## 4 Discussion  
First of all, there are several limitations in this study. First, this paper is based on a small group of patients, which might be biased on concluding some of the results. Further larger group of cohort studies may needed for significant conclusions. In addition, the heterogeneity of the underlying diseases may inherently introduce a degree of bias into the results. Conventional TMJ TJR procedures using autogenous materials, such as costochondral grafts, have been the gold standard for a long time. The benefits of using autologous graft materials involve its growth potential, biologic compatibility, and remodeling capacity. Thus, these procedures are expected in the treatment of TMJ ankylosis in young patients. Despite these merits, donor site morbidity and unpredictable growth have limited wider use of these grafts in other patients [3; 4; 5; 6]. To overcome the problems of the conventional autologous graft, various alloplastic TMJ TJR systems were developed. However, those early type of TMJ TJR were unsuccessful as the Vitek or Kent prostheses appeared to have issues with various foreign body reactions and mechanical fragility [12]. Recent technological advances have made the alloplastic TMJ TJR materials and procedures more reliable and a feasible treatment option for restoring TMJ functions in non-salvageLMJ conditions. Recently, the TMJ concept(r) (TMJ concepts Inc., Ventura, CA, USA) system and the Biomet Lorenz system have been popularized as standard systems representing customized and ready-made stock systems, respectively [11; 15; 22; 23; 25; 26]. Current alloplastic prosthesis systems in TMJ TJR have neither donor site morbidity, nor long-term temporary maxilla-mandibular fixation, reducing both operation time and hospitalization period. In addition, current TMJ TJR system offers the ability to correct facial asymmetry and early commencement of physical therapy for restoration of normal jaw function [27]. The TMJ concept(r) system as a 3D-printed customized alloplastic TMJ TJR system has many merits as compared to a ready-made one. It may be safer for the patient by the need for less bone removal and that it is easy to set the components in the physiologically appropriate position [28]. However, as our study showed, ready-made system also worked well without any major complications. Similarly, several studies comparing different TMJ replacement systems showed no relevant difference [24; 26]. From an economic aspect, as patient-specific custom system is too expensive and the Korean national health care system currently supports only the ready-made system, the ready-made system is a more practical choice for Korean patients [13; 29; 30]. Figure 4: Overlapped radiographic images (CBCT; frontal view) of the pre-operative and post-operative natural side of the condyle. The image was selected at the largest dimension of the condyle in frontal plane. (**A**) Pre-operative radiograph on the natural side of the condyle. (**B**) Post-operative radiograph on the natural side of the condyle. (**C**) Overlapped image (pre-operative: orange line, post-operative: blue line). Besides the cost issue of the device, in terms of longevity of the prosthesis, potential wear debris could aggravate destructive tissue reactions and affect finite durability of the prosthesis. Based on orthopedic experience, the desired lifetime of such a device should be at least 20 years [27]. Some patients in this study concerns about another replacement surgery in the future because they are still young. Therefore, they are instructed to do more care about chewing and clenching to avoid wearing the alloplastic TMJ TJR. One of the disadvantages of the current alloplastic TMJ TJR system includes the limitation of condylar movement with limited translational and lateral movements [2]. The current designs of stock alloplastic systems require significant amount of bone removal to fit the prosthesis.