Purpose: We hypothesized that glenohumeral joint stability will decrease with increasingly larger humeral head defects.

Method: Humeral head defects were created in 9 cadaveric shoulders to simulate Hill Sachs defects. Defects represented 1/8, 3/8, 5/8, and 7/8 of the radius of the humeral head. Secondary factors included abduction angles of 45 degrees and 90 degrees, and rotations of 40 degrees internal, neutral, and 40 degrees external. Specimens were tested at each defect size sequentially from smallest to largest and at each of 6 conditions for all abduction and rotation combinations. Using a 6 degree-of-freedom robot, the humeral head was translated at 0.5 mm per second until dislocation in the anteroinferior direction at 45 degrees to the horizontal glenoid axis.

Results: ANOVA demonstrated significant factors of rotation (p< 0.001) and defect size (p< 0.001). In 40 degrees external rotation, there was significant reduction of distance to dislocation compared with neutral and 40 degrees internal rotation (p< 0.001). The 5/8 and 7/8 radius osteotomies demonstrated decreased distance to dislocation compared to the intact state (p< 0.05 and p< 0.001 respectively). There was no difference found between abduction angles. Post hoc analysis determined significant differences for each arm position. There was decreased distance to dislocation at the 5/8 radius osteotomy at 40 degrees external rotation with 90 degrees of abduction (p< 0.05). For the 7/8 radius osteotomy at 90 degrees abduction, there was decrease distance to dislocation for neutral and 40 degrees external rotation (p< 0.001). For the same osteotomy at 45 degrees abduction, there was decreased distance to dislocation at 40 degrees external rotation (p< 0.001). With the humerus internally rotated, there was never a significant change in the distance to dislocation.

Conclusion: Glenohumeral stability decreases at a 5/8 radius defect and was most pronounced in 40 degrees external rotation and at 90 degrees abduction. At a 7/8 radius humeral defect, there was further decrease in stability at both neutral and external rotation. Internal rotation always maintained baseline glenohumeral stability.

Purpose: Previous research has reported that increasing the posterior tibial slope through an opening wedge osteotomy results in an anterior shift in the position of the tibia relative to the femur. However, the effect of this on anterior cruciate ligament (ACL) strain remains insufficiently understood. The purpose of this study was to examine the relationship between tibial slope and tibial translation, as well as between tibial slope and ACL strain. It was hypothesized that increasing the posterior tibial slope would result in an increase in anterior tibial translation thereby increasing strain in the ACL.

Methods: Five cadaveric knees were subjected to a randomized experimental design study. One knee was excluded due to failure of a strain gauge during experimentation, resulting in data for four knees. The femoral and tibial portions of the knee were potted with PMMA and fixed using fixation pins. An anterior-based osteotomy was performed with no osteotomy plate present. A strain gauge was then placed in the anteromedial bundle of the ACL. Each knee was mounted at a flexion angle of 15° and loaded with various combinations of A-P loads (18N, 108N, 209N) and axial loads (216N, 418N), according to the study design. Osteotomies of 5mm and 10mm were then performed and measurements of strain and tibial translation were taken after each according to the study design. Tibial slopes were determined through lateral fluoroscopic imaging.

Results: As posterior tibial slope increased, anterior tibial translation increased as anticipated. However, contrary to expectations, as posterior slope increased, ACL strain decreased. One explanation for this result could be that by performing the osteotomy, the insertions sites of the ACL were being moved closer together resulting in increased ACL laxity. At higher slope angles, translation levels off, suggesting constraint of some tissue besides the ACL.

Conclusions: Although increasing the tibial slope through opening wedge osteotomy leads to an anterior tibial translation, there is no increase in strain on the ACL. Further studies are needed to examine the effect of opening wedge osteotomy on other soft tissue restraints of the knee.