Comparison of Replacement Prostheses for Segmental Defects of Bone. Different Porous Coatings for Extracortical Fixation.

The extent of extracortical bone-bridging and ingrowth into porous-coated prostheses for the stabilization of segmental defects was studied in a canine model. Initial fixation of the implant was achieved using bone cement. Autogenous bone grafts were applied over the porous-coated segmental portion of the prosthesis to stimulate the ingrowth and formation of bone. At twelve weeks, bone-bridging and ingrowth occurred uniformly in both the titanium fibermesh and the cobalt-chromiummolybdenum beaded prostheses. Maximum formation of osseous tissue over the implants occurred at two to four weeks. More bone formed in the posterior aspect of the prosthesis. At twelve weeks, 26 per cent of the porous space of the titanium fibermesh prosthesis and 47 per cent of the porous space of the cobalt-chromiummolybdenum beaded prosthesis were filled with bone. The torsional strength and stiffness of the prosthetic midsection that contained a conical coupling joint were increased significantly due to bone-bridging and ingrowth. The cortical bone that was apposed to the segmental prosthesis showed an increase in porosity. The use of bone cement did not appear to impede new-bone formation extracortically. The initial stability ofthe implant and the application of sufficient autogenous bone grafts are two important factors that contribute to the ultimate stable fixation of an implant by extracortical bone formation. CLINICAL RELEVANCE: Segmental replacement for bones and joints is indicated in patients who have had resection of a musculoskeletal tumor, severe trauma, or non-union; for salvage of a failed joint arthroplasty; and for certain metabolic bone diseases that involve massive osseous defects. Prosthetic replacement using a custommade metal prosthesis has been plagued with complications, mainly involving loosening of the stem and fracture. The proposed concept for the design and fixation C No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject ofthis article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was Public Health Service Grant CA 2375 1 awarded by the National Cancer Institute, Department of Health and Human Services. t Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic, Rochester, Minnesota 55905. Please address requests for reprints to Dr. Chao. of implants may provide an improved outlook for limb and joint-salvage procedures. Restoring the continuity of a major long bone after segmental resection in a patient who has had a bone tumor or has sustained severe trauma is one of the most difficult problems in orthopaedic surgery . Autogenous or allogeneic grafts of bone have been commonly used to replace and reconstruct such defects”2’222’3338. One alternative to bone-grafting is replacement with a custom-made prosthesis, fixed with cement527’29 3’ . However, the use of a prosthesis that has been fixed with cement can be complicated by failure of the cement, fracture of the prosthetic stem, loosening at the interfaces, and osteoporosis caused by stress-shielding7’5. To avoid these problems, porouscoated prostheses have been used. Clinical trials of titanium segmental prostheses that have a fibermesh porous-coated body and stem were recently reported’4”7’20. Although the early clinical results were encouraging, loosening and fracture of the stem still occurred. These prostheses are fabricated with a smaller-diameter stem to accommodate the porous layer. This results in a reduction in the fatigue strength of the stem, which, together with the weakening of the material that is caused by the sintering process, was responsible for fracture of the stem9’7. In addition, the initial stabilization of the implant through pressfitting of the porous-coated stem in the intramedullary canal, without cementing, may be inadequate to ensure consistent ingrowth of bone for biological fixation4’23. A better method to obtain secure fixation of segmental defect-replacement prostheses is needed. In the present study, the behavior and performance of two types of segmental defect-replacement prostheses were critically assessed in an animal model. Each experimental prosthesis contained a porous surface layer that was confined to the region of the segmental portion; there was no porous coating on the stem. Bone cement was used for initial fixation. Long-term fixation resulted from bone-bridging across the cortex into the porous space of the porous-coated shoulder of the implant, enhanced by the application of an autogenous bone graft. This type of fixation is defined as extracortical bone-bridging fixation in segmental defect-replacement prostheses. Fia. 1-A FIG. I-B COMPARISON OF REPLACEMENT PROSTHESES FOR SEGMENTAL DEFECTS OF BONE 161 VOL. 70-A, NO. 2, FEBRUARY 1988 Figs. 1-A and 1-B: The two types of porous-coated segmental defect-replacement prostheses that were used. Fig. I -A: The unassembled prosthetic components consist of a solid stem on one end and a conical coupling on the other end. Fig. I-B: The assembled and cement-fixed prosthesis replacing the middle one-third of the canine femoral diaphysis. Limited clinical series and animal experiments have successfully demonstrated the efficacy of such an approach2’3’9’6”7’20’23. However, the majority of these experimental prostheses were made of Ti-6Al-4V alloy and had a porous coating of titanium fibermesh. The evolution of the process of extracortical bone-bridging, the strength of its biomechanical fixation, and the incorporation of an autogenous bone graft in the presence of a cemented stem have not been examined in carefully controlled experimental studies. Furthermore, it is important to examine the results of another type of well established porous-coating material (cobalt-chromium-molybdenum beads) in the design of segmental defect-replacement prostheses to achieve extracortical bone-bridging fixation. The main objectives of the present study were: ( 1) to investigate the roentgenographic, histomorphological, and biomechanical characteristics of extracortical bone-bridging fixation after the insertion of a segmental defect-replacement prosthesis, (2) to examine the effect of cement fixation of the prosthetic stems on extracortical bone formation, incorporation, and ingrowth, (3) to explore cortical boneremodeling after successful bone-bridging and ingrowth, and (4) to compare the results of the use of two types of porous-surface structures (titanium fibermesh and cobaltchromium-molybdenum beads) under the same conditions. A short-term (twelve-week) animal experiment was carried out to compare the behavior of the prostheses during the most critical period of initial incorporation of the graft. Materials and Methods Eight mongrel dogs, weighing approximately fifteen kilograms, were used. Age was estimated by observing dosure of the proximal tibial growth plate28; the approximate range was eight months to two years. The dogs were randomized, and bilateral segmental replacement of a segment of the middle of each dog’s femur with one of the two types of prosthesis was performed. The results were compared in paired fashion.