A Simulator Design for Manipulation Training of Cervical Spine

Background

Physiotherapists receive long-term and detailed training to develop their skills on treatment methods of cervical musculoskeletal impairments. The most important reasons for the long training period are the presence of vital organs in the region and the widespread use of manual therapy (MT) techniques in the cervical spine. MT interventions are comprised of externally applied passive movements for facet joint alignment and soft tissue mobilization. The purpose these interventions are to archive a clinically significant decrease in neck pain and an increase in neck mobility. The passive movements with slow velocity are called mobilization techniques. Usually, mobilization techniques are easier to learn and do not have any important risk of complications.

The interventions applied to adjust the involved cervical segment, with a single high velocity and low amplitude thrust are known as manipulation techniques. Manipulation of the cervical spine (MCS) has advantages over conventional physical therapy modalities in the immediate increase in the range of motion and decrease of symptoms (headaches, stiffness) despite the risk of complications in a broad spectrum from simple spasms to arterial dissection (may cause quadriplegia or death). The presence of serious risks, increase the course duration of manipulation techniques relative to other techniques.

During the long training period, physiotherapy students study on each safety component (extrinsic factor) of their manipulation skills; the appropriate pre-manipulative position of the patient head and cervical spine, the angular displacement of the head and each cervical segment during the manipulation and appropriate thrust velocity. If there is a mismatch between extrinsic factors and the patient’s condition, MCS can cause adverse effects or complications instead of the desired results. In the classical MT training, students can only have subjective feedback; manual or verbal instructions of expert practitioner about their practices. The numerical amplitude of extrinsic parameters are unknown. This is the main limitation of the classical training model and there is a need for measurement, demonstration and verification of extrinsic parameters in an objective way. Also student physiotherapist develop their manipulation skills on each other before real patient experience. Even if they are healthy and classmates, first manipulation experience on human may exaggerate unreal perception of students about risk factors; baseless negative feedback of their classmates may sometimes contribute to distrust in themselves. This way of thinking, can lead student physiotherapist to avoid developing their manual skills on each other.

Patient simulators are commonly used in medicine and nursing for demonstration of normal physiological functions, pathological conditions and skill training before real patient experience. However, there are only experimental simulator designs in the literature for MT training. There is a need for fully functional systems for classroom settings which can help physiotherapy students to gain experience before performing techniques on healthy volunteers, malingerer or real patients. In Turkish physiotherapy practice, Orthopedic Manual Medicine is the most common and conventional MT method which was first introduced by Dr. James Cyriax in 1954. All MCS techniques of Dr. Cyriax can be reduced to different variations of two skill components which can be measured by transducers. These two parameters are traction force and angular displacement of head and neck which should stay in physiological limits for patient safety. Excessive traction force or excessive angular displacement of spinal segments in one or more directions can cause complications. Analysis of raw data to determine whether the limits are exceeded or not can give objective feedback about the safety of manipulation. The purpose of this study is to design and manufacture an objective skill assessment simulator for training of cervical spine manipulation skills according to Cyriax.

Methods

Traction Force and Angular Displacement Feedback System

A suitable traction force and angular displacement feedback system for Dr. Cyriax’s manipulation method is configured as a classical biophysical signal acquisition and processing system which consist of a traction force transducer; a traction force amplifier; an angular displacement transducer, a data acquisition device, and a data processing device.

Dr Cyriax had classified the amplitude of spinal traction force in a subjective way (Grade A, B, C) and there is insufficient data in the literature for magnitude equivalence. Mechanical traction systems are alternate methods of manual traction and 10% - 15% of patient’s body weight or traction forces between 2. 27-8. 14 Kgf (5-40lb) are used in clinical practice. S-type load cells are suitable for mounting on a rotating surface and can measure traction force. A 500N HC-C3 S-type (Zemic, Etten-Leur/ Netherlands) load cell is used as a force transducer. The transducer range is selected over the clinical range (200N) for excessive traction force simulations. A bar graph threshold option is included in the software for normal traction range guidance. Load cell signal is amplified by an instrumentation amplifier (LT1167) with 500x adjusted gain. Load cell calibration is done by M2 class reference mass set.

Dr Cyriax described the range of angular displacement by physiological barrier which is a subjective range between the anatomical barrier and the articular luxation. In an electro-goniometric measurement study physiological cervical rotation range is reported between 70° and 90°. This range was future be validated by using a motion capture system on real patients for an alternate manual manipulation technique. A single turn 5V/360° analog encoder, Opkon MRV-50 (Opkon Electronics İstanbul/Turkey) is used to measure the angular displacement. The transducer range is selected over the clinical range for simulation of excessive rotational displacement. Also a physiological limit guidance is provided by adjustment screws which can limit rotation in reported range for both sides.

