Relationship between rotator cuff muscle function and shoulder abduction function in patients with posterior superior rotator cuff tear: a dynamic biomechanical study / 中华创伤杂志

ABSTRACT

Objective:To evaluate the relationship of rotator cuff muscle function with shoulder abduction function after posterior superior rotator cuff tear via dynamic biomechanical study.Methods:By using the customized dynamic shoulder biomechanical testing system, seven freshly frozen cadaveric shoulders were used to stimulate shoulder abduction at 90° under four statuses (1) intact rotator cuff with activation (normal rotator cuff group); (2) posterior superior rotator cuff tear with activation (posterior superior rotator cuff tear with activation group); (3) posterior superior rotator cuff tear with posterior superior rotator cuff deactivation (posterior superior rotator cuff tear with deactivation group); (4) none rotator cuff tissue above the geometric rotation center of the humeral head with deactivation (global tear group). The peak and stable value of middle deltoid force were used to evaluate biomechanical status in different rotator cuff tear conditions during shoulder abduction procedure. The peak subacromial pressure, average subacromial pressure, subacromial contact area, and subacromial force were used to evaluate subacromial pressed conditions under different rotator cuff tear conditions. The peak and stable ratio of glenohumeral contact force/middle deltoid force were used to evaluate shoulder stability under different rotator cuff tear conditions.Results:During dynamic abduction at 90°, the peak and stable value of middle deltoid force were (42.1±8.7)N and (29.9±7.4)N in normal rotator cuff group, (45.7±10.3)N and (30.5±7.2)N in posterior superior rotator cuff tear with activation group, and (48.4±13.4)N and (29.9±4.8)N in posterior superior rotator cuff tear with deactivation group (all P>0.05). But the peak and stable value of middle deltoid force were (69.7±9.7)N and (53.7±8.9)N in global tear group, significantly increased compared with other three groups (all P<0.05). The elevated middle deltoid force increased the subacromial contact pressure between glenohumeral head and acromion. The peak subacromial pressure, average subacromial pressure, subacromial contact area, and subacromial force were (0.40±0.05)MPa, (0.22±0.03)MPa, (7.71±5.09)mm 2, and (1.66±1.06)N respectively in normal rotator cuff group, (0.41±0.05)MPa, (0.26±0.07)MPa, (12.71±11.35)mm 2, and (2.93±2.46)N respectively in posterior superior rotator cuff tear with activation group, and (0.50±0.12)MPa, (0.26±0.07)MPa, (17.29±9.11)mm 2, and (4.09±1.46)N respectively in posterior superior rotator cuff tear with deactivation group (all P>0.05). However, the peak subacromial pressure, average subacromial pressure, subacromial contact area, and subacromial force were (3.64±1.70)MPa, (0.98±0.49)MPa, (47.63±11.91)mm 2, and (45.48±23.86)N respectively in global tear group, significantly higher than those in other three groups (all P<0.05). The peak and stable ratio of glenohumeral contact force/middle deltoid force were 2.24±0.30 and 2.46±0.13 in normal rotator cuff group, 2.21±0.19 and 2.52±0.08 in posterior superior rotator cuff tear with activation group, and 2.03±0.14 and 2.42±0.16 in posterior superior rotator cuff tear with deactivation group (all P>0.05). However, the peak and stable ratio of glenohumeral contact force/middle deltoid force were 1.40±0.14 and 1.52±0.41 in global tear group, significantly higher than those in other three groups (all P<0.05). No significant differences of the above parameters were observed in posterior superior rotator cuff tear with activation group, posterior superior rotator cuff tear with deactivation group and global tear group (all P>0.05). Conclusions:After posterior superior rotator cuff tear, rotator cuff muscle function does not affect the whole abduction function of shoulder. When the size of rotator cuff tear involves the whole superior humeral head rotation center, the normal abduction function of shoulder will be significantly impaired.