INFLUENCING FACTORS OF PRIMARY TUBEROSITY STABILITY IN TRAUMA-ARTHROPLASTY OF THE SHOULDER

Abstract

The aim of our biomechanical study was to find out whether the prosthetic design, especially of the metaphyseal part, and the type of tuberosity fixation influences the primary stability in shoulder arthroplasty.

Materials and Methods: 16 fresh frozen human cadaveric humeri were dissected until only the rotator cuff remained. A four-part fracture was simulated by osteotomy. In a first step two types of shoulder prosthesis (open stem versus closed stem) were used. The Tornier Aequalis prosthesis (open stem) using suture fixation and the Mathys Affinis fracture prosthesis (closed stem) using cable fixation were implanted according to standard techniques. The specimens were then loaded into a servo-pneumatic testing device in 25° of abduction. In 20 consecutive cycles traction of 40 Newton was alternating exerted on the subscapularis and infraspinatus tendon, while a continuous force of 40 Newton was exerted on the greater tuberosity to simulate the pull of the m. supraspinatus. The motion of the fragments was recorded by 2 high speed cameras. The following parameters were investigated : Failure of osteosynthesis, intertuberosity motion, motion lesser tuberosity-shaft, motion greater tuberosity-shaft, motion metaphysis-shaft. After completing the first series the cable fixation exposed to be more stable. In a second series we compared cable versus suture fixation in the Affinis fracture prosthesis to find out whether the stability was depending on the prosthesis design.

Results:

• Series 1: The intertuberosity motion was significantly lower in the cable prosthesis. The tuberosity-shaft motion was significantly lower in the cable group for greater and lesser tuberosity. The metaphysis - shaft motion did not significantly differ in both groups.

• Series 2: The intertuberosity motion was significant lower when the tuberosities were fixed by cable. The tuberosity-shaft motion was significantly lower when cable fixation was used. The metaphysis-shaft motion was not significantly diverse.

Conclusion: Highest primary stability of tuberosity fixation in trauma-arthroplasty of the shoulder was measured in cable fixation and closed stem. We proved that the kind of fixation was the most important factor determing the stability. Metaphyseal design was less important.