Preliminary studies on the effects of direct current on the bone/porous implant interfaces

Implant fixation by bone ingrowth

The term osseointegration referred originally to an intimate contact of bone tissue with the surface of a titanium implant; the term bone ingrowth refers to bone formation within an irregular (beads, wire mesh, casting voids, cut grooves) surface of an implant. The section dealing with the historical background describes the development of macroporous, microporous, and textured surfaces with an emphasis on the evolution of porous and textured metal surfaces. The principal requirements for osseointegration and bone ingrowth are systematically reviewed as follows: i) the physiology of osseointegration and bone ingrowth, including biomaterial biocompatibility with respect to cellular and matrix response at the interface; ii) the implant surface geometry characteristics; iii) implant micromotion and fixation modes; and iv) the implant-bone interface distances. Based on current methods of bone ingrowth assessment, this article comparatively reviews and discusses the results of experimental studies with the objective of determining local and systemic factors that enhance bone ingrowth fixation.

Effect of noninvasive low intensity ultrasound on bone growth into porous-coated implants

Noninvasive low intensity ultrasound has been shown to be an effective means of accelerating bone fracture healing in both animal and clinical studies. An in vivo canine study was designed to determine if noninvasive low intensity ultrasound could influence the rate and extent of bone growth into porous-coated implants. Twenty-two pairs of fully porous transcortical implants were inserted bilaterally into the femora of 12 dogs. In each dog, one femur served as a control and the other was subjected to daily ultrasound stimulation for 2, 3, or 4 weeks. Overall, the ultrasound-stimulated implants demonstrated an 18% increase in bone ingrowth compared with their contralateral controls (p = 0.02). Noninvasive low intensity ultrasound had its greatest effect in the first 2-3 weeks of stimulation. At 2 and 3 weeks, the ultrasound-stimulated implants showed 21 and 16% more ingrowth than their respective contralateral controls. Because noninvasive low intensity ultrasound had a positive effect on bone ingrowth in this experimental investigation, further research is suggested to assess the clinical potential for application to noncemented porous-coated total joint replacements.

The quantification of bone tissue regeneration after electromagnetic stimulation

In this study a titanium implant, the bone harvest chamber (BHC), was used to evaluate the effect of electromagnetic stimulation on osteogenesis. The BHC was inserted with a minimum of surgical trauma in the proximal tibial metaphysis in six adult lop-eared rabbits. Bone anchorage occurred after 4 weeks. After implant incorporation bone tissue was harvested at 3-week intervals with the implant in situ without killing the animal. The regenerated bone tissue was analysed by means of microradiography and densitometry. A test group and a control group each comprised six rabbits. The test group was stimulated with a 72-Hz electromagnetic field. Bone tissue was harvested from each tibia six times during the stimulation time and twice after the stimulation had been turned off. The control group had the same harvest procedure performed from one leg. Results showed that electromagnetic stimulation can maintain constant high osteogenetic activity. After the electromagnetic stimulation was turned off the osteogenetic activity diminished rapidly and osteogenesis was significantly lower than during stimulation.

Field distributions in the rat tibia with and without a porous implant during electrical stimulation: a parametric modeling

Expeditious post-operative ingrowth of bone is necessary for clinically successful fixation of porous joint prostheses. Electrical or electromagnetic fields to stimulate bone growth into porous implants have been used; however, they produced nonconvincing data. This was partially attributable to the lack of quantification of the localized electric fields produced in the pores of the implants. Therefore, this study set out: i) to quantify the local electric field values induced into the surface pores of nonconducting implants by "capacitive" coupling and to determine the magnitude of the macroscopically applied capacitively coupled electrical currents to induce specific electric field amplitudes in the pores, ii) to identify the important dielectric properties of the implant-tissue interface, and iii) to create the basis for successfully applying electrical fields in an animal model to stimulate bone ingrowth. A finite element method was used to calculate the electric field gradients and current densities present in a rat tibia modeled with a porous intramedullary implant when capacitively stimulated. Results indicated that while the current density in the pores are reduced in comparison to the region just outside the pore by about one order of magnitude, a significant current density still exists in the pore region. Furthermore, the presence of the implant increases the current densities in the trabecular bone while decreasing these values in the cortical bone. Replacing the trabecular bone in the pore by saline increases the current density in the pore by three-fold, but decreases the voltage gradient by a similar factor.

Cementless total hip arthroplasty: femoral remodeling and clinical experience

Cementless fixation of arthroplasty components has proved to be viable. The physiology of ingrowth is increasingly understood, and many of the factors influencing this phenomenon have been elucidated. The effect of ingrowth fixation on bone remodeling is less understood. The effects of varying ingrowth surfaces and the distribution of these surfaces influence remodeling and are critical to clinical success. Clinical studies with femoral components of different design show that while both yield good clinical results and exhibit similar remodeling patterns, the degree of remodeling may be influenced by design criteria.

Calcium phosphate-coated porous titanium implants for enhanced skeletal fixation

Porous titanium fiber implants for cementless skeletal fixation by bone ingrowth were treated with a calcium phosphate coating applied by a plasma flame-spray technique. In a paired experiment, treated and control implants were inserted in the humeri and olecranons of 36 adult dogs for periods of 1, 2, 4, and 6 weeks. After the animals were sacrificed, a biomechanical evaluation of the strength of skeletal fixation of the implants and a histologic evaluation of bone ingrowth was done. The mean shear strength of skeletal fixation at four weeks for the calcium phosphate-coated implants was 24% greater (P less than .01) than for paired controls. No difference in strength of fixation between treated and control implants was present at other time periods. The osteoconductive properties of the ceramic coating were demonstrated by bone forming in direct contact with the calcium phosphate coating on the metal fibers of the treated implants. No significant increase for the volume of bone ingrowth was established for treated implants compared to paired controls at any time period.

Historical review of porous-coated implants

The implementation of cementless joint replacement prostheses is proving to be a significant advance in orthopedic surgery. The porous-coated implant, stabilized by the biologic fixation resulting from bone ingrowth, is one of several types of cementless devices. A historical review of porous materials reveals that the concepts underlying biologic fixation date back decades. It was the initial concerns about the long-term performance of bone cement that, in the early 1970s, stimulated several groups more actively to pursue the development of porous systems. As a result of their efforts, there are currently many different porous coatings and prosthetic designs undergoing clinical investigation, with generally encouraging results. However, it is becoming clear that the importance of porous-coated prosthetic devices is not that they will serve to replace cemented prostheses entirely, but rather that they offer viable alternatives for the treatment of certain populations of patients. An understanding of the evolution of porous-coated implants can help to identify gaps in our knowledge and areas that require additional study, to direct future design modifications of implants, and to foster effective implementation of the prostheses.