Use of agarose culture to measure the effect of transforming growth factor beta and epidermal growth factor on rabbit articular chondrocytes.

Nonneoplastic cell lines are unable to grow in soft agar. However, concomitant treatment of these cells with epidermal growth factor and transforming growth factor beta confers upon them anchorage independence. Since articular chondrocytes are unique as normal diploid cells that do have the capability of growing in soft agar, we tested whether transforming growth factor beta and epidermal growth factor could affect DNA synthesis and matrix production. In the presence of epidermal growth factor (5 ng/ml) concentrations of high-performance liquid chromatography-purified transforming growth factor beta at concentrations of 0.05-15 ng/ml induced a dose-dependent increase in DNA, to nearly double that of control cultures. A half-maximal effect was seen with transforming growth factor beta, 0.1 ng/ml, and epidermal growth factor, 5 ng/ml. Neither compound alone was mitogenic. In contrast, either transforming growth factor beta or epidermal growth factor alone was able to decrease synthesis of glycosaminoglycans and collagen. The data demonstrate that transforming growth factors can affect the behavior of nonneoplastic cells by modulating cell replication and the biosynthesis of two principal matrix components. In addition they support the hypothesis that these growth factors may play a role in the physiology of nonmalignant cells.

Efficacy of plasma-sprayed tricalcium phosphate in enhancing the fixation of smooth titanium intramedullary rods.

The osseous tissue response to intramedullary rods plasma-sprayed with TCP was investigated. TCP-sprayed smooth titanium rods and unsprayed controls were bilaterally implanted into the medullary canals of rabbit tibiae. Rabbits in the short-term study were sacrificed at 3 weeks; rabbits in the long-term study were sacrificed at 12 weeks. Pull-out tests were performed and histological sections prepared. The TCP-sprayed titanium rods exhibited significantly higher pull-out strengths than unsprayed contralateral rods at both 3 and 12 weeks, suggesting the bonding of bone to the TCP layer. The TCP-sprayed titanium rods at 3 weeks showed significantly greater osseous tissue response than unsprayed contralateral controls. At 12 weeks, the osseous tissue response was greater than at 3 weeks, and, surprisingly, the response to TCP-sprayed and unsprayed titanium rods was comparable.

Potential errors inherent in quantitative densitometric analysis of orthopaedic radiographs. A study after total hip arthroplasty.

We sought to determine whether x-ray densitometry could be used to provide quantitative information about the growth and resorption of bone in a retrospective analysis of the radiographs of patients who had a non-cemented hip prosthesis. We examined variables that were likely to be uncontrolled in routine orthopaedic radiographs for their effect on radiographic (film) density. Changes in film lots, exposure, target distance, field variability (heel effect), and femoral orientation were independently analyzed for radiographs of precisely oriented femora of cadavera and a radiology step-wedge. Despite attempts to normalize these variables, we found that retrospective comparison of serial radiographs of patients for densitometric analysis of change in the bone was prone to significant error. In uncontrolled radiographs, non-uniformity of exposure, intensity of the radiation field, target distance, and deviation in femoral rotation will be likely to result in substantial variations in the apparent thickness or density of the femoral cortex and make quantitative comparisons of serial radiographs unreliable.

Corrosion between the components of modular femoral hip prostheses.

We studied the tapered interface between the head and the neck of 139 modular femoral components of hip prostheses which had been removed for a variety of reasons. In 91 the same alloy had been used for the head and the stem; none of them showed evidence of corrosion. In contrast, there was definite corrosion in 25 of the 48 prostheses in which the stem was of titanium alloy and the head of cobalt-chrome. This corrosion was time-dependent: no specimens were corroded after less than nine months in the body, but all which had been in place for more than 40 months were damaged. We discuss the factors which may influence the rate of these changes and present evidence that they were due to galvanically-accelerated crevice corrosion, which was undetected in previous laboratory testing of this type of prosthesis.

In vitro simulation of contact fatigue damage found in ultra-high molecular weight polyethylene components of knee prostheses.

