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Acoustic transmission in normal human hips: structural testing of joint symmetry
Medical Engineering & Physics 25 (2003) 811–816 www.elsevier.com/locate/medengphy
Acoustic transmission in normal human hips: structural testing of joint symmetry Kevin S.C. Kwong a,∗, Xiaolin Huang a, Jack C.Y. Cheng b, John H. Evans c b
a Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China Department of Orthopaedics and Traumatology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong, China c Centre for Rehabilitation Sciences and Engineering, Queensland University of Technology, Brisbane, Australia
Received 14 October 2002; received in revised form 28 March 2003; accepted 26 May 2003
Abstract An acoustical technique has been developed for the measurement of structural symmetry of the hip joints. A mild vibratory force was applied to the sacrum and sound signals were picked up at both hips by a pair of microphones installed in two stethoscopes. These stethoscope–microphone assembles were calibrated to achieve a difference in relative sensitivity of less than 0.2 dB. The relative transmission of sound signals was analysed and compared between both hips by a dual-channel signal analyser. Twentyseven healthy adults, 20 healthy pre-school children and 19 normal neonates were tested. Results from these three groups showed high coherence of the sound signals and that the discrepancy between both hips was smallest in the frequency range of 200–315 Hz. For normal neonates, the sound signals maintained a high coherence (g2 ⬎ 0.97) and small discrepancy (D ⬍ 1.25 dB) between both hips. This study has shown that the acoustical technique provides a practical structural testing for bony symmetry of the hips and the results offer a baseline for further investigation into developmental dysplasia of the hip (DDH) in neonates. Clinical screening for DDH is still problematic in developing countries. 2003 IPEM. Published by Elsevier Ltd. All rights reserved. Keywords: Hip joint; Joint symmetry; Acoustic transmission; Structural testing
1. Introduction
2. Acoustic techniques
For centuries, auscultation has been a clinical skill of listening for sounds within the body, and it has been well developed in clinical examination and diagnosis of cardiac and respiratory diseases. This basic clinical skill utilizes the principles of sound production and its transmission properties in human tissues for the understanding of disease states in man. When set in motion, human joints emit vibrations, which can be felt through manual contact or through auscultation by the use of a stethoscope over the joint. The observation of these sub-clinical signs was described as vibration emission and it has been claimed to have diagnostic value [1,2].
The stethoscope can be considered as an acoustic instrument and it is perhaps the very first device of this nature available to the medical profession. Early acoustical study of the mechanical disorders of the peripheral joints involved solely the use of the stethoscope for auscultation. The earliest recording of sounds emitted from human joints was reported by Blodgett [3] who detected the sound generated within the knee joints through auscultation. The sounds were recorded with respect to the joint position, and it was found that both the quantity and quality of the emitted sound were related to the pathology of the joint as well as the age. Walters [4] described the joint sounds obtained through auscultation as smooth, rough or grating. It was found that the degree of grating was related to clinical signs and symptoms and that there was a steady trend of increase in severity with age. With the advancement of electronics, microphones in association with amplifying and filtering systems
∗
Corresponding author. E-mail address: [email protected] (K.S.C. Kwong).
1350-4533/$30.00 2003 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1350-4533(03)00113-9
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K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
became available for recording joint sounds. A more systematic recording of joint sounds was made by Steindler [5] who used a cardiophone (modified microphone) to pick up sounds emitted from the knee joint. The amplified signals were then suitably filtered and displayed on an oscillogram together with information of the joint position and a time-trace. The intensity and characteristics of the joint sounds were studied for a range of joint conditions. Peylan [6] used a type of “throat microphone” which picked up the joint sounds from contact with the skin. The system consisted of an amplifier with independent control of the amplification of high and low frequencies. Direct auscultation was made for a range of rheumatoid and related joint conditions. Various types of characteristic joint sounds were detected, which were found to have explainable anatomical and pathological backgrounds. In the 1960s and 1970s, there was no significant advancement in the auscultation of joints except in the methods of storage and analysis of the sound signals. The microphone was still the only feasible sensor for joint sounds. Chu et al. [7] reported the design of a double microphone assembly for noise cancellation. The characteristic acoustic signals for normal, rheumatoid and degenerative knees were recorded and analysed by a frequency analyser. The peak magnitude and bandwidth of the power spectra were also found to correspond to the severity of the disease. The technique enabled the determination of an acoustic “signature” of the joints, which was found to be related to the types and severity of the disease conditions. However, Mollan [1] critically investigated the various technical aspects associated with acoustic technique and evaluated its capability for the detection of vibration emission from peripheral joints. It was found that vibrations emitted from joints were in the extremely low range of audio frequency. In this range, acoustic technique suffered from inherent deficiency in limited dynamic sensitivity and limited frequency response of the apparatus, particularly that of the microphone. It was also difficult to exclude ambient noise in a clinical situation. Mollan et al. [8] also commented that acoustic registration of joint sounds would be problematic if these limitations were not appreciated.
