Mark L. Palmer

Posterior Glenohumeral Subluxation: Active and Passive Stabilization in a Biomechanical Model*

We examined the role of the glenohumeral and coracohumeral ligaments as well as the forces provided by the rotator cuff muscles, the long head of the biceps, the anterior and middle deltoids, and the pectoralis major in the stabilization of the glenohumeral joint in the posterior direction. Simulated muscle forces were mechanically applied to eight shoulder specimens. The humeroscapular position for testing simulated the 90-degree forward-flexion (humerothoracic) position used clinically for the so-called jerk test, which is the most clinically important position with regard to posterior instability of the shoulder. Experiments were performed with a variety of configurations of ligamentous and capsular cuts, humeral rotation, and levels of muscle force. Stability was investigated by measuring the force required to subluxate the humeral head a specified amount from its reduced position. Of the muscles and ligaments tested, the subscapularis muscle contributed the most to this subluxation force. The coracohumeral ligament was an effective contributor in neutral humeral rotation, and the inferior glenohumeral ligament was an effective contributor in internal humeral rotation. The long head of the biceps was found to reduce the subluxation force in certain positions. CLINICAL RELEVANCE: It is widely agreed that a complex interaction of passive and active stabilizing structures and forces is necessary for clinical stability of the shoulder. The present study identified the contributions of ligaments and muscles to posterior stability of the shoulder in the position of greatest clinical importance—posterior subluxation with the shoulder in forward flexion.

Lateral transmission of force is impaired in skeletal muscles of dystrophic mice and very old rats

Non‐technical summary  The force developed by a single fibre in frog muscles is transmitted laterally to the muscle surface with little or no loss. To demonstrate this phenomenon in mammals, a ‘yoke’ apparatus was developed that attached to the surface of whole, parallel‐fibred muscles and permitted measurements of the lateral transmission of forces. We then demonstrated that for wild‐type mice and rats longitudinal and lateral transmission of forces in muscles were not different. In contrast, for skeletal muscles of dystrophic mice and very old rats, in which the dystrophin‐associated glycoprotein complex (DGC) of fibres was disrupted, the forces transmitted laterally were impaired severely. We conclude that during contractions of skeletal muscles, an intact DGC is essential for the lateral transmission of force and disruptions of the DGC lead to sarcomere instability and contraction‐induced injury.

The Relationship Between Anterior Tibial Acceleration, Tibial Slope, and ACL Strain During a Simulated Jump Landing Task

BACKGROUND Knee joint morphology contributions to anterior cruciate ligament (ACL) loading are rarely considered in the injury prevention model. This may be problematic as the knee mechanical response may be influenced by these underlying morphological factors. The goal of the present study was to explore the relationship between posterior tibial slope (which has been recently postulated to influence knee and ACL loading), impact-induced anterior tibial acceleration, and resultant ACL strain during a simulated single-leg landing. METHODS Eleven lower limb cadaveric specimens from female donors who had had a mean age (and standard deviation) of 65 ± 10.5 years at the time of death were mounted in a testing apparatus to simulate single-limb landings in the presence of pre-impact knee muscle forces. After preconditioning, specimens underwent five impact trials (mean impact force, 1297.9 ± 210.6 N) while synchronous three-dimensional joint kinetics, kinematics, and relative anteromedial bundle strain data were recorded. Mean peak tibial acceleration and anteromedial bundle strain were quantified over the first 200 ms after impact. These values, along with radiographically defined posterior tibial slope measurements, were submitted to individual and stepwise linear regression analyses. RESULTS The mean peak anteromedial bundle strain (3.35% ± 1.71%) was significantly correlated (r = 0.79; p = 0.004; ß = 0.791) with anterior tibial acceleration (8.31 ± 2.77 m/s-2), with the times to respective peaks (66 ± 7 ms and 66 ± 4 ms) also being significantly correlated (r = 0.82; p = 0.001; ß = 0.818). Posterior tibial slope (mean, 7.6° ± 2.1°) was significantly correlated with both peak anterior tibial acceleration (r = 0.75; p = 0.004; ß = 0.786) and peak anteromedial bundle strain (r = 0.76; p = 0.007; ß = 0.759). CONCLUSIONS Impact-induced ACL strain is directly proportional to anterior tibial acceleration, with this relationship being moderately dependent on the posterior slope of the tibial plateau.

