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Category Archives: Hip

Femoral Neck Modularity in THA: Not a Bridge Too Far?

Femoral neck modularity in total hip arthroplasty design is receiving increased citation in the clinical literature. The advantages of these systems include a potential reduction in the occurrence of femoro-acetabular impingement and its consequences as well as an ability to optimize leg length, version and offset. While modular femoral neck systems have been successfully employed for a variety of patient skeletal pathology, they are not without clinical concerns. Their structural compromise at metal-metal interconnections due to cyclic microdisplacements, component disassociation in vivo, and increased potential for metallic wear debris generation and corrosion have all been reported along with tissue reactivity surrounding the tapers. Whether the cited benefits of deformity and biomechanical correction, tissue preservation and revision options outweigh these risks is a consideration in their selection. This handout describes a laboratory basis for the evaluation of modular femoral neck designs and suggests a thought process for the joint arthroplasty surgeon when considering their employ.

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The High Performance Modular Hip: What a Surgeon Should Know

Modularity in total hip arthroplasty design has received increased citation in the clinical literature. The advantages of these systems include off-the-shelf flexibility for customizing proximal and distal canal filling, preservation of soft tissue structures, biomechanical restoration of offset, version and leg length, as well as accommodating difficult situations of femoral deformity and bone loss. Both mid-stem and distal neck modular femoral systems have been successfully employed for a variety of patient skeletal pathology. However, they are not without clinical concerns. The maintenance of anatomical stability within the femoral canal, structural compromise at metal-metal interconnections due to cyclic microdisplacements defined as fretting, decoupling of components in vivo, and increased potential for metallic wear debris generation and corrosion have all been reported. This handout describes a laboratory basis for the evaluation of modular femoral stem designs and suggests a thought process when considering their employ.

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Do Thin Acetabular Shells Increase the Disassociation Risk of Ceramic Liners?

The procedural standard for the fixation of metallic acetabular shells is under-reaming and impaction. Recently, concerns have arisen regarding achieving and maintaining secure ceramic liners when thin shells are deformed during normal insertion. Failure to achieve an adequate ceramic taper lock has been associated with clinical disassembly and liner fracture. Additionally, this phenomenon has been hypothesized as a possible cause of articular “squeaking”, leading to revision in a small number of patients. This study evaluates the influence of shell deformation on the locking mechanism integrity of contemporary modular acetabular designs employing ceramic liners.

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Enhanced Cross-Linked Polyethylene Modular Acetabular Designs: Performance Characteristics

Modular acetabular designs enjoy widespread use in THA procedures. Their mechanical performance has continually increased through improved locking mechanism assemblies. Recently, a number of enhanced cross-linked polyethylenes have emerged whose commonly heralded benefit is a reduction in polymer wear due to increased cross-linking concurrent with minimized oxidation. These processes, however, change the chemical structure of the polymer affecting both static mechanical and fatigue properties, including a decrease in the resistance to crack propagation. This has significant ramification on the long-term clinical integrity of modular acetabular components where enhanced cross-linked polyethylenes are employed and reduced liner thicknesses are advocated. This study evaluated the locking mechanism integrity for three, contemporary, modular acetabular designs, which employ both conventional and enhanced cross-linked polyethylene liners.

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Do Thin Acetabular Shells Increase the Disassociation Risk of Ceramic Liners?

The procedural standard for the fixation of metallic acetabular shells is under-reaming and impaction. Recently, concerns have arisen regarding achieving and maintaining secure ceramic liners when thin shells are deformed during normal insertion. Failure to achieve an adequate ceramic taper lock has been associated with clinical disassembly and liner fracture. Additionally, this phenomenon has been hypothesized as a possible cause of articular “squeaking”, leading to revision in a small number of patients. This study evaluates the influence of shell deformation on the locking mechanism integrity of contemporary modular acetabular designs employing ceramic liners.

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Fracture of Highly Crosslinked UHMWPE Acetabular Liners

Four well-placed crosslinked UHMPWE acetabular liners of distinct design were retrieved subsequent to fracture failure of the rim. In each case, electron microscopy of the fracture surface revealed fatigue initiation markings associated with a stress concentrating feature in the rim. Fracture surface investigation demonstrated that crack propagation in all cases followed contours of tensile stresses. FEA simulations were performed to examine whether direct rim loading could be the cause of the observed failures.

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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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