Ann Thorac Surg 2007;83:1513-1516
© 2007 The Society of Thoracic Surgeons
New Technology
Measurement of Non-Physiological Movement in Sternal Instability by Ultrasound
Doa El-Ansary, BAppSc (Phty)a,*,
Gordon Waddington, PhDa,
Roger Adams, PhDb
a Department of Physiotherapy, School of Health Sciences, University of Canberra, Canberra, Australia
b School of Physiotherapy, Faculty of Health Sciences, University of Sydney, Sydney, Australia
Accepted for publication October 23, 2006.
* Address correspondence to Mr El-Ansary, c/o Roger Adams, Faculty of Health Sciences, School of Physiotherapy, University of Sydney, 2006, Australia (Email: doa.el-ansary{at}canberra.edu.au).
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Abstract
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Purpose: Sternal instability, a complication arising for some patients after sternotomy for cardiac surgery affects their later quality of life and cost of care. However, there are currently few guidelines for its diagnosis, quantification, and monitoring. Ultrasound equipment with associated software for calculating selected video-monitor distances provides one way of quantifying the extent of sternal separation.
Description: This study evaluated the validity and reliability of an ultrasound measurement made by attaching the head of the unit to an extensible stand. First the procedure was tested with bony sterna, and second in the examination of the chests of 8 patients with sternal instability.
Evaluation: Reliability estimation of the ultrasound measure on bony sterna gave ICC (2, 1) values >0.99, and reliability estimates for the sternal separation measure in the patient group were ICC (2, 1) values between 0.90 and 0.93.
Conclusions: Therefore gap measurements taken by ultrasound can objectively reflect the extent of bony separation occurring in a group of cardiac surgery patients experiencing sternal instability.
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Technology
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Midline sternotomy is the most common incision used in cardiac surgery due to its ease of its performance and optimal exposure of the heart [1–4]. Despite the advantages of sternotomy there is some incidence of sternal wound complications such as sternal nonunion and instability. Preferential use of the internal mammary artery as a conduit has been reported to result in an acute reduction of sternal perfusion [4, 5].
The terms "sternal instability" are applied in the literature to describe nonphysiologic or abnormal motion of the sternum after either bone fracture or disruption of the sternal wires reuniting the surgically divided sternum [3]. Separation of the two sterna may be total, involving the entire sternum, or partial, being limited to a portion of the sternum, usually the caudal third due to its lesser blood supply [3, 6]. Sternal instability is characterized by clicking and excessive motion with the resultant pain and discomfort typically creating difficulties in the performance of activities of daily living [3, 7]. In addition, patients often report feeling fearful and state that their "chest is going to fall open."
Early detection, monitoring, and measurement of sternal instability are needed to determine optimal treatment options. In addition, for those patients who can not have their sternum surgically repaired, diagnosis and monitoring of the extent sternal instability is important in establishing guidelines for modification of activities of daily living, as well as in determining appropriate exercises within the cardiac rehabilitation program.
In previous research, a 5-point scale for manual examination of sternal instability was assessed and was found to have good reliability [7]. Detecting very small changes in a patient’s sternal condition would promote better monitoring, but this requires a measuring system with greater sensitivity than the manual method. Using a computed tomographic is one possibility, but it has been demonstrated to have a relatively low sensitivity (0.67) for diagnosing mediastinitis in patients with sternal pain or instability, and it adds little information to the information available from the clinical presentation of the patient [8]. Another possible measuring system, ultrasound imaging, has gained popularity as an objective form of measurement in several areas of medicine [9, 10]. However, reliability information regarding its use in examining sternal separation has not been previously reported.
Accordingly, the aims of this study were to: (1) evaluate the validity and reliability of ultrasound measures using bony sterna and (2) evaluate the reliability of ultrasound measurement of sternal separation in patients with sternal instability.
Material and Methods
The Human Research Ethics Committee of the University of Sydney gave their approval for conducting this study.