An in vitro simulation of fatigue loading of ultra-high molecular weight polyethylene (UHMWPE) knee components was carried out on a knee simulator and on a rolling and sliding wear tester. Tibial components for the knee simulator were gamma-sterilized, implantable components taken from manufacturing inventory. The rolling/sliding UHMWPE discs were machined from bar stock and either gamma sterilized in air and accelerated aged, or left as non-sterilized (controls). Cracking and delamination of samples that had been gamma sterilized in air and aged were observed in both types of tests. Contact fatigue damage was visible in as few as 150,000 cycles using the knee simulator at loads of 122 N (275 1b). The rolling/sliding samples showed signs of damage in as few as 130,000 cycles with an estimated stress of 15 MPa and 25 per cent sliding. However, cracking and delamination were not generated in the never-sterilized or recently sterilized controls. UHMWPE that has been gamma sterilized in air and aged is shown to be susceptible to contact fatigue damage. These results are important to the interpretation of in vitro total knee replacement simulations used to assess the performance of tibial bearings.

An analysis of hylamer and polyethylene bearings from retrieved acetabular components.

Hylamer and conventional polyethylene acetabular liners of the same design, revised for a variety of reasons, were examined and compared to assess the performance of Hylamer as a bearing material. Clinical damage modes, linear wear rates, oxidation levels, and mechanical properties were measured. In both series, many liners were retrieved for dislocation. Wear/osteolysis was the most common reason for retrieval in the Hylamer series, while none of the conventional polyethylene liners were retrieved for this reason. Nearly all liners exhibited abrasion, burnishing, scratching, and creep. The Hylamer liners had more cracking, delamination, and pitting. The Hylamer liners had an average linear wear rate of 0.32 mm/year, while the conventional polyethylene liners had an average wear rate of 0.20 mm/year. Due to sample size, no statistical difference in wear rate was noted between the two groups. In general, both the Hylamer and conventional polyethylene showed oxidation peaks subsurface, resulting from their exposure to gamma radiation in air. Liners with elevated oxidation had decreased ultimate tensile strength, elongation, and toughness. For given oxidation levels, the corresponding mechanical properties of Hylamer appeared lower than those of conventional polyethylene. The ultimate tensile strength values ranged from 14 to 33 MPa for Hylamer and 19 to 32 MPa for conventional polyethylene. Elongation ranges were 19% to 350% (Hylamer) and 80% to 375% (conventional). The Hylamer retrievals in this study gave initial indications of performance; Hylamer appeared to behave similarly, but not superiorly, to conventional polyethylene, in the early functional period with respect to clinical wear and clinical performance. Both Hylamer and conventional polyethylene liners were degraded by gamma sterilization in air, with Hylamer liners demonstrating greater property changes.

Interfaces between host and uncemented femoral hip prostheses.

The authors analyzed 200 uncemented hip prostheses that were retrieved. They determined the type and extent of host-implant interfaces, but found little correlation between subjective pain relief and the histology of the interfaces. Wear debris, bone resorption and looseness were related to pain, although even with these the relationship was not statistically significant.

In vivo oxidation in retrieved highly crosslinked tibial inserts.

The current study determined (I) the environment where oxidation in a series of retrieved, HXL UHMWPE tibial inserts occurred (in vivo or postexplant); and (II) the effect of fabrication variables (irradiation source, irradiation dose) and thermal processing after irradiation (annealing or remelting) on oxidation resistance. Hypotheses examined are (1) HXL UHMWPE tibial inserts have potential to oxidize in vivo, and (2) annealed HXL UHMWPE oxidizes at a higher rate in vivo than remelted HXL UHMWPE. Highly crosslinked UHMWPE tibial inserts (87), received by an IRB-approved retrieval laboratory from 20 surgeons at 10 institutions across the U.S., were analyzed from 2005 to 2011. Fourier transform infrared spectroscopy was used to determine oxidation and trans-vinylene index for each retrieved insert. Measured oxidation that was maximum subsurface was found in 56% of all HXL tibial inserts. This maximum oxidation correlated significantly with in vivo time, trans-vinylene index, and thermal processing after irradiation. Articular oxidation rate correlated with crosslinking irradiation dose and thermal processing after irradiation. Retrieved below-melt annealed tibial inserts had significantly higher articular oxidation rates than remelted tibial inserts (p < 0.001). Articular oxidation rates correlated positively with cross-linking irradiation dose and postirradiation thermal processing. Edge oxidation correlated with postirradiation thermal processing. Oxidation of HXL UHMWPE may have clinical implications for tibial inserts, since loss of UHMWPE toughness resulting from oxidation has led to fatigue damage in gamma-sterilized tibial inserts.

In vivo oxidation in remelted highly cross-linked retrievals.

