The histopathology of periprosthetic tissues has been important to understanding the relationship between wear debris and arthroplasty outcome. In a landmark 1977paper, Willert and Semlitsch (1) used a semiquantitative rating to show that tissue reactions largely reflected the extent of particulate debris. Notably, small amounts of debris, including metal, could be eliminated without “overstraining the tissues” but excess debris led to deleterious changes. Currently, a plethora of terms is used to describe tissues from metal-on-metal (M-M) hips and corroded modular connections. We reviewed the evaluation and reporting of local tissue reactions over time, and asked if a dose response has been found between metal and tissue features, and how the use of more standardized terms and quantitative methodologies could reduce the current confusion in terminology. Methods. The PubMed database was searchedbetween 2000 and 2015 for papers using “metal sensitivity /allergy /hypersensitivity, Adverse Local Tissue Reaction (ALTR): osteolysis, metallosis, lymphocytic infiltration, Aseptic Lymphocytic Vasculitis-Associated Lesions (ALVAL), Adverse Reaction to Metal Debris (ARMD) or pseudotumor/ pseudotumour” as well as metal-on-metal / metal-metal AND hip arthroplasty/replacement. Reports lacking soft tissue histological analysis were excluded. Results. 131 articles describing M-M tissue histology were found. In earlier studies, the terms metal sensitivity / hypersensitivity /allergy implied or stated the potential for a Type IV delayed type hypersensitivity response as a reason for revision. More recently those terms have largely been replaced by broader terms such as ALTR, ALVAL and ARMD. ALVAL and metal hypersensitivity were often used interchangeably, both as failure modes and histological findings. Several histology scoring systems have been published but were only used in a limited number of studies. Correlations of histological features with metal levels or component wear were inconclusive, typically because of a high degree of variability. Interestingly, there were very few descriptions that concluded that the observed reactions were benign / normal or anticipated i.e. regardless of the histological features, extent of debris or failure mode, the histology was interpreted as showing an adverse reaction. Discussion. There is now an expanded set of terms to describe tissues but they lack clear definitions and typically do not use quantitative histological data to describe a wide range of periprosthetic reactions to metal. Lower limits of inflammation, necrosis or re-organization that represent a “normal” reaction to surgery and/or small amounts of wear debris are not clearly defined and are rarely discussed. The widespread adoption of the term “adverse” in the present tissue lexicon implies a cause and effect relationship between metal wear and corrosion products and histological features even though this has yet to be determined. The use of quantitative histological scores rather than subjective histological descriptions is imperative to improve the understanding and reporting of the range of periprosthetic reactions. In particular, a new lexicon that allows for a level of tissue reaction that is not misinterpreted as adverse is required

Artificial joints have been increasingly used in the treatment of physically disabled people who suffer from joint diseases such as osteoarthritis and rheumatoid arthritis. Ultra high molecular weight polyethylene (UHMWPE) is commonly used in hard-on-polymer joints as an impact-absorbing material for artificial hip joints because of its very low friction coefficient, high wear resistance, impact strength, and biocompatibility. However, particles generated by excessive wear and fatigue can cause osteolysis, which may lead to loosening. This has led to recent interest in metal-on-metal joints, which can provide better wear properties than hard-on-polymer joints, leading to reduced osteolysis. However, during gait, metal-on-metal joints are exposed to greater impacts than hard-on-polymer joints. These impacts can cause severe pain in patients who have undergone hip replacement arthroplasty. In previous work, we proposed a double-shell metal-on-metal artificial hip joint in which a single garter spring was inserted between the inner and outer acetabular shell of an impact relief device[1]. A garter spring is usually used by loading a compression stress from the outside to the center axis. The acetabular shell is composed of two layers as shown in Fig.1. In the current work, the performance of single and dual garter springs was investigated using static compression and free-fall type impact tests. Static compression tests were conducted on a conventional vise to examine the deformation of various kinds of garter springs under uniaxial loading. Free-fall impact tests, on the other hand, were conducted on a free-fall type impact test machine as shown in Fig. 2. The impact relief ability of the garter springs under impact loading was examined, and the maximum impact load and maximum impact load arriving-time were estimated[2]. The relief ability was also investigated for smaller and larger diameter garter springs with a three-pitch angle, and the maximum applied load was determined by taking into account the applied load on actual hip joints. Static compression test results indicated that some kinds of garter spring could withstand vertical loads of over 6000N, which is estimated to be equal to maximum vertical load during jumping. The pitch angle increased with an increase in the compression load and the shape of the coil ring deformed from a circular to ellipsoidal shape as the compression load increased, which may lead to a reduction in impact load and an increase in impact relief time. The impact test results for a single spring indicated that the maximum impact load decreased in reverse proportion to the maximum impact load arriving-time. A smaller diameter garter spring provided less maximum impact load and longer arriving maximum load time. In the case of dual garter springs, which have smaller and larger diameter garter springs, the springs offered a lower maximum impact load and a longer impact load arriving-time than a single spring