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J D Bobyn

Publications and source records attributed to J D Bobyn.

At least 37 records · Page 2Linked to original sources
PubMed1999

Characteristics of bone ingrowth and interface mechanics of a new porous tantalum biomaterial.

We have studied the characteristics of bone ingrowth of a new porous tantalum biomaterial in a simple transcortical canine model using cylindrical implants 5 x 10 mm in size. The material was 75% to 80% porous by volume and had a repeating arrangement of slender interconnecting struts which formed a regular array of dodecahedron-shaped pores. We performed histological studies on two types of material, one with a smaller pore size averaging 430 microm at 4, 16 and 52 weeks and the other with a larger pore size averaging 650 microm at 2, 3, 4, 16 and 52 weeks. Mechanical push-out tests at 4 and 16 weeks were used to assess the shear strength of the bone-implant interface on implants of the smaller pore size. The extent of filling of the pores of the tantalum material with new bone increased from 13% at two weeks to between 42% and 53% at four weeks. By 16 and 52 weeks the average extent of bone ingrowth ranged from 63% to 80%. The tissue response to the small and large pore sizes was similar, with regions of contact between bone and implant increasing with time and with evidence of Haversian remodelling within the pores at later periods. Mechanical tests at four weeks indicated a minimum shear fixation strength of 18.5 MPa, substantially higher than has been obtained with other porous materials with less volumetric porosity. This porous tantalum biomaterial has desirable characteristics for bone ingrowth; further studies are warranted to ascertain its potential for clinical reconstructive orthopaedics.

Animals↗
PubMed1999

Effect of flexibility of the femoral stem on bone-remodeling and fixation of the stem in a canine total hip arthroplasty model without cement.

The purpose of this study was to compare, with regard to fixation of the implant and femoral bone resorption, two fully porous-coated stems of different stiffnesses in a canine total hip arthroplasty model. A bilateral arthroplasty was carried out with insertion of a titanium-alloy stem (which had stiffness properties comparable with those of the canine femur) on one side and with insertion of a composite stem (which was three to fivefold more flexible than the canine femur) on the contralateral side. Eight femora were evaluated at six months and eight, at eighteen months after the operation, to determine the extent of bone ingrowth, periprosthetic cortical area, intracortical porosity, and bone-remodeling. Despite the markedly greater flexibility of the composite stems, no significant difference could be detected (with the numbers available), with regard to the overall degree of femoral stress-shielding, cortical area, or cortical porosity, between these stems and the stiffer, titanium-alloy stems at either time-period. However, the composite stems had less bone ingrowth and more formation of radiopaque lines than did the titanium-alloy stems. At eighteen months, the values for bone ingrowth were 9.7 +/- 5.38 percent (mean and standard deviation) for the composite stems compared with 28.1 +/- 5.31 percent for the titanium-alloy stems (p = 0.003). Furthermore, the histological sections from the femora containing a composite stem showed radiopaque lines indicative of fibrous ingrowth approximately threefold more often than did those from the femora containing a titanium-alloy stem (p = 0.02).

Alloys↗
PubMed1998

Femoral cement grading in total hip arthroplasty.

The current gold standard for primary total hip arthroplasty is a cemented femoral component combined with a porous-coated acetabular component. Barrack and colleagues described a femoral cement mantle grading system which is increasingly being used to evaluate surgical technique and to compare arthroplasty results. The immediate postoperative radiographs of 100 primary total hip arthroplasty cases from five community surgeons were assessed by three observers to evaluate the overall quality of cement technique, the interobserver variability in cement mantle grading, and specific characteristics of the grading system. All three observers agreed on the grading in only 73% of the x-rays (anteroposterior view, 69%; lateral view, 77%). Compared with reports in the literature from specialized hip arthroplasty centers, a very high proportion of the cases had grade C mantles. To gain a balanced perspective of the global effectiveness and longevity of cemented total hip arthroplasty it is important that follow-up studies be reported upon from representative cross sections of the orthopaedic community.

