Michael C. Dahl

The efficacy of using vibrometry to detect osteointegration of the Agility total ankle

Arthritis is a chronic, debilitating disease affecting one in six people in the United States annually. One of the most promising surgical treatments is total joint replacement. After decades of development, some joint replacement (arthroplasty) systems such as the hip and knee enjoy high success rates while others, particularly newer ones for the ankle, have disappointing survival rates. The goal of this study was to investigate, develop, and test a methodology to assess implant osteointegration, specifically for the talar component of a total ankle prosthesis. A vibrometry technique using Doppler ultrasound was developed to non-invasively determine osteointegration clinically. This methodology was evaluated via preliminary experimentation, along with another validation methodology, to access whether design criteria have been met in order to initiate a clinical study of the technique. Bench-top and cadaveric testing demonstrated that the Doppler ultrasound technique could distinguish the level of osteointegration between loose and fixed implant components. The laser vibrometry technique, used for the validation of the ultrasound technique intraoperatively, was also shown to be functional and indicative of the ultrasound techniques testing results. This methodology can provide a much needed tool to determine the integration of implants non-invasively in the clinical and surgical setting, thus allowing each patients rehabilitation program to be monitored and tailored to maximize the osteointegration and survival rate of their total joint replacement.

A comparison of the shock-absorbing properties of cervical disc prosthesis bearing materials

Background Data Cervical arthroplasty offers theoretical advantages over traditional spinal fusion, including elimination of adjacent segment disease and elimination of the risk of pseudoarthrosis formation. Initial studies of cervical arthroplasty have shown promising results, however, the ideal design characteristics for disc replacement constructs have not been determined. The current study seeks to quantify the differences in the shock absorption characteristics of three commonly used materials in cervical disc arthroplasty. Methods Three different nucleus materials, polyurethane (PU), polyethylene (PE) and a titanium-alloy (Ti) were tested in a humidity- and temperature-controlled chamber. Ten of each nucleus type underwent three separate mechanical testing protocols to measure 1) dynamic stiffness, 2) quasi-static stiffness, 3) energy absorption, and 4) energy dissipation. The results were compared using analysis of variance. Results PU had the lowest mean dynamic stiffness (435 ± 13 N/mm, P < .0001) and highest energy absorption (19.4 ± 0.1 N/mm, P < .0001) of all three nucleus materials tested. PU was found to have significantly higher energy dissipation (viscous damping ratio 0.017 ± 0,001, P < .0001) than the PE or TI nuclei. PU had the lowest quasi-static stiffness (598 ± 23 N/mm, P < .0001) of the nucleus materials tested. A biphasic response curve was observed for all of the PU nuclei tests. Conclusions Polyurethane absorbs and dissipates more energy and is less stiff than either polyethylene or titanium. Level of Evidence Basic Science/Biomechanical Study. Clinical Relevance This study characterizes important differences in biomechanical properties of materials that are currently being used for different cervical disc prostheses.

Kinematic and fatigue biomechanics of an interpositional facet arthroplasty device

BACKGROUND CONTEXT Although approximately 30% of chronic lumbar pain can be attributed to the facets, limited surgical options exist for patients. Interpositional facet arthroplasty (IFA) is a novel treatment for lumbar facetogenic pain designed to provide patients who gain insufficient relief from medical interventional treatment options with long-term relief, filling a void in the facet pain treatment continuum. PURPOSE This study aimed to quantify the effect of IFA on segmental range of motion (ROM) compared with the intact state, and to observe device position and condition after 10,000 cycles of worst-case loading. STUDY DESIGN/SETTING In situ biomechanical analysis of the lumbar spine following implantation of a novel IFA device was carried out. METHODS Twelve cadaveric functional spinal units (L2-L3 and L5-S1) were tested in 7.5 Nm flexion-extension, lateral bending, and torsion while intact and following device implantation. Additionally, specimens underwent 10,000 cycles of worst-case complex loading and were testing in ROM again. Load-displacement and fluoroscopic data were analyzed to determine ROM and to evaluate device position during cyclic testing. Devices and facets were evaluated post testing. Institutional support for implant evaluation was received by Zyga Technology. RESULTS Range of motion post implantation decreased versus intact, and then was restored post cyclic-testing. Of the tested devices, 6.5% displayed slight movement (0.5-2 mm), all from tight L2-L3 facet joints with misplaced devices or insufficient cartilage. No damage was observed on the devices, and wear patterns were primarily linear. CONCLUSIONS The results from this in situ cadaveric biomechanics and cyclic fatigue study demonstrate that a low-profile, conformable IFA device can maintain position and facet functionality post implantation and through 10,000 complex loading cycles. In vivo conditions were not accounted for in this model, which may affect implant behavior not predictable via a biomechanical study. However, these data along with published 1-year clinical results suggest that IFA may be a valid treatment option in patients with chronic lumbar zygapophysial pain who have exhausted medical interventional options.