compared by digital filters implemented on a signal analyser. Acoustic techniques outlined in previous paragraphs exemplify what could be described as measurement of acoustic emission. However, these techniques depend very much on the intensity and quality of sounds emitted from the joints under investigation. In this study, we have developed an acoustical technique for the measurement of relative acoustic transmission across both hips of the test subjects while they were subject to an external vibratory force applied at the sacrum. This was a test of symmetry (or asymmetry) of the bony structure between both hips, based on a modified system analysis approach. The merit of this approach is that it allows direct comparison of the sound signals transmitted across both hips regardless of the measure of the input vibratory force. Hence, the discrepancy (asymmetry) between both hips could be computed from the relative acoustic transmissibility, defined as follows. Referring to Fig. 1, the acoustic transmissibility across the hip joint is defined as the ratio of acoustic response signal (Output) in Pa to the vibratory force (Input) in N: Acoustic transmissibility ⫽ Output / Input; Since the input is cancelled out from the following
3. Acoustic transmissibility By the suitable use of measuring microphones installed in stethoscopes, we successfully acquired sound signals transmitted across the hip joints by matching the acoustic impedance of the transducer with the sound emitted from the joints. This stethoscope–microphone (S–M) assembly was also found to be insensitive to ambient noise. Sound signals picked up from both hips were objectively measured in units of Pascal (Pa), and
Fig. 1. (a) Schematic diagram of instrumentation for testing on adults (S–M = stethoscope–microphone assembly). (b) Experimental set-up for adults.
K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
expression, the measurement of the vibratory force is not needed. Relative acoustic transmissibility ⫽ [Output (R) / Input] / [Output (L) / Input] ⫽ [Output (R) / Output (L)] The signal analyser processed the output acoustic signals from both hips and measured the relative acoustic transmissibility in terms of the ratio of the power of the sound signals from both sides. From the relative acoustic transmissibility, the discrepancy (D) between both hip joints can be expressed in decibel (dB): Discrepancy (D) ⫽ Abs{20 log[Output (R) / Output (L)]} dB where “R” and “L” denote right and left, respectively; and “Abs” denotes absolute values. The definition of discrepancy was based on the hypothesis that perfectly symmetrical bony structure of the hips would present an equal transmission of sound signals between both hips. Hence the discrepancy (D) = 0 dB, as a result of [Output (R)/Output (L)] being equal to unity. The discrepancy therefore objectively evaluates the degree of asymmetry of the bony structure between both hips. This study was the first attempt of applying the concept of structural testing in the measurement of bony symmetry in human joints by acoustic means. It was also the first attempt to establish a baseline of the relative acoustic transmissibility between both hips for normal adults, pre-school children and neonates. Based on this concept, an acoustic technique has been established for the screening of developmental dysplasia of the hip (DDH) in neonates. It was found that the technique was able to achieve a sensitivity of 100% in the detection of unilateral DDH [9]. It is expected that future work of a similar nature will benefit from the concept of relative acoustic transmissibility outlined in this paper.
813
the vibratory force through a plastic probe, when testing the adults (Fig. 1a,b). A hand-held exciter (Bruel & Kjaer, Type 5961) was used in tests for pre-school children and neonates (Fig. 2a,b). The transduction system comprised of a pair of identical microphones (Bruel & Kjaer, Type 4136) installed separately in the tubes of two stethoscopes (Littmann, 3M, adult-size) for picking up sound signals (Fig. 3a,b). This was the S–M assembly placed over the greater trochanters on both sides. The S–M assembly was placed in position by applying a 2.5 cm wide elastic rubber band, which was wrapped round the pelvis with a predetermined elastic tension, at the level of the greater trochanters. The use of the elastic band was to ensure that the S–M assembly maintained an effective area of contact with the skin. The tension of the rubber band also ensured an adequate pressure of the stethoscopes over the skin for an optimal acquisition of sound signals emitted from the hip joints. The pressure also helped to exclude as much ambient noise as possible. Care had been taken to ensure that equal pressure of the stethoscope was applied over both greater trochanters, by maintaining a consistent tension throughout the rubber band. However, the actual pressure of contact was not measured. The measuring microphones were used in conjunction with microphone preamplifiers (Bruel &
4. Materials and methods The instrumentation used in this study consisted of three systems. An excitation system delivered a mild vibratory force of less than 1 N (RMS) at the centre of the sacrum of the test subjects. This vibration level was safe in accordance with BS 6841 and BS 7085. This system also included a signal generator built within a dual-channel signal analyser (Bruel & Kjaer, Type 2148), which generated a pink noise with a flat frequency spectrum across a logarithmic frequency scale. The noise signal was fed to a power amplifier (Bruel & Kjaer, Type 2706) to supply the electrodynamic exciter (Bruel & Kjaer, Type 4801), which was used to deliver
Fig. 2. (a) Schematic diagram of instrumentation for testing on preschool children and neonates (S–M = stethoscope–microphone assembly). (b) Experimental set-up for pre-school children and neonates (photograph taken on a dummy).
