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Tag Archives: DePuy Orthopaedics

Highly Crosslinked Polyethylenes: Hopes vs. Realities

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2003 more than 800,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to ultra-high molecular weight polyethylene (UHMWPE) debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for 20 to 30 years and represents the gold standard against which contemporary material improvements will be measured over time. Beginning in 1997, the FDA approved a series of UHMWPE’s with elevated crosslinking for use in prosthetic joints. Their stated benefit is to dramatically decrease the generation of UHMWPE wear debris, confirmation of which finds support in wear simulator reports for hip and knee components. The commercial adaptation of these new UHMWPE’s has been aggressively marketed particular to acetabular components well in advance of now emerging short-term clinical reports. What follows is a descriptor of the clinical evolution of UHMWPE bearing surfaces: the good, the bad and the hopeful.

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Highly Crosslinked Polyethylenes: Hopes vs. Realities

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2003 more than 800,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to ultra-high molecular weight polyethylene (UHMWPE) debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for 20 to 30 years and represents the gold standard against which contemporary material improvements will be measured over time. Beginning in 1997, the FDA approved a series of UHMWPE’s with elevated crosslinking for use in prosthetic joints. Their stated benefit is to dramatically decrease the generation of UHMWPE wear debris, confirmation of which finds support in wear simulator reports for hip and knee components. The commercial adaptation of these new UHMWPE’s has been aggressively marketed particular to acetabular components well in advance of now emerging short-term clinical reports. What follows is a descriptor of the clinical evolution of UHMWPE bearing surfaces: the good, the bad and the hopeful.

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The Influence of Antibiotics on the Fatigue Life of Acrylic Bone Cement: Assuring Clinical Structural Integrity – Series II

Aseptic loosening attributed to cement fracture and the subsequent disruption of fixation interfaces remains a major long-term failure mode of cemented arthroplasty. Knowledge of the fracture strength of bone cement, especially in fatigue, is an important indicator of cement integrity and the potential for fixation failure. In current practice, orthopaedic surgeons may choose to utilize antibiotics in bone cement for prophylaxis or treatment of a known infection. However, the antibiotics, bone cements and mixing methodologies employed lead to variability in the quality of the end product. To date, several orthopaedic manufacturers have received Food and Drug Administration (FDA) 510[k] clearance for their pre-packaged antibiotic-loaded bone cement for use in the second stage of a two-stage revision situation where the initial infection has been cleared. This availability provides a more uniform cement mix with known mechanical and elution characteristics at the 0.5 to 1.0g level of antibiotic per 40g of polymer powder. This study evaluates the influence of antibiotic inclusion on the porosity, strength and fatigue life of six contemporary bone cements.

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New Polys for Old: Contribution or Caveat?

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2001 more than 600,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to polyethylene debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for 20 to 30 years. Despite this obvious success, polyethylene debris is increasingly identified as the principle causative factor of bone loss and failure of TJA procedures. The conditions of occurrence, severity and clinical consequences suggest limitations on the in vivo integrity of contemporary arthroplasty designs. Material remedies inclusive of carbon reinforced polyethylene (Poly II), heat pressing and Hylamer with its variants have been advanced as alternatives with negative clinical results. Contemporary attempts to improve longevity (i.e., post-processing heat stabilization, peroxide additives and increased radiation with remelting) focus on the benefit of increased cross-linking while minimizing free radical creation. With increasing patient longevity and activity levels a search for the ultimate polymer is important. Variations on an already successful theme hope to lead to its optimization in this century. However, these “new polys” offer no direct clinical evidence to demonstrate their efficacy. It is known that reprocessing techniques will alter mechanical characteristics. Contemporary improvements in laboratory evaluative capabilities suggest significant reduction in component wear, but past experiences, in these regards, have not always predicted their clinical viability.

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The Effects of Walking Gait on UHMWPE Damage in Unicompartmental Knee Systems: A Finite Element Study

There is increasing re-interest in the use of unicompartmental knee replacement as a remedy of choice for isolated compartment disease, although there is debate regarding its role as a temporizing or definitive procedure. The popularization of mini-incision surgery with claims of reduced pain, shorter hospitalization, more rapid rehabilitation, more normal knee function and decreased cost are positive arguments for the procedure. However, the damage observed in ultra-high molecular weight polyethylene (UHMWPE) component retrievals is the result of high cycle fatigue loads, which act on the polymer insert during daily ambulation and suggests a material limitation in their use. The study reveals the influence that four different modular unicompartmental knee design geometries have on stresses that are associated with abrasion and delamination of the polymer insert and suggests their efficacy in clinical use..

