Eli Patten

Corrosion-Induced Fracture of a Double-Modular Hip Prosthesis: A Case Report

Modularity is an important aspect of total hip replacement design. A modular Morse taper is commonly used to attach the femoral head to the femoral stem. Newer double-modular designs incorporate a second interface at the neck-stem junction. Increased modularity purportedly allows the surgeon to more closely restore patient anatomy, such as limb length, lateral offset, and femoral anteversion, and to better balance the soft tissue to achieve optimal biomechanics. However, modularity also increases the number of mechanical junctions that may lead to fretting (micromotion)1, corrosion2,3, and ultimately fracture. We have evaluated a double-modular PROFEMUR Z stem (Wright Medical Technology, Arlington, Tennessee) after catastrophic failure due to a fractured neck component at the neck-stem junction. This retrieval allowed us to analyze the mechanical failure mechanisms associated with a double-modular design. We hypothesized that modularity at the neck-stem junction resulted in fretting and crevice corrosion, which led to crack initiation and, on the application of an overload event, to fracture. The patient was informed that data concerning the case would be submitted for publication, and he consented. In June 2006, a thirty-year-old man with rheumatoid arthritis who was 6 ft and 6 in (2 m) tall, weighed 242 lb (109.8 kg), and had a body mass index of 29 kg/m2 underwent an uncomplicated right total hip arthroplasty with a cementless PROFEMUR Z hip stem with a long, straight neck (no varus/valgus offset) and a ceramic-on-ceramic articulation. The ceramic-on-ceramic modular implant was considered to be indicated in this particular patient because of his young age, high body mass index, active lifestyle, and profession. A modular stem was used in an effort to reproduce the anatomic femoral anteversion, offset, and lower-limb length to optimize hip abductor and biomechanical function. The hip functioned well until April 2008, when …

Tradeoffs amongst fatigue, wear, and oxidation resistance of cross-linked ultra-high molecular weight polyethylene

This study evaluated the tradeoffs amongst fatigue crack propagation resistance, wear resistance, and oxidative stability in a wide variety of clinically-relevant cross-linked ultra-high molecular weight polyethylene. Highly cross-linked re-melted materials showed good oxidation and wear performance, but diminished fatigue crack propagation resistance. Highly cross-linked annealed materials showed good wear and fatigue performance, but poor oxidation resistance. Moderately cross-linked re-melted materials showed good oxidation resistance, but moderate wear and fatigue resistance. Increasing radiation dose increased wear resistance but decreased fatigue crack propagation resistance. Annealing reduced fatigue resistance less than re-melting, but left materials susceptible to oxidation. This appears to occur because annealing below the melting temperature after cross-linking increased the volume fraction and size of lamellae, but failed to neutralize all free radicals. Alternately, re-melting after cross-linking appeared to eliminate free radicals, but, restricted by the network of cross-links, the re-formed lamellae were fewer and smaller in size which resulted in poor fatigue crack propagation resistance. This is the first study to simultaneously evaluate fatigue crack propagation, wear, oxidation, and microstructure in a wide variety of clinically-relevant ultra-high. The tradeoff we have shown in fatigue, wear, and oxidation performance is critical to the materials long-term success in total joint replacements.

Costs of a more spacious home after remodelling by hermit crabs

Architectural creations occur throughout the animal kingdom, with invertebrates and vertebrates building structures such as homes to maximize their Darwinian fitness. Animal architects face many trade-offs in building optimally designed structures. But what about animals that do not build, and those that only remodel the original creations of others: do such secondary architects face similar trade-offs? Recent evidence has revealed that hermit crabs—animals well known for opportunistically moving into remnant gastropod shells—can also act as secondary architects, remodelling the shells they inherit from gastropods. Remodelling has only been found among terrestrial hermits (Coenobita spp.), not marine hermits. Here we investigate the potential trade-offs Coenobita compressus faces from remodelling by subjecting its remodelled and unremodelled homes to controlled engineering crush tests, which parallel the homes being crushed by predators. While remodelled homes are significantly more spacious and lightweight than unremodelled homes, we find that the homes attain these beneficial qualities at a cost: a reduced resistance to being crushed. Hermit crabs may therefore only remodel their homes to thresholds set by the bite force of their predators. Our results suggest that, like primary animal architects, which face trade-offs when optimizing architectural designs, secondary animal architects face trade-offs when remodelling such designs.

Delamination of a highly cross-linked polyethylene liner associated with titanium deposits on the cobalt-chromium modular femoral head following dislocation

Retrieval studies of total hip replacements with highly cross-linked ultra-high-molecular-weight polyethylene liners have shown much less surface damage than with conventional ultra-high-molecular-weight polyethylene liners. A recent revision hip replacement for recurrent dislocation undertaken after only five months revealed a highly cross-linked polyethylene liner with a large area of visible delamination. In order to determine the cause of this unusual surface damage, we analysed the bearing surfaces of the cobalt-chromium femoral head and the acetabular liner with scanning electron microscopy, energy dispersive x-ray spectroscopy and optical profilometry. We concluded that the cobalt-chromium modular femoral head had scraped against the titanium acetabular shell during the course of the dislocations and had not only roughened the surface of the femoral head but also transferred deposits of titanium onto it. The largest deposits were 1.6 microm to 4.3 microm proud of the surrounding surface and could lead to increased stresses in the acetabular liner and therefore cause accelerated wear and damage. This case illustrates that dislocations can leave titanium deposits on cobalt-chromium femoral heads and that highly cross-linked ultra-high-molecular-weight polyethylene remains susceptible to surface damage.