Acetabulum↗
PubMed1997

The effect of heat treatment on the fatigue strength of microknurled Ti-6Al-4V.

Microknurling, a high pressure surface indentation technique, was devised as an alternative to traditional heat-bonded porous coatings found on many orthopedic implants designed for fixation by tissue ingrowth. Heat-bonded porous coating can cause at the surface of an implant stress concentrations that reduce fatigue strength. However, microknurling may reduce stress intensification without eliminating it. Thus the purpose of this work was to explore surface thermal/mechanical processing of Ti-6Al-4V to improve the fatigue strength of microknurled specimens via the production of a Ti-6Al-4V dual microstructure. The latter consists of a surface layer of equiaxed grains known to be effective against crack initiation and a bulk microstructure of lamellar grains that possesses optimum fatigue crack propagation resistance. Rotating-bending fatigue tests showed that such a microstructure had some benefits, but this was offset by the reduction in compressive strains imparted to the surface by the heat treatments needed to obtain this microstructure.

Alloys↗
PubMed1997

Kinematics of the MATCO hip simulator and issues related to wear testing of metal-metal implants.

Metal-metal hip implants have been used clinically in Europe to reduce the risk of wear particle induced osteolysis. Joint simulator devices could provide useful information for design improvement of the modern generation of metal-metal hip implants. Early wear results for metal-metal hip specimens were obtained using a MATCO hip simulator. A detailed kinematic analysis was developed for the MATCO simulator and applied to two of the wear experiments to predict the starting surface motion, contact zone and lubricant film thickness. It was shown that points on cup surfaces were not subjected to a reciprocating interaction with the head during wear at the beginning of testing but as wear proceeded, it was suggested that, in some cases, reciprocating interaction did occur on the cup surface. Comparison between simulator and in vivo kinematics suggested a more realistic representation for cup than for head wear. In the simulator, the Hertzian contact zone moved in a circular path over the cup surface and changed in size in correspondence with the applied load. Elastohydrodynamic lubrication was considered to be possible in the simulator, with estimated fluid film thickness as great as 0.1 micron. However, such thick films were not likely to have occurred at the start of the two wear tests which were examined in detail, although some mixed film lubrication might have accounted for the relatively low wear of one of the specimens. The inclusion of kinematic details, contact mechanics and elastohydrodynamic lubrication analysis in simulator testing protocols and in design of metal metal hip implants was recommended.

Elasticity↗
PubMed1997

Fatigue fracture of a forged cobalt-chromium-molybdenum femoral component inserted with cement. A report of ten cases.

Ten patients who had had a total hip replacement with a forged cobalt-chromium-molybdenum femoral prosthesis (Precoat or Precoat Plus) inserted with cement were seen with a fatigue fracture of the stem an average of fifty months (range, nineteen to seventy-four months) postoperatively. The average age of the patients was sixty-one years (range, forty-three to seventy-three years), and the average weight was ninety-six kilograms (range, seventy to 130 kilograms). Eight patients had had a primary total hip replacement, and two had had a revision; all of the acetabular components had been inserted without cement. Radiographs that had been made before the fracture were available for four of the eight hips that had had a primary replacement; all four had radiographic evidence of debonding of the cement mantle from the proximal end of the stem. This probably caused exaggerated cantilever bending stresses on the proximal aspect of the stem as the distal end of the stem was well fixed. The radiographs of both hips that had had a revision demonstrated a non-union of the greater trochanter, which had resulted in separation at the cement-bone interface at the proximal portion of the femur before the fracture. Scanning electron micrographs of five of the ten fractured prostheses demonstrated a fatigue fracture that began near the anterolateral corner of the prosthesis, through characters that had been etched on the implant with a laser. Metallurgical analysis indicated subsurface voids or inclusions, or both, immediately under the region that had been etched. This finding is consistent with thermal changes to the microstructure of the alloy that probably caused a focal reduction in the material strength. A high proportion (seven) of the ten stems had a poor cement mantle. Also, of the seven small stems that were used, six had been implanted in patients who weighed more than eighty kilograms, so there was relative undersizing of the prostheses. Early debonding of the proximal end of a Precoat femoral prosthesis from the cement mantle may occur as a result of a thin cement mantle, leading to loosening and possibly to early fatigue fracture of the stem if the distal portion of the stem remains solidly fixed in the distal portion of the cement column. On the basis of our experience, we recommend that patients who have radiographic evidence of a debonded Precoat femoral component should be informed of the risk of fatigue fracture of the stem and be followed closely even though there may be no symptoms of loosening of the femoral component.

