Effects of flexural rigidity of plates on bone healing.

The effect of bone-plate flexural rigidity on the strength of healing bone was explored. Plates with similar shapes and a range of flexural rigidities (from 1.4 to 16.8 Nm2) were prepared from fiberpolymer composites or selected from among commercially available metal plates and applied to the anterolateral surface of osteotomized femora in dogs. At sixteen weeks, the plated femora and contralateral control femora were removed Bone specimens (composed of cortex and callus) were removed from the area beneath the plate and from the opposite side of the same femur as well as from the anterobateral surface of the contralateral control femur. The strengths of these specimens were determined in bending tests. Bone strength increased with decreasing rigidity of the plate, and the effects were more marked in the specimens taken from the side of the femur opposite that of the plate The results indicate that control of the bending * Read at the Annual Meeting of the Orthopaedic Research Society, Las Vegas, Nevada, February 3, 1977. t Division of Orthopedic Surgery, University of Utah Medical Center, Salt Lake City, Utah 84132. Please address reprint requests to Dr. Dunn. t National Bureau of Standards, Washington, D.C. 20234. § University of Utah Research Institute, Salt Lake City, Utah 84108. 1 Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 841 12. loads to which bone is subjected during healing may be an important aspect of fracture treatment. CLINICAL RELEVANCE: Initial fracture healing appears to be enhanced by rigid internal fixation. However, the ultimate strength of the bone depends not only on this phase of fracture healing but also on the remodeling that occurs. Traditional metallic plates have been shown to have unfavorable effects on bone remodeling because they prevent the underlying bone from being subjected to the stresses that are normally present. Remodeling studies have demonstrated that when the normal forces to which living bone is exposed are removed, decreased cortical thickness and increased osteoporosis ensue. These changes have been shown to be a function of plate flexibility or stiffness. The present study, utilizing flexible plates made from fiber-reinforced composite materials, indicates that the presence of more normal stress may be beneficial in early fracture healing as well as remodeling. We suggest that so-cabled stress-shielding caused by rigid metallic plates can be diminished through the use of plates constructed from more flexible composite materials. Bone is a living tissue which responds to its mechanical environment. It has been shown that mechanical stresses are needed to minimize undesirable remodeling of FIG. I EFFECTS OF FLEXURAL RIGIDITY OF PLATES ON BONE HEALING 867 VOL. 61-A, NO 6. SEPTEMBER 1979 bone, but the effects of loading and movement during fracture healing have been explored less thoroughly’3. The optimum mechanical environment for fracture healing has not been delineated, a fact that is well demonstrated by present-day clinical use of such widely divergent treatments as cast-bracing and internal fixation. We studied the response of fracture healing to various mechanical environments provided by bone plates with a wide range of flexural rigidities, and in this report we describe our experimental approach and initial findings. In studies of bone remodeling, it has been noted that use of conventional metal fixation plates results in a bone plate-bone mechanical mismatch in which the relatively rigid plates ‘ ‘shield ‘ ‘ the bone from the stress to which it is ordinarily exposed. It has been demonstrated that this socabled stress-shielding results in bone remodeling causing cortical thinning or osteoporosis, or both2’9”#{176}”3. It has also been shown that the magnitudes of these changes are functions of flexural stiffness of the plate. For exampie, Woo and co-workers, using intact canine femora, compared the effects of a conventional metallic plate with the effects of a fiber-reinforced-polymer composite plate which was geometrically similar to the metallic plate but was one-tenth as stiff. Less cortical thinning was observed with the more compliant plate. Similarly, Tonino and associates noted less remodeling beneath an even more compliant non-reinforced-polymer plate than beneath a metal plate with which it was compared. The effect of flexurab stiffness of the plate on fracture healing has been explored to some extent”36. Fracture healing is a complex continuing process, but to facilitate understanding it can be considered as composed of phases. The first phase is characterized by proliferation of callus and the second, by bone remodeling. When considering the effects of plates in bight of these healing phases, one can hypothesize two conflicting requirements during the service life of a plate. First, it must have sufficient flexural rigidity to prevent excessive motion at the fracture site, to maintain alignment and allow initial fracture healing (calbus proliferation) to occur. Subsequently, the plate should have sufficient compliance to minimize stress-shielding and the undesirable aspects of remodeling such as cortical thinning and bone resorption. We performed this preliminary study to explore the effect on fracture healing of varying the flexural rigidity of bone plates from about 4 to 40 per cent of the rigidity of the bone to be plated. It is possible to measure many aspects of the biological response of bone to the mechanical environment present during fracture healing. We chose to determine bone strength because it is of direct clinical interest. Our findings indicate that within the limits explored, decreasing the flexurab rigidity of the plate increased the strength of the healing bone. Materials and Method One class of materials available for the fabrication of internal fixation devices is composed of polymers reinforced with continuous fibers 1,3.6. 5,13 These materials can be used to produce devices such as bone plates by laminating layers of the materials in a heated die under pressure. A spectrum of mechanical properties can be achieved in devices of similar geometric configuration by using different materials and lamination patterns. For our project, geometrically similar plates with a range of flexural rigidities were obtained. Some were selected from clinically used stainless-steel plates, while others were fabricated from commercially available graphite-polysulfone and glass-epoxy laminates as well as from laboratoryprepared graphite-polypropylene composites . All composite materials were tested for biocompatibility in agaroverlay tissue-culture tests and were shown to produce responses similar to those produced by samples of the USP Negative Control Plastic Reference Standard . Examples of the fabricated plates and of a stainless-steel plate are shown in Figure 1. Examples of the plates used to stabilize the femoral osteotomies. Top graphite-polysulfone; middle glass-epoxy; bottom conventional 316L-type stainless steel. The plates were approximately I 15 millimeters long, and depending on the properties desired they were fabricated with widths between about eleven and thirteen millimeters and thicknesses between 2.5 and 6.3 millimeters. All plates were approximately 1 15 millimeters long, with six screw-holes located in the same positions as in the standard steel plates. The desired spectrum of flexural rigidity was achieved not only by varying the plate materiabs but also by varying the plate widths from eleven to thirteen millimeters and the thicknesses from 2.5 to 6.3 millimeters. The flexural rigidities ranged from I . 45 to 16.8 Nm2, while the axial stiffnesses were more nearby the same. The eleven plates that were more flexible had axial stiffnesses that ranged from approximately I . 4 x bO” to 2.8 x lO Nm2, and the six stiffer plates had axial stiffnesses that ranged as high as 10.8 x l0 Nm2. The flexural rigidities were determined by four-point bending tests, while the axial stiffnesses were determined in compression tests. The flexural rigidities were calculated using the