Pin-bone Interface Research Articles

The Effect of Rigidity on Fracture Healing in External Fixation

Knowledge of the basic biomechanics of external fixation is necessary to obtain the full benefits of the technique for bone fracture treatment. The rigidity of external fixation, including pin-bone interface stresses, is discussed and bone healing and remodeling under different fixation stiffnesses and fracture gap conditions are described. The rigidity of fixation ultimately depends on the biomechanical characteristics of the fracture, the accuracy of reduction, and the amount of physiologic loading. Comparative experiments using a canine tibial fracture model have suggested that fixation rigidity is important in early bone healing and in the prevention of pin loosening. Bone union can be achieved under external fixation through different pathways, ranging from callus-free gap healing under a rigid neutralization configuration to direct-contact healing with periosteal new bone formation under axially dynamized stable fixation. Cortical reconstruction by secondary osteons seems to be important for the ultimate strength of the bone union.

Structural behavior of the halo orthosis pin-bone interface: biomechanical evaluation of standard and newly designed stainless steel halo fixation pins.

The structural response of the halo orthosis pin-bone interface to transverse loading was evaluated on an Instron testing machine using fresh cadaver calvarium sections. Commercially available stainless steel (control) pins and newly designed stainless steel experimental pins were evaluated. Cyclic loading tests and load-to-failure tests were performed. Of the many designs tested, one pin demonstrated an improvement in structural properties at the pin-bone interface compared with the control pin. Furthermore, the new pin design was more resistant to insertional torque reduction when subjected to cyclic loading after insertions at 4 and 6 in-lb. Both the control and experimental pins exhibited improved structural behavior at 8 in-lb of insertional torque compared to the currently recommended 6 in-lb.

Guidelines for external fixation frame rigidity and stresses.

Using results from FEM analyses and experiments as references, analytical methods are applied to develop simple approximate formulas to relate frame rigidity, maximal pin stresses, and peak pin-bone stresses in external fracture fixation (EFF) configurations in axial loading to the most important frame, pin, and bone parameters. It is found that, in a realistic range, the parameters can be adapted to vary the frame rigidity from about 13 N/mm to 17,000 N/mm, thereby reducing the maximal stresses in the pins and at the pin-bone interface by a factor of 140. In particular, when compromises have to be established in the frame characteristics in order to ensure a flexible configuration and limit the stress values at the same time, the formulas presented can provide useful guidelines. The side-bar separation and the pin modulus, in particular, can be adapted to decrease the rigidity, while only moderately increasing the stresses, thereby reducing changes for pin failure, pin-bone loosening, and pin-tract infection. A nomogram is presented for a quick reference to estimated relations between frame characteristics, rigidity, and stresses. It is believed that this material may be of use in EFF design and applications in clinical and animal experimental trials.

Stresses at the pin-bone interface of external fixators: A study employing reflective photoelastic coatings

Stresses at the pin-bone interface of external fixators: A study employing reflective photoelastic coatings

Parametric analyses of pin-bone stresses in external fracture fixation devices.

Problems occurring in the use of external fixators for bone fractures include pin-bone interface necrosis, infection, and loosening. These may be initiated and enhanced by pin-bone interface stress levels. Based on stress data from finite element method (FEM) models, an analytical "closed-form" model of the local pin-bone configuration in long-bone fracture fixation is developed. This model is relatively simple and useful for routine applications in combination with clinical studies and animal experiments. Although approximate, the most significant stress predictions in the pin-bone structure compare well with more sophisticated FEM analysis results. The analytical model is used for extensive parametric analyses, investigating the effects on the pin-bone interface stress distribution of frame configuration parameters, pin diameter and modulus, bone dimensions, and elastic characteristics. The results indicate that these stresses may reach very high levels under unfavorable circumstances but can be drastically reduced by increasing the bending rigidity of the pin, reducing the side-bar separation and applying full-pin configuration in favor of half-pins.

The mechanical performance of the standard Hoffmann-Vidal external fixation apparatus.

A standard Hoffmann-Vidal quadrilateral fixation system for tibial fractures was analyzed using different geometric and material variations of the basic configuration. The fixation stiffness properties were quantitated to provide objective comparison. The results have shown that the bone-pin interface is the least-stiff link in the entire structure, particularly under the anterior-posterior bending mode. Rigidity of the device can be substantially improved by increasing the number of pins, using full threaded pins with a larger diameter, decreasing side connecting-rod distances, and increasing pin-separation distances in each pin group. Symmetrical tightening of the compression screws by hand is sufficient to produce compression of bone at the fracture site. The use of titanium pins tends to reduce stiffness, but using a frame made of titanium can significantly decrease the weight of the apparatus without decreasing its stiffness.