function measureInsertionsRing(obj)
  % The insertions ring is the intersection of the plane fitted to the
  % insertions' landmarks with the humeral head fitted sphere.

  insertionsArray = struct2array(obj.landmarks.insertions.ring)';
  insertionsPlane = Plane();
  insertionsPlane.fit(insertionsArray);

  ringNormal = Vector(insertionsPlane.normal);
  humerusCenterToPlanePoint = Vector(obj.center, insertionsPlane.point);

  % The ring's center is the projection of any point of the insertions' plane
  % on the vector starting at the humeral head center and with the same 
  % direction as the insertion's plane normal direction.
  ringCenter = obj.center...
    + (ringNormal.dot(humerusCenterToPlanePoint) * ringNormal.vector);

  humerusCenterToRingCenter = Vector(obj.center, ringCenter);
  ringRadius = sqrt(obj.radius^2 - humerusCenterToRingCenter.norm()^2);
    
  obj.insertionsRing.center = ringCenter;
  obj.insertionsRing.radius = ringRadius;
  obj.insertionsRing.normal = ringNormal;
end