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PubMed1991

Evaluation of contact stress in metal-backed patellar replacements. A predictor of survivorship.

Significant wear-through and dissociation failures of metal-backed, point and line contact patellar replacements are associated with excessive contact stresses on the polyethylene-bearing surfaces. Analytical mathematical contact-stress analysis was used to evaluate various patellar component geometries under loading conditions consistent with walking, stair descent, and deep knee bends, respectively. Typical point- and line-contact patellar-surface geometries exceeded the manufacturer's recommended maximum permissible compressive stress level of 10 MPa by a factor greater than three, which also exceeds the yield stress of ultra-high, molecular-weight polyethylene. A metal-backed, rotating-bearing, area contact geometry patellar replacement maintained safe contact stress levels at less than half of the maximum permissible compressive stress level. These contact stress analyses predict early fatigue failure of all-polyethylene or metal-backed, point- and line-contact patellar replacements, while predicting long-term survival of area-contact, rotating-patellar replacements. Clinical evaluation of these various implants support the conclusions of these analyses. Contact stress analysis should precede any clinical use of patellar implants to avoid predictable failure mechanisms.

Biomechanical Phenomena↗
PubMed1993

Difficulties with bearing dislocation and breakage using a movable bearing total knee replacement system.

In 1980, the low contact stress, or New Jersey, system was introduced with movable high-density polyethylene bearings between the metallic femoral and tibial components. This system was developed to deal with two principal concerns of conventional knee replacement systems: loosening and wear. A federal Drug Administration review was required, and the study was performed in two phases. From 1981 until 1984, components were cemented in 40 knees, which then were evaluated for an average of seven years after surgery. From 1984 until 1986, 16 knees were selected for cementless fixation and were followed for an average of five years. The cemented group had a failure rate of 10% (four knees), two from tibial component loosening, one from bearing dislocation, and one from infection. Failure was determined by the necessity for revision. The uncemented group had a failure rate of 31% (four knees from bearing fracture and one from knee pain of uncertain source). Bearing failure was attributable to the entrapment of the subluxed lateral bearing. Sacrifice of the anterior cruciate ligament predisposes the system to bearing subluxation and failure.

Aged↗
PubMed1996

[Is Hylamer a suitable substance for use in endoprosthetics?].

UNLABELLED: In the present study, original UHMWPF implants obtained from different manufacturers were evaluated using Differential Scanning Calorimetry (DSC), Infrared Spectroscopy and Wide Angle X-ray Scattering (WAXS). DSC measurements revealed clear differences in the crystallinity of the implants with figures varying between 58.2% and 73.2%. The highest level of crystallinity (73.2%) was found in the Hylamer material. The results of IR spectroscopy confirmed these findings WAXS investigations showed that Hylamer has both a orthorrhomboid and a monoclinic phase. Remelting was followed by a disappearance of the ortherhomboid phase. CLINICAL RELEVANCE: The use of Hylamer that has been processed by secondary companies is not recommended. The product Hylamer makes clear how the original crystal structure of UHMWPE can be altered. The clearly increased crystallinity in Hylamer also increases the risk of implant failure due to brittleness.

Crystallization↗
PubMed2004

Quantifying the effect of resin type and sterilization method on the degradation of ultrahigh molecular weight polyethylene after 4 years of real-time shelf aging.

Alternative sterilization methods including ethylene oxide, gas plasma, and gamma-radiation in an inert environment were implemented in the late 1990s, to limit oxidative degradation of ultrahigh molecular weight polyethylene (PE). There was also a simultaneous transition to PE resins that did not contain calcium stearate. Shelf storage duration of PE inserts following gamma-irradiation in air has been correlated to poor clinical performance and increased wear. This study aimed to determine how sterilization method and resin type influenced degradation of PE after 4 years of real-time shelf aging. It was hypothesized that gamma-irradiation and stearate containing resins would incur significantly more degradation than nonradiated, stearate-free resins. Gamma-irradiated PE samples in air and nitrogen had a significantly increased density and oxidation index, compared to nonirradiated PE after 4 years of shelf aging. Alternative sterilization methods such as ethylene oxide and gas plasma appeared to have significantly less oxidation regardless of PE resin type. A partial correlation demonstrated that density and oxidation index were not correlated (r(2) = 0.079) when examining the influence of sterilization method. The data supported that after 4 years of real-time shelf aging, the type of sterilization method had a larger influence on PE degradation than resin type.

