← Part 3 · Persona resurfacingPART 4 / RESECTION ACCOUNTING

Measure cartilage. Set the cut.

VERIFICATION BEFORE CORRECTION

Check what was removed before changing what comes next.

Inspect the existing cut planes, then work through the spot-the-error exercises below. Compare each surface separately with the intended component thickness.

The original articular surface defines the target.

Affected sideCartilage

Native knee · extension

Loading anatomy…

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Cut plane & mm ticks Original cartilage envelope

Orange shows the planned cut; green shows the reconstructed cartilage surface.

Inspect a femoral surface

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Surface close-ups are illustrative; cartilage depth is not measured from this model.

9 MM TOTAL · EACH SURFACE

Measure the unworn side.

Use that thickness to reconstruct the worn side.

Distal femur

Unworn cartilage thickness

Planned bone cut

Target bone resection— mm

9 mm − cartilage thickness

Posterior femur

Unworn cartilage thickness

Planned bone cut

Target bone resection— mm

9 mm − cartilage thickness

Assumes matching native cartilage thickness on the worn and unworn sides. Depth is measured beneath exposed bone, with no bone erosion.

Measurement details

Distal and posterior values are independent. Inputs change the illustrative cartilage layer and its green envelope. The starting layer is an assumed 2 mm, not a measurement of this specimen. Planned bone cuts independently move the cut planes and rigid Persona component; CAD dimensions remain unchanged.

If cartilage remains, add its thickness when measuring from the current articular surface. Bone erosion requires a separate correction.

The removed specimen is thinner than the planned cut depth by the saw kerf. For example, 2 mm native cartilage gives a 7 mm bone cut; a 1 mm kerf leaves a 6 mm bone specimen.

Persona technique · resection and saw-kerf accounting. Matching cartilage thickness is the assumption used here.

SPOT THE ERROR

Four checks before the next cut.

Choose an exercise

Clinical question

Account for the missing surface

Illustrative component: 9 mm. Specimen thickness includes any retained cartilage on that specimen. Enter only lost cartilage below; do not count retained cartilage twice.

Measured specimen (mm)Saw kerf (mm)Lost cartilage (mm)Pre-existing bone loss (mm)

Positive difference: more surface thickness accounted for than the component replaces. Negative: less. This is arithmetic, not a recommendation to recut or release tissue. Kerf and component thickness must be verified for the actual instruments and implant.

Return to the model ↑

Verify → identify → reassess

Record medial and lateral distal and posterior measurements separately. Check actual cuts, component seating and impingement before interpreting trial balance. An acceptable extension space alone does not verify the posterior reconstruction.

These are constructed teaching exercises, not patient cases. The model uses the existing anatomy and a fixed 9 mm accounting example. It does not predict forces, individual laxity or the clinical correction required. A target comparison is reversible onscreen; removed bone is not.

Next in Chapter 3 · preserved tibial landmarks →

Sources and assumptions

Kinematically Aligned Total Knee Arthroplasty Using Calipered Measurements, Manual Instruments, and Verification Checks
Alexander J. Nedopil, Stephen M. Howell, Maury L. Hull · Personalized Hip and Knee Joint Replacement, Chapter 24 · Springer · July 1, 2020. Supports wear/kerf accounting and serial verification; examples here are adapted for teaching.

Cartilage thickness can be accurately measured intraoperatively in total knee arthroplasty: A step further in calipered kinematic alignment
Giancarlo Giurazza, Stefano Campi, Michael T. Hirschmann, Edoardo Franceschetti, Andrea Tanzilli, Pietro Gregori, Michele Paciotti, Biagio Zampogna, Rocco Papalia · Journal of Experimental Orthopaedics · January 26, 2025 · 12(1):e70155. Supports assessing cartilage rather than assuming every surface is 2 mm. The model’s equal worn/unworn native thickness remains an assumption.

Course cross-reference: difficult varus/valgus knees, slides 29–37 (Crawford): verify earlier resections before compensating at a subsequent step.

Continue to Part 5 · Tibial reconstruction →