## 1 Introduction  
The temporomandibular joint (TMJ) often loses its function due to various causes. In severe and non-salvageable cases, it often needs total joint replacement (TJR). Numerous procedures for TJR of the TMJ (TMJ) TJR) are performed with either autologous or alloplastic materials with varying degrees of success [1, 2]. The costochondral graft is popular as a conventional TMJ TJR procedure, which is standard in young patients [3, 4, 5, 6]. Historically, alloplastic TJR has been an effective and reliable treatment option in orthopedic surgery [7, 8]. For the TMJ, similar alloplastic TJR systems were introduced for the surgical management of end-stage TMJ diseases. Furthermore, recent evolution of imaging technologies have made this procedure more reliable and durable as a treatment option for restoring TMJ functions [9, 10, 11]. In the past, the use of alloplastic materials for TMJ TJR was avoided due to various foreign body reactions to the Vitek or Kent prostheses [12]. To overcome these problems, progressive approaches, such as the TMJ Concepts system and the Biomet Lorenz system, were developed and approved for TJR of TMJ by the Food and Drug Administration (FDA) in the United States. The Korean Ministry of Drug and Safety also approved the use of the Biomet Lorenz system for TMJ TJR. To date, this is the only approved system for TMJ TJR in Korea [13]. The Biomet Lorenz system is composed of a glenoid fossa and a ramus condylar component. The glenoid fossa component is formulated with ultra-high molecular weight polyethylene and the ramus condylar component is constructed from a cobalt-chromium-molybdenum alloy. The glenoid fossa component is fixed into the bony glenoid fossa with several positional titanium screws. Clinical indications for TMJ TJR have been suggested and revised according to development of the system of TMJ TJR [14; 15; 16]. The success and longevity of an alloplastic TJR of the TMJ are directly attributed to stability in situ of the prostheses, biocompatibility of the prostheses, mechanical, and the reasonable indications of the surgery [17]. A predictable and satisfactory outcome of the reconstruction can only be expected under the appropriate diagnosis and indications. Mandibular movement, as centered by the TMJ, is delicate and complex and can be only achieved by sophisticated and harmonious bilateral movement. Therefore, an unbalanced condition between the TMJs may cause harmful effects, resulting in degenerative changes in either joint. Different from the natural TMJ, the rotation center of an alloplastic TJR system is lower to compensate for the natural translational movement of the condyle, because it only allows rotational condyle movement. Lateral movement of the alloplastic TJR side is also limited, as the responsive muscles cannot attach to the artificial condyle and the condylar neck area. Therefore, unilateral alloplastic TJR of TMJ may cause unexpected degenerative changes of the natural joint side and/or adverse effects of alloplastic joint side, because physiological actions at both condyles are quite different during mandibular movement. However, only a few studies have assessed the effects of unilateral TMJ total joint replacement. [PERSON] et al. [18] and [PERSON] et al. [19] evaluated patients requiring unilateral TMJ reconstruction with a patient-fitted total joint prosthesis and the risk for development of post-surgical contralateral TMJ pain and dysfunction leading to total joint reconstruction. [PERSON] et al. [20] investigated stress distribution on the TMJ prosthesis and contralateral natural TMJ with finite-element analysis. These studies examined patient-customized prostheses for TJR of TMJ. Ready-made stock prostheses are generally affordable to patients rather than patient-customized ones although they provide limited sizes and shapes for correction of defects.