Types of prostheses The various combinations of materials being used for THA are continuously being studied in an effort to improve prosthetic performance. The original 1962 Charnley THA successfully utilized a metal femoral component and a high-density polyethylene acetabular component, and that advancement in materials paved the way for more modern materials over the past 50 years (Sonntag et al., 2012). However, during this time active patient populations have tended to experience inflammatory responses and early prosthetic loosening secondary to wear debris with metal-on-plastic designs (Sonntag et al., 2012). The development in the 1990s of alternatives to polyethylene has reduced the number of implants that loosen and may cause ultimate THA failure. The newer materials are ultrahigh molecular weight polyethylene, a cross-linked variant of this ultrahigh molecular weight polyethylene, cobalt chromium metal alloys, and alumina ceramics (Sonntag et al., 2012). Active patients of today need a combination of materials that allow for friction-free movement, a low wear rate, and a minimal inflammatory response in the patient’s tissues. Patients may preoperatively request their physical therapist’s opinion or guidance regarding the type of prosthesis. Given the individual factors that must be taken into account, it is not possible to make generalized recommendations about which type of prosthesis might be appropriate; however, the physical therapist should be familiar with the features and the potential indications and risk factors associated with the different prosthetic types. Currently prostheses fall into two main categories: hard-on-hard bearings, such as metal-on-metal (MoM) implants and ceramic-on-ceramic (CoC) implants, and hard-on-soft implants, such as metal on conventional noncross- linked ultrahigh molecular weight polyethylene (Rajpura, Kendoff, & Board, 2014). MoM implants promise decreased friction and wear and reduced joint dislocations because the MoM design allows for the use of a larger femoral head component. However, there are some drawbacks to these MoM implants, one of which is the well- publicized metallic ion release that may produce a systemic inflammatory response (Konttinen & Pajarinen, 2013). Debris is believed to be released from the articular surfaces of the femur and acetabulum as a result of abrasion, fatigue, and corrosive Fixation methods Once the orthopedic surgeon chooses the appropriate prosthetic components for the individual patient, the surgeon must decide on fixation of the implants. Currently there is the option of using a cement or uncemented fixation of the implants. A cemented joint arthroplasty is one in which the prosthesis is adhered to the surrounding bone with bone cement. This bone cement is typically a combination of a powder polymethylmethacrylate copolymer and a liquid polymethylmethacrylate monomer (Saleh et al., 2016). An uncemented joint arthroplasty is one in which a porous or textured prosthesis is pressed into place (ingrowth prosthesis, press-fit prosthesis) and then is gradually fixed into place with the patient’s own bony ingrowth. Currently the trend in the United States is toward uncemented THAs. The authors of a study in 2013 found that 86% of THAs in the United States are uncemented, and that this appears to be a trend worldwide (Troelsen, Malchau, Sillesen, & Malchau, 2013). Advantages of an uncemented THA are a potentially longer prosthetic life and the elimination of cement-related side effects, such as cracking of the cement and loosening of the implant, especially of the femoral component. Traditionally with an uncemented hip, weight bearing was restricted for, on average, the first 6 weeks after surgery to partial weight bearing, with the rationale being that this allowed for reduced risk of loosening of the prosthesis. More recently the trend has been toward weight The resurfacing option When advanced hip degeneration is established and is directly causing pain, decreased function, and poor quality of life, an
wear on the implants. Previously the debris was thought to have originated from the cement used to secure the implants, but with the movement to cementless metal implants it was considered that the debris might be from the metal surfaces and not from the cement. These debris or particles are released into the soft tissues surrounding the implant where they are phagocytosed by macrophages. However, the macrophages are unable to degrade the particles, leading to persistent stimulation of the macrophages and chronic inflammation. This increases the presence of foreign-body giant cells, granulomas, and ultimately, osteoclast-driven periprosthetic bone loss and aseptic loosening of the implant (Konttinen & Pajarinen, 2013). Ceramic-on-ceramic (CoC) implants have an extremely low wear rate compared to metal-on-polyethylene implants and provoke a minimal inflammatory response (Porat et al., 2012). However, CoC implants are recognized for long-term squeaking beyond what might occur with MoM implants. This squeaking is a primary reason for revision surgery when a CoC implant was used in the primary THA (Matar, Restrepo, Parvizi, Kurtz, & Hozack, 2010). The squeaking could be related to poor accuracy of implant placement leading to posterior impingement and subsequent anterior edge loading of the joint surfaces (Sariali, Klouche, & Mamoudy, 2014). The combination of ceramic-on-metal is another model proposed where the advantages of CoC and MoM are combined. The authors of a recent study found that this combination of implants allowed for excellent clinical outcomes with limited chromium release at 3 years (Cadossi, Mazzotti, Baldini, Giannini, & Savarino, 2016). Another option is the combination of a newer, more resilient form of polyethylene material combined with metal that reduces the risk of metal ion and debris release while providing the option of longevity (Rajpura et al., 2014). What is clear is that there is no one current implant coupling that is widely accepted and that research into optimal materials and combinations of materials is ongoing (Sonntag et al., 2012). The demands that the patient is likely to place on the prosthesis should be a factor to consider when discussing the different types of implant options currently available. bearing as tolerated for those who are not cemented during surgery (Markmiller, Weiss, Kreuz, Ruter, & Konrad, 2011). Advantages of a cemented THA over an uncemented THA are instant stability and fixation of the prosthesis that allows for immediate full weight bearing, faster recovery time, a decreased need for surgical accuracy when preparing the bony ends for the prosthetic implants, and the ability to glue the implants safely into position regardless of the patient’s bone quality. Consequently, cemented THAs are more common in older patients and in those with osteoporosis. Troelson and colleagues report that the rate of revision is lower in those over 75 years of age when cement is used. Possible but rare complications of a cemented THA include allergic or inflammatory reactions to the bone cement. Eventual loosening or cracking of the cement may occur due to osteolysis; this is one reason that the uncemented option is favored (Abdulkarim, Ellanti, Motterlini, Fahey, & O’Byrne, 2013). This is a particular concern with cemented acetabular components (Chen, Pigott, & Klatt, 2013). In the ongoing search for the perfect THA, attempts are being made to utilize the best combination of fixation methods by using a hybrid total hip replacement. With a hybrid THA, a mix of cemented and uncemented components are used; however, long-term studies on the merits of prosthetic performance are forthcoming.
alternative option to a THA for some patients is hip resurfacing. Increasingly, younger, more active patients are choosing the hip
Page 144
Book Code: PTNY3622B
EliteLearning.com/Physical-Therapy
Powered by FlippingBook