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Instrumented implants in orthopaedics
Instrumented
implants in orthopaedics
P.J. Sell Biomedical Engineering Unit, North Stoke on Trent, ST4 7NY, UK Received
April
1988,
accepted
July
Staffs Polytechnic.
c/o Medical
Institute,
Hartshill
Rd,
1988
ABSTRACTS This brief paher discusses the needfor instrumentation in orthopaedic implants, and examines areas for further development. It is suggested that the future of this technology may have major clinical applications.
Keywords:
Instrumented
implants,
orthopaedics,
The use of implants in orthopaedic surgery has increased with the development of systems for internal fixation of fractures and joint replacement, the numerous types of prosthesis available having been developed in collaboration with engineers. In mechanical enginering a fundamental concern is the soundness of loaded structures’. A rational approach to the improvement and development of implants should include the measurement of forces acting across them in both static and dynamic modes of loading. On this basis alterations in the structure or material of the implant can be undertaken in an objective manner, and so eliminating trial and error in arriving at the most effective system. The loads carried by implants have in the past been calculated from indirect measurements using body weight, external force measurement and geometry. The number of reported cases of implant failure bears witness to the less than satisfactory results obtained by this method’; implant failure in prosthesis is nothing short of a disaster. The application of engineering techniques to measure strain in implants would provide valuable information with both clinical and research benefits. Strain is the change in unit length in a material subjected to load and its measurement, which is fundamental to the practical field of experimental stress analysis, can be achieved using mechanical, optical, pneumatic, acoustic or electrical strain gauges. Other techniques to arrive at a value for strain include mathematical modelling in the form of finite element analysis. In the biological setting the electrical strain gauge initially appears the most attractive, although there are considerable technical difficulties to be overcome. Electrical strain gauges are instruments that respond to strain in the surface to which they are bonded with an associated change in some electrical characFoil gauges respond with a teristic of the gauge. change in resistance. These consist of a small section grid or filament (produced by a photographic engraving process) mounted on paper or plastic. This is bonded to the structure to be analysed and connected to an instrument to measure the small 0 1989 Butterworth 0141-5425/89/0201
& Co (Publishers) I I-02 $03.00
strain
research and
gauges
changes in resistance which occur when strain is applied. Semiconductor gauges have a strain sensitivity an order of magnitude greater than that of foil. They consist of silicon or germanium crystals whose electrical properties are modified by doping with boron or arsenic atoms. These gauges exhibit the piezoresistive effect, in which the resistance changes with changes in strain independently of rate of deformation. Static strain can therefore be measured but this requires a current from an external source. Piezoelectrical strain gauges generate a free electrical surface charge proportional to stress. However the duration of this process is finite and therefore these gauges can only be used for dynamic measurements. In comparison with foil gauges, semiconductor gauges are more difficult to handle, have a narrower band for linear strain measurement and are more sensitive to temperature variations. In terms of biocompatibility semiconductor gauges present a possible hazard due to leakage of the doping agent. Measuring surface strain under laboratory conditions is straightforward and instrumentation is commercially available. At the present time, data can be acquired from implanted strain gauges by telemetry or by ‘hard wires’. The ‘hard wire’ techniques involve percutaneous leads to link the gauges to the instrumentation - this has a number of drawbacks. There is the hazard of infection as the wires create a sinus between implant necessitating regular microbiological and skin, screening and removal at the first sign of infection. Patients have to be connected to the instrumentation and this limits their mobility. Patient acceptability of this method would be low in the long term. Telemetry is the transmission of information to a separate receiver3. In 1903 Eintoven transmitted ECGs over a distance of 1.5 km using the public telephone system and the first use of radiotelemetry was in 1921 when Winters transmitted heart sounds from on board ship. Implantable and ingestable devices were first described by McKay and Jacobson4 and Farrer et al5 in 1957. The invention of the transistor in 1948 gave birth