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K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
sion was completed within about 10 min for each subject. Twenty-seven healthy adults, 20 healthy pre-school children and 19 normal neonates were recruited for testing. Informed consent had been obtained from the adult subjects and the parents of the children and neonates. For the adult group, there were 9 females and 18 males. The average age was 22.9 years (range from 20 to 28 years), and the average body weight was 58.3 kg (range from 43.5 to 80.1 kg). Among the pre-school children, there were 9 females and 11 males. The average age was 3.56 years (range from 2 to 7 years), and the average body weight was 15.2 kg (range from 12 to 24.5 kg). For the neonates, there were 9 females and 10 males. The average age was 7.0 days (range from 3 to 20 days), and the average body weight was 3.34 kg (range from 2.8 to 4.6 kg). All subjects were free from current medical or orthopedic conditions and any known history of bone or joint disease, as well as any visible anatomical pathology or abnormality in the musculoskeletal system.
5. Results
Fig. 3. (a) Stethoscope chest-pieces and measuring microphones. (b) Stethoscope–microphone assemblies.
Kjaer, Type 2633) to improve signal-to-noise ratio. The data acquisition system was a portable signal analyser (Bruel & Kjaer, Type 2148), with a dual-channel digital filter program (Bruel & Kjaer, Type 7667), which acquired the sound signals and processed the relative acoustic transmissibility in 1/3 octave frequency bands, designated by the centre frequencies, fc, defined by fc = 10n / 10, where n = 20–27. The bandwidth was defined as f2⫺f1, where f1 and f2 were the low and high cut-off frequencies, respectively and f2c = f1 × f2. Coherence and discrepancy were measured during the testing. The coherence (g2) assessed the degree of linear relationship between the sound signals from both hips and hence the validity of the measurement of discrepancy. Coherence was defined according to the following expression:
Tables 1 and 2 depict the means and standard deviations of the coherence and discrepancy between both hip joints for the adult group. For the adult group, the coherence was above 0.99 in the frequency range from 200 to 315 Hz. The discrepancy between the two hips was smaller than 1.5 dB in the same frequency range. Tables 3 and 4 show the results for the pre-school children. A generally lower coherence was found, and the discrepancy was generally greater when compared with the measurements on adults. Even so, the best results were below 1.6 dB in the frequency range of 200– 315 Hz. Tables 5 and 6 show the results for the neonatal group. For the normal neonates tested, the coherence was higher than that of the pre-school children (p ⬍ 0.05) though it was slightly lower than that of the adults (p ⬍ 0.05, except for 200 Hz). The discrepancy between the hips was found to be less than 1.25 dB in the frequency range of 200–315 Hz. The results of repeated measurements of coherence of the sound signals and the discrepancy between both hips suggested that the technique was repeatable and reliable.
Coherence, g2 ⫽ |GRL|2 / GLL· GRR,
6. Discussion
where GLL and GRR are the auto-spectra of the acoustic signals from the left and right hips, and GRL is the complex cross-spectrum of the acoustic signals from both sides. Both coherence and discrepancy were calculated as the average of at least five repeated measurements, each of which lasted for about 10 s. The whole test ses-
In this study, the human body was considered as a passive mechanical transmission system and that sound energy itself did not induce any change in the bone’s and joint’s mechanical properties. The vibratory force was easily applied to the sacrum. The S–M assemblies
K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
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Table 1 Mean and standard deviation of the coherence in 27 healthy adults Frequency (Hz) Mean SD a
100 0.91 0.15
125 0.94 0.08
160 0.97 0.05
200 0.99 0.02
250 1.00a 0.02
315 0.99 0.02
400 0.97 0.06
500 0.82 0.23
250 1.27 0.75
315 1.45 0.91
400 2.15 1.55
500 2.92 2.06
250 0.94 0.06
315 0.91 0.05
400 0.86 0.14
500 0.8 0.14
250 1.4 0.95
315 1.56 1.22
400 2.45 2.54
500 3.38 3.59
250 0.97 0.04
315 0.97 0.04
400 0.95 0.04
500 0.91 0.11
250 1.08 0.78
315 1.24 0.59
400 2.86 1.54
500 3.13 2.18
Subject to rounding up error.
