The Effect of Traction Position in Cervical Traction Therapy Based on Dynamic Simulation Models

This study describes the development of a cervical traction therapy simulation model that evaluates two types of the traction positions, namely the sitting position and the inclined position. An anatomically correct human skeleton model and two mechanical traction device models were constructed in simulations using a physics engine. The anterior and posterior intervertebral separations were measured at both positions with a series of traction forces (60N to 200N) and traction angles (10°  to 40° ). The result suggested that the sitting position caused the subject to lean forward and as a result led to excessive anterior compression when traction angle is over 20 degrees. The inclined position creates greater intervertebral separations on both the anterior and posterior sides than the sitting position. This suggests that the inclined position may be more effective in increasing intervertebral separation than the sitting position.


Introduction
Neck pain is a very common problem in the general population. In the "The Empowerment of People with Neck Pain: Introduction", Haldeman et al. [1] states that "most people can expect to experience some degree of neck pain in their lifetime". According to Judovich et al. [2], neck pain accounts for 15% of all soft tissue problems and 26% to 71% of the adult population experienced an episode of neck pain or stiffness in their lifetime. In an extensive literature review "The Burden and Determinants of Neck Pain in the General Population" [3], it was suggested that about 30% -50% of the general adult population experience neck pain that lasts for 12 months. There are many options that can help ease the pain on the neck. Cervical traction therapy, also known as non-surgical spinal decompression therapy, is one of the possible treatment options frequently included in rehabilitation programs.

Cervical Traction Therapy
Cervical traction therapy refers to any medical procedure that applies force along the inferior-superior axis of the spine to extend the cervical spine vertebrae. Its purpose is often to straighten the back, to relieve pressure on the spine and to increase blood flow to the injured area. For decades, traction therapy has been widely employed in nonsurgical therapies and rehabilitation to treat chronic neck pain caused by herniated discs and other injuries at the cervical spine region.
Cervical traction therapy can be mainly divided into manual traction and mechanical traction. A manual therapy is performed by a trained physical therapist or chiropractor with the patient usually lies flat in bed. In a mechanical traction therapy, the patient's head is attached to a head halter and is gently pulled by a motorized traction machine at a specific degree. Traction position varies in mechanical traction. In this case, the patient can sit upright on a chair at 90˚, referred to as the sitting position, or at an inclined angle, known as the inclined position.
Cervical traction therapy has been widely adopted in clinics and rehabilitation centers. Over the year, many studies have demonstrated its positive effects on cervical and lumbar spine-related pain [4] [5] [6]. However, some review studies [7] [8] [9] pointed out that further research is needed, since there is not enough conclusive evidence to fully support the contribution of the therapy. Therefore, it is important to conduct a qualitative study on the mechanics of the therapy.

Dynamic Simulation
In recent years, along with computer technology advancement, multi-body dynamic simulation engines are now able to realistically simulate real world mechanical systems based on the laws of physics. These advanced physics-based models helped researchers to generate animated simulation of physical events with high degree of precision and flexibility [10] [11]. Physics-based modelling is particularly popular among researchers as it allows the user to interact with the model in real-time with the virtual world and to adjust the various mechanical properties of the model to match the target test subjects. Several studies [11] [12] [13] have developed multi-body cervical spine to study the human head-neck response to impact loading.

Purpose of This Research
The purpose of this research is to better understand the complex cervical spine biomechanics and its behavior during a cervical traction therapy. In order to ve-rify the efficacy of cervical traction, it is necessary to first identify the various factors that cause intervertebral separations between the C2 and C7 vertebrae in a cervical traction therapy. While previous studies have examined how different traction angles and forces affect the efficacy [14], it is important to point out that traction position also plays an important role in the efficacy of cervical traction due to the mass distribution of the body. The two most common traction positions are the sitting and supine positions. Previous research studies [15] [16] have evaluated the two positions and concluded that supine position is more ef-

Cervical Spine Model
The cervical spine component was created based on an anatomically correct human skeleton 3D model retrieved from BodyParts3D/Anatomography [17] and it consisted of 8 rigid parts: a human skull and 7 pieces of the cervical vertebrae. In order to minimize processing load in the simulation, all the bones in the skull were combined to create a simple rigid part to represent the skull. All the bones that made up the cranium and the facial area were combined into one rigid body. The cervical vertebrae (C1-C7) were modeled separately and each cer- The positions of the sensors are shown in Figure 1. Once the sensors were attached to the designated positions of the vertebrae, they moved along with the vertebrae. By simply measuring the distance between sensors, the separation distance between each vertebra could be acquired.