Ltd J. Biomed. Eng. 1989, Vol. 11, March
111
Implantsin orthopaedics:P. J. Sell
to a second generation of telemetry devices, which permitted reductions in size, weight and power consumption. Third generation equipment has been available since the early seventies, utilizing CMOS integrated circuits and PCM coding techniques. These have improved reliability and reduced size but increased development cost. Their reduction in size allows less complex environmental protection and packaging. With either thick or thin film hybrid circuits and custom silicon integrated circuits, an hermetically sealed multi-channel transmitter would be no larger than a modern pacemaker. Biocompatibility has been shown to be acceptable for long term pacemaker implantation. Long term implantation studies require a stable dependable power source6. Primary batteries convert chemical or nuclear energy into electrical energy through an irreversible process. Major considerations are safety in the body environment, size, weight, availability, cost and service capacity. Lithium, mercuric or silver oxide/zinc batteries are commonly used in biotelemetry and there are several types of nuclear battery available. Over 2300 nuclear powered pacemakers were implanted between 1970 and 1976 with no failures reported in 1977’. Secondary batteries can be recharged, but for implanted telemetry devices a remotely coupled charging circuit is required. Biological batteries using dissimilar materials to form galvanic cells using body fluids as the electrolyte have been described. Electromagnetic energy is propagated by two means, induction and radio frequency radiation. The field strength of inductive coupling diminishes as the cube of the distance whereas RF radiation is inversely proportional to the distance, and as a result inductive coupling is effective over distances of only a few metres; RF radiation must be used for greater distances. Government regulations on the use of RF transmitters are more restrictive than for inductively coupled short range systems’ ’ . The small number of orthopaedic studies performed using strain gauges have employed a variety of methods with a wide range of orthopaedic implants *rg. Most commonly they have modified the implant so that the gauges are sealed within the aftered structure, and have either incorporated the telemetry device or connected the telemetry unit by flying leads r”*” Most studies have been short term. The problem of the environmental protection of gauges remains to be solved. If this could be overcome, then orthopaedic implants could be instrumented without altering their structure, so enabling more satisfactory strain readings to be obtained.
112
J. Biomed. Eng. 1989, Vol. 1 I, March
The engineering advances seen in the automotive and aerospace industries owe much to the information gained from experimental stress analysis, and the rational development of orthopaedic implants should follow the application of these techniques; the technology is available but at a high development cost. There may be useful clinical information to be gained as well, such as the monitoring of the progress of a bone graft fusion in spinal surgery or rational management of post-operative mobilization following hip prosthesis surgery. Perhaps the day will come when patients have check strain gauge readings taken from the implanted inductively powered telemetry unit at clinic attendance rather than check X-rays. ACKNOWLEDGEMENT Mr Snell is a Surgicraft
Spinal Research
Fellow.
REFERENCES I. Neubert HKP. Strain Gauges-Kinds and Usage. London: Macmillan 1967: 1. BM. A method of management of the 2. Wroblewski Fractured stem in total hip replacement. Clin Orthop 1979; 141: 11-3. HP. Artifact free measurement of biological 3. Kimmich parameters, a historical review and layout of modern developments. In: A Handbook On Biotetemetry and Radiotracking. Oxford: Pergamon 1980: 3-20. Endoradiosonde. Nature 1957; 179: 4. Mackay RS,JacobsonB. 1239. VK, Baum J. Pressure-sensitive 5. Farrer JT, Zworkin telemetering capsule for study of gastro-intestinal motility. Science 1957; 126: 975. telemetry and 6. Ko WH. Power sources for implant stimulation systems. In: A Handbook on Biotelemetry and Radio-tracking. Oxford: Pcrgamon 1980: 225-45. 7. Parsonnet V. Cardiac Pacing and pacemaker VII, Power sources for Implantable pacemakers. 1 and II. Am Heart J 1977;94:517-28. A, Elfstrom G. Intravital wireless telemetry 8. Nachemson of axial forces in harrington distraction rods in patient with idiopathic scoliosis. J Bone and Jt Surg 1971; 53A: 445565. M, Goodman R. In uivo hip joint forces 9. Kilvington recorded on a strain gauged ‘english’ prosthesis using an implanted transmitter. Eng in Med 1981; 10: 175-87. in uivo loads from nail plate 10. Brown R et al. Telemetering implants. J Biomech 1982; 15: 815-23 11. D epartment of Trade and Industry. Minimum mandatory requirements for medical and biological telemetry devices. Publication W 6802.
