Design of a Mechanical Tensioner Unit for Chest Suspension in Postoperative Cardiac Neonates

Luke J. MacLean 1, 2; Jan Andrysek 1, 3; Vito Forte 1, 2

1. Institute of Biomaterials & Biomedical Engineering, University of Toronto; 2. The Hospital for Sick Children; 3. Holland Bloorview Kids Rehabilitation Hospital

Background

Newborns requiring cardiac corrective surgery also need a sternotomy. Sternal closure can create instabilities that can be mitigated by a Delayed Sternal Closure (DSC). The open chest reduces pleural pressure and complications, but an unsupported chest wall can still increase pressure. SickKids has begun chest wall suspension via tension sutures, but the approach has technical limitations. Chest suspension (CS) prevents X-ray imaging, lacks quantified tension, and leads to parental anxieties. A tensioner is a mechanism which controls and measures cable tension. An adjustable and radiolucent tensioner could compliment and improve the current technique.

Objectives

A device must be designed to support chest suspension. It should allow for tension quantification, chest radiography, and a professional aesthetic. The device must emulate the current surgical approaches, and be purely mechanistic (i.e. non-robotic) to allow for sterilization. The first goal will be to establish quantified design requirements to achieve these functions. Secondly, the device must be appropriately designed and constructed. Finally, the functionality of the tool will be verified by benchtop studies.

Methods

Design requirements are determined by survey interviews and a retrospective chart review. The survey defines the current methodology and translates its requirements to thresholds. The chart review studies the care of CS patients to show distinctions. The tool is built by T-slotted framing, 3D printing, and machining thermoplastics. Geometry and structural integrity of key parts are assessed analytically and by finite element analysis. The tool is validated by measuring tension with an electronic force gauge, temperature with a pyrometer, and deflections with an electronic level and the Aurora 3D system. X-ray imaging verifies lucency. Validation Interviews established requirements in orientations, quick-release, lucency, stiffness, heat transfer and tension. The chart review showed CS patients have twice the number of operations, critical care time, and open chest time, and certain diagnoses were more common. A metal frame was built and the rigidity was checked analytically, numerically and experimentally. A cantilever was machined out of Ultem to satisfy the stiffness, thermal and lucency requirements. A compensator-pulley system was created so to move the device for X-Rays without altering tension. A non-backdrivable gear system adjusts tension and is read by a spring gauge. The tensile transfer and twisting was measured, and heat transfer to the bed was captured with and without the device to show minimal difference.

Significance

The chest suspension device will standardize the technique and overcome the current technical limitations. The tool will improve complex-care to recovering newborns. Further, the methodology of chest suspension can be formally implemented and studied. The device will help to disseminate a standardized technique to other pediatric centers.