## 1 Introduction  
Surgical implants and artificial joint prostheses are commonly used in the treatment of fractures and joint diseases. However, patients receiving such therapies frequently develop joint contractures. It is believed that deep heating procedures are helpful in such circumstances [1]. Unfortunately, the use of short wave and microwave diathermy, in most cases, would result in overheating in the vicinity of the implants [2]. As a result, ultrasound diathermy has become an alternative for deep heating. Ultrasound produces heat through molecular vibrations at high frequency, which conduct or propagate energy throughout a medium. Such energy transmission in ultrasound treatments is minimally hindered by adipose tissue and has many benefits [3; 4]. Therapeutic ultrasound can be utilized as either a superficial or a deep heating modality depending on its operation frequency and power. In general, 3 MHz ultrasound is often adopted in superficial heating within 3 cm while 1 MHz ultrasound can be used to heat tissues at depths of 3-5 cm and is considered a deep heating agent [4]. In clinical applications, therapeutic ultrasound is used as a deep heating agent for the treatment of various musculoskeletal disorders [5; 6; 7]. The biophysical effects of therapeutic ultrasound on tissuesresult from two mechanisms: (1) thermal effects due to continuous acoustic wave, and (2) non-thermal effects due to pulse acoustic wave [8]. However, there are few studies [9; 10; 11; 12; 13] that take into account the presence of implants during ultrasound diathermy and discuss the influence of implants on the temperature field in the tissues near the implant. Early research [9; 10; 11] considered the possibility that ultrasonic energy was reflected at the metal implant. Such reflection could result in the focusing of ultrasonic intensity in such a fashion that \"hot spots\" could develop, resulting in overheating of the tissues. In vitro experimental results demonstrated that a large amount of ultrasonic energy is reflected at the tissue-metal implant surface [9]. The increase of intensity in the focal area has been measured and found to be appreciable. However, the increase in ultrasonic intensity does not lead to a selective temperature rise in the focal area or in the standing waves, since the presence of the metal implant, which has a higher thermal conductivity than that of the tissue, produces a markedly increased heat loss [10; 11]. These results suggested that it was possible to use ultrasound as a deep heating tool for tissues even with the presence of metallic surgical implants. Alternatively, [PERSON] et al. [12] conducted a series of in vitro experiments using a tissue-mimicking hydrogel phantom to investigate the influences of an implant on the temperature field in the composite system during ultrasound thermal therapy. The temperature history curves from these experiments revealed that a polyethylene implant led to a higher temperature in the hydrogel phantom as compared to the metal implant. However, the standing wave pattern in front of the implants was not observed in the transparent hydrogel phantom. In addition, some in vitro experiments showed that the intramuscular temperature was significantly higher (\(p<0.05\)) in groups with a presence of a metal plate [13]. From the literature review, some uncertainties remain about the effect of implants on ultrasound thermal therapies. With rapid progress in material science and manufacture technology, the surgical implant materials have improved. Advancements and developments in both the qualities and quantities for implant materials have promoted the salutary treatments very promising in recent years. In addition to metals [14; 15], a variety of polymers has been used as surgical implants or artificial joint prostheses [16; 17]. Recently, newer biocompatible materials such as zirconia, roxolid, surface-modified titanium, and hydrogels have also shown potential as implants [18; 19; 20].