Investigation of the morphology of longus colli among patients with neck pain using ultrasonography – a preliminary study

Objective: Longus colli provides stability to cervical spine during movements. In clinical practice, the clinicians teach exercises to longus colli to manage neck pain. The aim of this study was to investigate differences in thickness or anterior-posterior dimension (APD), shape ratio and contraction ratio of longus colli between subjects with neck pain and healthy controls. Materials and methods: A total of 12 subjects (6 with neck pain and 6 healthy controls) were recruited by predefined selection criteria. Ultrasound imaging of bilateral longus colli was conducted at the level C5-C6. Anterior-posterior dimension (APD) and lateral dimension (LD) were measured using Ultrasonography. Shape ratio (LD/APD) and contraction ratio (APD during contraction/APD during rest) were calculated. Mann-Whitney U test was used to compare the thickness of longus colli between healthy and neck pain subjects. Results and discussion: The results showed that the subjects with neck pain had smaller APD (Mean Rank=4.83, n=6) than healthy subjects (Mean Rank=8.17, n=6), U=16.00, z=-1.601 (corrected for ties), p=0.109, two tailed. Also, shape ratio showed higher values in neck pain subjects (Mean Rank=7.17, n=6) than normal subjects (Mean Rank=5.83, n=6), U=14.00, z=-0.641(corrected for ties) p=0.522, two tailed. In addition, contraction ratio was smaller in neck pain subjects (Mean Rank=4.83, n=6) than those healthy subjects (Mean Rank=8.17, n=6), U=8.00, z=1.601(corrected for ties) p=0.109, two tailed. Conclusion: There is a trend of reduced size of longus colli with the median of APD and smaller contraction ratio among patients with neck pain when compared to healthy controls.


Introduction
The cervical spine is surrounded by a complex arrangement of muscles that contribute to static and dynamic control of the head and neck. 1 Deep neck flexor provides stabilization of the cervical spine in anticipation of movements of the cervical spine. 1 Among the deep neck flexor muscles, the longus colli and longus capitis are considered as the primary stabilizers of the neck. 2 The longus colli muscle consists of three parts which originates from the anterior aspect of the vertebral bodies from C5 to T3, the anterior tubercle of the transverse processes of C3-C5 and the sides of T1-T3 vertebral bodies.3 The longus colli muscle inserts on the anterior sides of C2-C4 vertebraes, the anterior tubercle of the atlas and the transverse processes of C5 and C6. 3 The longus colli have a high density of muscle spindles, which are clustered and concentrated anterolaterally at the cervical spine.4 The dense spindles of longus colli are involved in enhancing stability of cervical spine as well as in maintaining cervical lordosis.4 Evidence had suggested that deep neck muscles were associated with neck dysfunction and cervical pain.5,6 Approximately 16.5% of women and 10.7% of men reported neck pain at least once a month.7 In cervical pain or neck pain, the superficial neck muscles masked the action of deep neck muscles in stabilizing the cervical spine. 2 This lead to inability of the deep neck muscle to counteract the loading imposed by cervical lordosis.8,9 A systematic review on low back pain speculated the possibilities of physical deconditioning and disuse of lumbar multifidus muscle.10 Hence, a similar pattern of disuse muscle atrophy and weakness of longus colli may exist among patients with neck pain.
Previous studies revealed reduced thickness of longus colli on ultrasound imaging.11,12 Other studies had reported structural changes and reduced size of deep neck muscle among patients with neck pain.12,13 The thickness of longus coli, which is also the anterior-posterior dimension (APD) is defined the inferior distance from the superior to inferior border of the longus colli.14 Meanwhile width or lateral dimension (LD) is defined as the distance from the lateral to medial border of the longus colli.15,16 Shape ratio which describes muscle shape is calculated as LD divided by APD (LD/APD).Shape ratio describes muscle shape.17 About the past studies conducted over longus colli muscle and neck pain, the researchers did not look into contraction ratio, APD and LD of longus colli during contraction.12,13,19 Till date, no studies had investigated the contraction ratio of longus coli among neck pain patients.However, a recent past study explored the contraction ratio of internal oblique and transverses abdominis among low back pain patients and concluded that contraction ration of the stabilization muscles were reduced among low back pain patients.18 Thus, the researchers in the current study developed the interest to investigate the contraction ratio values of the longus colli among subjects with neck pain.As longus coli is the muscle involved crucially in providing stability to cervical spine, the findings of the current study would help to interpret the stabilizing role of the muscle in providing stability to cervical spine among subjects with neck pain.Therefore, the current study measured the APD and LD of longus colli during rest and contraction using real time ultrasound imaging.
Nowadays, ultrasound technology is widely used, both in the aspect of diagnostic and rehabilitation purposes.6 Ultrasound is cost effective in assessing muscles and related soft tissue morphology, 6 in the measurement of muscle size of neck extensor muscle, 20 to visualize neck muscles and to obtain thickness values at rest and during different states of contraction for cervical multifidus.17 Study reported that ultrasound has high reliability in taking and interpreting images of longus colli.17 The objective of the current study is to compare the longus colli between healthy and neck pain subjects during rest and contraction.Thus, contraction ratio of the longus colli can be calculated.This study hypothesized that APD, LD and contraction ratio of the longus colli were significantly reduced in neck pain subjects when compared with matched healthy controls.

