Rotator cuff tears (RCT) have been shown to result in altered shoulder kinematics with disruption of the biomechanical synergy of the rotator cuff and deltoid muscles, which may be responsible for the correlation between RTCs and degenerative changes of the glenohumeral joint. The purpose was to evaluate the effect of an isolated full-thickness supraspinatus (SSP) tear on glenohumeral kinematics, contact mechanics, and quantify improvement following rotator cuff repair (RCR). The authors hypothesized that RCR would reverse the increased glenohumeral joint loads caused by a full-thickness SSP tear.
Ten fresh-frozen cadaveric shoulders (mean age: 63.1 ± 4.6 years) were tested using a dynamic shoulder simulator. A pressure mapping sensor was placed between the humeral head and glenoid. Each specimen underwent the following three conditions: (1) native, (2) isolated full-thickness SSP tear, (3) RCR. Maximum abduction angle (MAA) and superior humeral head migration (SHM) were measured using 3D motion tracking software. Cumulative deltoid force (CDF) and glenohumeral contact mechanics, including contact area (GCA) and contact pressure (GCP), were assessed at the resting position as well as at 15°, 30°, 45°, and 60° of glenohumeral abduction.
An a priori power analysis was performed to determine detectable differences in contact pressure given estimated standard deviations. Assuming a common standard deviation of 15kPa, a sample size of 6 specimens would provide 80% power to detect a 25kPa difference in pressure at an a level of .05. Repeated measures analysis of variance was performed to examine differences in MAA, SHM, glenohumeral contact mechanics, and CDF among the various testing conditions. When significant, post-hoc paired t tests with a Bonferroni corrected alpha were performed to determine which pairwise comparisons were statistically significant. The alpha level for all analyses was set at .05.
Compared to native, the SSP tear resulted in a significant decrease in MAA (Delta: -8.3°; P <. 001) along with a SHM of 6.4 ± 3.8mm, while significantly increasing CDF (Delta: 20.5N; P = .008), GCP (Delta: 63.1kPa; P < .001), and peak GCP (Delta: 278.6kPa; P < .001) as well as decreasing GCA (Delta: -45.8mm2; P < .001) at each degree of glenohumeral abduction. RCR reduced SHM to 1.2 ± 2.5mm, while restoring native MAA, CDF (Delta: 1.8N), GCA (Delta: 4.5mm2), GCP (Delta: -4.5kPa) and peak GCP (Delta: 19.9kPa) at each degree of abduction (P > .999, respectively).
In a dynamic biomechanical cadaveric model, increased glenohumeral joint loads due to a full-thickness SSP tear can be reversed with RCR. More specifically, RCR restored native glenohumeral contact area and contact pressure, while preventing superior humeral head migration and decreasing compensatory deltoid forces. These time-zero observations indicate that isolated full-thickness SSP tears should undergo repair, in order to reverse altered loading conditions and improve overall shoulder function. More importantly, this potentially prevents progressive cartilage degeneration, while preserving the native glenohumeral joint.