Table 2 Mean and standard deviation of the discrepancy in 27 healthy adults Frequency (Hz) Mean (dB) SD (dB)
100 4.54 4.35
125 3.26 3.75
160 2.11 1.94
200 1.4 0.96
Table 3 Mean and standard deviation of the coherence in 20 healthy pre-school children Frequency (Hz) Mean SD
100 0.76 0.16
125 0.86 0.19
160 0.91 0.13
200 0.94 0.07
Table 4 Mean and standard deviation of the discrepancy in 20 healthy pre-school children Frequency (Hz) Mean (dB) SD (dB)
100 4.57 3.9
125 4.19 3.11
160 2.35 1.88
200 1.38 0.85
Table 5 Mean and standard deviation of the coherence in 19 healthy neonates Frequency (Hz) Mean SD
100 0.95 0.06
125 0.97 0.05
160 0.98 0.04
200 0.98 0.03
Table 6 Mean and standard deviation of the discrepancy in 19 healthy neonates Frequency (Hz) Mean (dB) SD (dB)
100 2.25 1.49
125 1.76 0.78
160 1.17 0.76
200 1.12 0.76
were also securely placed over the greater trochanters, which were subcutaneous bony prominences on the lateral aspect of the upper end of the femoral shaft and could easily be located by palpation of the examiner. The results from the three groups showed that complete coherence (of unity) and nil discrepancy could not be obtained even in normal individuals. Naturally, we cannot expect perfectly equal and symmetrical anatomical structure on both sides of the human body. Hence, the sound signals obtained from both hips were bound to show some degree of difference. A mean discrepancy
of less than 1.45 dB was the greatest value that could be recorded on normal adult hips, within the middle range of the test frequencies (200–315 Hz). The slight difference in the anatomy of both hips, possible technical error in the measurement of sound signals and the system error (though expected to be very small) could explain for this discrepancy. The results of the preschool children were less consistent when compared with those of the adults and neonates. This could be explained by the fact that some of these children could not maintain a stable position throughout the test. The
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K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
results from the neonates revealed a wider frequency range of high coherence and smaller discrepancy. This may be due to the smaller body size of the neonates. The neonate’s bony structure was smaller and likewise the skin was thinner, and hence the transmission path was shorter. There would be stronger sound transmission across both hips and the signal-to-noise ratio was then higher than those of the adults and pre-school children. 7. Conclusion An acoustical technique has been developed primarily for the evaluation of the relative transmission of sound energy in both hip joints of a test subject, and it has been proved reliable and informative in providing an objective measure of the relative intensity of the transmitted sounds. It has also indicated that the most useful frequencies fall within the bands of 200–315 Hz, where the highest coherence and smallest discrepancy could be obtained in all three groups of subjects. These frequency bands are considered most effective in revealing the structural asymmetry of the hip joints under test. The results obtained in this study offer a baseline for further investigation of hip disorders associated with bone and joint asymmetry, with the anticipation that these defeats would be revealed by the measurement of discrepancy. A comprehensive investigation of controlled clinical study of a wider scale is required to assess the efficacy of this technique before clinical application. The most probable applications are early screening and diagnosis of hip disorders of the neonates. Criteria could be estab-
lished for a cut-off value such that discrepancy beyond such a threshold would be considered an indication of abnormality in the hip.
Acknowledgements This work was supported by a Competitive Earmarked Research Grant of the Research Grants Council of the Hong Kong SAR, China.
