Top 10: Free Downloadable 3D Knee Model and Other STL Files

As a complex joint and one of the largest joints in the body, the knee joint is a fascinating feature of the human form. This joint not only has the task of joining the femur (thigh bone), tibia (shin bone), and patella (knee cap), but also remaining flexible enough to allow for compound movements, such as running, jumping, dancing, kicking a soccer ball — the list goes on. The knee joint may have the greatest range of motion thanks to its non-interlocking form, but the muscles, joint, tendons, ligaments, menisci, capsule, and tendons must all work together to give the leg the rigidity it needs to support nearly the entire human bodyweight. 

Have you guessed this week's Top 10 featured uploads from the embodi3D® community? That's right, we're highlighting the marvel that is the human knee joint — bones, tissues, and all. Within the following sections you see stunning images and STL files of a 3D knee model (several, actually), as well as a number of other STL models of the lower limbs that you can download and create using your 3D printer.

If this topic is of particular interest, you may also want to check out a recent article in the online trade journal Manufacturing Tomorrow highlighting the use of 3D-printed knee models in pre-operative preparation. And, don't forget to check out our Extremity, Lower Leg library for more 3D-printable STL files like these! But, before you can make use of all the amazing tools offered on the embodi3D® website you need to register with embodi3D®. This community is absolutely free to join and members can upload, download, convert, and sell their CT-converted STL files.  

The Radiologist's Difficult Task of Imaging Knee Joints

There are a number of pitfalls and challenges when it comes to getting a stable image of the knee area, including:
• Variants: Multiple osseous and soft tissue normal variants
• Loose bodies on MR: Easily missed
• Partial voluming over convex surfaces: Morphology of trochlea, femoral condyles, and patella makes them particularly difficult to evaluate in 3 standard planes
• Imaging cartilage
○ T2 underestimates cartilage thickness since cortex and cartilage have similar signal
○ PD may have similar signal for cartilage and adjacent joint fluid, obscuring defects; fat saturation solves this.

#1. 3D Knee Model Showing the Muscles (in 3D-Printable STL Format)

Muscles acting on knee joint: Extensors (4 parts of quadriceps femoris) 
○ Rectus femoris (crosses both hip and knee joints, flexing hip and extending knee), ○ Vastus lateralis , ○ Vastus medialis ○ Vastus intermedius ( Extends knee)
Muscles acting on knee joint: Flexors
○ Biceps femoris (Flexes knee and also rotates tibia laterally; long head also extends hip joint)
○ Sartorius (Crosses both hip and knee joints, flexes both hip and knee joints, rotating thigh laterally to bring limbs into position adopted by cross-legged tailor)
○ Gracilis (Adducts thigh, flexes knee, and rotates flexed leg medially)
○ Semitendinosus (Crosses both hip and knee joints, extends hip, flexes knee, medially rotates flexed leg)

○ Semimembranosus (Crosses both hip and knee joints, extends hip, flexes knee, medially rotates flexed knee)
○ Popliteus (Flexes knee and medially rotates tibia at beginning of flexion)
– Innervation: Tibial nerve

Muscles acting on knee joint: Superficial flexors of knee
○ Gastrocnemius: (Flexes knee and plantar flexes ankle)
○ Plantaris (Flexes knee and plantar flexes ankle)
Muscles acting on knee joint: Internal rotators of leg
○ Popliteus, gracilis, sartorius, semitendinosus, semimembranosus
Muscles acting on knee joint: External rotator of leg
○ Biceps femoris
• Extensor mechanism
○ Quadriceps tendon and retinacula converge to inferior patellar tendon

#2. A Remarkable STL Model of the Skin Surface of the Knee 

This detailed STL file was converted from a CT scan through democratiz3D® and uploaded to the community for the benefit of all. 

#3. 3D Model of the Bones within the Knee (Femur, Tibia, Fibula, and Patella)

The knee is composed of 4 bones: the femur, tibia, fibula and patella. All these bones are functional in the knee joint, except for the fibula.

The femur is the longest and strongest bone in the human body. The tibia lies distal to the femur and medial to the fibula. The proximal end consists of medial and lateral condyles, an intercondylar area, and the tibial tuberosity that articulates with the medial and lateral condyles of the femur. Distally, the tibia articulates with the ankle. The distal and proximal ends of the tibia articulate with the fibula. In addition, the shaft of the tibia and fibula are connected with an interosseous membrane to form a syndesmosis joint.

The fibula does not articulate with the femur or patella. Furthermore, the fibula is not directly involved in weight transmission.

The patella is the largest sesamoid bone in the human body. This bone is flat, proximally curved, and distally tapered; however, the shape can vary. The posterior patella articulates with the femur, but the apex sits proximal to the line of the knee joint. The tendon of the quadriceps femoris completely encompasses the patella.

#4. CT Scan of the Knee Showing Articulations of the Condylar Joints

The knee joint articulations are two condylar joints between the femur and the tibia as well as a joint between the patella and the femur. Although the fibula is closely related to the knee joint but it doesn't share in articulation.

#5. Fracture of the Tibial Plateau (Converted into STL, 3D-Printable Format)

Fx of tibial plateau due to axial loading, ± rotational injury, ± valgus angulation. Most tibial plateau fx involve lateral plateau
○ "Split" component of fx describes fx line extending from articular surface to margin of metaphyseal cortex 

○ "Depressed" component is displaced below level of remainder of articular surface.

