Elias Bachmann

A larger critical shoulder angle requires more rotator cuff activity to preserve joint stability.

Shoulders with rotator cuff tears (RCT) tears are associated with significantly larger critical shoulder angles (CSA) (RCT CSA = 38.2°) than shoulders without RCT (CSA = 32.9°). We hypothesized that larger CSAs increase the ratio of glenohumeral joint shear to joint compression forces, requiring substantially increased compensatory supraspinatus loads to stabilize the arm in abduction. A previously established three dimensional (3D) finite element (FE) model was used. Two acromion shapes mimicked the mean CSA of 38.2° found in patients with RCT and that of a normal CSA (32.9°). In a first step, the moment arms for each muscle segment were obtained for 21 different thoracohumeral abduction angles to simulate a quasi‐static abduction in the scapular plane. In a second step, the muscle forces were calculated by minimizing the range of muscle stresses able to compensate an external joint moment caused by the arm weight. If the joint became unstable, additional force was applied by the rotator cuff muscles to restore joint stability. The model showed a higher joint shear to joint compressive force for the RCT CSA (38.2°) for thoracohumeral abduction angles between 40° and 90° with a peak difference of 23% at 50° of abduction. To achieve stability in this case additional rotator cuff forces exceeding physiological values were required. Our results document that a higher CSA tends to destabilize the glenohumeral joint such that higher than normal supraspinatus forces are required to maintain modeled stability during active abduction. This lends strong support to the concept that a high CSA can induce supraspinatus (SSP) overload.

Pedicle screw augmentation with bone cement enforced Vicryl mesh

Achieving sufficient mechanical purchase of pedicle screws in osteoporotic or previously instrumented bone is technically and biologically challenging. Techniques using different kinds of pedicle screws or methods of cement augmentation have been used to address this challenge, but are associated with difficult revisions and complications. The purpose of this biomechanical trial was to investigate the use of biocompatible textile materials in combination with bone cement to augment pullout strength of pedicle screws while reducing the risk of cement extrusion. Pedicle screws (6/40 mm) were either augmented with standard bone‐cement (Palacos LV + G) in one group (BC, n = 13) or with bone‐cement enforced by Vicryl mesh in another group (BCVM, n = 13) in osteoporosis‐like saw bone blocks. Pullout testing was subsequently performed. In a second experimental phase, similar experiments were performed using human cadaveric lumbar vertebrae (n = 10). In osteoporosis‐like saw bone blocks, a mean screw pullout force of 350 N (±125) was significantly higher with the Bone cement (BC) compared to bone‐cement enforced by Vicryl mesh (BCVM) technique with 240 N (±64) (p = 0.030). In human cadaveric lumbar vertebrae the mean screw pullout force was 784 ± 366 N with BC and not statistically different to BCVM with 757 ± 303 N (p = 0.836). Importantly, cement extrusion was only observed in the BC group (40%) and never with the BCVM technique. In vitro textile reinforcement of bone cement for pedicle screw augmentation successfully reduced cement extrusion compared to conventionally delivered bone cement. The mechanical strength of textile delivered cement constructs was more reproducible than standard cementing.

The best knot and suture configurations for high-strength suture material. An in vitro biomechanical study

PURPOSE Surgical knots are particularly challenged by high-strength suture material. It was hypothesized that sutures in a double-stranded looped configuration present mechanical advantages. METHODS This in vitro biomechanical study repeatedly tested 12 different knots with a static distraction material testing machine with a constant tensile speed. The cow hitch, its altered version, and conventional half hitches were also tested on bovine tendon. Suture material was braided polyblend non-bioresorbable polyester. Primary outcome was knot security (stiffness) at clinical failure (≥3mm displacement). Secondary outcomes were knot size and loop security. RESULTS Double-stranded looped knots were up to three times stronger than one and a half- and single-stranded knots. The cow hitch was stiffest (mean 185 [95% CI 172-197]Newton per millimeter [N/mm]) (p<0.001), followed by the Nice knot (169 [154-183]N/mm). It was stiffer than half hitches (65 [53-78]N/mm). These findings remained in tendons (82 [77-86] and 40 [32-49]N/mm, p<0.001). The cow hitch (7.6mm3) and Nice knot (6.1mm3) were smaller than half hitches (9.5mm3). Loop security did not differ between the cow hitch and Nice knot, but was higher in the cow hitch than half hitch (158 [120-196]N and 85 [57-113]N, p<0.001). CONCLUSIONS Double-stranded knot configurations with a loop on one side are mechanically stronger and stiffer, less bulky, and preserve applied tension during tying better than conventional knots. The best performing and technically most simple knots best suited to exploit enormous mechanical capabilities of modern high-strength suture material are the cow hitch and Nice knot. LEVEL OF EVIDENCE Not applicable due to the biomechanical nature of the study.

