Abstract
The humeral offset has a medial-lateral and anteriorposterior dimension and can be defined as the distance of the central axis of the humeral shaft and the center of rotation of the humeral head. When using a canal filling prosthetic stem, inserted in a collinear alignment with the long axis of the humeral shaft, the placement of the humeral head within the anatomic boundaries of the humeral osteotomy surface will be dictated by how closely the prosthetic stem-humeral head offsets match that of the patient’s natural anatomic offsets. Given the fact that there are several millimetres of variation in the medial-lateral and anterior-posterior humeral offsets among the normal patient population, it follows that in many cases the prosthetic offset will not precisely match that of each patient when a canal filling medullary component is properly inserted. This mismatch in the humeral offsets can result in malposition of the humeral head within the confines of the humeral osteotomy surface when using a centred Morse taper humeral head.
Iannotti and Williams have studied, using the Polhemus tracking system, the biomechanical consequences and tolerances for a malpositioned humeral head using a prosthetic reconstructed human cadaver model (JOR in press). A 4 mm or greater malposition of the humeral head, particularly in an inferior or anterior direction (most common malposition) will result in a measurable decrease in range of motion, abnormal humeral kinematics and subacromial impingement. In addition, humeral head malposition (anterior and inferior) can result in impingement of the non-articular portion of the humeral component, rotator cuff or proximal humeral metaphysis on the glenoid component. This impingement can result in abnormal glenoid component wear (type 2 wear) or premature glenoid loosening.
Solutions for management of humeral head malposition include: 1.) variable stem offsets (not currently available), 2.) humeral component with an eccentric Morse taper (DePuy – Global Advantage, Tomier – Aequalis, Zimmer – Bigliani/Flatow) or an equivalent design (Stryker – Anatomica), and 3.) under-sizing and shifting the humeral stem to a centred position. The last option, best for monoblock humeral components (original Neer design), requires impaction grafting or PMMA cement to stabilise the under-sized stem in the centred position. Whatever solution is used it is important, in most cases, to select a head size which matches the normal anatomy and center it within the confines of the normal humeral osteotomy surface area.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