Biocompatible Materials↗
PubMed2005

[Influence of intraocular lens material and design on the development of posterior capsule opacification].

Based on the literature of 1999-2005, we attempted to clarify the influence of material and design of an intraocular lens (IOL) on the development of posterior capsule opacification (PCO) and identify the future problems to be solved. The sharp edge design of an IOL has significantly reduced PCO in recent years. Histopathological studies showed that a sharp capsular bend was formed at the sharp posterior optic edge, and the migration of lens epithelial cells was obviously inhibited at the site. Experimental and clinical studies suggest that the sharper the edge is, the sharper the capsular bend, and the greater the preventive effect, regardless of IOL material. The sharp capsule bend appears to represent a physical hindrance, which may induce contact inhibition of cell movement. How material participates in the preventive effect should be clarified in a future study. Thus, at the moment, the formation of capsular bend at the posterior optic edge appears to be the decisive criterion to evaluate the influence of IOL material and design on the development of PCO. Optimization of design and material of an IOL, which facilitates capsular bend formation as sharply and quickly as possible, may reduce PCO to a clinically negligible level.

Biocompatible Materials↗
PubMed2005

[New methods for the prevention of posterior capsule opacification].

Even though tremendous advances have been made especially during the last 10-15 years in terms of surgical techniques and improvement of implant technology, posterior capsule opacification (PCO) still remains a serious long-term complication. New clinical and laboratory studies (especially of autopsy eyes) have improved our understanding of how IOL design and material influence PCO. Sharp edge optic designs of IOLs of various materials have been shown to significantly reduce secondary cataract. The application of pharmacological substances selectively into the capsular bag is now possible due to the development of the PerfectCapsule System for vacuum-sealed capsule irrigation. Major advances in other areas of biotechnology and immunology including gene therapeutic methods offer totally new approaches for the future in the elimination of lens epithelium cells from the capsular bag. This survey gives an update on current and future means and trends to reduce or prevent PCO formation.

Biocompatible Materials↗
PubMed2005

Degenerative mineralization in the fibrous capsule of silicone breast implants.

The formation of a fibrous capsule made of long collagen fibers surrounding breast implants represents an unavoidable phenomenon as the patient's reaction to the presence of a foreign body. Depending upon the size and shape of the implants and the chemicals percolating through the shell, this fibrous capsule is continuously remodeled. The compaction of the foreign debris in the vicinity of the silicone shell is followed by the loss of cellular activity, shrinkage and necrosis. Calcification is the ultimate step. These phenomena were illustrated in the analysis of 18 explanted breast prostheses after 20 or more years of implantation. The degenerative mineralization was shown in scanning electron microscopy and light microscopy. The minerals proved to be bone-like hydroxyapatite by X-ray diffraction and Solid State NMR analysis. Whatever the characteristics of any sophisticated new model of breast implant, phenomenon of mineralization might be minimized but it is very unlikely that it would be totally eliminated.

Biocompatible Materials↗
PubMed2006

Current status of the total artificial heart.

Although heart transplantation remains the gold standard for patients who remain in advanced heart failure despite optimal medical therapy, limited donor supplies allows for just >2000 transplant each year in the United States. Recent enthusiasm has developed for the role of mechanical circulatory support for this ever-growing population of sick patients. Although much attention has been directed toward ventricular assist devices, less information is available regarding the role of the total artificial heart. Indeed, efforts in this latter technology have allowed the relatively recent deployment of a variety of complete circulatory assist devices. The purpose of this article is to review the historical development, current use, and future role of total artificial hearts.