Subjects and Demography Data
This is a pilot cross sectional study conducted among a total of 12 subjects (six healthy subjects and six neck pain subjects).Subjects with bilateral neck pain were recruited at the physiotherapy department, from university teaching hospital.The subjects with neck pain were included if they were presented with neck pain of more than three months, a history of cervical disc herniation, cervical spondylosis, cervical spine fracture and post surgical conditions of the neck.Subjects were excluded if they presented with lower back pain, migraine and neck pain less than three months for neck pain group.The healthy subjects matched with age and gender was recruited from the volunteers who accompanied the patients to a university teaching hospital.The subjects completed the written informed consent form.Demography data including name, sex, height, weight, medical history and duration of pain were documented.The research was approved by ethics committee.

Ultrasonography Protocol
This study followed the protocol developed by Javanshir et al to measure the thickness of longus colli using ultrasonography.13 The muscle thickness of longus colli was measured by a real time diagnostic ultrasonography (Philips HD 11 system U/S scanner (Koninklijke Philips Electronics, Netherlands) using 12MHz, B mode, linear probe).Subjects were in full crook lying on an examination table with both arms crossed and head in neutral position.A folded towel was placed under the occiput, so the head lay 3-4 cm higher than the examination table.The top of the thyroid cartilage were palpated, 2 cm below this level was marked.The transducer was placed perpendicular to the vertical axis of the neck.The transducer was moved laterally 1 cm for each side.The image of longus colli were scanned and taken at the C5-C6 level.13 There was no overlap between the longus colli and the longus capitis muscle, as the longus capitis muscle runs more laterally and attaches to the anterior tubercle of C6. 19 The measurement was taken for right and left longus colli because previous study reported neck pain was frequently presented bilaterally or in the dominant arm.21 The thickness or APD, width or LD of the longus colli were identified and assessed as the superior to the inferior border for APD and width of the lateral to medial border for LD. 14 The subjects were asked to do cranio-cervical flexion movement to engage contraction of longus colli and three measurements were performed.Figure 1 indicates the anterior-posterior and lateral measurements of longus colli.The average of the three measurements was taken as final measure for data analysis.

Statistical analysis
Data were analyzed with Statistical Package for the Social Sciences version 17.0.Means and standard deviations were calculated.The skewness and kurtosis values revealed that all the data were not in normal distribution.The differences in the thickness and contraction ration of longus colli between healthy and neck pain subjects were assessed with Mann-U Whitney test.Cohen'd' effect size was calculated to understand the clinical effect of the study findings.Level of significance was set at p value less than 0.05.

Results
The demographic characteristics of the study subject are indicated in (Table 1).A total of 12 subjects (six neck pain and six healthy subjects of matched control) participated in this study.Ten subjects were right handed and two were left handed.There was no significant difference in the demographic characteristics of the subjects with neck pain and healthy controls.