BACKGROUND: Elimination of free radicals to prevent oxidation has played a major role in the development and product differentiation of the latest generation of highly cross-linked ultra-high molecular weight polyethylene bearing materials. In the current study, we (1) examined oxidation in a series of retrieved remelted highly cross-linked ultra-high molecular weight polyethylene bearings from a number of device manufacturers and (2) compared the retrieval results with findings for shelf-stored control specimens. The hypothesis was that radiation-cross-linked remelted ultra-high molecular weight polyethylene would maintain oxidative stability in vivo comparable with the stability during shelf storage and in published laboratory aging tests. METHODS: Fifty remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners and nineteen remelted highly cross-linked ultra-high molecular weight polyethylene tibial inserts were received after retrieval from twenty-one surgeons from across the U.S. Thirty-two of the retrievals had been in vivo for two years or more. Each was measured for oxidation with use of Fourier transform infrared spectroscopy. A control series of remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners from three manufacturers was analyzed with electron paramagnetic resonance spectroscopy to measure free radical content and with Fourier transform infrared spectroscopy to measure oxidation initially and after eight to nine years of shelf storage in air. RESULTS: The never-implanted, shelf-aged controls had no measurable free-radical content initially or after eight to nine years of shelf storage. The never-implanted controls showed no increase in oxidation during shelf storage. Oxidation measurements showed measurable oxidation in 22% of the retrieved remelted highly cross-linked liners and inserts after an average of two years in vivo. CONCLUSIONS: Because never-implanted remelted highly cross-linked ultra-high molecular weight polyethylene materials had no measurable free-radical concentration and no increase in oxidation during shelf storage, these materials were expected to be oxidation-resistant in vivo. However, some remelted highly cross-linked ultra-high molecular weight polyethylene retrievals showed measurable oxidation after an average of more than two years in vivo. This apparent departure from widely expected behavior requires continued study of the process of in vivo oxidation of ultra-high molecular weight polyethylene materials.

The impact of sterilization method on wear in knee arthroplasty.

Sterilization by gamma irradiation in air has been shown to have the potential to accelerate the oxidation of polyethylene components resulting in reduced mechanical properties. In the hip, it has been reported that the occurrence of delamination and cracking in retrieved bearings is significantly different when comparing components sterilized with gamma irradiation in air with components sterilized with ethylene oxide. Using a collection of 1635 retrieved polyethylene knee bearings, this study pursues a similar comparison of sterilization method with clinical wear in the knee. It confirms that retrieved polyethylene knee components that were gamma irradiated in air have a high incidence of delamination and cracking, leading at times to complete wear through of the bearing. Knee components sterilized with ethylene oxide showed no evidence of fatigue damage even after in vivo durations in excess of 15 years.

Bone ingrowth into dynamically loaded porous-coated intramedullary nails.

Research to determine whether porous-coated Vitallium intramedullary rods could be used to bridge segmental defects in long bones was performed using rabbit tibias as a suitable model for human bone growth. One-centimeter segments of the midshaft of tibias of mature rabbits were removed surgically and replaced with equal-sized segments of Silastic tubing to maintain leg length. A coated rod was inserted through the proximal tibial segment, through the tubing, and into the distal bone segment. The legs were taken out of plaster at 3 weeks, and x-rays were taken periodically until sacrifice. At sacrifice, 30 weeks postoperatively, the mechanical properties of both tibias from each rabbit were measured destructively in a torsional testing machine. The load at failure of the tibia with the segmental defect averaged 90% of the contralateral control tibia. Rod pullout data and electron-probe x-ray micro-analysis indicated that a substantial rod-endosteal bone bond existed due to ingrowth into the porous coating. The torsional data may have been somewhat obscured by the presence of bony callus which grew over the Silastic tubing and bridged the segmental defect exosteally in every case.

Osseointegration of titanium implants in total hip arthroplasty.

Osseointegration is defined as direct contact on the light microscopic level between living bone tissue and the implant. Using titanium screw dental implants in the jaw, a lasting interface under loaded conditions extending over a 20-year follow-up period has been demonstrated. This demonstration brings up the question whether a similar interface can be achieved in total hip arthroplasty (THA) between living bone and a titanium alloy implant under necessitated conditions of immediate loading. Two series of cases are reported. The first series used a femoral, press-fit, titanium alloy component and the second used a press-fit titanium acetabular component and redesigned femoral, press-fit, titanium alloy component. Both demonstrated a high percentage of good to excellent results. Roentgenograms showed that the geometrical changes in the redesigned femoral component gave early indications of a better fixation with loading in valgus, less subsidence, and less evidence of distal stress transfer. A two-and-one-half-year postoperative anatomic specimen study confirmed osseointegration to the press-fit titanium alloy femoral component. Multiple areas of contact between bone and metal without fibrous interposition were seen. Examination by electron microscopy supported the light microscopic findings. These findings support further use of smooth, press-fit titanium components in THAs without the need for porous coating, mesh, or other surface modifications.