Adult↗
PubMed1996

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.

Animals↗
PubMed1996

Effect of rotation on periprosthetic bone mineral measurements in a hip phantom.

The effect of rotation on the measurement of periprosthetic bone mineral content and bone mineral density using dual energy xray absorptiometry was studied in 9 anatomic specimen femora implanted with noncemented titanium hip prostheses. An apparatus was constructed to permit accurate femoral rotation through an arc of 30 degrees internal to 30 degrees external rotation in 5 degrees increments. Two scans were obtained at each increment of rotation for a total of 26 scans for each implanted femur. Change in bone mineral content and bone mineral density was measured for each of 7 contiguous periimplant regions (Gruen zones). The precision of the paired bone mineral density measurements was 1.7%. In all Gruen zones, the means of bone mineral content and bone mineral density measurements varied within 5% between 15 degrees internal and 15 degrees external rotation. Variation in limb rotation, as might occur with temporal studies of periimplant bone remodeling, can be tolerated within easily controllable limits without excessive error in bone mineral measurements.

Absorptiometry, Photon↗
PubMed1996

Comparison of alloys and designs in a hip simulator study of metal on metal implants.

Previous studies of metal on metal hip implants have shown that it is possible to obtain substantially lower volumetric wear than with metal on polyethylene pairings. To work toward design optimization, the gravimetric wear of serum lubricated, metal on metal implants was examined in a hip simulator apparatus. Seventeen implant specimens were fabricated from 3 alloys in various combinations of diameter and clearance and wear tested as many as 3 million cycles. Theory was developed to predict the thickness of elastohydrodynamic lubricant films, the linear wear, and wear zone geometry. To help interpret the data, implant diameter and clearance were combined into a single parameter called effective radius. Implants with larger effective radii (6-11 m) tended to experience lower wear, and theory suggested that this was a result of the correspondingly thicker elastohydrodynamic lubricant films providing an enhanced mixed film lubrication. As in most wear testing, the results were scattered but, overall, a low carbon, wrought alloy had lower wear than either a high carbon, cast alloy or a low carbon, commercial alloy. Because of the small number of specimens tested and possible variations in tribologic parameters such as calcium phosphate film formation and surface microgeometry, there was low statistical reliability of the results.

Alloys↗
PubMed1996

Engineering issues and wear performance of metal on metal hip implants.

A major concern in total hip arthroplasty is the generation of polyethylene wear particles at the articulating surfaces and resulting macrophage mediated periimplant osteolysis. There is renewed interest in metal on metal bearings as a solution to this problem in view of their potential for greatly improved wear performance. Using a commercially available hip simulator, the wear performance of metal on metal femoral head and acetabular cup combinations was evaluated and various parameters affecting metal on metal implant wear were identified. Nine implants custom manufactured from 2 medical grades of CoCrMo alloy (ASTM F1537-95 and F75-92) were tested within bovine serum as the lubricant to 3 million cycles (equivalent to approximately 3 years of service in vivo). The progressive wear of the components was determined by gravimetric methods at approximately every 300,000 cycles. The wear rates were characterized by an initial period of accelerated wear after which a lower steady state wear rate was observed for subsequent cycles. The presence of calcium phosphate films on the component surfaces, the microstructure of the lower carbon, wrought alloy, and increased effective radii (decreased diametral clearances) were identified as factors that may be favorable to improved wear performance. The extent of the effect on wear of each parameter, however, cannot be discerned at this point and necessitates a study in which parametric changes are more tightly controlled. The present study suggests that the use of metal on metal articulating surfaces may mitigate the problem of osteolysis by offering improved wear performance.

