This study is the first to describe the objective resistance provided by the ARK-JSD, supporting its use as a low-cost alternative to commercial trismus devices and an evidence-based alternative to stacked tongue depressors. The forces generated by the ARK-JSD all fall below the reported fracture threshold load for a healthy mandible (431–725 N) [11] and below normal bite forces tolerated by healthy dentition (83.9–1642.8 N) [12]. The amount of force exerted was much higher when the ARK-JSD was in the closed position, where the IID was low, and got lower as the IID increased. The force applied by stacked tongue depressors and ARK-JSD is concentrated over a smaller area than TheraBite®; however, the capacity to manipulate the position within the mouth is a distinct advantage of both of these rehabilitation options. Whilst the ARK-JSD is safe to use in a healthy population, the mandibular fracture and dental damage threshold for those recovering from head and neck cancer treatment are unknown and may be considerably lower. We have found that there are several key device reliability factors for clinicians to be aware of. The first is that there is clinically meaningful variation in the maximal force provided for each level of resistance. This may impact the amount of stretch that a patient is able to achieve and impair rehabilitation when swapping between devices, either due to device failure or when progressing between different levels of resistance. The second is the loss of force over time, with the hard ARK-JSD losing over 5.5 N. This is important given that stretching and strengthening generally require force to increase over time for clinical gains.

The ARK-JSD is an affordable trismus device that offers passive and active stretching exercises at three different levels of resistance. It was created to address some of the major barriers to accessing commercial trismus devices such as cost. Whilst existing literature supports the use of commercial trismus devices (e.g. TheraBite®) for people recovering from head and neck cancer [8], access to best-practice care is not always affordable or available. The ARK-JSD design is similar to the Engstrom device. The Engstrom device has been evaluated in comparison to TheraBite® and found to improve mouth opening by a mean IID increase of 6.4 mm [13] and 6.2 mm [14] over a 10-week period. Whilst ARK-JSD has not been evaluated in a prospective trial, the added benefit of three resistance levels that allows for the progression of exercise difficulty is promising. Its low cost and ease of construction using readily available materials make it accessible to patients regardless of socio-economic status, a critical factor lacking in most commercially available trismus devices.

Safety is a primary concern for medical devices, and there is a paucity of data regarding the force that jaw-stretching devices are capable of exerting. This is particularly important in devices, such as the ARK-JSD that rely on a resistance mechanism that is not controlled by the user. Hence, it is the responsibility of the prescribing clinician to understand the maximal force that the device can potentially deliver. Variables such as treatment modalities, age, smoking status, medications, and bone density will lower the dental or jaw fracture threshold, and, in most cases, the ‘safe’ force that can be applied to a jaw following head and neck cancer is unknown [15]. There are currently no simple methods to determine the safe force threshold based on individual patient factors. Instead, trismus therapy replicates guidelines from static stretching literature where the patient stops the intensification of stretches when (or ideally prior to) experiencing pain. In patients recovering from head and neck cancer treatment where sensation is often compromised, the pain signal for force overload may be impaired or absent thus increasing the risk of fracture. Whilst finite element modelling (FEM) based on individual computerized tomography (CT) data could be used to quantify fracture thresholds, this technology is not clinically available, validated, nor practicable.

The ARK-JSD is not designed to, nor would it be expected to meet the durability of commercially made devices. However, whilst the construction protocol was followed for assembling each ARK-JSD unit, there was substantial variation in the maximal force for each resistance level. The greatest variation was observed in the hard ARK-JSD (24.7–48.8 N) which overlapped with the medium frame. Construction variables include how tight a clinician wraps the tape around the device, the presence of any splintering of the tongue depressors in the process of construction, and the position of the wedge within the outer frame (Fig. 5). This variation may affect the ability of patients with trismus to continue to improve their IID over time and should be factored into any rehabilitation plan or if this device was subject to a clinical trial. Despite the variation, there were statistically significant and clinically meaningful differences in the resistance provided by the easy, medium, and hard ARK-JSDs, with sufficient within-level consistency to be effective in most clinical scenarios (Table 1).

There were several limitations inherent in this study including potential inconsistencies in device assembly and limited sample size, and that cyclic testing was not performed on all devices. Furthermore, there is no data regarding what constitutes a clinically meaningful difference in force for trismus therapy. The authors selected 5 N based on their clinical experience; however, this value requires validation. Determining the minimum amount of force change that is discernible in trismus and healthy populations would serve to improve the validity of these findings.