Design of MEMS Sensor System to Measure Spinal Fusion

Abstract

Being able to determine the formation of a solid, spinal fusion after lumbar surgery continues to be one of the most difficult issues facing spinal surgeons today. Patients are fitted with a spinal brace for three to twelve months after surgery, even though the spinal implant provides internal fixation. Such a brace is not only inconvenient, as it completely immobilizes the patient, but not in the patient’s long term interest. Studies have shown that immobility leads to muscle atrophy in the spine; therefore, the best option is to minimize the amount of time for bracing. Currently, surgeons rely on radiographs to view the fusion mass. Fusions are inherently difficult to view, as the spine’s transverse processes and the spinal hardware lie within the fusion region. Thus, surgeons use their best judgment and their professional experience to decide when a fusion is solid and the patient may remove their brace. According to one study, fusion actually occurs between eight and twelve weeks in sheep, as much as eight weeks before it was indicated in their radiographs. Radiographs are delayed because they can only determine fusion when the trabecular bone of the fusion has mineralized, which occurs after the fusion -2- agglomeration has reached full size and stabilization and can bear mechanical load. Immobilization causes another cost to the patient and society. While wearing the brace, the patient cannot return to work, drive, or perform everyday activities that constitute a good quality of life. Long term disability insurance costs are thus dramatically increases, making spinal fusion surgery the most costly orthopaedic procedure. With over 300,000 predicted spinal surgeries in 2005 in a market calculated at $2.4 to $3.1 billion dollars, and at an average cost of $34,000 per surgery, plus $20,000 for professional surgeon fees, and because worker disability payments are averaging 1% of the gross domestic product, major steps need to be taken to reduce overall costs where possible. Serious progress toward the goal of controlling costs could be made by reducing the cost of disability, mainly by allowing patients to return to work sooner. Because of the importance of allowing early patient mobility to avoid muscle atrophy, and because of the equal importance of reducing costs of spinal fusion, the goal of this research is to develop a more accurate and earlier means of detecting spinal fusion. This study has developed a better diagnostic tool for surgeons based on strain to address the problems associated with the current method of bracing and radiographs. As it is known from the literature that the amount of time for fusion is significantly less than it is indicated by radiographs, and patient outcomes would be -3- improved if a superior diagnostic method were developed, this study has designed and submitted a patent application to measure strain in vivo using a capacitive sensor and a transponder to send the signal via radio frequency to an external receiver. A handheld unit would be brought into proximity of the sensor and an initial strain level would be recorded. Then, during routine office visits, the handheld sensor would again be brought into proximity of the sensor to get additional strain level recordings. Over time, the level of strain should decrease and eventually plateau at a lower level. The hypothesis was that this would occur within eight to twelve weeks following surgery, at which time the fusion could be proclaimed solid and the patient’s external bracing removed. The report outlines the evolution of a design to measure the onset of spinal fusion using a battery-free, interdigitated capacitive strain sensor that will use supplied radio frequency power from an external, handheld receiver to activate the sensor. The output data will be subsequently analyzed on a computer.