Cost of Illness↗
PubMed2006

Effects of bone materials on the screw pull-out strength in human spine.

A three-dimensional finite element model simulating the threaded connections including detailed helix curve for the bone and surgical screw was constructed. Validation of the FE model was conducted by comparing the predicted screw pull-out strength in different foam materials against experimental study. The FE model was then further analyzed to investigate the interaction of bone material and purchase length on the screw pull-out strength. The results show that failure of the connection was due to bone shearing which occurred along a cylindrical surface determined by the outer perimeter of the screw. The cortical shell resists around 50% of the pull-out strength for a screw of 4mm in major diameter and 22 mm in length. The effects of purchase length on the pull-out strength were different for different bone material. It is the bone material that determines the stability of the inserted surgical screw. The significance of the purchase length on the pull-out strength of cortical screw will be much lower than that in cancellous bone screw.

Bone Screws↗
PubMed2005

Metal-on-metal resurfacing versus total hip replacement-the value of a randomized clinical trial.

This article describes a randomized clinical trial in young patients, comparing metal-on-metal cemented resurfacing hip replacement with cemented total hip replacement. The trial was stopped early, mainly because of a high incidence of failure of the cemented resurfacing acetabular component. The results reinforce the importance of clinical trials for evaluating the safety and efficacy of prosthesis designs before being used in a large cohort of patients. Although there may be advantages of resurfacing hip replacement, trials are also required to demonstrate it has a midterm success that reasonably approaches that of total hip replacement.

Adult↗
PubMed2003

Stress analysis of interbody fusion--finite element modelling of intervertebral implant and vertebral body.

OBJECTIVE: To investigate stresses in cage type interbody fusion systems during compressive loading.Design. The study uses finite element methods to investigate predicted stresses. Previously published experimental material properties are used as inputs to the numerical simulation. BACKGROUND: Interbody spinal fusion procedures using cage style intervertebral implants often cause subsidence failure of the vertebral endplate, resulting in potential pain and mechanical instability of the fusion system. METHODS: Finite element models were developed to simulate compressive load transfer between interbody implants and adjacent vertebral body. The vertebral body was modelled using separate finite element mesh regions for cancellous core and cortical shell, with the meshes tied together at the core/shell interface. Coulomb friction was implemented to model the contact between implants and vertebral endplate.Results. Simulation results predicted endplate stresses of approximately 12 times the nominal contact pressure due to differing deformation stiffnesses of the implant and endplate structures. Reduction of the cancellous core elastic modulus to simulate severely osteoporotic bone resulted in endplate stresses up to three times higher than the values for an intact cancellous core. CONCLUSIONS: In this study, finite element analysis was used to investigate the stresses in interbody fusion systems. Published vertebral loads corresponding to certain activities were shown to generate endplate stresses which approach and exceed the failure stress for cortical bone. Endplate stresses are strongly dependent on the modulus of the underlying cancellous core. RELEVANCE: Endplate subsidence failure can potentially occur at the corners of existing cage-type interbody implants under physiological compressive loads. Matching material properties between cortical endplate and implant does not guarantee optimal contact conditions, and overall bending stiffness should be assessed.

Compressive Strength↗
PubMed2003

Comparison of hydroxyapatite- and porous-coated stems in total hip replacement.

24 patients underwent bilateral hip replacement, using the SROM modular system (Depuy Johnson & Johnson, MA, USA) with the porous-coated sleeve in one hip and an HA-coated sleeve in the other. 20 patients were followed for at least 4 years with clinical and radiographic evaluations. We found no difference between the two sides as regards the time of disappearance of thigh pain and hip scores. Radiographs showed a buttress sign in 18 HA-coated and 15 porous coated stems. There were no differences in the serial bone-remodeling pattern around the proximal sleeve and measurement of wear volume between the two groups. In the short- to medium-term, our findings support the view that the hydroxyapatite-coated femoral stem has no advantages over those with porous coating.