Magnitude of Sarcomere Extension Correlates with Initial Sarcomere Length during Lengthening of Activated Single Fibers from Soleus Muscle of Rats

A laser-diffraction technique was developed that rapidly reports the lengths of sarcomeres (L(s)) in serially connected sectors of permeabilized single fibers. The apparatus translates a laser beam along the entire length of a fiber segment within 2 ms, with brief stops at each of 20 contiguous sectors. We tested the hypothesis that during lengthening contractions, when maximally activated fibers are stretched, sectors that contain the longer sarcomeres undergo greater increases in L(s) than those containing shorter sarcomeres. Fibers (n = 16) were obtained from the soleus muscles of adult male rats and the middle portions (length = 1.05 +/- 0.11 mm; mean +/- SD) were investigated. Single stretches of strain 27% and a strain rate of 54% s(-1) were initiated at maximum isometric stress and resulted in a 19 +/- 9% loss in isometric stress. The data on L(s) revealed that 1), the stretch was not distributed uniformly among the sectors, and 2), during the stretch, sectors at long L(s) before the stretch elongated more than those at short lengths. The findings support the hypothesis that during stretches of maximally activated skeletal muscles, sarcomeres at longer lengths are more susceptible to damage by excessive strain.

Shoulder labral pathomechanics with rotator cuff tears.

Rotator cuff tears (RCTs), the most common injury of the shoulder, are often accompanied by tears in the superior glenoid labrum. We evaluated whether superior humeral head (HH) motion secondary to RCTs and loading of the long head of the biceps tendon (LHBT) are implicated in the development of this associated superior labral pathology. Additionally, we determined the efficacy of a finite element model (FEM) for predicting the mechanics of the labrum. The HH was oriented at 30° of glenohumeral abduction and neutral rotation with 50N compressive force. Loads of 0N or 22N were applied to the LHBT. The HH was translated superiorly by 5mm to simulate superior instability caused by RCTs. Superior displacement of the labrum was affected by translation of the HH (P<0.0001), position along the labrum (P<0.0001), and interaction between the location on the labrum and LHBT tension (P<0.05). The displacements predicted by the FEM were compared with mechanical tests from 6 cadaveric specimens and all were within 1 SD of the mean. A hyperelastic constitutive law for the labrum was a better predictor of labral behavior than the elastic law and insensitive to ±1 SD variations in material properties. Peak strains were observed at the glenoid-labrum interface below the LHBT attachment consistent with the common location of labral pathology. These results suggest that pathomechanics of the shoulder secondary to RCTs (e.g., superior HH translation) and LHBT loading play significant roles in the pathologic changes seen in the superior labrum.

Effects of biceps tension and superior humeral head translation on the glenoid labrum

We sought to understand the effects of superior humeral head translation and load of the long head of biceps on the pathomechanics of the superior glenoid labrum by predicting labral strain. Using micro‐CT cadaver images, a finite element model of the glenohumeral joint was generated, consisting of humerus, glenoid bone, cartilages, labrum, and biceps tendon. A glenohumeral compression of 50 N and biceps tensions of 0, 22, 55, and 88 N were applied. The humeral head was superiorly translated from 0 to 5 mm in 1‐mm increments. The highest labral strain occurred at the interface with the glenoid cartilage and bone beneath the origin of the biceps tendon. The maximum strain was lower than the reported failure strain. The humeral head motion had relatively greater effect than biceps tension on the increasing labral strain. This supports the mechanistic hypothesis that superior labral lesions result mainly from superior migration of the humeral head, but also from biceps tension.

