Alan Kop

Corrosion of a hip stem with a modular neck taper junction: a retrieval study of 16 cases.

Since the early 1990s, there has been a dramatic increase in modular total hip designs ranging from a stem with a proximal taper and modular head, to a distal stem, double taper proximal neck, and modular head. Clinical advantages of the modular neck include intraoperative adjustment of leg length via the neck-head taper and femoral anteversion via the neck-stem taper. Sixteen cases of a double tapered cone, Margron hip prosthesis, were presented for retrieval analysis. Macroscopic inspection, corrosion testing, light microscopy, and scanning electron microscopy were conducted to elucidate mechanisms of failure. In this regard, 6 neck components showed significant fretting, and crevice corrosion of the neck-stem taper with an average implantation time of 39 months compared with the remaining retrievals, which showed no corrosion with and average time in situ of 2.7 months. This retrieval study demonstrates that even with a modern taper design and corrosion-resistant materials, increased modularity can lead to fretting and crevice corrosion, metal ion generation, and particulate debris that may contribute to periprosthetic osteolysis and loosening.

Proximal Component Modularity in THA—At What Cost?: An Implant Retrieval Study

BackgroundWhile modular femoral heads have been used in THA for decades, a recent innovation is a second neck-stem taper junction. Clinical advantages include intraoperative adjustment of leg length, femoral anteversion, and easier revision, all providing flexibility to the surgeon; however, there have been reports of catastrophic fracture, cold welding, and corrosion and fretting of the modular junction.Questions/purposesWe asked whether (1) the neck-stem junction showed the same degradation mechanisms, if any, as the head-neck junction, (2) the junction contributed to THA revision, (3) the alloy affected the degree of degradation, and (4) the trunion machine finish affected the degradation mechanisms.MethodsWe compared 57 retrievals from seven total hip modular designs, three cobalt-chromium-molybdenum and four titanium based: Bionik® (four), GMRS® (four), Margron® (22), Apex® (five), M-series® (five), ZMR® (two), and S-ROM® (15). Macroscopic inspection, microscopy, and micro-CT were conducted to determine the effects of materials and design.ResultsThe cobalt-chromium-molybdenum components showed crevice corrosion and fretting of the neck-stem taper, whereas the titanium components had less corrosion; however, there were several cases of cold welding where disassembly could not be achieved in theater.ConclusionsEven with modern taper designs and corrosion-resistant materials, corrosion, fretting, and particulate debris were observed to a greater extent in the second neck-stem junction. Titanium-based modular arthroplasty may lessen the degree of degradation, but cold welding of the components may occur.Clinical RelevanceDegradation of the second junction contributed to 8 cases of metallosis and two cases of aseptic lymphocyte-laminated vascular-associated lesions contributing to revision.

Heat treatment of Ti‐6Al‐7Nb components produced by selective laser melting

Purpose – The purpose of this paper is to describe a preliminary investigation into the heat treatment of Ti‐6Al‐7Nb components that had been produced via selective laser melting (SLM).Design/methodology/approach – Bars of Ti‐6Al‐7Nb were produced using SLM by MCP‐HEK Tooling GmbH in Lubeck, Germany. These bars were then subjected to a range of heat treatments and the resultant microstructure evaluated with respect to its likely effect on fatigue.Findings – It was found that the as received material consisted of an α′ martensitic structure in a metastable β matrix. Evidence of the layer‐wise thermal history was present, as were large (up to ∼500 μm) pores. Solution treatment at 955°C (below the β transus) did not completely disrupt this layered structure and is therefore not recommended. When solution treatment was performed at 1,055°C (above the β transus) a homogeneous structure was produced, with a morphology that depended on the post‐solution treatment cooling rate. It was concluded that the most prom...

Quantification of polyethylene degradation in mobile bearing knees: a retrieval analysis of the Anterior-Posterior-Glide (APG) and Rotating Platform (RP) Low Contact Stress (LCS) knee.

