In Vivo Static Retraction and Dynamic Elongation of Rotator Cuff Repair Tissue After Surgical Repair: A Preliminary Analysis at 3 Months

BACKGROUND: Rotator cuff repair is a common orthopedic procedure that provides pain relief for many patients, but unfortunately, an estimated 20% to 70% of repair procedures will fail. Previous research has shown that elongation (ie, retraction) of a repaired tendon is common even in patients with a repair construct that appears intact on magnetic resonance imaging. However, it is unknown how this repair tissue functions under dynamic conditions. PURPOSE: To quantify static retraction and maximum dynamic elongation of repair tissue after rotator cuff repair. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Data from 9 patients were analyzed for this study. During surgery, a 3.1-mm tantalum bead was sutured to the supraspinatus tendon, medial to the repair site. Glenohumeral kinematics were assessed at 1 week (static) and 3 months (static and during scapular-plane abduction) after surgery using a biplanar videoradiographic system. The 3-dimensional position of the bead was calculated relative to the tendon’s insertion on the humerus (ie, bead-to-insertion distance). Static retraction was calculated as the change in the bead-to-insertion distance under static conditions between 1 week and 3 months after surgery, and maximum dynamic elongation was calculated as the maximal positive change in the bead-to-insertion distance during dynamic motion relative to the start of motion. The magnitudes of static retraction and maximum dynamic elongation were assessed with 1-sample t tests. RESULTS: At 3 months after surgery, static retraction occurred in all patients by a mean of 10.0 ± 9.1 mm (P = .01 compared with no elongation). During scapular-plane abduction, maximum dynamic elongation averaged 1.4 ± 1.0 mm (P \u3c .01 compared with no elongation). Descriptively, dynamic elongation consistently took 1 of 2 forms: an initial increase in the bead-to-insertion distance (mean, 2.0 ± 0.6 mm) before decreasing until the end of motion or an immediate and substantial decrease in the bead-to-insertion distance at the onset of motion. CONCLUSION: Repair tissue elongation (static retraction and maximum dynamic elongation) appeared to be a common and significant finding at 3 months after arthroscopic rotator cuff repair. Dynamic elongation of repair tissue during scapular-plane abduction exhibited 1 of 2 distinct patterns, which may suggest different patterns of supraspinatus mechanical and neuromuscular function

The Mechanism of Abrupt Transition between Theta and Hyper-Excitable Spiking Activity in Medial Entorhinal Cortex Layer II Stellate Cells

Recent studies have shown that stellate cells (SCs) of the medial entorhinal cortex become hyper-excitable in animal models of temporal lobe epilepsy. These studies have also demonstrated the existence of recurrent connections among SCs, reduced levels of recurrent inhibition in epileptic networks as compared to control ones, and comparable levels of recurrent excitation among SCs in both network types. In this work, we investigate the biophysical and dynamic mechanism of generation of the fast time scale corresponding to hyper-excitable firing and the transition between theta and fast firing frequency activity in SCs. We show that recurrently connected minimal networks of SCs exhibit abrupt, threshold-like transition between theta and hyper-excitable firing frequencies as the result of small changes in the maximal synaptic (AMPAergic) conductance. The threshold required for this transition is modulated by synaptic inhibition. Similar abrupt transition between firing frequency regimes can be observed in single, self-coupled SCs, which represent a network of recurrently coupled neurons synchronized in phase, but not in synaptically isolated SCs as the result of changes in the levels of the tonic drive. Using dynamical systems tools (phase-space analysis), we explain the dynamic mechanism underlying the genesis of the fast time scale and the abrupt transition between firing frequency regimes, their dependence on the intrinsic SC's currents and synaptic excitation. This abrupt transition is mechanistically different from others observed in similar networks with different cell types. Most notably, there is no bistability involved. ‘In vitro’ experiments using single SCs self-coupled with dynamic clamp show the abrupt transition between firing frequency regimes, and demonstrate that our theoretical predictions are not an artifact of the model. In addition, these experiments show that high-frequency firing is burst-like with a duration modulated by an M-current

Mixed-mode oscillations in a three time scale system of ODEs motivated by a neuronal model

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