Highlights PNI is a pseudoneoplastic lesion caused by repetitive microtrauma. Shearing in perineal fascia stimulates fibroblastic proliferation. Reported almost exclusively in males, it can occur in females and is underdiagnosed. Cyclists and any saddle sport athletes (even seated lawnmowing) are at risk. Treatment is to remove the trigger; surgery can be considered in refractory cases.

The rotator cuff (RC) muscles are crucial in moving and stabilizing the glenohumeral joint, and tears can be functionally devastating. Chronic fatty and fibrotic muscle changes, which are non-responsive to surgical tendon repair, are a focus of contemporary research. The rabbit model recapitulates key biological features of human RC tears, but function and physiology are poorly characterized-limited force and stress data are inconsistent with literature norms in other mammalian species. Here, we present an improved method to assess the physiology of the rabbit SSP, and report values for healthy SSP architecture and physiology. Using female New Zealand White Rabbits (n=6) under 2% isofluorane anesthesia, the SSP was surgically isolated and maximum isometric force measured at 4-6 muscle lengths. Architectural analysis was performed, and maximum isometric stress was computed. Whole muscle length-tension curves were generated using architectural measurements to compare experimental physiology to theoretical predictions. Maximum isometric force (80.87± 5.58N) was dramatically greater than previous reports (11.06 and 16.1N, p<0.05). Architectural measurement of fiber length (34.25 ± 7.18mm), muscle mass (9.9 ± 0.93g), pennation angle (23.67 ± 8.32°), and PCSA (2.57 ± 0.20cm2) were consistent with prior literature. Isometric stress (30.5 ± 3.07N/cm2) was greater than previous reports of rabbit SSP (3.10 and 4.51 N/cm2), but similar to mammalian skeletal muscles (15.7-30.13 N/cm2). Previous studies underestimated peak force by ~90%, having profound implications for interpreting physiological changes as a function of disease state. Data presented here enable understanding the physiological implications of disease and repair in the RC of the rabbit.

Chronic rotator cuff tears can cause severe functional deficits. Addressing the chronic fatty and fibrotic muscle changes is of high clinical interest; however, the architectural and physiological consequences of chronic tear and repair are poorly characterized. We present a detailed architectural and physiological analysis of chronic tear and repair (both over 8 and 16 weeks) compared to age-matched control rabbit supraspinatus (SSP) muscles. Using female New Zealand White Rabbits (N=30, n=6/group) under 2% isofluorane anesthesia, the SSP was surgically isolated and maximum isometric force measured at 4-6 muscle lengths. Architectural analysis was performed, and maximum isometric stress was computed. Whole muscle length-tension curves were generated using architectural measurements to compare experimental physiology to theoretical predictions. Architectural measures are consistent with persistent radial and longitudinal atrophy over time in tenotomy that fail to recover after repair. Maximum isometric force was significantly decreased after 16 wks tenotomy and not significantly improved after repair. Peak isometric force reported here are greater than prior reports of rabbit SSP force after tenotomy. Peak stress was not significantly different between groups and consistent with prior literature of SSP stress. Muscle strain during contraction was significantly decreased after 8-wks of tenotomy and repair, indicating effects of tear and repair on muscle function. The experimental length-tension data was overlaid with predicted curves for each experimental group (generated from structural data), exposing the altered structure-function relationship for tenotomy and repair over time. Data presented here contribute to understanding the physiological implications of disease and repair in the rotator cuff