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Follow the patella.

Explore the patella and the Persona trochlea through flexion.

0°Flexion

Illustrative patellar tracking

Preparing the patella and knee…

PATELLA + TROCHLEA

Inspect the first 30°.

Use 0°, 10°, 20° and 30° to inspect the transition into the trochlea. Teal: trochlear centerline. Gold: patellar centerline. Amber dots: sampled proximity.

Patellar height Normal reference

↓ Patella bajaNormalPatella alta ↑

Lower ← normal reference → higher

Femoral component flexion 0° · reference0° ↔ 15° relative to the model reference

Gray: groove at 0° component flexion.
Teal: current groove.

Proximal landmark drop: 0.0 mm

Watch the trochlear entrance at 0–30° knee flexion.

Femoral valgus 6° · reference0° ↔ 12° valgus · reference 6°

Femoral rotation 0° · reference3° internal ↔ 5° external

Tibial component rotation 0° · reference10° internal ↔ 10° external · model-relative

Component position Centered3 mm medial ↔ 3 mm lateral

See through the patella Show near-contact samples

Preparing contact samples…

Lateral shift0.0 mm

Lateral tilt0.0°

Illustrative offsets from reference tracking.

Show tilt guide in sunrise

Gold: patellar axis · gray: untilted axis at the same position.

Reference tracking

Approved knee + Persona CAD. New illustrative patella and tendon geometry.

What this model shows

Full-extension contact and groove engagement are not interchangeable. The exact patellar height and contact in this Persona reconstruction have not been established from patient imaging. The patellar tendon attaches to the anterior tibial mesh in the modeled tuberosity region. Its centerline length is held fixed through motion for each height setting (44 mm at the reference setting, matching the illustrative 44 mm patellar pole-to-pole length). Lower or higher settings shorten or lengthen that constraint. This is a geometric reference, not a patient-specific radiographic measurement. The position is projected onto this fixed-length constraint and adjusted outward to reduce sampled implant intersections. The attachment location, shape and motion still require anatomical interpretation; muscle forces and retinacular restraint are not solved. Conventional Insall–Salvati ranges are below 0.8 for baja, 0.8–1.2 for normal and above 1.2 for alta, but the slider labels indicate direction rather than diagnosing a radiographic index. Tendons are hidden in sunrise view.

An authored path sampled from the Persona trochlea seeds the patellar pose, then the fixed tendon-length constraint determines its permitted position. This demonstration adds progressive lateral shift and tilt with internal femoral or tibial component rotation, or valgus above the 6° model reference, strongest in early flexion. Sampled posterior-surface intersections adjust the pose while retaining tendon length; the model does not attract a separated patella into contact. Amber samples are within 0.5 mm along the sampling direction after this adjustment. They are not a contact-area measurement, percentage of engagement, or pressure map. Valgus, rotation and translation move the implant relative to that path.

The tibial tray and bearing rotate together within the resection plane, with slope and depth preserved. The tibial bone and tendon insertion remain fixed. The model does not solve the compensatory bone rotation that may occur with component malrotation.

At the upper internal-rotation or added-valgus settings, the demonstration progresses smoothly to partial lateral escape of the patella (subluxation), with the lateral edge lifting into more pronounced tilt. External rotation and reduced valgus do not trigger that authored response. The added shift and tilt, including the subluxation transition, are prescribed teaching effects, not measured responses or clinical thresholds. The 6° reference is specific to this model. The model does not solve quadriceps forces or retinacular restraint. It is a geometric teaching illustration, not a clinical tracking prediction. The patella is newly constructed illustrative anatomy, not a patient scan or manufacturer patellar implant. Native patellar cartilage is retained.

Why femoral component flexion matters

Flexing the component can reduce the proximal reach of the trochlea, potentially delaying patellar engagement during early knee flexion. Less early guidance may contribute to instability.

Brar, Howell et al. (2016) modeled 20 femurs with component flexion and downsizing, finding average proximal-reach reductions of 4, 8 and 12 mm at 5°, 10° and 15°. Those values are not programmed into this Persona model.

Nedopil, Howell and Hull (2017) found 13 instability cases among 3,212 KA knees; cases had greater component flexion than matched controls (11° versus 5°). This is an association, not an individual risk threshold.

This control rotates the same-size Persona CAD around the model’s distal femoral pivot. Gray holds the groove at zero added sagittal flexion with the other placement settings retained. The readout measures the proximal landmark’s displacement along the femoral model axis. Downsizing, forces and a predicted engagement angle are not simulated. No extra lateral shift or tilt is prescribed from this slider; inspect the changed surface relationship and sampled contact.

Evidence and limitations

Cadaver studies show that component rotation can alter patellofemoral mechanics, but the response depends on implant design and experimental conditions. More external rotation is not assumed to be universally better. The additional-valgus response here is an authored scenario, not a validated rule that every increase causes maltracking.

Miller et al. · Optimizing femoral component rotation
Femoral rotation and joint kinematics · eight cadaveric knees

Patel et al. · Loaded MRI of ten normal knees: contact was present in extension and increased with flexion. Native-knee findings do not establish the correct engagement for this implant reconstruction.

Berger et al. · Component malrotation and patellofemoral complications: combined internal component rotation was associated with patellofemoral complications. This does not establish a universal threshold or validate this viewer’s response.

Verhulst et al. · Reliability of patellar-height measurements. Thresholds can vary by measurement method and population.

These studies provide context; they do not validate the trajectory, shape or displacement in this viewer.