TKA Sagittal Balance Lab

This is a conventional mechanical-alignment, measured-resection, anterior-referencing teaching framework [1]. Femoral rotation is set from anatomic landmarks, not from a tensioned gap, so it is not a balancing lever here; coronal balance is handled separately. Bone cuts are irreversible — resections only increase (2 mm increments) and bone is restored only with augments (5 mm increments). Constraint conversion is one-way in the model: after PCL resection and PS box preparation, returning to CR is unavailable [14]. PCL recession is a named teaching assumption: three equal grades produce a total 1.5 mm flexion-predominant effect. Clinical magnitude is unpredictable and extension may also change [4] [5] [12]. Aim for the thinnest available insert that balances (the model ladder starts at 9 mm). Sizing is generic: one femoral size is modelled as 2.5 mm A/P, essentially all posterior with anterior referencing [11]. The 1.5–3 mm target is a teaching band, not a universal validated threshold; published targets vary by compartment, technique and outcome [8] [15].

What this module does not model or validate:

• Not patient-specific and not validated. The gap model is linear — adequate for the relationships being taught, not for quantitative or patient-specific claims.
• Sagittal plane only. Coronal balance, mid-flexion instability, and patellofemoral tracking are out of scope; femoral rotation is set from anatomic landmarks and is not a balancing lever here.
• Only two positions are evaluated — 0° and 90°. Laxity between and beyond them is not represented.
• Generic implant ladder. Sizes, increments, and insert thicknesses are a teaching approximation within the range reported for contemporary systems, not any specific manufacturer's geometry.
• Individual ligament response is not predictable. The PCL recession grades and slope coefficients are population-level approximations; a given knee may respond differently, and extension may also change.
• The model-preferred plan is not a clinical optimum. It is the lowest-scoring legal state under this module's local weights; other plans may be equally reasonable.
• Overstuffed constructs (a negative gap) clamp the drawn band to a sliver while still reporting the true negative value.

• Anatomy artwork. The lateral-knee geometry is drawn from a commissioned labelled SVG. Extension and 90° flexion use separately authored femur, femoral-component, and patella paths; CR and PS use their own authored insert paths. The tibia, tray, inserts, and fibula share one coordinate system.
• Drawing scale. All linear motion derives from the artwork's own geometry — the templated size-5 component spans 174.52 SVG units for a modelled 64 mm A/P, giving 2.727 units/mm.
• Model constants are teaching approximations documented in Model assumptions, with their evidential basis in each control's info popup and in Evidence and references below.
• Software. Plain HTML, CSS, and JavaScript with no third-party runtime libraries or external requests; typography is the shared platform stack. See the site third-party notices.