Pigjaw Scaffold–Bone Interface Mechanical Testing

Introduction

Removing cancer from the jaw can leave a gap too large to heal on its own. Researchers are testing scaffolds that help new bone grow in that gap. The planned push-out test would measure force and displacement to estimate the scaffold–bone construct’s apparent stiffness and assess its mechanical integration.

Can a low-cost, 3D-printed holder keep the jawbone steady during a test without bending enough to throw off the results?

Initial Design

The PLA assessment used a supplied reference geometry with a central opening, a slotted annulus, and narrow connections to the surrounding plate. The thinner central web and interrupted load path made it important to look beyond the fixture’s overall dimensions when estimating stiffness.

The analysis used an assumed 250 N load and a 50 µm maximum annular-motion target. These were screening assumptions, not measured test conditions or validated requirements.

Isometric CAD view of the reference fixture with a raised central deck, circular opening, two curved annular slots, mounting flanges, and an underside relief — Reference fixture geometry used for the PLA assessment, showing the central annulus, curved slots, and mounting flanges.

Intended push-out setup with a porcine jaw specimen secured to the fixture and a red downward force F applied directly to the cylindrical scaffold inside the bone defect — Intended scaffold–bone push-out setup. The red force vector acts directly on the cylindrical scaffold, while the fixture supports the surrounding jawbone.

Simplified Model

I modeled the deck as an equivalent simply supported bridge to estimate global bending and shear compliance. This retrospective September 2026 calculation uses a short, linear-elastic beam under a central load, not an as-built model of the metal assembly.

Equivalent simply supported bridge with a central downward force and equal upward reactions at the two supports — Equivalent bridge model used to estimate global bending and shear deflection.

Geometry and assumptions

The model assumes a rigid base supporting the lower lands, idealized as simple supports at \(z=\pm30~\mathrm{mm}\). The equivalent section uses the remaining web thickness and subtracts the full slot envelope across the entire span.

Input	Symbol	Value
Assumed central load	\(F\)	\(250~\mathrm{N}\)
Effective support span	\(L\)	\(60~\mathrm{mm}\)
Remaining web thickness	\(h\)	\(30-15=15~\mathrm{mm}\)
Equivalent width	\(b_g\)	\(120-2(22.15)=75.7~\mathrm{mm}\)
PLA surrogate modulus	\(E\)	\(2000~\mathrm{MPa}\)
Poisson ratio	\(\nu\)	0.35
Rectangular shear factor	\(\kappa\)	\(5/6\)
Selected annular-motion target	\(\delta_{\mathrm{target}}\)	\(50~\mu\mathrm{m}\)

The section properties and isotropic-surrogate shear modulus are:

\[ \begin{aligned} A_g&=b_gh=1135.5~\mathrm{mm^2},\\ I_g&=\frac{b_gh^3}{12}\approx21290.6~\mathrm{mm^4},\\ G&=\frac{E}{2(1+\nu)}\approx740.7~\mathrm{MPa}. \end{aligned} \]

Reactions and nominal bending stress

\[ \begin{aligned} R_A=R_B&=\frac{F}{2}=125~\mathrm{N},\\ M_{\max}&=\frac{FL}{4}=3750~\mathrm{N\,mm},\\ \sigma_{g,\max}&=\frac{M_{\max}(h/2)}{I_g}\approx1.321~\mathrm{MPa}. \end{aligned} \]

This is nominal bridge stress, excluding annulus and ligament-root concentrations—not a fixture strength rating.

Bending and shear deflection

The span-to-thickness ratio is only 4, so both bending and transverse shear contribute to midspan motion.

\[ \begin{aligned} \delta_b&=\frac{FL^3}{48EI_g}\approx26.42~\mu\mathrm{m},\\ \delta_s&=\frac{FL}{4\kappa GA_g}\approx5.35~\mu\mathrm{m}. \end{aligned} \]

\[ \boxed{\delta_g=\delta_b+\delta_s\approx31.77~\mu\mathrm{m}} \]

The bridge estimate is below the selected 50 µm screen, but does not resolve local annulus or ligament deformation. Its midspan motion differs from the FEA’s approximately 131 µm annular maximum; the lower bridge result alone does not establish full-fixture compliance.

Material comparison

With geometry, supports, and load held fixed, the same bending-plus-shear model gives:

Trial material	\(E\) (MPa)	\(\nu\)	\(G\) (MPa)	Midspan motion (µm)
PLA surrogate	2,000	0.35	740.7	31.77
6061 aluminum	68,300	0.33	25,676.7	0.928
304 stainless steel	200,000	0.30	76,923.1	0.316

These are analytical material comparisons using trial isotropic properties, not metal FEA or measurements of the built assembly.

Finite Element Analysis

ANSYS modeled the full PLA fixture with annular pressure, compression-only backing, and idealized washer restraints, excluding bone, scaffold, and interface failure.

At the assumed load, the model predicted approximately 131 µm of maximum downward annular motion, above the selected target. Hand-calculation results are not directly interchangeable; local stresses need further assessment.

ANSYS vertical displacement contour of the loaded annulus in the supplied PLA reference fixture, with the millimeter legend and result readouts — Modeled vertical displacement of the loaded PLA annulus; contour values are shown in millimeters.

Displacement Comparison

At the same assumed 250 N load, the PLA bridge predicts 31.77 µm of midspan motion, while the final M2 FEA predicts 131.37 µm of maximum downward annular motion. The FEA exceeds the chosen 50 µm annular-motion target; the bridge’s smaller result does not establish that the complete fixture meets it.

On smaller screens, scroll the table horizontally to read all columns.

Reported displacement magnitudes at an assumed 250 N
Material and model	Motion (µm)	Response and model scope
PLA · analytical	31.77	Midspan bending plus shear; equivalent 60 mm simply supported bridge, 15 mm web.
PLA · final M2 FEA	131.37	Maximum downward motion on the loaded annulus; full reference fixture with backing and idealized hold-downs.
6061 aluminum · analytical	0.928	Same 60 mm bridge, with trial aluminum properties; no metal FE result.
304 stainless · analytical	0.316	Same 60 mm bridge, with trial stainless-steel properties; no metal FE result.

Metal Fixture

Following the PLA analysis, I built the metal push-out fixture shown here. The assembly includes the push-out plate, an adapter, and a threaded pusher connection.

Hand calculations also compared aluminum and stainless steel as stiffer material options for the same simplified geometry. These comparisons supported considering metal, but no metal FEA or physical qualification of the completed assembly is included in this analysis.

Photograph of the built metal push-out plate beside a reference CAD view of the plate mounted on an extrusion frame — Metal push-out fixture I built, shown alongside its CAD assembly.

Section view showing the threaded pusher rod, nut, and adapter connection — Section view of the threaded pusher connection.

Metal cylindrical adapter with a cross-hole beside a separate threaded pusher rod — Adapter and threaded rod for the push-out fixture.

Dimensioned Pigjaw fixture plate drawing showing the front and edge views, central opening, curved slots, and complete title block — Pigjaw fixture plate drawing.

Dimensioned push-in fixture drawing with front, end, and isometric views of the threaded pusher and cylindrical adapter, including the complete title block — Push-in fixture drawing.

Outcome

This work connected a simplified structural model, a more detailed numerical assessment, and a physical fixture build. The PLA results identified a compliance concern and showed why passing the bridge calculation was not enough to establish the full fixture’s behavior. They did not demonstrate fracture. The next step is to confirm the actual test requirements and measure the completed assembly’s motion under representative loading.