Hip Prosthesis↗
PubMed1995

Evaluation of bone ingrowth in proximally and extensively porous-coated anatomic medullary locking prostheses retrieved at autopsy.

Three proximally (40 per cent) and five extensively (80 per cent) porous-coated anatomic medullary locking femoral components were retrieved from seven cadavera at autopsy. Each component (with the surrounding, intact femur), was sectioned transversely at one-centimeter intervals. Backscattered scanning electron microscopy was used to evaluate circumferentially the interface between the bone and the porous surface of each section. Bone ingrowth was considered to be present within a field when bone was in contact with the outermost layer of the sintered beads, it was detected within the porous space, and it had penetrated the porous space to a depth of at least one bead diameter. All eight components had some bone growth into the porous space. A mean of 35 per cent of the surface of the implants had bone ingrowth. In the areas where bone was present, 67 per cent of the available porous space on the extensively coated stems and 74 per cent on the proximally coated stems contained bone. With both types of implants, the greatest amount of compact bone ingrowth was found at the level where the porous coating ended. Transverse sections obtained at this level frequently demonstrated that bone ingrowth had occurred circumferentially and that the ingrowth was continuous with and an integral part of the femoral cortex. These direct connections to the cortex could be predicted from the appearance of the radiographs. In the most proximal transverse sections of both types of implants, bone was most frequently connected to the medial side and corners of the implant.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1995

The susceptibility of smooth implant surfaces to periimplant fibrosis and migration of polyethylene wear debris.

The purpose of this investigation was to establish whether the tissue response and migration of polyethylene debris differed at noncemented smooth and porous implant surfaces. This was accomplished through 3 separate but closely related studies: (1) a canine cylindrical implant model with smooth and porous surfaces exposed to polyethylene debris; (2) a canine total hip arthroplasty model analyzing the interface between bone and femoral implants with various porous-coating configurations; and (3) a histologic analysis of autopsy-retrieved, human, noncemented hip prostheses with noncircumferential porous coating. The cylindrical implant model involved the placement of split cylinders, 1/2 porous and 1/2 smooth, into the distal femur and proximal tibia of 4 dogs. Four control implants and 10 test implants (chronically exposed to simulated polyethylene debris with a mean size of 4.7 microns) were examined histologically as long as 30 weeks after surgery. The canine hip study involved the study of 54 noncemented hip prostheses at periods of 1, 6, and 24 months. The prostheses possessed 4 different porous surface configurations: 1 with circumferential porous coating, 2 with noncircumferential coating, and 1 without porous coating. The human retrieval analysis involved the study of 7 cadaveric femora (age, 6 months-5 years) implanted with a straight titanium-alloy prosthesis possessing proximal pads of titanium fiber metal on the anterior, posterior, and medial aspects. With all implants in all 3 studies, there was the common finding of bone ingrowth at the porous implant surface and a fibrous interface or periprosthetic cavity around the portion of the implant that was smooth surfaced. The periprosthetic cavity typically was encapsulated by a thin continuous shell of trabecular bone. In addition, polyethylene debris was found to have preferentially migrated along the smooth implant surfaces. In the longer-term canine and human hip retrievals, polyethylene particles in the micron size range were present within histiocytes, whereas larger particles as much as 100 microns were found within foreign-body giant cells. Of importance for the implants from all 3 studies, with the exception of some pronounced cavities on the lateral aspect of the human hip prostheses, the periimplant cavities around the smooth surfaces were not detectable radiographically. This study clearly established a fundamental principle of relative barriers to particulate debris migration. Smooth implant surfaces are more susceptible than porous surfaces to the development of a fibrous tissue filled periimplant cavity and the subsequent migration of polyethylene wear debris.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1994

Fatigue behavior of titanium femoral hip prosthesis with proximal sleeve-stem modularity.