Human Skeleton Model
The human skeleton section of the model included the rest of the body below the cervical spine, and was modelled as separated rigid bodies. Similar to the cervical spine, all the 3D models used in this section were retrieved from BodyParts3D [17]. Each part was imported into 3D modelling software [18] for scaling and re-positioning with the traction equipment. The topmost part of the trunk was cut off at the first vertebra of the lumbar spine (T1) and this would serve as a support for the lowest part of the cervical spine (C7). The two arms were at-  The body segment mass data was configured based on the data from Zatsiorsky et al. with adjustment made by DeLeva [19]. The human body model was 1.74 m tall and weighted 73 kg. The mass of each bone was calculated based on the percentage listed in the body segment parameter data and is shown in Table   1. The combined head and neck was estimated to be 5.07 kg. It is worth noting this value includes the brain, the cervical spine and all the tissues around the neck.

Results
Simulation runs were performed using combinations of traction angles, forces

Intervertebral Separations vs. Time
The changes of the anterior and posterior separations are shown in Figure 4 and  Figure 8 shows the segmental separations, from C2 to C7 on the anterior side. The largest separations were at 10˚ and they gradually turned to negative as the traction angle increased. In all cases, the segment C5-C6 was extended the most, followed by the segment C4-C5 and then C6-C7. Overall, the inclined position achieved larger separation and lower compression. The posterior measurements are presented in Figure 9. Again, the segment C5-C6 achieved the highest separation, followed by C4-C5 and C6-C7. The inclined position is found to achieve larger separations than the sitting position. Furthermore, at 40˚, the segment C6-C7 in the sitting position achieved the smallest separation.

Traction Angles and Traction Forces
The separations caused by a combination of traction angles and traction forces were compared. Figure 10 shows the anterior separations in the sitting and inclined positions. The negative separation, which indicates that the spine was compressed, can be found in both the sitting and inclined positions. The cervical spine was always compressed further in the sitting position in all combinations of angle and force when compared to the inclined position. Figure 11 shows the posterior separations. On the posterior side, the inclined position was able to achieve a larger separation than the sitting position. When the traction force was small, the sitting position recorded a negative separation in the 30˚ and 40˚ cases.

Discussion
The small dips in the sitting position measurements in Figure 4 and Figure 5 are likely related to the forward leaning motion of the body during the traction therapy. In the simulation, we observed that when the subject's head was pulled by the halter, the body stayed in the chair due to the friction force between the lower body and the chair. At this moment, the hip turned and the upper body gradually leaned forward as the traction force increased. The motion stopped when the hip joints reached its limiting angle and could not rotate further.
However, the neck continued to bend and created a compression force on the anterior side of the cervical spine. On the other hand, in the inclined position, with the subject resting on a chair at an angle, gravity helped to keep the back of the body remain in contact with the chair. The hip was never turned and the neck did not over bend as in the sitting position. This observation also agreed with a previous study [21], which concluded that anterior separations become negative when traction angle goes beyond 20˚. Regarding the segmental separations in Figure 8 and Figure 9, the segment C5-C6 was extended the most, followed by the segment C4-C5 and then C6-C7. These results also agreed with a previous study [21]. The weight of the head and the halter may contribute to the negative separation in the 60N case in Figure 10 and Figure 11. In the sitting position, the head and the halter, with a combined weight of 5kg, exerted a constant downward force to the cervical spine. Since the traction force was at only

Conclusion
In