USB-1608FS DAQ card (Measurement Computing (MCC), Norton/USA) is used for traction force and angular displacement data acquisition.

The device was constructed by using CNC machine separate elements (ball bearing, bearing housing, connector shell and shafts), a head model (Enas CPR Prompt, Wisconsin/USA) and cervical vertebra models (3B Scientific A72, Budapest/Hungary). All elements are mounted on a stainless steel base. Two adjustment screws with rubber pads were placed to proximal side of the base to limit rotation as in physiological range.

Software

The software is developed under C# by using MCC Universal Library for users who are not familiar with technical computing. A/D input channels are read by AIn function and returned 16-bit integer count values are converted an equivalent single precision voltage value by ToEngUnits function. Sampling rate was set to 1KS/sec. Raw voltage measurements are converted to force and angular position data by calibration equations.

Final data is presented as xy line plot or bar graphic. XY line graph format is suggested for general skill pattern demonstration and analysis. Bar graph format is especially designed for physiological limit training sessions. Save graphic, save data and generate report functions are included for storage. The software is tested with the reference software Tracer DAQ 2. 3. 0 (MCC USA) for reference weights and angular positions until the same results achieved.

Skill Assessment

Software supports peak measurement results, generates a report with graphic and peak values for fundamental skill assessment. Additionally maximum traction zone, rotation zone and intersection zone markers are used for brief visual definition of skill deficits. Maximum traction zone is a horizontal rectangle at the level where traction force makes a peak and remains constant. Rotation zone is a vertical rectangle which represents the duration of rotation. The intersection of both zones, identifies the critical zone where any mistakes may cause complications.

Results

Force calibration was performed between 0-50 Kgf by using M2 class reference mass set. All data were analyzed by using linear regression. The linearity of force transducer is R=0. 995 and SEE=0. 910 is in the range. Angular position sensor has factory calibration. It has 10 bit, 5V/360° resolution.

Working Principles

Traction force on the head piece triggers the load cell, the output signal increases in relation to traction force. Then poor output signal is amplified 500 times and acquired by DAQ-Card.

Center of mass puts the head piece in a natural 90° (1. 25 V) position. Baseline can be adjusted by the software. At the end of traction, rotation of the head triggers rotary encoder. The output voltage increases for right side and decreases for left side. The output signal has adequate magnitude and directly acquired by the DAQ-Card.

Software Interface

Software interface consist of two group boxes and one graph area. DAQ-Card, acquisition and assessment operations can be performed by using controls in the group boxes. Peak and current value are shown by legends. Values of desired points can be accessible by mouse clicks. Skill report, data and graphic export functions are included.

Controls and graphic presentations; (a) xy line pilot for general skill assessment (b) bar graph for physiological threshold identification. Red line/bar represents force and blue line/bar represents angular displacement.

MCS performance data in normal physiological ranges and skill deficits samples are presented in figure 5. The ideal magnitudes of maximum traction zone, rotation zone and intersection zone is seen in figure 5a. MCS out of physiological limits and other skill deficits can be seen as; absence, level wedge or expansion and contraction of zones.

1. Demonstration of MCS in normal physiological ranges.
2. At the beginning of rotation, the practitioner pulls off the head and decreases traction force. This may cause spasm or more serious situation.
3. The practitioner begins at an insufficient steady state level of traction force. This practitioner may have fear of harming the patient.
4. The practitioner begins rotation as the traction force continues to increase.
5. The practitioner exceeded the physiological limit of traction.

Discussion

The purpose of this study was to develop an objective skill assessment simulator to overcome difficulties related to subjective skill assessment in MCS training. The final prototype is capable to acquire the traction force and angular displacement data from a human like mechanical system. The simulator provides the numerical magnitude and graphical presentations of two significant quantities for patient safety.

The skill deficits related to timing, traction force magnitude and angular displacement can be explainable and compared by using zones. This simulator can be used in various ways; to demonstrate skill pattern of trainers, to compare student trials with demonstrated pattern and to develop student improvement reports.

There are more than 50 physical therapy schools in Turkey and Orthopedic Medicine (Cyriax’s Concept) is a part of their curriculum. Physical therapy students first meet manual therapy by Cyriax’s Concept and generally they use it in their early clinical practices. The simulator can also help students to develop experience for future patient experience and overcome fear of harming the patient.

Future studies may focus on effect of simulator on training, improvement of simulator for physiological or biomechanical responses of cervical spine in healthy and pathological conditions and new simulator designs for other manual therapy concepts.

Conclusions

The purpose of this study was to develop a simulator which can give objective feedback. The current design provides the magnitude of traction force and angular displacement and enables objective pattern analysis. Future studies are needed to determine the effect of simulator on training and feature improvement of the simulator.