References [1] Mollan RAB. Vibration emission in bone and joints. MD thesis, Queen’s University, Belfast, 1981. [2] Kernohan WG, Beverland DE, McCoy GF, Shaw SN, Wallace RGH, McCullagh GC et al. The diagnostic potential of vibration arthrography. Clin Orthop 1986;210:106–12. [3] Blodgett WE. Auscultation of the knee joint. Boston Med Surg J 1902;146:63–6. [4] Walters CF. The value of joint auscultation. Lancet 1929;May 4:920–1. [5] Steindler A. Auscultation of joints. J Bone Joint Surg 1937;19(1):121–36. [6] Peylan A. Direct auscultation of the joints. Rheumatism 1953;9:77–81. [7] Chu ML, Gradisar IA, Railey MR, Bowling GF. Detection of knee joint diseases using acoustical pattern recognition technique. J Biomech 1976;9:111–4. [8] Mollan RAB, McCullagh GC, Wilson RI. A critical appraisal of auscultation of human joints. Clin Orthop 1982;170:231–7. [9] Kwong KSC, Huang X, Cheng JCY, Evans JH. New technique for early screening of developmental dysplasia of the hip: pilot study. J Paediatr Orthop 2003;23:347–51.
Acoustic transmission in normal human hips: structural testing of joint symmetry Kevin S.C. Kwong a,∗, Xiaolin Huang a, Jack C.Y. Cheng b, John H. Evans c b
a Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China Department of Orthopaedics and Traumatology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong, China c Centre for Rehabilitation Sciences and Engineering, Queensland University of Technology, Brisbane, Australia
Received 14 October 2002; received in revised form 28 March 2003; accepted 26 May 2003
Abstract An acoustical technique has been developed for the measurement of structural symmetry of the hip joints. A mild vibratory force was applied to the sacrum and sound signals were picked up at both hips by a pair of microphones installed in two stethoscopes. These stethoscope–microphone assembles were calibrated to achieve a difference in relative sensitivity of less than 0.2 dB. The relative transmission of sound signals was analysed and compared between both hips by a dual-channel signal analyser. Twentyseven healthy adults, 20 healthy pre-school children and 19 normal neonates were tested. Results from these three groups showed high coherence of the sound signals and that the discrepancy between both hips was smallest in the frequency range of 200–315 Hz. For normal neonates, the sound signals maintained a high coherence (g2 ⬎ 0.97) and small discrepancy (D ⬍ 1.25 dB) between both hips. This study has shown that the acoustical technique provides a practical structural testing for bony symmetry of the hips and the results offer a baseline for further investigation into developmental dysplasia of the hip (DDH) in neonates. Clinical screening for DDH is still problematic in developing countries. 2003 IPEM. Published by Elsevier Ltd. All rights reserved. Keywords: Hip joint; Joint symmetry; Acoustic transmission; Structural testing
1. Introduction
2. Acoustic techniques
For centuries, auscultation has been a clinical skill of listening for sounds within the body, and it has been well developed in clinical examination and diagnosis of cardiac and respiratory diseases. This basic clinical skill utilizes the principles of sound production and its transmission properties in human tissues for the understanding of disease states in man. When set in motion, human joints emit vibrations, which can be felt through manual contact or through auscultation by the use of a stethoscope over the joint. The observation of these sub-clinical signs was described as vibration emission and it has been claimed to have diagnostic value [1,2].
The stethoscope can be considered as an acoustic instrument and it is perhaps the very first device of this nature available to the medical profession. Early acoustical study of the mechanical disorders of the peripheral joints involved solely the use of the stethoscope for auscultation. The earliest recording of sounds emitted from human joints was reported by Blodgett [3] who detected the sound generated within the knee joints through auscultation. The sounds were recorded with respect to the joint position, and it was found that both the quantity and quality of the emitted sound were related to the pathology of the joint as well as the age. Walters [4] described the joint sounds obtained through auscultation as smooth, rough or grating. It was found that the degree of grating was related to clinical signs and symptoms and that there was a steady trend of increase in severity with age. With the advancement of electronics, microphones in association with amplifying and filtering systems
∗
Corresponding author. E-mail address: [email protected] (K.S.C. Kwong).
1350-4533/$30.00 2003 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1350-4533(03)00113-9
812
K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
became available for recording joint sounds. A more systematic recording of joint sounds was made by Steindler [5] who used a cardiophone (modified microphone) to pick up sounds emitted from the knee joint. The amplified signals were then suitably filtered and displayed on an oscillogram together with information of the joint position and a time-trace. The intensity and characteristics of the joint sounds were studied for a range of joint conditions. Peylan [6] used a type of “throat microphone” which picked up the joint sounds from contact with the skin. The system consisted of an amplifier with independent control of the amplification of high and low frequencies. Direct auscultation was made for a range of rheumatoid and related joint conditions. Various types of characteristic joint sounds were detected, which were found to have explainable anatomical and pathological backgrounds. In the 1960s and 1970s, there was no significant advancement in the auscultation of joints except in the methods of storage and analysis of the sound signals. The microphone was still the only feasible sensor for joint sounds. Chu et al. [7] reported the design of a double microphone assembly for noise cancellation. The characteristic acoustic signals for normal, rheumatoid and degenerative knees were recorded and analysed by a frequency analyser. The peak magnitude and bandwidth of the power spectra were also found to correspond to the severity of the disease. The technique enabled the determination of an acoustic “signature” of the joints, which was found to be related to the types and severity of the disease conditions. However, Mollan [1] critically investigated the various technical aspects associated with acoustic technique and evaluated its capability for the detection of vibration emission from peripheral joints. It was found that vibrations emitted from joints were in the extremely low range of audio frequency. In this range, acoustic technique suffered from inherent deficiency in limited dynamic sensitivity and limited frequency response of the apparatus, particularly that of the microphone. It was also difficult to exclude ambient noise in a clinical situation. Mollan et al. [8] also commented that acoustic registration of joint sounds would be problematic if these limitations were not appreciated.