Corrosion at the interface of cobalt-alloy heads on titanium-alloy stems.

The combination of a cobalt-alloy head on a titanium-alloy femoral hip stem is widely accepted for press-fit and biologic fixation applications. Examination of 30 components retrieved at periods of 0.5 to 66.9 months for histologic examination of tissue ingrowth revealed that 56.6% of the tapered connections between head and stem showed evidence of crevice corrosion leading to concerns of metal ion release and the potential failure of head to stem fixation.

A comparison of contact pressures in tibial and patellar total knee components before and after service in vivo.

Laboratory testing of total knee components indicates that many designs produce contact stresses that exceed the yield strength of ultra-high molecular weight polyethylenes (UHMWPEs). It is often assumed that the polyethylene component will creep and wear to become more conforming over time, thus reducing these stresses. To test this theory, retrieved polyethylene tibial and patellar components, which showed signs of increased contact area through in vivo deformation, were tested for contact stress against matching components using Fuji Prescale pressure-sensitive film. The results showed an inverse relationship between initial conformity and in vivo changes in contact stress. More conforming devices showed little or no change in contact stress, and less conforming components showed small decreases in contact stress as a result of creep and wear. Even with these changes, however, the contact stresses for nonconforming designs remained well above those for the more conforming devices as well as the uniaxial yield strength of UHMWPE.

The correlation between fusion defects and damage in tibial polyethylene bearings.

Recent reports of fatigue failures of polyethylene tibial and patellar components have led to investigation of the role of design and material properties in these failures. Earlier investigations, concluding that high contact stress designs suffered greater damage in service, could not account for some of the fatigue failures. The current study hypothesizes that this failure is related to variations in the material properties of the polyethylene due to incomplete consolidation of the powder during manufacture, resulting in fusion defects. Retrieved polyethylene components and samples of polyethylene stock were examined to gain insight into the relationship between fusion defects, component failure, material forming processes, and powder grade. Statistically significant correlations (p < 0.05) were observed between the extent of defects and cracking, delamination, total wear damage, and duration in vivo. These correlations indicate that components manufactured from material with fusion defects may be less resistant to fatigue than components formed of fully consolidated material.

Tibial insert undersurface as a contributing source of polyethylene wear debris.

Sixty-seven ultrahigh molecular weight polyethylene tibial inserts from cementless total knee arthroplasties were retrieved at autopsy and revision surgery and analyzed for evidence of articular and nonarticular surface wear after a mean implantation time of 62.8 months (range, 4-131 months). Polyethylene cold flow and abrasive wear on the nonarticular insert surface (undersurface) were assigned a wear severity score (Grade 0-4). The severity of articular wear was assessed quantitatively and graded. Corresponding prerevision radiographs were evaluated for evidence of tibial metaphyseal osteolysis and osteolysis around tibial fixation screws. Exact nonparametric conditional inference methods were used to establish correlations between different variables and the occurrence of tibial metaphyseal osteolysis. Severe Grade 4 wear of the tibial insert undersurface was associated with tibial metaphyseal osteolysis or osteolysis around fixation screws. Time in situ statistically was related to Grade 4 undersurface wear and tibial metaphyseal osteolysis. The occurrence of tibial osteolysis was not related statistically to articular wear severity, insert thickness, or implant type. The main articulation between the femoral implant and ultrahigh molecular weight polyethylene insert has been assumed to be the primary source of polyethylene debris contributing to osteolysis and total knee arthroplasty implant failure. The undersurface of the insert is an additional source of polyethylene debris contributing to tibial metaphyseal osteolysis. To lessen polyethylene debris produced at this modular interface, the tibial implant locking mechanism should fix the insert firmly to the metal backing to decrease relative micromotion. Because motion between the insert and metal backing may be inevitable, the wear characteristics of the inner tray surface should be optimized to minimize wear debris production at this other articulation.