Berton A. Rahn

The use of miniplates in mandibular fractures: An in vitro study

To test the stability of miniplate osteosynthesis in the mandible, a three-dimensional in vitro model was developed. Four clinically relevant fracture situations were simulated by osteotomies in polyurethane mandibles. The stability of the osteosyntheses was tested according to a reproducible procedure of unilateral loading under conditions of static equilibrium. The mandible was supported at the condyles in silicon rubber fossae and was held in position by traction on the coronoid processes in combination with preloading in the region of the incisors with a force of 10 N. Six points were tested to register the effects of unilateral loading on bending and torsion at the sites of the osteotomies. The following results were obtained. The preliminary results show that in cases of jaw angle fracture neither bending nor torsional forces are sufficiently controlled by miniplate-fixation in the commonly proposed position. Furthermore, it was confirmed that fracture treatment in the canine region requires two plates instead of one to resist displacement of the fracture fragments during function.

Long-term in vivo degradation and bone reaction to various polylactides. 1. One-year results

Injection-moulded pins from poly(L-lactide), poly(L/DL-lactide) (95/5%) were implanted in the cortex of the tibiae of sheep. The bone-implant interface was evaluated to observe whether there is any bone resorption caused by the implants. The molecular weight and crystallinity changes upon implantation were also measured. There was no net bone loss around the implants or sterile cyst formation in any of the animals implanted with polylactides up to 1 year. The new bone formed around the poly(L-lactide) and poly(L/D-lactide) pins was separated from the implants with a thin layer of connective tissue. For the implants from poly(L/DL-lactide), there was direct apposition of bone on the polymeric material. At 1 year of implantation, the implants were not completely resorbed, although the molecular weight of polylactides was reduced from 40,000-50,000 to 500-300. The crystallinity at 1 year was about 45% for poly(L/DL-lactide) and poly(L/DL-lactide) and 65% for poly(L-lactide), respectively, indicating the presence in the degraded material of thermodynamically stable crystals.

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.

Histology of the ligamentum flavum in patients with degenerative lumbar spinal stenosis.

Abstract The degree of calcification as well as the structural changes of the elastic fibres in the ligamentum flavum in patients with degenerative lumbar spinal stenosis were evaluated and the results were compared to those of patients without spinal stenosis. In 21 patients (13 male, 8 female) with lumbar spinal stenosis the ligamentum flavum was removed, histologically processed and stained. The calcification, the elastic/collagenous fibre ratio as well as the configuration of the fibres were evaluated with an image analyzing computer. As a control group, 20 ligaments of 10 human corpses were processed in the same way. The results were statistically analysed using the Mann-Whitney-Wilcoxon test (α = 0.05) and the t-test (α = 0.05). Nearly all the ligaments of patients with lumbar spinal stenosis were calcified (average 0.17%, maximum 3.8%) and showed relevant fibrosis with decreased elastic/collagenous fibre ratio. There was a significant correlation between age and histological changes (P < 0.05). In the control group we only found minimal calcification in 3 of 20 segments (average 0.015%). No relevant fibrosis was found and the configuration of elastic fibres showed no pathologic changes. The results of this study illustrate the important role of histological changes of the ligamentum flavum for the aetiology of lumbar spinal stenosis.

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. Variaous staining techniques were supplemented by polarized light microscopy. Both the transverese 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°. 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 consitituent, 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.

Bone regeneration with resorbable polymeric membranes : Treatment of diaphyseal bone defects in the rabbit radius with poly(L-lactide) membrane. A pilot study

Tubular poly(L-lactide) membranes with a pore size of 5-15 microns and a molecular weight of 70,000 Daltons were implanted into 24 New Zealand skeletally mature rabbits to cover 10-mm mid-diaphyseal defects of the radius of the forelimb. An identical defect on the contralateral limb was not treated with the membrane and served as a control. The animals were killed at 1, 2, 4, 8, 12, 24, 36, and 64 weeks after implantation, and radiographic and microscopic studies were conducted. The canals of the polymeric tubes were initially filled with blood. At 2 weeks, there was direct woven bone formation within the polymeric tube in continuity with the fragment cortices and its medullary canal. The formation of woven bone across the defect progressed until reconstruction of the defect had occurred at 6-8 weeks. The bone continued to remodel throughout the observation period of 64 weeks. By 12 weeks, bone within the lumen of the implant consisted of cancellous bone and cortical bone lining the membrane walls. At 24, 36, and 64 weeks, the implants were filled with cancellous bone and cortical bone in direct apposition to the polymer membrane. For one implant, the newly formed woven bone had only incompletely filled the defect at 8 weeks. This resulted in a nonunion with a residual gap of 0.5 mm and the appearance of mature bone. There was extensive bone formation along the intraosseous membrane in both control and implanted defects, although the untreated defects were rapidly filled with overlying muscle and soft tissues. The osseous activity of the untreated defects appeared confined to the bone ends by the interposed muscle and fibrous soft-tissue margins. The untreated defects resulted in radial-ulnar synostosis along the intraosseous membrane with cortical bone caps at the bone ends. The poly(L-lactide) membrane remained intact throughout the 64-week period without evidence of significant degradation. The membranes were encapsulated in a thin fibrous tissue. There was no histological evidence of acute or chronic inflammation associated with the implants.

