Allan M. Weinstein

Stress Analysis of Porous Rooted Dental Implants

A two-dimensional plane stress finite element analysis of porous rooted dental implants was performed. The results of this analysis were compared to results obtained from mechanical tests performed on actual implanted specimens. The appropriate selection of interface material properties was shown to be highly significant.

An evaluation of skeletal attachment to LTI pyrolytic carbon, porous titanium, and carbon-coated porous titanium implants

Porous titanium, carbon-coated porous titanium, and low-temperature isotropic (LTI) pyrolytic carbon transcortical implants were placed in the femora of mongrel dogs. Mechanical and histologic analyses were performed in specimens that remained in situ for six months. Qualitative histologic results indicated that the bone formed a direct appositional interface with as-deposited LTI carbon. The bone tissue response to the two systems with porous coatings was similar. Both systems showed little fibrous tissue interposition and a high degree of mineralized bone ingrowth. The ingrown bone was well organized. However, there was some evidence that the ingrown bone mineral differed significantly from the bulk bone mineral. The only difference between carbon-coated and uncoated porous systems was a significant increase in the percentage of bone ingrowth, with carbon-coated specimens having a 4% increase in bone volume. The strength of appositional attachment to LTI carbon was shown to be at least one order of magnitude weaker than bone ingrowth attachment to the porous systems. The interface shear stiffness of the two porous systems was equivalent; however, the attachment shear strength of bone growth into carbon-coated porous titanium was significantly increased compared with that of bone growth into the uncoated porous titanium. Correlation of the percentage of bone ingrowth and pushout strength was also found to be statistically significant, suggesting that the presence of the carbon coating enhanced bone ingrowth, which resulted in significantly increased shear strengths.

A canine composite femoral stem. An in vivo study

To evaluate a carbon fiber/polysulfone composite femoral stem, a press-fit unilateral hemiarthroplasty was performed in 17 greyhounds. The implant was designed to have strength and elastic properties commensurate with the proximal canine femur. The implant geometry was such that the naturally occurring internal cancellous structures of the proximal femur would be preserved and participate in load transfer from the implant to the bone. Animals were killed at one, five, ten, 16, and 24 months. At necropsy all the femoral stems were well fixed and functioning. All implants maintained their structural integrity. Radiographs and computed tomography scans showed a constructive bone remodeling response. Histologic analysis revealed a benign host tissue response, with few inflammatory cells observed. Both bone and fibrous tissue were observed at the implant-host tissue interface. Implants fabricated from carbon/polysulfone composites have the potential for use in load-bearing applications. An implant with appropriate elastic properties provides the opportunity for the natural bone remodeling response to enhance implant stability. Naturally occurring internal cancellous structures can be utilized for load transfer by femoral components. Press-fit devices with no physical or chemical bone-bonding mechanisms can attain long-term successful functional performance.

An Evaluation of Porous Alumina Ceramic Dental Implants

The fabrication, characterization, and clinical results of porous rooted alumina dental implants in animals are presented. Failures of all implants occurred in less than six months and were attributed to the presence of microporosity on the crown and cervical portion of the implant which prevented the establishment of an effective bialogical seal between the oral cavity and the alveolus.

Pyrolite carbon implants in the metacarpophalangeal joint of baboons.

A nonconstrained uncémented Pyrolite carbon prosthesis was evaluated for replacement of the metacarpophalangeal joint. Six prostheses were inserted into the long finger metacarpophalangeal joint of four baboons. Nine months after insertion, the prostheses and surrounding tissues were removed enbioc and evaluated radiographrca/ly, utilizing histologic and microradiographic analyses. The four Pyrolite carbon implants inserted without cement were well tolerated. Histological evidence of direct appositional bone fixation along the medullary stem was observed in one specimen, and a combination of bone fixation with an interposing fibrous tissue membrane was observed in another. There was no evidence of bone resorption around the implant stems, and functional fixation was obtained with all of the uncemented Pyrolite carbon implants. No foreign body reaction was observed in the soft tissues, and no evidence of intracellular particles was present. Two cemented implants (one Pyrolite carbon and one polyethylene and metal) were also evaluated; both showed evidence of bone resorption and/or gross implant loosening.This study has demonstrated the potential for biological fixation with Pyrolite carbon-stem med implants for prosthetic replacement of the metacarpophalangeal joint, which thus offers significant improvement as a material for joint reconstruction.

