The knee is an important joint for daily movement. In cases of trauma or fall, one may have to undergo surgery which might include the replacement of the knee (prosthesis). The prosthesis has been used as a form of clinical treatment for a long time. To date, researchers and clinicians are on the path of improving the prosthesis process to reduce pain and improve the functions of the knee.
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One of the problems that pose a great challenge to research in prosthetic work is strain/stress shielding; thus many studies have been conducted to find methods that will help reduce stress shielding. Because the manufacturers of materials used for this procedure are involved, they have continuously improved their products to reduce stress shielding (Mann, et al., 1991, p. 798).
The issue of stress shielding in prostheses has been raised by different researchers. The load transference that occurs with a normal gait and the loading forces would be transferred through the hip and knee joint. Considering that the weight of the implants is higher than that of the normal skeleton, the majority of the pains would be traversing through the implants only (Santhappan, et al., 2008, p. 13).
Thus, due to the latter, the bone is stress-shielded by the stress shielding is one of the reasons for continuous knee surgery. It results in pain to the patient and also has cost implications since the patient has to undergo surgery after every five to ten years (Ridzwan, et al., 2007).
Stems are used to stabilize the prosthesis while allowing reconstruction of the bones. There has been researching done on the different types of stems that used their design length and means of fixing as factors considered to determine the efficiency of the stems in reducing the stress/strain shielding (Shannon, et al., 2003).
In this paper, we review work that has been done about stress shielding and knee prosthesis, compare them and outline the recommendations given in these papers.
The prosthesis is an artificial joint that is used in knee replacement, made of ceramic, plastic, or metal combinations of materials. Apart from improving knee function, arthroplasty is used to relieve pain, as well. Some of the major conditions that lead to knee replacement include Osteoarthritis Rheumatoid arthritis, Trauma (damage to the knee due to an accident or a fall) (Silber, 1999). Total knee arthroplasty (TKA) is one of the methods that are used as a form of treatment for patients. It involves the femoral component, the tibial component, and at times the patella surfacing (Santhappan, et al., 2008, p. 14).
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Knee arthroplasty alters the mechanical loading of the knee joint making the surrounding bones adjust their structure and mineral components to meet the mechanical demands of the knee. Stemming is used in revision total knee Arthroplasty to protect the bones surrounding the implants (Murase, et al., 1983, p. 15).
Stress /strain shielding
After an implant, the removal of normal stress from the born reduces bone density; a process known as shielding. According to Walker, et al. (1981, p. 258), the bone changes its external shape and internal architecture in response to stress working on it. Due to the mechanical stress, it experiences bone model and remodels causes the bone to generate a minimal-weight structure in the opposite intensity to the applied stress (Lipperman & Gefen, 2006, p. 42).
According to Walker, et al., (1981, p. 259) reports of stress shielding during hip implants were common, because stress being subjected upon the femur was absorbed by the denser and harder object. However, without enough stress on the surrounding bone tissue, the reabsorption by the bone density results in the deterioration of the bone (ABR-Therapy, 2010).
Due to fluctuations of pressure and tension of the body’s visco-elastic features, loads and stress are transmitted through, hence allowing the passage of the stress/strain. When an individual has conditions that require implants or prostheses, materials used are usually harder or denser. These dense materials absorb the stress/strain transmitted in the prosthesis region. This causes the body areas around the prosthesis to disintegrate and be reabsorbed since it’s shielded from receiving enough stress/strain (ABR-Therapy, 2010).
According to ABR-Therapy (2010), bones need the pressure to maintain their normal structure. A prosthesis procedure acts as stress shielded causing the pressure around the bone to be non-uniformly distributed and thus disintegrate the bone. For patients with arthritis, shielding results in hardening and calcification of joints due to the reduction in the hydraulic stress transfers and bone marrow density reduction. With this knowledge, there is a constant need for the bones and surrounding tissue of a prosthesis procedure to be continually under strain/stress for healthy movement (Reilly, et al., 1982, p. 759).
The prosthesis used during total knee Arthroplasty may have a long or a short stem. Using the short stem, the prosthesis does not come into contact with the cortex of the bone. On the other hand, the long stems are fit to the tibial tray to offload the tibial subchondral bone. It is intended to increase the surface area relative to the long axis of the bone; thus increasing the stability between the bone and the implant interfaces (Luring & Gefen, 2006, p. 36).
