Spinal Fusion CAD

Project Overview

I created this project to strengthen my SolidWorks skills through a medical-device application. After becoming interested in spine-focused med-tech companies in Carlsbad, California, I wanted to move beyond basic CAD practice models to something combining anatomy, medical imaging, and implant design.

The goal was to design a conceptual patient-specific L4–L5 interbody fusion cage using anatomy derived from a public CT dataset, working through a full CAD workflow: CT segmentation and anatomical measurements, part modeling, assemblies, fixation geometry, technical drawings, and animation.

This project was completed independently after graduation as a learning exercise. It is intended for educational and design practice only and is not a clinically validated medical device. The geometry and design decisions should not be interpreted as recommendations for clinical use.

3D Slicing CT Scans

I also experimented with 3D Slicer’s measurement tools to better understand the anatomy and disc-space geometry. These served as a learning and cross-reference step; the dimensions driving the final CAD design were taken from the exported anatomy in SolidWorks.

Once the segmentations were complete, I exported L4 and L5 as STL files and imported them into SolidWorks, positioning them in an assembly that maintained their relative anatomy.

Defining the Cage Dimensions

With L4 and L5 inside SolidWorks, I measured the opposing endplates and L4–L5 disc space to establish the design envelope for the cage.

Measurements included:

Measurement	Value
L5 superior endplate AP depth	32.05 mm
L5 superior endplate width	42.45 mm
L4 inferior endplate AP depth	31.16 mm
L4 inferior endplate width	41.64 mm
Anterior disc-space height	9.38 mm
Posterior disc-space height	4.98 mm
Patient-left disc-space height	5.68 mm
Patient-right disc-space height	6.07 mm
Central disc-space height	10.01 mm

I intentionally did not make the cage footprint equal to the full measured endplate dimensions. Using the smaller L4 inferior endplate as the limiting reference, I targeted a footprint of approximately 36 mm mediolateral × 26 mm AP. Against the 41.64 mm × 31.16 mm L4 endplate, that gave roughly 2.82 mm of mediolateral and 2.58 mm of AP clearance per side, about a 2–3 mm perimeter inset.

The offset keeps the footprint inside the endplate boundary rather than at its maximum dimensions, reducing the risk of edge overhang while preserving a broad contact area. I kept refining it as the design evolved, using the patient anatomy and assembly fit as visual references.

Once I had established the dimensional envelope and intended clearance, I began building the cage.

Building the Base Geometry

• Sketched the initial cage footprint and used sketch fillets to create rounded corners appropriate for the overall implant form.
• Tapered the footprint so the anterior and posterior widths were different, using construction geometry and sketch relationships to maintain symmetry.
• Iterated on the footprint as the design developed, including increasing the corner radii to produce a smoother overall profile.

• Extruded the footprint to establish the initial solid body before modifying its superior surface.
• Created a side-profile sketch and used a Cut-Extrude for the wedge-shaped superior surface, with approximately 9.82° of slope so the cage height changes across its AP direction.
• Added fillets around the exterior edges and repeatedly checked the part against the L4–L5 assembly to evaluate its overall fit and proportions.

Fixation & Detailed Geometry

• Added three anterior fixation paths: one larger central fixation hole and two smaller lateral holes.
• The central fixation hole uses a stepped Ø5.00 mm / Ø4.00 mm geometry; the lateral holes use Ø3.50 mm / Ø2.50 mm. Both are conceptual proportions rather than a validated screw specification.
• Created angled reference planes to control the trajectories of the fixation holes rather than cutting them perpendicular to the anterior face.
• Positioned sketches on the angled planes and aligned their centers with the corresponding anterior-face geometry before creating the cuts.
• Used a shallow blind cut for the larger screw-head region followed by a Through All Cut-Extrude for the narrower screw-body path.
• The central fixation trajectory sits at approximately 39.82° relative to the inferior surface, the lateral trajectories at approximately 45.18°. Both angle into the adjacent vertebral bodies rather than running straight posteriorly.
• Returned to the L4–L5 assembly throughout the process to visually evaluate the trajectories against the surrounding anatomy.

• Added a lattice-inspired region to explore porous implant geometry and practice more advanced patterning workflows.
• Created a 1.00 × 1.00 mm rounded-square opening and cut it through the surface.
• Used Linear Pattern to repeat the feature across the cage, selectively skipping instances near the outer boundary and fixation geometry.
• Repeated the patterning process from additional faces so the intersecting cuts created an internal lattice-like structure rather than only a surface texture.
• I built this geometry manually rather than using a dedicated lattice-generation workflow, to practice Cut-Extrude, Linear Pattern, instance control, and repeated feature creation while approximating the structure of porous spinal implants.
• The lattice is a conceptual representation, not a manufacturing-ready porous architecture. In real interbody implants, porous structures support bone ongrowth/ingrowth and influence mechanical properties; I did not evaluate those characteristics here.

Material & Final Assembly

Final Design

These screens are interactive, move the models around to get a good look!

The completed project includes:

• Interactive 3D model of the cage
• Interactive 3D model of the L4–L5 assembly with cage and fixation screws
• Assembly animation
• Dimensioned engineering drawing with section views
• Final CAD imagery

View full drawing (PDF)

The engineering drawing documents the primary cage geometry, including its overall dimensions, wedge and footprint angles, lattice opening size, fixation-hole diameters and trajectories, and section views through both fixation-hole configurations.

Reflection

This project gave me experience working through a CAD workflow that started with medical imaging rather than a predefined engineering drawing. I had to interpret anatomy, decide which measurements mattered, translate them into design constraints, and continually evaluate the part within the anatomical assembly.

It also gave me hands-on practice with a wide range of SolidWorks tools, including sketch relationships, construction geometry, extrusions, cut-extrudes, fillets, reference planes, linear patterns, assemblies, mates, section views, materials, technical drawings, and animation.

The largest opportunity for improvement is moving from a conceptual model to a more clinically and mechanically informed implant. The lattice geometry, fixation system, cage footprint, and anatomical fit would all need considerably more engineering analysis and validation first. In a future iteration I would explore more advanced lattice-generation methods, improve the cage’s conformity to the patient endplates, and incorporate additional design and manufacturing constraints.

My next step is to get feedback from engineers and product designers working in spine and medical devices, to make my SolidWorks workflow more efficient and my design methodology more representative of actual patient-specific implant development.