compared by digital filters implemented on a signal analyser. Acoustic techniques outlined in previous paragraphs exemplify what could be described as measurement of acoustic emission. However, these techniques depend very much on the intensity and quality of sounds emitted from the joints under investigation. In this study, we have developed an acoustical technique for the measurement of relative acoustic transmission across both hips of the test subjects while they were subject to an external vibratory force applied at the sacrum. This was a test of symmetry (or asymmetry) of the bony structure between both hips, based on a modified system analysis approach. The merit of this approach is that it allows direct comparison of the sound signals transmitted across both hips regardless of the measure of the input vibratory force. Hence, the discrepancy (asymmetry) between both hips could be computed from the relative acoustic transmissibility, defined as follows. Referring to Fig. 1, the acoustic transmissibility across the hip joint is defined as the ratio of acoustic response signal (Output) in Pa to the vibratory force (Input) in N: Acoustic transmissibility ⫽ Output / Input; Since the input is cancelled out from the following
3. Acoustic transmissibility By the suitable use of measuring microphones installed in stethoscopes, we successfully acquired sound signals transmitted across the hip joints by matching the acoustic impedance of the transducer with the sound emitted from the joints. This stethoscope–microphone (S–M) assembly was also found to be insensitive to ambient noise. Sound signals picked up from both hips were objectively measured in units of Pascal (Pa), and
Fig. 1. (a) Schematic diagram of instrumentation for testing on adults (S–M = stethoscope–microphone assembly). (b) Experimental set-up for adults.
K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
expression, the measurement of the vibratory force is not needed. Relative acoustic transmissibility ⫽ [Output (R) / Input] / [Output (L) / Input] ⫽ [Output (R) / Output (L)] The signal analyser processed the output acoustic signals from both hips and measured the relative acoustic transmissibility in terms of the ratio of the power of the sound signals from both sides. From the relative acoustic transmissibility, the discrepancy (D) between both hip joints can be expressed in decibel (dB): Discrepancy (D) ⫽ Abs{20 log[Output (R) / Output (L)]} dB where “R” and “L” denote right and left, respectively; and “Abs” denotes absolute values. The definition of discrepancy was based on the hypothesis that perfectly symmetrical bony structure of the hips would present an equal transmission of sound signals between both hips. Hence the discrepancy (D) = 0 dB, as a result of [Output (R)/Output (L)] being equal to unity. The discrepancy therefore objectively evaluates the degree of asymmetry of the bony structure between both hips. This study was the first attempt of applying the concept of structural testing in the measurement of bony symmetry in human joints by acoustic means. It was also the first attempt to establish a baseline of the relative acoustic transmissibility between both hips for normal adults, pre-school children and neonates. Based on this concept, an acoustic technique has been established for the screening of developmental dysplasia of the hip (DDH) in neonates. It was found that the technique was able to achieve a sensitivity of 100% in the detection of unilateral DDH [9]. It is expected that future work of a similar nature will benefit from the concept of relative acoustic transmissibility outlined in this paper.