The role of insulin-like growth factor II in magnetic field regulation of bone formation

Abstract Musculoskeletal tissue is uniquely sensitive to biophysical input, as demonstrated by both mechanical and electrical stimulation experiments. However, the mechanism by which biophysical input couples to cellular processes is not well understood. The results presented in these studies suggest that these stimuli are bioactive due to stimulation of growth factor biosynthesis by musculoskeletal target cells. We chose insulin-like growth factors (IGFs) as the model growth factor, as the IGFs are capable of stimulating chemotaxis, proliferation, and differentiation of osteoprogenitor cells. These specific studies addressed whether short-term exposure to combined a.c. and d.c. magnetic fields (CMF) would increase production of IGF-II by both osteoblast-like cell cultures as well as rat fracture callus cultures. In vitro studies on human osteoblast-like cell cultures demonstrated statistically significant increases in IGF-II levels and DNA synthesis after only 30 min CMF exposure. In rat fracture callus explant cultures, IGF-II levels were increased at least two-fold dependent on the callus differentiation stage, and these results were comparable with the effect of the osteotropic agent, parathyroid hormone. In summary, these results suggest that the mechanism by which CMF, and other biophysical stimuli, regulate musculoskeletal repair is by modulation of endogenous growth factor (IGF-II) synthesis and secretion.

Clinical, Radiographical, and Histological Evaluation of Porous Rooted Polymethylmethacrylate Dental Implants

Forty-three porous rooted polymethylmethacrylate (PMMA) dental implants were inserted into twenty-four dogs. Successful implants were maintained for up to three years. Histological sections with the implants in situ of implants revealed a soft tissue-implant interface similar to natural teeth. Bone and fibrous tissue ingrowth into the pores attached the implant to the bone. Failures were attrabuted to mechanical weakness of the implant, thin buccal cortical bone, and excessive implant-gingiva interface.

Femoral Head Stresses Following Cortical Bone Grafting for Aseptic Necrosis: A Finite Element Study

A two-dimensional finite element model was used to compare the femoral head stress distribution of an intact femur with that of a femur following core biopsy and biopsy with cortical grafting. There was a decrease in superior and lateral femoral head stresses of up to 50% by an incorporated and properly placed cortical graft. Core biopsy and cortical grafting short of the subchondral plate produced stress-concentrating effects. A medially placed cortical graft did not stress-relieve the superolateral femoral head. Thus, if cortical grafting is to be undertaken, the graft should extend from the lateral cortex to the subchondral plate, transfixing the length of the necrotic segment. Because of deleterious effects on femoral head stress distribution, core biopsy without cortical grafting should be discouraged or only very cautiously used.

The effect of high pressure insertion and antibiotic inclusions upon the mechanical properties of polymethylmethacrylate.

The properties of self-curing polymethylmethacrylate as affected by the admixing of an antibiotic to the powder-liquid mixture and as affected by insertion technique demonstrated that the antibiotic agent may leach from the surface of the acrylic, but not necessarily from the interior. The compressive and tensile strength, determined on two different commercial formulations, suggests that the antibiotic additive is not detrimental to the mechanical properties. A high pressure insertion technique produces a significant improvement in the mechanical properties of the self-curing acrylic material.

The mechanical behavior of normal and osteoporotic canine femora before and after hemiarthroplasty.

Nutritional secondary hyperparathyroidism (NSH) was induced in adult beagle dogs through nutritional control by feeding them a diet containing high phosphorus (1.2%) and low calcium (0.12%). A control group was fed a diet containing 0.42% phosphorus and 0.54% calcium. An osteoporotic condition was produced by NSH, which was verified with histologic, histomorphometric, and biochemical analyses. A significant increase in i-PTH level was measured at both ten and 20 weeks after initiation of the diet in experimental animals when compared with control animals. A 25% reduction in volume fraction cancellous bone and increased osteoclastic activity was found in histologic sections from the femoral heads of the NSH animals when compared with control animals. No abnormally large osteoid seam was found in the cancellous bone of the experimental animals. The results of energy dispersive radiographic analysis and bone ash analysis revealed no significant differences in calcium-to-phosphorus or mineral-to-matrix ratios between the two groups. The results of in vitro mechanical testing indicated significant differences in the strain state for the NSH and control animal femora, with the highest strains being observed in the calcar region of the NSH femora. The presence of a Co-Cr-Mo alloy femoral head prosthesis was found to alter the strain distribution significantly in both the control and NSH femora.