THE RADIAL FOREARM FLAP - A BIOMECHANICAL STUDY OF DONOR-SITE MORBIDITY UTILIZING SHEEP TIBIA

The use of vascularized bone grafts to reconstruct extremity and mandibular defects is now commonplace in reconstructive surgery. Fibula, scapula, iliac crest, rib, and metatarsal as well as the radial forearm osseocutaneous flaps have all been utilized for this purpose. Troublesome spiral fractures of the distal radius are the most common fractures associated with the use of the distal radius as a vascularized bone-graft donor site. This study was proposed to investigate the effect of donor-site bone loss on the strength of the radius under torsional (rotational) loading. Previous clinical series and experimental studies have not examined this aspect of distal radius loading after harvesting the bone graft. Fifty pairs of sheep tibiae were utilized in the experiment. Five pairs were used in a pilot study and 45 pairs were used in the main experiment. Five pairs of human radii were used for the control in the pilot study. The pilot study attempted to make a comparison between the human radius and the sheep tibia for experimental purposes. For the biomechanical study of donor-site defects, four study groups were examined with random assignment and matched pairs. The control group (group 1) had no alteration to the bone. Each test condition included five matched pairs of sheep tibiae. Experiment 1 compared the difference in the depth of the osteotomy defect. In doing this, one-third of the total length of the bone was removed in each of the following specimens to include (1a) 30 percent of the cross-sectional area of the total bone, (1b) 37 percent of the cross-sectional area of the total bone, and (1c) 50 percent of the cross-sectional area of the total bone. In experiment 2, the osteotomy shape was varied. Instead of the ends of the cuts being squared, the ends were beveled or rounded. Experiment 3 compared different lengths of bone removed in the osteotomy defect and included the following: In experiment 3a the diameter of the sheep tibia was measured at the incisura fibularis. This dimension was one diameter of bone, and a one-diameter length of bone was removed. In experiment 3b, a two-diameter length of bone was removed. In experiment 3c, a three-diameter length of bone was removed. In experiment 3d, a four-diameter length of bone was removed. The results of this study indicated that in the comparison control group between the human radius and the sheep tibia, no statistically significant difference was noted between the two bones in the pilot study. Both bones had very similar values when tested with strengths against torsional loads. We therefore felt that the sheep tibia was an ideal bone source to mimic the human radius. The bone strength and maximum torque decreased 74 percent of the original value after removal of only one-quarter of the depth of bone. There was a statistically significant difference after removal of more than one-third of the depth. The beveled-end osteotomy was the strongest; however, it also weakened the bone more than 70 percent. The conclusions of this study are that sheep tibiae are ideal alternatives to the human radius in comparative studies. Removal of a portion of the distal radius as a vascularized bone graft results in significant weakening of the bone after only one-quarter of the diameter of the total bone has been removed. Stiffness of the donor bone is dependent on both the length and the depth of the bony defect left, and the beveled-cut osteotomy reduces the stress concentration effect, although by only 5 percent. This effect is minimal in preserving strength of the osteotomized bone. The distal radius remains an option in vascularized bone transfer, but in view of the significant weakening and problems incurred with fracture of this bone after its harvest, we feel that there are certainly better donor sites available in vascularized bone reconstruction.