Kronig isthmus is a band of resonance representing the apex of lung, also it's described as the narrow strap-like portion of the resonant field that extends over the shoulder, that connect the larger areas of resonance over the pulmonary apex in front and behind.
Boundaries Medially: Scalene
Laterally: Acromion process
Anteriorly: Clavicle
Posteriorly: Trapezius
References