RIB FIXATION DEVICE DESIGN
Summary
This project proposed the conceptual design of a bioabsorbable rib fixation device. The design process was informed by a clinical analysis of patient conditions, diagnostic criteria, and existing fixation systems, revealing stiffness-related limitations in current devices and highlighting the need for progressive flexibility of the device. In response, a hybrid rib fixation concept was developed, integrating biodegradable components to enable a gradual increase in the flexibility of the implant as the ribs heal.
Integrative Design of Biomedical Products
November, 2024
#rib_fracture
#medical_device_design
#technical_analysis
#biodegradable_materials
(Left) Bioabsorbable rib-fixation device concept sketch and (right) illustration of the device's behaviour as it degrades.
Problem Analysis
Clinical contex
Rib fixation devices are used to surgically stabilize severe rib fractures, particularly in flail chest cases—defined as three or more ribs fractured in at least two places (Moya et al., 2017). The procedure involves incision, muscle dissection, fracture localization, device attachment, and wound closure, while preserving the intercostal muscles.
Proper selection of device components (e.g., plate size and screw length) is essential for stability. Pre- and postoperative mobility and pulmonary hygiene are critical to reduce respiratory complications (Kocher et al., 2020).
Users
Rib fixation patients in the Netherlands have a mean age of 62 years, with 68% being male (Peek et al., 2022). Postoperative hospitalisation is associated with lower mortality (R. Zhu et al., 2020). Lifestyle factors may therefore influence recovery outcomes and the success of the intervention.
Involved tissues
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Rib bones are flexible structures with an elastic modulus of 7.5–11.9 GPa, varying by region and age (Stitzel et al., 2003). They expand 3–5 cm during breathing, protecting thoracic organs and enabling lung motion (G. Zhang et al., 2018). Flail chest recovery typically takes 8–12 weeks.
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Intercostal muscles, often affected in rib fractures, connect the ribs to surrounding tissues and are essential for rib cage expansion and contraction during respiration (Saumarez, 1986; De Troyer et al., 2005).
Problem Definition
The clinical need is to stabilize severe rib fractures without imposing long-term restrictions on rib flexibility and respiratory function. The core issue lies in the permanent stiffness of conventional plating systems, which can limit physiological rib motion. To address this, the proposed technical principle aims to increase the deformation capacity of the plating system so that it better aligns with natural rib mechanics.
Technical principle
This can be achieved by increasing strain (𝜀ₚ) and elongation (∆Lₚ), reducing the contact area (Aₚ), and lowering the elastic modulus (Eₚ) of the implant material.
Design Concept
Following, three main design criteria were defined for the design.
Inspired by insect moulting, the concept proposes a hybrid rib fixation device that balances stiffness and flexibility during healing. It combines an absorbable semiflexible plate with rigid, non-absorbable stiffening bridges and screws, making the system initially stiff and progressively more flexible. As the absorbable components degrade, contact area and structural constraint decrease, while the stiffening bridges maintain stability without significantly restricting rib motion.
Design motivation
Existing devices provide either fixed rigidity or flexibility, leading to risks such as respiratory insufficiency, implant loosening, and migration; fully bioabsorbable systems may also lack sufficient stiffness for flail chest stabilization. The proposed design enhances flexibility by using a lower Young’s modulus material, such as PLA (Xue et al., 2022), and by progressively reducing contact area, while initial stiffness is ensured through a curved crease geometry that increases moment of inertia (Raducanu et al., 2021).
The present conceptual design was further developed in the group phase of the course, leding to further characterisation and testing of the design. To know more about this project, request the full report here.
(Left) movement pattern of the lung in a flail chest and (right) rib-fixation intervention with plates.
Persona mapping: target user analysis
(Left) tissues involved in a flail chest and (right) healthy breathing pattern displaying lung displayment.
(Top) Free-body diagram of the rib fixation device; (bottom) mechanical analogy to a cantilever beam subjected to a distributed load.
Analytical expression for fixation device displacement as a function of breathing force, device–rib contact area, and material Young’s modulus.
Design criteria specifications.
(Left) Graphic references illustrating the design concept inspiration—namely insect moulting, bioabsorbable plates, and metal plates; (right) schematic representation of the device’s mechanical behavior during degradation.
(Left) existing rib-fixation plates flexibility and (right) their surgical intervention.
Strain and stress comparison between PLA (biodegradable) and titanium plating systems under equal force and contact area conditions.