Quantitation of Cartilage Strains in the Glenohumeral Articulation Immediately After Pushups

OBJECTIVES: Alterations in joint mechanics contribute to the onset and progression of cartilage degeneration. While shoulder kinematics have been investigated through range-of-motion activity with minimal loads, less is known about shoulder function during weight-bearing exercise, specifically in terms of cartilage deformation. This study used magnetic resonance (MR) imaging and three-dimensional (3D) modeling of the glenohumeral joint to measure cartilage thickness before and after upper-extremity exercise. A series of 30 standard pushups were hypothesized to produce measurable strain in the glenohumeral cartilage. METHODS: A total of 8 healthy subjects (5 male, 3 female; age 22-28 years) without history of shoulder injury or treatments were recruited for this IRB-approved study. Subjects were instructed to perform a series of 30 pushups with hands shoulder width apart, spine in neutral, legs extended, and elbows bent parallel to the sagittal plane. Before and after exercise, axial MR imaging was taken of the subject’s dominant shoulder (3.0 T; Siemens TruFISP). To establish baseline cartilage thicknesses, subjects avoided strenuous upper-extremity activity for 24 hours prior to testing and rested 45 minutes immediately before pre-exercise imaging. Pre- and post-exercise MR images were imported into solid modeling software. From manual segmentation of glenohumeral cartilage and subchondral bone, 3D cartilage thickness maps were generated for the glenoid and humerus, before and after exercise (cartilage thickness = nearest distance between points on cartilage surface and underlying bone). After spatial registration of pre- and post-exercise models, cartilage strain (thickness change post-exercise, divided by pre-exercise thickness) was measured at 18 locations on the humeral head and 9 on the glenoid, averaging all strain measurements within 2 mm of the sampled location. Strain across the entire glenoid or humeral surface was calculated by averaging sampled measurements. Student’s t-tests were used to compare pre- and post-exercise cartilage thicknesses. One-way ANOVA and Bonferroni post-hoc tests were used to determine differences in cartilage strain by region (p<0.05). All values are represented as mean ± standard error. RESULTS: Pre-exercise cartilage thickness averaged 1.33 ± 0.06 mm at the glenoid and 0.99 ± 0.06 mm at the humerus (Fig. 1). The humeral and glenoid cartilage thicknesses significantly decreased to 1.13 ± 0.08 mm and 0.81 ± 0.04 mm after exercise, respectively, resulting in an average glenoid cartilage strain of 15 ± 3% and average humeral cartilage strain of 17% ± 3%. Glenoid strain in the anterior region (19% ± 1%) was significantly greater than that of the central and posterior regions (15% ± 2%, 12% ± 2%, p<0.05). In contrast, there was no significant strain difference between the superior (15% ± 2%) and inferior glenoid (17% ± 4%) regions. Regional strains in the humeral head did not differ significantly in either the anterior-posterior (p=0.07) or superior-inferior directions (p=0.20). CONCLUSION: This study demonstrates that a common intensive upper-extremity exercise induces significant changes in glenohumeral cartilage thickness. Specifically, the anterior glenoid cartilage was observed to experience the highest strains. This strain distribution data can serve as benchmarks of healthy shoulder function in future studies of joint disease and cartilage degeneration.