Anatomically, lower trapezius transfer (LTT) may be better positioned for restoring the muscular force couple in the setting of irreparable posterosuperior rotator cuff tears (PSRCT) when compared to latissimus dorsi transfer (LDT). The purpose was to evaluate the effect of LTT and LDT on maximum abduction angle (MAA), superior humeral head migration (SHM), and cumulative deltoid forces (CDF) using a dynamic shoulder model. It was hypothesized that the LTT would better restore native glenohumeral kinematics when compared to the LDT.
Ten fresh-frozen cadaveric shoulders (mean age: 56.5±17.2 years) were tested using a dynamic shoulder simulator. MAA, SHM, and CDF were compared across four conditions: (1) native; (2) irreparable PSRCT; (3) LTT using an Achilles tendon allograft (mean graft thickness: 5.3 ± 0.5 mm), as previously described by Elhassan et al.; and (4) LDT. MAA and SHM were measured using 3-dimensional motion tracking. CDF was recorded in real time throughout dynamic abduction motion by load cells connected to actuators and was calculated as the summation of anterior, middle, and posterior deltoid forces.
A sample size of 6 specimens will provide 80% power to detect a 1° difference in shoulder angle at an a level of 0.05. Repeated measures analysis of variance was performed to examine differences in MAA, SHM, and CDF among the various testing conditions. When significant, post-hoc paired t tests with a corrected alpha using the Holm Bonferroni sequential correction method were performed to determine which pairwise comparisons were statistically significant. The alpha level for all analyses was set at 0.05.
Compared to the native state, the PSRCT resulted in a significant decrease (Delta: -24.1°; P < 0.001) in MAA, with a subsequent significant increase after LTT (Delta: 13.1°; P < 0.001) and LDT (Delta: 8.9°; P < 0.001). LTT achieved a significantly greater MAA than LDT (Delta: 4.2°; P = 0.004). Regarding SHM, both LTT (Delta: -9.4mm; P < 0.001) and LDT (Delta: -5.0mm; P = 0.008) demonstrated a significant decrease when compared to the PSRCT state. LTT also achieved significantly less SHM compared to the LDT (Delta: -4.4mm; P = 0.011). Further, only the LTT resulted in a significant decrease in CDF when compared to the PSRCT state (Delta: -21.3N; P = 0.048), while LTT and LDT showed similar CDF (Delta: -11.3N; P = 0.346). However, none of the techniques was able to restore MAA, SHM, and CDF of the native shoulder (P < 0.001, respectively).
LTT and LDT achieved a significant increase in maximum abduction angle along with significantly less superior humeral head migration compared to the PSRCT state. While LTT required significantly less cumulative deltoid forces compared to the PSRCT state, this was not observed for LDT. Further, LTT prevented loss of abduction motion and superior head migration more sufficiently when compared to LDT. In the challenging treatment of irreparable PSRCTs, transfer of the lower trapezius may restore native glenohumeral kinematics more sufficiently, potentially leading to improved postoperative functional outcomes.