813
the vibratory force through a plastic probe, when testing the adults (Fig. 1a,b). A hand-held exciter (Bruel & Kjaer, Type 5961) was used in tests for pre-school children and neonates (Fig. 2a,b). The transduction system comprised of a pair of identical microphones (Bruel & Kjaer, Type 4136) installed separately in the tubes of two stethoscopes (Littmann, 3M, adult-size) for picking up sound signals (Fig. 3a,b). This was the S–M assembly placed over the greater trochanters on both sides. The S–M assembly was placed in position by applying a 2.5 cm wide elastic rubber band, which was wrapped round the pelvis with a predetermined elastic tension, at the level of the greater trochanters. The use of the elastic band was to ensure that the S–M assembly maintained an effective area of contact with the skin. The tension of the rubber band also ensured an adequate pressure of the stethoscopes over the skin for an optimal acquisition of sound signals emitted from the hip joints. The pressure also helped to exclude as much ambient noise as possible. Care had been taken to ensure that equal pressure of the stethoscope was applied over both greater trochanters, by maintaining a consistent tension throughout the rubber band. However, the actual pressure of contact was not measured. The measuring microphones were used in conjunction with microphone preamplifiers (Bruel &
4. Materials and methods The instrumentation used in this study consisted of three systems. An excitation system delivered a mild vibratory force of less than 1 N (RMS) at the centre of the sacrum of the test subjects. This vibration level was safe in accordance with BS 6841 and BS 7085. This system also included a signal generator built within a dual-channel signal analyser (Bruel & Kjaer, Type 2148), which generated a pink noise with a flat frequency spectrum across a logarithmic frequency scale. The noise signal was fed to a power amplifier (Bruel & Kjaer, Type 2706) to supply the electrodynamic exciter (Bruel & Kjaer, Type 4801), which was used to deliver
Fig. 2. (a) Schematic diagram of instrumentation for testing on preschool children and neonates (S–M = stethoscope–microphone assembly). (b) Experimental set-up for pre-school children and neonates (photograph taken on a dummy).
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K.S.C. Kwong et al. / Medical Engineering & Physics 25 (2003) 811–816
sion was completed within about 10 min for each subject. Twenty-seven healthy adults, 20 healthy pre-school children and 19 normal neonates were recruited for testing. Informed consent had been obtained from the adult subjects and the parents of the children and neonates. For the adult group, there were 9 females and 18 males. The average age was 22.9 years (range from 20 to 28 years), and the average body weight was 58.3 kg (range from 43.5 to 80.1 kg). Among the pre-school children, there were 9 females and 11 males. The average age was 3.56 years (range from 2 to 7 years), and the average body weight was 15.2 kg (range from 12 to 24.5 kg). For the neonates, there were 9 females and 10 males. The average age was 7.0 days (range from 3 to 20 days), and the average body weight was 3.34 kg (range from 2.8 to 4.6 kg). All subjects were free from current medical or orthopedic conditions and any known history of bone or joint disease, as well as any visible anatomical pathology or abnormality in the musculoskeletal system.
5. Results
Fig. 3. (a) Stethoscope chest-pieces and measuring microphones. (b) Stethoscope–microphone assemblies.
Kjaer, Type 2633) to improve signal-to-noise ratio. The data acquisition system was a portable signal analyser (Bruel & Kjaer, Type 2148), with a dual-channel digital filter program (Bruel & Kjaer, Type 7667), which acquired the sound signals and processed the relative acoustic transmissibility in 1/3 octave frequency bands, designated by the centre frequencies, fc, defined by fc = 10n / 10, where n = 20–27. The bandwidth was defined as f2⫺f1, where f1 and f2 were the low and high cut-off frequencies, respectively and f2c = f1 × f2. Coherence and discrepancy were measured during the testing. The coherence (g2) assessed the degree of linear relationship between the sound signals from both hips and hence the validity of the measurement of discrepancy. Coherence was defined according to the following expression:
Tables 1 and 2 depict the means and standard deviations of the coherence and discrepancy between both hip joints for the adult group. For the adult group, the coherence was above 0.99 in the frequency range from 200 to 315 Hz. The discrepancy between the two hips was smaller than 1.5 dB in the same frequency range. Tables 3 and 4 show the results for the pre-school children. A generally lower coherence was found, and the discrepancy was generally greater when compared with the measurements on adults. Even so, the best results were below 1.6 dB in the frequency range of 200– 315 Hz. Tables 5 and 6 show the results for the neonatal group. For the normal neonates tested, the coherence was higher than that of the pre-school children (p ⬍ 0.05) though it was slightly lower than that of the adults (p ⬍ 0.05, except for 200 Hz). The discrepancy between the hips was found to be less than 1.25 dB in the frequency range of 200–315 Hz. The results of repeated measurements of coherence of the sound signals and the discrepancy between both hips suggested that the technique was repeatable and reliable.