As explained by Marmor (1976, p. 103) and the University of Washington, department of radiology, stress shielding narrates the life course of the prosthesis. The prosthesis would usually take 5-10 years depending on the patient and other external factors before its symptomatic nature resulting from loosening of the prosthesis due to enough stress shielding. Replacing the loose prosthesis with a new one that reaches the normal bone due to its length.
To protect the constrained autogenous bone stock increased use of TKA has been reported in most studies. Studies to analyze the efficacy of the stem arguments in the stability of the tibial components using in-vitro experiments have been done (Laskin, 2001, p. 95; Bourne and Finlay, 1986, p. 97; Jazrawi, et al., 2001, p. 759; Stern, et al., 1997, p. 46).
Research done by Jazrawi et al., (2001, p. 761) suggests that the use of long-stemmed prosthesis in hip and knee Arthroplasty may result in significant stress shielding about the length of the stem, resulting in significant osteopenia, progressive radiolucencies, and eventually loosening. This is in agreement with work by Completo, et al., (2009) which pointed to the minor effect in stress concentrations and shielding of the short stems.
To enhance stability, Lachiewicz and Soileau (2004, p. 527) suggest that implants with extended stems have been used during revision knee surgery. Stems transfer stress from the deficient plateau to the shaft. In a study done by Bourne and Finlay in 1986, reduction in the use of intramedullary stems for the revision of TKA was caused by the increasing stress shielding of the proximal tibia cortex. The use of extended stems without significant stress shielding was predicted by several authors through biomechanical testing, Reilly et al., (1982, p. 766) noted that if 60mm tibia stem was used with incomplete coverage, decreased proximal strain would be noted. Jazrawi et al., (2001, p. 765) noted that there would be no significant decrease in proximity tibial strain with the use of either cemented or uncemented stems.
Research done by Shannon et al., (2003, p. 30), on reducing bone loss due to stress shielding in the tibia through the interlocking screw fixation method, suggests that porous coating the interlocking stem for bony in-growth may be advantageous in preventing stress shielding associated with then distal fixation. This method was found to be a viable alternative to more common fixation techniques such as cement and press-fit with full bony in-growth.
According to Yoshii, et al., (1992, p. 436), tibial fixation requires a stem to prevent the implant from sinking due to compressive failure of the proximal tibial cancellous bone and stress. The tibial stem reduces the force at the prosthesis bone interface and ensures uniform distribution of the load/stress through the upper tibia. In cases of osteoporosis, stemmed components are used fat the points that are severe or in the cases where large bone defects or where bone grafts have been used. The stem used are of two categories; the short and the long stems. Stem lengths greater than 5cm are considered minimally beneficial in terms of stability. The use of very long-length stems is related to some degree of stress shielding of the tibial cortex along the length. The stems also help to support the tibial plateau in cases where fibrous membranes have developed; in this case, the stem prevents fatigue of the cantilever plateau (Silber, 1999).
Many studies have been done to suggest ways in which stress/strain shielding can be reduced.for as long as the materials used in the prosthesis are heavier than that of the normal skeleton, stress/ strain shielding will always be a problem in the tibial prosthesis. Many results from the different studies show that getting materials for a prosthesis that would are lighter or equal weight as the normal skeleton is an impossible task, the manufacturers of the materials used in the prosthesis can try to come up with products that are near the weight of the normal skeleton
The studies here show that the use of short stems has a lower stress/strain shielding factor as compared to that of long stems. However if one has to use a long stem for prosthesis, then it is recommended that they use it with a distal polymeric tip to help reduce the stress. Results also show that the design of the stem is a factor that should be considered in the prosthesis.
ABR-Therapy. 2010. Stress Shielding: ABR as the Method for Bypassing Stress Shielding. ABR- Therapy. Web.
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Bourne, R.B., & Finlay, J.B., 1986. The influence of tibial component intramedullary stems and implant–cortex contact on the strain distribution of the proximal tibia following total knee arthroplasty. An in vitro study. Clin Orthop Relat Res, 208, pp. 95–99.
Completo. A., Talaia. P., Fonnseca, F & Simeoes, J. A., 2009. Relationship of design features of stemmed tibial knee prosthesis with stress shielding and end-of-stem pain. Web.
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