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Highly Cross-Linked Polyethylene Modular Acetabular Designs: Performance Characteristics

Modular acetabular designs enjoy widespread use in THA procedures and demonstrate versatility in accommodating musculoskeletal acetabular pathologies. Their mechanical performance has continually improved through enhanced locking mechanism assemblies and shell-liner conformity. Recently, a number of “improved” polymers have emerged whose commonly heralded benefit is a reduction in polyethylene wear due to increased cross-linking concurrent with minimized oxidation. These processes, however, change the chemical structure of the polymer as well as affect the static mechanical properties and fatigue characteristics. Additionally, decreases in resistance to crack propagation due to the increased crystallinity have been reported. This is of concern as short-term failures of modular, acetabular components employing conventional polyethylene have been reported with liner separation and gross fracture in the absence of normal wear. This study evaluated the locking mechanism strength for three, contemporary, modular acetabular designs, which employ both conventional and highly cross-linked polyethylene liners..

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Assuring Cement Fixation: All Mixing Systems are NOT the Same

Aseptic loosening attributed to cement fracture and the subsequent disruption of fixation interfaces remains a major long-term failure mode of cemented arthroplasty. Knowledge of the fracture strength of bone cement, especially in fatigue, is an important indicator of cement integrity and the potential for fixation failure. Several manufacturers have advocated mixing devices to increase the strength of bone cement by reducing its porosity or pore diameter. This study evaluates different mixing methodologies as they influence cement porosity and the diametral tensile strength of cement specimens in both single cycle and fatigue. This serves as a means of establishing the effectiveness of these methods in producing optimal bone cement for use in joint arthroplasty.

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New Polys for Old: Contribution or Caveat?

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2000 more than 600,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to polyethylene debris-induced osteolysis has been in the single digits for most contemporary series, with some reports describing prostheses surviving for 20 to 30 years. Despite this obvious success, polyethylene debris is increasingly identified as the principle causative factor of bone loss and failure of TJA procedures. The conditions of occurrence, severity and clinical consequences suggest limitations on the in vivo integrity of contemporary arthroplasty designs. Material remedies inclusive of carbon reinforced polyethylene (Poly II), heat pressing and Hylamer with its variants have been advanced as alternatives with negative clinical results. Contemporary attempts to improve longevity (i.e., post-processing heat stabilization, peroxide additives and increased radiation with remelting) focus on the benefit of increased cross-linking while minimizing free radical creation. With increasing patient longevity and activity levels a search for the ultimate polymer is important. Variations on an already successful theme hope to lead to its optimization in this century. However, these “new polys” offer no direct clinical evidence to demonstrate their efficacy. It is known that reprocessing techniques will alter mechanical characteristics. Contemporary improvements in laboratory evaluative capabilities suggest significant reduction in component wear, but past experiences, in these regards, have not always predicted their clinical viability.

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Tibial Plateau Abrasion in Mobile Bearing Knee Systems During Walking Gait II: A Finite Element Study

The abrasion observed in ultrahigh molecular weight polyethylene (UHMWPE) total knee arthroplasty component retrievals is the result of high cyclical loads, which act on the tibial plateau during daily ambulation. This dynamic process influences in vivo component longevity and is dependent on the magnitude and distribution of contact stresses on the tibial plateau. Mobile bearing knee systems offer increased component conformity over their fixed plateau counterparts and thus diminish the magnitudes of these contact stresses. This study reveals the contact areas and stresses that are associated with tibial plateau abrasion in four mobile bearing knee designs during three highly loaded points in the walking gait cycle, and suggests their efficacy in clinical use.

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Classification of Mobile Bearing Knee Design: Mobility and Constraint

Restoration of normal knee joint function through surgical reconstruction is dependent upon load sharing between the implant and surrounding soft tissue structures. Mobile bearing knee designs offer the advantage of maximum conformal geometry while diminishing constraint forces to fixation interfaces through plateau mobility. The degree of mobility afforded by these designs in the anterior-posterior, medial-lateral and rotational directions defines the required interaction between soft tissue and design geometry to maintain a stable articulation. This study characterizes nine, contemporary mobile bearing knee designs in terms of the force generated during a prescribed displacement.

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