Retrieval studies implant

Abstract The biocompatibihty, biofunctionahty and biostability of a prosthesis, its degradation and the host response are determined throngh an implant retrieval stndy.This chapter focnses on varions aspects of analyzing retrieved biotextile implants. Sample preparation techniques, analytical test methods and different test standards are reviewed. Some precautions and the limitations of implant retrieval studies are also discussed. 

A biotextile retrieval study is the study of a biotextile implant that has been retrieved from an animal or human patient after a certain period of time. In animal trials, retrieval times are prescheduled and coincide with the time of sacrifice of the animal. For human patients this is obviously not possible, and the time of retrieval is unscheduled and occurs either at... 

Standards and animal models for implant retrieval studies... 

As mentioned earlier, biotextile retrieval studies can be carried out in animals (preclinical trials) as well as in human subjects (clinical trials). Retrieval studies using animal models are usually planned studies where the device to be tested is implanted in the animal for a fixed duration after which it is harvested and studied for the desired characteristics. 

Retrieval study samples from human beings are more difficult to obtain and are usually available as a result of an unscheduled reoperation or autopsy, either of which could be due to device failure or to some totally unrelated cause. Implants retrieved at reoperation are generally in a poorer condition since they are often collected in response to emergency surgery where the patient is the primary concern and the preservation and integrity of the biotextile explant is of limited interest. It is therefore mandatory to include with every clinically retrieved sample some explanation... 

Some other examples of animal models that are used in biotextile retrieval studies include implanting hernia repair meshes in the abdomen of piglets and mitral valve replacement in juvenile sheep or calves, and subcutaneous implants of valve materials in young rats for in vivo biocompatibility and calcification studies. 

Changes in the physical properties of the retrieved biotextile implant can suggest the in vivo behavior of the implant during its service life. The possible mode of failure can be determined by studying these changes. 

For all retrieval studies, unused control samples are crudal, especially when these studies are of fairly long duration. Procurement and/or storage of control samples are important aspects of retrieval studies. Unused control samples can either be obtained from the manufacturer if the equivalent products are still available, or alternatively, the unused controls can be removed from the implanted specimen in the operating room at the time of surgery and stored at room temperature in dark, moisture-free conditions. Since this second approach eliminates any batch-to-batch variations and changes due to storage, handling and sterilization procedures, it is believed to provide more reliable controls. 

Collecting all the relevant information and maintaining documentation is very important for retrieval studies. Information about the implant manufacturer, the dates of implantation and explantation, the reason for retrieval, the patient s or animal s age and sex, the subject s health and medical state and the condition of the biotextile implant at the time of explantation. 

On the other hand, retrieval studies in animals permit detailed monitoring of biocompatibility, biofunctionahty and biostabiUty of the implants at any desired and prescheduled interval of time. They also allow detailed studies of tissue-device interactions at the surface of the biotexthe since the surrounding tissue can be removed as appropriate. Animal models facilitate the study of certain complications such as caldfication in an accelerated time frame. Moreover, experimental conditions can be held constant in animal studies. However, animal models do not rephcate exact human physiological behavior, particularly for iU and injured patients with particular pathologies. 

The focus of this chapter has been the study of retrieved biotextile implants. While similar issues are relevant to all types of implantable devices, most of the tests and protocols described in this chapter are exclusively for biotextile implants. A biotextile retrieval study can serve a number of different purposes, such as ... 

An important further development of the retrieval study would be the simulation and modeling of in vivo performance of the biotextile implant. By simulating the various parameters such as blood flow and tissue-material interactions it may in the future be possible to predict such biological response variables as thrombosis, aneurysm formation and the rate of healing. 

Polymers that are suitable to be implanted in the body should not deteriorate unacceptably during their shelf-life or while in vivo, in addition, neither they nor any packaging materials should deteriorate before implantation (Hukins et al., 2008). One method that can be used to study the material s ageing properties is from retrieval studies in the human body (Dole el et al., 1989a), where implants are retrieved, after many years inside the human body, and the properties of their constituent materials investigated, or to implant it... 

Furthermore, often it is not feasible or practical to study the degradation mechanisms of polymers using only retrieval studies or by implanting them into animals, because of the timescales involved of the projected lifetime of the polymer, possibly of the order of 20 years (Mahomed et al., 2010a). One... 

McGee MA, Howie DW, Costi K, Haynes DR, Wildenauer Cl, Pearcy MJ, et al. Implant retrieval studies of the wear and loosening of prosthetic joints a review. Wear 2000 241 158-65. 

Many large-scale retrieval studies have been conducted involving hundreds of implants. Although corrosion of the devices is only one of many topics that are the subjects of retrieval analyses, this type of investigation has yielded the majority of the knowledge that has been amassed regarding corrosion in vivo. ASTM F 561, Practice for Retrieval and Analysis of Implanted Medical Devices, and Associated Tissues, governs the performance of implant retrieval analyses. 

A useful collection of symposium papers under the general title Corrosion and Degradation of Implant Materials has been published by the American Society for Testing and Materials. " In this, two separate studies of retrieved orthopaedic implants are presentedtogether with an account by Dumbleton " of the literature on the use, primarily in Scandinavia, of polyoxymethylene (Delrin) for compK>nents of total hip and total knee joint prostheses. 

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