Part 1
Bony sterna were used in the initial testing. To determine the reliability of ultrasound as a tool for measurement, two sterna were placed in a water bath at varying distances apart from each other. To allow repeat placement of the sterna at a given distance apart, metal discs were used as spacers. Vernier callipers with a manufacturer’s accuracy level of 0.02 mm (200 x 0.02 mm, [Mitre 10, Belconnen Act, Australia]) were used to determine the diameter of the discs. Ultrasound measures of the distance between the sterna were made by immersing the sterna below the water surface in a plastic bowl to approximately the same depth as the sternum is beneath the skin of a human chest. The borders of the sterna were traced onto the base of the plastic tub to ensure consistency of placement on the two separate occasions of measurement. The region corresponding to the midpoint of the second sternal notch was marked with a permanent marker and used as the reference point for measures. A randomly determined metal disc was then placed between the sterna at the level of the second sternal notch and the sterna were adjusted to touch the spacer. Once in position the spacer was removed and the resulting distance between the sterna was measured with ultrasound. This was repeated 3 times for each spacer in random order. An assistant entered the measures onto data sheets. One hour later the entire procedure was repeated (Table 1). The clinical ultrasound unit used in the study was a Dornier MedTech (MicroEnvision Ultrasound; Postfach, Germany) with the following specifications: 7-5 MHZ, 3-cm depth, gain 15, smoothing 2.
Part 2
Following an advertisement placed in the physiotherapy practice of the second author, 8 subjects volunteered for inclusion into the study, which was conducted over a 10-week period. Data collected included a record of the date and type of surgery, gender, age, the score on the sternal instability scale and documentation of pre-existing problems. All subjects had a history of a median sternotomy and a postoperative complication of sternal instability at 6 to 9 weeks after surgery, as documented in their medical record. Sternal instability was initially confirmed on physical examination by a cardiac surgeon and by a physiotherapist. In the sample there were seven subjects who underwent coronary artery bypass grafting (CABG) and one subject who had underwent a valve operation. All subjects reported discomfort, crepitus, and an unstable feeling of excessive motion that interfered with function or comfort. Written and informed consent was obtained from all patients prior to their participation in the study.
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Technique
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Using the manual sternal instability scale, the region of greatest instability along the sternum for each participant was identified by the chief investigator and marked with a water-soluble ink pen as a reference point for all later measurements. The ultrasound unit head was adapted by fixing it to a microphone stand to facilitate contact with the chest of a seated subject (Fig 1). The amount of sternal separation at different vertical points on the sternum could now be measured from the projected image using the proprietary software for the unit (Fig 2).
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Fig 1. Set up for measurement of sternal separation (elbow flexion with resistance by Theraband [Ausmedic Australia, Hornsby, Australia]).
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Fig 2. Ultrasound image of separated sternotomy margins: (A) and (B) represent the separated edges of the sternum. The separation from point A to point B is approximately 14.1 mm; 12.7 mm on the horizontal axis and 6 mm on the vertical axis. (St = sternum; XA = left sternal edge; XB = right sternal edge.)
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Thereafter, a series of movements was examined with each subject in a supported seated position. The movements included being seated in a chair with the arms by the side, pushing up with the arms to rise from a chair, flexion of the elbows against resistance (eg, Theraband), protraction of the shoulders, retraction of the shoulders, and elevation of both arms. To enable calculation of reliability indices for the procedure, movements were repeated 3 times, in random order.
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Results
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Measures of the metal spacers made by Vernier callipers (Mitre 10, Belconnen Act, Australia) and the gaps between the two sterna created by the same spacers and measured by ultrasound are presented in Table 1. Ultrasound measures differed from those taken by the Vernier callipers to a maximum of 0.30, 0.50, and 1.2 mm, respectively. Therefore the greatest difference between the ultrasound and Vernier measures was 1.2 mm, thereby establishing good validity for the measure. A linear regression equation was constructed to predict the ultrasound measure from the Vernier measure. The obtained regression equation accounted for 99.8% of the variance in the ultrasound scores and indicates the close correspondence between the two sets of scores. This regression line is shown in Figure 3. With three measures made at each of the bony separations determined by the spacers, a reliability coefficient could be computed. The form of the intraclass correlation coefficient (ICC) appropriate for sets of measures made by a single researcher, the ICC (2,1), was used as the index of reliability. Measured in this way, the ICC (2,1) reliability estimates for ultrasound measures made with bony sterna obtained on two occasions were both greater than 0.99.
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Fig 3. The regression line (solid line) and 95% confidence interval (dotted lines) used to predict the ultrasound (US) measure from the vernier (V) measure by the equation: US = 0.971 x V + 0.044.
