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Objectives

The design challenge is to design and prototype a new anchor. Our research team, which consists of bioengineers and orthopaedic surgeons, has developed a new concept for a shoulder anchor that maximizes the stability of anchorage into the bone, but uses a smaller footprint (requires less bone removal). Bearing in mind key design criteria of biocompatibility, matching the design solution to the material properties and geometry/anatomy of bone in the glenoid fossa, and ease of surgical deployment, the design team will design a new anchor, fabricate a prototype, and test the prototype on phantom (simulated) and cadaveric shoulders.

Final Design

The final design consists of 3 components. The primary component is the anchor which is comprised of two wings connected by a flexure hinge. The anchor is a cylindrical 2.8mm diameter tube with 2D cut outs. From pre-deployment, the anchor opens by 90° to its final state.

The cone tip is a 3.175mm diameter cylinder with a thickness of 2.5mm. The bottom face of the cone tip which points into the bone is flat. The other end of the cone tip takes the shape of an inverted cone. The cone side is aligned with the anchor to act as a guide during deployment. The final component is the wedge inserter which is a 2.8mm diameter tube with a v-shaped cut at the bottom to push the wings apart during deployment.

Surgically, whether through open surgery or arthroscopic surgery, the surgeon gains access to the glenoid, and drills a hole into the edge of the bone of the glenoid fossa. An anchor is then placed into the drill hole, and sutures, which are attached to the anchor, are used to tie the labrum securely in place. A variety of anchors exist, each with their own strengths and weaknesses.

More traditional anchors more firmly and stably attach to the bone, but require a significant volume of precious bone to be drilled away. Other

anchors require less bone to be drilled away, but attach less securely to the bone.

Anchor for Shoulder Instability

Bryan Brennan, Helen Tat, Delta Caraulia, Darren Deng

Background

Shoulder dislocation is an important clinical problem. The shoulder, a ball-and-socket joint, is the most flexible joint in the body. However, a

consequence of this increased flexibility is decreased stability. Decreased stability renders the shoulder at high risk for dislocation, which is defined as the ball of the joint coming out of the socket. In the case of the shoulder, this means that the head of the humerus comes completely out of the glenoid fossa. Because of the geometry and biomechanics of the shoulder, it is the most susceptible joint for dislocation. In fact, shoulder dislocations account for over 50% of major dislocations, often through traumatic events such as athletic injury or fall. Compounding the problem, once the shoulder has dislocated once, it increases the likelihood of another dislocation. Recurrent dislocations can cause permanent damage to the anatomy of the shoulder, leading to chronic joint instability, loss of shoulder function, and pain.

Wedge Inserter Anchor Cone Tip

(1. Initial Insertion, 2. Beginning of Deployment, 3. Mid Deployment 4. Fully Deployed 5. Inserter Removed )

Suture routes through all three components and begins at the cone tip where suture is bunched up in a knot to prevent pull-out. The single loaded suture then splits into two sutures and a surgeon then ties the labrum tear.

Steps:

1) drill hole into glenoid fossa

2) insert cone tip and anchor

3) place wedge inserter at a fix depth

4) deploy anchor by pulling up on suture to bring anchor and cone tip to wedge inserter

5) remove wedge inserter

6) tie labrum with suture

Test Setup

The final design of the anchor was tested in three different porosities of Sawbones synthetic bone. Porosity refers to the density of the bone specimen. After insertion into a pre-drilled hole in block of Sawbones, the following test setup was arranged on an Instron machine.

Summary of Performance Results

Each sample was run on the Instron twice: once to deploy the anchor and once to test the pull out strength.

Deployment (example of anchor in 15 PCF):

Pull-out Test (example of anchor in 15 PCF):

The anchors were tested in various simulated bone densities and types of deployment. The results are as follows:

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