Student Presentation -- Alex Drew
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Ph.D. Dissertation Defense, Tuesday September 18, 2018 -- Humerus Morphology and Mechanics: Informing Design of a Percutaneous Osseointegrated Docking System for Above Elbow Amputees

Eccles Institute of Human Genetics (EIHG) Gesteland-White Auditorium, 1:00 pm

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Speaker: Alex Drew. Advisor: Dr. Kent Bachus


Socket suspensions that fit over the residual limb of an amputee are the standard of care in prosthetic limb attachment. Unfortunately, complications related to pain, inconvenience, and lack of functionality lead to rejection rates as high as 65% in upper extremity amputees. Percutaneous Osseointegrated Docking Systems (PODS) for direct skeletal attachment of prosthetic limbs represent an alternative for those underserved by the current standard of care. PODS have been used in Europe for prosthetic attachment in above knee amputees since 1990 with limited surgeries occurring in the United States in recent years. Use in above elbow amputees has been less common, and is troubled with poor implant fit leading to high rates of complication, including aseptic loosening, and adverse cortical remodeling. Such complications highlight the challenges of designing an implant system capable of stable fixation at multiple amputation levels in the humerus.

Detailed morphologic evaluation of the humeral diaphysis revealed that proximal stabilization of an implanted stem is made difficult by the diverging canal of the humerus and progressive thinning of cortical bone proximally. Therefore we propose a novel implant design that leverages our recent experience with a press-fit porous coated implant for transfemoral use, and incorporates proximal interlocking screws for enhanced fixation. This design underwent preclinical mechanical testing of initial stability in cadaver humeri to evaluate construct stiffness, yield strength, and ultimate fracture strength. The contribution of interlocking screws to initial stability was also compared against press-fit fixation without proximal stabilization.

Ensuring the safety and effectiveness of this design required improved understanding of the loads experienced by the humerus during daily motion. Therefore the last aim of this dissertation characterized the dynamic mechanical forces experienced by the humerus during advanced activities of daily living (AADLs) representative of an active amputee population. Tensile, torsional and bending loads were estimated from inverse dynamic analysis in healthy volunteers. Results indicated that non-contact AADLs were capable of producing forces and moments at or above fracture loads reported in the humerus; motivating the development of overload protection devices that isolate the implant from damaging forces.

This dissertation is part of a larger effort funded by the Department of Defense Joint Warfighter Medical Research Program aimed at delivering transhumeral PODS technology to an FDA Early Feasibility Study. The overarching aim of this work is to inform the design and preclinical evaluation of that technology.