Background Routine qualitative observations of more than 850 polyethylene fixed and mobile bearings at our institution have noted minimal wear of mobile bearings. The APG mobile bearing is the most recent design variant of the LCS knee, allows multi-directional movement at the tibiofemoral articulation, and is posterior cruciate sparing. Even though it is difficult to perform, quantitative wear measurement is important in determining the likely longevity of new arthroplasty devices, and is especially relevant because of increasing numbers of new mobile bearing designs. Patients and methods We analyzed 10 retrieved APG and 7 retrieved RP tibial bearings (De Puy) with a mean implantation period of 33 (9–70) months. We used coordinate-measuring techniques to quantitatively determine linear penetration, and optical and scanning electron microscopy to assess wear mechanisms qualitatively. Results The mean total volume loss (superior and inferior articulations) of the APG and RP designs was 85 mm3/year and 77 mm3/year, respectively. Burnishing was the predominant wear mechanism, and to a lesser extent scratching, abrasion and pitting. Multidirectional scratching and abrasion were noted on the APG inserts inferiorly, whereas there was circumferential scratching on the RP inserts. Interpretation Our short-term results for the APG and RP mobile bearing designs are similar and compare more than favorably with reported values for fixedbearing designs. However, increased backside wear due to multidirectional movement may predispose the APG design to greater wear in the long term.

The Royal Perth Hospital method for the design and manufacture of titanium cranioplasty plates.

b o oyal Perth Hospital designs and manufactures 40–50 ustom-made titanium cranioplasty plates each year, and they ange in size from roughly 50 mm to complete bifrontal craial replacements. The method has evolved over the past ecade into a robust procedure that incorporates computer odelling, rapid-prototyping, and highly-skilled production ngineering. A high resolution helical multislice computed tomoraphic (CT) scan (1.0 mm slice thickness, 1.0 mm slice pacing) is taken of the patient. The CT data are stored and ransferred by PACS in DICOM format. The software packge Mimics (Materialise®, NV Leuven, Belgium) is used to reate a virtual three-dimensional model of the skull. Care s taken to remove the segments of any resorbed bone flap r other metalwork that will be excised during operation hen the titanium plate is implanted (Fig. 1A). The model is xported from Mimics in the .STL format. A rapid-prototype model of the defect is created from he .STL file using the Catalyst EX software (Stratasys®, den Prairie, Minnesota, USA) and a Dimension SST1200es hree-dimensional printer. The .STL file is imported into he software package FreeForm® Modeling PlusTM (Sens-

Catastrophic failure of 4 cobalt-alloy Omnifit hip arthroplasty femoral components

Background Femoral component neck fracture is an uncommon type of failure in total hip arthroplasty. We present a report on 4 retrieved cobalt-chrome femoral components that fractured at the neck, where we investigated the mechanisms of failure. Methods The 4 retrieved implants were analyzed with regard to their macro- and microstructures and the fracture surfaces were examined using electron microscopy. The medical record of each patient was also examined for any history of complications prior to failure of the implant. Results These fractures occurred immediately adjacent to the base of the modular head. Skirted modular heads were used in 3 of the 4 failed components. This constructs promotes corrosion. Cyclic fatigue-loading in combination with the material factors of course grain microstructure and extensive carbide precipitation along the grain boundaries were also identified as the cause of implant failure. Interpretation Our findings suggest that a solution annealing step could be introduced into the manufacturing process to improve the microstructure of the cobalt chrome alloy. We also advise caution in using a skirted modular head in combination with a device of known suboptimum microstructure, for a greater margin of safety.

Rupture of poly implant prothèse silicone breast implants: an implant retrieval study.

