# External femoral rotation of 2 degrees is associated with the lowest stuffing rates around the femur in functionally aligned TKA

## TL;DR

A 2-degree external femoral rotation in knee replacement surgery is linked to the least bone removal around the femur, improving surgical outcomes.

## Contribution

Identifies optimal femoral rotation angle to minimize stuffing rates in functionally aligned TKA.

## Key findings

- External femoral rotation of 2° is associated with the lowest incidence of femoral stuffing >2 mm.
- Lateral facet of the trochlea is understuffed, while the medial distal condyle is also understuffed.
- Lateral posterior condyle is over-stuffed in functionally aligned TKA.

## Abstract

Modifying femoral morphology after total knee arthroplasty (TKA) poses a potential risk for ligament‐imbalances and patella mal‐tracking. The purpose of this study was primarily to quantify TKA‐induced stuffing around the femur and secondarily assess the effect of femoral rotation (FR) on trochlear and condylar anatomy‐changes.

Knee anatomy‐modification was quantified in 69 robotic‐assisted TKAs utilising tibia‐based functional alignment (FA). Caliper‐measurements were performed on the medial (Med), central (Ctr), and lateral (Lat) sides of the following resection planes: anterior trochlea (AT), oblique trochlea (OT), distal condyles (DC), posterior condyles (PC) and tibia (TIB). The same caliper‐measurements were performed on the femoral components used to calculate bone‐implant differences and analyse possible patterns of postoperative trochlear anatomy‐modifications (TAM) and condylar anatomy‐modifications (CAM). Over‐ or understuffing analysis for different FRs and regression analysis were conducted to assess the effect of FR on CAM and TAM.

TAM results were Lat‐AT −3.2 mm ([95% confidence interval [CI]: −3.71 to −2.63], p < 0.001), Ctr‐AT 0.7 mm ([95%CI: 0.22–1.32], p = 0.02), and Ctr‐OT −1.7 mm ([95%CI: −1.85 to −0.93], p < 0.001) with stuffing > 2 mm in 60.9%, 39.1%, and 39.1%, respectively. CAM results were Med‐DC −3.6 mm ([95%CI: −4.14 to −3.05], p < 0.001) and Lat‐PC 3.0 mm ([95%CI: 2.48–3.38], p > 0.001) with stuffing > 2 mm in 78.3% and 63.8%. FR (3.8 ± 2.6°, range: −1.6° to 8.5°) affected mostly the anterior (r = −0.40, p < 0.001) and posterior (r = 0.71, p < 0.001) aspects of the knee but hardly the OT plane (r = 0.06, p = 0.624) and the trochlear groove to its full range of flexion (r = 0.21, p = 0.17). External FR 2° was associated with the lowest incidence of femoral stuffing > 2 mm and ≥ 4 mm.

FA‐typical modification‐pattern was a TAM with lateral facet understuffing, and CAM with medial distal understuffing and lateral posterior overstuffing. Trochlear groove height was non‐significantly affected by FR. FA with the current off‐the‐shelf implant induces the lowest stuffing rates when set in 2° external femoral rotation.

Level II.