Designing for Crosslinked UHMWPE Implants: Clinical Consequences of Stress Concentrations

Ultrahigh molecular weight polyethylene (UHMWPE) remains the polymer bearing of choice for total joint replacements (TJR) [1]. However, the long-term performance of this polymer has been limited by in vivo wear: UHMWPE wear debris generated in the joint space can travel into the periprosthetic bone, initiating osteolysis and implant loosening [2]. Crosslinked UHMWPE (through ionizing radiation) has demonstrated increased wear resistance [3], but at the cost of reduced fatigue crack propagation and fracture resistance [4]. Additionally, radiation processes can release free radicals which, when not eliminated through thermal treatment, can increase UHMWPE susceptibility to oxidation and mechanical embrittlement [5]. Such tradeoffs present clinical concerns when implant designs incorporate stress concentrations that experience elevated stresses under loading. These compromises are evaluated through the failure analysis of several crosslinked UHMWPE retrievals that fractured in vivo.Copyright

The Effects of Sliding, Rolling, and Rotation on Cross-Shear and Wear of UHMWPE in Knee Replacements

Wear debris from the ultra-high molecular weight polyethylene (UHMWPE) tibial bearing surface leading to loosening is still the main cause for revisions after 5 years for total knee replacements (TKR) [1]. Wear of UHMWPE is greatly increased under cross-shear motion, such as in the hip joint, due to how the molecules and crystalline regions can align in one direction and make the material weaker in the other [2]. The motion in the knee joint is mainly linear rolling and sliding, but there is rotation and medial-lateral sliding that introduce cross-shear [3]. Wear tests are typically performed with basic motion parameters and simplified geometry (pin-on-disk tests) or under gait simulation to test specific designs, but little is known about the effects of the different motions in the knee on UHMWPE wear. There is also disagreement over how to best quantify cross-shear and model how much wear it will cause [3–5]. This study investigates the effect on wear of the different individual and combined motions in TKR: sliding, rolling, and rotation, for a total of eight separate kinematic conditions. The different measures of cross-shear present in each case were calculated in a computer model and compared to the wear test results.Copyright

Profilometry Analysis and Improvements of a Novel Damage Scoring Method for Metal Bearing Surfaces of Shoulder Replacements

Characterizing the type and extent of in vivo damage to total joint replacements (TJR) is important for improving the success of arthroplasty outcomes, modeling damage modalities, and validating simulator studies. A method for quantifying the damage present on Cobalt Chrome (CoCr) humeral heads was developed in our lab to fulfill a much-needed gap in clinical knowledge regarding total shoulder replacements as well as metallic bearing surfaces [1,2]. A lack of inter-observer consistency with regard to severity classifications from our initial protocol [1] prompted several modifications to the method, which are tested and described here in this study. Also, since sub-micron scale ultra-high molecular weight polyethylene (UHMWPE) wear debris is linked to osteolysis and implant loosening, additional analysis with high magnification 3D optical profilometry was performed on a subset of damage modes with a long-term goal of correlating surface damage with propensity for osteolysis in TJR [3,4].Copyright

A Multi-Directional Tribo-System: Testing the Wear of UHMWPE Under Sliding, Rolling, and Rotation

A tribological testing system with up to four degrees of motion has been designed and built to simulate the wear of materials used in joint replacements. The machine is versatile enough to test simple pin-on-disk contact in planar sliding as well as ball-on-flat contact with more complex rotation and rolling motions. A wide range of materials, loads, and specimen configurations can be tested with this machine, making it a powerful tool for hypothesis-driven research. This tribosystem is robust, customizable, cost effective, and offers the first opportunity to investigate the fundamental wear mechanisms that arise from the four degrees of motion in total knee replacements. Basic design features, advantages, and preliminary validation will be discussed.Copyright

Double-Modular Hip Device Design Susceptible to Stress Corrosion Failure

Total hip replacements restore pain-free mobility to approximately 200,000 patients in the U.S. each year [1]. A typical hip system comprises a metal alloy stem, a femoral head (ceramic or metal alloy), and a polyethylene acetabular cup fit into a metal alloy backing. A modular press-fit Morse taper is commonly used to attach the femoral head to the stem. There are also more recent designs that incorporate a second interface at the neck-stem junction (Figure 1). Increased modularity in total hip replacement design allows the surgeon to intraoperatively preserve patient anatomy such as leg length and femoral anteversion and better balance the surrounding soft tissue for optimal biomechanics. However, modularity also increases the number of mechanical junctions and interfaces in the device which may lead to complications such as corrosion, wear, and fracture.Copyright