I-III involve lateral plateau only 
○ Schatzker I: Split fx with no depression (usually younger patients)
○ Schatzker II: Lateral split/wedge fx with depression of weight-bearing portion (usually older patients with osteoporosis)
○ Schatzker III: Focal depression of articular surface, no associated split (elderly, osteoporotic patients) 
○ Schatzker IV: Any medial plateau fx: Split, ± depression; may involve tibial spines; associated soft tissue injuries and poor prognosis
– Lateral plateau fx line that extends to medial articular surface adjacent to tibial spines but without depression or extension to metaphyseal cortex not
considered to involve medial plateau for classification purposes 

– Commonly associated with lateral collateral ligament complex or posterolateral corner injuries or proximal fibula fx
○ Schatzker V: Split fx of both medial and lateral plateau (bicondylar) ± depression
– Up to 1/2 have meniscal injuries, 1/3 anterior cruciate ligament avulsions
○ Schatzker VI: Bicondylar or unicondylar split fx with dissociation of metaphysis from diaphysis by transverse fx component

#6. CT Scan of the Knee Showing an LTP Fracture 

Findings of this CT scan include:
• Assists in diagnosis of radiographically occult fx 

• Confirms anatomic relationship of fx fragments in complex cases
○ Describe number, size, and location for fragments and fx lines
○ Accurate measurement of size and extent of plateau fragment depression 
• Surgical planning for either elevation of depressed fragments or for Schatzker type IV-VI fxs

#7. An MRI of the Menisci of the Knee (History of Injury)

These MRIs highlight a patient with a history of injury to the area.

We can clearly see the menisci and its analysis include:

Lateral meniscus
○ Overall configuration: Semicircular
○ Shape: Uniform, minimally and gradually enlarging from anterior to posterior
○ Normal recess: Peripheral, inferior at anterior horn
Medial meniscus
○ Overall configuration: Semilunar (C-shaped)
○ Shape nonuniform: Anterior horn similar in size & shape to LM but midbody is small, approximating an equilateral triangle; MM posterior horn is largest portion of MM, nearly 2x as long as anterior horn
○ Normal recess: Peripheral, superior at posterior horn
Meniscal "flounce": Buckling of a portion of meniscus, perhaps related to femorotibial subluxation 
Signal
• Generally uniformly low signal throughout
• Exceptions
○ Children and adolescents may have normal increased intrameniscal signal that does not extend to surface (due to rich vascular supply)
○ Adults may develop central degenerative changes seen as linear or globular signal that does not extend to surface and does not represent a tear
○ Various high signal clefts and dots can normally be seen in anterior horn LM at and near its root attachment, due to immediate adjacency of origin of ACL and divergence of longitudinal fibers at root; do not misinterpret as tear 

○ Peripheral portion of meniscus is quite vascular 
– Outer meniscal margin as seen by MR is usually not true periphery of structure: Meniscus signal in its peripheral vascular portion (10-30%) blends in with gray signal of the capsule
○ "Magic angle" may affect signal in posterior horn of LM in region of intercondylar notch

#8. Knee Ligaments and Muscles in a 3D-Printable Model (STL File)

This extraordinarily detailed 3D model of a 64-year-old male's knee shows the exquisite details of the muscles and ligaments.  

#9. Printable STL File of a Right Leg Bone Model

Fibrohistiocytic tumors represent a highly heterogeneous group of soft tissue neoplasms composed of cells exhibiting fibroblastic and histiocytic features.  The extremities are the most common site followed by the trunk, the pelvis, the head and neck region and the genital area. The differential diagnosis should exclude benign myxoid neoplasms, epitheloid types of MFS, carcinoma, melanoma, myoepithelial carcinoma, pleomorphic liposarcoma and pleomorphic rabdomyosarcoma.

#10. Total Knee Arthroplasty Completed with 3D-Printed Metal Condyles

Total knee arthroplasty (TKA): Replacement of femoral, tibial, and patellar articular surfaces

DIAGNOSTIC CHECKLIST:

• Keep in mind shape of polyethylene components; lucency of this shape in wrong location is hint of dislocation
• Periprosthetic fractures are easily missed; include them in your search pattern 
○ Increased risk for periprosthetic fracture with osteoporosis &/or tibial tubercle transfer.

Complications, other than malalignment
○ Patellar button dislocation from cement or metal backing.
○ Tibial polyethylene may dislocate from metal tray 

○ Stress shielding: Occurs in anterior and mid femoral metaphysis, seen on lateral radiograph
– Does not predict component failure 
○ Loosening: Change in position (tilt or subsidence) – Patellar button usually subsides superiorly. - Tibial component subsides inferiorly, usually with
medial trabecular compression
○ Infection
– Rare radiographic findings of serpiginous destruction
– MR: Lamellated hyperintense synovitis differentiates infectious from noninfectious synovitis

References

1. Manaster, B. J., & Crim, J. R. (2016). Imaging Anatomy: Musculoskeletal E-Book. Elsevier Health Sciences.

2. Castronovo, C., Arrese, J. E., Quatresooz, P., & Nikkels, A. F. (2013). Myxofibrosarcoma: a diagnostic pitfall. Rare tumors, 5(2), 60-61.

3. Blankenbaker, D. G., & Davis, K. W. (2016). Diagnostic Imaging: Musculoskeletal Trauma E-Book. Elsevier Health Sciences.