A novel way to dynamize a spatial frame and optimize fracture healing

Fracture site motion creates mechanical strains on the healing tissues which influences bone formation. Axial micro-motion maximizes dilatational strains, whereas shearing motions maximize deviatoric strains on the healing tissues. Dilatational strains optimize bone healing, deviatoric strains retard bone healing. Dynamization of external fixation using either an Ilizarov or Spatial Frame platform is used to increase loading on the limb which increases the mechanical stress and strain on the tissues to improve healing. The scientific literature does not address how dynamization of the spatial frame effects fracture site motion. The purpose of this study is to assess the effect of modified shoulder bolts incorporated into a spatial frame during dynamic loading.

Five identical two-ring spatial frame constructed were mounted on Sawbones tibias with an osteotomy performed distal to the tibial tubercle. Sinusoidal load was applied at a rate of 0.25 Hz. Axial force and displacement, in addition to motion of the proximal and distal tibia segments were recorded. Eight constructs were tested: 1) All struts of the Spatial Frame rigid, 2) Strut #1 loose, 3) Struts #1 and #3 loose, 4) Struts #1, #3 and #5 loose, 5) All struts loose, 6) All struts rigid with dynamization bolts on the proximal end, 7) All struts rigid with dynamization bolts on alternating sides, 8) Threaded rods between the rings with two millimeters of dynamization.

No difference in vertical displacement was observed between the Ilizarov and all struts locked. No significant difference in shear values between all struts locked and modified shoulder bolt struts was observed. Increase in vertical movement with the modified shoulder bolts was an average of 1.83 mm. Significant shear forces at the fracture site were observed with unlocking single or multiple struts of the spatial frame.

Modified shoulder bolts can be used for spatial frame dynamization without increasing shear motion.