Comparison of Longus Colli Muscle Size between Healthy and Neck Pain Subjects
The median values (M) and interquartile range (IQR) f of the thickness and contraction ratio of longus colli size were shown in (Table 2).The result indicated that APD during contraction for longus  were lesser than those of the healthy subjects (Mean Rank=8.17,n=6), U=16.00, z=-1.601(corrected for ties), p=0.109, two tailed.Although the above finding is not statistically significant, the observed effect size can be described as "medium" (r=0.462).
Similarly, the measurements of LD during rest and contraction of longus colli in neck pain subjects (Mean Rank=7.17,n=6) was higher than those of the healthy subjects (Mean Rank=5.83,n=6), U=14.00, z=-0.641(corrected for ties), p=0.522, two tailed with low effect size (r=0.185).
The Mann-Whitney U test was used to compare longus colli muscle between healthy (N=6) and neck pain subjects (N=6) during rest and contraction and the results are shown in (Table 3).Furthermore, results from Mann-Whitney U showed that shape ratio of longus colli during rest among neck pain subjects (Mean Rank=7.17,n=6) was higher than those of the healthy subjects (Mean Rank=5.83,n=6), U=14.00, z=-0.641(corrected for ties) p=0.522, two tailed with low effect size (r=0.185).Similarly,Mann-Whitney U indicated that shape ratio of longus colli during contraction was higher among neck pain subjects (Mean Rank=8.17,n=6) when compared with healthy subjects (Mean Rank=4.83,n=6), U=8.00, z = 1.601 (corrected for ties) p=0.109, two tailed.The observed effect size for this finding is described as "medium"(r=0.462).

Discussion
The current study investigated the changes in APD of the longus colli among patients of neck pain and healthy subjects during rest and contraction.Prior to the commencement of the data collection, the intra rater reliability of the procedure to measure thickness of longus colli was conducted.The reliability test showed excellent reliability for APD (ICC ranging between 0.86 and 0.97) and fair to excellent reliability (ICC ranging between 0.46 and 0.81).The reliability result for the longus colli measurement suggested that ultrasound could be a reliable tool for longus colli imaging.13 In this current study, APD was measured at rest and contraction.Deep neck flexor was shown to be more active in contraction to provide stability to cervical spine.12 Furthermore, increased size of APD was reported in cervical multifidus among asymptomatic populations during contraction (rest=0, 25% load= 8.11 ± 4.98, 50% load= 13.52 ± 8.23).9 Similarly, in the current study, the size of APD was shown higher in healthy subjects during rest and contraction.
Reduced in APD size might indicate muscle atrophy.12 Previous research suggested natural ageing process might be associated with physiological ageing of the cervical spine.22 However, the mean age of neck pain subjects for the current study were 42.17±8.11(Mean±SD)years.Hence, in this study, reduced size of APD might be as a result of structural changes in neck pain patients, but not due to physiological ageing.In this study, LD of the longus colli was measured to estimate the shape ratio as the size of LD was higher in neck pain patients than healthy subjects.This increased of LD was also reported earlier.The current result showed the neck pain subjects exhibited a similar reduced pattern of reduced contraction ratio.There were no other reported studies that had investigated the contraction ratio among neck pain patients.As this is the first study that had investigated contraction ratio among neck pain, the current study findings were not comparable with past studies.However, the clinical finding of reduction in contraction ratio was in line conceptually with the similar finding observed among patients with low back pain.25

Limitation of the study
The major limitation of the study is the small sample size.However, this study was structured as a pilot study; keeping the fact in mind that contraction ratio was attempted first time among neck pain patients.
In the future, this study should be conducted with a larger sample size.Secondly, the subjects did not receive any familiarization in cranio-cervical flexion movements which was used to contract deep neck flexor.Hence, there are possibilities that the subjects used the other muscles around the neck.Future studies may consider to familiarize the cranio-cervical flexion movements and to train deep neck flexor contraction prior to data collection.

Conclusion
This preliminary study shows a trend of reduced size of longus colli and smaller contraction ratio among patients with neck pain when compared to healthy controls.Further study with greater sample size is warranted to confirm the pattern of structural changes in longus colli among neck patients.
Contraction ratio is calculated by muscle thickness during contraction divided by muscle thickness during rest.It indicates whether an individual can sustain a stable phase during contraction.18

Table 1 :
Demographic characteristics of the subjects in mean and standard deviation (n=12)

Table 3 :
Comparison of anterior-posterior dimension (APD), lateral dimension (LD), shape ratio and contraction ratio of longus colli size between healthy and neck pain subjects The study done by Javanshir et al. also showed reduction in APD values among neck pain patients.However, studies studies done by Javanshir et al. measured APD during rest only.11,