Biomechanical investigation on refixation of tuberosities on shoulder prostheses

Elderly patients suffering from complex, non-reconstructable fractures of the proximal humerus are commonly treated by primary implantation of a shoulder endoprosthesis. One of the most critical factors for success or failure of treatment is still the refixation of the tuberosities. Using sheep infraspinatus tendons with attached tuberosities three different suture materials were investigated. For 2 of the suture materials 4 tests were accomplished and 5 tests were carried out for the third suture material. A material testing machine was used to perform cyclic loading tests (20 mm/min, F-min = 50 N, F-max = 100 N, respectively after 50 cycles: F-max + 50 N until failure). The results showed large variations in the average maximum forces (152.4 N for suture 1, 219.9 N for suture 2 and 452.3 N for suture 3). All tests showed a high initial lengthening and caused incision-like defects in the bone or tendon and led to failure and high displacement of the tuberosities. Due to these results suture materials have a limited usefulness for refixation of tuberosities as an increased risk of obstruction for bony consolidation can result.