Physiological Loading of the Coonrad/Morrey, Nexel, and Discovery Elbow Systems: Evaluation by Finite Element Analysis

PURPOSE Wear of polyethylene bearings represents a limiting factor in the long-term success of total elbow prostheses. Bearing stress is 1 factor contributing to accelerated wear. Physiological loading of total elbow prostheses and implant design influence upon bearing stresses have not been well described. This study evaluates bearing stresses in 3 commercially available implant designs under loads associated with daily living. METHODS Motion tracking from a healthy volunteer helped establish a musculoskeletal model to simulate flexor and extensor muscle activation at 0°, 45°, and 90° of shoulder abduction with a 2.3-kg weight in hand-forces and moments were measured at the elbow. Resulting physiological joint reaction forces and moments were applied to finite element models of 3 total elbow bearing designs (Coonrad/Morrey, Nexel, and Discovery) to evaluate contact area and polyethylene stresses. RESULTS Increasing shoulder abduction resulted in minimal changes to the elbow joint reaction force but greater joint moments. All implants showed greater peak stresses with increasing shoulder abduction-elbow varus. Discovery and Nexel achieved greater contact area (23% vs > 100%) and demonstrated up to 39% lower peak polyethylene stresses compared with the Coonrad/Morrey design. CONCLUSIONS Shoulder abduction results in a varus moment at the elbow. Newer bearing designs (Nexel and Discovery) provide a combination of higher contact area, improved load sharing, reduced edge loading, and lower stresses through elbow range of motion when compared with a cylindrical hinge-bearing design (Coonrad/Morrey). CLINICAL RELEVANCE Although the Coonrad/Morrey is a clinically successful prosthesis, our physiological loading model shows that Discovery and Nexel provide greater contact area, better load sharing and lower peak stresses. This may lead to a decrease in polyethylene wear rates and the eventual risks of osteolysis and aseptic loosening. Further studies are needed to determine how these findings translate clinically.

Effects of biceps tension on the torn superior glenoid labrum

The purpose of this study was to evaluate the role of the tension on the long head of the biceps tendon in the propagation of SLAP tears by studying the mechanical behavior of the torn superior glenoid labrum. A previously validated finite element model was extended to include a glenoid labrum with type II SLAP tears of three different sizes. The strain distribution within the torn labral tissue with loading applied to the biceps tendon was investigated and compared to the inact and unloaded conditions. The anterior and posterior edges of each SLAP tear experienced the highest strain in the labrum. Labral strain increased with increasing biceps tension. This effect was stronger in the labrum when the size of the tear exceeded the width of the biceps anchor on the superior labrum. Thus, this study indicates that biceps tension influences the propagation of a SLAP tear more than it does the initiation of a tear. Additionally, it also suggests that the tear size greater than the biceps anchor site as a criterion in determining optimal treatment of a type II SLAP tear.

Effect of ACL Laxity on ACL Loading: Dynamic Simulation Using Impulsive Loads

Noncontact ACL injuries are among the most common, potentially traumatic and costly sport-related injuries, approximately 200,000 injuries occurring each year. Female athletes exhibit a trend toward higher rates of ACL injuries suggesting the influence of gender-specific characteristics. ACL laxity, a direct manifestation of hormonal cycling is a crucial differentiating factor between males and females that influences the strength and compliance of ACL when stressed. However, the mechanism through which ACL laxity implicates within an injury is unknown. Therefore, using FE methods, we tested the hypothesis that a lax ACL fails to adequately restrain the tibia during dynamic activities, resulting in lower intersegmental decelerations and subsequently, higher strain in the ACL. A 2D computational model of bones, meniscus, cartilage and ACL in sagittal plane was created. An impulse load was applied for 100ms (peak:1200N at 35ms) to simulate drop landing. Multiple simulations were run with varying ACL laxity. We observed that as stiffness increased from 50% to 150%, intersegmental deceleration increased by 10.9% and ACL strain reduced by 19.4%. These results indicate that relative peak deceleration of tibia can be used to predict ACL strain and is a potential clinical measure for identifying female athletes at risk for injury.Copyright