Coherence, g2 ⫽ |GRL|2 / GLL· GRR,
6. Discussion
where GLL and GRR are the auto-spectra of the acoustic signals from the left and right hips, and GRL is the complex cross-spectrum of the acoustic signals from both sides. Both coherence and discrepancy were calculated as the average of at least five repeated measurements, each of which lasted for about 10 s. The whole test ses-
In this study, the human body was considered as a passive mechanical transmission system and that sound energy itself did not induce any change in the bone’s and joint’s mechanical properties. The vibratory force was easily applied to the sacrum. The S–M assemblies
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Table 1 Mean and standard deviation of the coherence in 27 healthy adults Frequency (Hz) Mean SD a
100 0.91 0.15
125 0.94 0.08
160 0.97 0.05
200 0.99 0.02
250 1.00a 0.02
315 0.99 0.02
400 0.97 0.06
500 0.82 0.23
250 1.27 0.75
315 1.45 0.91
400 2.15 1.55
500 2.92 2.06
250 0.94 0.06
315 0.91 0.05
400 0.86 0.14
500 0.8 0.14
250 1.4 0.95
315 1.56 1.22
400 2.45 2.54
500 3.38 3.59
250 0.97 0.04
315 0.97 0.04
400 0.95 0.04
500 0.91 0.11
250 1.08 0.78
315 1.24 0.59
400 2.86 1.54
500 3.13 2.18
Subject to rounding up error.
Table 2 Mean and standard deviation of the discrepancy in 27 healthy adults Frequency (Hz) Mean (dB) SD (dB)
100 4.54 4.35
125 3.26 3.75
160 2.11 1.94
200 1.4 0.96
Table 3 Mean and standard deviation of the coherence in 20 healthy pre-school children Frequency (Hz) Mean SD
100 0.76 0.16
125 0.86 0.19
160 0.91 0.13
200 0.94 0.07
Table 4 Mean and standard deviation of the discrepancy in 20 healthy pre-school children Frequency (Hz) Mean (dB) SD (dB)
100 4.57 3.9
125 4.19 3.11
160 2.35 1.88
200 1.38 0.85
Table 5 Mean and standard deviation of the coherence in 19 healthy neonates Frequency (Hz) Mean SD
100 0.95 0.06
125 0.97 0.05
160 0.98 0.04
200 0.98 0.03
Table 6 Mean and standard deviation of the discrepancy in 19 healthy neonates Frequency (Hz) Mean (dB) SD (dB)
100 2.25 1.49
125 1.76 0.78
160 1.17 0.76
200 1.12 0.76
were also securely placed over the greater trochanters, which were subcutaneous bony prominences on the lateral aspect of the upper end of the femoral shaft and could easily be located by palpation of the examiner. The results from the three groups showed that complete coherence (of unity) and nil discrepancy could not be obtained even in normal individuals. Naturally, we cannot expect perfectly equal and symmetrical anatomical structure on both sides of the human body. Hence, the sound signals obtained from both hips were bound to show some degree of difference. A mean discrepancy
of less than 1.45 dB was the greatest value that could be recorded on normal adult hips, within the middle range of the test frequencies (200–315 Hz). The slight difference in the anatomy of both hips, possible technical error in the measurement of sound signals and the system error (though expected to be very small) could explain for this discrepancy. The results of the preschool children were less consistent when compared with those of the adults and neonates. This could be explained by the fact that some of these children could not maintain a stable position throughout the test. The
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results from the neonates revealed a wider frequency range of high coherence and smaller discrepancy. This may be due to the smaller body size of the neonates. The neonate’s bony structure was smaller and likewise the skin was thinner, and hence the transmission path was shorter. There would be stronger sound transmission across both hips and the signal-to-noise ratio was then higher than those of the adults and pre-school children. 7. Conclusion An acoustical technique has been developed primarily for the evaluation of the relative transmission of sound energy in both hip joints of a test subject, and it has been proved reliable and informative in providing an objective measure of the relative intensity of the transmitted sounds. It has also indicated that the most useful frequencies fall within the bands of 200–315 Hz, where the highest coherence and smallest discrepancy could be obtained in all three groups of subjects. These frequency bands are considered most effective in revealing the structural asymmetry of the hip joints under test. The results obtained in this study offer a baseline for further investigation of hip disorders associated with bone and joint asymmetry, with the anticipation that these defeats would be revealed by the measurement of discrepancy. A comprehensive investigation of controlled clinical study of a wider scale is required to assess the efficacy of this technique before clinical application. The most probable applications are early screening and diagnosis of hip disorders of the neonates. Criteria could be estab-
lished for a cut-off value such that discrepancy beyond such a threshold would be considered an indication of abnormality in the hip.
Acknowledgements This work was supported by a Competitive Earmarked Research Grant of the Research Grants Council of the Hong Kong SAR, China.
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