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.

The purpose of this study is to elucidate the possibility of an ideal joint alignment after monofocal lengthening of tibia in achondroplastic patients. In 10 cases of the alignments of knee and ankle joints of tibias in which plane radiographs were examined.Unilateral fixators were applied to both tibias,after lengthening in the normal manner, deformity was corrected manually in a single procedure without anesthesia. In order to determine the amount of angle to be corrected, a line was first drawn on the radiograph from the center of the knee joint to the center of the ankle joint. (This line is named the Knee-ankle line: KAL).

Next we drew a line along the ankle joint and measured the angle between this line and KAL. We also drew a line across the tibial plateau and measured the medial angle between this line and KAL. We tried to align the ankle joint perpendicular to KAL and the medial angle between the tibial plateau and KAL at 87 degrees , instead of trying to align the axis of the tibial shaft perfectly straight. The medial angles between the line across the tibial plateau and KAL were corrected to 86 degrees in average, with a range from 84 to 90 degrees, and the medial angles between the line across the ankle joints were corrected to 87 degrees in average, ranging from 80 to 90 degrees in result.

In conclusion, joint alignments of tibias in achondro-plastic patients were able to be corrected successfully without any complications using our monofocal lengthening technique. And severe varus deformities of tibias can be corrected even with monofocal lengthening technique by trying to correct the alignments of knee and ankle joints rather than trying to straighten tibial shafts.