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When the ultrasound head was applied to the chest of seated subjects, the sternal separations measured at the point of maximum opening for each subject were averaged over three measures (Table 2). The separation measures obtained during various tasks show a range over subjects, from the narrowest gap of 5.5 mm up to the largest at 35 mm, and give an indication of what exercises may require caution during rehabilitation. For three subjects, shoulder retraction resulted in the greatest amount of sternal separation, and for a further three subjects the task associated with the maximum separation was bilateral arm elevation. Because three repeated measures were made in each of the movement conditions, reliability could be calculated. Test-retest reliability of ultrasound sternal separation measurement made after different upper limb movements was between ICC (2,1) values of 0.90 and 0.93 (Fig. 4).
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Fig 4. Reliability of ultrasound sternal separation measures under different movement condition. The error bars show the upper and lower limits of the 95% confidence interval for the estimates of the intraclass correlation coefficient (ICC) (2,1).
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Comment
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Ultrasound measurement of sternal separation was found to have high reliability. Similar results to those reported in this study have been found by Yip and colleagues [9] who recorded ICC (2,1) values between 0.93 and 0.96 when assessing the reliability of ultrasound measurement in the bladder. In addition, Rankin and colleagues [10] noted ICC (2,1) values of 0.92 upon evaluation of ultrasound reliability in measurement of the cross-sectional area of the anterior tibial muscle group. Therefore, ultrasound seems to produce measures of high reliability in various settings.
With a modification to support and stabilize the ultrasound unit head, this study has demonstrated that left to right gap measurements made by the ultrasound unit can be taken to accurately reflect the true extent of sternal separation in a group of post-sternotomy patients experiencing sternal instability. In recent years the reduction in cost of clinical ultrasound units has translated into their increased availability and usage in the health care system. A computed tomographic scan has been reported to add little information to the clinical presentation of the patient [8]. Ultrasound is a noninvasive tool that could be useful for detecting very small changes in sternal separation and the movement conditions under which these occur. Using ultrasound, it will be possible to determine how functional movements or tasks affect the amount of sternal separation in a given patient with sternal instability. Furthermore, the technique could be used to assess what physical activities might serve to strengthen relevant muscles minimize excessive motion and thus reduce the symptoms experienced by the patient. Being able to accurately measure the degree of sternal separation at rest and during movement may serve to promote better monitoring of patients with sternal instability and enable future research to gain information about the effects of different upper limb and trunk movements on the healing sternum.
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Disclosures and Freedom of Investigation
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The equipment used in this study included a commercially available Clinical Ultrasound Unit (Dornier MedTech [MicroEnvision]), which was located in the second author’s practice. In addition, the authors had full control of the design of the study, methods used, outcome measurements, analysis of data, and production of the written report.
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Footnotes
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Disclaimer The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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References
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- El Gamel A, Yonan NA, Hassan R, et al. Treatment of mediastinitis: early modified Robicsek closure and pectoralis major advancement flaps Ann Thoracic Surg 1998;65:41-46.[Abstract/Free Full Text]
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- Robicsek F, Fokin AA, Cook J, Bhatia B. Sternal instability after midline sternotomy J Thorac Cardiovasc Surg 2000;48:1-8.
- Kouchoukos NT, Wareing TH, Murphy SF, Pelate C, Mashall WG. Risks of bilateral internal mammary artery bypass grafting Ann Thorac Surg 1999;49:179-187.
- Fokin AA, Robicsek F, Masters TN, Fokin A, Reames MK, Anderson JE. Sternal nourishment in various conditions of vascularization sternum Ann Thorac Surg 2005;79:1352-1357.[Abstract/Free Full Text]
- Moore KL. Clinically oriented anatomy. 5th edit. Canada, Baltimore: Williams and Wilkins; 2005.
- El-Ansary D, Adams R, Toms L, Elkins M. Sternal instability following coronary artery bypass grafting Physiother Theory Pract 2000;16:27-33.
- Bitkover CY, Cederlund K, Aberg B, Vaage J. Computed tomography of the sternum and mediastinum after median sternotomy Ann Thorac Surg 1999;68:858-863.[Abstract/Free Full Text]
- Yip SK, Sahota D, Chang AM. Determining the reliability of ultrasound measurements and the validity of the formulae for ultrasound estimation of postvoid residual bladder volume in postpartum women Neurological Urodyn 2003;22:255-260.
- Rankin G, Stokes M. Reliability of assessment tools in rehabilitation: an illustration of appropriate statistical analyses Clin Rehabil 1998;12(3):187-199.[Abstract/Free Full Text]
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