To investigate whether a long bone cortex of well-defined thickness can be regenerated by using an anatomically designed membranous resorbable "tube-in-tube" implant and to establish the functions of membranes in the healing of segmental diaphyseal bone defects
larger than the "critical size."
Bone healing in segmental diaphyseal defects larger than the critical size in the sheep tibiae covered with a single porous tubular membrane or implanted with anatomically shaped porous double tube-in-tube membranes was evaluated. Membranes with different pore structures were applied alone and/or in combination with autogenous bone graft.
Healing of segmental diaphyseal bone defects
in animals can be enhanced by covering the defects with resorbable polylactide membranes. Based on the results of bone healing in defects ten millimeters long in the rabbit radii, it was suggested that the membrane prevents muscle and soft tissue from invading the defect and maintains osteogenic cells and osteogenic substances within the space covered with membrane, thus promoting new bone formation. The functions of membranes may differ, however, depending on the size and the location of the defect and on the experimental species used. Bone defects
larger than the critical size may not heal at all, even if membranes are used. The critical-size defect is defined as the smallest bone defect that does not heal spontaneously when covered with polymeric membranes
. To heal such defects, it is mandatory that membranes are used in combination with autogenic bone graft and/or a suitable bone substitute. If bone graft is used to fill the defect, the structure and geometry of the covering membrane will determine whether the graft will be vascularized and/or nourished from the surrounding soft tissue and, in consequence, survive. It can be appreciated that bone healing in areas of good vascularity should be more efficient than bone healing in poorly vascularized areas. The influence of all these factors on healing of bone in segmental diaphyseal defects covered with membranes is not known.
Four-centimeter-long diaphyseal segmental defects in the tibiae of six- to seven-year-old Swiss mountain sheep were covered with resorbable membranes from poly(L/DL-lactide). In Group 1, a single microporous external membrane was used. In Group 2, one microporous membrane was inserted into the medullary cavity at the cut ends of the tibiae (internal membrane), and the other microporous membrane was placed on the outer surface of the cortex (external membrane). In Group 3, a single microporous external membrane was also laser-perforated to produce openings with a diameter in the range of 800 to 900 micrometers. In Group 4, the defect was filled with autogenous cancellous bone graft and covered with a single perforated membrane. In Group 5, one perforated internal membrane was inserted into the medullary cavity at the cut ends of the tibiae, and the other perforated membrane was placed on the outer surface of the cortex. Group 6 was identical to Group 5, except that cancellous bone graft was placed in the space between these two membranes.
There was no bone healing in Groups 1, 2, 3, and 5. Only in Groups 4 and 6 did the defects heal. In Group 4, new bone was dispersed across the "medullary canal" formed by the membrane. In Group 6, the new bone had grown into the space between the outer and inner membranes, forming the "neocortex."
The resorbable polymeric implant consisting of two concentric perforated membranes (the tube-in-tube implant) used in combination with cancellous bone graft to treat segmental diaphyseal defects in sheep tibiae allows for the reconstitution of the "neocortex" with well-defined thickness. The primary functions of polymeric membranes
in the healing of bone defects
larger than the critical size are optimizing the contact between the soft tissues and bone graft to avoid its excessive resorption, allowing adequate graft vascularization/nutrition from the surrounding soft tissue, maintaining the graft in the required location, and providing a substrate for osteogenic cells. The interface between the soft tissues and bone graft seems to be a predominant factor in determining graft survival and functionality. Such an interface may be provided by the perforated polymeric membranes