Background There is evidence that specific variants of scapular morphology are associated with dynamic and static posterior shoulder instability. To this date, observations regarding glenoid and/or acromial variants were analyzed independently, with two-dimensional imaging or without comparison with a healthy control group. Therefore, the purpose of this study was to analyze and describe the three-dimensional (3D) shape of the scapula in healthy and in shoulders with static or dynamic posterior instability using 3D surface models and 3D measurement methods. Methods In this study, 30 patients with unidirectional posterior instability and 20 patients with static posterior humeral head subluxation (static posterior instability, Walch B1) were analyzed. Both cohorts were compared with a control group of 40 patients with stable, centered shoulders and without any clinical symptoms. 3D surface models were obtained through segmentation of computed tomography images and 3D measurements were performed for glenoid (version and inclination) and acromion (tilt, coverage, height). Results Overall, the scapulae of patients with dynamic and static instability differed only marginally among themselves. Compared with the control group, the glenoid was 2.5° ( P = .032), respectively, 5.7° ( P = .001) more retroverted and 2.9° ( P = .025), respectively, 3.7° ( P = .014) more downward tilted in dynamic, respectively, static instability. The acromial roof of dynamic instability was significantly higher and on average 6.2° ( P = .007) less posterior covering with an increased posterior acromial height of +4.8mm ( P = .001). The acromial roof of static instability was on average 4.8° ( P = .041) more externally rotated (axial tilt), 7.3° ( P = .004) flatter (sagittal tilt), 8.3° ( P = .001) less posterior covered with an increased posterior acromial height of +5.8 mm (0.001). Conclusion The scapula of shoulders with dynamic and static posterior instability is characterized by an increased glenoid retroversion and an acromion that is shorter posterolaterally, higher, and more horizontal in the sagittal plane. All these deviations from the normal scapula values were more pronounced in static posterior instability.

Purpose Patient‐specific instruments (PSIs) are helpful tools in high tibial osteotomy (HTO) in patients with symptomatic varus malalignment of the mechanical leg axis. However, the precision of HTO can decrease with malpositioned PSI. This study investigates the influence of malpositioned PSI on axis correction, osteotomy, and implant placement. Methods With a mean three‐dimensional (3D) model (0.8° varus), PSI‐navigated HTOs were computer simulated. Two different guide designs, one with stabilising hooks and one without, were used. By adding rotational and translational offsets of different degrees, wrong placements of PSI were simulated. After 5° valgisation of the postoperative mechanical axis, the distance between joint‐plane and osteotomy screws, respectively, were measured. The same simulations were performed in a patient with varus deformity (7.4° varus). Results In the mean 3D model, the postoperative mechanical axis was within 3.9°–4.5° valgus with mean value of 4.1° ± 0.1° (correct axis 4.2° valgus). Surgical failure concerning osteotomy occurred in 17 of 76 HTOs. Significantly safer screw placement was observed using PSI with stabilising hooks (p = 0.012). In the case of the 3D model with 7.4° varus deformity, the postoperative mechanical axis was within 3.2°–3.9° valgus with mean value of 3.8° ± 0.2° (correct axis 3.9° valgus). Surgical failure concerning osteotomy occurred in 3 of 38 HTOs. Screws were always within the safety distance. Conclusion The clinical relevance of the presented study is that malpositioning of a PSI within the possible degrees of freedom does not have a relevant influence on the axis correction. The most vulnerable plane for surgical failure is the sagittal plane, wherefore the treating surgeon should verify correct guide placement to prevent surgical failure, particularly in this plane. Level of evidence III.

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Our findings suggest that the anisotropy of the mesh becomes more relevant with the increase of the defect size. Additionally, according to our results Optilene showed the closest deformation to the natural distensibility of the abdomen while Infinit should be carefully used due to its excessive compliance.