Osteopromotion by a β-Tricalcium Phosphate/Bone Marrow Hybrid Implant for Use in Spine Surgery

Study Design. Experimental animal study. Objective. Evaluate osteopromotive properties of a &bgr;-tricalcium phosphate (TCP) implant with different osteogenic substances in an animal study. Summary of Background Data. Current research in spine surgery is focusing on resorbable bone implants because of the high morbidity after iliac crest graft harvesting. Therefore, several osteoconductive scaffolds are combined with osteoinductive substances. Methods. In 14 sheep, a critical size defect was performed on both tibiae. The sheep were randomized into 3 groups (4 sheep each) and a control group (2 sheep). In the treatment groups, the defects were filled with the &bgr;-TCP scaffold impregnated with either venous blood, bone marrow from sternal aspiration, or a concentrated mononuclear cell suspension derived from sternal bone marrow aspiration. The sheep in each group were euthanized 6 and 12 weeks after the operation; the investigation included quantitative computerized tomography and histology. Results. The best bone formation occurred in the bone marrow group after 6 and 12 weeks, whereas no difference was found between the cell and venous blood groups. Only the bone marrow group showed bone formation inside the scaffold after 6 weeks. We conclude that a &bgr;-TCP scaffold filled with bone marrow aspiration creates a biologic resorbable bone substitute with high osteopromoting capacity. Surprisingly, no better bone formation occurred in the concentrated cells group, which may be a result of technical reasons and needs to be further evaluated. Conclusions. The combination of &bgr;-TCP and bone marrow has superior osteopromotive properties to venous blood or concentrated mononuclear cells and can be used effectively as a substitute to iliac crest graft.

A paradigm of delayed union and nonunion in the lumbosacral joint : a study of motion and bone grafting of the lumbosacral spine in sheep

In a group of nine sheep (Group A), it was noted that when small, interlocking cancellous and cortical cancellous bone grafts are placed posteriorly on the lumbosacral spine, union always occurred in the interlumbar motion segments and almost never occurred at the lumbosacral joint. One of the main differences in these two areas is the amount of motion that occurs at each level with flexion and extension. Because nonunion following bone grafting for arthrodesis of the spine is a serious clinical problem, we have studied the amount of motion seen at the interlumbar and lumbosacral joints in sheep to ascertain how much motion is compatible with union and how much is associated with nonunion. In vivo studies were carried out in eight sheep (Group B), and five normal spine segments were studied in vitro to determine normal motion in this species. To simplify the complex in vivo motion that occurs at the lumbar motion segments, the simple linear displacement and strain of the fusion mass (consisting of fibrous tissue and bone grafts placed on and between the laminas posteriorly) was measured with the spine in flexion and extension. When the displacement and strain at the interlaminar level of the L6-S1 joint was measured, the linear displacement was found to be 5.2 mm and the associated linear strain 36%. The displacement at the L5-L6 interspace was 1.2 mm, and the strain 10%. The stiffness of the L5-L6 joint (which always fused) and the L6-S1 joint (which did not fuse, with one exception) were also studied. In a third group of four animals (Group C), internal fixation of the lumbosacral joint was attempted in addition to bone grafting. In this group bone grafts were put only across the lumbosacral joint. Two of these animals developed serious problems in the first 2 weeks after surgery (one fractured its ilium, and the other developed a deep infection) and were killed before the arthrodesis could become solid. The two other sheep in this group were killed at 6 months, and it was noted that the one fixed with distraction rods united its lumbosacral joint, whereas the one fixed with plates did not unite.

Stabilization of the short sagittal split osteotomy: in vitro testing of different plate and screw configurations

Using a 3-dimensional in vitro model, the stability of different types of osteosynthesis for the short sagittal osteotomy was tested. The following four groups of plate and screw configurations were evaluated: Group I: Fixation with miniplates using monocortical screws only, Group II: Fixation with miniplates, but with two of the screws engaging both fragments, Group III: Same as group II with an additional position screw, Group IV: Fixation with 3 position screws. The stability obtained with miniplate fixation using monocortical screws only (group I) was by a factor of 2.9 less than position screw fixation (group IV), which is considered to be an approved standard. In order to increase the stability of miniplate fixation, the screws in the area of overlapping bone should engage both fragments.

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

Summary 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 >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.