the fracture using implants requires contact surfaces between implant and bone. Such contact has been observed to induce bone porosis first seen at one month after surgery. Bone loss in the vicinity of implants has hitherto been explained as being induced by mechanical unloading of the bone (stress protection). Experiments in sheep, dogs, and rabbits combining intravital staining of blood circulation and polychrome fluorescent labeling of bone remodeling leads to the conclusion that early bone porosis in the vicinity of the implants is the result of internal remodeling of cortical bone and is induced by necrosis rather than by unloading. This theory is favored by the evidence that (1) the bone porosis is of a temporary nature, an intermediate stage in internal bone remodeling; (2) the pattern of the remodeling zone is closely related to that of the disturbed circulation, and not to that of unloading; (3) plastic plates may produce more porosis than steel plates; and (4) improved blood circulation using modified plates resulted in reduced porosis. The clinical relevance of these findings is related first to the temporary weakening of the bone, and second to the possibility of sequestration. Sequestration may be the result of intensified remodeling activity in the presence of inflammation or infection.

Animals↗
PubMed1987

Direct and indirect bone healing after operative fracture treatment.

The knowledge of the entire spectrum of differing healing patterns should help the surgeon to judge the progress of fracture healing. To be able to recognize an abnormal course and deviating patterns will help the surgeon to avoid risks and to improve the mechanical and biologic conditions necessary for successful fracture treatment.

Fracture Fixation↗
PubMed1986

Laser Doppler flowmetry for bone blood flow measurement: correlation with microsphere estimates and evaluation of the effect of intracapsular pressure on femoral head blood flow.

Laser Doppler flowmetry (LDF) was used to measure bone blood flow in the rabbit femoral head and femoral condyles. To correlate the LDF output signal blood cell flux to in vivo blood flow, simultaneous measurements using LDF and 85Sr-labeled microspheres were made in an adult rabbit model. There was no correlation between the two methods for blood flow in the femoral condyles and the correlation between the two methods for blood flow in the femoral head does not achieve statistical significance. An LDF signal of 0.4 V was approximately equal to a microsphere measured flow rate of 0.4 ml blood/g bone/min. The strength of the correlation in the latter case may have been affected by (a) large arteriovenous shunts, (b) inadequate mixing of the microspheres with a left ventricular injection, and (c) insufficient numbers of microspheres present in the bone samples with which to satisfy the mathematical requirements of the microsphere method. When LDF was used to evaluate the effect of elevated intracapsular pressure on femoral head blood flow in skeletally mature rabbits, femoral head subchondral bone blood flow declined with increasing intracapsular pressure from a baseline value of 0.343 +/- 0.036 to a value of 0.127 +/- 0.27 at 120 cm of water pressure. The decline in femoral head blood flow was statistically significant at pressures of 40 cm of water or higher (p less than 0.001), and evaluation of sections of the proximal femora made from preterminal disulphine blue injections confirmed these findings. Intracapsular tamponade has an adverse effect on femoral head blood flow beginning well below central venous pressure and should be considered in the pathophysiology of posttraumatic and nontraumatic necrosis of the femoral head. Laser Doppler flowmetry was easy to use and appears to be a reproducible technique for evaluating femoral head blood flow, offering distinct advantages over the microsphere technique for measuring bone blood flow. Further studies of the in vivo calibration of the LDF method for bone blood flow are necessary for the method to have potential for clinical application.

Animals↗