Introduction

In oblique olecranon fracture, fracture line begins in the trochlear notch and proceeds distally to the dorsal cortex of the ulna. We have experienced a nonunion of reverse oblique fracture.

The most important issue in the assessment of fracture healing is to acquire information about the restoration of the mechanical integrity of bone. Many researchers have attempted to monitor stiffness either directly or indirectly for the purpose of assessing strength, as strength has been impossible to assess directly in clinical practice. The purpose of this study was thus to determine the relationship between bending stiffness and strength using mechanical testing at different times during the healing process. Unilateral, transverse, mid-tibial osteotomies with a 2-mm gap were performed in 28 rabbits. The osteotomy site was stabilized using a double-bar external fixator. The animals were divided into four groups (n=7/group/time point; 4, 6, 8 and 12 weeks). A series of images from micro-computed tomography of the gap was evaluated to detect the stage of fracture healing and a 4-point bending test was performed to measure stiffness and strength. Formation of cortex and medullary canal at the gap was seen in the 12-week group and would represent the remodeling stage. In addition, the relationship between stiffness and strength remained almost linear until at least 12 weeks. However, stiffness recovered much more rapidly than strength. Strength was not fully restored until the later stages of fracture healing. However, the current study demonstrated that stiffness could be monitored as a surrogate marker of strength until at least the remodeling stage.

The most important issue in the assessment of fracture healing is to acquire information about the restoration of the mechanical integrity of bone. Echo tracking (ET) can noninvasively measure the displacement of a certain point on the bone surface under a load. Echo tracking has been used to assess the bone deformation angle of the fracture healing site. Although this method can be used to evaluate bending stiffness, previous studies have not validated the accuracy of bending stiffness. The purpose of the present study is to ensure the accuracy of bending stiffness as measured by ET. A four-point bending test of the gap-healing model in rabbit tibiae was performed to measure bending stiffness. Echo tracking probes were used to measure stiffness, and the results were compared with results of stiffness measurements performed using laser displacement gauges. The relationship between the stiffness measured by these two devices was completely linear, indicating that the ET method could precisely measure bone stiffness.

Ring frames have the advantage of allowing progressive correction. However, the available frames for complex deformities are heavy and bulky leading to poor compliance by patients. Also, the mounting procedure requires considerable expertise and skill. On the other hand, a unilateral external fixator has the advantages of less bulk and a lighter weight. Thus, it causes less disability and can achieve better patient compliance even with bilateral application. However, previous unilateral fixators have had various limitations with respect to deformity correction, such as restricted placement of hinges, restricted correction planes, and a limited range of correction angles. In addition, it was impossible to achieve progressive correction while fixation was maintained. To overcome these disadvantages of existing unilateral fixators, we developed a new fixator for gradual correction of multi-plane deformities including translational and rotation deformities. This unilateral external fixator is equipped with a universal bar link system. The link is constructed from three dials and two splines that are connecting the dials. The pin clamps are able to vary the direction of a pin cluster in the three dimensional planes. The system allows us to correct angulation, translation, rotation, and the combination of the above. In addition, open or closed hinge technique is available because the correction hinge can be placed right on the center of rotational angulation (CORA), or at any desired location, by adjusting the length of the link spline. By increasing the spline length, the virtual hinge can also be set far from the fixator. Gradual correction can be performed by rotating the three dials using a worm gear goniometer that is temporarily attached. A 3D reconstructed image of the bone is generated preoperatively. Preoperative planning can be done using this image. Mounting parameters are determined by postoperative AP and lateral computed radiography images. These postoperative images are matched with the pre-operative 3D CT image by 2D and 3D image registration. Then, the fixator can be virtually fixed to the bone. By performing virtual correction, it is possible to plan the correction procedure. The fixator is manipulated by rotating each of the three dials to the predetermined angles calculated by the software. Static load testing disclosed that the fixator could bear a load of 1700 N. No breakage or deformation of the fixator itself was recognized. Mechanical testing demonstrated that this new fixator has sufficient strength for full weight bearing, as well as sufficient fatigue resistance for repeated or prolonged use. The results of clinical application in patients with multi-plane femoral deformities were excellent, and correction with very small residual deformity was achieved in each plane.

Aims: The purpose of this study was to clarify how the mechanical characteristics of the lengthened bone changes with time by means of the analyses using the CT based finite element method. Methods: CT images were obtained from the bilateral tibiae of five patients who had undergone unilateral tibia. The average time interval from completion of lengthening to CT scanning was 30 months. There were two patients had CT examinations twice. The analyses were made using the Mechanical Finder®(Mitsubishi Space Software, Osaka, Japan). 3-D finite element models were made from axial CT images of the whole tibiae. The models were 3mm tetrahedron elements for a cancellous bone and 3 nodal-points shell elements with a thickness of 0.3 mm for a cortical bone. The uni-axial compressive load was applied on the tibial plateau, while the distal part of the tibia was fully restrained. The elastic moduli at the middle of the lengthened bones and the maximum principal strains were calculated using the elastic analysis. Results: The elastic moduli of the lengthened bones were significantly smaller than those of the contra-lateral bones, while the maximum principal strains of the lengthened bones tended to be larger. The ratios of the elastic modulus disparity between the lengthened bone and the paired contra-lateral bone to the elastic modulus of the contra-lateral bone decreased significantly with time. Likewise, the ratios of the maximum principal strains calculated as above decreased identically. Conclusions: The results indicated that the stiffness of the lengthened bone got closer to that of the contra-lateral with time, which means the lengthened bones were in the process of modeling during these follow up time intervals.

Pin clamp motion was continuously monitored using a displacement sensor as patients walked with a dynamic fixator applied. Patients with a shaft fracture, nonunion or lengthening of the tibia were monitored, all of whom were in the stage of dynamization. The Hifixator equipped with a ball bearing mechanism on the inner surface of its dynamic pin clamp was used as a dynamic external fixator. The aim of this study was to estimate the magnitude of movement and the type of deformation occurring at the fracture site or callus generated after distraction osteogenesis. The actual motion of the bone fragment has components with six degrees of freedom, which are transferred to the pin clamp. The magnitude of the displacement of the pin clamp along the shaft is expressed by an equation involving these six components. If the pin clamp has a sufficiently smooth sliding surface and a small clearance between it and the shaft, and the pin clusters are sufficiently rigid during walking, the amount of the displacement can be expressed by the linear combination of these components.

Accuracy of the measurement was evaluated using a bone model fixed with a Hifixator mounted with a displacement sensor, by performing dynamic loading tests with axial, bending and torsional forces The measured values agreed well with the theoretical values when the rigidity of the bone model was high. The displacement was recorded versus time during more than twenty cycles of walking with weight bearing of the patients. The rhythm of walking was controlled with a metronome set at 0.5 Hz. The displacement curve had an oscillatory component synchronized with a heel strike and a toe off, a time dependent component expressed by shifting of the baseline, and an irreversible component during a non-weight bearing period after walking. The three components were analyzed with a simple Voigt model.

In all patients, both the amplitude of the oscillatory component and the time dependent component expressed as retardation time decreased as healing proceeded, and by the time of fixator removal the irreversible component had disappeared. This method was useful for quantitatively evaluation the viscoelastoplascity of the healing site.