Adult↗
PubMed2000

Effect of low-friction ion-treated femoral heads on polyethylene wear rates.

Polyethylene wear is a major contributor to osteolysis and subsequent aseptic loosening of prosthetic components in total hip arthroplasty. Use of ion implantation as a surface modification to the metallic bearing component of orthopaedic implants may be an effective means of reducing wear debris at the bearing interface. In July 1991, low friction ion treated femoral heads were introduced. This study evaluates the effect of the low friction ion treated femoral head on polyethylene wear. Fifty-five total hip arthroplasties (53 patients) with low friction ion treated femoral heads followed up a minimum of 3 years were matched with 55 total hip arthroplasties (47 patients) without low friction ion treated femoral heads for the same postoperative period. Socket wear was evaluated radiographically. Case matching and strict inclusion criteria were used to control for known factors influencing polyethylene wear. These criteria included: (1) cases matched for gender and age within 2 years; (2) diagnosis limited to osteoarthritis or avascular necrosis of the femoral head only; (3) femoral head diameter limited to 26 or 28 mm only; (4) hydroxyapatite coated femoral stem of the same design and a metal backed socket of the one of two designs with the same polyethylene insert; and (5) minimum followup of 3 years. The linear wear rate of polyethylene was 0.161 +/- 0.095 mm per year in the group without the low friction ion treated heads and 0.116 +/- 0.101 mm per year in the low friction ion treated group. The volumetric wear rates were 74.5 +/- 44.3 mm3 per year for the group without the low friction ion treated heads and 57.8 +/- 51.1 mm3 per year for the low friction ion treated group. Assuming the sensitivity of these measurements can detect these small differences in wear accurately, these results suggest low friction ion treated prosthetic heads are useful in reducing polyethylene wear at 3-year minimum followup.

Adult↗
PubMed2002

Long-term followup of nonmodular total knee replacements.

The purpose of the current study was to evaluate and determine the mechanism and etiology of failure of components that failed in long-term followup of Anatomic Graduated Component total knee replacements. The authors previously reported the survivorship of 4583 Anatomic Graduated Component total knee arthroplasties done during a 17-year period. The current study was done to evaluate the etiology and cause of failure of the components that failed. There were six (0.18%) failures of the femoral component. There were 21 tibia components that failed (0.46%). Twelve tibial components failed because of an osteonecrotic lesion in the medial tibia plateau. The clinical survival rate with revision or loosening of one or more components was 98.9% at 15 years. No component was revised for polyethylene wear or osteolysis. This total knee replacement has proved to have minimal wear and excellent longevity with time with no revisions between 10 and 15 years despite having nearly flat-on-flat geometry and retaining the posterior cruciate ligament. The most common cause for revision (12 tibial components) was because of an osteonecrotic lesion in the medial tibia plateau, which developed after surgery and led to loosening.

Adult↗
PubMed2002

Unicompartmental knee arthroplasty for younger patients: an alternative view.

Unicompartmental knee arthroplasty has been a controversial operation for the past 3 decades. Many surgeons in the 1980s and 1990s found little or no indication for the procedure. Others were enthusiastic for its use in selected patients with osteoarthritis who had unicompartmental disease. Initially, unicompartmental knee arthroplasty was thought to be appropriate for the elderly patient who was sedentary. With the advent of minimally invasive techniques, indications have expanded to include its use in younger patients (especially females) as an alternative to osteotomy or tricompartmental knee arthroplasty. Advantages over osteotomy include higher initial success, greater longevity, and fewer early complications. If done conservatively, salvage is not difficult. The extent of safe postoperative activity levels has yet to be established. Failure rates seem to be higher in heavy active males. A metallic interposition hemiarthroplasty in the form of a McKeever or Macintosh prosthesis has been available for 50 years as a unicompartmental arthroplasty, but with limited use. It still may have a role in selected patients as a conservative temporizing procedure.

Adult↗