Date Approved

12-2018

Graduate Degree Type

Thesis

Degree Name

Engineering (M.S.E.)

Degree Program

School of Engineering

First Advisor

Dr John Farris

Second Advisor

Dr Prince Anyalebechi

Third Advisor

Dr Lindsay Corneal

Academic Year

2018/2019

Abstract

A rotator cuff tear is a common injury that affects many elderly people. These tears vary in severity, with moderate to severe tears requiring surgery. In rotator cuff surgery, bone anchors are used to re-attach the supraspinatus tendon to the humerus. This surgery is difficult in many older patients because aging bones become osteoporotic, or soft, and thus inadequate for securing bone anchors. Studies confirm that osteoporotic bone is a contributing factor of implant failure. A finned sheath concept was designed, developed, and tested to ensure securement of the bone anchors during and after rotator cuff repair. The sheath interacts with existing bone anchors and expands its fins into the soft bone. The expanding fins push against the bone’s cortical layer and prevent failure.

Based on the limits on properties method and clinical feedback, polyether ether ketone (PEEK) was chosen as the optimum material. Due to the expense of injection molded PEEK, five prototype sheaths were 3D printed in acrylonitrile butadiene styrene (ABS) to test in an osteoporotic bone model. For comparison, five anchors and five sheath assisted anchors were inserted into the osteoporotic bone model. The anchors were pulled out by the sutures included in the Arthrex SwivelLock anchor kit using a MTS tensile machine. The ultimate tensile strength and failure mode were recorded for each. Four of the five anchor trials had a failure mode of pulling out of the bone model. All five of the sheath trials had a failure mode of the suture breaking. Without the sheath, the anchor pullout force was comparable to published literature. With the sheath, the sutures broke at approximately an order of magnitude greater than the sheath pullout force.

Additional calculations and finite element analyses were completed to determine the factors for adequate fixation. The first factor is an interference fit of 0.05 mm between the sheath and the anchor, and the sheath and the bone. The second factor is the expanding fins design. These fins were analyzed through a Solidworks simulation to prove that adequate fixation is possible with four fins. The analyses and calculations used to determine the two factors demonstrate that the tendon would tear before the sheath would fail.

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