M. In viva records of hip loads 12. English TA, Kilvington using a femoral implant with telemetric output (A preliminary report). J Biomed Eng 1979; 1: I 11-5.
implants in orthopaedics
P.J. Sell Biomedical Engineering Unit, North Stoke on Trent, ST4 7NY, UK Received
April
1988,
accepted
July
Staffs Polytechnic.
c/o Medical
Institute,
Hartshill
Rd,
1988
ABSTRACTS This brief paher discusses the needfor instrumentation in orthopaedic implants, and examines areas for further development. It is suggested that the future of this technology may have major clinical applications.
Keywords:
Instrumented
implants,
orthopaedics,
The use of implants in orthopaedic surgery has increased with the development of systems for internal fixation of fractures and joint replacement, the numerous types of prosthesis available having been developed in collaboration with engineers. In mechanical enginering a fundamental concern is the soundness of loaded structures’. A rational approach to the improvement and development of implants should include the measurement of forces acting across them in both static and dynamic modes of loading. On this basis alterations in the structure or material of the implant can be undertaken in an objective manner, and so eliminating trial and error in arriving at the most effective system. The loads carried by implants have in the past been calculated from indirect measurements using body weight, external force measurement and geometry. The number of reported cases of implant failure bears witness to the less than satisfactory results obtained by this method’; implant failure in prosthesis is nothing short of a disaster. The application of engineering techniques to measure strain in implants would provide valuable information with both clinical and research benefits. Strain is the change in unit length in a material subjected to load and its measurement, which is fundamental to the practical field of experimental stress analysis, can be achieved using mechanical, optical, pneumatic, acoustic or electrical strain gauges. Other techniques to arrive at a value for strain include mathematical modelling in the form of finite element analysis. In the biological setting the electrical strain gauge initially appears the most attractive, although there are considerable technical difficulties to be overcome. Electrical strain gauges are instruments that respond to strain in the surface to which they are bonded with an associated change in some electrical characFoil gauges respond with a teristic of the gauge. change in resistance. These consist of a small section grid or filament (produced by a photographic engraving process) mounted on paper or plastic. This is bonded to the structure to be analysed and connected to an instrument to measure the small 0 1989 Butterworth 0141-5425/89/0201
& Co (Publishers) I I-02 $03.00
strain
research and
gauges
changes in resistance which occur when strain is applied. Semiconductor gauges have a strain sensitivity an order of magnitude greater than that of foil. They consist of silicon or germanium crystals whose electrical properties are modified by doping with boron or arsenic atoms. These gauges exhibit the piezoresistive effect, in which the resistance changes with changes in strain independently of rate of deformation. Static strain can therefore be measured but this requires a current from an external source. Piezoelectrical strain gauges generate a free electrical surface charge proportional to stress. However the duration of this process is finite and therefore these gauges can only be used for dynamic measurements. In comparison with foil gauges, semiconductor gauges are more difficult to handle, have a narrower band for linear strain measurement and are more sensitive to temperature variations. In terms of biocompatibility semiconductor gauges present a possible hazard due to leakage of the doping agent. Measuring surface strain under laboratory conditions is straightforward and instrumentation is commercially available. At the present time, data can be acquired from implanted strain gauges by telemetry or by ‘hard wires’. The ‘hard wire’ techniques involve percutaneous leads to link the gauges to the instrumentation - this has a number of drawbacks. There is the hazard of infection as the wires create a sinus between implant necessitating regular microbiological and skin, screening and removal at the first sign of infection. Patients have to be connected to the instrumentation and this limits their mobility. Patient acceptability of this method would be low in the long term. Telemetry is the transmission of information to a separate receiver3. In 1903 Eintoven transmitted ECGs over a distance of 1.5 km using the public telephone system and the first use of radiotelemetry was in 1921 when Winters transmitted heart sounds from on board ship. Implantable and ingestable devices were first described by McKay and Jacobson4 and Farrer et al5 in 1957. The invention of the transistor in 1948 gave birth
Ltd J. Biomed. Eng. 1989, Vol. 11, March
111