Mechanical properties of Triclosan sutures: MECHANICAL PROPERTIES OF TRICLOSAN SUTURES

To avoid infections and wound healing disorders, Triclosan coated sutures have been invented. Little is known of these sutures regarding their tensile properties. Three different Triclosan coated sutures (Vicryl 1 plus, PDS 0 plus, Monocryl 3‐0 plus) were tested at several time points over 42 days regarding load to failure, strain, and stiffness compared to their non‐coated versions (Vicryl 1, PDS 0, Monocryl 3‐0). Four different measurement points were made. Suture loops were fixed in a material testing machine over two metal bars which were moved apart creating a stress to the fiber. Unpaired, two‐tailed t‐test were performed for each group (untreated and treated) while level of significance was defined at a level of p < 0.05. Vicryl 1 was significantly stronger on day 14 than Vicryl 1 plus (p = 0.033). On day 28, significant changes were found in PDS 0 which was weaker compared to PDS 0 plus (p = 0.039) and Vicryl 1 which was stronger than Vicryl 1 plus (p = 0.032). We have seen that Vicryl 1 plus sutures are significantly weaker according to loading to failure after 14 and 28 days, which might cause incisional hernias. PDS 0 sutures are used to reconstruct tendons, therefore a longer durableness might be of interest as re‐ruptures of tendons are problematic. Our in vitro findings support, the use of Triclosan coated PDS plus sutures and Vicryl sutures as they show a longer resistance.

Influence of the site of acromioplasty on reduction of the critical shoulder angle (CSA) – an anatomical study

BackgroundA large critical shoulder angle (CSA) >35° is associated with the development of rotator cuff tearing. Lateral acromioplasty (AP) has the theoretical potential to prevent rotator cuff tearing and/ or to reduce the risk of re-tears after repair. It is, however unclear which part of the lateral acromion has to be reduced to obtain the desired CSA. It was the purpose of this study to determine which part of the lateral acromion has to be resected to achieve a desired reduction of the CSA in a given individual.MethodsFirst, the influence of the exact radiographic projection on the CSA was examined. Second, the influence of anterolateral versus strict lateral AP on the CSA was studied in eight scapulae with different anatomic characteristics. Differences in CSA reduction were investigated using paired t-test or Wilcoxon test.ResultsScapular rotation in the sagittal and axial plane had a marked influence on the radiologically measured CSA ranging from -6 to +16°. Overall, lateral AP of 5/10mm reduced the CSA significantly greater than anterolateral AP of 5mm/10mm [5mm: 2.3° (range: 0.7°-3.6°) SD±0.8° vs. 1.2° (range: 0°-3.3°) SD±1.1°, p=0.0002]/[10mm: 4.8° (range: 2.1°-7°) SD±1.3° vs. 2.7° (range: 0°-5.3°) SD±1.7°, p=0.0001]. Depending on scapular anatomy anterolateral AP did not alter CSA at all.ConclusionsFor comparison of pre- and postoperative CSA, the exact orientation of the X-ray and the spatial orientation of the scapula must be as identical as possible. Anterolateral AP may not sufficiently correct CSA in scapulae with great acromial slopes and smaller relative external rotation of the acromion as the critical acromial point (CAP) may be located too posteriorly and thus is not addressed by anterolateral acromioplasty. Consistent reduction of the CSA could be achieved by lateral AP in all eight scapulae.

Rabbit Achilles tendon full transection model - wound healing, adhesion formation and biomechanics at 3, 6 and 12 weeks post-surgery.

ABSTRACT After tendon rupture repair, two main problems may occur: re-rupture and adhesion formation. Suitable non-murine animal models are needed to study the healing tendon in terms of biomechanical properties and extent of adhesion formation. In this study 24 New Zealand White rabbits received a full transection of the Achilles tendon 2 cm above the calcaneus, sutured with a 4-strand Becker suture. Post-surgical analysis was performed at 3, 6 and 12 weeks. In the 6-week group, animals received a cast either in a 180 deg stretched position during 6 weeks (adhesion provoking immobilization), or were re-casted with a 150 deg position after 3 weeks (adhesion inhibiting immobilization), while in the other groups (3 and 12 weeks) a 180 deg position cast was applied for 3 weeks. Adhesion extent was analyzed by histology and ultrasound. Histopathological scoring was performed according to a method by Stoll et al. (2011), and the main biomechanical properties were assessed. Histopathological scores increased as a function of time, but did not reach values of healthy tendons after 12 weeks (only around 15 out of 20 points). Adhesion provoking immobilization led to an adhesion extent of 82.7±9.7%, while adhesion inhibiting immobilization led to 31.9±9.8% after 6 weeks. Biomechanical properties increased over time, however, they did not reach full strength nor elastic modulus at 12 weeks post-operation. Furthermore, the rabbit Achilles tendon model can be modulated in terms of adhesion formation to the surrounding tissue. It clearly shows the different healing stages in terms of histopathology and offers a suitable model regarding biomechanics because it exhibits similar biomechanics as the human flexor tendons of the hand. Summary: The rabbit Achilles tendon full transection model can be used to study adhesion extent in a controlled way. It also mimics the biomechanics of human hand flexor tendons.