Ultrasonic indentation: a procedure for the noninvasive quantification of force-displacement properties of the lumbar spine.

BACKGROUND: Alterations in the normal force-displacement (FD) properties of spinal tissues have been associated with specific forms of pathology, such as degenerative disk disease. Unfortunately, few current procedures exist that assess spinal FD properties in an accurate, reliable, and noninvasive manner. Consequently, the clinical relevance of the relation between spinal disorders and spinal FD properties is not fully understood. OBJECTIVE: To investigate the accuracy and reliability of spinal FD measures obtained through use of a procedure in which real-time ultrasonic imaging (ultrasonic indentation) is used during load-controlled external indentation. SETTING: McCaig Centre for Joint Injuries and Arthritis Research, University of Calgary. METHODS: The bench-top accuracy and reliability of ultrasonic indentation were assessed by cyclic indentation of a spring-mounted platform. These data were compared with criterion data derived from a materials testing machine. A porcine preparation was then used to assess the accuracy of ultrasonic indentation-generated estimates of vertebral displacement in comparison with a criterion of optically tracked displacement. In addition, previously unreported parameters relating to indentation accuracy (frame deflection and off-axis loading) were characterized. RESULTS: Reliability of ultrasonic indentation ranged between 0.99 and 1.00 (intraclass correlation coefficient). Error values in force, displacement, and stiffness ranged from 0.81% to 13.62% over varying experimental conditions. CONCLUSIONS: Ultrasonic indentation is a unique procedure that is capable of assessing, noninvasively, FD properties of spinal tissues, including vertebral displacement in the indentation plane. The results of this study suggest that ultrasonic indentation is a potentially useful technique for quantifying spinal FD properties in vivo.

Sources of variation in spinal indentation testing: indentation site relocation, intraabdominal pressure, subject movement, muscular response, and stiffness estimation.

BACKGROUND: Force-displacement properties of spinal tissues assessed by blunt indentation are thought to have clinical relevance; however, numerous variables with respect to spinal indentation have yet to be identified or characterized completely. OBJECTIVE: To identify and quantify, where possible, previously unidentified or incompletely characterized variables with respect to spinal indentation. DESIGN: Multiprotocol design. METHODS: Four experiments were performed: (1) Twelve asymptomatic subjects were indented with concurrent electromyography during conditions of rest, held inspiration, increased intraabdominal pressure and lumbar extension. (2) Changes in the recumbent position of 12 subjects were measured while a series of movements was performed in restrained and unrestrained conditions. (3) Ten clinicians attempted to locate, and to relocate, a subcutaneous anatomical landmark through visualization/palpation and ultrasonic imaging. (4) Performances of 3 methods of force-displacement curve modeling were compared with respect to stiffness estimation. RESULTS: (1) Spinal stiffness increased significantly in a minority of subjects awaiting indentation and in a majority of subjects during increases in intraabdominal pressure. (2) Changes in subject position were significantly reduced by a restraint system. (3) With respect to interclinician error in locating and relocating an indentation site, there was significant improvement with the use of ultrasonic visualization. (4) The error associated with linear techniques used to model curvilinear force-displacement data plots increased with increasing linear intervals. CONCLUSION: Several sources of variation in spinal indentation were identified: indentation site relocation, intraabdominal pressure, subject movement, muscular response, and stiffness estimation. These variables, which have been unaccounted for in previous indentation studies, might be responsible for the change or lack of change in force-displacement properties between preintervention and postintervention indentation trials.

Ultrasonic quantification of osseous displacements resulting from skin surface indentation loading of bovine para-spinal tissue.

OBJECTIVE: To validate an ultrasound-based technique which quantifies uni-planar subcutaneous displacement of an osseous object resulting from an externally applied load. BACKGROUND: Many spinal conditions are thought to be characterized by aberrant vertebral displacements yet the invasive nature of many investigative techniques has left the clinical significance of this relation incompletely understood. METHOD: Six bovine bone/paravertebral tissue preparations were indented by one of two ultrasonic transducers (5 and 7 MHz) fitted to an electromechanical actuator. The resulting osseous displacement along the principal indentation axis was calculated by subtracting the change in transducer/bone distance between ultrasonic images collected at tissue contact and maximal load from the change in actuator displacement. A dial gauge contacting the bone was used as a displacement criterion measure. RESULTS: Using the 7 MHz transducer, the mean error of the technique was 6.74% (SD=3.98) while the mean error associated with the 5 MHz transducer was 12.73% (SD=7.49). CONCLUSIONS: This non-invasive technique is capable of quantifying subcutaneous uni-planar bone displacement with an accuracy comparable to similar invasive techniques over a comparable displacement range. RelevanceThis non-invasive technique may be beneficial in assessing the significance of vertebral displacements in conditions such as hypermobility and osteoarthritis, as well as in studies of manipulative therapy.