Implantsin orthopaedics:P. J. Sell
to a second generation of telemetry devices, which permitted reductions in size, weight and power consumption. Third generation equipment has been available since the early seventies, utilizing CMOS integrated circuits and PCM coding techniques. These have improved reliability and reduced size but increased development cost. Their reduction in size allows less complex environmental protection and packaging. With either thick or thin film hybrid circuits and custom silicon integrated circuits, an hermetically sealed multi-channel transmitter would be no larger than a modern pacemaker. Biocompatibility has been shown to be acceptable for long term pacemaker implantation. Long term implantation studies require a stable dependable power source6. Primary batteries convert chemical or nuclear energy into electrical energy through an irreversible process. Major considerations are safety in the body environment, size, weight, availability, cost and service capacity. Lithium, mercuric or silver oxide/zinc batteries are commonly used in biotelemetry and there are several types of nuclear battery available. Over 2300 nuclear powered pacemakers were implanted between 1970 and 1976 with no failures reported in 1977’. Secondary batteries can be recharged, but for implanted telemetry devices a remotely coupled charging circuit is required. Biological batteries using dissimilar materials to form galvanic cells using body fluids as the electrolyte have been described. Electromagnetic energy is propagated by two means, induction and radio frequency radiation. The field strength of inductive coupling diminishes as the cube of the distance whereas RF radiation is inversely proportional to the distance, and as a result inductive coupling is effective over distances of only a few metres; RF radiation must be used for greater distances. Government regulations on the use of RF transmitters are more restrictive than for inductively coupled short range systems’ ’ . The small number of orthopaedic studies performed using strain gauges have employed a variety of methods with a wide range of orthopaedic implants *rg. Most commonly they have modified the implant so that the gauges are sealed within the aftered structure, and have either incorporated the telemetry device or connected the telemetry unit by flying leads r”*” Most studies have been short term. The problem of the environmental protection of gauges remains to be solved. If this could be overcome, then orthopaedic implants could be instrumented without altering their structure, so enabling more satisfactory strain readings to be obtained.
112
J. Biomed. Eng. 1989, Vol. 1 I, March
The engineering advances seen in the automotive and aerospace industries owe much to the information gained from experimental stress analysis, and the rational development of orthopaedic implants should follow the application of these techniques; the technology is available but at a high development cost. There may be useful clinical information to be gained as well, such as the monitoring of the progress of a bone graft fusion in spinal surgery or rational management of post-operative mobilization following hip prosthesis surgery. Perhaps the day will come when patients have check strain gauge readings taken from the implanted inductively powered telemetry unit at clinic attendance rather than check X-rays. ACKNOWLEDGEMENT Mr Snell is a Surgicraft
Spinal Research
Fellow.
REFERENCES I. Neubert HKP. Strain Gauges-Kinds and Usage. London: Macmillan 1967: 1. BM. A method of management of the 2. Wroblewski Fractured stem in total hip replacement. Clin Orthop 1979; 141: 11-3. HP. Artifact free measurement of biological 3. Kimmich parameters, a historical review and layout of modern developments. In: A Handbook On Biotetemetry and Radiotracking. Oxford: Pergamon 1980: 3-20. Endoradiosonde. Nature 1957; 179: 4. Mackay RS,JacobsonB. 1239. VK, Baum J. Pressure-sensitive 5. Farrer JT, Zworkin telemetering capsule for study of gastro-intestinal motility. Science 1957; 126: 975. telemetry and 6. Ko WH. Power sources for implant stimulation systems. In: A Handbook on Biotelemetry and Radio-tracking. Oxford: Pcrgamon 1980: 225-45. 7. Parsonnet V. Cardiac Pacing and pacemaker VII, Power sources for Implantable pacemakers. 1 and II. Am Heart J 1977;94:517-28. A, Elfstrom G. Intravital wireless telemetry 8. Nachemson of axial forces in harrington distraction rods in patient with idiopathic scoliosis. J Bone and Jt Surg 1971; 53A: 445565. M, Goodman R. In uivo hip joint forces 9. Kilvington recorded on a strain gauged ‘english’ prosthesis using an implanted transmitter. Eng in Med 1981; 10: 175-87. in uivo loads from nail plate 10. Brown R et al. Telemetering implants. J Biomech 1982; 15: 815-23 11. D epartment of Trade and Industry. Minimum mandatory requirements for medical and biological telemetry devices. Publication W 6802.
M. In viva records of hip loads 12. English TA, Kilvington using a femoral implant with telemetric output (A preliminary report). J Biomed Eng 1979; 1: I 11-5.