Modular hip prostheses are increasing in variety and utilization. Component stability, high endurance limit, and minimal particulate debris generation are critical for long-term clinical success. The purpose of this study was to characterize the fatigue response and evaluate the in vitro potential for component motion and wear of the S-ROM¿, a Ti-6Al-4V hip prothesis with a modular design based on a Morse taper connection. A fatigue jib was designed to simulate fixation of the device at the sleeve-bone interface only with distal support mainly against the lateral endosteal cortex. Two series of tests were performed in air at room temperature: one with direct vertical loading (to produce high bending moments in the coronal plane) and one with a compounding loading angle directed at 15 degrees out-of-plane (to include torsional physiological loads). Applied loads using a servohydraulic test machine ranged from 5 x BW (body weight) to 9 x BW (1 x BW = 73 kg, approximately 160 lb) at 10 Hz on an Instron apparatus. No mechanical failures were observed on the 11-mm size stems below 6 x BW for in-plane vertical loading, and at or below 7 x BW for out-of-plane loading. Using displacement monitoring with a sensitivity of 35 mum, no measurable slippage or relative motion was detected between the stem and sleeve when they were properly assembled. Examination of the contact areas with scanning electron microscopy releaved random surface modification (an indication of fretting or burnishing) with occasional evidence of transfer of material between stem and sleeve.(ABSTRACT TRUNCATED AT 250 WORDS)

Alloys↗
PubMed1994

Concerns with modularity in total hip arthroplasty.

Modularity is being diversified in total hip prostheses to increase surgical latitude in optimizing implant fixation and adjusting hip biomechanics. However, several problems have been clearly identified with implant modularity. First generation metal-backed acetabular components have shown deficiencies in the locking mechanism, the congruency and extent of polyethylene liner support, and polyethylene thickness, all of which have been implicated in accelerated polyethylene wear and failure. Evidence of screw motion against the metal backing, release of particulate material, and focal osteolysis have also been observed. At the head/neck junction evidence of corrosion and fretting has been documented with both similar-metal and mixed-metal taper combinations. Femoral prostheses with other sites of modularity present additional concerns with regard to mechanical integrity and generation of particulate debris by fretting. The modular junctions of three hip prostheses, the S-ROM, Infinity, and RMHS, were subjected to wet environment high cycle mechanical testing in a worst-case loading scenario. Preliminary results at relatively low loads up to three times body weight indicated gross stability of the modular junctions with evidence of minor fretting damage. Analysis of water solutions surrounding the modular junctions after ten to 20 million loading cycles yielded counts of one to three micron sized particles totalling several hundred thousand to several million. It is unknown what quantity of particulate material is sufficient to cause macrophage-mediated osteolysis or whether the debris from modular junctions can cause third-body wear of the articulating surfaces. Modular hip prostheses should be examined under stringent test conditions in order to characterize their fretting behavior and establish their mechanical limitations.

Biomechanical Phenomena↗
PubMed1993

Mechanical compatibility of noncemented hip prostheses with the human femur.

It is generally accepted that more flexible implants are needed to reduce stress shielding and postoperative thigh pain. However, there is no detailed information on the stiffness of currently used implants relative to the human femur. The purpose of this study was to determine the stiffness characteristics (bending, torsional, and axial) of human femora relative to commercially available prostheses as a first step in assessing the mechanical compatibility of the implants. This was achieved by computerized tomography scanning of a collection of human femora from proximal to distal at 10 mm intervals, digitizing the cross-sectional contours, and calculating the stiffness characteristics of each section using standard beam theory. The results show that significant stiffness mismatches exist, especially for larger stem sizes and for stems fabricated from cobalt-chrome alloy. Interestingly, certain implant stiffness values are lower than those of the femur for stems up to 15 mm in diameter, substantially so if the implant is made from titanium alloy and incorporates design features that reduce area and moments of inertia. The data suggest that only larger implant sizes need to be adjusted for increased flexibility compared with current stands.

Aged↗