Development of a design process for a patient-specific temporomandibular joint prosthesis

The temporomandibular joint (TMJ) has multiple vital functions (e.g. chewing, breathing, sucking, swallowing,…) which are all derived from protrusion, retrusion, and lateralization of the mandible and opening and closing of the mouth. These are complex combinations of rotations and translations, which is a unique feature in the human body, as well as it being a bilateral joint acting as one unit. Temporomandibular disorders (TMDs) are common, partially because the TMJs are the most regularly used joints on a daily basis. However, not all TMDs need a total joint replacement (TJR), since they can often be treated using non-invasive or minimally invasive treatments. Pain, chewing disorders and speech disorders can sometimes not be treated with simple therapies or tissue-sparing surgeries, drastically affecting the quality of life. An end-stage TMJ could then be replaced by either an autogenous or an alloplastic (prosthetic) TMJ replacement. Indications for prosthetic replacement include TMJ ankylosis and end-stage joint diseases resulting from trauma, infection, degenerative arthrosis, cancer or surgery. The decision to replace the affected joint is based on the severity of the reduced quality of life, mainly related to food intake and pain. While stock prostheses may reduce pain and aid mouth opening (±1 cm), they do not naturally function in alignment with the healthy, contralateral joint because they have not been adapted based on the patient’s anatomy, nor do they allow for proper (lateral) grinding movements. This is because the lateral pterygoid muscle (LPM) is often sacrificed during condylectomy and not re-attached. To upgrade a TMJ stock prosthesis to an optimal joint, all physiological movements that are required for the abovementioned functions of a normal TMJ should be restored on both the replaced and contralateral (healthy or replaced) sides.

The first objective of this research was to develop such a novel prosthesis with state-of-the-art materials and features, such as a design based on the patient’s anatomy (patient-specific design) and the reintegration of the lateral pterygoid muscle. The materials currently used in the medical device industry were reviewed. Multiple options were considered and evaluated, out of which Ti6Al4V Grade 23 ELI and Crosslinked UHMWPE with Vitamin E were chosen as the best options available. A closer investigation of surface optimization methodologies also lead to the use of specific surface treatments which optimize tissue adhesion (in)to the implant and to the selection of a specialized coating to improve the wear resistance and to decrease the friction inside the joint.

Next, the features necessary for functional reconstruction were developed. The area of the implant touching the bone is shaped to perfectly fit the patient’s anatomy. But also, the patented re-attachment of the lateral pterygoid was implemented, together with general design features that improve the ease-of-use during surgery, and more importantly, the ideal outcome for the patient. The prosthesis’ functionality was further improved by a reverse-engineering tool that can semi-automatically design the fossa component based on the input of the desired post-operative mandibular motions for a specific patient. Moreover, a statistical analysis was performed to systematically assess the results of the abundance of research concerning osseointegration using the unique benefits of additive manufacturing of medical devices.

To validate the safety and the performance of the newly developed TMJ prosthesis, dubbed “TMJ Parametro” (CADskills BV, Belgium), an extensive computer analysis of its mechanics was performed. This analysis showed that the TMJ Parametro is a safe working implant, in which ingrowth would be able to occur over time. Furthermore, its behaviour during use was proven to be similar to that of a healthy reference person and in line with that of other patient-specific TMJ prostheses currently on the market.

Next, the wear was analysed by implanting the proposed prosthesis in a number of sheep. These animal trials showed that the prostheses exhibit acceptable wear resistance and the addition of the specialized coating significantly improved long-term condylar surface smoothness.

To improve the rehabilitation of patients that have undergone a total temporomandibular joint replacement, a novel device was developed which could aid the patient to exercise according to a specific protocol: by inserting custom TMJ exerciser bars into the patient-specific arch bars, rubber bands with different resistances can be applied, allowing the patient to vary the difficulty and type of exercise based on a prescribed training protocol. To evaluate the range-of-motion, a novel dynamic CT imaging protocol was developed to track a patient’s (post-operative) jaw movements.

Early in vivo results (1 year after surgery) of a small sample group of patients showed promising outcomes of the developed, novel prosthesis, including both high improvements in mandibular movements and decreases in pain scores.