Effects of repetitive subfailure strains on the mechanical behavior of theinferior glenohumeral ligament

The mechanical response of the inferior glenohumeral ligament to varying su bfailure cyclic strains was studied in 33 fresh frozen human cadaver should ers. The specimens were tested as bone-ligament-bone preparations represent ing the 3 regions of the inferior glenohumeral ligament (superior band and anterior and posterior axillary pouches) through use of uniaxial tensile cy cles. After mechanical preconditioning, each specimen was subjected to 7 te st segments, consisting of a baseline strain level I 1 (400 cycles! alterna ting with either I (group A, 10 shoulders), 10 (group B(, 13 shoulders), or 100 (group C, 10 shoulders) cycles at increasing levels (L2, IS, L4) of su bfailure strain. Cycling to higher levels of subfailure strain (L2 L3, L4) produced dramatic declines in the peak load response of the inferior glenoh umeral ligament for all specimens. The group of ligaments subjected to 100 cycles of higher subfailure strains demonstrated a significantly greater de crease in load response than the other 2 groups, ligament elongation occurr ed with cyclic testing at subfailure strains for all 3 groups, averaging 4. 6% +/- 2.0% for group A, 6.5% +/- 2.6% for group B, and 7.1% +/- 3.2% for g roup C. Recovery of length after an additional time of nearly I hour was mi nimal. The results from this study demonstrate that repetitive loading of t he inferior glenohumeral ligament induces laxity in the ligament, as manife sted in the peak load response and measured elongations. The mechanical res ponse of the ligament is affected by both the magnitude of the cyclic strai n and the frequency of loading at the higher strain levels. The residual le ngth increase was observed in all of the specimens and appeared to be large ly unrecoverable. This length increase may result from accumulated microdam age within the ligament substance, caused by the repetitively applied subfa ilure strains. The clinical relevance of the study is that this mechanism m ay contribute to the development of acquired glenohumeral instability which is commonly seen in the shoulders of young athletes who participate in rep etitive overhead sports activities.