Articular Surface Of Humeral Head Research Articles

Total shoulder arthroplasty: Some considerations related to glenoid surface contact

The area of prosthetic surface contact is an important surgeon-controlled variable in total shoulder arthroplasty and is related to the geometry of the glenoid and humeral articular surfaces and their relative positions. This study explores some of the factors that affect joint surface contact area. We measured the humeral and the glenoid articular surface angles in the superior-inferior and anteroposterior planes for two prosthesis systems representative of those in common clinical use. On the basis of these data we determined the range of glenohumeral positions providing full glenoid surface contact, a condition in which all of the articular surface of the glenoid component is in contact with the articular surface of the humeral component. We found a wide variability in the range of glenohumeral positions providing full glenoid contact with different prosthetic combinations. Some combinations do not even offer full glenoid surface contact with the joint in the centered position, for example, when the center of the humeral head articular surface is opposed to the center of the glenoid articular surface. The maximal range of glenohumeral positions providing full glenoid surface contact was 117° for a combination with a small radius of head curvature and a large articular surface angle. The relative positions of the humeral and glenoid articular surfaces also have a major influence on the joint contact area. Some combinations offer full glenoid surface contact only in a position of humeral abduction with respect to the scapula. Loss of full glenoid surface contact between the glenoid and humerus may allow for unwanted translations of the humeral head on the glenoid in the direction where contact is lacking. Furthermore, in positions where full surface contact is lacking, humeral bone or soft tissue may make unwanted contact with the glenoid. These results suggest that the design of the humeral articular surface and the surgical procedure should maximize full glenoid surface contact in functionally important positions.

Management of Shoulder Instability

I nstability of the glenohumeral joint falls into two categories, traumatic and atraumatic. Both have a variety of nonoperative and operative treatments. In general, traumatic means that a stabilizing structure is torn or broken. In atraumatic instability, a specific trauma may not be identifiable-rather, the mechanisms that together stabilized the shoulder no longer do so; for example, a weakening of muscles may allow a loose but previously functional joint to become unstable. We discuss below these categories of instability in terms of the evaluation, nonoperative management, and surgical management with its corresponding postoperative rehabilitation. We distinguish instability from laxity. Instability is the inability to maintain the humeral head articular surface centered in the glenoid fossa. Laxity is the amount of joint motion allowed by the joint capsule and the configuration of the joint itself. A relatively large amount of motion may be allowed by a particular joint configuration and capsule, while still maintaining the humeral head articular surface centered in the glenoid fossa: laxity does not necessarily imply instability. 2

Humeral head prosthetic arthroplasty: Surgically relevant geometric considerations

The relationship of the humeral head prosthesis to the humerus is a critical determinant of the result in glenohumeral arthroplasty. With canal-fitting humeral prostheses, the position of the component is largely dictated by the location of the reamed medullary canal. This study explores the geometric relationships of a surgically defined humeral reference, the "orthopedic axis." This is the axis of a cylindric reamer or press-fit prosthetic stem inserted to the appropriate depth for the humeral prosthesis. The orthopedic axis provides a reference for measuring surgically important geometric features of the normal humeral articular surface and comparing them with those available with humeral prosthetic components. In 10 cadaveric proximal humeri, we measured the following seven parameters in a radiologic projection of the humerus on the plane transverse to the orthopedic axis: the surgically-determined reamed diameter of the humeral canal, the diameter of curvature of the humeral head articular surface, the effective humeral neck length, the combined head and neck length, the subtended angle of the humeral joint surface, the anterior/posterior offset of the center of the humeral head, and the biceps-articular surface angle. We then determined the prosthetic geometry of a canal-fitting humeral component necessary to match the stem size, head diameter, head and neck length, and effective humeral neck length. To examine the effect of changing component version, we determined the maximal angle of anteversion and retroversion achievable by rotation of the component about the orthopedic axis without compromising the tuberosities. In the maximal possible anteversion or retroversion, the combined head and neck length changed by only 2 mm. Thus the effect of component version of a press-fit prosthesis on glenohumeral soft-tissue tension is small. The study suggests that the surgeon controls relatively few important variables in a canal-fitting humeral arthroplasty. Kinematics of the arthroplasty are controlled primarily by soft-tissue releases and the selection of the prosthetic head-neck length.