Background: Poly Implant Prothèse implants were recalled in Australia in April of 2010 following concerns of higher than expected rupture rates and the use of unauthorized industrial grade silicone as a filler material. Although subsequent investigations found that the gel filler material does not pose a threat to human health, the important question of what caused a relatively modern breast implant to have such a poor outcome compared with contemporary silicone breast implants is yet to be addressed. Methods: From a cohort of 27 patients, 19 ruptured Poly Implant Prothèse breast implants were subjected to a range of mechanical tests and microscopic/macroscopic investigations to evaluate possible changes in properties as a result of implantation. New Poly Implant Prothèse implants were used as controls. Results: All samples, explanted and controls, complied with the requirements for shell integrity as specified in the International Organization for Standardization 14607. Compression testing revealed rupture rates similar to those reported in the literature. Shell thickness was highly variable, with most shells having regions below the minimum thickness of 0.57 mm that was specified by the manufacturer. Potential regions of stress concentration were observed on the smooth inner surfaces and outer textured surfaces. Conclusions: The high incidence of Poly Implant Prothèse shell rupture is most likely a result of inadequate quality control, with contributory factors being shell thickness variation and manufacturing defects on both inner and outer surfaces of the shell. No evidence of shell degradation with implantation time was determined.

Oxidation of Second Generation Sequentially Irradiated and Annealed Highly Cross-Linked X3™ Polyethylene Tibial Bearings

Since the first use of ultra-high-molecular-weight polyethylene as a bearing material, research and development efforts have sought to improve wear resistance, increase longevity and lessen the potential for debris mediated adverse tissue responses. A series of second generation sequentially cross-linked and annealed tibial bearings were analysed after several bearings sent for routine retrieval analysis showed oxidative degradation including subsurface whitening, cracking and gross material loss. Evaluation incorporated visual and white banding assessment, mechanical testing and spectroscopy analysis. Whilst visual observation and white banding assessment confirmed oxidative changes, a decrease in mechanical properties and increasing ketone oxidation index as a function of time in vivo suggest time dependent oxidative degradation. Clinically relevant degradation of the sequentially cross-linked and annealed tibial bearings was observed.

What are the predictors for bead shedding in porous-coated hip and knee arthroplasties?: An implant retrieval study.

Bead shedding has been reported in both hip and knee arthroplasties and can result in increased polyethylene wear and metal ion release. The rates of bead loss were recorded for each device type, and the interfacial bead-substrate shear load to failure and contact areas of the beads were determined. The 3 components with the highest rate of bead loss, namely, S-Rom, Omnifit, and Optifix acetabular shells, recorded the lowest shear loads to failure. Percentage bead-substrate contact area ranged between 7% and 37% and was the most important factor affecting bead shedding. Other factors influencing bead loss in decreasing order of importance were bead fusion, bead size, component type (acetabular or knee), component loosening, and time in situ.

Cranial reconstruction using allogeneic mesenchymal stromal cells: a phase 1 first-in-human trial

Cranioplasty is necessary for patients that have undergone craniectomy following trauma, stroke or other causes of elevated intracranial pressure. This study assessed the effectiveness of treating cranial defects with allogeneic mesenchymal stromal cells (MSC) on a ceramic carrier and polymer scaffold, to produce viable bone and healing of a cranial void. Patients underwent a baseline computed tomography (CT) scan for construct design. Two sets of interlocking moulds were three‐dimensional printed to enable shaping of two polymer meshes, which formed the boundaries of the construct corresponding to restoration of the skull interna and externa. In vitro expanded donor MSC were seeded onto ceramic granules in a good manufacturing practices facility. The inner mesh was placed in theatre, followed by the cell‐loaded granules, and the outer mesh. Patients were followed‐up at 3, 6 and 12 months and cosmesis assessed visually, while bone formation was assessed by CT scans at 1 day, 3 months and 12 months. Manufacture of the construct and surgery was uneventful for all three patients. Initial cosmesis was excellent with no complications. New bone formation was demonstrated by analysis of CT data; however, bone resorption was noted in all 3 cases on the 12‐month CT scan. The lack of rigidity of the construct in an environment with continuous pulsatile movement may be preventing the formation of solid bone. It is possible to produce a customized allogeneic MSC construct for cranial reconstruction to replace cranial bone with good cosmesis, using a combination of medical computer modelling, rapid‐prototyping and tissue engineering.