From d8705b76eed87b22dda5fca7727be66c905b96b1 Mon Sep 17 00:00:00 2001
From: Themis Skamagkis <themisskamagkis@gmail.com>
Date: Mon, 15 Jun 2026 13:28:45 +0200
Subject: [PATCH] [SolidMechanics][FEM] Fix
 FastTetrahedralCorotationalForceField stiffness matrix assembly

---
 .../fem/elastic/FastTetrahedralCorotationalForceField.inl       | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Sofa/Component/SolidMechanics/FEM/Elastic/src/sofa/component/solidmechanics/fem/elastic/FastTetrahedralCorotationalForceField.inl b/Sofa/Component/SolidMechanics/FEM/Elastic/src/sofa/component/solidmechanics/fem/elastic/FastTetrahedralCorotationalForceField.inl
index 759cf36ae87..5d0ba93ddb6 100644
--- a/Sofa/Component/SolidMechanics/FEM/Elastic/src/sofa/component/solidmechanics/fem/elastic/FastTetrahedralCorotationalForceField.inl
+++ b/Sofa/Component/SolidMechanics/FEM/Elastic/src/sofa/component/solidmechanics/fem/elastic/FastTetrahedralCorotationalForceField.inl
@@ -556,7 +556,7 @@ void FastTetrahedralCorotationalForceField<DataTypes>::buildStiffnessMatrix(
     for(sofa::Size i=0; i < nbEdges; ++i )
     {
         const auto& edge = edges[i];
-        dfdx(3 * edge[0], 3 * edge[1]) += -edgeDfDx[i];
+        dfdx(3 * edge[0], 3 * edge[1]) += -edgeDfDx[i].transposed();
         dfdx(3 * edge[1], 3 * edge[0]) += -edgeDfDx[i];
     }
 }