Treatment of massive rotator cuff tears can be challenging. Previous studies with irreparable rotator cuff tears showed good clinical results of tendon healing with the arthroscopic insertion of a protective biodegradable spacer balloon filled with saline solution between the repaired tendon and the acromion [1,2], but so far no scientific evidence has showed how the device alters pressures over the repaired tendon. This biomechanical study investigated the effects of a spacer inserted in the subacromial space on pressures over the repaired rotator cuff tendon in passive motion cycles typical for post-operative rehabilitation routines.

Six human cadaveric shoulders were prepared with the humerus cut 15cm below the joint and embedded in a pot, while the scapula fixed at three points on a plate. A rotator cuff tear was simulated and repaired using a suture anchor and a Mason-Allen suture. The specimens were then mounted on a custom-made pneumatic testing rig to induce passive motion cycles of adduction-abduction (90–0°) and flexion-extension (0–40°) with constant glenohumeral and superior loads and tension is exerted on the supraspinatus tendon with weights. A pressure sensor was placed between the supraspinatus tendon and the acromion. After pressure measurements for 15 cycles of each motion type, the InSpace balloon (OrthoSpace, Inc, Israel) was inserted and the specimens tested and pressure measured again for 15 cycles. Statistically significant changes in peak pressures were then measured before and after balloon.

Peak pressures were measured near 90 degrees abduction. No statistical differences were observed for internal-external rotation before and after balloon-shaped subacromial spacer was inserted. Mean pressures in abduction-adduction were significantly reduced from 121.7 ± 9.5 MPa to 51.5 ± 1.2 MPa. Peak pressures after repair were 1171.3 ± 99.5 MPa and 1749.6 ± 80.7 MPa in flexion-extension and abduction-adduction motion, respectively, and significantly decreased to 468.7 ± 16.0 MPa and 535.1 ± 27.6 MPa after spacer insertion (p<0.0001).

The use of the spacer above the repaired tendon reduced peak pressures and distributed them more widely over the sensor during both abduction-adduction and flexion-extension motions and therefore can reduce the stress on the rotator cuff repair. The InSpace system may reduce the pressure on the repaired tendon, thus potentially protecting the repair. Further studies to investigate this phenomenon are warranted, in particular relating these changes to shoulder kinematics following tear repair and spacer insertion.

This review is aimed at clinicians appraising preclinical trauma studies and researchers investigating compromised bone healing or novel treatments for fractures. It categorises the clinical scenarios of poor healing of fractures and attempts to match them with the appropriate animal models in the literature.

We performed an extensive literature search of animal models of long bone fracture repair/nonunion and grouped the resulting studies according to the clinical scenario they were attempting to reflect; we then scrutinised them for their reliability and accuracy in reproducing that clinical scenario.

Models for normal fracture repair (primary and secondary), delayed union, nonunion (atrophic and hypertrophic), segmental defects and fractures at risk of impaired healing were identified. Their accuracy in reflecting the clinical scenario ranged greatly and the reliability of reproducing the scenario ranged from 100% to 40%.

It is vital to know the limitations and success of each model when considering its application.